A Burl

A burl or bur or burr (in the UK) is a deformed growth on a tree trunk or branch. Here is a description of the growth from the Wikipedia article entitled, "Burl."

A burl results from a tree undergoing some form of stress. It may be caused by an injury, virus or fungus. Most burls grow beneath the ground, attached to the roots as a type of malignancy that is generally not discovered until the tree dies or falls over. Such burls sometimes appear as groups of bulbous protrusions connected by a system of rope-like roots. Almost all burl wood is covered by bark, even if it is underground. Insect infestation and certain types of mold infestation are the most common causes of this condition. 

In some tree species, burls can grow to great size. The largest, at 26 ft (7.9 m), occur in coast redwoods (Sequoia sempervirens) and can encircle the entire trunk; when moisture is present, these burls can grow new redwood trees. The world's second-largest burls can be found in Port McNeill, British Columbia. One of the largest burls known was found around 1984 in the small town of Tamworth, New South Wales. It stands 6.4 ft (2.0 m) tall, with an odd shape resembling a trombone. In January 2009, this burl was controversially removed from its original location and relocated to a public school in the central New South Wales city of Dubbo.

This particular burl was in a grove of trees on Vancouver Island, Canada.  The wood from burls is prized for its remarkable grain and hardness. Unfortunately, illegal cutting of burls is a major problem in some areas of the world.