Gradient analysis in a riparian landscape: contrasts among forest layers

We used standard vegetation sampling techniques to characterize species associations in a riparian forest of the Buffalo National River, Arkansas, USA. Species associations were influenced by environmental gradients dominated by pH and elevation, but secondary gradients differed among forest layers. Distinct groups of sampling plots within each vegetation layer were strongly supported, but the number and composition of the clusters did not correspond well among forest layers. The independence of forest layers may be promoted by response to different patterns of environmental factors operating at different levels in the forest. Traditional methods of plant community ecology rely on the dominance of overstory trees to define species associations and establish plant community boundaries, however species associations in the overstory are not necessarily good predictors of understory associations. Ecosystem management relies on accurately identifying components of the forest landscape, but traditional use of canopy dominants as community indicators may be inaccurate and imprecise for the majority of forest species. (C) 1997 Elsevier Science B.V.