This section provides a rudimentary introduction to the procedures VANHP employs in the collection of field data and development of ecological community classifications. A more detailed explanation and discussion of these procedures will be presented in a future document.
VANHP ecologists are committed to a rigorous, quantitative approach to vegetation sampling, analysis, and classification, from the use of standard sampling protocols in the field to the application of numerically intensive, state-of-the-art methods of data analysis. This emphasis is consistent with both a long history of vegetation investigations in North America and Europe and the contemporary development of national standards for vegetation classification and of a national plots database (Jenningset al. 2003; http://vegbank.org/vegbank/index.html). Quantitative data explicitly recognize differences in abundance and biomass among coexisting plant species. Moreover, data collected from sample units of known, standard size provide a consistent basis for comparing stands of disparate vegetation and facilitate sharing of data among users to an extent not possible with mere species lists or data collected from an area of indefinite size. Patterns in composition and structure of ecological communities are dependent on the scale of observation, and a sampling protocol that specifies a particular scale reduces both the likelihood of obscuring patterns across multiple scales and sampler bias in determining whether particular species are recorded. Similarly, numerical methods of analysis offer objective means of generating and evaluating a classification of vegetation and enable the detection of patterns that might otherwise remain unapparent. The collection and analysis of quantitative data to support ecological classification should be viewed neither as panaceas nor ends in their own right. Rather, they are important as tools to further biodiversity conservation, but they provide a critical objective, scientific context for doing so.
Data collection follows standard protocols developed and refined by VANHP over the past 15 years. The fundamental components of these protocols are identical within the agency and consistent with sampling techniques employed by a wide range of other users (e.g., Peet et al. 1998). For inventory purposes and most contract projects, data are collected from plots 400 m2 in forests and woodlands and 100 m2 in shrubland and herbaceous vegetation. On natural area preserves and other managed areas that require permanently marked plots for long-term monitoring, modular plots up to 1000 m2 that contain nested subplots are sampled more intensively to provide information about vegetation structure, composition, and species richness at multiple spatial scales. In all cases, within each plot all vascular plants present are recorded and the total individual cover of each taxon (defined as the vertical projection of all above-ground biomass) is estimated and assigned to one of nine cover classes representing a range of percentage values. Vegetation structure is assessed by estimating the cover of each woody species at six vertical (height) strata, and stem diameters are measured for all woody stems => 2.5 cm at breast height (1.4 m). A standard set of environmental data is collected, including elevation, aspect, degree of slope, coverage of different types of surface substrate, soil characteristics, and qualitative measures of soil moisture and hydrology. A soil sample is routinely collected from each plot in order to document soil chemistry. All sampling locations are recorded using a global positioning system (GPS) unit and mapped using ArcView GIS (ESRI 1992-2002). Frequently, the sampling protocol also includes photographic documentation and cursory examination of tree increment cores to deduce stand age and history. Click here for a copy of the VANHP standard plot data collection field form and Instructions (requires the free Adobe reader).
Data Analysis and Classification
Prior to 1997, data analysis and classification were conducted using traditional Braun-Blanquet methodology, in which a tabular array of vegetation samples and species is iteratively rearranged to isolate samples of similar composition and species of similar affinities. A narrow focus on individual landscapes or self-contained data sets often resulted in the proliferation of largely redundant vegetation types that overlapped considerably in composition, but differed in name and local distribution. In addition, significant community occurrence records were referenced under a statewide classification system based on putative classes of soil fertility (Rawinski 1992). The fine-scale approach employed for individual landscapes and the trophic-based, coarse-scale approach adopted statewide recognized classification units at different conceptual scales that were not fully integrated.
Since 1997, numerical methods have been employed to generate classifications, and increasing emphasis is now placed on circumscribing units across their full distributional range in Virginia. This new approach makes use of several integrated applications to manage, manipulate, and analyze data. Microsoft Access provides a database platform for permanently archiving digital compositional data, environmental data, and metadata, as well as a powerful and flexible tool for efficiently assembling any desired set of plots for analysis. Adhering to the principle that the recognition of vegetation types should be based on total floristic composition, VANHP ecologists employ cluster analysis (primarily agglomerative hierarchical) to generate classifications. Subsequently, ordination (chiefly non-metric multidimensional scaling) is performed to assess the classification and identify those environmental gradients and site conditions most strongly associated with variation in species composition. Both cluster analysis and ordination are implemented in PC-ORD (Version 4.36; McCune and Mefford 1999). Once potential vegetation types are identified, several summary statistics are calculated to evaluate the consistency and distinctiveness of the type and to aid in selecting nominal taxa. Species-specific values for constancy (the proportion of plots assigned to a vegetation type in which a species occurs), fidelity (the degree to which a species is restricted to a particular type), and mean cover identify the most characteristic and dominant species for each type. Macros, written in Microsoft Visual Basic code and executed in Excel, automate the computation of these statistics and allow several types or multiple clustering levels to be evaluated rapidly. Additional macros facilitate the interface between data tables and queries in Access and spreadsheets in Excel. An additional software application, VTAB Ecosystem Reporter, is used to update obsolete botanical nomenclature, merge taxa into higher-level groupings and compute mean cover values for large data sets.
The nomenclature of community types is similar to standards adopted for the USNVC, which uses the scientific names of up to six characteristic species. Although they cannot serve as complete surrogates for detailed descriptions, the names of community types are constructed to facilitate both distinguishing among types and identifying them readily in the field. As a rule, species are listed in descending order of importance and structural position (i.e., overstory species are listed first, followed by understory species, then herbs and low shrubs). Nominal species in the same stratum are separated by a dash (-) while different strata are separated by a slash (/). Species listed in parentheses are less constant, but locally important, in a type. When two species are listed within parentheses, it means that either one or both may be important in a given stand. The typical physiognomy (i.e., forest, woodland, shrubland, etc.) and, for tidal wetland communities, hydrologic regime are included at the end of the formal community type name. A common name equivalent is not a strict translation of Latin names of nominal species, but instead usually refers to the ecological group name and contains a compositional or geographic modifier. As examples,
Zizania aquatica - Pontederia cordata - Peltandra virginica - Polygonum punctatum Tidal Herbaceous Vegetation
Freshwater Tidal Marsh (Wild Rice - Mixed Forbs Type)
Acer rubrum - Fraxinus americana - Fraxinus nigra - (Betula alleghaniensis) / Veratrum viride - Carex bromoides - Forest
Central Appalachian Basic Seepage Swamp