PARENT SESSION
Symposium #16: Thresholds and non-linear responses in ecosystems: Understanding, sustaining, and restoring complex rangelands.
Sponsored by ESA Rangeland Ecology Section
Organized by: D.D. Breshears and J. Herrick.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Lecture Hall
Runoff and erosion thresholds: Implications for rangeland degradation and restoration.
BRESHEARS, DAVID1, LUDWIG, JOHN2, MARTENS, SCOTT3, BEESON, PETER1, WILCOX, BRADFORD4, ALLEN, CRAIG5, 1 2 3 4 5
ABSTRACT- In arid and semiarid ecosystems, ecological and hydrological processes are tightly coupled in many ways. These couplings can produce threshold-type responses that have important implications for rangeland degradation and restoration. Here we highlight how runoff and erosion processes are interrelated with vegetation pattern. Research in a variety of arid and semiarid ecosystems highlights three important general trends. (1) There are functional units at the scale of vegetation patch types, often related to canopy patches of woody plants, bare patches within intercanopy areas, and herbaceous patches within intercanopy areas. These functional units differ with respect to generation and storage potential for runoff and erosion. (2) The heterogeneity and connectivity of these patch types, in conjunction with their relative proportions and spacing determines larger-scale behavior related to hillslope-scale runoff. Under some conditions, small changes in the proportions of these patch types can produce large, threshold type responses at the hillslope scale. (3) The redistribution of water and nutrients in these systems plays a critical role in determining vegetation pattern, which in turn modifies runoff and erosion. Concentration of water and nutrients at patch scales can increase vegetation, whereas redistribution of water and nutrients over larger scales can result in losses that decrease vegetation. Patch-scale relationships for runoff and erosion can trigger threshold responses that are critical for understanding and managing rangeland degradation and restoration.
KEY WORDS: hydrology, threshold, rangelands, runoff and erosion