Influence of Overstory Removal 

 and Western Spruce Budworm 

 Defoliation on Growth of 

 Advance Conifer Regeneration 

 in Montana 



Clinton E. Carlson 

 Wyman C. Schmidt 



INTRODUCTION 



Mature conifer forests in the Northern Rocky Moun- 

 tains usually have an uneven-aged understory of shade- 

 tolerant conifers, primarily because frequency of surface 

 fires has decreased dramatically since the early 1900's 

 (Amo 1980; Marsden 1983). Larch/Douglas-fir (Larix 

 occidentalis Nutt JPseudotsuga menziesii var. glauca 

 [Beissn.] Franco) forests on subalpine fir (Abies lasiocarpa 

 [Hook.] Nutt.) habitat types (h.t.'s) (Pfister and others 

 1977) are no exception, usually supporting an extensive 

 understory of subalpine fir, Engelmann spruce (Picea 

 engelmannii Parry), and Douglas-fir. These are produc- 

 tive habitat types. Pfister and others (1977) rank Abies 

 lasiocarpa IClintonia uniflora h.t.'s as moderate to highly 

 productive compared to Douglas-fir h.t.'s. Even-aged 

 silviculture often is recommended for these forests, 

 wherein following harvest the advance regeneration is 

 destroyed during site preparation and the stand is subse- 

 quently regenerated to serai conifers (Schmidt and others 

 1983). At times, however, foresters may elect to culture 

 the advance regeneration instead of regenerating a new 

 even-aged stand. Few data exist concerning the perform- 

 ance of advance regeneration after commercial harvest in 

 larch/Douglas-fir forests in the Northern Rockies, and 

 with the current interest in uneven-aged management 

 (implying a significant component of advance shade- 

 tolerant species) it is of considerable interest to know 

 something about its expected growth. Furthermore, 

 western spruce budworm {Choristoneura occidentalis 

 Freeman) is present over much of the Northern Rockies 

 (Johnson and Denton 1975), so we also need to under- 

 stand more about the effects of the insect in stands of 

 advance regeneration. 



Research in forest types other than the larch/Douglas- 

 fir in the Western United States indicates that advance 

 regeneration will release after harvest of merchantable 

 trees. In the spruce/fir type, height growth of Engelmann 

 spruce and subalpine fir advance regeneration accelerated 

 following harvest (McCaughey and Schmidt 1982). Re- 

 generation in clearcuts responded better than in partial 

 cuts, whereas understory trees in the controls did not 

 release. Other studies documenting release of conifer 

 advance regeneration in the Western United States 



following overstory removal include: California red fir 

 {Abies magnifica A. Murr.) (Oliver 1986); grand fir (Abies 

 grandis [Dougl.] Forbes) (Ferguson and Adams 1980; 

 Seidel 1980); California red fir, white fir (Abies concolor 

 [Gord. & Glend.] Lindl.), and Douglas-fir (Helms and 

 Standiford 1985); subalpine fir, white spruce (Picea 

 glauca [Moench] Voss), and black spruce (Picea mariana 

 [Mill.] B.S.P.) (Johnstone 1978). Most of these studies 

 compared growth prior to clearcutting to postharvest 

 performance rather than investigating effects of overstory 

 removal per se. The purpose of our study was to develop 

 equations to predict postharvest height and diameter 

 growth of advance subalpine fir, Engelmann spruce, and 

 Douglas-fir regeneration in productive subalpine fir habi- 

 tats and to assess impact of western spruce budworm on 

 the regeneration. 



METHODS 



Field Study Design 



The study was conducted at the Intermountain 

 Research Station's Coram Experimental Forest in north- 

 western Montana from 1973 to 1984. This study was part 

 of a much larger research and development program deal- 

 ing with logging systems, utilization, and biological 

 responses (Barger 1980). Elevation in the study area 

 ranges from 4,400 feet mean sea level (m.s.l.) in block 1 to 

 5,000 feet m.s.l. in block 2. Predominant aspect is east, 

 slopes generally are steep — greater than 50 percent — and 

 annual precipitation is 25 to 35 inches. Habitat type 

 (Pfister and others 1977) over most of the area is Abies 

 lasiocarpa IClintonia uniflora . A western larch/Douglas- 

 fir cover type occupied the area prior to treatment. 

 Stands were two-storied, with dominant and codominant 

 trees averaging about 25 inches diameter breast height 

 (d.b.h.) and 130 feet tall. Average diameter of the under- 

 story was about 3 to 4 inches d.b.h. and height about 25 to 

 35 feet. 



Four treatments, randomly assigned within two blocks, 

 were tested: (1) removal of all overstory and all under- 

 story trees larger than 7 inches d.b.h.; (2) removal of 

 about 50 percent of the overstory and removal of under- 

 story trees larger than 7 inches d.b.h.; (3) removal of all 



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