Geomorphic gradients of landforms and associated soil catenas in the Central Desert region of Baja California produce complex but repeatable patterns of plant community distribution and a diversity of plant functional groups distributed unequally over the gradient. We measured plant diversity and made three assessments of plant dominance in six plant communities associated with a gradient of geomorphic landforms. Dominance was assessed by plant cover, aboveground biomass, and aboveground water mass held by vegetation. Perennial plant cover was highest on northeast-facing rocky pediment slopes (54.6%) and lower bajada slopes (45.7%), and lowest on creosote bush flats (19.2%). The highest aboveground biomass of any community (25.07 t ha−1) was present in wash woodlands heavily dominated by Prosopis glandulosa. This was more than ten times the biomass present in the creosote bush flats. The highest aboveground water mass contained in vegetation was present on the northeast- and southwest-facing rocky pediment slopes which each held more than 30.0 t H2O ha−1. This water mass was heavily contained in just two species of arborescent stem succulents, Pachycormus discolor and Fouquieria columnaris, neither of which utilizes crassulacean acid metabolism (CAM). High amounts of standing water mass on upper and lower bajada slopes were held by the arborescent cactus Pachycereus pringlei. The changing pattern of relative dominance of plant functional groups across this gradient of desert landforms reflects the morphological and ecophysiological traits associated with each group, and indicates strong assembly rules of community composition.
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Vol. 61 • No. 1