LAUSR

Abstract Humans that inhabited the glacial refugia in the Cape Floristic Region are assumed to have harvested and consumed plant underground storage organs (USOs) as an important carbohydrate source during the Middle Pleistocene. Botha et al. ( 2020 , this issue), and others have demonstrated the rich resource that such plants might offer to early human gatherers. However, such studies do not correct for the likely reduction in plant productivity and USO yields related to much reduced CO2 concentrations typical of Pleistocene glacial periods. To assess the need for such a correction, we grew Oxalis pes-caprae, a species harvested by the human gatherers, at a range of CO2 concentrations from glacial to current. Decreasing CO2 concentrations to glacial levels almost halved the mass of bulbs produced, while biomass was preferentially allocated to roots and shoots, which were not significantly decreased relative to plants grown at current CO2 concentrations. This would have major implications for estimating the potential calorific return to foraging effort, as modelled using anthropological data and assuming that USO yields would be similarly affected. The model suggests that an almost doubled foraging effort would have been required at glacial compared to current conditions. This highlights the need to consider plant growth conditions and particularly atmospheric CO2 concentrations when extrapolating current anthropological studies to the past, which could be a major gap in understanding human dietary preferences in glacial conditions.