Late Quarternary montane vegetation dynamics in Bwindi-impenetrable forest, Central Africa
Mubwindi Swamp in the Rukiga Highlands of south west Uganda is today surrounded by dense stands of moist lower montane forest. A survey of the present-day vegetation combined with an investigation of the surface pollen spectra indicate pollen accumulating on the swamp surface mainly result from taxa within the catchment. This survey also highlights that a combination of climatic, edaphic, demographic and human factors control the present-day composition and distribution of montane forest. Mubwindi Swamp comprises deep accumulations of peat-rich deposits. Pollen data from three cores of sediment provide a partial record of montane vegetation history since at least 43,000 yr B.P. This record is broken by at leat two, and possibly three, sedimentary hiati. Twelve radiocarbon ages, charcoal data from two cores using combination of three methods, and investigations into particle size characteristics place this record of montane vegetation dynamics within a wider palaeoenvironmental debate concerning Pleistocene climates, forest refugia, and human induced impacts on montane forests. Pollen deposited before and soon after the last glacial maximum represents a vegetation composition very different to that of the modern vegetation within the Mubwindi Swamp catchment. Specifically there were increased amounts of Artemisia, Ericaceae, Faurea and Stoebe. However, some elements found within the modern vegetation persisted during these periods, these include Ilex, Olea, Podocarpus and Zanthoxylum. These taxa may have persisted possibly because of favourable topography or soils. The uppermost 5m of deposits in core MB6 from the site are dated to the last 2100 years, and thus provide an opportunity, so far unique for Uganda, and much of central Africa, to record variations in the composition of montane vegetation during the late Holocene. Results of analyses of pollen indicate that lower montane forest has been present in the catchment for Mubwindi Swamp throughout the last 2100 years, despite extensive clearance in other parts of the region over the same period. A transition to a more open - and possibly drier - form of forest is apparent from around 680 yr B.P. This montane forest composition is thought to stem from a recent period of climatic change to less humid conditions. This change is followed by an increased occurrence of pollen from plants presently associated with areas of degraded forest from around 200 yr B.P. and a decrease in pollen from important sources of timber. Some recovery of timber trees is apparent from around 50 yr B.P. Changes in forest composition around 200 years ago could be interpreted as representing slightly increased humidity. However, in view of the nature of these changes, a low level of human impact was probably the most important causal factor. The gazetting of Bwindi-Impenetrable Forest as a Forest Reserve in the early 1930s may have facilitated a recovery of timber trees during the present century.