Ecological Classifications

NatureServe Explorer reports information on ecological communities and systems as defined for terrestrial, marine and (in the future) freshwater ecosystems. Developed by NatureServe and its natural heritage member programs in collaboration with federal, international, academic and state partners, these classifications provide a systematic way of describing and assessing ecological diversity. They address the need for subnational, national and international classification standards for ecosystems, while allowing for classification at a scale fine enough to be used to understand, manage, and protect natural resources on a local or site-by-site basis. The standards can be applied to define ecosystems anywhere in the world; but the emphasis of the classifications are for types that occur in North, Central and South America.

TERRESTRIAL

International Vegetation Classification

The International Vegetation Classification (IVC) (formerly called the International Classification of Ecological Communities or ICEC) is based on vegetation as it currently exists on the landscape. Landforms, soils, and other features are not directly considered as part of the classification criteria, but ecological and biogeographical information help guide the structure of the classification. Because of conservation objectives, classification efforts focus on natural and semi-natural types of vegetation, and NatureServe Explorer reports only these types. However, the system can be used to classify any vegetation as it currently exists across the landscape, including natural, modified, agricultural and urban vegetation types.

The classification system has won broad acceptance within the NatureServe network as well as among our partners. In North America, the IVC consists of the U.S. National Vegetation Classification (USNVC) and the Canadian National Vegetation Classification (CNVC). The IVC has also been developed for the Caribbean region and many parts of Latin America. The upper levels of the IVC cover the entire globe, mid-levels are comprehensive across the western Hemisphere and various other parts of the globe, and lower levels are most complete in the U.S. and Canada. Partnerships with other compatible classification efforts, such as those of the Braun-Blanquet tradition, will facilitate its use in Europe and other regions.

Standard References

Website: www.natureserve.org/explorer

Faber-Langendoen, D., T. Keeler-Wolf, D. Meidinger, C. Josse, A. Weakley, D. Tart, G. Navarro, B. Hoagland, S. Ponomarenko, J.-P. Saucier, G. Fults, E. Helmer. 2012. Classification and description of world formation types. Part I (Introduction) and Part II (Description of formation types). Hierarchy Revisions Working Group, Federal Geographic Data Committee, FGDC Secretariat, U.S. Geological Survey. Reston, VA, and NatureServe, Arlington, VA.

D. Faber-Langendoen, T. Keeler-Wolf, D. Meidinger, D. Tart, C. Josse, G. Navarro, B. Hoagland, S. Ponomarenko, J.-P. Saucier, A. Weakley, P. Comer. 2012. Guidelines for a Vegetation - Ecologic Approach to Vegetation Description and Classification. Hierarchy Revisions Working Group, Federal Geographic Data Committee. FGDC Secretariat, U.S. Geological Survey, Reston, VA, and NatureServe, Arlington, VA. (in prep).

Guiding Principles

1. Types are based on full floristic and growth form composition, with supporting ecological and biogeographic information.


2. Natural and cultural vegetation have separate treatments.


3. Vegetation is characterized using plot data, with supporting ecological data.


4. Vegetation is classified based on existing vegetation criteria, organized around ecological factors and biogeography:
      diagnostic and dominant species
      diagnostic and dominant growth forms
      full floristic composition


5. Hierarchy is structured using physiognomy, floristics, ecology.


6. Levels of the hierarchy are both independent (i.e. no one level is more fundamental than another), and inter-connected (patterns at one level coordinate with other levels).


7. Changes to the classification are coordinated by a Panel, through a peer review process, responding to changes from individuals or work groups, who may either conduct original analyses, or integrate information available in the literature.

Defining the Classification Hierarchy

Currently the International Vegetation Classification and the U.S. National Vegetation Classification share the same principles and structure, as described below. A first version of the hierarchy was developed in 1997-1998, which was revised in 2008 (see U.S. National Vegetation Classification (USNVC). The most current units of the IVC are maintained on the NatureServe Explorer website, with USNVC units also posted on the usnvc.org website.

The revised hierarchy addresses the following issues, among others: uses vegetation criteria to define all types (de-emphasizing explicit abiotic criteria, such as hydrologic regimes in wetland types), b) provides a clear distinction between natural (including semi-natural) and cultural vegetation wherever these can be observed from broad growth form patterns (rather than combining natural and cultural vegetation initially and separating them at lower levels), c) for natural vegetation, defines the upper levels based on broad growth form patterns that reflect ecological relationships (rather than detailed structural criteria, which are more appropriate lower down in the hierarchy), d) provides a new set of middle-level natural units that bridge the large conceptual gap between alliance and formation, e) integrates the physiognomic and floristic hierarchy levels based on ecologic vegetation patterns, rather than developing the physiognomic and floristic levels independently and then forcing them into a hierarchy, f) provides detailed standards for plot data collection, type description and classification, data management and peer review of natural vegetation, and g) for cultural vegetation provides an independent set of levels that addresses the particular needs of cultural vegetation.

Criteria for Natural Vegetation

Floristic and physiognomic criteria are the primary properties of natural vegetation used to define all units of the classification. The choice of how these criteria are used is integrated with ecological and biogeographic considerations. The IVC includes criteria for all of the new and revised levels, as shown in Table 1.

Table 1. Summary of IVC Revised Hierarchy Levels and Criteria for Natural Vegetation

The variety of vegetation criteria can be summarized as follows (FGDC 2008, see also Mueller-Dombois and Ellenberg 1974, p. 154-155):

Physiognomic and structural criteria include 1) Diagnostic combinations of growth forms; 2) Ecological patterns of either dominant growth forms or combinations of growth forms (growth forms of similar ecological (habitat) and dynamic significance, or growth forms of similar geographical distribution), and 3) Vertical stratification (layering) of growth forms (complexity in structure as produced by arrangement of growth forms).

Floristic criteria include 1) Diagnostic combinations of species (differential and character species, constant species, dominant species), 2) Ecological combinations of species (indicator species of similar ecological (habitat) and/or dynamic significance, species of similar geographical distribution), 3) Vertical stratification (layering) of species (species patterns found in the dominant growth forms or strata, species patterns found between strata (overstory/understory), and 4) Numerical relation criteria (community coefficients, such as indices of similarity among plots within a type).

Too often vegetation classifiers rely solely on physiognomic or floristic criteria. The IVC focuses on being a scientific natural vegetation classification, using multiple vegetation criteria to achieve the most natural groupings of vegetation types as possible. The revised IVC is more likely to be compatible with other multi-factor ecosystem classifications. Still, habitat factors (e.g., climate, soil type) or management activities are not an explicit part of the hierarchy; rather, they are used to help interpret the patterns expressed through the vegetation (Fig. 1).

Figure 1. Vegetation classification criteria (from FGDC 2008)

The diagram portrays the five vegetation criteria used to classify vegetation at all levels of the IVC hierarchy. These criteria are arranged from the most fine-scaled on the left to the most broad-scaled on the right. The five criteria are derived from stand attributes or plot data (inside oval) and reflect the ecological context (outside oval) of the stand or plot. The ecological context includes environmental factors and biogeography considered at multiple scales, as well as natural and human disturbance regimes.

The question is how to structure these criteria across the different levels of the hierarchy. The criteria are weighted more towards physiognomy-ecologic at higher levels, combinations of physiognomy and floristics at mid levels, (guided by biogeographic, climatic and other ecological factors), and more strongly floristic-ecologic criteria for the lowest levels (see Table 1). The intent is to provide as "natural" a classification of vegetation as possible. An example of the types across the levels is provided in Table 2.

Table 2. Example of the IVC Hierarchy

Criteria for Cultural Vegetation

The cultural vegetation hierarchy consists of eight levels (see Table 3). These levels are different from the natural vegetation hierarchy, by providing an additional physiognomic level (level 4), placing less emphasis on broad-scale, biogeographic and climate patterns, but still providing for multiple scales of floristically and physiognomically defined agricultural and developed vegetation types.

Table 3. Hierarchy for Cultural Vegetation with Examples

Upper level (physiognomic-ecological) units:

1. Cultural Class: A cultural vegetation classification unit of high rank (1^st level) defined by a characteristic combination of dominant growth forms adapted to relatively intensive human manipulations, as reflected in relatively rapid changes in structure and/or composition.
2. Cultural Subclass: A cultural vegetation classification unit of high rank (2^nd level) defined by combinations and degree of herbaceous versus woody growth forms.
3. Cultural Formation: A cultural vegetation classification unit of high rank (3^rd level) defined by whether or not canopy structure of dominant growth forms is annually converted or heavily manipulated / harvested.
4. Cultural Subformation: A vegetation classification unit of intermediate rank (4^th level) defined by the spatial structure of the vegetation, including whether in swards, rows, and degree of manipulation to the canopy.



Mid-level (physiognomic-floristic) units:

5. Cultural Group: A cultural vegetation classification unit of intermediate rank (5^th level) defined by a common set of growth forms and many diagnostic plant taxa sharing a broadly similar region and climate, and disturbance factors.
6. Cultural SubGroup: A cultural vegetation classification unit of intermediate rank (6^th level) defined by a common set of growth forms and diagnostic species (taxa) preferentially sharing a similar set of regional edaphic, topographic, and disturbance factors.



Lower-level (floristic) units:

7. Cultural Type: A vegetation classification unit, of moderately low rank (7^th level) defined by one or more dominant or co-dominant species, as well as habitat conditions, and physiognomy.
8. Cultural Subtype: A vegetation classification unit, of low rank (8^th level) defined on the basis one or more dominant or co-dominant species, in conjunction with a characteristic set of associated species, habitat conditions and physiognomy.

Floristic and physiognomic criteria are the primary properties of cultural vegetation used to define all units of the classification, but assessed in light of human activities that govern these properties. Thus, choice of how these criteria are used should be evaluated in light of human management needs. Excluded from these criteria are properties from outside the current vegetation, such as explicit habitat factors (e.g., climate, soil type) or land use activities (e.g., grazed pasture versus ungrazed pasture), except as these are expressed in the vegetation cover. Some types are difficult to place in terms of natural versus cultural vegetation (e.g., forest plantation, pastures), and the user may need to look in both parts of the hierarchy to determine the type’s location. The broad criteria for classifying cultural vegetation may be summarized as follows (Table 3):

1. Growth form criteria
1. Diagnostic patterns of growth forms
2. Ecologic and managed patterns of growth forms
• Growth forms of similar management significance (e.g., crop types)
• Growth forms of similar ecology and habitat
3. Vertical stratification (layering) of growth forms
2. (crop or managed species) criteria
1. Diagnostic combinations of species/crop or managed types
2. Ecologic and managed combinations of species/crop or managed types
• Species of similar management significance (e.g., crop types)
• Species of similar ecology and habitat
3. Vertical stratification (layering) of species

All type concepts based on these criteria should be derived from field observations, in which the crop or managed species, growth forms, and their abundance, along with the field observation record, overall vegetation structure, and habitat setting are described. These field data provide the fundamental information for the description of types. All types at all levels should be described and characterized. Initially, the new upper and mid levels may have only brief characterizations, but shall be elaborated over time.

Understanding Ecological Community Names

Plant species that are dominant (cover the greatest area) and diagnostic (found consistently in some vegetation types but not others) are the foundation of alliance and association names. At least one species from the dominant and/or uppermost stratum is included in each name. The following guidelines apply to alliance and association names:

• A hyphen ("-") indicates species occurring in the same stratum.
• A slash ("/") indicates species occurring in different strata.
• Species that occur in the uppermost stratum are listed first, followed successively by those in lower strata.
• Order of species names generally reflects decreasing levels of dominance, constancy, or indicator value.
• Parentheses around species name indicate species less consistently found either in all associations of an alliance, or in all occurrences of an association.

Alliance names include a term relevant to the class or formation in which they belong (e.g., "Forest," "Woodland," "Grassland", "Marsh"), and, if needed, can be followed by the word "alliance" to distinguish them from associations. The lowest possible number of species is used for an alliance name, up to a maximum of four.

Examples of alliance names:

• Pseudotsuga menziesii Forest Alliance
• Fagus grandifolia - Magnolia grandiflora Forest Alliance
• Pinus palustris / Quercus spp. Woodland Alliance
• Andropogon gerardii - (Calamagrostis canadensis, Panicum virgatum) Grassland Alliance

Association names include a term relevant to the class or formation in which they are classified. The lowest possible number of species is used in an association name. Up to four species may be necessary to define types with very diverse vegetation, relatively even dominance, and variable total composition.

In cases where diagnostic species are unknown or in question, a more general term (such as "Prairie Forbs" is currently allowed as a "placeholder." An environmental or geographic term (for example, "Northern"), or one that is descriptive of the height of the vegetation ("Dwarf"), can also be used as a modifier when such a term is necessary to adequately characterize the association. When confidence in the circumscription of the association is low, the name is followed by the term "[Provisional]".

Examples of association names:

• Abies lasiocarpa / Vaccinium scoparium Forest
• Metopium toxiferum - Eugenia foetida - Krugiodendron ferreum - Swietenia mahagoni /Capparis flexuosa Forest
• Rhododendron carolinianum Shrubland
• Quercus macrocarpa - (Quercus alba - Quercus velutina) / Andropogon gerardii Savanna

Development Status of the International Vegetation Classification and NatureServe Explorer Data

• In North America, the IVC is being widely applied by NatureServe, federal, academic and state partners through the USNVC and CNVC classifications. Many mapping and classification products are available (see www.natureserve/publications).