Seasonality and foliar nutrient dynamics of a tropical rain forest in Costa Rica

Through the examination of seasonal patterns of leaf litter nutrient cycling we may be better able to predict the response of tropical ecosystems to future environmental change. This work examines how seasonal variability in litter nutrient dynamics affects ecosystem function of a wet tropical forest in Costa Rica. I investigated the importance of litter nutrient dynamics through two experiments manipulating forest floor litter and observations of nutrient resorption. The response to litter application was explained by the phosphorus (P) concentration of added litter. The response was not related to total P inputs, or to the concentration or inputs of nitrogen (N). The quality of leaf litter inputs is determined at the time of abscission via nutrient resorption (withdrawal of nutrients during senescense). Examination of foliar nutrient concentrations in nine common canopy tree species showed a mean resorption efficiency of 44% for P and 46% for N. Therefore, as much as 2.5 kg/ha/yr of P and 50 kg/ha/yr of N are recycled internally by the vegetation. Among these nine species, senesced leaf nutrient concentrations correlated positively with those of the green leaves. Therefore, variability in the quality of leaf litter inputs is likely a reflection of green leaf nutrient concentrations. Modeled shifts in the relative importance of these species at the stand-level indicate that a change in species dominance could have a major impact on stand-level nutrient economy by significantly altering annual inputs of leaf N and P to the forest floor. Green and senesced leaf nutrient concentrations as well as nutrient resorption efficiency for the nine species were significantly higher in the wet versus the dry season. Low nutrient resorption during the dry season could reflect either incomplete resorption due to water stress or lower plant demand during this period. A more detailed evaluation of foliar N and P for two species (Pentaclethra macroloba and Laetia procera) showed that green and senesced leaf nutrient concentrations as well as resorption efficiency changed significantly over the short-term. This variability in foliar nutrients and resorption efficiency was not related to climate over the four-month study period. It is possible that variability in nutrient resorption is driven by species-specific changes in phenological demands. This idea is supported by the different temporal patterns of foliar nutrients of P. macroloba and L. procera.; Results from this study demonstrate the dynamic nature of wet tropical forests. From the soil pool to the leaves of canopy trees, nutrients varied significantly both over the short-term and seasonally. Species differed in their resorption efficiency, foliar nutrient concentrations, as well as their response to temporal variability in climate. Should land-use and climate change be accompanied by a shift in species dominance, both the pattern and magnitude of nutrients cycled in litterfall would likely change. With it, the functioning of tropical forests would be altered, as well as feedbacks on atmospheric carbon, water and energy balance.