Modelling the effects of CO2 on C3 and C4 grass competition during the mid-Pleistocene transition in South Africa

Palaeoenvironmental reconstructions of the interior of South Africa show a wetter environment than today and a non-analogous vegetation structure in the Early Pleistocene. This includes the presence of grasses following both C 3 and C 4 photosynthetic pathways, whereas C 3 grasses decline after the mid-Pleistocene transition (MPT, c. 1.2–0.8 Ma). However, the local terrestrial proxy record cannot distinguish between the potential drivers of these vegetation changes. In this study we show that low glacial CO 2 levels, similar to those at the MPT, lead to the local decline of C 3 grasses under conditions of decreased water availability, using a vegetation model (LPX) driven by Atmosphere–Ocean coupled General Climate Model climate reconstructions. We modelled vegetation for glacial climates under different levels of CO 2 and fire regimes and find evidence that a combination of low CO 2 and changed seasonality is driving the changes in grass cover, whereas fire has little influence on the ratio of C 3 :C 4 grasses. Our results suggest the prevalence of a less vegetated landscape with limited, seasonal water availability, which could potentially explain the much sparser mid-Pleistocene archaeological record in the southern Kalahari.