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Title: Invasion of shrublands by exotic grasses: Ecohydrological consequences in cold vs. warm deserts

Author
item WILCOX, B.P. - Texas A&M University
item TURNBULL, L. - Arizona State University
item YOUNG, M.H. - University Of Texas
item Williams, Christopher - Jason
item RAVI, S. - University Of Arizona
item Seyfried, Mark
item BOWLING, D.R. - University Of Utah
item Scott, Russell - Russ
item GERMINO, M.J. - Idaho State University
item CALDWELL, T. - Desert Research Institute
item WAINWRIGHT, J. - University Of Sheffield

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2011
Publication Date: 3/1/2012
Citation: Wilcox, B., Turnbull, L., Young, M., Williams, C.J., Ravi, S., Seyfried, M.S., Bowling, D., Scott, R.L., Germino, M., Caldwell, T., Wainwright, J. 2012. Invasion of shrublands by exotic grasses: Ecohydrological consequences in cold vs. warm deserts. Ecohydrology. 5: 160-173.

Interpretive Summary: Across the globe, many savannas and woodlands are undergoing conversion to grasslands due the invasion of foreign grass species. Here we summarize the current state of knowledge concerning the hydrological and ecological consequences of this conversion for the desert regions of North America. We surveyed and summarized the peer-reviewed scientific literature and used mathematical models to arrive at the following conclusions. When shrublands are invaded by grasses, many changes take place: rooting depths, canopy cover, species heterogeneity, water use and fire regimes are radically altered. Obviously, then, grass invasion has the potential to alter key ecohydrological processes. With respect to the processes of runoff and erosion, we find that grass invasion influences cold deserts and warm deserts in different ways. In cold deserts, runoff and erosion will increase following invasion; in particular, erosion on steep slopes will be greatly accelerated following burning. In addition, evaporation will be lower and infiltration will be higher which after several decades could affect groundwater levels. For warm deserts grass invasion may actually reduce runoff and erosion (except for periods immediately following fire), and is likely to have little effect on either evaporation or soil water. Significant gaps in our knowledge do remain, primarily because there have been no comprehensive studies measuring all components of the water and energy budgets at multiple scales. How these changes may affect regional changes like weather patterns are still uncertain.

Technical Abstract: Across the globe, native savannas and woodlands are undergoing conversion to exotic grasslands. Here we summarize the current state of knowledge concerning the ecohydrological consequences of this conversion for the cold deserts (Great Basin, Colorado Plateau) and the warm deserts (Mojave, Sonoran, Chihuahuan) of North America. Our analysis is based on a synthesis of relevant literature, complemented by simulation modeling with a 1-D, soil-water-redistribution model (HYDRUS-1D) and a hillslope runoff-and-erosion model (MAHLERAN). When shrublands are invaded by grasses, many changes take place: rooting depths, canopy cover, species heterogeneity, water use and fire regimes are radically altered. Obviously, then, grass invasion has the potential to alter key ecohydrological processes. With respect to the processes of runoff and erosion, we find that grass invasion influences cold deserts and warm deserts in different ways. In cold deserts, runoff and erosion will increase following invasion; in particular, erosion on steep slopes (>15%) will be greatly accelerated following burning. In addition, evapotranspiration will be lower and soil water recharge will be higher—which after several decades could affect groundwater levels. For warm deserts grass invasion may actually reduce runoff and erosion (except for periods immediately following fire), and is likely to have little effect on either evapotranspiration fluxes or soil water. Significant gaps in our knowledge do remain, primarily because there have been no comprehensive studies measuring all components of the water and energy budgets at multiple scales. How these changes may affect regional energy budgets, and thus weather patterns, is not yet well understood.