Common (Translated Scientific) Name: Mesomorphic Shrub & Herb Vegetation Class
Colloquial Name: Shrub & Herb Vegetation
Hierarchy Level: Class
Type Concept: Shrubs and herbs with broadly mesomorphic (including scleromorphic) growth forms (including broad-leaved, needle-leaved, and sclerophyllous shrubs, some types of rosette shrubs, and forb and graminoid herbs) dominate this type. Vegetation structure is typically moderately open to closed canopy, with irregular horizontal canopy spacing and variable height, but typically <5 m, and where mesomorphic trees have <10% cover and mesomorphic shrub and herb growth forms have the majority of cover compared to xeromorphic or cryomorphic growth forms. But in tropical upland savanna regions, trees may have up to 40% cover, are <8 m tall, and the vegetation has a substantial graminoid layer. Climates range from tropical humid to (seasonal) dry to boreal and subalpine, with fairly moderate moisture and temperature conditions. Substrate moisture conditions vary from dry to wet. Vegetation includes upland grasslands, shrublands, open tree savannas, wetland emergent marshes, bogs and fens.
Diagnostic Characteristics: Shrubs and herbs in non-tropical savanna regions are at least 10% cover, mesomorphic trees <10% cover, and the majority of cover is composed of mesomorphic shrub (broad-leaved, needle-leaved, sclerophyllous, and rosette shrubs) and herb (forbs and graminoids) growth forms compared to xeromorphic or cryomorphic shrub and herb growth forms. The vegetation structure has irregular horizontal canopy spacing. In tropical upland savanna regions, trees typically have up to 40% cover, are <8 m tall, and the vegetation has a substantial graminoid layer.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: This class includes both non-treed grasslands and shrublands, and open tree savannas, where tree canopy cover is typically <10%. Non-tropical tree savannas, defined by a strong graminoid layer (>50% in UNESCO 1973) and some level of open tree cover (10-25% cover in UNESCO (1973), 10-30% cover in Nelson (2005), 10-50% cover in Curtis (1959)), often closely resemble open upland grasslands and shrublands, especially in grassland regions. Thus, in this particular realm of vegetation, the open tree canopy, in combination with the ground layer, helps define savanna vegetation, distinct from forests or woodlands (e.g., Nelson 2005). But in many other regions and vegetation types (e.g., forested swamps and bogs, longleaf pine woodlands (Peet 2006), California oak woodlands (Barbour et al. 2007), subarctic woodlands), the role of canopy closure varies. Thus, in non-tropical regions, we generally place tree savannas, where trees >5 m have >10% cover, with woodland and forest. However, as Dixon et al. (2014) and others suggest, there may be good ecological reasons to treat upland tropical tree savannas, with up to 40% tree cover, trees <8 m tall, and a substantial graminoid layer, as part of 2. Shrub & Herb Vegetation Class (C02), and we allow for that option here.
Croplands with a spontaneous ground layer of "weeds," where the annual rotation of crops prevents a persistent ground layer of herbs, are treated in 7. Agricultural & Developed Vegetation Cultural Class (CCL01). If agricultural practices are abandoned, these sites may succeed to more persistent "ruderal" grasslands and shrublands (and forests) and would be placed in this class.
Where grasses and shrubs overtop low trees ("brush prairie"), the stands are placed here in Shrub & Herb Vegetation. These situations may often occur in grassland or cool semi-desert regions (e.g., North American Great Plains or Great Basin). Also challenging are some scrub trees, such as juniper (Juniperus spp.) or mesquite (Prosopis spp.), which can form shrub-like stands, and are typically <5 m tall at maturity in parts of their range. Where forms are not easily defined, woody plants equal to or >5 m at maturity are considered trees (FGDC 2008). In the United States, there are shrubby junipers (Juniperus communis, Juniperus pinchotii) that should be treated as shrubs. But pygmy conifers can potentially make 5 m and are treated as trees (Pinus edulis, Juniperus scopulorum, Juniperus monosperma, Juniperus occidentalis, Juniperus osteosperma).
There is a group of ephemerals that are generally mesomorphic, which can, under optimal rainfall conditions, grow on sand, including desert sands and pavement. They are adapted to deserts through their seed characteristics. Typically they do not occupy extensive deep shifting sands, only margins of sandsheets and more stable sands. They rely on seedbanks that may persist for decades in the sand and germinate under optimal winter or summer episodes of heavy rains. These are treated here as part of the mesomorphic shrub and herb class. Examples of psammophytic species include some sand verbenas (Abronia spp.) and several Asteraceae (e.g., Dicoria canescens, Palafoxia spp., etc.).
Similar NVC Types:
C04 Polar & High Montane Scrub, Grassland & Barrens, note: Distinguishing cryomorphic vegetation from mesomorphic can be a challenge in both arctic and alpine regions. Here we allow mesomorphic extensions into the cryomorphic climatic zone to be retained in the mesomorphic class (e.g., boreal mesomorphic tall willow shrublands along arctic drainageways are excluded from Cryomorphic Vegetation), whereas prostrate or dwarf willow species are treated as part of the cryomorphic class.
CCL01 Agricultural & Developed Vegetation, note: Agricultural grasslands in this class may be regularly mowed, hayed or grazed, and have an atypical "cut" structure periodically throughout the season, and may be dominated by exotic grasses. Developed grasslands in this class have a very regularly, often very low, cropped structure because of regular cutting or mowing (even weekly during the growing season).
C03 Desert & Semi-Desert, note: Desert grassland growth forms currently are not distinguished from temperate (mesomorphic) grasslands, and, where these grasslands lack xeromorphic shrubs, there may be no growth forms that provide xeromorphic criteria. In addition, the transition from dry temperate grassland to cool or warm semi-desert may be challenging, and rely more on floristics than growth forms. Desert grasslands lacking many xeromorphic associates in the sense of those in the southwestern United States (New Mexico, Texas, or Arizona) may be considered monsoonal and, thus, could be considered mesomorphic grasslands. Further review is needed.
C01 Forest & Woodland, note:
C06 Open Rock Vegetation, note: Sparsely vegetated sand dunes are similar to this class, but the vascular species present are typically mesomorphic, with adaptations to rooting in sand (psammophytic); crustose lichens and ferns are typically absent. Unstable scree may also contain scattered mesomorphic herbs and little to no nonvascular vegetation, but for now are retained in this class.