Can be harvested from specific trees in the game.
For example some south of the bramin stables in NCR.
click and hold on a tree. If its a tree that shows you a hand symbol as the first option you can harvest from this tree.Go to the icon, portraying a bag.
Then pick an hatchet or a primitive tool from your inventory and you will get 5 wood with 2 minutes cooldown time.
Also if u still in the same position of this tree for some minutes it will grow again, so that you might get wood from it again.