An atmospheric oxygen concentration above 13% allows lightning to create wildfires. The rise in O2 was the handicraft of plants: their exhaust and increasing pile of biomass created the conditions for wildfire to become a significant feature of terrestrial ecology.
Fire and the vegetation that feeds it, in concert with the atmospheric CO2 and O2 cycles, developed into a feedback mechanism regulating the oxygen level. Besides destroying vegetation, fire alters soil conditions, which negatively affects plant growth in the short term.
Depending upon aridity, an atmospheric oxygen level beyond 25% can inspire widespread wildfires. Moist air suppresses combustive tendencies, albeit only to a degree.
Combustion has much the same role as respiration in carbon cycling, though, of course, fire is faster and the energy produced is almost entirely heat. Fire creates charcoal, which results in a much higher rate of carbon burial than otherwise.
The Late Palaeozoic era – the Carboniferous and Permian periods – was a time of high humidity, with lush vegetation. Oxygen levels reached over 35%. Wildfires raged.
The Carboniferous coincided with the evolution of conifers: plants adapted to a fire ecology. Fire resistance included thicker bark, deeper embedding of vascular tissue, and sheathes of fibrous roots surrounding the stem.
Numerous seed-bearing plants, such as species of pine, only germinate after a forest fire cracks their seed coats. This is a strategic withholding, so that new plants germinate only after an open habitat becomes available.
Fires did not ravage forests worldwide owing to the evolution of fire-resistant traits and the swampy biome covering vast tracts of land. The humid coolness of the Carboniferous also helped keep fires in check.
Geology seemed to be in cahoots with other changes favorable to vegetation. Continental drift altered global weather patterns and climate, rendering them more conducive to plant growth.