When temperatures dip well below zero Fahrenheit, especially if they fall precipitously, things go bump in the night. Frozen lakes and ponds emit ominous groans, snaps and booms that reverberate through the ice. Wood siding and old knee joints might creak. And if soil moisture is high and snow cover sparse, the soil can freeze deeply, causing the earth to shift in a harmless, localized cryoseism, or “frost quake” that produces a nerve-rattling bang.
If you live in a wooded area, you’ve probably heard trees snap, crackle and pop during a deep freeze. It’s an eerie sound on an otherwise still night. Native peoples from northern regions have always been quite familiar with this sound, and historically, some even named a winter month in honor of it. The Lakota call February cannapopa wi, “moon when trees crack from the cold.” The Arapaho consider December the tree-cracking time, while for the Abenaki, the tree-popping month is January.
I once found a reference in a novel to exploding trees. In the book, a lost teenager survives a northern winter so cold that some trees burst into smithereens as if dynamited. I’d seen plenty of winters with minus-40 degree nights but had neither seen nor heard of exploded trees. What did this author know that I didn’t?
After much research, I uncovered the truth. Fiction is sometimes fictional – who knew? As I thought, trees don’t blast themselves to splinters. But since that first reference I’ve encountered the exploding-tree myth a few more times. Although trees don’t blow up, they can go pop in the night for various reasons.
As we all know, when water freezes, it expands. Some freeze-pops occur when water which has collected between narrow-angle trunk unions freezes. Such freeze-thaw cycles within narrow forks help open this kind of union more, creating or extending cracks down the trunk and increasing the likelihood the tree will eventually split. It’s one reason why, whenever possible, these kinds of defects should be corrected when a tree is young.
Live wood can also freeze-pop due to its high moisture content. Luckily, sap is not pure water. It’s endowed with antifreeze in the form of sugars and to a small extent, dissolved minerals.
Up to a point, the more sugar, or any solute for that matter, that gets mixed with water, the lower the freezing point becomes. Each additional solute further depresses the freezing point, meaning salt and sugar together can melt ice better than plain salt (though I don’t recommend it). This phenomenon is due to something known as the “Colligative Property of Solutions,” as everyone no doubt recalls from General Chemistry. (I’m actually trying to forget such trivia in hopes that one day there will be sufficient head-space to remember stuff like where I put my glasses and car keys.)
There comes a point when even sugar- and mineral-fortified sap will freeze and expand. This can sometimes rupture the bark of a tree, resulting in a visible crack as well as an audible one. In many cases, frost cracks close with no long-term ill effects, but sometimes they become perennial. Rarely, a frost crack will send a piece of bark flying. This is more likely to happen to species like bigtooth aspen or balsam-poplar, especially when they are young.
Since it’s a weak point, a previous frost crack may pop open again in cold spells. Then each spring and summer the tree makes callus (“repair”) tissue in an attempt to cover the injury, resulting in a raised lip along the seam. Such trees have reduced timber value and an increased potential for decay to set in.
There’s nothing one can do for frost-cracked forest trees in terms of prevention or treatment. You can protect young landscape and fruit trees, however, with light-colored trunk wraps, or even a coat of interior-grade white latex paint, on the lower trunks. Wraps should be removed promptly in spring, and cracks or other wounds should never be coated.
In truth, trees do occasionally detonate – every time someone places explosives in them. An old acquaintance of mine worked as a contractor for the US Forest Service in Oregon in the 1980s to make habitat for cavity-nesting birds. To create snags, he climbed live mature spruce and fir trees, drilled a hole in the trunk halfway up and inserted dynamite, which was detonated from a safe distance away. I’m pretty sure he did this work when it was not below zero.
Photo of Conifers in winter courtesy Wikimedia user Olga Ernst.