I love sawing burls and selling them, and over the years I have accumulated many different species and varieties of grain patterns - spalted, eyes, flames, and curls. Every individual burl is unique and has its own character. It seems that burls are caused by broken branches, insect damage, and many other sorts of distress to the tree. Does anybody know exactly how a burl forms and possibly how to force a tree to start forming one? It also seems that in my area (NJ), most maple trees in cemeteries are absolutely littered with burls - what could cause this? Something to do with underground nutrients for the tree? Also, why do some trees produce sphere-like burls and some almost pure burl, from the roots, to the trunk, to the branches?
(Sawing and Drying Forum)
From contributor J:
I did a casual search of this once and found a lot who claimed they knew, but none were backed up by research. In addition to the insect and physical damage you mentioned, some think burls may be related to genetics or some type of fungus. Others have made observations that there are an abundance of burls in specific areas or groves, suggesting possible genetic susceptibility, but it seems to me that could just as easily be from localized damage from insect, bird, or fungus. (Maybe burls are caused by cosmic rays or fairy dust!)
The possible soil and chemical means you suggested is interesting, as is the cemetery observation; I suspect no one has studied the contribution of minerals from bones or other substances from buried mammals!
If you do find some real science concerning burl formation, please pass it on! I have black cherry, maple, oak, yellow poplar, and sassafras with burls and/or burl figure on my property and am also interested in propagating burls. I want to set up some controlled experiments including deliberate bark and under-bark damage, perhaps by injection. Another thing that occurred to me to try was to extract small plugs from an existing burl and implant them into an unaffected tree, a type of grafting.
This document is well worth reading for anyone interested in figured wood. On skimming, it appears to be an excellent reference defining the various types of figure encountered in wood and the underlying wood structure.
However, concerning burl formation, the author indicates what I've found elsewhere, stating "the ultimate cause of figure in living trees is largely unknown" and goes on to list various possible causes that have been suggested, some already noted in this thread. The author does mention the considerable interest in figure formation in the 1930's and '40's and includes references that would be interesting to track down. Also discussed are a number of things reported to affect the growth (not the initiation) of burled wood as well as information that appears to debunk some common misconceptions.
Of special interest is the chapter "Artificial Production of Figure", near the end, stating that while it would be desirable to artificially induce figuring in trees, "few successful attempts ... have been recorded." Some attempts and techniques are given, including one, shown in figure 52, a method to create "crinkle" by binding a tree with bamboo and cord.