Afforestation of Tropical Pasture Only Marginally Affects Ecosystem-Scale Evapotranspiration

Abstract

Evapotranspiration (ET) from tropical ecosystems is a major constituent of the global land–atmosphere water flux and strongly influences the global hydrological cycle. Most previous studies of ecosystem ET have been conducted predominantly in tropical forests, and only few observations cover other tropical land-use types such as pastures, croplands, savannas or plantations. The objectives of our study were: (1) to estimate daily, monthly, and annual ET budgets in a tropical pasture and an adjacent afforestation site, (2) to assess diurnal and seasonal patterns of ET, (3) to investigate environmental controls of ET, and (4) to evaluate the soil infiltration potential. We performed eddy covariance measurements of ecosystem ET in Sardinilla (Panama) from 2007 to 2009. Daily ET (2.6 ± 1.0 mm day⁻¹) was significantly lower in the pasture compared to the afforestation site (3.0 ± 0.9 mm day⁻¹). The highest ET was observed during the wet–dry transition period in both ecosystems. However, differences in daily ET between sites were relatively small, particularly during the wet season. Radiation was the main environmental control of ET at both sites, however, we observed considerable seasonal variation in the strength of this control, which was stronger during the wet compared to the dry season. In 2008, total annual ET was only slightly higher for the afforestation (1114 mm y⁻¹) than the pasture site (1034 mm y⁻¹). Our results suggest that afforestation of pasture only marginally increases ecosystem-scale ET 6–8 years after establishment. Differences in soil infiltration potentials between our sites seem to explain this pattern.