Gradient Models, Gradient Analysis, and Hierarchical Structure in Plant Communities

Oikos 78: 23-30. Two general models of plant community structure, the community-unit and the continuum, have dominated the thinking of American community ecologists. Hy- potheses derived from these and other models of plant community structure rarely have been tested, however. Traditionally, analyses of gradient structure have focused primarily on whether or not the boundaries of species response curves are clustered, which does not provide a complete picture of gradient structure. In this study, we statistically analyzed three characteristics of plant community structure along gradi- ents (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 vegeta- tion structure 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 the southern Great Plains, USA. Boundaries of species distributions were clustered in 10 of 42 cases, modes of species response curves were clustered in 19 of 42 cases, and species distributions exhibited hierarchical 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 the community-unit. These results confirmed the need to explore alternative models of gradient structure, and suggested that more than one model of vegetation structure may be needed to represent community structure along gradients. B. W. Hoagland and S. L. Collins, Dept of' Botany and Microbiology, Unic. of Oklahoma, Norman, OK 73019, USA (uresent address of B WH: Oklahoma ,Vatural Heritage Inventory, Unit.. of' Oklahoma, 111