Forests, it's often said, are the lungs of Earth. Tall trees stretch skyward to find sunlight, sucking water up from their roots and carbon dioxide in through their leaves to photosynthesize.

Like any chemical reaction, this elemental exchange works best within an optimal temperature range, outside of which a tree's photosynthetic machinery can break down.

With global temperatures rising, a new study combining tree canopy measurements and global satellite data has found a small fraction of canopy leaves in tropical forests might be reaching their upper limit.

"This study shows that there are times and places where tropical forest leaves are surpassing their critical temperature thresholds," says senior author and tropical ecologist Gregory Goldsmith of Chapman University in California.

Research shows tropical trees can keep photosynthesizing up to 46.7 °C (116 °F) on average. But the world doesn't warm evenly, and neither do forest canopies. Heat tolerance varies among species and populations, and possibly even within leaves of the same tree, Goldsmith says.

To understand if some leaves are nearing temperatures that are too hot for photosynthesis, a team led by Northern Arizona University ecoinformastician Christopher Doughty pulled data from NASA's ECOSTRESS sensor, which measures land surface temperatures as it whizzes around Earth aboard the International Space Station.

Satellite measurements taken between 2018 and 2020 were validated with data from tower-mounted sensors recording temperatures in the upper canopies of five forest sites in Brazil, Puerto Rico, Panama, and Australia.

"You need both the ground and satellite-based observations to understand the temperatures of these tropical forest canopies," Goldsmith explains, "because for leaf temperatures, it's really not the averages that are important; it's the extremes."

The analysis found canopy temperatures peaked around 34 °C on average during dry periods, though some leaves exceeded 40 °C (104 °F). A small fraction of all leaves – about 0.01 percent – surpassed critical temperatures at least once a season.

"Although infrequent, the occurrence of extreme temperatures may have a catastrophic effect on the physiology of a leaf and may be thought of as a low-probability, high-impact event," the researchers write in their paper.

Based on what we know of plant physiology, trees may close the bean-shaped pores on their leaves, called stomata, to conserve water as temperatures rise. However, doing so might expose leaves to damaging temperatures since the leaves can no longer cool down through transpiration.

In times of drought when soils are parched, heat might intensify for water-stressed trees, as soil moisture also impacts canopy leaf temperatures.

"Believe it or not, we don't know terribly much about why trees die," Goldsmith says. Our understanding of the interactive effects of heat and drought, water and temperature, is particularly thin, he adds.

To simulate future conditions, the team modeled the responses of tropical forest to warmer temperatures and periodic droughts, using data from three warming experiments in which temperature sensors were taped to individual leaves in upper forest canopies.

The model simplifies forest dynamics with a host of assumptions, but nevertheless, simulations suggest that up to 1.4 percent of upper canopy leaves might get too hot to photosynthesize under future warming scenarios.

If warming exceeds 3.9 °C, which is possible under worst-case scenarios, it might be hotter than forests can withstand. Leaves and trees could potentially start dying off, triggering a huge loss from these important carbon stores.

The researchers stress, however, that there is a considerable degree of uncertainty in their results, which means we might have time to act yet, reducing emissions and curbing deforestation to protect tropical forests.

"It is still within our power to decide the fate of these critical realms of carbon, water, and biodiversity," they conclude.

The study has been published in Nature.