II. TREE SELECTION 



Rings of trees have told many stories of the past. By their mere 

 enumeration the historian has built up our realization of great events 

 injhuman progress; by more careful counting the forester has dis- 

 covered the dates of ancient destructive fires; by changes in the rings 

 ecologists have determined historic changes in lakes and rivers and 

 settled questions of legal ownership. The present study of climate 

 and solar activity uses the accurate dating and width of rings over 

 wide geographical areas and into times long past for several purposes, 

 but chiefly to derive an understanding of that immensely complex 

 process by which climatic forces reach the earth and distribute them- 

 selves about it. This, it is hoped, will eventually lead to safe long- 

 range prediction of climatic conditions. In the present approach to 

 the subject, the recent development of technique is given first, and 

 this chapter deals with the selection of trees for climatic study. 



SPECIES 



Pines — The western yellow pine is perhaps the best tree for climatic 

 studies, on account of its precision and length of record and its wide 

 distribution. It is normally a dry-climate tree and does well in a 

 sandy soil, for its thick bark prevents evaporation from the trunk 

 and thus enables it to live when other trees could not survive. Thus 

 it endures relatively trying conditions and has little competing vegeta- 

 tion, so that the Arizona forest is said to be the largest "pure" stand 

 in the country. It can be injured by too much moisture in the soil, 

 and draining then improves it. Its age is very favorable, reaching 

 over 500 years. It is commonly free from burns and defects and its 

 rings are very readable. The immense area over which the yellow 

 pine grows adds to its value in this study, as its use avoids the com- 

 plexities arising from the use of different species. For all these reasons 

 it is considered the standard tree. 



The Scotch pine of north Europe is very similar, but not usually 

 so large. However, this is because the European regions have been 

 cut over so much that very old trees are rare. The white pine in the 

 Appalachian Mountains cross-identifies very well. The pines in 

 eastern Massachusetts are less satisfactory, probably because the 

 region is too much cultivated. Very old hemlocks in the Green 

 Mountains of Vermont have rings extraordinarily like those of the 

 western yellow pine and almost as perfect in cross-identification. 



White pines in the Yellowstone are good, and a few white or limber 



pines near Flagstaff give records that are readable, but the locations 



in which they grow are so rugged and variable that a complete test has 



not been made of them. The foxtail pine at high altitudes sometimes 



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