A drought-induced pervasive increase in tree mortality across Canada's boreal forests

Drought-induced tree mortality is expected to increase worldwide under projected future climate changes(1-4). The Canadian boreal forests, which occupy about 30% of the boreal forests worldwide and 77% of Canada's total forested land, play a critical role in the albedo of Earth's surface(5) and in its global carbon budget(6). Many of the previously reported regional-scale impacts of drought on tree mortality have affected low-and middle-latitude tropical regions(2) and the temperate forests of the western United States(3), but no study has examined high-latitude boreal regions with multiple species at a regional scale using long-term forest permanent sampling plots(7-9). Here, we estimated tree mortality in natural stands throughout Canada's boreal forests using data from the permanent sampling plots and statistical models. We found that tree mortality rates increased by an overall average of 4 : 7% yr(-1) from 1963 to 2008, with higher mortality rate increases in western regions than in eastern regions (about 4.9 and 1 : 9% yr(-1), respectively). The water stress created by regional drought may be the dominant contributor to these widespread increases in tree mortality rates across tree species, sizes, elevations, longitudes and latitudes. Western Canada seems to have been more sensitive to drought than eastern Canada.