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27 1922 LIBRARY

THE SCIENTIFIC

MONTHLY

JULY, 1922

SOME ASPECTS OF THE USE OF THE ANNUAL RINGS OF TREES IN CLIMATIC STUDY

By Professor A. E. DOUGLASS

UNIVERSITY OF ARIZONA

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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,

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.

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