Site Classification
Within any biogeoclimatic subzone or variant, a recurring pattern of sites occurs, reflecting the variety of site features and soil characteristics that occur across the landscape. These ecosystems are described by the site classification of BEC. Three units are formally recognized in the BEC site classification: site association, site series, and site type. Two other units, site phase and site variation, are not formally included in the classification, but may be used to further subdivide site series.
Site association is the basic unit of site classification, but site series is the unit most commonly used by operational field staff.
A site association includes all ecosystems capable of developing vegetation belonging to the same plant association (or, in some cases, subassociation) at the climax or near-climax stage of vegetation development. In other words, a site association is a group of related ecosystems physically and biologically similar enough that they have or would have similar vegetation at maturity.
Since a site association can contain ecosystems from several climates, it can be somewhat variable in its environmental conditions. Therefore, a site association is divided into site series within subzones and variants. Although a site association occurs on ecologically equivalent sites, the site series in it may occupy different positions on the scale of relative moisture and nutrients in different biogeoclimatic subzones or variants. For example, a site association that occupies sites that are drier relative to others in a wetter variant, may be found in areas that are wetter relative to other sites in a drier variant.
Site series, then, are subdivisions of site associations and include all sites within a biogeoclimatic subzone or variant that are capable of producing the same mature or climax vegetation unit (plant association). Site series are described in the Regional Field Guides to Site Identification. Site and soil conditions, and the vegetation community, are used to identify site series.
Site types are subdivisions of site series that are distinguished by edaphic differences that are considered significant to the management of the site. Site types reflect the compensating effects of various site conditions within a uniform climate which, in different combinations, produce similar vegetation. For example, slope position and soil texture can combine in various ways to produce the same soil moisture regime. Of all the ecosystem units, they are uniform in the largest number of environmental characteristics, however, few site types have been defined.
The site phase can be used to subdivide site series or site types. Site phases could be based on soil particle size classes, slope classes, aspect, parent materials, soil climate, or bedrock geology. For example, sites with shallow soils over bedrock and with coarse gravelly soils may have the same climax or mature vegetation unit, but require very different management approaches. Use of the phase also allows more consistent prediction of ecosystem response to management treatments.
In some cases, the site variation is used to describe vegetative trends or floristic features that diverge from the central concept of the plant association. Usually such variation is related to short-term successional factors and it involves recent stand history. Variations could be recognized on the basis of stocking, species composition of the tree stratum, understory structure and composition, etc.
Site units provide the "local" level of ecological integration.