A palaeoreconstruction of late holocene fire history and vegetation dynamics from a savanna ecosystem at Mapungubwe National Park, Limpopo Valley, South Africa.

Abstract

Palaeoenvironmental studies facilitate the understanding of palaeoclimatic trends that have been difficult to understand within the African context. Multi-proxy climatic reconstructions are spatially and temporally limited in southern Africa due to the shortage of continuous, chronological and high-resolution sequences and the scarcity of suitable terrestrial archives such as wetlands, peatlands and lakes; alternative archives such as swamps, isolated pans and springs have been used in savanna ecosystems. Due to the ecological importance of savannas, there is a need to understand long-term ecosystem processes in these environments, particularly in southern African savannas where there is a lack of records representing the late Holocene, Mapungubwe National Park (MNP) located in the lower Limpopo Valley represents one such site. Situated within the savanna biome and characterised by Colophospermum mopane Bushveld; the UNESCO World Heritage Site (inscribed in 2003) is of biodiversity, cultural and archaeological signsificance. A palaeoreconstruction of MNP was conducted using a multi-proxy fossil based approach. Pollen, charcoal analysis and radiocarbon dating were applied to determine past vegetation change and fire history. Two sites were analysed to infer palaeoenvironmental dynamics at MNP, Croc Pan (CP) and Leokwe Rock Pool (LRP). The AMS radiocarbon dating indicated that the sediments represented young sediments falling within the very late Holocene epoch, with the CP and LRP sequences dating back to ca.1700 cal. AD and ca. 1900 cal. AD, respectively. However; chronological uncertainties were acknowledged due to the limitations of dating young records. Analyses show that the ca. 1700-1865 cal. AD period in the MNP record was broadly contemporaneous with the Little Ice Age (LIA; ca. 1500-1800 AD); declines in arid woodland savanna taxa and Cyperaceae indicated cool, dry conditions; approaching ca. 1800 cal. AD warm wet conditions persisted. The charcoal signal indicated a high frequency of regional fire activity. Between ca. 1865-1945 cal. AD, the charcoal record continued to reflect a regional fire signal; declines in aquatics, riparian and warm savanna taxa at the beginning of this period implied reduced moisture availability however, peaks in Cyperaceae suggested incidents of increased local moisture and changes in local hydrological conditions. The reduction in Cyperaceae and pteridophytes between ca. 1945-1960 cal. AD reflected drying conditions whilst reduced microscopic and macroscopic charcoal suggested inconsistent local and regional fire activity. Between ca. 1960-1985 cal. AD, a transition from warm, dry conditions indicated by increases in Euphorbiaceae undiff. and warm savanna woodland elements to warm, wetter local conditions marked by sharp peak in Cyperaceae was observed. The influx of macroscopic charcoal during this period is suggestive of more frequent local fire activity. Evidence of warm woodland vegetation such as Combretaceae undiff., Capparaceae undiff. and Fabaceae undiff. accompanied by the inconsistent occurrence of Cyperaceae allude to warm temperature and variable moisture conditions from 1985 to present. The regular increase of macroscopic charcoal is suggestive of an increased frequency of local fires, coincidently, this period of the record overlaps with the known fire regime for MNP. The increase in macroscopic charcoal towards the end of the Mapungubwe record can also be associated with human activity. The pollen records for MNP reflects that the region has generally been characterised by warmer temperatures and drier conditions during the late Holocene; the vegetation dynamics of MNP have not been directly influenced by fire activity as the record generally indicates regional fire activity. These results show similar trends to other savanna ecosystems in the Limpopo Valley, excluding sites with grassland traits. Pollen and charcoal signals from the MNP records have improved the understanding of tree-grass coexistence and the impact of fire on the dynamics of the savanna ecosystem in the Limpopo Valley. This record has therefore contributed to a greater understanding of late Holocene vegetation change and the fire history of the greater Limpopo Valley.