The impact of including export limitation in photosynthetic models from leaf to global scales

Photosynthesis is the largest gross carbon flux between Earth's terrestrial surface and the atmosphere, enabling the sequestration of approximately one-third of anthropogenic carbon dioxide (CO2) emissions. Models of leaf-level photosynthesis provide the foundation for estimating CO2 uptake by plants from leaf to global scales, yet there is considerable uncertainty associated with these estimates. Currently, photosynthesis is thought to be limited by either electron transport or carboxylation. As atmospheric CO2 rises, a third limitation, export limitation caused by triose phosphate utilization, is possible, but its implications have received little attention despite its potential importance under future climate conditions. Here, we explore the uncertainty around the response of photosynthesis to CO2 under future climate by examining the impact of including representations of export limitation in both leaf-level and land surface models. We find that under most current environmental conditions, export limitation does not alter A/Ci curve fitting estimates. However, we find that export limitation does limit photosynthesis estimates from leaf and global-scale models. Export limitation is most prevalent under high CO2, low temperatures, and high light, reducing global ecosystem carbon simulated by the Community Land Model (CLM) by 5-9 Pg by 2100, most notably in high latitudes. Overall, our results suggest that the current representation of export limitation in models limits photosynthesis and may become a more important photosynthetic constraint in estimates of future carbon cycling.