A tree hollow or tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree. These are predominantly found in old trees, whether living or not. Hollows form in many species of trees, and are a prominent feature of natural forests and woodlands, and act as a resource or habitat for a number of vertebrate and invertebrate animals (or geocaches).
The cache you are looking for is in one of these tree holes. There is no need to climb.
Please replace as found and enjoy.