Vegetation Classification and Mapping Project Report - USGS

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.
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