Tropical forests are facing increasing risks of exposure to critical temperature thresholds

A leaf has a critical temperature above which photosynthesis starts to break down, and the tree grows less and is more likely to die.⁴ This study asks how close the canopies of tropical forests are to that line, and how fast they are closing on it.^1,3 Nina van Tiel and Gaston Lenczner share first authorship; Mukund P. Rao is the third author.^2,3 Charlotte Grossiord and Devis Tuia of EPFL in Switzerland supervised it.³ Christopher B. Field edited it for PNAS.³

The method joins two kinds of data. Satellites give the maximum surface temperature of the canopy.¹ Range maps say where each of 208 tropical tree species grows, for which a critical temperature had been catalogued.^1,4 The gap between the two is the thermal safety margin.¹ The study covers South America, Southeast Asia and Central Africa from 2001 to 2020, then carries the trend forward with Earth System Models under medium-to-high emission scenarios.¹

The species tolerate a lot of heat: their critical temperature averages 46.1 °C.¹ But the margin fell everywhere, by 0.4 °C a decade across the tropics and 0.5 °C in South America.¹ The area where the canopy grew hotter than the average critical temperature rose from 43 million to 57 million hectares over the twenty years, 4% of the area studied.^1,4 Measured against the most sensitive species, it was 10%.¹ The projections put the area past the average threshold at 83 million hectares by 2050 and 160 million by 2100.^1,4 A Down To Earth report of the study gives more than 93 million hectares for 2050;⁵ the abstract and the EPFL release both say 83 million.^1,4

The authors point to the carbon the forests take up and to their biodiversity as what is at stake.³ In the EPFL release, Grossiord says the method can find the places at risk, and that within one forest more heat-tolerant species may replace others; how fast that can happen is unknown.⁴ Tuia adds that rare species may matter more to how a forest works than their numbers suggest.⁴ The 208 species are a small share of the more than 15,000 in tropical forests, and the team plans to add species and to look at Mediterranean forests in Europe.⁴

For a model of a forest, the paper is a recipe. A satellite temperature record, species range maps and a table of measured heat limits together give a map of where a forest's trees are nearest their limit.