Landscape-scale patterns of eastern spruce budworm outbreak risk: Defoliation onset vs. tree mortality
The stand-level risk of both defoliation and mortality due to the spruce budworm (Choristoneura fumiferana, Clem., SBW) is mainly linked to the quality of the resource such that stands dominated by the primary host species, balsam fir (Abies balsamea [L.] Mill.), are considered the most vulnerable. At the landscape scale, insect dispersal suggests that both configuration and resource availability should influence the onset and subsequent mortality of an outbreak. However, outbreak risk at this scale is yet to be quantified for the SBW, one of the most studied forest pest defoliators in the world. We aim to determine the risk of both SBW defoliation onset and resultant tree mortality at a landscape scale due to factors that have been associated with SBW defoliation at the stand scale. We used Cox proportional hazard models to quantify the relative risk of different landscape configuration and species composition patterns on defoliation onset and risk of mortality of the current SBW outbreak in Eastern Canada. In particular, we measured the risk associated with primary host proportion, relative amounts of secondary host and hardwood trees to fir trees, the structure of balsam fir in the landscape, and the structure of the forest overall. We show that defoliation onset is more related to the configuration of the landscape than to the species composition while the risk of ensuing mortality increases as the proportion of the primary host increases. Defoliation onset risk is increased by a more complex configuration of fir patches in the landscape. Mortality risk, however, is unaffected by fir configuration but is reduced where there is more black spruce (Picea mariana [Mill.] B.S.P.) or a decrease of balsam fir in the landscape. The usefulness of survival analysis for assessing risk for insect outbreaks has been overlooked and can quantify the effect of relevant factors to guide management and mitigation strategies. The results of our analysis suggest that forest management strategies should avoid clear cuts and the development of fir monocultures and instead maintain or increase the proportion of black spruce and improve forest connectivity across large forested landscapes to delay defoliation onset and reduce SBW-related mortality.