Vegetation and climate dynamics during the last glacial period in the fynbos-afrotemperate forest ecotone, southern Cape, South Africa

Abstract

Despite the southern Cape's great climatic and botanical significance (occupying the transition between the temperate and subtropical circulation systems and forming part of a global biodiversity hotspot), palaeoenvironmental data for this region of southern Africa is limited. This study presents pollen, charcoal and sedimentological data preserved in the Vankervelsvlei wetland, situated in the modern year-round rainfall zone at the ecotone between the Fynbos and Afrotemperate Forest biomes. Combining optically stimulated luminescence and radiocarbon dating techniques, it was possible to establish a chronology for a sediment sequence spanning the last 140,000 years, the longest record yet produced in the region. The data suggest that MIS 5d was relatively warmer (low fynbos pollen percentages and Pentzia-type pollen) than later MIS 5, MIS 4 and most of MIS 3 (∼96–37 ka), which were characterised by decreased temperatures (dominance of ericaceous fynbos). The pollen data indicate a complex response to the change from interglacial to glacial conditions, and suggest an important threshold is crossed in regional ecological dynamics. We postulate that during MIS 5d increased summer rainfall under warmer conditions may have offset increased potential evapotranspiration, allowing for the development of more extensive forests. During its early stages of development Vankervelsvlei was more open (increased aquatics and coarse sediment), trapping more longer-distance pollen (Podocarpus). As the mire became more closed, local elements dominated; a succession that is reflected in significant changes in the pollen assemblage, as Podocarpus remains only in trace percentages, but pollen of Canthium and Morella, which occupy nearly identical climatic niches as Podocarpus, increase in abundance. It is suggested that drought stress remains limited during the last glacial period as a result of reduced temperatures, compensating for what may have been a more seasonal winter-dominated rainfall regime, and that changes in the pollen record relate to vegetation succession and the development of the wetland rather than to major changes in moisture availability. Due to the virtual absence of palaeodata from the southern Cape covering MIS 5 to MIS 3, the establishment of this record provides an important contribution to the overall palaeoenvironmental history of the region.