POSTFIRE DIVERSITY PATTERNS IN 2 SUBASSOCIATIONS OF CALIFORNIAN COASTAL SAGE SCRUB

Abstract. Coastal sage scrub exhibits floristically dissimilar subassociations on opposing aspects in the Santa Monica Mountains of southern California. Southward‐facing (Venturan I) and northward‐facing (Ventaran II) subassociations were sampled at two scales (1 m2 and 625 m2) each spring for five years following a fire. Mature (10‐ and 28 yr‐old) sites of these subassociation types were also sampled in the study area. The subshrub and herbaceous components that were most prevalent on northerly slopes following the first post‐burn year contributed the most cover for all ages examined. Aspect‐related differences in species richness were most apparent at the smaller sampling scale where they were equal the first post‐burn year and varied inversely thereafter. At this scale, richness values were two times higher on older north‐facing sites, largely due to the herbaceous component. South‐facing sites displayed progressive decreases in equitability through time. This pattern reflects gradual reduction in overall species richness combined with progressive site domination by fewer species, as manifested by increasing dominance concentration values. By contrast equitability on north‐facing sites fluctuated little through time, though dominance concentration tended to decrease. Higher richness, equitability, and overall cover with time after burning on north‐facing sites is associated with the relatively mesic habitats upon which this subassociation develops. Post‐burn patterns of species richness and stratal differentiation on south‐facing slopes are consistent with those reported for other mediterranean‐type shrublands, i.e. peak richness was attained the first two years following a fire with subsequent decrease to a constant level. 1990 IAVS ‐ the International Association of Vegetation Science