Impact of soil disturbance by forest harvest on soil properties and tree growth in paludified black spruce stands

Background/Question/Methods The clay belt region of eastern Canada supports an important forest industry. In this region, black spruce stands are generally slow growing and frequently paludified, and are harvested by careful logging in order to minimize soil disturbance. Careful logging, by providing protection to soils, may contribute to a long-term decline in black spruce forest productivity by promoting post-harvest paludification, a natural process during which a forest stand gradually turns into a forested peatland via the accumulation of a thick organic layer and water table rise. These new soil conditions create unfavorable conditions for the humification of organic matter and contribute to reduce tree growth. While it is now established that paludification of black spruce stands on the clay belt is promoted by current harvest practices, research is needed to identify harvest methods that could delay post-harvest paludification and restore forest productivity. The objective of our research was to evaluate how soil disturbance created by alternative harvest methods, i.e. unrestricted machinery circulation during summer and winter clearcutting, affects soil fertility, and the regeneration and growth of black spruce compared to careful logging. Results/Conclusions Preliminary results show that at the tree scale, tree growth was positively correlated to soil N_tot, Ca_exch, Mg_exch, and pH, and negatively correlated to C:N in all harvest types. At this scale, the growth rate of individual black spruce was similar among harvest methods. At the stand scale, however, tree regeneration and growth was favored by summer clearcuts in contrast to winter clearcuts and careful logging. While stocking was similar among harvest methods, there were 3 times more trees >3m, and 5 times more trees >4m in summer clearcuts than in winter clearcuts and careful logging.These differences could be attributed to a greater abundance of favorable microsites created during summer clearcuts. Thus, at the stand scale, summer clearcuts favored black spruce regeneration and growth compared to winter clearcuts and careful logging, and could be used to delay paludification and restore forest productivity in paludified stands on the clay belt region of eastern Canada.