Document Details

Balancing Disturbances in Forest Management
Nealis, Vince G.
The purpose of this project is to examine the effect of stand-level modifications resulting from removal of lodgepole pine to mitigate losses from mountain pine beetle, on the risk of disturbance to residual Douglas-fir from the western spruce budworm. Outbreaks of the western spruce budworm (wSBW), Choristoneura occidentalis, are characteristic of western forests. These outbreaks have become more extensive in the past century, apparently associated with human-induced changes to the landscape (Swetnam and Lynch 1993). In Oregon, for example, selective removal of Ponderosa pine, decreased grazing and increased fire suppression resulted in a more mixed-conifer forest with a continuous canopy that significantly increased its susceptibility to wSBW (Powell 1994). Defoliation by wSBW has been mapped in British Columbia since 1909 but there has been a dramatic increase in the area of defoliation beginning in the late 1970s in the southern interior. This outbreak peaked at nearly 1 million ha in 1987 (Parfett et al. 1994). Following a brief period of decline in the mid-1990s, the area of outbreak began increasing again and has exceeded 500 000 ha annually since 2003 (Maclauchlan et al. 2004). More than 750 000 ha were defoliated in 2006. Significantly, the recent outbreak is not confined to the areas of historical defoliation in the southern interior but occurs also in the central interior as far north as Quesnel. This indicates the entire range of Douglas-fir in the province is at risk. In BC, outbreaks of wSBW occur principally on Douglas-fir. Nearly all trees in all age classes within an area of outbreak are impacted. Repeated defoliation results in mortality of early and advance regeneration and significant reductions in growth and form of intermediate and dominant trees. These impacts persist for several years after the actual outbreak declines, affecting both current and future inventory (Alfaro et al. 1982). Long-term volume losses may exceed 50%(Alfaro and Maclaughlan 1992). Short-term management options to protect the forest are restricted to treatment with the biological insecticide, Btk. Typically; only a small proportion of an outbreak can be treated (Maclaughlan et al. 2004). Forest managers seek solutions to pest problems that can be effectively integrated with existing forest practices at the stand level and result in sustainability in the long-term at the landscape level. Adaptive modification of local silvicultural practices referenced to landscape-level disturbance patterns offers a cross-scale approach to forest pest management planning. In the case of western spruce budworm, there is increasing evidence that stand structure and interactions between site-specific tree condition and weather-related growth rates significantly influence the dynamics of budworm outbreaks and the nature and severity of their impacts. The causative pathways behind the association of weather and outbreaks operate at a meso- and micro-scale via the intimate relationship between budworm and its tree host (Thomson 1979, Nealis and Nault 2005). Understanding these host-plant relationships at the individual and stand levels offers the most promising opportunities for adaptive management of risk. Douglas-fir often grows in association with lodgepole pine in the southern interior of BC, both as mixed forests at the stand level and as part of the diverse mosaic of forest types at the landscape level. As mountain pine beetle infestations threaten these forests, forest managers have responded with accelerated, selective harvesting of lodgepole pine, either to salvage damaged stands or to reduce future susceptibility of stands to mountain pine beetle. This prescription is applied to stands with at least 30% lodgepole pine and so results in stands with significantly reduced stem density and more homogeneous species composition, often leading in mature Douglas-fir. This residual forest is the inventory following the bark beetle outbreak and so rep ...
Report Number
Executive Summary
Western Spruce Budworm Report
Balancing Disturbances (Presentation)

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