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2-6 I . I I I I I . -. I . I I . I . I I . One climatic factor of great importance because of the agricultural activity in the basin is the length of the frost-free period. The frost-free period is the average number of days each year between the last freezing temperature in the spring and the first frost in the autumn. While the frost-free period does not completely define the growing season for all crops and grasses, it is a general indicator of the most favorable period. The length of the frost-free period in the basin is about 30 days in the higher elevations of the Rocky Mountains, about 140 days on the Great plains, and about 180 days in the Interior Highlands. BIOLOGICAL AND ESTHETIC CHARACTERISTICS Terrestrial Biological Resources Ecosystems of the Missouri River Basin were originally dominated by of grasslands in the prairies and plains region, and by forests, shrublands, mountain grasslands, and alpine tundra in the mountainous regions. Gallery forests have developed along the major rivers and streams, especially in the Middle Missouri and Lower Missouri subbasins. During the last few years many grasslands of the plains and prairie region have been converted to cropland where soils, topography, and climate favorable to farming exist. Although river bottomland forests in many parts of the basin, have been eliminated or have deteriorated, many still afford key habitat for native plants and animals. Sizable acreages of natural grasslands also persist in several areas. The shrubland, woodland, and forest land of the mountains have not been subjected to the disturbances such as those which have occurred on the prairies and retain more of their natural characteristics. Native plants and animals in the Missouri River Basin have been exposed to a succession of changing environments in the geological past, and only a few known endemic species have managed to survive the extremes of the ice age.