Diameter Growth 



Diameter growth of individual trees in the West Fork plots increased promptly and sig- 

 nificantly following thinning. Average diameter growth of trees on all four thinned plots either 

 equalled or exceeded growth of trees on the unthinned plot (see table 3). Analysis of the individ- 

 ual trees shows that when trees of equal initial diameter are compared, the growth per tree was 

 better on the thinned plots than on the unthinned. A multiple regression analysis relating basal 

 area and diameter before thinning to 10-year growth after thinning, shows that growth response 

 of individual trees is related to the basal area stocking of the plots prior to thinning (fig. 1). 

 For example, 6-inch trees ranged from 0.64-inch d.b.h. growth per decade on the unthinned 

 plot to a high of 1.06 inches for the same period on the heavily thinned plot (plot 1). Therefore, 

 in this individual tree class, the trees in the heavily thinned plot showed a 65-percent greater 

 d.b.h. growth than those on the check plot. 



Table 3. --Ten-year growth per acre, crop trees only (West Fork) 



Treatment 



and 

 plot number 



D.b.h. : 



Height 



: Basal area : 



Volume 







Inches 



Feet 



Sq. ft. 



Cu. ft. 



D+4 



(1) 



1.6 



10.9 



16.0 



467 





(2) 



1.1 



9.6 



10.0 



298 



Crown 



(3) 



1.0 



10.3 



9.5 



289 



Crown 



(4) 



1.4 



8.6 



12.5 



361 



Unthinned 



(5) 



1.0 



10.1 



8.9 



316 



The individual tree relationship is not evident in the means because of the widely differing 

 range and distribution of crop -tree initial diameters. The varied length of the curves in figure 

 1 shows the range of initial diameters. The broad range of crop-tree diameters on the unthinned 

 plot (plot 5) results in a larger mean than the means of any of the thinned plots. The low means 

 for plots 2 and 3 are the result of rather restricted distribution (short curves) shown for these 

 two plots . 



Diameter growth can be visualized in another way by considering the change in the range 

 of plot means. For example, in 1949 the largest mean diameter (plot 5, unthinned) was 13 per- 

 cent greater than the smallest (plot 1, D-l-4). In 1959, at the end of the 10-year growth period, 

 the largest mean diameter (plot 5, unthinned) was only 6 percent greater than the smallest (now 

 plot 2, D4-4). Thus, the range of plot means had been narrowed from 13 to 6 percent. The dif- 

 ference between means of plots 1 and 5 was reduced from 13 percent in 1949 to 1 percent in 

 1959. Thus, the initial difference between the means of these two plots had virtually disappeared. 

 The mean crop-tree diameter of the thinned plots will soon surpass the mean diameter for the 

 unthinned plots if the present growth rates continue. 



Diameter growth of trees in the thinned plots in Pattee Canyon increased much more 

 obviously because of the extreme overstocking which existed in the stand before thinning. The 

 unthinned plots show the marked effect of prolonged overstocking (table 4). 



4 



