By the late 1960s the Germans (leaders in scientific approaches to forestry) had published research showing that 40% of the progeny of two curly trees would be curly. (If it was a dominant gene all the progeny would be curly.) That is why when you find a curly tree in the woods you should look at all the other maples in the immediate neighborhood. They all came from the same parent stock so curly trees do occur together– and that has probably lead to the myth that where the tree grows causes it to be curly.

The heredity is a lot like the gene that causes curly hair on people. Anything from a slight wave to real kinky. Note that not all the hair on a person has the same amount of curl, likewise, wood in different parts of the same tree have different amounts of curl! Some of the extremely curly trees are curly all the way up into the limbs but most are not.

In wood curl shows up as some form of wave pattern to the fibers. The wave can vary in amplitude (height) and frequency (spacing). The higher the amplitude the more the curl will show as a stripe when stained or even just finished. Curl can also run in different directions. Peel the bark on a curly tree and the curl can be like a washboard with the waves going in and out on the radius (toward the heart of the tree) OR it can be a wave that is 90 degrees to the radius or in the plane of a tangent (much more subtle because you have to look at the fibers of the wood to see them waving back and forth) . Most curl has some of both components.

There are also two kinds of curl that are not hereditary. The “crotch grain” wood occurs (naturally) in the crotch where a tree forks or a big limb comes off. It is caused by wrinkling of the wood fibers as the two limbs both increase in diameter. Each year a new annual ring tries to squeeze into the remaining space. The correct term for this kind of curl in compression wood. The same thing happens at the swell where the trunk flows in to the roots. If you look at a smooth barked tree you can see wrinkles on the outside of the stump swell where the wood is being compressed.

True burl wood is not curl. Burl is formed by growths on the tree that look like huge warts. They are caused abnormal division of advantageous growth cells. Those growth cell normally serve as a way for a tree to sprout new limbs when that is to the trees "advantage." You will see new growth sprouting from the trunk of a tree when cutting other trees exposes the trunk to sunlight. A burl happens when these cells take on a cancer like growth spurt.

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