"On average", no forest produces or consumes any CO2 or O2 unless the biomass is somehow sequestered, for example buried under volcanic ash or on the bottom of anoxic lake which would typically prevent it from decomposing.
The forest binds some amount of carbon and oxygen for a time being but then it is released back when it burns or decomposes, etc. It is really insignificant when compared with steady flow of carbon from fossil fuels.
That isn't completely true. A well managed forest will sequester some carbon when it burns. Note well managed in there, that means a small fire every year, a large fire gets hot enough to burn all the carbon, but a small fire will burn only part of the carbon and leave the rest as charcoal. Moral: shoot smokey the bear and let foresters start the fires they want to if only they were allowed to.
Of course the above is a generalization, and as all of them false in some way. Ask your local forester what applies to the forest in your area, but don't try to apply it to a forest in the next neighborhood as that might be different.
To sequester carbon it would have to be prevented from decomposing. It is not enough for the trees to turn to charcoal during fire. If what you said was true "well managed" forest would be standing atop of layers of charcoal which obviously is not true.
Instead what I suspect happens is that the charcoal from fires weathers, crumbles and becomes part of the soil. Soil is feed for other organisms.
Anecdotal evidence suggests to me that European spruce stores more CO2 than it sheds way over 80 years old as a spruce grows in mass significantly decades after reaching that age.
Anecdotal evidence tells me that spruce trees are really good at suppressing undergrowth due to their acidic needless so claiming a mature first is a net emitter makes sense to me. The trees may be growing, but nothing else is.
That’s not true. Methane is a significantly more potent greenhouse gas than CO2, so if the tree is absorbing CO2 but the decomposing needles and leaves are emitting methane, it could easily be a net GHG emitter.
If a tree keeps growing, but also gets more successful at suppressing growth of it's neighbors, then it can be a net emitter overall even though it's a net absorber considered individually.
No, if anything individual trees do start to become emitter as they age due to low growth and decay, how ever in forests old dying trees are quickly replaced by new ones.
