Ecologist Akhil Javad felt the thrill of fieldwork quickly fade when he was faced with the prospect of scaling trees over five times his height. But for some of the trees he was studying in India’s Western Ghat mountains, that was the only way to take their temperature.
So, Javad got climbing. Sensors that he placed on leaves in the upper canopy are providing unprecedented insights into how tropical forests are weathering global warming. The findings suggest that the trees may be in better shape than scientists thought, he and colleagues report in the February Global Change Biology.
In the summer, which lasts from March through June in the region, daily high temperatures in the mountains can cross 37° Celsius and are projected to rise by about 4 degrees Celsius in the next 60 years. That could be a problem for trees, since leaves can get much hotter than the surrounding air.
As the temperature of a leaf rises, its ability to harness sunlight to make sugar and oxygen becomes less efficient. On average, when leaves surpass 46.7° C, their photosynthetic machinery shuts down, lab studies have shown. When that happens, trees don’t get the energy they need. Many trees in the tropics are already experiencing temperatures beyond that average limit.
But what matters most is length of exposure, says ecologist Deepak Barua of the Indian Institute of Science Education and Research Pune. The new work is the first to study cumulative heat exposure in tropical trees, he says.
On a 6-hectare plantation that grows crops such as vanilla, cocoa and cashew, Javad placed 14 sensors on the leaves of 12 trees — a mix of rose sandalwood, ironwood, Ceylon boxwood and Kindal.
It wasn’t all smooth sailing, though. Sensors would often get dislodged by strong winds. One sensor was destroyed by wild langur monkeys. Another time, a thunderstorm wiped out a week’s worth of data. Perpetually repositioning, replacing and checking sensors was “a pain,” says Javad, now of the University of Leeds in England.
Such constant troubleshooting is one of the main reasons people avoid studying cumulative heat exposure, which makes this work even more valuable, says Andy Leigh, a plant ecologist at University of Technology Sydney who was not involved in the research.
“It’s really important for crazy scientists like us to get up trees and fight off monkeys, to put [sensors] on leaves,” Leigh says. “Otherwise, we really aren’t going to be able to understand how our natural systems … are going to be looking after us in the future.”
The sensors continuously recorded leaf and air temperatures throughout the four summer months in 2023. Meanwhile, in the lab, the team determined that species-specific critical thresholds for the area’s trees ranged from 43.37° C to 45.5° C, slightly below the average. Thirty minutes of continuous exposure to these temperatures can permanently damage leaves, the researchers found. But leaves crossed those temperature limits for less than 10 minutes a day, sensor data showed.
Trees in these forests will continue photosynthesizing with minimal reduction in efficiency despite rising temperatures, the findings suggest. However, exposure to elevated temperatures even for a few minutes a day could affect other aspects of tree health, like reproduction and drought resistance, Leigh notes. Her team is now gauging the extent of heat’s indirect impacts in Australia.
Barua is encouraged by the growing interest in heat exposure. Despite the logistical challenges, he expects more researchers will put temperature sensors to work in ecosystems across the world.