CLIMATIC RECORDS IN THE TRUNKS OF TREES 



BY A. E. DOUGLASS 



DEAN COLLEGE OF LETTEIIS. 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 effects 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 photographs 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 ])art. 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 

 struggle 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. Clavenger, "Effect of the Soot in Smoke on Vegetation," Bulletin No. 7, Smoke Investigation, Mellon Inet.. Pittsburgh, Pa 



30 in 



— 20m. 



DotTed liDe; RaiVrfall. 



So I id / /■ ne : T re e G ro.wTh 

 18170 I 



I6la0 I I8l90 



10 'n 



Oin 



30'" 



ZO>n 



10 1 1. 



DoTted line ; Rainfall 



line.'C 

 18 170 



Solid line .■Calculated Rainfall. 



1 I a 180 



I6l90 



19100 



IQIOI Oin 



CO.MP.ARISON OF 43 YE.\RS OF RAINFALL AND TREE GROWTH AT PRESCOTT, ARIZONA 



. ..^ 



Fig. 1 — In the second diagram, the "Calculated Rainfall" is obtained from the Tree Growth by multiplying its value each year by three terms: 

 First, a general cocfiicicnt changing mean tree gro ''l' to me.nn rn^nfall: ^ecof' a «mall factor correcting for age of tree, and third, a "conserva 

 tion" factor to correct for preservation of moisture fallen in previous years. This conservation factor is very nearly the formula for "accumulated 

 moisture" reversed. 



732 



