Earth models overestimate water stress in tropical forests

Written by AZoCleantechNovember 23, 2020

Forests could help fight climate change by absorbing carbon dioxide (CO₂) during photosynthesis and retaining it in their biomass (roots, tree trunks, etc.). Currently, forests absorb around 25-30% of man-made CO2₂ emissions.

Some rainforest regions, such as the Amazon, store excess carbon in their biomass than any other ecosystem or forest. However, when forests become subject to water stress (lack of adequate water in the soil and / or very dry air), forests will slow down or stop photosynthesis. This would result in more CO₂ in the air and can cause tree mortality.

Existing models of the Earth system used to predict climate show that the Amazon rainforest is extremely sensitive to water stress. As the atmosphere is expected to become drier and warmer in the future with climate change, causing increased water stress, this could have serious repercussions not only on the survival of the forest, but also on its ability to store CO₂.

If the forest is unable to thrive in its current capacity, climate change could accelerate significantly.

Columbia Engineering scientists set out to examine whether this was true, whether these forests were truly sensitive to water stress as indicated by the models. In a study recently reported in Advances in science, discuss their finding that these models substantially exaggerated water stress in tropical forests.

The team learned that while the models reveal that increased dryness of the air seriously reduces the rates of photosynthesis in some regions of the Amazon rainforest, the results of the observational data contradict this: in some very humid regions, forests increase. even the rates of photosynthesis in reaction to drier air, on the other hand.

To our knowledge, this is the first basin-level study to show that, contrary to what models show, photosynthesis is actually increasing in some of the very humid regions of the Amazon rainforest during limited water stress..

Pierre Gentine, Associate Professor of Earth and Environmental Engineering and Earth and Environmental Sciences, Columbia University

“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. When trees get stressed, they generate more efficient leaves that can more than compensate for water stress“Added Gentine, who is also affiliated with the Earth Institute.

Gentine and her former PhD student Julia Green used data from the Intergovernmental Panel on Climate Change’s Coupled Model Intercomparison Project 5 (CMIP5) models and integrated them with machine learning methods to establish the modeled sensitivity of photosynthesis in tropical regions of the Americas to air dryness and soil moisture.

Then, they performed a similar analysis, but this time using observational remote sensing data obtained from satellites instead of model data, to learn the observational sensitivity comparison. To match their findings with smaller-scale processes that could describe them, the researchers then used the flow tower data to understand their results at the leaf and canopy level.

Previous studies have indicated that towards the end of the dry season, greenery in the Amazon basin increases when both the air and soil are drier, and some have correlated this with increased photosynthesis.

But prior to our study, it was still unclear whether these results translated into a wider region effect and had never been linked to dry air other than light. Our results indicate that current models overestimate the carbon losses in the Amazon rainforest due to climate change. Therefore, in this particular region, these forests may actually be able to sustain photosynthesis rates, or even increase them, with some warming and drying in the future..

Julia Green, postdoctoral researcher, Laboratory of Climate and Environmental Sciences, France

However, according to Gentine and Green, this sensitivity has been established using only current data and, if dryness levels increase to levels that are not currently observed, this could change. Indeed, the team discovered a tipping point for more severe drought stress episodes where the forest could not preserve its level of photosynthesis. Thus, notes Gentine and Green, “our results are certainly no excuse not to reduce our carbon emissions. “

Gentine and Green analyzed issues related to the water stress of vegetation in the tropics. Green is currently trying to create a water stress indicator using remote sensing data (a dataset that can be used to recognize when a forest is under stressful conditions), measuring the effects of water stress on carbon uptake in plants. and associating them with 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. 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..

Pierre Gentine, Associate Professor of Earth and Environmental Engineering and Earth and Environmental Sciences, Columbia University

Journal reference:

Green, JK, et al. (2020) Amazon rainforest photosynthesis increases in response to atmospheric drought. Advances in science. doi.org/10.1126/sciadv.abb7232.

Source: https://www.engineering.columbia.edu