Date of this Version
From the highlands of central Mexico entirely across the United States and northward into Canada extends the great midcontinental area of grassland. From this central mass, prairie extends westward across Wyoming into eastern Utah, and southwestward through northern New Mexico into northern Arizona. Other grasslands in the northwest cover most of southern Idaho, a part of northern Utah, large areas in eastern Oregon and Washington, and recur in British Columbia. The Pacific prairie occupies the Great Valley of California, and the Desert Plains grassland much of southern Arizona and New Mexico and southwestern Texas. Together they constitute the Grassland or Prairie Formation, which is the most extensive and most varied of all the natural units of vegetation of the North American continent. In fact, grassland formerly covered 38 per cent of the land surface of the United States (Shantz & Zon 1924).
Throughout the entire prairie or grassland formation the climate is more favorable to grasses than to trees or shrubs or indeed any other type of vegetation. But within the vast range of grassland climate there are marked differences in degrees of favorableness or unfavorableness to growth even for species of grassland, as is illustrated especially by differences of precipitation and relative rates of evaporation. Temperature and length of growing season are of less importance, since all of the grasslands seem to lie well within summer temperature limits favorable to growth of the grass life form. Since rainfall decreases and evaporation increases from east to west in the great midcontinental area, there have resulted several different types or associations of prairie, each limited in extent by a distinctly different minor grassland climate. These associations have been determined and their approximate boundaries delimited after long study by Dr. Frederic E. Clements, ecologist for the Carnegie Institution of Washington. They are known as Tall-grass Prairie, True Prairie, and Mixed Prairie, respectively.
In correlation with the amount of precipitation, grasses fall rather naturally into three groups: tall grasses, such as big bluestem and slough grass; mid grasses, as June grass and prairie dropseed; and short grasses, illustrated by buffalo grass and blue grama. Prairie everywhere owes its character to the most important or dominant grasses. These are called dominants since they largely control the abundance, vigor of growth, and often the very existence of other species. This control is exerted through their effects upon the water supply, light, and other factors of the surroundings or environment. Most of these dominants are bunch-formers although some propagate by rhizomes, and rarely by stolons, to form a dense sod.
Prairie is not merely land covered with grass. It is a complex and definite organic entity with interrelated parts developed and adjusted throughout very long periods of time. Prairie is the handiwork of climate and of soil. Vegetation is not only closely adjusted to these agencies but is an expression of them. It is quite as proper to speak of prairie soil and prairie climate as of prairie vegetation. Prairie may be considered from many points of view; a most important one is that of the species of which it is composed. Although a prairie is distinguished by its dominant species, subdominant and secondary species are also important. Several years of study over an area of 60,000 square miles in the true prairie of the Missouri Valley have shown that there are about 10 dominant or controlling species which make up the general background of vegetation. In addition, a group of 25 minor grasses and sedges of uplands was determined.