122 CLIMATIC CYCLES AND TREE-GROWTH 



It will be noted that this inner group does not include the trees from 

 San Bernardino Mountain, which show a remarkable double sunspot 

 cycle. These were not included because they seemed to represent an 

 extreme condition of some sort which should be studied by itself. 

 A preliminary analysis of the Sierra Nevada mean curve shows a 5, 

 10, 20 year cycle, very strong in early half (1700-1800), a 5.8, 11.8, 

 22.8 year cycle, strong in the late half (after 1800), and a 7±, 14-year 

 cycle growing strong in the late half. 



METEOROLOGICAL AREAS: THE PROBLEM OF COMBINATION 



Use of trees in outlining meteorological areas — Very few weather 

 records reach 100 years in length, and they are apt to be at widely 

 scattered places, subject to different conditions, such that the records 

 can not be combined advantageously, but a forest gives a vast number 

 of long records in some definite region. With proper care we do not 

 need to mix records of different types. No doubt we have exaggera- 

 tions, and in young trees we have a smoothing-down of variations. 

 In terms of thermometer and measuring-rule, our values are not of the 

 highest precision, but as seen from the viewpoint of actual growing 

 vegetation the tree record is hard to surpass. 



Disadvantages — While we have as yet no substitute for the length 

 of record given by the trees, the chief difficulty is that the reaction 

 of trees to certain weather elements that physical conditions make it 

 easy for us to measure (temperature, precipitation, etc.) is not every- 

 where proportionate to these causes and under certain conditions may 

 be fundamentally changed, as, for example, in the reaction to moisture 

 in wet climates. The differences between the zones as shown below 

 is perhaps in part an illustration of this. That investigation is as yet 

 unfinished. 



Problem of combination — Meteorological reports are collected in 

 various districts which are political subdivisions, and are not outlined 

 by weather conditions. When the student begins to combine areas in 

 order to get general averages, he is confronted at once by the problem 

 of combination, for before combining he has to find out what areas 

 it is safe to combine without losing valuable material. The error of 

 too large combination kept meteorologists from admitting solar effects 

 in weather for a score of years. 



Tree-record combinations — In work with the western groups the 

 general experience has been that trees in the same forest are very 

 much alike and may be combined without loss, if care is taken to use 

 trees exposed to similar conditions of soil-moisture. Thus the groups 

 were formed. In combining groups the guides have been: (1) geo- 

 graphical outlines of zones, (2) obvious similarity in smoothed curves 

 which probably is equivalent to phase similarity in cycles, and (3) 



