Rapid warming in boreal region since the mid-20th century has led to widespread changes in the growth dynamics of these high-latitude forests, particularly through shifts in the climatic drivers of growth of several tree species. One such species is black spruce (Picea mariana) - the most dominant and widespread tree species in the North American boreal region. Here, we investigate the variable responses of black spruce to rapid climate change across its range, in terms of local productivity and climatic drivers of growth. We predict that warming temperatures will promote growth at the northern limits where forest extent and tree growth are thought to be temperature-limited and reduce growth at the southern limit due to increased evapotranspirative drought stress. We took tree ring samples of 800 trees from four sites across 15° of latitude, effectively encompassing the complete latitudinal extent of black spruce in Western Canada. Archival climatic data from stations near all sites was used to determine climate-growth relationships at each site over the period of 1945-2006. In addition, δ13C isotope analysis was performed on a subset of trees from each site to investigate moisture stress and CO2 fertilization signals.
Results/Conclusions
The growth trends of three sites showed significant increases over time, with higher rates of annual growth since ~1975. On the contrast, our mid-latitude site has shown declining annual growth. We found that the increases in growth at our two northern sites were driven largely by increasing temperatures and reduced winter precipitation, which both contribute to extending the short growing season. Neither of these sites exhibit significant trends in 13C isotope signatures. Growth at our southernmost site was responsive to precipitation variables that influence moisture availability at the start of the growing season, indicating sensitivity to drought conditions during bud-break. The mid-range site showed positive growth responses to precipitation and negative responses to temperature over the growing season, which also points to summer drought stress. Patterns in 13C isotope signatures at the southernmost and mid-range site indicate increases in moisture stress over time. While we propose that year-to-year changes in growth at the southernmost site is driven by temperature-induced evapotranspirative demands, the declining growth over time at the mid-range site is likely a consequence of local drought stress due to permafrost thaw dynamics. Understanding the variability in within-species growth responses to climate change is crucial to predicting boreal forest resilience.