Interaction of Fire and Community Development in Chaparral of Southern California

Abstract

Fire is an ecosystem property rather than an exogenous force in southern California chaparral, and it interacts with processes of drought—mediated canopy development, production, and mortality to affect stability of community composition. Where species that must reproduce from seed, such as Ceanothus crassifolius or Ceanothus oliganthus, are predominant, composition can be altered by a single fire with little or no recruitment after initial postfire establishment. Water balance apparently regulates subsequent leaf area development; after 15—22 yr of postfire growth, foliage biomass in monospecific C. crassifolius stands in this study had approached a maximum that was unrelated to incident solar radiation and insensitive to initial population density over a 10—fold range. Thus, establishment success, above that required for canopy closure, should have little effect on the foliage biomass that sustains combustion. After canopy closure, total biomass accumulated at an accelerating rate through at least two decades with aboveground net primary production as great as 12—13 Mg · ha—1 · yr—1. C. crassifolius mortality was substantially less than predicted from growth rates and the —3/2 power model of Yoda et al. (1963), and there was no approach to a common asymptotic density by stands of disparate initial density. With low deadwood biomass and absence of ground fuels, c. crassifolius cannot sustain burning in the absence of wind, steep slopes, or exceptionally low live—fuel moisture. Increased Ceanothus abundance in multispecies communities with Adenostoma fasciculatum or Salvia mellifera alters biomass structure and could modify subsequent fire effects even if foliage area fully redevelops in concert with site water balance. Rare, low—intensity fires can devastate Ceanothus chaparral, that reproduces only from seed. Salvia mellifera and Eriogonum fasciculatum can occupy resulting openings in the canopy, and their abundant deadwood and compact biomass can readily spread low—intensity fires, thereby perpetuating the degraded community. Productive stands within a chaparral association are probably subject to especially severe fires that limit nutrient accumulation and may also limit subsequent productivity. Copious nitrogen volatilization during burning is promoted by high nitrogen concentrations in foliage and fine woody biomass of Ceanothus and heavy leaf litter of Quercus dumosa and C. crassifolius. The communities most prone to severe fires also accumulate and cycle nitrogen and phosphorus rapidly.