A few weeks ago, one of the kids at Crow’s Path and I found a white ash (note the diamond shaped grooves etched in the bark) that had a strip torn out from the trunk. The missing section was about 6″ wide running a cork screw from the base of the tree right up the bole about 30′ up. While pondering about how the scar got there, we started looking for the missing chunks. We found the lion’s share about 20′ away from the tree – chunks of wood with bark still attached. Also visible in the scar is a thin seam running the length of the exposed area.
The tree was dead and has been for some time, while the crack was fresh (it wasn’t there three weeks earlier). The majority of the crack ran along the side of the tree facing southwest towards an open field (and the winter sun). We decided that the explosive event must have been due to frost cracking.
Ecological notes: Frost cracking occurs on super sunny days that are also super cold. When the sun was shining down on the white ash, the tree’s bark heated up rapidly (like your cat’s fur when it lays on the ledge next to the window). As the bark warmed it conducted heat inward and the core of the tree slowly heated as well. Wood’s not the greatest conductor of heat, so the process is a slow one.
If the tree happens to be in full exposure it’s got lots of time to conduct heat to the inner heartwood. In deeper woods, however, where there are lots of other trees around to shade each other as the sun moves across the sky, the bark cools as quick as it warms and the core is less likely to heat up. You probably well know the difference on a cold day of standing in the sun versus standing in the shade. As soon as you step into a shadow you’re hit with the shivers. This whole heating thing isn’t necessarily a problem, at least until the tree starts to cool.
As the tree heats up it expands, and when it cools it contracts. As soon as the sun sets or ducks behind another tree, our dead ash began to cool and cool rapidly, starting with the outside, that was only staying warm because it was in the sun. As the bark and outer wood cools (and cools rapidly) it contracts. If the tree cools too fast: kablooey! The tree can literally explode. There’s more to it than that, but that’s a start.
Recap of how frost cracking occurs:
1. Tree grows up to be big, with one side exposed to the south
2. Cold sunny day
3. Over a long period of time the bark warms, conducts heat to heartwood, which expands
4. Sun sets/casts a shadow, outer layers of the tree cool very quickly
5. As it cools it shinks around the expanded inner wood and kablooey!
Conditions conducive to frost cracking:
1. Larger diameter tree (stores more heat, can expand more)
2. Cold, sunny day
3. Dark barked tree (dark colors absorb more heat)
4. Smooth bark (rough bark, like a radiator, dissipates heat)
5. Tree growing in open
6. Defects in wood (doesn’t handle contraction nearly as well as healthy wood)
Where: Frost cracking typically occurs on the south, or sun-exposed surface of thinly barked trees. Edges of woods are great places to find this. I see it most often on species where Vermont is at the northern edge of its range (like red oak – I’ll post more on the crazy oak inside an oak healing pattern – aka frost ribs – on these trees later this winter).