“I still don't understand how the stuff picked up from the forest floor (not cutting down trees) is the major source of biomass use in the U.K. or anywhere else.”
The vast majority of woodland in the UK is commercial. Plant, grow for 50-70 years, harvest, repeat.
Spruce are planted at 2m spacing, as they grow they self – prune – the lower branches die off, the upper ones seek the light. At c. 10 years and 20 years they are thinned – take out the weak, forked, crooked. Some trunks go for paper making, brashings for biomass.
Structural timber must be straight and largely knot free, so that’s from the outside, near the base, but not the sapwood.
When the mature tree is harvested, the straight and knot free is structural. The knotty for doors, furniture (e.g. your IKEA), chipboard, OSB etc.
The rest, c. 20% is biomass, a by product with no other commercial use.
Commercial dense planting is good for timber production and CO2 sequestration, not so good for flora and fauna.
These days the planting is much more sensitive. I have blocks of dense Sitka, but also areas of broadleaves which have a much longer rotation, c. 100-150 years, a variety of ages – the mature are much less dense, so grass grows and deer graze. I have no watercourses but they are usually broadleaves too, to reduce acidity, insect habitat to benefit fish etc.
