Angolan montane forest-grassland mosaic

Part of a continent-wide archipelago of Afromontane vegetation, the Angolan Montane Forest-Grassland Mosaic ecoregion shows more affinities to faraway mountain chains than to surrounding lowlands. Open grasslands are widespread here and maintained by anthropogenic fires. Forest patches survive only in isolated ravines and at high, inaccessible elevations. These forests harbor rare endemic birds, but the large ungulate populations indigenous to grassland areas have likely been extirpated. This area is largely unknown biologically due to social unrest and civil war that stems from Angolan independence in 1975.

  • Scientific Code
  • Ecoregion Category
  • Size
    9,800 square miles
  • Status
  • Habitats

Location and General Description
This ecoregion comprises a number of small montane forest patches surrounded by grasslands and Protea savanna in the west-central highlands of Angola. The forest patches are restricted to the deep ravines or remote valleys of the highest mountains in the Huambo and Cuanza Sul provinces and an area of Afromontane forest mosaic further south, on the Serra da Chela in Huíla province. The ecoregion represents a small fragment of White’s (1983) Afromontane archipelago-like center of endemism, which consists of widely scattered "islands" of forest on mountain systems in southern, eastern, and western Africa (Werger 1978). The characteristic elements of the ecoregion’s fauna and flora are more closely related to other such Afromontane areas than to the surrounding Angolan biomes (Huntley 1974a, Dean 2000).

The ecoregion lies on the Marginal Mountain Chain of Angola, which is restricted to a narrow band running along the inland margin of the escarpment from 11° to 16° S. Residual land surfaces that possibly date back to the Gondwanan age (King 1963) form the highest points here, reaching 2,620 m on Mt. Môco, 2,582 m on Mt. Mepo, and 2,554 m on Mt. Lubangue (Huntley 1974a). The mountain chain is part of the Precambrian basement complex that comprises gneisses, gneissic granites and metamorphosed sediments (Barbosa 1970). The soils are deep and highly weathered.

Mean annual rainfall in the ecoregion is between 1,200 mm and 1,600 mm, increasing with elevation. Rainfall is concentrated in the summer months, although precipitation from the mists, which rise as the cold Benguela Current meets warmer tropical waters offshore, occurs through most of the year. The coolest months are July and August, when sub-zero temperatures are frequently recorded in the mountains (Huntley 1974a). Mean annual temperatures range from 17° to 20°C (Texeira 1968).

Pockets of forest survive mainly in deep, humid ravines and on isolated peaks higher than 1,800 m (Huntley and Matos 1994). The forest patches range from 1 to 20 ha in size and reach a canopy height of 8 m to 15 m. The dominant forest tree species is the yellowwood Podocarpus latifolius. Other common tree species include Polyscias fulva, Apodytes dimidiata, Pittosporum viridiflorum, Syzygium guineense afromontanum, Halleria lucida, Olea spp., and Ibex mitis. Hardly any grass grows in these shady forests, and they are less heavily overgrown with epiphytes than similar forests elsewhere in Africa (Huntley and Matos 1994). The canopy tends to be very irregular because of the steep and rocky slopes on which the forest patches are found.

Open grasslands with widely scattered trees and shrubs cover large areas of the highland plateau above 1,600 m and make up most of the ecoregion’s area. In well-drained areas, this vegetation is generally fire-prone and includes shrub species such as Philippia benguelensis, Erica spp., Protea trichophylla, Stoebe vulgaris, and Cliffortia sp. and grasses Themeda triandra, Tristachya inamoena, T. bequertii, Hyparrhenia andogensis, H. quarrei, Festuca spp., and Monocymbium ceresiiforme. On waterlogged plateaus, representative plants include Parinari capensis, Myrsine africana, Protea welwitschii, Dissotis canescens, Cyathea spp., Loudetia spp., Fimbristylis spp., and Xyris spp. (Huntley and Matos 1994).

The grasslands in the ecoregion are partly of edaphic origin and partly maintained by fire, much of this from anthropogenic origin. Though the forest vegetation is not very flammable, fires can intrude into the forest in hot, dry periods or when forests have been thinned through logging and grass has been able to grow. The abrupt boundaries of the forest fragments and their remaining distribution in ravines and moist south-facing slopes demonstrate that the extent of forests is largely determined by fire. This is also the case in other Afromontane areas in Africa, e.g. South Africa, where Afromontane forest has been highly fragmented for a very long time as a result of fire (Midgley et al. 1997). It has also been shown that the smaller the forest fragments are, the greater the potential impact of fire. In a study of Afromontane type forest in the Western Cape of South Africa, Bond and Euston-Brown (1993) found that some large, fire-sensitive forest tree species (including Podocarpus latifolius) were absent from very small forest fragments, and that smaller patches tended to be dominated by more fire-tolerant tree species.

The highest human population densities in Angola outside Luanda are encountered in the highlands of this ecoregion and adjacent parts of the Angolan Miombo Woodland Ecoregion. Population densities here exceed 30 people per km2 (Moyo et al. 1993) as a result of the high agricultural potential of the area.

Biodiversity Features
The montane forests of Angola are of great biogeographic interest as they are the sole surviving relics of a much larger moist forest biome that spread during more favorable past climatic conditions (Huntley 1974a). A few narrow endemic species and subspecies are found in the Angolan montane forests, and many elements of their fauna and flora are shared not with the surrounding ecoregions but with Afromontane vegetation occurring on mountain formations thousands of kilometers away. The ecoregion has been extremely poorly studied, with the exception of the avifauna that is described by Hall (1960). Stuart et al. (1990) list the Angolan montane forests among critical sites for biodiversity conservation.

Few large mammal species occur in the ecoregion. Huntley (1974b) describes Burchell’s zebra (Equus burchelli), eland (Taurotragus oryx), reedbuck (Redunca arundinum), oribi (Ourebia ourebi), and roan antelope (Hippotragus equinus) as being indigenous to the montane grassland areas, but it is unlikely that populations of these large mammals survive in the ecoregion. The mammal fauna of the forests and forest margins includes yellow baboon (Papio cynocephalus), red-footed squirrel (Funiscurius pyrrhopus), blue duiker (Cephalophus monticola), and bushpig (Potamochoerus porcus), but has been severely reduced through hunting (Huntley, 1974b). The small mammal fauna is poorly known. Two shrews species, Crocidura erica and Crocidura nigricans are considered near-endemic to this ecoregion. Current research indicates that both have a narrow distribution in this ecoregion and adjacent portions of the Angolan Miombo Woodland ecoregion, though further research may find that their ranges are larger.

Of the approximately 360 bird species found in the ecoregion, five are endemic or near-endemic. Boulton’s batis (Batis margaritae) is known only from here and the border area between Zambia and southern DRC. Swierstra’s francolin (Francolinus swierstrai, VU) is known only from a few montane areas in Angola, though it has not been established whether it is forest-dependent, as it has also been observed in other habitat types (Collar and Stuart 1985). It is listed as vulnerable due to decreasing and fragmented habitat (BirdLife International 2000). The near-endemic Angola cave-chat (Xenocopsychus ansorgei) is common but extremely localized, being limited to a few rocky hills and cliffs and surrounding forest habitat in four isolated areas, two of which fall into the ecoregion: eastern Namibe and Huíla in the southern outlier of the ecoregion, and on the inselberg of Mt. Soque in Huambo (Dean 2000). The rest of its distribution falls into the Angolan Scarp Savanna and Woodlands ecoregion. BirdLife International (2000) classifies it as lower risk/near-threatened because its habitat is inaccessible, despite having a localized distribution. The near-endemic grey-striped francolin (Francolinus griseostriatus) occurs mainly on the escarpment of the Angolan Scarp Savanna and Woodland ecoregion, but its range is very localized and it has been recorded in the Afromontane region (Dean 2000). It is listed as vulnerable and hunting is a threat for both francolin species (BirdLife International 2000). One other bird species, the Angolan slaty-flycatcher (Dioptrornis brunneus) is also shared between this ecoregion and the Angolan scarp forests.

There are a number of endemic and near-endemic bird subspecies that are thought to have evolved in the ecoregion after its isolation from other montane forest patches when the climate became drier. These subspecies are separated by more than 2,000 km from their nearest relatives in the mountains of Fernando Po, Cameroon, Ruwenzori, Tanzania, Ethiopia, and Malawi. These include long-billed pipit (Anthus similis moco), mountain chat (Oenanthe monticola nigricauda), mountain nightjar (Caprimulgus poliocephalus koesteri), western green tinkerbird (Pogoniulus coryphaeus angolensis), evergreen-forest warbler (Bradypterus lopezi boultoni), Margaret’s batis (Batis margaritae margaritae), African hill babbler (Pseudoalcippe abyssinica ansorgei), and thick-billed canary (Serinus burtoni tanganjicae). Other species with an Afromontane distribution include bar-tailed trogon (Apaloderma vittatum), scarce swift (Schoutedenapus myioptilus), orange thrush (Zoothera gurneyi) and African black swift (Apus barbatus sladeniae) (Dean 2000). The rare Fernando Po swift (Apus sladeniae) has been collected on Mt. Môco, and is considered data deficient by BirdLife International (2000).

The herpetofauna of the ecoregion is poorly documented. The link-marked sand racer (Psammophis ansorgii) is considered endemic and Marx’s rough-scaled lizard (Ichnotropis microlepidota) near-endemic, but research is severely lacking and data are likely to be inaccurate. Notable among the amphibians is the strict endemic Hyperolius erythromelanus, which is only known from collections within this ecoregion. The adder, Causus angolensis, has recently been reported to be endemic in this ecoregion (Broadley pers. comm).

Current Status
The forests of this ecoregion are highly fragmented as a result of fires, agriculture and woodcutting. The remaining forest patches seldom exceed 20 ha in size, and their total area is probably less than 200 ha (Huntley 1974a). The most extensive forest areas, at Mt. Namba, were exploited for timber during the colonial period and are devoid of pristine patches. Relatively undisturbed patches remain at Mt. Môco between 1,800 and 2,400 m elevation (Huntley and Matos 1994). However, due to the lack of data, it is not known how extensive the forest patches once were, and at what rate their extent and quality have changed since the 1970’s. No protected areas currently exist in this ecoregion. Unless drastic conservation efforts are implemented, it is possible that little or nothing will remain of the forest patches and their fauna (Huntley 1974a, Huntley and Matos 1994).

Types and Severity of Threats
The almost continuous civil war in Angola since 1974 has led to great instability, poor security, economic depression, massive displacement of the rural population and a lack of infrastructure and basic services. As a result, water, sanitation, health, energy and food are the most important items on the environmental agenda, against which most other issues, including conservation, pale in significance (Moyo et al. 1993). This, combined with the high population pressure in much of the ecoregion and the lack of protected areas, makes conservation of the forest fragments and surrounding grasslands a very daunting task.

Probably the greatest threat to the forest areas is from logging and other harvesting of forest products. Agriculture and fires are of less significance to the forest patches, which are already confined to steep, inaccessible slopes and ravines. It is thus likely that the quality rather than the extent of the remaining forest fragments is at risk.

The moist, often waterlogged, grassland areas are the least threatened as they are unsuitable for agriculture and not affected by fires. Better-drained areas are subject to frequent fires, though it is not known how the frequency and seasonality of the fires affects the vegetation, which is fire-adapted. Clearing for agriculture is probably the greatest threat to the Afromontane grasslands and savannas given the relatively high agricultural potential and the dense human population.

Because the ecoregion’s ecology has been so poorly studied, the effects of this habitat fragmentation and modification on the ecoregion’s fauna are not clear. Hunting is however considered to be a danger to the ecoregion’s mammals and birds, having already resulted in the virtual extinction of the larger herbivores.

Justification of Ecoregion Delineation
White (1983) mapped four areas of Afromontane vegetation in the Huambo and Cuanza Sul provinces and an area of Afromontane forest mosaic further south, on the Serra da Chela in the Huíla province. The extent of the ecoregion corresponds to the Bailundu Highlands portion of the Western Angola Endemic Bird Area (Stattersfield et al. 1998), although it was modified to the 1,200 m elevation contour (WWF 1998). The ecoregion is distinguished from surrounding areas since its fauna and flora share closer affinities to other Afromontane areas (Huntley 1974a, Dean 2000).

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Prepared by: Suzanne Vetter
Reviewed by: In progress