Evapotranspiration and energy balance of Brazilian savannas with contrasting tree density

Abstract

Tropical savannas including those of central Brazil (Cerrado) serve as an important regulator of energy and mass exchange with the atmosphere. However, the effects of interactions between grasses and deeply rooted trees and shrubs on energy and water fluxes of savannas are not well understood. In this study, eddy-covariance measurements were used to observe and compare the energy balance and evapotranspiration at two adjacent Cerrado sites differing in woody plant density. Latent energy flux (LE) and evapotranspiration (ET) for Cerrado Denso (8-10-m trees with a cover density of approximately 50% and an understory of grass and shrubs) were generally similar to or greater than for Campo Cerrado (widely scattered 3-4-m trees with a cover density of about 5% and an understory of grasses, shrubs, and sedges), with the greatest difference between the sites occurring during the dry season. Mean annual ET was 823 and 689 mm yr-1, and LE averaged 63.9 ± 11.7 and 53.5 ± 14.3 W m-2 at Cerrado Denso and Campo Cerrado, respectively. At both sites, ET reached a minimum at near the end of the dry season, indicating that reduced water availability constrained dry season ET at both sites. The Bowen ratio at both sites increased as soil moisture decreased, but with a stronger dependency at the Campo Cerrado site, presumably due to the dominance of shallow-rooted plants and the larger contribution of soil evaporation there. Energy partitioning, as indicated by the Bowen ratio, was also strongly influenced by variations in leaf area index (LAI). The strong similarity in the Bowen ratio-LAI relationship for the two sites suggests that LAI can be used to explain much of the observed temporal and spatial ET variability across seasons and variations in woody plant density in the Cerrado.