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The distributions of species and species groups of southern coastal sage scrub in Alta and Baja California are examined in relation to structural and environmental variables. Variables examined include vegetative structure, geographic and topographic position, soil physical characteristics and macronutrient levels, substrate, temperature, precipitation, and factors of disturbance (fire, light grazing, air pollution). Evapotranspirative stress during summer months seems to be a major factor influencing species distributions. Mean temperature of the warmest month appears to be a better predictor of this stress than seasonal or annual precipitation levels. Many of the dominant species also respond sensitively to differences in soil fertility and parent material, with 18 of the 21 dominant species examined showing significant preference for a single substrate among seven classes. Among soil nutrients, levels of organic and nitrate nitrogen appear as particularly useful predictors of species distributions. Of disturbance variables, regional levels of air pollutants are correlated most strongly with floristic differences. Multiple linear regression equations are presented permitting prediction of species' foliar cover levels from levels of environmental variables. Geographic segregation of floristic associations within the sage scrub flora is most marked along a northwest-southeast axis paralleling the Pacific coastline, although southern inland (Riversidian) associations show affinities with the northern coastal flora. Medium-scale patchiness ( = 0. 1 ha) in floristic composition occurs within the sage scrub due to local dominance by one or a small number of shrub species. The possible ecological significance of this is briefly discussed.

The facultatively drought—deciduous shrublands of coastal California and Baja California are lowest in species richness of the four Mediterranean—climate regions where this physiognomic type occurs. Alpha richness in the North American coastal sage scrub varies primarily with the abundance of herbaceous annual species. Herb levels in turn vary with differences in levels of precipitation, favorableness of temperature during the winter and spring growing season, shading by shrubs, soil nitrogen, and air pollution. Levels of herbaceous annuals are highest in the growing season following fire, and show a second pulse of abundance in stands 15—25 yr old. Mature stands of coastal sage scrub are typically low in species equitability, due at least in part to the shade—intolerance of the herbaceous understory and to reduced levels of soil nitrogen. Symbiotic nitrogen—fixing organisms are virtually absent from stands which have not burned in 20 yr or more. The pattern of postfire succession varies markedly with fire intensity; dominant shrubs sprout abundantly from root crowns only following less intense fires. While there are °50 widespread sage scrub species, more than half of the 375 species encountered in the present study of the sage scrub flora are rare in occurrence within the habitat range. In view of the reduction of the area of coastal sage scrub in California to 10—15% of its former extent and the limited extent of preserves, measures to conserve the diversity of the flora are needed.

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