Winter woods expose many details otherwise obscured by foliage. One of nature’s wonders made more visible are burls. With just a slightly more involved glance into the woods you start to see these tree bumps frequently. You may know burls are treasured by woodworkers for the interesting patterns revealed when shaped by lathe then sanded and shined to a lustrous finish. There is a monstrous burl high up in a tree on my way to work. Each day I observe it with admiration and wonder. As we hike through the woods, friends and family ask: “What caused that?”, “Will the tree survive if you cut it off?”
Seeking out the Answers
I don’t know the answers to these questions so I decide to hit the books. I first go to the tree bible A New Tree Biology by Alex Shigo. This reference contains the “facts, photos and philosophies on trees and their problems and proper care.” Seems to me this is the first place to look. I grab it from the shelf, look up burl in the index and head to the one page listed. There I find one sentence and a photo. The sentence is:
“We still know very little about burls on trees.”
Well, shit, if Alex Shigo can’t tell me about burls on trees…who in the world can? Could it be possible that the internet knows more about these than Mr. Shigo? My copy dates to 1995, perhaps Mr. Shigo has looked into burls since this edition was printed.
So I turn to good old Google to see what I can find. I search ‘Alex Shigo burls’. I find a review of forestry research that discusses the Successions of Organisms in Discoloration and Decay of Wood. In this review Shigo notes that Mistletoe infection sites have resulted in burl development in Hemlock (Tsuga), confirming what I have heard about damage and disease causing these misshapen spots on trees.
Next I find the Photoguide to the Patterns of Discoloration and Decay in Living Northern Hardwood Trees by Shigo and Larsen from 1969 for the US Forest Service. Geared towards those wanting to harvest their timber, this guide explains what causes various problems in hardwood trees and what to expect regarding the impact on quality and value of the wood. Here the authors explain burls can be found on all types of hardwoods and are similar to tumors. Tumors, of course, are abnormal masses of tissue growth that serve no function in the body. This seems to be the case with burls. These large swollen areas on trees often have the protective bark layer intact and when dissected show no sign of disease or decay inside the burl, unless the bark is damaged. The main effect of the burl is the irregularity in the wood grain desired by wood turners.
“The grain makes its way around the bases of branches growing from within; and the cut branch bases form the knots in wood. some trees, including oaks and redwoods, produce anomalous masses of buds that come to nothing but persist to form burls. Timber grows around the burls, and its grain may be all over the place. The grain may go this way and that, too, around the bases of trees and in parts of the buttress roots. Builders want straight-grained sod, for maximal strength and predictability. But makers of veneers, as well as turners, interested in decoration, love burl wood, and will pay hugely for it.” Colin Tudge The Tree A Natural History of What Trees Are, How They Live and Why They Matter
Burls, Galls and Tumors, a publication by the US Forest Service states that all three of these “may be caused by bacteria, fungi, insects, environmental stress, or genetic predisposition.” Though still no one seems to be exactly sure why trees form burls, this document goes on to explain trees of all ages are susceptible and really the only concern one should have with a burl on the tree is its lowering of timber value. Removing a burl, especially if small, may not harm the tree, but as with any damage caused to a tree (which may have caused the burl in the first place) this may create an entry point into the tree through which bacteria, insects and fungus may enter. You are better off just admiring the mysterious burl.
I think it terrific there are so many mysteries remaining in the natural world and a relief that experts (and google) sometimes don’t know the answers to the questions I have! This leaves us to wonder, explore, investigate, imagine and draw our own conclusions.