Plant Communities

Plant communities are groups of plants sharing a common environment that interact with each other, animal populations, and the physical environment.

Certain plant communities often occur together on the landscape due to shared environmental requirements.

This provides a way to organize biological information -- creating mappable units for land management and conservation planning.

Communities are often defined by dominant plant species, which provide useful habitat information for many animal species.

This provides an efficient starting point for biological surveys.

As the jurisdictional agency for plants in Pennsylvania, DCNR has developed a plant community classification for Pennsylvania through its partnership with the Pennsylvania Natural Heritage Program.

Plant Community Classification

An important role of the plant community classification is to define and standardize the concept for each plant community type.

Plant community descriptions provide information about:

• The common and rare species found within them
• Typical species composition
• Physical appearance and structure of vegetation
• The physical environment with which they are associated

These standardized types provide a common framework for ecologists, foresters, environmental planners, and others to use in a variety of ways, including:

• Vegetation mapping
• Ecological restoration
• Environmental planning
• Conservation

DCNR’s land managers use the Pennsylvania Natural Heritage Program’s Terrestrial and Palustrine Plant Communities of Pennsylvania: 2nd Edition (PDF) for describing forest and plant community types on state forest lands and park lands.

Pennsylvania currently has 127 documented plant communities, including 78 wetland and 49 terrestrial community types.

These community types can be organized or classified in several ways.

Classifying by Physical Appearance

One way is by “physiognomy,” which groups types by the physical appearance of the plant community, such as forest, open woodland, shrub land, or herbaceous (dominated by grasses and non-woody plants).

Forest and woodland types also can be further split by whether the dominant tree species are conifers (pines, hemlock, or spruces) or broadleaf deciduous (maples, oaks, birches, etc.).

Classifying by Environmental Conditions

Another way to classify communities is by “ecological groups,” which groups together plant communities that tend to occur in similar environmental conditions.

For example, while there are 78 different wetland plant community types in Pennsylvania, only five different types regularly occur in vernal pools and a completely different set of 13 plant communities are associated with peatland wetlands (bogs and fens).

Distribution and Extent

Plant communities vary in their distribution and extent across the Commonwealth.

Some plant communities are common and widespread across different regions.

For example, the Northern Hardwood Forest community is the most common type across the northern tier of Pennsylvania, while the Dry Oak -- Heath and Dry Oak -- Mixed Hardwood Forests are common across much of the Ridge and Valley and Pocono Plateau Regions.

Other plant communities are relatively uncommon and restricted to very specific habitats.

The Serpentine Grassland is limited to a handful of sites in the southeast corner of the state where serpentine bedrock is at or near the surface.

The unique chemistry of serpentine bedrock (high in magnesium and heavy metals such as nickel and chromium) result in a unique mix of serpentine-adapted species not seen anyway else in Pennsylvania.

Pennsylvania Plant Community Classification

The Pennsylvania plant community classification has been evolving over the past 35 years as the Pennsylvania Natural Heritage Program continues to survey and collect data across the Commonwealth.

In 2012, the wetland portion of the classification was updated and expanded, while the terrestrial portion of the classification is under review and revision.

The Pennsylvania classification includes crosswalks to the National Vegetation Classification, which allows you to see where the same or similar plant communities occur in other states.