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Viewpoint: Sustainability of Piñon-Juniper Ecosystems: A Unifying Perspective of Soil Erosion Thresholds

David W. Davenport, David D. Breshears, Bradford P. Wilcox and Craig D. Allen
Journal of Range Management
Vol. 51, No. 2 (Mar., 1998), pp. 231-240
DOI: 10.2307/4003212
Stable URL: http://www.jstor.org/stable/4003212
Page Count: 10
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Viewpoint: Sustainability of Piñon-Juniper Ecosystems: A Unifying Perspective of Soil Erosion Thresholds
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Abstract

Many piñon-juniper ecosystems in the western U.S. are subject to accelerated erosion while others are undergoing little or no erosion. Controversy has developed over whether invading or encroaching piñon and juniper species are inherently harmful to rangeland ecosystems. We developed a conceptual model of soil erosion in piñon-juniper ecosystems that is consistent with both sides of the controversy and suggests that the diverse perspectives on this issue arise from threshold effects operating 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 erosion. In piñon-juniper ecosystems with high SEP, the erosion rate is highly sensitive to ground cover and can cross a threshold so that erosion increases dramatically in response to a small decrease in cover. The sensitivity of erosion rate to SEP and cover can be visualized as a cusp catastrophe surface on which changes may occur rapidly and irreversibly. The mechanisms associated with a rapid shift from low to high erosion rate can be illustrated using percolation theory to incorporate spatial, temporal, and scale-dependent patterns of water storage capacity on a hillslope. Percolation theory demonstrates how hillslope runoff can undergo a threshold response to a minor change in storage capacity. Our conceptual model suggests that piñon and juniper contribute to accelerated erosion only under a limited range of site conditions which, however, may exist over large areas. /// Algunas ecosistemas de piñon-juniper tiene erosion que es acelerada, mientras en otras hay poca erosion. Por eso, hay controversia sobre el effecto de la invasion de estos arboles a la sustentabilidad de los suelos. Aqui, presentamos un modelo conceptual que es consistente con los dos lados de la controversia. Las perspectivas diferentes surgen por causa de diferencisas importantes en la potencia de erosion en los suelos. Se puede ver erosion de suelo como una funcion de (1) potencia de erosion (PE), la cual es afectada por caracteristicas de los suelos, el clima, y la geomorfologia; y (2) la cobertura vegetal. Modelaciones del USLE demuestran las interacciones de esas dos cosas. Por ejemplo, cuando el PE es alto, la erosion de suelo tiene mucha sensibilidad a los cambios de la cobertura vegetal. En esa caso, la proyeccion de erosion cambia mucho con muy pocos cambios del cobertura vegetal. Teorias de catastrofe y percolacion son muy util para apoyar nuestro modelo conceptual. Segun nuesto modelo, piñon y juniper causa una acceleracion de la erosion, solamente bajo unas condiciones muy limitadas, sin embargo esas condiciones puede exister sobre areas grandes.

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