Climate and CO2 modulate the C3/C4 balance and δ13C signal in simulated vegetation

Climate and atmospheric CO2 effects on the balance between C-3 and C-4 plants have received conflicting interpretations based on the analysis of carbon isotopic fractionation (delta C-13) in sediments. But, climate and CO2 effects on the C-3/C-4 balance and delta C-13 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 CO2 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 delta C-13 signal were primarily sensitive to temperature and CO2 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 delta C-13 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-delta C-13 signal is used to reconstruct the C-3/C-4 balance.