How are tree burls formed?
Dear Eldonna, the following should be informative. Sincerely, Anthony R. Brach
"burl: 1) A hard, woody outgrowth on a tree, more or less rounded in form,
from the entwined growth of a cluster
of buds. Such burls are the source of the highly figured
burl veneers used for purely
ornamental purposes. 2) In lumber or veneer, a localised severe
distortion of the grain generally
rounded in outline. "
"Burls in coast redwood are masses of stem tissue where elongation of the
many stem tips has not occurred. They are anatomically similar to tissues
in eucalyptus called "lignotubers"
by the Australians.
Burls develop from axillary buds in the seedling redwood. This "basal burl"
(sometimes called a bud collar) persists, growing larger throughout the
life of the tree. The dormant stem
tips continue slow growth and branching, but do not elongate. Unlike aerial
stems of redwood, burl tissue grows downward. It forms an enlarged mass
near the base of the tree above or
below the soil surface. Burl tissue overgrows the root tissues at the base
of the tree. Burls may also occur well up the main stem and on branches.
Adventitious (hanging) roots often develop from burls, particularly near
the soil, but no stems develop on redwood roots. "Stump" sprouts from the
burl are often incorrectly called
"root" sprouts, but there is no known instance in which stems have
developed from root tissue in redwood. Dormant buds rapidly elongate after
the biochemical dominance of the main
stem is removed (such as when the tree is cut). In undisturbed forest
stands, a few trees may sprout from the burl. These sprouts, growing under
the canopy of the main stem, are at a
physiological disadvantage and rarely reach large size. In this regard,
they are like branches which eventually die when severely shaded by other
Anthony R. Brach, Ph.D.
They are basically benign tree tumors. They occur when a twig bud fails to
grow normally, differentiating into the tissues needed for forming a limb,
and instead just multiplies and multiplies and multiplies its bud cells.
That's how you get the round growth with an irregular grain structure.
Many burls will sprout when placed into water, forming normal-looking
shoots. Apparently the water saturation somehow helps them "remember" that
they are, after all, limb buds.
Richard Barrans Jr., Ph.D.
Usually response to some sort of injury or invasion that triggers a response
in the cells surrounding the area, causing unusual growth - a little
analagous to cancer in animals but does not spread beyond immediate area and
usually doesn't harm the tree, also a little analagous to scar tissue.
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Update: June 2012