Grasslands and savanna

Natural grasslands, which occur mainly in temperate regions, and savanna (tropical grassland) represent climax vegetation. The temperate grasslands of the North American prairies, the Eurasian steppes, and the South American pampas differ from those of western Europe, which are artificial. European grasslands were created through the deliberate removal of the original climax forest vegetation. They are maintained today either as meadows, for cut hay, or as pasture, for grazing farm animals.

Grasslands (except in the pampas) occur naturally in areas where the annual rainfall does not exceed 30 to 40 inches (75 to 100 centimeters) and is no less than 6 to 10 inches (15 to 25 centimeters). Seasonal differences are marked in such areas. Climate is the most important factor that defines grasslands, although several other features influence the environmental conditions, such as grazing, human activity, fire, soil structure, and topography.

Turf-forming grasses (A) have creeping underground stems (rhizomes) with shallow, matted roots. Grasses that form bunches (B) have independent root systems and generally spread by means of their seeds. Natural meadowland (C) often forms “layers,” both above and below ground. Tall herbaceous plants (forbs) bear flowers above the height of the tallest grasses; shorter forbs grow at the level of the short grasses, often blooming in spring before the tall grasses grow; and at soil level mosses and lichens thrive in the leaf litter. The various roots and root systems are similarly stratified.

Grass adaptations

One of the most significant features of grasses that enables them to survive in their habitats is their method of growth. They have closely noded underground stems that continuously produce new leaves and large numbers of shoots. Growth occurs through cell division at the bases of the leaves and stems, rather than at the tips as in most other plants. The underground stems are a means of vegetative reproduction, which allows the plant to be closely cropped above ground by grazing animals, burned by fire, die down in cold weather, or lie dormant in times of drought. In addition, the fibrous root systems and underground rhizomes of grasses and the large tap roots of numerous forbs often grow down to 6.5 feet (2 meters), where they are able to use soil water during droughts. Where precipitation is less than 10 to 20 inches (25 to 50 centimeters), roots of grasses and forbs grow down to 3 to 4 feet (90 to 120 centimeters). During periods of drought, grass leaves curl, which reduces the leaf surface, which reduces transpiration. Corn and wheat plants have the same adaptation.

Annual grasses overwinter or survive drought as seeds. These seeds germinate in late winter and early spring, utilizing soil water before the perennial native grasses begin growth. This, coupled with frequent fires and overgrazing, results in introduced annual grasses dominating what were native perennial grasslands in the western United States.

Grasses increase their chances of dispersal by having light seeds, which are carried by wind or animals. This is more important for annuals—it is their only means of reproduction.

Mountain grasslands and savannas

Many artificial European grasslands have existed since late Neolithic times. Forests were cleared, often replaced by grasses and forbs (broad-leaved, flowering plants). These converted pastures and grasslands typically occur on shallow soils overlying acid (granitic) to alkaline (limestone) rocks. These grasslands are dominated by grass species of fescue (Festuca spp.), bent (Agrostis spp.), mat grass (Nardina stricta), and purple moor grass (Mor/ina caerulea), along with numerous forbs. Wild species of the mint family are found in the limestone mountains of southern France and northern Italy, species that have been selected for the perfume industry.

Mountain grasslands and savannas occupy vast landscapes in New Zealand, the high Andes of South America, and smaller highlands in East Africa, Australia, Mexico, and Asia. The mountain grasslands of New Zealand are dominated by species of snow tussock (Chinochloa spp.) at lower elevations, with hard tussock (Festuca novae-zelandiae), fescue (Festuca matthewsii) and bluegrass (Poa co/en-soi) above. One thousand yeards ago, many of these lands were occupied by forests and were converted with fire by the Maoris. These grasslands, in contrast with those of South America and Australia, did not support large herbivores, so common to most natural evolved grasslands in the world. The wet paramo of Colombia, Venezuela, and Ecuador and the dry puna of Peru, Bolivia, Chile, and Argentina are dominated by species of fescue (,Festuca spp.), reed grass (Calamagrostia spp.), bluegrass (Poa spp.), and needle-and-thread (.Stipa spp.). These grasses and associated forbs support the wild populations of alpaca and vicuna in the puna. The paramo and puna are used extensively for cattle- and sheepgrazing.

The Australian mountain savannas in New South Wales and Victoria, dominated by Poa caespitosa and Danthonia frigida, are grazed by sheep and cattle as well as more limited populations of kangaroo.

Artificial temperate grassland on the fringes of the Black Forest includes pasture set aside for grazing animals, and meadow (foreground). Meadow grass is cut and dried to make hay, for feeding to animals in winter.

Temperate grasslands

Prior to the arrival of Europeans, grasslands dominated vast areas of the central and western United States and the southern portions of the prairie provinces of Canada. Tail-grass prairies extended from southern Manitoba to Texas and westward to about the 100th meridian. Most of these lands are now home to crops and pastures. The original grasslands were dominated by big bluestem (Andropo-gon gerardii), switch grass (Panicum virgatum), Indian grass (Sorghastrum nutans), and species of needle-and-thread (Stipa spp.). Many species of forbs, both spring- and fallflowering, dotted these prairies, including species of the legume and sunflower families. The showy flowers of these forbs provided color to the unbroken sea of grasses.

The mixed-grass prairies that extend from southern Saskatchewan and Alberta south through the Dakotas to central Kansas, Oklahoma, and Texas are dominated by grasses and forbs, mostly less than 3 feet (1 meter) in height, where precipitation averages 20 to 25 inches (50 to 65 centimeters). These soils are not as fertile as those of the tail-grass prairie, yet they are highly productive for wheat production.

The wintry steppes of Mongolia (left) are natural grasslands, which support small herds of grazing ponies.

The high plains of Montana, Wyoming, Colorado to western Texas, and northern Mexico are still dominated by short-grass prairie, although large areas grow wheat with irrigation or dry-land farming. Short grasses and forbs, less than 15 inches (35 centimeters) in height, predominate. The sod grasses blue grama (Bouteloua gracilis) and buffalo grass (Buchlae dactyioides) and scattered forbs supported large herds of buffalo and antelope in the past and now are some of our major grazing lands. Precipitation is generally less than 15 inches (35 centimeters), often with long winters with limited snow.

Additional original grassland dominated central Washington, Oregon, southern Idaho, northern Utah, and Nevada. These lands, mostly in wheat originally, were dominated by native wheat fescue, rye, bluegrass, and nee-dle-and-thread grasses in addition to numerous forbs. The California grasslands of the central valley were once dominated by species of needle-and-thread {Stipa spp.), rye grass (Eiymus) and June grass (Koeleria cristaia), but now are converted to crops or to wild oats (Avena) and brome (Bromus). The desert grasslands of southern Arizona, New Mexico, and Mexico occur in cooler and moister uplands adjacent to the Sonoran and Chihuahuan deserts. Grasses and shrubs predominate.

Other major grasslands are found in eastern Europe, Ukraine, the savannas of East Africa, the pampas of Uruguay and Argentina, the Canterbury Plains of New Zealand, and the lowland Australian savannas. In summary, many of the present and former grasslands of the world are the present world’s breadbaskets in terms of crop and pasture production. Fire as well as drought played an important role in preventing tree and shrub invasion in those lands able to support woody rather than herbaceous vegetation.

Resembling a squadron of tanks, combine harvesters cut grain on the vast wheat fields, which have replaced the natural vegetation in Kazakhstan.

Types of savanna

Many savannas occupy regions between the equatorial rain forests and the hot deserts. Because of differences in rainfall, there are three main types of savanna. The moist savanna belt that borders the equatorial forests has 42 inches (107 centimeters) or more of rain each year with a dry season lasting 5 or 6 months. Elere the grasses may reach 10 feet (3 meters) in height. In the dry, savanna belt, with 24 to 42 inches (60 to 107 centimeters) of rain a year and a 7- to 8-month dry season, the grasses reach 5 feet (1.5 meters). The thorn savanna belt, with an annual rainfall of less than 24 inches (60 centimeters) and a dry season of more than 8 months, is the driest type. It has mainly annual grasses with some trees. The ground vegetation grows only to about 20 inches (50 centimeters).

Africa contains the largest single savanna region. It extends across West Africa, between the rain forests and the Sahara Desert, and sweeps through east central Africa, merging in the south into the Namib and Kalahari deserts. The African savanna includes vast tracts of each of the main savanna types.

In South America, the savanna include the llanos of Venezuela, along the Orinoco River, and the campos of Brazil, south of the Amazonian rain forest, two examples of moist savanna. Northeastern Brazil has a very dry thorn savanna with little grass.

The Australian savanna stretches across the northern part of the country. It is mostly dry savanna, some of which is difficult to differentiate from desert scrub. It is characterized by scattered Acacia trees, shrubs, and grasses.

Plant adaptations in savannas

Drought and fire, caused naturally or by farmers, have reduced the number of plant species in the savannas. The species that do occur show many adaptations to fire, and it is fire rather than climate or grazing animals that determines the stability of the vegetation.

Many trees have thick, corky, fire-resistant bark that is spongy and saturated with water after rains, as in the baobab (Adansonia digitata) and the bottle tree (Cavanillesia sp.), an adaptation that is also suited to survival during drought. Trees and shrubs also produce vast numbers of seeds, and many of the herbs have underground food storage organs.

The adaptations of grasses in temperate regions all apply equally to savanna species. One example is pampas grass (Cortaderia sp.) of South America, which is grown as a garden plant. In fact, gardeners are advised to burn off old leaves in spring in order to prevent the new shoots from being choked and to provide some fertilizer for the plant. Providing that the burning is rapid, the plant comes to no harm.

In Australia, the open temperate grasslands also support scattered shrubs and trees—mostly various species of Eucalyptus. Sheep crop the grass among the trees.

The soils of the tropical savanna are generally brown or black and more basic than those of the tropical forest. The dense roots of tussock-forming grasses add much organic matter to the soil.

In South American grasslands, the dominant species is often pampas grass, which produces its characteristic feathery plumes when it blooms. It is also grown as an ornamental plant.

Savanna plants

The Australian wooded savanna is dominated by gum trees—various species of Eucalyptus and Acacia spp. Like other savanna trees, they are fire-resistant. Even when the crown is severely damaged, shoots and suckers from the base and roots may ensure the tree’s survival. One species of Eucalyptus even requires fire to allow its seeds to germinate. Fire also ensures that seedlings have a fairly free area in which to grow with a minimum of competition.

Acacias grow in both the Australian and African savannas, although they are of different species. Together with acacias in Africa are the grotesque baobabs. Elsewhere in the African savanna, from Senegal to Uganda, are such fire-resistant genera as Terminalia and Isober-linia. Commiphora sp. is found with acacias in the densest savanna types, and the leguminous Coiophospermum mepane is found in the Zambezi region in the south.

Many grasses found in the savannas are widespread species. Kangaroo grass (Theme-da trianda) from Australia is known as red oat grass in Africa. Some tussock grasses (Poa spp.) occur in Australia and South America. In these areas, other typical tussock-forming savanna grasses include species of Sporoboius, Digitaria, Panicum, Setaria, Pennisetum, and Sorghum although they tend to form loose clumps rather than dense raised tussocks. Species of the genus Hyparrhenia (and others) have seeds that, when detached from the plant, respond to changes in humidity. Depending on whether it is wet or dry, a bristle (the awn) on the seed twists and untwists. This motion is sufficient to cause it to bury itself in the soil, where it can germinate with the next rains. This is also true of the porcupine or needle and thread grass species of Stipa in North America and Asia.

The African savanna is the home of vast herds of wildebeest, which migrate hundreds of miles in phase with the seasonal growth of the grass. The herds are, in their turn, a source of food for predatory animals and carrion-eaters.