GRADIENT MODELS, GRADIENT ANALYSIS, AND HIERARCHICAL STRUCTURE IN PLANT-COMMUNITIES

Two general models of plant community structure, the community-unit an d the continuum, have dominated the thinking of American community eco logists. Hypotheses derived from these and other models of plant commu nity structure rarely have been tested, however. Traditionally, analys es of gradient structure have focused primarily on whether or not the boundaries of species response curves are clustered, which does not pr ovide a complete picture of gradient structure. In this study, we stat istically analyzed three characteristics of plant community structure along gradients (1) pattern of boundaries of species distributions, (2 ) pattern of modes of species response curves, and (3) whether or not species distributions exhibit hierarchical structure. In combination, these characteristics yield eight different models of vegetation struc ture along gradients. To determine if vegetation corresponds to any of these models, we sampled species composition using belt transects of contiguous quadrats in a total of 42 wetland sites in Minnesota and th e southern Great Plains, USA. Boundaries of species distributions were clustered in 10 of 42 cases, modes of species response curves were cl ustered in 19 of 42 cases, and species distributions exhibited hierarc hical structure in all 42 cases. Results varied between sites. Overall , four models of community structure were supported. None of the sites sampled supported the models often associated with the continuum or t he community-unit. These results confirmed the need to explore alterna tive models of gradient structure, and suggested that more than one mo del of vegetation structure may be needed to represent community struc ture along gradients.