Effects of nitrogen application on growth and mortality of stemwood biomass in coastal forests of British Columbia

Abstract

Forest nutrient management (FNM) is implemented as a climate change mitigation strategy in coastal British Columbia, Canada. To help parameterize ecosystem carbon balance responses to nitrogen (N) application, we summarized 10-year response of stemwood biomass fluxes to single applications of urea across 68 unique sites. Adding 225 kg N ha⁻¹ increased gross growth and mortality in unthinned coastal Douglas-fir and western hemlock. The increase in net growth was only significant in Douglas-fir owing to high mortality in western hemlock. Responses of gross and net growth were greater in thinned vs. unthinned Douglas-fir. Gross growth response was similar in thinned and unthinned stands of western hemlock, while the mortality response was lower in thinned vs. unthinned stands, leading to significant responses of net growth in thinned western hemlock. Responses to 450 kg N ha⁻¹ were consistently greater than those to 225 kg N ha⁻¹, but nitrogen-use efficiency was lower at the higher dose. In a multiple linear regression analysis, responses increased with stand density index (SDI) and decreased with average tree height at age 50 years (H₅₀). We propose that the correlations with SDI and H₅₀ may represent variation in N demand and supply, respectively. We describe how this predictive model might be applied to improve performance of FNM across the region.