In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light. When drought conditions go away, the models assume a forest’s recovery is complete and close to immediate. That’s not how the real world works. – William R.L. Anderegg

SALT LAKE CITY — A global look at how long it takes forests to recover from drought is shattering old assumptions about how quickly the trees bounce back, with scientists learning it is a much longer process.

“This really matters because in the future, droughts are expected to increase in frequency and severity due to climate change,” says lead author William R.L. Anderegg, an assistant professor of biology at the University of Utah. “Some forests could be in a race to recover before the next drought strikes."

The study, published Thursday in the journal Science, found that living trees took an average of two to four years to recover and resume normal growth rates in the aftermath of a drought.

Because forest trees play a significant role in buffering carbon dioxide emissions by "storing" the carbon in woody tissue, researchers now fear that drought is compromising their role in a changing climate.

“If forests are not as good at taking up carbon dioxide, this means climate change would speed up,” said Anderegg, who performed much of the work on this study while at Princeton University.

Anderegg and other researchers used records from the International Tree Ring Data Bank to look at the recovery of tree growth after severe droughts at 1,300 forest sites that have played out since 1948. The rings tell the history of wood growth and also track carbon uptake of the ecosystem in which the tree is found.

The data showed that trees in a majority of the world's forests struggled for years after a drought had ended.

On average, trunk growth took two to four years to return to normal, according to the research, and long-lasting effects of drought were most prevalent in dry ecosystems, with scientists suspecting growth may be impaired due to lasting damage to vascular tissues that compromise water transport.

The correlation between tree growth and challenges caused by drought is further complicated by research that indicates certain regions are experiencing higher temperatures.

“The fact that temperatures are going up suggests quite strongly that the western regions of the United States are going to have more frequent and more severe droughts, substantially reducing forests’ ability to pull carbon from the atmosphere," Anderegg said.

The drought that hit the Southwest during 2000-2003 drives the point home, highlighting a deficit in precipitation comparable to earlier droughts, but with temperatures that were as much as 6 degrees hotter than long-term averages.

“It really seemed to make the drought lethal to vegetation where previous droughts with the same rainfall deficit weren’t,” Anderegg said.

The impact of delayed recovery from drought on carbon storage is not trivial. Over a century, carbon storage capacity in semi-arid ecosystems alone would drop by about 1.6 metric gigatons — an amount equal to about one-fourth of the entire U.S. emissions in a year.

“In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light. When drought conditions go away, the models assume a forest’s recovery is complete and close to immediate,” Anderegg says. “That’s not how the real world works.”

Anderegg added that the research reveals some startling information about forest health in general.

"The concern here is that the fate of forests around the world and here in the West is quite uncertain."

Anderegg co-authored the study with colleagues at Princeton, Northern Arizona University, University of Nevada–Reno, Pyrenean Institute Of Ecology, University of New Mexico, Arizona State University, U.S. Forest Service Rocky Mountain Research Station, NOAA Geophysical Fluid Dynamics Laboratory, and the Lamont-Doherty Earth Observatory of Columbia University.

The research follows that of a study released earlier this month that revealed trends in fire weather seasons across the globe have increased by nearly 20 percent and the burnable area has doubled over the last 35 years.

Scientists from the USDA-Forest Service, South Dakota State University, the Desert Research Institute and the University of Tasmania, Australia looked at climate-induced variations in global wildfire danger from 1979 to 2013.

That study, published in the international journal Nature Communications, found that wildfire activity is driven by three key factors: fuels, sources of ignition and weather, which is the largest "driver" of a regional burned area.

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