f'--. 



CLIMATIC RECORDS IN THE TRUNKS OF TREES 



BY A. E. DOUGLASS 



DEAN COLLEGE OF LETTERS, ARTS AND SCIENCES OF THE UNIVERSITY OF ARIZONA 



THE annual rings of trees have been found to dis- 

 play in their varying width a life-long record of 

 events historically important in the life of the tree. 

 Such events naturally have to do with favorable or 

 adverse weather conditions, interference by competing 

 vegetations, shade, drainage, pests, fires, and so forth. 

 Most of these eflFects are well known to the forester, thus 

 the result of forest fire is a matter of constant observa- 

 tion. Smoke near great iron manufactories in diminish- 

 ing the rings of trees has been studied with care, and 

 numerous interesting photogra])hs showing it have been 

 published by the Mellon Institute.* 



In the Geological Museum at Berlin one may see sam- 

 ples of pine, collected by the late Professor Potonie, some 

 grown in upland and some in swampy ground, showing 

 wonderfully diminished growth in the latter, due to the 

 excess of water. The effect of drainage in wet climates 

 is beautifully shown in a small section of Pinus sylvestris 



in the office of Professor Jelstrup, chief of the Norwegian 

 Forest Service, at Christiania. This little section shows 

 17 rings of annual growth in a radius of 15 millimeters 

 from the center. In that year trenches were dug drain- 

 ing the land and allowing the soil to dry in part. The re- 

 mainder of the radius of the section is 40 millimeters in 

 width, but contains only eight rings. The growth in- 

 creased five and a half times after drainage. 



But in the great fundamental questions of weather 

 conditions, nature has constructed immense laboratories 

 over the earth, some of which isolate effect of varied 

 rainfall in a beautiful manner. In regions where the 

 rainfall is really deficient, the tree makes a lifelong 

 stniggle against drought and, if other accidents are 

 largely absent, that struggle is the most prominent fea- 

 ture of the rings. Even if the other factors are present, 

 we must remember that the average of a group of trees, 

 sufficiently scattered in location, will practically eliminate 



'J. F. CUvenger, "Effect of the Soot in Smoke on Vegetation," Bulletin No. 7, Smoke Investigation, Mellon Inst , Pittsburgh, Pa. 



30m 



10 In. 



Gin 



30'" 



_ ZOin 



10 in. 



Gin 



COMPARISON OF 43 YEARS OF RAINFALL AND TREE GROWTH AT PRESCOTT, ARIZONA 



Fig. I In the tecond diagram, the Calculated Rainfall" is obtained from the Tree Qrowth by multiplying its value each year by three terms: 

 rirst, a general coefficient changinf; mean tree growth to mean rainfall: seconl. a small factor correcting for age of tree, and third, a "conscrva 

 tion factor to correct lor preiervation of moisture fallen in pravioua years This conservation factor is very nearly the formula for "accumulated 

 moisture reversed. 



m 



