How Burls Are Formed

Disease

• Burl formation, although as yet not fully understood, is often attributed to several diseases that bacteria, fungi or viruses cause. Mold infestations are held as the most common cause of tree burls. Experts believe that infections situated just under the tree bark trigger these abnormal growths. The tree's cells continue to grow at pretty much the same rate as the rest of the tree; however, the direction of growth is drastically changed.

Environmental Stress

• Different kinds of environmental stresses play a major role in burl formation, as evidenced by the prevalence of burl-producing tree species in areas subject to periodic fires or floods, such as the Mediterranean. Trees produce burls as a response to injuries from fire or flood damage or to continued irritation from wind or water erosion.

Other Physical Trauma

• Other factors that contribute to burl formation include insect damage, mechanical wounds from logging and other types of human or animal interaction that result in physical trauma to trees. In many cases, it is a combination of the factors -- disease, environmental conditions and human activity -- which causes the burl growth. Essentially anything that can injure or in any way affect a tree is a potential cause of burls.

Unformed Buds

• Some trees, on the other hand, have the genetic predisposition to produce burls, with or without any outside influence. Examples of these species include eucalyptus, cork oak, ginkgo biloba and the California redwood, all of which normally form underground burls, or lignotubers, in their seedlings. These burls are more like unformed buds -- clones of the parent tree -- which can in some instances resprout into a branch or even an entire tree when the primary stem gets injured. These types of burls also serve as storage sites for carbohydrates and minerals and as additional anchors for the tree.