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Some Amazon Regions May Resist Climate-Driven Drying Better Than Thought

Trees and other vegetation can help mitigate climate change, by taking in carbon dioxide during photosynthesis and storing it in stems, trunks, leaves and roots. Forests currently take in around 25 to 30 percent of human-generated carbon dioxide emissions, and the Amazon rain forest in particular stores huge amounts of carbon. But many scientists think that if warming climate causes conditions to become drier—a potential impact in many areas—some forests will slow down or even stop photosynthesis, leaving more CO2 in the air, and possibly killing trees.

Current models used for predicting the effects of warming climate show that the Amazon is very sensitive to water stress—an effect that could have vast implications for the forest’s storage of CO2, and possibly its very survival. But in a study published today in the journal Science Advances, researchers at Columbia Engineering report that models have been largely overestimating potential water stress in tropical forests.

The team found that, while models show that increases in air dryness greatly diminish photosynthesis rates in certain regions of the Amazon, observational data show the opposite: In certain very wet regions, the forests instead may even increase photosynthesis rates in response to drier air.

two farmers in a burned clearing
Much of the Amazon is predicted to become drier in the future, but trees may not suffer as much as previously thought; clearing of land may present a far greater danger. Here, farmers in a recently burned-over area at the edge of the forest near Pucallpa, Peru. (Kevin Krajick/Earth Institute)

“To our knowledge, this is the first basin-wide study to demonstrate how—contrary to what models are showing—photosynthesis is in fact increasing in some of the very wet regions of the Amazon rain forest during limited water stress,” said Pierre Gentine, an associate professor of earth and environmental engineering who is affiliated with Columbia’s Earth Institute. “This increase is linked to atmospheric dryness in addition to radiation, and can be largely explained by changes in the photosynthetic capacity of the canopy. As the trees become stressed, they generate more efficient leaves that can more than compensate for water stress.”

Gentine and his former PhD. student Julia Green used data from the Intergovernmental Panel on Climate Change’s Coupled Model Intercomparison Project 5 (CMIP5) models. They combined the data with machine-learning techniques to determine the modeled sensitivity of photosynthesis in the tropical regions of the Americas in relation to both soil and air moisture. They then performed a similar analysis, this time using observational remote sensing data from satellites in place of the model data, to see how the observational sensitivity compared. The team then used data gathered by sensors mounted on towers at various places in the forest to understand what was driving the results, at the canopy and leaf level.

Earlier studies have shown that the Amazon basin tends to green up toward the end of the dry season, when both soil and air are drier. Some researchers have linked this phenomenon to increased photosynthesis. “Before our study, it was still unclear whether these results translated to an effect over a larger region, and they had never been connected to air dryness in addition to light,” said Green, now a postdoctoral researcher at France’s Le Laboratoire des Sciences du Climat et de l’Environnement. “Our results mean that the current models are overestimating carbon losses in the Amazon rain forest due to climate change. [In] this particular region, these forests may in fact be able to sustain photosynthesis rates, or even increase [them], with some warming and drying in the future.”

Gentine and Green note, however, that forest sensitivity was determined using only existing data; if dryness were to increase to levels not currently being observed, that could change the picture. Indeed, the researchers found a tipping point for the most severe dry episodes, where the forest could not maintain its level of photosynthesis. So, they say, their findings are no excuse to not reduce carbon emissions.

Gentine and Green are continuing to investigate water stress in the tropics. Green is currently focusing on developing a water-stress indicator using remote-sensing data that would quantify the effects of water stress on plant carbon uptake, and relate that to ecosystem traits.

“So much of the scientific research coming out these days is that with climate change, our current ecosystems might not be able to survive, potentially leading to the acceleration of global warming,” said Green. “It was nice to see that maybe some of our estimates of approaching mortality in the Amazon rain forest may not be quite as dire as we previously thought.”

Adapted from a press release by Columbia Engineering.

Science for the Planet: In these short video explainers, discover how scientists and scholars across the Columbia Climate School are working to understand the effects of climate change and help solve the crisis.
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