Drought Damage from Climate Change Leads to Widespread Forest Death

From 2000 to 2003, the American southwest experienced a widespread drought that impacted forests throughout the region. Trees began to die-off and retreat. Now, scientists have created a new modeling tool to explain how and where trembling aspen forests died as a result of this drought.

In this latest study, the researchers wanted to address a longstanding disagreement over how climate change caused by the emission of greenhouse gasses will affect forest ecosystem. One impact of rising carbon dioxide levels is that trees will be benefitted by being able to use water more efficiently. A negative consequence, though, is the possibility of increasing droughts that can cause forest trees to die off. In fact, most current models of forests under climate change cannot predict when or where forests might die from temperature and drought stress.

The drought-triggered aspen die-off actually impacted about 17 percent of aspen forests in Colorado, as well as in parts of the western United States and Canada. In order to create an accurate model, the researchers focused on the physiology of how drought kills trees.

Drought causes damage to the vascular system that transports water throughout the tree. The researchers were actually able to identify a threshold over which drought conditions start to degrade an aspen's water-transporting vascular system. Then, the researchers incorporated this information into a model designed to predict drought-induced forest mortality. They tested the model against regional forest mortality observations from scientific forest plots, aerial surveys and satellite measurements.

So what did they find? Future projections revealed that in a world of continuing high greenhouse gas emissions, the threshold for widespread drought-induced vascular damage would be crossed and initiate widespread tree deaths on average in the 2050s. However, lower greenhouse gas emission scenarios did not always lead to widespread tree mortality.

"Finding the thresholds in planet physiology after which climate stress causes tree mortality will allow us to resolve uncertainty over the fate of forest ecosystems in a changing climate," said William Anderegg, one of the researchers, in a news release. "But most importantly, a lot rides on human decisions to slow climate change. The clock is ticking on the future of these forests."

The findings are published in the journal Nature Geoscience.

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