Researchers collect leaves to measure carbon storage in an Andean cloud forest.

JAKE BRYANT

Tropical forests have been one of Earth’s best defenses against rising carbon dioxide levels. The trees suck carbon from the atmosphere as they grow, and researchers estimate that, despite ongoing deforestation, tropical forests hold more carbon than humanity has emitted over the past 30 years by burning coal, oil, and natural gas. But scientists have worried that the ability of tropical forests to act as carbon sinks will diminish and ultimately reverse with continued global warming, as trees stressed by heat and drought die and release their carbon.

Today in Science, researchers report that measurements of carbon storage and growing conditions for some 500,000 trees around the world suggest some tropical forests, particularly in Africa and Asia, will—if left intact—continue to sequester large amounts of carbon even as global temperatures rise. But only up to a point. “There are certain levels where forests can’t respond,” says William Anderegg, a forest ecologist at the University of Utah. If warming reaches 2°C above preindustrial levels, the study finds huge swaths of the world’s tropical forests will begin to lose more carbon than they accumulate. Already, the hottest forests in South America have reached that point.

Trees, with their long lives and massive woody trunks, are particularly good at storing carbon. But just how much carbon tropical forests can capture as the planet warms depends on the balance between tree growth spurred by higher atmospheric carbon levels and tree stress and death caused by rising temperatures and increasing drought. “It comes down to a tug of war,” Anderegg says, “between the benefits of carbon dioxide and the potential impact of climate change.”

To see which side might ultimately win, a global team of more than 200 researchers measured more than half a million trees in 813 forests in 24 countries. The team, led by tropical ecologist Oliver Phillips of Leeds University and his postdoc Martin Sullivan, calculated how much carbon the different forests now store based on the height, diameter, and species of each tree. Researchers also looked at how carbon storage varied from place to place using data from 590 long-term monitoring plots.

To forecast how carbon accumulation might change in the future, the researchers assumed that the hottest forests, which are mostly in South America, are bellwethers of the future. By comparing carbon storage in forests across the range of climates, they could use space as a proxy for time. They analyzed how changes in temperature and precipitation might affect carbon storage, looking for those changes that best explained what they observed in the forests. (The analysis takes into account differences in the forests’ mix of tree species.)

Previous studies had suggested the lowest temperature a forest experiences at night has the biggest impact on its long-term carbon storage capacity, perhaps because warm nights cause trees to boost respiration and release more carbon. But this study found that the maximum daytime temperature is most important, perhaps because on hot days trees slow their carbon dioxide intake to reduce water loss through pores in their leaves.

The study showed that, overall, the forests now take up more carbon than they lose. But it found that at a tipping point—when the average daily maximum temperature during the warmest month of the year rises to 32.2°C—long-term carbon storage capacity declines steeply and carbon loss increases. The decline is even greater in drier forests, notes Sullivan, now at Manchester Metropolitan University, likely because the lack of water makes trees more vulnerable to stress and death.

The team calculated that, worldwide, each 1°C increase in maximum temperature reduces carbon storage in tropical forests by 7 billion tons (roughly equivalent to total U.S. carbon emissions over 5 years), although much of that loss is currently offset by increased growth. If global temperatures rise 2°C above preindustrial levels, however, 71% of tropical forests will be pushed past the thermal tipping point, the researchers found. Carbon losses would be four times greater, with South America seeing the largest loss.

The “massive data compilation … allows us to draw conclusions with much higher confidence than individual studies would allow,” says Julia Pongratz, a climate scientist at Ludwig Maximillian University of Munich. But ecosystem ecologist Lara Kueppers of the University of California, Berkeley, worries the study might be too optimistic in forecasting that cooler forests, especially in Asia and Africa, will continue to accumulate large quantities of carbon as they warm. It’s not clear whether those forests will behave like their counterparts in South America, she notes, or that they can adapt to the speed of human-induced climate change. “I don’t have confidence that forests are going to be able to adjust on the time scale they will need to,” she says.

Other researchers see the findings as a wake-up call for action, noting the world has already warmed about 1°C above preindustrial levels. “Even though tropical forest sinks will weaken, conserving them is still better than not having them at all and turning them into carbon sources,” says Richard Betts, a climate modeler specializing in the global carbon cycle at the University of Exeter. “It is not too late,” he adds, “to avoid the most severe impacts.”

source: sciencemag.org

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