Birdseye in Sugar Maple
The birdseye pattern in wood is much like maple sugaring and
fall foliage. We're familiar with it, we value it, and we
know a tremendous amount about it. And yet, ultimately, it
remains a mystery. For the better part of a century, wood
scientists have been beating up on each other's explanations
of what causes it. Hell, there isn't even agreement on how
to spell it. I've seen it written as the simple possessive,
"bird's eye" and I've seen it written as the hyphenated
possessive, "bird's-eye." But I prefer yet another
common spelling, the one-word elegance of "birdseye."
Birdseye is a relatively rare kind of grain pattern or figure
resulting from localized distortions in the growth rings.
Imagine using a dull number two pencil to push through the
bark and into the wood of a tree. If you and the pencil were
strong enough, this action might distort the growth rings,
resulting in small, conical indentations protruding from the
inner bark into the tree across several growth rings of wood.
On veneer and plain-sawn boards, such indented growth rings
appear as tiny burl-like structures that resemble birds' eyes.
The distribution of birdseye within the tree varies considerably.
It may be found through the entire bole and even in branches
or it may be restricted to one side of the stem or in irregular
patches scattered throughout. Birdseye is most commonly found
in sugar maple wood but a similar pattern has also been described
in other species including red maple, yellow birch, white
ash, and black walnut.
So, scientists seem to agree that birdseye figure
from these conical indentations which result from wood fibers
actually growing in different directions. What remains a mystery
is why they do so.
Birdpeck has always been among the most popular hypotheses
attempting to explain the cause of birdseye. But many wood
technologists have described how woodpecker or sapsucker wounds
in live tissue usually heal over, whereas birdseyes actually
continue to grow into the tree. Another common but equally
insufficient explanation is that birdseyes result from little
eruptions of formerly dormant buds within the tree, buds that
inexplicably sprouted into rather than out of the tree. But,
if birdseyes were born of such wayward buds, they would -
like all stems and branches - contain a dark central pith
and, you guessed it, birdseyes have no pith.
Several more obscure explanations have been suggested over
the years - from fungi deactivating the cambium to suppression
early in a tree's life to soil texture and fertility - but
none has been fully substantiated. The mystery lives.
Fortunately, most of us can still sleep not knowing. For
now, we're content observing birdseye wood and the trees it
comes from and the sites they grow on. We think that if we
can make enough careful observations, we might be able to
predict from exterior features which trees contain birdseye
figure. After all, if logs from such trees are otherwise salable,
and they contain significant amounts of birdseye figure, they
can be worth even more money.
For my own fascination with this process, I
blame and thank David Paganelli, Orange County Forester in
central Vermont. He did what most good foresters do: he took
me for a walk in the woods and he changed the way I look at
trees. In this case, it was sugar maple trees. He was showing
me a stand in which he had supervised a logging job and as
we scouted around looking at standing trees and examining
stumps and remaining parts of cut trees, we found evidence
of a surprising number of birdseye maple trees. We looked
at bark patterns and colors. We looked at stem shapes and
sizes. We assessed stocking and composition. All the while
we tried to findcorrelations with birdseye.
I don't think we ever agreed on bark features, but I sure
do see what Dave means about that "coke-bottle"
shape of the tree's lower bole. Paganelli showed me how unlike
the flared bottom of a "normal" sugar maple tree,
some birdseye stems have a particular constriction just above
the root flare -- much like the constriction between the bottom
and the middle of the old-fashioned coke bottles.
Although the coke-bottle test is not foolproof, it is preferable
to more invasive and damaging methods of looking for birdseye.
While our woods certainly don't need marauding bands of hatchet-wielding
birdseye hunters hacking away at perfectly good trees, we
could all benefit from paying attention to cut logs and observing
how outer appearances translate into interior wood features.
We just might solve a mystery.
Michael Snyder is the Chittenden County Forester in Vermont.