Researchers use satellite data to uncover signs if a forest might be in trouble.
By Nathan Collins
Monteverde, Costa Rica. (Photo: Donna Boley/Flickr)
Figuring out how global climate change will impact tropical rainforests is one of the most important and difficult questions facing scientists today. Tropical rainforests are home to enormous biodiversity and a vast store of carbon—the latter of which could be released into the atmosphere should those forests be destroyed, to disastrous results. Now, researchers have identified a potential early warning sign by watching forests from space.
“An overview of tree mortality events across continents and climate zones suggests that drought and elevated temperatures can bring forests to a threshold for massive die-off,” Jan Verbesselt, Marten Scheffer, and their colleagues write in Nature Climate Change. “For tropical forests, there is an extra dimension to such mortality events, as they could tip forests into a fire-dominated savannah state from which recovery is difficult,” a possibility that several recent studiessuggest.
“Drought and elevated temperatures can bring forests to a threshold for massive die-off.”
The question is, could scientists see such events coming? While a number of researchers have analyzed individual biological processes that could lead to a major die-off, Verbesselt, Scheffer, and their colleagues looked to a more holistic measure of forest health — simply put, whether, at any spot in the tropics, there was land that qualified as rainforest, and whether such spots experienced what’s called “critical slowing down.” In this context, critical slowing down means that forests will take longer to recover from disturbances such as fires or droughts—and the slower the recovery gets, the more likely a forest is to be on the precipice of long-lasting collapse.
To measure recovery times, the team tracked monthly changes in forest density, as measured by satellites and remote radar sensors, then modeled those changes as a function of average annual precipitation, temperature, and soil quality, among other factors.
The team found that, in tropical rainforests around the world, recovery time “increases markedly [for forests where] mean annual precipitation falls below around 1,500mm,” or about five feet of rain—a pretty low number for rainforests. Recovery times also increase with temperature, except, for some reason, not in African tropical rainforests.
While the study has some limitations, including the fact that satellites have only been tracking forests for about 15 years, the results suggest “independent support for the idea that such forests may have a tipping point for collapse at drying conditions,” Verbesselt, Scheffer, and their team write.