Environmental change in montane southwest Uganda: a pollen record for the Holocene from Ahakagyezi Swamp

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Abstract
The environmental history of montane southwest Uganda over the last c. 12 300 years is described using data from a 22.84-m long core of sediment from Ahakagyezi Swamp, Rukiga Highlands. These data include plant microfossils and macrofossils, charcoal and nine radiocarbon dates. According to evidence from plant fossils and radiocarbon dates, vegetation in the Ahakagyezi catchment was very different to the expected climax today until c. 10500 BP. The surface of the swamp was dominated by sedges, whilst the adjacent hillsides were largely devoid of trees. Changes in pollen suggest that a form of dry montane scrub was replaced by moist, lower montane forest from c. 11000BP. Subsequent fluctuations in pollen indicate that, once established, the composition of moist, lower montane forest varied during the Holocene and that unreversed forest reduction commenced in the Ahakagyezi catchment at c. 800 BP. On the surface of the swamp itself Syzygium-dominated swamp forest increased in extent from c. 5800 BP, and more rapidly from c. 3100 BP, before beginning a decline c. 1300 BP. The most likely reason for the presence of montane scrub in the catchment, rather than moist lower montane forest, prior to c. 11000 BP (i.e., a period that is believed to precede agricultural activity in the region) is a climate somewhat drier than the present. The spread of moist lower montane forest is believed to indicate the onset of climatic conditions broadly similar to those of today (i.e., the beginning of the Holocene). The causes of Holocene changes in forest composition and extent are most probably due to one or a combination of the following factors: natural succession; regional climate change; and the impact of human activity.