Forest Preserve District of Cook County, Illinois
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Tree Rings
Nature Bulletin No. 516-A   February 2, 1974
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Every one of our native trees keeps its own diary of climatic changes or other events that affect its growth. Each year a page is added which faithfully records whether that was a lean year or a fat one. Each year, beneath the bark, the tree adds a layer of wood to its trunk which becomes that much larger in diameter. When conditions are ideal, the layer is thick. When there is a severe drought, or a plague of insects that destroy most of its leaves in early summer, or some other trouble, the layer will be thin. If the tree is cut down with a saw, those layers appear on the stump as a series of concentric rings called Annual Rings or Tree Rings.

Chicago's Museum of Science and Industry displays a section of the trunk of a California redwood cut in 1933. It is about nine feet in diameter and has 1384 rings. That tree started to grow only 549 years after the birth of Christ. In contrast, John Muir tells about a wind- twisted stunted pine, stubbornly clinging to a rock ledge near the timber line on a mountain, that was barely three feet high and four inches in diameter but had 255 annual rings.

If you examine the stump of an oak or a walnut cut while it was alive and sound, you will see at the center a small core of pith formed when the tree was a sapling. Then comes a cylinder of dark dense wood -- the heartwood -- with annual rings which are often very narrow near the core because as a youngster it grew in the shade of older trees and did not get enough light. Surrounding that is a collar of lighter-colored wood -- the sapwood -- with a smaller number of rings. Beyond that and just inside the rough outer bark is a spongy layer of inner bark called the phloem.

But you will not see, because it is so very thin, the most vital part of all. Between the sapwood and the inner bark there is a single layer of living cells, the cambium, which has the magical property of producing, each year, a layer of sapwood on the inside and a layer of inner bark on the outside. The wood formed each spring consists of light-colored thin-walled cells. Toward the end of the growing season, the cells formed are smaller and have darker thicker walls. The springwood and summerwood form that year's ring and their difference in color distinguishes it from the similar one a year older.

In slow growing trees, with dense fine-grained wood, such as an oak, the annual rings are generally much narrower than those of fast- growing species such as the sycamore and poplars. Further, in any tree, the thickness of each ring is affected, for good or bad, by one or more factors: (a) the precipitation of rain and snow; (b) the amount of sunlight it gets; (c) the fertility of the soil and whether it is aerated or badly compacted; (d) temperatures; (e) the length of the growing season; (f) fires; (g) insect infestations and diseases.

In a tree, it is the sapwood that conducts water and dissolved nutrients, taken from the soil by the roots, up through the trunk and branches to the leaves. The food, mostly sugars, manufactured by the leaves is carried down to the trunk and roots by the sieve-like inner bark or phloem. As the tree grows, the older rings of sapwood are gradually filled with a hard substance, called lignin, and become heartwood. It many species, such as the oaks, walnuts and sugar maple the heartwood contains other substances that cause it to be darker, tougher and more durable than the sapwood. In others, such as the sycamore, there is very little difference between the two.

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