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Modeling phenology and outbreaks of the western spruce budworm Nealis, Vince G.
Abstract: The western spruce budworm, Choristoneura occidentalis, is the most destructive insect on Douglas-fir in BC. Damaging outbreaks have become chronic in the IDF zone, especially in the southern interior, since the 1970s (Maclauchlan et al. 2006). More than 500 000 ha have been defoliated every year since 2004. The striking characteristic of these recent outbreaks is their extension over the entire ecological range of Douglas-fir in the central and southern interior of the province. This represents a dramatic northward extension of damage into the Cariboo region as well as upward in elevation to mixed Douglas-fir/spruce/lodgepole pine forests and is unprecedented in historical times. One indicator of this concern to forestry was the largest Btk spray program ever conducted for this insect in BC in 2006. ( A strong influence of weather and climate on western spruce budworm outbreaks has long been suspected. Trees in dry, stressed areas seem to be at greatest hazard to western spruce budworm (Maclaughlan et al. 2006) and the observation of defoliation in specific and shifting elevation bands across the landscape is common. Early, survey-based models invoked weather-driven variation in the synchrony of insect emergence and bud-burst as a driving factor for these outbreak patterns (Thomson et al. 1984). A recent application of this early model suggests that the absence of outbreaks in coastal Douglas-fir since the 1930s is related to a warming trend in the coastal climate which has desynchronized the critical relationship between budworm feeding and bud-flushing dates of coastal Douglas-fir (Thomson and Benton 2008). Similarly, an improvement in synchrony associated with current climate conditions in the BC interior may be contributing to the exceptional severity, duration, and extent of the outbreaks there. If so, not only has the hazard profile for western spruce budworm changed over an extensive area, but many of the forests at risk have little or no historical signal of previous losses in their growth profiles. Todays depletions are new, and perhaps unaccounted, reductions in expected volume projections. As future timber supply in the wake of the mountain pine beetle will rely more heavily on non-pine species, increased protection from spruce budworm may be required for interior Douglas-fir not only in mature but in juvenile stands. Non-timber values are also at risk from increased budworm disturbance as Douglas-fir plays an important role in hydrology and wildlife ecology in dry, interior forest and range ecosystems. Assessment of these potential changes now will provide more management options for mitigation and adaptation. This project will develop a seasonal, process-oriented phenology model for western spruce budworm that can be applied to questions of current and future risk of BCs Douglas-fir forests to damaging outbreaks, and for aiding operational decisions around control. Process models describe data using key mechanisms or processes that determine the intrinsic structure and behavior of a system. By comparison, empirical models, often used in forestry, for example, to estimate hazard, emphasize statistical relationships with more emphasis on descriptive than causative structure. A process model for phenology will use parameters which describe eco-physiological relationships between weather variables and insect development and survival to analyze and predict target events such as seasonal occurrence of life stages and rates of change in population densities for any given location. Because the parameters of the process model are general, they remain relevant for new conditions, whether these be the need to predict target events in different locations or times, or in different weather regimes likely under climate change. Process models are thus flexible tools of great use at multiple spatial and temporal scales which characterize the information challenges of
Nealis, Vince G.. 2009. Modeling phenology and outbreaks of the western spruce budworm. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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