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Gap Analysis of the Actual Vegetation of California: The Southwestern Region

Frank W. Davis, Peter A. Stine, David M. Stoms, Mark I. Borchert, and Allan D. Hollander

Gap Analysis is a method of conservation risk assessment that evaluates the protection status of plant communities, animal species and vertebrate species richness by overlay of biological distribution data on a map of existing biological reserves. The National Biological Survey has undertaken a national Gap Analysis that is being conducted by individual states but that will eventually produce regional and national assessments. Given California's size and complexity, we are conducting separate Gap Analyses for each of the state's 10 ecological regions, as delineated in The Jepson Manual. Here we summarize our findings on the distribution of plant communities and dominant plant species in the Southwestern Region of California, exclusive of the Channel Islands. We tabulate and discuss regional distribution patterns, management status and patterns of land ownership for 76 dominant woody species and 62 natural communities. Nineteen of 62 mapped communities appear to be at risk, as determined by their poor representation in existing reserves, parks or wilderness areas. Communities restricted largely to the lower elevations, such as non-native grasslands and coastal sage scrub types, are clearly at considerable risk. A majority of the lands at these elevations have already been converted to agricultural or urban uses and most of the remaining lands are threatened with future urbanization. Areas that appear to be of highest priority for conservation action based on agreement between our analysis and a recent assessment by The Nature Conservancy include the Santa Margarita River, San Mateo Creek, Miramar Mesa, Santa Clara floodplain near Fillmore, Sespe and Piru Canyons, and Tejon Pass.

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