Very true about the usable trunk of the tree only representing a fraction of the total biomass.
Also true about the significant constraints of biomass potential compared to our current energy draw. According to the notion of solar carrying capacity, wasn't Europe at it's full limit by about 1700? With about 100 million continental population, and very modest per person energy budgets? Effectively every acre of the continent was drawn to the max, either as woodlot or field, wood for charcoal for smithing etc, and nobody had imagined an automobile or train or engine yet.
I have read that in the 70's or 80's Sweden had estimated wood for automotive gasification would take them about 3 years before reaching sustainability limits.
In Canada, ignoring the long transportation required, we are probably in a lot better position. But bottom line there's no simple follow up to the car culture, and the petrochemical industrial culture.
Regarding root and tops biomass, it's pretty volatile, as long as it's part of a healthy forest ecosystem, it's roughly carbon stable. But, if the land is ever cleared and farmed, massive amounts of organic carbon are lost. Apparently there are clear signs of the uptake and loss of carbon in ice cores which match very well with the black death sweeping through Europe, as farmland reverted to dark forest and vice versa.
That's why I am attracted to the idea of just extracting the volatile hydrocarbons from wood, and incorporating the charcoal in soil, then it's outside of the cycle long term.