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A tree trunk or trunk is the main wooden axis of a tree.
The trunk has several roles. It supports the crown of a tree and it connects the roots of the tree to the tree's leaves.
The trunk is covered with bark of varying kinds (dependent on the type of tree). Beneath the bark, there are two sets of vessels or pipes that the tree uses to transfer liquids. These vessels or pipes form around each side of the cambium, which is a layer of cells found immediately beneath the bark. The vessels or pipes are:
- The xylem is used for the flow of water and nutrients up from the roots to the leaves.
- The phloem is used to transfer sugars manufactured by the leaves from the leaves down to the roots.
Annual rings in tree trunks
When the cambium begins to work in spring, its woody vessels increase in size, as well as thinning. They are preparing to carry the tree sap which is used to form buds and leaf growth in spring. The wood at this stage is known as spring wood. When this activity slows down and ceases, the vessels shrink in size and are smaller and thicker. This is known as summer wood. The differences between spring wood and summer wood forms tree rings. These rings form annually, thus allowing dating of the tree's age when accessed.
Tree rings form only on in trees which experience the sudden growth spurt in spring due to temperature changes (or, in some cases, drought). While rings do form in trees that do not experience this sudden growth and cessation cycle, the rings are much harder to see in trees that grow at a steadier pace, such as tropical rainforest trees.
As the tree grows, the woody vessels and cells that once transported the sap grow old and die. When this happens, they darken due to being encased in resins or tannins produced by the tree. Once this happens, the dead part forms what is known as heartwood. For the purposes of harvesting trees to produce timber, it is the heartwood that is of greatest use.
There is another cambium besides that producing the sap conducting vessels. The other cambium produces the protective layer of bark around the outside of the trunk, the layer most visible to the human eye and the one we can touch (or hug, if so inclined). The bark type varies greater according to species, from thin and papery to tough, thick and gnarled.
Tree trunk utility to species beyond the tree
The tree trunk is the principal part of trees used for the production of timber. When cut down, the tree trunk is usually termed a log.
In some cases, the sap is retrieved from the tree trunk to feed humans, as in the case of collecting maple syrup.
Tree trunks can be inhabited by animals such as birds, insects, small mammals. This may be under or on the bark, inside the trunk when holes are created or form naturally or in the nexus between trunk and branches.
Some plant species will grow on or around tree trunks, using them as a form of support. Sometimes this is symbiotic, sometimes harmless and sometimes harmful.