Radial growth and δ 13 C responses of black spruce to climate

Background/Question/Methods

In predicting the impacts of environmental change on ecological systems, accounting for individual variation and local environmental heterogeneity is essential. However, this is often neglected in dendroecological research of forest ecosystems, where examining population responses to climate removes the individual variation attributed to local environmental conditions.  In this study, we use a novel combination of multivariate statistics and traditional dendroecology methods to analyze growth-climate responses of individual trees within sites and determine how responses vary between landscape positions and regions in the boreal forest. Previous research in the boreal forest has largely emphasized negative growth responses to climate in forest species and habitats characteristic of drier or warmer landscape positions. Here, we examine black spruce, a dominant tree species typical of cool and moist habitats in the boreal forest. We explored relationships between climate and tree-ring growth and assessed the variation in responses of individual trees within sites, and between landscape position and stand characteristics across four different regions in Alaska and Yukon Territory. As environmental conditions are known to influence the relative amount of ¹³C to ¹²C in individual tree rings, we also used stable carbon isotope analysis on a subsample of trees to verify the observed growth-climate responses. 

Results/Conclusions

We assessed growth-climate responses of 421 black spruce trees throughout the northern boreal forest. In all regions, trees exhibited reduced radial growth in response to warm temperatures in the previous growing season and current spring, which we interpret as a signal of drought stress. This signal is supported by our preliminary stable carbon isotope results, where δ¹³C in tree rings was positively associated with spring and summer temperatures.  We found considerable variation in the growth responses of individual trees within sites. Landscape position and stand characteristics were relatively weak predictors of the variability in radial growth responses of black spruce to climate, explaining ≤10% of the variation in any region. Our results indicate that future warming, particularly in spring, is likely to result in drought stress and a reduction of black spruce radial growth independent of region, landscape position, or stand characteristics. The occurrence of negative growth responses to temperature, even in cool and moist habitats, suggests that drought stress limitations may be more widespread in the northern boreal forest than previously anticipated, indicating broad sensitivity of ecosystem processes and services to climate change across a diverse range of habitat types.