WEAVE: Climatic Controls of Tropical Rainforest Productivity

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9629245 Oberbauer Tropical rain forests (IRF), the worlds most productive terrestrial ecosystems, are a major component of the global carbon budget. The lack of a comprehensive understanding of the relation between climate and TRF productivity is a major impediment for realistically modeling the role of TRF in global carbon budgets under current and possible future climate scenarios. Recent results from fie1d work in Costa Rica, supported by other modeling studies, show that tree growth in TRF varies significantly on a year-to-year basis. This variation is not related to simple annual averages of climatic variables, and is therefore hypothesized to result from the interaction of these variables on a sub-annual scale. To determine the basis for this major ecosystem response will require a combination of controlled observations and modeling at physiologically relevant time scales, which is the goal of this project. This research will address four central hypotheses: 1) Forest-level productivity in TRF varies substantially among years. 2) Variation in annual aboveground production is driven by the combined effects of subannual variation in radiation and temperature, with episodic soil moisture limitation. 3) Stand-level production in TRF varies significantly along within-forest edaphic gradients. 4) TRF productivity is significantly affected by variation in speeies composition. To assess forest-level productivity and its relation to climate, annual production of tree biomass, fine litter, fine roots, and coarse wood debris will be measured. The study-plots will be replicated across the main topographic and edaphic gradients present in the old-growth TRF at the La Selva Biological Station in Costa Rica. In parallel with the plots, tree functional responses to climatic factors will be determined at short and medium time scales for species of trees from different functional groups. Short-term studies will be used to assess the relation between whole-tree performance and local microclimate. Th e response functions developed from this work will be further tested and calibrated on a series of individuals measured monthly. These functional responses will then be used to parameterize a top down model of forest productivity, based on the relationship between canopy carbon gain and intercepted radiation. Results of both the empirical studies of forest productivity and the forest-level process model, will potentially have wide applicability for understanding regional variations in TRF productivity, predicting responses to global climatic change, and for managing human-influenced tropical ecosystems.