Tucson Standard

In the late 2000s, Scott Saleska noticed unusual changes in the Amazon rainforest. In 2005, a significant drought hit the region. Two years later, Saleska, a University of Arizona professor in the Department of Ecology and Evolutionary Biology, published research using satellite images that showed increased green growth in large areas of the Amazon following the drought. However, field researchers observed plants browning and some dying due to the drought.

Research published today in the journal Nature clarifies this discrepancy. Shuli Chen, a doctoral candidate in ecology and evolutionary biology working with Saleska, is the lead author. Chen and Saleska collaborated with Antonio Nobre from Brazil's National Institute for Space Research, who was studying how landscape topography and groundwater tables interact with forests using satellites.

The team used 20 years of data from 2000 to 2020, including drought data from 2005, 2010, and a more extensive drought in 2015-2016. They aimed to understand how drought affects different regions of the Amazon rainforest. Their findings reveal that local forest environments and tree properties influence regional responses to drought. This work focuses on local environmental factors rather than broad climate trends.

In southern parts of the Amazon rainforest over rock formations known as the Brazilian Shield—with relatively fertile soil and shorter trees—drought response depended on groundwater access. Trees with shallow water table access "greened up" during droughts, while those over deeper water tables experienced more foliage browning and tree death. Conversely, northern Amazon areas dominated by tall trees with deep roots on less fertile soil were more resilient to drought regardless of water table depth.

This new understanding provides a framework for conservation decisions and improved predictions of forest responses to future climate changes. It also highlights that the most productive forests are at greater 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."

The research team used remote sensing satellite data to track forest canopy health by measuring greenness and photosynthetic activity. They examined how variations in non-climatic factors such as water table depth, soil fertility, and overall forest height affect resilience during droughts.

For trees with shallow water table access in southern Amazon's fertile soils, droughts result in increased growth due to better oxygen uptake when roots are exposed as water recedes slightly but doesn't disappear entirely. Additionally, these trees receive a photosynthetic boost from extra sunlight.

Trees growing over deeper water tables rely on rainwater and are more vulnerable during droughts.

Slow-growing trees in northern Amazon have adapted to harsh conditions with their tall canopies, deep roots, and relatively infertile soil making them hardy against droughts.

"Our results are not just important for the Amazon; it's important for the whole world because the rainforest has a significant stock of our carbon," Saleska said. "If that carbon is lost—because trees burn or are deforested—that adds to carbon dioxide in the atmosphere which makes global warming even worse."

The Amazon rainforest also plays an essential role in Earth's hydrologic cycle through an "atmospheric river" carrying substantial amounts of water vapor above its rainforests from east to west across South America.

"If you imagine either cutting down or losing those trees because they're vulnerable...you basically threaten integrity [of] whole system," said co-author Nobre.

Saleska emphasized that preserving biodiversity requires understanding this kind of information: "If we care about preserving biodiversity...knowing this kind [of] information is really critically helpful."