As the theory goes, sap - which is a liquid - expands when it freezes. If it expands far enough, fast enough, the tree would not have time to react, and explodes. Many people report hearing loud bangs in the forest during cold snaps, said to be the sound of trees exploding.
(Of course, this could all simply be the work of the Splintercat!)
There are two conditions under which trees are known to explode for sure, beyond a doubt. The first is in the case of lightning. When lightning strikes a tree, the sap instantly boils and vaporizes from the heat. This sudden catastrophic increase in pressure causes the tree to explode.
The second condition is during forest fires. This is particularly the case for the eucalyptus tree in Australia, which produces a very flammable oil.
But can the same phenomenon happen just because of a drop in temperature?
The physics of cold causes the wood of the tree to shrink. This alone can cause the wood to split along fault lines or weaknesses. Trees in temperate regions are particularly prone to this kind of splitting or breakage.
The cold and shrinkage also makes the wood brittle. Trees normally flex in a high wind. If a wind kicks up during an unusually cold spell, the trees - unable to flex - could easily fall or split.
Some people report that branches break "because of the cold." More often, the branches which have been made brittle by the cold actually break because of wind, or the accumulated weight of snow and ice. Cold alone is not likely going to be enough to break a branch.
However, trees exploding simply due to the freezing of their sap seems extremely unlikely. For one thing, sap doesn't freeze as quickly as it boils during a lightning strike. Wood is an excellent insulator, and even if the weather suddenly changed for the worse, it would take several hours before the sap inside the tree even began to freeze. Hours in which the wood of the tree would have time to adjust to the increase in pressure.
Furthermore, trees are adapted to handle cold temperatures in winter. Trees have been on our planet for a very long time, and they have had a lot of winters to acclimate. Some trees distribute the water from the sap throughout their sapwood. Other species add more substances to the sap, so that it acts as an antifreeze. In most species the sap gets thicker and more viscous, less watery, and therefore less inclined to freeze.
If you have ever heard a roof beam pop with a temperature change, you know that it can be a loud, startling sound. This sound happens when wood changes temperature, and it happens to trees, too.
The most likely scenario for "exploding winter trees" is that someone hears the sound of a falling branch or a tree popping like a roof beam. They look around, spot a tree which was damaged by a lightning strike ages ago, and attribute the sound to that tree having exploded.
The best evidence against the theory that trees explode in the cold? The robust and thriving northern forests of Vermont, Maine, Canada, and Alaska.
Photo credit: Flickr/Alastair Thompson