The Disappearance of an Area of Prairie in the Cook County, Illinois, Forest Preserve District

Abstract

Typical prairie and characteristic forest occur within a few feet of each other. 'Climatic factors cannot be used to explain the differences. Drainage, fire, and grazing are factors in determining which shall be present, forest or prairie, but taken seplarately or together, they do not give the answer to the question. Cowles (1929) states that the presence of forest or prairie rests on factors starting in the past and continuing to the present with cumulative differences, especially in soil. 'This is only a partial truth. In his lectures Cowles often pointed out that tree seeds do not germinate and seedlings survive in prairie. Soil studies have afforded evidence that 'over long periods of time with climatic changes, forest has invaded prairie and prairie has invaded forest. Devious patterns of forest and prairie soil occur in areas from which forest has been removed. Speaking in terms of a few centuries, prairies are doubtless 'as persistent as Cowles (1929) assumed. There was no evidence of any invasion of prairie by woody plants in 1905 through 1915 in the area shown in Fig. 1, and there is no reason to assume that there had been any appreciable change in the wood's edge for at least a century, or perhaps much longer. The area is a part of a practically continuous prairie, reaching west nearly to the Mississippi River, interrupted only by stream-skirting forest and groves. The tract, shown in the photographs, out to the tip of the forested parts contains about 'three acres of prairie. It is located east of the Ch'icago Zoological Park (Brookfield, Illinois), between First Avenue and the Des Plaines River, anid south of Thirty-first Street. Approximately two acres of the prairie area were purchased by the Cook County Forest Preserve in 1917. The limits of the original forest can be determined by the location of the liarge old oaks and other trees along the south and east, but not 'the north, margin of