Role of high fire frequency in destruction of mixed chaparral.

Abstract

Postfire regeneration of Adenostoma and Salvia dominated chaparral was studied in adjacent sites burned by wildfire once, twice, or three times in the past six years, at intervals of 1 , 4, and 6 yr prior to the study. The following combinations of burning were: 1 yr ago (previous burn was >20 yr); 1 and 6 yr ago; 1, 4, and 6 yr ago; 4 and 6 yr ago; and 6 yr ago. Marked changes in community composition occurred as fire frequency increased. On the site that burned only once in the past several decades the dominant shrubs Adenostoma fasciculatum and Salvia mellifera established abundant seedling populations in the first growing season after fire. Seedling establishment the first season after fire was 62% (S. mellifera ) to 83% (A. fasciculatum) less on the site that had also burned 6 yr previously and zero on the site that had burned 1 , 4, and 6 yr ago. The suffrutescent (chamaephyte) Lotus scoparius, likewise showed the lowest seedling establishment on the site burned the most frequently, however, it did best on the site burned 1 and 6 yr ago. Although Adenostoma fasciculatum resprouts after fire, there was some mortality after each fire and thus the number of resprouting shrubs diminished as fire frequency increased. Postfire annual species, in general, were abundant on all sites burned 1 yr ago, regardless of the prior fire frequency. Non-native species such as Brassica nigra, Bromus spp., and Schismus barbatus were poorly represented on the 1 yr old burn that had not been repeatedly burned in recent years. In general, as fire frequency increased, these non-native weeds increased dramatically. It is suggested that high fire frequency has played an important role in the establishment of these weeds and conversion from shrub-dominated to herbaceousdominated ecosystems in the California landscape. California chaparral is a fire-type vegetation that not only regenerates rapidly after wildfires, but many species are dependent upon such disturbance for regeneration (Horton and Kraebel 1955; Hanes 1 97 1 ; Keeley and Keeley 1988). It seems reasonable, however, that different fire regimes may generate different patterns of postfire community composition, due largely to the fact that different regimes favor different plant species. Gill (1973) distinguished three components of fire regime: fire frequency, fire intensity, and fire season. Fire frequency plays a dominant role in determining vegetation structure and vegetation structure largely determines fire intensity. In California chaparral, Zedier et al. (1983) described the impact of two fires only a year apart, on postfire regeneration of the woody vegetation at several mixed chaparral sites. The effect of this extreme event was to nearly eliminate * Reprint requests to second author. Madrono, Vol. 40, No. 3, pp. 141-147, 1993 This content downloaded from 207.46.13.86 on Sat, 15 Oct 2016 04:22:51 UTC All use subject to http://about.jstor.org/terms