A burl is thought to be a tree's reaction to some kind of stress, often disease, fungus, or an injury, but I can't find any conclusive studies. The burl grows faster than the rest of the tree, and the wood is softer. Because the burl causes distortions in the grain of the wood, they are often used to make decorative objects, sometimes producing beautiful patterns.

In redwoods, burls serve the purpose of growing new trees. Redwood burls contain shoots for new growth, but remain dormant as long as the tree is alive. When the tree dies, and falls to the ground, the burls sprout into new redwood trees.