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, such as heating of the soil and seed bank, and indirect effects of fire, such as a temporary reduction in competition by shrubs or herbivory by mammals. I conducted an experiment in burned and adjacent unburned chaparral to assess the relative importance of these mechanisms to seedling emergence and mortality of four functional plant groups: shrubs, subshrubs, perennial herbs, and annual herbs. I assessed direct effects of burning by comparing experimental plots in a burned area to plots in adjacent, unburned chaparral that were cleared of shrubs. In the unburned chaparral, competition with shrubs was manipulated by removing aboveground vegetation; herbivory by large mammals was manipulated using fence exclosures. The direct effects of burning enhanced emergence of shrub and subshrub seedlings, but reduced emergence of annual herbs. Shrub removal alone did not affect seedling emergence, but did improve survivorship of annuals. Exclusion of mammalian herbivores improved the survivorship of annual herbs and all seedlings combined. No single factor explained the high abundance of perennial herbs in the burn area; my results suggest that the combined removal of shrubs and herbivores following fire enhanced their growth and establishment. Thus, both the direct and indirect effects of fire contributed to the high postburn recruitment of seedlings in maritime chaparral, but different mechanisms were responsible for the increase in specific plant groups.