FORESTS:

Warming is killing older Western tree stands, study finds

This article first ran in ClimateWire.

Trees in the United States' oldest forests are dying at faster and faster rates due to climate change, according to a new study that says global warming could reshape tree stands across the West.

That is because new trees are not sprouting fast enough to replace the dying trees, finds the research, published last week in the journal Science.

Over the past two to three decades, the death rate for trees in the West's old-growth forests has doubled, a phenomenon the new study pins on climate change's effects on the region's water cycle. As the West has warmed, snowpack has decreased and spring snowmelt has come earlier in the year, extending the summer drought.

"If current trends continue, forests will become sparser over time," said Phillip van Mantgem, a U.S. Geological Survey ecologist and the study's lead author. "Average tree ages will eventually decrease by half."

In practical terms, that suggests that the region's forests could see fewer, smaller trees, a change with far-reaching implications.

"In the future, forests might store less carbon than they do at present," said van Mantgem. "It introduces the possibility that Western forests could become net sources of carbon dioxide to the atmosphere, further speeding up the process of global warming."

Over time, the loss of trees could also make forests less desirable habitats for some species, like spotted owls, and increase the risk of forest fires. Warmer temperatures are also preferred by insects and pathogens, like the bark beetles that have ravaged trees in the West.

"It's very likely that mortality rates will continue to rise," said Nathan Stephenson, a USGS research ecologist and an author of the study.

Stephenson and his colleagues -- a mix of federal and academic scientists -- examined old-growth forest plots at 76 sites scattered across Arizona, California, Colorado, New Mexico, Oregon, Washington and southwestern British Columbia.

Multiple species involved in die-off

The increase in tree death was evident, to varying degrees, at all the sites, home to a mix of trees and located at differing elevations. Pines, firs and hemlocks were among the species included in the study.

The researchers examined a range of possible culprits, ruling out things like insect infestations, forest fires, increased competition between trees growing near each other, and even old trees falling and crushing younger ones, before finally settling on climate change.

"One possible cause is that trees are under more drought stress," Stephenson said. "The other is that warmer temperatures are often better for insects and pathogens that like to eat trees. And it's also possible both could be acting on these forests."

Steven Running, an ecologist at the University of Montana who was not involved in the study, said the work "methodically ... crawled through every other possible interpretation" before pinning the blame on climate change.

"These forests have grown up in a water-limited climate that is now getting even more arid with global climate change, shorter winters and longer, drier summers," he said. "This study gives us a glimpse of how fundamentally forest ecosystems may change in the coming century."

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