The trunk of a tree is composed of several basic parts. The bark of the tree is probably one of the most familiar parts of a tree. The bark is an outer covering of dead tissue, which protects the tree from weather, disease, insects, fire and mechanical injury.Bark characteristics can vary greatly from tree to tree and can be useful in identification.he next portion of the trunk is called the vascular cambium. The cambium is a very thin layer of living tissue which produces new phloem to its outside and new xylem to its inside. The cambium is most active in the spring and early summer when most tree growth is taking place.