Common (Translated Scientific) Name: Bur Oak - Quaking Aspen / Western Wheatgrass Woodland Division
Colloquial Name: North American Great Plains Forest & Woodland
Hierarchy Level: Division
Type Concept: This division is composed of aspen, oak and mixed hardwood woodlands found throughout much of the Great Plains, from central Kansas to the Canadian aspen parkland region. Deciduous trees dominate most stands. Trees are typically short to medium in height, and the canopy can vary from open to closed (10-100%). Quercus macrocarpa is common across much of eastern part of the range; Populus tremuloides and Betula papyrifera are most abundant northward, and scattered in the southern parts. Fraxinus pennsylvanica and Ulmus americana are abundant in ravines and draws in the central and southern parts of the range. The shrub stratum can be nearly absent but is typically moderate to dense. Amelanchier alnifolia, Corylus spp., Elaeagnus commutata, Prunus americana, Prunus virginiana, Ribes spp. (including Ribes oxyacanthoides), Rosa spp. (including Rosa arkansana, Rosa woodsii), Salix spp., Shepherdia argentea, Symphoricarpos albus, Symphoricarpos occidentalis, and small trees are common. The understory is typically dominated by shrubs, grasses and sedges more tolerant of shade, but may also be common in the surrounding prairies. Among these are Andropogon gerardii, Calamagrostis canadensis, Festuca spp., Pascopyrum smithii, Sorghastrum nutans, and Sporobolus heterolepis. Most stands occur on the landscape where water accumulates and where there is some protection from fire, i.e., in ravines, near rivers or ponds, or on mesic slopes. Northward, where the woodlands approach the boreal forests, trees are more common and this type is more widespread on the landscape. Stands occur on a variety of soils, though fine-textured soils are more common. Fire, drought, and grazing are important drivers of the system.
Diagnostic Characteristics: This type is distinguished from the grasslands adjacent to it across much of its range by having <10% tree cover. In the north, other forests can be adjacent but they are typically dominated by coniferous trees.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: Recognition of Great Plains Forest & Woodland as its own division, with a single macrogroup, is controversial. The Great Plains generally have a lower diversity of species than other temperate biomes in North America. In earlier drafts of the USNVC, the single macrogroup of this division was included with eastern North American forests, resulting in a division that extended from the Atlantic Coast westward to Alberta, Montana, Wyoming and Colorado. A major reason for recognizing this division is that there are only a handful of tree species that extend from either 1.B.2.Na Eastern North American Forest & Woodland Division (D008) or 1.B.2.Nb Rocky Mountain Forest & Woodland Division (D194), and yet these few trees species are spread across a large biogeographic and climatic region. They also contain a ground layer that may be rather distinct from the Eastern or Rocky Mountain forests and woodlands. Currently this division includes Great Plains forests and woodlands that share tree species with eastern or boreal forests, but it may be that several Pinus ponderosa or Juniperus scopulorum associations in the Great Plains could be added. We have not had an opportunity to fully review this possibility. All that said, there is another approach (C. Lea pers. comm. 2015) to this issue, namely to extend the Rocky Mountain division further east into the Plains, thereby reducing the westward fingers of the Eastern forests (though it does not address the Great Plains to boreal transition issue in the aspen parkland). That approach avoids the rather problematic issue of this division not having diagnostic species, though it doesn't address the added ecological and biogeographic variability of extending those divisions. It may also be that the Eastern forest division itself, even without the Great Plains component, is too heterogeneous, and realignment of that division might change the perspective here. A fuller review of these proposals are needed.
Where the northwestern Great Plains transition to mountains, this type may appear similar to early-successional stands of Rocky Mountain types at the lowest montane elevations where those occur.
Similar NVC Types:
D008 Eastern North American Forest & Woodland, note: "D328 could conceivably be merged with this division, as all of the species are found there, but the turnover is so high, and the understories rather different, that for now it is treated separately."
D011 Eastern North American-Great Plains Flooded & Swamp Forest, note: Mesic terraces of D328 may resemble floodplain types.
D023 Central North American Grassland & Shrubland, note: "When oak and aspen woodlands in D326 are fire-maintained, they may resemble stands of D023, but D023 has tree cover <10%."
Physiognomy and Structure: This division is dominated by deciduous trees, though the tree cover can vary from 10-100%. Trees are short to medium-tall (5-10 m tall). The shrub layer is usually moderate to dense with most shrubs 0.5-2 m tall. The herbaceous layer is also typically moderate to dense and dominated by a mix of graminoids, forbs and shrubs.
Floristics: Quercus macrocarpa is dominant in the eastern part of the range; Populus tremuloides and Betula papyrifera are abundant northward, especially in the aspen parkland regions, and more scattered in the southern parts. Fraxinus pennsylvanica and Ulmus americana are abundant in ravines and draws. Trees that can be locally common or abundant include Juniperus scopulorum in the western Great Plains, Populus balsamifera in the far northern Great Plains, and Tilia americana and Juniperus virginiana in the eastern Great Plains. The shrub stratum can be nearly absent but is typically moderate to dense. Amelanchier alnifolia, Corylus spp., Elaeagnus commutata, Prunus americana, Prunus virginiana, Rosa spp. (including Rosa acicularis, Rosa arkansana, Rosa blanda, Rosa woodsii), Salix spp., Shepherdia argentea, Symphoricarpos albus, Symphoricarpos occidentalis, and small trees are common. Crataegus spp., Juniperus horizontalis, and Cornus sericea can be locally common. The understory is typically dominated by low shrubs, grasses, sedges or forbs, some of which may be common in the surrounding prairies, particularly if woodlands are allowed to burn under natural fire regimes. Common grasses found in more open, fire-maintained stands include Andropogon gerardii, Calamagrostis canadensis, Festuca spp. (including Festuca altaica), Pascopyrum smithii, Sorghastrum nutans, and Sporobolus heterolepis.
Dynamics: This type occurs in a landscape where fire was historically common, in combination with periodic droughts. Fire restricts this type within much of its range, limiting the trees to protected places on the landscape. Fire was used by aboriginal peoples for maintaining open grassland. A reduction in fire frequency typically allows this woodland type to spread into surrounding prairies, though it also allows the tree canopy of established stands to close with a reduction in the prairie flora in the understory. In the northeastern Great Plains, conditions are more favorable for tree growth and fire is necessary to maintain this type. A significant reduction in fire frequency allows the woodlands to succeed to forests and the prairie plants are replaced by a forest understory.
Periodic drought and grazing also help maintain the open canopy of this type with intense grazing by bison, elk, and other grazers to limit woody regeneration. The combination of natural fire regimes and grazing pressures, especially in the northern part of the range, contributed to a dynamic natural landscape tension between parkland and prairie, with trees advancing in times of greater moisture and/or less grazing. Native wildlife (e.g., buffalo wallows) also created areas of exposed mineral soil that acted as seed beds for aspen, willow, etc. seedling establishment (Bird 1961).
Environmental Description: Across much of its range, stands occur on landscape positions that receive more water than the surrounding landscape, i.e., in ravines or canyons, near rivers and lakes, and on mesic, typically north-facing slopes. In the aspen parklands of the southern Canadian provinces, Montana, North Dakota and northwestern Minnesota, this type is common on flat or rolling topography. In these areas, the evapotranspiration rate and precipitation are more favorable to tree growth and trees are not as restricted to protected landscape positions. See Zoltai (1975) and Hogg (1994) regarding climatic balance between parkland and boreal forest in Canada. Stands can occur on a variety of soil textures but fine-textured soils are more common. The aspen woodlands in the parkland region are more mesic than stands found in the rest of the Great Plains and wet-mesic or even wet pockets are common.
Geographic Range: This type is found throughout the central and northern Great Plains from Kansas and Colorado north to southeastern Alberta, southern Saskatchewan, southwestern Manitoba, northern North Dakota and northwestern Minnesota. It may occur in Oklahoma.
Nations: CA, US
States/Provinces: AB, CO, IA, KS, MB, MN, MT, ND, NE, OK?, SD, SK, WY
|US Forest Service Ecoregions (2007)|
Southern Rocky Mountain Steppe - Open Woodland - Coniferous Forest - Alpine Meadow Province
Western Great Plains Section
Confidence Level: Moderate
Confidence Level Comments:
Concept Author(s): Faber-Langendoen et al. (2015)
Author of Description: D. Faber-Langendoen and J. Drake
Version Date: 13Jan2016
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