Natural communities are recurring assemblages of plants and animals found in particular physical environments. New Hampshire has a fascinating and complex variety of natural communities, from tidal marshes to alpine meadows, riverbanks to mountain slopes, streams to lakes. Each type of natural community has a unique set of environmental conditions that support certain species that have adapted to those conditions. For example, a rich hardwood forest typically has a canopy of sugar maple and white ash underlain by dutchman’s breeches, blue cohosh, and certain other plants, animals, and microbes. This natural community occurs on moist soils enriched with nutrients, and many of the species present grow only under these conditions. Pitch pine/scrub oak barrens, in contrast, develop on extremely dry sand and gravel deposits, and are characterized by drought- and fire-resistant pitch pine, scrub oak, low-bush blueberries, and a variety of rare moths and butterflies that feed on these plants.

The NH Natural Heritage Bureau (NH Heritage) has been developing a classification of New Hampshire’s natural community types since 1986. This effort is an outgrowth of natural community classification work undertaken in the northeastern United States by The Nature Conservancy, and ties into similar classifications under development by state natural heritage programs across the country.

Each natural community type is distinguished by three characteristics:

1. a definite plant species composition;

2. a consistent physical structure (such as forest, shrubland, or grassland); and

3. a specific set of physical conditions (such as different combinations of nutrients, drainage, and climate conditions).

Natural communities include both wetland types (e.g., red maple basin swamp) and uplands such as woodlands (e.g., rich red oak-sugar maple/ironwood talus forest/woodland) and forests (e.g., hemlock - beech - oak - pine forest). Some types are rather broadly defined and serve as catchall categories, while others are more precise. A broadly defined community type, such as "shrub swamp," is likely to have greater differences in species composition from one example to another than those in narrowly defined types, such as red pine forests. Further, boundaries between natural community types can be either discrete (and therefore easily identified in the field) or gradual (thus making some areas difficult to map).

Natural community types are usually defined in terms of plants because they are easy to study, often compose the physical structure to which most other organisms respond, and are sensitive indicators of physical and biological factors that influence many types of organisms. Since plant assemblages often correspond closely to other groups of organisms, they can be used as coarse filters that include many of the species and processes in a naturally community, even if they have not been specifically identified.

Classifying natural communities enables ecologists, land managers, and others to communicate effectively and to make management decisions regarding ecological systems. Many classifications exist that define vegetation or other land units. The classification of natural communities in New Hampshire is based on data from more than ten years of ecological research by ecologists with NH Heritage and The Nature Conservancy, plus extensive reviews of scientific literature. These data have been compiled and arranged into natural community types in part through the use of ordination and other statistical methods. Most state natural heritage programs continually update their classifications and cooperate with The Nature Conservancy's regional and national ecologists to define natural community types that are comparable across state lines.

The names of natural community types generally begin with the dominant or most characteristic plant species, and may include the name of a landscape feature or vegetative structure that is typical of that community type. For example, black gum - red maple basin swamp refers to a basin swamp (a specific landscape feature, as opposed to a streamside swamp) with black gum and red maple in the canopy. In addition, like all Society of American Foresters forest cover types, forested natural communities may have considerable overlapping species and other characteristics, but they contain distinct and diagnostic combinations of species and physical characteristics. For example, the red spruce-northern hardwood natural community has considerably more red spruce in the overstory, and is generally higher in elevation, than the standard northern hardwood forest (sugar maple - beech - yellow birch forest natural community) despite many species that occur in both.

Exemplary Natural Communities

Exemplary natural communities include all examples of rare types (such as a rich mesic forest) and high-quality examples of common types. High-quality natural communities have greater potential to contain or achieve natural dynamics that are characteristic of the original community types and are useful for recognizing areas of high biodiversity significance today. A forested natural community need not be "old growth" to obtain exemplary status. Typical exemplary forested natural communities have a variety of characteristic species, natural regeneration within forested gaps, multiple age classes, diverse structural characteristics abundant standing and fallen woody debris, intact soil processes, and little direct evidence of human disturbance. Exemplary natural communities represent the best remaining examples of New Hampshire's flora, fauna, and underlying ecological processes.

Damage caused by natural disturbances, such as the 1998 ice storm, do not preclude any natural community from being designated exemplary. Ice storms, blow-downs, and fires are among the suite of natural processes influencing natural community dynamics. We take their effects into account when assessing natural communities, but if the community displays appropriate attributes, including minimal human influence, then we are likely to classify it as exemplary.

Relationship of Natural Communities to Other Classifications

In the following paragraphs, several other classification systems are contrasted with the natural community classification used by NH Heritage.

At a national level, The Nature Conservancy has published a National Vegetation Classification System (Grossman et al. 1998; Anderson et al. 1998) that uses a formal classification hierarchy emphasizing differences in both vegetation structure and floristics. This system is periodically updated to include new information from more specific natural community classifications developed at the state level, such as the New Hampshire natural community classification. The Federal Geographic Data Committee has adopted a vegetation classification standard derived from the National Vegetation Classification for use by federal agencies, and future development of the classification is expected to be a collaborative effort (Grossman et al. 1998). This system cross-references classifications produced and maintained by all state Heritage programs.

While natural community names can be similar to the names of Society of American Foresters (SAF) forest cover types (Eyre 1980), natural communities are defined using a broader range of considerations. SAF forest cover types are primarily based on dominant tree species, while natural communities are based on all species, the structure of these species, and the specific physical environment. Trees are often subtle indicators of their environments. A number of natural communities can be distinguished based largely on trees, and in some cases differences in tree composition are the main difference between two community types. However, some trees are so broadly adapted that their presence does not precisely indicate site conditions (e.g., white pine or red maple). Differences in tree canopy composition may also primarily relate to cutting or other disturbances.

For example, there are four SAF spruce-fir cover types that correspond to the "montane spruce-fir forest" natural community type. These different cover types primarily relate to stand disturbance history or the successional stage rather than to major environmental differences. The four cover types also do not differentiate between upland spruce-fir forests and spruce-fir swamps. When one considers understory species and soils, upland spruce-fir forests are markedly different from the red spruce/Sphagnum basin swamp natural community. In fact, the differences between these two natural communities are more dramatic than the internal differences between the four SAF spruce-fir cover types. SAF cover types are, however, useful for timber management.

Natural community types and the U.S. Forest Service’s Ecological Land Types (ELTs), which to date have been defined only for National Forest lands, are not easily comparable for three primary reasons. First, ELTs are mapped at units of 100 or more acres, so some natural communities occur as smaller patches within various ELT types. Second, ELTs do not reflect major differences in soil nutrient status while natural communities do. Third, ELTs describe fine-scale soil characteristics that may have silvicultural significance but sometimes have no corresponding floristic expression.

A classification scheme frequently used in wetland and aquatic systems was produced by Cowardin et al. (1979) for the U.S. Fish and Wildlife Service (USFWS). In the USFWS system, wetlands and deepwater habitats are defined by their vegetation, substrate, and frequency of flooding in a hierarchy that emphasizes flooding regimes and attributes of vegetation at a coarse scale (e.g., vegetation structure, life form, persistence, etc.). This classification system is useful because of its applicability to broad geographic regions and because it can be readily applied in conjunction with aerial photograph interpretation. It was the basis for wetland typing in the National Wetland Inventory mapping effort.

Natural community types can typically nest within the hierarchical structure of the USFWS system. In addition to the flooding regimes and coarse vegetation characteristics used to distinguish USFWS types, however, NH Heritage's natural community classification also considers factors such as nutrient regime, water source, and geomorphic setting, as indicated by specific differences in floristic composition. For example, under the USFWS system, red maple/Sphagnum saturated basin swamps and red maple-black ash/swamp saxifrage seepage swamps would both be considered saturated, palustrine broad-leaved deciduous forested wetlands. This grouping does not reflect important differences between the two communities, including differences in species composition (ground cover by Sphagnum versus forb species), nutrient levels (species indicative of nutrient-poor versus minerotrophic conditions), water sources (upland runoff versus groundwater seepage), geomorphic settings (basin depression versus headwater seepage area), and soils (deep peat versus shallow peat over silt). The natural community classification provides additional detail regarding ecological conditions and processes that helps clarify the distribution of biological diversity across the landscape.