Land-use Changes Alter Energy and Water Balances on an African BrachiariaPasture Replacing a Native Savanna in the Orinoco llanos

Abstract

The seasonal changes in the energy balance after the substitution of a herbaceous savanna by a Brachiaria field located in the Orinoco lowlands were assessed over an entire year using the eddy covariance technique. Simultaneously, an herbaceous savanna was monitored as a control. This work provides evidence that the vegetation replacement lead to different patterns of energy and water balance. The seasonal trends of the latent heat flux (λE) to available energy (Ra) ratio tended to decrease as senescence increased due to seasonal influence of air humidity mole fraction deficit and soil water content on leaf area index (LAI) and surface conductance (gs).  Therefore, the partitioning of the available energy depended on both climatological (i.e., solar radiation, volumetric soil water content and air humidity mole fraction deficit) and biological variables (i.e., conductance behavior and LAI) which were stress-induced. For the wet season, the seasonally averaged daily λE in the Brachiaria field (i.e., 0.8 ± 0.1 mm d-1) was 1.3-fold higher than that in the herbaceous savanna (i.e., 0.6 ± 0.1 mm d-1) (Mann-Whitney U-test). For the dry season, the value was 2.7 ± 0.6 and 2.2 ± 0.4 mm d-1, respectively, these means values were not significantly different. In the Brachiaria and herbaceous savanna stands, the annual evapotranspiration was 731 and 594 mm year-1, respectively, and the annual ratio of evapotranspiration to precipitation was 0.52 to 0.42 respectively. In Brachiaria field, the deep drainage was relatively lower (43% of total precipitation) than that in the herbaceous savanna stand (53%) leaving a similar amount of water to increase soil storage. The current shift in land cover decrease deep drainage and increased λE by water uptake from a pasture with high belowground phytomass and LAI.