I've always been fascinated by odd-shaped trees. While hiking in Banff National Park, I spotted this deformed pine growing on the bank of the Bow River. It has a rather unusual knobby trunk which is due to burl (burr) formations.
Burls are formed by stress, usually a fungal infection but sometimes due to a virus or an injury. Many burls actually grow below the ground, so you don't see them. On reading a little more about this topic, I was surprised to find that some trees have massive burls, the largest ever documented being in a redwood tree, measuring 26 ft (7.9 m).
Burls yield unusually patterned wood which is sought after by furniture makers and wood sculptors. Because of the unpredictable nature of its grain, burl wood can be hard to work with as it can unexpectedly chip of crack.