Effects of overstory paper birch (Betula Papyrifera March) on physiology, growth, and biomass responses of under-planted conifer seedlings

This thesis tested whether two- and three-year old under-planted Abies /asiocarpa (Hook.) Nutt., Thuja plicata Donn ex. D. Don, Pseudotsuga menziesii var. glauca (Beissn.) Franco, and Picea glauca x P. engelmannii (Moench) Voss seedlings responded to changes in light quantity and spectral quality (measured by red to far-red ratios) under five densities of overstory paper birch (Betula papyrifera Marsh.) on a site in the interior cedar hemlock (ICHmk2) subzone, about 80 km east of Williams Lake, British Columbia. The five B. papyrifera densities were 0 (clearcut), 400, 800, 1200 stems ha-1 , and unthinned (> 10,000 stems ha-1 ). Generally, light quantity and red to far-red ratio increased three-fold in the clear-cut compared to the unthinned paper birch treatment. Conifer photosynthesis and percent foliar nitrogen was greater on the clear-cut compared to other paper birch densities. Total biomass was three-fold greater in the clear-cut grown seedlings of A. lasiocarpa, P. menziesii, and P. glauca x P. engelmannii, while for the most shade tolerant species, T plicata, the difference was two-fold. Similarly, relative growth rate increased for all species as birch density decreased. Results showed that A. lasiocarpa, P. menziesii, and P. glauca x P. engelmannii were more plastic in their photosynthetic and growth responses to increasing light compared to T plicata. Growth response to variation in light may be an important factor when deciding on appropriate densities of overstory paper birch for different under-planted conifer species.