|Wilcox, Bradford -|
|Turnbull, Laura -|
|Young, Michael -|
|Ravi, Sujith -|
|Bowling, David -|
|Germino, Matthew -|
|Caldwell, Todd -|
|Wainwright, John -|
Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2011
Publication Date: April 2, 2012
Citation: Wilcox, B.P., Turnbull, L., Young, M.H., Williams, C.J., Ravi, S., Seyfried, M.S., Bowling, D.R., Scott, R.L., Germino, M.J., Caldwell, T.G., and Wainwright, J. 2012. Invasion of shrublands by exotic grasses: Ecohydrological consequences in cold versus warm deserts. Ecohydrology. 5:160-173. Interpretive Summary: Exotic grasses are altering native savannas and woodlands across the globe. Our synthesis of the literature indicates knowledge is scant regarding how such plant community transitions affect hillslope- to watershed-scale hydrologic processes such as overland flow, erosion, and groundwater recharge. Hydrologic models were used to evaluate the ecohydrological consequences of native-shrubland-to-grassland conversions occurring on cold and hot deserts in the western United States (US). Hydrologic responses to these large-scale community transitions differ for cold and hot deserts. Simulations indicate runoff and erosion are likely to increase for cold deserts, particularly for steeply sloping sites with an alien grass fire cycle. The opposite is likely following grassland invasions in hot deserts of the US. Grass invasion will likely lead to lower evapotranspiration and higher soil water recharge in cold deserts, which, over several decades, may influence groundwater levels. In warm deserts grass invasion likely has little affect on evapotranspiration fluxes or soil water. The results of this study highlight important research gaps concerning the potential ecohydrological implications of grass invasions in shrublands and provide simulations of likely ecohydrological responses to grass invasions in cold and hot deserts of the western United States.
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.