These classification systems share a geographically based hierarchical system that begins with large regions broadly similar in climate and landform that are further subdivided into progressively smaller mapped units nested within. An essential concept of the hierarchical approach is that the ecological factors defining a larger scale unit (like climate) also control the ecological potential of units nested below.

Natural Resources Conservation Service System

The Natural Resources Conservation Service (NRCS) is the national pioneer in developing a classification and mapping system. The largest scale units are Land Resource Regions (LRR). There are 20 LRRs in the continental US; Montana includes three of these: Northern Great Plains, Western Great Plains, and the Rocky Mountains. Within each LRR are a few Major Land Resource Areas (MLRA), mapped and described here. Land Resource Units (LRU) form the next subdivision followed by Statsgo units that are groupings of similar soils; there are about 700 in Montana. Definitions and a scale hierarchy for NRCS types are available as is detailed Montana soil information.

Soil series are the most commonly detailed level of information but these are not mapped for much of Montana, more common are soil map units that contain up to three soil series in a repeating pattern too complex to map individually. Ecological sites (or ecosites) correspond to the earlier range site units and are based on broad textural, chemical, and slope factors; each soil series is assigned to an ecosite. Ecosites are a nested level within precipitation zones and MLRAs. Since soil map units can be composed of a few soil series, there may also be a few different ecosites within a map unit.

Forest Service Ecological Classification System

The Forest Service has led the effort in developing the National Hierarchy of Ecological Units. Regional and subregional descriptions and delineations exist across the United States. Several states have also implemented landscape scale classifications statewide. Site level information is typically only available within Forest Service boundaries.

The eight possible levels within the hierarchy each have a distinct scale; unit delineations within the levels are based on the most important ecological factors operating at that scale. At the largest ecoregional scale (Bailey 1995), units have a size of thousands or millions of square miles and are dominated by broad patterns in climate and landform. Subregional scale ecological sections (McNab and Avers 1994) and subsections (Nesser et al. 1997) are based on mid-scale patterns of climate, geology, and landform. The smallest landscape scale units like landtype associations are only measured in 10’s to a few thousands of acres and have quite uniform patterns of soils and vegetation. These have only been described in Montana for Forest Service land.

Environmental Protection Agency

James Omernik and the EPA have developed a regional and subregional scale classification and maps for the United States. Their approach is similar to other efforts in integrating climate, geology, landform, soils, and the living ecosystem to create map units that describe similar types in a four level nested hierarchy. There are 15 Level I units that are subdivided into 52 Level II and 104 Level III delineations. A recent collaborative effort for Montana has resulted in a detailed map and description of Level IV units statewide.

Classification Units and Scale

Other Land Classification and Mapping Systems

The US Geological Service (USGS) maintains a National Land Cover Dataset (NLCD) that includes land mapping using 21 categories derived from late 1980’s to early 1990’s data for Montana. The Gap analysis project also created a 1990’s era map of Montana vegetation using 50 types; this was followed with several other regional product enhancements. Wetlands have been classified and mapped throughout the country by the National Wetlands Inventory (NWI), however Montana coverage is incomplete.