A non-linear tree mortality increase across the northeastern north american boreal forest due to multiple climate drivers

Background/Question/Methods

Climate change is increasing pressure on forests globally. In addition to modifying disturbance regimes such as fires, there is also an increase in the frequency and severity of droughts. Extended temperature and drought-induced reductions in growth and increases in mortality have been reported across the north american boreal forest. While low-water availibity has been identified as the main mortality driver in western Canada, increased mortality in the east despite abundant precipitation lacks a clear explanation. Our objectives are thus to 1) estimate the historical mortality rates (1970-2014) of the six most abundant species in the boreal forest of Quebec (Betula papyrifera, Picea glauca, Picea mariana, Populus tremuloides, Pinus banksiana and Abies balsamea); 2) establish and quantify the relationship between the mortality rates of these species and key climatic variables. Using data derived from more than four decades of provincial forest inventories as well as the national meteorological station network, mortality probability was evaluated as a function of time and different climate metrics via general additive mixed effect models (GAMM).

Results/Conclusions

Overall, the spatiotemporal patterns of mortality rates show great interspecific divergence. However, all the species under study show a significant increase in the probability of mortality in at least one region of the study area. With the exception of P. banksiana, all species showed significant sensitivity to at least one climate metric. B. papyrifera was the most climate-sensitive species. The mortality patterns observed suggest a considerable negative impact of climate change on the Quebec boreal forest in recent decades, thus demonstrating its vulnerability despite generally abundant precipitation.