Climate and CO 2 modulate the C 3 /C 4 balance and δ 13 C signal in simulated vegetation

Abstract. Climate and atmospheric CO 2 effects on the balance between C 3 and C 4 plants have received conflicting interpretations based on the analysis of carbon isotopic fractionation (δ 13 C) in sediments. But, climate and CO 2 effects on the C 3 /C 4 balance and δ 13 C signal are rarely addressed together. Here, we use a process-based model (BIOME4) to disentangle these effects. We simulated the vegetation response to climate and CO 2 atmospheric concentration ( p CO 2 ) in two sites in which vegetation changed oppositely, with respect to C 3 and C 4 plants abundance, during the Last Glacial Maximum to Holocene transition. The C 3 /C 4 balance and δ 13 C signal were primarily sensitive to temperature and CO 2 atmospheric partial pressure. The simulated variations were in agreement with patterns observed in palaeorecords. Water limitation favoured C 4 plants in case of large negative deviation in rainfall. Although a global parameter, p CO 2 affected the δ 13 C signal differently from one site to the other because of its effects on the C 3 /C 4 balance and on carbon isotopic fractionation in C 3 and C 4 plants. Simulated Plant functional types (PFT) also differed in their composition and response from one site to the other. The C 3 /C 4 balance involved different competing C 3 and C 4 PFT, and not homogeneous C 3 and C 4 poles as often assumed. Process-based vegetation modelling emphasizes the need to account for multiple factors when a palaeo-δ 13 C signal is used to reconstruct the C 3 /C 4 balance.