A global database of geo-referenced drought and heat-induced tree mortality events reveals climatic drivers of forest die-off.

Abstract:

Earth’s forests face grand challenges in the Anthropocene. Warming, drought, and their interactive effects have been mechanistically linked to the death of trees. In addition to chronic warming elevating background rates of tree mortality, extreme heat and drought events lead to pulses of forest die-off. Over the last forty years, hundreds of peer-reviewed studies have carefully documented particular climate-induced tree mortality events on every forested continent. Despite the vital importance of forests to global environmental services (including C and water cycling, biodiversity, and human economies), the fate of historical forests in a warming world remains difficult to predict, partly because broad-scale patterns and trends of climate associated with tree mortality have been difficult to quantitatively describe without a collective global database of tree mortality events. Here, we present a precisely geo-referenced database of global observations of climate-induced tree mortality built from peer-reviewed studies. Spanning all forested biomes, six continents, and over forty years of observations, our database represents the most complete (>1,300 plots) and precisely georeferenced record of climate-induced tree mortality (see abstract image). We encourage broad application of our open-access database for myriad uses—including validating spatially-explicit models predicting tree mortality and improving detection and monitoring for future mortality events. For example, using TerraClimate, a dataset of climate and water balance for global terrestrial surfaces, we defined multiple climate thresholds associated with these tree mortality events; here we show strong global signals for temperature and drought anomaly-related metrics preceding, during, and after mortality events. Under two warming-scenarios (+2℃, +4℃), the frequency of potentially lethal conditions—of the magnitude associated with historic mortality events documented in our database—will markedly increase. Our initial findings are alarming–many of Earth’s historical forests likely are imperiled by further warming and drought.