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Regeneration in Thinned and Unthinned Uneven-Aged Interior Douglas-fir Stands
Marshall, Peter L.
The Interior Douglas-fir (IDF) biogeoclimatic ecosystem classification (BEC) zone of British Columbia is found on low- to mid-elevations in the south-central interior of the Province and is characterized by forests dominated by interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco). On the driest parts of this zone, Douglas-fir is frequently found growing in an uneven-aged, fire-dominated, sub-climax. The zone extends into Alberta, Montana, Idaho, Oregon, and Washington (Hope et al. 1991). The IDF zone covers approximately 5% (4.3 million ha) of British Columbia (Ministry of Forests 1995). The IDF is structurally complex due to a history of disturbances from partial cutting, insects, and fire; this is particularly true in the drier parts of its range. Many pockets exist within stands where the density of saplings is quite high. Individual tree growth rates within these pockets are generally low. This contributes to low stand growth rates, low tree and stand vigour, slender form and can result in many years being required to produce large dbh trees. Losses to bark beetles, defoliators and snow press are common. Uneven-aged interior Douglas-fir stands form an important portion of the harvest in the Southern Interior Region. This forest type also provides recreation opportunities, views from travel corridors, and ungulate winter range. Interior Douglas-fir stands are structurally complex due to a history of disturbances from partial cutting, insects, and fire. Guidance under General Wildlife Measures for mule deer in the Cariboo requires the maintenance of these complex Douglas-fir stands. Despite the local importance and complex dynamics of this forest type, relatively little research effort has been focused on improving predictions of its dynamics. Sufficient quantities of regeneration are required to maintain the existing structure of these stands. Much of the regeneration in this type occurred historically following light surface fires; however, the historical fire regime has been drastically altered over the last century. It is uncertain whether the disturbance introduced by partial cutting and insects will promote natural regeneration in sufficient quantities to maintain stand structures through time. Two long-term studies involving uneven-aged Douglas-fir stands were established in the Knife Creek Block of the UBC Alex Fraser Research Forest in the late 1980s. The Knife Creek Block is located in the IDF dk3 subzone (Hope et al. 1991). The landscape is gently rolling with an elevation of approximately 1000 m. Most of the plots are located on zonal (mesic) sites, with some plots on slightly drier sites. However, between-site variation is minimal and does not warrant any changes in site classification. Douglas-fir is by far the most prevalent species, accounting for approximately 90% of the trees in the plots. Other tree species present are lodgepole pine (Pinus contorta var. latifolia (Engel.)), spruce (Picea glauca (Moench), Picea engelmanni (Parry) and their crosses), white birch (Betula papyrifera (Marsh.)) and trembling aspen (Populus tremuloides (Michx.)). Establishment details are described in Marshall (1996) and Marshall and Wang (1996). The trees on the plots in both of these studies are stem mapped and have been re-measured several times, most recently following the 2003 growing season. The first study consists of three different stand structures (6 plots). Partial harvesting took place on these plots in 2005. The second study is a replicated pre-commercial thinning experiment involving three treatments cut in 1991, and a control (24 plots). Lodgepole pine trees on these plots were killed by mountain pine beetle in 2001-2004. To date, the focus of these studies has been on the growth of the residual trees. The aim of this project is to take advantage of the variety of stand structures and treatments included in the two studies described above to relate the quantity and quality of
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