Forests of the Ngorongoro Highlands

Forests of the Ngorongoro Highlands

Mention the word 'Ngorongoro' and everyone immediately thinks of the magnificent volcanic crater which is full of large mammals and other wildlife. But the Crater is just one part of the vast Ngorongoro Conservation Area which incorporates a rich variety of habitats.

The eastern rim of the Ngorongoro Crater, at an altitude of about 2500 metres, is regularly shrouded in mist and low cloud, for it receives moist air drifting in from Tanzania's Indian Ocean coast. Clouds and the resultant precipitation are formed by the upward movement of warm air rising from the Rift Valley, cooling as it ascends to the Ngorongoro highlands. These moist conditions have given rise to a broad band of evergreen forest which cloaks the eastern rim and its seaward slopes, as well as some of the isolated peaks.

This cloud forest is characterised by abundant epiphytic lichens, orchids and mosses which grow on the boughs of giant trees, and supports a flora and fauna quite distinct to that of the surrounding countryside. The veils of Old Man's Beard lichen (Usnea spp.) which hang from the trees lends an air of mystery and intrigue to this mountain world. The composition of the forest changes with altitude, as different tree species and plant communities dominate at particular elevations. In the middle reaches (such as at the Lodware entrance gate) Croton megalocarpus and Croton macrostachys are dominant and conspicuous, with Albizia gummifera and Celtis africana also common. On the rim itself, the larger tree species include Nuxia congesta, Ficus thonningii, Cassipourea malosana and Bersama abyssinica, with leafy shrubs such as Crassocephalum mannii and Vernonia auriculifera common. Higher still, on the slopes of Olmoti, Empakaai and Lemagarut at over 3000 metres, Hagenia abyssinica and Hypericum revolutum are the dominant trees, with Erica arborea common in places. Groves of African bamboo (Arundinaria alpina) grow in places, with the largest stands below Makoromba peak.

The understory of closed-canopy forest receives little light, and ferns are among the dominant plants in this damp and shady environment of decomposing logs, fungi, moss and leaf litter. A greater diversity of soft-stemmed plants thrive on the fringe of the forest, with flowering herbs often concentrated along the roadsides.

In common with uplands around the world, there are some spectacularly beautiful flowers in the Crater highlands. When in bloom, the clumps of Delphinium leroyi, with their slender white trumpet-shaped flowers are an arresting sight, while the closely related Clematis brachyiata (both species belonging to the Ranunculaceae family) decorates the landscape with masses of small white flowers or fluffy seed heads. Orange-flowered Leonotis nepetifolia and L. mollissima offer abundant nectar to five species of sunbird - Golden-winged, Tacazze, Malachite, Bronze and Eastern Double-collared - which are amply rewarded for their role as pollinators. Orange-flowered Begonia, as well as delicate blooms of Impatiens cling to steep road verges cloaked in moss and ferns, and this is also the favoured habitat for carpets of beautiful blue-flowered Lupinus princei. Favouring more sunlight are the leguminous Crotalaria agatiflora with its lemon-yellow pea flowers, Cynoglossum coeruleum, and the impressive Kniphofia thomsonii; both of these plants are pollinated by nectar-seeking sunbirds.

Several species of epiphytic orchids cling to the branches of big trees, with Aërangis thomsonii and Rangaeris amaniensis among the commoner species. These, and other epiphytes such as ferns and lichen, often form such a dense mass that their weight brings down the very branches on which they are dependent. Spectacular when in flower, the yellow-flowered Senecio hadiensis is another plant to be seen draped over taller trees, but this is a liana not a epiphyte. The fissured bark of mature Nuxia congesta trees provides ideal conditions for these tenants. The fruits of Bersama abyssinica split to reveal four scarlet seeds which are relished by Schalow's Turaco, African Green-Pigeon and Silvery-cheeked Hornbill, which otherwise enjoy the bounty of figs provided by Ficus thonningii. A rich bird community is dependent upon the cloud forest and among the species which are more or less limited to this habitat are Olive Pigeon, Lemon Dove, African Wood-Owl, Montane Nightjar, Bar-tailed Trogon, Cinnamon-chested Bee-eater, Brown-backed Woodpecker, Mountain Greenbul, Cabanis' Greenbul, African Hill-Babbler, Ruppell's Robin-Chat, Cape Robin-Chat, White-starred Robin, Olive Thrush, White-eyed Slaty-Flycatcher, White-tailed Blue-Flycatcher, Brown Woodland-Warbler, Evergreen Forest-Warbler, Cinnamon Bracken-Warbler, Hunter's Cisticola, Chestnut-throated Apalis, Brown-headed Apalis, Bar-headed Apalis, Brown Parisoma, Montane White-eye, Black-fronted Bushshrike, Tropical Boubou, Waller' Starling, Baglafecht Weaver, Red-faced Crimsonwing, Abyssinian Crimsonwing, Yellow-bellied Waxbill, Grey-headed Nigrita, Oriole Finch, Streaky Seedeater, African Citril, Yellow-crowned Canary and Thick-billed Seedeater.

For the montane flora and the birds associated with it, as well as hosts of other creatures, the Ngorongoro highlands are an ecological island. Mount Meru and Kilimanjaro possess similar communities but are over 100km to the east, and separated from Ngorongoro by the semi-arid Maasai steppe. That the same plants and wildlife occur in such separate localities reveals that this part of East Africa - and the continent as a whole - was much cooler and dryer during previous climatic regimes. As the climate gradually became warmer and wetter, so the plants and animals which had evolved to flourish in such conditions retreated upwards to an elevation that best suited them. And as the intervening lowlands warmed up, so they were invaded by acacias, gazelles, rollers and other appropriately-adapted species. This oscillation of climatic regimes separating and reuniting plant and animal communities has occurred repeatedly on our planet and has been the driving force for speciation.

-Duncan Butchart-

Posted: Plants by CC Africa, Date: 21 November 2006