42 CLIMATIC CYCLES AND TREE-GROWTH 



Averaging — The sums are usually made on an adding machine 

 and the divisions by slide-rule. Once or twice an average by weight 

 has been made. If some character is recognized that makes the record 

 in one tree better than that in another, a suitable weight can be 

 included in the standardizing process by placing the standardizing 

 line at a different ordinate. In the table the same effect has been 

 produced by repeating the same tree in two or more lines, giving it 

 double or more weight. 



CYCLE PLOTS 



Uses of tree-growth curves — There are three main purposes in 

 producing tree-record curves and certain advantageous characters 

 vary in these uses. They follow. 



Cross-identification — Curves for this purpose must display certain 

 special characters like single small rings or drought groups of small 

 rings, which from their extreme and unusual character are likely to 

 extend over a considerable district. The single small deficient ring 

 is the best characteristic to use in dating. Good years seem to spread 

 their effects over a longer period of time and are not definite. 



Skeleton plot — In consequence, a special "skeleton" curve has 

 sometimes been successfully used in cross-dating. Such curve is a 

 long, narrow strip of coordinate paper, dated or numbered as usual 

 and showing only the dates of very small microscopic or absent rings, 

 which are indicated by vertical lines whose conspicuousness is propor- 

 tional to the deficiency of the rings. No other rings are represented in 

 these plots. Two of these skeleton curves from different trees, one 

 known and the other unknown as to date, can be moved slowly past 

 each other until similarity of spacing discloses identity in dates. 



Plotting climatic curves — By comparison of growth-curves the 

 climatic origin of many tree variations is established; hence these 

 curves need to show all the individual years. The scale should not 

 be too great, as then it is difficult to compare two plots. Therefore, 

 the ordinary form, consisting of points connected by straight lines, 

 made on such a scale that slopes dominate, is the more convenient. 

 It has been found most advantageous to use coordinate paper whose 

 smallest divisions are 2 mm. and whose major fines are spaced at 5 

 (not 10) of these small divisions. On this paper the smallest horizontal 

 division commonly represents one year and rather commonly 2 vertical 

 centimeters represent 1 mm. of tree-growth. 



Cycle plots — These are the curves arranged specially for studying 

 the cycles. At first it was thought that the usual unsmoothed plots 

 just described were well adapted for this purpose, but it was noticed 

 that in searching out some cycle with the periodograph, or cyclograph 

 as it will usually be called in this volume, several possible settings 



