Background: Ecological sites are the basic component of a land-type classification system that describes ecological potential and ecosystem dynamics of land areas. All land/land use types are identified within the ecological site system, including rangeland, pasture, and forest land. An ecological site is defined as a distinctive kind of land with specific soil and physical characteristics that differ from other kinds of land in its ability to produce a distinctive kind and amount of vegetation and its ability to respond similarly to management actions and natural disturbances. Lands are classified considering discrete physical and biotic factors. Physical factors include soils, climate, hydrology, geology, and physiographic features. Biotic factors include plant species occurrence, plant community compositions, annual biomass production, wildlife-vegetation interactions, and other factors. Ecological dynamics, primarily disturbance regimes, such as grazing; fire; drought; management actions; and all resulting interactions are also a primary factor of ecological sites. Information and data pertaining to a particular ecological site is organized into a reference document known as an Ecological Site Description (ESD). ESDs function as a primary repository of ecological knowledge regarding an ecological site. ESDs are maintained on the NRCS Ecological Site Information System (ESIS), which is the repository for information associated with ESDs and the collection of all site data.
Forage Suitability Groups
Pasture land is addressed through interpretive reports know as Forage Suitability Group Descriptions (FSGDs). FSGDs provide a soil and plant science basis for conservation planning on livestock operations where forage crops are grown. They identify adapted forage species, yearly forage production estimates, and distribution of production during the growing season. FSGDs are composed of groups of soils and ecological sites having similar potentials and limitations for forage production. These reports can be accessed on the Ecological Site Information System (ESIS).
Soils and Ecological Sites
A fundamental concept of ecological sites is their direct linkage to soil types, specifically the soil map unit components of the National Cooperative Soil Survey; therefore ecological site occurrence on a given landscape area will be readily discernable and fixed. This allows for highly accurate mapping of ecological sites, as detailed soil mapping is readily available. Therefore, ecological sites can be accurately delineated even when displaying a varying range of plant communities resulting from past and current disturbances factors. Existing vegetation can provide a clue as to what ecological site occurs on a given area, but sites can only be definitively delineated by the linking of the soil map unit components.
State and Transition Models
State and transition models are the key component of ESDs, as they depict and organize information regarding the ecological dynamics of an ecological site. States are stable, long term ecological conditions that are produced on a site due to the interactions of the biotic, physical, and disturbance factors. States are composed of usually several plant community phases, which vary based on species composition and production. Expression of community phases can be, and often is, dynamic on a particular ecological site location due to the interaction of all ecological factors. This interaction of ecological factors resulting in different plant community phases is termed community pathways. Ecological sites will also display multiple states, with the change from one state to another being non-reversible without significant management inputs. A native shrubland state being converted to cropland and then converted to introduced pasture grasses is an example of three distinct states. Transitions are the drivers and mechanisms of changes between states and the ESD will describe how these function. Management actions, such as conservation practice implementation, grazing management, and other land use decisions are a significant part of the described state and transition model.