INTRODUCTION 13 



with reference to the forest border. It had been quickly recognized in the 

 big sequoias between 1915 and 1918 that there was a great difference in mean 

 ring thickness and in sensitivity of ring series, depending on the location of 

 the individual trees with reference to character of water supply. High sensi- 

 tivity was found in trees growing on the steep slopes of the uplands. Large 

 rings, low sensitivity and complacent sequences were found in the swampy 

 basins. The mean growth in the basins sometimes was four times as great 

 as that on the uplands and ridges. So relation to water supply was studied 

 in the pines about Flagstaff and it was realized that trees near a constant 

 water source had larger rings and less sensitive sequences than those growing 

 where there was little or no chance for accumulation and conservation of 

 water. A special search for particularly favorable locations in the Arizona 

 regions was made by Dr. Glock and the writer in 1934, with the result of find- 

 ing sites where the sensitivity was so high as to introduce many difficulties 

 from absent rings. Interesting series of this sort were obtained from Defi- 

 ance Plateau between St. Michaels and Ganado at elevations near the lower 

 forest border. The soil is exceedingly thin and here and there sandstone 

 shows above the ground. Plate 11 shows the stump of a tree that had been 

 blown down at this site, whose roots penetrate the cracks between large 

 blocks of sandstone. In these trees the sensitivity of the record was ex- 

 ceedingly high. 



One other term in the environment of Arizona trees was observed at a 

 sufficiently early date to appear in Volume I of Climatic Cycles and Tree 

 Growth (page 22), namely, the nature of the soil; and our present knowledge 

 is only the beginning of a complex subject. Near Flagstaff there are roughly 

 two chief soil types: one has formed on limestone with a varying admixture 

 of sandstone products, and the other rests on volcanic rock. The former 

 rocks decompose into a porous soil from which moisture readily escapes and 

 the rocks themselves possess deep cracks, as may be seen in many places 

 about Flagstaff. The volcanic rock decomposes into a clay soil which holds 

 water very tightly but usually in small quantity. When the two soils are 

 adjacent and somewhat similar in contours as at points near Fort Tuthill, 

 five miles south of Flagstaff, the ring records on the lava soils are relatively 

 complacent. In the deeper soils and higher precipitation of the forest interior 

 the growth seems larger but still complacent. In locations where the soil is 

 topped by a granular mulch that aids conservation, the growth in dry years 

 may remain small while that in wet years may increase greatly. This results 

 in a sensitive record, as in OL-12, the dissected tree referred to above, which 

 grew in the "cinder" area northeast of Flagstaff. (Measures of ring growth 

 in this area made at the Museum of Northern Arizona by Mr. John C. 

 McGregor show large growth.) Thick rings in wet years occur also in the 

 granitic areas near Prescott, Arizona, as shown in specimen PR-62 in Plate 

 10B. 



Significance of Cross-Identity and Cycles in Tree-Ring Records — It is evident 

 at once that cross-identity depends on similarity in the records of different 



