Global warming accelerates drought-induced forest death

Nathan McDowell: Everybody knows trees die when there's a drought, if there's bark beetles or fire, yet nobody in the world can predict it with almost any accuracy. So that means we don't understand it. And that means we can't make predictions that are relevant to policymakers about how they can plan ahead for consequences on their citizens. Sound of birds chirping William Pockman: The Sevilleta site is part of a network of sites funded by the National Science Foundation Long-Term Ecological Research Program. Plants don't live and die in three years, landscapes and ecosystems change over 5, 10, 20-year periods. And this network of LTER, as they're called, sites, provide a consistent — a long-term set of data to capture some of those events that you couldn't predict are going to happen. Sound of sprinklers McDowell: We started a study at the Sevilleta where we both droughted trees and irrigate some plots as well, as well as we have control ones, and we compare them to get an understanding of, how much does drought, in the form of a precipitation reduction, affect trees? As everybody could expect, if you reduce the precipitation, trees die.

but the irrigation has revealed also that trees can do really well when it rains a lot, but they may also be more vulnerable to a subsequent drought. So climate predictions suggest for sure that along with more droughts, we should also have more heavy rainfall periods. Those things may actually set up these trees for failure during the next drought. Pockman: We measure sap flow through the trees, so we measure how much water is actually being used by — by the trees. We can measure it at the more — in a more detailed way by clamping a little chamber onto the leaves and measuring how much CO2 is going into or out of leaves. We can sample parts of the plant to understand how water is transported through — through those tissues. Rob Pangle: We'll do multiple sampling across years in different parts of the season to compare and contrast long-term effects versus seasonal effects within a year . McDowell: So it's not just precipitation, a lot of predictions from much of the world say precipitation may not change that much, in the next 100 years, however there's always been variability from year to year from decade to decade in rainfall that has caused tree death. What is changing now is the temperature is going up.

The warmer the air is, the more water it can hold. The more water it can hold, the more it sucks out of the earth after a rain event, out of the plants and out of the soils. Pockman: Because we're not working in a greenhouse with plants in pots, our field manipulations provide great power to understand what actually happens to real plants. McDowell: From experiments like this, from modeling analyses, from observations around the world, all of the data sets point in one direction, Globally, in Europe, Asia, North America, Canada, all of the models are predicting that by 2100, most forests should be heavily, heavily disturbed, if not gone. That's really why we need to keep refining our understanding, not only so we can make better predictions to enable us to adapt, but also to understand how we might deal with these things in the short-term..