The interpretation of a budworm-induced impact on fir 

 between 1960 and 1983 in the unthinned stand appears 

 to be reasonable and is corroborated by yearly observa- 

 tions of defoliation at Lubrecht (Fellin 1984). 



The relative growth responses (growth ratios) of both 

 Douglas-fir and ponderosa pine demonstrate the sig- 

 nificance of the thinning and the influence of budworm 

 (fig. 6). Current relative radial growth (ratio 1) of 

 Douglas-fir in the thinned stand was significantly 

 greater than in the unthinned stand (p < 0.05); pon- 

 derosa pine had a modest but nonsignificant (p < 0.05) 

 increase (table 3). Prior to thinning (ratio 2), relative 

 growth of Douglas-fir actually was slightly greater in 

 the unthinned stand than in the thinned stand, but the 

 difference was barely significant (p < 0.05), whereas 

 ponderosa pine growth was not statistically different 

 between the stands. 



Relative radial stem growth (ratios 1 and 2) of 

 Douglas-fir was significantly less (p < 0.05) than pon- 



derosa pine in the unthinned stand (table 4). Relative 

 growth of fir also was significantly less (p < 0.05) than 

 pine in the thinned stand prior to treatment (ratio 2). 

 Nevertheless, posttreatment relative growth (ratio 1) of 

 fir in the thinned stand was not different from the rela- 

 tive growth of pine; here Douglas-fir radial growth 

 accelerated and, in a relative sense, nearly equaled that 

 of pine. 



Both Douglas-fir and ponderosa pine responded dra- 

 matically to thinning. Radial growth rates of both spe- 

 cies in the thinned stand after thinning (ratio 1) were 

 significantly greater (p < 0.05) than before thinning 

 (ratio 2) (table 5). Radial growth of Douglas-fir increased 

 57 percent, whereas pine increased 38 percent. In the 

 unthinned stand, however, growth of Douglas-fir did not 

 change between the periods (ratio 1 vs. ratio 2, p < 0.05), 

 whereas growth of ponderosa pine increased significantly 

 (p < 0.05) during the most recent period. 



1.4 



„ PP, THINNED 



PP, CONTROL 



1.0 



0.5 



0.0 



DF, THINNED 

 lSl ^*-^^T^ t- ** DEF=15 % 







DF, CONTROL 

 DEF= 43 % 



1 



RATIO 2 

 61-»70 

 51-^60 



RATIO 1 

 73~»82 

 61-WO 



RATIO PERIOD, YEAR 



Figure 6.— Influence of thinning on radial 

 growth of ponderosa pine and of Douglas-fir 

 infested with western spruce budworm. Thin- 

 ning relieved the budworm pressure on 

 Douglas-fir, resulting in accelerated radial 

 growth. The nonhost ponderosa pine 

 exhibited accelerated radial growth before 

 thinning and in the unthinned stand, demon- 

 strating the "biological thinning" induced by 

 the budworm. PP = ponderosa pine; DF = 

 Douglas-fir; DEF = defoliation; 61 = 1961: 

 70 = 1970. 



Table 3.— Comparison of radial growth ratios by species 

 between the thinned and unthinned stands. 

 Lubrecht Experimental Forest 





Thinned 



Unthinned 





Prob. 



Ratio 1 DF 



1.13 



0.74 



3.10 



0.004 



PP 



1.35 



1.22 



1.00 



.329 



Ratio 2 DF 



.72 



.80 



-2.08 



.045 



PP 



.98 



.98 







.998 



6 



