Climate-mediated lodgepole pine tree growth response to thinning and fertilization in interior British Columbia

Abstract

In the interior British Columbia, forest managers use thinning to reduce inter-tree competition and soil fertilization to enhance the soil nutrient status for improved tree growth. In recent decades, thinning has been promoted as a tool to increase tree resilience to drought. However, fertilization may modulate tree response to drought by altering biomass allocation from below- to above-ground depending on soil moisture availability. Here, we analyzed two long-term thinning and fertilization trials set up in fire-origin lodgepole pine (Pinus contorta Douglas ex Loudon) stands in two biogeoclimatic subzones in interior British Columbia that differ in soil productivity. Both trials featured untreated control, fertilized-only, thinned-only, and the combination of thinning and fertilization treatments. Drought Index (i.e., Standardized Precipitation Evapotranspiration Index, SPEI) was estimated for each site and used as a covariate in a linear mixed effects model to assess the impacts of thinning and fertilization on tree growth sensitivity to drought. Tree growth was increased by fertilization at the very dry-cold site, and more so in the thinned plot than the unthinned plot. The opposite effect of thinning on tree growth was found at the dry-cold relatively fertilized site. Tree growth was less sensitive to drought in the thinned-only plots at both sites. Our results, however, showed increased tree growth sensitivity to drought under the thinning and fertilization combination treatment at the less dry and relatively more productive site. The results suggest that the combination of thinning and fertilization can increase tree growth on very dry-cold and dry-cold sites and may be a suitable strategy to mitigate drought stress on tree productivity.