(Inside Science) -- Trees around the world are dying earlier and decaying faster than they did as recently as several decades ago. And rising temperatures and droughts could accelerate those trends as climate change continues to ramp up.
The findings, reported in two recent scientific papers, call into question how well forests of the future will be able to store carbon and fight climate change. So-called “natural climate solutions” that pull carbon from the air into living plants and soil have been promoted as a relatively uncontroversial, shovel-ready means to slow climate change while countries figure out how to reduce their fossil fuel use. This summer, a group of scientists based at ETH Zurich in Switzerland set off a global media frenzy when they asserted in the journal Science that forest restoration was “our most effective available climate solution.” (The researchers have since revised the claim to “one of the most effective carbon drawdown solutions.”)
Even as governments, corporations and billionaires pour money into forest conservation and tree-planting programs, however, scientists worry about how those same forests will fare in the future. Computer models, experiments, and field and satellite observations provide reasons to believe that elevated carbon dioxide could supercharge photosynthesis and enhance tree growth. But they also suggest that the hotter temperatures and the stronger droughts and storms brought on by the greenhouse gas could stress and ultimately kill trees. When dead trees decompose, the carbon in their tissues is usually released back into the atmosphere, exacerbating climate change.
When it comes to forest growth, “there’s a tug of war going on between the beneficial effects of elevated carbon dioxide and the harmful effects of climate change,” said William Anderegg, a biologist at the University of Utah in Salt Lake City who coauthored both studies.
In a paper published last week in the Proceedings of the National Academy of Sciences, Anderegg and colleagues analyzed field data from almost 700 research plots in intact forests in all three of Earth’s major forest biomes: tropical, temperate and boreal. The researchers limited their study to forests that had been measured at least three times over a decade or more, documenting back-to-back periods of tree growth and death that could be compared.
Both tree growth and tree death accelerated everywhere between 1955 and 2018, the scientists found. So even if trees are growing faster thanks to carbon dioxide fertilization, the benefits will be short-lived, Anderegg suspects, as trees die younger in a warmer, stormier and more drought-stricken world. The authors also suggest that elevated carbon dioxide itself could be driving some of the amped-up tree death by increasing competition between fast-growing trees. “The mortality trend is increasing quite rapidly,” Anderegg said. “That raises the very real prospect that the land could flip to carbon source.”
In the second study, published this week in the same journal, Anderegg and colleagues built a computer model to represent how trees take in carbon dioxide and release water through small pores in leaves called stomata, how trees move water from soil to leaves, and how climate’s effects on the availability of those molecules determine rates of photosynthesis, tree growth and forest death. They then combined their model with climate change predictions made by other computer models to simulate 30 years of growth and death in 20 different U.S. forest types.
The researchers subjected their virtual forests to two conditions: In one, trees were not allowed to alter their physiology to acclimate to climate change, for example by reducing the number of water-losing stomata. In the other, trees and forests could adjust to grow optimally in whatever conditions the climate models predicted.
Without acclimation, the beneficial effects of carbon dioxide outweighed the negative effects of heat and drought only 55 percent of the time. With acclimation, that number jumped to 71 percent.
The results suggest that trees are perched “right on a knife edge” between the boost they may get from elevated carbon dioxide and the harm they’re likely to suffer from climate change, said Anderegg. And he noted that the simulations omitted factors that also harm trees, such as insects, diseases and fire, possibly making his team’s results overly optimistic.
Christian Körner, a plant scientist at the University of Basel in Switzerland, applauded both papers for examining how long carbon will actually remain locked up in forests; most studies look only at how fast carbon enters trees, not how fast it exits, he said. “They put their finger on the right point.”
Körner also praised the first study’s use of field data. The findings bolster results he and his colleagues published earlier this year based on growth rings of trees growing in the Altai Mountains of Russia and the Pyrenees of Western Europe, he said.
The modeling study, by contrast, is “a fantasy story,” Körner said, because it assumes that trees grow faster when carbon dioxide is elevated. Several research projects that flooded natural forests with carbon dioxide, including one that Körner and colleagues ran in a Swiss forest, suggest that the situation may be even worse, because forest growth slowed quickly as soil nutrients were depleted. Körner believes nutrient limitations will always prevent trees from taking full advantage of the carbon dioxide feast, but other scientists are unsure if this will be true everywhere, and leading computer models differ in how strongly carbon fertilization and nutrient limitation affect forest growth.
Ecologist Sean McMahon of the Smithsonian Environmental Research Center in Edgewater, Maryland shares Körner’s enthusiasm for the first study. “It’s a pretty careful approach to a complex topic,” he said, that “demonstrates the incredible power of long-term datasets.” He added that data from tropical forests in Africa and Asia would have contributed even more insight. (The first study’s tropical forest data came only from the Americas.)
McMahon also lauded the second paper. “It’s an important advance in how we appreciate the direct and indirect effects of increasing carbon dioxide,” he said. While he agreed with Körner that it remains unclear how much carbon fertilization accelerates tree growth, he said the study’s assumptions are reasonable given what scientists currently know about how plants work.
While forests globally are still socking away carbon today, that could change sooner than scientists expected, said Anderegg. “We’re starting to see warning lights flashing.”