Although grassland is the characteristic vegetation of the Great Plains, contact with forests and woodlands occurs at the boundaries of the region, and significant areas of transition between woodland and grassland vegetation exist. Trees are also associated with river systems and various physiographic features within the Plains.
Tallgrass prairie is the dominant vegetation of the eastern Great Plains. In Kansas and Oklahoma a broad transition zone occurs where the tallgrass prairie contacts the deciduous forest of eastern North America, resulting in a mosaic vegetation combining elements of both plant communities. Where prairie dominates and the tree canopy cover is less than 50 percent, the vegetation is called savanna. The trees most commonly associated with savannas are various species of oaks. Bur oak (Quercus macrocarpa) dominates the savannas of the northeastern Great Plains. Post oak (Quercus stellata) and blackjack oak (Quercus marilandica) are the primary tree species in the savanna region, called the Cross Timbers, of Kansas, Oklahoma, and Texas. The balance between tallgrass prairie vegetation and woody plants in savannas was maintained historically by fire. Trees and other woody species increase in number in savannas where fire is suppressed.
In Manitoba, Saskatchewan, and Alberta, where tallgrass and mixed prairie contact the boreal forest of the north, a transition zone, called the Parkland Belt, occurs. This is a region of grasslands interspersed with groves of trees. Bur oak is the most common tree in the eastern portion of this zone and is replaced by aspen (Populus tremuloides) to the west. Isolated, islandlike occurrences of aspen/bur oak parklands occur to the south in North Dakota.
In Montana, Douglas fir (Pseudotsuga menziesii) forest occurs on the plains east of the Rockies in association with isolated mountains such as the Highwood and Bearpaw. Disjunct occurrences of ponderosa pine (Pinus ponderosa) occur as forest and savanna in association with landforms such as the Pine Ridge Escarpment in western Nebraska and the badlands of western North Dakota.
The Black Hills of western South Dakota and adjacent Wyoming are forested with a unique assemblage of trees and other plants dominated by Rocky Mountain species but also including species from the deciduous forest of eastern North America and the boreal forest of Canada. A similar mixing of elements from these three forest types occurs in the Niobrara River valley of north-central Nebraska.
In northeastern New Mexico and adjacent parts of Colorado and Oklahoma, areas of woodland dominated by piñon pine (Pinus edulis) and juniper (Juniperus spp.) occur in association with escarpments, canyons, and other physiographic features of volcanic origin. Juniper savanna often occurs where the shortgrass prairie comes into contact with these woodlands.
At the southern reaches of the Great Plains in Texas the shortgrass prairie mingles with mesquite trees (Propsis glandulosa) to form mesquite savanna. Where the Southern Plains blends into the Texas Hill Country, a mosaic of woodland and grassland occurs with oaks and junipers being the important trees.
In addition to these transition areas at the boundaries of the Great Plains, forest vegetation penetrates the Plains along many river systems. A thin band of eastern deciduous forest dominated by oaks and hickories (Carya spp.) borders the Missouri River and its tributaries in eastern Kansas and Nebraska. The diversity of trees and other plant species diminishes upstream. In Oklahoma Cross Timbers savanna extends westward into the Plains along the Cimarron and Canadian Rivers.
Woody vegetation associated specifically with the floodplain zone of streams and rivers is termed riparian forest. Riparian forests occurred historically along much of the major rivers like the Missouri, Platte, and Arkansas. Cottonwood (Populus deltoides) was the main tree of these forests and often was the only tree along the western reaches of these rivers. Isolated, western occurrences of cottonwood were sometimes given the name "Big Timbers." Other frequently associated riparian trees include willows (Salix spp.), hackberry (Celtis occidentalis), boxelder (Acer negundo), and elm (Ulmus spp.). The Osage orange tree (Maclura pomifera), widely planted for hedges and windbreaks by pioneers and farmers, originally occurred in riparian habitat along the Red River in Oklahoma and Texas.
Occurrences of woodlands add significantly to the biodiversity of the Plains, having more associated species of birds, mammals, and other animals than the adjacent grasslands. These areas were also important to the Native peoples of the Plains, providing firewood, lodge poles, bow wood, and other aspects of their material culture, as well as providing sheltered places for camps.
James H. Locklear Nebraska Statewide Arboretum
Babour, M. G., and W. D. Billings, eds. North American Terrestrial Vegetation. New York: Cambridge University Press, 1988.
Great Plains Flora Association. Flora of the Great Plains. Lawrence: University Press of Kansas, 1986.
Küchler, A. W. Potential Natural Vegetation of the Coterminous United States. New York: American Geographical Society, 1964. Map.