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INTRODUCTION The primary purpose for developing a plant community classi¤cation is to provide information on plant communities useful to conservation planning and ecological management (Grossman et al. 1994). Plant community classi¤cations de¤ne groups of plants that share biotic similarities and abiotic, system process, and structural characteristics. This is done by grouping plants by species composition and community structure, overlain on physical features and ecosystem parameters. In Arkansas, the blackland ecosystem includes prairie-savanna, woodland, and forest types (Foti et al., this volume). These plant communities are being classi¤ed using the system developed by the Nature Conservancy (Grossman et al. 1998) in cooperation with state, federal, and academic partners (Foti et al. 1994). The Nature Conservancy classi ¤cation is a modi¤ed version of the UNESCO vegetation classi¤cation system (UNESCO 1973). This national classi¤cation has been developed to present a consistent framework for conserving and stewarding biodiversity. An objective of the Nature Conservancy and many state-based natural heritage programs is identi¤cation and conservation of representative examples of all natural plant community types (TNC 1996). Plant communities can also be used as a coarse ¤lter approach in planning the conservation of biological diversity. Plant community descriptions with ecosystem information can be useful in developing management regimes that maintain biodiversity across the landscape by incorporating relatively large-scale ecosystem process models during the planning process. Ecosystem transition models can be used in the restoration of degraded natural communities. The spatial arrangement of plant communities on the landscape can be used to interpret gaps in the landscape picture where plant communities are 8 A Plant Community Classi¤cation for Arkansas’s Blackland Prairie Ecosystem Douglas Zollner, Scott Simon, and Thomas L. Foti no longer extant. With this information it may be possible to conserve much of the natural diversity of an area through strategic conservation planning and ecological management. As part of a conservation initiative for Arkansas’s blackland ecosystem that includes land acquisition, ecological restoration, and stewardship , plant community descriptions were developed across the range of abiotic variation. The plant community descriptions include spatial models of community relationship to other plant communities, soil, moisture, topography, and ¤re regimes. A model of ecosystem dynamics was also developed. METHODS The national classi¤cation has seven levels of hierarchy: class, subclass , group, subgroup, formation, alliance, and association. These levels are described below. The ¤rst ¤ve levels are broad physiognomic classes and the ¤nal two are based on plant species composition (Grossman et al. 1998). At the highest level there are seven classes: (1) forest areas dominated by trees over 5 m tall with overlapping crowns; (2) woodland areas dominated by open stands of trees over 5 m tall with a shrub and/or herbaceous understory usually present; (3) shrubland areas dominated by shrubs over 0.5 m tall; (4) dwarf-shrubland areas dominated by low growing shrubs; (5) herbaceous vegetation areas dominated by herbs and grasses; (6) nonvascular vegetation areas dominated by lichens, bryophytes, or algae; and (7) sparse vegetation areas where the vegetative cover is nearly absent. Subclasses are divisions within each class based on predominant leaf phenology: evergreen, deciduous, and mixed for wooded areas and perennial gramminoid, perennial forb, hydromorphic rooted vegetation , and annual gramminiod or forb for herbaceous vegetation. The group level of the hierarchy is de¤ned by a combination of climate , leaf morphology, and leaf phenology. Categories within each subclass may be based on climate (tropical, temperate, subpolar). For wooded types categories may be based on leaf type (broad-leaved, sclerophylous , needle-leaved). For herbaceous types categories may be based on the presence or absence of tree, shrub, or dwarf shrub canopy coverage of less than 25 percent. Subgroups divide natural or near-natural (mildly altered) vegetation from converted or cultivated vegetation. Formations within each subgroup are based on crown shape (rounded, cylindrical, conical), crown height (tall, medium-tall, and so forth), elevational zone (alpine, montane, lowland), and hydrologic regime (after Cowardin et al. 1979). An alliance is a group of plant communities having the same priA Plant Community Classi¤cation 111 mary dominant species and similar physiognomy. It is an aggregation of plant community types. The alliance can be considered equivalent to the cover type used in most USGS Gap Analysis Programs (GAPs) to standardize mapping. Descriptions of alliances for the southeastern United States are available from Weakley et al. (1999). An association is an assemblage of plant species with a de¤ned species composition and physiognomy that repeats across landscapes as...

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