Bark can be obtained by players with 45 Woodcutting by cutting a hollow tree, which are found east of Canifis, granting 82.5 Woodcutting experience for an unsuccessful cut, or 357.7 if bark is obtained. It can take a while to obtain just one piece of bark.
They can also be used to light torches to gain 100 Firemaking experience. When you light the bark in the torches you regain 10 Prayer points.
While cutting these with the inferno adze, the player might find themselves getting 105 bonus Firemaking experience on unsuccessful attempts. It's not known if it was intended by Jagex or just missing Hollow Trees with Adze update.