New Insights into Seasonal Carbon Exchanges in the Amazon Rainforest

Tropical South America (mostly characterized by Amazonia) contains Earth's largest tropical forest. This region has significant influence on the global carbon budget and carbon-climate feedbacks, both in present and future climate. Circa year 2000, it was thought that the region was a carbon sink during the wet season, and a source during the dry season due to dry soil. Data from a few eddy-covariance flux towers  opposed this paradigm and showed uptake during the dry season, and efflux in cloudy, rainy periods. Many models replicated this behavior at large scales, although data constraints remained sparse. New observations from eddy-flux towers, satellites, and data-constrained inverse models suggest that the original paradigm of uptake during the wet season and efflux during the dry season does, in fact, hold across the region as a whole. We identify several physical mechanisms and processes that are implied by these new data, and these are conspicuously different from previous hypotheses used to explain seasonality in the region. As the net flux is the small residual between very large uptake (photosynthesis) and efflux (respiration/decomposition) signals, the net flux in any given month is close to zero, and relatively small month-to-month changes in either component flux can easily change the sign of the net carbon flux. These findings have implications for mechanistic understanding and process modeling of tropical carbon dynamics and our ability to predict the interaction between climate and carbon dynamics under future climate conditions.