Intrinsic water-use efficiency (iWUE) reflects how efficiently plants assimilate carbon relative to water loss at the leaf level. While widely studied using carbon isotope and gas-exchange measurements, most existing knowledge is derived from local observations.

To address this gap, a research team led by Prof. FU Zheng at the Institute of Geographic Sciences and Natural Resources Research (IGSNRR) of the Chinese Academy of Sciences, in collaboration with international partners, has developed a global isoscape of leaf carbon isotope discrimination for C₃ plants spanning 2001 to 2020. Building on this framework, the team quantified global patterns and long-term trends in leaf-level iWUE, examined the respective roles of atmospheric and soil drought, and evaluated the performance of an optimality-based ecological model.

The study was recently published in Nature Communications.

The researchers found that iWUE is generally higher in cold, dry regions than in warm, humid ones, with the global average iWUE increasing over the past two decades. Distinct differences were observed across biomes: grasslands exhibit relatively high iWUE but slower growth rates, while evergreen broadleaf forests show lower iWUE values but faster increases.

iWUE typically rises as water stress intensifies, though this enhancement weakens under more severe stress conditions. Notably, vapor pressure deficit (VPD) was found to exert a far broader influence on iWUE than soil moisture.

The ecological optimality model successfully captured the large-scale spatial patterns of observed iWUE and confirmed VPD as the dominant driver of plant responses to water stress. However, it tended to overestimate both absolute iWUE levels and their long-term increases.

Collectively, these findings offer a global perspective on plant carbon-water relations. They also suggest that intensifying water stress may limit further improvements in plant water-use efficiency, with implications for the long-term stability of the terrestrial carbon sink.