Dynamics of montane forest in central Africa during the late Holocene: a pollen-based record from western Uganda

According to the results from a number of palaeoecology-based studies, montane forest appears to have been present across a broad range of altitudes in western Uganda throughout much of the Holocene (before increased levels of human impact over the past 2000 years or so resulted in extensive clearance). Critically for pollen-based studies of vegetation history, clearance has limited investigations into the late-Holocene history of montane forest, and thus the impact of small amplitude, sub-Milankovich variations in climate and low levels of human activity. Mubwindi Swamp in the Rukiga Highlands of southwest Uganda is today surrounded by dense stands of moist lower montane forest. The uppermost 5 m of organic-rich deposits from Mubwindi Swamp are dated to the last 2100 years, and thus provide an opportunity, so far unique for Uganda, to record variations in the composition of lower montane forest 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. A transition to a more open, and possibly drier, form of forest is apparent from around 710 BP. It seems likely that this transition was at least partly driven by climate. There is an increased occurrence of pollen from plants presently associated with areas of degraded forest from around 200 BP and a decrease in pollen from important sources of timber. These changes 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.