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 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:
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:
C02 Shrub & Herb Vegetation, note: "Desert grasslands typically include xeromorphic shrubs and trees, which place them in the Xeromorphic class. Environmental drivers and stressors can cause desert grasslands to cross thresholds into desert scrub. They may also cause dry mesomorphic grasslands with small-leaved shrubs to appear similar to xeromorphic grasslands (e.g., shortgrass prairie in the North American Great Plains)."
C04 Polar & High Montane Scrub, Grassland & Barrens, note: "In montane deserts, there may be a transition from cool semi-desert to alpine that may be difficult to distinguish."
C01 Forest & Woodland, note:
C06 Open Rock Vegetation, note: "This class is restricted to open rock vegetation where nonvascular vegetation is at least 10% and where vascular plants, if present, are mesomorphic."
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.
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).