There are countless reasons why your tree may be losing its bark. First off, it could be animals such as deer, which love eating barks off trees; it could also be disease or possibly the species of the tree, which will seasonally peel off the bark.
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There could be many reasons why the bark is off the tree. It could be an animal, a disease or it may just be the species of tree. Birch tree's bark peels off seasonally. It could also be insects, perhaps termites.