The left side of this trunk bark has fallen off due to sun damage which killed tissue, leaving a weakness in the trunk.
In botany, trunk (or bole) refers to the main structural member of a tree that is supported by and directly attached to the roots and which in turn supports the branches. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the trunk to the top, depending on the species. The trunk is the most important part of the tree for timber production.
Trunks can be vulnerable to damage, including sunburn.