Ecological Studies in the Tension Zone between Prairie and Woodland

Upon the completion of a series of investigations in southeastern Washington and adjacent Idaho (23), in which there was found to be a direct relation between the available water content of the soil and the evaporating power of the air as succession progressed from gl'assland through scrub to forest, the senior writer moved to Minneapolis, Minnesota. Here such excellent conditions of prairie invaded by scrub and followed by forest offered themselves for study that the services of the junior author were enlisted and an analysis of the situation attempted. Later this study was supplemented by similar worK carried on at Lincoln, Nebraska. The time-worn controversy as to the origin of the prairies and the absence of trees from this great area of grassland has been a favorite theme for the layman and amateur as \yell as for the scientist. However, we need not concern ourselves here with a general discussion of the literature. A comprehensive bibliography including papers up to 1911 is given by Shimek (17) ; and Clements (3,1916) summarizes the successional work in the Prairie-Plains Climax. Notwithstanding the fact that the exact methods of permanent quadrat and factor measurements of modern ecology are rapidly replacing the earlier type of observation and deduction, few quantitative data looking towards a solution of the prairie-forest problem are yet available. Thornber (20, 1901) while studying the structure of the prairie formation at Nebraska City, Nebraska, maintained stations in prairie, Z anthoxylurn-Corylus thicket and woodland. Since the non-available water of the soil was not determined, his moisture content readings are of little value. Numerous thermometer and psychrometer readings showed the prairie to have the highest temperature and the lowest humidity. Likewise, the soil temperature of the 4 THE BOTANICAL SURVEY OF NEBRASKA praIrie (at 10 em.) was always the highest and showed a greater daily range (10-20° F.) than either of the other plant communities. Harvey (9,1908) working in South Dakota, summarizes the climatic and edaphic conditions during each of the seasonal aspects. Unquestionably his data on the soil moisture are the most valuable and this appears to be about the (jnly factor reading taKen in the field. The data on temperature , wind, mean relative humidity and relative evaporation , taken from the U. S. Weather Bureau at Yankton or calculated from data thus obtained, must have only a general significance as regards actual habitat conditions. The non-available soil moisture was determined …