Classification of Root Systems of Forbs of Grassland and a Consideration of Their Significance

Abstract

Plants other than grasses constitute an important part of the native grasslands of North America. M\4ost 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 ileed 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. Forbs accommodate themselves not only to the climate but also to the environment imposed upon them by the grasses. On lowland prairie most forbs are tall. They grow even more rapidly than the grasses and may for a long time or continuously overtop them. Many other species flourish oni upland true prairie where they form extensive societies. Much competition occurs between grasses and forbs, not only for light but also for water and nutrients. Roots of most grasses occupy only the first 4 to 6 feet of soil. Usually forbs are more deeply rooted. Their roots have rates of growth not unlike those of grasses and depths of 3 to more than 4 feet may be attained during the first growing season and 8 to 15 feet at maturity. Hence, they may absorb partly below the root zone of grasses and thus lessen root competition. Perhaps 90 percent of the abundant forbs, like most prairie grasses, are not only perennial but have a life span extending over a long period of years. The root habits of a large number of species of forbs have been examined during a period of several years throughout a wide territory from western Iowa, through Nebraska and Kansas, to the mountains of Colorado. Nearly all of the root systems studied were those of mature perennial plants, undisturbed by grazing. Nearly always 3 to 8 plants of a species were examined. Drawings were made simultaneously with the excavating of the roots and always to exact measurements. In the drawings the roots were arranged as nearly as possible in their natural position in a vertical plane. At each examination the work extended over a field sufficiently large, sometimes several square miles in extent, so that the effects of any local differences in soil texture or structure were largely eliminated.