Land-use changes alter radiative energy and water vapor fluxes of a tall-grass Andropogon field and a savanna–woodland continuum in the Orinoco lowlands
José San José (1, 2), Rubén Montes (3), John Grace (4), Nina Nikonova (1) and Anaís Osío (1)
1. Centro de Ecología, Instituto Venezolano de Investigaciones Científicas, Apartado 21827, Caracas 1020-A, Venezuela / 2. Corresponding author () / 3. Departamento de Estudios Ambientales, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, Venezuela / 4. School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JN, U.K. / Received May 10, 2007; accepted December 10, 2007; published online January 2, 2008
Summary
Changes in land use in the Orinoco lowlands affect the daily trends of energy and water vapor fluxes. We analyzed these fluxes
along a disturbance gradient beginning from a cultivated tall-grass Andropogon field (S1) and extending over three savanna sites with increasing woody cover over herbaceous vegetation. The savanna sites
encompass a herbaceous savanna (S2), a tree savanna (S3) and a woodland savanna (S4). In the wet season, there were differences
in the radiation budget: seasonally averaged albedo for S1 (0.17) exceeded that of S2–S4 (0.13–0.14). Eddy covariance fluxes
indicate that the partitioning of the daily net radiation (Rn) into sensible and latent heat (λE) fluxes depends on land use. During the wet season, evapotranspiration (i.e., λE) over
the S1–S4 sites accounted for a variable fraction of Rn (i.e., 0.75, 0.52, 0.67 and 0.68, respectively). Therefore, the Bowen ratio was typically below 1. As the dry season progressed,
the λE/Rn ratio decreased markedly with increasing air and canopy temperatures and air humidity mole fraction deficit. The maximum
evaporation rate over the S1–S4 sites was 3.2, 2.5, 3.5 and 4.1 mm day–1, respectively, and the annual values were 721, 538, 771 and 732 mm year–1, respectively, equivalent to 49, 65, 52 and 88% of the rainfall. Soil water content fell from a maximum above 0.28 in the
wet season to 0.030, 0.026, 0.030 and 0.028 m3 m–3 at sites S1–S4, respectively, in the dry season. Leaf area index was greatly reduced as herbaceous vegetation dried out.
Keywords:
eddy covariance, energy balance, savannas.
