Increased fire activity under high atmospheric oxygen concentrations is compatible with the presence of forests

Throughout Earth's history, the abundance of oxygen in our atmosphere has varied, but by how much remains debated. Previously, an upper limit for atmospheric oxygen has been bounded by assumptions made regarding the fire window: atmospheric oxygen concentrations higher than 30-40% would threaten the regeneration of forests in the present world. Here we have tested these assumptions by adapting a Dynamic Global Vegetation Model to run over high atmospheric oxygen concentrations. Our results show that whilst global tree cover is significantly reduced under high O-2 concentrations, forests persist in the wettest parts of the low and high latitudes and fire is more dependent on fuel moisture than O-2 levels. This implies that the effect of fire on suppressing global vegetation under high O-2 may be lower than previously assumed and questions our understanding of the mechanisms involved in regulating the abundance of oxygen in our atmosphere, with moisture as a potentially important factor. This study shows that fire activity under high atmospheric oxygen concentrations does not remove or prevent regeneration of present-day global forests, contradicting a long-term assumption used to define the upper limit of oxygen through time.