Species boundary clustering in zoned vegetation was examined along six wetland–upland coenoclines from four prairie marshes in eastern South Dakota that differed in size, permanence, water quality (salinity), topography, and exposure to waves. Vegetation structure and composition were measured from small, contiguous quadrats running through concentric vegetation zones. Rates of vegetation change along coenoclines were estimated using direct gradient analysis, detrended correspondence analysis, and a statistical technique that detects clustering of species' borders. All wetlands exhibited visually distinct zones, but quantitative analysis revealed that vegetation fomed a virtual continuum along the gradient in small wetlands, while near the shorelines of large wetlands both vegetation composition and structure were sharply discontinuous between marsh and meadow zones. This lower border of the meadow zone apparently was sharpened by exposure to waves and ice scour. The upper meadow zone border with prairie vegetation was less distinct but coincided with a sharp change in topography and in subsoil texture. Interspecific competition appeared less important than physical environmental factors in affecting zonation discreteness. The meadow–prairie zone contact approximates the upper limit of influence from wetland processes and is proposed as the wetland–upland border.
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