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Invasion of trees into grassland is primarily a phenomenon of plant competition. It involves the relation of the invaded and the invader to light and other atmospheric factors of the habitat, both physical and biotic. But perhaps even to a greater degree the edaphic factors, and especially water in dry climates, are concerned. Consequently to understand the reasons for plant distribution and the changes resulting from natural successions, a knowledge of the parts of the plants underground and of the interrelations of the root systems is of fundamental importance.
The great tension zone or ecotone between the deciduous forest formation of the eastern United States and the grassland formation that centers west of the Missouri River occurs, in part, in eastern Nebraska. It offers a convenient and fascinating field for the study of the age-long struggle between grassland and woodland for possession of the territory bordering the Missouri River. Here the many factors of soil and climate, controlled by limited humidity above ground and scarcity of water beneath, offer conditions under which it is just possible for sturdy pioneer trees to ecize but never to develop to the stature of their kind under more congenial conditions of growth. Against their invasion Andropogon and other grasses com- pete most severely and in the main successfully.