Organization of an Assemblage of Early Successional Species on a Soil Moisture Gradient

Abstract

To determine to roles of prior evolutionary divergence and competitive displacement of species in successional community organization, and aspects of the colonizing strategy, 6 co—occuring annuals were grown together and alone on a soil moisture gradient. The species had similar board vegetative and reproductive responses to the gradient. Interspecific competition reduced the niche breadth and similarities of the species. Competitive and gradient stresses were generally absorbed through plasticity, although Polygonum pensylvanicum and Chenopodium album showed some mortality. Response centers of Polygonum, Ambrosia artemisiifolia, and Chenopodium, competitively subordinate species, shifted due to interspecific competition. Amaranthus retroflexus and Setaria faberii, the leading competitors, already slightly offset from each other when grown alone, did not shift response centers. Competition delayed the start of flowering in Setaria, Chenopodium, Polygonum and Ambrosia. Competitive success did not correlate with response breadth or overlap. Polygonum and Abutilon behaved differently in different competitive environments, indicating the existence of competitive refugia. Refugia may be shifted, reduced, or maintained depending on the competitors present. Coexistence may depend on: (1) differences in response to the moisture gradient and other niche dimensions previously investigated, and (2) on heterogeneous and unpredictable distributions of neighbors. Thus, competitive refugia will be unthreatened at some times and places.