Gradient Trends in the Streamside Forest of Central Illinois

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Abstract
The streamside forest vegetation in the Sangamon River basin of east-central Illinois was studied by direct gradient analysis. A gradient of river flood frequencies was used to combine data from 3 vegetational gradients and to study gradient trends in species richness, diversity, evenness, dominance, and the Gaussian curve model of species distribution. Species richness, diversity and evenness increased from minima at the streambank to maxima in mid-coenocline areas and then regressed slightly in the unflooded portion of the flood-frequency gradient. Dominance was strongest at the extremes of the gradient. Tolerance to flood and moisture conditions are suggested as the primary sorting factors controlling the distribution of species in the streamside forest. Historical disturbances, are also important and represent an alternative hypothesis. The Gaussian curve model was determined to statistically fit 20 of the 27 spp. for which there were sufficient data for analysis. Species distribution modes were distributed on the gradient apparently at random. Minor species were distributed independent of the dominants.