Spruce Budworm Outbreaks Research Articles

Expression of the β-glucosidase gene Pgβglu-1 underpins natural resistance of white spruce against spruce budworm.

Periodic outbreaks of spruce budworm (SBW) affect large areas of ecologically and economically important conifer forests in North America, causing tree mortality and reduced forest productivity. Host resistance against SBW has been linked to growth phenology and the chemical composition of foliage, but the underlying molecular mechanisms and population variation are largely unknown. Using a genomics approach, we discovered a β-glucosidase gene, Pgβglu-1, whose expression levels and function underpin natural resistance to SBW in mature white spruce (Picea glauca) trees. In phenotypically resistant trees, Pgβglu-1 transcripts were up to 1000 times more abundant than in non-resistant trees and were highly enriched in foliage. The encoded PgβGLU-1 enzyme catalysed the cleavage of acetophenone sugar conjugates to release the aglycons piceol and pungenol. These aglycons were previously shown to be active against SBW. Levels of Pgβglu-1 transcripts and biologically active acetophenone aglycons were substantially different between resistant and non-resistant trees over time, were positively correlated with each other and were highly variable in a natural white spruce population. These results suggest that expression of Pgβglu-1 and accumulation of acetophenone aglycons is a constitutive defence mechanism in white spruce. The progeny of resistant trees had higher Pgβglu-1 gene expression than non-resistant progeny, indicating that the trait is heritable. With reported increases in the intensity of SBW outbreaks, influenced by climate, variation of Pgβglu-1 transcript expression, PgβGLU-1 enzyme activity and acetophenone accumulation may serve as resistance markers to better predict impacts of SBW in both managed and wild spruce populations.

Multicentury reconstruction of western spruce budworm outbreaks in central British Columbia, Canada

The western spruce budworm (Choristoneura occidentalis Freeman; WSB) is a widespread and destructive defoliator of commercially important coniferous forests in western North America. In British Columbia, Canada, Douglas-fir (Pseudotsuga menziesii) is the most prevalent host, and records show that significant outbreaks have primarily occurred in the southern interior over the last century. In the central interior of British Columbia, particularly in the Cariboo Forest Region, the most recent outbreak of western spruce budworm, which is in its second decade, has been severe and widespread. It is perhaps due to this long outbreak, which is unprecedented in duration since detailed surveys began in the 1950s, that there is an anecdotal perception that the western spruce budworm is expanding its range northward into the region, affecting Douglas-fir near the edge of its northern distribution. However, very little is known about the long-term outbreak history of western spruce budworm in the central interior of the province. In this study we used 11 Douglas-fir host chronologies and 2 regional Pinus non-host chronologies to reconstruct western spruce budworm outbreaks over a 435-year period. Standard dendrochronological techniques were used to develop tree-ring chronologies at each site and the program OUTBREAK was used to detect outbreak periods. We reconstructed 12 outbreaks starting in early 1600s with a mean return interval of 30years. Sensitivity analysis illustrated that including light, moderate and severe intensity thresholds modified the number of reconstructed outbreaks, their duration and mean return interval. Sub-regional chronologies based on biogeoclimatic subzones and variants (BEC units), revealed strong synchrony of some outbreak events, but not all. Wavelet analysis of the sub-regional chronologies demonstrated the quasi-periodic behavior of western spruce budworm over the entire record, which averaged 32-years. By reconstructing WSB dynamics over 400years we demonstrated that outbreaks observed over the last 40years in the Cariboo Forest Region are not unprecedented.

Partial windthrow as a driving process of forest dynamics in old-growth boreal forests

As climate changes, boreal forest ecosystems may become subject to disturbances that were previously uncommon in some regions. In recent decades, large tracts of northeastern boreal forest of Canada have been affected by different types of climatic events causing a lot of partial and some total stand mortality. Since these disturbances may become more important drivers of forest dynamics, there is a need to document their impact on forest structure. The objectives of this study were to describe temporal dynamics of partial windthrows and determine the effect of partial windthrow on stand composition and understory vegetation. The study was conducted in the North-Shore region of Quebec (Canada). Eighteen plots in closed forests were paired with 18 adjacent windthrow areas, in which trees experienced similar edaphic and climatic conditions. Dendroecological analyses, combined with vegetation sampling, were conducted on each site to determine stand structure and vegetation development through time. Significant increases in balsam fir and shade-tolerant species were observed in windthrow gaps. Tree mortality in windthrown stands was a slow process until the mid-1990s, a period during which spruce budworm defoliation may have played a role in weakening trees and making them more vulnerable to partial windthrow. Greater mortality observed following the mid-1990s was most certainly related to a regional storm. The initial composition of stands plays an important role in driving postwindthrow succession, as balsam fir is more susceptible to treefall. As opposed to stand-replacing windthrow and spruce budworm outbreaks that generate various postdisturbance responses, partial windthrow appears to only create opportunities for pre-established balsam fir to undergo release in gaps.

Spatial variability of spruce budworm defoliation at different scales

Aerial survey records of defoliation by spruce budworm (Choristoneura fumiferana (Clem.)) in New Brunswick, Canada taken annually from 1965 to 1992, were analyzed at four scales (66×66km, 10×10km, 2×2km, 1×1km) to determine patterns and spatial variability of defoliation. Spruce budworm defoliated 92% of the 7.4 million ha study area in at least one year. Cluster analysis was used to group similar defoliation patterns, and 51% of the variation in the 28-year record was described by 28 representative defoliation patterns, at 1×1km cell size. Individual defoliation patterns were grouped into five spruce budworm outbreak classes (termed A–E), which each covered 12–27% of the area. These differed substantially in defoliation duration and severity: classes A–B had only 1–3years of moderate–severe defoliation and 42–75% cumulative defoliation; class C had 8years and 127% cumulative defoliation; and classes D–E had 13–16years and 170–208% cumulative defoliation. Given the degree of ‘averaging out’ of high defoliation occurrences that cause the largest impacts, larger 10×10km and 66×66km cells were inappropriate for impact analysis. Defoliation variability was highest at 20–60% mean defoliation than at lower or higher defoliation levels. Distributions of area in each class in 1×1km cells for 66×66km areas throughout the province showed that with the exception of several small coastal areas, four or five of the above outbreak classes occurred in all areas. The wide variation in spruce budworm outbreak patterns is important in making management decisions that differentiate insecticide or salvage treatments based on defoliation severity, and emphasizes the importance of accurate annual aerial survey or remote sensing-based defoliation assessment.

Growth Response of Northern White-Cedar (Thuja occidentalis) to Natural Disturbances and Partial Cuts in Mixedwood Stands of Quebec, Canada

Northern white-cedar (Thuja occidentalis) is a species of high commercial and ecological value, the abundance of which has been declining since the middle of the 19th century. Very little information regarding its silviculture in mixedwood stands is currently available, even though a significant portion of wood resources comes from these stands. The present study is a retrospective analysis of white-cedar growth in partially harvested mixedwood stands of western Quebec, Canada. Eight stands distributed across two regions were analyzed. Dendrochronological approaches examined long-term diameter growth for sample white-cedar trees and stems of associated species. These approaches were used to reconstruct stand characteristics at the time of harvesting, together with local harvesting intensity. The study demonstrated white-cedar’s capacity to maintain good growth for long periods of time and at large tree sizes. Accession to the upper canopy positions occurs through repeated episodes of suppression/release, most of which seem to be associated with spruce budworm (Choristoneura fumiferana) outbreaks. White-cedar response to partial harvesting varies with tree size, residual basal area and species composition. Growth response was generally stronger for small trees, even though large trees still maintained the best diameter growth. Growth of white-cedar was negatively affected by an increase in softwood proportion in basal area. Growth responses to harvesting could be sustained for a period of 20 years.

Drought-triggered western spruce budworm outbreaks in the interior Pacific Northwest: A multi-century dendrochronological record

Douglas-fir forests in the interior Pacific Northwest are subject to sporadic outbreaks of the western spruce budworm, a species widely recognized as the most destructive defoliator in western North America. Outbreaks of the western spruce budworm often occur synchronously over broad regions and lead to widespread loss of leaf area and decrease in growth rates in affected stands. In spite of the ecological and economic significance of this defoliator, the mechanisms controlling this species’ population dynamics are still not fully understood. We used dendrochronological methods to reconstruct three centuries of western spruce budworm outbreaks at thirteen sites along a transect running from central Oregon to western Montana and compared the outbreak reconstructions with regional drought history. The reconstructions reveal repeated western spruce budworm outbreaks that sometimes persisted more than a decade and were significantly synchronous among sites. Synchrony was higher in the second half of the record, possibly due to changes in forest composition and structure brought about by land use practices. Across stands and regions, there was a moderately strong relationship between initiation of synchronous outbreaks and periods of transitional climate conditions in which moisture availability was below average prior to outbreak initiation, but above average in the first few years of an outbreak. There was a weak relationship between cessation of outbreaks and one or more years of high moisture availability. Outbreaks tended to occur near the end of droughts. The association between climatic variability and outbreak dynamics observed across this transect indicates that climate is an important driver of western spruce budworm outbreaks. Other factors that we did not test, but that may influence outbreak dynamics include stand structure, forest composition, predation, and phenological synchrony between larvae and host trees. Future changes in western spruce budworm outbreak dynamics will be determined by a combination of changing climate, interactions with other disturbance agents, and changing forest composition and structure. Our results suggest that western spruce budworm outbreaks will likely intensify if drought frequency increases in the future.

Periodicity of western spruce budworm in Southern British Columbia, Canada

The western spruce budworm (WSB), ChoristoneuraoccidentalisFreeman), a defoliator of conifers in western North America, causes severe timber losses to forests. In British Columbia, Canada, where the main species damaged is Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, outbreaks of C. occidentalis have been recorded since 1909. However, there is little information on the frequency of outbreaks of this defoliator for previous centuries. This information is needed to establish baselines defining the historic range of variability of this disturbance, to calculate potential depletions in timber supply from defoliation, and to refine forest management plans. Also, precise estimates of budworm recurrence are needed to assess potential ecosystem changes and possible departures from the historic range of this disturbance due to global warming. We used dendrochronology and time series analysis to determine past frequency of spruce budworm outbreaks in southern BC and found that, since the 1500s, outbreaks have been periodic, with a mean return interval of 28years (95% Confidence Interval 21–35years). No data was available before the 1500s. We found the number of outbreaks per century, since the 1800s, was fairly constant, with 3–4 outbreaks per century.

Influence of environmental factors and spatio-temporal covariates during the initial development of a spruce budworm outbreak

Recurrent and synchronous spruce budworm (SBW) outbreaks have important impacts in boreal and sub-boreal forest ecosystems of North America. This study examines the early phase of an outbreak that was developing across a 268,000 km2 area over a period of 9 years (2003–2011). The territory was subdivided in 225 km2 cells, and the relative influence of forest composition, elevation, forest age, average degree-days and soil drainage were examined during three development phases of the outbreak: initial epicenter location, relatively long-distance spread (cell-to-cell expansion), and expansion inside individual cells (within-cell expansion). The results indicate that elevation is the most determinant variable for initial epicenter location. Other variables that were identified as important for outbreak development by previous studies, such as forest composition and average degree-days, were not so important during this phase. However, forest composition and average degree-days were important factors during the cell-to-cell and within-cell expansion phases. Separating outbreak development in distinct phases also allowed to integrate phase-specific spatial and temporal covariates that were highly significant in the models, such as distance from previous year defoliations during the cell-to-cell expansion phase, and the proportion of defoliated stands during the preceding year for the within-cell expansion phase. Overall, this study provides limited evidence that patterns of SBW outbreak expansion could be altered by reducing host tree species abundance in the forest [mainly balsam fir (Abies balsamea) in this region]. More generally, this study suggests that the influence of environmental variables on SBW outbreak development is clearly phase-dependent, and that this landscape-level, process-based approach could be useful to forecast insect outbreak development in forest ecosystems.

Impact of Whole-Tree and Cut-to-Length Harvesting on Postharvest Condition and Logging Costs for Early Commercial Thinning in Maine

Regenerating clearcuts dating from the spruce budworm outbreak of the 1980s are beginning to reach merchantable size throughout northern Maine and would benefit from commercial thinning, but there is no consensus among foresters and the logging industry about how such stands can be efficiently thinned. This study investigated silviculturally effective, operational solutions to implement early commercial thinning treatments. Comparisons between two wholetree (WT) and two cut-to-length (CTL) systems were made in terms of residual stem damage, retention of downed woody material, product utilization, and unit cost of production. Results show significantly more crop trees removed (P 0.030) and more high severity wound area per plot (P 0.011) for the WT systems. There was no difference in the number of stems wounded per plot at the high severity level between harvest methods (P 0.312). Harvest-generated and retained downed wood material volume averaged 511.9 and 205.6 ft/ac for CTL and WT plots, respectively. Round wood production was the same for CTL and WT plots (average 30 tons/ac), but more than four times more biomass was produced from the WT operations. Production costs were not significantly different between harvest methods due in part to high machine productivity and increased biomass production for the WT systems.

The influence of forest composition and climate on outbreak characteristics of the spruce budworm in eastern Canada

The spruce budworm (Choristoneura fumiferana Clemens) is perhaps the single most important disturbance agent in Canada’s eastern forests. Climate and forest composition are dominant factors in spruce budworm outbreak dynamics through their direct influences on the pest, its natural enemies, and its hosts, and through their influence on the multitrophic interactions that are important in outbreak dynamics. A combination of four climate variables, three forest composition variables, and one location variable explained 60% of the multivariate variability in outbreak characteristics (duration and severity) in eastern Canada. Outbreak duration was most strongly influenced by April–May accumulation of degree-days; outbreak severity was most strongly influenced by the extreme maximum temperatures of April–May. The basal area of balsam fir had a stronger influence than that of black spruce on duration and on severity. Both outbreak characteristics declined in more northerly locations. Under a projected future (2011–2040) climate scenario the largest increases in outbreak duration and severity are predicted to occur on the Gaspé Peninsula and the north shore of the St. Lawrence River (Quebec). The largest decreases in duration and severity are predicted to occur in southern Ontario and along the Bay of Fundy in New Brunswick and Nova Scotia. The predicted average change in outbreak duration is around –1.3 years. The predicted average change in outbreak severity is only slightly different from zero (around –1.5% defoliation).

Impact of salvage logging on stand structure and beetle diversity in boreal balsam fir forest, 20 years after a spruce budworm outbreak

Spruce budworm outbreaks represent the most important natural disturbance in the boreal balsam fir (Abies balsamea (L.) Mill.) forest of Canada, killing trees over wide areas and thus generating large amounts of dead wood. This disturbance is part of the dynamic processes occurring within balsam fir forests, and its effects on biodiversity need to be understood in order to move closer towards sustainable forest management. We described stand structure and used beetles as biodiversity indicators to compare the ecological value of salvaged stands, managed afterwards with three different silvicultural treatments, with unsalvaged stands, 20years after the last spruce budworm outbreak. Our experimental design involved five replicated stands of each of the four disturbance types for a total of 20 stands. A total of 3124 Coleoptera belonging to 47 families and 269 taxa were captured over 3months of sampling. Results showed that habitat attributes still differed 20years after a spruce budworm outbreak between unsalvaged and treated salvaged stands, such as more abundant and diverse dead wood in unsalvaged stands. Salvage logging and silvicultural treatments led to different beetle communities than in unsalvaged stands, even though abundance and richness did not change significantly. Most indicator species identified were saproxylic and associated with unsalvaged stands, suggesting salvage logging generated the largest changes to the original ecosystem while additional effects of the silvicultural treatments were less pronounced. This may result from the 67% reduction in downed dead wood volumes (e.g., 138m3/ha in unsalvaged stands vs 46m3/ha in managed stands). Even if this volume of dead wood is still relatively high, it is less diverse than in unsalvaged stands and includes almost no recent dead wood. Salvage logging affected saproxylic beetle communities, suggesting that the retention of naturally disturbed stands may help in implementing ecosystem management in boreal balsam fir forest and move closer towards sustainable management.

Re-examining wood supply in light of future spruce budworm outbreaks: A case study in New Brunswick

Forest protection (spraying of biological insecticide), salvage harvesting, and strategic re-planning are typical mitigation options to reduce wood supply impacts caused by spruce budworm (Choristoneura fumiferana [Clem.]) defoliation. However, all such measures are expensive and difficult to implement, and decisions about if and to what extent such measures should be applied hinge on the magnitude of benefits and associated costs. We used an integrated forest estate pest-impact planning model to quantify harvest impacts for three plausible outbreak scenarios, and the effect of these typical mitigation strategies for 3.0 million ha in New Brunswick. Cumulative harvest reductions, relative to the no defoliation case, reached 18% and 25% by 2052 under moderate and severe defoliation patterns, respectively. We demonstrate that up to 30% to 50% of these projected reductions could be avoided through foliage protection treatments, depending on the outbreak scenario. Salvage and re-planning mitigated harvest losses by up to 20% in the short term (20 to 25 years), but had little benefit over the long run (40+ years). Even with aggressive implementation of all mitigation measures, significant harvest impacts (10% reduction from 2017 to 2042) were unavoidable, regardless of outbreak scenario.

The Relative Importance of Natural Disturbances and Local Site Factors on Woody Vegetation Regeneration Diversity across a Large, Contiguous Forest Region

Stand-level diversity after natural disturbance can potentially differ across a large, contiguous forest region despite being dominated by the same canopy species throughout as differences in disturbance types and local site conditions can regulate species distribution. Our main objective was to examine the relative importance of natural disturbances (spruce budworm (Choristoneura fumiferana) outbreak, windthrow, and their interaction) and local site factors (climate, physiography, and stand structure and composition variables) on woody vegetation diversity among three, physiographically distinct locations across a large, contiguous forest region. Seventy-six Abies balsamea-Betula spp. stands affected by natural disturbance were compared and analysed using canonical ordination methods, diversity indices, and ANOVA. Different combinations of factors were important for vegetation re-establishment at each location. Differences in alpha (α), beta (β), gamma (γ), Shannon’s H’, and evenness (J) diversity indices were observed among locations across the study region. Our findings indicate that while certain processes are important for maintaining canopy dominance by Abies balsamea and Betula spp. throughout the region, different combinations of factors were important for creating variation in woody species diversity among locations that resulted in greater woody species diversity at the regional scale.

Mate‐finding allee effect in spruce budworm population dynamics

AbstractAllee effects can cause populations to decline due to decreasing population growth rates with decreasing density and play a major role in population dynamics. Mate‐finding failure, a common mechanism contributing to demographic Allee effects, is usually difficult to demonstrate because of the arduous nature of sampling individuals at very low densities. In a rising outbreak of the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), we used caged and tethered virgin females in traps to measure mating success over population densities ranging from deep endemic to outbreak conditions. We found that mating success increased with increasing population density, and that at endemic population densities, females experienced difficulties attracting males and mating, demonstrating for the first time a mate‐finding Allee effect in the spruce budworm. The relationship between population density and mating success is nonlinear. As population density increased, the proportion of mated females eventually reached a plateau and mating success was not 100% even at the highest moth densities, probably due to female reluctance to mate and perhaps interference competition by males for access to females. Both laboratory‐reared and wild females were equally effective in synthesizing pheromone, attracting males, and mating. Our results strongly suggest that a mate‐finding Allee effect is involved in maintaining low‐density spruce budworm populations below an Allee threshold where they fail to grow. Factors such as changes in predation pressure and immigration could help populations overcome this Allee threshold.

Is the closed‐crown boreal forest resilient after successive stand disturbances? A quantitative demonstration from a case study

AbstractQuestionRecent studies revealed by indirect evidence that lichen–spruce woodlands in the closed‐crown boreal forest are an alternative stable state of black spruce–feathermoss stands. This paper aims to demonstrate and quantify black spruce forest cover regression following successive wildfire events in the closed‐crown forest zone during the 20th century. A case study in north‐eastern North American closed‐crown forest zone is presented.LocationThe study area is at the centre of the closed‐crown boreal forest zone of eastern Canada (49°59′ N, 71°59′ W).MethodsA burned area of 25 ha was selected in which three tree cohorts were visible. The first cohort contains partly buried trunks and stumps of a stand destroyed by a forest fire in 1920, the second contains standing dead trees from a 1995 forest fire and the third contains seedlings established after the 1995 fire. Fires were dated by scar analyses of living trees at the edge of the site. An array of sample plots was established in autumn 1997 to evaluate each cohort in terms of stocking and tree density.ResultsBlack spruce stocking decreased significantly between cohorts, from 43.5% in Cohort 1 (all coniferous trees) to 25.6% in Cohort 2 and 11.5% in Cohort 3 (2011). Jack pine showed an inverse tendency, from 0% in Cohort 1 (not determined) to 0% in Cohort 2 and 3.8% in Cohort 3. Black spruce stems per hectare also dropped significantly from a minimum of 1322 in Cohort 1 to 1088 in Cohort 2 and 976 in Cohort 3.ConclusionsThis study is the first quantitative demonstration that lichen–spruce woodlands in the closed‐crown boreal forest zone are an alternative stable state of black spruce–feathermoss stands. Stocking levels and densities markedly declined after each of the two fires (1920 and 1995) that both occurred after spruce budworm outbreaks (1910 and 1980) and are most likely responsible for the shift from a closed‐crown to an open woodland.

Regeneration and stand development following a spruce budworm outbreak, spruce budworm inspired harvest, and salvage harvest

Spruce budworm ( Choristoneura fumiferana (Clemens)) (SBW) outbreaks are a major disturbance that influence stand dynamics and succession in balsam fir ( Abies balsamea (L.) Mill.) dominated forests of eastern Canada. We used stand and regeneration data collected from five plots in each of one immature and five mature stands before, during, and 30 years following the last major SBW outbreak in the Cape Breton Highlands to examine the role of stand and regeneration characteristics in shaping future stand development. Comparisons were also made between regeneration in four SBW outbreak stands versus two SBW salvage stands, with and without a subsequent precommercial thinning, and with 25 plots that underwent SBW emulation harvest. In mature unharvested balsam fir stands, species composition 30 years following the SBW outbreak was closely related to predisturbance species composition, and in immature fir stands, hardwood composition increased from 0% to 4%–27%. Species composition in harvested stands varied depending on whether intolerant hardwoods had been precommercially thinned, where thinned stands had 30% less hardwood 30 years postdisturbance than unthinned stands. Seedling density decreased by 17%–85% from 1979 to 1989 in all SBW outbreak stands, but average seedling height increased by 17%–500% as live canopy cover decreased from an average of 50% to 4%. Results suggest that advanced regeneration should be protected during harvest of balsam fir dominated stands, post-outbreak precommercial thinning will increase tree growth, and live tree retention can help develop late-seral structural characteristics in second-growth stands.

Long-term effects of thinning on growth and yield of an upland black spruce stand

Knowledge of the long-term effects of silviculture treatments is crucial to forest management. The long-term effects of thinning, a common and widely used silviculture treatment, is little documented for upland black spruce ( Picea mariana (Mill.) Britton, Sterns & Poggenb.) stands. We revisited a partial cutting experiment installed in 1961 in a 65-year-old unmanaged upland black spruce stand. The aim was to document the long-term effects of thinning on tree and stand growth and to complete previously published results of the first 15 years of response to thinning by determining its influence in terms of merchantable volume. Free thinning was applied following three intensities: 0%, 25%, and 50% of total basal area removal. The retrospective analysis of growth rings showed that the response over the first 15 years was less significant when determined in net merchantable volume instead of net total volume. Heavily thinned plots, nonetheless, showed a net stand merchantable volume increment 33% greater than that of the unthinned plots. In the longer term, a spruce budworm ( Choristoneura fumiferana Clemens) outbreak affected the site; nevertheless, the heavily thinned plots maintained a superior tree growth rate and did not show senescence mortality like the other plots, allowing stand volume to catch up to that of the unthinned plots after 33 years. Results suggested that thinning upland black spruce stands may be useful in mitigating reductions in volume production associated with growing stands to longer rotations as called for by certain ecosystem-based management approaches.

Growth and mortality of balsam fir- and spruce-tolerant hardwood stands as influenced by stand characteristics and spruce budworm defoliation

Balsam fir (Abies balsamea (L.) Mill.)-tolerant hardwood and spruce (Picea sp.)-tolerant hardwood stands were compared along a gradient of hardwood composition (12–82%). Using permanent sample plot data, influences of stand hardwood content, past spruce budworm (Choristoneura fumiferana Clem.) defoliation, insecticide spraying, and stand and site characteristics on survivor growth and mortality of the fir–spruce (spruce budworm host species) component of these stands were determined. Balsam fir basal area, spruce basal area, and percent hardwood were significantly related to fir–spruce survivor growth. Balsam fir mixedwood plots had four to five times higher fir–spruce cumulative mortality (3.7–13.3m2ha−1) than spruce mixedwood plots (1.0–2.9m2ha−1) from 1990 to 2004, and sustained higher wind-related mortality after the budworm outbreak ended in the late 1980s to early 1990s. A mortality regression tree model indicated that initial balsam fir basal area, % tolerant hardwood basal area, maximum cumulative defoliation from 1983 to 1987, and spruce budworm outbreak zone (north, central, south New Brunswick) all significantly influenced fir–spruce mortality. Results demonstrated differences in stand dynamics between these stand types, with lower spruce budworm vulnerability and higher longevity and more stable structure of spruce mixedwood than of fir mixedwood.

Comparison of black spruce (Picea mariana) radial growth reduction in different soil moisture regimes during a spruce budworm (Choristoneura fumiferana) outbreak

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.

Dendrochronological reconstruction of spruce budworm (Choristoneura fumiferana) outbreaks in southern Quebec for the last 400 years1This article is one of a selection of papers from the 7th International Conference on Disturbance Dynamics in Boreal Forests.

It is argued that spruce budworm ( Choristoneura fumiferana (Clemens)) (SBW) outbreaks have tended to be more frequent, severe, and spatially synchronized since the beginning of the 20th century. However, few studies have assessed the long-term (>200 years) variations in SBW outbreak dynamics. We reconstructed the SBW outbreak history at the northern limit of the temperate forest in southern Quebec using dendrochronological material from old buildings and five old-growth stands. Our regional tree-ring chronology (1551–1995) represents one of the longest and most replicated insect outbreak reconstructions in North America. Nine potential outbreaks were identified (1976–1991, 1946–1959, 1915–1929, 1872–1903, 1807–1817, 1754–1765, 1706–1717, 1664–1670, and 1630–1638) with three additional uncertain outbreaks (1647–1661, 1606–1619, and 1564–1578). Results suggested that southern Quebec has experienced frequent and synchronized outbreaks throughout the last 400 years. Although outbreak frequency was higher during the 20th century (approximately 30 years) as compared with the 1660–1850 period (approximately 50 years), similar or even higher outbreak frequency might have occurred prior to 1660 (approximately 28 years). We did not find any evidence that the recent outbreak dynamics in southern Quebec is outside its historical range of the last 400 years. Previous studies based on living trees may have underestimated outbreak frequency and synchrony prior to 1900.