Trees Absorb Less Carbon in Warming World Than Experts Have Assumed

Trees may not be the planetary saviors people have been counting on in a warming climate.

A new study shows that while trees certainly help counteract rising temperatures, they are absorbing 3.4 percent less carbon than had been assumed in models used in the  Intergovernmental Panel on Climate Change reports. More CO2 in the atmosphere means more warming.

According to the study, published in the Proceedings of the National Academy of Sciences, it all comes down to how the trees react to sunlight.

One of the basic assumptions has been that warmer conditions created by climbing levels of the greenhouse gas CO2 would extend the growing period. The longer season would, theoretically, allow trees to absorb more carbon through photosynthesis.

In the first half of the season, that appears to be true. Earlier and warmer springs let trees leaf out sooner. But late in the season, trees can't do much with the extra warmth. Bill Bauerle, an ecophysiologist at Colorado State University and lead author of the study, told InsideClimate News that trees' carbon intake drops way off in late summer and early fall, even when temperatures stay high.

"Plants are not able to capitalize on that (added warmth) as much as we thought," he said. "The trees look perfectly healthy, and you'd think they'd be physiologically active, but they're not."

Bauerle first noticed that trees weren't very active late in the summer while in South Carolina. It was a very warm September, and he had set up devices to measure the local trees' gas exchange system—how much CO2 they were taking in, and how much water they were putting out. He found that the trees' biomechanical activity had basically shut down, despite the warmth.

He and the team, which included researchers from Canada and Sweden, also looked at leaf levels in Colorado and supplemented that data set with published measurements taken elsewhere, including California.

They found that after the summer solstice—the longest day of the year, on or around June 21—photosynthesis gradually decreases. By late summer, no matter what the temperatures are, trees start to get ready for winter vacation. They're sequestering only about 20 percent of the carbon they had sequestered during the peak growing period.

Bauerle is confident that the team's findings are solid and that correcting the current formula for trees' carbon-absorption capacity will help improve the accuracy of future climate models. The corrections will have the biggest impact on carbon-intake estimates for the boreal forests of the Northern Hemisphere, where day length fluctuates the most.

Lisa Welp, a post-doctoral scholar at the Scripps Institution of Oceanography in San Diego who has studied boreal forest in Alaska and Siberia since 2000, said she wished the study had addressed springtime carbon absorption, too. But she said the research does help further the understanding of how trees respond to climate change.

"Just because temperatures warm up, if there's not enough sunlight, you're kind of stuck," said Welp, who has a Ph.D. in environmental science and engineering and works in the carbon dioxide research group at Scripps.

Bauerle doesn't want to leave the impression that trees aren't a key part of the formula for mitigating the impacts of climate change, or that continued deforestation won't make matters much worse.

"Trees are still a very good thing," he said. "We should still plant trees. But we need to realize there's a limit to what they can do."

Timber Use and CO2 Levels

When forests are cleared, how the tree products are used has a big influence on atmospheric CO2 levels, according to a new study out of the University of California, Davis.

If the felled trees are turned into solid wood products, such as timber for construction, much of the trees' carbon is effectively sequestered for decades. But if the wood is used for bioenergy or turned into pulp for paper, almost all of the carbon is quickly released back into the atmosphere.

Lead author J. Mason Earles, a doctoral student with the UC Davis Institute of Transportation Studies, said previous models assumed that when forests were cleared, all the carbon was released immediately. But the study, published earlier this month in the journal Nature Climate Change, found that 30 years after a forest is cleared, the amount of carbon that might remain in storage varies from 0 percent to 62 percent.

The researchers looked at how 169 countries harvest their forests. In Canada, the U.S. and Europe, forests are used primarily for solid wood products. In tropical forests of the Southern Hemisphere, energy and paper production are the main uses.

The findings have implications for the biofuel industry and could enter into land-use decisions around the globe. If the U.S. were to increase incentives for corn-based ethanol production, for example, less profitable crops, such as soybeans, might shift to other countries. And if those nations cleared forests for crop production, how those tree products would be used would affect the global CO2 equation.

Giant Firestorms a New Phenomenon

Officials with the National Weather Service said last week that much of the country faces increased odds of a warmer-than-usual summer. That warmth, combined with continued drought, has created an elevated fire risk across much of the American West, which has already experienced wildfires this spring.

Fire there is nothing new; it has been a part of the region's ecology for millenniums. But a new study has found that the massive blazes that have destroyed millions of acres in recent years are unprecedented in the long-term record, especially in the Southwest.

Researchers at Southern Methodist University in Dallas and the University of Arizona studied tree-ring records that go back more than 1,500 years. They wanted to see if today's hot, dry climate alone is responsible for the megafires.

They found a common trigger for fires: a couple of wet years followed by an exceptionally dry one. But the tree-ring record showed no signs of the intense, canopy-destroying blazes that have occurred in recent decades. More than a century ago, low-severity surface fires were the norm.

Extreme droughts caused by climate change may be setting the stage for fires, but their severity is probably much greater because of a century of livestock grazing and firefighting, concluded the study, published in the scientific journal The Holocene. Decades of fire suppression have allowed dead material to build up on forest floors and live vegetation to become dense and overcrowded. The accumulated fuels make the forests more vulnerable than ever to extreme droughts.

Study co-author and fire anthropologist Christopher I. Roos, from SMU, said climate change has made it urgent that we fix these problems in the forests.

"The forests may be equipped to handle the climate change, but not in the condition that they're currently in," Roos said. "They haven't been in that condition before."