Tropical forest canopies get hotter than expected, putting wildlife at risk

New research finds that the canopies of tropical forests get significantly hotter than the surrounding air. As global warming ups temperature extremes around the world, scientists worry that this means the treetops of rainforests and the wildlife that live in them could fare poorly in the future. They found high temperatures could also diminish tropical forests’ abilities to remove greenhouse gases from the air, reducing their effectiveness as carbon sinks at a time when the world needs them the most.

To better understand how the temperatures at the tops of trees in a rainforest compare to surrounding air temperatures and how this might affect the functioning of the trees themselves, researchers at Florida State University in the U.S. trained thermal cameras on a tract of rainforest in Barro Colorado Island in Panama.

A study published recently in Ecosphere reveals that the canopies they looked at exceeded the maximum air temperature by as much as 7 degrees Celsius. For context, the Paris Agreement is attempting to stave off the worse effects of climate change by keeping warming under 2 degrees Celsius.

Overall, they found that canopies could become 40 percent warmer than the surrounding air. The researchers write that this could have dire implications not only for the trees themselves, but also for the plants and animals that spend their lives in their treetops.

“There are so many different organisms that use and partition the canopies in different ways,” said study coauthor Stephanie Pau, an assistant professor at the University of Florida. “For example, ant species in tropical forests are extremely sensitive to bark temperatures. The diversity of canopy dwelling species will be affected by canopy temperatures — and it’s not just how hot it will get, but it’s also the temperature variability within a canopy.”

When the researchers looked at the effects of temperature hikes on the overall primary productivity of forests – in other words, the amount of carbon they absorb from the atmosphere via photosynthesis – they discovered that this ability started dropping off if a canopy reached a temperature between 28 and 29 degrees Celsius. And if a canopy temperature exceeded 31 degrees Celsius, its productivity began to decline altogether.

This weakened productivity is worrisome to the researchers who say that it could have big implications on the health of tropical forests, which evolved to survive in only a narrow band of temperature range. They say that even small changes could have big impacts on the world’s tropical forests. And as they’re considered the “lungs of the planet,” trouble for rainforests could spell trouble for the world.

“Tropical forests are among the most productive places on Earth — they account for about 30 percent of terrestrial productivity,” Pau said. “These forests are vulnerable to climate change and climbing temperatures.”

The study’s findings indicate the canopies of tropical forests warm 1.4 degrees for every 1 degree of air temperature change. Yet, Pau and her colleagues write that canopy temperatures are rarely taken into consideration in climate change analyses.

She warns that this could be a big mistake.

“When people talk about climate change, we usually hear about air temperatures,” Pau said, “but we should keep in mind that tropical forest canopies are even more vulnerable than air temperatures might indicate.”

 

Citation: Pau, S., Detto, M., Kim, Y., & Still, C. J. (2018). Tropical forest temperature thresholds for gross primary productivity. Ecosphere, 9(7), e02311.

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