A tree is a woody perennial, a plant that lives and continues to grow for more than a year. It consists of three basic sections: the roots, the stem (or trunk), and the leaves. The root system grows down into the soil, anchoring the tree to the ground and absorbing water and minerals to nourish the tree. The leaves produce energy for the tree through photosynthesis and respiration. The trunk is the main structure of the tree and connects the roots to the branches and leaves.

There are several layers that create the trunk of a tree. The outer layer of bark protects the tree from damage. The layer just beneath the outer bark is the phloem, also called inner bark. The phloem is a layer of living cells that transports the food made by the leaves down to the other parts of the tree. The cambium is the growing layer—it divides the phloem from the xylem and produces new cells for both types of tissue. The xylem is mostly made of dead cells and can be separated into two sections: young xylem and old xylem. Young xylem, also called sapwood, conducts water up from the roots to leaves as high as 100 meters off the ground. As xylem ages, the cells die and it becomes old xylem, also called heartwood. Old xylem no longer conducts water but remains as the central wood of the trunk.

Each year, as the cambium creates new xylem cells and as the inner layer of young xylem becomes old xylem, the tree trunk grows in diameter. The layers of growth, preserved as rings, record a history of the tree. Typically, one ring is seen for each year of growth, though for some trees it can be very difficult to distinguish the rings. Not only can the tree's age be determined by the number of rings, but the size and shape of the rings can also give information about the environmental conditions that existed throughout the tree's life. For example, good growing conditions (such as ample water and sunlight) produce wider rings, while less favorable conditions produce thinner rings. Very old trees can provide data on climate conditions for the past 2000-3000 years, and fossilized trees can extend this record back even further.

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