Growing up in Western Colorado, William Anderegg has fond memories of countless hiking, fishing, and camping trips in the Rockies. But when he returned as an adult, he found a changed landscape. “A lot of these forests, just within my lifetime, had really died off,” says Anderegg, a post-doc at Princeton University.
While decades of research have given scientists a pretty good understanding of how trees grow, we still don’t really know much about how they die. “We don’t have a very good monitoring network of tree mortality, but the pieces of the puzzle that we can see don’t look very good” Anderegg says. Tree deaths have hit North America particularly hard, affecting multiple forest types and tens of millions of acres. Widespread deaths of trembling aspen, the most widely distributed tree species across North America, have “stretched from Arizona to Alberta, and seem to have been entirely triggered by drought and climate change,” he says.
As the climate warms and droughts become more frequent and severe, predicting how trees will respond to the stress becomes ever more important. Trees take in water from the soil around their roots, and try to hold on to it against the pull of gravity and transpiration, like competing ends of a spring; as drought progresses, the soil gets drier, the atmosphere gets hotter, and the spring is pulled tighter and tighter. In a new study published today in Nature Geoscience, Anderegg and his colleagues measured water flow in tree branches to find the threshold at which water flow was reduced enough that aspen trees would succumb to drought. Using this threshold, they made predictions about past mortality events and found that they matched observations about 75 percent of the time.
Atmospheric water demand—the amount that the hot, dry atmosphere pulls water from the leaves of trees—had an even more important role in tree death than the amount of rain.
“It predicts where trees died relatively well, which give us confidence that it’s going to be useful for the future,” Anderegg says. Next, they compared the mortality threshold they found to a set of six climate projections based on emissions scenarios with varying levels of precipitation. They found that in high-carbon emissions situations, deadly droughts will likely occur on an almost yearly basis after the 2050s.
The team was surprised to find that atmospheric water demand—the amount that the hot, dry atmosphere pulls water from the leaves of trees—had an even more important role in tree death than the amount of rain. This is significant, according to Anderegg, because while precipitation levels varied across all six climate projections, temperatures consistently climbed, making the atmosphere hotter and drier, further heightening atmospheric water demand.
“It’s not a pretty picture, but it doesn’t mean that these forests are doomed,” Anderegg says. In lower emissions scenarios, not all of the climate projections resulted in drought-induced aspen deaths. “We do control some of the future of these forests, particularly in taking action early to address climate change,” he says. “We’re at this place where the past starts to fork, and we can choose to really protect these forests or not.”
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