Species-specific responses of foliar nutrients to long-term nitrogen and phosphorus additions in a lowland tropical forest

1) The concentration, stoichiometry, and resorption of nitrogen (N) and phosphorus (P) in plant leaves are often used as proxies of the availability of these growth-limiting nutrients, but the responses of these metrics to changes in nutrient availability remains largely untested for tropical forest trees. 2) We evaluated changes in N and P concentrations, N:P ratios, and resorption for 4 common tree species after 13 years of factorial N and P additions in a lowland tropical forest in Panama. 3) Chronic P addition increased foliar P concentrations, decreased P resorption proficiency, and decreased N:P ratios in three locally common eudicot tree species (Alseis blackiana, Heisteria concinna, Tetragastris panamensis). The increase in foliar P involved similar proportional increases in organic and inorganic P in two species and a disproportionately large increase in inorganic P in A. blackiana. 4) Nitrogen addition did not alter foliar N concentrations in any species, but did decrease N resorption proficiency in H. concinna. 5) A fourth species, the palm Oenocarpus mapora, demonstrated remarkably static foliar nutrient concentrations, responding only with a marginal decrease in P resorption proficiency under N plus P co-addition. 6) Synthesis: Collectively, these results suggest that adjustment of N:P ratios can be expected in eudicots exposed to elevated P, but foliar N appears to already be at optimal levels in these lowland rainforest tree species. The complexity of species-specific responses to altered nutrient availability highlights the difficulty in predicting future responses of tropical forest trees to a changing world.