Demographic Studies of the Chaparral Shrub, Ceanothus Megacarpus, in the Santa Ynez Mountains, California

Survivorship patterns and the development of plant community structure were studied in 11 stands of chaparral dominated by Ceanothus megacarpus in the Santa Ynez Mountains, Santa Barbara County, California. Mature plants of C.megacarpus are not fire—resistant and do not resprout following fires; buried, dormant seeds germinate in response to fires, often forming dense, nearly pure, even—aged stands. Self—thinning through intraspecific competition begins about 5 years after fire, presumably when the shrub canopies begin to overlap. Thinning proceeds during the next 10 to 15 years depending on the density of shrubs. Study sites of varying density and age show that the size—frequency distributions of the living shrubs change through the thinning process as a result of mortality which is concentrated among the smaller individuals. Thinning often reduces stand densities by 50% or more; the mortality is greater in stands of higher initial density. Actively thinning stands show a slope of —1.23 in a double—logarithmic plot of stand density versus mean weight per plant, determined by dimension analysis. During thinning, the horizontal pattern of shrubs becomes less clumped. Eventually self—thinning ceases at a point when the stand has reached a high, relatively constant level of biomass. The stand is then dominated by relatively few, large individuals and net primary production is balanced by the death of older plant parts.