When in drought: Researchers map which parts of the Amazon are most vulnerable to climate change

In the late 2000s, Scott Saleska noticed something strange going on in the Amazon rainforest. In 2005, a massive drought struck the region. Two years later, Saleska—a University of Arizona professor in the Department of Ecology and Evolutionary Biology—published surprising research that used satellite images to find that the drought resulted in more green growth in large swaths of the Amazon. On the other hand, field researchers saw plants turn brown and some die in response to the drought.

New research published in the journal Nature reveals what caused the scientific mismatch. Shuli Chen, a doctoral degree candidate in ecology and evolutionary biology who works with Saleska, is the lead author.

Chen and Saleska teamed up with Antonio Nobre, an Earth scientist at Brazil's National Institute for Space Research, who was using satellites to detect how landscape topography and groundwater tables interact with forests.

The trio and their co-authors from Brazil, the U.S. and the United Kingdom used 20 years of data—from 2000 to 2020, which includes drought data from 2005, 2010 and a more widespread drought in 2015 and 2016—to tease out how drought impacts the most biodiverse forest on Earth, which spans an area twice the size of India, and is one of the world's largest carbon sinks.

They found that different regions of the Amazon rainforest respond to drought differently because of differences in local forest environments and differences in the properties of trees themselves. This work goes beyond wide-scale climate factors and homes in on how local environments drive drought response, Chen said. The team created maps to illustrate their findings.

In the southern reaches of the Amazon rainforest, mostly over rock formations that geologists call the Brazilian Shield—with relatively fertile soil and forests with shorter trees—drought response was controlled by access to groundwater. Trees with access to shallow water tables "greened up" during drought, the researchers found, while trees over deeper water tables experienced more foliage browning and tree death. In contrast, the northern Amazon, dominated by what geologists call the Guiana Shield—home to tall trees with deep roots and less fertile soil—was more drought-resilient regardless of water table depth.

This new understanding of regional differences provides a framework for conservation decisions and improved predictions of forest responses to future climate changes, according to the researchers. It also warns that the Amazon's most productive forests are also at the greatest risk.

"It's like we brought a blurry image into focus," Chen said. "When we talk about the Amazon being at risk, we talk about it as if it were all one thing. This research shows that the Amazon is a rich mosaic in which some parts are more vulnerable to change than others, and it explains why. This is key to understanding the system and ultimately protecting it."

Building the mosaic

The research team used remote sensing satellite data—which relayed forest canopy health by measuring greenness and photosynthetic activity—to track how variations in non-climatic factors including water table depth, soil fertility and overall forest height affect forest resilience in the face of drought.