How does overstory retention affect above- and below-ground resource availability and seedling performance?: implications for silvicultural systems choice in the ESSF

In subalpine forests, low light, low soil temperature, and low nitrogen (N) availability may co-limit tree growth. We used greenhouse and field experiments to quantify the effects of these factors on the growth of Picea engelmannii and Abies lasiocarpa seedlings from the subalpine forests of British Columbia. For approximately 500 nitrogen fertilized and unfertilized eight-year old planted seedlings from shelterwood, patch cut, and clear-cut stand treatments, growth was examined as a function of soil temperature, light, and stand treatment. In a greenhouse, growth and photosynthesis were examined at root temperatures representing the range of field growing season temperatures (5, 10, and l5~C). Fertilizer effects on diameter growth in the field were not significant indicating that the seedlings were not N limited. In multiple regression, temperature and light (both partial p<O.OOOl) combined to explain 55% and 42% of the variation in diameter growth for P. engelmannii and A. lasiocarpa, respectively. Picea engelmannii was more sensitive to low soil temperatures than A. lasiocarpa as jndicated by 1) the steeper slope of the growth versus temperature response in the field, and 2) two times greater Relative growth rate (RGR) at l5~C than 5~C root temperature in the greenhouse compared to similar growth rates for A. lasiocarpa at all three root temperatures. No significant effect of soil temperature on net photosynthesis was observed in the field measurements. However, results from the greenhouse study show that net photosynthesis was limited over the range of the temperature treatments. Light-saturated rates of net photosynthesis for P. engelmannii in the l5~C treatment were 1.3 and 2.5 times greater than rates in the 10~C and 5~C treatment respectively. In high elevation areas, light and soil temperatures covary and strongly limit seedling growth such that seedlings growing in partial-cuts could be considered limited by both light and temperature. Growth, root development, photosynthesis, and foliar N concentrations of seedlings were all affected by their proximity to residual mature trees, and especially for P. engelmannii seedlings. These responses have significant implications for efforts to reforest partial-cut areas in subalpine forests.