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The Eastern Arc Coastal Forests (Arabuko-Sokoke Forest and Shimba Hills National Reserve)

Date of Submission: 12/02/2010
Criteria: (x)
Category: Natural
Submitted by:
Kenya Wildlife Service
State, Province or Region:
Coast Province
Ref.: 5507
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Description

Coordinates:

Arabuko - Sokoke: S3 20 E39 55

Shimba Hills: S4 15 E39 25

Arabuko-Sokoke lies a few kilometres inland on the Kenyan coast, between the towns of Kilifi and Malindi and some 110 km North of Mombasa. The Shimba Hills are a dissected plateau that ascends steeply from the coastal plains, some 30 km south-west of Mombasa and just south of Kwale town.

Both Arabuko-Sokoke Forest Reserve and National Park and Shimba hills National Reserve combined hold the largest coastal forests in East Africa, both sites are important for conservation of birds and are listed as one of the twenty-five biodiversity hotspots in the world. The sites should therefore be listed for serial nomination with other coastal forests along the Eastern Arc Coastal forests.

The Eastern Arc Mountains and Coastal Forests hotspot runs along the Tanzanian and Kenyan coasts from the border with Somalia to the north to that with Mozambique to the south. By the early 1990s, there were about 175 forest patches in the Coastal Forest Mosaic (Kenya 95, Tanzania 66) covering an area of 1,360 km2 (Kenya 660 km2, Tanzania 700 km2) (Burgess et al.2000). The two largest coastal forests are both in Kenya (Arabuko-Sokoke, minimum area 370 km2; Shimba, minimum area 63 km2) (WWF-EARPO 2002), while in Tanzania there are no coastal forests larger than 40 km2 (WWFUS 2003b).

It is the largest extant fragment of the forests that once covered much of the East African coast, and whose remnants constitute the East African Coastal Forests. Arabuko-Sokoke 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 (Robertson & Luke 1993). Three very distinctive forest types, each with its-own special flora and fauna, correspond to these soil types.

The forest has three main forest types: - these include the mixed forest (7,000 ha) in the east, on grey sands. This habitat 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. 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 (formerly) Brachylaena huillensis. (This last tree, much in demand for the carving trade, has been almost logged out from much of the forest.) 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. Between these 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 surrounding escarpment rises from around 120 m elevation to c. 300 m across the bulk of the plateau, and as high as 450 m at Marare and Pengo Hills. The underlying rocks are the Triassic Shimba Grits and (in the north-central part near Kwale town) Pliocene Magarini Sands (Schmidt 1991, Blackett 1994). Rainfall ranges from 900-1,200 mm per year, and rivers flowing from the hills supply fresh water to Mombasa and to the Diani/Ukunda area, immediately to the east.

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, Kwale and Longomwagandi, are situated within the National Reserve (the Kaya forests have spiritual and ceremonial significance to the Mijikenda people of the Kenya coast). A fenced elephant corridor connects the Shimba hills with Mwaluganji Forest Reserve to the north.

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 (Virani 1993). 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) (Drewes 1997). Robertson & Luke (1993) list 50 plant taxa from Arabuko-Sokoke that 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 (Collar & Stuart 1988). 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 (Fanshawe 1995), including 25 of Kenya's 30 African East Coast biome species (24 regularly). In terms of birdlife the area has the Southern Banded Snake Eagle, Fischer's Tobacco, which are near threatened are resident in the forest, Sokoke Scops Owl and Spotted Ground Thrush are Endangered, 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, Scaly Babbler and are regionally threatened.

Shimba Hills is rich in flora and fauna and hosts the highest density of African Elephant (Loxodonta africana) in Kenya. Kenya's only population of Sable Antelope (Hippotragus niger), which was the major reason for incorporating grassland areas into the National Reserve. The restricted Black-and-rufous Elephant Shrew (Rhynchocyon petersi) also occurs here together with a distinctive race of the Bushy-tailed Mongoose (Bdeogale crassicauda omnivora). The forest also holds substantial numbers of the small unique mammals. The rare fruit bat, (Myonycteris relicta) has been collected here (Bergmans 1980). Two frog species, Afrixalus sylvaticus and Hyperolius rubrovermiculatus, are endemic to the Shimba Hills forests and are believed to be endangered. The little known and range-restricted Caecilian Boulengurula also occurs (Duff-MacKay 1980, W.R. Branch 2004). The butterfly fauna is very diverse, with some 295 species (35% of Kenya's total), including the rare Acraea aubyni and Neptis rogersi, endemic Charaxes acuminatus shimbaensis.

The flora of the hills is exceptionally rich and important. A total of 1,100 plant taxa are recorded (Luke, 2000), 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 (Luke 2000). Notable tree species include Diospyros shimbaensis, Cephalosphaera usambarensis, Pavetta tarennoides, Synsepalum kassneri, Bauhinia mombasae, Phyllanthus sacleuxii and undescribed species of Polyceratocarpus and Uvariodendron (Robertson & Luke 1993).

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, Fishers Turacco, Spotted ground thrush, Sokoke Pipit, East coast Akalat and Plain Backed Sunbird.

Justification of Outstanding Universal Value

(x): The grasslands of Shimba hills are of outmost 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. While, 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.

Statements of authenticity and/or integrity

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 in cooperate 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 Kararacha extension (2,700 ha) to the south-east.

Conservation issues

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 (ASFMT). 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 bird 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 (FitzGibbon et al. 1995, Kanga 1996); 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 (Fanshawe 1992). 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.

Comparison with other similar properties

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 are rich in terms of species diversity and endemism, are important for the conservation of birds, reptiles, plants and mammals. As Shimba hills hold the only Sable antelope, they are comparable to Ruma National Park in Kenya that holds Kenya's only population of the Roan antelope.