Savanna soils are closely linked with climate and tend to reflect the local seasonal rainfall pattern. Soils in the savanna are commonly leached, ferralitic soils. These are similar to soils of the rainforest, but not as intensely weathered. Soil development shows a marked seasonal pattern. During the wet season, the excess of precipitation over potential evapotranspiration means that leaching of soluble minerals and small particles will take place down through the soil. These nutrients are deposited deep within the soil. By contrast, in the dry season E is less than P, and so silica and iron compounds are carried up through the soil and precipitated close to the surface.
There are many types of vegetation in savannas, including grasses, trees and scrub. All, however, are (adapted to drought) and (adapted to fire). Adaptations to drought include deep taproots to reach the water table, partial or total loss of leaves, and sunken stomata on the leaves to reduce moisture loss. Adaptations to fire include very thick barks and thick budding that can resist burning, with the bulk of the biomass being below ground level to aid rapid regeneration after a fire. The growth tissue in grasses is located at the base of the shoot, close to the soil surface. This is the opposite of shrubs where growth occurs from the tips. This means that burning, and even grazing, grass encourages growth. The warm, wet summers allow much photosynthesis and there is a large net primary productivity of 900 g/m2 per year. This varies from about 1,500 g/m2 per year where the region borders the rainforest, to only about 200 g/m2 per year where the area becomes savanna scrub. In contrast, the biomass varies considerably (depending on whether it is largely grass or wood) with an average of 4,000 g/m2. Typical species in Africa include the acacia, palm and baobab trees and elephant grass, which can grow to a height of over 5 m. Trees grow to a height of about 12 m and are characterised by flattened crowns and strong roots. The nutrient cycle also illustrates the relationship between climate, soils and vegetation. The store of nutrients in the biomass is less than that in the rainforest because of the shorter growing season. Similarly, the store in the litter is small because of fire. Many of the nutrients are stored in the soil so that they are not burnt and leached out of the system. The role of fire, whether natural or man-made, is important as it helps to maintain the savanna as a grass community. It mineralises the litter layer, kills off weeds, competitors and diseases, and prevents trees from colonising relatively wet areas.
The African savanna has the largest variety of herbivores, with more than 40 types of grazers, including giraffes, zebras, gazelles, elephants and wildebeests. Selective grazing allows for a categorisation depending on the height at which they eat. For example, the giraffe feeds from the top of the tree, the rhinoceros eats the lower twigs and the gazelle survives on the grass beneath the trees. These animals, as well as birds, will migrate to search for water and fresh pastures during the dry season. Carnivores, including lions, cheetahs and hyenas, are also supported in the savanna. Other fauna creatures include locusts, which can decimate large areas of grassland with devastating speed, and termites, which aerate the soil and break down up to 30 kg of cellulose per hectare each year. In some areas up to 600 termite hills per hectare can be found, which has a significant effect on the upper horizons of the soil.
The value of has long been recognised by people who make use of it on a daily basis for traditional medicine, food, building materials and traditional clothes. The savanna ecosystem provides Swaziland society with a wide range of goods and services. Many services, such as fuel wood and medicine, are not consumed as goods but are used as part of servicing the wider community (such as pollination, erosion control and flood control). In the past 20 years, game farming has replaced cattle ranching in many parts of southern Africa. This has occurred purely for economic reasons, as game farming is much more profitable. Indigenous games, such as impala and kudu antelopes, are far more suited to surviving in Africa’s drier, marginal landscapes than cattle. These indigenous species are less susceptible to diseases, require far less water and do not impact negatively on the vegetation. An important direct, but non-consumptive, use of ecosystems is nature-based tourism or ecotourism. Swaziland is generally recognised as a country of great scenic beauty and between 1989 and 1995 over 250,000 tourists per annum have visited Swaziland. Tourists spend about £2 million per annum in the eight largest reserves. This is almost certainly an underestimation as fees are higher in the private reserves. Furthermore, this does not take into account the spin-off benefits of nature-tourism, nor does it reflect the number of jobs that are created.
Savannas can be harsh ecosystems, with drought and fire dictating what species can survive. They are very seasonal ecosystems that appear bountiful in the wet season and quite desolate in the dry season. They contain a large range of plants and animals, a factor that has attracted much human interest. Savannas are important for biodiversity, but they are under increasing pressure from growing populations. Whether they can be protected and preserved is debatable, especially in LEDC regions where the natural environment is seen as a source of free resources.