Thunderstorms are a major driver of tree death in tropical forests

Trees in tropical forests are dying at an increased rate, with consequences for biodiversity, carbon storage, and the global climate. While deforestation is the primary cause of forest loss, intact forests are also experiencing a rise in tree death. Drought, higher temperatures, and fires have been the leading suspects, but a new paper led by Evan Gora, a forest ecologist at Cary Institute of Ecosystem Studies, identifies an underappreciated threat: thunderstorms, which are becoming more frequent with climate change.

Not to be confused with hurricanes or cyclones, these convective storms tend to be short-lived but powerful, with tree-toppling winds and lightning. In a perspective paper in Ecology Letters, Gora and colleagues lay out the case for why such storms could be a major driving force behind the rising death toll of tropical trees. As they become more common in the warming tropics, thunderstorms are a growing threat to trees and the carbon they store.

“Tropical forests have massive effects on global climate. They're like the lungs of the Earth, and we're seeing trees in them dying at higher rates than in the past, and the composition of forests is changing, too,” said Gora. “That could be really problematic for the future of not just tropical forests, but for the planet.” 

Understanding what’s causing the trends in tree death is critical to guiding decisions about which tree species to plant or conserve in a forest, so that forest managers can ensure forests continue thriving and storing carbon long into the future.

“Being in the forest during a tropical storm is unforgettable,” said coauthor Vanessa Rubio, a forest ecologist in Gora’s lab at Cary Institute. “As the storm quickly builds, the sky darkens, humidity changes drastically, and strong winds shake the trees. Then, thunder and lightning come. Leaves and branches fall to the ground, rain pours down, and your instinct is to get back to the field station as quickly as possible.” 

Despite their obvious danger to people, storms had been overlooked and understudied as a potential culprit in tree mortality trends. But when the team reanalyzed data from previous studies on tropical forest carbon stocks, they found that storms were at least as good as drought and temperature in explaining the patterns of tree mortality and forest carbon storage. 

“We were surprised to find that storms may be the largest single factor causing tree death in these forests, and they’re largely overlooked by research into carbon storage in the tropics,” said Gora. “Our estimates suggest that storms are responsible for 30 to 60% of tree mortality in the past, and that number must be increasing as storm activity increases by 5 to 25% each decade.”

The team also added storms to the largest plot-based study of forest biomass carbon dynamics to date. That study had previously concluded that when temperatures go above a certain threshold, tropical forests experience a fast decline in carbon stocks. “But when you add storms, that relationship goes away,” said Gora. “It basically shows that you have to include storms, or you might not get the answers right.”

Storms and droughts are not mutually exclusive, the scientists note — the same forests can experience both high storm activity and drought stress. They found high convective storm activity across the southern Amazon, where water stress is also high and patterns of change are among the most extreme.

“During my studies on threats to tropical forests, my professors, our textbooks, and even overall climate policy never mentioned small, convective storms as a potential source of forest mortality,” said coauthor Ian McGregor, a Cary Institute forest ecologist in Gora’s lab. “I don't remember seeing them in global climate models used to inform climate policy. Given our findings, however, it's clear we need a more thorough understanding of these storms to have more accurate climate models, and thus more effective policy.” 

There are good reasons why scientists have overlooked storms until now. Temperature and water stress can be monitored with meteorological stations and readily connected to long-term forest plot data. It is much harder to detect storms and track their highly localized damage. Mortality caused by thunderstorms is not easily detected via satellite, and it’s not practical for researchers on foot to survey large forested areas frequently enough to pinpoint the damage caused by a specific storm. 

Gigante, a project led by Gora and co-author Adriane Esquivel-Muelbert from the University of Birmingham, offers one way to overcome these challenges. The project combines a lightning location system, drone scouts, and on-the-ground experts to sample large areas of tropical forest frequently. With these tools, they are starting to quantify when, where, and why tropical trees are dying, and which species are most affected.

Understanding current and future threats to tropical forests is crucial to informing long-term conservation and restoration efforts. 

“If we make decisions about which species to plant or conserve based on an incorrect understanding of what's actually killing these trees and which species are most vulnerable, those forests won’t reach their full potential,” said Gora. Storms are most deadly to mature trees, so the consequences of misguided reforestation efforts might not be known until decades after the trees are planted.

“However,” Gora continued, “if we can build a more holistic picture of what’s driving forest change, we can be a lot more confident in guiding forest management practices for long-term sustainability.”

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Authors

• Evan M. Gora - Cary Institute of Ecosystem Studies, Smithsonian Tropical Research Institute

• Ian R. McGregor - Cary Institute of Ecosystem Studies

• Helene C. Muller-Landau - Smithsonian Tropical Research Institute

• Jeffrey C. Burchfield - University of Alabama, Huntsville

• KC Cushman - Oak Ridge National Laboratory

• Vanessa E. Rubio - Cary Institute of Ecosystem Studies

• Gisele Biem Mori - National Institute for Amazon Research, Universidade do Estado de Mato Grosso

• Martin J. P. Sullivan -  Manchester Metropolitan University

• Matthew W. Chmielewski - University of Louisville

• Adriane Esquivel-Muelbert - University of Birmingham

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Funding was provided in part by the National Science Foundation (NSF) grants DEB-2213245 and DEB-2241507 to EMG, and NE/W003872/1 to MS and EMG. AE-M was further funded by the Royal Society Standard Grant RGS\R1\221115 ‘MegaFlora’, the UK Research and Innovation/Natural Environment Research Council (NERC) TreeScapes NE/V021346/1 ‘MEMBRA’, the NERC/NSF Gigante NE/Y003942/1, and the Foundation for Research on Biodiversity/Centre for the Synthesis and Analysis of Biodiversity ‘Syntreesys’.

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Cary Institute of Ecosystem Studies is an independent nonprofit center for environmental research. Since 1983, our scientists have been investigating the complex interactions that govern the natural world and the impacts of climate change on these systems. Our findings lead to more effective resource management, policy actions, and environmental literacy. Staff are global experts in the ecology of: cities, disease, forests, and freshwater.

 

 

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