Physiological and isotopic ( d 13 C and d 18 O ) 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 (DCp). These offsets could be attributed to a breakdown in the relationship between DCp and the ratio of intercellular to ambient CO2 partial pressures (ci/ca) in P. pinnatum, and to variation among species in the leaf-to-air vapour pressure difference (v). Thus, a plot of v·TE against ci/ca showed a general relationship among species. Relationships between d 18O of stem dry matter and stomatal conductance ranged from strongly negative for S. macrophylla to no relationship for T. grandis. Results suggest interspecific variation among tropical tree species in relationships between stable isotope ratios (d 13C and d 18O) and the gas exchange processes thought to affect them. Key-words: carbon isotope ratio; leaf N concentration; oxygen isotope ratio; transpiration; tropical tree; water-use efficiency.