Drought has emerged as a major threat to the world's forests. A study shows that tree mortality in Canada's boreal forests has increased by nearly 5% per year — much higher than expected — owing to water stress from regional warming.

1found that annual mortality rates increased from less than 0.5% per year to values as high as 4.0% per year, with a significantly greater increase in Western Canada. This study and other recent reports of widespread droughtinduced tree mortality 2 suggest that the world may already be entering a new era of tree death and forest transformation — processes that will reduce the contribution of forests in removing anthropogenic carbon dioxide emissions from the atmosphere. This could lead to further acceleration of the rate of increase in atmospheric carbon dioxide concentration and potentially even more drought stress for forests. A recent analysis of inventory data of the world’s forests 3 revealed a significant increase in the mass of carbon accumulating in the dead wood of boreal forests, mainly because of tree mortality from insects and wildfires. The study by Peng and colleagues 1