Grasslands have adorned our planet for the past 40 million years, a very short interval in Earth's natural history. More tolerant of drought, prolonged cold and poor soil conditions, grass began to replace woodlands during the Oligocene Period, leading to the rise of the Tertiary megafauna. Since that time, wildfires, high winds, glaciation and the grazing of massive herds of bison have played a crucial role in the evolution and maintenance of North America's grasslands; similar events have occurred across the globe (though the mammalian grazers vary with location).
Modern grasslands are found in a wide variety of landscapes where tree invasion is impaired by climate or soil conditions. Alpine and Arctic tundra grasslands occur at altitudes or latitudes where the annual growing season is too short to permit tree growth and survival. Many grasslands, like those across our High Plains, lie in the rain shadow of major mountain ranges; unlike trees, grasses can tolerate semiarid environments, where annual precipitation is below 20 inches. Grasslands also develop in areas receiving adequate precipitation but where the depth or nature of the soil is inadequate to retain the moisture; sandy shorelines, glades and karst areas are prime examples.
Of course, man has produced his own grasslands (lawns, parks, farms, strip mines, etc.) in areas that would otherwise support woodlands. If human activity is abandoned in these areas, the temporary grasslands are soon covered by shrubs and trees.
