At the upper elevations of Bwindi Impenetrable National Park, above approximately 2,300 metres, the dense mixed forest of the lower slopes gives way to a distinctive vegetation community dominated by Hagenia abyssinica and Hypericum revolutum — the hagenia-hypericum zone that characterises Afromontane forest across the highland rim of East Africa. This upper forest belt is a visually distinctive ecosystem, its gnarled hagenia trees draped in mosses and lichens against a backdrop of yellow-flowered hypericum scrub, and it supports a specialist biological community that includes several species found nowhere at lower elevations in the park.
Hagenia: the African rose tree
Hagenia abyssinica is one of the most distinctive trees of the East African highlands, easily identified by its large, bipinnate leaves with white woolly undersides, its clusters of reddish-pink flowers produced in drooping panicles, and its deeply furrowed bark that develops a coppery-red tone in mature individuals. The tree is dioecious — male and female flowers are borne on separate trees — and both forms are visible in the hagenia zone of Bwindi’s upper sections, the female trees distinguished by larger, more pendant flower clusters.
Hagenia trees in the upper Bwindi zone typically grow to 10–20 metres, shorter than the mixed forest trees of the mid-altitude zone but with a more spreading crown that gives the hagenia forest a parkland quality — widely spaced, large trees with an open understorey that allows more light to reach the ground than the dense canopy below. The epiphytic load on mature hagenia trees is extraordinary: the bark and major branches of old individuals are often invisible beneath layers of mosses, liverworts, orchids, and lichen that transform each tree into a hanging garden.
The medicinal properties of hagenia were noted earlier in the context of traditional pharmacology: the dried flower heads contain compounds with anthelmintic (de-worming) activity that were used across highland East Africa for tapeworm treatment before pharmaceutical alternatives became available. The tree is also significant in the cultural traditions of highland communities as a marker tree for boundaries and sacred sites, its distinctive appearance making it a reliable landmark in the landscape.
Hypericum: the tree-heather of the highlands
Hypericum revolutum — the African St. John’s wort or highland hypericum — is the dominant shrub and small tree of the upper forest margins, forming dense thickets in disturbed areas, along trails, and at the boundary between the closed forest and the subalpine zone above. It is familiar to anyone who has trekked in the Rwenzori Mountains, the Virungas, or the Aberdare highlands of Kenya — the yellow flowers and resinous, slightly aromatic leaves are a consistent sensory signature of East African high-altitude environments.
In the upper Bwindi zone, hypericum grows both as a dense understorey shrub beneath the hagenia canopy and as an open heath community on the more exposed ridges and slopes above the forest’s uppermost limit. The transition from closed hagenia-hypericum forest to open hypericum heath is abrupt in places, creating distinct edges that are often associated with higher bird diversity — edge habitats typically support species from both adjacent zones simultaneously, making them productive for observers willing to walk the boundary between ecosystem types.
Specialist wildlife of the upper forest zone
The Ruhija sector of Bwindi, which accesses the park’s eastern and higher-altitude sections, provides the best access to the hagenia-hypericum zone for gorilla trekkers and birdwatchers. The sector hosts several gorilla families that range into the upper forest belt, and trekking to find them in this zone provides a visually and experientially distinct environment from the lower forest of Buhoma or Nkuringo.
The birdlife of the upper Bwindi zone includes several Albertine Rift endemic species that are specifically associated with the hagenia-hypericum habitat. Rwenzori colobus (black-and-white colobus adapted to the highland forest) range through the upper canopy. The Grauer’s broadbill, Handsome francolin, and Kivu ground thrush are among the bird species that reach their highest densities in this upper forest zone. The Ruhija area road, which traverses hagenia-hypericum habitat for several kilometres, is one of the most productive single birding transects in Uganda and can produce 15–20 specialist highland species in a morning walk.
Gorilla use of the upper forest zone
Mountain gorillas in Bwindi range into the hagenia-hypericum zone seasonally, with the timing linked primarily to the availability of specific food resources. The bamboo shoot emergence in the bamboo zone — which typically borders the hagenia-hypericum zone at higher elevations — attracts gorilla families that make extended forays into the upper forest during the brief period when the protein-rich shoots are available. The hagenia zone itself provides bark, epiphytic plants, and fruit from hagenia and hypericum that supplement the gorilla diet during periods when lower forest resources are less abundant.
Trekking to gorillas in the upper forest zone is physically more demanding than lower-altitude treks because the increased elevation (the upper Bwindi zone reaches 2,400–2,600 metres) reduces oxygen availability and because the vegetation structure — dense hypericum scrub and moss-covered hagenia forest floor — creates challenging movement conditions. The reward is a gorilla encounter in a visually dramatic environment that is completely different in character from the closed lower forest, with the additional benefit of spectacular views over the forest canopy to the valley systems and, on clear days, toward the Rwenzori Mountains to the north.
