152 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



straight line by eliminating all effects except those of climate. This process has been 

 carried so far, especially by the corrections for age and longevity, that we have apparently 

 eliminated and even reversed certain differences between the remote past and the present 

 which are really due to a difference in climate. The reader can not be too strongly re- 

 minded of this fact. The curves of tree growth as here presented show with great exact- 

 ness the cycles which are of less duration than the periods covered by the curve; but they 

 do not show the possible differences that may exist between the mean climate of the present 

 and of 3,000 years ago. 



Average growth 

 per decade in mm. 



25.00 



20.00 . 



1 2 3 4 .5 6 7 8 9 11 1.3 15 17 19 21 23 25 27 29 31 No of group 

 250 450 650 850 1050 1250 14.50 1650 18.50 20.50 2250 2450 26-50 28-50 30.50 320 Average age 



15.00 



10.00 



5.00 



0.00 



15.00 



10.00 



Growth during first 250 

 years of life of trees. 



Growth between 250 

 and 650 years of age. 



Growth between 650 

 and 1050 years of age. 

 Growth between 1050 

 and 1450 years of age. 



Growth between 1450 

 and 1850 years of age. 



Growth during first 

 1050 years of age. 



Fig. 37 .—Sequoia iiaaliiiigtoniana : Corrective Factor for Longevity. 

 (See Table B, p. 301.) 



An examination of the curve of the sequoias corrected for age and longevity, the solid 

 line of figure 38, shows that as a whole it rises from left to right in a quite unexpected fashion, 

 especially in the later or right-hand portions. Just where the most marked rise begins 

 it is hard to say, but from about 1200 a. d. onwards it is plainly visible, and from 1500 a. d. 

 onward it is highly marked. This rise in the curve seems at first sight to indicate that 

 the climate of California has been growing distinctly moister during the past 600 or 700 

 years, which would be most interesting and important if it should prove to be a fact. 

 Yet other evidence points in the contrary direction. In the first place, the major changes 

 in California, as distinguished from the minor ones, appear on the whole to agree with 

 those in New Mexico and Arizona, and in that region there are strong indications of greater 

 aridity at present than during long periods in the past. In California itself there is not 

 much evidence on this point. Yet the distribution of young seciuoias is too important a 

 matter to be overlooked. In three different locaUties, during the summers of 1911 and 

 1912, we investigated the number of young sequoias as compared with old, and as com- 

 pared with the number of young trees of other species. We found that young trees are 

 abundant in moist places, such as valley bottoms, the flat tops of ridges, and hollows 



