Global C4 distribution estimate constrained by observations and optimality theory

Abstract

Abstract Photosynthesis of C4 plants responds to climate change differently than the more common C3 plants, due to their unique anatomic and biochemical characteristics. The different response is expected to cause a change in global C4 distribution, however, current C4 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 C4 distribution. We found that global C4 coverage stabilized at 11.2% of the vegetated land surface during 1992 to 2016, as a net effect of C4 grass decrease due to elevated CO2 and C4 crop increase, mainly from maize expansion. Using an emergent constraint approach, we estimated that C4 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 C4 dynamics, our study sheds insight on the critical and previously underappreciated role of C4 plants in modulating the global carbon cycle in recent history.