Date of this Version
As a memorial to Dr. Frederic E. Clements, it is a pleasure to present this paper on the midcontinental grasslands of North America. Born and reared at Lincoln, Nebraska, he developed a remarkable interest in and love for the prairie. He has often stated to the first author that of all the vegetation of North America, he liked the grassland best. Clements was the first to recognize the Mixed Prairie as a distinct plant association and to describe its nature and range and the grouping of the dominants ( 1920).
When one examines the prairies of western Iowa and then proceeds some distance westward, he is impressed by several changes. The vegetation is reduced in height, it becomes less dense, ant1 takes on a distinctly more xeric impress. These changes result from gradually increasing unfavorable water relations as the vegetation of True Prairie gives way to that of Mixed Prairie. Along the Nebraska-Kansas interstate line, for example, the 33-inch annual precipitation near the Missouri River decreases westward at an average rate of about an inch every 15 miles. Conversely, rate of evaporation rapidly increases.
In Nebraska, the transition from True to Mixed Prairie occurs over three types of topography. South of the Platte Valley Lowland it has been traced over the nearly level portion of the Loess Plain Region (Fig.1) . Northward, the transition takes place in the greatly dissected Loess Hills and Plains. East of the northern Sandhill Region, it occurs on the Holt-Pierce Plain and Boyd Shale Plain.