Tropical forests will struggle in a fast-warming world

Plants in tropical forests are by nature resistant to heat, yet that resistance has its limits. If temperatures in the tropics warm too much too fast, say scientists, local trees’ ability to absorb and store atmospheric carbon will suffer. That in turn will accelerate the rate of climate change.

Up to a certain point of warming, “tropical forests are surprisingly resistant to small temperature differences,” says Martin Sullivan, an ecologist at the University of Leeds who specializes in tropical forests.

Sullivan and his colleagues on an international team of researchers measured more than half a million trees in more than 810 forests in the tropics in order to assess how much carbon forests growing under different climatic conditions could store. They have found that temperature increases have a direct bearing on the outcome.

Tropical forests continue to store large degrees of carbon under high temperatures, the researchers say. Trees can handle heat up to around 32 degrees Celsius during daytime; beyond that point, though, they are increasingly less able to store large amounts of carbon. Each temperature degree increase above this critical threshold causes trees to release four times as much carbon dioxide as below the threshold, the experts say.

“If we limit climate change [trees] can continue to store a large amount of carbon in a warmer world,” explains Sullivan, who was the lead author of a new study published in the journal Science. Failing that, however, tropical vegetation will lose much of its capacity for carbon storage.

A global temperature increase of 2°C above pre-industrial levels would cause nearly three-quarters of tropical forests above the specific heat threshold identified by Sullivan and his colleagues. “Any further increases in temperature will lead to rapid losses of forest carbon,” he notes.

That’s bad news because tropical forests store a quarter century’s worth of our fossil fuel emissions in their trees alone, the experts observe. And it isn’t just warming air temperatures that are a direct threat to tropical forests. So are the increased risks of prolonged droughts from climate change and fragmentation from forest clearing.

“Our results suggest that intact forests are able to withstand some climate change,” says Prof. Beatriz Marimon, an expert at the State University of Mato Grosso in Brazil who studies some of the world’s hottest tropical forests in central Brazil. “Yet these heat-tolerant trees also face immediate threats from fire and fragmentation. Achieving climate adaptation means first of all protecting and connecting the forests that remain,” she adds.

These findings are in line with those of another study, published earlier this year and spearheaded by the University of Leeds, which indicates that forests such as those in the Amazon are fast losing their ability to absorb CO2 from manmade greenhouse gas emissions. The reason is that they are becoming so saturated with carbon that they may turn carbon neutral within just a decade and a half.

Trees suck up CO2 from the air through the process of photosynthesis. Plants grow faster in carbon-rich environments, yet the rate of tree loss (through deforestation, fire or droughts) can supersede a forest’s ability to absorb CO2. This will especially impact forests in the Amazon where the rapid loss of forest cover driven by land clearing and climate change means that local forests are becoming less effective at carbon sequestration.

“The striking implication from all this is that it appears the tropical carbon sink may be disappearing, first in Amazonia, then in Africa, which means that we really must reassess global targets for reducing greenhouse gas emissions,” warns forest ecologist Andy Marshall.