Vegetation Classification and Mapping Project Report - USGS

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USGS-NPS Vegetation Mapping Program Colonial National Historical Park Vegetation Classification and Characterization The vegetation classification used to map seven mid-Atlantic NPS units in Virginia was developed through successive approximations. The initial classification from 2001 (Fleming 2001) was improved upon by two additional analyses, in 2003 and in 2006, each progressively using a larger regional dataset, with the objective of producing a more robust classification. All plot data collected in mid-Atlantic national parks as of November 2002 were combined into a regional data set of 1342 plots from throughout the Virginia Piedmont and Coastal Plain and from selected NPS units in Maryland and the District of Columbia. The resulting preliminary vegetation classification was reviewed by NPS ecologists and Natural Heritage Program ecologists from Virginia, Maryland, and West Virginia. In December 2006, with the addition of plot data collected since 2002 from Virginia, Maryland, and West Virginia, a regional dataset of 2,250 plots was used to develop the final vegetation classification for the project. All data were examined using a combination of cluster analysis, ordination, and tabular (statistical) analysis. In general, the analytical process was designed to progressively fragment the large datasets into more workable subsets, using cluster analysis to identify groups, statistical analysis to validate those groups, and lastly ordination studies to examine compositional gradients between groups and correlations with important environmental gradients. In practice, the process was iterative as increasingly finer groups were identified and analyzed. The general steps included 1) data preparation and transformation, 2) numerical classification (cluster analysis), 3) summary statistics, 4) gradient analysis (ordination), and 5) assignment of classification units to the standard (crosswalking to USNVC). Each of these steps is outlined below. Data Preparation and Transformation Plot data collected during field work were combined with existing data from throughout the Mid- Atlantic Coastal Plain and Piedmont using databases created with Microsoft Access 2000. The final dataset consisted of 2,250 plots (1,452 upland and palustrine wetland plots; plus 798 tidal plots). Since individual plant taxa are not always identified to the same taxonomic level in plot sampling, botanical nomenclature for the whole analysis dataset was reviewed and standardized. As a rule, taxa were treated at the highest level of resolution possible, but treatment at the subspecific level was not always possible and a few groups of species had to be merged into "pseudospecies." For example, various plots had Polygonatum biflorum, Polygonatum biflorum var. biflorum, or Polygonatum biflorum var. commutatum; these were merged at the species level. Species richness was calculated for each plot using all taxa (including unidentified species) rooted within plot boundaries. However, taxa identified only at generic or higher levels (e.g., “Carex sp.” or “unidentified woody seedling”) were deleted from the dataset prior to analysis to eliminate "noise" and potentially erroneous correlations between generic entities. 32
USGS-NPS Vegetation Mapping Program Colonial National Historical Park Prior to analysis, most environmental variables were transformed, either to normalize frequency distributions or to assign numeric values to categorical variables. Topographic position and slope were converted to ordinal variables (Table 4). Aspect was transformed using the cosine method of Beers et al. (1966), using the formula A' = cos (45º - A) + 1, where A' = transformed aspect and A = aspect in degrees. The Beers transformation is a commonly used formula for the conversion of the circular measure of slope aspect in degrees into linear values that can be used in correlation and regression analysis. Beers transformation yields values between 0 and 2 that are used to relate aspect to topographic moisture and solar exposure. Drier, solar exposed slopes (SW, 225 o ) have the lowest values and moist, sheltered slopes (NE, 45 o ) the highest transformed values. Surface substrate values were converted to decimals and arcsine transformed to normalize their distributions. A synthetic Topographic Relative Moisture Index (TRMI) was calculated for each plot using a procedure modified from Parker (1982). TRMI is a scalar index ranging from 0 (lowest moisture potential) to 50 (highest moisture potential) and is computed by combining three topographic variables that potentially influence water runoff, evapotranspiration, and soil moisture retention: • Slope inclination (10-point scale; per Parker [1982]) • Aspect (20-point scale) = Beers-transformed aspect X 10 • Topographic position (20-point scale; per Parker [1982]) Normally slope shape would be included as an additional 10-point scale but, unfortunately, data on slope shape were not consistently collected from the plots in this study. Because of this omission, as well as assumptions of the formula that may not apply as well to Piedmont and Coastal Plain topography as to montane topography, TRMI as calculated for this study, should be regarded as strictly experimental. Soil samples collected from plots were oven-dried, sieved (2 mm), and analyzed for pH, estimated nitrogen release (ENR), phosphorus (P), soluble sulfur (S), exchangeable cations (calcium [Ca], magnesium [Mg], potassium [K], and sodium [Na] in ppm), extractable micronutrients (boron [B], iron [Fe], manganese [Mn], copper [Cu], zinc [Zn], and aluminum [Al], in ppm), cation exchange capacity (CEC; m.e.q./100g), total base saturation (%TBS), and percent organic matter (%OM). Chemical analyses were conducted by Brookside Laboratories, Table 4. Ordinal values for topographic position and slope inclination used in data analysis. Topographic Position Slope Inclination basin/depression = -1 0–3º = 1 (flat) plain/level/bottom = 0 4–6º = 2 (gentle / undulating) toe slopes = 1 7–13º = 3 (sloping / rolling) lower slope = 2 14–20º = 4 (moderate / hilly) middle slope = 3 21–40º = 5 (steep) upper slope = 4 41º+ = 6 (very steep) crest = 5 33
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Vegetation Classification and Mapping Project Report - USGS

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