Growth dynamics of two chaparral shrub species with time after fire

The effect of stand age on growth dynamics of two chaparral shrubs, Adenostoma fasciculatum and Ceanothus greggii, was studied in southern California. It was asked whether shoots of young shrubs grow faster than those of old ones, and if on young shrubs more shoots develop per square meter of canopy projection area than on old ones. These measurements were made in ten chaparral stands representing six different ages ranging from 6 to 87 years. Measurements taken biweekly throughout the 1984 growing season indicated no decline in growth rates or biomass of new shoots with increasing stand age. Variability in measurements was as high among even-aged stands as among stands of differing ages. Measured values were very low compared to earlier studies, probably because of very low rainfall in 1983-1984. It has often been assumed that chaparral shrubs become senescent in the long absence of fire (Hanes 1971; Parsons 1976; Rundel and Parsons 1979, 1980). Recent reviews, however, have concluded that there is no clear evidence in favor or against a decline (sensu Nooden 1988) at the shrub level in chaparral (Zedier and Zammit 1989; Hilbert and Larigauderie 1990). Two studies using different approaches have focussed on the possibility of a decline with time after fire. A demographic study of even-aged stands of Ceanothus megacarpus by Montygierd-Loyba and Keeley (1987) suggested that shrub mortality between 8 and 55 years after fire is related more to intraspecific competition than to shrub decline. Mortality appeared to be more frequent in smaller plants. A study of Adenostoma fasciculatum by Rundel and Parsons (1979) showed that in stands older than 60 years, shrub size and leaf area index were reduced compared to shrubs found in younger stands, suggesting a decline of this shrub with age. The goal of our study was to assess the effect of stand age on growth dynamics of two chaparral shrubs, Adenostoma fasciculatum, a resprouter, and Ceanothus greggii , an obligate seeder. A common assumption in California chaparral is that C. greggii is shorter-lived than A. fasciculatum and dies within 30 to 40 years after fire (Keeley 1975). There is, however, only limited support for this assumption. Zammit and Zedier (pers. comm.) found that the number of C. greggii shrubs per ha declined with increasing stand age, but Keeley Madrono, Vol. 37, No. 4, pp. 225-236, 1990 This content downloaded from 157.55.39.154 on Thu, 08 Dec 2016 05:12:59 UTC All use subject to http://about.jstor.org/terms 226 MADRONO [Vol. 37 Table 1 . Location, Years Since the Last Fire, Elevation, Precipitation (from October 1983 то April 1984), and Soil Types for the 10 California Chaparral Stands. All stands were on north-facing slopes. Stand age Precipi(in years Elevation tation since fire) Location (m) (mm) Soil 6 Warner Springs 1091 113 Sheephead 13a Warner Springs 1164 113 Sheephead 13b Warner Springs 1285 113 Sheephead 13c Warner Springs 1285 113 Sheephead 32 Warner Springs 1150 113 Sheephead 56a Warner Springs 1188 113 Sheephead 56b Sky Oaks 1408 292 Tollhouse 56c Sky Oaks 1552 292 Sheephead 82 Warner Springs 1006 113 Sheephead 87 Sky Oaks 1415 292 Sheephead (1975) documented that С. greggii can be an important part of chaparral even 90 years after fire. We expect shoot growth rate and shoot biomass production per m2 projected canopy area to be a measure of general shrub vigor, and hypothesize that if decline is taking place with age, these variables will be lower in old than in young shrubs. Such a decline should occur especially in C. greggii if it is shorter-lived than A. fasciculatum.