Factors Influencing the Distribution of Species of Californian Coastal Sage Scrub

Abstract

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.