GRASSLAND

area with the dominant vegetation being grasses and grasslike plants, often with scattered trees or shrubs. Grasslands were, until extensive agriculture, the largest single biome type in the world. Climatic conditions vary from semiarid to subhumid habitats, marked by periodic precipitation and a topography that ranges from level ground to gently undulating slopes. The environments are too dry to support forest growth but not arid enough to prevent the development of nonwoody, long-lived plant species such as grasses, sedges, herbs, and small shrubs. This kind of environment once dominated nearly half of the Earth's total surface. Grasslands can develop in succession from bare, dry land or from watery areas. As a dry landmass gradually becomes overgrown with an insulating plant cover over the surface, the first lichens and mosses, and then herbaceous species, appear. Watery environments are first dominated by submerged plant species, and then floating reeds as the water level recedes with a rising substrate. The receding water forces the plant cover to become more dependent on climatic conditions such as soil, temperature, and precipitation levels. The periodic precipitation of grasslands varies seasonally and annually. Temperate grasslands average between 250 and 750 mm (10 and 30 inches) of rain each year, while tropical and subtropical grasslands have an annual precipitation rate of between 500 and 1,500 mm. The amount of rainfall determines the nature and extent of grassland inhabitants and soil formations. Periods of drought are common and often prolonged, having an adverse effect on species development, particularly in the warmer climates. Soil conditions in grasslands create an abundance of organic matter at all depths. The rate of precipitation in temperate grasslands allows plants to release nutrients slowly into the ground over long periods of time, so that annual crops are produced. Soil colours range from black near the forest margin to chestnut and brown. Tropical soils are reddish and yellowish in colour, owing to a low organic content, as leaching is more severe in humid climates than in temperate ones. Tropical soils are therefore more subject to nutrition depletion than temperate soils. The high temperatures and dense rainfalls cause organic matter in the tropics to decompose rapidly, and only low levels of humus can accumulate. Water regimes in grassland areas influence both soil texture and structure. Vegetative cover is modified according to whether the type of soil is sandy or peaty; thus, sandy soil permits water to percolate through the layers, causing a low rate of water runoff and evaporation. Sandy soils thus permit a large quantity of grassy roots because their moisture content is so great. The length of the growing season in grasslands is a function of both precipitation and temperature variation. Growing seasons in hot climates vary between 120 and 190 days each year. In more temperate climates, the growing season commences when the average temperature reaches about 5 to 10 C (40 to 50 F) and extends from 100 to more than 165 days. Wind and fire play important roles in controlling organic development in grassland regions. Wind speeds tend to be high because of the low vegetative cover, and lightning-induced fires sweep across open plains up to the forest boundaries. Fire and other disturbances such as grazing are important factors in encouraging new growth of grassland vegetation. They also help to maintain the integrity of the grassland ecosystem by preventing the establishment of trees and shrubs. The dominant plant forms-grasses and sedges-are particularly well adapted to compete with herbaceous species (also called forbs) because the grasses can provide a dense plant cover that prevents the less-hardy forbs from taking root and maturing. Grasses are categorized according to the size of their growth and their formation in the soil. Tall grasses occur in tropical areas; midgrasses with flowering stems are found in temperate regions; and short grasses are found in arid lowlands. Turf grasses spread along the ground by budding on lateral stems below the surface of the soil, and bunch grasses develop horizontally. Many of the animal species found in the grasslands are small and inconspicuous, and many others live underground. Small predators include rodents, lizards, frogs, snakes, and large insects such as grasshoppers, termites, and locusts. Reptiles are often predators of insects, while snakes eat the rodents and other small vertebrates. Rodents are credited with causing widespread destruction of the grassy ranges because they modify the speciation of plant cover and expose the soil surface to erosion. The predators of rodents and snakes are usually birds such as owls, hawks, and eagles. Small soil animals-e.g., springworms and roundworms-are also important to the ecology of the grasslands. Microorganisms found in the soil are mainly microflora, which dwell among the root systems of grasses and herbs. They are eaten by worms, who also aid in providing waste materials and decomposing matter for soil nutrition. The grasses and herbs nourished by the resulting humus are in turn eaten by grazing larger animals such as giraffes, gazelles, zebras, kangaroos, buffaloes, and elephants. Some of these species congregate in herds or colonies for protection against carnivorous animals-i.e., leopards, lions, and foxes. Predatory birds are found in the grasslands, but herbivorous birds such as larks and grouse are also common. The grassland ecosystem is stratified so that the uppermost layer consists of the tallest grasses and forbs. A lower layer is made up of short grasses and herbs, and along the ground surface lie the mosses and lichens, or algae on watery surfaces. The appearance of the grasslands changes seasonally, so that it is drab and lifeless in winter, a grayish colour in early spring as the old flora is replaced by green shoots, and in summer and early autumn the landscape takes on a bluish or whitish colour, when the dominant grasses and legumes blossom. Late autumn brings yellowish flowers into foliage, which fade as the temperature cools. About one-half of the energy received from sunlight is absorbed by the flora and stored underground, where approximately 85 percent of the total biomass of the grasslands is found. The rate of organic decomposition due to the activity of microorganisms on dead plant matter is such that the turnover of the biomass occurs about every two years on the surface and every four years underground. area in which the vegetation is dominated by a nearly continuous cover of grasses. Grasslands occur in environments conducive to the growth of this plant cover but not to that of taller plants, particularly trees and shrubs. The factors preventing establishment of such taller, woody vegetation are varied. Figure 1: Principal regions where significant areas of natural grassland occur. Grasslands are one of the most widespread of all the major vegetation types of the world (Figure 1). This is so, however, only because human manipulation of the land has significantly altered the natural vegetation, creating artificial grasslands of cereal crops, pastures, and other areas that require some form of repetitious, unnatural disturbance such as cultivation, heavy grazing, burning, or mowing to persist. This discussion, however, concentrates on natural and nearly natural grasslands. Jeremy M.B. Smith Additional reading The grassland ecology of North America is discussed in Lauren Bowen, Grasslands (1985). Jeremy M.B. Smith