FACTORS CONTRIBUTING TO POSTFIRE SEEDLING ESTABLISHMENT IN CHAPARRAL - DIRECT AND INDIRECT EFFECTS OF FIRE

1 In the chaparral significant seedling establishment occurs only foll owing fire. Mechanisms proposed to explain this abundant recruitment a fter fire include: direct heating of the soil and seed bank, and a tem porary reduction in competition or herbivory. 2 I tested hypotheses re garding the relative importance of these mechanisms to different plant functional groups using field experiments conducted in burned and nea rby unburned chaparral. Competition was manipulated by removing shrubs from mature chaparral, and mammalian herbivory manipulated using smal l cage exclosures. 3 Burning and reduction in herbivory were primarily responsible for the postburn 'flush' of seedlings, although plant fun ctional groups differed in their responses to fire. 4 Shrub seedling d ensity was enhanced only by herbivore exclusion. The abundance of subs hrubs and annual herbs was positively affected only by burning. Perenn ial herbs increased both with burning and herbivore exclusion. 5 The e ffects of variation in fire intensity or soil heating were also invest igated, by comparing unmanipulated plots in the burn to areas in the b urn which had been cleared of shrubs prior to the fire. 6 Areas in the burn with reduced soil heating had higher overall seedling densities, cover and biomass; perennial herbs were the only functional group una ffected by variation in fire intensity. 7 Comparison of the results of this study to those from similar experiments in maritime chaparral su ggest that inconsistencies between the responses of chaparral communit ies to fire may be due primarily to differences in species composition .