New study details global warming impact to forests

‘We expect to see widespread declines in forest productivity’

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Red beetle-killed lodgepole pines in the White River National Forest near Frisco, Colorado. @bberwyn photo.

Staff Report

FRISCO — The vulnerability of the world’s forests to global warming has been widely underestimated, a group of scientists concluded after taking a hard look at all the scientific data on forest mortality.

“We expect to see widespread declines in forest productivity, changes in the species composition and dominance patterns of forest trees, a shift to smaller-sized trees, and reductions in forest extent in some regions,” said U.S. Geological Survey researcher Craig Allen, adding that, even forests in wetter parts of the world are going to be affected by rapidly warming global temperatures.

For the study, scientists with the USGS, the University of Arizona, and Los Alamos National Laboratory teamed up to assess more than 400 research studies on forest mortality. They also included observational and experimental data, as well as modeling results to conclude that forest die-off to0date represent only the beginning of an increasing phenomenon of such mortality episodes.

These tree mortality events will result primarily because of the combination of droughts with warmer temperatures due to projected climate change. The researchers use the term “hotter drought” to indicate the integrated effects of drought and warmer temperatures associated with climate change.

Despite numerous compensatory processes that commonly allow trees to survive drought stress, during hotter droughts, warmer temperatures increase stress and mortality risk for trees both directly through many physiological impacts and indirectly through higher risks from pests and disease.

“This synthesis leads us to conclude that the future broad-scale vulnerability of forests globally is being widely underestimated, including the vulnerability of forests in wetter regions,” Allen said.

The scientists emphasized that their research is not saying that forests globally will collapse concurrently or that most forests in existence today are at risk of disappearing during this century. Instead they anticipate major reorganizations in forest ecosystems due to more tree mortality in coming decades from increasingly extreme hotter droughts.

Even when growing tree mortality doesn’t result in wholesale changes to forests, it can affect internal forest processes, including species diversity, radiation fluxes, as well as biogeochemical processes and associated carbon sequestration.

The paper, On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene, was published in Ecosphere, and authored by Craig D. Allen, USGS; David D. Breshears, University of Arizona, Tucson; and Nate G. McDowell, Los Alamos National Laboratory.

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