- Researchers at the University of Arizona analyzed 20 years of satellite data to understand how different Amazon forest ecosystems respond to drought. They found that variations in water-table depth, soil fertility and tree height influence forests’ response to droughts.
- In the southern Amazon, experts observed a strong relationship between groundwater availability and the forests’ drought resilience. But the situation was more complex in the northern Amazon, where drought vulnerability depended on a combination of factors, including water availability, soil fertility and tree height.
- The study suggests scientists may have overestimated the risk of drought-related tree death — and the amount of carbon released into the atmosphere as a result — in the southern Amazon. However, long droughts, like the current one across the Amazon, can push these forests to the brink of collapse.
- The researchers created a map of drought resilience across the Amazon Basin, which shows that forests at high risk of deforestation are also most vulnerable to drought. These forests also play a key role in regional weather patterns by feeding the “atmospheric river” that brings rainfall to major agricultural areas.
The Amazon’s most fertile, productive forests, which are critical for supplying Brazil’s agricultural region with rainfall, are also the most vulnerable to drought, according to recent research that has mapped drought resilience across the basin.
An international team of researchers analyzed 20 years of satellite data to understand why some forests might be more resilient to drought than others and focused on the underlying mechanisms behind different responses of forests across the basin.
The study, published in Nature, used water-table depth, soil fertility and tree height to define different intact forest ecotopes in the Amazon and looked at how much growth and photosynthesis was happening in these ecotopes during “once-in-a-century”, major drought events in 2005, 2010 and 2015.
Results showed that forests behaved differently according to their position in the basin.
In the southern Amazon, where soil fertility tends to be high, researchers found a strong relationship between groundwater availability and the forests’ resilience to drought. Shallow water tables, where soil water is plentiful even in the dry season, protected the forests from the harmful effects of droughts. This allowed forests to thrive during short droughts, by taking advantage of the additional sunlight and increased availability of oxygen to their roots. The result was an overall greening effect in shallow-water-table forests during moderate droughts.
Forests’ responses to drought were more complex in the northern Amazon, where the soils are relatively infertile. Here, forests growing on shallow water tables were more vulnerable to drought, reversing the relationship the researchers found in the southern Amazon. But taller trees, which tend to have deeper root systems, were able to access deep groundwater reserves, making them more resilient than shorter trees in this part of the Amazon Basin.
Mapping vulnerability
The “local environment and tree properties interactively affect the [forest’s] response to drought,” Shuli Chen, lead co-author of the study and researcher in ecology and evolutionary biology at the University of Arizona, tells Mongabay. Chen explains that the environmental characteristics of different regions promote different plant strategies. For example, the southern Amazon’s fertile soils have favoured fast-growing trees, whereas harsher conditions in the northern Amazon mean trees tend to grow more slowly and invest more in traits that protect them from drought.
Variation in local soil characteristics and tree growth strategies create “a rich mosaic” across the Amazon Basin, Chen says. By analyzing these characteristics independently, the researchers created “an integrated product that shows [forest] resilience at very fine scale,” she remarks.
The results give scientists a “detailed spatial distribution of vulnerabilities,” says Scott Saleska, study co-author and professor of ecology and evolutionary biology at the University of Arizona. This map could help to “direct our attention to the places that would be the priority to preserve.”
The study reveals a delicate interplay between soil characteristics, plant traits and climate that explains why previous studies investigating forests’ responses drought have produced conflicting results. Studies of satellite imagery had suggested an overall positive “greening” response to drought, whereas field studies of individual forest plots had reported tree browning and death.
These seemingly contradictory results emerged because field studies have tended to focus on forests with deeper water tables, which are less resilient to drought according to Chen and Saleska’s study. Across the Amazon Basin as a whole, shallow-water-table forests were more likely to experience greening during droughts, which was detected by large-scale studies of satellite imagery.
“It’s like we had a blurry image for the Amazon Basin, that we have now brought into clearer focus,” Chen says.
“This study is very useful for us to start unraveling the specific mechanisms by which climatic extremes — and also global climate change — may affect the functioning of the world’s largest tropical forest,” says David Lapola, tropical ecologist at the University of Campinas in Brazil, who was not involved in the study. He suggests that monitoring water-table depth in key locations across the Amazon Basin could be used “as an early warning signal for critical ecological transitions in the region.”
Scientists have warned that the Amazon forest may be close to an ecosystem tipping point — driven by climate change, deforestation and forest degradation — beyond which it will transition from lush tropical forest to dry, degraded savanna. Lapola says he would like to see the drought response patterns identified in this study reproduced in computer models to investigate “how they change projections of the Amazon tipping point.”
Good and bad news for the Amazon’s future
Shallow-water-table forests are less studied but make up about half of the Amazon Basin. As the research suggests that forests of this type in the southern Amazon may be more resilient to drought than previously thought, this could mean scientists have overestimated the risk of drought-related tree deaths — and the amount of carbon released into the atmosphere as a result — in this region.
However, there are limits to forests’ resilience, experts warn. Long droughts can deplete even the most plentiful groundwater supplies, pushing forests to the brink of collapse. “The protective power of shallow water tables are diminished as the drought gets longer,” Chen explains.
“Now, we see this happening before our eyes,” Saleska adds. The most recent Amazon drought, which began in 2023, has been long enough to break down the protection enjoyed by shallow-water-table forests.
The map of drought resilience shows that the most vulnerable forests are close to the Arc of Deforestation and largely sit outside protected areas. These forests are also the most important for generating the “atmospheric river” that carries moist air to other parts of Brazil, including major agricultural areas in the south.
“It’s not just about the Amazon forest, it’s also about the larger-scale hydroclimate,” Chen explains. The combined effects of deforestation and drought threaten the whole hydrological system, which has far-reaching impacts, she says.
Citation:
Chen, S., Stark, S. C., Nobre, A. D., Cuartas, L. A., De Jesus Amore, D., Restrepo-Coupe, N., … Saleska, S. R. (2024). Amazon forest biogeography predicts resilience and vulnerability to drought. Nature. doi:10.1038/s41586-024-07568-w
Banner image: Image courtesy of Marielle N. Smith.
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