SCOTT SIMON, HOST:
If a tree explodes in a forest and there's no one there to hear it, did it really explode? A social media post warning of this winter weather phenomenon went viral earlier this week as people across the country prepared for this big winter storm. Bill McNee, a forest health specialist with the Wisconsin Department of Natural Resources, joins us now from Oshkosh. Thanks so much for being with us.
BILL MCNEE: Thank you for having me.
SIMON: The trees don't really explode, do they?
MCNEE: That term of an exploding tree is kind of a sensationalized headline that gets a lot of attention. In reality, the more appropriate term would be cracking tree. When it gets cold enough, the sap inside a tree will actually freeze. Most of the time, it's a liquid that has all sorts of sugars and things dissolved inside it. When that sap reaches that critical freezing temperature, which is usually well below 32 degrees, it will expand. Just like water will expand when it freezes, the sap is mostly water, and the sap expands. This puts tremendous pressure on the inside of the tree. And if the tree is physically unable to handle that pressure, it cracks, and that's how it reduces the pressure on the inside of the tree. This usually does not kill the tree, but it can be a safety hazard. So if these large cracks appear in trees that are around people or buildings or vehicles, an arborist should look at this tree and determine if it is a safety hazard.
SIMON: What's the sound like?
MCNEE: As you can imagine, it takes a lot of pressure to cause wood to crack. So it would not be uncommon to hear a large cracking sound, a large popping sound that almost sounds like a gunshot, but it is actually the wood physically cracking.
SIMON: Any kind of trees more vulnerable than another?
MCNEE: Maple trees get most of the attention, given how common they are and how susceptible they are to cracking. Most hardwood trees that lose their leaves in the winter will crack if it gets cold enough. And conifers, such as pine trees or spruce trees, can also crack as well. When the temperature gets down to about minus 20, that's when the sap freezes and you get this cracking occurring.
SIMON: That's really cold, isn't it?
MCNEE: The Upper Midwest gets these cold temperatures every few years, so trees are used to this. They handle it. They may have a physical deformation, but they probably will live through it.
SIMON: Is there anything people can do to avoid their trees cracking?
MCNEE: Realistically, the most practical thing for a homeowner with a small tree to do is wrap the tree with tree wrap. It's sort of like painter's tape or duct tape that you wrap around the base of the tree up to the first set of lower branches. This can help provide some insulation and hopefully reduce the likelihood that that small tree is going to reach that critical temperature at which it cracks.
SIMON: Bill McNee of the Wisconsin Department of Natural Resources. We wish you and your colleagues and everyone in Wisconsin a good weekend. Thanks so much, sir.
MCNEE: Thank you for having me. Transcript provided by NPR, Copyright NPR.
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