GeoM;
I tend to disagree with your opinion here. Not that I have any hard figures but I have been heating with wood my whole life. I happen to live in hardwood country so we burn a lot of oak, hickory, ash, etc.
We do have a substantial woodlot and with regular, necessary harvesting that keeps the timber healthy I find I can heat with wood on a completely renewable basis from logging waste [tops & culls] combined with deadfalls.
Even after an intense logging of the woodlot where I can see the huge holes in the canopy from harvesting the big guys after removal of a huge amount of biomass within 2 years the canopy has filled in and I believe the biomass is replenished in a couple years. I heat pretty much exclusively with tops or dead falls and seldom kill a tree for heat.
The other thing with a woodlot is it produces more cash from logging than the same area in crops. It also requires little or no input to do so.
Harvesting trees is quite important to a healthy timber if done properly. It opens up the canopy and allows sunlight to reach smaller trees and you remove trees that would eventually die and fall to the ground to rot. The bulk of the minerals and nutrients are in the leaves so hauling off the logs and firewood does not appreciably deplete the soil nutrients. We basically utilize about 50 acres of trees for heating simply because it is the easier part for me to get to than the rest of the farm. Just that 50 acre plot can produce $10,000 to $15,000 in logs in a harvest. Depending on weather, etc we can rotate harvest on that plot from 5 to 10 years. Not numbers from a theory but real world stuff.
Oh, we harvest a couple of deer per year from the timber, too which increases the value of the woodlot. We probably use about 5 cords a year altho I don't measure it just keep the wood shed full from year to year and it holds maybe 35 cords completely full.
I guess my point is that done properly on a suitably sized wood lot wood heat is completely sustainable and more. Just the view from here.
Cheers.
TomW