5.0 A Standard National Vegetation Classification System

It is proposed that the vegetation classification system developed and
implemented by The Nature Conservancy be further refined for use in the
NPS/NBS Vegetation Mapping Project as the standard for a national
vegetation classification system. The national vegetation classification
system proposed below is reviewed by specifications for a national
vegetation classification standard and specific NPS/NBS program
objectives. Further recommendations for the refinement of the proposed
system will be considered throughout the project, and modifications will
be implemented as appropriate. Additional recommendations will be
generated during the pilot applications of this system in the field.

5.1 Characteristics of the National Vegetation Classifications System

**5.1.1 Based on Existing Vegetation**

The national vegetation classification system focuses on existing
vegetation rather than potential natural vegetation, climax vegetation,
or physical habitats (see Section 3). The vegetation types covered in
the classification range from the short-lived to relatively stable and
persistent plant communities. The classification includes natural,
seminatural, modified, and cultural vegetation. The temporal and spatial
variation in communities is an intrinsic property of the vegetation
itself and, therefore, critical to the protection of biodiversity and
landscape dynamics. Not restricting the classification to stable
vegetation types ensures the units are appropriate for inventory and
site description, and provide the level of detail required to build
ecological and landscape models.

**5.1.2 Combined Physiognomic--Floristic Classification Approach**

The national terrestrial vegetation system is hierarchical and combines
physiognomy at the highest levels of the hierarchy and floristics at the
lowest levels. This classification approach was chosen to allow the
characterization of vegetation patterns at multiple scales. The combined
physiognomic floristic system allows identification of units from both a
divisive (\"top-down\") and agglomerative (\"bottom-up\") approach. The
top-down approach allows the use of physiognomic distinctions to help
map vegetation, to stratify sampling, and, where floristic information
is lacking, to delimit vegetation units. The bottom-up approach requires
that plot sampling and floristic analysis are the primary means for
defining communities. Where physiognomy is variable, the bottom-up
approach can also be used to help determine the important physiognomic
distinctions.

The basic unit of the vegetation classification is the \"community
element\" which is defined as an individual plant association or
repeating complex of plant associations. These associations have
definite floristic composition, uniform physiognomy, and represent
uniform habitat condition (see Flahault and Schroter 1910). The
community element concept is similarly related to the plant association
concept used in the Zurich-Montpellier tradition (see above). These
floristic units are characterized as patterns of co-occurring species
that recur either in space or time under similar environmental
conditions.

In the field, community elements are recognized as structurally and
floristically homogeneous stands of vegetation that occur in a
relatively uniform environmental setting[^1] As a result of the
individual species distribution patterns (the continuum concept) and the
environmental complexity across the landscape, there is considerable
variation within a community type across environmental gradients and the
landscape. The vegetation communities can be defined as homogeneous
stands of vegetation on the ground, but individual occurrences of a
particular plant association will vary in species compositions and
structure.

The floristic units are arranged under a hierarchy based on physiognomic
characteristics of their dominant vegetation. This physiognomic
hierarchy is a modification of UNESCO (1973) and Driscoll, *et al*.
(1984), and utilizes the physical form of the dominant vegetation to
organize the floristic units (see below).

**5.1.3 Role of the Environment**

An underlying assumption of national vegetation classification system is
that vegetation is the best and most easily measured assimilator of
complex environmental and historical site conditions. Although the
classification units are defined by vegetation only, the concept of a
community as an ecological unit includes all the biological and physical
diversity associated with that specific vegetation type. For example, a
herbaceous woodland \"serpentine barren\" plant community (scientific
name: *Pinus \[virginiana, rigida\]/Schizachyrium scoparium* alliance)
actually describes the unique geologic setting in which it is found, the
rare insects associated with the vegetation, and the fire disturbance
history that maintains the community.

The community elements of the national vegetation classification system
are related to a set of environmental factors rather than to a
particular site. This ensures a consistent ecological meaning for the
community level of the classification across a broad geographic range.
Environmental parameters are measured with the floristic units to
develop this correlation with the ecological reality. When the
classification is mapped across a site, the distribution of community
elements provides a basis for interpreting the ecological and land use
processes across the landscape.

5.2 Description of the Levels of the Terrestrial Vegetation
Classification Hierarchy

The national vegetation classification system has seven levels. The top
level of the hierarchy identifies whether the community is terrestrial,
aquatic, or subterranean. For the classification of natural and
seminatural terrestrial vegetation, the next four levels describe
physiognomic characteristics, and the last two levels describe the
floristics. The levels are

System

Physiognomic class

Physiognomic subclass

Formation group

Formation

Alliance

Community element

**5.2.1 System Level**

The top division of the classification hierarchy separates vegetated
communities (Terrestrial System) from those of unvegetated deep-water
habitats (Aquatic System) and unvegetated subterranean habitats
(Subterranean System). The Terrestrial System of the national hierarchy
is very inclusive. It includes the vegetation of uplands, the emergent
and rooted submerged vegetation of lakes, ponds, rivers, and marine
shorelines, and the sparsely vegetated and nonvegetated communities. In
relation to Cowardin *et al*. (1979), this system includes those
portions of the palustrine, lacustrine, riverine, estuarine, and marine
systems that have rooted vegetation.

Communities of the Aquatic System lack rooted vegetation and are
generally described as having fish, macroinvertebrates, algae, and
corals. The Aquatic System includes the nonvegetated (faunal) and
vegetated communities of the Cowardin *et al*. (1979) marine, estuarine,
riverine, and lacustrine systems beyond the limits of rooted vegetation.
The

Subterranean System includes terrestrial cave communities which are
generally described

using the dominant fauna.

There are different hierarchical divisions below each of the three
systems. The hierarchy for the Terrestrial System is structurally
complete. It has six levels, with four physiognomic levels (physiognomic
class, physiognomic subclass, formation group, and formation) and two
floristic levels (alliance and community element). The hierarchical
levels of the Aquatic and Subterranean classification systems are in
different stages of development, and the marine component is also near
completion.

For the purpose of the NPS/NBS Vegetation Mapping Project, the Aquatic
System (e.g., freshwater streams and rivers, lakes, reservoirs) will be
classified and mapped at a coarser level of detail than the communities
in the Terrestrial System (see Section 5.2.4.2 below).

**5.2.2 Physiognomic Levels**

The physiognomic portion of the national vegetation classification
hierarchy is a modification of the UNESCO world physiognomic
classification of vegetation (1973) and incorporates some of the
revisions made by Driscoll *et al*. (1984) for the United States.

The UNESCO vegetation classification system uses physiognomy (outward
appearance) and structure of the vegetation to define the units. It is
intended to provide a comprehensive framework for the preparation of
vegetation maps at a scale of 1:1 million or smaller. The system was
designed to include all natural and seminatural vegetation, but
\"cultural\" vegetation (wheat fields, vineyards, etc.) is not included.

The UNESCO physiognomic system was incorporated as the physiognomic base
for the hierarchy for the following reasons:

> • It is one of the few classification systems already in place that
> could be employed with relatively little research and development
> cost.
>
> • It is already the product of an international group of experts. As a
> result, it is worldwide in coverage and a more readily acceptable
> product than local and single-authored systems. Parts or variants of
> the system are presently being used by different United States and
> international agencies.
>
> • It is ecologically meaningful.
>
> • It is a hierarchical system that was designed for classification and
> mapping at multiple scales.
>
> • The structure of the system makes it open-ended; units can be added
> as needed.

***5.2.2.1 Modifications to the UNESCO Hierarchy***

The UNESCO system has now been modified and refined to provide greater
consistency at all hierarchical levels and includes additional
physiognomic types. Several limitations of the UNESCO hierarchy
prevented an unmodified application to the national vegetation
classification system. As an example, there was little supporting
information to explain the criteria used to define each hierarchical
level. In addition, the same criteria were used at different levels to
define the units. Finally, there were several vegetation formations,
such as wetlands, that were not adequately represented in the original
UNESCO system.

In particular, the \"subclass level\" of UNESCO has been modified to
better conform to the Federal Geographic Data Committee\'s standards for
vegetation classification. The UNESCO system has also been adjusted by
including more explicit hydrological modifiers at the formation level.
The hydrologic modifiers introduced by Cowardin *et al*. (1979) were
explicitly adopted since these have been used extensively to map
wetlands across the United States (Appendix 9.3). The levels are
outlined in the following sections. See Appendix 9.1 for a complete
version of the hierarchy.

***5.2.2.2 Physiognomic Class***

The Physiognomic Class is based on the structure of the vegetation. This
is determined by the height and relative percentage of cover of the
existing tree, shrub, dwarf shrub, and herbaceous strata (Figure 1).
This level has nine mutually exclusive classes:

Forest Woodland Sparse Woodland

Shrubland Sparse Shrubland

Dwarf Shrubland Sparse Dwarf Shrubland

Herbaceous Sparse Vascular/Non-Vascular

![](media/image1.png){width="6.208333333333333in"
height="4.520833333333333in"}

**Figure 1. Percent Canopy Cover of Vascular Vegetation**

***5.2.2.3 Physiognomic subclass***

The Physiognomic Subclass is determined by the predominant leaf
phenology of classes defined by a tree, shrub, or dwarf shrub strata
(evergreen, deciduous, mixed evergreen-deciduous), the average
vegetation height for types defined by the herbaceous stratum (tall,
medium-tall, short), and particle size of the substrate for sparsely
vegetated and nonvascular communities (e.g. consolidated rocks,
gravel/cobble, sand accumulations, mud flats).

Examples:

•Evergreen forest

•Deciduous forest

•Mixed Evergreen--Deciduous forest

•Tall grassland

•Medium-tall grassland

•Short grassland

•Sparsely vegetated sand accumulations

***5.2.2.4 Formation Group***

The units for the Formation Group are based largely on a combination of
climate, leaf morphology, and leaf phenology. In addition to climate and
leaf characteristics, the groups for the sparse woody classes (i.e.,
sparse woodland, sparse shrubland, and sparse dwarf shrubland) are
defined by the dominant lower stratum.

Examples:

•Temperate evergreen needle-leaved woodland

•Broad-leaved evergreen sparse shrubland with a dominant herbaceous
stratum

•Polar short grassland

***5.2.2.5 Formation***

The Formation represents an ecological grouping of vegetation units
based on broadly defined environmental factors such as elevation and
hydrologic regime, and additional structural factors such as crown
shape, and life-form of the dominant lower stratum.

Examples

•Tropical or subtropical seasonal montane evergreen forest

•Seasonally/Temporarily flooded medium-tall grassland

•Needle-leaved evergreen woodland with rounded crowns

•Broad-leaved evergreen sparse shrubland with tall graminoids

**5.2.3 Floristic Levels**

***5.2.3.1 Alliance***

The Alliance is a physiognomically uniform group of plant associations
(see Community Element below) sharing one or more diagnostic species
(dominant, differential, indicator, or character), which, as a rule, are
found in the uppermost strata of the vegetation (see Mueller-Dombois and
Ellenberg 1974).

The Alliance is roughly equivalent to the \"cover type\" of the Society
of American Foresters (Eyre 1980), although it is not restricted to
describing forest cover. The Alliance may be finer in detail than a
cover type when the dominant species extend over large geographic areas
and varied environmental conditions. The Alliance is also similar in
concept to the \"Series.\" Alliances, however, are described by the
diagnostic species for *all* existing vegetation types, whereas series
are restricted to climax types and are described by the primary dominant
species (see Pfister and Arno 1980).

Examples

•*Acer rubrum* --- *Liquidambar styraciflua* Forest Alliance

•*Pseudotsuga menziesii* Woodland Alliance

•*Juniperus osteosperma* Sparse Woodland Alliance

See Appendix 9.1 for the list of known Alliances in the United States.

***5.2.3.2 Community Element***

The Community Element is the finest level of the classification system.
For the Terrestrial System, the community element is defined as an
individual plant association or repeating complex of plant associations.
These associations have definite floristic composition, uniform
physiognomy, and represent uniform habitat condition (see Flahault and
Schroter 1910, Third International Botanical Congress 1910). This basic
concept has been used by most of the schools of floristic classification
(Braun-Blanquet 1965, Westhoff and van der Maarel 1978). The plant
association concept applies to existing vegetation regardless of
successional status.

The definition of the community element can be clarified with the
following points:

> • \"Habitat\" refers to the combination of environmental conditions
> and ecological processes influencing the community.
>
> • Uniformity of physiognomy and habitat conditions may include
> patterned heterogeneity (e.g., hummock/hollow).

• As a rule, community elements recur over the landscape.

> • The scale of the community element varies. Among other factors, the
> variation is determined by the size and apparent homogeneity of the
> occurrences across the landscape, the amount of data that has been
> collected, and the interpretation of these data by the field experts.
>
> • The community element may be composed of a complex of plant
> associations that constitutes a functioning ecological unit if the
> plant associations always occur together (e.g., prairie pothole).
>
> • The terms \"community element\" and \"plant association\" are both
> used to refer to the community element.

The community element is differentiated from the Alliance level by
additional plant species, found in any stratum, which indicate finer
scale environmental patterns and disturbance regimes. This level is
derived from analyzing complete floristic composition of the vegetation
unit when plot data are available. In the absence of a complete data
set, approximation of this level is reached by using available
information on the dominant species, indicator species, and
environmental modifiers.

Examples

•*Acer rubrum*-*Liquidambar styraciflua*-*Populus heterophylla* Forest

•*Pseudotsuga menziesii/Festuca idahoensis* Woodland

•*Juniperus osteosperma*/*Stipa comata* Sparse Woodland

See Appendix 9.2 for examples of community elements organized under the
classification hierarchy.

**5.2.4 Cultural Land Cover**

***5.2.4.1 Agricultural Land Cover***

The national vegetation classification system classifies agricultural
land cover using the Federal Geographic Data Committee (FGDC) Vegetation
Subcommittee\'s recommended system for Cultivated Vegetation (Table 3).
The FGDC system is still under development, and the national
classification system will evaluate further changes that may be made to
these classes.

+-----------------------------------------------------------------------+
| **Table 3. Federal Geographic Data Committee Classification of        |
| Cultivated Lands**                                                    |
|                                                                       |
| **Herbaceous**                                                        |
|                                                                       |
| Row crop                                                              |
|                                                                       |
| Close grown                                                           |
|                                                                       |
| **Shrub**                                                             |
|                                                                       |
| Fruit/Leaf/Nut shrubs                                                 |
|                                                                       |
| Fruit vines                                                           |
|                                                                       |
| **Tree**                                                              |
|                                                                       |
| Fruit and nut trees                                                   |
|                                                                       |
| Christmas tree plantations                                            |
+-----------------------------------------------------------------------+

***5.2.4.2 Urban Land Cover and Water***

The national vegetation classification system presently classifies and
maps urban land cover and water at a coarser level of detail than
natural and seminatural vegetation types. The system employs the land
use and land cover (LULC) classification system developed by Anderson
*et al*. (1976) for attributing Urban and Water dominated land cover.
Urban, or \"built up\" land, water classes are attributed at Level II of
Anderson\'s system (see Table 4). Anderson\'s LULC system is a widely
accepted system used throughout many federal, state and local agencies.
It was developed for use with remote sensor data.

+-----------------------------------------------------------------------+
| **Table 4. Anderson\'s Land Use and Land Cover Classification         |
| System**                                                              |
|                                                                       |
| This portion of the Anderson *et al*. system was adopted for the      |
| national vegetation classification system to map cultural land cover  |
| (Anderson *et al*. 1976).                                             |
|                                                                       |
| [Level I]{.underline} [Level II]{.underline}                          |
|                                                                       |
| 1\. Urban or Built-up Land Residential                                |
|                                                                       |
| Commercial and Services                                               |
|                                                                       |
| Industrial                                                            |
|                                                                       |
| Transportation, Communications,                                       |
|                                                                       |
| and Utilities                                                         |
|                                                                       |
| Industrial and                                                        |
|                                                                       |
| Commercial Complexes                                                  |
|                                                                       |
| Mixed Urban or Built-up Land                                          |
|                                                                       |
| Other Urban or Built-up Land                                          |
|                                                                       |
| 2\. Water (nonvegetated portion)                                      |
|                                                                       |
| Streams and Canals                                                    |
|                                                                       |
| Lakes                                                                 |
|                                                                       |
| Reservoirs                                                            |
|                                                                       |
| Bays and Estuaries                                                    |
+-----------------------------------------------------------------------+

5.3 Nomenclature Standards

Each Alliance and community element is assigned a name based on the
scientific names of the diagnostic species that have a high degree of
constancy. To ensure consistency of plant species nomenclature, the
plant species names follow the standards developed by Kartesz (1994).
Provisional community names are updated as additional information
becomes available.

In the Alliance and community element names, plant species used in the
name occurring in the same stratum are separated by the \"-\" symbol,
and those occurring in different strata of the vegetation are separated
by the \"/\" symbol (e.g., *Quercus macrocarpa/Corylus cornuta-Corylus
americana* Woodland). In those cases where the diagnostic species are
unknown or in question, environmental modifiers or broad vegetation or
geographic modifiers are used as placeholders until the diagnostic
species become known with more certainty (e.g., *Pinus palustris-Pinus
echinata/Schyzachyrium scoparium* Serpentine Woodland).

As a rule, the diagnostic species for Alliances are consistently present
(constant) in the community elements within the Alliance and the
diagnostic species for community elements are consistently present in
occurrences of the community. There are, however, certain situations
where a diagnostic species is not consistently present in community
elements within an alliance or in occurrences of a community element.
When this happens, the species that are not consistently present in the
community element or occurrences are placed in parentheses. For example
\"*Pinus ponderosa-(Pinus flexilis)* Alliance\" means *Pinus ponderosa*
is present in most of the associations while *Pinus flexilis* is not.

Some alliances have also been documented in which the associations share
two diagnostic species, but neither of the diagnostic species are
consistently present in all associations. In this situation both of the
diagnostic species names are put in parentheses. For example, \"*Pinus
(ponderosa-flexilis)* Alliance\" means that both species are not
necessarily present in all of the associations, but at least one of them
is present.

5.4 Development of the National Vegetation Classification System

**5.4.1 Development of the Floristic Classes (Alliances and Community
Elements)**

Development of the floristic classes (Alliances and community elements)
is an iterative qualitative and quantitative process. The majority of
the floristic units presently defined in the classification system are
the result of rigorous qualitative assessments due to the lack of
quantitative data. The long-term goal for the national classification
system is the determination of all floristic units through the
quantitative analysis of consistent plot data. Field data (species lists
and environmental information) will be prioritized and collected over
time in order to verify the classification of many provisional types.

***5.4.1.1 Qualitative Assessment***

Qualitative assessments of existing information are completed to
identify and describe provisional community elements. This process
includes the compilation of existing state classifications and
vegetation information from the literature and other sources. The
vegetation units are placed into the physiognomic hierarchy based either
on qualitative or quantitative description of structure and species
composition. Alliances and community elements are named and described
based on the qualitative assessment of patterns of diagnostic species.
Groups of ecologists are required to develop and review these
classification units. Problematic classifications and high-priority
elements are targeted as a focus for future data acquisition and
quantitative analyses to refine the classification of these types.

***5.4.1.2 Quantitative Analysis***

The process of quantitative assessment of the floristic elements
includes the compilation and assessment of existing stand table and
summary data on the community element and related types across the
entire range of occurrence. Collection of additional field data is often
required to support a robust analysis of the community. The resulting
classifications are then sent out for peer review by appropriate experts
(federal, state, and academic ecologists). Throughout this process the
goal is to ensure consistent quality control of the data and application
of the quantitative techniques.

Stand and summary data appearing in journal articles and published and
unpublished reports are used extensively for the development of
community elements. For a reference to a particular plant association to
be included in the analysis, its source must provide location
information, description of methods, species lists and quantitative
measure of species abundance values. Primary data are collected by the
Natural Heritage network and other researchers on community types that
are undersampled and of high priority. Data collection is carried out by
Natural Heritage and Conservancy ecologists using a standard relevé
methodology (Sneddon 1992, Bourgeron *et al*. 1991 and see above).

Compiled data are assembled into a single file and transformed
mathematically to a common abundance scale. The element classification
process is implemented using quantitative approaches of ordination,
clustering, and correlation depending on the information available.
Three multivariate analysis programs, TWINSPAN, DECORANA, and CANOCO,
are particularly useful in examining the floristic patterns and their
relationships to measured environmental variables (Hill 1979, Hill and
Gauch 1980, ter Braak 1990).

Despite their utility in synthesizing large data sets, many of the
analytical programs identify vegetation patterns that are statistically
but not ecologically meaningful. The quantitative analysis to determine
vegetation patterns must be carried out under the guidance and review of
experts who have a practical understanding of the ecological
relationships in the field.

***5.4.1.3 Confidence Levels***

Each community type is assigned a \"confidence level\" that is
determined by the amount and type of information available and the
analysis methods used to define it (Table 5). These confidence levels
help to identify where additional information will be required for the
refinement of the classification. As additional field data become
available, the classification is updated and the confidence levels
reevaluated.

+-----------------------------------------------------------------------+
| **Table 5. Confidence Levels Assigned to Each Community**             |
|                                                                       |
| Confidence levels are assigned to each community based on the type    |
| and amount of information used to classify the type as indicated      |
| below.                                                                |
|                                                                       |
| > 1 --- STRONG                                                        |
| >                                                                     |
| > Classification is based on quantitative analysis of verifiable data |
| > (species lists and associated environmental information) collected  |
| > in the field. Information is based on occurrences that can be       |
| > relocated.                                                          |
| >                                                                     |
| > 2 --- MODERATE                                                      |
| >                                                                     |
| > Classification is based on qualitative assessment of published      |
| > field data or field data that are of questionable quality, that     |
| > include limited numbers of samples, or have not been quantitatively |
| > analyzed.                                                           |
| >                                                                     |
| > 3 --- WEAK                                                          |
| >                                                                     |
| > Classification is based on anecdotal information or community       |
| > descriptions lacking data.                                          |
+-----------------------------------------------------------------------+

***5.4.1.4 An Example of the Development of Floristic Classes --- Pine
Barrens***

To refine the classification of pine barrens communities and help
identify conservation and management priorities, The Nature Conservancy
initiated a classification and mapping project at the Waterboro barrens
in York County, Maine. This project involved the collection of data on
all communities in a single pine barren site and relating these data to
the information available on pine barren communities at a regional
scale.

Local (Intensive) Analysis of a Single Pine Barren Site

Waterboro is an expansive pine barren site which occurs on sandy,
nutrient poor, outwash soils in southern Maine (Harris 1991). The mosaic
of communities that occur within the site exhibit a wide range of
composition and structure. This reflects the complex of climate,
terrain, hydrology and historical factors present at the site.

A set of stereo aerial photos was obtained for the Waterboro Barrens
site. Boundaries of vegetation units were delineated on the photos using
the criteria of texture (smoothness or coarseness of the image), tonal
contrast, and topographic location (Avery 1977), and these boundaries
were transferred to a 1\" by 500\' topographical map. Three 10 m x 10 m
plot samples were taken from representative areas within each vegetation
type identified from photointerpretation. Particular attention was paid
to the pitch pine -- scrub oak vegetation types. Within each plot, one
nested 5 m x 5 m quadrat was used to sample the understory vegetation
and two 1 m x 1 m quadrats were used for sampling herbaceous vegetation.
Information collected for each plot followed The Nature Conservancy
standards reviewed above (e.g., species composition and abundance, soil
texture, slope ). A preliminary community classification was developed
from this information, and a community map of the site was produced.

Regional (Extensive) Analysis of Each Community Type

Plot data for each community type occurring within the Waterboro Barrens
complex were then compared with data from similar communities across the
region. The analysis of the pitch pine -- scrub oak community, for
examples, benefited from a large data set (224 samples) that was
assembled from published (Olsvig 1980, Ollson 1979, Milne 1985,
Patterson 1984, McIntosh 1959) and unpublished literature (Pesiri,
Latham, Tucker, Seichab, Harris, State Natural Heritage Program field
forms for Massachusetts, Connecticut, Pennsylvania, New Hampshire, and
Rhode Island). Each sample was collected from a standard plot placed in
a vegetation association exhibiting a canopy of pitch pine over an
understory of scrub oak and contained lists and abundance of all species
(bryophytes and lichens excluded). Each sample was transformed into a
common, four-category abundance scale to normalize the data. An arranged
species-by-sample table illustrating patterns of floristic association
was produced using TWINSPAN (Hill 1979), and the floristic patterns were
circulated widely among state ecologists for review. Based on the
discussions and comments of the reviewers, the table was manually
rearranged until an agreement was reached on the ecological meaning of
the floristic associations.

For the pitch pine--scrub oak communities, the relationships between the
floristic patterns and the ecological variables were examined
quantitatively using CANOCO. Although the only consistent environmental
data available for every sample were latitude, longitude and elevation,
the CANOCO analysis confirmed that these variables explained a large
proportion of the variation in the data. This was reconfirmed through a
DECORANA ordination followed by a nonparametric correlation (Spearmans
rank) between the axis scores and the environmental variables. A
reassessment of the existing pine barrens literature, in light of the
new classification scheme, was also very useful in elucidating the
relationships between vegetation and environment.

This process was repeated for other communities found within the
Waterboro Barrens complex until each sample was classified within the
national context. The overall species composition and diagnostic
species, associated environmental variables, typical structure, and the
range of physiognomic expressions were examined and the distribution,
range, and global rank of each community was determined. The information
was then used to refine the classification attributes on the vegetation
map with the regional classification names.

**5.4.2 Arranging the Floristic Units under the Physiognomic Levels of
the Hierarchy**

Once defined, the floristic units are fit into the physiognomic
structure of the hierarchy based on their physiognomic expression across
all stands. In some cases, communities may exhibit different
physiognomic expressions without a concurrent shift in species
composition. In these cases, the physiognomic group is determined by the
most common expression of the community as opposed to a theoretically
stable expression. Where floristic and physiognomic groupings do not
correspond, precedence is given to the floristic relationships over the
physiognomic structure. Types that present more than one physiognomic
expression are cross-referenced in the hierarchy.

**5.4.3 Adding New Vegetation Types Identified during the Course of the
NPS/NBS Mapping Project**

The present classification is a dynamic product that has been developed
through the continuous review of literature, communications with local
and regional experts, directed field examinations, and some quantitative
analyses. All of the units have been derived through consistent
application of classification rules using available vegetation data and
associated environmental information. The NPS/NBS vegetation mapping
project will provide a large amount of additional information that will
allow further refinement of the classification. The classification
system will evolve to reflect the growing body of knowledge concerning
the biology, ecology, and geography of the different vegetation types.
Many new vegetation types may be added to the classification and some
current types may be split into new types, while others may be lumped
together.

For new types to be added to the classification, they must contain
significantly different biotic composition, be associated with different
environmental conditions, and be documented to recur across the
landscape. They also must be compared to information on related types
from a rangewide perspective to ensure it is not a local variant of a
community already classified. For the NPS/NBS vegetation mapping
project, suspected new types will be evaluated, qualitatively or
quantitatively assessed depending on the level of available data.

It is the responsibility of The Nature Conservancy to review the process
for the classification of all new types. The recent designation of a
special Ecological Society of America panel for Vegetation
Classification should provide assistance in the formal review of new
vegetation types in the near future.

***5.4.3.1 Qualitative Assessment***

When a vegetation type is discovered that may be new to the
classification, it will be reviewed using the process described above
for qualitative assessment of the floristic classes (Section 5.4.1.1).
New data collected for the suspected new type and any existing plot data
from the park will be assessed by The Nature Conservancy regional
ecologists. The type will be placed under the classification hierarchy
and compared to the information available for closely related types. If
the type is still suspected to be new, it will be described by the
regional ecology staff. This description will be circulated to the state
Natural Heritage ecologists, other regional ecologists, and other
experts. If the experts agree there is sufficient reason to believe the
type is new, it will be named and assigned a confidence level of \"3 ---
weak\" or \"2 --- moderate\" depending on the amount of available data.

The information generated on the new type will be disseminated from the
Conservancy regional ecologist to all field ecology and
photo-interpretation teams in each of the parks that could potentially
contain the new type. The photo interpreters will incorporate the new
type into their photo-interpretation keys at the park level (if this has
not been done already). New types will be reviewed by the experts,
classified and described before the final maps are produced for the park
in which it was discovered.

***5.4.3.2 Quantitative Assessment***

If there is significant interest or need to quantitatively assess a new
vegetation type or group of types, the process described in Sections
5.4.1.2 and 5.4.1.3 will be followed. The literature and other sources
of data (including existing plot data from parks) will be searched, and
all available stand table data will be compiled and assessed. Additional
primary data will be collected where necessary. The entire data set will
be analyzed, and the results will be reviewed by the experts on the
type.

5.5 Ecological Considerations and Variability

Plant communities need to be recognized over the entire range of
environmental variability (Austin 1991, Bourgeron *et al*., 1994). The
identification of community elements is performed to identify distinct
floristic patterns with a clear ecological meaning. Thus, an important
step to the classification itself is the identification of ecological
factors that determine the vegetation patterns. Vegetation types are
characterized by the co-occurrence of individual species as constrained
by environmental features (e.g., climate, geomorphic, and edaphic
factors), the dynamics of biotic processes (e.g., immigration,
emigration, competition), and disturbance. The relationship between
these factors and the vegetation patterns is often complex.
Interpretation of the ecological meaning of the units is completed, in
part, through qualitative understanding of the ecology, and wherever
possible the quantitative analysis of correlations between species and a
set of environmental factors. To understand these relationships, the
literature on community processes, plant demography, reproductive
biology, physiology, geography, must be consulted.

**5.5.1 Homogeneity**

Although some ecologists have identified shortcomings in the restriction
that plant associations must be defined from homogeneous units (Noss
1987), floristic and physiognomic uniformity has been generally widely
accepted as a valid criterion in the definition of floristic units
(Mueller-Dombois and Ellenberg 1974). The criterion for homogeneity is
particularly important when sampling vegetation for use in numerical
analysis. On the landscape, however, existing vegetation is often
transitional in nature. As a rule the national vegetation classification
system does not presently recognize transitional areas or ecotones
between two types as distinct elements in the classification. Two
special exceptions to the classification rules deserve explicit
attention: community complexes and gradients.

***5.5.1.1 Community Complexes***

Communities often occur as a fine-grained mosaic of interrelated, but
distinct, floristic associations. Classification of these community
complexes can be problematic as many, especially those with intrinsic
microtopographical variation, are inseparable in any definable or useful
fashion. These situations may occur in both random unpatterned fashion
or as small-scale patterned heterogeneity (e.g., hummocks and hollows in
bog situations might share some species, but have largely different
dominants). When these situations occur, the complexes of plant
associations are defined as a single community element. In these cases
the patterning is described as attributes of the community complex.

***5.5.1.2 Gradients***

The composition of most communities reflects the distribution of
individual species over multiple environmental gradients (Austin and
Smith 1989). Deciding the optimal place along the major gradients to
partition the continuum of change is one of the fundamental questions of
classification theory. While in some cases the data are naturally
clustered, in others several possible divisions of the data are
justifiable. The final choices as to where to draw the line between
related communities are driven by interpretation of the patterns by
field experts and the objectives of the research.

**5.5.2 Disturbance**

Disturbance processes have a profound influence on the character and
composition of vegetation. Broad-scale natural disturbances such as
hurricanes, fire, flooding, avalanches, and disease as well as chronic
small-scale disturbances such as hydrologic variation, tree-fall, animal
digging, and herbivory often explain the variations in existing
vegetation better than many of the traditionally measured ecological
factors. A number of anthropogenic disturbances, such as clearing,
plowing, grazing, development, and nutrient enrichment, have also
affected existing vegetation patterns. These anthropogenic disturbances
may simulate natural disturbances, create entirely new disturbance
regimes or alter natural disturbance regimes (e.g. fire suppression).
Often only circumstantial evidence is available to estimate the
disturbance regime associated with a particular vegetation type.

Some disturbances, whether natural or anthropogenic, can cause
alterations in the structure and composition of an occurrence of a
community. If the disturbance is severe enough to alter the structure
and floristic composition of a community on the ground, the
classification of that unit may change. Following a catastrophic fire,
for example, a Jack Pine/Blueberry Forest (Scientific name: *Pinus
banksiana/Vaccinium* spp. Forest) may become a Jack Pine (Northern Pin
Oak)/Little Bluestem Sparse Woodland (Scientific name: *Pinus banksiana
(Quercus elipsoidales)/Schizachyrium scoparium* Sparse Woodland).

In contrast, some disturbance regimes may alter the structure or
composition only moderately and the community may still fall within the
range of acceptable variation for the type. Since most communities are
identified by groups of diagnostic species rather than single diagnostic
species, small-scale disturbances that cause minor changes in the
floristic composition of the type are often not severe enough to change
the classification of the unit. For example, selective logging
techniques may extract Jack Pine from occurrences of a Jack
Pine/Blueberry Forest. If the rest of the species composition of the
community remain, the loss of only the Jack Pine may not be enough to
consider the community as a different type.

**5.5.3 Succession**

Successional stages are treated like any other existing vegetation type.
Once the structure and composition of a community reaches a stable state
that is physiognomically and floristically different from its previous
successional stage, it is considered a different community in the
classification. In developing the classification, particular emphasis is
placed on understanding how the species composition relates to a
particular successional process.

Floristic analysis of many successional vegetation types can reveal that
a type is an unusual physiognomic expression of an existing community
element. In the eastern region, for example, the *Quercus ilicifolia*
(shrub oak) thickets that develop in areas of frequent fire share an
identical species composition with the *Pinus rigida* (pitch pine) --
*Quercus ilicifolia* (scrub oak) barrens with which they typically
occur. Both of these types are considered subcommunities of the major
community element, though only the fire-maintained type does not contain
unique species.

5.6 The Relationship Between the National Vegetation Classification
System and Other Classification Systems

The national vegetation classification system was developed with the
knowledge that it would need to be related to other major classification
approaches. Cross-references to other major classification systems are
currently being developed. In the southeastern United States, The Nature
Conservancy is completing the classification, description, and keys to
the national forests. Included in the description of each type is a list
of the Society of American Foresters (SAF) Covertype (1980) with which
it is associated. For example, the Longleaf Pine/Little Bluestem-Blazing
Star Woodland from the national classification (Scientific name: *Pinus
palustris/Schizachyrium scoparium-Liatris pycnostachya* Woodland) would
be found within the \"Longleaf Pine\" SAF covertype. Additional
crosswalks that are being documented include the Kuchler Potential
Natural Vegetation classification (1975), the Classification of
Wetlands, and Deepwater Habitats of the United States (Cowardin et al.
1979), Brown, Lowe, and Pase (1980), and others.

**5.6.1 An Example of Crosswalking: The Relationship between the Brown,
Lowe, and Pase Classification and The National Vegetation Classification
System**

The Brown, Lowe, and Pase system was developed for use in the southwest,
with special emphasis on Arizona. A later version was expanded to
include all of North America (Brown *et al*. 1979, 1980). The mechanics
of crosswalking the Brown, Lowe, and Pase (1980) classification system
to the national vegetation classification system have been completed for
all of the communities that occur on the Gray Ranch site in New Mexico.

The Brown, Lowe, and Pase classification and the national vegetation
classification system combine physiognomy and broad climatic patterns in
the upper levels of the hierarchy, though the factors may be treated at
different hierarchical levels. For example, both systems separate
wetlands from uplands, but the Brown *et al*. system does this at the
second level (Vegetation level) of the hierarchy whereas the national
classification does so at the fourth level (Formation level). Both
systems also identify coarse physiognomic classes such as forest,
woodland, scrubland, etcetera. The Brown, Lowe, and Pase classification
calls this level of the hierarchy the \"Formation-type\" and this is
recognized as the \"Class\" level in the national classification.

The major difference between the two systems is that the Brown, Lowe and
Pase classification recognizes a Regional Formation or Biome level which
is based on \"distinctive evolutionary history within a given
formation.\" These biomes tend to be centered in particular geographic
regions or provinces (Brown 1982). The national classification does not
make such regional distinctions. The national classification, which is
physiognomic at the highest levels, is geographic only to the extent
that physiognomy reflects local ecological factors.

The Brown, Lowe, and Pase classification has two floristic levels, both
which tend to be coarser in scale than the national classification. The
Series level of the Brown, Lowe, and Pase generally represents the
dominant species at climax and are often named by the dominant genus
(i.e., Pine series). This level is much broader than the Alliance level
of the national classification. The lowest level of Brown, Lowe, and
Pase (1980) system, called the association, is generally identical to
the Alliance level used in the national classification. For example,
Brown, Lowe, and Pase (1982) describe a *Juniperus deppeana* association
that is equivalent to the national classification\'s *Juniperus
deppeana* Alliance. In a few cases, Brown, Lowe, and Pase divided the
vegetation into associations that correspond to one, or a related group
of associations, from the national classification. For example, the
national classification contains a single *Pinus ponderosa* Alliance
which has roughly forty associations within it. One of the associations
within the *Pinus ponderosa* Forest Alliance is the *Pinus
ponderosa/Quercus gambelii* association. Brown, Lowe, and Pase describe
two associations: (1) The *Pinus ponderosa* association, which is nearly
equivalent to the *Pinus ponderosa* Alliance in the national
classification except that it does not include *Pinus ponderosa/Quercus
gambelii* types; and (2) the *Pinus ponderosa/Quercus gambelii*
association, which is equivalent to the *Pinus ponderosa/Quercus
gambelii* association in the national classification (Table 6).

**Table 6. Example of a Crosswalk between the Brown, Lowe, and Pase
Classification and the National Vegetation Classification System**

The \"\<\" and \"=\" symbols identify the relationship between the
floristic units from each system.

BROWN, LOWE, AND PASE NATIONAL CLASSIFICATION

+-------------------------------+---+----------------------------------+
| **Biographic Realm**          | \ | **System**                       |
|                               | < |                                  |
| Nearctic                      |   | Terrestrial                      |
|                               | = |                                  |
| **Vegetation**                |   | **Class**                        |
|                               |   |                                  |
| Upland                        |   | Forest                           |
|                               |   |                                  |
| **Formation Type**            |   | **Subclass**                     |
|                               |   |                                  |
| Forest and Woodland           |   | Evergreen Forest                 |
|                               |   |                                  |
| **Biome**                     |   | **Group**                        |
|                               |   |                                  |
| > Cold Temperate Forests and  |   | Temperate and Subpolar           |
| > Woodlands                   |   | Needle-Leaved Evergreen Forest   |
|                               |   |                                  |
| **Regional Formation**        |   | **Formation**                    |
|                               |   |                                  |
| > Rocky Mountain (=Petran)    |   | Evergreen Needle-Leaved Woodland |
| > Montane Conifer Forest      |   | with Rounded Crowns (Upland)     |
|                               |   |                                  |
| **Series**                    |   | **Alliance**                     |
|                               |   |                                  |
| > Pine Series                 |   | *Pinus ponderosa* Alliance       |
|                               |   |                                  |
| **Association**               |   | **Association**                  |
|                               |   |                                  |
| *Pinus ponderosa*             |   | *Pinus ponderosa/ Quercus        |
|                               |   | gambelii*                        |
| *Pinus ponderosa/Quercus      |   |                                  |
| gambelii*                     |   |                                  |
+-------------------------------+---+----------------------------------+

5.7 Current Status of the National Vegetation Classification System

**5.7.1 State Coverage**

The data used to generate the communities in the national vegetation
classification come from a wide variety of sources. The national
classification is primarily based on communities described and tracked
by individual state Natural Heritage programs. The combined expertise of
these programs has contributed substantially to the generation of the
national vegetation classification. The national classification
currently covers all of the United States except Alaska and Hawaii, and
work is underway to incorporate these states.

**5.7.2 Regional Coverage**

The information on most vegetation types identified in the state Natural
Heritage program classifications has been synthesized to describe
national elements. In addition, some elements have been derived from
rigorous analysis at the regional level. The western, midwestern,
eastern, and southeastern regions have now completed provisional
regional classifications (Allard 1990, Bourgeron and Engleking 1993,
Bourgeron and Engleking 1994, Faber-Langendoen 1993, Sneddon and Metzler
1992, Sneddon *et al*. 1994). The majority of the floristic units in
these classifications are based on qualitative assessment of available
data. Approximately 20 percent of the elements are the result of
quantitative analysis (see Table 7). Each regional classification is now
organized under the national vegetation classification hierarchy. There
remains some redundancy in the Alliances and community elements listed
in the regional classifications, as the evaluation of communities that
occur in more than one region has not been fully completed.

The regions vary in the degree of refinement and the total number of
community elements identified (Table 7). The variation among regions in
the number of floristic units is due to differences in the amount of
available community information, the diversity of habitats, and the
overall geographic coverage among regions. The differences also reflect
the classification approach adopted by the regions to develop their
units and the levels of financial support for classification work. For
example, the eastern region has recently been supported by the NBS Gap
Analysis program to generate a list of all Alliances in the region. A
comprehensive list of Alliances (126) was completed as a result of this
project, though the list of community elements is not complete for this
region. This region expects to have classified approximately 400
community elements upon completion. In contrast, the western region has
worked primarily from the bottom up and has identified alliances by
grouping known plant associations.

+-----------------------------------------------------------------------+
| **Table 7. Number of Floristic Units Identified in each Region**      |
|                                                                       |
| [East]{.underline} [Midwest]{.underline} [Southeast]{.underline}      |
| [West]{.underline}                                                    |
|                                                                       |
| Alliances 126 203 367 520                                             |
|                                                                       |
| Communities 70 471 230 2,010                                          |
+-----------------------------------------------------------------------+

**5.7.3 National Coverage**

The number of units currently identified for each level of the
classification hierarchy is presented in Table 8. The physiognomic
levels of the hierarchy are still being tested and refined. Significant
structural modification of the physiognomic levels of the hierarchy is
not expected. The addition of several new formations is pending review
by the group of national and regional ecologists which comprise the
\"national ecology team.\" The greatest fluctuation in the number of
units identified under each level of the hierarchy is expected to be in
the floristic levels. It is estimated that perhaps as many as 1,500
additional community elements may be identified as the classification is
refined.

+-----------------------------------------------------------------------+
| **Table 8. Number of Units Currently Identified for Each Level of the |
| Classification Hierarchy**                                            |
|                                                                       |
| Class 9                                                               |
|                                                                       |
| Subclass 33                                                           |
|                                                                       |
| Formation Group 103                                                   |
|                                                                       |
| Formation 254                                                         |
|                                                                       |
| Alliances\* 1,216                                                     |
|                                                                       |
| Community Element\* 2,781                                             |
|                                                                       |
| \*The numbers of Alliances and community elements represent a simple  |
| total of the units identified in each regional classification. As a   |
| result, communities that occur in more than one region may be counted |
| more than once. All of the regional floristic units that are          |
| suspected to cross regional boundaries are currently being evaluated  |
| to create a more consistent national list of vegetated terrestrial    |
| communities.                                                          |
+-----------------------------------------------------------------------+

Up to this time, the approach to refine the national vegetation
classification system has been prioritized to those types that have been
identified as rare at the state level and then proceed to the more
common types. As part of a project supported by the United States Fish
and Wildlife Service, an initial survey of the rare communities of the
conterminous United States has been completed (Grossman *et al*. 1994).
Each of the 371 rare communities identified in the report has been
placed into the national hierarchy and duplication among regions
rectified. Descriptions have been written for each type and confidence
levels assigned.

**5.7.4 Gaps in the Classification**

***5.7.4.1 Gaps in State-Level Information***

The degree of community information varies considerably among states.
Some states lack classifications for their communities altogether, while
others have classifications that are at a coarser level than the
national vegetation classification standard. Others may have
classifications but inventory efforts for communities have not been
extensive. As a result, the national vegetation classification contains
more information in some states than others. In the eastern region,
additional information is needed from Maryland, Rhode Island, Virginia,
and West Virginia to fill gaps in the national vegetation
classification. Additional information is needed from Alabama, Georgia,
and Texas to refine the southeastern portion of the national vegetation
classification. In the midwestern region, the states of Iowa, North
Dakota, and South Dakota require additional inventory and classification
work to refine the national vegetation classification. Although
additional community information from the states listed above is needed
to refine the national vegetation classification, efforts have been made
to supplement the information from the state Natural Heritage programs
with information from other sources available for communities in these
states. The National Park Service vegetation mapping project will allow
additional information to be collected in many of these states. Many of
the vegetation units identified in parks in these states will be treated
as new types and will be fit into the national vegetation classification
using the process for adding new classes described above.

***5.7.4.2 Types Still in Need of Basic Work***

Although the classification includes vegetation from all of the
physiognomic classes (forests, woodlands, shrublands, etc.), there is a
greater amount of information available for some vegetation classes than
others. In general, more is known about the forest, woodland, and
shrubland classes than about herbaceous and sparse woody classes (sparse
woodland, sparse shrubland, sparse dwarf shrubland). Comparatively
little is known about the sparsely vegetated communities. In addition,
the degree of confidence associated with upland types in the
classification is generally higher than for wetland types. The
classifications for communities that occur as complexes or in zones are
also in need of further work.

[^1]: . Structural uniformity is assessed by evaluating all layers of
    the vegetation, not just the canopy. Floristic homogeneity is
    assessed by evaluating the general uniformity and consistency in
    species composition, especially with respect to the dominants
    (Mueller-Dombois and Ellenberg 1974).