Mapping the global distribution of C4 vegetation using observations and optimality theory

Plants with the C₄ photosynthesis pathway typically respond to climate change differently than more common C₃-type plants, due to their distinct anatomical and biochemical characteristics. The different responses are expected to drive changes in global C₄ and C₃ distributions. However, current C₄ distribution models may not predict this response as they do not capture multiple interacting factors and in many cases lack observational constraints. Here, we used a global database of plant photosynthetic pathways, satellite observations, and photosynthetic optimality theory to produce a new observation-constrained global estimate of C₄ distribution. We found that global C₄ coverage decreased from 17.7% to 17.1% of the land surface during 2001 to 2019, as a net effect of C₄ natural grass decreases due to elevated CO₂ favoring C₃-type photosynthesis, and C₄ crop increases, mainly from corn (maize) expansion. Using an emergent constraint approach, we estimated that C₄ contributed 19.5% of global photosynthetic carbon assimilation, a value within the range of previous estimates (18-23%) but higher than the ensemble mean of dynamic global vegetation models (14 ± 13%). By improving the understanding of recent global C₄ cover and productivity dynamics, our study sheds insight on the critical and underappreciated role of C₄ plants in the contemporary global carbon cycle.