Forest Investment Account (FIA) - Forest Science Program
FIA Project M086045

    Balancing Disturbances in Forest Management
Project lead: Nealis, V.G. (Natural Resources Canada)
Contributing Authors: Nealis, Vince G.; Turnquist, Rod; Noseworthy, Meghan; Waring, Vince
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
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 represents critical future timber supply in the depleted forests. The objective of this proposal is to evaluate and quantify the response of populations of the western spruce budworm on Douglas-fir in mixed stands following selective removal of lodgepole pine in terms of outbreak characteristics of direct importance to forest health. Established methodologies and results available from current ecological research will be adapted to address the question of immediate to mid-term forest health in post-beetle stands and the challenge of designing alternative management regimes that balance and mitigate both current and future disturbance from insects. Accelerated harvest of lodgepole pine in response to the mountain pine beetle provides stand-level modifications of forest structure and a unique opportunity to examine the impact of this forest practice on risk of damage from the spruce budworm to remaining trees. This proposal will take advantage of recent advances in our understanding of the population ecology of budworms and current forestry practices associated with mitigation of a specific disturbance, to develop a basis for integrated, adaptive management of mixed forests subject to different disturbance regimes. Specifically, we know that rates of increase in budworm populations are strongly influenced by relative phenology of spring emergence of the insects and bud-flush, previous levels of defoliation, stand density, and the impacts of late-larval natural enemies. Each of these variables can be expected to change with altered stand structure as stand-level microclimate, foliage volumes, and the diversity of natural enemies are altered. Methods exist for measuring reliably each of these processes at the population level and so should be able to detect significant differences in altered stands. There are reasons to hypothesize both increased or decreased risk of budworm outbreaks following selective harvesting of lodgepole, but no way of providing useable guidelines without stand-level measurements. In 2006, 10 pairs of plots (logged vs intact) were established near Clinton, Merritt, and Princeton, BC. Plot measurements were taken, recent damage history quantified, and budworm populations monitored with respect to density, stage-specific rates of change, mortality factors and defoliation. Results have been tabulated and descriptive summaries prepared. A literature search on both budworm outbreaks and the effects of forestry practices on outbreak behavior has been completed. In the second year, the population measures will be continued and reports will be prepared.
Related projects:  FSP_M075045


Executive Summary (22Kb)
Western Spruce Budworm Report (37Kb)
Balancing Disturbances (Presentation) (2.4Mb)
Understanding Insect Disturbances (LINK Article, Vol. 9, Issue 1)

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Updated August 16, 2010 

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