There are not many undisturbed tropical rain forests remaining in the world, and those that survive tend to be remote and difficult to access. Consequently, there’s a lack of data on their adaptability to droughts.
“We don’t really have long-term information on tropical forests,” said Jérôme Chave, research director of the Evolution and Biological Diversity Laboratory at the University of Toulouse. “Also, we don’t have a lot of information on undisturbed forests, forests that have not been degraded or partly logged.”
Chave was part of a research team that sought to answer two central questions: What are intact tropical rain forests’ past responses to drought, and what are their likely future drought tolerances? Their research, published in the Proceedings of the National Academy of Sciences of the United States of America, shows that intact forests in Africa, the Americas, and Asia are all experiencing long-term moisture loss and diminished tolerances.
Expanding the Data
Much previous data surrounding intact rain forests are accurate but lopsided, explained Chave. Although some areas, particularly in Central America, are really well studied, most intact rain forests are very remote and don’t have much, if any, ground data. And that, said Chave, has led to a biased view of how tropical forests are changing through time. Other data come from optical remote sensing, but the tropics are cloudy, and clouds can block sensors, leaving a lot of gaps in the data collected.
Chave and his colleagues opted to analyze intact rain forests with long-term remote sensing data from microwave signals. They created a C band radar data set at a resolution of 25 kilometers covering continuous monthly data from 1992 to 2018 in the three regions (Africa, the Americas, and Asia). With these data, the team evaluated the relationship between signal intensity and occurrences of droughts.
“Basically, the water is emitting waves,” explained Chave. The signals “are sensing the amount of water on the surface of the Earth. And when we are sensing over a forest, we are detecting how much water there is in the ecosystem.”
The researchers considered both the resistance (the scale of changes during and before a drought) and resilience (the capacity to bounce back from droughts) of the forests studied.
The American forests’ decline in radar signal was the largest, at 93%, followed by Asia at 88% and Africa at 84%. The radar signals also aligned with a cumulative water deficit index (a measurement of water availability) and surface and air temperatures. The signals suggest that the ability of these intact rain forests to survive future droughts is poor.
“The main result of our study is that the droughts are not easy to recover [from] for a forest at this scale,” said Chave. “So the next question that we tried to address is, How much can they take? How resilient are they to these changes?”
The researchers looked at the forests’ resistance and resilience during the 2 years before and 2 years after a drought to account for postdrought effects such as increased mortality and reduced growth. Their findings documented a long-term trend of reduced moisture, particularly in the Amazon, driven by droughts’ cumulative effects. The effects “add up,” Chave said. “It’s not like the forest has been recovering through time.”
William Anderegg, an associate professor of biology and director of the Wilkes Center for Climate Science and Policy at the University of Utah, said the study provides some new and valuable perspectives. Anderegg was not involved in the research.
“The big-picture finding that the Amazon and tropical forests in the Americas are substantially more vulnerable and that there are these accumulating damages of multiple droughts in many of these tropical forests, that’s incredibly important, and it also fits quite well with a lot of recent literature,” he said.
However, Anderegg pointed out that the radar research would benefit from on-the-ground corroboration. “It’s always going to be a limitation in a remote sensing study that you don’t fully know what’s driving the signal on the ground. What are the structural changes in rain forests that this data set is picking up?”
Chave and his fellow researchers are working on verifying other data sets to account for landscape changes brought on by events like storms and floods. They also want to use the same methods to examine disturbed tropical forests and other environments.
Ultimately, said Chave, “we’re just scratching the surface. Everybody’s talking about tropical forests. Everybody is aware of the threats to these forests, but yet we still know so little, and what we know is coming from very nonrandom areas on the planet, which is kind of sobering.”
—Danielle Beurteaux (@daniellebeurt), Science Writer