Gathered from trees. A tree will produce X amount of wood every 60 seconds, where X depends upon the type of tree (small trees produce a few wood, large dead trees produce many).
Using an Axe will produce 2x the amount of wood from a tree. The refresh rate doubles to 120 seconds.
Using a [Fine Steel Axe]? will produce 3x the amount of wood from a tree. The refresh rate triples to 180 seconds.