Canada's Vegetation According to the CNVC Upper Hierarchy
Canada’s vegetation is very diverse – ranging from warm temperate grasslands and forests, to cool boreal and mountain forests, to cold treeless arctic and alpine tundra, to freshwater and marine aquatic vegetation. The CNVC uses the upper levels of the United States National Vegetation Classification (USNVC) taxonomic hierarchy to describe, in general terms, the vegetation of Canada.
According to the CNVC/USNVC hierarchy, there are 18 Formations (global vegetation units) occurring in Canada that are subdivided into 33 North American Vegetation Divisions. An overview of the natural vegetation of Canada follows, linked from the six broad Formation Classes at Level 1 of the hierarchy. The first four levels of the proposed CNVC hierarchy for natural and semi-natural vegetation are presented diagrammatically here.
Level 1 (Formation Class) of the CNVC/USNVC hierarchy focuses on the adaptation of plant growth forms to the main ecological drivers of water availability and climatic temperature. Formation Class: A vegetation classification unit of high rank (1st level) defined by a characteristic combination of dominant growth forms adapted to a very basic set of moisture / temperature regimes. Class names are based on the very broad growth forms (mesomorphic, xeromorphic, cryomorphic, lithomorphic, hydromorphic) that correspond to basic global moisture/temperature regimes.
The Formation Class also considers how physiognomic patterns relate to ecological factors known to affect vegetation. Physiognomy is defined as “the visible structure or outward appearance of a plant community as expressed by the dominant growth forms, such as their leaf appearance or deciduousness” (Fosberg 1961). Some physiognomic forms relate clearly to distinct ecological habitats, e.g., hydromorphic (e.g. floating aquatic plants) versus non-hydromorphic vegetation. Still, it is not always straightforward to assign ecological significance to a growth form, e.g., the tree growth form cannot be ascribed to a specific environment or habitat.
Formation Classes recognized by the CNVC/USNVC at the highest taxonomic level include:
Forest and Woodland (Mesomorphic Tree Vegetation)
The Forest and Woodland Formation Class refers to “tree-characterized” vegetation, i.e., stands where trees are distributed throughout the stand with at least 10% overall ground cover. Trees play an important role in ecological processes and influence the distribution of other plant species in these communities. Tree species tend to be mesomorphic, i.e., not strongly adapted to drought, cold or aquatic conditions.Two Subclasses are recognized for Canada: Temperate Forest and Boreal Forest.
Shrubland, Grassland, Meadow, and Non-forested Wetland (Mesomorphic Shrub & Herb Vegetation)
This Formation Class includes “shrub- and herb-characterized” vegetation that is mesomorphic (i.e., not strongly adapted to drought, cold or aquatic conditions). In the case of shrub-characterized vegetation, the shrubs or dwarf-shrubs are distributed throughout the stand, providing a consistent layer, even if of low cover. Similarly, in herb-characterized vegetation, herbs (grasses or forbs) provide a consistent, well-distributed ground cover, even if of low cover. The shrubs or herbs are important in ecological processes and influence the distribution of other plant species. Only one Formation Subclass is recognized for Canada: Temperate and Boreal Shrubland and Grassland.
Semi-Desert Scrub and Grassland (Xeromorphic Scrub & Herb Vegetation)
Vegetation in this Formation Class has a predominance of plant species with adaptations to prevent water loss. Stands may be shrub- or herb-characterized, and are found in desert climates. Only one Formation Subclass is recognized for Canada: Cool Semi-desert Scrub and Grassland.
Polar and High Montane Vegetation (Cryomorphic Shrub & Herb Vegetation)
The “Polar & High Montane” Formation Class comprises arctic and alpine tundra vegetation. Plant species have adaptations to withstand cold temperatures and growing-season frosts. Generally, they are also adapted to survive in areas of high wind or soil instability. Dwarf-shrubs, grasses and forbs characterize this vegetation. Two Formation Subclasses are recognized for Canada: Polar Tundra and Temperate and Boreal Alpine Vegetation.
Aquatic Vegetation (Hydromorphic Vegetation)
Aquatic vegetation occurs in ponds, shallow parts of lakes, along slowly moving rivers, and in estuaries and shallow marine coastal areas. Plant species are generally rooted in or under water and have emergent, floating or submerged growth forms. Most species are herbaceous forbs or graminoids. Two Formation Subclasses are recognized for Canada: Saltwater Aquatic Vegetation and Freshwater Aquatic Vegetation.
Nonvascular and Sparse Vascular Rock Vegetation (Lithomorphic Vegetation)
This vegetation is characterized by plant species with adaptations for living on rock surfaces or on rocky substrates. These include some grasses and forbs but nonvascular cryptogams – lichens, mosses and liverworts – predominate. Vascular plants, when present, are of very low cover. Two Formation Subclasses are recognized for Canada: Mediterranean, Temperate and Boreal Nonvascular and Sparse Vascular Vegetation and Polar and High Montane Nonvascular and Sparse Vascular Vegetation.