30 CLIMATIC CYCLES AND TREE-GROWTH 



Practical application — The practical method of handling mean 

 sensitivity is to take the sum of all the changes in 10 years without 

 regard to sign and divide by the sum of the 10 years' growth. This is 

 the way it has been used in the limited mathematical tests. As a 

 matter of fact, high sensitivity in a ring sequence is often apparent 

 to the eye, as anyone can see in the illustrations, and in much exploring 

 work the eye estimates have been the practical and rapid way for 

 using this criterion in judging between ring records. 



SELECTION WITHIN RECORD 



The recognition of the preferable parts of a sequence of rings comes 

 from an understanding of the natural divisions of a tree's ring system 

 due to age and the recognition of the various kinds of errors and 

 difficulties in the rings themselves. Most important of all perhaps is a 

 knowledge of the meaning of rings in terms of their environment. 

 This last part of the subject is discussed in Chapter VIII. 



PARTS OF A TREES RECORD 



All parts of a tree 's record are not equally useful. For purposes of 

 description a good record may roughly be divided into infancy, youth, 

 maturity, and age. These are largely recognized by the size and 

 character of the rings. 



Infancy rings — These are most easily found in the sequoia and 

 consist of a central series of extraordinarily large rings, sometimes 

 2 cm. in width, 10 to 50 in number, showing practically no variation 

 except a successively diminishing size. They are very soft and in very 

 old trees often disappear, leaving a conical hole extending to some 

 height from the ground up into the tree. This is probably the explana- 

 tion of the rather common central hole, sometimes untouched by fire, 

 as shown by study of stumps. This was formerly attributed to other 

 causes, but some recent identification of the central parts of very old 

 trees described above under "Vertical uniformity" have favored this 

 view. 



Youth — The youth of a tree is evidenced by large complacent 

 rings, usually largest in the center and outwardly growing regularly 

 smaller. Speaking from an economic point of view, the tree at this 

 time has to build a large trunk in order to support the growing top 

 and resist wind. It is true, as Antevs pointed out, that at this stage 

 the tree shows large, less sensitive rings. In the yellow pine this 

 period is likely to be 20 to 40 years, but even in these immature rings 

 in many trees cross-identification is perfect almost to the center. 

 This is not always so, and often it is best to drop the inner 20 rings. 



The youth rings of the sequoia cover perhaps 300 to 800 years. 

 It is the region where the rings are large and show a gradual diminu- 



