Diversity Relations and Succession in Californian Coastal Sage Scrub

Abstract

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.