Site Quality Evaluation and Classification

Site quality is the inherent biological capacity of an area to be productive. Site quality can influence: species composition (through competition), response to disturbances, the ease or difficulty of getting desired regeneration, cultural practices, and ultimately, the growth and yield of a stand. Therefore, the success or failure of forest management depends, to a large extent, on correct evaluation of forest sites or sound judgment in fitting species to sites (Smith 1962).  For these reasons, foresters need to understand the relationships between forests and their physiographic environment, particularly in the southern Appalachians, where there are strong localized relationships between site quality and species composition.

Southern Appalachia is extremely heterogeneous. Soil, topography, climate, and biotic factors interact to produce a multitude of forest sites. Superimposed on this complex physiography is a long and complex land-use history. This variation poses a great challenge to land managers seeking to characterize the land (Van Lear 1990). Despite these difficulties, or perhaps as a result of them, several land classification systems have been developed in the southern Appalachians.

Forest site classification had its origins near the turn of the 20th century, shortly after forestry began in the United States. The early history of site classification dealt with finding an appropriate expression of site quality, because it was generally recognized that productivity was a basic criterion for delineating sites. Despite its limitations, site index, which uses tree growth as a measure of site quality, has been and remains the most commonly used measure of site quality. Decades of soil-site studies established the rather obvious fact that site quality was related to soil and site variables that affect water and nutrient availability. Compiling factors related to site quality, while a necessary first step, did little to spatially delineate units of land with differing growth potential. Early on, foresters attempted to use general purpose Soil Conservation Service (SCS) soil survey maps to delineate sites of different quality. However, this approach proved unsuccessful, because site index varied widely within mapping units and responses to management often did not coincide with soil series (Van Lear 1991).  It should be noted that site index was developed and continues to be used primarily in reference to wood productivity and thereby indirectly to economics. 

The importance of landform in classifying forest sites was not widely recognized in the South until the late 1970s. Landform naturally integrates climatic, hydrologic, soil, and vegetative variables. Most importantly, landforms, such as coves and ridges, are readily mapped. Multifactor physiographic classification systems, which separate landscape components on the basis of geology, topography, and soils into visually identifiable landtypes have greatly increased understanding of forest-site relationships (Van Lear 1991).

Ecological classification is similar to multifactor physiographic classification except it places greater emphasis on vegetation. Late successional or near-climax plant communities are identifes as are the seral communities that preceed them. Such a system provides a broader ecological basis for interpreting patterns and processes of ecosystems. Classification systems that integrate the major ecosystem components of landforms, soils, and vegetation provide a relatively sound basis upon which to make long-term decisions that make both ecologic and economic sense. However, as Van Lear (1991) notes: "Neither landscape ecosystem classification, nor any other system, is a panacea that will solve all land classification problems" (Van Lear 1991).