Region III Quarterly

Volume 3 - No. 4

October, 1941

counting tree rings
COUNTING TREE RINGS. Courtesy Laboratory of Anthropology.


By Dr. Gordon Baldwin,
Junior Park Archeologist,
Boulder Dam National Recreational Area.

There is frequently a lot of doubt concerning the age of some women - and maybe that's as it should be - but the element of uncertainty has been eliminated entirely in determining the age of certain species of trees. Facials, henna packs, eyebrow plucking, and 9-day diets are among the veils that screen femininity's time cycles, but the diaries of trees are recorded in rings that can be translated into years. You can "guess" that a certain woman is "past 40", and then regret that you were not more conservative - or diplomatic. There is no need of guessing about the age of trees, though. Tree-ring dating has become such an exact science that the ruins of early Indian dwellings can be dated by studying the annual rings found in the timbers taken from such ruins. And so, putting women aside for the moment, there can still be romance and adventure in the life of a scientist!

Tree rings are the material record of the growth of trees year by year. The study, however, encompasses a great deal more than that. It requires a fairly broad knowledge of such widely spaced fields as astronomy, meteorology, botany, geology, and archeology. The idea was conceived back in 1901 by Dr. A. E. Douglass, of the University of Arizona. Originally the study of tree rings began in an attempt to solve certain astronomical problems related to solar change, such as sun spots, and their indirect effects on the earth's weather. If sun spots affected our weather, either by producing more or less moisture, Dr. Douglass reasoned that there should be some evidence of this fact in living things. Since trees are the earth's oldest living occupants, he turned to them as a possible source of information.

Each year a tree puts on a ring around the exterior, just beneath the bark. The width of this ring is directly proportioned to the amount of rainfall during the preceding year, being wider if the supply of moisture is above the average, and narrower if it is not. Thus the rings not only tell us the age of the tree, but they record as well the abundance or the scantiness of rainfall, year by year. This is not true of all types of trees. In the Southwest, the studies have included pine, pinyon, and Douglas fir. Even in this section of the country, cottonwood and other trees growing in conditions of constant supply of moisture, such as along stream channels and in lowlands, have not proved satisfactory. These trees do not record variations in rainfall by ring growth.

The Southwestern United States is ideal for the study of tree ring growth because there are two rainy seasons each year. One comes in winter when slow rains soak the lowlands, and snow falls in the higher altitudes. Much of the moisture thus derived sinks into the ground where it is available to trees. The second rainy season comes during the height of summer when the country is drenched by local downpours lasting at the most, only a few hours. Most of the summer rain runs off, thus contributing very little to tree growth. It follows, then, that in times of abundant winter rainfall or snowfall, the trees will have access to more moisture stored in the ground, and will grow more rapidly and that in times of scanty rainfall there will be comparatively little growth. Additional factors determine tree growth, such as the nature of the soil in which the tree is rooted, altitude, early and late frosts, ultra-violet radiation, and other elements. But these, for the moment, are not particularly important to our present field of interest.

Dating trees by annual rings in the Southwest is made possible because the rainfall is not the same from year to year; some years are considerably wetter than others. Looking at any series of rings, then, one will usually see a pattern of large, intermediate, and small rings - giving what Dr. Douglass has referred to as the "tree's fingerprint." If the moisture were constant year to year, each ring would be approximately as large as its predecessors, and we could do little more than tell the age of the tree. Since it is the winter rains and snows which are largely responsible for the size of a tree's rings, and these winter storms generally prevail over rather large areas, it follows that all trees growing in similar environments in the same general region should register an abundance or a lack of moisture in the same way. On this basis rests one of the important angles of tree-ring dating, namely, the ability to match the record of one tree against that of another.

Suppose a tree springs up in a forest and is permitted to grow unmolested for 100 years. At the end of that tinne another seedling sprouts beside it and these two trees continue growing for another 100 years. At that time the older of the two trees, now 200 years' old, is cut for lumber. The younger tree continues growth for another century, when it, too, is felled. Now, in examining the stumps it will be found that the ring pattern of the first tree during the last century of its growth will "cross-date" that of the other. The century which the two trees had in common serves to tie the ring records together to form a single 300-year series although each tree was only 200 years' old when cut. We call this "building a tree-ring chronology." By applying this principle on a large scale, beginning with living trees and shingling back, as it were, with older and older material obtained from ruins, Dr. Douglass has built a tree-ring calendar which falls just short of 2,000 years; in fact it goes back to ll A. D.

That is how tree rings enable us to date accurately many of the prehistoric cliff dwellings and pueblos throughout our Southwestern United States. Archeologists can take a small fragment of wood or even charcoal and, by comparing these large and small rings with the sequence of the 2,000-year chronology, can determine the exact year in which the beam was cut. Fortunately some of the timbers used as beams in roof and floor construction in ancient ruins can still be found.

Of course, perfectly sound specimens of timbers are desirable but even fragments or pieces of charred wood will often suffice as long as the ring structure is well preserved. It is little wonder, then, that the archeologist pays as much attention to scraps of wood or charcoal which he finds in ruins, as he does to pottery, basketry, stone implements, and textiles, for from this lowly material is derived information on that all-important factor, time.

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Date: 17-Nov-2005