Weather basically determines where a grassland can or cannot exist. An area that receives at least 40 inches (101 centimeters) of rain a year is where a forest would grow. An area receiving less than 10 inches (25 centimeters) a year would be called a desert. Any area where the rainfall is between that of a forest and that of a desert is a grassland. Some grasslands can even live where forests can be supported.
Grasslands began to appear in North America after the Rocky Mountains formed, about 39 million years ago. The formation of the Rocky Mountains cut off moist Pacific coast winds to forests inland, thus causing them to die out. The new, drier climate would perfectly support a temperate grassland. Other grasslands were formed from strong winds that took moisture from forests and cleared land for strong grasses to grow. Natural fires, caused by lightening, also helped to clear once heavily forested areas for grasslands to thrive. Fires help grasslands to grow. Grass plants in a grassland are almost unaffected by fires because seventy percent of the plants are virtually underground in the form of roots and rhizomes. Rhizomes are the grasses underground network of stems. As a fire rages through the area, it destroys trees, plants, and dead plant material that could slow grass production and eventually kill the plant. After a fire, the grasses grow stronger and healthier, and remain dominant.
Drying climates and fires helped to create the worlds first grasslands. Ancient humans had a hand in helping to spread grasslands, as well. Hunters living in the old Stone Age probably burned grasslands so they could attract the grazing animals they hunted closer to their homes. Four thousand years ago farmers in Europe chopped down most of the trees in order for large blankets of green grass to grow for their sheep and cattle herds. Today, these sheep and cattle still graze where large forests once lived, and still could live if humans had allowed it.