Apparently, little research has been done to confirm the cause or causes of burls. Available sources suggest that a burl could be caused by many environmental factors but the biology of burls on trees is not well known. To be sure, burls and galls may serve as secondary infection avenues for insects and diseases, but as a rule they do not appear to be harmful to most trees and maintain protective bark.
These tree trunk infections called "burls" look like bumps or warty growths probably caused as a result of environmental injury. Cambial growth is hyper-stimulated as a way for the tree to isolate and contain the injury. Almost all burl wood is covered by bark, even when underground.
Often, a tree that has developed burl wood is still generally healthy. In fact, many trees with burl wood will go on to live for many years. Still, burl wood in vulnerable spots or with off-shooting growth can become so large and heavy that they create additional stress on a tree and can cause the tree to break apart.
Even though not much is know about the cause of burls, it should be assumed that proper tree management that improves tree health can help reduce the occurrence of burls or make the presence of a burl less of a problem. Burls certainly should not be removed from a living tree bole since that would expose a large decay producing wound or completely kill the tree. Boles can be removed if located on prune-able branches or limbs using proper pruning methods.
Not All Burls are Bad:
Burls can yield a very peculiar and highly figured wood, one prized for its beauty by many and sought after by people such as furniture makers, artists, and wood sculptors. There are a number of well-known types of burls. Quality burl wood often comes from redwood, walnut, buckeye, maple, baldcypress, teak and other species. The famous birdseye maple superficially resembles the wood of a burl but is something else entirely.