2.1. Phytogeography of the country
2.2. Utilization patterns of forest species
2.3. Threats
The vegetation represents a transition between the moist tropical rainfall to the south of the continent and the dry Savanna to the north. Two major vegetation types have been identified, namely: the Guinea Savanna and the Sudan Savanna (Bojang, 2000). Currently 43% of the country’s land area (505,300 ha) is under forest cover with woodland accounting for 10%. The remaining 67 000 hectares are savannah woodland and mangrove forests (Bojang, 1999).
Fig. 2: Rainfall distribution pattern in The Gambia (SERTG, 1997 in Bojang, 2000)
The vegetation type is strongly associated with edaphic factors. One hundred and fifty (150) plant species have been identified in The Gambia (Bojang, 1999).
In the woodland areas, particularly in the western division, the following species are common: Daniellia oliveri, Khaya senegalensis, (restricted to moist areas), Pterocarpus erinaceus, Terminalia albida, Parkia biglobosa, Prosopis africana, Piliostigma thonningii, Combretum micranthum and Bombax buonopozense.
For the riparian woodland, the vegetation is generally denser due to the increase in available moisture (higher water table). Trees on these sites would normally grow to large sizes and include Khaya senegalensis, Detarium senegalensis, Pterocarpus erinaceus, Parinari excelsa, Dialium guineense, Parkia biglobosa, Erythrophleum guineense, Elaesis guineensis, Terminalia albida, Cassia sieberiana and Lannea velutina. Also present are common climbers and creepers, including Landdolphia heudelotii and Saba senegalense.
Disturbed woodlands with shrub understorey is typically fairly open with trees seldom 15 m tall. The common tree species are: Combretum micranthum, Terminalia albida, Cordyla africana, Cassia sieberiana, Oxytenanthera abyssinica and Sterculia setigera. S. setigera is however dominant in the eastern part of the country.
Mangrove swamps are found on the banks of the River Gambia and its tributaries. The common species are Rhizophora racemosa and Avicennia africana. Most of the mangroves are used as fuel wood and by small-scale artisans. The mangroves are home to a variety of fish species.
Forests are important natural resources containing numerous species of flora providing food and medicine, habitat for numerous species of fauna with great potential for honey and wax production (Bojang, 1999). They also contribute to an integrated live-support system to many Gambians for the upland and coastal forest.
Medicinal plant species
Trees and shrubs are commonly used in traditional treatments and for other uses. With the cultural belief that “Every plant grown on the surface of the earth has a medical property/use”, it is impossible to provide a complete list of the uses to which most of these plants may be put (Bojang, 1999). Over 28 plant families contain species used for medicinal purposes in The Gambia. Species locally known for their medicinal uses include: Annona senegalensis, Moringa oleifera, Securidaca longepedunculata, Lophira lanceolata, Combretum spp. Guiera senegalensis, Terminalia macroptera, Waltheria lanceolata, Sterculia setigera, Bridelia micrantha, Cordyla pinnata, Afzelia africana, Cassia sieberiana, Tamarindus indica, Ficus spp., Khaya senegalensis, Lannea acida, Voacanga africana, Nauclea latifolia, Vernonia spp., Kigelia africana.
Food tree species
A majority of women, particularly those in the Western Division, rely on the natural forests for their subsistence. Wild fruits and leaves from trees and shrubs, many of which are under exploited from a variety of plants (some of which are either supplementary or primary food source), provide good nutritious supplements to diet such as protein, vitamins and roughage and also medicinal values at certain times of their growing period. Bojang (1999) recorded 24 plant species in the forests widely in use as food by the local people. Principal species are Adansonia digitata, Anacardium occidentale, Annona senegalensis, Balanites aegyptiaca, Bombax costatum, Borassus aethiopum, Ceiba pentandra, Detarium senegalensis, Detarium microcarpum, Dialium guineense, Elaeis guineensis, Ficus gnaphalocarpa, Moringa oleifera, Parinari excelsa, Parinari macrophylla, Parkia biglobosa, Sclerocarya birrea, Spondias mombin, Tamarindus indica, Ziziphus mauritiana. The author has also recorded 24 less widely used ones.
Wood products
The natural forests provide 80 to 85 % of the primary domestic energy for more than 84 percent of the total population in the form of fuel wood.
The total sawn timber volume in The Gambia was 4,630,000 cm3 or 5.5 cm3/ha (National Forest Inventory of The Gambia, 1983 quoted by Bojang, 2000). The major contributory species are shown in Table 1. With the exception of the mangrove species, Rhizophora and Avicennia, which are not “popular timber species”, total sawn timber volume was 2.482 million cm3 in 1983.
Table 1: Most common tree species in terms of sawtimber volumes
|
Species |
Volumes |
|
Pterocarpus erinaceus |
569,000 cm3 (23%) |
|
Daniella oliveri |
314,000 cm3 (13%) |
|
Cordyla africana |
312,000 cm3 (13%) |
|
Borassus aethiopum |
280,000 cm3 (11%) |
|
Khaya senegalensis |
269,000 cm3 (11%) |
Other uses
From the environmental point of view, forest cover contributes among many other benefits, to the protection of soil against desiccation, wind erosion, to the improvement of rainfall percolation, rainfall distribution and temperature moderation. The existence of mangrove forests on either side of the river banks up to 250 kilometers upstream, play significant role in maintaining stability of these river banks and provide habitat for many migratory birds and a life support system to many other plants, mammals and animals.
The upland forests continue to provide energy, construction and building material, food and local medicine to both rural and urban settlements. As such, it significantly contributes to the socio-economic development of the people of these areas in providing resources, job opportunities and income. However, the local commercialisation of forest products, such as fuel wood, timber, fruits etc., contributes to the unsustainable exploitation of the resources.
The coastal forests, including the mangrove forests, also provide the local communities with wood products for construction and energy as well as the needed protection against erosion of the riverbanks as well as breeding grounds for the many varieties of fish and oyster. The mangroves, which provide natural habitat for oysters, also provide many Gambians, mostly women, with some source of subsistence. However, the results of the coastal resources assessments under the climate change scenarios indicate a possible inundation of most of the mangroves on the coastal areas. This is probably due to a rise in the sea level. The Gambia would be adversely affected.
Forest resource endowment of Gambia has been fairly distributed but seriously degraded, mainly through human influence (illegal felling and frequent bush fires) and to a limited extent by climate variations (drought, stress, precipitation and rainfall distribution). However, the unsustainable local commercialisation of forest products (both wood and non-wood products), are the main destructive activities of our forest resources.
Prior to independence from Britain, The Gambia was endowed with dense forests. However, with the increase in human and livestock population, the equilibrium that had existed between man and nature started to decline at a rate faster than it could be regained. Unsustainable forms of exploitation by man, such as the extraction of wood and non-wood products, were the cause. The expansion of agricultural activities and frequent incidence of bush fires and grazing pressure also affected the conditions of the forest. As a result, most of the forest lands have undergone changes, more often to secondary succession, resulting to less dense forests, poor regeneration potential, lower growth, incidence of undesirable grass species and lower plant species diversity.
Agriculture impact on forest lands
Approximately 33% of The Gambian territory, excluding fallow lands, is put into active agricultural use. While crop rotation, and to some extent intercropping, still is widely practiced throughout The Gambia, long fallow periods are coming to an end. For instance, in 1983, the previously practiced or traditional 20-year fallow period after 4 years of cropping, was observed to have dropped to one or two years in some parts of The Gambia. Reduced soil fertility has been the result. Indeed several fields no longer experience fallow periods at all. Consequently, there has been severe pressure on forestlands leading to fast disappearance of forest resources from the high rate of deforestation. This rate of deforestation is now estimated at 3.11% annually (Bojang, 1999).
The 1983 forest inventory gave indications of some significant changes in the forest situation in respect of forest types, area and land use, but with no significant change in total forest area. The open and closed forest types declined in quality by more than 80% between 1968 and 1983 (Table 2).
The reduction in forest area did not result in any increase in savanna but in increase of cultivated area from 274,100 ha in 1980 to 336,000 ha in 1988. Based on the information available from the 1980 aerial photographs and 1988 satellite imaginary, indications are that, no changes occurred in areas of high and low mangrove forest types. However, more up to date information may be available after the analyses of currently obtained forestry inventory data.
Table 2: Changes in forest types in The Gambia between 1980 and 1988.
|
Forest types |
Years |
Percentage Change (%) |
|
|
1980 |
1988 |
||
|
Open forest |
62,700 |
52,300 |
- 16.6 |
|
Closed forest |
28,000 |
16,200 |
- 42.1 |
|
Low Mangrove |
51,900 |
51,900 |
0 |
|
High mangrove |
15,000 |
15,000 |
0 |
|
Savanna |
369,900 |
369,900 |
0 |
|
Cultivated area |
274,100 |
336,000 |
+22.6 |
No data is available to indicate how many plant species are extinct or near extinction (Bojang, 1999). It is however known that several species of high economic value are threatened. Due to the transformation of the ecosystems, some species might have been replaced by the more tolerant.
Livestock (grazing) impact on forest tree species or populations
Livestock production system is reported to be predominantly and traditionally based upon seasonal transhumance, utilizing communal lands and any other unmanaged forestlands. Large cattle herds graze in the plateau bush in the wet season, while the wetlands provide additional fodder resources in the dry season (www.afrol.com). The dry season represents a bottleneck for animal husbandry, especially because of the low nutritional value of the dried grasses. However, the pasture is normally sufficient for Gambian herds, in the absence of bushfires.. Grazing and trampling of large livestock herds causes substantial stress on the geo-ecological environment.
Forest exploitation (timber and non timber products)
The following species are currently over-exploited: Pterocarpus erinaceus, Khaya senegalensis, Prosopis africana, Borassus aethiopum and Milicia regia.
Other types of threats
Due to frequent bush fires and stress from drought, many species even in their natural habitats, are subjected to threats of reduction in species diversity, from being endangered and total extinction.
