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.
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| HIERARCHY FOR NATURAL VEGETATION | VEGETATION CLASSIFICATION CRITERIA | ECOLOGICAL CONTEXT | SCIENTIFIC NAME | COLLOQUIAL NAME |
|---|---|---|---|---|
| Upper Levels | Predominantly physiognomy | |||
| 1 Formation Class | Broad combinations of general dominant growth forms. | Basic temperature (energy budget), moisture, and substrate/aquatic conditions. | Mesomorphic Tree Vegetation Class | Forest & 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 Subclass | Temperate & 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 Formation | Cool 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 Division | Vancouverian Cool Temperate 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 Macrogroup | Southern 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 Group | Cascadian 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 - Pinus ponderosa/ Carex geyeri Woodland Alliance | Oregon White Oak - Ponderosa Pine / Geyer's Sedge 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 regimes | Pinus ponderosa - Quercus garryana / Balsamorhiza sagittata Woodland | Ponderosa Pine - Oregon White Oak/ Arrowleaf Balsamroot 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
