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

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 ¹³C) in sediments. But, climate and CO ₂ effects on the C ₃/C ₄ balance and δ ¹³C 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 ( pCO 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 ¹³C 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, pCO ₂ affected the δ ¹³C 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 ¹³C signal is used to reconstruct the C ₃/C ₄ balance.