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Treefall in the Mount Tom Group Selection Silvicultural Systems Trial in Central British Columbia

Author(s) or contact(s): M.J. Waterhouse
Source: Southern Interior Forest Region
Subject: Silvicultural Systems
Series: Extension Note
Other details:  Published 2013. Hardcopy is available.


Mountain caribou (Rangifer tarandus caribou) are both provincially and federally listed as Threatened. British Columbia has 98% of the entire global population of this ecotype (approximately 1900 animals) (Hatter 2006). These caribou require older forests in the Engelmann Spruce-Subalpine Fir biogeoclimatic zone (ESSF) because arboreal lichens (winter forage) are often abundant in the tree canopies. Clearcut harvesting not only removes arboreal lichens, but also a condition of low lichen abundance could persist for more than a century in the regenerated forest (Stevenson et al. 2001). On the other hand, the group selection silvicultural system provides continuous lichen-bearing habitat through space and time (Waterhouse et al. 2007).

The group selection silvicultural system is one of the 'modified harvesting' prescriptions described in the Cariboo-Chilcotin Land Use Plan (CCLUP) Mountain Caribou Strategy (Youds et al. 2000). One risk associated with using a group selection silvicultural system is decreased stand stability due to a large area of exposed edge (Ruel 1995). This causes economic loss and increases the risk of tree mortality due to bark beetles, which are endemic to the ESSF. Moreover, it could also damage caribou feeding habitat if treefall and tree mortality are excessive. The Quesnel Highland research trial was developed to measure the lichen and stand stability response to the group selection 'modified harvesting' prescriptions (Waterhouse et al. 2007). Over a 10-year period, a low amount of treefall (< 1 %/yr) was documented in the forest matrix around group selection openings that were 0.03 - 1.0 ha. Other shorter-term studies from across the British Columbia interior have documented low rates of treefall within the forest matrix during the first few years of partial cutting (Coates 1997; Huggard et al. 1999; Quesnel and Waters 2001; Waterhouse and Armleder 2004; Waterhouse 2009).

In the Quesnel Highland trial, the amount of treefall on edges of the openings was not measured, but results from other trials in British Columbia indicated that there was an elevated amount of treefall on edges of small openings in some cases (Huggard et al. 1999; Waterhouse 2009) but not in others (Quesnel and Waters 2001). Compared to the forest matrix, freshly created edges of openings are particularly sensitive to treefall because the trees have developed in the context of a stand, and the stand collectively reduces wind penetration (Ruel 1995). The size of the opening could also affect the amount of treefall on an opening edge. In clearcuts, the treefall rate increases on edges exposed to stronger, more turbulent wind (DeWalle 1983), with more wind fetch distance (Steinblums et al. 1984; Burton 2001), and set perpendicular to the wind (Ruel 1995). The placement of openings in relation to topographic features and soil conditions is a key factor in determining stability (Stathers et al. 1994). Also, the tree species, size, spatial arrangement, and condition determine the stability of the stand, particularly on opening edges (Ruel 1995).

The Mount Tom adaptive management trial is an extension of the Quesnel Highland trial at an operational scale (Waterhouse 2011). The trial covers > 4000 ha of caribou habitat near Wells, B.C., and the many blocks cover a range of geographic positions and ecosystems. Within each block, the openings range in size (0.1 - 1.0 ha) and have irregular shapes, reflecting an operational approach to harvesting. This trial provided the opportunity to examine the windfirmness of edges around irregularly shaped openings and to determine if treefall rate was affected by opening size. It also provided the opportunity to examine opening placement and characteristics of the fallen trees. This could have implications for the current CCLUP Mountain Caribou Strategy (Youds et al. 2000) regarding the range of acceptable opening sizes, and could provide edge management strategies.

Specific objectives were to:
- compare rates of treefall among (a) the forest matrix of each block, (b) the 10-m forested edges around openings, and (c) the forested controls,
- compare rates of treefall on the perimeters among openings of different sizes, and
- describe the attributes of the treefall (decay class, size class, species, crown class, and direction of fall).

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Updated March 25, 2013