104 



CLIMATIC CYCLES AND TREE-GROWTH 



process. It is also immediately evident that these latter sensitive 

 trees give short-period variations far more accurately and effectively 

 than the complacent trees. These types, as well as the following one, 

 are illustrated in Plate 3 and figure 3. Yellow pines in the dry climate 

 of Arizona at so low an altitude that they have the utmost difficulty 

 in getting water to prolong life become extraordinarily sensitive. In 

 the same tree one finds some rings several millimeters across and others 

 microscopic in size or even absent. 



Mean sensitivity — Mean sensitivity, which expresses this different 

 quality in the trees (page 29) depends in large part on the relative 



q Growth above average 



u Average growth 



x Growth below average 



Fig. 11 — Land contours and annual growth of sequoias in Redwood Basin 



response of trees to climatic influence and so long as there are no large 

 changes of ring-size due to injury, it gives a good criterion of climatic 

 effects in trees. Such appears to be the meaning of figure 12, in which 

 the 10 Prescott trees used in the original rain comparison are plotted 

 with respect to ring-size and other features, including calculated mean 

 sensitivity. The first curve shows them arranged in order of ring- 

 size. The second curve, apparent mean sensitivity, estimated by 

 inspection only, shows that such estimates may be too much affected by 

 ring-size to be of value. Curve 3 shows that sensitivity is independent 

 of ring-size. Curve 5 shows that correlation with rainfall had a slight 



