One does not grow trees "commercially", really. You find trees in forests, out in the forest. You don't need to "grow" them commercially (you need to manage your forest resources, replant, or protect seedlings, clear deadwood, etc). Lumber doesn't come out of greenhouses, nor does it need to, we just need to attack the existing resource with some moxy. Briar is the exact same. Grows all over the mediterranean. Take some, leave some behind for next year (next decade).
The briar cutters I talk to express exactly what Alejo mentions above - there are lots of trees, lots of roots, and few men willing to do the arduous work. If you want a briar burl that is, say, 50 years old (the plant is too small to harvest until it is 30 or so), you are talking about a plant that has been growing since 1970. So, okay. Even if they milled every single big enough heath tree in the Med for 40 years (and yeah, they did), there's still lots coming at any given time.
You pay for incredible grain, you pay for outrageous size, but buying briar, it's no problem at all in terms of the ecological side. I can get 30 or 300 blocks shipped from any number of mills. But it's tough dirty work in terrible terrain, not the sort of thing too many guys grow up dreaming about doing.