Physiological and isotopic (δ13C and δ18O) responses of three tropical tree species to water and nutrient availability

Water-use efficiency and stable isotope composition were studied in three tropical tree species. Seedlings of Tectona grandis , Swietenia macrophylla and Platymiscium pinnatum were grown at either high or low water supply, and with or without added fertilizer. These three species previously exhibited low, intermediate and high whole-plant water-use efficiency ( TE ) when grown at high water supply in unfertilized soil. Responses of TE to water and nutrient availability varied among species. The TE was calculated as experiment-long dry matter production divided by cumulative water use. Species-specific offsets were observed in relationships between TE and whole-plant ¹³C discrimination (Δ¹³C_p). These offsets could be attributed to a breakdown in the relationship between Δ¹³C_p and the ratio of intercellular to ambient CO₂ partial pressures ( c _i/ c _a) in P. pinnatum , and to variation among species in the leaf-to-air vapour pressure difference ( v ). Thus, a plot of v · TE against c _i/ c _a showed a general relationship among species. Relationships between δ ¹⁸O of stem dry matter and stomatal conductance ranged from strongly negative for S. macrophylla to no relationship for T. grandis . Results suggest inter-specific variation among tropical tree species in relationships between stable isotope ratios ( δ ¹³C and δ ¹⁸O) and the gas exchange processes thought to affect them.