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For A Hundred Million People

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THE SCIENTIFIC

MONTHLY

JULY, 1922

SOME ASPECTS OF THE USE OF THE AN-
NUAL RINGS OF TREES IN CLIMATIC STUDY

By Professor A. E. DOUGLASS

UNIVERSITY OF ARIZONA

I. AFFILIATIONS
TATURE is a book of many pages and each page tells a fas-

cinating story to him who learns her language. Our fertile valleys and craggy mountains recite an epic poem of geologic conflicts. The starry sky reveals gigantic suns and space and time without end. The human body tells a story of evolution, of competition and survival. The human soul by its scars tells of man's social struggle.

The forest is one of the smaller pages in nature's book, and to him who reads it too tells a long and vivid story. It may talk industrially in terms of lumber and firewood. It may demand preservation physiographically as a region conserving water supply. It may disclose great human interests ecologically as a phase of plant succession. It may protest loudly against its fauna and parasites. It has handed down judicial decisions in disputed matters of human ownership. It speaks everywhere of botanical language, for in the trees we have some of the most wonderful and complex products of the vegetable kingdom.

The trees composing the forest rejoice and lament with its successes and failures and carry year by year something of its story in their annual rings. The study of their manner of telling the story takes us deeply into questions of the species and the individual, to the study of pests, to the effects of all kinds of injury, especially of fire so often started by lightning, to the closeness of grouping of the trees and to the nearness and density of competing vegetation. The particular form of environment which interests us here, however, is climate with all its general and special weather conditions. Climate is a part of meteorology, and the data which we use are obtained largely from the Weather Bureau. Much helping knowledge needed from meteorology has not yet been garnered by that science. For example, the conditions for tree growth are markedly different on the east and west sides of a mountain or on the north and south slopes. The first involves difference of exposure to rain-bearing winds, and the second means entirely different exposure to sun and shade. The latter contrast has been studied on the Catalina Mountains by Forrest Shreve. Again, the Weather Bureau stations are largely located in cities and therefore we can not get data from proper places in the Sierra Nevada Mountains of California, where the Giant Sequoia lives. Considering that this Big Tree gives us the longest uninterrupted series of annual climatic effects which we have so far obtained from any source, it must be greatly regretted that we have no good modern records by which to interpret the writing in those wonderful trees, and, so far as I am aware, no attempt is yet being made to get complete records for the future.

1 Address of the President of the Southwestern Division of the American Association for the Advancement of Science, Tucson, Arizona, January 26, 1922.

In reviewing the environment, one must go another step. One of the early results of this study was the fact that in many different wet climates the growth of trees follows closely and sometimes fundamentally certain solar variations. That means astronomical relationship. It becomes then an interesting fact that the first two serious attempts to trace climatic effects in trees were made by astronomers. I do not know exactly what inspired Professor Kapteyn, the noted astronomer of Groningen, Holland, to study the relation of oak rings to rainfall in the Rhineland, which he did in 1880 and 1881 (without publishing), but for my own case I can be more explicit. It was a thought of the possibility of determining variations in solar activity by the effect of terrestrial weather on tree growth. This, one notes, assumed an effect of the sun on our weather, a view which was supported twenty years ago by Bigelow.

But the possible relationship of solar activity to weather is a part of a rather specialized department of astronomical science, called astrophysics. And there is a great deal of help which one wants from that science, but which one can not yet obtain; for example, the hourly variations in the solar constant. I would like to know whether the relative rate of rotation and the relative temperatures of different solar latitudes vary in terms of the 11-year sunspot period. These questions have to do with some of the theories proposed in attempting to explain the sunspot periodicity.

We do not know the cause of the 11-year sunspot period. Here then is work for the astronomers.

Yet another important contact has this study developed. The rings in the beams of ancient ruins tell a story of the time of building, both as to its climate and the number of years involved and the order of building. This is anthropology. It will be mentioned on a later page.

Viewed through the present perspective, there is one way of expressing the entire work which shows more clearly its human end, a contact always worth emphasizing. If the study works out as it promises, it will give a basis of long-range weather forecasting of immense practical value for the future and of large scientific value in interpreting the climate of the past. This statement of it carries to all a real idea of the central problem.

II. YEARLY IDENTITY OF RINGS The one fundamental quality which makes tree rings of value in the study of climate is their yearly identity. In other words, the ring series reaches its real value when the date of every ring can be determined with certainty. This is the quality which is often taken for granted without thought and often challenged without real reason. The climatic nature of a ring is its most obvious feature. There is a gradual cessation of the activity of the tree owing to lowered temperature or diminished water supply. This causes the deposition of harder material in the cell walls, producing in the pine the dark hard autumn part of the ring. The growth practically stops altogether in winter and then starts off in the spring at a very rapid rate with soft white cells. The usual time of beginning growth in the spring at Flagstaff (elevation 7,000 feet) is in late May or June and is best observed by Dr. D. T. MacDougal's “Dendrograph,” which magnifies the diameter of the tree trunk and shows its daily variations. This spring growth depends upon the precipitation of the preceding winter and the way it comes to the tree. Heavy rains have a large run-off and are less beneficial than snow. The snow melts in the spring and supplies its moisture gradually to the roots as it soaks into or moves through the ground. There is evidence that if the soil is porous and resting on well cracked limestone strata, the moisture passes quickly and the effect is transitory, lasting in close proportion to the amount of rain. Trees so placed are sensitive" and give an excellent report of the amount of precipitation. Such condition is commonly found in northern Arizona over a limestone bed rock. If the bed rock is basalt or other igneous material the soil over it is apt to be clay. The rock and the clay sometimes hold water until the favorable season is past and the tree growth de

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pends in a larger measure on other factors than the precipitation. For example, the yellow pines growing in the very dry lava beds at Flagstaff show nearly the same growth year after year. It is sometimes large, but it has little variation. Such growth is “complacent.

Yearly identity is disturbed by the presence of too many or too few rings. Surplus rings are caused by too great contrast in the seasons. The year in Arizona is divided into four seasons, two rainy and two dry. The cold rainy or snowy season is from December to March, and the warm tropical summer, with heavy local rainfall, occurs in July and August. Spring and autumn are dry, the spring being more so than the autumn. If the snowfall of winter has not been enough to carry the trees through a long dry spring, the cell walls in June become harder and the growing ring turns dark in color as in autumn. Some trees are so strongly affected that they stop growing entirely until the following spring. A ring so produced is exceptionally small. But others near-by may react to the summer rains and again produce white tissue before the red autumn growth comes on.

This second white-cell structure is very rarely as white as the first spring growth and is only mistaken for it in trees growing under extreme conditions, such as at the lowest and dryest levels which the yellow pines are able to endure. Such is the condition at Prescott or at the 6,0007,000 ft. levels on the mountains about Tucson. A broken and scattered rainy season may give as many as 3 preliminary red rings before the final one of autumn. In a few rare trees growing in such extreme conditions, it becomes very difficult to tell whether a ring is formed in summer or winter (that is, in late spring or late autumn). Doubling has become a habit with that particular tree a bad habit—and the tree or large parts of it cannot be used for the study of climate.

But let us keep this clearly in mind: This superfluous ring formation is the exception. Out of 67 trees collected near Prescott, only 4 or 5 were discarded for this reason. Out of perhaps two hundred near Flagstaff, none have been discarded for this reason. Neodly a hundred yellow pines and spruces from northwestern New Mexico have produced no single case of this difficulty. The sequoias from California, the Douglas firs from Oregon, the hemlocks from Vermont and the Scotch pines from north Europe give no sign of it. On the other hand, 10 out of 16 yellow pines from the Santa Rita Mountains south of Tucson have had to be discarded and the junipers of northern Arizona have so many suspicious rings that it is almost impossible to work with them at all. Cypress trees also give much trouble.

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