Answer: Cracks and splits in tree trunks are fairly common and may occur for various reasons, but are usually not a significant threat to the tree. Typically, there's not much you can do about them once they occur.
One of the most common reasons for cracks and splits on tree trunks is frost cracking. Frost cracks occur during cold winter weather. The inner and outer wood in a tree's trunk expands and contract at different rates when temperatures change. When winter temperatures plummet below zero, especially after a sunny day when the tree's trunk has been warmed by the sun's rays, the different expansion rates between the inner and outer wood can cause such a strain in the trunk that a crack develops. Frost cracks occur suddenly, can be several feet long, and are often accompanied by a loud, rifle-shot sound. Frost cracks at a point where the trunk was physically injured in the past.
Maples and sycamores are very prone to frost cracks.
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