Grassland

I

Introduction

Grassland, ecosystem in which grasses and other forage plants form the dominant type of vegetation. Temperate grasslands occur in the interiors of landmasses where continental climatic conditions promote cold winters, hard frosts, and long, dry summers. Summer drought, occasional fires, and intense grazing of the sward by animals are the main factors preventing the growth of trees. The North American prairie, South American pampas, Eurasian steppe, and South African veld constitute the world’s major natural temperate grasslands. Semi-natural grasslands exist in temperate regions where climatic conditions are suitable for the establishment of forests but where management by burning, mowing, cultivation, or grazing keeps the land free of trees (for example, most of agricultural Europe). Tropical grasslands, or savannahs, develop in regions with marked wet and dry seasons and are the subject of a separate entry in this encyclopedia.

The diversity of vascular plants can be quite high in grassland areas (from 50 to 200 species depending on the latitude), but usually two or three species form upwards of 60 per cent of the biomass above ground level. The major type of vegetation which ultimately develops under a given set of climatic conditions is termed the climax vegetation type, so managed grasslands, deflected from their normal course of successional development, are often referred to as plagioclimaxes.

Because rainfall tends to be relatively low, soluble nutrients remain in the soil and productive grasses promote the development of organically rich topsoils, so grassland soils tend to be inherently fertile. As a result, most temperate grasslands have either been given over to grain crops or used to support domestic grazers, particularly cattle and sheep. In the process, large native grazers and their predators have been virtually eliminated.

II

Climate

Mean annual precipitation in most temperate grasslands ranges from 250 to 750 mm (10 to 30 in) with a marked periodicity in rainfall both from season to season within the year and between years. Annual periods of drought lasting several weeks to several months are common, often accentuated by cyclic or unpredictable climatic conditions resulting in strings of consecutive dry years. To a large extent, rainfall determines the nature and extent of natural grasslands through its effect of soil-moisture since trees are usually out-competed by grasses where upper soil layers are moist for part of the year but deeper layers remain consistently drier. On the eastern steppe of Mongolia, rainfall averages 20 mm (˜ of an inch) in January and February, rising to 40 mm (1y in) in April, 100 mm (4 in) in June, 180 mm (7 in) in July, 100 mm (4 in) again in September, then back down to 20 mm (˜ of an in) in November and December. In north-western America the gross pattern is only slightly different with rainfall peaking earlier in the year in April and May (130 mm/5 in), often followed by particularly dry conditions in June (60 mm/2 in). A brief recovery to 100 mm (4 in) is typical for July after which there is a gradual fall-off to around 20 to 50 mm (1 to 2 in) between November and February.

In contrast, the annual pattern of temperature variation in Mongolia is much more dramatic than in north-west America. Temperatures may average out at a drought-inducing 28^o C (82° F) on the Mongolian steppe in July, but they then fall well below zero (32° F) for most of the period between December and mid-March, and may average as low as -10^o C (14° F) in January. Temperatures on the American prairie are much more equable, averaging 32^o C (89° F) in July and August and only falling to 12^o C (54° F) in December and January. These differences show the importance of rainfall patterns in determining the balance between forest and natural grassland rather than temperature.

III

Flora

Grass and grass-like plants (such as sedges), collectively referred to as graminoids, are the most characteristic plants of temperate grasslands, but broadleaved herbs, known as forbs, may also be common. Woody species are also found in some areas but, unlike herbs, these maintain perennial stems above the soil surface and may live through several growing seasons. Occasionally, colonies of non-seed-bearing plants, particularly algae, mosses, and lichens can also be found. Grassland plants often have characteristic times of growth and development, with those requiring more water developing earlier in the season and drought-resistant species growing later.

Graminoids typically make up less than 20 per cent of the species present in grasslands but they may comprise 90 per cent or more of the above-ground biomass. The success of these plants seems to be the result of two factors: they form dense swards in which seedlings of other plants have difficulty in becoming established; and they grow from the base of the leaves and shoots allowing continuous growth even when the tips have been cut, burnt, or chewed off by grazing animals. Growth zones in the stem also help shoots bent over by trampling eventually to regain a semi-vertical position. In addition, some grasses can continue growing during periods of drought by increasing the rate at which they extract moisture from the soil, while others have the ability to enter a dormant state before complete desiccation occurs. Grasses typically reproduce and spread by sending out stems below the soil-surface (rhizomes) or, more rarely, above it (stolons). Under the most extreme conditions, severe drought or over-winter survival can only be achieved by producing seeds, in which case the plants are known as annuals. Annuals may predominate in disturbed areas of grassland and are often the most characteristic plants of the ephemeral herbaceous cover of extreme environments such as deserts.

Under mild grassland conditions, usually near the boundaries of forests, forbs may be particularly abundant, but as conditions become drier, grasses increasingly gain the upper hand. Some forbs manage to cope with arid conditions by storing water in their tissues, or by surviving as dormant underground organs, or by sending down roots much deeper than the surrounding grasses. Forbs exhibit a variety of root-types including dense networks, taproots, corms, bulbs, and roots capable of producing shoots. The latter type are particularly prevalent in the temperate grasslands of Eurasia and in some areas may even dominate over grasses. Forbs with such root systems, including spurges, white-top, thistle, and morning glory (see Bindweed), are among the most persistent and troublesome arable weeds in many parts of the world.

As annual rainfall decreases from west to east across Russia, and east to west across North America, the types of grass dominating the vegetation change. The American prairies are traditionally divided into three broad types: shortgrass prairie, in which drought-tolerant plants rarely grow more than 50 cm (20 in) tall; mixed grass prairie, in which they grow to between 50 and 150 cm (20 and 50 in); and moist, eastern tallgrass prairie where they may reach 300 cm (120 in) in height. The steppes of Eurasia can be divided into western forest steppe, in which patches of fescue and feather grasses alternate with stands of oak, birch, and pine; open steppe characteristic of central Russia, which contains no trees except along sheltered river valleys; and southern semi-desert where steppe grasses gradually give way to sagebrush and desert vegetation.

Grassland plants and those of adjacent biomes respond to cyclic periods of drought followed by periods of more humid weather. During wetter years on the prairie, the woodlands to the east send seedlings into the tallgrass, the tallgrass encroaches on the mixed grass, the mixed grass encroaches on the shortgrass, and the shortgrass sends runners into the desert. As shown by studies of tree-rings, however, severe drought every 22 years or so interrupts the run of humid conditions and the pattern of colonization is reversed. At this point, fires sparked by lightening often take a hand, sweeping across millions of hectares, fertilizing the soil with ash, and favouring the subsequent development of fire-resistant perennials. By these mechanisms, the natural grassland, forest, and desert biomes of the region coexist in a state of dynamic equilibrium mediated by cyclic changes in climate and infrequent, but often catastrophic, episodes of natural disturbance.

IV

Fauna

Many herbivorous, insectivorous, omnivorous, and carnivorous birds inhabit grasslands, including larks, longspurs, meadowlarks, starlings, partridges, finches, pipits, pratincoles, cranes, eagles, hawks, and owls. Smaller non-avian animals include mice, shrews, voles, lemmings, ground squirrels, suslik, gopher, prairie dogs, bobac marmots, rabbits and hares, and a wide variety of insects, especially grasshoppers. The large animals of temperate grasslands, at least until recent times, included bison, pronghorn, wolves, coyotes, and lynxes (North America); saiga antelope, deer, gazelles, wild horses, wild asses, foxes, and wolves (Eurasia); llamas and guanaco (South America); and kangaroos and dingoes (Australia).

Inconspicuous as they may be, small animals are key components of temperate grassland ecosystems. Insects break down plant material, fertilize the soil, provide food for small mammals and birds, and spread seeds and pollen. The presence and activities of small mammals are often crucial to the functioning of grassland environments. At one time, for example, prairie dogs on the high plains of North America outnumbered the global population of humans, with colonies stretching for hundreds of square kilometres beneath the plains. The burrowing of these small rodents helps to aerate the soil and their grazing keeps certain aggressive grasses in check; it has long been appreciated that areas with prairie dogs tend to support a greater diversity of plants than areas without. Hawks, coyotes, and other predators tend to concentrate near prairie dog colonies to exploit the abundance of small game. Rattlesnakes use their tunnels to escape either the heat of the day or the cool of night and winter and burrowing owls often lay their eggs in abandoned burrows during spring. The young owls fledge in July at which time prairie dogs often reappear and live alongside them.

The most conspicuous animals of temperate grasslands, at least until very recently, were large mammalian grazers. In tropical forests, most of the production of plant material occurs high up in the canopy out of the reach of large, heavy animals. Nearly all the production of grasslands, however, typically ranging between 0.5 and 1.5kg/sq m per year, occurs at or near ground-level and is thus directly accessible to animals living on the solid, two-dimensional plain of the ground. Consequently, there are no barriers to the evolution of large grassland animals and, indeed, some of them have become huge. Most famous are the large grazers and browsers of the savannah grasslands of Africa, which include elephants, rhinoceroses, giraffes, buffalo, zebras, and numerous species of antelope, but temperate grasslands once had their characteristic megafauna too. Less than 10,000 years ago, mammoths, camels, and giant ground sloths thrived in the grasslands of central North America. As the climate became warmer and less stable, boreal forests formed to the north, deciduous forests to the east, and grasslands to the west, accompanied by enormous increases in the numbers of pronghorn, bison, and their main predators, coyotes and bobcats (or wildcats). Bison, in turn, had a dramatic effect on the prairie grasses, which evolved extremely robust root systems to withstand the impact of their penetrating hooves and tearing method of eating. Because bison ate mainly grasses, they also gave a competitive chance to plants such as shrubs, thereby maintaining the plant diversity of the prairies.