Release of a Thinned 

 B u d worm-In tested 

 Douglas-fir/Ponderosa 

 Pine Stand 



Clinton E. Carlson 

 Robert D. Pfister 

 Leon J. Theroux 

 Cars E. Fiedler 



INTRODUCTION 



Douglas-fir (Pseudotsuga menziesii var. glauca 

 [Beissn.] Franco) forests of the Northern Rocky Moun- 

 tains are vulnerable to the western spruce budworm, 

 Choristoneura occidentalis Freeman, (Johnson and 

 Denton 1975). Certain site and stand factors, such as 

 stand density, are hypothesized to influence vulnerabil- 

 ity (Carlson and others in press). While studying densi- 

 ties of mixed conifer stands in western Montana, we 

 noted striking contrasts in defoliation between two 

 adjacent stands; these stands were within an active bud- 

 worm outbreak. Douglas-fir in one stand, thinned about 

 10 years previously, looked relatively healthy, whereas 

 firs in the nearby unthinned stand were heavily 

 defoliated. Substantial reduction of radial growth of host 

 trees usually is associated with outbreaks of budworm 

 (Williams 1966; Johnson and Denton 1975; Brubaker and 

 Greene 1978; Alfaro and others 1982). We therefore 

 questioned (1) whether defoliation had depressed radial 

 stem growth of Douglas-fir before thinning and (2) 

 whether radial growth had recovered since the thinning. 

 This paper presents the results of a study concerning 

 relationships of current budworm defoliation and of peri- 

 odic radial increment between the thinned stand and the 

 unthinned stand. 



Western spruce budworm is a serious pest of forests in 

 western North America (Johnson and Denton 1975; 

 Fellin and Dewey 1982), particularly in unmanaged 

 stands (Carlson and others in press). Although this 

 defoliating insect does not cause significant impact in 

 most even-aged serai conifer stands less than 20 years 

 old, host stands older than 20 years at times are 

 damaged (Schmidt and others 1983). Intermediate-aged 

 (20 to 80 years) Douglas-fir/ponderosa pine (Pinus 

 ponderosa Dougl.) stands have developed since the early 

 1900's, probably due to increasingly effective fire sup- 

 pression (Dickman 1978; Arno 1980). Selective harvest- 

 ing practices also contributed to the establishment and 

 development of these stands (Schmidt and others 1983). 

 Intermediate-age stands now occupy many thousands of 

 acres in the Northern Rocky Mountains and appear to 

 be highly suitable habitat for budworm. 



Reducing stand density and creating unistoried condi- 

 tions in immature Douglas-fir stands may decrease sus- 

 ceptibility to western spruce budworm (Carlson and 

 others in press), but this hypothesis has not been exten- 

 sively field tested. In unthinned natural host stands, 

 susceptibility to western spruce budworm was found to 

 increase as density of host species increased (Fauss and 

 Pierce 1969; Despain 1981; Carlson and Theroux 1982). 

 Similarly, increasing crown closure (that is, density) of 

 balsam fir (Abies balsamea [L.] Mill.) in Maine resulted 

 in better survival of spring-dispersing larvae (Kemp and 

 Simmons 1979); in Quebec, defoliation of balsam fir 

 increased as the basal area and number of stems of fir 

 increased but decreased as the percentage of nonhost 

 (hardwoods) increased (Crook and others 1979). 



METHODS 



Two stands, one thinned, the other unthinned, were 

 selected for study (fig. 1) about 30 miles east of 

 Missoula, MT, in the Blackfoot River drainage. Habitat 

 type (Pfister and others 1977) in both stands was 

 Pseudotsuga menziesii/Symphoricarpos albus h.t., with 

 minor inclusions of Pseudotsuga menziesii/Vaccinium 

 caespitosum h.t. The thinned stand (160 acres), on State 

 land near the northern border of Lubrecht Experimental 

 Forest, had been treated in 1973. Sixty percent of the 

 volume removed was Douglas-fir, 35 percent ponderosa 

 pine, and 5 percent western larch (Larix occidentalis 

 Nutt.). The treatment was a best-tree thinning with 

 removal of a few old (200+ years) veteran overstory 

 Douglas-fir and ponderosa pine, leaving a mixed stand of 

 dominant and codominant conifers with species composi- 

 tion in approximately the same proportion as the volume 

 removed. Currently there are about 214 trees per acre, 

 equivalent to a 14- by 14-foot spacing (table 1). 



The unthinned stand (200 acres) is located about one- 

 half mile southwest of the thinned stand on similar 

 aspect (NW), elevation (4,000 feet m.s.L), and slope (5 to 

 10 percent) as the thinned stand. The thinned stand is 

 77 years old, the unthinned is 86. In all respects, except 

 for the thinning and slight age difference, the two 

 stands are similar. The percentage of Douglas-fir and 



