Demographic Structure of Ceanothus Megacarpus Chaparral in the Long Absence of Fire

Abstract

Wildfires have had a major influence on the struc? tural and functional adaptations that have evolved in species of mediterranean-type ecosystems. In Califor? nia chaparral, for example, shrubs regenerate after fires from basal sprouts or seed germination cued to the first postfire season. Certain species of Ceanothus and Arctostaphylos produce abundant seed throughout their lifespan, and the seeds lie dormant in the soil for many years until germination is stimulated by fire. These species do not resprout after fire, and thus are often referred to as obligate seeding shrubs. In the absence of fire, these species neither rejuvenate their canopy from basal sprouts as sprouting shrubs do, nor do they recruit new seedlings. The demographic patterns for one such species, Ce? anothus megacarpus Nutt. ssp. megacarpus, have been documented in studies by Schlesinger and Gill (1978, 1980). This shrub is distributed on the coastal front of the Coast Ranges from Santa Barbara County to San Diego County, often forming nearly pure, even-aged stands dating back to a previous fire. As with other obligate-seeding species, seedling populations are high in the first postfire year, but nil in later years. Mortality in the 1st yr is typically quite high. The causes of mor? tality probably include drought, predation, and com? petition from the dense herbaceous vegetation. After the 1st yr, mortality is relatively low during the first decade. However, as the shrub canopy closes in, Schle? singer and Gill (1978) found another sharp increase in mortality, and they attributed this stand-thinning to increased competition for water (Schlesinger and Gill 1980). After stand-thinning in the second decade, mortality in Ceanothus megacarpus was shown to level offonce again. Schlesinger and Gill's studies, however, did not extend much beyond the second decade. It is not known what happens after this time. Anecdotal observations in the literature suggest that after ?30-40 yr another sharp increase in mortality is to be expected due to stand senescence.