2.3 Ecosystem Units

Within ecosection and biogeoclimatic units, local and vegetation developmental level units termed ecosystem units, are defined. Ecosystem units are generally derived from the site series classification of BEC, by being further differentiated according to more specific site conditions (thus defining more homogeneous site units) and structural developmental stages (thus defining more homogeneous vegetation structural stages) (Figure 2.2). Additional attributes, such as seral community type or stand composition, can be added to map symbols to serve the needs of a particular client.

Figure 2.2 Hierarchy of TEM classification levels

2.3.1 Site series

Variation in site conditions encountered within a biogeoclimatic unit is accommodated within the site classification of BEC. The site series describe all land areas capable of supporting specific climax vegetation. This can usually be related to a specified range of soil moisture and nutrient regimes within a subzone or variant, but sometimes other factors, such as aspect or disturbance history, are important determinants as well. Ecologically similar site series occurring under more than one climatic regime (e.g., in more than one subzone or variant) are grouped together to form a site association (see Meidinger and Pojar, 1991 for more details). A classification of site series for most of the biogeoclimatic units of the province has been developed by the BC Ministry of Forests and is presented in regional field guides.

2.3.2 Site modifiers

Ecosystems with the same vegetation potential are grouped and classified to the site series level. However, compensating effects of different environmental characteristics can result in some site series having a wide range of physical site conditions. In TEM, this variation is dealt with by defining the "typical" conditions for a site series (RIC, 1997b) and then using site modifiers (see Table 3.2), a set of descriptive terms for certain site conditions, to describe conditions outside those considered typical. The typical environmental conditions were determined by reviewing each of the Ministry of Forests Regional Field Guides and selecting the "typical" characteristics of each site series.

2.3.3 Vegetation developmental units

While the site series describes site potential, actual stand conditions will vary considerably, depending on disturbance history, stand age, species composition, and chance. Many study areas will contain a complex of early to late seral and climax vegetation units. The level of detail required in descriptions of seral communities will be largely determined by the survey objectives and sampling intensity. Several attributes, outlined below, can be used to describe seral and structural variation in plant communities. Section 3.2.3 describes the standard coding to be used for each attribute in more detail.

The structural stage is the only mandatory vegetation developmental unit. The more detailed modifiers and seral community types will only be used to serve specific project objectives.

Structural stages

For studies emphasizing structural habitat characteristics, the structural stage category will generally be sufficient to describe seral variation within a site series. Structural stages describe the existing dominant stand appearance or physiognomy for the ecosystem unit, and are derived from the seral and stand structure classifications recommended by Hamilton (1988), and Oliver and Larson (1990). Stand structure substages and additional modifiers can be used to better differentiate non-forested categories (e.g., forb-dominated versus graminoid-dominated herb stage) and forested categories (e.g., single storied, multi-storied, coniferous versus broadleaf forests). Forested structural stage modifiers and stand composition modifiers are useful for developing wildlife and silvicultural interpretations, and will be used wherever specific project objectives require them.

Seral community types

Within BEC, the seral association describes present vegetation where the plant association is not in a climax or near-climax state. Seral associations represent non-climax plant associations belonging to the successional sequence of ecosystems within one or more site series. A formal, correlated classification of seral associations has not yet been developed for the province, although efforts are under way in some of the forest regions.

In mapping projects requiring differentiation of successional communities, a less formal approach will generally be taken in describing seral vegetation. Seral community types will be defined, describing more generalized seral units dominated by a similar group of species, often in the upper strata (tree and/or shrub layers in the case of forest and shrub communities), but being more variable in understory composition. By examining site and soil characteristics, and identifying soil moisture and nutrient regimes, it should be possible to identify the site series to which the seral community type belongs (e.g., site potential). However, seral communities typically span a much broader range of site characteristics than do site series, and thus the same seral community type may belong to the successional sequence of more than one site series.

The data collected in mapping projects and used to develop preliminary seral community types will be useful in eventually developing a correlated provincial classification of seral associations. Such a classification would be developed within the site series framework, with associations being differentiated using a diagnostic combination of species.

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