The trunk is the main stem or "main woody axis of a tree". In the lumber trade a severed trunk is a log. In botany it means the main structural member of a tree that is directly connected to the roots and which supports the branches. The trunk is also often called the bole. The trunk is covered by the bark, which protects the tree against damage and often differs markedly from the bottom of the trunk to the top, depending on the species. The trunk, or bole, is the most important part of the tree for timber production. The tree trunk can be separated into five different layers: the heartwood, the sapwood, the vascular cambium, the inner bark, and the outer bark.