Named so for the bulbous belly button of burl that seems a ridiculous mockery of the solemn sentinels that stand silentious in the forest. (And no, I'm not sure if "silentious" is used correctly, but I like the way the word sounds)
It's Burl. When a tree is threatened with disease and the like it will sometimes grow around the blight forming a knot of live tissue which continues to grow like a tumor. Sometimes it's fatal to trees (when it inhibits nutrient uptake and/or transfer, such as growth in the roots), sometimes it just plain looks cool.