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Volume 86, Issue 2 p. 308-319
Special Feature

ECOHYDROLOGICAL IMPLICATIONS OF WOODY PLANT ENCROACHMENT

Travis E. Huxman

Travis E. Huxman

Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721-0088 USA

E-mail: [email protected]

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Bradford P. Wilcox

Bradford P. Wilcox

Rangeland Ecology and Management, Texas A&M University, College Station, Texas 77843 USA

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David D. Breshears

David D. Breshears

Earth and Environmental Sciences, Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 USA

Present address: School of Natural Resources, and Institute for the Study of Planet Earth, University of Arizona, Tucson, Arizona 85721-0043 USA

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Russell L. Scott

Russell L. Scott

United States Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center, Tucson, Arizona 85719 USA

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Keirith A. Snyder

Keirith A. Snyder

United States Department of Agriculture, Agricultural Research Service, Jornada Experimental Range, Las Cruces, New Mexico 88003-0003 USA

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Eric E. Small

Eric E. Small

Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399 USA

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Kevin Hultine

Kevin Hultine

Department of Biology, University of Utah, Salt Lake City, Utah 84112 USA

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William T. Pockman

William T. Pockman

Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131-1091 USA

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Robert B. Jackson

Robert B. Jackson

Department of Biology and Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina 27708-0340 USA

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First published: 01 February 2005
Citations: 561

Corresponding Editor. C. R. Hupp. For reprints of this Special Feature, see footnote 1, p. 275

Abstract

Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotranspiration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change—such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes.