Location and General Description
This northern latitude forest-savanna mosaic occupies the eastern half of White’s (1983) Guineo-Congolian/Sudanian phytogeographical regional transition zone. Forming the northern border of the Congo watershed, it begins east of the Cameroon highlands and extends east through the Central African Republic, northeastern Democratic Republic of Congo and into southwestern Sudan and a sliver of north-western Uganda. The Ubangi and Uele Rivers demarcate the central and eastern borders with the northeastern Congo rain forest, while the Bar al Ghazall, part of the Upper Nile drainage, delineates the transition to the Saharan flooded grasslands to the east.
This ecoregion lies in the tropical savanna climate zone (Faniran and Jeje 1983). White (1983) describes the entire region as climatically transitional between the Sudanian and Guineo-Congolian regions. Most of the region is characterized by a single wet season and a single dry season. Forested areas exhibit high dry season relative humidity. Mean annual precipitation ranges locally from about 1,200 mm to 1,600 m per year. Latitude plays a major role in the general decline in average precipitation as the ecoregion grades into the Sudan/Sahel savanna. This ecoregion experiences small seasonal temperature fluctuations, with rainy season mean daily maximum temperatures of 31° to 34°C and dry season mean daily minimum temperatures of 13° to 18°C (White 1983).
Most of the ecoregion sits on a dissected plateau that averages about 500 m in elevation, and rises to 700 m or more towards the montane rain forests of the Cameroon Highlands. Successive tecto-metamorphic events of Late Precambrian age are largely responsible for the granite-greenstone belts that form the bedrock beneath much of this ecoregion. Outcrops of these rocks rise abruptly from the plains as inselbergs. Later uplifts and corresponding sagging during the early Tertiary created the northern equatorial plateau, the extensive dissected plains that characterize much of the current topography and which forms the northern boundary of the Congo River Basin (Cahen 1984, Faniran and Jeje 1983).
Central Africa’s active seismic history exposed many rock types that provided parent material for diverse soil catenas. Oxisols, the extremely weathered soils of low fertility that dominate the humid tropics, underlie the central and eastern portions of the ecoregion. The pervasiveness of oxisols provides some evidence of central Africa’s fluctuating climatic past; they develop in hot and wet conditions typical of the rain forest interior but now underlie both humid forest and savanna (Faniran and Jeje 1983, Kingdon 1989). Western CAR is underlain by relatively new and unweathered entisols, and central Cameroon, by a mixture of oxisols and ultisols, highly weathered soils that often contain a fragipan (Brady and Weil 1999).
The plethora of soil series that have developed partially explain the abrupt shift from rain forest to open grassland (Cole 1992). The transition from the equatorial forest to northern latitude savannas was most probably gradual throughout the early Pleistocene. Two main factors sharpened the transition zone. Initially, about 50,000 years ago, fires became a frequent disturbance. Secondly, for the last 3,000 years humans have burned clearings for farms and livestock, further reducing tree densities and creating wooded grasslands (Hopkins 1992). These historic land use patterns reflect plant-soil interaction. Nutrient-poor oxisols traditionally have been left under forest or slash-and-burn agriculture (Brady and Weil 1999).
The mosaic nature of this ecoregion is also the result not only of underlying soil conditions, but also of dramatic climatic fluctuations that have affected Africa for millions of years, including significant changes over the last five thousand years. In periods of glaciation in higher latitudes, the climate of Africa has become cooler and drier and forests have retracted; conversely during interglacials, it has become warmer and wetter and forests have expanded. Over the past two thousand years, it has been a relatively warm and wet phase during which, were it not for human activities, the forests would have expanded at the expense of the savannas (Maley 1994). However, there are some indications of a drying of climate over the past three decades.
Forest, woodland and secondary grassland intergrade in patterns controlled by annual precipitation, duration of water stress and the severity of dry-season fires and human activity (Cole 1992, Longman and Jenik 1992). Vegetation common either to the Sudanian or Congolian provinces characterizes much of the region. Lowland rain forests of the drier, semi-evergreen type typical of the periphery of the Congo basin, intergrade with secondary grassland and wooded grassland, that in turn form the basis of the higher latitude Sudanian province (Millington et al. 1992, White 1983). Gallery forests along rivers provide corridors for forest species far into the northern grasslands. Furthermore, monitoring how the interfacing ecosystems advance and retreat can offer insight into the nature and rate of environmental change over time, and into the causes of this change (Furley 1992). Such changes between domination by forest or savanna woodland habitats are believed to have occurred many times over the past few million years (Kingdon 1989, Maley 1994).
Gallery forests are the dominant forest type in the northern Congolian forest-savanna mosaic, growing wherever there are watercourses, sufficient rain, available groundwater and low fire frequency (Mayaux et al. 1999). Widespread gallery species include Berlinia grandiflora, Cola laurifolia, Cynometra vogelii, Diospyros elliotii, Parinari congensis, and Pterocarpus santalinoides. Along the border between the Guineo-Congolian zone and this ecoregion, Gilbertiodendron dewevrei, a common rain forest evergreen forms extensive, mono-dominant stands in well-drained red-clay soils with high soil water capacity in the Ubangi and Ule regions (White 1983).
The remnants of lowland rain forest are floristically indistinguishable from the drier type of peripheral semi-evergreen rain forest (White 1983). The canopy trees in these stands do not stand bare for lengthy period of time. Up to 70 percent of the canopy species may loose their leaves for very short periods, often regenerating leaves on some branches while loosing foliage on others. Species restricted to these drier forests and widespread across this ecoregion include Afzelia africana, Aningeria altissima, Chrysophyllum perpulchrum, Cola gigantea, Morus mesozygia, and Khaya grandifoliola, a shade intolerant tree that regenerates only in natural forest gaps. The lowland rain forest complex also includes some species more typical of the moister rain forests of the Congo Basin such as Piptadeniastrum africanum and Sterculia (Eriobroma) oblonga. Several of the tree species, including Canarium schweinfurthii, Ricinodendron heudelotii, and Terminalia superba, (a valuable timber species that regenerates on abandoned agricultural land), are characteristic of secondary rain forest (White 1983).
The forests and woodlands give way to expansive, relatively moist wooded grasslands that characterize this ecoregion (Mayaux et al. 1999, Justice 1997). Most of the tall grass savannas are fire-climax communities that generally grow on well-drained soils. Woody complexes regenerate on these grasslands when burning is halted and seed trees are available (White 1983). Edaphic grasslands also occur where soils are either too dry or too waterlogged to support trees. Common graminids, many growing taller than two meters, include Andropogon spp., Hyparrhenia spp. and Loudetia spp. Fire-adapted woodland trees grow in varying densities, depending on fire frequency and site history. Trees restricted to relatively arid northern wooded grasslands, most of which are Sudanese near-endemics, include Afzelia africana, Butyrospermum paradoxum, Daniellia oliveri, Maranthes polyandra, Parkia biglobosa, Piliostigma thonningii, Psuedocedrela kotschyi, Pterocarpus erinaceus, Stereospermum kunthianun and Terminalia spp. Trees common throughout both northern and southern secondary grasslands include Annona senegalensis, Burkea africana, Combretum collinum, Hymenocardia acida, Pariniari curatelifolia, Stereospernum kunthianum, Strychnos spp. and Vitex spp.
In the relatively arid corners in the northeast (Cameroon) and northwest portion (southwest Sudan) of the ecoregion, the transitional Isoberlinia spp. dominated Sudanian woodlands and wooded savannas characterize the flora where cultivation has not drastically altered the system (White 1983, Mayaux et al. 1999). Other important trees include Daniellia oliveri and Erythrophleum africanum. Where human population remains sparse in central Cameroon and the Central African Republic, patches of dense dry forest remain, dominated by Isoberlinia doka with Afzelia africana, Burkea africana, Anogneissus leiocarpus, Terminalia spp. and, Borassus aethiopum. Toward the northern reaches of this ecoregion Vitex doniana is a Sudan near-endemic. Acacia woodlands appear intermittently (Stuart et al. 1990, White 1983). Grasslands that are probably secondary in origin comprise the continuous herbaceous strata.
Central Africa has experienced repeated climatic fluctuations that have caused rain forest and savanna expansion and contraction at least since the late Pleistocene tectonics defined the Congo Basin (Faniran and Jeje 1983). It has been speculated that significant climatic shifts have occurred more than 20 times in less than 10 million years (Kingdon 1989). Plants and animals adapted, migrated or became extinct with each climatic oscillation. Widespread and highly adaptable organisms that survived in islands of habitat and adapted to spatial fluidity were favored (Kingdon 1989). During dry periods, savanna communities invaded far into the Congo Basin. The relatively moist riparian forests become isolated from one another and formed forest island communities in the savanna matrix (White 1983, Kingdon 1989, Smith et al. 1997). Paleobotanical evidence suggests that rain forests grew more extensively on the deeper lowland and low slope soils during wetter periods, with transition woodland forming an ecotone to Isoberlinia woodlands on shallower upland soils (White 1983). The forests also expanded north during these wetter periods (Kingdon 1989). The periods of isolation from the interior forests created diverse riparian communities with incomplete species overlap between each other and with the central Basin forests (Kingdon 1989, White 1983, Dzwonko 1994).
Due to their great physical diversity, geographic range and high percentage of ecotonal area, the gallery forests may have provided the loci for differentiation and speciation during these massive climatic and habitat fluctuations. Forest savanna mosaics continue to provide incubators of tropical biodiversity. Smith et al.(1997) looked at interbreeding populations of a common passerine, the little greenbul (Andropadus virens), which lives in both dense rain forest and in the forest-savanna ecotone of the Cameroon mosaic. They found that forest dwellers showed significant morphological and behavioral differences from edge dwellers despite continually shared genes. The results suggest that forest savanna ecotones, with their characteristically diverse habitat complexes may be vital centers for differentiation and speciation in the tropics. The role of savanna areas in bird and plant speciation is further outlined in papers by Fjeldså (1994) and Fjeldså and Lovett (1997).
Some current grassland dwellers appear to have evolved from rain forest predecessors, and some forest species show evidence of savanna origins. Kingdon (1989, 1997) shows how the Chlorocebus genus of monkeys radiated and differentiated largely following climatic shifts. Similarly, red-headed lovebirds (Agapornis pullaria), woodland seed eaters common to the northern and eastern forest-savanna mosaics, are most likely descended from open-canopy species. Opportunistic species such as olive baboons (Papio anubis) continue to actively adapt and expand their range. .
While some fauna illustrate the shifting nature of this mosaic, other species are able to maintain parts of the habitat mosaic. The forest sub-species of elephant (Loxodonta africana cyclotis) is adapted to the forest and its lifestyle assists forest regeneration by the spreading of large seeds. In contrast the savanna sub-species (Loxodonta africana africana) occupies the savanna woodlands where it denudes trees and suppresses sapling growth, effectively creating a more fire-prone system (Hopkins 1992, Belsky and Amundson 1992). In some parts of the ecoregion, for example Garamba National Park, the elephants seem to be intermediate between the forest subspecies, Loxodonta africana cyclotis, and the savanna subspecies (IUCN/UNEP 1987, IUCN 1992).
This ecoregion thus provides a unique set of habitats and resources that supports moderate levels of diversity, including many species with broad distributions in tropical Africa (Millington et al. 1992). The red-flanked duiker (Cephalophus rufilatis) inhabits forest patches within the savanna matrix across the Guineo-Congolian/Sudanian transition zone. Widespread mammals in these savanna forest mosaics include the black rhinoceros (Diceros bicornis longipes, CR) (now however restricted to a few individuals remaining in Cameroon), giant eland (Taurotragus derbianus) and in the eastern sector, bongo (Tragelaphus eurycerus). Other animals typical of forest-savanna mosaics include waterbuck (Kobus ellipsiprymnus), Buffon’s kob (Kobus kob), patas monkey (Cercopithicus patas), roan antelope (Hippotragus equinus), buffalo (Syncerus caffer), and hippopotamus (Hippopotamus amphibius). Typical northern savanna animals include northern savanna giraffes (Giraffa camelopardalis congoensis). Top predators are lion (Panthera leo), the leopard (Panthera pardus) and the Nile crocodile (Crocodylus niloticus), which is present in northern waterways across the region.
The mole rat (Cryptomys ochraceocinereus) is one of the few near-endemic mammals in this ecoregion. The rare Pousargue’s mongoose (Dologale dybowskii) is also nearly endemic, living here and in the Victoria Basin Forest-Savanna Mosaic (IUCN 1992). There is one near-endemic bird species, the forest ground-thrush (Zoothera oberlaenderi). This ecoregion claims a number of herpetile endemics. Strictly endemic amphibians include Mauda River frog (Phrynobatrachus albomarginatus), Bouda River frog (P. scapularis), the Bamileke Plateau frog (Rana (Amnirana) longipes), Hymenochirus boulengeri and Ptychadena ingeri. Reptiles include the strictly endemic Sudan beaked snake (Rhinotyphlops sudanensis), Ichnotropis chapini and Helophis smaragdina.
The ratio of forest to savanna in this area has fluctuated considerably over time, driven by long-term climatic changes, the vagaries of rainfall and dry season length as well as by shifts in human disturbance patterns. Species distributions have likewise shifted over time (Kingdon 1989). The vegetation in this ecoregion also reflects a long history of land use in which people have significantly modified the ecosystem and thus, in part, determined the ability of the area to support various species complexes. People have burned and cultivated, herded livestock and extracted resources and conducted warfare in the northern Congolian forest-savanna mosaic region for at least 3,000 years. Local agriculturists and pastoralists have historically been part of the "natural" disturbance cycle, participating in the maintenance of this vast mosaic. Shifting agriculture, practiced at low intensity, permits natural secondary succession of rain forest species; the burn cycles and fallow periods maintain soil fertility and secondary forest growth. But as population increases, traditional slash and burn agriculture becomes untenable. Population pressure has shortened the average fallow period from 20 years to as little as 5, and this is insufficient for forest to regenerate and soils to regain fertility.
Excessive burning slowly alters the native vegetation, causing some poorly-adapted species to disappear. Large-scale clearing and high intensity fires alter the light, temperature and hydrologic regimes considerably, transferring dominance from woody plants to grasses, and creating conditions such as degraded soil in which non-pioneer trees can no longer regenerate (Longman and Jenik 1992). Frequent fires set at the edge of gallery forests alter the community, replacing riparian species with tall grasses and Isoberlinia woodlands dominated by Lophira lanceolata, Daniellia oliveri, and Parkia biglobosa (Sayer et al. 1992). The edges of drier type peripheral semi-evergreen rain forests growing among this grass-dominated zone have been severely degraded: regular grassland fires consume litter and small woody plants, creating a different structure from larger forest patches. The relative openness of the canopy at the forest edge allows invasion by savanna species such as Afzelia africana and Anogneissus leiocarpus, and by oil palms (Elaeis guineensis). Over the last several centuries the fire regime has intensified, and farms, pastureland and urbanization have replaced much of the vegetation associated with the forest-savanna mosaic, with the loss of some forest dependent animal species.
Compared to known centers of endemism, this ecoregion has received little scientific attention. Even in the case of well-studied species, confusion persists about their conservation requirements, making broad habitat protection critical. The evolutionary history of the little greenbul illustrates the potential value in broad habitat and species protection. The long history of the gallery forests suggests that they may serve as both refuge for rain forest species and mode of conveyance and dispersal. If gallery forests thus facilitate survival despite climate change, they should be included in the system of protected areas
The protected areas in this ecoregion are few and widely dispersed. Unprotected patches of suitable but imminently threatened habitat remains scattered throughout this region. For example, extensive fragmentation threatens the endemic plant communities of the Cameroon’s Adamaoua Plateau, which is largely unprotected (Stuart et al. 1990). The focus of many of the protected areas has been to protect the remaining populations of large game species, often favoring the Isoberlinia woodlands over forest habitat. Garamba National Park in the Democratic Republic of Congo and Mbem-Djerem National Park in Cameroon are the only two areas that conserve the forest savanna transition zone.
Types and Severity of Threats
Increasing human population, poverty, the ongoing civil wars in Sudan and the Democratic Republic of Congo, strife between government and rebel groups in the Central African Republic and armed incursions by well-armed poaching gangs from the Sudan, mean that the northern forest-savanna mosaic faces increased threats. Hunting of animals for food, including from within protected areas, occurs in all areas, as does deforestation.
Political instability has propelled floods of transnational refugees, as well as provided incentive for widespread poaching, exacerbating negative human impact on the natural systems. Warring rebel factions poach valuable game and timber to buy munitions (CIA 2000); mass migrations of refugees further tax fuelwood, wildlife in the form of bush meat resources, water, and soils. Ongoing economic, political and social instability have drained the already limited conservation budgets, and parks and protected areas are particularly susceptible to poaching.
The distribution of large mammals has been drastically reduced in recent times, providing evidence of the impact of habitat conversion and overhunting. Hunting camps are found far into the bush, with ivory and other valuable animal products targeted to pay for weapons as well as food. Gallery forests are logged for timber, even though high transportation costs marginalize the profitability of commercial logging ventures. Cutting wood for fuel and charcoal production threatens woodlands and forests where populations become too dense. Refugee camps produce intense local pressure on the environment, and severely degrade their local natural resources.
Climate change is also implicated in the increase of grassland in proportion to forest. Across much of the ecoregion, average rainfall levels have dropped precipitously since the 1970s. Careful monitoring of the dynamics of the forest/grassland ecotones, and the relative advance and retreat of the adjoining habitats, may offer insight into the nature and rate of climate change (Furley 1992). Regions closer to the forest zone will continue to see increased human population in response to desiccation.
Justification of Ecoregion Delineation
This ecoregion is based on the ‘mosaic of lowland rain forest and secondary grassland’ vegetation unit of White (1983). It is distinct from the Guinean Forest-Savanna Mosaic ecoregion as it is largely separated by the Cameroonian Highlands, which acts a range limit for several forest-savanna mosaic taxa. The southern boundary is defined by the transition to more continuous forest cover. The northern boundary was verified with 1 km classified land cover data derived from AVHRR satellite imagery (Loveland et al. 2000).
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Prepared by: Illisa Kelman
Reviewed by: In progress