Physiography of the Prairies and North-Eastern Australia

Abstract

THE much-debated problem why the prairies of the United States are treeless is, according to an article by Mr. B. Shimek in the Bulletin of the State University of Iowa, new series, No. 35, essentially one for the botanist, since, despite variation in surface-conditions, there is comparative uniformity in the flora throughout the area. Summarising the available evidence, the author concludes that exposure to evaporation, as determined by temperature, wind, and topography, is the primary factor in the development of the treeless condition, and that the flora persists in the exposed areas because of its xerophytic character. On the other hand, rainfall and drainage, although important as determining the amount of moisture in air and soil, are only a secondary factor, as they may be equal in the forested and treeless areas; while the nature of the soil and the geological formation affect the matter only so far as they induce conservation of water. Prairie-fires were an effect rather than a cause, and when they did act in the latter sense were but local, while seed-dispersal, although accounting for the growth of plants, will not explain the origin and presence of the flora as a whole. Finally, such agencies as the bison and the action of the sea do not enter into the problem at all.