Increasing drought stress in the southwestern United States could lead to widespread diebacks of aspen forests by the mid-twenty-first century, according to a study published online in Nature Geoscience.

During droughts, as plant demand for water outstrips supply, bubbles of air can be introduced into plant vessels, severely reducing the flow of water and ultimately leading to mortality. While droughts are expected to become more intense and frequent in response to a warming climate, current models are not able to predict forest dieback.

William Anderegg and colleagues used field measurements taken during the summers of 2010, 2011, and 2013 to test water flow in the branches of aspens, and discovered a critical threshold above which drought conditions reduced water flow in the trees enough to cause mortality. They incorporated this threshold into a detailed plant hydraulics model and were able to predict historical aspen forest die-offs with 75% accuracy, supporting the utility of the threshold for predicting tree mortality. Running this model using climate data from six general circulation models, they find that high greenhouse-gas emissions could create drought conditions that exceed the threshold throughout much of the southwestern United States by the 2050s.