At the upper elevations of Bwindi Impenetrable Forest and throughout the high forest of the Virunga mountains, one tree species defines the landscape in a way that no other can. Hagenia abyssinica — known variously as African rosewood, African redwood, or simply Hagenia — forms the structural framework of the high montane forest above 2,400 metres where mountain gorillas spend much of their lives. Its massive, spreading form, covered in hanging moss and epiphytic growth, draped with the wispy grey-green lichen that hangs like smoke from its branches, is the visual signature of mountain gorilla habitat across the Albertine Rift.
Hagenia is not a well-known tree internationally, but it deserves attention from anyone interested in the ecology of mountain gorilla habitat. It is the tree species most strongly associated with the landscape type that mountain gorillas depend on, it has a long history of medicinal use across highland East Africa, and its presence or absence in a mountain forest is one of the most reliable ecological indicators of the high-altitude zone where the gorillas preferentially live. This guide explores the biology, ecology, and cultural significance of this extraordinary tree.
Identifying Hagenia in the forest
Hagenia abyssinica belongs to the family Rosaceae — the same family as roses, apples, and cherries — which is unexpected for a large tropical forest tree. It is a medium to large tree reaching 10 to 20 metres in height with a characteristically broad, spreading crown that creates a substantial canopy in older individuals. The bark is fibrous, reddish-brown, and deeply furrowed in mature trees — the source of the common name African redwood, though the timber is not closely related to the true redwood of California.
The leaves are pinnately compound — arranged as multiple leaflets along a central rachis rather than as a single undivided blade. Each leaflet has a distinctly toothed margin and a somewhat leathery texture. The leaves have a characteristic shape that, once learned, is instantly recognisable even at a distance: the combination of the compound arrangement and the toothed leaflets against the reddish twigs is distinctive in the forest at Bwindi’s upper elevations.
Hagenia is dioecious — male and female flowers are borne on separate trees. The flowers are small and numerous, arranged in long drooping clusters (panicles) that hang from the branch tips. Male flowers are yellowish; female flowers are pinkish-red. The flowering season in the Virunga and Bwindi highland forests typically occurs between August and January, and the hanging flower clusters are among the most visible flowering displays of any tree in the highland forest community. When in flower, a large Hagenia is recognisable from a considerable distance by its distinctive pinkish-red floral fringe.
The hagenia-hypericum forest type
Hagenia does not occur alone. It is the defining species of a distinctive forest type — the hagenia-hypericum forest — that represents the uppermost closed-canopy forest zone in the Albertine Rift mountains. In this forest type, Hagenia forms the canopy with a second characteristic tree, Hypericum revolutum (commonly called St John’s wort, though it is a large tree rather than the familiar low-growing herb of temperate gardens). Together these two species create a distinctive, open-structured forest with a rich understorey of herbs, ferns, and shrubs.
The Hypericum trees of the highland forest are notable for their bright yellow flowers — among the most vivid yellow of any large African forest tree — and for their aromatic oils that give the tree a distinctive scent when the leaves or bark are crushed. Both Hagenia and Hypericum are adapted to the moist, cool, frequently misty conditions of the high montane environment: both tolerate waterlogged soils and both can cope with the intense cold that occurs during clear nights at altitudes above 2,500 metres when temperatures drop close to freezing.
The hagenia-hypericum forest type is the preferred habitat of mountain gorillas across their range. The gorillas’ preference for this forest type over the bamboo zone below or the afroalpine moorland above reflects both the food resource distribution — the forest offers a year-round mix of leaves, herbs, fruit, and bark — and the shelter conditions, where the dense moss-draped Hagenia canopy provides relatively dry sleeping conditions compared to the open alpine zone.
Hagenia as gorilla food
Mountain gorillas interact with Hagenia trees primarily as shelter and sleeping site providers rather than as food plants. The massive, spreading branches of old Hagenia trees create stable platforms where gorillas build their overnight nests — bending small branches together to create a compressed sleeping platform. The bark and leaves of Hagenia are consumed occasionally but are not major dietary components. The tree’s primary value to gorillas is structural: the large branches and spreading crown create the forest architecture that gorillas use as both a travel route and a sleeping site.
The nesting function is ecologically significant. Gorillas rebuild nests every night, selecting new sites rather than reusing old ones. A single habituated gorilla family of ten individuals creates ten new nests per night — 3,650 new nest sites per year. In the hagenia-hypericum forest, this nesting activity creates disturbance to the vegetation, opens canopy gaps where nests are built in the understorey, and deposits the substantial quantities of gorilla dung that enrich soil nutrients at nest sites. The gorilla’s interaction with Hagenia is thus a subtle but consistent ecological influence that shapes the forest structure at the scale of decades and centuries.
Medicinal use of Hagenia across highland Africa
Hagenia abyssinica has one of the longest and most documented histories of medicinal use of any African tree species. The female flowers — specifically the dried heads of female Hagenia flowers — have been used as an anthelmintic (anti-worm treatment) across highland East Africa for centuries. The active compounds, particularly the phloroglucinol derivatives protokosin and kosotoxin, are toxic to tapeworms (Taenia species) and were the basis for a widely used traditional treatment administered as a decoction.
The Hagenia anthelmintic treatment was documented by European physicians in Ethiopia and East Africa in the nineteenth century and was brought into colonial medical practice as an official pharmaceutical remedy known as Kousso (from the Ethiopian name for the plant, Kosso). It was listed in pharmacopoeias in Europe and North America until synthetic anthelmintics replaced it in the mid-twentieth century. The effectiveness of the traditional treatment was real — protokosin is a genuinely potent antiparasitic compound — though the therapeutic margin was narrow enough that overdose was a risk in traditional use.
Hagenia and the conservation of highland forest
Hagenia forest outside protected areas has been severely reduced across the Albertine Rift through agricultural expansion and timber extraction. The tree produces a durable, attractive timber that was historically used for construction and furniture; this commercial value drove selective extraction that removed the largest trees from unprotected highland forests throughout Uganda, Rwanda, and Ethiopia. Within protected areas like Bwindi and the Virunga parks, mature Hagenia with girths that represent centuries of growth survive in numbers that are otherwise rare in the landscape.
The moss and lichen coverage on old Hagenia trees is itself a conservation value. Old growth Hagenia supports a diversity of epiphytic mosses, liverworts, and lichens that are absent from younger trees and from trees in disturbed forest. These epiphyte communities provide habitat for specialised invertebrates and microorganisms that are poorly understood but represent genuine biodiversity that is effectively irreplaceable once lost. The protection of old-growth Hagenia in Bwindi and the Virunga parks is thus a conservation achievement that extends well beyond the headline gorilla story, preserving an entire community of organisms that would otherwise disappear from the highland landscape.
Finding Hagenia on your gorilla trek
The hagenia-hypericum forest where gorilla families spend much of their time is the destination of your trek, not the starting point. As the trail climbs through lower-elevation mixed forest and bamboo zones, the vegetation character changes — trees become larger and more moss-covered, the air becomes cooler and moister, and the first Hagenia trees begin to appear with their characteristic reddish bark and compound leaves. When you notice these changes, you are entering the zone where gorilla encounters are most likely.
Ask your ranger guide to identify Hagenia as you enter the upper forest. The guide will almost certainly know the tree by its local Rukiga or Swahili name (in many areas it is called mwino or omushasha) and will be able to point out distinctive features. Old Hagenia trees that show signs of gorilla nesting — flattened branch depressions, accumulated dung beneath the tree, bark marked by gorilla feeding — are worth pausing at even before the group is found. They tell you, in physical evidence, that the gorillas you are about to encounter live here. This is their home, not a stage set, and the trees that define it deserve the same attention as the animals themselves.
