Date of this Version
Plants other than grasses constitute an important part of the native grasslands of North America. Most of these are forbs, a few are shrubs and half-shrubs. Probably 85 to 90 percent of the forbs are perennial and most are as deeply rooted and many far more deeply rooted than the grasses. Their number varies considerably from place to place. In the prairie of the central part of the North American Lowland and on the hard lands and sandy soils of the Great Plains they compose, perhaps, between 5 and 15 percent of the vegetation. They are least abundant in the drier habitats.
A recent summary of the community root habits of grasses and an interpretation of the findings included a discussion of soils and climate of each of these extensive areas (Weaver 1958). Hence this need not be repeated here. Suffice it to say that in mixed prairie westward the drier air and decreased water content of soil become limiting factors for growth. Forbs decrease gradually in number, size of individuals, and in the extent of their groupings in societies. This condition prevails despite the fact that the more mesic species of prairie are largely replaced by more xerophytic ones of the Great Plains.