So how, exactly, does global warming kill forests?

Global warming has killed half a billion trees across the U.S.

Global warming is killing forests around the world. @bberwyn photo.

Staff Report

Scientists tracking massive forest die-offs say a new study may help forest managers learn how to predict which trees will succumb to global warming — and what the implications are for the global carbon balance.

“There are some common threads that we might be able to use to predict which species are going to be more vulnerable in the future,” said University of Utah biologist William Anderegg, explaining that recent tree-killing droughts in the western U.S. were marked more by elevated temperature than by a lack of rainfall.

“These widespread tree die-offs are a really early and visible sign of climate change already affecting our landscapes,” Anderegg said.

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Forest mortality has been stunning in recent years. After combing forest studies, Anderegg and his fellow researchers estimated that 225 million trees died across the Southwest in a 2002 drought. The Texas drought of 2011 killed about 300 million trees and the recent California drought killed another 12 million. And some research suggests that many forests won’t be able to recover from such mega-disturbances.

The new findings, published in Proceedings of the National Academy of Sciences
will help climate scientists account for changes in carbon dioxide storage by the world’s forests. Globally, trees absorb CO2 equal to about 25 percent of anthropogenic carbon dioxide emissions. Dead forests don’t absorb CO2, and also release some of it back to the atmosphere as the tree decompose.

Anderegg said the quest to include that information in climate models spurrted the search to understand the physiological reasons trees die during droughts. After looking at the results of more than 30 other studies, covering 475 tree species and more than 760,000 individual trees, the researchers tallied mortality rates for each species, then compared those to 10 tree physiological traits, searching for commonalities.

The traits included typical tree characteristics, such as wood density, rooting depth and basic leaf characteristics, such as whether the species was an evergreen or a deciduous tree. Other traits concerned the hydraulics of how water moves through trees. Three traits significantly predicted tree drought mortality, all related to hydraulics.

During a hot drought, evaporation from leaves drives higher water demand. At the same time, if water is harder to come by due to drought, the tree’s roots have to pull harder to draw scant water from the soil, increasing the tension on the pipes that transport water from the roots to the leaves.

At a certain point, the tension on the pipes throughout becomes so great that bubbles of air enter the pipes and block the flow of water. It’s called an embolism.

“It’s a little bit akin to a tree heart attack,” Anderegg said. “You can actually hear this on a hot summer day if you stick a microphone up to a tree … you can hear little pings and pops as these pipes get filled with air.” The blocked pipes lead to tree death.

“With drought projected to increase, being able to predict tree death is imperative to understanding changes in Earth’s forests,” said Liz Blood, program director in the National Science Foundation Division of Environmental Biology, which funded the research. “This study offers important new insights into how and why trees die from drought, and which trees are at greatest risk.”



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