Zambezian Baikiaea woodlands
The Zambezian Baikiaea woodlands lie along a zone where deep Kalahari sands occur in a broad belt along the Angolan-Namibian border across to Zimbabwe, supporting dry deciduous forest dominated by Rhodesian Teak (Baikiaea plurijuga). The hot, semi-arid climate and nutrient-poor soils render this region unsuitable for farming, and thus it has retained some of its natural vegetation. Over 160 mammal species are found here, including ungulates and large predators. However, human settlements occur along the Kunene, Kwando and Zambezi rivers, and the valuable Rhodesian Teak is sought after for the timber trade. The instability promoted in Angola by Russian financed Cuban troops, and hostilities between Angola and Namibia in the Caprivi Strip have contributed to the degradation of this ecoregion.
Location and general descriptionendemism and coincides largely with White’s Zambezian dry deciduous forest and scrub forest. This ecoregion forms a belt on deep Kalahari sands along the Angola-Namibia border, extending in a straight line to southwestern Zimbabwe. A portion of this ecoregion extends northwards, along the Zambia-Angola boundary. It is defined and shaped by a number of factors. The limits of the Kalahari sand delineate the east and west extent of this belt, while the southern boundary is limited by frost, and to the north, as rainfall increases the vegetation turns into evergreen Cryptosepalum forests and Miombo woodland. Around the Barotse floodplain, seasonal waterlogging or flooding suppresses tree growth, and Baikiaea woodlands give way to grasslands. While the distribution of the forest, woodland, savanna, and grassland elements is partly determined by edaphic and climatic factors, disturbance factors such as fire, logging, and agriculture play an increasing role in the spread of secondary savanna and grassland.
The area lies on an extensive plain of 800 metres (m) to 1000 m in elevation and is drained by the Okavango, Kwando, and Upper Zambezi Rivers and their numerous tributaries. The ecoregion overlies deep Kalahari sands of aeolian origin. Fossil dunes deposited in the Pleistocene and extensive dambos (shallow, seasonally inundated pans or vleis) that have formed in river valleys and dune troughs are a characteristic feature of the ecoregion. Soils are very deep and free-draining with virtually no clay or silt. They absorb all the incidental rainfall or receive lateral seepage water and remain moist throughout the year. Thus they are able to support forest and woodland vegetation despite the low rainfall in parts of the ecoregion. Because of their extremely low clay content, these soils only hold nutrients where there is organic matter. Exposure of the soil surface to the sun through clearing and burning of the vegetation destroys much of the organic matter, and such areas tend to remain bare.
The ecoregion experiences a hot, semi-arid climate. Mean annual rainfall ranges from less than 400 millimetres (mm) in the drier southwest to more than 600 mm in the eastern parts in Zimbabwe. Annual rainfall increases to about 800 mm in the northernmost parts of the ecoregion in Angola and Zambia, and is strongly concentrated from November to April. The mean maximum temperature is between 27° and 30°C and the mean minimum temperature ranges from about 9° to 12°C.
The Zambezian Baikiaea Woodland ecoregion represents a transition from moist southern savanna woodlands to dry southwestern deserts. Based on an analysis of the distribution of woody species occurring in Zambia, White recognized three centers of endemism within the Zambezian regional center of endemism: the Katangan, Zambezi, and Barotse. The Barotse region is comprised of Baikiaea vegetation and associated Loudetia spp. grasslands. It is defined by species confined to Kalahari sand, and the dominant tree species, Zambezian teak or mukusi (Baikiaea plurijuga), is endemic to it. The following species are also restricted to this center (and endemic or near-endemic to this ecoregion): Acacia fleckii, Alchornea occidentalis, Baphia massaiensis subsp. obovata, Brachystegia bakerana, Dialium engleranum, Erythroxylum zambesiacum, Grewia schintzii, Hannoa chlorantha, and Lonchocarpus nelsii.
There is considerable floristic variation in Baikiaea vegetation, particularly towards the edges of the ecoregion. Nearly all the species are deciduous, but there is considerable variation from species to species and from year to year. Baikiaea plurijuga is the dominant tree species characterizing the ecoregion, though logging and fires have removed it from many areas and the boundary with surrounding woodlands and savanna communities is often difficult to recognize. In well-developed Baikiaea communities, species of Brachystegia and Julbernardia and Colophospermum mopane (species typical of miombo and mopane woodlands) are totally absent. Baikiaea plurijuga is the sole dominant tree, forming a rather dense, dry, semi-deciduous forest with trees up to 20 metres in height. There is a dense and shrubby understory of Combretum engleri, Pteleopsis anisoptera, Pterocarpus antunesii, Guibourtea coleosperma, Dialium engleranum, Strychnos spp., Parinari curatellifolia, Ochna pulchra, Baphia massaiensis subsp. obovata, Diplorhynchus condylocarpon, Terminalia brachystemma, Burkea africana, Copaifera baumiana and Bauhinia petersiana serpae. Lianas and climbers are also common in the understory, including Combretum elaeagnoides, C. celastroides, Dalbergia martinii, Acacia ataxacantha, Friesodielsia obovata, and Strophanthus kombe. Smaller shrubs are scattered beneath the thicket. The herb layer is only conspicuous during the rainy season. Grasses vary from sparse to dense and include Leptochloa uniflora, Oplismenus hirtellus, Panicum heterostachyum, and Setaria homonyma. Other conspicuous herbs are Aneilema johnstonii and Kaempferia rosa. Epiphytes and mosses are virtually absent.
Forest patches dominated by Ricinodendron rautanenii are strongly associated with the Baikiaea plurijuga forests but found on more alluvially influenced soils towards the rivers, with trees up to 12 m tall. These patches are particularly frequent in the vicinities of the lower Cubango, Cuito, and Kwando Rivers and their numerous tributaries in Angola, southern Zambia, and the Caprivi Strip.
Among the interesting phenomenon in the ecoregion are dwarf forests of Baikiaea plurijuga: 1 to 1.5 m in height, which forests are found in the Sesheke District in western Zambia. These forests are situated on dambos and are surrounded by normally sized Baikiaea forest. The dwarf Baikiaea have a peculiar growth form in response to impeded water drainage in the lower soil strata, which results in reduced aeration of the soils.
The Baikiaea forests and woodlands are easily penetrated by fire, especially in the late dry season, and if there is a significant amount of grass and shrubby undergrowth. After frequent fires, a dense shrub layer develops, which is dominated either by shrubs and climbers or by grasses and herbs. When fire damage is severe or after cultivation, Baikiaea plurijuga can disappear completely, as it is rather sensitive to fire and does not regenerate easily in frequently burned sites.
This ecoregion is an area of moderate species richness for most taxonomic groups. While floristically it falls within its own center of endemism (as discussed above), the fauna of the area has low levels of endemism as it largely represents a merging of elements from the southern savannas, the arid southwest and the miombo woodlands.
More than 160 mammal species occur in the ecoregion. These include several large predator species such as lion (Panthera leo), leopard (P. pardus), Painted hunting dog (Lycaon pictus), cheetah (Acinonyx jubatus), and spotted hyena (Crocuta crocuta). Ungulates include zebra (Equus burchelli), roan antelope (Hippotragus equinus), sable antelope (H. niger), bushbuck (Tragelaphus scriptus), kudu (T. strepsiceros), sitatunga (T. spekei), reedbuck (Redunca arundinum), impala (Aepyceros melampus subsp. melampus), common duiker (Sylvicapra grimmia), oribi (Ourebia ourebi), steenbok (Raphicerus campestris), eland (Taurotragus oryx), blue wildebeest (Connochaetes taurinus), buffalo (Syncerus caffer), hartebeest (Alcelaphus buselaphus), giraffe (Giraffa camelopardalis), tsessebe (Damaliscus lunatus), waterbuck (Kobus ellipsiprymnus), puku (K. vardoni), and lechwe (K. lechwe). Other mammals of interest are elephant (Loxodonta africana), black rhinoceros (Diceros bicornis), white rhinoceros (Ceratotherium simum) (both now rare in the ecoregion), hippopotamus (Hippopotamus amphibius), and honey badger (Mellivora capensis).
With more than 400 recorded bird species, this ecoregion’s avifauna is characterized by moderately high species richness but low endemism. According to Winterbottom’s zoogeographical subdivision of southern Africa, the Baikiaea woodland mosaic makes up most of the southeastern parts of a transition zone from the Central Highlands District to the Southwest Arid Kalahari Woodlands District. The ecoregion’s avifauna is largely derived from the districts lying to the north and south. A variety of habitats exists in the ecoregion and directly adjacent to it, which boosts bird diversity. The many rivers, dambos, and wetlands form an important network of aquatic habitats for resident and migrating birds.
Baikiaea woodlands are the preferred habitat of Bradfield’s hornbill (Tockus bradfieldi), which is near-endemic to the ecoregion and fairly common. Southern ground hornbill (Bucorvus leadbeateri) also occurs here. The ecoregion provides essential habitat for the rare and vulnerable black-cheeked lovebird (Agapornis nigrigenis), which is confined to medium-altitude mopane woodland in South Zambia and extreme northern Zimbabwe. A. nigrigenis occurs only where the mopane woodland is contiguous with Baikiaea dominated woodland. This lovebird spends the dry season in the mopane woodland (though not evergreen, Colophospermum mopane stays green far into the dry season) and feed on the young leaves of Pterocarpus antunesiana in the Baikiaea woodlands in the rains. A. nigrigenis populations are highly localized within available habitat. This species was massively exploited in the 1920s due to its popularity with the pet trade, and it seems that its populations have never fully recovered. Although it is officially protected, it is still subject to illegal trapping.
The ecoregion has a rich variety of raptor species including Secretary bird (Sagittarius serpentarius), White-backed vulture (Gyps africanus), lappet-faced vulture (Torgos tracheliotus), white-headed vulture (Trigonoceps occipitalis), hooded vulture (Necrosyrtes monachus), lesser kestrel (Falco naumanni), Dickinson’s kestrel (F. dickinsoni), African hobby falcon (F. cuvierii), bateleur (Terathopius ecaudatus), tawny eagle (Aquila rapax), martial eagle (Polemaetus bellicosus), and African hawk eagle (Hieraaetus spilogaster). Riparian vegetation supports Pel’s fishing owl (Scotopelia peli).
In the wetlands and riverine areas within the ecoregion, a great number of water-birds are found. Two rare and threatened species are noteworthy: wattled crane (Bugeranus carunculatus) and slaty egret (Egretta vinaceigula, VU) which have a wide distribution but are confined to floodplains and are sensitive to disturbance.
The ecoregion is home to 87 recorded reptile species, including seven species of amphisbaenids or worm-lizards of the genera Zygaspis, Monopeltis, and Dalophia, two of which are near-endemic.
One amphibian is also near-endemic to the ecoregion, the Khwai River toad (Bufo kavangensis).
On the whole, this ecoregion is rather sparsely settled by man, with fewer than five people per km2 in most areas. In the least populated areas, population densities are probably less than one person per km2. As a result of the low human population density and the arid nature of the environment, much of the habitat has not been modified or fragmented. However, especially in Zambia, Angola, and Zimbabwe, timber harvesting together with frequent wildfires from slash and burn practises have significantly reduced the area of mature Baikiaea woodland and forest.
Close to eight percent of the ecoregion is covered by ten protected areas falling into all five countries where Baikiaea woodland is found. There are three protected areas in Angola, Bikuar and Mupa National Parks, and Luiana Partial Reserve, four in Namibia, Mudumu Nature Reseve, Mahango and Caprivi Game Reserve, and Popa game park, and Kazuma Pan and Hwange national parks in Zimbabwe plus Simoa Ngwezi National Park in Zambia. In addition to these parks where Baikiaea woodland is the dominant vegetation type, Chobe National Park in Botswana and Khaudom National Park in Namibia have some areas of Baikiaea woodland in their northeasternmost parts. Most of the West Zambezi Game Management Area (GMA), which extends over 38,070 km2 in the southwestern corner of Zambia, falls into the ecoregion. Although the aim is to manage the area for game and enforce strict control of hunting via a licensing system, there is little game left, and protection of intact habitat or biodiversity is not really an objective of the GMA. Mudumu, Mahango, and Hwange are among areas in the southern African subregion (this excludes Angola and Zambia) listed as Globally Important Bird Areas.
The level of management and effective protection varies greatly among parks in the region. In Angola, more than 25 years of almost continuous civil war (exacerbated by arrival of Cuban troops backed by Russia} devastated wildlife resources due to the breakdown of protected area management as park wardens left their posts for economic or security reasons. The former protected areas became open spaces for poachers, commercial timber felling, human settlement, and cultivation. On the other hand, Hwange National Park, the largest in the ecoregion, is very well researched and managed and also has good tourist infrastructure. The Ministry of Environment and Tourism in Namibia has initiated a comprehensive planning program for the Caprivi Game Reserve, Mangetti Game Camp, Khaudom, Mudumu, Mamili, and Mahango under the Northeast Parks Project.
Among the protected areas of the ecoregion, fauna and flora is well represented, with some of the largest and most stable populations of large mammal species in the region. These include huge herds of elephant and buffalo, as well as endangered predators such as cheetah, leopard, and wild dog. Black rhinoceros and white rhinoceros have been reintroduced to Hwange, and their numbers are slowly increasing in the intensive protection zone within the park.
Types and severity of threats
Poaching is a serious and widespread problem in this ecoregion, even within protected areas. Resources for anti-poaching operations are often limited. In Angola and the Caprivi Strip, the long civil war and military operations along the Namibia-Angola border have significantly worsened the poaching situation. Military firearms from these operations are acquired in Zambia, where they present a real threat to poaching-control efforts. Commercial poaching by outsiders is a big problem in Sioma Ngwezi and the surrounding GMA, with very little game remaining in the latter. Cross-border smuggling of wildlife products in this remote area, where security levels are low, is also a major concern for wildlife management and protection.
Annual migration routes of animals in protected areas are often blocked by park borders, international boundaries, and human settlements (particularly along rivers). None of the parks in the ecoregion cover the entire migratory ranges of animals such as wildebeests and elephants, and protection in surrounding areas including Zambia’s West Zambezi Game Management Area is insufficient or non-existent. Several protected areas do not extend to rivers, where animals migrate in search of drinking water. Game from the Sioma Ngwezi National Park migrates eastward to the Zambezi, which does not fall within the national park, although the West Zambezi Game Management Area extends as far as the river. Similarly, game from the Caprivi and Hwange Parks migrate to the Cuando and Gwai Rivers respectively. Along these rivers are settlements where crop damage, livestock attacks by predators, and concern about game as hosts for tsetse fly cause conflict between conservation efforts and farmers in the area surrounding game reserves. Illegal hunting is difficult to control in these areas, and commercial poachers from other areas are thought to use settlements near rivers as a base for their operations in the dry season. Cattle fences (e.g. those erected in Botswana between the Caprivi Strip and the Okavango Delta in 1995 to control the spread of cattle lung disease) can cause increased rates of mortality when animals are cut off from grazing and water resources.
Timber logging is a threat to the Baikiaea woodland and forest habitats, as well as to Baikiaea plurijuga as a species. Annual production of mukusi timber peaked at 100,000 cubic meters (m3) in the 1930s and again in 1964 with the construction of railway lines. Since the mid-1970s, logging has declined to around 20,000 m3 per year, largely due to a decline of harvestable timber. In Zambia, a recent inventory found no more exploitable reserves in the prime teak forest areas of Sesheke District, and the same applies to Zimbabwe outside protected areas. Such exploitation has destroyed large forest areas, with little hope of recovery because opening the forest results in the invasion of grasses and fires. However, pressure on timber resources in the ecoregion is increasing with rising South African and Namibian timber demands. Recently, the conservation status of Baikiaea plurijuga in Zambia was revised and changed from category LR-nt (lower risk, near threatened) to VU (vulnerable, with population reductions of at least 20 percent over the last three generations) as a result of exploitation and habitat destruction. The status of Baikiaea in the other countries – especially in Angola and Namibia, where the most extensive stands occur – is not clear at this time.
Clearing for agriculture also affects the Baikiaea woodlands and forests, though this is limited in large parts of the ecoregion by the low levels of rainfall. The nutrient-poor sandy soils necessitate shifting cultivation. Uncontrolled bushfires are common and frequent in the ecoregion, and this makes the forests and woodlands particularly vulnerable to logging and clearing, as regeneration of forest vegetation, and B. plurijuga in particular, is hindered.
Presently, tourism development in some areas is unregulated, and much of it is in the form of fishing and safari hunting. In Zambia, both types of development have been observed without adherence to restrictions and licences. The Sioma Ngwezi National Park has no distinct boundary, no official entry point and no tourism infrastructure, and hence does not generate any revenue with which to support its conservation activities. There seems to be a general trend of selling licenses for tourism development and timber logging to people, including foreigners, living outside the region where conservation is taking place. Among local communities, this is causing resentment and a lack of cooperation, which is prerequisite for effective wildlife and natural resource management in areas outside reserves. There has been a recent trend toward community-based wildlife conservation and management programs in Zimbabwe, Zambia, and Namibia.
Tsetse flies occur in the Baikiaea belt in Zambia and the Caprivi Strip. Spraying against them has been known to affect bird life and other fauna. Barnes noted that pesticides used annually in the spraying against mosquito and tsetse in the Caprivi Strip (DDT and dieldrin) were polluting the Kavango River, mainly due to the practice of rinsing equipment and disposing of leftover chemicals. However, according to Simasiku et al., tsetse control in the West Zambezi GMA has more recently been carried out using traps.
Justification of ecoregion delineation
Lying within the Barotse center of the Zambezian Regional Centre of Endemism, the Zambesian Baikiaea Woodlands ecoregion follows White’s ‘Zambezian dry deciduous forest and secondary grassland.’ It is dominated by Baikiaea vegetation associated with deep Kalahari sands. The southern boundary is limited by frost and drier desert, while the northern boundary borders Cryptosepalum forests and miombo woodland. The ecoregion is primarily defined as the center of endemism for Baikiaea vegetation, dominated by the endemic Zambezian teak or mukusi.
This ecoregion is 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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