Title: Classification of Vegetation Communities of Maryland

Classification of Vegetation Communities of Maryland: First Iteration

A Subset of the International Classification of Ecological
Communities: Terrestrial Vegetation of the United States
March 2004

Background of the National Vegetation Classification

photo of Hudsonian CarpetThe purpose of the classification system is to provide a complete standardized listing and description of all vegetation types that represent the variation in biological diversity at the community level, and to identify those communities that require protection (Grossman et al. 1994).  The classification system focuses on existing vegetation rather than potential natural vegetation, “climax vegetation” or physical habitats.  The vegetation types described in the classification range from the ephemeral to the stable and persistent. Recognizing and accommodating this variation is fundamental in protecting biological diversity.  A classification that is not restricted to static vegetation types ensures that the units are useful both for inventory/site description, and as the basis for building dynamic ecological models.  The classification will be consistent throughout the United States and elsewhere at appropriate scales for conservation planning, the management and long-term monitoring of ecological communities and ecosystems, and will have applications as a vegetation data layer in landscape and ecosystem characterization.

The national vegetation classification includes all existing vegetation, whether natural or cultural, but the Maryland Natural Heritage Program has emphasized vegetation types that are considered "natural" since they are the focus of biodiversity conservation. Therefore, this document is limited to types classified as natural and semi-natural vegetation. All natural vegetation types occur spontaneously without regular human management, maintenance, or planting, and generally have a strong component of native species. Natural types include a range of naturalness, namely, "natural (narrowly defined)," "semi-natural" and "modified" vegetation, which together reflect differences in the natural and anthropogenic disturbance regimes, but all types have a strong component of native species (see below). Natural vegetation, narrowly defined, includes plant communities that appear not to have been modified by human activities. The term semi-natural can include "plant communities where the structure of vegetation has been changed through human activities, but where the species composition is natural" (van der Maarel and Klötzli 1996), though some authors would restrict it to "vegetation in which the dominant or constant species are undoubtedly native species and the structure of the vegetation conforms to the structure of presumed natural vegetation" (Birks 1996). The first definition allows for more deviation from a strictly natural condition. Regardless, the use of the term "natural" in this classification is broadly inclusive. Apart from this broad distinction, issues of naturalness are dealt with through a quality ranking process of actual community occurrences rather than through classification concepts.

The National Vegetation Classification is a combination of physiognomic and florisitic systems. It has been developed for terrestrial vegetation; that is, all upland terrestrial vegetation and all wetland vegetation with rooted vascular plants. In relation to Cowardin et al. (1979), terrestrial as defined here includes those portions of the palustrine, lacustrine, riverine, estuarine, and marine systems that have rooted vegetation. Classification of this vegetation (i.e. the Terrestrial System) is distinct from that of unvegetated deep-water habitats (Freshwater and Marine Systems) and unvegetated subterranean habitats (Subterranean System), all of which will have their own classification systems (e.g. Lammert et al. 1997). The rationale for coupling physiognomic and floristic systems has developed over many years (e.g., Rubel 1930, Whitaker 1962, Ellenberg 1963, Webb et al. 1970, Westhoff 1967, Beard 1973, Werger and Spangers 1982, Borhidi 1991). These studies have found a good correlation between floristics and physiognomic classifications of the same vegetation. In the United States, Driscoll et al. (1984) recommended the development of a joint system using the physiognomic units of UNESCO (1973) and the floristic units of habitat types, of which an example has been provided by Dick-Peddie (1993) for New Mexico. Vankat (1990) developed a physiognomic-dominance type classification for forest types in North America. Strong et al. (1990) in Canada also proposed a combined physiognomic-floristic approach.

The combined physiognomic/floristic system used here allows identification of units from both a "top-down" (divisive) and "bottom-up" (agglomerative) approach. The top-down approach allows the use of physiographic distinctions to help map vegetation, to stratify sampling, and to delimit vegetation units where floristic information is lacking. A bottom-up approach employs plot sampling and floristic analysis as the primary means for defining associations. Where physiognomy is variable, the bottom-up approach can also be used to help to determine the important physiognomic distinctions. The relationships between physiognomy and floristics are not always simple; when they do not correspond, precedent may be given to the floristic relationships over the physiognomic structure. The basic unit of inventory, the plant association or community element, is uniform in structure, composition, and habitat. The uniformity of the plant community makes the comparison and identification of protection priorities more objective than would be possible at more heterogeneous scales. The plant association is a suitable unit for conservation planning because it encompasses all the layers of vegetation in a stand, reflects ecological and human-caused processes including management activities, and is a repeating unit in different landscapes. From a site-based perspective, there may be many different community types at a given location. In fact, it is relatively rare that a site contains only a single community type. However, community elements tend to combine in predictable ways to create repeatable landscape mosaics. Thus, the particular mosaic of community elements present at a site and their distribution across the landscape provide information that is fundamental to any type of ecological land management.



The top division of the classification hierarchy separates vegetated communities (Terrestrial System) from those of unvegetated deepwater habitats (Aquatic System) and unvegetated subterranean habitats (Subterranean System). The Terrestrial System of the national hierarchy is broadly defined and includes vegetation of uplands as well as emergent and rooted submerged vegetation of lakes, ponds, rivers, and marine shorelines. It includes, then, the portions of the Cowardin et al. (1979) palustrine, lacustrine, riverine, estuarine, and marine systems that have rooted vegetation. Communities of the Aquatic System lack rooted vegetation and are generally dominated by fish or invertebrates or floating vegetation. The Aquatic System includes non-vegetated (faunal) and vegetated communities and 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 dominated by fauna. There are different hierarchical divisions below each of the three Systems. The hierarchy for the Terrestrial System is now complete (Grossman et al. 1998). It has seven levels: the five highest levels are physiognomic (physiognomic class, physiognomic subclass, formation group, formation subgroup, and formation) and the two lowest levels are floristic (alliance and community association). The hierarchical levels of the Aquatic and Subterranean Systems are in development. The levels of the classification system below the System level are listed in Figure 1 below, and described in the following sections (see also Table 1).

FIGURE 1. Vegetation Classification System  









Physiognomic levels


Floristic levels




The physiognomic portion of the Conservancy’s 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 used physiognomy (outward appearance) and structure of the vegetation to define the units. It was intended to provide a comprehensive framework for the preparation of vegetation maps at a scale of 1:1,000,000 or smaller. The system was designed to include all natural and semi-natural vegetation, and excluded modified or “cultural” vegetation (wheat fields, vineyards, etc.). The UNESCO hierarchy is fairly complex, and little information is provided that explains the criteria used to define each of the four hierarchical levels. The same criteria are sometimes used at different levels to define the units. The Conservancy modified the UNESCO system to improve the consistency and clarity of the classification hierarchy while maintaining its ecological meaning, and to make it applicable for classification and mapping at scales of 1:24,000 or larger. To improve the classification of wetland formations, the Conservancy further modified the UNESCO system by including more explicit hydrologic modifiers at the formation level. The hydrologic modifiers introduced by Cowardin et al. (1979) were chosen since these have been used extensively to map wetlands across the United States. However, these also were modified before inclusion into the hierarchy.

Physiognomic class
The physiognomic class is based on the structure (see Figure 2) of the vegetation. This is determined by the height and relative percentage of cover of the dominant life-forms: tree, shrub, dwarf-shrub, herbaceous and nonvascular. This level has seven mutually exclusive classes: forest, woodland, shrubland, dwarf-shrubland, herbaceous vegetation, nonvascular vegetation, and sparse vegetation.

Figure 2. Physiognomic Class Structure


Trees with their crowns overlapping (generally forming 60 percent to 100 percent cover)


Open stands of trees with crowns not usually touching (generally forming 25 percent to 60 percent cover)


Shrubs generally greater than 0.5 meters tall with individuals or clumps overlapping to not touching (generally forming greater than 25 percent cover). Vegetation dominated by woody vines is generally treated in this class.


Low-growing shrubs, usually less than 0.5 meters tall, with individuals or clumps to not touching (generally forming greater than 25 percent cover, with trees and tall shrubs generally forming less than 25 percent cover)


Herbaceous plants dominant (generally forming at least 25 percent cover, with trees, shrubs, and dwarf-shrubs generally forming less than 25 percent cover)


Nonvascular cover (bryophytes, non-crustose lichens, and algae) dominant (generally forming at least 25 percent cover)

Sparse Vegetation

Abiotic substrate features dominant. Vegetation is scattered to nearly absent and generally restricted to areas of concentrated resources (total vegetation typically forming less than 25 percent cover)

Physiognomic subclass
The physiognomic subclass is determined by the predominant leaf phenology of classes defined by a tree, shrub or dwarf-shrub stratum (evergreen, deciduous, mixed evergreen-deciduous), the persistence and growth form of herbaceous and nonvascular vegetation, and particle size of the substrate for sparse vegetation (e.g., consolidated rocks, gravel/cobble). Examples include: Evergreen forest, Deciduous forest, Deciduous shrubland, Perennial graminoid vegetation, Consolidated rock sparse vegetation.

The group (or formation group) generally represents a grouping of vegetation units based on leaf characters, such as broad-leaf, needle-leaf, microphyllous, and xeromorphic. These units are identified and named with broadly defined macroclimatic types to provide a structural-geographic orientation, but the ecological climate terms do not define the groups per se. Examples include: Temperate or subpolar needle-leaved evergreen forest, Cold-deciduous forest, Cold-deciduous shrubland, Temperate or subpolar grassland, Sparsely vegetated cliffs.

The subgroup (or formation subgroup) represents a distinction between natural vegetation, including natural, semi-natural and some modified vegetation, and cultural vegetation (planted/cultivated). Each formation is divided into either a natural subformation group or a cultural subformation group. The classification presented here only includes planted/cultivated subgroups in some (forested) classes. Examples include: Natural temperate and subpolar needle-leaved evergreen forest; Cultural temperate and subpolar needle-leaved evergreen forest (e.g., pine and spruce plantations)


The formation represents a grouping of community types that share a definite physiognomy or structure and broadly defined environmental factors, such as elevation and hydrologic regime. Structural factors such as crown shape and lifeform of the dominant lower stratum are used in addition to the physiognomic characters already specified at the higher levels. The hydrologic regime modifiers were adapted from Cowardin et al. (1979; see Appendix IV), and are somewhat more explicit in defining vegetation units. With or without an organized hierarchy as presented here, the formation is a widely used vegetation concept (Whittaker 1962, Schrader-Frechette and McCoy 1993). Examples include: Rounded-crowned temperate or subpolar needle-leaved evergreen forest, Seasonally flooded cold-deciduous forest, Semipermanently flooded cold-deciduous shrubland, Tall sod temperate grassland, Cliffs with sparse vascular vegetation.


The alliance is a physiognomically uniform group of plant associations (see below) sharing one or more diagnostic species (dominant, differential, indicator or character), which, as a rule, are found in the dominant and/or uppermost strata of the vegetation (see Mueller-Dombois and Ellenberg 1974). The alliance level includes existing (not just "climax" or potential) vegetation types.

The concept of an alliance is similar to a "cover type." An alliance is equivalent to a cover type when the dominant species also have diagnostic value. The alliance may be finer than a cover type when the dominant species extend over large geographic areas and varied environmental conditions especially when a diagnostic species occurs in different climate zones or in both upland and wetland situation. The concept for the alliance is also similar to the concept of 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 include:


The association (or plant association) is the finest level of the classification system. For the terrestrial system, plant association is defined as "a plant community of definite floristic composition, presenting a uniform physiognomy, and growing in uniform habitat conditions" (Flahault and Schroter 1910). This basic concept has been used by most schools of vegetation classification (Whittaker 1962, Braun-Blanquet 1965, Westhoff and van der Maarel 1978). In this traditional sense, the plant association concept applies to existing vegetation regardless of successional status. The concept of an association is also used in habitat type methodology developed by Daubenmire (1952), but in that system it is restricted to climax or late seral vegetation (Pfister and Arno 1980).

The plant association 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 or environmental modifiers, and their hypothesized indicator species. The Conservancy will primarily use the plant association as the level at which community inventory and conservation action are aimed.

While this definition of a plant association is still generally accepted as an international standard, a few clarifications of the use of the definition for the classification system may be helpful:

  • "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 occur repeatedly over the natural 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, wooded dune and swale complex).

A few examples include:

  • Picea rubens - (Tsuga canadensis) / Rhododendron maximum Saturated Forest
  • Acer saccharum - Betula alleghaniensis - Prunus serotina Forest
  • Quercus (falcata, alba, velutina) / Gaylussacia baccata - Vaccinium pallidum Forest
  • Morella cerifera - Rosa palustris / Thelypteris palustris var. pubescens Shrubland
  • Hudsonia tomentosa / Panicum amarum var. amarulum Dwarf-shrubland
  • Calamagrostis canadensis - Phalaris arundinacea Herbaceous Vegetation
  • Cakile edentula ssp. edentula - Chamaesyce polygonifolia Sparse Vegetation

TABLE 1. Illustration of the Classification System Hierarchy.






Deciduous Forest

Evergreen Woodland

Evergreen Shrubland


Cold-deciduous Forest

Temperate or Subpolar Needle-leaved Evergreen Woodland

Temperate Broad-leaved Evergreen Shrubland






Lowland or Submontane Cold-deciduous Forest

Rounded-crowned temperate or Subpolar Needle-leaved Evergreen Woodland

Tidal Broad-leaved Evergreen Temperate Shrubland


Fagus grandifolia – Quercus rubra – Quercus alba
Forest Alliance

Pinus pungens – (Pinus rigida)
Woodland Alliance

Morella cerifera – Rosa palustris
Tidal Shrubland Alliance


Fagus grandifolia – Quercus alba – Liriodendron tulipifera – Carya spp.

Pinus (pungens, rigida) / Quercus ilicifolia / Gaylussacia baccata

Morella cerifera – Rosa palustris / Thelypteris palustris var. pubescens Tidal Shrubland