Savannah (grassland)

I

Introduction

Savannah (grassland), also savanna, open tropical grassland sometimes with a scattering of shrubs and trees. The word savannah is derived from the 16th century-Spanish word zavanna, meaning “treeless plain”, but today it is applied to a more diverse set of landscapes from the truly treeless grasslands of parts of South America and the tree-dotted grasslands of Africa, to the lightly wooded savannahs of northern Australia. The biome occurs in areas where rainfall is sufficient to prevent the establishment of desert vegetation but too low to support rainforests. They are held between these two extremes by climate, grazing, and fire. Savannahs are among the most spectacular of biomes from both a landscape and wildlife perspective.

II

Climate

Climatically determined savannahs, typical of western and south-western Africa, develop in areas subject to pronounced wet and dry seasons and where mean annual rainfall varies between 100 and 400 mm (4 and 16 in). Temperatures are high throughout the year but typically 60 per cent to 90 per cent of the year's rain falls in a short period of a few months. The vegetation varies from open-canopy forests with a grassy lower level to classic savannahs in which grasses are dominant. Where rainfall is higher, for example in eastern Africa, the characteristic vegetation is maintained by fires that burn back woody vegetation, check the invasion of shrub and tree seedlings, and stimulate new grass growth. Savannahs can also develop under very different climates if soil conditions favour the growth of grasses over trees. Soil-determined savannahs include the llanos of Venezuela and the campos cerrados of Brazil. The latter is characterized by a hard crust in the soil horizon formed by the accumulation of iron oxides. Grasses grow in the soil above the crust but trees can only establish themselves where cracks in the crust allow roots to reach deeper groundwater.

The annual patterns of temperature and rainfall variation in northern Africa and northern Australia are broadly similar, although wet months and dry months are reversed because of the positions of each region in the Northern and Southern hemispheres respectively. In northern Africa, temperatures are high throughout the year but typically peak at an average of 28^o C (82° F) from February to March and reach a low of 16^o C (61° F) in December and January. Rainfall, however, is much more variable. Less than 80 mm (3 in) of rain falls each month between August and March (usually considerably less, and often none), but this shoots up to 350 mm (14 in) or more in June and July. In Australia, rainfall is spread over a longer portion of the year, averaging over 100 mm (4 in) in all months between October and April, but the year is interrupted by a pronounced and prolonged dry period between May and August. Mean monthly temperatures remain around 22^o C (72° F) throughout the year. On the campos cerrados of Brazil, rainfall is more evenly distributed than in Africa or Australia but there are usually two sharp dry periods, one in April and another from October to November. Temperatures average around 25^o C (77° F) throughout the year.

III

Flora

The most common plants of the savannah are flat-leaved grasses which may grow up to 1.5 m (5 ft) in height. One or two species usually dominate a given area, typically forming a continuous carpet that inhibits the growth of smaller herbs. Shrubs, baobabs, and acacia trees are often widely scattered within the matrix of grasses of Africa, while the open woodland savannahs of Australia tend to be dominated by eucalypts and paperbarks.

In the savannahs of western and south-western Africa, the nature of the vegetation varies with rainfall. Only grasses manage to survive the dry season where precipitation is between 100 and 200 mm (4 and 8 in) per year. In areas where rainfall reaches 300 mm (12 in), the soil holds enough water to sustain various species of shrub through dry months as well. Solitary baobabs and acacias are common in areas receiving between 300 and 400 mm (12 and 16 in) of rain, but in areas receiving over 400 mm (16 in) enough moisture remains in the soil throughout the year for trees to grow more densely and shade out grasses beneath their canopies.

Many savannah grasses, for example, Pennisetum, have well-developed root systems and underground rhizomes in which they store carbohydrates allowing rapid re-growth after the dry season. Indeed, in some regions, over two thirds of the vegetation biomass is located beneath the soil surface. Red oat grass, however, implements a different type of survival strategy. Red oat seeds have long, corkscrew-shaped awns that expand and contract in response to the wet and dry periods at the end of the rainy season. Contraction in response to drying of the atmosphere and ground has the effect of screwing the seed into the soil where it remains protected from the searing temperatures of the dry season until the rains return. Grasses become drier, more fibrous, and less palatable to grazers as the dry season takes hold, and many also furnish their leaves with tiny particles of silica which makes them even less attractive as fodder. Nevertheless, even with these deterrents, plants such as red oat grass are subject to heavy grazing and usually cropped from their maximum height of 1.5 m (5 ft) to dense tufts around 50 cm (20 in) high. Grasses manage to survive this pressure, as well as the periodic fires that whip across the savannah, mainly because their leaves and stems grow from the base rather than from the tip.

Trees, however, grow from the tip, and are easily killed by fire or grazing animals, especially when young. Adult trees tend to be less vulnerable but can still be damaged or killed by intense fires or the attention of browsing animals such as giraffes and elephants. Nevertheless, in most areas of African savannah, trees do manage to survive as scattered individuals within the extensive matrix of grass. Baobabs are characteristic, their bulbous trunks consisting mostly of water-storing cells enabling them to survive prolonged periods of drought. Acacias, in contrast, do not store water, leaving many species to survive on dried-up river beds where water can only be reached by deep, penetrating roots. Acacias tend to be smaller than baobabs and relatively short-lived (up to 40 years). Defensive thorns and leathery leaves act as deterrents to herbivores, while their thick bark laced with fire-retardant chemicals affords some protection against fire.

IV

Fauna

African savannahs are home to elephants, buffalo, giraffes, zebras, ostriches, numerous species of antelope, and many carnivores and scavengers such as lions, crocodiles, leopards, cheetahs, hyenas, hunting dogs, and vultures. Australian savannahs support large numbers of herbivorous kangaroos, wallabies, and emus, with dingoes being the dominant predators. The fauna of Central and South American savannahs is relatively poor, consisting of grazing rodents, two species of deer, and two species of rhea.

In tropical forests, most of the production of plant material occurs high up in the canopy out of the reach of large, heavy animals, but nearly all of the production of savannah grasslands occurs low down and is thus directly accessible to animals living on the solid plain of the ground. Consequently, there are no equivalent barriers to the evolution of large animals on grasslands, and the tropical grasslands of Africa in particular have come to host some of the largest land animals on the planet.

All of the vertebrate grazers of the African savannah eat grasses, but each species targets different parts of the plant or matrix and forages in a slightly different way. Because of this, many species manage to coexist with minimal competition. Zebras crop the grass close to the ground allowing the more delicate gazelles to reach the low-growing, succulent herbs underneath. Thomson's gazelles manage to survive by grazing relatively small territories all year round, whereas zebra and wildebeest exploit the freshest growth by migrating across the continent in pursuit of the rains. During the wet season in eastern Africa, when the equatorial rainbelt is in the south and the grass is growing most vigorously, wildebeest herds feed on the southern Serengeti Plain of Tanzania. The females all give birth to their calves within a few days of each other at around this time. This provides easy pickings for the numerous predators of the plains, but so many calves are produced that supply rapidly outstrips demand and ensures that many young animals survive. When the southern plains become overgrazed at the end of the wet season, wildebeest migrate first to the north-western Serengeti then to the north-eastern part in pursuit of the best grasses. Many rivers lie in the path of the wildebeest as they make their round-trip migration of the Serengeti, and the attempts of the herd to cross en masse provides a temporary glut of food for the resident populations of Nile crocodiles. See also Serengeti National Park.

Red-billed queleas (see Weaverbird) form huge migratory flocks that actually migrate around the savannah in the opposite direction from grazing mammals. This is because queleas are seed-eaters and most savannah plants flower and set seed at the end of the wet season in July and August. At this time, the earth has begun to bake hard preventing most of the seed from becoming buried, so huge numbers of seeds, up to 400 kg per hectare (357 lb per acre), lie around loose on the surface. As the equatorial rainbelt and wildebeest move north, millions of queleas head south to where the rains have long since passed and where the supply of loose seeds is greatest. Many other seed-eaters, particularly beetles and rodents, also gorge themselves on this seasonal glut of energy-rich food.