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Drought’s impact on forests is incorrect on climate models

Using computer modeling, scientists have always assumed trees bounced back from droughts and other weather extremes fairly quick. But that assumption wasn’t quite on the mark, according to a new study of drought’s impact on the world’s forests.

In a new study of forests around the world, lead author William Anderegg, an assistant professor of biology at the University of Utah, who performed much of the work in this study while at Princeton University, says trees feel the lingering effects from a drought for up to four years.

“Drought is always (thought of) as a light switch: when it’s dry, trees grow slowly, but the moment the rains come back and the soil gets wetter, it’s like the trees recover perfectly and almost immediately,” he said. But that assumption is not true, and it becomes a race for a tree to recover before the next drought hits.

It is well known that forest trees play an important role in removing carbon dioxide from the atmosphere, incorporating the carbon into their woody tissues. But the findings suggest that stress on the trees from drought sets back growth for several years, leading the scientists to believe they store less carbon during these periods than climate models suggest.

“If forests are not as good at taking up carbon dioxide, this means climate change would speed up,” says Anderegg. Co-authors in the study included his colleagues at Princeton University, 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.

Study used the International Tree Ring Database
The researchers used records from the International Tree Ring Database to aid them in the study. Because the recovery period from drought for individual tree species is largely unknown, the scientists carefully measured tree stem growth after droughts dating back to 1948 at over 1,300 forest sites around the planet, using records from the database.

Using Dendrochronology  or tree-ring dating  we can get a snapshot of climate change.

Using Dendrochronology, or tree-ring dating, we can get a snapshot of climate change.
National Science Foundation


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. Now even though we don’t know exactly how drought causes its long-term effects, the scientists do have an idea. They postulated three possible ways this can happen.

The study suggests: “Loss of foliage and carbohydrate reserves during drought may impair growth in subsequent years. Pests and diseases may accumulate in drought-stressed trees. Lasting damage to vascular tissues could impair water transport.”

Canada’s vast boreal forest impacted by drought
The researchers used 18 different methods of measuring the impact of drought on forest trees. They found that all the methods gave similar results. Anderegg said, “It seems like trees are pretty sensitive to drought and they just take a while to recover.” Two of the trees most affected by drought were pines and aspens, two common species of trees in the great boreal forest that stretches across Canada, as well as the forest lands of northwestern U.S.

A vast swath of boreal forest supports many kinds of wildlife in Canada.

A vast swath of boreal forest supports many kinds of wildlife in Canada.
PEW


This knowledge is of particular importance to foresters estimating timber harvest rates, and to scientists studying climate models to estimate greenhouse gas emissions. “A lot of people are concerned,” said Anderegg. “They’ve started to see some pretty big drought effects in Canada’s boreal forest. There’s been documented increasing tree mortality rates. (And) our study adds to these concerns.”

Interestingly, Anderegg suggested suppressing wildfires was not the best idea in managing our forests. He says that because many wildfires are suppressed, the forests are denser than they should be. “That leaves them vulnerable to drought, large forest fires, and insect outbreaks. Things like thinning out the density of trees would certainly increase resilience.”

Drought in western regions of U.S.
As for how precipitation is intertwined with drought conditions, Anderegg says, “Drought, especially the type that matters to forests, is about the balance between precipitation and evaporation. And evaporation is very strongly linked to temperature.” He added, “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.”

The above study was published in the journal Science on July 31, 2015, entitled: Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models

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We are deeply saddened to announce the passing of our dear friend Karen Graham, who served as Editor-at-Large at Digital Journal. She was 78 years old. Karen's view of what is happening in our world was colored by her love of history and how the past influences events taking place today. Her belief in humankind's part in the care of the planet and our environment has led her to focus on the need for action in dealing with climate change. It was said by Geoffrey C. Ward, "Journalism is merely history's first draft." Everyone who writes about what is happening today is indeed, writing a small part of our history.

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