RINGS 31 



tion. Cross-identity carries through it usually with perfect ease. It 

 is not always easy to recognize the end of this period. In rare cases a 

 tree gets down to small growth in 200 years. It is possible that tests 

 of mean sensitivity would provide a means of judging. In addition, 

 actual climatic change enters here as a variable. A considerable 

 number of the dated trees started near 300 b. c. and show the reduc- 

 tion in ring-size near 400 to 600 a. d. It is probable that there was a 

 climatic drying at about that time which helped these trees to reduce 

 ring-growth. 



Maturity and age — Maturity in pines and sequoias covers the 

 time from the attainment of full height to the decay at the top which 

 indicates old age. During this period the rings have their best sensi- 

 tiveness, though almost equal sensitiveness may last into old age, when 

 the rings become smaller and possibly a trifle less sensitive and yet a 

 trace more erratic. That is, there are longer periods with little varia- 

 tion, broken by a little more frequent complete disappearance of a 

 ring from the sample under study. The growth has gone to some 

 other part of the circumference. These are the unusual cases. It has 

 never seemed desirable to discard the outer parts of a tree so long as 

 the rings were certainly identified. 



RING ERRORS 



Superfluous rings — The one fundamental quality which makes tree 

 rings of value in the study of climate is their yearly identity. This is 

 sometimes disturbed by the presence of too many or too few rings. 

 Superfluous rings are due to doubling. This is a climatic phenomenon 

 to which some trees are especially liable, probably from their location 

 and rapid growth. But let us keep clearly in mind that superfluous 

 ring formation is the exception. Out of 75 trees collected near 

 Prescott, only 4 or 5 were discarded for this reason. Out of hundreds 

 near Flagstaff, none have been discarded on this account. 



Nearly 200 yellow pines and spruces from northwestern New Mexico 

 have produced no single case of this difficulty. The sequoias from 

 California, the Douglas firs from Oregon, the hemlocks from Vermont, 

 and the Scotch pines from north Europe give no sign of it. On the 

 other hand, 10 out of 16 yellow pines from the lower levels of the 

 Santa Rita Mountains south of Tucson have had to be discarded, and 

 the junipers of northern Arizona have so many suspicious rings that 

 it is almost impossible to work with them. Cypress trees also give 

 much trouble. Trees whose extra rings can not be exactly identified 

 are always excluded in part or as a whole. 



Missing rings — The other difficulty connected with yearly identity 

 is the omission of rings. Missing rings occur in many trees without 

 lessening the value of the tree, unless there are extensive intervals 



