Bwindi Impenetrable Forest:
conservation importance and vegetation change

Conservation importance

The importance of conserving Bwindi and other forests in western Uganda has been explained by Butynski (1984) and Struhsaker (1987). Detailed comment here is limited to aspects relating to forest plants.

Photo 1. An unidentified Memecylon (Melastomataceae) species.

Although tree species diversity of Bwindi Impenetrable Forest is low compared with high diversity rain forest, it is important not only as a representative of the Afromontane centre of endemism for plants (Photo 1), but also for animals restricted to this habitat (Butynski, 1984; Howard, 1991) (Tables 1 and 2). A 1ha plot surveyed for trees >10 cm dbh (diameter at breast height) in Amazonian rain forest in Peru, for example, contained 275 species, representing 50 families (Peters et al., 1989), compared to only 45-50 tree species >10 cm dbh in 1 ha of Bwindi Impenetrable Forest at 2000-2200 m asl, and only 20 tree species per ha in forest at 2400 m asl (Howard, 1991). Bwindi Impenetrable Forest contains tree genera endemic to Afromontane forest, and many tree species that typify Afromontane rain forest are also represented.

Although Lovoa swynnertonii (Meliaceae) is the only tree species listed as endangered, Bwindi Forest contains a number of tree species not found elsewhere in Uganda, or represented in Uganda only in Kabale-Rukungiri.

Photo 2. Fruit of Allanblackia kimbiliensis (Clusiaceae).

(Although Allanblackia kimbiliensis (Photo 2), Brazzeia longipedicellata, Grewia mildbraedii, Strombosiopsis tetrandra, Maesobotrya floribunda (plus Chrysophyllum pruniforme, which Howard (1991) has since recorded from Budongo and Itwara forests) were thought to be confined to Ishasha Gorge (Hamilton, 1991), this is collecting surveys. Allanblackia probably an artefact of previous plant

Bwindi Forest is a Pleistocene refugium containing not only plants typical of Afromontane forest but also representatives of the Guineo-Congolian flora, such as the secondary forest tree Musanga leo-errerae (Cecropiaceaea), the shrub Agelaea pentagyna (Connaraceae), herbs such as Ataenidia and Marantochloa (Marantaceae) and parasitic plants such as Thonningia sanguinea (Balanophoraceae).

Bwindi Forest is a major catchment area and a source of water to surrounding rural communities, and through Lakes Edward and Mutanda via the Nile to the Mediterranean. It can also provide economic benefit from non-consumptive uses of the forest (e.g. eco-tourism) and consumptivuses. Consumptive uses may be of resources meeting basic needs of the surrounding community (e.g. plant resources) or on a wider scale (e.g. genetic values of wild relatives of crop and forage plants, and chemical structures for new pharmaceuticals).

To facilitate informed decision-making, plant use and forest conservation policy have to be seen against the background influences of climate and human disturbance of forest ecosystems. Both have had a major influence on African vegetation in the past and will continue to do so in the future, perhaps even more so with the effects of global warming and human population increase.

Climate change

Massive oscillations in the Pleistocene climate, caused by expansions and shrinking of the polar ice-caps, resulted in long, cool, dry periods alternating with shorter, warmer, moist periods. Equatorial forests, as indicators of world climatic conditions, are believed to have expanded outwards from, or shrunk into, Pleistocene refugia. Detailed pollen analysis from cores taken in the Rukiga highlands near to Bwindi Forest has provided evidence of vegetation dynamics and climate change over the past 40,000-50,000 years, including forest expansion around 10,600 BP into the Ahakagyezi catchment along the Ishasha river south-east of Bwindi Forest (Taylor, 1990).

During the most recent glacial phase (pre-12,000 BP), forests were restricted to a few refugia, later expanding outwards with moister, warmer conditions (Hamilton, 1981). Hamilton (1981) has stressed the importance of conserving forests which retained forest cover during the earlier arid phase. Bwindi Impenetrable Forest is considered to be one of these refugia in Uganda. The decline of forest tree species-richness across Uganda from west to east is considered to be indicative of this, and forests in western Uganda, particularly those of Bwindi-Kayonza and Bwamba are thus considered to have the highest national conservation priority (Hamilton, 1981; Howard, 1991).

People and vegetation change

Dating of archaeological remains from Matupi Cave in the eastern Ituri Forest, Zaïre, indicates human occupation 32,000-40,700 years ago (Van Noten, 1977). Similar data for the Rukiga highlands are not available, but it is likely that, like the Mbuti hunter-gatherers in the Ituri region, Batwa Pygmy people originally occupied the forests and savanna of south-west Uganda and northern Rwanda.

From their study of Mbuti hunter-gatherer subsistence in the Ituri Forest, Hart and Hart (1986) suggest that it is unlikely that hunter-gatherers would have lived independently in the forest interior, as for five months of the year virtually no nutritionally important wild edible plants are available, honey is not abundant and, although game meat is available, it has a low fat content. From field observation, the density and species abundance of edible wild plants (the principal species being Myrianthus holstii (fruits) and Dioscorea spp. (ebikwa) tubers) appear to be even lower than in the Zaïre lowland forest studied by Hart and Hart (1986). None of the Guineo- Congolian zone edible fruit trees (e.g. Irvingia, Ricinodendron heudelotii) that are major food sources to Mbuti people occur in Bwindi Forest. It appears likely therefore that Batwa subsistence would have been dependent on plant and animal resources of savanna and wetlands, in addition to those of forest.

However, Batwa hunter-gatherers may have manipulated forest and savanna vegetation. Although there is no direct evidence from the Rukiga highlands on this, it may be that Batwa hunter-gatherers achieved this through the use of fire. Fire would have been used seasonally in forest during honey hunting and possibly in savanna to attract game.

Fire could also have been used as a tool in forest during dry periods, to create disturbance and stimulate production of Dioscorea tubers. Dioscorea climbers are most commonly found in secondary forest or forest margins (Hart and Hart, 1986; this study). Hunter-gatherers in southern Africa, for example, use fire as a tool to increase below-ground production of edible Iridaceae corms (Deacon, 1983). "Fire-stick farming" is also thought to have been used by hunter-gatherers in forests in New Guinea for edible resources, including yams (Dioscorea spp.) (Groube, 1989). Dioscorea tubers are thought to have been a major food resource of Mbuti Pygmy peoples in the past (Tanno, 1981). In the Ituri Forest, Mbuti Pygmy population density was approximately 1 person per km². The hunter-gatherer population density in the Rukiga highlands in the past is unknown, but was probably no higher than this. It would be expected therefore that with low human densities, their impact on vegetation would have been localized and subtle compared to the clearing of forest by agriculturists. Butynski (1984) estimated that the Batwa Pygmy people accounted for less than 0.5% of the total population. This would be consistent with a population density of (less than) 1 person per km² today.

Photo 3. Iron smelting technology introduced into the Rukiga highlands c. 2000 yr ago remains essentially unchanged today by blacksmiths (omuhesi) in this area. Wooden bellows (omuzuba), made from Polyscias fulva (omungo) wood and the clay tuyère (encheru).

Pollen analysis has not only provided evidence of shifting forest cover in response to climate change over the past 40,000-50,000 years, but also of the clearing of forests in the Rukiga highlands. Although previously thought to have started before about 4800 years ago (Hamilton et al., 1989), a reassessment of core material suggests that clearing took place after about 2200 years ago (Taylor, 1990), coinciding with the influx of Bantu-speaking agriculturists with iron-smelting technology (Van Noten, 1979) (Photo 3). Farming was probably based on finger millet (Eleusine coracana), sorghum (Sorghum sp.) and possibly cow-peas (Vigna sp.) and pigeon peas (Cajanus cajan) which originate from the Horn of Africa. Agriculture was established in Rwanda (and probably the Rukiga highlands) by about 2000 years ago, resulting in more permanent settlements and concentrating the effects of human occupation on the surrounding vegetation due to burning, clearing and cutting of fuel (for iron-smelting (Photo 4) and household use and other purposes).

This would also have stimulated trade in forest products (e.g. bush meat) for cultivated starches between Batwa and Bakiga. Since then, a wider range of crops has gradually been introduced from Central and South America (sweet potatoes, tomatoes, cassava, pineapples, chilli peppers, groundnuts, potatoes and tobacco), south-east Asia (bananas, sugarcane), south-central China and northern India (tea), the near East (peas), Central Asia (carrots) and the Mediterranean (cabbages). Agricultural production occupies 83% of the population of Uganda, accounts for nearly all export earnings, and contributes 60% to GDP (World Bank, 1986).

Photo 4. Polyscias fulva (omungo) tree (Araliaceae) favoured for wooden bellows (omuzuba) used in traditional iron-smelting technology.

In 1921, the Ugandan population was 3 million people (Howard, 1991), and by the year 2000 it is projected to be 23.8 million (Bulatao et al., 1990). At the same time, the area of forest that formerly would have been used for harvesting of plant resources has rapidly decreased, due to agricultural clearing and burning. Harvesting intensity therefore concentrates on the remaining vegetation, ultimately focusing on species within core conservation areas.

Extensive transformation of the Rukiga highlands landscape has occurred since the 1900s, due to natural population increase and migration from Rwanda, where population density is 480 people per km² of arable land (Balasubramanian and Egli, 1986). Between 1948 and 1980, the population of the Kabale and Rukungiri districts increased by 90%, from 396,000 to 752,000 (Butynski, 1984). Today, the Rukiga highlands are one of the most densely populated areas of Uganda, with population densities in the DTC area surrounding the forest ranging from 102-320 persons per km² (Figure 2, page 5) (data from an unpublished DTC 1991 census). Intensive agriculture by a high density of rural farmers has resulted in removal of indigenous woody plants, shorter and shorter fallow periods and a reduction in species diversity. Situated in one of the most densely populated areas of Uganda, with a 115 km long boundary surrounded by almost 100,000 people (Anon, 1992) Bwindi Impenetrable National Park became an island in a sea of rural farmers, gold-miners and pit-sawyers (Photos 5-8).

Photo 5. Bwindi Forest, intact apart from disturbance due to tree falls (canopy gaps) and fire.			Photo 6. The impact of farming on forest: fields in what was forest in 1950.
Photo 7. Species-selective over-exploitation and gap formation: pitsawing.			Photo 8. Disturbance to river valley forest due to illegal panning of alluvial gold.