Decoupling of stomatal conductance, transpiration and photosynthesis in terrestrial plants under elevated temperature: a meta-analysis.

Elevated temperatures pose challenges to stomatal conductance, which regulates transpiration and photosynthesis. However, the coupling of stomatal conductance, transpiration and photosynthesis may shift with warming. Here, we synthesize evidence from a meta-analysis of 207 studies to assess leaf physiological responses to warming. On average, the responses of stomatal conductance are highly variable, exhibiting no consistent directional trend, whereas transpiration increases and photosynthesis decreases, reflecting a shift towards transpirational cooling. Stomatal conductance declines until temperatures exceed 5 °C above ambient, whereas transpiration remains stable under warming <3 °C. Beyond these two thresholds, both stomatal conductance and transpiration increase with further warming. The sensitivity of stomatal conductance, photosynthesis, and water-use efficiency to warming varies substantially among plant functional types, with distinct responses across life forms, phylogenetic groups, and photosynthetic pathways. Higher mean annual temperature amplifies the positive responses of stomatal conductance and transpiration to warming, whereas greater mean annual precipitation mitigates the warming-induced declines in photosynthesis. Elevated CO2 exacerbates warming-induced declines in photosynthesis, while drought constrains transpirational cooling. Collectively, these findings highlight a progressive decoupling of stomatal conductance, transpiration and photosynthesis with warming, revealing complex trade-offs between plant water use, thermal regulation, and carbon assimilation.