The LANDFIRE Environmental Site Potential (ESP) layer represents the vegetation that could be supported at a given site based on the biophysical environment. ESP is used in LANDFIRE to inform the existing vegetation and fuel mapping processes. Map units are based on NatureServe's Ecological Systems classification, which is a nationally consistent set of mid-scale ecological units (Comer and others 2003). LANDFIRE's use of these classification units to describe environmental site potential differs from their intended use as units of existing vegetation. As used in LANDFIRE, map unit names represent the natural plant communities that would become established at late or climax stages of successional development in the absence of disturbance. They reflect the current climate and physical environment, as well as the competitive potential of native plant species. The LANDFIRE ESP concept is similar to that used in classifications of potential vegetation, including habitat types (Daubenmire 1968; Pfister and others 1977) and plant associations (Henderson and others 1989).