Vegetation in East Africa: Types, Characteristics, and Distribution
Explore the diverse vegetation of East Africa, from lush equatorial forests to dry savannahs and semi-deserts. Learn about their characteristics, distribution, and ecological significance.
VEGETATION
East African vegetation zones are closely linked to climatic regions. As with climatic zones, differences in the amount and seasonal distribution of precipitation constitute the most important influence on the development of vegetation. Moving across the region from the wet into drier climates, the sequence of vegetation changes from tropical rainforest to savannah, semi-desert, and finally desert.
Types of vegetation in East Africa
FORESTS
There are three main types of forests in East Africa.
They include:
1. Equatorial forests or tropical rainforests
2. Montane forests
3. Mangrove forests.
4. Riverine forests
5. Woodland forests.
Equatorial forests.
These forests are found in many localities of East Africa which experience heavy rainfall and hot temperatures. In Uganda the forests include; Mabira, Budongo, Bugoma, Semliki, Kibale and other localities. In Kenya, they include Gongeni, Umba River valley forests, Mida-Gedi, WituKanyas and others found in Kiisi, Kisumu, Kakemega and Eldoret. While in Tanzania, they are found at Bukoba, Mpanda, and in the Lake Rukwa region.
Characteristics of equatorial forests.
1. The forests are thick and luxuriant e.g. Bwindi Impenetrable forest derives its name from its thickness.
2. Thousands of different species of trees and other plants are found in the tropical rainforests. The wealth of vegetation includes tree species such as mahogany, rosewood, green heart, ebony, ironwood, cedar and other plants such as palms, climbers, creepers, ferns and orchids. In the Congo Basin and Amazon inforest areas the vegetation is extremely rich and diverse. This profusion or abundance is a direct result of equatorial climatic conditions of heavy rainfall, hot temperatures and ample sunlight.
3. The trees are evergreen throughout the year. Absence of seasonal climatic change results in some plants being in flower, others in fruit and others in leaf-fell at one and the same time. No resting period is imposed by drought or cold as in the case of savannah or deciduous forests.The trees have broad-leaved, dark green colour due to the need of releasing excess water through transpiration and need for food formation through photosynthesis. The leaves also have drip tips to shed off excess water e.g. leaves of Mvule trees in Bugoma, Budongo and Mabira.
4. The trees tend to be tall, the tallest reaching over 60 meters due to adequate rainfall and competition to acquire enough sunlight e.g. Mahogany and Mvule trees in Bugongo and Mabira forests.
5. The ground is mainly covered by mosses and ferns due to the heavy rainfall which leaves the ground wetand moist.
6. Where part of the virgin forest has been cleared for either timber or to create room for cultivation, a less luxuriant forest growth takes over. This called a secondary forest and consists of shorter trees and dense undergrowth.
7. Many trees develop buttressed roots from the lowest few meters of the trunk and can stretch out 10 meters or more to support their great height of the trees e.g. the Mvule trees in Kashoha-kitomi forests. In addition the top heavy because nutrient-poor rain forest soils lead to fragile, shallow root systems.
8. The trees consist of hard woods e.g. Mvule and Mahogany trees in Budongo and Bugoma forests.
9. Trees in tropical rainforests are not found in pure stands of a single species. They are highly heterogeneous. In the Congo Basin area the vegetation is extremely rich and diverse. Timber trees occur abundantly and include teak, ebony, African cedar, mahogany, iroko, and redwood. Among the native fruit trees are banana, coconut palm, and plantain. In the Amazon rainforests, there are over 300 tree species including rosewood, ebony, mahogany, greenheart, palms and rubber.
Montane forests
These are found in the highlands or mountain areas such as on the slopes of mountains like Kilimanjaro, Kenya, Elgon, Rwenzori, Meru and others. There are different forest zones on the mountains. Tropical rainforests merge into temperate forests and finally bamboo forests.
The tropical rainforests are thick and luxuriant and consist of a wide variety of tree species such as Mahogany, Mvule, Rosewood, Greenheart and others. The individual tree species are widely scattered in the forest. The forests are ever green due to the heavy rainfall which is distributed throughout the year and the hot temperatures. The trees tend to be tall, the tallest being over 60 metres.
The trees have straight, smooth-backed trunks and a few branches before they crown. The crowns of the trees tend to form two or three distinct layers of canopies. The canopies shut out the light from reaching lower levels resulting into little or no under growth. Many trees develop buttressed roots from the lowers few meters of the trunks to support their great weight and height. There are many climbing plans which twine from tree to tree e.g. lianas. The trees consist of mainly hard woods.
The tropical rainforests are replaced by temperate forests with increase in altitude. These forests reflect characteristics similar to other temperate forests else where. The trees are evergreen and occur in pure stands, confined to only a few species such as pine, podocarpus, camphor and cedar. A single species may occur over a large area. The trees grow straight, tall and slender although their height decreases with increase in altitude. The tree leaves are tiny and needle-shaped with small surfaces, which prevent excessive loss of water by transpiration. The trees yield cones instead of fruits. These have scales close together and seeds, which are thick, coated to survive in a region deficient in heat and moisture. Trees have shallow roots to survive the thin podzol soil. The trees grow close to each other and of moderate density.
Temperate forests are replaced by bamboo forests. These consist of trees which are up to 20 m high with stiff stems. The stems are segmented (like the sugarcane stems) and hollow. The trees grow in pure stands and are ever green with needle shaped leaves.
Mangrove forests
These are found along the coastal areas of Kenya and Tanzania especially in estuaries and bays at the mouth of Rivers Pangani, Rufigi, Ruvuma, Tana and Sabaki. Other areas include coastal areas of Tanga, Lindi, Mtwara, Dar-es-Salaam, Lamu, Malindi, Mombasa and Shimoni.
Characteristics of mangrove forests
· The trees are evergreen throughout the year with broad leaves to release excess water by transpiration. The trees are short usually ten metres and below.
· There are few tree species consisting of mainly coconut palms and pines. The trees have tangled stilt-like or aerial roots that are exposed during low tides.
· Some mangrove roots extend above the water. The trees tend to be closer together forming a moderate density.
Riverine forests.
These are found along river courses such as along Katonga, Kagera, Nile, Kafu, Ruvuma and Rufigi Rivers. They are characterised by tall evergreen trees. The forests are dense limiting light on the forest floor resulting in little under growth. There are many climbing plants such as lianas which twine from tree to tree and attain long length.
Factors influencing the distribution of natural forest vegetation in East Africa.
Climate.
This factor influences the distribution of natural forest vegetation in the following ways: Equatorial forests grow under conditions of heavy rainfall of distributed throughout the year, and where temperatures are hot all the year around. This explains why the 1500 per annum, which is well hot, wet, humid conditions in the Lake Victoria basin have given rise to the growth of forests such as Mabira, Malabigambo and those in the Ssese Islands.
The montane forests grow under conditions of moderate to heavy rainfall of over 1000 mm per annum, which is also well distributed throughout the year but where temperatures are cool. Montane forests on Mt Rwenzori and Mt Elgon grow on the windward slopes where there is heavy rainfall and less on the leeward slopes which are relatively dry. On Mt. Kenya, the forests grow more on the south and south-eastern slopes and less in the North West.
The mangrove forests thrive under climatic conditions of heavy rainfall and hot temperatures found in coastal areas.
Altitude
This refers to the height above sea level. Equatorial forests thrive in areas of low altitude areas of less than 1,500 meters above sea level. The montane forests grow in areas of higher altitude ranging between 2,000- 3,500 meters above sea level e.g. Mt Elgon and Rwenzori forests. Mangrove forests on the other hand thrive in areas of very low altitude at coastal areas at sea level.
Drainage
Rivers with constant flow of water such as the Nile, Katonga, Kafu and Kagera have facilitated the growth of riverine forests along their channels. Mangrove forests require shallow and muddy salty, swampy or poorly drained areas such as those found in coastal areas of East Africa. Both montane and equatorial forests thrive in well-drained soils.
Soils.
All natural forests require the presence of fertile soils to certain their growth e.g. Mabira forest in Mukono and Malabigambo forest in Rakai for example are favoured by the presence of fertile feralitic-clay-loam soils which montane forests on Mts. Elgon and Muhavura are favoured by the presence of fertile volcanic soils.
Human Influences
The activities of man have greatly affected the distribution of natural forests. Human influences such as gazetting forest reserves such as Sango Bay in Rakai and Mabira in Mukono as well as forests gazetted as national game parks e.g. Bwindi Impenetrable, Mt. Elgon forest, Mt. Rwenzori forest and Semliki forest have favoured the growth of forests. On the other hand, activities such as deforestation to create room for settlement and farming e.g. clearing of Bugala Island forests for oil palm growing by BIDCO, lumbering activities in search of timber in Budongo forest, road construction, mining, quarrying and others. All the above factors have reduced the land area under the natural forests. This is most notices in the Lake Victoria Basin.
Problems associated with land use in equatorial forests regions.
· There are many different species of trees in one region making the selection and felling of valuable species very difficult e.g. in Budongo and Bugoma forests.
· Once the forest cover has been removed to create room for cultivation, the soils rapidly deteriorate and tend to be thin unable to sustain arable farming e.g. parts of Mabira and Kibale forests.
· The dense forests act as breeding grounds for dangerous pests and diseases such as mosquitoes causing malaria and tsetse flies causing sleeping sickness. This limits such areas for settlement e.g. some of the Buvuma Island forests in Lake Victoria.
· The dense vegetation is very difficult to clear making the development of transport and communication lines difficult and expensive e.g. Mabira forest along the Kampala-Jinja road.
· Low levels of technology evidenced by the use of hand saws in lumbering activities e.g. in Zoka forest.
· Insufficient capital to purchase modern machinery to carry out lumbering activities e.g. in Bugoma forest.
· The dense vegetation cover is difficult to clear. This has resulted into sparse settlements and shortage of labour e.g. around Wiceri and Zoka forests.
· There is uncontrolled exploitation of the forest resources leading to their depletion e.g. Budongo forest.
· Tourism as a land use is facing a problem of poaching and poorly developed transport routes e.g. in Kibale and Mt. Elgon forests.
SAVANNAH VEGETATION
The term savannah is used to refer to a wide range of plant communities. This type of vegetation lies between the tropical forest zone and the desert areas. The amount of precipitation varies from the fringes of the tropical forest to the desert areas and hence the nature of vegetation. The savannah vegetation can thus be divided into three major categories:-
1. Savannah woodland
2. Savannah grassland
3. Dry bush scrub.
Savannah woodland.
This type of vegetation occurs near the tropical forest zone where rainfall is heavier usually between 1,000 and 1,500 mm and the dry season does not last for more than three months. The vegetation is best developed in Tanzania where it is refereed to as the Miombo woodlands. Miombo is the Swahili word for Brachystegia, a genus of tree comprising a large number of species. Miombo woodlands form a broad belt across south-central Africa, running from Angola in the west to Tanzania to the east. In Tanzzania, the Miombo woodlands cover areas of Songea and Lindi in the south, as well as Mbeya, Tabora, Rukwa and Kigoma in the cenral and western parts of the country. Other areas in East Africa with woodlands include West Nile areas of Mt Kei and Otzi, northern Uganda regions of Mts. Kadam and Napak, parts of Lake George-Edward region, and some coastal areas in Kenya as well as parts of southern Kenya.
Characteristics of savannah woodlands.
A more or less continuous cover of trees characterizes this type of savannah vegetation. The trees are short ranging between 8-16 meters tall and umbrella shaped. The trees are deciduous, shedding off their leaves in the dry season. Many trees are drought resistant e.g. acacia and the baobab. These trees are adapted to the severe dry season, which characterize such areas. The baobab trees for example have swollen trunks in which they store water during the dry season while some trees have thick barks like the acacia trees to prevent water loss. The thick barks also make the trees be fire resistant. Most trees have small leaves to reduce transpiration. The leaves of the trees are small, waxy and sometimes thorny- like, to reduce the rate of transpiration. The roots of the trees are long and extend to tap any underground water. More light reaches the ground than in forest areas, so there is a denser growth of grass and shrubs on the ground.
Savannah Grassland.
This mainly occurs where annual rainfall ranges between 750 to 1000mm, which is excessive for grass but inadequate for forest growth. The resultant vegetation is thus grassland with few scattered trees. This type of vegetation can be seen in the Nyika plateau of Kenya, rift yalley floor of western Uganda, parts of
northern, eastern and western Uganda such as areas of Soroti, Kamuli, Iganga, Nakasongola, Masindi, Gulu and Mubende. In Tanzania it occurs around Bukoba, Tanga, Rukwa, Mara and Serengeti plains.
Characteristics of savannah grasslands
Grasses form the dominant plant life. Elephant grass and spear grass are the dominant grass species. The grasses vary in height depending on the amount of rainfall from about one meter or so tall, to the three-four meters in case of the elephant grass. There are few scattered deciduous trees within the grasslands. These trees are umbrella shaped and have small leaves to reduce the rate of transpiration. The trees are also deciduous. The grasses dry up during dry season and look brown while in the wet season they are green. Where rainfall is low the vegetation consists of tough tufted grass. Savannah regions appear extremely barren in long dry season.
Dry bush and scrub.
This type of vegetation occurs on the fringes with the arid and semi-arid areas where rainfall is low, less than 750 mm per annum but more often between 250-500 mm per annum. It occurs in Kaabong, Moroto, Abim, Amuria, Isingiro, Kiruhura and Buliisa in Uganda, Turkana, Marsabit, Mandera, Wajir areas of Kenya, and while in Tanzania it is found in Dodoma, Morogoro and parts of Lindi. Characteristics of dry bush and scrub Bush thorny trees 5-10 meters tall are densely interlaced with each other with scrub growing between them. The grasses are very short grass, more tufted with bare ground between scattered thorny bushes occurs. The few trees there are drought resistant with very small leaves or thorns instead of leaves to reduce the rate of transpiration. The trees have long tap roots to reach the underground water supplies. Others such as the baobab have swollen trunks to store water for use during the long dry season.
How the savannah vegetation has adapted to the alternate wet dry climate.
Most savannah plants have a way of adopting themselves to the alternate wet-dry climate. Plants like the acacias have long tap roots, which reach down to the moist rock layers for below the surface in search of ground water. Trees like the bamboo and baobab trees store water in their swollen trunks and in this way, they are able to survive the dry season. Other plants are capable of retaining water in their roots.
Thorny bushes and scrubs have thorns instead of leaves. This helps to reduce the rate of transpiration. Savannah species like the eucalyptus produce seeds, which are fire resistant as bush fires are common in the dry season. Most trees shed their leaves during the dry season. If they did not do this they would literally dry up as transpiration proceeds. Many stems of savannah plants are photosynthetic.
The advantage of this is that they do not need to wait for leaf growth before photosynthesis can start in the short wet season. The more shallow-rooted species compete more successfully on less absorbent surfaces, where they can quickly use the water before it evaporates. Some species have twisting leaf stems to shield them from direct sunshine. During the dry season the leaves of the grasses turn yellowish-brown and die but the roots only remain dormant (sleeping) until the on-set of the wet season and they start growing.
Factors responsible for the distribution of savannah vegetation in East Africa.
Physical factors.
The amount of rainfall and its distribution has a great bearing on the distribution of savannah vegetation in East Africa. In areas where rainfall is heavier usually between 1000 to 1500 mm per annum and the dry season does not last for more than three months and where temperatures, are hot throughout the year, usually above 20°C savannah woodlands are found. The woodlands are best developed in Tanzania where they are referred to as Miombo woodlands.
The grasslands occur where annual rainfall ranges between 750 to 1,000 mm per annum with alternate wet and dry seasons and hot temperatures above 20° C. The dry bush savannah occurs on the fringes of the arid areas where rainfall is below 750 mm per annum with a long dry season and where temperatures rise above 20° C.
The excessively hot temperatures in the dry seasons result in excessive evaporation and scarcity of water. Grasses wither and trees shed off their leaves. This explains why the vegetation is adapted to such climatic conditions e.g. the baobab trees on Mt. Kadam and Napak with swollen trunks to store water for use during the long dry season. Most dry bush and scrub savannah has thorns instead of leaves to reduce water loss through transpiration e.g. in Kaakong and Kotido. The alternate wet and dry seasons result in savannah grassland regions being green in the wet season and brown in the dry season. In addition to the above, savannah vegetation requires low humidity in case of the grasslands and dry bush and scrub savannah while the woodlands require moderate humidity.
There is a close relationship between the nature of soils and the resulting vegetation. The woodlands and grasslands thrive in areas of sandy loam soils of medium fertility e.g. the grasslands in Gulu Lira and Dokolo, and the woodlands in Kiboga and Kilak. The dry savanna thrives in areas of strongly weathered sandy soils of low soil fertility and low water retention capacity e.g. in Kaabong, Kotido and Tanga.
The woodlands and grasslands savannah vegetation thrives in areas with well-drained soils under conditions of fresh water while dry bush savannah thrives in areas with poor drainage.
Savannah vegetation grows in the low altitude areas below 1,300 metres above sea level. Most plateau areas in Uganda with savannah vegetation in Soroti, Kamuli, Iganga, Nakasongola, Masindi, Gulu lie between 900-1000 m above sea level.
Human factors
In East Africa, fires are widespread especially during the dry season. Some fires are started accidentally but others are deliberately set. In pastoral communities of the Bahima in Ankole and Karamajong in north eastern Uganda, fires are set out to burn off old grass and stimulate the growth of new shoots for their animals to feed on when the rains begin. Although burning may be good in the short run, in the long run, it has led to the growth of fire resistant grasses and shrubs which are short and stunted which bare soil between them. Thus savannah grasslands turn over time into dry bush and scrub savannah.
The pastoralists in East Africa cover extensive grazing lands e.g. the Karamajong and Bahima cover large areas of north eastern Uganda and Ankole while the Turkana cover large areas of north western Kenya. These pastoralists keep large herds of cattle which lead to overgrazing and hence the deterioration of vegetation. In Masai region of southern Kenya, cattle need as much as two hectares each to live comfortably but one or less is available for each head of Masai stock and thus the grass is nibbled on the roots. Savannah grasslands have been turned into dry savannah vegetation consisting of very short grass, more tufted with bare ground between in Isingiro, Mbarara, Kiruhura and Karamoja.
The increase in population leads to more and more land being needed for both settlement and farming. Today most of the savannah vegetation especially grasslands in Kamuli, Tororo, Iganga and Mubende has been cleared for this purpose. Derived types of savannah showing remnant species to have replaced a previous closed type are also evident in the Lake Victoria basin and northern Uganda.
Man's activities such as clearing of forest have had an effect on the climate e.g. it reduces the moderating effect of vegetation on climate resulting in low rainfall recieved. In Uganda for example, forests have been cut in Mpigi, Kiboga, Hoima and Mityana and there is a marked reduction in rainfall. This reduced rainfall means that a less extensive vegetation cover has to be supported in the areas mentioned.
Areas on the Miombo woodlands and in parts of Masindi, Koboko, Yumbe and Moyo are infested with tsetse flies. One way of eradicating this pest is to clear away the entire bush so that the fly has no shed in which to rest. This method has been used which has led to the deterioration of the woodlands in the above mentioned areas.
In East Africa, one most widely used source of energy in the rural areas is firewood. Trees in the savannah woodland and grassland regions have therefore been depleted for firewood e.g. on Mt Kadam, Mt Napak and Miombo woodlands. Similar to the above, the trees and grasses have been over exploited for timber and thatching grass for houses in rural areas such in Kumi, Amuria, Kaberamaido and Katakwi leading to their deterioration.
Land use in savannah regions.
Savannah is the dominant vegetation type in East Africa. The resources in these lands are vital to the livelihood of millions of rural and urban people living in and around them. Mainly used as pastures and for subsistence farming they also provide a multitude of products including food, energy, shelter, medicines and a number of invaluable environmental and spiritual services. As earlier stated, savannah regions experience hot temperatures throughout the year. Rainfall varies according to location from 250 mm to 1,500 mm per annum and temperatures are over 20° C throughout the year. These climatic conditions can give rise to various land uses as seen below:-
Cultivation:
Because of the moderate rainfall with alternate wet and dry seasons, seasonal crops are mainly grown e.g. millet, maize, sorghum, potatoes, beans and others in Soroti, Kumi and Kamuli.
Animal rearing:
The savannah grasslands and dry bush provide extensive areas for animal grazing e.g. the nomadic tribes like the Masai in southern Kenya and northern Tanzania.
Lumbering:
The savannah woodlands consist of a continuous cover of trees thus giving rise to-lumbering activities e.g. in the Miombo woodlands.
Wild game conservation:
The vegetation provides natural habitant for many species of wildlife, which forms the basis of the thriving tourist industry particularly in East Africa. To see game in its natural state, one must go to big game reserves and parks such as Ngorongoro in Tanzania and Tsavo Park in Kenya.
Settlement:
Savannah regions are easily settled by man and therefore have moderate to high population densities. It is only near the desert and semi-desert fringes that rainfall is very low that population issparse.
The wetter zones provide numerous useful trees such as oil palm, shear butter trees and kola nut trees. Other land uses in the savannah regions include transportation, bee keeping and charcoal burning.
Problems associated with land use in savannah regions.
Savannah regions have been occupied and utilised by human beings in East Africa for thousands of years. However, currently type and intensity of land-use are changing due to changes in the socio- economical, political, and technological set-up in this region leading to increased land degradation in many areas. The temporal assessment of vegetation shifts and the underlying changes of land-use pattern, fire regime, browsing/grazing by mammalian herbivores and termites are of primary importance for the sound economical and ecological management of these dominating vegetation types in Tanzania. Currently, below are the major problems facing land use in the savannah regions:-
The low and unreliable rainfall is a major climatic limitation to arable farming and animal rearing. The long dry season limits the range of crops grown to millet, sorghum in Pader, Kitgum and Ajumani and crop failures are common.The low rainfall also results into scarcity of pasture and water for the animals. The natural pastures are poor and not nutritive for the livestock. This leads to animals moving long distances in search of water and pasture resulting in loss of weight, poor quality animals and even their death. This has affected the Masai pastoralists in southern Kenya, Bahima pastoralists in Ankole and Karamajong pastoralists in Kotido and Moroto.
The dry season is long up to six months in Kaabong, Kotido, Turkana and Marsabit which results into scarcity of vegetation. The land is left bare and serious soil erosion by wind, and by water on the on set of the wet season. This reduces the fertility of the soil for arable farming.
Soil erosion is also caused by overstocking and overgrazing by cattle e.g. in Kiruhura, Isingiro, Abim, Moroto and Nakapiripirit. The vast herds of wild game such as the wildebeests in Serengeti and Masai- Mara parks leads to overgrazing, soil erosion and soil exhaustion.
Fires tend to be wide spread during the dry season. Some fires are started accidentally but others are deliberately set. Hunters sometimes burn the bush to drive out wild animals. Fires are also started by cultivators to clear the land for growing crops and pastoralists to burn off the old grass in anticipation of the wet season. Bush fires have the effect of destroying variable trees thus affecting the lumbering industry in Morongole and Timu woodlands in Kaabong, lead to the migration of wildlife e.g. from Matheniko Wildlife Reserve thus affecting the tourist industry, affect bee keeping and result into growth of fire resistant grasses which are not nutritious to the animals. In addition, the land is left bare and soil erosion sets in.
Uncontrolled cutting of trees leads to the depletion of the woodlands thus affecting the lumbering industry. As more trees are removed for fuel, and over grazing reduced the productivity of grasslands, the heavy rains gullies and the wind blows away the surface soil. Where the savannah is not farmed, there are usually more trees, suggesting that grass may not be the natural climactic climax vegetation.
Prevalence of diseases e.g. the tsetse fly in the Miombo woodlands causing sleeping sickness in humans and nagana in animals. Other diseases include wide-spread malaria.
The harsh climate and low productivity of the soil make settlement difficult. In addition, the tsetse flies in areas of Miombo woodlands, Yumbe and Moyo as well as parts of Masindi result in low population, and the low soil productivity.
The sparse settlement results into low market for products e.g. honey and other crops and remoteness of the regions because of the limited transport routes.
Wildlife conservation as a land use is faced with a problem of poaching e.g. Kidepo Valley National Park in Kaabong, and encroachment resulting from increased demand for land for both settlement and farming e.g. Pian Upe Wildlife Reserve in Nakapripiripirit.
The poorly developed transport routes in the region affect land uses such as settlement, tourism, and lumbering. Remoteness is therefore a serious problem in Moroto, Kotido and Kitgum.
Generally, there is inadequate capital to develop farming systems such as ranches in Masailand, Isingiro, and Kiruhura, developing transport routes and setting up irrigation schemes e.g. in Nakapiripirit and Abim in north eastern Uganda.
DESERT AND SEMI-DESERT VEGETATION
About two-thirds of Kenya is covered by semi-desert vegetation e.g. areas of Turkana, Masabit, Mandera, Wajir and Lodwar. In Uganda, semi-desert vegetation is confined to some areas in Karamoja such as Kaabong and Kotido. It is also in small pockets in north eastern Tanzania. Desert vegetation occurs in the Chalbi desert east of Lake turkana in Kenya.
The vegetation consists of mainly woody plants, mostly bushes, acacias and dwarf shrubs that are widely spaced. Occasional sparse grass occurs when rain permits. With the sparse vegetation cover, the soils are often truncated by sheet wash and there are extensive stony areas as well as areas covered by blown sand.22 15The greatest inhibiting factor where desert vegetation is concerned is the extreme limited moisture (usually less than 250 mm). Normal rainfall is so scanty and so erratic that slow and sparse plant growth is the result.
The common plant species are cacti, thorn bushes, course grasses, thorny acacia and flowering herbs. Desert plants are highly xerophytic and have various ways of with standing long periods of drought. Many plants are succulent i.e. they can store water in their tissues. Many succulents have fleshy stems and some have swollen leaves. The Cacti for example absorb large quantities of water during the infrequent rains and store it in their trunks. This causes the trunks to swell up. This moisture is lost slowly during the long dry season through transpiration.
Transpiration does not take place from the leaves but rather from the stems. The stomata through which water is lost close during the day and only open nocturnally or during night periods. The steams are thick and most of them waxy to reduce the rate of water loss. Many plants have tiny, thorny leaves, which are often waxy, hairy, varnished, leathery or needle-shaped. Many end up with thorns. These help to keep transpiration to the minimum e.g. the cactus. The roots are either very long to tap under-ground water supplies like the acacia roots which exceed 15 meters or they spread out over very wide areas near to the surface to take the maximum advantage of any rain or due e.g. the creosote plants.
Many plants produce seeds with thick coats which by protecting the seed it enable it to lie dormant for years until a little rain falls and they germinate. They however soon wither from the surface and lie dormant till the next rain. Many plants are ephemerals and can complete their life cycle in a very short time of two-three weeks. Very hot temperatures and a high diurnal range as such as 28° C make continuous plant growth difficult. Increased salinity also impedes plant growth. However, there are certain plants known as the halophytes that survive best in saline soils.
SWAMP VEGETATION
This type of vegetation occurs in permanently and seasonally wet or flooded areas in ill-drained valleys. There are several types of swamps or wetlands in east Africa. Natural lakes and lucustrine swamp orwetlands occur around lake shores e.g. Sango Bay, Buyaga Bay, Kachindo, and Lwamunda around the shores of Lake Victoria. Along the shores of Lake Kyoga, wetlands can be seen in Gogonyo, Mulondo, Kidera, Amolator, Lwampanga and Akokoro.
Riverine swamps occur along river valleys like Katonga, Kagera, Mpologoma, Sezibwa, Nile and Kafu. Most of these swamps consist of water loving plants such as papyrus, sedges and palms. Papyrus plants are evergreen and about 1 to 3 m high and have a woody, thick creeping under ground stems. The leaves are long and sharp, and the upright flowering stems are naked, soft, and triangular in shape. The lower part of the stem is as thick as a human arm when mature. Sedges consist of grass-like flowering plants with triangular stems.
Mangrove swamps occur at Tanga, Mombasa, Dae es-Salaam, Mtwara, and Kilwa along the East African coast, The vegetation consists of trees and shrubs that grow in shallow and muddy salt water waters especially in creeks and estuaries. The plants have aerial or arching roots that are exposed during low tides. Some mangrove roots extend above the water.
ALPINE OR MOUNTAIN VEGETATION
On mountains such as Kilimanjaro, Kenya, Elgon and Rwenzori, various vegetation zones can be identified Usually the foot of the mountain slopes are covered by dry bush savannah, which merges into savannah grassland and finally merge into woodland.
The savannah woodland merges into tropical rain forests, which are evergreen. Bamboo and temperate forests replace the tropical rain forests. The trees are shorter 9-12 meters in height where mist prevails trunks are covered with mosses and lichens. The trees further become shorter with increased height. The bamboo and temperate forests are replaced by health and moor land. This vegetation consists of grasses, shrubs and flowers becoming more and sparser at upper levels. Above 4500 meters, plant growth is not existent due to very cold temperatures.
Influence of altitude on vegetation on mountains.
Changes in altitude bring about changes in the amount of rainfall received, temperature, humidity, soils and drainage hence leading to various vegetation zones. The amount of rainfall increases with increase in altitude while temperatures decrease with increase in altitude. On the lowest slopes below 2,000 meters above sea level, rainfall ranges between 500 to 1,500 mm per annum and temperatures are hot usually above 20°C. These conditions have given rise to the growth of savanna vegetation. The lowest slopes are therefore covered with dry bush and scrub savannah, which merges into grassland savannah and finally woodland savannah.
Between 2,000 to 3,000 meters above sea level, the amount of rainfall also increases to over 1,000 mm per annum and it is received throughout the year. Temperatures are still hot thus above 20° C. Soils are deep, fertile, and well drained and support vegetation that is more luxuriant. These conditions give rise to the growth of tropical rainforests or equatorial forests. The forests are evergreen and denser on the windward slopes where there is more rainfall.
Between 2500-3500 metres above sea level both the amount of rainfall and temperatures decrease to below 20°C. The trees of the tropical rainforests become shorter and shorter and the branches closer to the ground. The tropical rainforests merge into temperate. On Mount Kenya species of camphor, cedars and poda carpus exist. Only plants that can withstand the cool temperatures grow.
The plants have cones instead of fruits and thick barks to withstand cold conditions. The temperate forests are replaced by bamboo forests. The trees are evergreen with segmented, hollow stems.Beyond 3,500 meters above sea level, rainfall is also very low, temperatures are low and soils are thin and infertile. This zone is limited to heath and moorland, which can withstand such cold temperatures.
The vegetation includes giant heather, giant groundsel, and giant lobelia. There are many woody plants, with small trees and shrubs pre-dominating these regions. Beyond the height of 4,500 metres above sea level, temperatures are too low for plant growth. Snow and bare rocks are found.
Other economic activities carried out on mountains of East Africa include mining at the foothills e.g. limestone and cobalt mining at the foothills of Mt. Rwenzori, quarrying for the provision of building materials, fish trapping from rivers and irrigation farming e.g. on the slopes of Mount Elgon with water derived from Manafwa and Doho rivers.
VEGETATION IN AFRICA
African vegetation zones are closely linked to climatic zones, with the same zones occurring both north and south of the equator in broadly similar patterns. As with climatic zones, differences in the amount and seasonal distribution of precipitation constitute the most important influence on the development of vegetation. Moving across the continent into drier and drier climates, the typical sequence of vegetation is from tropical moist forest to moist savanna, dry savanna, semi-desert, and finally desert.
Relationship between climate and natural vegetation.
In the equatorial climate region, rainfall is heavy usually more than 1,500 mm per annum. The rainfall is also received throughout the year with no marked dry season. Temperatures are hot throughout the year the average being 26-27° C. The humidity is high as well as a dense cloud cover.
These climatic conditions have given rise to the growth of equatorial forests. This vegetation is characterized by being thick and luxuriant, many different plants and trees species grow. The trees include Mvule, Mahogany, ebony and rosewood while other plants include climbers, creepers and lianas. The vegetation is evergreen due to the rainfall received all the year round. The trees grow to great heights of about 60 meters in an attempt to reach the sunlight. The trees have broad leaves in order to releases excess water through transpiration.
In the savannah type of climate, there are distinct wet and dry seasons. The amount of rainfall varies from the fringes of the equatorial forest to the desert areas from 1,500 mm to 250 mm per annum. Temperatures are hot throughout the year the average being 25°C. The above climatic characteristics have given rise to three types of savanna vegetation. Where the rainfall is between 1,000 to 1,500 mm and the dry season is short, the natural vegetation is savanna woodland. Trees of 8-16 meters in height form an almost continuous plant life. As more light reaches the ground, there is a dense growth of grass. The trees shed their leaves in the dry season. The leaves of the trees are small to reduce the rate of transpiration. Some trees have swollen trunks in which they store water for use during the long dry season.
Where the rainfall ranges between 750 to 1,000 mm per annum, the natural vegetation is grassland dotted with trees. Because of the alternating wet and dry seasons, there are alternating periods of green and brown covers of the grasses. The grasses are tall about one meter and more in height. At the fringes with the deserts where rainfall is low ranging between 250 to 500 mm per annum, the resultant vegetation is dry bush and scrub consisting of thorny bushes and very short grass. The plants have thorns to reduce the rate of transpiration, as water is scarce.
Arid and semi-arid climatic regions are characterized by having low and unreliable rainfall of less than 250 mm per annum. Temperatures are very high usually over 30° C. The cloud cover is very low and the regions experience a high diurnal temperature range. The resultant vegetation is thus cacti, bunch grass, small woody plants, and patches of short dry grass with outbursts of flowering plants that grow during the brief rains. The plants have long tap-roots to pierce far and wide in search of water. Most plants usually complete their life cycle within a short period after the rain before the soils completely dry off. Other plants such as the baobab trees have swollen trunks in which they store water for use during the long dry season.
The Mediterranean climatic regions such as northern Tunisia, Morocco and Libya and the Western Cape Province of South Africa experience cool wet winters, warm dry summers and moderate rainfall. Temperatures in these areas are from warm to hot in the high sun season with high evaporation rates and are mild in the low sun season with reduced evaporation rates. These regions have thus been called 'winter-rain and summer dry' climates.
The resulting vegetation Mediterranean consists of forests, woodlands, and shrub. The forests are evergreen consisting of oak, pine and cider, and shrubs, which are interspersed with xerophytes. The trees have thick backs and small shinny waxy leaves to reduce water loss and long tap roots to pierce everywhere in search of water. The trees are composed of broadleaf evergreen trees, such as the oak and mixed sclerophyll forests. The trees adapted to the distinctive climatic regime of summer drought and cool moist winters with only sporadic frost.
The Mediterranean shrub lands are composed of dense thickets of evergreen sclerophyll (which means 'hard-leaved' in Greek) shrubs and small trees. Scrublands are most common near the seacoast, and are often adapted to wind and salt air off the ocean. The vegetation is generally not luxuriant because precipitation is only moderate. Trees range in height from 3-5 meters. The trees grow at widely spaced intervals and therefore provide little shade. Most trees are highly xerophytes i.e. drought resistant. The trees have small leaves with waxy, hairy or glossy surfaces, which are efficient in reducing transpiration in the dry summer season.
The trees have thick and often gnarled barks to reduce the rate of transpiration e.g. the cork oak. The thick barks to reduce transpiration. Many trees have long tap-roots, which grow deep in search of under ground water supplies. In some cases, many trees have bulbous roots which store water. Plant growth is fastest in winter when the westerlies bring rainfall. Growth is checked during the long dry summers. Many of the shrub species are sweet scented. Their scent is strongest in the long dry summer. The hot, dry summers make much of the region prone to fires, and lightning-caused fires occur with some frequency. Many of the plants are pyrophytes.
Montane climate is experienced in high altitude areas such as Kilimanjaro, Rwenzori and Kenya. These areas posses a number of micro-climates. A rise in altitude brings about climatic and vegetation changes e.g. a rise in altitude brings about a fall in temperature. The amount of rainfall received also increases up to ascertain height before it begins reducing.
Similarities in latitudinal and altitudinal distribution of vegetation zones over the earth's surface.
Mountain regions possess a number of microclimate instead of one single climatic type. Broadly speaking, a rise in altitude brings about climatic and vegetation features comparable to the effect of an increase in latitude.
Effects of latitude on vegetation in Africa.
Vegetation types change as one moves from the equator to the poles or from the lower latitudes to the higher latitudes.
The tropical climate occurs in the lowlands between 5°N and S of the equator e.g. in the Amazon Congo basins. In these regions, rainfall is heavy and well distributed throughout the year.
The temperatures are also high throughout the year the average being 26°-27°C. The combination of high temperatures and heavy rainfall throughout the year has given rise to the growth of equatorial rainforests,which are characterised by being evergreen.
The tropical climate occurs between 5° to 15° north and south of the equator. Here rainfall is less than in the equatorial climate and there is distinct wet and dry season. This type of climate has given rise to savannah vegetation. This type of vegetation is best developed north and south of the Congo basin, in west and east of African plateaus, parts of Brazil and north and east Australia.
Between latitudes 30° to 45° N and S of the equator, rainfall is generally low but received throughout the year. Temperatures are lowered due to increased distance from the equator. The climate here varies from Mediterranean to temperate climate depending on the location. Mediterranean vegetation mainly occurs on the western sides of landmasses in the temperate latitudes e.g. around the Mediterranean Sea, South West Africa and central California. The coniferous forest occurs across North America from coast tocoast.
The tundra climate is best developed at around latitude 65°. It is characterised by very low rainfall of less than 250 mm and very cold temperatures especially in winter. This has given rise to tundra type of vegetation, which consists of mainly lichens, mosses, sedges and flowering shrubs. Main areas with this are covered by type of vegetation are areas fringing the Arctic Ocean in these continents of Eurasia and North America. At the poles temperatures are extremely low and plant growth is inhibited. These areas ice sheets.
Effect of altitude on vegetation.
On mountains such as Kenya, Rwenzori and Kilimanjaro, altitude determines the climate in terms of precipitation and temperatures received. Temperatures decrease with increase in altitude until the summits are reached. The vegetation is thus distributed in zones similar to that from the equator to the poles. This is explained below-
The lowest slopes are covered by both equatorial and savannah climates. On many mountains, the savannah vegetation emerges into tropical rainforests due to increased rainfall with increased altitude. With increased altitude, the temperatures became lower and rainfall decreases.
These conditions give rise to cool temperate climate and the growth of temperate vegetation between 2,500 to 3,500 meters above sea level. Beyond 3500 meters above sea level, the vegetation is limited to plants adapted to cold conditions. The vegetation is heath and moorland and it is similar to the tundra vegetation in the higher latitudes. Above 4500 meters above sea level, temperatures are very cold and no vegetation exists. These areas are covered by ice caps.
