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2.1. Phytogeography of the country
2.2. Utilization patterns of forest species
2.3. Threats to Forest Genetic Resources and Impact

2.1. Phytogeography of the country

Kenya is rich in biological diversity. The country harbours over 6000 species of higher plants (including 2000 trees and shrubs). Recorded species of butterflies are 875, of birds, 1079 and 379 species of mammals (KIFCON, 1994). Most of these species of fauna are associated with forest and woody vegetation. Further more the forest contains 50% of the nation’s tree species, 40% of the larger mammals and 30% of the birds (KIFCON, 1994).

Kenya’s forest estate totals 2.4 million ha or just under 3% of the country (KIFCON, 1994). As observed in table 1 above, four ecological zones (Fig.1) are defined in the country with different vegetation cover type and floristic composition. In addition woodlands, bushlands and wooded grasslands cover about 38.5 million ha. Farmlands and settlements take up some 10.02 million ha while forest plantations area occupies about 136,000 ha.

Acacia and Commiphora spp dominates the desert thorn-scrub lands. Grazing is nomadic and is the main occupation of the local people. The low montane forests in the ASALs are dominated by tree species and in some cases such trees form closed forest vegetations. The main tree species include: Juniperus procera (cedar), Podocarpus gracilior, Olea africana, Olea hochstetteri, Lawsonia inermis, Combretum molle, Casipourea malosana, Diospyros abyssinica and Teclea simplicifolia.

The deciduous woodland occurs throughout the ASALs and is dominated by Acacia tortilis. Other more notable species include: Hyphaene ventricosa, Salvadora persica, Acacia nubica on the Northwest and northern Kenya and Commiphora and Acacias in the southern parts.

Deciduous and evergreen thorn bush constitute another extensive vegetation cover type. The main species in the north include: Acacia reficiens, Acacia senegal, Euphorbia sp., Pappea capensis and Combretum molle.

The dominant species of the shrubland vegetation are Acacia mellifera, Acacia senegal, Acacia reficiens and Acacia tortilis in the more northerly parts. In the south, Acacia reficiens and Commiphora sp. are the dominant species.

2.2. Utilization patterns of forest species

Table 2 gives a list of tree species and their different purposes.

Table 2: Different uses of forest species

Category of use

List of mains species used

Medicinal plant species

Schlebera alata, Myrsine africana, Acacia nilotica, Juniperus procera, Warbugia ugandensis, Balanites aegyptiaca, Erythrina abyssinaca, Combretum molle, Carissa edulis, Acacia nubica, Ziziphus mucronata, Senna didymobotrya, Salvadora persica.

Food tree species

Balanites aegyptiaca, Cordia sinensis, Grewia spp., Ziziphus mucronata, Vangueria infausta, Carissa edulis, Ximenia Americana, Lannea schweinfurthii, Lannea alata, Balanites glabra, Ziziphus mauritiana, Tamarindus indica, Berchemia discolar, Adansonia digitata, Sclerocarrya birrea, Dobera glabra

Fodder tree species

Acacia gerrardii, Acacia tortilis, Acacia mellifera, Strychnos henningsii, A. senegal, Maytenus heterophylla, Achyranthes aspera, Olea europaea, Acacia brerispica, Acacia etbaica, Grewia tembensis, G. similis, Rhus natalensis.

Timber species

Melia volkensii, Combretum schumanii, Combretum zeyheri, Terminalia pruniodes, Terminalia kilimandscharica, Terminalia brownii, Azadrachta indica (neem), Newtonia hilderbrantii, Pinus caribaea

Fuel wood species

Acacia elatior, A. mellifera, Newtonia hilderbrandtii, Parkinsonia aculeate, Eucalyptus camaldulensis, E. tereticornis, Combretum molle, Prosopis spp

2.3. Threats to Forest Genetic Resources and Impact

External intervention of natural forest and woodlands do impact on the state of the forest genetic resources in a positive or negative manner. Usually interventions induce certain kinds of changes to genetic processes that in turn affect the evolution and sustainability of forest ecosystems. Changes in genetic variation are attributable, in short term, to changes in the basic evolutionary processes, which are characterised by:

The above evolutionary processes affect the level and distribution of genetic variation and the present state of the genetic resources is a result of their joint efforts. Human activities have influences on the above processes that affect genetic variations of the forest resources under management and use.

The major threats thus on forest and woodlands resources in Kenya include:

The following human activities are more specifically elaborated to show the impact caused on forest genetic resources.

Kenya forests have numerous endemics as well as threatened species. Some 150 internationally threatened wood plants in the country have been identified (Beentje, 1988). In addition, 60 inland forests and 65 coastal forests are known to harbour threatened plant species (KIFCON, 1994).

Demography impact on forestlands

As population increases forestland has been cleared to settle people near major towns. Forests near towns are more threatened since the population increase and density are higher than in the rural areas therefore requiring more land for expansion. While the rural population has a growth rate of 2.1%, that of the cities and major towns is above 4%. New settlements in dry areas are followed by immediate clearing of the indigenous vegetation before residents start growing trees in their now smaller compounds. This scenario cause heavy losses of populations of tree species through clearing. Currently, some 465,000 ha of land is taken up by settlements as towns and expansion of this type of land use is increasing fast.

Impact of agriculture on forest lands

Agriculture is the main stay of the national economy. It contributes about 30% of the GDP, produces most of the national food requirements, generate 60% of the foreign exchange earnings and provide about 70% of Kenya’s agro-based industrial raw materials and almost all employment opportunities in the rural areas.

Changes of land use from natural vegetation to farming as a result of increasing human population and the need for more food have greatly reduced the dry forest areas. Pressure on woodlands for cultivation, intensive grazing and settlement has continued to increase at an alarming rate.

Excisions of forestland to create land for farming have become a common practice and alarmingly notable. Over the last decade or two, the rates of loss of closed forestland stood at 5.000 ha/annum and 19,000 ha per year when other types of dry woody vegetation types are included. Loss of closed forestland has increased with a proposed excision of over 80,000 ha in 2001 alone. This will mean serious impact on forest genetic resources. FAO estimates that between 1990-2000, Kenya lost 931,000 ha (93,000 ha/annum) of its forest cover (FAO, 2001).

Such excisions are scattered, some large, others small, while others are done in small and isolated forest stands. This is likely to affect the genetic structure of the forest populations and thus leading to reduction of population sizes, few individuals to seed and produce pollen needs. The source of migration of genes may be cut off causing random losses of variation by drift. Random drift of alleles will severely reduce genetic variation that may be significant for future adaptability for several generations. It is feared that the later case could be enhanced for those small populations in isolated forest stands being cleared without any considerations of the danger of erosion of forest gene pools.

The situation in woody vegetation in dry areas is even worse since larger areas are being cleared to enable sustaining higher production of crops under the poor conditions for efficient agricultural practice. Furthermore even where land is left as fallow, recovery rate is very slow and the fallow period has become less and less as more population continue to migrate into the drier areas of the country.

Presently some 10.02 million hectares is estimated to fall under farmland form of land use, both in mainly medium and high potential land areas. The rate of expansion of agricultural land during the last decade has been about 100,000 ha/annum. This land has come from forest land but more so from the woodland vegetation types indicating that threats on wild tree genetic resource are real now and in the future.

Impact of livestock (grazing) on forest tree species and populations

Livestock in the arid and semi-arid areas (ASALs) includes the donkeys, camels, cattle, sheep and goats. The dry zone support 50% of the cattle, 55% of the goats, 75% of the sheep and nearly all the camels in Kenya. In numbers, Kenya has 12 million cattle, 19 million sheep and goats, 875,000 camels, 18 million poultry and 114,000 pigs.

The people living in the ASALs are mainly pastoralists, and derive their livelihood from keeping livestock. Crop farming is rare except along the permanent course of the river Tana. Apart from providing a livelihood, livestock is also a symbol of social status. With the increasing population, there has been a trend towards sedentarisation of communities. This has greatly interfered with the traditional grazing patterns. Migration of people from high potential areas into drier lands in search of space for cultivation is not only causing degradation of vegetation and causing erosion but is also changing socio-economics and general environmental situations.

It is observed that grazing in forest and woodlands by livestock mostly affect regenerating species especially of those more palatable. This results in a thinning effect thus changing the population densities of the palatable species. Grazing on such species could affect genetic drift. Grazing especially along riverine systems has been found to cause physical impact on the grazed sites by altering stand structure and environmental conditions. This is a common feature along rivers in the dry areas of northern Kenya and especially along the River Tana where there is a great concentration of pastoral communities. Such changes could affect populations of potential regeneration of plants and forest-dwelling herbivores and therefore, have a negative impact on richness in biodiversity.

Water in the dry lands is generally scarce, only a few rivers are perennial streams, the lakes are generally saline due to high evaporation and the major sources of reliable water are wells and bore holes established at strategic watering points. Unfortunately watering points have turned out to centres of vegetation and environmental degradation due to concentration of high population pressure and grazing.

Forest exploitation (timber and non timber products)

Use of trees is restricted to a few species in different forest/woodland types for various requirements and according to traditional uses and this has tended to place unsustainable utilization on such tree and shrub species. Such utilization concentrating on a few species (eg. Dalbergia melanoxylon, Melia volkensii, Acacia mellifera, Branchylaena huillensis, Acacia senegal, Hyphaenie copressa, Tamarindus indica, Carissa edulis, among others), has put a number of species in the endangered category. This is a serious problem especially where no measures are being put in place to revert such trend.

Exploitation in forest has been through selective cutting and there are a lot of isolated poaching activities. This leads to depletion of quality timber species especially where cutting is not controlled. Operations at forest level that changes the environmental conditions of the forest or woodland will affect relative genotype viability and can also affect pollinator behaviour and other inter-species interaction.

Under the restricted cutting or logging will directly impact on genetic resources of the target commercial species by changing their population age and population density. Selection favours certain genotypes over others and some genes would be lost at higher rates than would be expected by drift alone. Very strong selection of a tree species, e.g. the case of Prunus africana, Dalbergia melanoxylon or Milicia excelsa could reduce population sizes below recuperation levels. The impact will be more on reduced production and survival of these species. This observation is presently apparent and common.

It is also apparent that uncontrolled cutting of a certain species or high selective cutting will also affect other non-commercial species and thus decrease biodiversity of the forest dependent plants and animal species. In isolated populations of forest stands the effect as stated earlier will be even worse.

Natural forests and woodlands are important sources of non-timber forest products. More important are collection of bamboo poles, fruits, fodder, honey production, herbal medicine, bush meat, etc. In dry areas fodder collection during dry and drought periods become destructive where trees and shrubs are cut down to access canopy foliage for animals. Collection of parts of trees (bark, roots, leaves, and branches) can be destructive as is common with several species (e.g. Fagara macrophylla, Prunus africana etc) where whole tree deaths are common when large portions of stem barks are removed for herbal medicine.

Collection of non-timber forest products will have varying effects on genetic evolution processes depending on the part of the plant harvested or collected for use. Harvesting of reproductive parts of a plant, tree or shrub, such as fruit or seed, reduces the effective size of pool of reproductive parents. Intensive harvesting of fruits and seeds cause severe genetic impact. Direct selection of e.g. a certain variety of a species will affect reproductive traits but the strongest genetic impact is on mating system and gene migration depending on the harvested parts.

Harvesting non-productive parts such as leaves or bark for medicine, as is the case with fagara spp and Prunus africana, will have indirect selection effects on viability and reproductivity of affected individuals. Harvesting of whole tree/shrub for products other than timber, as is the case of shrubs cut by pastoralists to provide fodder, implies that the population size is remarkably reduced. This will have a serious genetic impact. In particular genetic drift will be affected thus causing random losses of variation. Reproduction capacity of the affected population is affected and this may impact on the genetic viability by reducing it and therefore affecting future adaptability of future generations.

Other types of threats

Droughts in the dry areas are common but rarely do they result in massive deaths of tree species since most of the species are adapted to such conditions. Natural disasters are not common in Kenya. The most other important threats to forests and woodland species in Kenya is forest and bush fires. The incidences of forest fires are very high especially during the drought periods. About 10,700 ha of plantations and natural forests are lost annually as a result of fire. In the process the Forest Department has been losing annually close to US$505,000 through loss of forest produce and suppression costs. This cost does not include bush fires in dry areas where timber production is not a management priority and therefore not put into cost in the computation of losses. The incidences of fire are more prevalent in dry woodland ecosystems and fire management is an important issue in Kenya but is less given the attention it deserves.

Fire may cause variable effects depending on the intensity and extent in space of the fire. Severe fire may have the same effect as clearing a forest especially where large patchy openings are created as a result of fire. The patterns and sizes of such openings versus the forest cover influences genetic diversity. The major impacts are heavy mortality on the burnt species thus reduced population sizes and this would increase genetic drift. For isolated populations this means the migration rates of seed and pollen exchange are affected. Sources of migration could even be cut off, thus reducing the effectiveness of pollinators.

Adverse fire may directly affect biotic dispersal agents and this may decrease migration of genes between populations. Migration may increase if the migration vectors are abiotic. A more devastating fire may affect traits that could have a direct bearing effect on fire resistant species and such action would have direct selection that indiscriminately remove all such genotypes.

List of threatened species and populations

At the species level it has been estimated that 39 species are currently rare, 32 are vulnerable, 20 are endangered and 1 species is feared extinct. Using local herbarium and a recent field species collection it has been established that about one third of all rare species (39 species) occur in the coastal forests. Table 3 shows general distribution of rare, vulnerable and endemic species nation-wide. Table 4 shows the case for the coastal region alone. More recently, the National Museums of Kenya in collaboration with WWF has concentrated its field survey in the coast and hence more information on this area is available (Robertson and Luke, 1995). Table 5 lists species that are known to be threatened and require urgent attention.

Table 3: Nation-wide distribution of rare, endangered and endemic tree species in Kenya

Forest Reserve

Total Species

Vulnerable or Endangered

Strict Endemic

Coastal Forests




Taita Hills




Central Kenya (dry forests)




North East Kenya (dry bushland)




Central Kenya (moist forests)




Central Kenya (riverine forest)




North Kenya (Central Hills)




Cost (outside forests)




Coast (littoral thickets)




Table 4: Distribution of species threats in the Coastal region

Forest Area

Total rare species

Vulnerable or endangered

Simba hills






Pangani rocks



Witu Forest



Cha Shimba Rocks



Tana River









Table 5: List of species or population of species threatened

Ecological zones

Threats at species levels

Threats at population level

Causes of threats

Semi-humid to semi-arid

Albizia gummifera, Diospyros abbysinica, Newtonia hilderbrandtii

Albizia gummifera, Trichilia metica, Brachylaena huillensis, Barassus aethiopum

Over exploitation for firewood, charcoal burning, construction, canoe and beehive making, animal browsing, poor regeneration, fruit gathering

Semi - arid

Dalbergia melanoxylon, Newtonia hilderbrandtii, Carissa edulis

Melia volkensii, Diospyros mespiliformis, Mimusops fruticosa, Cordia sinensis, Tamarindus indica

Same as above


Strychnos spinosa, Hyphaene compressa, Populus ilicifolia

Acacia sengal, Salvadora persica, Grewia tenax, Acacia rovumae

Same as above

Very arid

Hyphaene compressa

Lawsonia inermis, Acacia tortilis

Same as above

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