Study: Droughts, heat will kill more forests than expected

Forests worldwide are vulnerable to growing risks of drought- and heat-induced tree mortality and forest die-off because of a rapidly warming Earth, according to just-published research in the scientific journal Ecosphere.

The paper is an invited “ESA Centennial Paper” to commemorate the 100th anniversary of the Ecological Society of America.

Researchers from the USGS, University of Arizona, and Los Alamos National Laboratory assessed more than 400 research studies on forest mortality to help answer the question about whether forests will be more or less vulnerable to projected climate change in the future, a subject of significant scientific debate.

Their in-depth assessment of diverse results from observational, experimental and modeling studies concludes that forest die-off events to date 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,” said Craig D. Allen, a USGS forest expert and lead author of the research.

The scientists emphasize 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.

“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,” Allen said.

The authors note that even when amplified tree mortality does not cause species range changes or shifts in forest biome boundaries, broad-scale tree mortality fundamentally affects a diverse suite of environmental processes and ecosystem services, including forest community and ecosystem dynamics, the diversity of species, radiation fluxes, biogeochemical processes and associated carbon sequestration, and global earth system consequences and feedbacks.

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. Breshears and Allen are presenting results from this paper at the ESA Annual Conference this week.