Regeneration and ecophysiology of white spruce in aspen-dominated boreal mixedwoods

The ecophysiology of white spruce (Picea glauca (Moench) Voss.) and the use of shelterwood silvicultural systems to regenerate white spruce were examined in aspen (Populus tremuloides Michx.)-dominated boreal mixedwoods in central Alberta. Photosynthetic responses to light and temperature in potted seedlings grown for two years in open sites and in the understory of aspen suggested that white spruce has a strong ability to acclimate to low light environment by increasing photochemical efficiency and decreasing light compensation and saturation points. Understory seedlings also had higher light-saturated net photosynthetic rate, transpiration, stomatal conductance and dark respiration than open-grown seedlings in spring and especially in autumn when night frosts were frequent. In saplings, photosynthesis commenced in early April despite frozen soil and night frosts and stopped abruptly in late October when minimum air temperature dropped to {dollar}{lcub}-{rcub}10sim{lcub}-{rcub}15spcirc{dollar}C. Existence of an overstory canopy greatly reduce the fluctuation of net photosynthesis of understory saplings during periods of drought and high temperature in the summertime compared to saplings on open sites. In spring and fall, when overstory aspen is leafless, white spruce saplings are photosynthetically active and able to use the high light resources despite low soil temperature (in spring) and night frosts.; In regeneration trials, microclimates, seed germination and seedling establishment in the first two years after planting were compared under various levels of canopy residual: two shelterwood seeding cuts (low and high canopy residuals), one conventional clearcut and one control (uncut). Each overstory treatment was subdivided into three site preparation treatments (blading, mixing and control). The partial canopy of the shelterwoods offered a more benign environment: increased light transmission and soil and air temperatures in comparison with the control, and less environmental extremes (higher humidity, cooler maximum, warmer minimum and reduced risk of night frosts) compared to the clearcut. Shelterwoods had earlier budflush, higher terminal leader survival, greater height growth, and better seed germination and recovery of photosynthesis than the clearcut and the control. Effects of site preparation were also apparent, especially in soil temperature and seed germination. Seedlings in shelterwoods can be expected to establish better than those in clearcuts and uncut stands.