Coastal Forests of Kenya (Arabuko Sokoke Forest and Shimba Hills National Reserve)

Description

The coastal forests of eastern Africa are ranked among the 35 identified world’s biodiversity hotspots due to the concentration of many endemic species and habitat loss. They originally extended from southern Somalia to northern Mozambique, an area roughly corresponding to the Swahilian regional centre of endemism that hosts a large number of unique plant species and assemblages.

Arabuko Sokoke is the largest extant fragment of the forests that once covered much of the East African coast. It was proclaimed a Crown Forest in 1932 and gazetted in 1943, covering an area of 39,100 ha. The Kararacha extension (2,700 ha) to the south-east, which includes important tracts of key habitats, was added in 1968. Part of the forest, containing sections of the three main habitat types, was gazetted as a strict nature reserve (covering 4,300 ha) in the late 1960s. Average annual rainfall ranges from 900 mm (in the relatively dry and scrubby north-west) to 1,100 mm (in the east). The relatively flat eastern section lies on Pleistocene lagoon sands and clays, separated by a wide band of apparently riverine sandy deposits from the ridge of red Magarini sands that forms the western part of the Reserve.

The forest has three main forest types:

1. Mixed Forest (7,000 ha) in the east, on grey sands. This vegetation type is relatively dense, tall and undifferentiated, with a diversity of tree species. Characteristic trees include Combretum schumannii, Drypetes reticulata, Afzelia quanzensis, Dialium orientale, Hymenaea verrucosa and Manilkara sansibarensis.

2. The Brachystegia woodland (7,700 ha) runs in a strip through the approximate centre of the forest, on white, very infertile soil. This relatively open habitat is dominated by Brachystegia spiciformis. In the west, on red Magarini sands, is Cynometra forest and thicket, dominated by Cynometra webberi with Manilkara sulcata, Oldfieldia somalensis and  Brachylaena huillensis. The transition between white and red soil is sudden, and marked by a chain of seasonal ponds. There are two areas of relatively tall Cynometra forest, with a canopy height of up to 20 m, in the north (3,300 ha) and the South (6,000 ha) of this zone.

3.  Cynometra thicket. This is a lower, scrubbier formation of intermediate Cynometra (11,300 ha) with a canopy height of 7-8 m. The dry north-western part of the Reserve is covered by a low, dense, and often almost impenetrable Cynometra thicket (2,300 ha), with the canopy no more than 5 m high.

The Shimba Hills National Reserve is in Kwale County, km from Mombasa and 15 km from the coast. The reserve is an area of coastal rainforest, woodland and grassland.

The Shimba Hills were gazetted as National Forest in 1903, being one of the few large areas on the south coast that was still well forested. Grassland areas were incorporated in 1924, and several subsequent extensions took place to bring the reserve to its present size. The hills have a heterogenous mosaic of vegetation, including grassland, scrub and exotic plantations as well as forest. Estimated from aerial photographs that 44% (i.e. 9,500 ha) of the total area was forested, and a further 8,000 ha were forest/scrub formations. Grassland or grassland/scrub covered 3,400 ha, the remainder being plantations (600 ha) and other cover. The hills certainly hold one of the largest coastal forests in East Africa after Arabuko-sokoke, (Kahumbu 2000). There are at least six major forest types, including tall Milicia forest on the deep soils on the plateau top (in Longomwagandi and Makadara forests, and near Kwale town), and on the western escarpment Afzelia Erythophoeum forest, covering much of the eastern and southern escarpment; Paramacrolobium forest on particular steep scarp slopes to both east and west; and Manilkara-Combretum forest in the lower, western sector of the plateau. The biggest single patch of forest is the south-western sector, including Mkongani North and West. Further east and north, the forest breaks up into a complex mosaic interspersed with scrub and grassland. Very few forest patches are entirely isolated from each other, however, as corridors of forest or forest/scrub formations remain. At-least two Kayas, that have spiritual and ceremonial significance to the Digo people, Kwale and Longomwagandi, are situated within the National Reserve.

 Wildlife in the area

Arabuko-Sokoke is rich in rare wildlife, especially among the fauna. Six taxa of butterflies endemic to the East African coast are present, as well as three rare, near endemic mammals: Golden-rumped Elephant shrew (Rhynchocyon chrysopygus), Ader's Duiker (Cephalophus adersi) (found only in Sokoke and Zanzibar) and the distinctive Sokoke Bushy-tailed Mongoose (Bdeogale crassicauda omnivora). There is also a population of the African Elephant (Loxodonta Africana), and African Golden Cat (Felis aurata), rare in Kenya, may occur. Unusual reptiles include the Green Keel-bellied Lizard (Gastropholis prasina), and the forest is exceptionally rich in amphibian, including coastal endemics such as Bunty's Dwarf Toad (Mertensophryne micrannotis). At least 50 plant taxa from Arabuko-Sokoke are globally or nationally rare. Arabuko-Sokoke has been ranked by Bird Life International as the second most important forest for bird conservation in mainland Africa. Six globally threatened species, and five out of the seven species in the East African Coastal Forests Endemic Bird Area, occur. Clarke's Weaver bird is known only from Arabuko-Sokoke and the little-studied Dakatcha woodland, while the Sokoke Scops Owl is only known from this forest and one other site in north-east Tanzania. More than 230 bird species are recorded, including 25 of Kenya's 30 African East Coast biome species. The area holds endangered birds including Southern Banded Snake Eagle, Fischer's Turaco, Sokoke Scops Owl, Spotted Ground Thrush, the Sokoke pipit, East Coast Akalat, Amani Sunbird, Plain-backed Sunbird and Clarke's Weaver are vulnerable while Arye's Hawk Eagle, African Crowned Eagle, African Pitta and Scaly Babbler are regionally threatened.

The Shimba Hills National Reserve is rich in flora and fauna and hosts the highest density of African elephants (Loxodonta africana) in Kenya. Kenya's only population of Sable Antelope (Hippotragus niger) is found here, which was the major reason for incorporating grassland areas into the National Reserve. Other mammals found in the hills include. The restricted Black-and-rufous Elephant Shrew (Rhynchocyon petersi), a distinctive race of the Bushy-tailed Mongoose (Bdeogale crassicauda omnivora) andthe rare fruit bat, (Myonycteris relicta). Two frog species, Afrixalus sylvaticus and Hyperolius rubrovermiculatus, are endemic to the Shimba Hills forests while the little known and range-restricted Caecilian Boulengurula also occurs there. The butterfly fauna is very diverse, with some 295 species (35% of Kenya's total), including the rare Acraea aubyni, Neptis rogersi and the endemic Charaxes acuminatus shimbaensis.

Justification de la Valeur Universelle Exceptionnelle

Arabuko Sokoke forest provides a significant natural habitat for the conservation of rare and endangered species; these include the Golden-rumped Elephant shrew and Ader's Duiker where the only populations of these two species occur. This forest also serves as an important bird area in that it supports populations of the Clarke's Weaver bird (vulnerable) and the Sokoke Scops Owl (endangered) that are restricted to this forest.

The grasslands of Shimba hills are of utmost importance to Kenya as grasslands are rapidly declining in most parts of the country and hold Kenya's only population of the sable antelope, two endangered frog species (Afrixalus sylvaticus and Hyperolius rubrovermiculatus) and symbolize an outstanding representation of plant diversity (1,100 plant taxa recorded in the area), this area is therefore very important and significant for in-situ conservation.

Criterion (ix): Arabuko Sokoke is the largest extant fragment of the forests that once covered much of the East African coast. It was proclaimed a Crown Forest in 1932 and gazetted in 1943, covering an area of 39,100 ha. The Kararacha extension (2,700 ha) to the south-east, which includes important tracts of key habitats, was added in 1968. Part of the forest, containing sections of the three main habitat types, was gazetted as a strict nature reserve (covering 4,300 ha) in the late 1960s. Average annual rainfall ranges from 900 mm (in the relatively dry and scrubby north-west) to 1,100 mm (in the east). The relatively flat eastern section lies on Pleistocene lagoon sands and clays, separated by a wide band of apparently riverine sandy deposits from the ridge of red Magarini sands that forms the western part of the Reserve.

The Shimba Hills were gazetted as National Forest in 1903, being one of the few large areas on the south coast that was still well forested. Grassland areas were incorporated in 1924, and several subsequent extensions took place to bring the reserve to its present size. The hills have a heterogenous mosaic of vegetation, including grassland, scrub and exotic plantations as well as forest. Estimated from aerial photographs that 44% (i.e., 9,500 ha) of the total area was forested, and a further 8,000 ha were forest/scrub formations. Grassland or grassland/scrub covered 3,400 ha, the remainder being plantations (600 ha) and other cover. The hills certainly hold one of the largest coastal forests in East Africa after Arabuko-sokoke. There are at least six major forest types, including tall Milicia forest on the deep soils on the plateau top (in Longomwagandi and Makadara forests, and near Kwale town), and on the western escarpment Afzelia Erythophoeum forest, covering much of the eastern and southern escarpment; Paramacrolobium forest on particular steep scarp slopes to both east and west; and Manilkara-Combretum forest in the lower, western sector of the plateau.

Criterion (x): At least 50 plant taxa from Arabuko-Sokoke are globally or nationally rare. Arabuko-Sokoke has been ranked by Bird Life International as the second most important forest for bird conservation in mainland Africa. Six globally threatened species, and five out of the seven species in the East African Coastal Forests Endemic Bird Area, occur. Clarke's Weaver bird is known only from Arabuko-Sokoke and the little-studied Dakatcha woodland, while the Sokoke Scops Owl is only known from this forest and one other site in north-east Tanzania. More than 230 bird species are recorded, including 25 of Kenya's 30 African East Coast biome species. The area holds endangered birds including Southern Banded Snake Eagle, Fischer's Turaco, Sokoke Scops Owl, Spotted Ground Thrush, the Sokoke pipit, East Coast Akalat, Amani Sunbird, Plain-backed Sunbird and Clarke's Weaver are vulnerable while Arye's Hawk Eagle, African Crowned Eagle, African Pitta and Scaly Babbler are regionally threatened.

The flora of Shimba Hills is exceptionally rich and important. A total of 1,100 plant taxa are recorded, around 280 of which are endemic to Shimba Hills area and nearly a fifth considered rare globally or in Kenya. This qualifies Shimba as a centre of Plant Diversity, according to the criteria of the association pour l'Etude Taxonomique de la Flore d'Afrique. Notable tree species include Diospyros shimbaensis, Cephalosphaera usambarensis, Pavetta tarennoides, Synsepalum kassneri, Bauhinia mombasae, Phyllanthus sacleuxii, Vangueriopsis shimbaensis and undescribed species of Polyceratocarpus and Uvariodendron.

The forest is an important bird area and rich in forest bird fauna, including three threatened and two restricted-range species, and holds 18 of Kenya's 30 East Coast biome species. The grasslands hold localized species such as Red-necked Spurfowl, Croaking Cisticola and Zanzibar Red Bishop. The bird species of concern include Southern Banded Snake Eagle, Fischer's Turaco, Spotted ground thrush, Sokoke Pipit, East coast Akalat and Plain Backed Sunbird.

Déclarations d’authenticité et/ou d’intégrité

Arabuko-Sokoke Forest Reserve and National Park and Shimba Hills National Reserve are all protected by Kenya Laws. Shimba Hills was among the first gazetted National Forests in 1903 as one of the few large areas on the south coast that was still well forested. It was later extended in 1924 to incorporate the grassland areas. Arabuko-Sokoke was proclaimed a Crown Forest in 1932 and gazetted in 1943, covering an area of 39,100 ha. Subsequent extensions followed over the years to eventually cover Karacha extension (2,700 ha) to the south-east.

Arabuko-Sokoke is rich in biodiversity, but of particular importance is the exceptionally high degree of endemism. This, together with the forest's large area of continuous woody vegetation (most remaining coastal forests cover only a few hundreds of hectares, sometimes much less) gives it a very high conservation value. The forest is managed jointly by the Forest Service and the Kenya Wildlife Service under a Memorandum of Agreement, through the Arabuko-Sokoke Forest Management Team. The arrangement brings together these two institutions with the Kenya Forestry Research Institute and the National Museums of Kenya. Extensive licensed logging has occurred in the past, with noticeable negative effects on wildlife species and those in birds are documented (Fanshawe 1995). Licensed selective logging continues on a smaller scale, along with licensed collection of dead wood for fuel. Both these practices have proven difficult to police, and regular poaching of valuable trees continues to be a major problem. Brachylaena huillensis, which is preferred for the carving and construction industry, has been severely affected, as have timber species such as Pleurostylia africana. Illegal hunting, mainly of duiker and elephant shrews, is evident. Other species threatened by poaching are the dikdik and the giant forest Hog, although its impacts have not been determined. Local people use forest products for many purposes, including fuel wood and medicinal plants, and collect water at the seasonal pools. The forest is surrounded by agriculture on all sides.

The Shimba Hills forests have suffered drastic habitat modification over many years from commercial extraction of timber. Milicia excelsa has been a particular target, while Combretum schumannii and Afzelia quanzensis have been heavily exploited in the drier forest to the west. Commercial exploitation has now largely stopped. Regeneration of some of the logged-over forests seems to be prevented by the large, increasingly confined elephant population. The translocation of the elephants is expected to increase regeneration of the species affected. The continued early burning of the plateau grasslands is important to provide grazing for the Sable Antelope and other large herbivores. Burning may also help to check the advance of the exotic weed Lantana camara, which has invaded many of the clearings.

Comparaison avec d’autres biens similaires

Both these Kenyan coastal forests are similar to other forests in Tanzania that lie along the coast i.e., Usambara and Pugu hills, not only because they are listed as one of the biodiversity hotspots but because they are rich in terms of species diversity and endemism, are important for the conservation of birds, reptiles, plants and mammals.