The Amazon rainforest’s environment is not uniform—nor is its response to drought. While some portions of the forest brown and die in the face of dry spells, others first grow greener.
Seeing this response during a drought that hit the Amazon in 2005, then again in 2010 and 2015–2016, surprised Scott Saleska, an ecologist and evolutionary biologist at the University of Arizona. So he teamed up with a group of researchers in Brazil, Canada, the United Kingdom, and the United States to investigate.
The resulting study, published in Nature, suggested that geography and water table depth play a role in the forest’s vulnerability and resilience to drought.
Brazilian coauthors Antonio Nobre and Adriana Luz had already developed a tool to remotely measure water table depth. But water depth was just one of the three main parameters the team looked at, said Shuli Chen, the study’s first author and a doctoral researcher working with Saleska at the University of Arizona. Intrinsic characteristics of trees, such as height and root depth, also play a part. And so do soil fertility and texture. “Combining these dimensions, we looked at how trees adapt to harsh environments, including drought situations,” she said.
North and South Differences
The southern Amazon region contains layers of richness. “It’s got silted and clay material in the surface,” said Sociedade Brasileira de Geologia vice president Nely Palermo, who did not participate in the study. “And deeper down there is carbonate—so much so [that] the region has oil and gas.”
Some trees in the southern Amazon have access to both this fertile soil and shallow water tables, which they can use to survive—and even grow more greenery—for short drought periods. But they eventually brown and die as droughts prolong and repeat. Trees over deeper water tables, on the other hand, have come to rely on rainwater, so they are more likely to brown and die earlier on in a drought.
With more nutrients available, southern Amazon trees have more height variability and tend to grow more quickly. In good times, this gives them a certain competitive edge over trees that invest more in drought resilience than in quick growth, Chen explained. But that edge comes with drawbacks.
“Trees on the most fertile soils and with highest productivity are actually under higher risk.”
“These [southern] trees have poor adaptation to drought and overlay with a region where human activities pose the highest risk to them,” Chen said. “Trees on the most fertile soils and with highest productivity are actually under higher risk.”
Jorge Figueiredo, a geologist at the Universidade Federal do Rio de Janeiro who did not take part in the study, said the southern Amazon forest sits on the Solimões sedimentary basin. It’s especially nutrient-rich, he said, due to the mix of fragments resulting from the lifting of the Andes mountain chain and the waters of the Amazon River. “It’s not connected to the Amazonian craton that forms the [geomorphological regions of the] Guiana and Central Brazil shields,” he explained.
Trees north of the forest, in the Guiana shield, are less vulnerable to drought regardless of water table depth, and despite the region’s less fertile soil.
One reason for this is that these trees are more dense, said coauthor Bruce Nelson, a remote sensing researcher at Brazil’s Amazonian research institute, the Instituto Nacional de Pesquisas da Amazônia (INPA). Because they grow more slowly, these trees have narrow vessels in their xylem—the tissue that transports water and nutrients from a plant’s roots to its stem and leaves. This trait “provides greater resistance to embolism, which is death by the cavitation of a certain percentage of the xylem vessels,” he said.
Tracking Photosynthesis
Chen and her colleagues analyzed remote sensing data from between 2000 and 2020 from the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard NASA’s Terra and Aqua satellites, as well as data from NASA’s Orbiting Carbon Observatory-2 satellite. By viewing images of light reflected from tree canopies and emitted by chlorophyll molecules, they could study trees’ capacity to photosynthesize.
The team used Landsat observations to analyze what happened in the droughts of 2005, 2010, and 2015–2016 in the degraded forest. To look at the last drought in the series, they also used Landsat data with 30-meter resolution to study a region farther north, near Manaus.
Celso Silva Jr., a biodiversity conservation researcher at the Universidade Federal do Maranhão who did not take part in the study, said he wonders how other factors of forest degradation, such as wildfires and selective logging, might affect the forest’s responses to drought as shown in the study. He noted that another recent study analyzing the response of the Amazon forest to the 2023 drought concluded that the greening of some parts of the forest happened not as a response to drought alone, but also to forest fires. “The study is really robust, but I feel this information is missing,” he said.
“Losing those trees will directly affect Brazil’s breadbasket, as less rains will hit the country’s agriculture.”
Chen acknowledged that drought is only one of the threats these trees face, as the southern Amazon overlaps with a region known as the Arc of Deforestation. “Losing those trees will directly affect Brazil’s breadbasket, as less rains will hit the country’s agriculture,” Chen observed. Evapotranspiration from the rainforest is an essential component of the rainfall that stretches from northern Brazil down to the La Plata River basin, benefiting agricultural activity across South America.
“Brazil needs to stop deforestation in [the southern Amazon] as fast as it can—it’s the least protected region of the forest” in terms of environmental regulation, she added.
—Meghie Rodrigues (@meghier), Science Writer
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