Climate of the Past www.clim-past.net 1814-9324 1814-9332 5 3 2009 10.5194/cp-5-431-2009 http://www.clim-past.net/5/431/2009/ http://www.clim-past.net/5/431/2009/cp-5-431-2009.html http://www.clim-past.net/5/431/2009/cp-5-431-2009.pdf 431 440 2009-08-07 Climate and CO₂ modulate the C₃/C₄ balance and &delta;¹³C signal in simulated vegetation O. Flores olivierflores@free.fr E. S. Gritti D. Jolly CEFE, UMR 5175 CNRS, 1919, route de Mende, 34293, Montpellier cedex 5, France ISEM, UMR 5554 CNRS/Univ. Montpellier II, Case 61, 34095 Montpellier cedex 5, France Climate and atmospheric CO₂ effects on the balance between C₃ and C₄ plants have received conflicting interpretations based on the analysis of carbon isotopic fractionation (&delta;¹³C) in sediments. But, climate and CO₂ effects on the C₃/C₄ balance and &delta;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 CO₂ atmospheric concentration (<i>p</i>CO₂) in two sites in which vegetation changed oppositely, with respect to C₃ and C₄ plants abundance, during the Last Glacial Maximum to Holocene transition. The C₃/C₄ balance and &delta;¹³C 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 C₄ plants in case of large negative deviation in rainfall. Although a global parameter, <i>p</i>CO₂ affected the &delta;¹³C signal differently from one site to the other because of its effects on the C₃/C₄ balance and on carbon isotopic fractionation in C₃ and C₄ plants. Simulated Plant functional types (PFT) also differed in their composition and response from one site to the other. The C₃/C₄ balance involved different competing C₃ and C₄ PFT, and not homogeneous C₃ and C₄ poles as often assumed. 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