130 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



Table 3a. 



diagram at the bottom of the figure. The groups of trees from Q to U have been included 

 for the sake of completeness, but they may be disregarded not onlj' because the number 

 of trees in the last three groups is small, but because all the trees in these five groups are 

 somewhat abnormal specimens, selected for cutting because of the fact that they had 

 grown to large size in spite of their youth. With the older groups of trees this type of 

 selection has had little influence, and beyond the age of 200 years we maj' safeh' paj- no 

 attention to it. Each of the sinuous hues in figure .30 is comparable, as has been said, 

 to the single straight line of figure 27. It represents the average growth of each of the 

 various groups during a particular decade. The successive lines represent the growth 

 during successive decades up to the tenth, as indicated by the 

 large figures on the right-hand side. Then there is a skip to the 

 fifteenth and another to the twentieth decade. The small figures 

 in parentheses at either end of each line indicate the dates for 

 the particular decade and group there represented. The other 

 small figures show how great a growth in inches was made during 

 the respective decades by the terminal groups of each curve, that 

 is, by group A^ as shown on the right of each of the 12 curves, 

 and by group U for the first ten decades, Q for the fifteenth, and 

 L for the twentieth, as shown on the left. In each of the curves 

 the actual figures derived from measurements are indicated by 

 the fine line, while the heavy Une represents the results obtained 



by smoothing according to the formula - — ^ — . The process of 



smoothing is the simple one of taking the average of three suc- 

 cessive points of the curve and plotting it for the middle point. 



From an examination of figure 30 it appears that the oldest 

 group of trees (A*) began to grow about 1520, and grew on an 

 average only 0.15 inch during the first decade. The next group 

 (A^), which began life about 70 years later, grew about 0.29 inch 

 during the first decade; the third group, 10 years younger, grew 

 0.41 inch; the fourth group (B) 0.45, etc., until we come to the 

 youngest group (U) which grew 1.03 inches. Down to the tenth 

 decade all the cm'ves have a distinct slope from left to right, 

 indicating that up to the age of 100 years the trees which are 

 now old grew more slowly than those which have not yet attained 

 great age, and most of which are not destined to attain such age. 

 By the time the fifteenth decade is reached the difference between the rate of growth of 

 old trees and younger trees has decreased notably, and in the twentieth decade it has 

 disappeared. On the whole there is a steady decrease in the contrast between old trees 

 and young from the first decade to the fifteenth or later. The object of the correction for 

 longevity, here as everywhere, is to reduce curves like those now under discussion to 

 straight, horizontal lines. If the oldest trees be taken as the standard, the greatest correc- 

 tion will be applied to trees of group U, that is, young trees during their first decade, and 

 the correction will diminish to zero with the oldest group. A'; it will also diminish as the 

 trees increase in age until it becomes zero at the age of nearly 200 years. In practise I have 

 found it advisable to treat the curves as straight, sloping lines and to assume that the 

 divergence of each curve from a straight line is due merely to accidents and to the fact 

 that the number of trees is not sufficient to eliminate accidental effects. 



Returning now to figure 29, let us take the corrective factor for longevity as determined 

 by the process just outlined and apply it to the curve already obtained bj' the application 

 of the other corrective factor, that is, the factor for age. By so doing we raise the early parts 



