Forest Preserve District of Cook County, Illinois
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Tree Secrets in Tree Rings
Nature Bulletin No. 660   De3cember 16, 1961
Forest Preserve District of Cook County
John J. Duffy, President
Roberts Mann, Conservation Editor

A huge tree inspires people with feelings of awe and respect. It stands there, imperturbable, rooted in the very spot where, by some chance, it sprouted and grew from a little seed. For centuries that monarch has been slowly growing taller and more massive. It appears likely to continue there long after we are dead and gone.

We wonder what occurred during those centuries. What people, of what races, came that way; what children, perhaps, played beneath it? What competition did it have from lusty neighbors and why was it the only one that survived? What droughts, floods, tornadoes, fires, attack by insects, and other vicissitudes did that sturdy old giant endure.

The outward appearance of an old tree may indicate what happened to it during recent years but seldom more than that. Beneath the bark, however, each tree keeps a record: the concentric rings formed by its growth each year. If it is cut down with a saw, those rings can be counted and compared. From their number we know its age; and from them, with some "educated guesses", we can improvise the saga of that tree.

In bulletin No. 516 we explained how a tree grows and forms those annual rings. Also why the thickness of each ring depends not only upon the species -- such as a slow-growing oak or a fast-growing sycamore -- but upon other factors.

They vary in width according to the amount of precipitation each year, the temperatures and the length of the growing season, the fertility of the soil and its physical condition -- porous or compacted -- and whether or not the tree suffered serious damage from fire, storms, defoliating insects, or some other cause.

All trees are truthful but some kinds tell their stories more plainly than others. Willows, or any tree that grows near an always ample supply of water, tell us very little; life has been so easy for them that their annual rings are all practically alike. That is true of most cottonwoods. Oaks, hickories, walnuts and sugar maples leave complete records but, because those are slow-growing trees, they are rather difficult to read and interpret. Pines are the best.

In 1947 a big white oak in one of the preserves was removed because it was dead and a public hazard. It had 430 annual rings. Those made in 1934 and 1930 were so narrow they scarcely could be distinguished. Weather bureau records show that those were years of extreme drought. Those made in 1909, the early 80's and 1857, abnormally wet years, were extremely wide.

Defoliation of a tree by insects also retards its growth and causes the ring formed that year to be thin. In 1938 some of the big elms in forest preserve areas were attacked during late spring by cankerworms and stripped of their leaves. The same thing happened in 1939 and '40. Recently, one of those elms became afflicted with Dutch elm disease and had to be removed. Its stump revealed that the growth rings formed during those three years were much narrower than the adjoining ones.

To a skilled interpreter, the annual rings reveal much about the history of a tree and the place where it grew, such as the hidden scar from a fire long ago, or a double ring caused by a dry winter and spring followed by a wet summer.

Scientists specializing in the analysis of growth-rings in trees that grew in all parts of the U. S. have arrived at some important conclusions. One is that there is no regular drought-rainfall cycle. Another is that a drought affecting all parts of the country has not occurred and probably never will. Most important, they find that extreme droughts were always followed by plentiful rainfall, and that the trees say the same thing over and over: the climate of the United States is not changing.

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