9781422277522

Australian outback and the East African savannah, an open grassy plain dotted with trees.

How Grasslands Survive Even in an ideal climate, grasslands cannot survive without regular disturbance. The most common is fire, started by lightning during a dry period. Unless grasslands are periodically burned, shrubs and trees invade them. If the trees are far enough apart the area is savan- nah, a form of grassland, but when the trees grow closely the grass cannot get enough sunlight to grow, and woodland takes over. Fire kills off tree saplings, but grass quickly recovers from burning. The other factor that keeps grasslands open is grazing . Herds of millions of wildebeest in Africa eat and trample tree seedlings before they can grow. (In the past, herds of bison did the same job on the North American prairies.) The grazers eat the grasses, but this does not kill the grass plants. Some grasslands were created and maintained by people from pre- historic times. The North American prairies were kept open by Native American hunters. Observing that animals came to feed on the fresh shoots after a natural fire, they realized that they could attract animals to hunt by starting fires themselves. Other grasslands are maintained as pastures by people grazing livestock. They are classified as man- made or semi-natural, as opposed to wild or natural grasslands. Why Grasslands are Fertile Grasses grow in places with moderate rainfall, so the nutrients in the soil are not washed away by heavy rain. They build up deep, rich top- soil, and their roots hold the soil in place: even after a flood, the nutri- ents remain. Natural grasslands feed huge herds of grazing animals,

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Grasslands