Connecting the dots: elucidating the relationship between spruce budworm population dynamics and defoliation

The objective of this study was to determine if the stand‐level soil moisture regime had a significant effect on the reduction in black spruce (Picea mariana (Mill.) B.S.P.) radial growth during the most recent spruce budworm (Choristoneura fumiferana Clem.) outbreak in the boreal forest region of northeastern Ontario. We collected a stratified random sample of co-dominant black spruce trees from three moisture regimes and compared the reduction of radial growth during a spruce budworm outbreak between dry, moist, and wet stands. We focused on the most recent outbreak from 1975–1987, which we dated by dendrochronological analysis of black spruce increment cores from the Romeo Malette Forest near Timmins, Ontario. Samples collected from dry and moist sites showed significantly greater maximum radial growth reduction than those from wet sites. Mean growth reduction over the entire outbreak was not significantly different among moisture regimes but followed the same trend. We found no evidence of spatial autocorrelation in the growth reduction response, suggesting that the moisture effect was not confounded by location.

The Canada warbler (Wilsonia canadensis) is one of many common neotropical migrants whose populations are in decline across their range. Influences of habitat loss and degradation on breeding or wintering grounds have been postulated as possible causes, but few empirical data exist to support a specific cause. Based on previous studies linking abundances of Canada warbler and spruce budworm (Choristoneura fumiferana), we hypothesized that the Canada warbler may be influenced by a persistent decline in spruce budworm throughout the bird's breeding range, a hypothesis that has received little attention. This hypothesis makes 5 predictions: 1) budworm outbreaks and warbler detections should be spatially and temporally coincident; 2) the relationship between Canada warbler and spruce budworm outbreaks should be similar to relationships for other warblers known to be spruce budworm associates; 3) the relationship should be stronger than for warblers lacking an association with spruce budworm; 4) because temporal trends of both spruce budworm and Canada warblers have varied throughout Canadian provinces, declines in Canada warblers should be seen only in provinces where spruce budworm also declined; and 5) variation in Canada warbler abundance should reflect variation in supply of preferred habitat for the spruce budworm if habitat rather than budworm abundance is the key. Our analyses supported predictions 1–4, suggesting that Canada warbler may be even more closely associated with spruce budworm than are known associated species, a phenomenon noted in the literature but previously unexplained. Prediction 5 was not supported, because budworm habitat (area of mature and older balsam fir [Abies balsamea] and white spruce [Picea glauca]) remained constant in Ontario while warbler abundance declined. Although the correlative nature of these results precludes inference of a causal relationship between the declines of the Canada warbler and spruce budworm, we postulate that potential links may exist directly, where spruce budworm outbreaks provide elevated levels of insect prey items for breeding Canada warblers, or indirectly through changes in forest structure and composition following outbreaks. These results have implications when considering long‐term trends in Canada warbler populations, because it may be impossible to alter population trends for species linked to the timing and magnitude of spruce budworm outbreaks.

Spruce budworm (SBW) outbreaks are a major disturbance in North American forests. In Quebec, Canada, the recent history of SBW outbreaks at the local scale is well-known. Studies at the Québec scale nonetheless remain rare despite the need to better understand the dynamics of SBW outbreaks at a larger scale. This study aimed to reconstruct the spatiotemporal dynamics of SBW outbreaks during the 20th century across the insect's range in southern Quebec. To this end, we sampled 83 stands throughout southern Quebec. These stands were selected according to their age and the presence of black, white, and red spruce. In fact, spruce, unlike balsam fir, survives the SBW outbreaks and can record them in these growth rings. In each stand, cores were taken from 20 spruce trees. The dendrochronological series of more than 1,600 trees were analyzed, and we identified, through the k-means grouping of stands, the spatial patterns of tree growth for the three previously documented 20th century SBW outbreaks. The outbreaks were not homogeneous across the distribution range of the insect. Two groups of stands showed early- (1905–1930) and late-century (1968–1988) outbreaks of high severity and a mid-century (1935–1965) outbreak of moderate severity. This pattern is explained mainly by the presence of the insect within the balsam fir–yellow birch and balsam fir–white birch bioclimatic domains, areas where outbreaks tend to be most severe because of the abundance of balsam fir, the main SBW host species. However, these two models differ in terms of the duration of outbreaks. A third, more northern, cluster of stands experienced lower severity outbreaks over the 20th century, a pattern explained by a lower proportion of balsam fir trees in these landscapes. Our study shows that, on the one hand, these three groups of stands are defined by outbreaks of specific duration (an outbreak period beginning when more than 20% of the trees are affected and ending when <20% of the trees are affected) and severity (in terms of percentage of affected trees), and on the other hand they are spatially distinct and subject to different climatic conditions.

Spruce budworm (Choristoneura fumiferana) outbreaks cannot be prevented, but the amount of damage that occurs can be managed. Tree species, stand age, hardwood content, and drainage class, as well as the outbreak severity and length, determine the amount of tree mortality during budworm outbreaks. Silviculture and forest management can be used to reduce the incidence of the most damaged stand types across the landscape. The amount of defoliation in mixed balsam fir-hardwood stands is strongly negatively related to hardwood content, especially with hardwoods > 40%. The Spruce Budworm Decision Support System (DSS) links models of stand and forest response to budworm outbreaks and inventory interpretation to a GIS, and can be used to evaluate effects of outbreaks and management on forest structure and timber supply. An example using the Spruce Budworm DSS for a portion of the Fundy Model Forest indicated that losses from a future budworm outbreak could be reduced 34% by directing harvesting and silviculture towards conversion of one-half of the most vulnerable stand types into low susceptibility or non-susceptible species. Key words: decision support system, vulnerability, silviculture, insect damage

Salvage logging during spruce budworm outbreaks increases defoliation of black spruce regeneration

In the southern boreal forest of eastern Canada, black spruce (Picea mariana (Mill.) BSP) is subjected to both defoliating insect and fire disturbances. As black spruce depends on its aerial seed bank for postfire regeneration, reduction of cone crop during a spruce budworm (Choristoneura fumiferana (Clem.)) outbreak opens a vulnerability window during which stand regeneration could be hindered in the event of a fire. To assess the long-term effect of spruce budworm outbreak on black spruce reproductive potential, cone production and viable seed bank were estimated using cone crop surveys and germination trials in black spruce – lichen woodland stands that sustained different levels of defoliation during the 1980s. Black spruce cone crop was significantly related to the defoliation history of the stands (R2 = 0.89), but not to stand age, basal area, or tree density. Black spruce stands damaged by severe defoliation showed a smaller number of cones, a higher incidence of insect-damaged cones, and a viable seed bank 3 to 17 times smaller than a lightly defoliated stand. The vulnerability window for black spruce regeneration following a spruce budworm outbreak may be as long as 20 years in the study area because black spruce seed bank in heavily defoliated stands has not yet replenished. Our work supports conclusions from stand reconstruction studies that suggest closed-crown spruce–moss stands convert to open lichen woodlands as a result of weak postfire regeneration caused by successive insect and fire disturbances.

Climate change is altering insect disturbance regimes via temperature‐mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest insects are occurring in Europe and North America. The degree to which northern forest ecosystems are resilient to novel disturbance regimes will have direct consequences for the provisioning of goods and services from these forests and for long‐term forest management planning. Among major ecological disturbance agents in the boreal forests of North America is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abiesspp.) and spruce (Piceaspp.). Northern expansion of this defoliator in eastern North America and climate‐induced narrowing of the phenological mismatch between the insect and its secondary host, black spruce (Picea mariana), may permit greater defoliation and mortality in extensive northern black spruce forests. Although spruce budworm outbreak centers have appeared in the boreal black spruce zone historically, defoliation and mortality were minor. Potential increases in outbreak severity and tree mortality raise concerns about the future state of this northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity compared with their occurrence in more diverse, southern balsam fir forest landscapes that have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient cycling could alter ecosystem dynamics and replace black spruce by more productive mixed‐wood forest, or by less productive ericaceous shrublands. Long‐term monitoring, manipulative experiments, and process modeling of climate‐induced phenological changes on herbivorous insect pests, their host tree species, and natural enemies in northern forests are therefore crucial to predicting species range shifts and assessing ecological and economic impacts.

Dermal absorption of triclosan following short- and long term exposure in an ex vivo human skin model

In the boreal forest of Quebec, 80% of harvested black spruce (Picea mariana [Mill.] BSP) stands regenerate naturally. In the remaining 20%, forest regeneration is ensured by planting seedlings and these plantations are expected to increase future forest yields. However, predictions of future yields using the information from very young plantations in this ecosystem may have low accuracy. To compare juvenile growth in plantations versus naturally regenerated stands, and also to evaluate the impacts of spruce budworm Choristoneura fumiferana (Clemens) defoliation periods during the juvenile phase, annual height and volume growth were calculated from measurements on nine black spruce stands. The nine sites consisted of two plantations and seven post-fire, naturally regenerated stands. Of the seven naturally regenerated sites, four were epidemic and 3 endemic. The term “epidemic” refers to stands regenerated during known spruce budworm defoliation periods. The term “endemic” refers to stands regenerated during periods without severe defoliation. For height and volume growth, 5 different methods were used and compared. The annual height and volume growth of all black spruce showed a sigmoid trend, characterized by low values in the first years after germination, followed by an exponential acceleration and finally oscillation around a horizontal asymptote. The black spruce stands established during endemic periods were more productive than those established during epidemic periods, but plantations were the most productive. Key words: black spruce, juvenile growth, plantation, post-fire stands, spruce budworm outbreak, volume, yield

Simulations of the effects of changes in mean fire return intervals on balsam fir abundance, and implications for spruce budworm outbreaks

Response of understory plant community of boreal mixedwood stands to fire, logging, and spruce budworm outbreak

A stand and landscape comparison of the effects of a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak to the combined effects of harvesting and thinning on forest structure

Balsam fir (Abies balsamea) is the most vulnerable species to the spruce budworm (Choristoneura fumiferana), one of the most devastating defoliators in the world. For decades, pest managers have advocated for reducing its abundance in the landscape to minimize losses to the spruce budworm (SBW). Although reduction of fir occurred during the endemic phase of the SBW cycle, there is little information about the extent to which this general principle (reduction of fir) was applied during an outbreak and whether it occurs at both stand and landscape levels. The objective of this paper is to compare the effect of insect and harvest disturbances on forest structure during the 1970–80s outbreak in Québec. We evaluate whether, (i) forest management activities targeted fir forests and whether patch size of host species influences management or SBW disturbance, (ii) SBW outbreaks and logging have similar or divergent effects on forest composition. Although data are from an earlier outbreak, they are at a scale rarely studied and will be useful in guiding decisions made at larger scales in the current and future outbreaks. Our results show that spruce was targeted preferentially by harvesting (up to 69% of plots) during the outbreak period, while it represented less than one third of plots defoliated by the SBW. On the other hand, fir stands represented up to 75% of plots that were defoliated by the SBW but less than 35% of plots that underwent harvesting. Harvesting targeted large blocks of spruce forest more than large blocks of fir-dominated forest while the opposite was observed for the SBW. In terms of regeneration, SBW tends to reduce fir and favor spruce recruitment, along with non-host species, whereas the opposite tendency was observed following harvesting. In terms of spatial organization of stands, our results support the suggestion that small stands of fir and large stands of spruce undergo the least SBW damage. Thus, in order to attenuate SBW impacts in the future, efforts should be made to ensure that spruce recruitment is favored and that its abundance increases at both the stand and landscape scale.

This study examined the interactions between thinning and soil drainage classes on the resistance of balsam fir, Abies balsamea (L.) Mill, white spruce, Picea glauca (Moench) Voss, and black spruce, P. mariana (Mill.) BSP, to spruce budworm, Choristoneura fumiferana (Clem.), defoliation 1 year after treatment. To estimate host tree resistance, foliage production and larval foliage consumption were determined to generate an index of resistance quantifying the amount of residual foliage available for photosynthesis after insect defoliation. Significant interactions on tree resistance and foliage chemistry were detected between thinning and soil drainage in balsam fir. Drainage class affected spruce budworm performance, foliar chemistry and balsam fir resistance to spruce budworm, whereas no effect was found in white spruce. Thinning had a significant effect on the foliar chemistry of balsam fir and white spruce, but no effect on black spruce. Thinning reduced balsam fir resistance to spruce budworm defoliation. This response is due to increased defoliation linked to reduction in concentrations of certain monoterpenes, and a decrease in foliage production, except on hydric drainage, demonstrating the importance of drainage class to tree resistance. The results suggest that the use of Bacillus thuringiensis might be required when conducting stand thinning during spruce budworm outbreaks.

Spruce budworm (Choristoneura fumiferana) is the main defoliator of conifer trees in North American boreal forests, affecting extensive areas and causing marked losses of timber supplies. In 2017, spruce budworm affected more than 7 million ha of Eastern Canadian forest. Defoliation was particularly severe for black spruce (Picea mariana (Mill.) B.S.P.), one of the most important commercial trees in Canada. During the last decades, intensive forest exploitation practices have created vast stands of young balsam fir (Abies balsamea (L.) Mill.) and black spruce. Most research focused on the impacts of spruce budworm has been on mature stands; its effects on regeneration, however, have been neglected. This study evaluates the impacts of spruce budworm on the defoliation of conifer seedlings (black spruce and balsam fir) in clearcuts. We measured the cumulative and annual defoliation of seedlings within six clearcut black spruce stands in Quebec (Canada) that had experienced severe levels of defoliation due to spruce budworm. For all sampled seedlings, we recorded tree species, height class, and distance to the residual forest. Seedling height and species strongly influenced defoliation level. Small seedlings were less affected by spruce budworm activity. As well, cumulative defoliation for balsam fir was double that of black spruce (21% and 9%, respectively). Distance to residual stands had no significant effect on seedling defoliation. As insect outbreaks in boreal forests are expected to become more severe and frequent in the near future, our results are important for adapting forest management strategies to insect outbreaks in a context of climate change.