Pattern and Biological Microsite Effects in Two Shrub Communities, Southern California

Abstract

Strip transects of 100 contiguous, aligned quadrats were sampled during the spring blooming in a creosotebush (Larrea tridentata) semidesert and a chamise (Adenostoma fasciculatum) chaparral, with species covers in quadrats recorded. Techniques of pattern analysis included reciprocal averaging ordination of quadrats, measurements of pattern diversity and pattern periodicity, and species association and contagion. In both communities the first ordination axis expressed a strong pattern of differentiation of the herb flora from shrub centers to openings; second and third axes expressed other responses of herb species to shrub species and shrub cover. The pattern axis represents a principal direction of niche difference to which most species responded and along which several pairs of congeners were separated. Pattern periodicities were 6 m (weakly defined) in the semidesert and 9 m in the chaparral; pattern diversities were 1.4 and 1.8 half—changes. Quadrat species richness was highest in the transitions between shrub clumps and openings in the semidesert, but higher in the openings and lower under the shrubs of the chaparral. Overall alpha diversity resulted from roughly comparable contributions of point or small—quadrat diversity, pattern diversity of more common species, and rare species. The importance of biological modification of microsites for population function and niche relationships is suggested by species responses to the primary pattern axis, the responses of some herb species to particular shrub species, and the indicated allelopathic effects in the chaparral.