SPECIAL FEATURE: VEGETATION SURVEY Classification and description of alluvial plant communities of the Piedmont region, North Carolina, USA Elizabeth R. Matthews, Robert K. Peet, & Alan S. Weakley Keywords Cluster analysis; Fluvial geomorphology; Ordination; Random forests; US National Vegetation Classification; Wetlands. Abbreviations CCA = canonical correspondence analysis; NVC = United States National Vegetation Classification; GIS = geographic information system Nomenclature: Weakley (2010) Received 29 September 2010 Accepted 1 June 2011 Matthews, E.R. (corresponding author, lizmatthews03@gmail.com); Peet, R.K. (peet@unc.edu) & Weakley, A.S. (weakley@unc.edu): Curriculum for the Environment and Ecology, CB#3275, University of North Carolina, Chapel Hill, NC 27599-3280, USA Abstract Questions: What are the alluvial vegetation types of the North Carolina Piedmont? How is species composition related to site conditions? Location: Catawba, Yadkin-PeeDee, Cape Fear, Neuse and Tar-Pamlico River Basins, North Carolina Piedmont region, southeast USA. Methods: We recorded 194 alluvial vegetation plots. Vegetation types were derived using flexible b-hierarchical cluster analysis and random forests classifiers to reassign misclassified plots. We used canonical correspondence analysis to show the relationship between species composition and key environmental variables. Results: Twelve forested vegetation types and two herbaceous types were distinguished, nested within a hierarchical classification structure of five higher- level groups. The five mega groups describe broad geomorphic–floristic condi- tions, whereas the narrower vegetation types characterize finer-scale floristic variation. Floristic variation is related to stream order and soil chemistry (pH and Ca:Mg ratio), as well as soil texture variables (percentage sand and percentage clay). We present a summary of floristic composition and structure, environ- mental setting and geographic distribution for each of the 14 vegetation types. Conclusions: We suggest recognition of 14 alluvial vegetation types in the North Carolina Piedmont. In comparing our vegetation types with the commu- nity concepts currently recognized in the US National Vegetation Classification, some of our types fit well within recognized NVC associations, whereas others deviate sharply from established types, suggesting the need for reworking the currently recognized NVC alluvial type concepts. Introduction Riparian ecosystems are home to diverse plant commu- nities, in part due to the diversity of habitats found in this landscape. Floodplain plant communities, in particular, have long been known to be among the more species-rich terrestrial habitats (Nilsson et al. 1989; Gregory et al. 1991; Naiman et al. 1993; Brown & Peet 2003). In addition to the ecological value of these communities, floodplain vegetation provides many ‘ecosystem services’ to the human population, including filtration of pollu- tants, flood and erosion control, fish and wildlife habitat, and a variety of recreational opportunities. However, few pristine riparian ecosystems remain, particularly in North America (Sharitz & Mitsch 1993). Many of these land- scapes have been converted to agriculture, damaged by impoundments or degraded by the invasion of non-native species. Due to the ecological significance of floodplain ecosys- tems and their current imperiled status, there is signifi- cant interest in conservation and restoration of these habitats, and North Carolina, in particular, is a hotspot for riparian restoration in the southeastern United States (Sudduth et al. 2007). However, informed management decisions and restoration project design and evaluation require detailed information regarding the composition and structure of natural alluvial plant communities, in addition to an understanding of the environmental Applied Vegetation Science 14 (2011) 485–505 Applied Vegetation Science Doi: 10.1111/j.1654-109X.2011.01150.x r 2011 International Association for Vegetation Science 485