Investigating C3/C4 plants competition using carbon isotopes and optimality principles

Understanding the mechanisms underlying changes in carbon isotope discrimination (Δ¹³C) in C₃ and C₄ plants is critical for predicting the C₃/C₄ fraction in mixed ecosystems. Variations in Δ¹³C are closely related to changes in the stomatal limitation of photosynthesis (i.e. the ratio of leaf internal to ambient partial pressure of CO₂, c_i/c_a), which are in turn determined by environmental variables, but also depend on the pathway of carbon assimilation. For instance, isotopic fractionation during the diffusion of CO₂ through the stomata primarily influences Δ¹³C in C₄ plants, while fractionation during Rubisco carboxylation has a stronger imprint on Δ¹³C in C₃ plants. As a result, C₃ plants are depleted in ¹³C compared to C₄ plants. Isotopic measurements can thus be used as tracers of physiological processes in plants.

Here we implement Δ¹³C formulations for C₃ and C₄ plants in the optimal P model to investigate the abundance of C₃ and C₄ plants at different locations across the globe. We first test model predictions of Δ¹³C (and hence c_i/c_a) for the two carbon pathways against a large network of isotopic measurements from leaves. We then predict the expected mean Δ¹³C in soil organic materials after plants decomposition using maps of C₃/C₄ plants distribution and assess model predictions with real isotopic measurements. Based on our results, we propose a model to predict the competition of C₃/C₄ plants as a response to environmental variations in different ecosystems.