Written by Brad Schultz, Extension Educator, Winnemucca, Nevada
Currently, the most widely accepted classification system for rangelands is the Ecological Site.
|A sagebrush grassland vegetation community. Photo courtesy of Rachel Frost.|
An ecological site is a unique, identifiable, and repeatable patch of vegetation and soil on a landscape. Each ecological site is the product of the environmental factors that influence the development of the soil and vegetation, including disturbance regimes. An ecological site has specific homogeneous biological, physical, and chemical characteristics and has the potential to produce a distinct mix of plant species with similar amounts of annual biomass of vegetation. The basic premise is that a specific soil type based on depth, texture, horizonation, water-holding capacity, pH, salinity, and other factors is inhabited by specific plant species. The soil, therefore, is the foundation of an ecological site. If the foundation undergoes a dramatic change, the structure above will have a corresponding change.
On rangelands, ecological sites form the basic classification unit for categorizing different plant communities and their associated soils. A key concept for an ecological site is the potential natural community (PNC). The PNC is a complex concept that includes the typical disturbance regime that affects the ecological site. Many rangeland plant communities endure a periodic stand-replacing disturbance — for example, fire — that does not dramatically alter the soil but decreases or removes some plant species and lets others immediately increase. The first plant community that establishes after a stand-replacing disturbance usually undergoes vegetation change over time. If the evolved disturbance regime has an average of 50 years between events the community that typically is present after 40 to 50 years of succession is considered the potential natural community. Vegetation on an ecological site, therefore, is not static, and several community phases of the PNC may develop and eventually succeed one another. Rangeland scientists have developed a conceptual approach called state and transition models to help describe changes in community composition, vegetation structure, and ecological function with regard to management actions and environmental condition.
The biological, physical, or chemical conditions that create an ecological site can also change. When their change is sufficient to add, remove, or change the intensity, frequency, or duration of the ecological processes — for example, plant competition, hydrology, disturbance regime — that maintain an ecological site, the site can degrade to a different ecological site.
Each ecological site has defined components that are described by range management specialists and soil scientists. The complete assemblage of these components into one document forms an ecological site description (ESD). Ecological site descriptions are intended to be clear descriptions of the features that characterize the site, making it unique and different from other ecological sites.
The format for ecological site descriptions changes with time because research and management regularly create new knowledge about the environmental variables and ecological processes that affect the formation of distinct soils and their associated PNC. Important ecological and environmental components of updated ESDs are: 1) the major land resource area (MLRA) in which the ecological site occurs, 2) physiographic features, 3) climatic features, 4) influencing water features, 5) representative soil features, and 6) plant communities. Except for the MLRA, the ESD will provide substantial detail for each parameter. The intent of an ESD is to provide landowners and managers with the environmental and ecological information they need to develop management plans, management goals, and management actions for the rangelands they manage. The content of an ESD provides the end users much of the information they need to understand the potential productive capability of their rangeland, the constraints that limit management options, and potential hazards that may occur.
ESDs are developed and housed by the USDA Natural Resources Conservation Service. Recently updated versions are often available on the Internet. Older versions, especially those still in previous formats, usually are not available electronically but can be obtained from state and field level offices of the Natural Resources Conservation Service. More detail about ESDs can be found in Chapter 3 of the NRCS National Range and Pasture Handbook.
Ecological site descriptions are used to make decisions about how to manage rangelands. Data and information about ecological sites are combined with other resource information to improve management decisions. One of the first steps is to identify and map the soils and ecological sites on a heterogeneous landscape. Once the soils and ecological sites are mapped, managers can assess the vegetative composition and annual production of the seral communities relative to the potential plant communities that could occupy the site.
Ecological sites are distinct units of the soil-vegetation complex, but many management questions and issues occur across landscapes occupied by several ecological sites. Interrelationships among ecological sites can be analyzed at different spatial scales and the results applied at the scale appropriate for the management questions and issues being addressed. Boltz and Peacock (2002) describe six uses of ecological site descriptions: 1) describing the interactions among soils, vegetation, and land management; 2) a foundation to assess the condition of current resources and monitor changes; 3) a framework to assess management opportunities and predict the outcome of management decisions; 4) a framework for identification of knowledge gaps in vegetation dynamics; 5) a common framework for communication of resource information among disciplines, agencies, and organizations; and 6) a framework for transferring experience and knowledge.
Boltz, S. and G. Peacock. 2002. Ecological Sites: Understanding the Landscape. Rangelands 24(5):18-21.
Feature photo: USDAgov / Foter / Creative Commons Attribution 2.0 Generic (CC BY 2.0)