CLIMATIC CYCLES AND TREE-GROWTH 



VOLUME II 



I. INTRODUCTION 



In a dry region the dominating physical factor in tree-growth is 

 moisture. It is impossible for anyone to realize how vital it is 

 without actual residence — a mere trip through a desert is far from 

 sufficient, for it lacks the time element. One must live in it by night 

 and by day, in rainy and in dry season, in drought and in wet cycle. 

 One must see the burning sun, the sparse shrubs, the clear skies, the 

 striking colors of earth, rock, and sky, without the green of vegetation, 

 followed by the strong primitive atmospheric colors when the sun is 

 just below the horizon; he must see the round green cedars and the 

 ever watchful isolated pines of higher elevations; he must see green 

 valley bottoms and herds escaping from sight through deep range 

 grass at one time, and later on he must travel through cactus wastes 

 and dead cattle lying beside dried-up water-holes. And all this must 

 be lived with to afford full realization. The visitor from wet climates 

 does not sense it all for the first year or two, for day by day he 

 unconsciously expects a change, as has always happened in his pre- 

 vious experience. But after a year or more he is able to realize the 

 excessive value of moisture and even to recognize the evidence of 

 climatic changes. 



This was the approach in the present study of climate and trees. 

 Many investigators have come to the study of growth variations 

 from other viewpoints. For example, a large number think of them 

 in terms of pests, for economic necessity has demanded their study, 

 especially in wet climates, where timber is abundant and they are 

 nature's agents for maintaining an equilibrium. It is true that the 

 relation of the abundance of animal life, even pests, to climatic con- 

 ditions is receiving more and more consideration, but the supreme 

 r61e of rain in a dry climate needs to be a matter of constant 

 experience in order to bring appreciation of the relation of tree-growth 

 to moisture in the Southwest. 



AFFILIATIONS 



At the outset this work was recognized as on the borderland 

 between astronomy, meteorology, and botany, and as needing help 

 and information from each with some expectation of ultimate return. 

 To some degree this return is realized in the present volume, which 

 gives for the astronomers some evidence of a real history of solar 



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