Future Decline in the Buffering Effect of Canopy Water on Ecosystem Water‐Use Efficiency in Tropical Forests

Abstract Tropical forests are crucial for mitigating climate change through carbon sequestration and maintaining hydro‐ecological balance by regulating vegetation‐climate feedbacks. However, their ecosystem water‐use efficiency (WUE) is anticipated to decline due to intensifying water stresses, including atmospheric aridity and soil drought (SD). While highly isohydric tropical forests demonstrate resistance to water stress, as evidenced by buffered decreases in canopy water, the influence of canopy water dynamics on WUE under increasing water stress remains unexplored. Here, we integrated various satellite‐derived, observational, and reanalysis data sets, CMIP6 projections, and Community Land Model historical (2001–2015) and future (2086–2100) simulations. We defined a canopy water amount index that successfully replicated the spatiotemporal patterns of canopy water amount. Additionally, we developed a two‐stage Partial Least Squares Regression approach to elucidate how canopy water variations regulate the WUE decline induced by water stress. Our findings revealed that WUE in tropical forests was more adversely affected by atmospheric aridity than by SD. Canopy water variations buffered the WUE decline by 18% under increasing atmospheric aridity and by 9% under increasing SD during 2001–2015, primarily by alleviating stomatal conductance restrictions and enhancing nitrogen availability for photosynthesis. However, these buffering functions are predicted to weaken significantly under future escalating water stresses, with an 88% reduction expected under rising atmospheric aridity during 2086–2100. Collectively, our results underscore the heightened risks of plant hydraulic failure and its potential to destabilize carbon‐water feedbacks and tropical forest resilience.