Relationship of NH Natural Communities to Other Classification Systems
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
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.