CARBON SEQUESTRATION IN BRITISH COLUMB~NSUB~OREALCUT BLOCKS by Thomas G. Pypker BSc., McMaster University, 1997 THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE lll NATURAL RESOURCES MANAGEMENT ©Thomas G. Pypker, 2001 THE UNIVERSITY OF NORTHERN BRITISH COLUMBIA April, 2001 All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. UNIVERSITY Of i~ORTHERN BRinSH COLUMBIA LIBRARY Prince George, BC Abstract During the summers of 1999 and 2000 measurments of component and ecosystem-level C0 2 fluxes were taken in seven cut blocks aged 0, 2, 3, 5, 6, 9 and 10years since harvest in the Aleza Lake Research Forest, near Prince George, British Columbia. A Bowen ratio energy balance (BREB) system was placed within a 5 to 6 year-old planted interior spruce (Picea glauca x Picea engelmanniz) clearcut to measure the ecosystem C0 2 flux during the 1999 and 2000 growing seasons. To substantiate the BREB measures of the ecosystem C0 2 flux a second method, called the component model, was established based on instantaneous measurements of component C0 2 fluxes (conifer, herbaceous plant, woody shrub and below ground) scaled up to the ecosystem-level. Both approaches indicated inter-annual variation in the growing season C0 2 fluxes. The 1999 growing season ecosystem C02 flux measurements estimated the clearcut to be a sink of -20 ± 43 g C m·2 and -86 g C m·2 from 27 June to 3 September, using the BREB method and component model respectivly. In contrast, in 2000 the growing season C0 2 flux during the same timeperiod was a source of 43 g C m·2 (BREB method) and 66 ±44 g C m ·2 (component model) during the same period. In 2000, an addtional 50 days of measurement, from 24 May to 20 September, indicated an even larger source of C0 2 totaling 143 ±57 and 103 g C m·2 using the BREB method and component model, respectively. However, regardless of the approach used, both years would have been sources of C0 2 if additional measures were taken for the entire year. The component fluxes were dominated by the C0 2 source from below ground (338 and 466 g C m·2 in 1999 and 2000, respectively) and the C0 2 sink provided by the deciduous plants (-382 and -365 g C m·2 in 1999 and 2000 respectively) . The conifer photosynthetic C0 2 uptake for the 5 to 6 year-old clearcut was meager in comparistion (-47 and -57 g C m·2 in 1999 and 2000, respectively). During the 2000 growing season, the below ground C0 2 flux and above ground biomass was measured in 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years and one mature stand. All sites were in the SBS wkl 08 or 07 except the 0 and 9 year-old sites that were of the sub series 01. The growing season below ground C0 2 flux was similar for all cut blocks, including the mature forest. Furthermore, there was little change in above ground biomass carbon uptake between the different aged cut blocks. Hence, when biomass was incorporated with the below ground C0 2 fluxes all cut blocks were sources for C02• Therefore, cut blocks within theAleza Lake region (SBS wk1 07 or 08) are sources of C0 2 for at least 10 years after harvest because the conifer component of the C02 flux is incapable of surmounting the loss of C0 2 from below ground. ll TABLE OF CONTENTS Thesis structure and overview ............................................................................................................................ ix CHAPTER 1- INTRODUCTION ............................................................................... 1 Introduction ........................................................................................................................................................... 1 References ........ ........ ..... .. ... ... ............................................................ .... ............ ..... ........ .... ... ....... ........ .... ......... 8 CHAPTER 2-BREB AND COMPONENT MODEL ESTIMATES OF GROWING SEASON C02 FLUX .............................................................................................. 13 The growing season carbon balance of a sub-Boreal clearcut five and Six years after harvest using two independant approaches to measure C0 2 flux •....••.....•.........••••.••................................................................... 13 Abstract .................... ... ........ ..................................................................... .. ... .............. ........ ............ ........ ........ 13 Introduction .......................... ............................................................................ .. ...... ............... .................. ..... 15 Methodology .... ... ................ .... ...... ............................................................. ....................... ....... .......... ...... ....... 20 Results ........... ........ ................... ... .......... ......... .. .. .. .. ..... ..................... ..................... ............... .............. ........ ..... 30 Discussion .................................... .... ....... ................................................. ... ........... .......................................... 36 References ... .... ......................................................................................................... ................. .............. ........ 43 Figures and Legends ....... ... .... ......................................... ... ....... .... ... ............ ... .... ........... .......... ................ ...... 59 CHAPTER 3: BELOWGROUND C02 FLUXES .................................................... 76 Below ground C0 2 Flux From Cut Blocks of Varying Ages in Sub-Boreal British Columbia ................. 76 Abstract ... .... ........... ........ ............................................................ ........ ............ .. ....... ... .. .. .............................. ... 76 Introduction .. ................................................................. ...... ................... ....... ... ............................... .. .. .. ......... 77 Material and Methods .. ........................................................................... ........... .. ............... ..... .... ....... .......... 80 Results ................... ................. ..... .. .................. ..... .................................. ... ... ..... ..... ................. ...... .................. 84 Discussion ........... .................. ... ... ............................................................ .. .. .... ............. ... ....... ............ .............. 86 References ............ ............ .................................... ..................................................... ..... ... ...................... ........ 9 1 Figures and Legends .... ..... ............ ... .. ....... ......... ......................... .............. .................... ......... .. .......... .... ........ 99 CHAPTER 4- SUMMARY AND CONCLUSIONS ................................................ 109 Summary and conclusions ............................................................................................................................... 109 Appendix 1..•.................................................................................••••••••................•...........••••...............•.................. I Appendix II. ..........................................................................................................................................................II LIST OF TABLES AND FIGURES Page Numb:r Chapter 2 Table 1a- Model regressions for 1999 .............................................................52 Table 1b- Model regressions for 2000 ............................................................. 53 Table2a- Biomass data for 1999......................................................................... .54 Table 2b- Biomass data for 2000 ...................................................................... 55 Table 3 - Growing season C02 flux estimates for 1999 ................................ .56 Table 4- Growing season C02 flux estimates for 2000 .................................57 Table 5- Comparison of the C02 flux for 1999 and 2000 ........................... .58 Figure 1 - Rainfall and Air temeprature for 1999 and 2000 ......................... 62 Figure 2 a,b - Soil temeperature and moisture for 1999 and 2000 ............... 63 Figure 3 a,b - Plant respiration rates in 1999 .................................................... 64 Figure 3 c-d - Plant respiration rates in 2000 ................................................... 65 Figure 4 a-c - Plant photosysnthesis in 1999 ....................................................66 Figure 4 d-f- Plant photosynthesis in 2000 ...................................................... 67 Figure 5 - Below ground respiration rates in 1999 and 2000 ......................... 68 Figure 6 a,b- Daily BREB and componet model C02 fluxes in 1999 .........69 Figure 6 c,d - Daily BREB and componet model C02 fluxes in 2000 ....... ..70 Figure 7 - BREB and component model C02 flux for 1999 ....................... .71 Figure 8 - BREB and component model C02 flux for 2000 ....................... .72 Figure 9 - Below ground C02 flux for 1999 .................................................... 73 Figure 10- BREB C0 2 flux estimates for 1999 and 2000 ........................... ..74 Figure 11 - Daily net photosynthsis for 1999 and 2000 .................................75 Figure 12- Below ground C02 flux for 1999 and 2000 ............................... ..76 Chapter 3 Table 1- Models of each cut block's below ground respiration .................. 96 Table 2- Biomass data for each cut block ....................................................... 97 Table 3 -Correlation between cut block and tower soil temperature ........ 98 Figure 1 - Instantaneous below ground C02 fluxes .................................... 101 Figure 2 - Soil temperature for each cut block throughout the season ... .1 02 Figure 3 - Relationship between soil temperature and respiration ........... 103 Figure 4- Soil moisture for each cut block throughout the season ......... .104 Figure 5- Total above ground biomass ......................................................... 105 Figure 6 - Cumulative below ground C02 flux ............................................ .1 06 Figure 7 - Above ground biomass and the below ground C02 flux ........ .1 07 Figure 8- Net ecosystem C02 flux ................................................................ .108 v ACKNOWLEDGMENTS The author wishes to thank his supervisor, Dr. Art Fredeen for his guidence, patience and instrution during data collection, analysis and writing. Furthermore, the author wishes to thank Shawn Dagenais, Benita Kaytor, Tyson Von den Steinen and Sandra Wawryszyn for their assistance in field sampling and laboratory analysis and Dr. Chris Hawkins and Dr. Peter Jackson for their help in research design. I would like to extend special thanks to Dave Coopersmith for providing our project with the Bowen ratio and Li-6200 systems. Funding for this study was provided by NSERC. I would also like to thank MikeJull, Dr. Paul Sanborn and Canadian Forest Products Ltd. for their assistance in finding suitable research sites. VI GLOSSARY Word BREB - Bowen ratio energy balance akg- K C Heat capacity of moist soil am· K Cd- Specific heat of dry mineral soil akg- K Cw- Specific heat of water akg- K 1 Cr - Heat capacity of air 1 ) 1 ) 3 5 - 1 1 1 ) 1 ) d- depth (m) G - Ground heat flux ('W ~ H - Sensible heat flux ('W ~ Kc - eddy diffusivity coeficient for C0 2 (m2 s·1) Kc eddy diffusivity coefficient for latent heat (m 2 s·1) K H - eddy diffusivity coefficient for sensible heat (m 2 s·1) LE - Latent heat flux ('W ~ Lv - Latent heat of vaporization Q ':- -Net radiation ('W akg- 1 ) ~ S - Stored energy below ground ('W ~ T- absolute air temperature (K) z - height (m) P- Bowen ratio (dimensionless) .. Vll £ - Ratio of the molecular weight of water vapour to dry air (dimensionless) em- Water content of soil on a mass basis (kg (H 20) kg-2 (soil)) p- Air density (kg m·3) . . ~ p v ~ Water vapour density (kg m·3) Vlll I THESIS STRUCTURE AND OVERVIEW For presentation purposes this thesis is organized into four chapters. Each chapter has a distinct purpose in addressing the overall question regarding carbon sequestration in subBoreal cut blocks. The two middle chapters (2 and 3) are stand alone papers while the first acts to introduce the topic of carbon sequestration and the last attempts to tie the results from the study together. The four chapters are: • Chapter 1 - Introduction • Chapter 2- BREB and Component Model Estimates of Growing Season C0 2 Flux • Chapter 3 - Below ground C0 2 Fluxes of Seven Cut Blocks and One Mature Stand • Chapter 4 - Conclusions and Summary Chapter 1 identifies the importance of studying carbon fluxes from cut blocks and provides a basic overview of the methods used to identify the size of the C02 fluxes from the seven cut blocks investigated. Chapter 2 examines, in detail, the size of the carbon flux over two growing seasons (27 June to 3 September 1999 and 24 May to 20 September 2000) in a 5-6 year-old clearcut. Two methods are contrasted and compared to identify the size and direction of the C0 2 flux for each growing season. Furthermore, ~ magnitudes of the C0 2 flux components were established for both growing seasons. It was noted in 1999 that the below ground portion of the C0 2 flux was substantial, causing the clearcut to ultimately be a source of C02• Chapter 3 investigates the impact below ground C02 fluxes have on cut block carbon budgets. The C0 2 flux and above ground biomass uptake for seven cut blocks aged 0, 2, 3, 5, lX 6, 9, and 10 years of age were compared and contrasted. Ultimately chapter 3 indicates how below ground C0 2 fluxes change with time since harvest and using the biomass values, estimates the carbon budget for each cut block. Chapter 4 serves as a concluding chapter. It compares and contrasts the results from chapters 2 and 3, and discusses the implication of the results on current global carbon budgets and management practices. X Chapter 1- Introduction INTRODUCTION In December of 1997, 160 countries, including Canada, signed the Kyo to protocol. Under the agreement Canada committed to reducing greenhouse gases emissions by 8% below 1990 levels by 2012. Recently, several nations including Canada, have argued that they should be given credit for C0 2 uptake by forest and agricultural processes in greenhouse gas emission calculations. While Canada argued for the implementation of the forest carbon credits during recent meetings at The Hague, there is little agreement within the scientific community regarding the costs and benefits of reforestation and afforestion on atmospheric C01 levels (e.g. Schulze et al. 2000). Proponents for carbon credits point to evidence of a possible C0 1 sink in the Northern Hemisphere (Schimel1995; Keeling et al. 1996; Myneni et al. 1997), arguing that part of the carbon sequestration is due to C0 2 uptake from regenerating forests. There is little doubt that forests absorb C0 2, and for a least part of their life cycle, take up more C0 2 than they lose. Furthermore, the terrestrial biosphere contains a large portion of the active carbon pool ~ (Falkowski et al. 2000) and how it interacts with the atmosphere will have an impact on C0 1 concentrations. However, for carbon credits to have any merit, the sinks created from timber harvesting and ~ i must be greater than if the forests were left undisturbed. Forests in British Columbia's Boreal and sub-Boreal forests may be harvested on a rotating basis that will result in a finite opportunity for C02 uptake. During the early years cut blocks are assumed to be sources of C0 2 because of reduced photosynthetic uptake. Both the time required for cut blocks to switch from a source to a sink 1 for C0 1, and the size of the C0 2 flux from the cut block during its early years, is critical to understanding the magnitude and the direction of fluxes from regenerating forests . Numerous complex models have attempted to quantify the C0 2 flux from forested stands (e.g. Birdsey et al. 1993; Burschel et al. 1993; Kolochugina & Vinson 1993b; Kolochugina & Vinson 1993a; Kurz & Apps 1993; Kurz & Apps 1994; Kurz et al. 1995; Turneret al. 1995; Cohen et al. 1996; Kurz et al. 1996), but they are based on broad assumptions and often do not properly incorporate disturbance events such as fire, insect outbreak, or timber harvesting (e.g.Burschel et al. 1993; Kolochugina & Vinson 1993b). Disturbance is of great importance for the forests around the world including those of British Columbia. Models of British Columbian forests that attempt to incorporate fire, insect outbreat and timber harvesting, are forced to rely on very broad assumptions. For example, Kurz et al. (1996), assumed that areas burned were evenly distributed among the four ecoclimatic provinces (Boreal, Cordilleran, Interior Cordilleran and Pacific Cordilleran), ignoring the fact that each region has greatly differing fire intervals. Likewise, timber harvesting was divided up evenly between 6 administrative units, ignoring the fact that different tree species, forest types, and age-classes are harvested at greatly varying intensities across those units. Post disturbance, assumptions are equally broad. For example, it is generally assumed that after a disturbance, forests advance through four stages: regeneration, immature, mature and over mature (Kurz & Apps 1994). During the first stage, regeneration, the site is recovering from disturbance and is assumed to act as a source of C0 1 for approximately 10 y (Kurz & Apps 1994). However, there are few in situ studies that actually verified the size and duration of the source of C02 from young regenerating stands. If timber harvesting results in forests that act as large sources of C0 2 over longer periods of time, 2 then the overall size of the col uptake from harvested stands would diminish and the argument for carbon credits would be weakened. Ecosystem-level C02 flux measurements for forest ecosystems following harvesting are still relatively rare. There have been several tower based ecosystem C02 flux studies in the subBoreal and Boreal forests (Fan et al. 1995; Black et al. 1996; Baldocchi et al. 1997;Jarvis et al. 1997; Goulden et al. 1998; Lindroth et al. 1998; Hollinger et al. 1999), but apart from the 14 day study by Valentini et al. (2000), none document C0 2 fluxes from young regenerating Boreal cut blocks or clearcuts. In the month of July 1996, Valentini et al. (2000) measured the C0 2 exchange above a 12 year-old regenerating Boreal forest in central Siberia and found it to be a small sink for C0 2 (-0.09 g C d·1) during two measurement periods Guly 6-15 and 21-26). Price & Black (1990), measured summertime C01 fluxes from a 22 year-old low-productivity coastal Douglas-fir cut block on Vancouver Island and found that on many days during the summer the nighttime flux of C02 negated any gains during the day. Therefore, information on C0 2 fluxes from young sub-Boreal cut blocks is greatly needed to validate assumptions used in large-scale models and to give a better understanding of the environmental controls on C02 fluxes in disturbed northern forests. This is especially critical if we are to contemplate moving to shorter rotation ages in Northern BC. There are currently three common methods for measuring the ecosystem C02 flux from tower based instruments: Aerodynamic, Bowen ratio, and eddy covariance. In this study we employed the Bowen ratio energy balance method, which, as with other tower based C02 flux methods, is based on a number of assumptions and restrictions. 3 The Bowen ratio method for measurement of ecosystem C0 2 flux The Bowen ratio energy balance (BREB) method attempts to distribute the net radiation (Q':-) into the sensible heat (H), latent heat (LE) and ground heat (G) terms using the Bowen ratio (p = H/LE) in the equation: H=(Q*-G) (1 + /3) (1) (For a full description of how the fluxes are partitioned see the methodology section in Chapter 2) The Bowen ratio energy balance (BREB) method is underlain by two fundamental assumptions. First, the eddy diffusivity coefficients for H, LE and C02 (Fc) must be equivalent (KH=KE=Kc). For this to be true, eddies moving entities up or down, assumed to be analogous to molecular diffusion, do not discriminate between the movement of sensible heat, H 20, or C0 2• Second, that there are no large fluctuations in radiation or wind speed/direction during the sampling period. Hence, conditions such as intermittent cloud negatively impacts the ability of the BREB method to measure C0 2 fluxes from a site because fluxes are temporally averaged over 20 to 30-minute intervals (Price & Black 1990; Steduto & Hsiao 1998). Nighttime measurements and the periods surrou.nding dusk and dawn are also problematic for the BREB method. The BREB method relies on strong eddies to move the entities from or to the ground. During inversions, air temperature and C0 2 and H 20 concentrations stratify, frequently resulting in an overestimate of C02 losses from a site (Price & Black 1990). At dawn and dusk the Bowen ratio typically approaches -1 causing equation 4 (1) to be indeterminate because the denominator approaches zero (Angus & Watts 1984). Furthermore, energy storage is sometimes ignored in the BREB method because the amount of heat energy stored in the canopy is negligible during the day. However, at dawn and dusk the values for LE and Hand net radiation (Q'') are small and the change in stored energy over time is relatively large resulting in inaccurate readings (Tanner 1960). The dependence on unidirectional eddies further inhibits the use of the BREB. Tall canopies produce turbulent eddies that results in counter-current transport within the canopy (Baldocchi et al. 1988; Oke 1992). For the BREB to be accurate the sensors must be sufficiently higher than the canopy to avoid being influenced by wake eddies. Therefore, the BREB ideally should be used on short canopies. The BREB is further limited by other restrictions on the physical parameters of a research site. There should be no divergence or convergence of airflow, such as from complex terrain. If such complex terrain is present the movement of fluxes to and from the ground will not be consistent across a site which may result in over or underestimates of BREB fluxes . Furthermore, at night during inversions, the air may flow downhill causing C0 2 respired during the night to by pass the sensors. Instrumentation should be within a homogenous stand of sufficient size to prevent advection of H, LE or F c from adjacent stands. If the fetch length (distance from adjacent stands to the BREB system) is not sufficient, sites upwind may influence the sensors. This will provide false readings and ultimately lead to inaccurate flux predictions. In general the fetch should be equal 5 to or greater than lOOm for every min instrument height (Horst & Weil1992; Stannard 1997). In cases of extreme advection even a large fetch length may not be sufficient to prevent influences from adjacent fields. This is especially the case with the advection of sensible heat to a site. In these situations, the Bowen ratio becomes negative and the KE/KH departs from unity 01 erma et al. 1978). Such advection events are often associated with high winds in areas that are semi-arid to arid 01 erma et al. 1978). To prevent inaccurate estimates of fluxes, wind speed should be measured to detect instances of extreme advection and the data rejected during these intervals. Extremely dry conditions preclude the use of the BREB method because LE is small and consequently, estimates of LE become inaccurate (Angus & Watts 1984). As Bowen ratios increase energy is increasingly dissipated by sensible heat loss and decreasingly via LE. The measured values of LE then begin to approach their error values. However, the BREB approach is a robust method for measuring fluxes when sufficient consideration is given to the constraints and assumptions mentioned (Tanner 1960; Spittlehouse & Black 1979; Held et al. 1990; Price & Black 1990; Steduto & Hsiao 1998). In addition, the BREB approach compares favourably with other techniques for the measurement of latent and sensible heat (Dugas et al. 1991; Rana & Katerji 1996; Ham & Knapp 1998). Substantiating BREB C0 2 flux estimates by scaling up from component C0 2 fluxes To substantiate as well as augment the Bowen Ratio system estimates of the ecosystem C0 2 flux, we utilized a completely independent approach that we have called the component model. An LI-6200 was used to measure the instantaneous component C0 2 fluxes of conifer and 6 deciduous plant photosynthesis and resp1rauon, and below ground resp1rat1on. The instantaneous component C0 2 flux measurements were extended temporally using multiple regressions based on instantaneous and continuous microclimate variables and scaled to ecosystem level using foliar biomass measurements made across the growing season. Thesis purpose In the summer of 1999 and 2000 we employed a Bowen ratio system in a 5 and 6 year-old clearcut within the Aleza Lake Research Forest. The Bowen ratio and component model C0 2 flux estimates of the 5 and 6 year-old clearcut C0 2 flux were done in conjunction with measures of below ground C0 2 fluxes and above ground biomass in seven cut blocks aged 0 to 10 years. The BREB method and component model measurements of two growing seasons, coupled with the measurements of C0 2 fluxes from multiple aged stands provided a unique view of the carbon status of young cut blocks in the Aleza Lake region. The overall intent of this thesis research was to establish when cut blocks in the Aleza Lake region of British Columbia revert to being sinks for C0 2 after harvesting, and to establish the magnitude and importance of the component C0 2 fluxes (deciduous, conifer and below ground) to the overall ecosystem C0 2 flux in these systems. 7 References Angus, D. & Watts, P. 1984: Evapotranspiration- How good is the bowen ratio method? Agricultural Water Management 8, 133-150. Baldocchi, D., Hicks, B. & Meyers, T. 1988: Measuring bioshere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69, 1331-1340. Baldocchi, D., Vogel, C. & Hall, C. 1997: Seasonal variation of carbon dioxide exchange rates above and below a Boreal forest. Agricultural and Forest Meteorology 83, 147170. Birdsey, R. A., Planinga, A. J. & Heath, L. S. 1993: Past and prospective carbon storage in United States forests. Forest Ecology and Management 58, 33-40. 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Journal of Applied Meteorology 17, 330-338. 12 Chapter 2-BREB and Component model estimates of gro w in g season C0 2 flux THE GROWING SEASON CARBON BALANCE OF A SUB-BOREAL CLEARCUT FIVE AND SIX YEARS AFTER HARVEST USING TWO INDEPENDANT APPROACHES TO MEASURE C0 2 FLUX Abstract While northern forests, including those in northern and central British Columbia, have come under increasing harvesting pressure in recent decades, the impact of this on global atmospheric carbon dioxide (COJ concentration is poorly understood. It is often assumed that after forest harvesting, clearcuts remain a source for C0 1 for many years. However, few studies have actually quantified the in situ fluxes of C0 2 from young clearcuts. To help address this issue, a Bowen ratio energy balance (BREB) system was deployed in a sub-Boreal clearcut (SBS wk1 08) 5 and 6 years after harvesting, approximately 80 km east of Prince George, British Columbia (54°01'30" N and 122°07'30" W). The BREB system continuously monitored net ecosystem C0 2 fluxes from the clearcut from 27 June to 3 September 1999 and 24 May to 20 September 2000. A second independent method was also used to estimate ecosystem C0 2 flux (the component model approach). In the second approach, component fluxes including plant (spruce seedling and representative deciduous species) and below ground fluxes of C0 1 were measured separately, correlated with microclimate variables, and then scaled to the ecosystem level using regression equations coupled with biomass estimates from the clearcut. 13 The two approaches predicted very similar flux during the daytime periods but their estimates diverged at night. Over the study periods BREB and component model approaches predicted that the clearcut was a small sink for C0 2 in 1999 (27 June to 3 September) and a source in 2000 (24 May to 20 September). Specifically, in 1999, the BREB method and the component model estimated the site to be a sink of -20 ± 43 g C m·2and -86 g C m·2, respectively. In contrast, in 2000 the clearcut was a source of 143 ±57 and 103 g C m·2 using the BREB and Component model, respectively. The main components of the C0 2 flux within the regenerating clearcut were below ground respiration and deciduous plant photosynthesis. The conifer seedlings were only a minor component in overall C0 2 flux over the growing season, removing only 47 and 57 g C m ·1 in 1999 and 2000 respectively. In contrast, the herbaceous and woody shrubs removed 376 g C m ·1 in 1999 and 365 g C m ·2 in 2000. The below ground respiration counterbalanced much of the carbon uptake during the study period by releasing 338 (1999) and 587 (2000) g C m·2• The small overall sink predicted for the clearcut in 1999 for the approximately 2.5 month growing period would likely be surmounted by the below ground respiration if annual C0 2 fluxes are considered. For example, an additional68 g C m ·2 was added to the atmosphere from Sept. 3 to 23 (based on below ground respiration data only) after the deciduous plants had senesced. This source alone would nearly be enough to push the clearcut from a sink to a source for C0 2• Therefore, even though there was variability in the carbon uptake during the two growing seasons, the young clearcut was a source of C0 2 for both years. 14 In summary, the results suggest that while sub-Boreal clearcuts in the SBS wk1 08 may already be sinks for C0 2 over some growing seasons. However, based on previous work, the clear cuts will continue to be a source for C0 2 for more than 5 years if nongrowing season sources of CO J are considered. In addition, the results clearly show that the deciduous component of these northern clearcuts (often collectively called brush in the forest industry) is very important for at least the first 6 years after harvesting in counterbalancing the large C0 2 efflux from below ground. Introduction Within the last two centuries, anthropogenic C0 2 emissions have increased the level of atmospheric C0 2 from 280 ppm to over 360 ppm (Keeling et al. 1996). This rise in atmospheric C0 2 has largely been the result of three sources; burning of fossil fuels, cement manufacture and landscape modification. Landscape modification alone explains approximately 50% of the rise in atmospheric C0 2 prior to 1980 (Woodwell et al. 1983) and 23% of the rise in the 1980s (Schimel1995). Most of the rise in C02 resulting from landscape modification can be accounted for in timber harvesting (Harmon et al. 1990) and conversion of forests to agriculture and pasture (Tans et al. 1990). Attempts to quantify the sources and sinks of C0 2 to the atmosphere have unveiled a large missing carbon sink thought to reside in the Temperate Zone of the Northern Hemisphere (Tans et al. 1990; Ciais et al. 1995). While the size of the missing sink is reasonably well established (1.5 to 2.0 Pg C yr·1 or approximately 50% of anthropogenically derived CO~ its 15 location has not been resolved (Post et al. 1990; Dale et al. 1991; Sarmiento 1993; Dixon et al. 1994). The inability of researchers to pinpoint the area(s) responsible for the accumulation of carbon displays a lack of information about the carbon in the atmosphere and its interaction with the biosphere. Several complex large-scale models have been created in an attempt to estimate the fluxes of carbon to and from the atmosphere (Birdsey et al. 1993; Burschel et al. 1993; Heath et al. 1993; Kolochugina & Vinson 1993a; Kolochugina & Vinson 1993b; Melillio et al. 1993; Kurz & Apps 1994; Turner et al. 1995; Cohen et al. 1996). While some of the models benefit from large databases describing stand age and vegetation (i.e. Kurz et al. 1996), many have the difficult task of estimating C02 fluxes without basic information on the vegetation cover types present. To fill in the gaps, modelers must make assumptions about the carbon budgets of the forests by extrapolation from a limited number of study sites (i.e. Birdsey et al. 1993; Kolochugina & Vinson 1993a; Turner et al. 1995). Disturbance events, such as disease, insect outbreak, forest fire and timber harvesting, are common in many of the world's forests and affect the accumulation of carbon to varying degrees. Currently, none of the models account for all disturbance effects or are based on very broad generalizations (i.e. Burschel et al. 1993; Kolochugina & Vinson 1993b; Kurz et al. 1996). Hence, black-box models (a black-box model is one where x predicts y but the processes involved are not understood) are created with little understanding of the processes involved. If black-box models are to incorporate actual processes (i.e. become grey-box models), there will be a need for greater empirical data. 16 The obvious impact of timber harvesting on forest carbon budgets is the removal of tree carbon and the loss of tree photosynthesis. With C0 2 uptake reduced, it is generally assumed that forest ecosystems will become a source for carbon to the atmosphere and only when the trees or associated vegetation have been reestablished will the sites revert back to being a carbon sink (Kurz & Apps 1994). The environment (both climate and microclimate) that the seedlings grow in is crucial in determining when a clearcut will change from a source to a sink of C0 2• For example, waterlogging or drought-stress may restrict forest regrowth, prolonging the length of time the site acts as a source for C0 2• The impact soil has on the overall carbon budget is poorly understood because information regarding the world's soil carbon pools are limited. Currently Post et al. (e.g.1982) and Zinke et al. (e.g. 1984) are the main sources for information on world soil carbon stores. A pool of between 65 to 104 Pg carbon, or eight times the amount of carbon stored in the plant biomass, is stored in the soils of the Boreal and sub-Boreal forests. How clearing of forests impacts the below ground respiration may well determine when sites convert from being a source to a sink for carbon. It is often assumed that soil carbon is reduced in the years following a clearcut because of reduced liner from the canopy (Covington 1981; Federer 1984; Kawaguchi & Yoda 1989; Yarie 1993; Olsson et al. 1996; Pennock & van Kessel1997) . However, most studies indicate only a small change in soil carbon stores following tree harvest unless the site is exposed to an intense burn Gohnson 1992). Studies differ on the effect tree harvesting has on below ground respiration; some indicate an increase in respiration (Ewel et al. 1987a; Gordon et al. 1987; Lytle & Cronan 1998), while others indicate no change in respiration (Fernandez et al. 1993; Toland & Zak 1994) and still others note a reduction in respiration (Edwards & Ross17 Todd 1983; Weber 1990; Chang & Trofymow 1996; Striegl & Wickland 1998). The varying responses indicate that forest soils do not respond to clearcutting in a simplistic manner. Sitespecific conditions may well control how an area's carbon budget responds to disturbance. The main microclimate controls on the below ground C0 2 flux are moisture and temperature (Kucera & Kirkham 1971; Fernandez et al. 1993; Striegl & Wickland 1998). The microorganisms responsible for decomposition of organic matter generally demonstrate a positive relationship to moisture and temperature. However, if temperature or moisture becomes excessive respiraton may decrease. Therefore, the microclimate changes resulting from forest harvesting will influence how belowground respiration responds. The removal of trees lowers evapotranspiration and may increase solar radiation at the soil surface (Lewis 1998). The change in the soil's microclimate may increase and or decrease soil moisture and temperature, depending on factors such as time of year, slope, aspect and vegetation. The harvesting of trees may lower below ground respiration because root respiration can constitute over 50% of the predisturbance below ground respiration rates (E wel et al. 1987b; Fernandez et al. 1993; Londo et al. 1999). Furthermore, silvicultural practices employed may influence the respiration rates of the soil. Burning slash may remove much of the soil organic layer, and if severe enough, kill the soil microorganisms (Pietikainen & Fritze 1993; Chang & Trofymow 1996). Other impacts of timber harvesting, such as soil compaction due to machinery, may result in reduced aeration of the site leading to a decrease in soil microorganisms and/ or aerobic respiration (Chang et al. 1995). Hence, it is not surprising that 18 previous studies have not demonstrated a consistant response of soil respiration to timber harvesting. H ow vegetation and soils are affected by climatic change and forest management will influence the carbon budget of a regenerating clearcut. Rapid regrowth of vegetation following forest harvesting may mitigate the efflux of C0 2 from below ground respiration. However, if management, climate change or microclimate factors negatively impact revegetation of a clearcut, the C0 2 budget could be dominated by below ground respiration, thus delaying a site's transition from source to sink for carbon. In this chapter, I attempt to quantify the growing season net C02 flux for a sub-Boreal clearcut 5 and 6 years after harvesting in central British Columbia using two independent methods. The Bowen ratio energy balance (BREB) method is contrasted with another independent estimate whereby component fluxes are individually measured and modeled using microclimate data and scaled to the ecosystem level using biomass information for the site. There has been very little research on the overall fluxes of C0 2 from very young disturbed forest sites. It is the intent of this chapter to not only establish whether sub-Boreal clearcut sites in the SBS wkl 08 are likely to be a source or a sink for C0 2, but also to determine the relative importance of the component fluxes from soil and plants on the overall carbon budget of such sites. 19 Methodology The site characteristics C0 7 flux measurements were from 27 June to 3 September 1999 and 24 May to 20 Sept. 2000, in a clearcut in the University of Northern British Columbia/University of British Columbia Aleza Lake Research Forest (54°01 '30" Nand 122°07'30" W). An 84.15 ha stand, in the subBoreal spruce wet cool sub-zone that was sub-hygric and nutrient rich (SBS wk1 08), was harvested in 1994. Prior to harvesting the clearcut contained a combination of hybrid white spruce and subalpine fir (!!hies lasiocarpa) with an understory of Ribes lacustre, Lonicera involucrata, Viburnum edule, Oplopanax horridus, and Rubus idaeus. After mounding, the cut block was planted with two year-old hybrid spruce (Picea glauca x Picea engelmanniz) with a smaller component of lodgepole pine (Pinus contorta var.latifolia) in spring, 1995. The soil is classified as anOrthic Luvic Gleysol (Arocena & Sanborn 1999). Based on the site prescription and a vegetation survey conducted in 1999, the cut block contained 1200 spruce seedlings ha-1 along with a variety of natural deciduous plants dominated by Epilobium angustifolium, Spiraea douglasii spp. menziesii, R. idaeus, R.lacustre, Comus canadensis, L. involucrata, Equisetum arvense and a variety of grasses dominated by Calamagrostis canadensis. The site is cool and wet and has a relatively high snowfall when compared to other areas in the central plateau region of sub-Boreal British Columbia (mean annual air temperature- 1.7 to 5°C) (Meidinger & Pojar 1991). The site was well suited for the BREB approach. The regenerating canopy was short, with vegetation rarely exceeding 1.2 m in height. The dominant wind direction was from the southwest and the forest edge was well over 400 m from the tower in that direction. The tower had a minimum of 300m of fetch in all directions, satisfying the recommended 100m of fetch 20 for every meter in height suggested by others (Horst & Weil 1992; Stannard 1997). In addition, the site was relatively flat with a slope of approximately 1% overall. The 100 x 100 m measurement area was subdivided into a 20 x 20m grid network and the BREB tripod placed in the center grid square in 1999 and in the adjacent grid square to the south in 2000. Gridlines were also used as a trail network to prevent excessive trampling of the site. Measurement of ecosystem C0 2 flux Ecosystem C0 2 flux was measured using a commercial Bowen ratio system (023/ C0 2, Campbell-Scientific, Edmonton, Alberta) and Bowen ratio energy balance (BREB) approach. The C0 2 gradient was measured by means of a closed path CO/ H 20 infra-red gas analyzer (IRGA) (Li-6262, LI-COR, Lincoln, Nebraska} connected to intake tubes situated in arms off the instrument tripod. In 1999, the top arm of the Bowen ratio system was 2.5 m above ground level and the bottom arm 1.4 m directly beneath it. During the 2000 field season the top arm was increased to 2.8 m and the bottom arm 1.48 m directly beneath it. It is recognized that the arms are within the roughness layer of the canopy, but if the arms had been placed above the roughness layer, the fetch would have been inadequate in several directions. However, while not ideal, others have successfully measured C0 2 and H 20 fluxes within the roughness layer above the canopy (Raupach & Legg 1984; Denmead & Bradley 1985; Price & Black 1990). Flow rates through the system tubing were maintained at 700 to 800 ml min·'. Every two minutes the air drawn in through the intake tubes was reversed between the reference and 21 sample cells of the IRGA and 40 seconds was allowed for the pump to purge the IRGA between measurements. Average C0 2 gradients were calculated every 20 minutes. At the beginning of every hour, one cell of the IRGA was scrubbed to establish the absolute concentrations of C0 2 and H 20. The system was calibrated, at a minimum, two times per week. The energy balance for the site was determined based on BREB system measures of net radiation (Q''") and its components sensible heat (H), latent heat (LE) and ground heat flux (G). The Q ':- was measured every lOs with a tripod mounted Q7 REBS net radiometer (CampbellScientific) and averaged over a 20 minute interval. To establish the temperature gradient (sensible heat flux), two chromel-constantan fine wire thermocouples (75 !J.m) were placed at the end of each of the two arms. The ground heat flux was estimated using two heat flux plates (HFT-3, Campbell Scientific) each inserted 6 em below the surface of the ground in conjunction with thermocouple probes (TCAV, Campbell Scientific) placed 4 em above and 2 em below each plate. The four leads of the thermocouple averaged the temperature changes between 2 and 8 em of depth within the soil column. Calculation of energy and C0 2 fluxes All BREB calculations were performed using the computer program SPLIT (PC208 Software, Campbell-Scientific). In brief, soil heat flux (G) was calculated in three steps. First, soil characteristics were used to calculate the heat capacity of the moist soil (CJ: Cs = Pb(cd + Bmcw ) (1) 22 where Pb is the bulk density of the soil, cd is the specific heat of a dry mineral soil, 8mis the water content on a percent mass basis and Cw is the specific heat of water. Second, the amount of energy stored between the thermocouples (K. m 2 s·1) below ground (S) over each 20 minute interval was then calculated: where 6 T 5 is the change in soil temperature, dis the depth of the sensors and t is time. G was then calculated by summing the energy stored below ground (S) with the heat flux values provided by the heat flux plates (GJ. The movement of latent heat (LE), sensible heat (H), and C0 2 (F c), through the atmosphere above the clearcut were calculated based on the following equations: LE = -LvKE8p v I 8z H = - pCPKH8T I az (4-6) Fe = -Kc8pcl 8z Where KE, KHand Kc are the eddy diffusivities for water vapour, heat and C0 2 respectively, Pv is water vapour density, Pc is C0 2 density, pis air density, Cr is the specific heat of air, Tis the absolute air temperature and z is height. Using the universal gas law equations 4 and 6 can be transformed to: LE = -L K PM ,JJw V E TR az (l -S) F=K PMcac c TR 8z 23 where Pis pressure, Mv is the molecular weight of water, w is the mole fraction of water to air, R is the universal gas constant and Me is the molecular weight of C0 2 • The BREB system estimates the fluxes ofH, LE and Fe through a number of steps beginning with the simplified energy balance equation: Q* = H + LE + G (9) the similarity assumption (KH=KE=Kc) and the Bowen ratio. The Bowen ratio, defined as sensible heat divided by latent heat, was calculated by: fJ = H I L£ = CP8T I L v&8w (10) where aT is the temperature gradient Lv is the latent heat of vaporization of water, £ is the ratio of molecular weight of water vapour to dry air and 8pv is the water density gradient. Sensible heat was substituted in the energy balance equation with PLE solving for LE: LE = (Q*-G) I(l + /3) (11) The eddy diffusivity coefficient for the C02 flux (Kc) was calculated using the following equatiOn: K c = K H = (z, - z, )H I(T, - T, )pCP (12) The fluxes of C0 2 were corrected using the equations established by Webb et al. (1980). In keeping with biometeorological convention, positive flu;xes of F 0 LE and H indicate movement away from the surface and negative toward the surface. 24 Other meteorological measurements Further meteorological conditions were measured at the site. Wind speed and direction were estimated using a wind sensor placed at the top of the tower (at 2.8 min 1999 and 3.0 min 2000) (R.M. Young 03002-10 Wind Sentry, Campbell Scientific). Light levels were taken using a quantum sensor (Li-Cor quantum sensor, Li-Cor). In 1999, four thermocouples (chromelconstantan), two at 10 em and two at 15 em depth, were inserted into the soil to increase the spatial sampling of soil temperature for modeling purposes. In 2000, the four thermocouples were placed at 10 em beneath the surface to improve soil temperature sampling at that depth. The soil and light measurements were taken every minute and then averaged over 20 minutes. Air temperature and relative humidity were measured with a single probe (HMP35C, Campbell Scientific) every 10 seconds and then averaged over 20 minutes. In 2000, a rain gauge was added to the site (TE-525M, Campbell-Scientific) to continuously measure rainfall. During the 1999 field season rainfall values were taken from a meteorological tower located approximately 7.5 km away. All the data were stored on two data loggers (21X, Campbell Scientific) and transfered to a field-portable laptop computer as required. The meteorological data was compared to historical data provided by the Ministry of Forests, from 1952 to 1980 and 1993 to 1998. Conifer photosynthesis At the beginning of each field season, the average heights of the spruce seedlings were calculated and 20 seedlings that were within ± one standard deviation of the mean were randomly selected. A one-year-old branchlet and either a 3 year-old, 2 year-old or a second 1 year-old branchlet were selected for photosynthesis measurements on either the northern or 25 southern side of the seedlings in 1999. After finding the bulk of the biomass was in the needles less than or equal to 1 year of age (See Appendix I), and that there was little difference in the photosynthetic rates between those needles, only 1 year-old needles were selected in 2000 for sampling. Photosynthesis and respiration measurements were made using a portable closed gas-exchange system (LI-6200, Li-Cor) on a weekly basis. Respiration measurements were conducted during the day by placing the branchlet in the dark and measuring the corresponding respiration rates. Microclimate variables of light (J.lmol PAR m·2 s·1) (Li-Cor quantum sensor, Li-Cor) , relative humidity, air temperature and atmospheric water vapour content were measured by the LI6200 while soil temperature (Reotemp instruments, San Diego, California) and soil moisture (Nie-Co-Product Nieuwkoop B.V., Aalsmeer, Holland) were made independently but simultaneous with gas-exchange measurements. These microclimate variables were later used in regression models to establish relationships that were used to predict carbon uptake (see below). Deciduous plant photosynthesis and respiration The deciduous plants on site leafed out around 31 May in both 1999 and 2000. Throughout the field season photosynthetic and respiration rates of E. anguSt:ifolium, S. douglasii spp. menziesii, and L. involucrata, were measured with the Ll-6200 (as above) under varying microclimate conditions. The plants were randomly selected on each measuring day within the study area. Area-based photosynthesis and respiration were based on one side of the leaf area as determined by leaf traces on transparent acetate sheets. 26 Soil C0 2 flux measurements Six pairs of PVC collars (9 .55 em in diameter) were placed in randomly selected grid squares throughout the measurement area. Measurements were taken using a Li-6200 with the soil chamber attachment (6000-09, Li-Cor) as in Norman et al. (1992). During the soil measurements both soil temperature (6000-09TC, Li-Cor) and moisture (as above) were taken at a depth of 10 em. Biomass On July 7-8 and August 17-18 1999, the above ground living biomass for the deciduous plants was destructively sampled in 24 randomly selected 1 m 2 plots within the measurement grid. During the 2000 field season, 12 randomly selected 0.5 m 1 plots of above ground living biomass were destructively sampled six times across the growing season. The increase from two sampling dates in 1999 to six in 2000 was an attempt to better characterize the changes in leaf biomass throughout the growing season. On 25 September 1999, 20 randomly selected P. glauca x P. engelmannii seedlings were destructively sampled. The seedling branchlets were further divided up by individual needle age classes. In early May and late September of 2000, a further 20 seedlings were destructively sampled. All the biomass samples were dried at 65 °C for 72 h and weighed to the nearest 0.01 g. 27 System flux estimates based on component fluxes Extrapolations from instantaneous component fluxes to the ecosystem scale were made using regression relationships between component gas-exchange and microclimate data. The ecosystem flux was divided into four components: conifer, herbaceous deciduous plant, woody deciduous plant (shrub) and below ground respiration. Multiple regressions were established using the best subset method. The significant variables (p=0.05) found using this method were used in the models to predict fluxes. The regressions provided estimates of the C0 2 fluxes in llmol C0 2 m·2 s·1• The rates were given in m 2 of leaf surface area or soil surface area. El The component model required four steps to scale from the leaf to the clearcut. First, the leaf area for each component was correlated to biomass (conifer, herbaceous, and woody plant). Second, the relationship between leaf area and biomass was used in conjunction with the estimates of leaf biomass of the clearcut (g ~ taken throughout the study period to estimate the leaf area index for each plant biomass component. Third, the estimate of C0 2 uptake was made using the regressions and the data from the micometeorological tower for each component. Finally, the estimates from the regressions were multiplied by the leaf area calculated in the second step, producing the C02 flux estimates for the different plant components. When modeling the plant photosynthetic and respiration rates it was assumed that 15% of net primary production (NPP) of the leaves was.lost to stem respiration (Ryan et al. 1994; Levy & Jarvis 1998). The below ground C02 flux estimates were already in m 2, hence, the C0 2 flux was estimated based on the regressions and the micrometeorological values given by the tower. Upon completing the flux estimates for each component, the overall flux of the site was estimated for every 20 minutes using: 28 F e = F conifers + Fherbaceousp/ams + F wood_l'plams + F beloll'ground Corrections of BREB data In order to predict a continuous C0 2 flux for the cut block various corrections of the BREB data were necessary. The clearcut itself, with its shallow slope and large area of fairly uniform terrain and vegetation, was suitable for the BREB approach. However, the data did suffer from the typical problem of poor estimates for dusk, dawn, some nighttime periods as well as rainy days (Tanner 1960). The problem periods around dusk and dawn were common but occurred over a short time period, often for less than an hour. Hence, these values were systematically replaced by extrapolating from BREB C02 fluxes immediately surrounding the problem periods (Baldocchi et al. 1997; Ham & Knapp 1998). In contrast, during longer periods of equipment failure or extended rain events, a regression between light and the C0 2 fluxes for the days adjacent to the problem period were created and used as a predictor of the C0 2 fluxes. The regression was then used to fill in the gaps in the data (Ham & Knapp 1998). Of the total measurement days, 30% and 9% required the use of regressions to fill problem or lost data in 1999 and 2000, respectively. Nighttime C0 2 fluxes were occasionally over estimated because of stratification of the atmosphere as evidenced from high C0 2 concentrations and low wind speeds at the tripod level. Stratification is problematic because the assumption of equivalence of eddy diffusivity coefficients may not hold true 01 erma et al. 1978; Angus & Watts 1984). In addition, the small Q':· value associated with nighttime flux predictions can further exacerbate the problem by inaccurately predicting the eddy diffusivity coefficient resulting in an exaggeration of the C0 2 29 flux (Price & Black 1990). The Component model estimates of ecosystem flux were used in place of BREB nighttime estimates when stratification occurred. Error analysis The insturment errors were used in conjuction with the standard errors of nighttime component model estimates and the daytime regression estimates to produce 95% confidence limits for the BREB method. Error analysis for the BREB method was based on the instrument errors with the exception of the soil heat flux plates. Due to the spatial variablility of the soil, the error was assumed to be 20%. Results Physical environment The summers of 1999 and 2000 were cool with few episodes of warm weather (Fig. 1). The mean daily air temperatures for each month ranged between 7.5 oc (May) to 15 oc (August) in 1999 and 8.5°C (September) to 14.5 oc Guly) in 2000. The monthly mean temperatures were significantly cooler than the historical record (1952-1980) in May 1999, August 2000 and July and September in both 1999 and 2000 (p = 0.005). The rainfall in 2000 was significantly greater than the historical record (1952-1980) in July and significantly greater than the record from 1993-1998 in July, August and September (p=0.05). The rainfall data for 1999 could not be compared because of damage to the rain gauge from late June to August 14, 1999. However, when the 2000 meteorological data is compared to that of 1999 there was no statistical difference in air temperature (Fig. 1). The rainfall for the months of May and September were similar for 1999 and 2000, but the rainfall in the months of June and August may have been 30 greater in 2000. While not statistically significant, the greater rainfall in 2000 may have translated into higher soil moisture levels (Fig. 2). Microclimate variables and the ecosystem component C0 2 flux The component fluxes correlated well with microclimate variables for 1999 and 2000 (Table 1a,b). Conifer net photosynthesis correlated well with the microclimate variables oflight, soil temperature and air temperature in 1999 (R 2 = 0.76) and microclimate variables oflight, soil temperatures and relative humidity in 2000 (R 2 = 0.72) (Table 1a,b). In 1999, predictions of photosynthesis in herbaceous plants, based on photosynthetic measurements on E. angustifolium, required two separate regressions. Light, soil moisture and soil temperature were used to predict daytime C0 2 fluxes. A second equation for the herbaceous plants was created for estimations subsequent to 7 August 1999 following a marked decrease in photosynthesis at that time. In 2000 a single equation adequately represented daytime col uptake by herbaceous plants. Woody shrubs were represented by S. douglasii spp. menziesii in 1999 and L. involucrata in 2000. Unlike the other plants, the regression for S. douglasii spp. menziesii represented the C0 2 flux for both day and night. Instantaneous measurements of dark respiration for the plant species in 1999 and 2000 correlated significantly with air temperature alone in three of the four plant species (P. glauca x P. engelmannii , E. angustifolium and, L. involucrata) (Fig. 3 a-e). The relationship was hyperbolic or linear with maximum respiration rates ranging from 2.5 ~ 4.5 ~ C m·1 s·1 for L. involucrata to C m·2 s·1 for P. glauca x P. engelmannii and E. angustifolium. 31 In contrast to 1999, the net photosynthesis in, P. glauca x P. engelmannii, L. involucrata and E. angustifolium appeared to correlate more strongly with light level in 2000 (Fig. 4). The 2000 instantaneous C0 2 flux for P. glauca x P. engelmannii and E. angustifolium saturated at approximately 500 ).lmol PAR m·2 s·' and had compensation points of about 70 )lmol PAR m· s·'. Photosynthesis of L. involucrata exhibited a parabolic relationship to light with maximum 2 rates at approximately 1000 ).lmol PAR m·2 s·' and compensation point below 40 )lmol PAR m· 2 s·'. The maximum instantaneous C0 2 flux typically resided at 13 ).lmol C m·2s·' for P. glauca x P. engelmannii, 16 ).lmol C m·2s·' for E. angustifolium and 12 ).lmol C m·2s·' for L. involucrata. The instantaneous below ground C0 2 fluxes correlated with soil temperature at 10 em depth in 1999 and 2000 (Fig. 5 a, b). A correlation between instantaneous below ground C0 2 flux and soil moisture was significant in 1999 only (p=0.05). Instantaneous below ground C0 2 flux ranged from 1.2 )lmol C m·2 s·' at 3.2 °C to 9.2 ).lmol C m·2 s·' at 15.9 oc. The relationship was essentially linear in 1999 and exponential in 2000 with R 2 values of 0.66 and 0.7, respectively (Table 1a,b). Biomass Based on biomasss, the deciduous plants were a more important sink for C0 2 than the coniferous plants for both 1999 and 2000. The deciduous plants began to leaf out on approximately June 1 and by the middle of August all the plants had accumulated over 189 g C 2 m · (not including allocation to stem biomass of woody shrubs in 1999) of aboveground biomass in 1999 and 27 6 g C m·1 in 2000(T able 2a, b). By the end of the growing season the 32 conifers had only allocated 10.6 and 18.6 g C m·2 of new growth to stems and needles in 1999 and 2000, respectively. Of total carbon allocated to above ground plant parts, > 90% was resident in deciduous plant above ground biomass both years. The BREB method and component model estimates of ecosystem C0 2 fluxes The component model agrees with the BREB method during the day, but their estimates frequently diverged at night. Typical C0 2 flux estimates for the clearcut from the component model and BREB methods are shown for 4 July and 29 July 1999 and 15 June and 17 August 2000 (Fig. 6a-d). For sake of contrast, and to illustrate the problems with nighttime measurement, the BREB nighttime fluxes were not corrected using component model data in these figures. However, by substituting the component model nighttime estimates for poor predictions by the BREB method, the predicted C0 2 efflux from the site is much diminished resulting in both approaches predicting similar daily C0 2 losses/ gains for the clearcut (Fig. 7,8). After making nighttime corrections to the BREB data, both the component model and BREB approaches predicted the clearcut to be a sink from 27 June to 3 September 1999 (Table 3) and a source from 24 May to 20 September 2000 (Table 4). Component C0 2 fluxes indicated that herbaceous and woody plants absorbed the bulk of the 'Carbon removing -212 and -165 g C m·2 respectively during the 1999 study period and -315 and -172 g C m·2 respectively in the 2000 study. In comparison, the conifers only removed an estimated -47 and -96 g C m ·1 in 1999 and 2000 respectively. Below ground respiration acted as a constant C0 2 source throughout the season. From 27 June to 3 September 1999 and24 May to 20 September 2000 33 1 the roots and soil flora and fauna released 338 and 686 g C m· respectively , thereby counteracting most or all of the carbon sink generated via plant photosynthesis. Below ground C0 2 flux Below ground C0 2 efflux was sizable both during both growing seasons (between 4 to 8.2 g C m ·1 d·') (Table 3-4) and for measurement dates following the the last BREB measurement in 1999 (between 3.2 to 4.2 g C m·2 d·') (Fig. 9). Following the 1999 BREB measurement period, below ground respiration for Sept. 4 through 23, a time when herbaceous plants were without foliage and woody plants mostly senescent, contributed a further 68 g C m·2 to the atmosphere (Fig. 9), enough to convert the site into an overall source for C0 1• Change from sink for C0 2 to source for C0 2 in early August The clearcut experienced a divergence between the BREB and component model estimates of daily ecosystem C0 2 flux in early August for both growing seasons (Fig. 7,8). Prior to the down turn in photosynthetic uptake of C0 2 in 1999, the BREB approach predicted mean daytime C0 2 fluxes (mean = -0.07 mg C m·2 s·') that were only 0.01 mg Cm·2 s·' greater (more positive) than the component model (mean = -0.08 mg C m·1 s·'). After 7 August 1999 the mean daytime C02 fluxes were -0.046 and -0.079 mg C m·1 s·' for the BREB and component model, respectively. This resulted in a significant change in mean difference between the two approaches of 0.033 mg C m·2 s·'. Hence, a new equation was created with a negative relationship to day of year and time of day was created to estimate the daytime col flux of the herbaceous plants after 7 August 1999. Similarly, a weaker divergence occurred in 2000. Prior to 10 August 2000, the BREB method and component model averaged -0.09 and -0.074 mg C 34 m -2 s-1, respectively. This resulted in a mean difference of -0.016 mg C m -2 s-'. Following 10 August 2000 the BREB method averaged -0.033 mg C m-2 s-1 and the component model averaged -0.049 mg C m-2 s-1 resulting in a mean difference of 0.016 mg C m -1 s-'. The change in early August is significant for both 1999 and 2000 at p < 0.001 at a 99% confidence level. Comparison of Ecosystem C0 2 Flux estimates from 1999 and 2000 Because of the extended measurement period in 2000, a direct contrast of growing seasons between years was not possible. However, in the shared interval from 27 June to 3 September, the clearcut was a sink in 1999 and a source in 2000 (Table 5). The size of the sink for the summer of 1999 was calculated at -86 g C m-2 using the component model and -20 ±43 g C m2 using the BREB method. In contrast, the 2000 data indicates the clearcut to be a source of 43 g cm-2 using the component model and 65 g C m-2 with the BREB method. The BREB daily C0 2 flux estimates for 1999 were generally lower (greater sink) than those in 2000 (Fig. 10). Both years had similar daily photosynthetic rates for the season (Fig. 11) and cumulative CO 1 uptake for the individual plant components (Table 5). The seasonal totals of photosynthetic C0 2 uptake were -424 C m -1 in 1999 and -422 g C m-2 in 2000. Herbaceous plants and conifers fixed more total C0 2 (increase of 21 and 9.3 g C respectively) in 2000 than 1999, while the woody shrubs had a slight drop in C0 2 uptake (decrease of 31.6 g C ~ In both 1999 and 2000, the deciduous plant contribution to ecosystem photosynthesis was much larger than the contribution from the conifers seedlings (Table 5). 35 The difference in the seasonal ecosystem C02 flux appeared to be driven largely by the below ground C0 2 fluxes to the atmosphere in the summer of 2000 (Fig. 12). From 27 June to 3 2 September 1999, the cumulative below ground C0 2 flux was estimated at 338 g C m· while over the same interval in 2000 the estimate was 466 g C m·2 , a 38% increase. There was no statistical difference between the soil temperatures at 10-cm depth for the same periods for 1999 and 2000 (p=O.OS, data not shown). Discussion BREB flux estimates The component model and BREB method compared well. Both methods predicted the clearcut to be a sink in 1999 and a source of C02 in 2000. The daytime estimates were similar (Fig 6a-d) and the daily fluxes were very comparable for both 1999 and 2000 (Fig. 7,8). The two approaches diverged in early August during both 1999 and 2000, with the BREB method predicting the site to be a larger source for C0 2 than the component model. In 1999, the model for the herbaceous plants was adjusted by incorporating day as a negative influence on C0 2 uptake, but it was unable to immediately bring the two approaches into agreement. The problem associated with the component model is its reliance on measured leaflevel C0 2 flux that were only measured once or twice every two weeks. A rapid change in photosynthetic uptake due to senescence, flowering and seed set, heat sttess or frost damage cannot be accommodated by the component model approach. As a result, the component model estimated the clearcut to be a larger sink for C0 2 than the BREB method. 36 Both approaches predict the site to be a sink for C0 2 for the 1999 growing season (Table 3). The component model's estimate is likely too large because of its inability to interpolate fluxes during the two weeks subsequent to Aug. 7. Prior to Aug. 7, the difference between the BREB method and the component model was only 0.011 mg C m ·1 s-1, but after Aug. 7 the difference increased to 0.033 mg C m-2 s-1• While this appears to be a small difference, it was significant enough to change the site from a sink of -20 ±43 g C m -2 with the BREB method to -86 g C m -2 using the component model. Though it was necessary to dismantle BREB instrumentation prior to the onset of winter, components of the ecosystem flux, such as below ground respiration, would have undoubtedly continued and thus, contributed to the annual carbon budget of the cut block. For example below ground respiration emitted an estimated 68 g C m-2 between Sept. 3 and Sept. 23 in 1999 (Fig. 9) and this loss of C0 2 would continue at a reduced rate under the snowpack (Coxson & Parkinson 1987; Sommerfeld et al. 1993; Clein & Schimel1995; Evans et al. 1998). The snow insulates the soil, buffering it against the colder air temperatures thereby allowing for greater below ground respiration than would be generally indicated by air temperature (Bleak 1970; Moore 1983). A study conducted during the winters of97 /98 by Evans et al. (1998) within the Aleza Lake Research Forest at a site with an identical site series (SBS wk1-8) observed winter flux that ranged between 0.603 to 0.772 g C m-1 d-1• Others·have found similar winter C0 2 flux that ranged between 0.409 to 0.736 g C m-2 d-1 (Sommerfield et al. 1993) and 0.204 to 0.818 g C m-1 day- 1 (Coxson and Parkinson 1987) in southeastern Wyoming and southwestern Alberta respectively. Using the values from Evans et al. (1998) the modest sink from 27 June to 3 September results in loss of C0 2 from the site, roughly be estimated to be between 83 to 37 121 g C m·2 yr·1• The 1999 estimate contains four specific assumptions: C0 2 losses range between 0.603 to 0.772 g C d- 1 under the snowpack (Nov. 1 to April 30); fluxes linearly decreased to the winter flux in the fall (Sept.4 to Oct 31); fluxes decreased from the winter flux to a net uptake of -1.41 g C d-1 in the Spring (May 1 to June 1) when the site flushed (-1.41 g C d-1 is the average daily C02 flux from June 27 to July 5); after June 1 the flux remained at -1.41 g C d-1 until June 27 when the BREB flux estimates resumed. The loss of C0 2 in 2000 is only exacerbated by the winter C0 2 flux. The winter losses of C0 2 alone would raise the flux to approximately 229 to 265 g C m·2 yr" 1• The estimates demonstrate that the loss of C0 2 during the winter months is enough to surmount any gain by the plants during the summer months for this clearcut at this age. The component flux The conifer C0 2 uptake was similar to rates measured by others under a variety of conditions (Watts & Neilson 1978; Bassman 1989; Man & Lieffers 1997), typically ranging between 8 and 12 flmol C m·2 s·1 depending on the microclimate. The deciduous plants had much higher leaf area specific photosynthetic rates than the conifers, with rates typically ranging from 6 and 16 flmol C m·2 s·1 depending on the species and the micrometeorological conditions. Instantaneous below ground C0 2 fluxes were similar to those found by some researchers (Edwards & Sollins 1973; Ewel et al. 1987; Gordon et al. 1987; Russell & Voroney 1998), but higher than seen by others (Weber 1990; Fernandez et al. 1993; Lytle & Cronan 1998; Striegl & Wickland 1998) for various types of forests and cut blocks. The higher below ground C0 2 flux observed in this study may have been the result of a trend of higher respiration rates with increasing latitude (Valentini et al. 2000). In a study of European forests, Valentini et al. (2000) 38 found northern areas to have a greater ecosystem respiration rate, even though their soil temperatures were much lower. The Overall C0 2 flux in comparison to other northern forests The observed BREB method fluxes of 0.0545 to 0.213 mg C m·2 s·1 in this study, are similar to those found in the literature for young clearcuts and some mature stands. For example, Price 2 1 & Black (1990), found daily C0 2 fluxes to be between 0.0545 to 0.136 mg C m· s· for a young juvenile Douglas-fir stand. The main difference between the Douglas-fir stand (Price and Black, 1990) and the planted spruce clearcu studied here was that the former was a source for C0 1 for a longer period of the day. This may be the result of the shorter day length and and drier conditions at the lower latitude Douglas-fir site. Another study on a 12 year-old Boreal cut block in Siberia during July 1996, averaged -0.104 g C m·2 d-1 over a 14 day measurement period 0f alentini et al. 2000). In contrast, during the month of July at the Aleza Lake clearcut, ecosystem C0 2 flux in 1999 averaged -1.23 and -1.72 g C m·2 d-1 and in 2000 averaged 0.142 g C m·2 d·1 and 0.439 g C m·2 d·1 using the BREB method and component model, respectively . Therefore, the Aleza Lake cut block differed from the 12 year-old Siberian forest in both summers, being a greater sink in 1999 and a greater source in 2000. The maximum BREB C0 2 uptake rates in the clearcut were ~ mg C m·2 s·1 but it averaged approximately -0.114 and -0.12 mg C m·2 s·1at midday (27 June to 3 September) for 1999 and 2000 respectively. Mature stands in the Boreal forest tend to have equivalent or lower flux of C0 1 when compared to the clearcut in this study. Values of fluxes in high latitude forests range from maximum values of -0.155 mg C m·2 s·1 in an ecotonal Boreal forest 39 (Hollinger et al. 1999) to a minimum of -0.049 mg C m·2 s·1 in a Siberian larch forest (Hollinger et al. 1998). Maximum C0 2 uptake for Canadian Boreal forests average between :::::;-0.084 mg C m·2 s·1 Garvis et al. 1997) at a southern Saskatchewan black spruce site to :::::;-0.095 mg C m-2 s-1 in a black spruce-lichen woodland near Scheferville, Quebec (Fan et al. 1995)and a jack pine forest in central Saskatchewan (Baldocchi et al. 1997). A deciduous aspen site within the Boreal forest attained net C0 2 uptake rates of : : :;-0.24 mg C m-2 s-1 (Black et al. 1996) and this is closer to the maximum values found in this study. Deciduous plants performed the majority of the C0 2 uptake in the clearcut and they tended to have a greater ability to absorb C0 2 over the short term than the conifer seedlings. However, the growing season for deciduous plants is shorter because they are not able to take advantage of climatically favourable days in spring and autumn. Many northern forests often have a negative carbon budget. Studies indicate that northern forests are often tenuously balanced between being carbon source or sink. For example, Goulden et al. (1998), demonstrated that a Boreal black spruce site was a source for C02 over the span of one year (:::::;70 g C m-2 October 1994 to October 1995 and :::::;20 g C m-2 between October 1995 to October 1996), but a sink over a year from October 1996 to October 1997 (:::::;.- 10 g C m-1_). Furthermore, Lindroth et al. (1998), found that a forest in Sweden lost :::::;90 g C 2 m- between June 1, 1994 and May 31, 1995 and :::::;60 g C m -2 during the same period in 199596. 40 The Seasonal Ecosystem Flux for 1999 and 2000 The estimated ecosystem C0 2 flux for 27 June to 3 September 1999 and 2000, demonstrated a switch from source to sink for C0 2 then back to source again over the growing seasons. The variation demonstrated in the two years is not uncommon as illustrated by Goulden et al. (1998) above. Clearly, shifts in climatic conditions can impact the size of the ecosystem CO_, flux . For example, changes in the length of the growing season (Goulden et al. 1998) rainfall (Grieu et al. 1988; Baldocchi 1997; Cienciala et al. 1997), and soil/ air temperature 01 apaavuori et al. 1992; Harrington et al. 1994) can alter the photosynthetic C0 2 uptake and/ or the below ground C02 efflux from year to year. The difference in the overall ecosystem C02 flux, between 1999 and 2000, is not the result of reduced photosynthetic uptake. The modeled photosynthetic C0 2 uptake was remarkably uniform for 1999 and 2000 (Fig. 11). Furthermore, the biomass present on the site was very similar between the two years. The biomass values from 1999 totaled 235 g C m -1 in aboveground biomass compared to 276 g C m -2 in aboveground biomass in 2000 (woody shrub values for leaf biomass only). The main difference was an 18.6 g C m -1 increase in the aboveground conifer biomass and a 38 g C m-2 increase in herbaceous plants biomass. In contrast, the component models estimated a decrease in photosynthetic uptake of C02 of only 1.91 g C m-2 from 1999 to 2000 (27 June to 3 September): While there are discrepancies between the biomass and component model C0 2 flux estimates, the results do indicate that there was little change in the photosynthetic C0 2 uptake over the two growing seasons. 41 The below ground C0 2 flux increased greatly from 1999 to 2000 (Fig. 12). The component model, based on instantaneous measurements across the growing season, estimated the below ground C0 2 flux to be 338 g C m-2 in 1999 and 466 g C m-2 in 2000 (27 June to 3 September); an increase of 128 g C m-2• There were no distinguishable differences between the soil temperatures and an increase in root respiration seems unlikely given that there was only a small difference in aboveground biomass between years. While not statistically different, the soil moisture levels in 1999 were consistently lower in the mid summer months when compared to 2000 (Fig. 2b). Soil moisture was a significant variable in predicting below ground C0 2 fluxes in 1999. When soil moisture becomes limited its influence on the below ground C0 2 flux becomes greater (Londo et al. 1999), and in extreme situations soil moisture, demonstrates a greater importance than soil temperature (Parker et al. 1983). Summary Over the two study periods that roughly coincided with the growing season, the clearcut in this study acted as a small sink for C0 2 in1999 and source of C0 2 in 2000. However, if estimates for below ground fluxes for the entire year are considered, losses of C02 from the soil would easily exceed that taken up through photosynthesis at the site for both growing seasons. The conifer seedlings, because of their small biomass, were only a small contributor to the overall carbon budget for the clearcut. It was the deciduous plants that acted as the primary sink for C0 2 for the clearcut, even after 4 and 5 years of conifer growth in the field. Hence, if our goal is to minimize the accumulation of greenhouse gases in the atmosphere in the years immediately after harvest, mechanical (brushing) or herbicidal removal of deciduous ("noncrop") vegetation should avoided. 42 References Angus, D . & Watts, P . 1984: Evapotranspiration- How good is the bowen ratio method? 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(1984): Worldwide organic carbon and nitrogen data, Oak Ridge National Laboratory. 51 Table 1a- Multiple regression equations for prediction of component C0 2 fluxes below ground respiration and photosynthesis by plant type, day of year and time of day) in a 5 year-old clearcut in the Aleza Lake Research Forest, British Columbia Multiple linear regressions were established using the best subset method. Standard error of the estimate respresented by (SE). Component Flux Below ground Time of Year Jun. 27 to Sept. 23 Regression Equation (!lmol C m·2s-1) R = -0.953-0.354':-A+0.0198 ':-D 0.66 SE 0. 978 Conifer (Day) Jun. 27 to Sept. 3 P" = 1.61 + 0.22':-A + 1.8 ':-ln(B)-0.323 ':-c 0.76 2.74 Conifer (Night) Jun. 27 to Sept. 3 R = -0.056':-C / (1-0.0191""C) 0.72 0.915 Herbaceous Plants (Day) Jun. 27 to Aug. 7 P n = 7.89 + 0.666':-A + 1.88':-ln(B) + 0.0247':-C-0.0416':-E- 0.00729l""F 0.53 3.30 Herbaceous Plants (Night) Jun. 27 to Sept. 3 R = 0.534+0.13rA-0.16l""C-0.020YG 0. 81 0.465 Herbaceous Plants (Day) Aug. 7 to Sept. 3 P" = 70.4 + 2.48':-ln(B) + 0.496':-c -0.406':-E 0.78 +0.161G 2.87 W oody Plants Jun. 27 to Sept. 3 P" and R = 4.72 + 1.26':-ln(B)-0.239 ':-c 1.77 Key : A - soil temperature C - air temperature (0 C) D- soil moisture (%) E-day of year F- time of day G - relative humidity (%) ::·all regression equation variables significant at the 0.95 level 52 R2 0.77 Table 1b - The relationship between microclimate variables and the below ground C0 2 flux and photosynthesis and respiration in conifer, herbaceous plant and woody shrub components of a 6 year-old clear-cut within the Aleza Lake Research Forest, British Columbia. Multiple regressions were established using the best subset method. Standard error of the estimate respresented by (SE). Component Equation ().!mol C m·2s-1) R2 SE Below ground R = -0.092-3.647(Allo) 0.70 1.33 Conifer (Day) Pn = 0.613-0.0776A-0.019275B+0.2438C-17.12D-0.00000763(B) 2 0.54 3.04 Conifer (Night) R = -(0.0912C)/(1-0.0089C) 0.48 0.777 Herbaceous Plants (Day) Pn = 3.23-3.88ln(B)+0.0524E 0.72 2.79 Herbaceous Plants (Night) R = -(0.912C)/(1-0.0155C) 0.51 0.900 Woody Shrubs (Day) Pn = -10.9-0.0141B+O.OOOOOS(B/+0.553C-0.459F+0.0162G 0.78 1.57 Woody Shrubs (Night) R = -(0.0576C)/(1-0.0069C) 0.45 0.362 A- Soil Temperature (10 em depth) B - Light ().!mol PAR m·1 s·1) C - Air Temperature (0 C) D- Relative Humidity(%) E- Day of Year F - Absolute Humidity (O.lkPa H 20) G- Soil Moisture (%) ::--all regressions significant at the 0.95 level 53 Table 2a- Herbaceous and woody biomass for deciduous vegetation early Guly 78/ 99) and late (Aug. 17-18/99) in the growing season in a 5 year-old clearcut in the Aleza Lake Research Forest, British Columbia. The woody shrub estimates represent leaf biomass only. July 7-8 August 17-18 Biomass Change gm-2 gm-2 gm-2 Conifer':34.9 45.5 10.6 Herbaceous Plants 95 125 30 Woody Shrubs 48 64 16 Total 178 234 56.6 .. ·•esumated by subtractmg the we1ght of the new growth from the total weight in September Component 54 Table 2b- The amount of aboveground biomass present on May 10 and August 6, 2000, and the change in biomass between these dates, for a 6 year-old clearcut within the Aleza Lake Research Forest, British Columbia Component May 10 gm·2 45.5 Conifer -::Herbaceous Plants Woody Shrubs 140 Total 186 '··No bwmass present m the spnng 55 August 6 gm·2 64.1 163 234 461 Biomass Change gm·2 18.6 163 94 276 Table 3.- Comparison of growing season estimates of C0 2 flux from primary components using the component model from 27 June to 3 September 1999 with the BREB estimates in a 5 year-old clearcut at the Aleza Lake Research Forest, British Columbia. Component Model BREB Method gCm·2 gCm·2 Below ground 338 - Conifers -47 - Herbaceous Plants -212 - Woody Plants -165 - TOTAL -86 -20 ±43 Components 56 Table 4- Comparison of growing season estimates of C0 2 flux from primary components using the component model from 24 May to 20 September 2000 with the BREB estimates in a 6 year-old clearcut at the Aleza Lake Research Forest, British Columbia Components Component Model BREB Method gcm-2 gcm-2 Below ground 686 - Conifers -96 - Herbaceous Plants -315 - Woody Shrubs -172 - Total 103 143 ±57 57 Table 5 - The BREB method and component model estimates of the growing season CO 2 flux in a 5 to 6 year-old clearcut, over a comparable interval, from June 27 to September 3, in the Aleza Lake Research Forest, British Columbia Component 1999 2000 gcm-2 gcm-2 Component Model BREB Component Model BREB Below ground 338 - 466 - Conifers -47 - -58 - Herbaceous Plants -212 - -232 - Woody Shrubs -165 - -133 - Total -86 -20 ±43 43 66 ±44 58 Figures and Legends Figure 1 - The 1999 and 2000 growing season mean daily air temperature and rainfall contrasted with the historical mean daily air temperature and rainfall (1952-1980) for a 5-6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 2 a,b- The measured mean soil temperature (a) and moisture (b) in 1999 and 2000 for a 5-6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 3 a-e - The daytime respiration rates for coniferous and deciduous plant species in 1999 (Fig 3 a-b) and 2000 (Fig. c-e) for a 5 and 6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 4 a-f- The net instantaneous net photosynthetic C02 uptake in P. glaucaxP i~ S. douglasii spp. menesizii, L. involucrata and E. angustifolium versus light for the growing seasons of 1999 (Fig. 4 a-c) and 2000 (Fig. ~ for a clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 5 a-b- Instantaneous measures of below ground efflux of C0 2 for the growing seasons of 2000 (a) and 1999 (b). Measurements were taken in a 5-6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. 59 Figure 6 a-d - A comparison of C0 2 flux estimates of the BREB method and component model for selected days in 1999 (Fig. 6 a-b) and 2000 (Fig. 6 c-d) for a 5-6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 7- A comparison between the BREB method and component model estimates of C0 2 flux for 27 June to 3 September 1999. Measurements from a 5 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 8- A comparison between the BREB method and component model estimates of the C0 2 flux for 24 May to 20 September 2000. Measurements were made in a 6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 9- The estimated below ground C02 flux for 27 June to 23 September 1999. In the 20 days following the last BREB measurement the site was estimated to lose a further 68 g C ~ Estimated flux for a 5 year-old clearcut located in the Aleza Lake Research Forests, British Columbia. Figure 10- The BREB ecosystem C02 flux estimates for a young clearcut (age 5/6 years since harvest) during the 1999 and 2000 growing seasons in the Aleza Lake Research Forest, British Columbia. 60 Figure 11-The component model estimates of daily photosynthetic C0 2 uptake for the 1999 and 2000 growing seasons Gune 27 to September 3) in a 5-6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. Figure 12- The daily below ground C02 flux during the 1999 and 2000 growing seasons Gune 27 to September 3) in a 5 to 6 year-old clearcut located in the Aleza Lake Research Forest, British Columbia. 61 (a) 140 120 E' 100 .s 80 (ij LL. c:: 'iii a::: 60 40 20 0 M J J Month A s ==:;'1952-80 Rainfall 11999 Rainfall --2000 Rainfall - · - · 1952-80 Temperature · · · · · · 1999 Temperature --2000 Temperature C ::- data missing for July 1999 Figure 1 62 18 16 14 12 10 8 6 4 2 0 u ~ ....Cll ::I ....C'CI Cll c. E Cll 1- .... ~ -.... 20 18 Q)..c ::::::10. 16 ro-oI 14 ~ .... Q) E Eo Q)-rr-..., ~ Q) Q.() :-=CU - 0() (J)o (b) 12 10 ~ 8 6 4 5/1 ~ 6/1 l p2 ,777 ! B £ ~ • 2000 1999 0 0 0 2 0 ' ! 7/1 8/1 9/1 10/1 Date 0~ Q)"~ 0 ::::::1 (/) i ~ 0""0 ·--oo ~ - (f) ..Y. II N - 0> ..Y. 100 80 60 40 • (c) 2000 1999 0 tff t!HH i 2 H1 20 0 05/1 06/1 07/1 08/1 Date Figure 2 a,b 63 09/1 ?f 10/1 10 (a) 1999 P. glauca x P. engelmannii • •• 8 6 I • • •• • • •• • •• • .. ••• • • 4 2 c - 0 . • • ..I •• ., ... CJ) ON 1U E •- I I- ·a.. 0 CJ) (]) (!: 0 E :i __.. 10 8 5 0 10 15 20 (b) 1999 25 30 35 E. angustifolium 6 4 2 0 0 5 10 15 20 Air Temperature (oC) Figure 3 a-b 64 25 30 35 6 5 (a) P. glauca x P. enge/mannii 2000 4 3 2 1 0 c - ...... I en ON · - -ro L- I E ·a..() enQ) 0::: 4 3 •• • • ~ 10 15 0 E 1 -0 ~ • • 710 15 6 (e) E. angustifolium 35 • •••••• • • 2000 • • 30 25 20 (b) L. involucrata 2 • • •• • • :.i. • 20 25 30 35 30 35 2000 5 4 3 2 1 0 'Y"' 'Y 10 'Y 15 Figure 3 c-e 20 25 Air Temperature (oC) 65 30 ~ 25 20 15 10 (a) 1999 5 0 P. glauca x P. engelmannii -5 ~~ 30 ~ CJ) CJ) 25 C NCJ) 20 ~ en E 15 u 10 ..c I ~ -5 ..c0... 0 c :t __. E cu- 0 ~ ~ 500 1000 ~ ~ 1500 2000 ~ 2500 ~ (b) 1999 • • 5 0 0 25 20 15 10 500 1000 1500 2000 (c) 1999 ..,. 5 0 -5 ~~ 0 E. angustifolium ~ ~ 500 1000 Light Figure 4 (a-c) ..,. ~ ~ ~ 1500 2000 2500 (J.tmOI PAR m- 2 s- 1 ) 66 67 12 (a) 2000 10 8 >< ::J ~ w ..--...- 0 N 0 "'0 c ::J 0 C) ~ ~ 0 Q) co I CJ) N I E 0 0 E ::i. _.. • 6 • 4 • •• • •• • 2 0 2 12 • • • 4 12 10 8 6 • • • • • • • •• • •• • 14 16 18 (b) 1999 10 8 0 6 oo DO 4 2 0 2 4 Figure 5 a-b 6 8 10 12 14 Soil Temperature (°C at 10 em depth) 68 16 18 20 (a) 4 July 1999 24 • BREB 12 16 20 0 12 16 20 0 · o · Component 16 8 -8 X - ::::J ' +N--. 0 . .Cn. -16 0 ~ E E Q) +-' 0 w 8 0 0 en E ~ en - (.) 4 (b) 29 July 1999 24 16 - 8 -8 -16 0 4 Figure 6 (a-b) 8 Time of day {h) 69 50 (c) 15 June 2000 40 • BREB Component 0 30 20 X ::J - lJ_ 0 N () E Q) en >. en ~ 0 u w 10 ...- ...I N I en E 0 -10 0 () 4 8 12 16 20 0 12 16 20 0 0 - E :::1. 50 (d) 17 August 2000 • BREB 40 0 30 Component 20 10 0 -10 0 4 8 Hour Figure 6 c-d 70 5 >< ::J q:: 0 N 0 4 - ......I "'0 E N (]) I en E >. en 0 +-' 0 (.) w >. ·co 0 - 0) 3 ___.,.._ 2000 BREB · -o · · 2000 Component 0 0 \ 2 1 0 6 -1 .0 . .. 6 -2 0 6 -3 05/1 06/1 07/1 08/1 Date Figure 8 71 09/1 10/1 7 X :::J tt:: w u 0 N ""0 c :::J 0 L.. (.9 3: 0 ...... ""0 I N I E () _. 0') 6 5 4 3 Q) aJ 2 6/1 7/1 8/1 Date Figure 9 72 9/1 10/1 6 ~ >< :::::J 0 ·· o 4 LL ~ BREB 2000 BREB 1999 N o.::, 2 E -o Q) "il 1i5 E - ~ (.) 0 w (.) 0> 0 0 <9 •. . oO. o ·o o' · · · · ~ -2 ~ . 6' 0 . .q . 0 ~ 0 ·: -.- -3 2000 1999 C/)"'7 .... "'0 -4 ct E -5 0 0 ~ - u E Q) 0) -6 .... CJ) >-. CJ) 0 (.) w -7 -8 -9 ~~ ~~~~~~~~~~ 7/1 6/1 8/1 Date Figure 11 74 ~~~~~ 9/1 10/1 9 ~ ~ --+- 8 0 ""-;N ...- . 0 • 2000 1999 7 (.) "'0 "'0 ~ c E 6 '- .9 5 5 (.) 0) 3: 0 Q) OJ 4 3 ~ 6/1 ~ 7/1 8/1 Date Figure 12 75 ~ ~ 9/1 10/1 Chapter 3: Belowground C0 2 Flu x es BELOW GROUND C0 2 FLUX FROM CUT BLOCKS OF VARYING AGES IN SUB-BOREAL BRITISH COLUMBIA Abstract Instantaneous measures of below ground C0 2 fluxes were made in a mature stand and seven cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years after harvest in sub-Boreal forest of Central British Columbia (SBS wkl), Canada from May to October, 2000. The cut blocks aged 3, 5, 6, and 10 were located in the 08 variant (very nutrient rich, sub-hygric), the 2 year-old cut block was in the 07 variant (nutrient rich, sub-hygric) and the 0 and 9 year-old cut blocks were in the 01 variant (poor to nutrient rich, sub-mesic to sub-hygric). All cut blocks were replanted to hybrid spruce (Picea glauca x Picea engelmanniz) within two years of harvest and the deciduous vegetation was never herbicided or brushed. Instantaneous (each cut block) and continuous (one cut block) measures of soil temperature and moisture were made and later used to predict below ground C0 2 flux from 24 May to 20 September 2000. Instantaneous below ground C0 2 flux ranged from between 2 1-lmol C m-2 s-1 in the spring to highs of 10 1-lmol C m- 2 s-1 during mid-summer. Cumulative seasonal below ground CQ2 flux ranged from between 658 and785 g C m-2 for the cut blocks aged 3 years or older, while the 2 year-old and new cut block produced the high (861 g C m-l and low C0 2 fluxes (560 g C m-l respectively. Below ground C0 2 flux in the cut blocks positively correlated with soil temperature and the amount of 76 biomass present on site. Only a few cut blocks demonstrated a significant relationship between soil moisture and the instantaneous below ground C0 2 flux. The estimated net ecosystem C02 fluxes for the cut blocks were calculated by subtracting the biomass gained on each cut block from the cumulative seasonal below ground C0 2 flux. All the cut blocks were net sources of C0 2 with values ranging between 372 g C m·2 at the 3 yearold cut block to 540 g C m·2 at the 10 year-old cut block. There was no correlation between below ground C0 2 flux or the estimates of net ecosystem C0 2 flux and cut block age. Introduction Since pre-industrial times, land-use change has been associated with approximately 50% of the rise in atmospheric C0 2 prior to 1980 (!i! oodwell et al. 1983) and 23 % of the rise during the 1980's (Schimel1995). The increase of C0 2 from land-use change has been largely associated with timber harvesting (Harmon et al. 1990) and conversion of forestlands to pasture (Tans et al. 1990). Boreal!sub-Boreal regions in Canada are estimated to hold 65 to 104 Gt of carbon within the soil or eight times the amount stored in the plant biomass (Post et al. 1982; Apps et al. 1993). Hence, the impact timber harvesting has on below ground carbon stores in sub-Boreal/ Boreal forest may have a great impact on the current levels of atmospheric C02• Individual studies comparing a cut block against a mature forest's below ground C0 2 flux have found conflicting results: some studies indicate a decrease in C02 fluxes (Edwards & Ross-Todd 1983; Weber 1990; Chang & Trofymow 1996; Striegl & Wickland 1998), others demonstrate an increase 77 (Ewel et al. 1987a; Gordon et al. 1987; Lytle & Cronan 1998) and still others show no change in below ground C0 2 fluxes (Fernandez et al. 1993; Toland & Zak 1994). However, few studies have investigated the below ground C0 2 flux from a series of cut blocks that vary in age. Ewel et al. (1987a) found an increase in the below ground C0 1 flux from a 9 year-old to 29 year-old slash pine plantation in Florida. In British Columbia, it is assumed that cut blocks remain a source for C0 1 for at least 10 years after harvest (Kurz & Apps 1994). Below ground C0 2 flux is positively correlated to soil temperature and soil moisture (Kucera & Kirkham 1971; Fernandez et al. 1993; Striegl & Wickland 1998). Impacts on these microclimate variables from disturbance may result in higher or lower below ground col flux. Timber harvesting typically results in higher soil temperatures (Lewis 1998; Londo et al. 1999) and often a decrease soil moisture (McCaughey 1989; Londo et al. 1999). The soil moisture in a clear-cut is frequently reduced because of higher surface temperatures, but the loss can be partially offset by reduced transpiration from plants because of lower plant biomass. Hence, the amount of soil moisture at each cut block will be depend on the relative reduction in transpiration to increased evaporation. Below ground C0 2 fluxes result from two main sources: root respiration, and the decomposition of organic matter and associated respiration of soil flora and fauna. Timber harvesting has the potential to impact both of the above sources. Root respiration is assumed to represent up to 55% of the below ground C0 2 production in a forested site (Ewel et al. 1987b; Fernandez et al. 1993; Andrews et al. 1999). The removal of the trees results in the death of the tree roots and at least a temporary decrease in root respiration. Furthermore, the 78 reduction in roots results in a decrease in fine root turnover and the subsequent release of C0 2 from root decomposition. The silvicultural practices employed may influence the below ground col flux through the modification of the moisture or organic matter content of the soil(Mallik & Hu 1997). Burning slash may remove much of the soil organic layer, and if severe enough, kill the soil microorganisms (Pietikainen & Fritze 1993; Chang & Trofymow 1996). Other impacts of timber harvesting, such as soil compaction due to machinery, may result in reduced soil aeration of the cut block leading to a decrease in soil microorganisms and/ or aerobic respiration (Chang et al. 1995). It is the intention of this paper to investigate the effect of time since harvesting of sub-Boreal forest on below ground C0 2 evolution. Most studies researching the impact of timber harvesting on below ground C02 fluxes have focused on a single or few clearcut(s) of the same age in the few years initially following harvest (e.g. Edwards & Ross-Todd 1983; Gordon et al. 1987; Toland & Zak 1994; Mallik & Hu 1997; Striegl & Wickland 1998). While they provide information on the initial effects of harvesting and the impacts of different types of site preparation, they do not provide insight into how below ground C0 2 fluxes change over the complete forest reestablishment period. When one couples this lack of information with the increasing pressure to harvest northern areas and the large below ground carbon stores in Boreal and sub/Boreal forests, the need to understand the source/ sink relationships of C0 2 in clearcuts becomes vital. Thus, it is the intent of this paper to demonstrate the changes in below ground C02 flux and biomass accumulation in the 10 years following forest harvesting and reestablishment of sites in sub-Boreal British Columbia. 79 Material and Methods Study Site We made instantaneous below ground C0 2 flux measurements and sampled above ground biomass from May to October 2000, in seven cut blocks of various ages and one mature stand. The cut blocks were all located within a 10 km radius of each other, and resided in, or immediately adjacent to the University of British Columbia and University of Northern British Columbia Aleza Lake Research Forest (54 °01' Nand 122°07' W). All cut blocks were located within the sub-Boreal Spruce zone (SBS wk1) as described by the Ecosystem Classification of British Columbia, and classified as both cool: mean air temperature = 1.7 to 5 °C, and wet: relatively high snowfall compared to other regions within the central plateau region of subBoreal BC (Meidinger & Pojar 1991). The snowfall typically accumulates by November and melts by the end of April/ early May. During the winter of 1999/ 2000 the cut blocks were covered in snow by November and soils did not freeze. Soils at the cut blocks were all clay rich and classified as Ortho Luvic Gleysols (Arocena & Sanborn 1999). The cut blocks were winter logged and of varying ages since harvest (0, 2, 3 5, 6, 9, and 10 years). A non-harvested mature stand, adjacent to the 3 year-old cut block, was selected to represent mature forest within the research forest area. Each of the cut blocks were planted with hybrid white spruce (Picea glauca x Picea engelmannit) with the exception of the 5 and 6 yearold cut blocks which had some inclusions of lodgepole pine (Pinus contorta var. latifolia). There was some variation in the planting density of the cut blocks: the cut blocks aged 3 and 9 had planting densities of 1600 stems ha·1, the 5 and 10year-old cut blocks had a planting density of 1400 stems ha-1 and the 2 and 6 year-old cut blocks had a planting density of 1200 stems ha-2 • 80 The cut blocks aged 2, 3, 5, 6 and 10 years were of the site series 07 and 08 (nutrient rich to very nutrient rich, sub-hygric). The 0 and 9 year-old cut blocks was located in sites series 01 (poor to nutrient rich, sub-mesic to sub-hygric). The cut blocks experienced some differences in harvest and post harvest treatments: 0, 2, and 3 year-old cut blocks had single wildlife trees retained throughout the cut block, the 5 year-old cut block had several wildlife tree patches, and the remainder of the cut blocks were clearcut. Slash was piled but not burned at the 0 year-old cut block (harvested February, 2000), piled and burned at the 2 yr-old cut block and broadcast burned at all the older cut blocks. Prior to harvest, the cut blocks were composed mainly of hybrid spruce (P. glauca x P. engelmanniz), paper birch (Betula papyri/era), and sub-alpine fir (Abies lasiocarpa). Below ground C0 2 Flux Measurement In seven of the eight cut blocks, eight pairs of PVC collars (9.55 em in diameter) were placed along a 70 metre east-west transect at 10 meter intervals. In the 6 year-old cut block collars were randomly placed throughout a measurement area of 1 haas in Chapter 2. All collars located within cut blocks were a minimum of 20 meters away from wildlife trees or tree patches and all collars were a minimum of 30 meters way from the edge of the cut block or forested stand. Below ground C0 2 flux measurements were made using a portable infra-red gas exchange system (Li-6200, Lincoln, Nebraska, USA) with soil chamber attachment (600009, Li-Cor) as in Norman et al. (1992). Soil temperature (6000-09TC, Li-Cor) and moisture (kg H 20/kg dry soil) (Nie-Co-Product Nieuwkoop B.V., Aalsmeer, Holland) were taken simultaneously with the instantaneous C0 2 flux measurements at a depth of 10 em. 81 Biomass Sampling 2 On May 5 -10 and August 6-14,2000, a total of 40 randomly placed 0.5 m samples per cut block were sampled for total above ground deciduous biomass and separated into woody shrub and herbaceous plant. Conifer biomass was sampled by destructively harvesting 20 randomly selected seedlings within ± one standard deviation of the mean seedling height at each cut block. All biomass samples were dried for 72 h at 65 ac and weighed. Soil temperature measurement instrumentation Continuous soil temperature was monitored with a data logger (21X, Campbell-Scientific, Edmonton, Alberta) and four thermocouples (chromel-constantan) inserted at 10 em depth at the 6 year-old cut block. Soil temperatures were recorded every 1 minute and averaged over 20 minute intervals. Total Below ground and Ecosystem C0 2 Flux Estimates The below ground C0 2 flux was estimated for the growing season from 24 May to 20 September 2000 for all cut blocks. For each cut block, multiple regressions were established relating the measured soil temperature and soil moisture to the instantaneous below ground C0 2 flux. A second set of regressions were established between the recorded soil temperatures at the 6 year-old cut block and the instantaneous soil temperature measurements at each cut block to provide a continuous estimate of soil temperatures for each cut block. Using the estimated soil temperatures at each cut block and the soil moisture measurements 82 taken approximately every two weeks, the cumulative seasonal below ground fluxes were estimated for each cut block. The estimated seasonal ecosystem C0 2 flux accounted for growth of vegetation. The biomass sampled in early May was subtracted from the early August measurement to allow for a prediction of above ground biomass gains. It was assumed that there was an equivalent amount of biomass stored below ground as there was above ground(Broderick 1990). Of the total biomass sampled 50% was assumed to be carbon (e.g. Kawaguchi & Y oda 1989; Gower et al. 1997; Steele et al. 1997; Slaughter et al. 1998). Finally, total gain in biomass carbon was subtracted from the cumulative seasonal below ground C0 2 flux to determine if the cut blocks were sources or sinks of C0 2• Following the convention of ecosystem C0 1 flux papers, a positive value indicates C02 loss to the atmosphere while a negative value indicates the uptake of col from the atmosphere. Error analysis Error analysis for the cumulative below ground C0 2 flux and the estimated ecosystem C0 2 flux were based on 95% confidence limits produced from the standard error of the estimate (SE) for the the regressions and the standard deviations for the biomass samples. To produce a more accurate estimate of the error associated with the ~ ground C02 flux, theSE was totaled on a biweekly basis and the error was estimated as follows: Error = 1.96 * ~L Where x is the total SE for each biweekly period 83 Results Growing season below ground C0 2 fluxes The below ground C0 2 flux peaked in July for all cut blocks with the lowest values corresponding to measurements taken in May and October. The flux ranged from approximately 2 ).!mol C m·2s-1 in October to highs between 8 and 10 ~ C m·2s-1 in July for cut blocks aged 2 yrs to mature (Fig. 1 a-h). The newest cut block was the only exception with maximum below ground flux reaching only 6 ).!mol C m·2s·1 in July . Soil Temperature All cut blocks had similar soil temperatures, which were different to those in the mature stand (Fig. 2 a-h). The mature stand had soil temperatures similar to those in the cut blocks earlier in the growing season, but as the summer progressed the cut block soil temperatures rose a greater amount. For example, the mature stand had soil temperatures of only 12.27 oc in early August, while the cut blocks had temperatures ranging from 18.2°C in the 9 yr-old cut block to 13.2°C in the newest cut block. Below ground C0 2 flux and soil temperature Below ground C0 2 efflux correlated well with linear relationships to soil temperature (10 em depth) (Fig 3 a-h). The relationships were marginally but significantly (p=O.OS) improved for some cut blocks by adding a linear relationship to soil moisture (Table 1). The relationships appear to be fairly strong for the cut blocks aged 2 years to Mature, but the newest cut block had a low R2 of only 0.24. Fluxes followed the seasonal change in soil temperature for all cut blocks except the new cut block. 84 Soil Water Content The cut blocks and the mature stand had soil moisture (kg H 20/ kg dry soil) values ranging between 98% at the 3 year-old cut block to 41 % at the newest cut block (Fig. 4). The values were fairly consistent for each individual cut block throughout the summer, with none of the cut blocks experiencing severe drought or flooding. There was no obvious relationship between cut block age and soil moisture. Above ground biomass Age of the cut block had a significant effect on the relative proportion of above ground biomass found in conifer, woody shrubs and herbaceous species (Fig. 5). The younger cut blocks had a higher proportion of the biomass allocated to deciduous species while the older cut blocks (9 and 10 year-old), not surprisingly, had a greater proportion of the biomass in conifer biomass. The newest cut block had very little biomass present on site. The cut blocks aged 3, 5, 6, 9, and 10 year-old were not significantly different from one another. However, the 0 and 2 year-old cut block were significantly different from the others (p=0.05). Above ground biomass from spring to mid-August increased by 28 g C m·2 at the newest cut block to a high of 446 g C m·2 in the 2 year-old cut block (Table 2). With the exception of the newest cut block, all the sites had above ground biomass gains over 165 g C m ·2 across the season. 85 Cumulative seasonal below ground C0 2 flux Using the modeled soil temperature for each site (Table 3), the cumulative seasonal below ground C0 2 flux for all cut blocks ranged from 560 g C m·2 at the newest cut block to 861 g C m ·2 at the 2 yr-old cut block (May 24 to September 20, 2000) (Fig. 6). While these represent the extremes, the bulk of the cut blocks and the mature stand had cumulative seasonal below ground C0 2 flux of approximately 709 g C m·2• There was a correlation between total above ground biomass on each cut block and the cumulative seasonal below ground C0 2 flux (Fig. 7). As the biomass increases, there was a positive increase in the cumulative seasonal below ground C02 flux (R 2 = 0.87). Estimated net ecosystem C0 2 flux for each cut block The estimated net ecosystem C0 2 efflux varies from cut block to cut block (Fig. 8) . The 10 year-old cut block had the greatest net ecosystem C0 2 loss (538 g C cut block had the lowest (372 g C ~ ~ while the 3 year-old The magnitude of the estimated ecosystem C0 2 loss from each cut block was in the order of 5 > 10 > 0 > 9 > 6 > 2 > 3. Discussion Effect of soil temperature and moisture on below ground respiration The positive correlation between soil temperature and in some cases soil moisture (Table 1) has been demonstrated by others (Kucera & Kirkham 1971; Fernandez et al. 1993; Striegl & Wickland 1998). The relationship between soil temperature and the instantaneous below 86 ground C0 2 flux appeared largely linear in all cases. Researchers in the past have used either non-linear (Toland & Zak 1994; Striegl & Wickland 1998; Londo et al. 1999) or linear (Mathes & Schriefer 1985; Mallik & Hu 1997) equations to explain below ground C0 2 fluxes , but in this case the linear equations fit best. The effect soil moisture has on the below ground C02 flux is generally believed to be parabolic (Londo et al. 1999). As soils approach saturation or drought conditions, the below ground C0 2 flux generally decreases (Kucera & Kirkham 1971; deJong et al. 1974; Londo et al. 1999). However, the relationship between soil moisture and instantaneous below ground C02 flux in our study was either statistically insignificant or weakly linear. The range of soil moisture values is low for all cut blocks and never approaches the extremes of drought or saturation. Had the cut blocks been moisture limited, the impact of soil moisture on the below ground C0 2 flux may have been more evident. Parker et al. (1983) in a study on soil respiration in the Chihuahuan Desert, found soil moisture to have a greater impact on below ground C0 2 flux than soil temperature. The greater dependence of below ground C0 2 flux on soil moisture in the Chihuahuan Desert is believed to have been the result of moisture being more limiting to roots or soil organisms. Therefore, the weak or lack of a dependence of below ground C0 2 flux on soil moisture at cut blocks in the Aleza Lake Research Forest may have been due to the limited range of soil moisture content present, or the relatively high wetness of the subzone. Effect of cut block age and root respiration on below ground C0 2 flux Apart from the newest cut block, all the cut blocks have fairly similar cumulative below ground C0 2 flux for the season (Fig. 6). However, the 0, 2 and 5 year-old cut blocks were statistically 87 different. Initially, this seems to contradict results from Ewel et al. (1987a), in a study involving two slash pine (Pinus ellottit) plantations aged 9 and 29-years-old in Florida where below ground C0 2 flux was higher in the older plantation. The cut blocks in this study are much slower growing and are younger than the plantation in Ewel et al. (1987a). It is suggested by Ewel et al. (1987a), that the increase in below ground C0 1 flux in their study was due to greater root activity in the older stand. This relationship is weakly supported by the data in this study. As the biomass present on the cut blocks increased, so did the cumulative seasonal below ground C0 1 flux (Fig. 7). The new cut block had the lowest cumulative seasonal below ground C0 2 flux and above ground biomass and the 2 and 5 year-old cut blocks had the greatest cumulative seasonal below ground col flux and above ground biomass. Following this reasoning the mature stand with its greater biomass should have had the greatest cumulative seasonal below ground C0 1 flux. However, the mature stand also had much cooler soils. Had the soil temperatures been higher, the mature forest 's below ground C0 1 rates may have been much higher due to greater root activity. While the sample size is too small to confidently draw a strong conclusion, the importance of roots on below ground C0 2 production is well noted by other researchers (Ewel et al. 1987b; Bowden et al. 1993; Fernandez et al. 1993; Thierron & Laudelout 1996; Boone et al. 1998). Therefore, assuming increases in above ground biomass result in a proportional increase in below ground biomass, the difference in biomass present on the cut blocks likely contributed to the below ground C0 2 flux. 88 Cumulative seasonal below ground respiration Cumulative seasonal below ground C0 2 flux, between May 25 and September 20, totaled between 560 g C m"2 in the new cut block to 853 g C m-2 in the 2 year-old cut block (Fig. 7), with values ranging from 2.78 to 8. 97 g C m -2 d-1• These value are similar to a study by Russell & Voroney (1998) of the below ground C0 2 flux in a Boreal aspen forest (values ranged between 0.61 to 9.34 )-!mol C m·2 s·1) and others in an oak forest (Edwards & Sollins 1973) slash pine plantation (Ewel et al. 1987a) and Alaskan white spruce forest and clearcut (Gordon et al. 1987). However, other studies in an Eastern Ontario aspen forest (Weber 1990), a coniferous and deciduous forest in Maine (Fernandez et al. 1993), a spruce-fir forest in Maine (Lytle & Cronan 1998) and a jack-pine lichen woodland (Striegl & Wickland 1998) have predicted C0 2 flux that were lower than those found in this study. The greater below ground C0 2 flux may be due to the nutrient rich, moist soils found in the sub zones. Valentini et al. (2000), in a study of ecosystem C0 2 flux found that northern European forest ecosystems are dominated by respiration. They found that northern coniferous, deciduous and mixed forests in Europe have similar photosynthetic C02 uptakes to their southern counterparts, but respiration rates increased with latitude even though the forests typically had lower soil temperatures. It is postulated that this may be the result of higher soil moisture values (Grace & Rayment 2000). The estimated net ecosystem C0 2 flux The increase in above ground biomass may have promoted below ground C0 2 flux, but the biomass accumulation helped counteract the increased C0 2 flux from the cut blocks. The estimated net ecosystem C0 2 flux ranged from 371 C m ·2 to 538 g C m -2 in the 3 year-old and 89 10 year-old cut blocks respectively. The cut blocks with high cumulative seasonal below ground C0 2 flux also had greater above ground biomass (2 and 3 year-old cut blocks) . The size of the sink provided by the above ground biomass acted to augment the increase in below ground C0 2 flux, but the greater biomass acted as a larger sink. The increase in carbon sequestration by the plants appears to counter act the increase in below ground respiration, frequently reducing the loss of C0 2 from the cut block. Northern harvested cut blocks have slow regrowth and appear to be sources of C02 even ten years after harvest. The conifers are still not of sufficient size to balance the C0 2 lost from below ground. Deciduous plants may contribute to the below ground C0 2 flux through root respiration, but they also increase the amount of carbon absorbed on young regenerating cut blocks, thereby reducing the size of the net ecosystem C02 loss. It appears that up to 10 years after harvest, plant biomass and soil temperature, not the cut block age, dictate the size of the below ground and ecosystem C0 2 flux. Therefore, it may be important to support the deciduous plants in cut blocks in the years after harvest to reduce the amount of C0 2 lost to the atmosphere. 90 References Andrews, J ., Harrison, K., Matamala, R. & Schlesinger, W. 1999: Separation of root respiration from total soil respiration using carbon-13 labeling during free-air carbon dioxide enrichment (FACE). Soil Science Society of AmericaJournal63, 1429-1435. 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Thierron, V. & Laudelout, H. 1996: Contribution of root respiration to total efflux from the soil of a deciduous forest. Canadian Journal of Forest Research 26, 1142-1148. Toland, D. E. & Zak, D. R. 1994: Seasonal patterns of soil respiration in intact and clear-cut northern hardwood forests. Canadian Journal of Forest Research 24, 1711-1716. Valentini, R., Matteucci, G., Dolman, A., Schulze, E.-D., Rebmann, C., Moors, E., Granier, A., Gross, P ., Jensen, N ., Pilegaard, K., Lindroth, A., Arelle, A., Bernhofer, C., Grunwald, T., Aubinet, M., Ceulemans, R., Kowalski, A., Vesala, T., Rannik, 0., Berbigier, P., Loustau, D., Guomundsson, J., Thorgeirsson, H., Ibrom, A., Morgenstern, K., Clement, R., Moncrief, J ., Montagnani, L. & Jarvis, P . 2000: Respiration as the main determinant of carbon balance in European forests. Nature 404, 861-864. Weber, M.G. 1990: Forest soil respiration after cutting and burning in immature aspen ecosystems. Forest Ecology and Management31, 1-"14. Woodwell, G. M., Hobbie,]. E., Houghton, R . A., Meliilo,J. M., Moore, B., Peterson, B. J. & Shaver, G . R . 1983: Global Deforestation: Contribution to Atmospheric Carbon Dioxide. Science 222, 1081-1086. 95 Table 1- Multiple and single regression equations for instantaneous below ground C0 2 flux using soil temperature with or without moisture for cut blocks aged 0, 2, 3, 5, 6, 9, 10 and mature. Measurements of soil temperature, moisture and instantaneous below ground C0 2 flux for the regressions were taken in the Aleza Lake Research Forest, May to October, 2000. Regression Rl Regression Rl 0 1.837 + 0.235T 0.19 0.443+0.216T +0.0293M 0.24 2 -1.66 + 0.727T 0.40 -1.66+0.727T 0.40 3 0.091+0.441T 0.46 0.091 +0.441 T 0.46 5 -0.719 + 0.573T 0.50 -4.67 +0.592T +0.0583M 0.54 6 -1.631 + 0.571 T 0.67 -1.631+0.571T 0.67 9 -0.398 + 0.459T 0.62 -2.67 + 0.459T + 0.0447M 0.58 10 -0.229+0.473T 0.58 -0.233 + 0.47 4T 0.58 Mature 0.4019+0.545T 0.41 2.05 + 0.653T-0.0397M 0.43 Site Age T - temperature (0 C) M- moisture(%) 96 Table 2- Above ground conifer, herbaceous, woody shrub and total biomass in spring (May 5-10) and late summer (August 6-14), and the change in biomass from spring to summer, at 7 cut blocks in the Aleza Lake Research Forest. Late Summer Biomass (g m·2) Spring Biomass (g m·2) Cut Block Age Biomass Change gm·' Conifer 0 0 2 3 5 6 9 10 4.99 14 49 45.5 185 205 Herbaceous':- Woody Shrubs 3.73 - - - 91.9 86.8 158 140 33.8 85.7 Total Biomass Conifer 3.73 0 96.9 101 208 185.5 219 290 8.07 28 74 64 240 207 ::-No herbaceous plants present in spring 97 Herbaceou Woody Shrubs 20.3 12.1 323 127 108 163 184 126 208 262 280 234 43.3 122 Total Biomass 32.4 28.6 539 418 452 462 468 455 442 317 254 277 249 164 Table 3 - Correlation of soil temperature at a meteorological tower in the 6 year-old cut block with instantaneous soil temperatures measurements made at each cut block coincident with below ground C0 2 flux determinations. Site Age Correlation to Tower Measurement Rz 0 0.93 2 0.83 3 0.77 5 0.88 6 0.89 9 0.77 10 0.87 Mature 0.94 98 Figures and Legends Figure 1- The instantaneous below ground C0 2 flux for a mature stand and 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years since harvest from May until the end of October, 2000 in the Aleza Lake Research Forest. The symbols and lines represent the measured instantaneous and modeled below ground C0 2 fluxes , respectively. The bars on the symbols represent a 95% confidence interval Figure 2- In situ soil temperature (10 em depth) for a mature stand and 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years since harvest from May until the end of October in the Aleza Lake Research forest. Vertical lines represent a 95% confidence interval. Figure 3- Relationship between in situ soil temperature (10 em depth) and below ground C0 2 flux for a mature stand and 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years since harvest in the Aleza Lake Research Forest from May until October, 2000. The solid line (-) represents the modeled below ground C0 2 flux. Figure 4- In situ soil moisture for a mature stand and 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years since harvest from May until the end of October in the Aleza Lake Research forest. Vertical lines represent a 95% confidence interval. Figure 5 - The total above ground biomass partitioned into conifer, herbaceous and shrubs in each of the cut blocks measured within the Aleza Lake Research forest. 99 Figure 6- The estimated net below ground C0 2 efflux from a mature stand and seven cut blocks aged 0, 2, 3, 5, 6, 9 andlO years since harvest from May 24 to September 20,2000 in the Aleza Lake Research Forest. Figure 7- The relationship between the net above ground biomass and cumulative seasonal below ground C0 2 flux (May 24 to September 20, 2000) at 7 cut blocks aged 0, 2, 3, 5, 6, 9, and 10 years since harvest in the Aleza Lake Research Forest. Figure 8 - The estimated net efflux of C0 2 from May 24 to September 20, 2000, in 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years since harvest in the Aleza Lake Research Forest. 100 12 ~ (a)O yr 12 8 8 ~ 4 >< 0 5/1 ON 0 12 -LL :::J 7/1 4 • 11/1 9/1 (c) 3 yr I.... 0> ~ 0 (1) co CJ) :::J 0 (1) c co ........ c co ........ ~ CJ) 4 0 0 5/1 E 0 - ~ 12 8 4 CJ) c f N I Q 7/1 11/1 9/1 (d)5 yr 8 c- 8 :::J 0 5/1 12 "'0 0 1 7/1 9/1 4 11/1 0 5/1 (e)6 yr 12 . 8 0 5/1 7/1 9/1 11/1 0 5/1 12 8 8 4 4 0 5/1 7 7/1 9/1 11/1 (f) 9 yr 4 (g) 10 yr 12 y • 7/1 9/1 5/1 Date 101 7/1 9/1 11/1 (h) mature 1 0 11/1 Figure 1 a-h 0 7/1 9/1 2 11/1 20 15 15 10 10 5 5 5/1 ..c ....... 20 Q) 15 E (.) 10 0 5 c.. "'0 -"'r" ~ - :::::::10 ....... cu ~ Q) c.. E 20 (a) 0 yr 7/1 9/1 11/1 5/1 20 (c) 3 yr 15 7/1 5/1 20 (e) 6 yr 9/1 0 11/1 5/1 20 I- 10 10 0 5 5 7/1 5/1 20 (g) 10 yr 9/1 11/1 9/1 11/1 9/1 11/1 9/1 11/1 (d) 5 yr 15 10 10 5 5 7/1 Figure 2 a-h 9/1 7/1 (f) 9 yr 7/1 11/1 5/1 20 (h) mature 15 5/1 9/1 5 15 C/) 7/1 10 15 Q) (b) 2 yr 11/1 5/1 Date 102 7/1 (a) 0 yr 16 12 . " ~ • 8 4 0 >< ::::::! N 0 (/) 8 4 E 0 "'0 c..::::::! "";'" L.. N 0 0> ~ I 0 0 OJ 0 Q) (/) ::::::! 0 Q) c ro +-' c ro +-' (/) c 12 8 4 8 4 16 20 16 (c) 3 yr 12 LL 0 0 ~ E 16 4 8 12 (e) 6 yr --- 12 16 12 0 4 8 20 12 16 4 Figure 3 a-h 8 12 8 12 (d) 5 yr 16 20 \l 0 8 12 16 20 0 4 8 16 (h) mature 12 12 16 20 12 16 20 0 4 8 4 20 • 0 4 16 (f) 9 yr 12 (g)10yr 8 4 0 16 '· 8 4 0 0 8 4 ~.... ::i. 0 16 12 ~ 0 (b) 2 yr 16 12 0 8 4 16 20 0 0 4 8 Soil Temperature at 10 em (°C) 103 - 100 (a) 0 yr 100 80 80 60 60 40 40 5/1 cf2. 100 7/1 9/1 11/1 5/1 (c 3 yr 100 ~ 80 80 :en ~ 60 60 40 Q) · - I.... :::J 0 CJ) 0""0 ~ 6> _..::s::. ·- .._ ~ON 40 5/1 7/1 9/1 11/1 (e) 6 yr 100 60 40 60 5/1 9/1 11/1 (d) 5 yr ~~ ~ 7/1 9/1 ~ 11/1 ~ (f) 9 yr 80 40 7/1 9/1 11/1 (g) 10 yr 100 80 80 60 40 60 40 7/1 9/1 11/1 104 7/1 ~ 5/1 Date Figure 4 a-h ~~~~~~~~ 5/1 100 5/1 ~ ~~~~~~~~ 5/1 ~ 100 0) :::=., 80 7/1 9/1 11/1 ~ (g) mature 7/1 9/1 11/1 (J) (J) ro ·-E 0 ~ L__...___.j 600 -o E 500 O>E0 400 <( - - 200 0 100 c (J) :J (J) o ro - • Conifer Herbaceous - • Shrubs 700 Q) > ·- 300 o.c .c 0') ro I- Cut Block Age (yr) Figure 5 105 X :::J \;:: 0 1000 N 0 "'0 c :::J 0 ~ 0') ~ 0 Q) en -- - 750 0 500 N I -- E 0') .._.. ·=, Q) > +-' ro '" 250 :::J E :::J 0 0 Figure 6 ' 0 2 . I.. 3 5 6 9 Cut Block Age (yr) 106 10 M >< 900 :::::1 - LL 0 • N 0 0 c :::::1 0 '- ... 800 "'0 \7 ..- • N I E 3: 0 700 0 0) Q) (() Q) > co ~ D • 0) .._ 600 :::::1 0 0 \7 . D ... • :::::1 E 0 yr-old 2 yr-old 3 yr-old 5 yr-old 6 yr-old 9 yr-old 10 yr-old 500 0 100 200 300 400 Aboveground Biomass (g biomass m-2 ) Figure 7 107 500 600 >< ~ 700 - LL 0 N -.. 600 () ....... E Q) ...... 0 500 en Q) >. en en N 400 0 I c en ro ,..--L-- "'0 Q) ...... ro E ...... en w ,---'----, , '' II 1-· I { '-• - 100 0) ,--'-- r - '---- E 300 w () (.) r-'-- ,..--L-- r - '---- ' 200 0 I ~ 0 2 ~ 3 5 6 Cut Block Age (yr) Figure 8 108 9 10 Chapter 4- Summary and Conclusions SUMMARY AND CONCLUSIONS The C0 2 budget of young regenerating sub-Boreal forests of varying ages since harvest The common assumption that young regenerating forests only remain a source of C0 2 for approximately 10 years after harvest may not be applicable for sites in the SBS wk1 that are sub-hygric. The Bowen ratio data and the component model demonstrate that a clearcut of 5 to 6 years in the SBS wk1 08 is a source of C0 2 over the course of a year. Similar results from a Boreal forest study in Siberia found that a 12 year-old forest was only weakly a sink of C0 2 in July 01alentini et al. 2000a). Therefore, if the young Siberian forest is only a weak sink during the middle of summer, it is likely to be a source of C02 over the course of a year. Seedlings in sub-Boreal and Boreal forests are slow growing because of a shorter growing season and harsher winters as compared to cut blocks in the south. The assumption that seedlings enter their exponential growth phase after 10 years (Kurz & Apps 1994) appears to be false when considering SBS wk1 08 cut blocks. In the 9 and 10 year-old cut blocks observed in chapter 3, the conifers still are not the dominant contributor to the overall C0 2 uptake. The photosynthesis of deciduous plants dominate the C0 2 uptake component of ecosystem flux in regenerating stands at Aleza Lake on the sites studied for at least 10 years. For all cut blocks aged 0 to 10 years, the bulk of the biomass was found in the herbaceous and woody shrubs. The photosynthetic C0 2 uptake using the component model for the 5 to 6 year-old clearcut found the herbaceous plants and woody shrubs to take up 376 and 365 g C m·2 for 109 1999 and 2000, respectively (27 June to 3 September). The deciduous plant photosynthesis dwarfs the conifer C0 2 uptake of only 47 and 58 g C m -2 during 1999 and 2000 respectively . The conifers became more important as the cut blocks aged, but the increase in conifer biomass appeared to come at the expense of deciduous plants. The older cut blocks did not have a greater accumulation in above ground biomass when compared to the younger cut blocks. Hence, in young clearcuts, biomass sequestration did not appear to improve as the cut blocks aged. However, no measurements of change in below ground biomass with clearcut age were made. Below ground respiration was the dominant C0 2 flux in the cut blocks at Aleza Lake. In all cut blocks the below ground carbon loss exceeded any carbon gains by the plants. The dominance of the respiration in northern areas has recently been reported by Valentini et al. (2000) for a latitudinal series of European forests. Hence, even though northern regions have cooler temperatures, the respiration rates increased. This may be the result of increased soil moisture or available organic matter (Grace & Rayment 2000) or greater below ground soil flora or fauna. Timber harvesting did not result in greater below ground losses of C02 when compared to the mature forest. The below ground C0 2 flux for the mature harvest was similar to all cut blocks even though its soil temperatures were slightly lower. The mature stand would have the greater root biomass relative to the cut blocks. Therefore, it is possible that the increase in 110 root respiration in the mature stand was offset by the negative impact of lower temperatures on the below ground C0 2 flux. Discrepancies between the ecosystem C0 2 method results The results indicate that sub-hygric SBS wk1 cut blocks that are < 10 years of age in the Aleza Lake region are likely sources of C02 annually. However, there were discrepancies between the result of the BREB method, component model and the estimated ecosystem C0 2 fluxes found in chapter 3. The BREB method and the component model for 2000 estimated growing season ecosystem col flux to be 143 ±57 and 103 g c m·2, respectively. In contrast, the estimated ecosystem C02 flux predicted a loss of 424 g C m·2• The component model and the estimated ecosystem C02 flux both used similar cumulative below ground C0 2 flux equations. Therefore, the discrepancy is the result of Component model and the biomass based net ecosystem estimtes in Chapter 3. The component model estimated the C0 2 uptake to be 583 g C m·2 while the net ecosystem C0 2 flux estimated a gain of 276 g C m·2 based on the biomass values. The discrepancy is not surprising as the two methods use two fundamentally different approaches to estimating plant carbon sequestration. Discrepancies between different techniques are not uncommon when attempting to quantify the C0 2 flux from a forest. Norman et al. (1997), examined the differences in below ground C0 2 flux using five different soil chambers and a sonic anemometer. They found that multipliers ranging from 0.93 (sonic anemometer) to 1.45 were needed to bring the different methods in agreement, thus demonstrating a large variance in the results from process based 111 instruments. At an old aspen stand within the Canadian Boreal forest, two research groups set out to measure the net ecosystem exchange of C0 2• Yang (1998), using an eddy covariance system, estimated the net primary productivity (NPP) to be -708 g C m·2 • In contrast, two studies in the same old aspen stand that used allometric equations to estimate NPP and net ecosystem exchange (NEE) of carbon found lower C02 fluxes: Gower et al. (1997), estimated above ground components to sequester -352 g C m·2 y·1 and Steele et al. (1997) estimated the carbon allocation to roots to be -52 g C m·2 y" 1 for a total of -404 g C m·2 y" 1• As with the results of our study, the biomass based estimates are lower than the modeled or tower based estimates of NPP of C0 2• Biomass estimates of C02 uptake may have lower NPP values because the carbon lost to leaf turnover, flowers, seed production and root exudates are not accounted for. However, the BREB method and component model C0 2 flux estimates have their inherent flaws as well. The BREB method is susceptible to a variety of measurement errors and is hampered in non-homogenous stands. The component model has errors associated with the C02 uptake of each of the components that would be compounded when scaled up to the ecosystem level. It is difficult to predict the exact size of the C0 2 flux, and the size of the error for each method, because all methods have relatively large errors associated with them (Kimball et al. 1997). However, while the various estimates for the C0 2 flux for cut block at Aleza Lake varied, they all indicate the cut block was a source of C0 2 for at least six years. Implications for timber harvesting and the Kyoto Protocol Young regenerating forests in the Aleza Lake region slowly recover resulting in a prolonged period of C0 2 losses to the atmosphere. If losses of C02 from young regenerating sub-Boreal 112 forests are to be avoided, silvicultural practices must consider the important components of the C0 2 flux. In the years immediately after harvest the presence of deciduous plants should not be discouraged (e.g. removal through brushing) except in the extreme situations where conifer seedlings would otherwise be greatly suppressed. If overall productivity of the cut blocks is suppressed, the flux of C02 from below ground will dominate, providing an even larger source of C0 2 to the atmosphere. However, planted spruce seedlings are slow growing in the cut blocks studied, and are consequently incapable of providing the sink required to switch cut blocks from sources to sinks of C0 2 10 years after harvest. The implication for reforestation and afforestation carbon credits to industrialized nations such as Canada and the USA are weakened by the slower recovery of forests found in the SBS wk1 08. If forests in sub-Boreal British Columbia are cut on set rotation times then the time available for carbon sequestration will be shortened. The results from this study indicate that harvested stands in the SBS wk1 08 are sources for C02 for at least 10 years and likely more. The longer time it takes to change from source to sink casts doubt on the assumption that clearcuts require only ten years to become change from a source to a sink of C0 2• For forestry related activities to be used as carbon credits, the total sinks and sources must be accounted for. The longer a cut block remains a carbon source, the greater the impact of forest harvesting on the size of the source of C0 2 related to forestry activities. This will weaken the argument for taking credits for carbon sequestration in northern forests . 113 Potential implications of global warming on cut block carbon budgets If global warming occurs as predicted, the greatest impacts will likely be seen at higher latitudes (Melillio et al. 1993). It has been demonstrated that northern forest C02 budgets are sensitive to length of growing season and rising temperatures (Lindroth et al. 1998). The seedlings would benefit from increased temperature, as they will be photosynthetically active for a longer period of the year, but with increasing air temperatures comes greater evaporation and soil moisture loss. However, because Aleza Lake forests are located in a wet ecosystem (wk1), the impact may be smaller than for more xeric northern sites. While there is no concrete evidence that cut blocks in the Aleza Lake region would benefit under global warming, it is likely that the longer growing season and sufficient water supply would promote plant C0 2 uptake and possibly a earlier change from source to sink of C0 2 for cut blocks in the SBS wk1 08 . A more rapid change from source to sink for C02 for young regenerating forests is not certain. There is a fear among scientists that global warming will enhance decomposition of organic matter resulting in an uncontrollable feedback system of runaway greenhouse gases and global warming. Below ground C0 2 fluxes are closely linked to temperature (Kucera & Kirkham 1971; Fernandez et al. 1993; Striegl & Wickland 1998) and if the relationship is maintained over time, the losses of below ground carbon may dwarf any gains by improved plant C0 2 uptake. However, there is some doubt that the below ~ C02 flux will cause unrestrained C0 2 losses from decomposition and root respiraton to the atmosphere with increasing temperatures. Giradina & Ryan (2000) demonstrated that long-term below ground C02 fluxes are not limited by soil temperature. They suggest below ground C0 2 fluxes will not rapidly increase with rising global temperatures alone, postulating there may be other limiting factors 114 such as nutrient quality. This would be of tremendous importance in northern soils, as they tend to have a greater portion of their carbon stored below ground. However, if the below ground C0 2 flux is enhanced, carbon uptake that results from more favourable temperatures and longer growing season and may be counteracted by increased respiration. Summary The results of the two field seasons indicate that a spruce-dominated clearcuts in sub-hygric areas of the Aleza Lake Research Forest are sources of C0 2 for longer than the 10 years assumed by models (Kurz & Apps 1994). However, the size of the C0 1 source appears to vary from year to year in accordance with large inter-annual variability in the below ground C0 2• Deciduous plants contribute the bulk of photosynthetic C0 1 uptake on young regenerating cut blocks at Aleza Lake, with conifers, even at 10 years, contributing a lesser share. Future Research To better understand how harvesting sub-Boreal forest affects atmospheric C0 2 concentrations, the full breadth of impacts must be investigated. First, research is needed to understand when cut blocks in northern areas convert from sources to sinks of C0 2• The longer a cut block remains a source for C02 the greater the impact rotation time has on the size of the sink generated by the new forest over time. A larger span of years after harvest, ranging from 0 to 30, is needed to establish the time in which cut blocks change from source to sink. Ideally this would be repeated at multiple sites of the same age using different independent measurement techniques (e.g. eddy covariance) to confirm the results. Second, the full range of impacts, positive or negative, from road building, harvesting, transportation 115 and storage time of carbon in the wood products must be considered. Third, the influx of people into northern areas results in forested areas being cleared for agricultural, commercial (ie. sesmic lines) or residential uses. How the conversion of forested lands to other uses proceeds, will increasingly be of importance in northern regions. Lastly, it is necessary to understand what controls the C0 2 flux within a regenerating forest. The imminent effects of global climate change may drastically change the source/sink status of many forests. We need to understand if northern forests are going to become stronger or weaker sinks/ sources if we are to attempt to control the rising levels of atmospheric C0 2• 116 References Fernandez, I. J., Son, Y., Kraske, C. R., Rustad, L. E. & David, M. B. 1993: Soil carbon dioxide characteristics under different forest types and after harvest. Soil Science Society of AmericaJournal57, 1115-1121. Giradina, C. & Ryan, M. 2000: Evidence that deomposition rates of organic carbon in mineral soil do not vary with temperature. Nature 404, 858-861. Gower, T., Vogel, J. G., Norman, J. M., Kucharik, C. J. & Steele, S. J. 1997: Carbon distribution and above ground net primary production in aspen, jack pine, and black spruce stands in Saskatchewan and Manitoba, Canada. Journal of Geophysical Research 102, 29,029-29,041. Grace, J. & Rayment, M. 2000: Respiration in balance. Nature 404, 819-820. Kimball, J ., Thornton, P ., White, M. & Running, S. 1997: Simulating forest productivity and surface-atmospheric carbon exchange in the BOREAS study region. Tree Physiology 17, 589-599. Kucera, C. L. & Kirkham, D. R. 1971: Soil respiration studies in tallgrass prairie in Missouri. Ecology 52, 912-915. Kurz, W. & Apps, M. 1994: The carbon budget of Canadian forests: A sensitivity analysis of changes in disturbance regimes, growth rates, and decomposition rates. Environmental Pollution 83, 55-61. Lindroth, A., Grelle, A. & Moren, A.-S. 1998: Long-term measurements of Boreal forest carbon balance reveal large temperature sensitivity. Global Change Biology 4, 443-450. 117 Melillio,J. M., McGuire, A. D., Kicklighter, D. W., III, B. M., Vorosmarty, C.J. & Schioss, A. L. 1993: Global climate change and terrestrial net primary production. Nature 363, 234-240. Norman,]. M., Kucharik, C.J., Gower, S. T., Baldocchi, D. D., Crill, P.M., Rayment, M., Savage, K. & Striegl, R. G. 1997: A comparison of six methods for measuring soilsurface carbondioxide fluxes. Journal of Geophyisical Research 102, 28,771-28,777. Steele, S., Gower, S., Vogel,]. & Norman,]. 1997: Root mass, net primary productiong and turnover in aspen, jack pine and black spruce forests in Saskatchewan and Manitioba, Canada. Tree Physiology 17, 577-587. Striegl, R. G. & Wickland, K. P. 1998: Effects of a clear-cut harvest on respiration in a jack pine-lichen woodland. Canadian Journal of Forest Research 28, 534-539. Valentini, R., Dore, S., Marchi, G., Mollicone, D., Panfyorov, M., Rebmann, C., Kolle, 0. & Schulze, E.-D. 2000a: Carbon and water exchanges of two contrasting central Siberia landscape types: regenerating forest and bog. Functional Ecology 14, 87-96. Valentini, R., Matteucci, G., Dolman, A., Schulze, E.-D., Rebmann, C., Moors, E., Granier, A., Gross, P.,Jensen, N., Pilegaard, K., Lindroth, A., Arelle, A., Bernhofer, C., Grunwald, T., Aubinet, M., Ceulemans, R., Kowalski, A., Vesala, T., Rannik, D., Berbigier, P., Loustau, D., Guomundsson, J., Thorgeirsson, H., Ibrom, A., Morgenstern, K., Clement, R., Moncrief, J., Montagnani, L. & Jarvis, P. 2000b: Respiration as the main determinant of carbon balance in European forests. Nature 404, 861-864. Yang, P. 1998: Carbon dioxide flux within and above a Boreal aspen forest. PhD, University of British Columbia, Vancouver. 118 APPENDIX I Carbon allocation in the stems and needles of interior spruce seedlings from a 5 year-old clearcut in the Aleza Lake Research Forest, BC in 1999. The biomass was sorted according to the age of the needles. Needle Age-class 1 yr-old 2: 1 yr-old < TOTAL Stems (g) 67.3 135 202 I Needles (g) 137 40.2 177 APPENDIX II 21X program for the Bowen ratio system Program: 01 : Bowen-ratio manual. C02 program entered directly 1 Table 1 Programs Sec. Execution Interval Make measurements . 01 : Pl7 Panel Temperature 01 : 3S : [ RefTemp C] 02: Pl3 01 : 02 : 03 : 04 : OS : 06: 07: 08: 1 1 8 2 3S 33 1 0 . 0000 03 : Pl4 01: 02 : 03: 04 : OS : 06 : 07 : 08 : Thermocouple Temp (DIFF) Rep 1 S mV slow Range 4 IN Chan 2 Type E (Chromel - Constantan) 33 Ref Temp Loc 32 Loc : [:del ~ TC] 1 Mult 0 . 0000 Offset 04: P3S 01 : 33 02: 32 03: 34 OS: P91 01 : 24 02 : 30 Flag 4 is reset Then Do P2 01: 02: 03 : 04: OS : 06: Volt (D IFF) Reps SOO mv slow Range IN Chan Loc : Mult Offset 06: 07 : P89 2 4 6 10 1 0.0000 Thermocouple Temp (S E ) Rep S mV slow Range IN Chan Type E (Ch romel-Constantan ) Ref Temp Loc Loc : [ : TC lower] Mult Offset Z=X-Y X Loc Y Loc z Loc I f Flag/Inchan If X< = >F II from the 023/C02 system 01 : 02: 03 : 04 : 08: 09: P2 01: 02: 03 : 04: 05: 06 : 10 4 - 500 30 1 5 6 10 1 0 .0 000 P95 X Loc < F Then Do Volt ( DI FF) Rep 5000 mv slow Range IN Chan Loc : Mult Offset End Compute differential C02 and H20 10: P86 01 : 3 Do Call Subroutine 3 11 : P86 01 : 27 Do Set low Flag 7 12 : P87 01 : 0 02 : 2 Beginning of Loop Delay Loop Count 13 : P36 01 : 10 -02 : 26 -03 : 41 -- Z=X•Y X Loc y Loc z Loc 14: P33 01 : 41 -02 : 24 -03 : 41 -- Z=X+Y X Loc y Loc z Loc 15 : P86 01: 7 Do Call Subroutine 7 16: P35 01: 28 -02 : 21 -03 : 30 -- Z=X-Y X Loc y Loc z Loc 17: P36 01 : 30- 02 : 18 -03: 30 -- Z=X•Y X Loc y Lo c z Loc 18: P86 01 : 17 Do Set high Flag 7 19 : P95 End 20: P86 01 : 8 Do Call Subroutine 8 21 : P94 Else III 22 : P86 01 : Do Call Subroutine I 23 : P95 End If v al v es just switched or the system setFLAG 9 high. 24 : P9l I f Flag I IN chan 01 : 11 Flag l is set 02: 30 Then Do 25 : P86 01 : 19 Do Set high Flag 9 26 : P94 Else P9 1 I f Flag liNehan 27 : 28 : 01 : 13 Flag 3 is set 02 : 19 Set high Fla g 9 P95 End Generate Ou tput Array every 20 minutes P92 01 : 0 02: 20 03 : 10 I f time is minutes into a m1nute 1nterval Set high Flag 0 {output) PBO 01 : 1 Set Active Storage Area Final Storage Area 02 : 21 Array ID or location 31 : P77 01 : 110 Real Time Day/Hour -Minute 32: P78 01 : Reso1utlon High Resolution 33: P71 01 : 5 02 : 36 Av erage Reps Loc 34 : P70 01: 7 02 : 21 Sample Reps Loc 35: P86 01 : 29 Do Set low Flag 9 36: P71 01 : 3 02 : 32 Av erage Reps Loc [AVG TC upper] 29 : 30 : IV is in manual control ( FLAG 5 high ) 37 : P71 01 : 3 02 : 33 Average Reps Loc [AVG TC lower] 38: P91 01 : 15 02 : 3 0 I f Flag / INchan Flag 5 is set Then Do 39 : P86 01 : 2 Do Call Subroutine 2 40 : P94 Else Perform an automatic scrub at the top of the hour 41 : P92 01 : 0 02 : 60 03 : 14 I f time is minutes into a minute interv al Set high Flag 4 42: P9 1 0 1: 18 02 : 14 I f Flag / INchan 43 : Pl8 01 : 0 02 : 600 03 : 45 Time Tenths o f seconds int o minute (maximum 600 ) Mod/by Loc : Valve switching Flag 8 is set Set hJ.gh Flag 4 l.S synch r on ized every 4 minutes 44 : P92 01 : 0 02: 4 03 : 30 I f time is minutes into a minute interval The n Do 45: P86 01: 21 Do Set low Flag 1 46: P86 01 : 42 Do Set high Port 2 47 : P86 01 : 22 Do Set low Flag 2 48: P86 01 : 13 Do Set high Flag 3 49: P86 01 : 9 Do Call Subroutine 9 50 : P95 End 51: P92 01 : 2 02 : 4 03 : 30 I f time is minutes into a minute interval Then Do 52 : P86 01 : 41 Do Set high Port 1 53: P86 Do v 01: 12 Set high Flag 2 54: P86 01 : 13 Do Set high Flag 3 55: P86 01 : 9 Do Call Subroutine 9 56: P95 End 57: P89 01 : 02: 03 : 04 : 45 3 400 23 I f X<; >f X Loc >; F Set low Flag 3 58 : P95 End 59: 01 : P96 30 Serial Output SM192/SM716/CSM1 60: P End Table I • 2 Table 2 Programs 01: 10 Sec. Execution Interval 01 : P30 01: 50 00 02 : 44 F 02 : P21 01: 1 02 : 44 Analog Out CAO Chan mv Loc 03 : P10 01: 9 Battery Volt a ge Loc : [BATT VOLTAGE] 0 4: Pll 01 1 02 03 1 04 1 05 1 06 0 . 0000 Temp 10 7 Probe Rep IN Chan Excite all reps w/EXchan Loc : [SE T amb C] Mult Offset 05 : P4 01 1 02 5 03 2 04 3 05 15 06 5000 07 8 08 . 001 09 0.0000 Excite /Delay /Volt( SE ) Rep 5000 mV slow Range IN Chan Excite all reps w/EXchan 3 Delay (un its .Olsec ) mV Exc itation Loc : [RH fraction] Mult Offset 06 : P56 01: 1 02 : 2 Saturation Vapor Pressure Temperature Loc Loc : [SVP kPa] 07 : P36 01 : 8 02: 2 Z; X*Y X Loc Y Loc Z;f z Loc VI 08 : 09 : 10: 11 : 03:2 z Loc [e amb kPa] P38 01 : 2 02: 23 o<· 3 Z=X/Y X Loc y Loc z Loc [ :H2 0 mM/M] P37 01 : 3 02: 100 0 03: 3 Z=X*F X Loc F z Loc : [ :H20 mM / M] PI 01 02 03 04 05 06 Volt ( SE ) Rep 5 00 mv slow Range IN Ch an Loc : [ :Rn Mult Offset 4 3 4 1 0 . 0000 P89 01 : 4 02 : 3 03 : 0 04 : 30 If X< = >F X Loc Rn >= F Then Do net radiometer (positive ) 12 : P37 01 : 4 02 : 8 . 96 03 : 4 Z=X*F X Loc F z Loc Else 13 : P94 net radiometer (negative ) 14 : P37 01 : 4 02 : ll. 21 03 : 4 Z=X*F X Loc F z Loc 15 : P95 End 16 : PI 01 02 03 04 05 06 Volt ( SE I Reps 2 50 mV slow Range 3 IN Chan 9 Loc : 5 Mult 1 0 . 0000 Offset Soil heat flux #1 Z=X*F P37 17 : X Loc 01 : 5 F 02 : 43.7 03 : 5 z Loc Soil heat flux #2 18: PI 01 2 02 3 Volt (SE ) Reps 50 mv sl ow Range VII 03 04 05 06 19: 10 6 1 0 P37 01 : 6 02 : 42 . 6 03 :6 IN Chan Loc : Mult Offset Z=X*F X Loc F z Loc measure soil temperature 20 : P14 01 1 02 1 03 3 04 2 05 35 06 7 07 1 08 0 . 0000 Thermocouple Temp (D IFF ) Rep 5 mV slow Range IN Chan Type E (Chromel - Constantan) Ref Temp Loc Loc : [ : Tsoil Mult Offset 21 : P3 01 02 03 04 05 06 1 1 21 16 . 75 .2 Pulse Rep Pulse Input Chan Low level AC; Ou tpu t Hz. Loc : [ :Wind speed J Mult Offset 22 : P5 01 02 03 04 05 06 07 08 AC Half Bridge 1 Rep 5 5000 mV slow Range 4 IN Chan 2 Excite all reps w/EXchan 2 5000 mV Excitation 17 Loc: [ : Wind dir] 355 Mult 0 . 0000 Off set 23: P92 01 : 0 02 : 20 03: 10 If time is minutes into a minute interval Set hlgh Flag 0 (output) 24 : P80 01: 3 02 : 13 Set Active Storage Area Input Storage Area Array ID or loca ti on 25: P71 01 : 02 : 7 Average Rep Loc Tsoil 26 : P31 01 : 7 02: 13 Z=X X Loc Z Loc 27: P35 01 : 13 02 : 12 03: 14 Z=X - Y X Loc Y Loc Z Loc 28: P31 Z=X VIII 01: 13 02: 12 X Loc z Loc 29: P80 01: 1 02: 22 Set Active Storage Area Final Storage Ar ea Array ID or location 30: P77 01 : 110 Real Time Day/Hour - Minute 31: P71 01: 6 02: 1 Average Reps Loc 32 : P70 01: 2 02 : 13 Sample Reps Loc 33 : P70 01: 2 02 : 8 Sample Reps Loc 34: P69 01: 02: 03 : 04 : 05 : Wind Vector Rep Samples per sub- interval Polar Sensor/(S, Dl, SDl ) Wind Speed/East Loc Wind Direction/North Loc 35: p * 3 Table 3 Subr outines l 60 00 16 17 End Table 2 Scrub subroutine for IRGA zero 01 : 01: PBS 1 02: 01 : 02: 03: 01: 02: Beginning of Subroutine P91 21 30 P30 36.7 54 04 : P30 01 : 42 02 : 55 05 : 06: Subroutine Number I f Flag I INchan Flag 1 is reset Then Do Z=F F z Loc Z=F F z Loc Z=F P30 01: 17835 02: 56 F P30 01: 14724 02 : 57 F z Loc Z=F z Loc ******Enter local pressure in kPa for F below 07: 08: Z=F P30 01 : 98 02: 23 F P30 Z=F z Loc IX 01 : . 1 0132 02 : 4 3 F z Loc 09 : P38 01 : 43 02: 23 03 : 43 Z=X /Y X Loc Y Loc Z Loc 10 : P86 01: 11 Do Set high Flag 1 During first pass switch upper arm into re f ere n ce cell, lower arm into sample cell, and set scrub valve on 11 : P86 01: 42 Do Set h igh Po rt 2 12: P86 01: 44 Do Set high Port 4 13: P8 6 01: 22 Do Set low Flag 2 14: P86 01 : 9 Do Call Subroutine 9 15: P95 End 16 : P32 01 : 46 z Loc 17: P2 01 02 03 04 05 06 Volt IDIFF ) Reps 5000 mv slow Range IN Chan Loc : Mult Offset 18 : P86 01 : 3 Do Call Subroutlne 3 19 : P89 01 : 02 : 03: 04 : If X< = >F X Loc >= 2 5 6 10 1 0 .000 0 Z=Z+l 46 3 50 10 Set high Flag 0 (ou tput ) 20 : P8 0 01 : 3 02 : 10 Set Actlve Storage Area Input Sto rage Area Array ID or location 21: P89 01: 02: 03 : 04 : If X< = >F X Loc 46 4 40 19 F < F Set high Flag 9 22: P71 01 : 2 02 : 10 Average Reps Loc 23 : P86 Do X 01 : 27 Set low Flag 7 24 : P87 01 : 0 02 : 2 Beginning of Loop Delay Loop Count 25 : P35 01 : 56- 02 : 10 -- Z=X - Y X Loc y Loc 03 : 26 : 2 6-P38 01 : 56-02 : 26-03 : 26 - - z Loc Z=X /Y X Loc y Loc z Loc 27 : P36 01 : 26 - 02 : 10 -03 : 24 -- Z=X*Y X Loc y Loc z Loc 28 : P37 01 : 2 4- 02 : - 1 03 : 24 - - Z=X*F X Loc 29 : P31 01 : 24 - 02 : 41- - Z= X X Loc z Loc 30 : P86 01 : 7 Do Call Subroutine 7 31 : P31 01 : 28 -02 : 21 -- X Loc 32 : P86 01 : 17 Do Set high Flag 7 33 : P95 End 34 : P91 01 : 28 02 : 30 I f Flag l iNehan 35 : P91 01 : 10 02 : 30 Flag 0 (ou tput ) is set The n Do 36 : P86 01 : 24 Do Set low Fla g 4 37 : P30 01 : 0 02 : 46 Z=F F z Loc 38 : P86 01 : 43 Do Set high Po rt 3 39 : P86 01 : 9 Do Call Subroutine 9 p z Loc Z=X z Loc Flag 8 is reset Then Do I f Flag li Nehan XI 40: P95 End 41 : P95 End 42 : P95 End Manual va l ve control 43: P85 01 : 2 B ~ i of Sub routine Subroutine Number 44: P86 01 : 11 Do Set hi g h Flag 1 45 : P91 01: 12 02 : 41 I f Flag I INch an 46 : P91 01 : 22 02 : 42 I f Flag l iN eh an 47 : P91 01 : 16 02 : 45 48 : P91 01 : 26 02 : 46 Flag 6 is reset Set high Port 6 49: P86 01 : 9 Do Call Subrou tine 9 50 : P95 End 51 : P85 Beginning o f Subroutine 01 : 3 Subr outine Number 52 : Volt ( DIFF ) Rep 5 5000 mv slow Rang e 8 IN Cha n 40 Loc : 05 : . 01221 Mult 06 : 0 . 0000 Offset 53: Z; X+F P34 01 : 40 X 02 : 273 . 15 F 03 : 53 z Loc 54 : P38 01 : 53 02 : 54 03 : 18 Z;X/Y X Loc y Loc z Loc 55 : P38 01 : 53 02 : 55 03 : 19 Z;X / Y X Loc y Loc z Loc P2 01 : 02 : 03 : 04 : Flag 2 is set Set high Port 1 Flag 2 is reset Set high Port 2 I f Flag / INchan Flag 6 is set Port 5 Set ~ I f Flag l iNehan 1 XII 56 : P95 End Apply LI-COR coefficients to C02 and H20 57 : PBS Beginning of Subroutine 01 : 7 Subroutine Number 58: P36 01: 41- 02 : 43 03: 41 -- Z;X*Y X Loc Y Loc z Loc 59: P91 01: 27 02 : 30 I f Flag I IN chan Flag 7 is reset Then Do ****LI-COR C02 polynomial**** 60 : 61: Polynomial Rep X Loc F (X) Loc P55 01 02 41 03 28 04 0 05 150.94 06 7.6016 07 7 .7 366 08 -1. 0380 09 0 . 0669 cs P94 Else co Cl C2 C3 C4 ****LI - COR H20 polynomial**** 62 : P55 01 1 02 42 03 29 04 0 05 6.275 06 3.0456 07 - 0 . 0038 08 0 . 000 0 09 0.000 0 Polynomial Rep X Loc F(X) Loc Cl C2 C3 C4 C5 63 : P95 End 64 : P95 End 65 : P85 Beginning of Subroutine co 01: 8 Subroutine Number 66: P31 01: 30 02: 38 67 : P31 01 : 31 02 : 39 68: P31 01 : 10 z;x X Loc z Loc z;x X Loc z Loc z;x X Loc XIII 02: 36 z Loc 69 : P31 01: 11 02: 37 Z=X X Loc z Loc 70: P91 01: 12 02 : 30 I f Flag I INchan Flag 2 is set Then Do 71 : P87 01 : 0 02: 4 Beginning of Loop Delay Loop Count 72 : P37 01 : 36 -02: - 1 03 : 36 -- Z=X*F X Loc F z Loc 73: P95 End 74: P94 Else 75 : P95 End 76: P95 End 77: P85 Beginning of Subrout: i ne 01 : 9 Subroutine Number 78: P22 01: 3 02: 0 03: 2 04:0 Excitation with Del a y EX Chan Delay w/EX (units= . Olsec) Delay after EX (units= . Olse c ; mV Excitation 79: P86 01 : 51 Do Set low Port: 1 80: P86 01 : 52 Do Set low Port 2 P86 Do Set low Port 3 81 : 82 : 01: 53 P86 Do Set low Port 83: P86 Do 0155 Set: low Port 5 84: P86 01 : 56 Do Set low Port 6 85: P95 End 86 : p End Table 3 4 Mode 4 Output Options 01: 00 Tape/Printer Option 02 : 00 Printer Baud Option XIV A Mode 10 Memory Allocation 01 : 69 Input Locations 02: 75 Intermediate Locations Mode 12 Security 01 : 00 0000 02 : (OSX - 0) Security Option Security Code Input Location Assignments (with comments) Key: T=Table Number E=Entry Number L=Locatlon Number E L 4 1 Loc : [SE T amb C] 6 2 Loc : [SVP kPa] 7 2 z Loc [e amb kPa] 8 3 z Loc [ :H2 0 mM/M] 9 3 z Loc [:H20 mM/M] 10 4 Loc : [ :Rn J 12 4 z Loc 14 4 z Loc 16 5 Loc 17 5 z Loc 18 6 Loc 19 6 z Loc 20 7 Loc [ : Tsoil 5 8 Loc [RH fraction] 3 9 Loc [ BATT VOLTAGE] 6 10 Loc 8 10 Loc 17 10 Loc 28 12 z Loc 26 13 Z Loc 27 14 z Loc 21 16 Loc : [ : Wind speed 22 17 Loc: [:Winddir] 54 18 z Loc 55 19 z Loc 31 21 z Loc 7 23 z Loc 9 23 z Loc 27 Z Loc 24 28 24 z Loc 25 26 z Loc 26 26 z Loc 60 28 F (X) Loc 3 32 ' Loc : [ :del air TC] XV