Illustration by Adelaide Tyrol
Most people tend to call any crack in a tree trunk a “frost crack.” But then, most people don’t tend to slice open those trees to see inside. Walter Shortle does. As a research plant pathologist with the Northeastern Research Station, USDA Forest Service in Durham, New Hampshire, Shortle worked with renowned tree pathologist Alex Shigo, who dissected more than 15,000 trees from 1959 to 1985.
“It’s a bad name,” says Walter Shortle. “Virtually all of the stem defects that we tend to call ‘frost cracks’ are caused initially by something other than frost.”
Hundreds of those tree dissections were made specifically for the purpose of tracing these so-called frost cracks to their point of origin within the tree. “Every time we’ve dissected these trees,” says Shortle, “we’ve traced the cracks to some injury or wound. Frost may contribute to cracks – by making them larger – but it’s seldom the initial cause.”
No matter how cold the winter night, those cracks would not be there without the wound. According to Shortle, cracks begin with almost any kind of injury. Fire scars, logging damage, pruning wounds, rodent gnawings, deer rubs, even root rot or branch stubs all create areas of weakness. These localized spots of exposed sapwood – not just cracked bark, but openings through to wood – are all it takes to start a crack in the stem. Left open, that exposed sapwood will likely dry and shrink, resulting in small cracks or checks. In time, decay organisms infect cracked wounds, which then become wet.
As Shortle explains, this is when freezing air can become important because when water in that wet wood freezes, it expands. “It is logical that the force of rapid freezing could make the cracks larger,” he says. “And that process could repeat itself with repeated events of rapid cooling.” With such forces at work, something has to give, and typically it is that weak spot – the first minor crack – that lets go.
When it does, it is often accompanied by a sharp, loud report like a rifle shot echoing through the cold winter night. “That rifle sound is real. Cracking wood does make a loud sound,” says Shortle. “We think it is the sound of wood ripping at some previously formed minor crack.” In his text, A New Tree Biology, Alex Shigo (Shortle’s mentor) wrote that “cold temperature may pull the trigger for the crack” but it is some wound that “loads the gun.”
But as Shortle is quick to point out, the cracking story does not end there, and freezing is not the only thing that can pull that trigger. “Further enlargement of the crack is likely due to the force of inrolling ribs of wound wood.” You’ve seen these – ridges of odd wood running vertically up the trunk along the edges of a crack or seam. The growth pressure of these ribs, says Shortle, “is a key force in many young, fast-growing trees with small basal wounds.” Formation of this wound wood around the initial wound and along the subsequent stem cracks usually leads to wound closure. But not always. Says Shortle, “Sometimes the rib turns inward, keeping the wound aerated and favoring internal rot.”
That only worsens the situation. Further enlargement of the crack can then also be caused by wind twisting that already cracked and decay-weakened stem. “A cracked cylinder would not do well twisted,” says Shortle. “Sure, you hear those rifle shots on calm, cold nights,” he says, “but you may also hear them on windy summer nights and think they are just forked branches breaking off. It could be stems cracking.”
This all means that even small and minor wounds can become large and major problems for trees when they are subjected to the forces of drying, freezing, and wind. What begins as just a small nick at the root collar can become a crack or seam extending up the trunk. A flush-cut pruning wound can open a gash in the side of your most prized shade tree, providing an entry court for insects and disease. And, all of this can happen even after the initial wound has closed. That’s right; a seam in a young tree can close and remain closed for decades before opening again later in the mature stem.
Understanding so-called frost cracks provides yet another reason not to beat up trees. We can’t just blame it on frost. If frost really were the cause, wouldn’t all the trees in the vicinity of a cracked tree be cracked? “That’s the important thing,” says Shortle. “Cracks are associated with injuries and infections, which can be controlled, at least in part, by silviculture. We can’t control frost, but cracks are caused by wounding, and we can control that.”
Michael Snyder is a forester and freelance writer in Vermont.