306 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 19 31 



FUNDAMENTALS 



In any study of tree rings, cross dating between different trees is 

 of the first importance. This means the careful identification of 

 each ring in different trees and the location and correction of all 

 mistakes in each. Thus dates are carried from tree to tree and the 

 exact year of growth of each ring is firmly established. Upon this 

 depends directly the great precision in dating rings which many 

 peoi^le, I am sure, fail to realize. 



But cross dating does more than this. A single tree tells its own 

 story, which may contain accidental errors. But when many trees 

 agree with each other in successive variations over long intervals 

 of time, then some common factor which continuously influences the 

 whole forest is emerging. It is safe to regard this as climatic in 

 character. Pests and fires and falling trees are local or temporary 

 and reveal their identity. High ridges, steep slopes, and bottom 

 lands produce effects on trees, but such effects may be identified by 

 comparison of trees in different surface conditions. Climate, with 

 its story of limited change from year to year, is the factor which 

 emerges when large numbers of trees are compared. 



As yet the interpretation of the story told by rings is only partly 

 understood. A few very limited localities reveal their secrets. In 

 northern Arizona and New Mexico, the semiarid pueblo area of the 

 archeologists, conditions point rather clearly to rainfall as the con- 

 trolling factor among the yellow pines, for that area includes the 

 lower forest border which separates the successful forest from the 

 deserts. Actual tests at Prescott on the western edge of this same 

 lower border show a very close relationship between tree growth and 

 rainfall. The similarity of growth curves in different portions of 

 this border, hundreds of miles apart, is amazing. 



As one goes nearer the center of the forest the rings become less 

 sensitive, that is, more complacent or equal in growth. The changes 

 from ring to ring are less abrupt. At the uppermost limits of the 

 forest, near 9,000 feet in elevation, the pines have lost the principal 

 changes due to rainfall and have acquired other less marked varia- 

 tions, doubtless largely dependent on temperature. It was the 

 preference of the prehistoric Indians for locations on the lower and 

 drier forest border which made the dating of their ruins much 

 easier than might have happened. 



The giant Sequoia likewise has proved a tree of the greatest im- 

 portance, for cross dating can be carried continuously through al- 

 most every tree in all the groves. The rings are more complacent 

 than those of the Arizona pines, but the sequences are so long that 

 it is always possible to include and identify a number of telltale 



