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

Other literature type English OPEN
Flores, O. ; Gritti, E. S. ; Jolly, D. (2009)

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