To gain an understanding of plant community assembly in natural conditions we used five small islands located within the Mississippi River to observe natural assembly patterns. Elevations were determined on each island along six transects spanning from fore water edge to zee water edge for both 2007 and 2008, and corrected with the nearest river gage elevation to link island elevation with hydrological patterns of flooding frequency and duration. Along each transect, two 1m x 2m herbaceous vegetation plots were placed at every 1m deviation in elevation, with one randomly placed plot in the center of each elevation zone and the other plot positioned to ensure maximum diversity. We examined the composition and diversity along the disturbance gradient at three scales: transect, whole island, and island complex; blocked by island side and year. We expected to find higher diversity in the middle of the disturbance gradient, with greater composition under low disturbance and more stress under higher disturbance, supporting the shifting limitation model. We further expected to find the greatest dissimilarity among plots in the middle elevation, due to the increased chance colonization of species, less affected by disturbance stress and competition.
Results/Conclusions
Drastically different hydrology in 2007 and 2008 led to obvious topographical differences for islands when compared across years, and significant changes in vegetation diversity and composition. In 2007, dissimilarity of plots within islands generally increased with increasing elevation change, suggesting a composition gradient related to disturbance. The opposite was seen when comparing plots among islands, suggesting within-island interspecific controls and among-island neutral dynamics. Along the disturbance gradient, dissimilarity was highest at middle elevations, supporting our prediction. Following the major flood of 2008, dissimilarity of plots at lower elevations increased to nearly 100%, suggesting stochastic vegetation composition, while upper elevation plot dissimilarity changed little. In 2007, diversity indices showed the expected humped relationship at whole island and island complex scales, but not at the individual transect scale, which generally showed an increase in diversity along the gradient from high disturbance to low disturbance. We interpret this as further evidence of the stochastic nature of composition in the middle of the gradient. Diversity patterns were more difficult to interpret following the 2008 flood, as fewer plots had vegetation and overall diversity decreased significantly. Both diversity and composition patterns were different based in island side.
