112 CLIMATIC CYCLES AND TREE-GROWTH 



variation in the complacent type; (3) sensitive, meaning limited water- 

 supply from lessened rainfall and greatly diminished ground-water; 

 (4) shadow or sensitive surges, meaning very great variations in slow- 

 growing trees, such as come near the lower (dry) margin of the forest; 

 and (5) erratic, meaning immense variations in water-supply, causing 

 some rings to be omitted, while others are very large. 



Missing rings — This occurs more often in old age of the trees and 

 on very dry ridges, where the moisture is not likely to stay in the 

 ground nearby. 



Merging rings — These occur in the pines in dry periods. It does 

 not usually mean close grouping. It occurs normally in the junipers 

 and pinyons without close grouping. It probably does not usually 

 mean close grouping in the big sequoias, but in coast redwood it does 

 indicate it. 



Gross rings — Gross rings in the sequoias are understood to mean 

 root success with a slight climatic relationship, and to point toward 

 certain variable conditions of grouping. 



Lightning scars — Lightning scars are easily recognized in the tree 

 section, but not in the core. They are climatic and occur in torrential 

 summer-type storms. 



Fire injury — This also is easily recognized in the section. Such 

 fires are usually started by lightning and so become climatic in inter- 

 pretation. 



CHANGING RING-SIZE 



The change with age is always conspicuous in the diminishing size 

 from center to back. Rings growing smaller and then larger to a 

 marked degree, in Arizona, mean drought. Badly compressed outside 

 rings mean shallow and perhaps denuded soil. Probably soil denuda- 

 tion is better indicated when the compression lasts 50 or 100 years. 

 Drainage of soil and relief from too much shade are of rare occurrence, 

 but when they do come, are recognized by a very considerable change 

 that is fairly quick and practically permanent. Reinforced rings mean 

 wind whose season of occurrence may sometimes be estimated. 



Climatic variations — Outside the various effects mentioned above, 

 the further variations from year to year are mostly climatic. If 

 several trees over some area can be cross-identified, it helps in the 

 climatic interpretation. But the normal average tree in all ages, 

 judged from large numbers of prehistoric beams and many fossils 

 examined and measured, is practically free from other disturbances, 

 and most of its variations, apart from age changes, can be taken as 

 climatic. So also the smoothed curve and its cycle analysis tell a story 

 of climatic variations. 



