Abstract
The density, biomass, and pattern of shrubs on a desert piedmont in southern California were measured in a series of plots from which sheet- flow and stream-channel runoff from adjacent mountains have been excluded for 45 yr. The plots were compared with undisturbed adjacent areas in which soil moisture is derived from both runoff and incident precipitation. Lorrea tridentata and Ambrosia dumosa showed significantly lower density in areas of drainage diversion. There were no differences in mean shrub biomass for A. dumosa, but in areas of drainage diversion, mortality of L. tridentata was concentrated among larger individuals. The remaining population of smaller shrubs showed a more aggregated pattern than the population in undisturbed sites. Biomass per hectare was lower for both species in areas of drainage diversion. Desert shrub communities depend on soil moisture recharge during periods of overland runoff. Sources of runoff are important to consider in studies of competition and pattern of desert shrubs.
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