VIEWPOINT - SUSTAINABILITY OF PINYON-JUNIPER ECOSYSTEMS - A UNIFYING PERSPECTIVE OF SOIL-EROSION THRESHOLDS

Many pinon-juniper ecosystems in the western U.S. are subject to accel erated erosion while others are undergoing little or no erosion. Contr oversy has developed over whether invading or encroaching pinon and ju niper species are inherently harmful to rangeland ecosystems. We devel oped a conceptual model of soil erosion in pinon-juniper ecosystems th at is consistent with both sides of the controversy and suggests that the diverse perspectives on this issue arise from threshold effects op erating under very different site conditions. Soil erosion rate can be viewed as a function of (1) site erosion potential (SEP), determined by climate, geomorphology and soil erodibility; and (2) ground cover. Site erosion potential and cover act synergistically to determine soil erosion rates, as evident even from simple USLE predictions of erosio n. In pinion-juniper ecosystems with high SEP, the erosion rate is hig hly sensitive to ground cover and can cross a threshold so that erosio n increases dramatically in response to a small decrease in cover. The sensitivity of erosion rate to SEP and cover can be visualized as a c usp catastrophe surface on which changes may occur rapidly and irrever sibly. The mechanisms associated with a rapid shift from low to high e rosion rate can be illustrated using percolation theory to incorporate spatial, temporal, and scale-dependent patterns of water storage capa city on a hillslope. Percolation theory demonstrates how hillslope run off can undergo a threshold response to a minor change in storage capa city. Our conceptual model suggests that pinon and juniper contribute to accelerated erosion only under a limited range of site conditions w hich, however, may exist over large areas.