A Model of C <sub>4</sub> Photosynthetic Acclimation Based on Least‐Cost Optimality Theory Suitable for Earth System Model Incorporation
Helen G. Scott, Nicholas G. Smith
Abstract
Abstract Empirical studies have shown that plant photosynthetic responses to environmental change can vary over time due to acclimation, but acclimation responses are often not included in Earth System Models. Photosynthetic least cost theory can be used to develop models of photosynthetic acclimation that are simple and testable. The theory is based on the idea that plants will acclimate to minimize the ratio of carbon costs to photosynthetic assimilation rate (Prentice et al., 2014, https://doi.org/10.1111/ele.12211 ). Formulations of this theory have been developed for C 3 plants, but not C 4 plants, which account for over 20% of global photosynthesis and are over‐represented among widely grown crops. Here, we use photosynthetic least cost theory to derive a model for C 4 photosynthetic acclimation to above‐ground abiotic conditions. We then compare our model's responses to a similar model of C 3 photosynthetic acclimation and find that C 4 photosynthesis has the highest simulated advantage over C 3 photosynthesis in hot, dry, and low CO 2 environments. We find that this advantage predicts C 4 abundance globally, but that the shallower CO 2 response of C 4 as compared to C 3 photosynthesis will reduce C 4 plant competitiveness under future conditions, despite higher temperatures. We also show that an acclimated model predicts similar or faster rates of C 4 under all conditions than a model that does not consider acclimation, suggesting that Earth System Models (ESMs) are underestimating future C 4 carbon uptake by not including acclimation. Our model is designed for easy incorporation into such ESMs.