Phytolith-based environmental reconstruction from an altitudinal gradient in Mpumalanga, South Africa, 10,600 BP–present

Abstract

Studying vegetation change across biome boundaries provides insight into vegetation resilience. In this study, shifts in grassland composition are reconstructed from sediments in three wetland sites across altitudinal gradient from 2128 to 897 m.a.s.l., representing a gradient from the grassland biome to the grassland/savanna boundary in the Mpumalanga region, north-eastern South Africa. Phytolith records from Verloren Valei (dated from 10,600 BP), Graskop (dated from 6500 BP) and Versailles (dated from 4500 BP) are used to reconstruct shifts in grassland composition and vegetation change. Phytolith morphotypes are used to construct environmental indices that are correlated with pollen main ecological groups, charcoal and δ13C and C/N ratio. The results are compared to available regional paleoclimate data. Both Verloren Valei and Graskop have been dominated by grassland, but Versailles show a stronger influence of bushveld/savanna pollen. Phytolith data suggest that grassland composition was stable at Versailles and Graskop, but grassland at Verloren Valei has changed significantly over time. The early Holocene was dominated by a Pooideae/Chloridoideae C3 and C4 grassland, probably a remnant of the earlier Pleistocene cool-dry conditions. After 8500 BP grassland composition changed gradually to a Chloridoideae and Panicoidea dominated C4 grassland BP, and finally a moist Cyperaceae and Panicoidea dominated C3/C4 grassland after 4000 BP. This shift possibly occurs as a delayed response to the warmer and wetter conditions of the mid Holocene optimum at this high altitude site. The results suggest that the grassland/savanna boundary has remained stable over time, indicating considerable resilience of grasslands to climate change. This resilience may be related to the turnover of species within the grassland biome, as indicated by shifts between 8500 and 4000 BP at Verloren Valei.