There are multiple things involved here. If it were possible to harvest trees similar to the natural way forests replenish, with the canopy dotted with spots of old trees, which die making way for under-story seedlings to replace them, the harvesting would not need to be an issue.
Hence, you would want to harvest the large trees within a 100 foot square with a minimum of 400 feet between harvests. If the square of circle is not clear cut then it would open up the opportunity for young offspring of the removed tree to regrow.
There is another issue regarding the burning of the wood as well. Most current systems are simple bio-mass feed of chop/pellet/manufacturing sawdust(mill-waste) to the bottom of a furnace like firebox. If instead the waste were burned in a fluidized bed meaning that the mass is suspended in the updraft air feed would be an improvement.
If we went further, such that the firebox was surrounded by the bio-mass as in a gasification system with the final byproduct of charcoal, fed to the fluidized bed the burn would be both more efficient and complete.
Coupled with say 12-15 foot above the ground, 100 foot diameter, harvesting, the saplings would be preserved and the greatest part of the mass would be collected. Done in a patch work with 400 feet separating harvests would be a very smart idea.
At least this source would be fossil fuel free and would allow for no more then a 25% reduction while providing for a 25% supplement to the power utility feed stocks...
If this were employed via commercial/industrial farming techniques with 5 year rotations.(Meaning every 5 years you could go back in and harvest the next 100 foot plot, would mean that every plot would be about 25 years old when harvested and the youngest stock would be no less then 5 years. By the same token since you would return to the zone of the old harvest, if there were a problem it could be resolved with something like an air drop reseeding.)
The best part is if the harvesting was done via a blimp/zeppelin platform then the ground does not need to be disturbed. Done properly this is a good alternative. The desire would be to employ biomass digestion, the harvesting of methane and then the gasification of the remains. Providing a two part solution with one portion going to transportation and the second part going to power generation would be a very effective solution set and could be applied to all bio-mass sources.