RELATIVE IMPORTANCE OF FACTORS CONTRIBUTING TO POSTFIRE SEEDLING ESTABLISHMENT IN MARITIME CHAPARRAL

Several mechanisms have been proposed to explain the postburn flush of seedlings in the chaparral; these include direct effects of fire, suc h as heating, of the soil and seed bank, and indirect effects of fire, such as a temporary reduction in competition by shrubs or herbivory b y mammals. I conducted an experiment in burned and adjacent unburned c haparral to assess the relative importance of these mechanisms to seed ling emergence and mortality of four functional plant groups: shrubs, subshrubs, perennial herbs, and annual herbs. I assessed direct effect s of burning by comparing experimental plots in a burned area to plots in adjacent, unburned chaparral that were cleared of shrubs. In the u nburned chaparral, competition with shrubs was manipulated by removing aboveground vegetation; herbivory by large mammals was manipulated us ing fence exclosures. The direct effects of burning enhanced emergence of shrub and subshrub seedlings, but reduced emergence of annual herb s. Shrub removal alone did not affect seedling emergence, but did impr ove survivorship of annuals. Exclusion of mammalian herbivores improve d the survivorship of annual herbs and all seedlings combined. No sing le factor explained the high abundance of perennial herbs in the burn area; my results suggest that the combined removal of shrubs and herbi vores following fire enhanced their growth and establishment. Thus, bo th the direct and indirect effects of fire contributed to the high pos tburn recruitment of seedlings in maritime chaparral, but different me chanisms were responsible for the increase in specific plant groups.