Although considerable attention has been focused on competition among wetland plants, surprisingly few studies have considered the role of facilitation in structuring freshwater wetland plant assemblages. Positive interaction among plants has been investigated thoroughly in saline marshes, which display a definite, predictable pattern of species zonation, with facilitation playing an important role during colonization of bare or disturbed patches of marsh. The few studies that have investigated facilitation in freshwater marshes have implicated the processes of sediment oxygenation and sediment stabilization. Whereas both processes are known to be influenced by many freshwater hydrophyte species, they have not been considered widely as mechanisms of interspecific facilitation. In the tussock-forming rush Juncus effusus L., late-Spring collapse of culms away from the tussock center results in a morphology that increases light availability atop tussocks and appears to establish a unique spatio-temporal component to the effect of Juncus on neighbors. In previous work, shading around the tussock periphery, influenced largely by this collapse of culms, was shown to suppress production and species richness of neighbors; however, various plant species have been observed to grow directly upon the Juncus tussocks in the void left by collapsed culms. To quantify this latter phenomenon, vascular plant abundance was evaluated on tussocks of Juncus effusus in four Mississippi (USA) beaver wetlands to determine the likelihood and nature of positive interactions between Juncus and neighbors. Tussocks in three of these wetlands were examined for tussock microtopography and diameter, relative water depth atop the tussock, and colonization by other plant species. Species richness of colonizing plants correlated positively with tussock diameter in two of these wetlands, and no colonizers were present in the third. Tussock microtopography and elevation differed significantly among wetlands, but species richness was unrelated to relative water depth atop the tussocks. In the fourth beaver marsh, two sets of surveys were conducted to determine whether the species assemblages upon Juncus tussocks differed from those across the entire wetland. Plant assemblages on Juncus tussocks, and on elevated substrata in general, were significantly different from assemblages not associated with elevated surfaces. Thirteen species were associated exclusively with elevated rooting substrata, ten of which were exclusive to Juncus effusus tussocks. The present results suggest that small-scale spatial and temporal shifts occur in the effects of Juncus on neighboring plants. Effects shift from primarily competitive shading interactions resulting from a dense shoot canopy to facilitative interactions atop tussock mounds after mature culms collapse. Facilitation likely is mediated through characteristics of the tussock mound, such as provision of a stable rooting substratum or oxygenation of sediments by the Juncus effusus root system.
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