More than 2,900 species of vascular plants from some 730 genera in 159 families grow in the Great Plains. The vast majority (all but 81 species in 8 families) are angiosperms (flowering plants). For plants the Great Plains has one of the most stressful climates: hot summers, cold winters, frequent droughts, with dramatic seasonal and annual variation. Consequently, the plants that dominate the region are herbaceous grasses and forbs (wildflowers) derived from the plant families of temperate regions rather than trees or plants from families of tropical regions. The families with the most genera and species are sunflowers (Asteraceae), with more than 100 genera and 430 species, grasses (Poaceae, 76 genera, 260 species), sedges (Cyperaceae, 13 genera, 220 species), and legumes (Fabaceae, 34 genera, 158 species, or 46 genera, and 178 species if Caesalpinaceae and Mimosaceae are included in the Fabaceae). The precise numbers are subject to revision as taxonomists improve their understanding of the flora, but the pattern is unlikely to change. Other groups with numerous species are mustards (Brassicaceae), lilies (Liliaceae), umbels (Apiaceae), mints (Lamiaceae), and the penstemon family (Scrophulariaceae).
Compared to North American deserts and forests, Plains ecosystems formed very recently. Consequently, there are only about 100 endemic species and no endemic genera or families. The most common endemic species are composites (Asteraceae, 27 species), and legumes (Fabaceae, 21 species, especially Astragalus). Surprisingly, there are no endemic grasses.
Plants that are naturally rare in this region tend to occur in restricted habitats, especially at the western edge of the region, or in scattered microhabitats, such as salt marshes. However, reduction of formerly widespread ecosystems such as tallgrass and midgrass prairies to tiny fragments is putting many previously common species at risk of extinction. In addition, changes in land use, such as continuous grazing and reduction of fire, pose threats to some species: If a plant decreases under grazing, or requires fire to prosper, it may decline even if the area is otherwise undisturbed and well managed. Historically, both grazing and fire occurred episodically, and some species relied on the changing conditions.
Approximately 12 percent of all Plains species have recently been introduced to the region: plants from the eastern United States (previously excluded by drought, recurrent fire, and absence of tree cover), plants from the western United States (previously unable to cross the mountains), and plants from Europe, Asia, and occasionally Africa and South America brought by humans. Some of these, such as Salsola (tumbleweeds), are able to invade natural ecosystems, but most introduced plants remain associated with humans. These can be expected to steadily increase in number and diversity where human impact increases. The introduced plants form the basis for an ongoing evolution of the flora, in some cases changing in response to Plains conditions, and in other cases hybridizing with their native relatives to form new varieties and possibly species.
Great Plains flora is dominated by perennial herbs, plants that die back to the roots each winter then resprout in the spring. Trees are abundant on the edges of the region and in the larger river valleys of the eastern Plains. Beyond these areas, trees and shrubs, because they are more injured by prairie fires than herbs (especially grasses), were historically found only in areas protected from fire, such as near cliffs or on islands and at bends in rivers. A few shrubs–lead plant (Amorpha canescens), for example–survive well within the open grassland. Others, such as sumac (Rhus) and dogwoods (Cornus), rapidly increase in unburned prairie.
Kathleen H. Keeler University of Nebraska-Lincoln James H. Locklear Nebraska Statewide Arboretum
Great Plains Flora Association. Flora of the Great Plains. Lawrence: University Press of Kansas, 1986.