Global C4 distribution estimate constrained by observations and optimality theory

Abstract Photosynthesis of C 4 plants responds to climate change differently than the more common C 3 plants, due to their unique anatomic and biochemical characteristics. The different response is expected to cause a change in global C 4 distribution, however, current C 4 distribution models are inadequate to predict that as they are based on a temperature-only hypothesis and lack observational constraints. Here, we used a global database of photosynthetic pathways, satellite observations and a photosynthetic optimality theory to produce a new observation-constrained estimate of C 4 distribution. We found that global C 4 coverage stabilized at 11.2% of the vegetated land surface during 1992 to 2016, as a net effect of C 4 grass decrease due to elevated CO 2 and C 4 crop increase, mainly from maize expansion. Using an emergent constraint approach, we estimated that C 4 contributed 12.5% of global photosynthetic carbon assimilation, a value much lower than previous estimates (~ 20%) but more in line with the mean of an ensemble of dynamic global vegetation models (14 ± 13%). By improving the understanding of recent global C 4 dynamics, our study sheds insight on the critical and previously underappreciated role of C 4 plants in modulating the global carbon cycle in recent history.