Journal Article

Land-use changes alter radiative energy and water vapor fluxes of a tall-grass <i>Andropogon</i> field and a savanna–woodland continuum in the Orinoco lowlands

José San José, Rubén Montes, John Grace, Nina Nikonova and Anaís Osío

in Tree Physiology

Volume 28, issue 3, pages 425-435
Published in print March 2008 | ISSN: 0829-318X
Published online March 2008 | e-ISSN: 1758-4469 | DOI: http://dx.doi.org/10.1093/treephys/28.3.425
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

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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

Journal Article.  0 words. 

Subjects: Plant Sciences and Forestry

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