The vegetation of any area can be described at various scales and in varying degrees of detail – the two are often correlated. Any description requires information, which is generally obtained through an inventory or survey of some sort – either by remote sensing (aerial photography, satellite imagery, etc.) to generate maps, or by field survey for purposes of detailed description.
For a large area, the vegetation can usually only be described generally, e.g., the broad vegetation formations, such as forest, woodland, shrubland, grassland, and wetland, along with the dominant cover species, e.g., white spruce forests, alder shrublands. An inventory of a large area would involve sampling – describing the vegetation and site features for certain very small areas (i.e., sample plots / relevés) and extrapolating to the larger area through mapping or observation.
The vegetation of a small area can be inventoried in a reasonable period of time, although even in such cases, certain limits are placed on the inventory, e.g., only sampling the vascular plants and other plant life forms growing in and on the soil. For the data that is used in the CNVC, the following criteria are placed on the sample plot inventory:
- sample size: approximately 100 m² - 400 m² for forests; smaller for non-forested vegetation
- recording, by structural layer within the sample stand, of presence and estimated ground cover for all vascular plants and other plant life forms growing in and on the dominant substrate (usually the soil)
- description of habitat features, including, among others, soil moisture and nutrient regime, slope, aspect, elevation, soil material and rooting zone particle size, humus form, mesoslope position, and any other features pertinent to understanding the ecological 'drivers' influencing the development of the vegetation being sampled
- noting stand features, such as age or successional development, and location (i.e., geo-coordinates & eco-regional distribution)
Each province and territory collects vegetation plot data to their own standards, however, the methods are similar enough that they can be correlated for a Canadian classification. An example of a provincial data collection manual is available here.
Sample plot locations can be chosen in various ways, e.g., randomly located, within map strata, or selective. The latter is generally used in plant community description for classification purposes. Vegetation stands uniform in the vegetation cover and the habitat conditions are selected and a sample plot is located within the stand. The result is a set of sample plots for a vegetation type that quantify the range of floristic composition within the type, and of site conditions that repeatedly produce the type on the landscape . The plots are classified using a classification analysis procedure.