TREE GROWTH AND RAINFALL— A STUDY OF 

 CORRELATION AND METHODS^ 



By WALDO S. CLOCK 

 Macalesier College 



The purpose of the present study is to test critically the covariation 

 of tree growth and rainfall — tree growth as measured by the thick- 

 nesses of growth layers on increment cores. Three desiderata were 

 the bases of the study. First, an altitudinal zone was to be selected 

 above the region of violent fluctuations of soil moisture but below 

 the region of excessive accumulation and possible carry-over from 

 season to season. This would avoid the forest-border zone where 

 temporary depletion of soil moisture during the growing season could 

 bring growth to a halt temporarily and possibly cause multiplicity of 

 growth layers during that season. 



Second, the number of trees sampled was to be held to a minimum 

 in order to avoid excessive duplication of record and to avoid inclusion 

 of trees from habitats so diverse that the merged record would become 

 blurred. The number, however, was to be sufficiently large to absorb 

 any differences in relative growth-layer thicknesses from tree to tree 

 due to slight variations in site factors local to the individual trees. 



Third, the trees were to be selected in the field on the basis of 

 ecologic principles, after which each core, unless marred by accident 

 or disease, would enter into the group record whether or not the 

 relative thicknesses of its growth layers closely agreed with those of 

 the other cores. 



The writer is aware - of the shortcomings and the possible mis- 

 representation inherent in the use of rain-gauge records taken some 



1 Crateful acknowledgment is made to Dr. A. Wetmore and to^ the Smith- 

 sonian Institution, which supported the entire project. To Dr. R. Sidwell grati- 

 tude is due for courtesies extended in the field. Herbert Cross, of Macalester 

 College, was of much assistance not only in the preparation of the figures but 

 also in the lively interest he evinced in the problem. Rainfall data from 1931 to 

 1946 were obligingly supplied by the Weather Bureau office in Albuquerque, 

 N. Mex. 



2 Bot. Rev., vol. 7, pp. 649-713, 1941 ; Journ. Forestry, vol. 40, PP- 614-620, 

 1942. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 18 



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