TREE SELECTION 13 



center of the group. Such eccentricity is rarely harmful to the ring 

 sequence unless very conspicuous, but it may mean erratic or slanting 

 growth and therefore is to be avoided as a rule. While the Arizona 

 pines are naturally isolated, the sequoias are habitually close-grouped; 

 but in spite of this the latter tree rarely shows any effect that can be 

 attributed to nearness of other trees, unless two are almost in con- 

 tact. But in the coast redwoods close grouping is doubtless an 

 important cause of its failure to cross-identify. 



The big tree is surrounded by dense vegetation in the basins and 

 loose vegetation on the ridges; the coast redwood has a jungle about 

 it; the yellow pines, however, wherever they grow, have sparse or 

 actually deficient vegetation about them. 



Ridge and basin selection — This is a question of soil moisture and 

 underground drainage, most important factors in the life of the tree, 

 for while other influences may alter groups of rings and completely 

 spoil parts of the record, the moisture-supply in the soil may change 

 the character of the entire record or even make it totally useless. The 

 evident topographic features which control the situation are of course 

 hill and valley, but to make it more specific by naming the extremes, 

 it is called ridge and basin. Ridge and basin sequoias cross-identify 

 perfectly, but there is a great difference in their immediate response to 

 climatic changes, so that the ridge trees show much smaller average 

 growth with vastly greater differences from year to year. This goes 

 so far that the ridge trees nearly always omit many rings in the radius 

 one chooses to study. Only by accurate cross-identification can these 

 omitted rings be determined and correct dating carried past them. 



In the yellow pines, ridge and basin contours have the same effect, 

 producing quick-growing, complacent trees in the latter and slow- 

 growing, sensitive trees in the former. With these facts in mind one 

 can usually pick the kind of tree desired. 



Bedrock and soils — Lavas and clay soils give usually a small 

 complacent growth to the Arizona pines, while limestone and the 

 porous soil above it give more sensitive growth, which may be increased 

 in size by a richer soil. 



Pines and altitude — The Arizona yellow pines at low levels, such as 

 5,000 feet, are so sensitive to rain that rings are frequently doubled 

 by the two rainy seasons. This characteristic nearly disappears in 

 1,000 and 2,000 feet of greater elevation, where the most usable 

 records are found. At still greater heights the accuracy of the rainfall 

 record diminishes, as soil and air moisture are more permanent and 

 the tree in its type of ring record becomes more like the California 

 yellow pine and sequoia. 



East and west mountain slope — In the southwestern part of the 

 United States, the winter storms coming from the west supply nearly 



