Common (Translated Scientific) Name: Mesomorphic Tree Vegetation Class
Colloquial Name: Forest & Woodland
Hierarchy Level: Class
Type Concept: Trees with broadly mesomorphic (including scleromorphic) growth forms (including broad-leaved, needle-leaved, sclerophyllous, palm, and bamboo trees, and tree ferns) characterize this type. Vegetation structure typically has irregular horizontal spacing. The mesomorphic tree canopy is typically >10% cover and often exceeds 5 m in height, and often has both a mature (overstory) and regeneration layer, except in tropical upland savanna regions, where trees typically have >40% cover, are >8 m tall, and the vegetation lacks a substantial graminoid layer. Climates range from humid tropical to boreal and subalpine, with fairly moderate moisture and temperature conditions. Substrate moisture conditions vary from dry to wet. Vegetation includes tropical, temperate, and boreal forests and woodlands.
Diagnostic Characteristics: Mesomorphic tree growth forms (broad-leaved, needle-leaved, and sclerophyllous trees, palms, bamboo trees, and tree ferns) have >10% canopy cover, a spontaneous, irregular horizontal canopy spacing, and overtop other growth forms, except in tropical upland regions, where trees typically have >40% cover, are >8 m tall, and the vegetation lacks a substantial graminoid layer.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: Plantations, ruderal, and native forest and woodland stands are typically placed in this class. Some plantation forests can be placed here rather than in 7. Agricultural & Developed Vegetation Cultural Class (CCL01) despite having a canopy layer that may be very regular (spaced in planted rows), if the tree regeneration, shrub and ground layers develop spontaneously with irregular horizontal spacing, similar to native forests (i.e., regular human intervention does not occur for these layers). However, where plantations have highly regular horizontal structure, with regular human intervention in all layers (e.g., through cutting, spraying, ground-layer mowing), they may better be placed in 7. Agricultural & Developed Vegetation Cultural Class (CCL01).
Similarly, tree-dominated vegetation that has a natural, irregular horizontal spacing in the tree layer, but highly regular shrub or herb layers because of human intervention, should be placed in 7. Agricultural & Developed Vegetation Cultural Class (CCL01). For example, shade coffee, cacao, and other woody agricultural crops grown under native (or exotic) tree species and in which all non-treed strata layers are subject to regular human intervention should be placed in that class.
Included in this class are low-statured trees that overtop other growth forms. These include tree sapling and seedling stands with trees <2 m, where the tree growth form is at least 10% cover and overtops other growth forms. These include subarctic woodlands and coastal dwarf-tree woodlands that are <2 m but are single-stemmed with definite crowns. Conversely, treed bogs and fens, with low stunted trees and conical canopies, and otherwise sharing the moss and dwarf-shrub layers of open bogs and fens, are placed with those types. Where trees share the same strata as grasslands and shrublands (e.g., tallgrass brush prairie of 1-3 m, with trees <2 m), the vegetation is placed in the shrub- and herb-dominated classes.
Tree cover ranges from 10-100%, and is inclusive of what are called forests, woodlands, and tree savannas, at least in the non-tropical regions. The class limit of 10% tree cover (for mesomorphic tree growth forms) follows that of FAO's Global Forest Resources Assessment 2000 (FAO 2001). We exclude stands of trees planted primarily for agricultural production (such as fruit tree or forest plantations) and typically agroforestry, as well as those in developed lands, such as cities and roadsides (they are placed in Class 7, Formation 7.A.2). However, unlike FAO, we do not include what FAO calls "temporarily unstocked areas" (such as clearcuts, burnt areas) unless the tree growth form is actually present at 10% cover--even if in seedling or sapling form--and exceeds the height of other growth forms.
Other classifications may limit forests and woodland to a narrower range of tree cover. For example, forests and woodlands are sometimes defined as 25-100% cover, and distinguished from tree savannas with 10-25% cover which are included under broadly defined grassland and shrubland classes (Driscoll et al. 1984, FGDC 1997, Grossman et al. 1998, Minnesota DNR 2005b). UNESCO (1973) required that tree savannas have a strong herbaceous layer (>50% graminoid cover). Still, a variety of definitions of tree savanna have been published, from the 10-25% cover noted above, to 10-30% cover in Nelson (2005), 10-40% in UNESCO (1973), and 10-50% cover in Curtis (1959). Blue oak, Engelmann oak, and other woodlands in California Mediterranean woodlands are typically between 10 and 60% cover, and California State ecologists place these in Forest & Woodland (Barbour et al. 2007). However, apart from tree savannas, there are many other forest and woodland types and regions where the degree of canopy closure plays less of a role in defining types, for example, forested swamps and bogs, longleaf pine woodlands (Peet 2006), eastern pine barrens, and subarctic woodlands.
Thus, given the variety of situations, we have chosen a more inclusive definition of forest and woodland that encompasses most tree savanna concepts, and rely on lower levels of the hierarchy to make distinctions based on a combination of biogeography, ecology, and floristics. For example, in the context of the eastern Great Plains tallgrass prairie-forest border, it is common to recognize oak savannas, based on open tree cover levels (e.g., 5-30% tree cover in Nelson 2005), as separate from oak woodlands (30-80% cover) and oak forests (80-100% cover). Savannas may be distinguished from forests or woodlands, but these structural distinctions often correspond to lower-level units of the NVC hierarchy (e.g., group, alliance, and association). Still, Dixon et al. (2014) and others suggest that there may be good ecological reasons to treat upland tropical tree savannas, with 10-40% 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.
Similar NVC Types:
C02 Shrub & Herb Vegetation, note: "When grasses and shrubs dominate the ground layer and tree cover is near but >10%, is clumped, or overtopped by grasses and shrubs, there may be cases where assignment to this class is preferable. Short mesomorphic trees that do not overtop shrubs and herbs are placed in this class. However, in tropical upland savanna regions, stands may have 40% tree cover, where trees are <8 m tall, tree regeneration is sparse to absent, and there is a substantial graminoid layer."
CCL01 Agricultural & Developed Vegetation, note: "Forests found in urban parks and lawns may have an irregular horizontal spacing but a highly regular or mowed understory and are placed in this developed vegetation class. Forests, such as orchards and forest plantations, typically have either a very regularly spaced tree canopy (often pruned or trained) and/or plowed or regularly manipulated ground layer (e.g., tree-shaded shrub or herb crops)."
C03 Desert & Semi-Desert, note: "Occasionally xeromorphic trees may be sufficiently dense to form woodland stands (e.g., saguaro, Joshua tree, microphyllous-leaved mesquite woodlands, tropical thorn woodlands). The dominance of xeromorphic growth forms places these stands in the Desert & Semi-Desert class rather than Forest & Woodland."
Physiognomy and Structure: Growth Forms: Stands are dominated by mesomorphic trees, including broad-leaved deciduous (including mostly deciduous, winter-deciduous, facultatively-deciduous), broad-leaved evergreen (including mostly evergreen), needle-leaved (deciduous and evergreen), sclerophyllous trees, palms, bamboo trees, and tree ferns. Tree growth forms generally have a single main stem and more-or-less definite crowns, and may be as low as 2 m (scrub trees) (e.g., tropical cloud elfin forest, mesquite woodland). Where growth forms are not easily defined, woody plants equal to or >5 m at maturity are considered trees (FGDC 2008).
Structure: Typical stands have one or more of the tree growth forms exceeding 5 m in height, and the canopy has irregular horizontal tree stem spacing, with a minimum of 10% canopy cover (8 m in height and minimum of 40% in tropical forests and woodlands, as low as 2 m in subarctic woodlands). A tree regeneration layer is often present, along with various associated growth forms (shrubs, herbs, nonvasculars). Stands of trees where the tree growth form is short (<5 m) in height but overtopping shrub and herb growth forms, may also be placed in this class.
Dynamics: Fire, wind, and insects are most common; other pathogens, e.g., root- and heart-rots may be significant in creating openings in canopy.