| The land surface has significant influence over variability in the global atmospheric CO2 growth rate and the tropics, especially tropical South America, has been identified as a region of particular import. The Amazon rainforest is the largest tropical forest in the world, and contains up to 10% of terrestrial biomass. Gross fluxes of CO2 (photosynthesis and respiration) are massive, and slight variability in these large components can impose a net CO2 flux that is felt globally. In the tropics, seasonality in day length and temperature are minimal. The dominant signal is annual wet and dry seasons, caused by the oscillation of the Intertropical Convergence Zone (ITCZ) northward and southward during the year. Interannual variability is imposed by the El Nino-Southern Oscillation (ENSO), which can influence large-scale circulation patterns globally.;A positive correlation between El Nino and the atmospheric CO 2 growth rate has been noted, and a canonical explanation has evolved. In this canon, El Nino results in decreased precipitation over Amazonia, which results in decreased photosynthetic uptake, often at a lag of 6-12 months. Decreased precipitation results from less cloudiness, which can also increase solar forcing at the surface. This will result in warming, which can enhance respiratory processes that release carbon to the atmosphere. Therefore, there are two pathways (reduced photosynthesis and/or increased respiration) whereby an El Nino event can lead to a net release of CO2 from the land to the atmosphere.;There is no question that tropical forest function has decoupled, to some extent, from annual cycles of wet and dry. Were this not the case, the forest could not survive a dry season. But our physical understanding of this system, as represented by numerical models, has had difficulty reproducing observed behavior. Uncertainty also arises from a dispute surrounding what mechanisms drive variability in Amazonia.;The Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is an international research collaboration that ran officially from 1995--2005, and has provided a wealth of observational data from a formerly data-poor region. We have been able to use this data to address some of the uncertainty in the canonical explanations of surface ecophysiology in tropical South America.;From observational studies, we are able to identify mechanisms that have been observed to facilitate forest function through seasonal drought. Using surface-atmosphere exchange data from a observation tower in the Tapajos River National Forest, Brazil, as an evaluation metric, we can incorporate these mechanisms, singly and combined, into numerical models. By doing so, we identify both a deep soil that provides a reservoir for storing water, as well as rooting systems that can access this stored water, as requirements for maintaining forest function in the model. When these are incorporated into a numerical model, we demonstrate an ability to capture annual cycles and interannual as well as diurnal variability in our simulations.;Next, we extend the analysis across vegetation and moisture gradients. Maintaining our comparison to surface observation sites, we show that physiological function and annual cycles of surface-atmosphere exchange of energy, water, and carbon are a function of both annual rainfall and the characteristics (length, severity) of annual drought. We demonstrate an ability to capture mean seasonal cycles across these gradients in our computer models.;Finally, having demonstrated an ability to capture mean behavior at multiple observation sites, we extend the analysis across a large spatial domain and over time that includes multiple ENSO cycles. We find that on the scale of tropical South America, there is a net efflux of carbon during the wet season and uptake during seasonal drought. Radiation explains the most variability in ecophysiological function over the wettest regions (implying light-limitation), with water playing a larger role in areas where annual precipitation is less. There is variability in the response to moisture and light in the forest nearer the forest-savanna boundary, suggesting an interdependence of processes. Regional response to ENSO is heterogenous. During the 1997--1998 El Nino, canonical behavior was observed; precipitation decreased, and there was a basin-wide efflux of CO2 in a combination of photosynthetic and respiratory processes. In the 1987 El Nino, the response was more heterogenous, with regional patterns of both uptake and efflux. This suggests that variability around seasonal cycles of precipitation, as well as magnitude of the anomaly, combine in complex ways to determine large-scale carbon status.;We anticipate that this research will have implications for understanding of present climate, as well as predictions for the future. (Abstract shortened by UMI.)... |
