Table 4.— Comparison of radial growth ratios between species in the 

 thinned and unthinned stands, Lubrecht Experimental Forest 







Thinned 



Unthinned 







DF 1 



PP 1 "t" Prob. 



DF PP 



"t" 



Prob. 



Ratio 1 



1.13 



1.35 -1.49 0.146 



0.74 1.22 



-4.35 







R at i n 

 rid 1 1 U <£ 



.72 



.98 -4.11 



.80 .98 



-3.28 



0.003 



1 DF = 



Douglas-fir; PP = ponderosa pine. 









Table 5. 



— Comparison of pre- and posttreatment radial growth ratios by 





species in the thinned and unthinned stands, Lubrecht 







Experimental Forest 













Ratio 2 



Ratio 1 











(before thinning) 



(after thinning) 



"t" 



Prob. 



Thinned 



DF 1 



0.72 



1.13 



3.96 







stand 



PP 1 



.98 



1.35 



3.04 



0.005 



Unthinned DF 



74 



.80 



-.88 



.383 



stand 



PP 



.98 



1.22 



2.20 



.038 



'DF = Douglas-fir; PP = ponderosa pine. 



The poor periodic radial increment of Douglas-fir in 

 the unthinned stand and in the thinned stand prior to 

 thinning is attributed to a combination of stand density 

 and defoliation by western spruce budworm. The insect 

 has been present at relatively high levels (obvious defoli- 

 ation on Douglas-fir) in the vicinity of the study area 

 since the late 1950's (Fellin 1984), and we have every 

 reason to believe that fir in the thinned stand and 

 unthinned stand was equally affected by budworm. 

 Without budworm, we would expect the relative growth 

 of fir to be similar to the pine, a notion supported by the 

 radial growth patterns between 1935 and 1960 (figs. 3, 4). 



Thinning appeared to alleviate budworm pressure; 

 defoliation was much reduced in the thinned stand and 

 residual trees, both host and nonhost accelerated in 

 radial stem growth. We do not know whether absolute 

 budworm populations were reduced in the thinned stand, 

 but we suspect so. Thinning, however, may have stimu- 

 lated the crowns of the residual Douglas-fir, resulting in 

 production of high amounts of foliage. Given a constant 

 budworm population per tree, the relative budworm 

 population per tree (budworms per unit foliage) in the 

 thinned stand would have decreased, resulting in a lower 

 defoliation percentage. Alternatively, opening the stand 

 probably would increase dispersal losses of the larval 

 stages, also resulting in lower defoliation. 



Furthermore, the defensive chemistry of fir may have 

 been enhanced by the thinning. Cates and others (1983) 

 showed that terpene profiles of Douglas-fir are altered 

 by stress. As moisture stress increased, bornyl acetate 

 decreased, causing more favorable substrate for bud- 

 worm. Presumably, the thinning at Lubrecht decreased 

 tree stress, augmented the defensive chemistry, and 

 reduced susceptibility of the Douglas-fir to western 

 spruce budworm. 



Whatever the reasons, radial growth of Douglas-fir in 

 the thinned stand recovered and nearly equaled the thin- 

 ning response of the nonhost ponderosa pine. Given that 

 the stands are relatively old for thinning (about 80 

 years), this response should be encouraging to land 

 managers because we would expect similar or better 

 responses in younger stands. 



Ponderosa pine apparently benefited from the bud- 

 worm outbreak. In the unthinned stand, current 10-year 

 periodic radial increment increased dramatically. This 

 accelerated growth of pine in the unthinned stand indi- 

 cates that the budworm was acting as a biological thin- 

 ning agent. This presumed natural "thinning" effect has 

 been observed elsewhere for western spruce budworm 

 (Carlson and McCaughey 1982), but this study presents 

 more definitive data supporting such an effect. 



Although this study shows the influence of thinning 

 on budworm defoliation and host tree growth, some cau- 

 tion is advised. This was a case study between two 

 stands; there was no replication to include other thinned 

 and unthinned stands. Furthermore, the sample size was 

 small. Thus, the results may be applicable only to the 

 stands from which the data were collected. In our opin- 

 ion, however, the results are broadly applicable. Based 

 on the literature, the biology of the budworm and its 

 hosts, and data from this study, we conclude that thin- 

 ning the 80-year-old stand reduced the pressure of bud- 

 worm on Douglas-fir and significantly improved the 

 growth of both ponderosa pine and fir. At least, this is a 

 strong working hypothesis that should be valuable for 

 researchers and land managers until further testing is 

 possible. 



7 



