Field of Science

Frost cracks in trees

Frost cracking of trees has been in the news because it does become more prevalent in very cold weather, but frost cracks are generally misunderstood, even by "tree experts". When water freezes, it expands, a rare and fortunate quality.  Most solids are denser than their liquid phase, and if this were true for water, ice wouldn't float and the other consequences are quite horrifying (read about Ice 9 in Vonnegut's novel the Cat's Cradle).  So when the water in sapwood freezes and expands it can exert enough force to forcibly crack a tree vertically, often with a loud, sharp retort.  However the ultimate cause of the crack, often determining whether the wood of a tree will or will not crack, is a prior injury to the bark and vascular cambium that often could have occurred years if not decades before. The wound healing that follows such an injury to a tree produces a radially aligned weak zone in the wood, one that extends vertically far beyond the original injury.  TPP knows this because in a former research life he spent a couple of years dissecting damaged trees under the guidance of Alex Shigo, a renown tree pathologist (his publications on trees for general consumption can be bought here).  All frost cracked trees showed evidence of prior injury, and in urban settings the most common cause of injury is lawn mowers.  Pay attention to your trees people; whacking them with your darned lawn mower isn't a good thing, nor is lawn for that matter.  Such injuries greatly limit the life span of your trees. Once frost cracked, the crack may heal over at their surface, but internally the crack remains, and it weakens the tree especially with regards to twisting forces from the wind.  So the number of trees cracking in very cold weather only makes evident the number of trees with prior wounds.  On our campus before they started putting mulch around the base of new plantings, nearly 100% of the trees were basally damaged by lawn mowing.  It is doubtful that cracks mainly occur on the SW side of trees as the wounds are more randomly placed, but real data is lacking and to say it generally occurs on one side of trees is nothing more than a tree fable. 

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