102 CLIMATIC CYCLES AND TREE-GROWTH 



sist of immense masses of warm air laden with moisture. When these 

 pass over a large mountain, they are thrust up in the air and start the 

 storm. When there is not enough motion in the air to draw in distant 

 moisture, clouds form directly over the valley, evidently composed 

 of moisture from the valley. As the day goes on and the air gets a 

 general motion, these clouds are carried forward and contribute to the 

 rainfall in adjoining localities.* 



OTHER CLIMATIC CORRELATIONS 



Several factors may enter into the tree-rings at the same time; for 

 example, rainfall, temperature, length of growing-season, and direct 

 solar stimulation. These may be isolated in two ways. We may 

 select and study a special region, as northern Arizona, where nature 

 has chosen out some one factor and made it preeminent, as rainfall. Or 

 we may isolate certain relationships as in any other investigations, by 

 using large numbers of observations, that is, many trees, and averag- 

 ing them with respect to one or another characteristic. 



Temperature — Undoubtedly temperature and the resulting length 

 of growing-season enter tree-growth. At high elevations this becomes 

 the controlling factor. Probably that is the reason the Upper Flag- 

 staff group, FLH, shows departures from the usual curve of that area. 

 But there is no evidence that temperatures affect the lower pine 

 growth to any important degree, nor the sequoia growth, especially 

 in the southern groves, for sequoias at the highest and coldest levels 

 promptly respond to increased water-supply by enlarged growth, as in the 

 case of D-31, referred to below in connection with sequoia topography. 



Wind — Reinforced rings (see page 32) are interpreted as due to 

 wind or other pressure exerted in a constant direction. In the pre- 

 historic material from the ruins northeast of Flagstaff, such rings 

 rather plainly indicate exceedingly strong spring gales from the west 

 or southwest, if we can judge by conditions at the present day. 



TOPOGRAPHY 



The broad effects of topography were encountered and recognized 

 in large measure while searching for the oldest sequoias. Almost at the 

 start it was realized that size is far from a final indication of age, for 

 nearness of water alters the rate of growth profoundly; for example, 

 it is possible to assign 2,500 years as the approximate time it took 

 the General Grant tree, which has no running water near it, to reach 

 its present immense diameter of close to 30 feet. But about 3 miles 

 west, near a running brook, is a stump which is over 25 feet in diam- 



*In Tucson we have perfectly clear views of the Santa Rita Mountains 40 miles south and 

 7,000 feet higher than the city, the Rincons 20 miles east, the Catalinas 20 miles north, also 

 close to 7,000 feet higher, the Casa Grande and other mountains 50 miles northwest, and the 

 Tucsons, 15 miles west, and so on. Cloud formations are easily seen. 



