Succession after Fire in the Chaparral of Southern California

Abstract

Extensive sampling of chaparral with 10—m line intercepts in the San Gabriel and San Bernardino Mountains of southern California revealed site—oriented vegetative characteristics and successional patterns. Of the 78 species encountered, few were widespread and abundant; most were local and infrequent. The most widespread and abundant species were long—lived rootcrown sprouters. Adenostoma fasciculatum (chamise), the most frequently encountered shrub, occurred on 71.4% of the sites and composed one—fifth of all shrubs in the study. The second most common species, Quercus dumosa (scrub oak), appeared on 32% of the sites sampled. Many non—sprouting subshrubs and woody species were restricted to elevations below 3,000 ft (ca. 900 m), sunny exposures, and young stands. Chaparral succession, both in composition and rate of change, is influenced most by aspect, particularly north— and south—facing slopes. Next in importance is the influence of coastal and desert exposure. Elevation is a factor that may compensate for coastal—desert exposure or aspect. Percentage of slope is least important. The rate of succession after fire in coastal chaparral is slowest on south—facing slopes below 3,000 ft. Early stages of shrub succession are characterized by a mixture of chaparral seedlings, resprouts, and seedlings of coastal sage subshrubs. Most of the present—day chaparral on south—facing slopes is a coastal sage—chaparral subclimax due to frequent fire. On fire—free sites a chamise—chaparral climax community develops within 30 years after fire. The fastest succession rate is in coastal chaparral on north—facing slopes above 3,000 ft. The profusion of coastal sage subshrubs is missing, and vigorous, tall—growing sprouting species are abundant. The death of large, short—lived species in stands older than 25 years allows pockets of seral species to develop in the climax scrub oak—chaparral. Chaparral succession is not a series of vegetational replacements, but a gradual ascendance of long—lived species present in the pre—fire stand. The pattern of chaparral succession on desert exposures differs from that on coastal exposures. Slope aspect is less important, but proximity to the Pacific Ocean is more important than on coastal exposures. Fire favors the sprouting species of chaparral over woodland and forest communities bordering the chaparral communities. Fewer chaparral species occur on desert exposures than on coastal ones. Seedling and mature shrub mortality rates are lower in desert than in coastal chaparral. Succession after fire in desert chaparral is slow, and the climax community is composed of large shrub specimens with subshrubs clustered around their skirts and a canopy broken by intershrub spaces. Chaparral stands older than 60 years often are decadent, especially chamise—chaparral. Old stands are characterized by a high proportion of dead wood, little annual growth, and no new seedling development. Various phytotoxic substances may account for the loss of vitality and lack of regeneration. Maintenance of vigorous chamise—chaparral is shown to be dependent on fire. A re—evaluation of current fire—exclusion and suppression practices is needed. The present fire—exclusion policy is probably the least desirable one to insure the perpetuation of chamise—chaparral.