Tree Rings - What they Are and Why they Vary

Tree Rings - What they Are and Why they Vary
Notes by J. Zimmerman, Ph.D.

How can you estimate how old a tree was? And can you estimate what kind of climate variations occurred during the life of a tree? The answer lies in those light and dark rings that you see on a tree stump or a sawn log.

What is a Tree Ring?

A tree ring is simply a layer of wood produced during one tree's growing season. Tree rings have many uses.

A cross-section of a tree often shows a distinct pattern of concentric tree rings. You can see such rings on a stump or on a fallen tree truck that has been sawn through to clear a trail.

It is very interesting to count the tree rings, so that you can estimate the age of the tree. For example, I counted rings of a newly fallen redwood recently (see the highlights for November 2001 of Redwoods and Waterfalls: A Year of Walking in the Forest of Big Basin: The Berry Creek Falls Loop).

Each tree ring marks a line between the dark late wood that grew at the end of the previous year and the relatively pale early wood that grew at the start of this year. One annual ring is composed of a ring of early wood and a ring of late wood.

The growth occurs in the cambium (the thin, continuous sheath of cells between bark and wood). In spring, the cambium begins dividing. This creates new tissue and increases the diameter of the tree at two places:

  1. Outside the cambium. The outer cells become part of the phloem. The phloem carries food produced in the leaves to the branches, trunk, and roots. Some of the phloem dies each year and becomes part of the outer bark.
    [Phloem is pronounced "FLO-em." The word is derived from phloos, a Greek word for bark.]

  2. Inside the cambium. The inner cells become part of the xylem. These cells contribute most of a tree's growth in diameter. The xylem carries water and nutrients from the roots to the leaves. These cells show the most annual variation: [Xylem, which is a useful word for Scrabble games, is pronounced "ZI-lem." The word is derived from xylon, a Greek word for wood.]

Other features of tree rings include:

What Causes Variations in Tree Rings?

When conditions encourage growth, a tree adds extra tissue and produces a thick ring. In a discouraging year, growth is slowed and the tree produces a thin ring. Much of the variation in tree rings is due to variations year-to-year in:

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