RINGS 



29 



records, so that the average of the remainder will be improved. If 

 some approach is made to a numerical value of this conformity, then 

 it may be used to obtain a weighted mean. This was done in the case 

 of the four best sequoias selected for dating comparisons with Arizona. 

 This was a long process, but its application did not make enough 

 difference for one to feel that its universal use is necessary. 



MEAN SENSITIVITY 



Another criterion which helps in selecting the best record has 

 come into practical and important use, even though the computation 

 of numerical values is a refinement not usually applied. It is called 

 mean sensitivity (see also p. 104) and is an inherent character in each 

 individual. It may be denned as 



Moist 

 upland 



Dry 

 limate 



mm. 

 1.00 



0.11 



1.00 0.33 



1.00 0.64 



the difference between each two 



successive rings divided by their 



mean. The quotients are arranged 



in groups of 10 or some other 



number of years, and listed as the 



mean sensitivity of that period. 



Plate 3 shows the appearance of 



rings of different sensitivity. The 



first section (B) came from a 



sequoia which grew in a swampy 



basin about 15 miles east of the 



General Grant National Park. 



The tree had a "complacent" 



growth, with all rings of nearly 



the same size. Its mean sensitivity 



is 0.11. The second is a sensitive 



sequoia which grew near the top 



of the mountain, 800 feet higher 



up, with a limited water-supply and therefore more dependent on 



the moisture of each year as it came. Its rings have more character 



and individuality, and the changes from ring to ring are much more 



evident. The mean sensitivity is 0.33. The third is a hypersensitive 



dry-climate yellow pine near Prescott, one of the 10 used in the curves 



of Prescott tree-growth already described. It grew near the lowest 



limit of the yellow pine. Some of its rings, such as 1841 and 1857, 



are so small as to be found with difficulty. Its variations from year to 



year are extremely large, and its mean sensitivity is 0.64. 



The way these variations in sensitiveness look in plotted curves 

 is shown in figure 3, in which the curves of growth of these three trees 

 show percentage departures, each from its own mean. The different 

 character resulting from the different environment is at once apparent 

 to the eye. 



1840 1850 I860 



Three types of sensitivity 



Fig. 3 — Mean sensitivity and soil moisture 



