Yes I am also bothered by the "oxygen myth" and work at dispelling it
every chance I get. As you say the influence of net forest
photosynthesis on atmospheric O2 is trivial. There are about
380,000,000 teramoles (tera =10^12) of O2 in the atmosphere representing
nearly 1/4 of dry air. The small amount produced in global
photosynthesis each year (approx 8,400 teramoles O2 per year) is nearly
balanced by the amount consumed in respiration/decay. The current
atmospheric O2 level accumulated by photosynthesis slightly exceeding
respiration over geologic time. A sudden change in land use, even
catastrophic will have a neglible effect on this large pool of O2.
Additionally, the atmospheric O2 concentration is well "buffered". An
increase in global photosynthesis (or decrease in global respiration)
would increase the atmospheric O2 concentration (slightly) allowing
diffusion of O2 to greater depths in soils and water which, in turn,
would increase the rates of aerobic respiration again lowering the
atmospheric O2 concentration. Small increases in atmospheric O2 would
probably increase the incidence of fires (the atmosphere would be more
flammable!) which would ultimately result in a lowering of the
For more numbers and references I would refer your students to
Schlesinger's book "Biogeochemistry. An analysis of global change"
published by Academic Press.
All the best,