New research shows droughts can reduce tree growth for up to four years.
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This can have devastating effects because forests in general have a huge role in soaking up carbon dioxide.
The sight of drought-stricken trees, their wiry frames bare of leaves or needles, is hardly rare in the forests around Flagstaff. But that assumption wasn’t quite on the mark, according to a new study of drought’s impact on the world’s forests.
‘In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light, ‘ says William Anderegg, assistant professor of Biology at the University of Utah, and lead author of the report.
Researchers gathered tree ring data from over 1,300 sites across the globe to measure growth in periods after severe droughts that have occurred since 1948, and found that for the majority of the forests they studied, trees suffered years-long effects post-drought. Canada’s vast boreal forest impacted by drought The researchers used 18 different methods of measuring the impact of drought on forest trees. He added that if forests are not properly storing carbon dioxide, this indicates that climate change would accelerate.
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.
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.
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.
Tree rings give us a good source of information on rate of growth during times of extremes in climate, as well as a record of carbon uptake in the ecosystem where it grew.
They have also made a classification of the most drought-affected species of trees.
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The researchers are unsure of exactly how these droughts cause long-lasting harm to trees, but suggest it could have something to do with factors like: loss of foilage and carbohydrate reserves that later hinder growth; pests and diseases that build up in drought conditions; and lasting damage to the trees’ vascular tissues caused by lack of water. “The fact that temperatures are going up suggests quite strongly that the western regions of the U.S. are going to have more frequent and more severe droughts, substantially reducing forests’ ability to pull carbon from the atmosphere”. 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. “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”.
