By Steve Roark
Volunteer, Cumberland Gap National Park

While hiking in the woods you may stumble across a tree with a gnarly, wartlike bulge growing on the trunk or upper branch. These are called burls, and while not particularly pretty on the outside, the inner grain is gorgeous for woodworkers to create some beautiful work.
Burls are caused by a disruption of growth hormones on the surface of tree, causing wood cells to grow excessively. This is similar to what a tumor does in us, but does not seem to actually harm the tree. The growth is faster than healthy wood, causing the round bulges you see. The cause the disrupted growth is varied, sometimes from an injury or infection, but more often from a bud that should have formed a branch, but for whatever reason went rogue and started growing every which way.
As mentioned, burls don’t particularly add to the aesthetic beauty of a tree, but they certainly add character and make the tree stand out in the forest. I’ve seen them vary in size from softballs to medicine balls. They can occur on any species of tree, but I see them on oaks most often. Burls are vastly more interesting on the inside, because the wood grain pattern forms swirls, curves, and all manner of undulations. Some burls trap buds inside that also begin to grow weird and form “eye spots” that make the wood even more beautiful and unique. They are valued by woodworkers to make bowls and table-counter tops. The wood is valued enough that burls have been stolen by chain sawing them off trees, even if national parks.