Photo courtesy of VT Dept. of Forests, Parks & Rec.
Knots in trees occur where a branch has been surrounded by continued growth of the bole. When the bole is sawn into lumber, or peeled to make veneer, the branch sections included in the lumber or veneer show up as knots. Whether a knot is red or black was determined back when the tree was growing.
If a branch and bole are both alive and growing, a live – or intergrown – knot is produced. These are often called red knots, especially in conifers, where they tend to appear reddish in color. Black knots, on the other hand, result when the limb has died and subsequent growth of the bole surrounds the dead limb. These knots are often black and may contain bark or pitch pockets separating the dead-limb wood from the living stem wood. Such black knots are more formally known as encased knots.
Although knots certainly are points of vulnerability on standing trees – they can be mechanically weak or may allow entry to decay-causing microbes, fungi, and insects – they are known and classified by their effect on sawn boards. Indeed, beyond differences in how they originate, red knots and black knots also influence the value of wood products. Except in specialty products like “knotty pine” paneling and cabinetry, knots are typically considered to be undesirable defects in wood.
Yes, knots are wood. They are sawn sections of branches, dead or alive. It’s just that they cause and are defined by an interruption of continuity – and a change in direction – of the fibers in wood. In black knots, the bole fibers are not continuous with the fibers of the encased knot. Because the branch inside was dead at the time the stem wood grew around it, the result is a very poor connection between the old branch wood and the new stem wood. But in red knots, the branch had been alive when the stem wood grew around it, and therefore the annual rings of growth in the branch (knot) are completely intergrown with the stem wood that surrounds it. The resulting connection is much tighter and stronger.
Owing to much chemistry and physics, knots also compromise the use of a sawn board containing them. Here again, the difference between red knots and black knots becomes important. Encased black knots are generally considered to be more serious defects in lumber than intergrown red knots. This is in part due to the dark discoloration and bark bits associated with black knots. But more importantly, they represent serious engineering flaws. Because black knots are not well connected with the surrounding wood, they undergo differential shrinkage when drying, loosening the knot and often leading to it falling out of the board completely. This leaves a knot hole and further compromises the board’s mechanical integrity and value.
Pruning of branches from standing trees has long been used to minimize the defects associated with knots. Picture the first log (from the ground to 16-18 feet) of a large standing tree in the forest, say, a white pine. It probably has no live branches and very few dead branches remaining. Yes, that tree had low branches way back when it was a seedling, sapling, and even a small sawtimber-sized tree. But as that tree grew taller and its lower branches were shaded by the tree’s crown and perhaps the surrounding forest canopy, those branches died. Still, those long-lost lower branches continue to affect the lumber that may be sawn from that log.
Here’s how: typically the boards sawn from its outermost portions are relatively free of knots. That’s because the tree stem continued to increase in girth after the dead branch broke or fell off, thus adding clear wood to the outside. But, as the sawyer removes boards, working toward what was the inside of the tree, he or she will encounter black knots where the tree continued to grow around branches that were dead but not yet fallen. Finally, the innermost boards from that log are most likely to contain red knots, from when the tree stem was growing around living branches when it was much younger and smaller in diameter. Because black knots are serious defects in white pine boards, foresters often recommend pruning away those dead stubs of remaining branches on the lower boles of white pine trees to allow more years (and inches) of clear, knot-free wood to accumulate.
Michael Snyder is the Chittenden (Vermont) County Forester.