Introduction
The vegetation of a place can be defined as a group of plants such as trees, shrubs, herbs and grasses that cover an area. Several factors determine the vegetation of a place. In East Africa the main factors influencing the distribution of vegetation types are:
1. climate eg rainfall, temperatures etc
2. Relief
3. Altitude
4. Soils
5. Man's activities
Climate.
The distribution of vegetation types in East Africa is closely related to that of climate. Climatic factors , particularly , rainfall and temperature, are very important for plant growth. Since these factors vary from place to place, the distribution of vegetation types also varies from place to place.
Rainfall amounts and distribution determine the type of vegetation in a place. Heavy and well distributed rainfall tend to give rise to forests, moderate rainfall produces grassland and little rain/ (less than 250mm) produces scrub or desert vegetation.
Relief.
On steep slopes, water drainage is so rapid that little water is left for plants hence thin vegetation. On gentle slopes, drainage is slower and water can easily penetrate the ground. This water becomes available to the plants hence thick vegetation.
Altitude.
In highland areas temperatures decrease with altitude which in turn affects the type of vegetation. Low temperatures discourage the growth of a dense rain forest.
Soils
Nearly all plants get their moisture and nutrients from the soil. Soil differences produce variation in distribution, for example , fertile soils give rise to more luxuriant vegetation while poorer soils favours poor vegetation types.
Man's activities
Man modifies the natural vegetation by felling trees for timber and fuel wood, by cultivation and settlement or by overgrazing etc. In East Africa, man has changed some of the forested areas into savanna or semi-desert or scrub vegetation.
MAJOR VEGETATIONS IN EAST AFRICA.
There are four main types of vegetation in East Africa, they are:
1. Forests
2. Heath and moorland
3. Savanna vegetation
4. Semi-desert and desert scrubs
5. Swamp vegetation
1. FOREST VEGETATION IN EAST AFRICA.
This type of vegetation was once more extensive than today. Due to man's influence, its extent has been far reduced. A forest is a dense cover of trees about 10 to 30m and over in height forming several layers or canopies. There are two major types of forests found in East Africa:
(a) natural forests
(b) plantation forests
(a) Natural forests.
The natural forest vegetation in East Africa include:
-Equatorial forests/tropical rain forests
-Montane forests/temperate
-Mangrove forests.
A. Equatorial or tropical rain forests
These forests are found around the shores of lake Victoria like Ssese islands, Maramagambo, Budongo, Bugoma, Kibale, Semiliki, Kalinzu, Mabira etc in Uganda; kissi in Kenya and Bukoba in Tanzania. Small patches of dense forest survive on the humid coastal plain. These forests are also referred to as low land forests.
Characteristics of Tropical rain forests.
§ They are evergreen
§ They are tall and giant
§ They are ever damp
§ They have little or no undergrowth
§ They have predominately hard wood trees
§ They have mixed species(not in pure stand)
§ They have lianas/climbers
§ They have thick buttress roots
§ They have broad leaves
§ The trees are umbrella shaped
§ Have three(3) layers (canopies).
B. Montane forests
Montane forests are found at altitude above 1800 metres. These forests are concentrated in highland areas of East Africa, for example Mt. Kenya's, Mt. Kilimanjaro, Rwenzori, Mt Elgon, Mt Meru etc. The leading species of trees in these forests include: podo carp, cedar and camphor. Above 3000 metres bambo forest dominates. At these altitudes, rainfall is usually high, averaging about 1500 m per annum or more.
Characteristics of montane forests.
§ Evergreen
§ Pure stands
§ Single layer
§ Small leaves
§ Tuff leaves etc.
§ Mangrove forests
§ Mangrove forests are concentrated along the coastal areas of East Africa.
Characteristics.
§ Short stems
§ Aerical roots
§ Broad leaves
§ Evergreen
§ Dense forests
MAP OF EAST AFRICA SHOWING FORESTS
Plantation forests.
This type of forests consists of trees planted by man. In general, natural forest land in the three countries is limited. Consequently, al three East African countries have taken up afforestration.
In Kenya, foresters are concentrating on the introduction of exotic species in artificial plantations. These are mainly soft woods, notably cypress and pine, but there are also significant areas of eucalyptus and wattle(acacia). About 160,000 ha are now under plantations, many of which are mature and yielding well. The main areas of exotic commercial plantations include: kitale, Nairobi, Nyeri, Eldoret, Kisumu, Londiani, Thomson's falls, Elburgon etc.
Plantations are less important in Uganda, although there are about 14,000 ha of pine plantations mainly in western region. Many small plantations for fuel and constructional poles are scattered throughout the country especially near towns.
Tanzania has introduced a plantation policy based largely on cypress and pine species suitable for higher altitudes.
Factors influencing the distribution of forest vegetation in East Africa.
Climate:- cool climate favours montane forests; warm and wet climate favours tropical rain forests and mangrove forests. Heavy rainfall favours the growth of all forests.
Soils:- salty soils favours mangrove forests, well drained fertile soils favour montane and tropical rain forests.
Altitude:- Montane forests grow in areas of high altitude with cool climate. Tropical rainforests grow in areas of low altitude with a hot and wet climate.
Drainage:- mangrove swamps or forests grow in salt marshy areas.
Man's activities:- this has greatly influenced the distribution of forests in terms of expense and characteristics. This is through bush clearing for wood, fuel , agriculture etc.
Government policy of gazetting forests, evicting encroachment on forests, replacing destroyed forests through re afforestation and planting new ones through afforestation etc.
Heath and moorland
This type of natural vegetation is found between 3500-4500 metres above sea level. The area is too cold to support many plant species. This vegetation type consists of short tufty grass, shrubs and flowers of the Alpine type and is referred to as Afro- alpine vegetation. It is similar to that found in the temperate lands. This is the kind of vegetation that is best adopted to low temperature conditions of the major highlands of East Africa, for example on Mt Elgon and Mt Kenya, special plant species like the giant lobelia and groundsel thrive.
Savanna vegetation. This type of vegetation covers half of East Africa's land area. It includes three main types:
(a) Savanna woodland(woodland savanna).
It is a more or less continuous cover of trees. The miambo woodland of central Tanzania is the most striking example of this type of vegetation. It is also found of some parts is northern Uganda. It occurs in areas where rainfall averages between (750-1000mm)
C. Characteristics
§ It has short and umbrella shaped trees.
§ The trees are deciduous or shed off leaves
§ It has a dense cover of grass, bushes and shrubs on the ground.
§ Trees have thick barks
§ The plant varieties are drought resistant
§ Many trees are also fire resistant
§ The tree leaves are small to reduce transpiration
§ The tree trunks are swollen for storing water etc.
§ The common tree species are the Acacia and the baobab.
(b) Savanna grassland
this consists of open grassland area with scattered trees. it occurs in areas where rainfall averages between(500-750mm). it is found in the Nyika plains of Kenya, rift valley floor of western Uganda, northern Uganda and the area around Bukoba.
D. Characteristics
Range from natural pastures 1m tall to elephant grass(3-4m) tall of Uganda.
§ Elephant grass being dominant species and spear grass.
§ Has scattered trees and bushes and the dominant tree being acacia.
§ The trees are deciduous
§ The trees are short and umbrella shaped
§ The tree leaves are small
§ Grasses dry up during the dry season.
(c) Dry bush and scrub(steppe savanna)
This type of vegetation occur in areas with about 375-650 mm of rainfall. It is found in north east Uganda and Kenya.
E. Characteristics
§ It has bushy thorny trees with scrub growing between thickets.
§ The grass is very short with thickets in between
§ The trees are short and drought resistant
§ The trees tend to be fire resistant
§ Trees with very thick trunks exist eg baobab trees.
Fig 2. Distribution of savannah and semi arid and arid vegetation.
Factors influencing the distribution of savanna vegetation in East Africa.
¨ Moderate and seasonal rainfall between (760-1000mm) for woodland savanna and (500-760mm) for grassland savanah low rainfall between (250-500mm) for steppe savanna.
¨ Low humidity especially for steppe and grassland Savannah.
¨ Fairly fertile soils for woodland and grassland and poor soils for steppe savanna.
¨ High temperatures above 26°c.
¨ Over stocking leading to over grazing by domestic and wild animals
¨ Cultivation and settlement
¨ Wild fires and bush fires
¨ Deforestation
¨ All the above factors change the initial vegetation characteristics to savanna grassland or to bush or scrub.
Importance of savanna vegetation to man.
¨ It is ideal for livestock keeping hence cattle ranching is common in these areas eg Ankole ranching scheme.
¨ It is a habitat for wild animals and this has facilitated wild life conservation eg Queen Elizabeth National park, lake Mburo national park etc.
¨ The wild life conservation on these areas has promoted tourism in East Africa.
¨ It is a source of fuel ie charcoal and firewood.
¨ Poles for building, fencing and other construction are obtained from it.
¨ It is occupied by rural farmers hence suitable for crop cultivation like cotton, maize, millet, cassava, sorghum etc. hunting takes place since the area is a good habitat for different animal species.
¨ Bee keeping /Apiery takes place in the woodland savanna.
¨ It is a source of medicinal plants.
¨ The grass obtained is used for thatching houses, urinals and seed beds.
¨ Sticks and fibres used for crafts and granary making are supplied by savanna vegetation.
F. Semi-desert and desert vegetation
This consists of dry bushland, thicket and scrub. It is found in areas receiving a mean annual rainfall below 375mm. The trees become stunted in growth and are about 1to 3 m in height. Together with shrubs ½-1m high become amore and more widely spaced over bare ground. Grass becomes rare, except in moist depressions. Acacia is the most common tree. this natural vegetation is found in north Kenya(Turkana district and west pokot district), the Nyika plateau area of the Taita-Taveta District and parts of machakos, kitui and Meru Districts in Kenya. It is also found in North East Uganda and some parts of central Tanzania as shown in fig 2.
G. Swamp vegetation
This type of natural vegetation is composed of papyrus and mangrove plants. Swamps are often found bordered by belts of rich grasslands found in low lying poorly drained areas. Extensive swamps are found in central and western Uganda, the Nyanza province of western Kenya and along the coast where mangrove swamps occur.
H. Montane vegetation
On the mountain the vegetation varies from bottom to the top. Thus vegetation belts (zones) exist on mountains and they are distinguished by their characteristics. These different vegetation zones merge into each other hence there is no clear demarcation between different vegetation types. The factors which play a play a part in such zonation include temperatures, moisture availability and slope steepness. The distribution of vegetation on the mountain can be shown below.
Savanna vegetation is found at the bottom of a mountain because rainfall this zone is between 875-1500mm and the temperatures are high ranging between 20oC-25o C.
Above the savanna is the tropical rain forests because the rainfall increases with increase in altitude to over 1500mm and the temperatures are still high. The soils are also deep and fertile.
Above 3500 metres rainfall decrease and the temperatures are low. The rain forests merge into bambo forest which grows under cool conditions.
Heat and moorland is the last major vegetation type high up on the mountain. Above this there is snow and bare rock.