The Franks Model CO ₂ Proxy: Do Assimilation Rate Values or Isotopic Corrections Optimize CO ₂ Reconstruction Accuracy?

Abstract Paleo‐CO 2 proxies are crucial for reconstructing past atmospheric CO 2 variations and understanding their impact on Earth's climate, especially in light of anthropogenic climate change. The mechanistic leaf gas exchange model of Franks et al. (2014), https://doi.org/10.1002/2014gl060457 utilizes the relationship between atmospheric CO 2 and chemical and morphological fossil leaf traits for CO 2 reconstructions. While the model has shown promising results, its applicability to different plant groups remains unclear, particularly whether there are consistent offsets in CO 2 estimates among major plant clades. In this study, we evaluate the performance of the model across different phylogenetic groups (ferns, gymnosperms, and angiosperms), with generic input values and recommended adjustments to isotopic measurement values, assimilation rates and the scaling of maximum to operational stomatal conductance, by applying the Franks model to living plants. Results show generally accurate CO 2 estimates, but high interspecies variations with generic values; a clade‐level bias is therefore unlikely. Consequentially, the corrections to isotopic measurement values to remove a phylogenetic effect do not show improvements across the three plant groups. Adjustments to the assimilation rate based on phylogeny, growth form and solar exposure result in a decrease in variation and an improvement in CO 2 estimates. While the minor change in stomatal conductance scaling values for woody angiosperms results in limited improvements in accuracy, precision decreases due to higher measurement uncertainty. Overall, the results of this study suggest that the Franks model has a high potential for accurately reconstructing past atmospheric CO 2 within a multitaxon study, especially with careful choice of assimilation rate values.