- Basic Concepts
- Vegetation Classification
- Climatic (zonal) Classification
- Site Classification
- Seral Classification
- Naming BEC Units
- Relationship between BEC & Ecoregion classification
1. Basic Concepts
The BEC system groups ecosystems at three levels of integration: regional, local, and chronological.
At the regional level, vegetation, soils, and topography are used to infer the regional climate and to identify geographic areas that have relatively uniform climate. These geographic areas are termed biogeoclimatic units.
At the local level, segments of the landscape are classified into site units that have relatively uniform vegetation, soils, and topography. Several site units are distributed within each biogeoclimatic unit, according to differences in topography, soils, and vegetation.
At the chronological level of integration, ecosystems are classified and organized according to site-specific chronosequences. To do this, the vegetation units recognized for a particular site unit are arranged according to site history and successional status.
In order to arrange ecosystems at the three levels of integration, the BEC system combines four classifications: vegetation, climatic (zonal), site, and seral. Vegetation classification is most important to developing the ecosystem classification. However, the climatic and site classifications are the principal classifications used in the application of the BEC system. At this time the seral classification has not been adequately developed.
2. Vegetation Classification
Vegetation of mature ecosystems is emphasized in Biogeoclimatic Ecosystem Classification as it is considered to be the best integrator of the combined influence of the environmental factors affecting a site. Vegetation units are determined by grouping plot data and then comparing the resulting units in a series of vegetation tables.
The outcome is a hierarchy of vegetation units with plant class being the most general. Classes are divided into plant orders, orders into plant alliances, alliances into plant associations and associations into plant subassociations. Plant associations are the basic unit of the vegetation classification hierarchy. Each unit is differentiated by a diagnostic combination of species. Tree species, or broad forest types, are emphasized at the upper levels of the hierarchy (classes/orders), and understory vegetation at the lower levels, however, floristic similarity is used to group units.
The vegetation classification is integral to development of the other classifications in BEC because vegetation is readily observed and described, and differences in vegetation reflect climatic, site and successional relationships. Plant associations and subassociations are important for determining biogeoclimatic subzones and variants (Climatic classification), and site associations (Site classification), respectively. Vegetation is used in the field to identify climatic or site units, but the vegetation hierarchy is in the "background".
3. Climatic (zonal) Classification
The climatic or zonal classification uses vegetation, soils, and topography to infer the regional climate of a geographic area. Areas of relatively uniform climate, are called biogeoclimatic units. As used here, climate refers to the regional climate that influences ecosystems over an extended period of time and can be expressed as statistics derived from normals of precipitation and temperature.
The geographic extent of biogeoclimatic units in the BEC system is inferred from the distribution of climax and late-seral plant communities on zonal sites. Zonal sites are those that best reflect the regional climate and are least influenced by the local topography and/or soil properties. They tend to have intermediate soil moisture and nutrient regimes, mid slope positions on gentle to moderate slopes, with moderately deep to deep soils and free drainage. Ecosystems that are influenced more strongly by local topography and physical and chemical properties of soil parent materials do not provide as clear a reflection of the regional climate.
Biogeoclimatic subzones are the basic unit of the climatic classification. Subzones are grouped into biogeoclimatic zones to create more generalized units, and subdivided into biogeoclimatic variants to create more specific or climatically homogeneous units. Biogeoclimatic subzones with similar climatic characteristics and vegetation on zonal sites are grouped into biogeoclimatic zones. In practice, this may be thought of as a geographic area within which the same vegetation unit (plant association) occurs on zonal sites. A subzone boundary occurs when a different plant association occurs on zonal sites.
A zone is a large geographic area with a broadly homogeneous macroclimate. A zone has characteristic webs of energy flow and nutrient cycling and typical patterns of vegetation and soil. We characterize zones as having a distinct zonal plant order; that is, the vegetation classification groups zonal plant associations in the category of plant order. Zones also have characteristic, prevailing soil-forming processes, and one or more typical, major, climax species of tree, shrub, herb, and/or moss.
Subzones may include significant climatic variation marked by small changes in the vegetation on zonal sites and differences in the vegetation on non-zonal sites. In these cases, the subzone may be subdivided into biogeoclimatic variants. Variants are generally recognized for areas that are slightly drier, wetter, snowier, warmer, or colder than that considered typical for the subzone. These climatic differences result in corresponding differences in vegetation, soil, and ecosystem productivity, although the changes in the vegetation are not sufficient to define a new plant association. The differences in vegetation are evident as a distinct climax plant subassociation (vegetation classification). Differences in vegetation are often changes in the proportion and vigour of certain plant species or variations in successional development or the overall sequence of vegetation over the landscape.
The biogeoclimatic phase accommodates the variation, resulting from local relief, in the regional climate of subzones and variants. Phases are useful in designating significant, extensive areas of ecosystems that are, for topographic or topoedaphic reasons, atypical for the regional climate. Examples are extensive areas of grassland occurring only on steep, south slopes in an otherwise forested subzone; enclaves of apparently coastal forest on moist, northeastern slopes in an interior, continental subzone; or valley-bottom, frost-pocket areas in mountainous terrain. The biogeoclimatic phase relates to local climate and hence is not a formal category in the classification, but phases can be identified and mapped for management or descriptive purposes.
The biogeoclimatic classification provides the "regional" level of ecological integration.
4. 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.
5. Seral Classification
Every year in B.C. forests are subjected to various disturbances including human activity (e.g., logging, land clearing, grazing), and natural agents (e.g., wildfire, wind, insects, disease). Disturbance changes the plant community, but, given enough time, the original plant community may recover. This process of change after disturbance is called succession and the different plant communities that occur during this process are seral stages.
The seral (often termed successional) classification in BEC is an integration of site and vegetation classifications with structural stage development. However, seral plant communities are poorly described in B.C due to a lack of sampling. Seral plant communities are highly variable, making successional pathways difficult to predict. Complex interactions among many factors, including disturbance type, severity, and frequency, vegetation present before disturbance, seed and bud banks on a site and sources of seeds in the vicinity, and weather following disturbance, all influence plant community development. Adding the factor of time into the equation greatly increases the number of plant communities that need to be described to develop an adequate seral classification.
Within the BEC system, seral plant communities are grouped into plant associations, just as mature communities, and may occur in several different variants or span several stages during succession. The concept of structural stages (Hamilton 1988; DEIF 1998) is used within the BEC system, as a framework for describing seral plant associations. This scheme uses seven stages, some of which are further divided to accommodate specific situations. See Chapter 1 (Site) in DEIF for detailed definitions of structural stages.
One site association can include a variety of disturbance-induced, or seral, ecosystems, but succession should ultimately result in similar plant communities at climax throughout the association. The use of plants from the climax plant association to name site associations does not mean that climax vegetation dominates the present landscape. Many ecosystems in the province reflect some form of disturbance and are in various stages of succession towards maturity. Site associations can be differentiated from one another by a range of environmental properties. These site properties are used to identify a site association in an early successional stage.
Seral units provide the "chronological" level of ecological integration.
6. Naming BEC Units
Biogeoclimatic Zones are usually named after one or more of the dominant climax species in zonal ecosystems (the Alpine Tundra Zone is a self-explanatory exception), and a geographic (e.g., coastal, interior) or climatic modifier (e.g., boreal, montane). Biogeoclimatic zone names are often referred to by a two- to four-letter acronym. For example, the Interior Cedar - Hemlock Zone is referred to as the ICH Zone and the Montane Spruce Zone is referred to as the MS Zone.
Subzone names are derived from classes of relative precipitation and temperature or continentality. The first part of the subzone name describes the relative precipitation and the second part describes either the relative temperature (Interior zones) or relative continentality (Coastal zones). For example, the ICHmc stands for the Moist Cold subzone of the Interior Cedar - Hemlock Zone. Subzone names are abbreviated as letter codes (Table 1).
TABLE 1 Subzone names and codes.
|FIRST PART:||Very dry||x|
Biogeoclimatic variants are given geographic names reflecting their relative location or distribution within the subzone. For example, the Interior Douglas-fir Dry Cool Subzone (IDFdk) has four variants: Thompson Variant, Cascade Variant, Fraser Variant, and Chilcotin Variant. Variant names are given number codes (e.g., ICHdk1), which in most cases reflect their geographic distribution within the subzone from south to north.
Forested site associations are named using one or two tree species, followed by one or two understory plant species present in the climax or late seral vegetation unit (plant association) on which they are based. While the species chosen for naming the site association are often abundant in the climax vegetation, less common but characteristic species are sometimes used to ensure that the site unit has a unique name within the provincial classification.
Site series names use the same names as the site associations to which they belong, preceded by the appropriate biogeoclimatic subzone or variant name (or code). Zonal site units are always numbered 01. Non-zonal forested site series are numbered from 02 to 29 sequentially in order of driest to wettest moisture regime and secondarily in order of poorest to richest nutrient regime.
Site types are named with a single edaphic modifier and are given a two-digit numeric code.
Site phases and variations are named according to their differentiating criteria and given a single, non-connotative code after the site series number. Phases are given a single lower-case letter code, while variations are indicated by a number in parentheses.
7. Relationship between BEC & Ecoregion Classification
In BC, two different, but complementary regional ecosystem classification systems exist. The Ecoregion Classification, developed by the B.C. Ministry of Environment, Lands and Parks, describes broad regional ecosystems based on the interaction of climate and physiography and is mostly used for general conservation and wildlife management purposes.
The Biogeoclimatic Ecosystem Classification (BEC) delineates ecological zones (biogeoclimatic units) by vegetation, soils, and climate, and is more commonly used in forestry and conservation. It also classifies ecosystems, within the ecological zones, based on the potential of the site at climax or mature successional stages.
For more information on how these two classification systems are used, see the brochure A Ecological Framework for Resource Management.