To understand how vegetation responds to temperature and how it coordinates carbon uptake with water loss is of great significance in global change ecology and Earth system science. Previous studies mainly focus on the optimal temperature of photosynthesis, however, the thermal response of transpiration remains poorly understood.

In a study published in Nature Plants on April 15, Prof. FU Zheng's team from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences, along with international collaborators, found that plants can sustain transpiration at higher temperatures than photosynthesis under heat stress, revealing a thermal decoupling between carbon uptake and water loss in terrestrial ecosystems.

Through global eddy covariance measurements, sap flow observations, remote sensing, and Earth system model simulations, the researchers systematically quantified the optimal temperatures of transpiration and gross primary productivity (GPP).

The results showed that the optimal temperature for transpiration was consistently higher than that for photosynthesis by approximately 1.8°C globally, with a more pronounced difference in forest ecosystems. As temperature increased, photosynthesis reached its peak and declined earlier, while transpiration continued to increase over a broader temperature range, contributing to leaf cooling, until it decreased beyond its optimal temperature.

Moreover, the researchers demonstrated that, although the optimal temperatures of transpiration and GPP were positively correlated, they exhibited systematic divergence, indicating that ecosystem carbon uptake was more sensitive to high-temperature stress than water loss.

Machine learning analysis identified growing-season maximum temperature as the dominant driver of both optima, and revealed that their difference was primarily regulated by vegetation water content. Earth system models reproduced the general spatial patterns but significantly underestimated both the magnitudes and their differences.

The study provides the first global-scale evidence of divergence between the optimal temperatures of transpiration and photosynthesis, and proposes a "dual optimal temperature framework" for understanding ecosystem carbon-water responses to warming.