Over the past 10 years, the death of forest trees due to drought and increased temperatures has been documented on all continents except Antarctica. This can in turn drive global warming by reducing the amount of carbon dioxide removed from the atmosphere by trees and by releasing carbon locked up in their wood.
A common myth is that higher levels of CO2 will be good for all vegetation. Unfortunately, those higher levels of CO2 are accompanied by higher temperatures and, in many places, drought and bark beetle infestation, which are bad news for trees — as study after study has documented (see links below).
The new study spells this out for the trembling aspen (Populus tremuloides), which goes by many other names, including the white poplar, though, as it turns out, the aspens apparently now have reason to tremble — manmade global warming. Fittingly, the aspen die-off is “called Sudden Aspen Decline or SAD.”
The study has a nice overview and notes with the key recent studies:
Forests are important carbon sinks, yet they are threatened by global change (1, 2). In the past decade, widespread forest mortality related to drought or temperature stress has been documented in multiple biomes and on all vegetated continents (3–6). In temperate North America, some of these events have been linked to “global change-type droughts,” defined as severe drought coupled with elevated summer temperatures (6–9). Such mortality events can radically transform regional land cover and effect biodiversity, fire risk, ecosystem function, land–atmosphere interactions, and ecosystem services (10–12). Furthermore, forest diebacks can lead to dramatic decreases in net primary production and carbon sequestration, driving these ecosystems to become CO2 sources and to have a positive feedback to climate warming (11, 13–17). Climate-mediated die-off of pine forests caused by insect outbreak in Canada led to estimated carbon emissions of 990 Mt CO2e (CO2 equivalent) over a 20-y period, equivalent to 5 y of Canada’s annual transportation sector emissions (200 Mt CO2e/y) (15)
There is every reason to think things will get much worse if we stay on our current emissions path (see Science: Second ’100-year’ Amazon drought in 5 years caused huge CO2 emissions. If this pattern continues, the forest would become a warming source).
This PNAS study explores the cause of the climate-induced forest die-off, and explains how droughts kill trees:
New research led by Carnegie researcher and Stanford University PhD student William Anderegg offers evidence for the physiological mechanism governing tree death in a drought….
Forests store about 45 percent of the carbon found on land. Their mortality can radically transform ecosystems, affect biodiversity, harm local economies, and pose fire risks, as well as increase to global warming.
Scientists had two competing theories for how forest trees die during a drought. One hypothesis proposed that the trees starved due to decreased photosynthetic activity. The other proposed that the system for transporting water within a tree was damaged beyond repair due to the stresses of the drought.
Without knowing which theory was correct, it was difficult for researchers to build models and make projections about the larger impact of drought-induced forest mortality.
The team focused their efforts on climate-induced die offs of trembling aspen trees in North America. They looked directly at both carbon starvation and water-transportation stress on ongoing forest deaths….
The aspen die-off, called Sudden Aspen Decline or SAD, began after severe droughts between 2000 and 2004 and affects about 17 percent of aspen forests in Colorado, as well as parts of the western United States and Canada. SAD continued through 2010, when the research was conducted.
“Large scale mortality events, such as we see with aspens, are the dynamite in ecosystem responses to climate change. We know that when they occur, they make a huge difference. But we are at the early stages of being able to predict occurrence,” said Field, director of Carnegie’s Department of Global Ecology and professor of biology and of environmental Earth systems science at Stanford.
The team found no evidence of significantly decreased carbon reserves in SAD-affected aspens. This undercuts the starvation theory, although it is possible that carbon starvation had occurred and already been rectified.
By contrast they did find notable losses of function in the tree’s water-transportation systems, especially in the roots. SAD-affected trees showed about a 70 percent loss of water conductivity. Potted trees exposed to a summer’s-worth of drought exhibited significant root mortality.
“Our study provides a snapshot of what future droughts could hold for the emblematic tree of the American West. Our results indicate an impaired ability to transport water due to drought damage plays an important role in the recent die-off of aspens,” William Anderegg said.
What’s particularly worrisome about this drought-induced tree mortality is that droughts — and especially the hot weather “global change-type droughts” — are projected to get considerably more frequent and intense (see “Must-read NCAR analysis warns we risk multiple, devastating global droughts even on moderate emissions path“).
Now that’s SAD.
- Is human-caused climate change killing the great forests of the American West?
- Science: Global warming is killing U.S. trees, a dangerous carbon-cycle feedback
- Nature on stunning new climate feedback: Beetle tree kill releases more carbon than fires
- Climate Change Blamed for Dead Trees in Africa
- Science: Drought drives decade-long decline in plant growth; NASA news release:
“These results are extraordinarily significant because they show that the global net effect of climatic warming on the productivity of terrestrial vegetation need not be positive “” as was documented for the 1980′s and 1990′s,” said Diane Wickland, of NASA Headquarters and manager of NASA’s Terrestrial Ecology research program”….
“This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth,” Running said”….
“The potential that future warming would cause additional declines does not bode well for the ability of the biosphere to support multiple societal demands for agricultural production, fiber needs, and increasingly, biofuel production,” Zhao said.