It’s hard to imagine a slow news day these days, but every so often we get one and perhaps that is why everyone is talking about “exploding trees” this week. Exploding trees? How can that happen?
Let’s begin with a more appropriate term, frost cracks. And yes, during extreme low temperature events in winter, some trees (not all) will develop longitudinal splits in the trunk. These splits often occur suddenly and are often accompanied by a rather jarring crack, akin to the sound of a discharged shotgun. And if you’re walking through the woods when this happens, it can be quite alarming.
Conventional wisdom says that frost cracks occur due to thermal contraction. In other words, on very cold days, the core of a tree remains relatively warm compared to its rapidly contracting outer cylinder and this restriction (of bark and outer vascular tissues) results in tension and splitting. But if this theory is true, then why don’t all trees suffer from frost cracks.
At this point I will defer to the late Dr. Hans Kubler (Department of Forestry, University of Wisconsin – Madison) who posited that trees experiencing frost cracks are influenced by the notch effect of earlier faults. Based on work done by Butin and Shigo (1981), it was discovered that frost cracks are most frequently associated with other, preexisting wounds such as old branch stubs or basal sprout stubs. In other words, cracks are not random but occur at the site of previous wounds and flaws.
Frost cracks will usually close during the growing season, but they often open repeatedly (sometimes annually) over the life of affected trees. It might be fun to set up one of those fancy game cameras to watch a previously identified frost crack…reopen. Now that would be something to see and hear!
Finally, there are no known or effective ways to either prevent or treat trees that have suffered from frost cracking. And while the injury looks serious, trees are quite capable of living with frost cracks, even if they reopen in subsequent years.
References
Butin, H. and A.L. Shigo (1981). Radial shakes and “frost cracks” in living oak trees. USDA Forest Service Research Paper NE-478.
Kubler H. (1983). Origin of frost cracks in stems of trees. Journal of Arboriculture 13(4): 93-97.
Photo credits: 1: Robert L. Anderson, USDA Forest Service, Bugwood.org; 2: Steven Katovich, Bugwood.org
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