Recruitment of seedlings and vegetative sprouts in unburned chaparral
Age structure of 12 stands of chaparral, unburned for 56—120 yr, was investigated. All shrubs produced discernible growth rings, and ring counts on stems from stands of known age, plus synchrony in annual growth ring width among species in the same stand, were taken as evidence that growth rings represented annual rings. Species that survive fire by vegetative regeneration from the root crown were capable, in the absence of fire, of continuously regenerating their canopy with basal sprouts. These multistemmed resprouting species had an uneven age structure, indicating continuous recruitment of new stems into the population. The predicted age distribution for resprouting shrub stem populations illustrated very different patterns for species within the same stand and for the same species in different stands. In most resprouting shrubs, <15% of the stems dated back to the first decade after fire, and in many species the majority of stems were <10 yr of age. This contrasts with obligate seeding shrubs that do not initiate new stems from the root crown, and for which, in undisturbed chaparral, 100% of the stems date back to the years immediately following the last fire. Seedling recruitment was abundant in seven of these old chaparral communities. Species with abundant seedling populations, ranging from 1000 to 37 500 individuals/ha, were Quercus dumosa, Q. wislizenii, Rhammus crocea, Prunus ilicifolia, Heteromeles arbutifolia and Cercocarpus betuloides. These species share the characteristic of regenerating after fire solely by resprouting, and seldom recruit seedlings during the first few decades after fire. Thus, it is concluded that these postfire obligate resprouters require long fire—free periods for successful reproduction and potential population expansion. For all but the last species, seedlings were most abundant in stands with a closed canopy and deep litter layer. Safe sites for establishment were not in gaps, although gaps may be important for successful recruitment of saplings into the adult population. For all sprouting species the actual age distribution of seedlings and saplings indicated that recruitment did not occur every year; mortality was high but most populations had some saplings that survived beyond the first decade. For all postfire seeding taxa, viz., Adenostoma, Arctostaphylos, and Ceanothus, no significant seeding recruitment was observed in these ancient communities. Seedlings of woodland tree species had colonized several of these old chaparral communities; however, even in stands a century old, successional replacement of chaparral was not imminent.