Impacts of insect outbreaks on tree mortality, productivity, and stand development

Abstract

Abstract-The impacts of insect outbreaks on tree mortality, productivity, and stand development in Canada are reviewed, emphasising spruce budworm (Choristoneura fumiferana (Clemens), Lepidoptera: Tortricidae) and mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae). Reduced growth and survival are a function of insect population and defoliation level. It is feasible to make short-term (annual) predictions of insect population and defoliation based upon sampling, but long-term, multi-year predictions are problematic. Given the historical record, it is expected that outbreaks will occur with relatively predictable frequency and basic host relationships and abiotic constraints will not change dramatically. However, the precision of predictions at fine scales is variable and reduced over time. Relationships between tree growth reduction, survival, and cumulative defoliation or beetle population level are available for major insect species. Understanding insect outbreak effects hinges on mortality, changes in interspecies competition, regeneration, and succession. Altered stand dynamics caused by insects can be interpreted for indicators such as wildlife habitat, old forest, riparian buffer cover, viewscapes, and connectivity. Anthropogenic changes are altering impacts via range expansions, northward shifts, and changes in forest composition. We can better understand effects of insect outbreaks and how best to ameliorate damage through a combination of empirical permanent plot studies, modelling, and manipulative experiments.

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…Integrating a range of forest disturbances into a DGVM could improve the accuracy of forecasting and modelling climate change effects on Canada's eastern boreal forest. For instance, insect damage (MacLean, 2016) and outbreaks of eastern spruce budworm (Choristoneura fumiferana) in particular (Zhang et al, 2014;James et al, 2017) represent significant forest disturbances by the way they temporarily alter forest structure by affecting specific tree growth, tree survival, regeneration, and succession. These disturbances can also have an important impact on fire activity by modifying fuel distribution and connectivity (James et al, 2017).…”

Spruce Budworm (Choristoneura fumiferana Clem.) Defoliation Promotes Vertical Fuel Continuity in Ontario’s Boreal Mixedwood Forest

“…It has long been recognized that spruce budworm preferentially attacks over-mature, dominant and co-dominant host trees and that, consequently, outbreaks can have major impacts on stand ages and tree heights of the host species [26]. In fact, our measurements suggest structural changes that would reduce tree height and stand age.…”

Cross‐scale effects of spruce budworm outbreaks on boreal warblers in eastern Canada

“…Annual defoliation less than 30% has little effect on tree growth or mortality (Erdle & MacLean, 1999; MacLean, 2016). This single category of budworm defoliation (moderate and severe combined) is the only spatially explicit measure of budworm incidence available on the national scale, and so more detailed explorations of the effects of low, moderate, and severe defoliation were not possible.…”

“…At a broader scale, the stratum‐level budworm variable was calculated for each year‐ t , as the average, across the previous 4 years, of the proportion of the forested area of each stratum‐ i that was affected by moderate or severe defoliation ($b_{2_{i,t}}=\sum _{y=1}^4{b}_{2_{i,t-x}}/4$). A 4‐year lag was chosen because local responses to budworm by warblers can continue over 4–5 years (Venier et al., 2009), and the effect of prolonged defoliation on forest health relates to the defoliation over the last 4–5 years (Erdle & MacLean, 1999; Gray & MacKinnon, 2006; MacLean, 2016). …”

“…Such outbreaks can affect hundreds of thousands of hectares of its host tree species (Simpson & Coy, 1999), primarily balsam fir ( Abies balsamea ), white spruce ( Picea glauca ), red spruce ( Picea rubens ), and black spruce ( Picea mariana ). Thus, spruce budworm is the most important of North America's native forest defoliators (Hardy, Mainville, & Schmitt, 1986; MacLean, 2016). …”