Correspondence analysis of functional groups in a riparian landscape

We used multivariate analysis and ordinations to characterize the composition and distribution of woody vegetation within the Ozark National Scenic Riverways (ONSR), Missouri, USA. The objectives of the study were to: 1) evaluate patterns of woody species distributions along existing environmental gradients; 2) determine if different classes of woody plants (i.e., dominant overstory trees, all trees, understory trees, and shrubs) responded similarly to the same suite of environmental variables; and 3) determine if discrete ecotonal and/or ecoclinal vegetation patterns were present across the landscape. Woody vegetation was sampled from 94 plots along 35 transects positioned at right angles to the river channel. Sample plots were analyzed with Detrended Correspondence Analysis (DCA), Canonical Correspondence Analysis (CCA), and TWINSPAN. Overall, woody vegetation was correlated with several environmental gradients, including elevation of the plot above the river, soil pH, soil moisture, and soil particle size. Responses to secondary gradients differed among the four classes of plants analyzed, however. CCA biplots of understory trees indicated that patterns of those species were strongly correlated with slope through the plot and sand content of soil. CCA biplots of shrubs showed that CCA axes were most strongly correlated with soil organic matter content, soil moisture, and silt content. Further, there was limited evidence for discrete assemblages of woody species, with the exception of streamside vegetation. Instead, mixing of woody species was observed across a broad transition zone. Because there is little correspondence between vegetation layers, our results demonstrate including plant classes other than a subset of canopy dominant trees can provide additional resolution in characterizing vegetation responses along complex environmental gradients.