HISTORICAL DECLINE OF COASTAL SAGE SCRUB IN THE RIVERSIDE-PERRIS PLAIN, CALIFORNIA

Abstract

Californian coastal sage scrub (CSS), which consists of dense stands of soft-leaved drought-deciduous subshrubs 0.5-1.5 m tall, has been extensively cleared for agriculture and urbanization (Westman 1981).The state of California has initiated a regionally focused conservation-planning process for natural communities, including CSS, in southern California (O'Leary et al. 1992).To protect two endangered species of CSS, the Stephens' Kangaroo Rat (Dipodornys $tephen$i) and the California Gnatcatcher (Polioptila californica), Riverside County has developed a habitat-conservation plan for CSS in the Riverside-Perris Plain, an area of rapid present and future urbanization.This has resulted in protection of CSS through purchase of private lands surrounding preexisting public lands, largely through political and economic incentives (Feldman 1995).Few studies have examined the landscape-scale dynamics of surviving CSS, particularly in relation to the invasion of exotic annuals introduced from the Mediterranean basin and Middle East since the late 18th century.While it is widely reported that exotic annuals have displaced indigenous herbaceous ecosystems (McNaughton 1968, Gulmon 1977, Heady 1988, Drake and Mooney 1986, Huenneke et al. 1990, D'Antonio and Vitousek 1992), relationships between the spread of these annuals and the dynamics of shrubland communities are not well understood.O'Leary and Westman (1988) and O'Leary (1990) demonstrated that CSS has been reduced by frequent fire, grazing, and the invasion of exotic annuals, as well as air pollution.These trends have special importance for the endangered California Gnatcatcher, whose habitat requirements include dense stands of CSS with high shrub-species diversity (Atwood 1993).Planning efforts have not taken into account whether CSS is a static system.From 1929 to 1934 the vegetation of California was inventoried by the Vegetation Type Map (VTM) Survey under the California Forest and Range Experiment Station (Minnich et al. 1995).During the survey, 78 plots of CSS were surveyed in the Riverside-Perris Plain.The objective of our study is to replicate the VTM for this area to quantify change of CSS over the past 60 years and to evaluate the role of exotics, specifically, whether the invasion of exotics is dependent on frequent burning or whether exotics promote frequent burning.The answer to this question affects whether management should focus on disturbance or the control of exotics.Addressing the question requires the examination of such processes associated in habitat change, including disturbance, grazing, competitive replacement, and air pollution.