Community Structure and Productivity in Ceanothus Chaparral and Coastal Sage Scrub of Southern California

Adjacent, 22—yr—old stands of evergreen chaparral and drought—deciduous coastal sages scrub at a low—elevation site near the ocean in the Santa Monica Mountains of California were analyzed. The chaparral is a pure stand of Ceanothus megacarpus and has an aboveground live biomass of 6482 g/m². The coastal sage scrub is co—dominated by Salvia leucophylla and Artemisia californica and has an aboveground live biomass of 925 g/m². Attached and standing dead wood comprise 15% and 21% of the total live and dead biomass in the chaparral and coastal sage scrub, respectively, Mean annual aboveground primary production in Ceanothus chaparral is 1056 g°m— ²°yr— ¹, while litterfall is 801 g°m— ²°yr— ¹. In the coastal sage scrub, mean annual primary production is 355 g°m— ²°yr— ¹ and litterfall, 199 g°m— ²°yr— ¹. The drought—deciduous shrubs begin leaf production 1—3 wk after the initial winter rains and continue for 5—7 mo; thereafter, the shrubs lose most of their leaves in the summer. Twig and flower/fruit production continues for several months after leaf growth has ceased. Artemisia and Salvia have no measurable growth for 2 and 4 mo, respectively. In Ceanothus, leaf production begins 3—4 mo after the first rains of winter, and may or may not continue through the summer. Wood production and flowering/fruiting coincide with foliage production. Ceanothus may show measurable aboveground growth for up to 11 mo of the year. Flower production does not occur each year; however, in a flowering year, the production of all reproductive structures is very high (151 g°m— ²°yr— ¹). The length of the measurable aboveground growing season in the two growth forms does not fully account for the great differences in their annual net primary production. The high net primary production in the Ceanothus chaparral is much greater than other evergreen, mediterranean—type ecosystems and most temperate shrub communities. This high productivity is attributed to a tolerance of low water potentials, high photosynthetic rates, and an efficient use of nutrients in C. megacarpus. The high production in Ceanothus chaparral and the potential for reproduction in the coastal sage scrub suggest that the commonly ascribed senscence of mature shrub stands may not occur in these communities.