COS 131-1 - Variable growth responses to climate change seen in boreal tree rings

Background/Question/Methods

In the boreal forests of the northern high latitudes, tree ring widths have been historically positively correlated with temperature; however, that relationship is becoming less notable with anthropogenic climate change (D’Arrigo et al., 2008). Given the highly heterogeneous nature of the boreal landscape, tree rings provide an annually-integrated look at the effects of site-specific variables. Using the NOAA International Tree Ring Database, we identified 53 sites (2191 trees, white and black spruce) and examined the relationship between annual tree growth and environmental variables, including annual modeled temperature and precipitation data (Climate Research Unit), site-specific soil conditions (SoilGrids), and topographic variables (DEM). We analyzed growth trends using linear mixed-effects models in order to determine the contribution of different variables to post-1980 growth trends. To analyze site heterogeneity, we used a Principal Components Analysis (PCA) of the site-specific factors to determine how sites related to each other, and to elucidate potential underlying patterns in temperature-growth correlations.

Results/Conclusions

Our results indicate that while average June temperature has a strong effect on growth, the effects of many site-specific variables are included in the model and moderate the effect of June temperature. Latitude and longitude had the strongest effects on post-1980 growth trends. May temperature had the strongest negative effect on growth trends, suggesting the risk of late spring frost damage in a warming North. The PCA of site-specific soil and climate variables showed three clusters of sites which align with sand, silt. and clay soil textures. A majority of sites with decreasing post-1980 growth-temperature correlations clustered around the sand loading vector, which is also associated with soil carbon and nitrogen; in contrast, a cluster of sites with increasing post-1980 growth-temperature correlation were associated with the clay loading vector. Our findings provide further detail into the pattern documented by many studies in the boreal region that find some sites experiencing increasing growth in response to warming, while other sites are stationary or decreasing growth. The amelioration of temperature limited growth in boreal forests due to climate change is very likely mediated by the complex heterogeneity of the landscape.