Natural Vegetation Classification

The categories of Natural (including ruderal) and Cultural vegetation are treated as separate hierarchies within the NVC.   The new 8-level natural vegetation hierarchy emphasizes physiognomy in an ecological context at three upper levels, and increasingly integrates biogeography and floristics at three middle levels (Table 1). The new middle levels bridge the large conceptual gap between alliance and formation, an important improvement over the 1997 hierarchy. The upper levels of the USNVC hierarchy are based on dominant and diagnostic growth forms that reflect environment at global to continental scales. The mid-levels are based on dominant and diagnostic growth forms and compositional similarity reflecting biogeography and continental to regional environmental factors. The lower levels (alliance and association) are based on diagnostic and/or dominant species and compositional similarity reflecting local to regional environmental factors. The relationship of the new hierarchy levels and classification criteria is depicted in the diagram below.

ClassificationNatural_d

The Standard defines natural (including ruderal) vegetation … as vegetation where ecological processes primarily determine species and site characteristics; that is, vegetation comprised of a largely spontaneously growing set of plant species that are shaped by both site and biotic processes (Küchler 1969, Westhoff and van der Maarel 1973).  The following table shows criteria and examples of the levels of the revised National Vegetation Classification hierarchy for natural vegetation.

Criteria and examples of the levels of the revised NVC hierarchy for natural vegetation
NVC LEVELVEGETATION CLASSIFICATION CRITERIAECOLOGICAL CONTEXTSCIENTIFIC NAME

COLLOQUIAL NAME
Upper LevelsPredominantly physiognomy
1
Formation Class
Broad combinations of general dominant growth forms. Basic temperature (energy budget), moisture, and substrate/aquatic conditions. Mesomorphic Tree Vegetation ClassForest & Woodland
2
Formation Subclass
Combinations of general dominant and diagnostic growth forms. Global macroclimatic factors driven primarily by latitude and continental position, or overriding substrate/aquatic conditions. Temperate & Boreal Forest & Woodland SubclassTemperate & Boreal Forest & Woodland
3
Formation
Combinations of dominant and diagnostic growth forms. Global macroclimatic factors as modified by altitude, seasonality of precipitation, substrates, and hydrologic conditions. Cool Temperate Forest & Woodland FormationCool Temperate Forest & Woodland
Middle Levels Physiognomy, biogeography, and floristics
4
Division
Combinations of dominant and diagnostic growth forms and a broad set of diagnostic plant species that reflect biogeographic differences. Continental differences in mesoclimate, geology, substrates, hydrology, and disturbance regimes. Pseudotsuga menziesii - Tsuga heterophylla - Tsuga mertensiana Vancouverian Forest & Woodland DivisionVancouverian Forest & Woodland
5
Macrogroup
Combinations of moderate sets of diagnostic plant species and diagnostic growth forms that reflect biogeographic differences. Sub-continental to regional differences in mesoclimate, geology, substrates, hydrology, and disturbance regimes. Calocedrus decurrens - Pinus jeffreyi - Abies concolor var. lowiana Forest MacrogroupSouthern Vancouverian Dry Foothill Forest & Woodland
6
Group
Combinations of relatively narrow sets of diagnostic plant species, including dominants and co-dominants, broadly similar composition, and diagnostic growth forms. Regional mesoclimate, geology, substrates, hydrology and disturbance regimes. Quercus garryana - Pinus ponderosa - Pseudotsuga menziesii Forest & Woodland GroupCascadian Oregon White Oak - Conifer Forest & Woodland
Lower Levels Predominantly floristics
7
Alliance
Diagnostic species, including some from the dominant growth form or layer, and moderately similar composition. Regional to subregional climate, substrates, hydrology, moisture/ nutrient factors, and disturbance regimes. Quercus garryana - Pseudotsuga menziesii / Toxicodendron diversilobum Forest & Woodland AllianceOregon White Oak - Douglas-fir / Pacific Poison-oak Forest & Woodland Alliance
8
Association
Diagnostic species, usually from multiple growth forms or layers, and more narrowly similar composition. Topo-edaphic climate, substrates, hydrology, and disturbance regimesPinus ponderosa / Carex inops - Festuca roemeri WoodlandPonderosa Pine / Long-stolon Sedge - Roemer's Fescue Woodland

References:

Küchler, A.W. 1969. Natural and cultural vegetation. The Professional Geographer 21: 383-385.
Westhoff, V. and E. van der Maarel. 1973. The Braun-Blanquet approach. In: R.H. Whittaker (ed.). Handbook of Vegetation Science. Part V. Ordination and Classification of Communities. Junk, The Hague