AFROALPINE VEGETATIONAL CHANGE IN THE RWENZORI MOUNTAINS: A NEW PLEISTOCENE-HOLOCENE POLLEN RECORD FROM LAKE AFRICA

Afroalpine ecosystems, scattered throughout the highlands of East Africa, host a unique and potentially threatened array of plant life. Due to their geographical isolation, Afroalpine ecosystems are expected to shrink in the face of increasing disturbance from changing climate. Although the cool, wet conditions in these areas mean that natural fires should be rare, recent increases in wildfire associated with landslides and flooding have imposed serious geohazards on local communities. While changing temperature, rainfall, and human activity may play a role in this environmental change, little information exists to understand this relationship. Elucidating how Afroalpine plant communities have responded to past climatic changes during previous warm intervals, such as the middle Holocene (8-5 ka), can help inform effective conservation measures in light of future climate change. To reconstruct past Afroalpine vegetation change, pollen analysis was conducted on a 13.3 kyr sediment core from Lake Africa (3895 m elevation) in the Rwenzori Mountains, Uganda. Pollen assemblages from the early Holocene show lower abundances of modern Afroalpine pollen types than late Holocene assemblages with at least one prominent modern taxon, Alchemilla, completely absent. Instead, Afromontane forest pollen dominated throughout the cool but wet early Holocene and into the wet mid-Holocene up to 5 ka despite warming. There is a gradual decrease in the relative abundance of pollen associated with lower Afromontane forests between 5 and 3.5 ka coincident with both cooling and drying trends observed from this time in East Africa. The modern Afroalpine vegetation community was established by 3 ka and remained relatively stable until today. The higher influence of Afromontane forest pollen during both the warm and humid mid-Holocene and cooler but humid early Holocene suggests that while warmer temperatures may be partly responsible for allowing mountain vegetative belts to migrate upwards, precipitation also likely plays a key role in determining Afroalpine vegetation distribution. These results indicate that the composition of the Afroalpine flora is likely to be modified in the face of expected future climatic disturbance, and that the lower boundary of the Afroalpine zone is likely to shift upwards in response to warming and drying.