Increasingly frequent droughts are affecting the rate at which the world's forests can capture carbon, compromising their ability to serve as a buffer between human-caused carbon emissions and climate change, researchers say.
Current climate models that assume forests can bounce back rapidly from drought need to be rethought in the light of new study findings, researchers at the University of Utah say.
Rather than the quick bounce-back, living trees need an average of two to four years to resume their normal growth rate after a drought ends and resume their full carbon-storing capability.
During that recovery period, they are less able to act as carbon sinks, the researchers explain.
Forest trees have a significant role in alleviating the impact of human-induced climate change by taking massive amounts of carbon dioxide emissions out of the atmosphere and incorporating the carbon into their trunks, branches and leaves.
"This really matters because in the future droughts are expected to increase in frequency and severity due to climate change," says lead study author and biology professor William R.L. Anderegg. "Some forests could be in a race to recover before the next drought strikes."
For the vast majority of the globe's tree species, their recovery rate from drought has been largely unknown, so the Utah researchers used tree ring growth records to measure it for droughts from 1948 to the present.
Records from more than 1,300 global forest sites gathered for the International Tree Ring Bank were analyzed in the study published in the journal Science.
In most of the world's forests, trees had years-long struggles to recover after droughts, they found.
The carbon storage capacity lost to drought conditions is anything but trivial, the researchers note; over the span of a century, carbon storage capacity in semi-arid ecosystems alone would be reduced by around 1.6 metric gigatons and amount equal to around a quarter of the entire U.S. emissions annually.
In what Anderegg describes as a vicious cycle, forests reeling from drought accelerate climate change, and climate change, in turn, kills more forests.
"In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light," Anderegg says. "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."
For that reason, current Earth climate system models should be reworked to include the impact of drought on forests, to provide more accurate predictions of the way in which drought might alter the global carbon cycle, the researchers say.
