Vegetation Classification and Mapping Project Report - USGS

USGS-NPS Vegetation Mapping Program
Colonial National Historical Park
Summary Statistics
Summary tables of compositional statistics were calculated to evaluate the adequacy of groups
recognized in cluster analysis and ultimately to assist in naming and describing the community
types. Total mean cover and total frequency across all plots were determined for every taxon.
To compute mean cover, cover class scores were converted to the midpoints of their respective
percent ranges, the midpoints were averaged, and resulting values were back-transformed to
cover class scores. For each taxon in each group under consideration, the following summary
statistics were then calculated:
• Frequency – the number of samples in a group in which a species occurs.
• Mean Cover – back-transformed cover class value corresponding to mean percent cover
calculated from midpoint values of cover class ranges. All samples assigned to a group were
considered when calculating mean cover, not just those in which a taxon was present;
absences were assigned a cover value of 0.
• Relative Cover – the arithmetic difference between mean cover (for a given group of
samples) and total mean cover (for the entire data set) (= Mean Cover – Total Mean Cover).
Expressed by plus or minus symbols, this value provides a relative approximation of how
much more, or less, abundant a particular species is in a community type compared to the
overall data set.
• Constancy – the proportion of samples in a group in which a species occurs, expressed as a
percentage (= [Frequency / Number of samples in group]×100).
• Fidelity – the degree to which a species is restricted to a group, expressed as proportion of
total frequency that frequency in a given group constitutes (= [Frequency / Total Frequency]
× 100). An accidental or nonnative species can have maximal (100) fidelity to a type if it
occurs in only one sample in the entire data set. As a result, fidelity alone can perform
poorly as a criterion for identifying characteristic species and distinguishing among types.
• Diagnostic Value (DV) (= [Constancy × Fidelity] / 100). A synthetic value indicating
species that are both frequent within and relatively restricted to a group of plots.
• Diagnostic Value Adjusted by Cover, Scaled (Adj DV [scaled]) (= [Diagnostic Value ×
Mean Cover] / 9). By dividing DV x Mean Cover by 9, the maximum possible cover value,
this statistic synthesizes information about frequency, diagnostic value, and mean abundance.
A species entirely restricted to a particular community type, occurring in every sample of that
type, and attaining maximum mean cover will have a Scaled Adjusted DV of 100 for that
type. Empirically, taxa most characteristic of a type will have a much lower Adjusted DV,
although the exact range of values in any given type or data set may vary considerably.
• Diagnostic Value Adjusted by Cover, Unscaled (Adj DV [unscaled]) (=Diagnostic Value X
2 relative cover ). An alternative, unscaled synthetic measure of adjusted DV, using relative cover
as the modifier of DV. Since cover classes form a logarithmic, rather than linear scale of
values, Unscaled Adjusted DV is a statistically more legitimate means of incorporating
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