The CNVC Hierarchy
The CNVC employs an 8-level hierarchy that was developed by an international group of scientists from the western hemisphere. The United States National Vegetation Classification (USNVC) uses the same hierarchy. In most cases the CNVC interprets the hierarchy levels the same as the USNVC, but there are some differences. The table below provides the CNVC definitional criteria for each level of the hierarchy, with examples. For a more detailed outline of CNVC guidelines for determining units at all hierarchical levels, click here. To view the completed classification (to date), or to search for specific units, see Explore the Classification.
The CNVC emphasizes the ecological context of vegetation conditions when developing units at all hierarchical levels. The upper three levels of the hierarchy, Formation Class, Formation Subclass, and Formation, use dominant and diagnostic growth forms as criteria to reflect environmental gradients at global to continental scales. The Division level uses dominant and diagnostic growth forms, as well as broad sets of diagnostic species that reflect continental-scale biogeography and environmental factors. The fifth level of the hierarchy, Macrogroup, uses plant species composition, abundance and/or dominance to reflect regional climate. The lowest three levels of the hierarchy, Group, Alliance, and Association, use species dominance, diagnostic indicator value, and overall compositional similarity to describe vegetation conditions that reflect local, site-scale environmental gradients. In the CNVC, Alliances and Groups are first- and second-order aggregations of Associations and Alliances respectively.
For vegetation on "azonal" sites at the levels of Macrogroup, Group, and Alliance, the CNVC uses the interpretive guidelines of the USNVC. The CNVC permits subtypes for Macrogroups, Groups, Alliances and Associations. Subtypes are units that describe vegetation conditions that are not distinct enough to be recognized as formal units at their respective levels.
| Hierarchy Level | Definition |
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Formation Class
(e.g. Forest & Woodland)
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A broad combination of general dominant growth forms that are adapted to basic moisture, temperature, and/or substrate or aquatic conditions.
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Formation Subclass
(e.g. Temperate & Boreal Forest & Woodland) |
A combination of general dominant and diagnostic growth forms that reflect global mega- or macroclimatic factors driven primarily by latitude and continental position or that reflect overriding substrate or aquatic conditions.
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Formation
(e.g. Boreal Forest & Woodland)
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A combination of dominant and diagnostic growth forms that reflect global macroclimatic conditions as modified by altitude, seasonality of precipitation, substrates, and hydrologic conditions.
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Division
(e.g. North American Boreal Forest & Woodland)
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A combination of dominant and diagnostic growth forms and a broad set of diagnostic plant species that reflect biogeographic differences in composition and continental differences in mesoclimate, geology, substrates, hydrology, and disturbance regimes.
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Macrogroup
(e.g. West-Central North American Boreal Forest)
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For upland vegetation that includes “zonal” vegetation: A regionally distinct subset of plant species composition, abundance and/or dominance, representing primary regional climatic gradients as reflected in vegetation patterns on circum-mesic (“zonal”) sites.
For “azonal” vegetation: A vegetation unit that contains moderate sets of diagnostic plant species and diagnostic growth forms that reflect subcontinental to regional biogeographic composition and subcontinental to regional mesoclimate, geology, substrates, hydrology, and disturbance regimes.
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Group
(e.g. Cordilleran Boreal Mesic Trembling Aspen – White Spruce Forest)
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For upland vegetation that includes “zonal” vegetation: An aggregation of Alliances within the regional vegetation defined by a Macrogroup (or subtype), with consistency in dominant and/or diagnostic species. Groups describe regionally generalized vegetation patterns attributable to ecological drivers such as edaphic or geological conditions within the Macrogroup (subtype), successional relationships within the Macrogroup (subtype), etc.
For “azonal” vegetation: A vegetation unit that is defined by a relatively small set of diagnostic plant species (including dominants and codominants), broadly similar composition, and diagnostic growth forms that reflect regional mesoclimate, geology, substrates, hydrology, and disturbance regimes
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Alliance
(e.g. Populus tremuloides (Picea glauca) / Shepherdia canadensis / Leymus innovatus)
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An aggregation of Associations, with consistency in dominant and/or diagnostic species, describing regionally repeating vegetation patterns at the local to sub-regional scale. Alliances are created by grouping Associations that are ecologically “related” into more generalized ecological units (e.g., successionally related Associations on similar edaphic conditions can be aggregated into more generalized Alliances).
For “azonal” vegetation: A vegetation classification unit containing one or more associations and defined by a characteristic range of species composition, habitat conditions, physiognomy, and diagnostic species, typically at least one of which is found in the uppermost or dominant stratum of the vegetation. Alliances reflect regional to subregional climate, substrates, hydrology, moisture/ nutrient factors, and disturbance regimes
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Association
(e.g., Populus tremuloides / Leymus innovatus)
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A plant community type with consistency of species dominance and overall floristic composition, having a clearly interpretable ecological context in terms of site-scale climate, substrate and/or hydrology conditions, moisture/nutrient factors and disturbance regimes, as expressed by diagnostic indicator species.
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