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When it comes to climate change and the Amazon rainforest, the news is never good. In 2019 and 2020, the news was dominated by images of burning forests and a land ravaged by climate change and illegal logging.
But there are still many things that scientists still don’t know about the mysterious Amazon.
A new study published Friday in Advances in science is a case in point. In the study, scientists reveal some bewildering ecological traits in the wetter regions of the Amazon, suggesting that the rainforest may be tougher than we think.
Neural networks – In the study, scientists examine the relationship between increased dryness of the air (also known as a vapor pressure deficit) and gross primary production in the Amazon.
Gross primary production refers to the total amount of carbon that plants in the forest “fix” or absorb during photosynthesis.
The scientists used observation data from a nine-year period between June 2007 and May 2016, feeding the data into a machine learning tool known as an artificial neural network. These networks have helped researchers analyze and compare their findings with existing models and simulations of how dry air changes the Amazon’s ability to act as a carbon sink.
“What I wanted to do was basically use different environmental predictors and fit them into a model so that we could predict changes in photosynthesis,” Julia K. Green, lead author of the study and researcher at the Laboratoire des Sciences du Climat et l’Environnement in France, he says Reverse.
Simulation v. Reality – Armed with their model, Green and his colleagues then compared typical predictions from Earth’s surface simulations with what is actually happening in the Amazon.
“These are the models we’re using to make predictions about climate change. And so our predictions can only be as good or as accurate as those models,” Green says.
The wetter parts of the Amazon boast trees with deep root systems. But the models show that vegetation in these areas can be stressed due to the “water deficit,” caused by drier air or soil, Green says. That water deficit can have a serious impact on photosynthesis.
“Plants have these little pores on their leaves called stomata. And what ends up happening is that in order to essentially absorb carbon dioxide, the plant has to open the stomata on their leaves to allow gas to be absorbed,” Green explains.
But because “the plant takes up some carbon dioxide, it loses water through that through the leaf surface, because the air is drier,” Green says. Consequently: “the drier the air, the more closed the stomata” to conserve water, he explains.
The researchers hypothesized – and so did their models – that closing stomata from dry air would reduce photosynthesis.
“What we were seeing is that in model simulations with just a little dryness, photosynthesis in these rainforest regions was really decreasing in the models. And we weren’t sure if this necessarily seemed realistic,” Green says.
This is where artificial neural networks came in: Green used their findings to come to a surprising conclusion: the simulation models were wrong.
“What we ended up seeing is that as the vapor pressure deficit increases, photosynthesis in these truly humid areas of the forest [increases]”he explains.
The reason has to do with the nature of the Amazon rainforest itself. The Amazon has a very “dynamic” forest canopy, says Green, which compensates for the closure of the stomata, counterintuitively increasing photosynthesis.
“After the air starts to dry out, what ends up happening is that the forest ends up losing a lot of these old leaves at the top of the canopy,” says Green. “The new, younger leaves end up replacing them. And these new, younger leaves have a much higher photosynthetic capacity than the leaves they are replacing.”
A natural resilience – The Amazon serves one of the most important carbon sinks in the world. And that carbon sink relies on photosynthesis to survive.
But these findings suggest that the Amazon has its own natural resilience during periods of drier air. But whether the findings suggest that the Amazon is better able to withstand extreme weather events, such as drought caused by climate change, is unclear, Green says.
“This study, while it is showing that forests are more drought tolerant to air than we thought and what is pictured, is not saying it should dry the air to levels we haven’t seen so far that will be fine,” Green says. .
Abstract: Earth system models predict that increased atmospheric and soil dryness will reduce photosynthesis in the Amazon rainforest, with major implications for the global carbon cycle. Using in situ observations, sun-induced fluorescence, and non-linear machine learning techniques, we show that, in reality, this is not necessarily the case: in many of the wetter parts of this region, photosynthesis and biomass tend to increase with increasing of atmospheric dryness, despite associated reductions in the conductance of the canopy to CO2. These results can be largely explained by the changes in the properties of the canopy, in particular, the new leaves reddened during the dry season have a greater photosynthetic capacity than the leaves they replace, compensating for the negative stomatal response to the increase in dryness. As atmospheric drought will increase with climate change, our study highlights the importance of reformulating how we represent the ecosystem photosynthesis response to atmospheric dryness in very humid regions, to accurately quantify the Earth’s carbon sink.
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