The influence of competition and species mixture on plantation-grown white spruce: Growth and foliar nutrient response after 20 years

A 20-year-old experimental white spruce plantation was used to identify key stand (neighbourhood competition) and soil (physical and chemical properties) factors influencing spruce growth (Periodic Basal Area Increment) and foliar nutrients. Total and species-specific competition was estimated using Hegyi's distance-dependent index for 39 individual spruce trees. Twelve trees, covering the range of total HCI (2 to 8) and aspen competition (0% to >75%), were selected for repeated (May through October) foliar sampling. Spruce PBAI declined approximately 10% for each additional unit of total HCI; species did not significantly affect this decline. Increasing aspen presence significantly influenced spruce foliar N (1.17% to 1.31%), P (0.15% to 0.23%), and K (0.68% to 0.88%), but led to declines in Ca (0.81% to 0.48%). Multiple linear regression indicated that soil carbon (partial r(2) = 0.386) and available soil moisture (partial r(2) = 0.131) together explained more of the variation in spruce growth than did competition factors alone (partial r(2) = 0.251). The results suggest that, at this stage of stand development, pre-commercial thinning operations should focus on density control and inter-tree spacing, while retaining an aspen component resulting in well-spaced, free-growing mixed stands of white spruce and aspen.