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C03 Xeromorphic Woodland, Scrub & Herb Vegetation Class

Type Concept Sentence: Cool and warm deserts and semi-deserts dominated by xeromorphic growth forms, including succulent (e.g., cacti, euphorbias) and small-leaved shrubs and trees, desert grasses and other xeromorphic growth forms, with an irregular horizontal canopy spacing that is often open to very sparse (1%) cover.

Common (Translated Scientific) Name: Xeromorphic Woodland, Scrub & Herb Vegetation Class

Colloquial Name: Desert & Semi-Desert

Hierarchy Level:  Class

Type Concept: Succulents, small-leaved shrubs and trees, desert grasses and other xeromorphic growth forms are dominant or characteristic in this type, which can include very open rocky or sandy desert types. Vegetation often has open and irregular horizontal canopy spacing, typically <5 m tall. Mesomorphic trees have <10% cover, and xeromorphic growth forms [including succulent trees and shrubs (e.g., cacti, euphorbias)] and small-leaved shrubs and trees, have the majority of cover compared to mesomorphic or cryomorphic growth forms. The herb cover varies from open to absent, with various growth forms, including ephemerals and succulent forbs. Climates include warm (tropical) to cool (temperate) arid to desert, with regular drought periods. Substrates are dry to moist, sometimes strongly haline-alkaline, or rarely, temporarily wet (e.g., desert washes). Vegetation includes cool and warm deserts and semi-deserts.

Diagnostic Characteristics: Xeromorphic tree (succulent tree, small-leaved tree) or shrub (succulent shrub, small-leaved shrub) growth forms have the majority of the cover compared to mesomorphic and cryomorphic shrub and herb growth forms, and mesomorphic trees have <10% cover. The vegetation has irregular horizontal canopy spacing, and may be as low as 1% cover.

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: In desert regions, the vegetation may be very sparse (at or below 1%). Some judgment is required to ensure how to sample and classify vegetation in stands where there is <1% vascular plant cover in parts, but not all, of the stand. That is, some deserts may have only patchy non-vegetated areas, whereas others may have extensive non-vegetated areas. More work is needed to define xeromorphic herbaceous growth forms, such as ephemerals, that may appear under rare high-precipitation years.

Semi-desert grasslands may be hard to identify in this class when woody plants are absent or uncommon, because there are no herbaceous growth forms currently recognized that are diagnostic for the xeromorphic class. Desert grasses may be xerophytic but not clearly xeromorphic. However, in many cases they will have strong floristic similarity to open desert shrublands and, if degraded, may be dominated by desert shrubs. More work is needed to determine if such growth forms can be specified (e.g., desert-blooming ephemerals).

Similar NVC Types: No Data Available
note: No Data Available

Physiognomy and Structure: Growth Forms: Stands are dominated by xeromorphic growth forms (succulent tree, small-leaved tree, succulent shrub, small-leaved shrub, some types of rosette shrub) and various associated herb growth forms. Drought-deciduous, including facultatively drought-deciduous, trees may be present. Other characteristics include opportunistic leaf-flush (e.g., ocotillo), high incidence of energy allocation to protective structures such as spines, and allocation to heat- or moisture-loss structures. The various herb growth forms currently recognized in FGDC (2008) are not exclusive to the xeromorphic class, and more work is needed to assess whether additional herbaceous growth form types should be recognized that may be distinctive for this class. For example, Crassulacean Acid Metabolism (CAM) physiology is common in desert plants, especially among succulents.

In addition some desert vegetation has biological crust, comprised of soil particles, cyanobacteria, algae, microfungi, lichens, and bryophytes. The crusts do not develop under a closed canopy of vascular plants. They do occur in all climates following disturbance of that cover. However, the most conspicuous development of crusts occurs in hot, cool, and cold semi-arid and arid areas where plants are widely spaced.

Structure: Stands are very open and have irregular shrub or herb horizontal stem spacing, often with prominent non-vegetated surfaces. Xeromorphic trees are present, occasionally moderately dense (10-60%). There is <10% cover of mesomorphic trees, and xeromorphic tree, shrub and/or herb growth forms have the majority of cover compared to mesomorphic or cryomorphic shrubs or herbs. Tree cover varies from sparse to moderately closed (1% to >60% cover). Nonvascular and dwarf-shrub growth forms may vary from 0 to 100%. At maturity, xeromorphic trees typically are >2 m, tall shrubs typically >0.3 m, and herbs are of any height, but mostly >0.3 m. A biological crust layer may also be present.

Floristics: No Data Available

Dynamics:  [Description needed.]

Environmental Description:  Climate: In temperate climates, the growing season is typically very dry, and in tropical climates, droughts are persistent, and rain is minimal. Soil/substrate/hydrology: Soils may be thin, rocky, saline/haline (salt pannes, playas).

Geographic Range: This type is found in the Desert divisions in Bailey''s (1989) Dry Domain.

Nations: No Data Available

States/Provinces:  No Data Available

Confidence Level: Moderate

Confidence Level Comments: No Data Available

Grank: GNR

Greasons: No Data Available

Type Name Database Code Classification Code
Class 3 Desert & Semi-Desert Class C03 3
Subclass 3.A Warm Desert & Semi-Desert Woodland, Scrub & Grassland Subclass S06 3.A
Subclass 3.B Cool Semi-Desert Scrub & Grassland Subclass S11 3.B

Concept Lineage: No Data Available

Predecessors: No Data Available

Obsolete Names: No Data Available

Obsolete Parents: No Data Available

Synonomy: = Desert Biomes (Quinn 2009)

Concept Author(s): Hierarchy Revisions Working Group, Federal Geographic Data Committee (Faber-Langendoen et al. 2014)

Author of Description: Hierarchy Revisions Working Group

Acknowledgements: No Data Available

Version Date: 08-03-16

  • 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.
  • Belnap, J., and O. L. Lange, editors. 2003. Biological soil crusts: Structure, function, and management. Second edition. Springer-Verlag, Berlin.
  • FGDC [Federal Geographic Data Committee]. 2008. National Vegetation Classification Standard (Version 2.0). FGDC-STD-005-2008. Vegetation Subcommittee, Federal Geographic Data Committee, Reston, VA. 126 pp.
  • 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.
  • Quinn, J. A. 2009. Desert biomes. Greenwood Press, Westport, CT.