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C06 Cryptogam - Open Mesomorphic Vegetation Class

The U.S. National
Vegetation Classification
Type Concept Sentence: Tropical, temperate, and boreal habitats are characterized or dominated by plant growth forms, such as lichen, bryophyte, alga, or fern, that have structural adaptations for living on stable rock surfaces or on unstable rocky substrates, such as cliffs, talus, scree, pavement, cobble, lava or boulderfields, and with associated mesomorphic grass, shrub and tree growth forms.
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Common (Translated Scientific) Name: Cryptogam - Open Mesomorphic Vegetation Class
Colloquial Name: Open Rock Vegetation
Hierarchy Level: Class
Type Concept: Nonvascular plants, especially bryophyte, lichen, alga or fern (including clubmosses and other vascular spore-bearing plants) growth forms are present, but are constrained by rock or other hard substrates, such as cliff, talus, scree, cobble, lava, and rock flats. Vascular growth form structure is largely determined by the rock fissures, etc., and is typically <10% cover, with irregular horizontal spacing. Climates vary and are often less determinative than presence of open rock surface. Substrates are typically dry to moist, but occasionally wet (e.g., seepage cliffs) and typically lack soil development of any kind. Stands typically contain crustose lichen and/or sparse vascular vegetation.
Diagnostic Characteristics: Lithomorphic growth forms dominate, including lichen (especially foliose and crustose lichens), bryophyte, alga, ferns (non-woody, including clubmosses, etc.), and crevice-dwelling (rupicolous) vascular plants. Vascular vegetation is <10% cover and is composed of mesomorphic vegetation. Vegetation structure may be patchy across the rocky surfaces.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: This class is defined by primarily nonvascular, often crustose lichen, ferns or very sparse other vascular vegetation that is controlled by the rocky substrate. The substrate can be classed as either stable (large boulders, outcrops, monoliths, etc.) or unstable (colluvium such as scree, talus or dense, alluvial cobble). Stable substrates prevent vascular plant roots from penetrating. Species adapted to such substrates have no roots and occupy the surface of the substrates, such as nonvasculars (lichens, mosses, liverworts), or are largely prevented from growing on the substrate, except in localized fissures or cracks. Whenever the vegetation is able to establish sufficient cover on the rocky surface (typically >10% cover), the growth forms of the vegetation determine the class. Vascular spore-bearing plants (ferns and others) are included in this class. Spikemoss (Selaginella) regularly adheres to cliff and outcrops, as do many species of ferns such as Polypodium, Cheilanthes, Pellaea, etc.

Lithology may be consolidated or unconsolidated, but the conditions of the substrate preclude development of soil and either strongly impede the penetration of roots or require specialized adaptations of roots to support the plant species. Lithomorphic Vegetation may be similar to desert or polar types where climate per se limits the overall plant growth, and substrate limitations are secondary.

Open sand dunes are excluded from this class in general as open sand dunes are an extreme condition of other classes (i.e., mesomorphic, xeromorphic), where the constant movement of sand (ablation, deposition, etc.) is at too extreme a pace for even the typical adaptations of psammophytic plants, such as creeping stolons, rhizomes, etc., to keep up.

Larson et al. (2000b) characterize cliffs as having a "high, steep, or overhanging face of rock," whereas rock outcrop is a "portion of bedrock protruding through the soil." Lava flows and colluvial deposits would lay over the soil. They include both consolidated and unconsolidated cliffs in their definition, including sand, gravel or loess, as well as rock cliffs. Here we restrict it to rock (or at least very rocklike) materials. Apart from stating that a cliff need only be "high enough that falling off will kill you," they suggest that even 3 m of height is sufficient to create suitable cliff habitat.

Defining this class to include sparse vascular vegetation stands that are limited in their cover because of substrate is an issue. For example, scree vegetation could be considered very similar to psammophytic (dune and other sandy) vegetation and be treated as an extreme form of mesomorphic vegetation [mostly or all in 2. Shrub & Herb Vegetation Class (C02)]. Scree is usually dominated by sparse vascular plants, has considerable soil development for vascular plant establishment, but has unstable substrate on which nonvascular vegetation cannot become well-established (C. Lea pers. comm. 2012). However, whereas dunes can have vegetation that greatly exceeds 10% cover, this is less common on scree; by-and-large, the vascular vegetation remains sparse, and nonvascular vegetation is typical. Talus is generally stable enough that it has significant nonvascular vegetation, whereas scree may be unstable and less commonly contains a strong nonvascular component; further review of this distinction is needed. Thus talus fits in this Open Rock Vegetation Class based on the nonvascular vegetation criteria, and scree fits in this class because of the sparse vascular vegetation, perhaps in combination with nonvascular vegetation.

Further review may suggest that this class can be improved by distinguishing (probably at the subclass level), the less "rocklike" vegetation (including vegetation on scree, badlands, pavement) from the true solid rock vegetation (C. Lea pers. comm. 2012). Scree and other loose rocky substrates (volcanic cinders) typically have enough soil development to support a dominant layer of disturbance-adapted, facultative mesomorphic shrubs and/or herbs (e.g., in the United States, Ribes spp., Prunus virginiana, and other forest, woodland, or shrubland shrubs in the central Rockies), and true lithomorphs (saxicolous lichens) are quite sparse, if present at all. Although the surface may be "rocky," the driving ecological factor is the availability of soil in quantity under the relatively shallow rock armoring and the absence of large, stable substrates for true lithomorphs (saxicolous lichens) to establish. Cobbles along rivers and streams may also not typically support lithomorphic growth forms, and may best be treated as sometimes sparse flooded types within mesomorphic formations. Badlands (hard clays) and desert pavements also tend to support facultative vascular species, as opposed to lithomorphs or lithophiles, and could be considered for the xeromorphic class. Given this narrower definition of open rock vegetation, a requirement for <10% vascular cover and nonvascular cover greater than vascular cover may be more appropriate, to separate them from sparse or occasionally sparse types (scree, volcanic cinders) in which vascular layers are clearly dominant and lithomorphic nonvascular is insignificant (C. Lea pers. comm. 2012).
Similar NVC Types:
C02 Shrub & Herb Vegetation, note: "In moist climates, cliff, talus, scree, and rock outcrops can contain inclusions of mesomorphic vegetation, but the lithomorphic substrate limits their cover. Where mesomorphic cover is >10% of total vascular vegetation (excluding ferns and allies), the vegetation is placed in this class."
C04 Polar & High Montane Scrub, Grassland & Barrens, note: "In either extreme alpine habitats or in polar deserts, the vegetation will be limited to growing on rocks and become so sparse as to qualify as lithomorphic vegetation. But where the vegetation typically has >10% cover of total vascular vegetation (excluding ferns and allies), and is dominated by cryomorphic growth forms, it belongs in this class. This class also includes stands in polar regions where the vascular vegetation has extensive moss or fruticose or foliose lichen (but not crustose lichen) and is on mineral soil (i.e., lichen tundra vegetation)."
C03 Desert & Semi-Desert, note: "Dry habitats with sparse vegetation on rocky surfaces. When xeromorphic plants have >10% cover on rocky substrates, or, if <10%, are the dominant growth present, stands are classified as xeromorphic vegetation."
Physiognomy and Structure: Growth Forms: Stands have varying levels of nonvascular growth forms (including bryophyte, lichen, alga), and vascular ferns, and vascular flowering vegetation is typically sparse to absent. Foliose and crustose (saxicolous) lichens may be particularly diagnostic. Vascular plants may have a variety of adaptations that allow them to thrive in these rocky habitats (sometimes referred to as rupicolous species).

Structure: The structure is very open to closed, with irregular spacing of nonvascular and fern vegetation, which can vary from absent (but vascular plants are at least sparsely present up to 10%) to continuous. Vascular plant cover is patchy, confined to crevices in the substrate, and is comprised of mesomorphic vegetation.
Dynamics: Disturbances are often strong drivers for stand rejuvenation and development in lithomorphic systems, whether exfoliation and rockslides from consolidated rock.
Environmental Description: Climate: Found on bare rock and in rock crevices of cliffs, scree, talus, lava, pavement, other rock outcrops, and cobble-gravel habitat, with soils typically absent. Most often in non-desert or non-polar climates where substrate features limit establishment of mesomorphic vegetation. Habitat with flat bedrock, but with thin soil layers, typically excluded.

Soil/substrate/hydrology: Organic soil component usually lacking, substrate either consolidated rock, or cobble, gravel, or stone with or without regular movement. Rocky vegetation can be difficult to map because of the small size of individual units, or difficulty in discerning vegetation, or a combination of the two. Vertical cliff faces are also difficult to characterize and map. Fine-scale classification of nonvascular vegetation is not generally well-developed.
Geographic Range: Azonal areas found in regions from tropical to polar zones.
Nations: CA, US
Omernik Ecoregions:
Plot Analysis Summary:
Confidence Level: Moderate
Confidence Level Comments:
Grank: GNR
Name:Database Code:Classification Code:
Class 6 Open Rock Vegetation C06 6
Subclass 6.A Tropical Open Rock Vegetation S02 6.A
Subclass 6.B Temperate & Boreal Open Rock Vegetation S04 6.B
Concept Lineage:
Obsolete Names:
Obsolete Parents:
Synonomy: = Cliff, Bluffs and Rock Outcrops (Wardle 1991) [Wardle's concept is applied to New Zealand.]
Concept Author(s): Hierarchy Revisions Working Group, Federal Geographic Data Committee (Faber-Langendoen et al. 2014)
Author of Description: Hierarchy Revisions Working Group
Version Date: 17Oct2014
  • Bailey, R. G. 1989. Explanatory supplement to ecoregions map of the continents. Environmental Conservation 16:307-309 with separate map at 1:30,000,000. USDA Forest Service, Washington, DC.
  • 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, and E. Helmer. 2015c. Classification and description of world formation types. General Technical Report RMRS-GTR-000. USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO.
  • Larson, D. W., U. Matthes, and P. E. Kelly. 2000b. Cliff ecology: Patterns and processes in cliff ecosystems. Cambridge University Press, Cambridge, MA.
  • Lea, Chris. Personal communication. Ecologist, formerly with National Park Service, USGS / NPS Vegetation Mapping Program, Denver, CO.
  • Wardle, P. 1991. Vegetation of New Zealand. Reprinted in 2002. The Blackburn Press, Caldwell, NJ.