Location and General Description
This small but distinctive ecoregion consists almost entirely of dense evergreen forest dominated by Cryptosepalum exfoliatum pseudotaxus, known locally as "mavunda." It falls into White’s (1983) Zambezian regional center of endemism and is mapped as ‘Zambezian dry evergreen forest.’ The two main blocks of Cryptosepalum forest are found to the north and south of the Kabompo River. Together they constitute the largest area of tropical evergreen forest in Africa outside the equatorial zone (Bingham 1995).
The ecoregion is found at 1,100 to 1,200 m elevation in the higher rainfall areas of the Kalahari sands of northern Barotseland in western Zambia, and marginally extends into Angola. The Kabompo River and its tributaries, which form part of the drainage basin of the Upper Zambezi River, drain the ecoregion. In the west, this ecoregion gives way to the Loudetia simplex-dominated grassland where seasonal waterlogging suppresses tree growth. The Barotse floodplain (in the Zambezian Flooded Grassland ecoregion) lies to the southwest. To the north and east the ecoregion gives way to miombo woodlands on better soils. Deep Tertiary Kalahari sands of aeolian origin underlie the ecoregion and determine the extent of Cryptosepalum forest. These sands can reach more than 60 m in depth and have relatively high water tables, although most of the dambos (shallow pans or vleis) are permanently dry, and there is an almost complete lack of permanent surface water (Oatley 1969; van Gils 1988). The lack of water combined with soils that are low in nutrients results in a very low human population density. Disturbance by natural fire is of little importance, but shifting cultivation can change Cryptosepalum forest to a more open and fire-prone vegetation type known as "chipya."
The ecoregion has a tropical savanna climate with mean annual temperatures between 20° and 22° C. The annual temperature range averages around 8° C. Mean maximum temperatures are 28° to 30° C, and minimum temperatures are around 7° to 8° C (Schulze and McGee 1978). Mean annual precipitation ranges from 800 to 1,200 mm. Three seasons can be distinguished: a hot, dry season from August to October; a hot, wet season from November to April; and a cool, dry season from May to July.
Cryptosepalum forest represents part of a floristic and physiognomic transition from Guineo-Congolian rain forest to Zambezian woodlands, and many of the plant species found are Guineo-Congolian or Afromontane linking species. Most are readily killed by fire and hence are normally absent from woodland. Except for a few emergents, the forest vegetation rarely exceeds 25 m in height, and it is relatively poor floristically, with a simpler structure than rain forest (White 1983). Cryptosepalum exfoliatum pseudotaxus ("mukwe") is the dominant, and sometimes only, canopy species. Guibourtia coleosperma is often co-dominant and is an important exploitable timber (van Gils 1988). Towards central Barotseland, Brachystegia spiciformis, B. longifolia and Colophospermum mopane become more frequent (Wild and Barbosa 1967). Other tall canopy trees include Brachystegia floribunda, Syzygium guineense afromontanum, Bersama abyssinica, Erythrophleum africanum and Combretum elaeagnoides (Werger and Coetzee 1978). The understory is often dense and tangled, and dominant liana species include Combretum microphyllum, Uvaria angolensis, Artabotrys monteiroae, and Landolphia. Diospyros undabunda is conspicuous where there is a dense thicket understory and epiphytic lichens are common. Little grass is found on the forest floor, which is predominantly covered in mosses in the denser forests. After clearing the forest, nearly impenetrable regeneration stages with numerous lianas, shrubs and small trees can develop (Werger and Coetzee 1978). The sparse herbaceous undergrowth makes it difficult for fires to penetrate, and fire is not an important disturbance factor in mature forest stands.
Although the flora and fauna of the Cryptosepalum dry evergreen forests is distinct from surrounding ecoregions, there is only moderate species richness for most taxonomic groups and low levels of endemism. The area is, however, relatively poorly known biologically, and species values are likely to be underestimated.
Little recent information is available on the mammal fauna of Cryptosepalum forests. Ansell (1960) states that none of the known mammals are endemic to the Cryptosepalum habitat, which shares most of its fauna with the surrounding Brachystegia woodland or the Guineo-Congolian forest. The only species near-endemic to the ecoregion is Lemniscomys roseveari, a species of striped African grass mouse known from two localities in southwestern Zambia-Balovale and Solwezi (van der Straeten 1980). The former is in this ecoregion and the latter falls within the Central Zambezian Miombo Woodland ecoregion.
The large mammal fauna includes a variety of predators and ungulates representing mostly widespread elements of the southern savanna mammal fauna, particularly those with a preference for densely wooded habitat. The Cryptosepalum forests provide prime habitat for yellow-backed duiker (Cephalophus sylvicultor) and blue duiker (C. monticola). Bushpig (Potamochoerus porcus) is also common. Darling (1960) also reported kudu (Tragelaphus strepsiceros) and wild dog (Lycaon pictus) from Cryptosepalum forest, and noted that wild animals in this relatively sheltered habitat were much less shy than in surrounding areas. The Cryptosepalum forest is used as a safe retreat by elephant (Loxodonta africana) and buffalo (Syncerus caffer) from the Kabompo-Mwinilunga areas.
Cryptosepalum forests have a distinct and relatively rich avifauna with 381 known species, comprising a mixture of moist evergreen forest species, elements of the Brachystegia woodland avifauna, and some widespread species. The highest levels of species richness are found where local habitat disturbance (such as shifting cultivation) has resulted in a mosaic of tree savanna, thicket, savanna woodland and forest (Oatley 1969).
Benson and Irwin (1965) regarded 15 bird species as being representative of dry evergreen forest (bird names have been updated according to Sibley and Monroe, 1993): red-throated crested guineafowl (Guttera edouardi kathleenae), barred long-tailed cuckoo (Cercococcyx olivinus), Ross’s turaco (Musophaga rossae), golden-rumped tinkerbird (Pogoniulus bilineatus), white-chested tinkerbird (P. makawai), Banis’s greenbul (Phyllastrephus cabanisi), purple-throated cuckoo-shrike (Camephaga quiscalina), Boulton’s batis (Batis margeritae), African crested flycatcher (Trochocercus cyanomelas), square-tailed drongo (Dicrurus ludwigii), blackfronted bushshrike (Telophorus nigrifrons), Perrin’s bushshrike (T. viridis), olive sunbird (Nectarinia olivacea), forest weaver (Ploceus bicolor) and black-tailed waxbill (Estrilda perreini). Many of these species are found in similar dry evergreen forest fragments elsewhere in Zambia and Angola (Winterbottom 1978).
Only the white-chested tinkerbird (Pogoniulus makawai) is endemic to the ecoregion. It is, in fact, known only from its type specimen found in 1964. Subsequent attempts to relocate it have been unsuccessful, even though the habitat is still intact and there are few obvious threats (Collar and Stuart 1985). It is a close relative of the common and widespread golden rumped tinkerbird (Pogoniulus bilineatus). The two species are found together in the Cryptosepalum forest, and Benson and Irwin (1965) considered P. makawai to be a rare relictual form which speciated from P. bilineatus during a dry period 12,000 years ago. Other taxonomists, however, believe P. makawai to be an aberrant individual of P. bilineatus, and its status as a species is still debated.
Benson and Irvin (1965) as well as Oatley (1969) draw attention to several other species of birds that coexist with closely related species in Cryptosepalum forests. These species generally have different, though often slightly overlapping, ecological niches and some have been observed together in the same bird parties. Examples are golden rumped tinkerbird (Pogoniulus bilineatus), yellow-fronted tinkerbird (P. chrysocomus) and white-chested tinkerbird (P. makawai); chinspot batis (Batis molitor) and Margaret's batis (B. margaritae); and goldentailed woodpecker (Campethera abingoni) and little spotted woodpecker (C. caillantii). This coexistence, along with the geographical distribution of certain subspecies populations (e.g. of Margaret's batis), suggests that speciation occurred in Cryptosepalum forest fragments when bird populations were isolated there during drier climate cycles.
The ecoregion supports 34 species of reptiles and 14 species of amphibians, with no endemics. Its reptile and amphibian fauna falls into a broad transition zone between the tropical fauna that has its centre in the Mozambique Plain, and the Cape fauna of southwestern South Africa. Relatively few reptiles inhabit evergreen forest, and those that do are most likely to be encountered at the forest edge in clearings where the sun penetrates (Poynton and Broadley 1978).
Although the ecoregion is relatively small and fragmented due to edaphic and climatic determinants, much of the habitat is still in a natural, undisturbed state. This is largely due to the almost complete absence of any surface water in the Cryptosepalum forests, preventing human settlement. Although the southern parts of the forests are utilized for fuelwood and timber by the inhabitants of the nearly-treeless Barotse floodplain and surrounding grasslands, it is generally believed that the impacts of this have not been severe and are limited to a few localized parts of the forest margins (Muleta et al. 1996, Collar and Stuart 1985). Cryptosepalum exfoliatum is not an economically important timber species, and the arable potential of the ecoregion is low. The soils are nutrient poor, necessitating shifting agriculture, and the native vegetation is difficult to clear. These factors, combined with its remoteness, inaccessibility, and low human population density (fewer than 5 people per km2), have prevented the destruction of most parts of this ecoregion.
Only one protected area, the West Lunga National Park, falls within this ecoregion. It is surrounded in the east, north and west by game management areas, which provide some protection to game in the form of hunting restrictions. Details of the park are found in WCMC’s Protected Areas Database (http:\\www.wcmc.org.uk). The large Lavushi Manda National Park falls within the Central Zambezian Miombo Woodland ecoregion, but includes some Cryptosepalum forest patches along rivers within miombo Brachystegia-Julbernardia woodland vegetation.
Types and Severity of Threats
This ecoregion does not appear to be seriously threatened, although a lack of knowledge prevents a detailed assessment. For reasons discussed above, habitat fragmentation and destruction have not yet occurred on a large scale, and are unlikely to do so in the short- to medium-term given the sparse (if growing) human population, lack of water, and poor agricultural potential.
Poaching is a general threat to wildlife in southwestern Zambia, even within protected areas. Lack of management, infrastructure and funds in protected areas make poaching very difficult to control, and although no data is available, West Lunga is unlikely to be an exception. However, due to their remoteness and impenetrability, the Cryptosepalum forests are probably less heavily poached than other, more open reserves in more populated areas.
Justification of Ecoregion Delineation
The linework for this ecoregion follows the ‘Zambezian dry evergreen forest’ vegetation unit of White (1983). Even though small in geographic extent, it is the largest area of tropical evergreen forest in Africa outside the equatorial zone (Bingham 1995), with distinct flora and fauna from surrounding ecoregions. Kalahari sands determine the extent of Cryptosepalum forest. However, further west the vegetation changes to the Western Zambezian Grasslands, as seasonal watterlogging suppresses tree growth. The southwest boundaries are bordered by the Barotse floodplain, while miombo woodlands lie to the north and east.
This ecoregion forms part of larger complex of Caesalpinoid woodland ecoregions that support wet and dry miombo, mopane, thicket, dry forests, Baikiaea woodland, and flooded grassland habitats, among others. The dominance of Caesalpinoid trees is a defining feature of this bioregion (i.e., a complex of biogeographically related ecoregions). Major habitat types (e.g., mopane and miombo) and the geographic separation of populations of large mammals are used to discriminate ecoregions within this larger region. All of these ecoregions contain habitats that differ from their assigned biome or defining habitat type. For example, patches of dry forest occur within larger landscapes of miombo woodlands in several areas. More detailed biogeographic analyses should map the less dominant habitat types that occur within the larger ecoregions.
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Prepared by: Suzanne Vetter
Reviewed by: In progress