Combined effects of photorespiration and fire strongly regulate atmospheric oxygen levels.

Atmospheric oxygen concentrations have remained remarkably stable over the past ~400 million years (Myr), suggesting the presence of robust regulatory mechanisms. Because of its sensitivity to oxygen, wildfire was traditionally assumed to control oxygen levels by limiting terrestrial vegetation; however, this feedback is nullified by high moisture levels in tropical ecosystems. Using vegetation modeling, we show that where oxygen-fire effects are dampened by high moisture, photorespiration becomes more effective through increased temperatures. Together, these processes interact to drive an 86% reduction in modeled global biomass when oxygen levels reach 35%. This coregulation imposes substantially tighter control of atmospheric oxygen than wildfire alone, providing previously unknown insights into the spatial and interactive feedbacks that may explain the remarkable stability of oxygen levels since the evolution of forests.