Grasslands

Grasslands are environments in which herbaceous species, especially grasses, make up the dominant vegetation. Natural grasslands, commonly called prairie, pampas, shrub steppe, palouse, and many other regional names, occur in regions where rainfall is sufficient for grasses and forbs but too sparse or too seasonal to support tree growth. Such conditions occur at both temperate and tropical latitudes around the world. In addition, thousands of years of human activity—clearing pastures and fields, burning, or harvesting trees for materials or fuel—have extended and maintained large expanses of the world's grasslands beyond the natural limits dictated by climate.

Precipitation in temperate grasslands (those lying between about 25° and 65° latitude) usually ranges from approximately 10-30 in (25-75 cm) per year. At tropical and subtropical latitudes, annual grassland precipitation is generally between 24-59 in (60-150 cm). Besides its relatively low volume, precipitation on natural grasslands is usually seasonal and often unreliable. Grasslands in monsoon regions of Asia can receive 90% of their annual rainfall in a few weeks; the remainder of the year is dry. North American prairies receive most of their moisture in spring, from snow melt and early rains that are followed by dry, intensely hot summer months. Frequently windy conditions further evaporate available moisture.

Grasses (family Gramineae) can make up 90% of grassland biomass. Long-lived root masses of perennial bunch grasses and sod-forming grasses can both endure drought and allow asexual reproduction when conditions make reproduction by seed difficult. These characteristics make grasses especially well suited to the dry and variable conditions typical of grasslands. However, a wide variety of grass-like plants (especially sedges, Cyperaceae) and leafy, flowering forbs contribute to species richness in grassland flora. Small shrubs are also scattered in most grasslands, and fungi, mosses, and lichens are common in and near the soil. The height of grasses and forbs varies greatly, with grasses of more humid regions standing 7 ft (2 m) or more, while arid land grasses may be less than 1.6 ft (0.5 m) tall. Wetter grasslands may also contain scattered trees, especially in Lush grassland growth in autumn in the Mallee region of New South Wales, Australia. Photograph by Bill Bachman. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
low spots or along stream channels. As a rule, however, trees do not thrive in grasslands because the soil is moist only at intervals and only near the surface. Deeper tree roots have little access to water, unless they grow deep enough to reach groundwater.

Like the plant community, grassland animal communities are very diverse. Most visible are large herbivores—from American bison and elk to Asian camels and horses to African kudus and wildebeests. Carnivores, especially wolves, large cats, and bears, historically preyed on herds of these herbivores. Because these carnivores also threatened domestic herbivores that accompany people onto grasslands, they have been hunted, trapped, and poisoned. Now most wolves, bears, and large cats have disappeared from the world's grasslands. Smaller species compose the great wealth of grassland fauna. A rich variety of birds breed in and around ponds and streams. Rodents perform essential roles in spreading seeds and turning over soil. Reptiles, amphibians, insects, snails, worms, and many other less visible animals occupy important niches in grassland ecosystems.

Grassland soils develop over centuries or millennia along with regional vegetation and according to local climate conditions. Tropical grassland soils, like tropical forest soils, are highly leached by heavy rainfall and have moderate to poor nutrient and humus contents. In temperate grasslands, however, generally light precipitation lets nutrients accumulate in thick, organic upper layers of the soil. Lacking the acidic leaf or pine needle litter of forests, these soils tend to be basic and fertile. Such conditions historically supported the rich growth of grasses on which grassland herbivores fed. They can likewise support rich grazing and crop lands for agricultural communities. Either through crops or domestic herbivores, humans have long relied on grasslands and their fertile, loamy soils for the majority of their food.

Along a moisture gradient, the margins of grasslands gradually merge with moister savannas and woodlands or with drier, desert conditions. As grasslands reach into higher latitudes or altitudes and the climate becomes to cold for grasses to flourish, grasslands grade into tundra, which is dominated by mosses, sedges, willows, and other cold-tolerant plants.