I've noticed that a large chunk of trees cannot be harvested - in fact, the ones that I specifically wanted to harvest:
The wood they provide is at zero:
But starting a new file it shows that they are harvestable. It appears I did something to make them unusable, since this is actually the second time it happens to me on the same area (maybe surrounding them with industry?)
Did I do something to render unusable, or is it just a bug?