Fire severity and plant age in postfire resprouting of woody plants in sage scrub and chaparral

Postfire resprouting by woody plants confers a marked advantage in rate of recovery over species that regenerate entirely from seed. However, the predictability of this advantage varies markedly between species, with some showing nearly 100% rootcrown survival and others often much lower. This study examined patterns of fire-caused mortality and tested the relative importance of fire severity and plant age between various shrubs and subshrubs characteristic of chaparral and sage scrub associations. Resprouting success varied from about 10% for Eriogonum fasciculatum to a high of 98% for Quercus berberidifolia. For most of the shrub species, skeletons of resprouting plants were significantly taller than those of dead individuals of the same species, indicating less biomass consumption, and thus lower severity fires were associated with higher resprouting success. This pattern was less strongly the case with sage scrub species. Shrubs and subshrubs, however, differed in the effect of aboveground plant age (as estimated by basal diameter) on resprouting success. For most chaparral shrubs, age was not related to resprouting success, whereas in four out of five subshrub species, including Artemisia californica, Eriogonum fasciculatum, Salvia leucophylla, and S. mellifera, the youngest plants exhibited the highest resprouting success. I hypothesize that the reason for this inverse relationship between age and resprouting in certain sage scrub species is that, as these subshrubs age, there is a tendency to loose the resprouting ability and the mechanism is quite possibly because adventitious buds responsible for sprouting become buried by woody tissues. Patterns of character evolution in these sage scrub lineages support the hypothesis that the herbaceous perennial mode of seasonal aestivation evolved early and lignification is a more recently derived trait and this may have negative effects on resprouting as the plant ages.