Stand basal area and temperature interact to influence growth in white spruce in southwest Alaska

Abstract

Warming temperatures are having a disproportionate effect on boreal ecosystems, influencing the establishment and growth of tree species across their respective ranges. However, less is known about how competitive interactions influence growth–climate relationships. We used tree-ring data from 26 study plots (836 trees) to investigate how the growth patterns of white spruce (Picea glauca) respond to variation in competition and climate in southwest Alaska. Using linear mixed-effects models, we compared growth against covariates for stand basal area, competition, climate (temperature, precipitation), and spruce beetle mortality. We characterized competition at the tree level using two distance-dependent competition indices: Hegyi index and a height advantage index. Furthermore, we assumed that stand-level competition increased with basal area. We found that high summer temperatures resulted in reduced growth in stands with higher basal area (i.e., the most crowded stands), but increased growth at sites with low basal area. Our results suggest that historically productive stands may show declines in growth under warmer conditions, with important implications for future stand structure and productivity.