Seasonal variation in energy balance and canopy conductance for a tropical savanna ecosystem of south-central Mato Grosso, Brazil

Tropical savanna (locally known as cerrado) comprises 24% of Brazil and is characterized by high temporal (climatic) and spatial (land cover) variation, biodiversity, and human activity. However, temporal variations in energy exchange are poorly understood, especially for mixed-grasslands (locally known as campo-sujo), making current and future patterns of energy balance highly uncertain. We used eddy covariance to measure latent (Le) and sensible (H) heat flux of a mixed-grassland, and linked meteorological and remote-sensing data to determine the controls on these fluxes. We hypothesized that (1) seasonal variations in H and Le would be large due to variations in precipitation, (2) ecosystem phenology, estimated using the Enhanced Vegetation Index (EVI), would be the best predictor of seasonal variation in Le, and (3) cerrado, transitional, and humid evergreen forests would have similar rates of average annual Le despite large seasonal variation in cerrado Le. We found that campo-sujo exhibits large seasonal fluctuations in energy balance that are driven by rainfall, and that responses to rainfall pulses are rapid and dynamic, especially during the dry season. Seasonal variations in the EVI did not affect energy fluxes; however, when energy fluxes were normalized with net radiation (Rn), the EVI was found to significantly affect the amount of available energy dissipated by H, Le, and G, indicating an important ground surface feedback on energy partitioning. Compared to other tropical ecosystems, cerrado exhibited substantially more seasonal variation in energy flux density than forested tropical ecosystems. For example, cerrado had lower rates of Le during the dry season, due to water limitations, but higher rates of wet-season Le than tropical forests, which were likely limited by radiation due to frequent cloud cover. Overall, these seasonal variations caused average annual rates of Le to be similar between cerrado, transitional, and humid evergreen forests.