14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



group, table 3 results. Among the species, PP means ponderosa pine, 

 FP foxtail pine, WF white fir, and DF Douglas fir. The interval of 

 years in either case ends on 1941. Table 3 is divided into three 

 groups : the first comprising trees east of the Pass, the second on the 

 Pass, and the third west of the Pass. 



In general, agreement declines with increasing distance, a distance 

 measured in feet. Site factors at the surface appear to the eye to 

 be nearly identical among the trees of any one group, but apparently 

 the factors do change within short distances in spite of appearances. 

 Proximity outweighs difference of species as well as presence or 

 absence of variability. Factors present at the immediate location of 

 the individual tree, or what may be called microsite factors, appear 



Table 3. — Holman Pass collection 

 Ratios of opposed trends 



to exert a strong measure of control on tree growth. That trees 

 separated by a distance of a mile or more do show a parallel agree- 

 ment of variation in a majority of years indicates the influence of 

 a gross factor uniformly variable within limits over the area. How- 

 ever, when it is remembered that the trend between two growth 

 layers on one tree compared with the trend between two growth 

 layers of the same date on another tree, no matter how remote, can 

 vary only in two directions, parallel or opposite, some allowance must 

 be made for accidental similarities. The same principle, of course, 

 holds true where visual comparisons are made in so-called cross-dating 

 because in the consideration of two growth layers of same date in 

 different trees one growth layer can only be thinner than, thicker 

 than, or of the same thickness as, the other growth layer. 



