One of the interesting things I saw last Saturday on my walk was this white pine that has died and all the bark is stripped off. The wood has grown in a helix. I've seen this before, but this time I wondered why bad enough to go hunting for answers.
You can see the two sine curves in two dimensions each on the YZ and XZ planes. Project them into XYZ space and there's your helix!
Have you ever watched squirrels chase each other around a tree in that pattern? Fun.
So the bottom line as to why... is... nobody is 100% sure.
It happens as a result of some kind of stress that causes uneven growth. There's some data to suggest that if there are strong prevailing winds, or if one side of the crown gets considerably more light it can occur.
It's NOT true that trees grow one way in the northern hemisphere and the other way in the southern. In fact, quite a few trees spiral one direction for a while and then change directions (so the deeper layers of wood will go the other way). They can even change more than once. Now that is weird! And that doesn't support the prevailing wind or sun theory.
Pinaceae (pine, cedar, fir, hemlock and spruce) are more likely to have the condition than are junipers or redwood. I've seen it on pines and cedar for sure.
If you want to read the best article I found about it, follow the link below to the Gymnosperm Database.
Then again, maybe it's the stress of the squirrel racetracks.
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