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
TREE SECRETS IN TREE RINGS
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
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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Update: June 2012