“Oxygen and Paleofires.
The level of atmospheric oxygen cannot rise indefinitely unless the frequency of forest fires becomes so excessive that plant life cannot persist. This has been pointed out by Watson et al. (27), who emphasize that fires serve as strong negative feedback against excessive O2 variation. Conversely, O2 cannot have dropped to such low values over Phanerozoic time that fires became impossible. Fossil charcoal, as evidence of paleofires, has been found for all times that trees have populated the land, and the lower limit for the production of charcoal has been estimated to be at about 13% O2 (28). By contrast, the upper limit for O2 is in dispute. On the basis of experiments on the ignition of paper strips at different oxygen levels and fuel moisture contents, Watson et al. (27) concluded that past levels of atmospheric O2 could never have risen above 25%. However, consideration of actual forest fires and the response of ecological disturbance to fires led Robinson (29) to conclude that greater O2 variation might occur and that, at any rate, paper is not a good surrogate for the biosphere. In fact, Robinson states paleobotanical evidence for a higher frequency of fire-resistant plants during the Permo-Carboniferous, supporting the idea of distinctly higher O2 levels at that time. “
http://www.pnas.org/content/96/20/10955.full
Atmospheric oxygen over Phanerozoic time
“On the basis of experiments on the ignition of paper strips at different oxygen levels and fuel moisture contents, Watson et al. (27) concluded that past levels of atmospheric O2 could never have risen above 25%. However, consideration of actual forest fires and the response of ecological disturbance to fires led Robinson (29) to conclude that greater O2 variation might occur and that, at any rate, paper is not a good surrogate for the biosphere.”
Yeah, I would say he was wise to revise his upper limit:
“Analyses of the gases in these bubbles show that the Earth’s atmosphere, 67 million years ago, contained nearly 35 percent oxygen compared to present levels of 21 percent.”
http://geology.com/usgs/amber/