C₄ photosynthesis and climate through the lens of optimality

CO₂, temperature, water availability, and light intensity were all potential selective pressures that determined the competitive advantage and expansion of the C₄ photosynthetic carbon-concentrating mechanism over the last ∼30 My. To tease apart how selective pressures varied along the ecological trajectory of C₄ expansion and dominance, we coupled hydraulics to photosynthesis models while optimizing photosynthesis over stomatal resistance and leaf/fine-root allocation. We further examined the importance of nitrogen reallocation from the dark to the light reactions. We show here that the primary selective pressures favoring C₄ dominance changed through the course of C₄ evolution. The higher stomatal resistance and leaf-to-root ratios enabled by C₄ led to an advantage without any initial difference in hydraulic properties. We further predict a reorganization of the hydraulic system leading to higher turgor-loss points and possibly lower hydraulic conductance. Selection on nitrogen reallocation varied with CO₂ concentration. Through paleoclimate model simulations, we find that water limitation was the primary driver for a C₄ advantage, with atmospheric CO₂ as high as 600 ppm, thus confirming molecular-based estimates for C₄ evolution in the Oligocene. Under these high-CO₂ conditions, nitrogen reallocation was necessary. Low CO₂ and high light, but not nitrogen reallocation, were the primary drivers for the mid- to late-Miocene global expansion of C₄ We also predicted the timing and spatial distribution for origins of C₄ ecological dominance. The predicted origins are broadly consistent with prior estimates, but expand upon them to include a center of origin in northwest Africa and a Miocene-long origin in Australia.