Caylor2005a - Ecosystems (2005) 8: 1732 DOI:...

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Tree Canopy Effects on Simulated Water Stress in Southern African Savannas Kelly K. Caylor, 1 * Herman H. Shugart, 2 and Ignacio Rodriguez-Iturbe 1 1 Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08540, USA; 2 Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904, USA A BSTRACT A coupled energy and water balance model is used to simulate the effects of large tree canopies on soil moisture and water stress across a series of sites spanning a regional moisture gradient in southern Africa. The model tracks evapotranspiration from Fve components of the land surface at each site—the tree canopy, the grass under and between tree canopies, and the bare soil under and between tree canopies. The soil moisture dynamics are sim- ulated at daily time steps and driven by a stochastic model of storm arrivals and storm depth. Evapo- transpiration is modeled using the Priestley-Taylor approach, with potential evapotranspiration scaled by soil moisture availability. The soil moisture un- der tree canopies is compared to the soil moisture between tree canopies, and differences in average annual soil moisture stress conditions are analyzed at each site. The spatial distribution of large trees has important consequences for small-scale soil mois- ture dynamics across the rainfall gradient. The re- sults indicate that tree canopies serve to reduce soil moisture stress of under-canopy vegetation in the middle of the rainfall gradient. At the dry end of the rainfall gradient, the effect of tree canopies on soil moisture is dependent on the amount of rainfall received in a given growing season. Key words: energy and water balance model; tree canopies; soil moisture; water stress; rainfall; southern Africa; Kalahari Transect. I NTRODUCTION Savannas have been variously deFned, but they are generally described as mixed life-form vegetation communities in which woody and herbaceous components codominate (Bourliere and Hadley 1970; Walter 1971; Sarmiento 1984; Tothill and Mott 1985). In addition to their characteristic structural heterogeneity, savannas typically expe- rience strong seasonality in rainfall, with pro- nounced wet and dry seasons and a high degree of interannual variability in rainfall (Huntley and Walker 1982). The conspicuous shared dominance of contrasting plant life-forms makes savannas in- herently interesting from an ecological perspective; moreover, their geographic and socioeconomic importance is underscored by the rapid growth in human populations that depend on these lands for wood products and livestock rangeland. Tropical savannas are distributed over an eighth of the ter- restrial surface and make up one-half of African land cover (Atjay and others 1987).
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Caylor2005a - Ecosystems (2005) 8: 1732 DOI:...

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