Life form and behavior type shape water use efficiency in tropical plants via leaf functional traits

Understanding how water use efficiency ( WUE ) responds to fluctuating light is critical for predicting ecosystem function under global change. In this study, we examined photosynthetic gas exchange and leaf functional traits across 48 tropical plant species in Hainan, categorized by life form (woody/herbaceous) and light-adaptation (sun/shade). We introduced a novel suite of parameters [e.g., maximum WUE ( WUE max ), rising slope ( R S ), and declining slope ( D S )] to quantify the dynamic WUE -light response curve. Our results revealed divergent water-use strategies where elevated CO 2 enhanced WUE max in shade-tolerant species but reduced it in sun-adapted species. This negative reaction might be caused by the closure of stomata in sun-adapted plants, and its inhibitory effect on photosynthesis is stronger than that on transpiration. Using EXtreme Gradient Boosting model, we identified mean tilt angle (MTA) and leaf nitrogen content (LNC) as direct regulators of WUE max . SHapley Additive exPlanation analysis further indicated this was a non-linear relationship, where moderate leaf inclination was associated with improved WUE max . We further uncovered a key physiological trade-off where WUE max was positively associated with mesophyll conductance ( g m ) but negatively correlated with stomatal conductance ( g s ) and photosynthetic capacity. This is because high g m ensures efficient CO 2 diffusion to carboxylation sites, whereas excessively high g s leads to substantial water loss. In conclusion, this study not only provides a novel framework for analyzing dynamic WUE but also reveals distinct water-use strategies across tropical plant functional groups. This finding underscores the necessity of incorporating such divergent, niche-specific strategies into projections of future ecosystem function. • A novel set of parameters was proposed for quantifying the light response curve of WUE . • Different functional groups of tropical plants exhibited divergent responses in WUE max to elevated CO 2 . • Physiological trade-off governing WUE max involves balance between g m , g s and photosynthetic capacity.