Nonlinear biofluvial responses to vegetation change in a semiarid environment

Abstract

The desertification of grassland communities in the Jornada del Muerto Basin, southern New Mexico, USA, has occurred in association with a series of geomorphic responses that have influenced the system of vegetation change. Rainfall simulation experiments indicate that the volume of runoff generated from basin surfaces and its ability to erode are greatly affected by the distribution of vegetation, which ultimately controls processes such as rainsplash erosion, soil infiltrability and crust development. Animal activities also influence rates of sediment movement from unvegetated surfaces by disrupting soil crusts and making loose sediment available for transportation by overland flow. Shrublands in the Jornada Basin have a patchier vegetation cover than grasslands, with vegetated areas (shrubs) being separated by unvegetated (intershrub) zones. The exposed intershrub surfaces are more vulnerable to erosion than the grass and shrub surfaces. Thus, water and sediment yields, calculated using rainfall simulation experiments, were higher for vegetated (shrub and grass) plots than they were for unvegetated (intershrub) plots. The runoff and erosion model, KINEROS2, predicts that at the base of a 100 m slope, shrubland surfaces shed seven times more runoff and 25 times more sediment than grassland surfaces. Evidence to support the prediction of higher rates of erosion in the shrubland can be found in the form of the extensive rill networks that are common in this community. The contraction of grasslands has been associated with elevated rates of erosion that have altered the morphology of the surface, lowering slopes between shrubs, and increasing the amplitude of the microtopography. Overall, the viability of the exposed soils for recolonization by grasses has been reduced, reinforcing the system of shrubland invasion and lending support to the use of state-and-transition models to describe ecologic responses to change within this environment. Combined, these results indicate that biophysical interactions in semiarid environments, such as the Jornada del Muerto Basin, are extremely complex and highlight the need for integrative investigations in these regions.