Some regions of the Amazon rainforest are more resilient to climate change than previously thought

Forests can help mitigate climate change by absorbing carbon dioxide during photosynthesis and storing it in their biomass (tree trunks, roots, etc.). In fact, forests currently absorb around 25-30% of our human-generated carbon dioxide (CO₂) emissions. Some rainforest regions, such as the Amazon, store more carbon in their biomass than any other ecosystem or forest, but when forests become subject to water stress (not enough water in the soil and / or the air is extremely dry) , forests slow down or stop photosynthesis. This leaves more CO₂ in the atmosphere and can even lead to tree mortality.

Current Earth system models used for climate predictions show that the Amazon rainforest is very sensitive to water stress. As the air is expected to become hotter and drier in the future with climate change, resulting in increased water stress, this could have major implications not only for the survival of the forest, but also for its CO storage.₂. If the forest is unable to survive in its current capacity, climate change could accelerate dramatically.

Columbia Engineering researchers set out to investigate whether this was true, whether these forests are indeed sensitive to water stress as the models have shown. In a study published today in Advances in science, report their finding that these models largely overestimated water stress in tropical forests.

The team found that while the models show that increased dryness significantly decreases the rates of photosynthesis in some regions of the Amazon rainforest, the results of the observational data show the opposite: in some very humid regions, forests instead, the rates of photosynthesis even increase in response to air.

“As far as we know, this is the first basin-level study that demonstrates that, contrary to what the models show, photosynthesis is actually increasing in some of the very humid regions of the Amazon rainforest during limited water stress,” he said. Pierre Gentine, associate professor of earth and environmental engineering and earth and environmental sciences and affiliated with the Earth Institute. “This increase is related to atmospheric dryness in addition to radiation and can be largely explained by changes in the photosynthetic capacity of the canopy. As 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 and combined them with machine learning techniques to determine what the modeled sensitivity of photosynthesis in tropical regions of the Americas was for both soil moisture than for dryness air. They then performed a similar analysis, this time using observational remote sensing data from satellites instead of model data, to see how observational sensitivity is compared. To correlate their findings to smaller-scale processes that could explain them, the team then used the flow tower data to understand their results at the canopy and leaf levels.

Previous studies have shown that there are increases in greenery in the Amazon basin at the end of the dry season, when both the soil and the air are drier, and some have linked this to increased photosynthesis. “But prior to our study, it was still unclear whether these findings translated into a wider region effect and had never been linked to air dryness other than light,” Green, who is now a postdoctoral researcher associate at Le Laboratoire des Sciences du Climat et de l’Environnement in France, he explained. “Our results indicate that current models are overestimating carbon losses in the Amazon rainforest due to climate change. Therefore, in this particular region, these forests may actually be able to sustain, or even increase, rates of photosynthesis with some heating and drying. in the future. “

Gentine and Green note, however, that this sensitivity was determined using only existing data and, if dryness levels were to increase to levels that are not currently observed, this may actually change. Indeed, the researchers found a tipping point for more severe drought stress episodes where the forest could not maintain its level of photosynthesis. So, say Gentine and Green, “our results are certainly not an excuse not to reduce our carbon emissions”.

Gentine and Green are continuing to look at issues related to the water stress of vegetation in the tropics. Green is currently focusing on developing a water stress indicator using remote sensing data (a dataset that can be used to identify when a forest is under stress), quantifying the effects of water stress on carbon uptake of plants and relating them to the features of the ecosystem.

“Much of the scientific research coming out these days is that with climate change, our current ecosystems may not be able to survive, potentially leading to acceleration of global warming due to feedback,” Green added. “It was nice to see that perhaps some of our estimates of approaching mortality in the Amazon rainforest may not be as disastrous as we previously thought.”

The study is titled “Amazon Rainforest Photosynthesis Increases in Response to Atmospheric Drought.”