Leaf nitrogen to phosphorus ratios of tropical trees: experimental assessment of physiological and environmental controls

We investigated the variation in leaf nitrogen to phosphorus ratios of tropical tree and liana seedlings as a function of the relative growth rate, whole-plant water-use efficiency, soil water content and fertilizer addition. First, seedlings of 13 tree and liana species were grown individually in 38-l pots prepared with a homogeneous soil mixture. Second, seedlings of three tree species were grown in 19-l pots at high or low soil water content, and with or without added fertilizer containing nitrogen, phosphorus and potassium. For plants grown under common soil conditions, leaf nitrogen to phosphorus ratios showed a unimodal, or hump-shaped, relationship with the relative growth rate. The leaf nitrogen to phosphorus ratio increased in response to low soil water content in three species, and increased in response to fertilizer addition in two of the three species. Across all species and treatments, the leaf nitrogen to phosphorus ratio was positively correlated with the water-use efficiency. The results suggest that the within-site variation among tropical tree species in the leaf nitrogen to phosphorus ratio may be caused by associations between this ratio and the relative growth rate. Modification of the soil environment changed the leaf nitrogen to phosphorus ratio, but underlying associations between this ratio and the relative growth rate were generally maintained. The observed correlation between the leaf nitrogen to phosphorus ratio and water-use efficiency has implications for linking nutrient stoichiometry with plant transpiration.