Forest Tent Caterpillar Outbreaks Research Articles

Diversity, Stability, and the Forecast Challenge in Forest Lepidopteran Predictive Ecology: Are Multi-Scale Plant–Insect Interactions the Key to Increased Forecast Precision?

I report on long-term patterns of outbreak cycling in four study systems across Canada and illustrate how forecasting in these systems is highly imprecise because of complexity in the cycling and a lack of spatial synchrony amongst sample locations. I describe how a range of bottom-up effects could be generating complexity in these otherwise periodic systems. (1) The spruce budworm in Québec exhibits aperiodic and asynchronous behavior at fast time-scales, and a slow modulation of cycle peak intensity that varies regionally. (2) The forest tent caterpillar across Canada exhibits eruptive spiking behavior that is aperiodic locally, and asynchronous amongst regions, yet aggregates to produce a pattern of periodic outbreaks. In Québec, forest tent caterpillar cycles differ in the aspen-dominated northwest versus the maple-dominated southeast, with opposing patterns of cycle intensity between the two regions. (3) In Alberta, forest tent caterpillar outbreak cycles resist synchronization across a forest landscape gradient, even at very fine spatial scales, resulting in a complex pattern of cycling that defies simple forecasting techniques. (4) In the Border Lakes region of Ontario and Minnesota, where the two insect species coexist in a mixedwood landscape of hardwood and conifers, outbreak cycle intensity in each species varies spatially and temporally in response to host forest landscape structure. Much attention has been given to the effect of top-down agents in driving synchronizable population cycles. However, foliage loss, tree death, and forest succession at stem, stand, and landscape scales affect larval and adult dispersal success, and may serve to override regulatory processes that cause otherwise top-down-driven periodic, synchronized, and predictable population oscillations to become aperiodic, asynchronous, and unpredictable. Incorporating bottom-up effects at multiple spatial and temporal scales may be the key to making significant improvements in forest insect outbreak forecasting.

Forest tent caterpillar (Lepidoptera: Lasiocampidae) across Canada, 1938–2001: II. Emergent periodicity from asynchronous eruptive anomalies

Abstract Using aerial sketch map data spanning more than 1 000 000 km2 across Canada, I use cluster analysis to show that outbreaks of forest tent caterpillar, Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), through the 20th century have occurred regularly every decade or so; however, there are two distinct aspects to the patterning of outbreaks. The dominant mode of variability is a nonrecurring pattern of singular spike anomalies, lasting just a few years, that are regional in extent but are not synchronised across the country. The regional time series derived from cluster analysis that are dominated by these singular spike eruptions exhibit extreme skewness and kurtosis, are not stationary in mean or variance, and are not amenable to classical time-series analysis. Although these regional-scale eruptive anomalies tend to occur periodically in aggregate, their central location always varies in an unpredictable manner, resulting in aperiodic local behaviour. Range-wide periodicity is thus an emergent property from asynchronous, aperiodic eruptions aggregated across regions. The second mode of variability is a low-amplitude fluctuation of weak periodicity that is weakly synchronised across the country. These observations support a hybrid cyclic–eruptive theory of outbreak occurrence that is not consistent with the simpler idea of spatially synchronised cycling.

Confronting the cycle synchronisation paradigm of defoliator outbreaks in space and time—Evidence from two systems in a mixed‐species forest landscape

Abstract Defoliators cause extensive damage in boreal and temperate forests of the world. Considerable effort has been invested to understand their individual population dynamics, and despite ample theorising, there is little empirical evidence on factors causing spatial synchrony of pest eruptions at landscape scales. We report on the landscape‐level effect of forest configuration and composition on the intensity of outbreaks of spruce budworm and forest tent caterpillar in a mixedwood boreal forest in northern Minnesota (USA) and adjacent Ontario (Canada), and how this is related to the degree of spatial synchrony in each species' outbreak cycling. Using a large spatiotemporal tree‐ring reconstruction of outbreak impacts across these two systems, we evaluate two contrasting theories governing defoliator outbreaks: harmonic oscillation (a.k.a. ‘clockwork’) and relaxation oscillation (a.k.a. ‘catastrophe’), each with consequences linked to top‐down versus bottom‐up influences on outbreak behaviour in time and space. We find synchrony varies temporally, among outbreak cycles and in direct proportion to cycle peak intensity; however, cycle peak intensities are distributed bimodally in time, and so, therefore, are synchrony coefficients. Spatially, the area where each pest species currently cycles with the greatest peak intensity and synchrony is where their preferred host trees are currently found in greatest proportion. Despite overall synchrony in cycling, we found, in both systems, a persistent negative spatial correlation among successive eruptive pulses of defoliation. Many of these eruptions failed to spread spatially and to coalesce with other spot eruptions to form extensive area‐wide outbreaks. Eruptions often fail to spread at the hardwood‐conifer interface, resulting in outbreak pulses that systematically bounce back and forth between landscape types, particularly when systems were cycling at low amplitude. These over‐dispersed spatial patterns of pulse impact are consistent with a contagious theory of eruption and outbreak spread. They could be considered consistent with harmonic oscillation theory only for populations cycling at different frequencies, with cycling frequency determined by host forest landscape structure. Synthesis. We find that defoliator outbreak dynamics across systems include spatiotemporal signatures of each theoretical paradigm—suggesting a hybrid approach will better characterise outbreak behaviour. Host concentration influences which paradigm dominates the spatial dynamics in any given forest landscape context. Because of the synchronising effect of host concentration on forest insect spatial dynamics, mixedwoods appear to be less prone to intense, synchronised defoliator attack than forests of pure hardwood or pure conifer.

Soil Springtail Communities Are Resilient to Forest Tent Caterpillar Defoliation in Quebec Mixed Hardwood Forests

Outbreaks of defoliator insects are important natural disturbances in boreal forests, but their increasing frequency under warming climate conditions is of concern. Outbreak events can shape ecosystem dynamics with cascading effects through trophic networks. Caterpillar defoliation can alter tree physiology, increase sunlight to the understory, and result in the deposition of large amounts of leaf litter and caterpillar frass to the forest floor. These modifications can thus affect soil organisms through direct (e.g., changes in soil temperature or moisture) or indirect (e.g., changes in detrital and root food webs) mechanisms. We assessed whether a recent (2015 to 2017) outbreak of the forest tent caterpillar (Malacosoma disstria) at the Lake Duparquet Teaching and Research Forest (Abitibi, QC, Canada) affected soil springtail communities, abundant microarthropods in forest soils. In 2018 and 2019, we sampled litter and soil (0–10 cm depth) at eight sites each in aspen-dominated (Populus tremuloides Michx) stands that were undefoliated or had a recent defoliation history. We found no significant difference in springtail abundance (specimens cm−2) or alpha diversity indices between undefoliated sites and those with defoliation history. However, we observed a transient change in springtail community composition 1 year after the outbreak (2018) with the absence of Folsomia nivalis, Anurophorus sp1, and Xenylla christianseni in sites with defoliation history, but no compositional differences were observed in 2019. Certain soil nutrients (P, C, Mg, Mn) were significant predictors of springtail community composition, but soil microbial biomass was not, despite its significant decrease in sites with defoliation history. Our results show that soil springtail communities respond in the short-term to the forest tent caterpillar outbreak with compositional shifts, but seem ultimately resilient to these events.

Forest Tent Caterpillar Outbreaks Drive Change in Ant Communities in Boreal Forests

Insect outbreaks are major drivers of natural disturbances in forest ecosystems. Outbreaks can have both direct and indirect effects on the composition of soil arthropod communities through canopy opening, nutrient addition and predator-prey interactions. In this study, we aimed to understand the effects of forest tent caterpillar (Malacosoma disstria; FTC) outbreaks through cascading effects on ant communities in both temperate and boreal forests in Canada. Pitfall traps and Berlese funnels were used to compare the ant communities, as well as the surrounding arthropod communities, between control and outbreak sites in boreal and temperate forests (in Quebec, Canada). Using the Sørensen dissimilarity index, we determined the alpha and beta diversity of the ant community. Other arthropods collected in the traps were counted to evaluate the richness and abundance of potential prey for the ants and other potential predators of the FTC. We used an indicator species analysis to examine the species associated with sites defoliated by the outbreak. In the boreal forest, we found that FTC outbreaks caused decreases in species richness and increases in the evenness of ant communities in defoliated sites. In the boreal forest sites, species composition varied significantly between control and outbreak sites. This pattern was driven in part by the presence of other predators. A similar, but weaker pattern was observed in the temperate forest. We saw no changes in the beta diversity in the boreal forest, but did see a significant decrease in the temperate forest between the outbreak sites and the control sites. Ant species in the boreal forest tended to exhibit a more marked preference for either control or previously defoliated sites than species in the temperate forest. Our study showed that disturbances such as insect outbreaks can drive changes in the ant community. While we saw small effects of outbreaks, manipulation experiments using resource addition could help us validate the mechanisms behind these relationships.

Forest landscape structure influences the cyclic‐eruptive spatial dynamics of forest tent caterpillar outbreaks

AbstractA fundamental question in forest insect ecology is the role of forest landscape structure, particularly the amount and spatial configuration of host tree species, in shaping the dynamics of recurring forest insect outbreaks. For forest tent caterpillar (FTC), independent studies do not converge on a singular conclusion, although all indicate that forest structure influences outbreak dynamics. These studies also vary in how they treat climate as a covariate. We evaluated the relative importance of host forest landscape composition and configuration, as well as climate, for their influence on FTC outbreak cycling in the twentieth century. We predicted that FTC outbreaks would exhibit greater synchrony and intensity within areas associated with higher abundance of host trees. We reconstructed FTC outbreaks from 1928 to 2006 using tree‐ring analysis within a well‐structured experimental landscape located in northwestern Ontario and northern Minnesota. Time‐series clustering and spatial nonparametric covariance were used to determine whether similarities in time series and patterns in spatial synchrony corresponded with land management history. Using constrained ordination, we compared statistical properties of outbreak time series to landscape variables representing host abundance, forest configuration, and climate. We found no evidence of climatic effects at the scale of this study, but a significant albeit small influence of landscape structure on outbreak dynamics. Outbreaks were more synchronous and more cyclic within managed zones containing a greater relative abundance of aspen and other hardwood host tree species, compared with the more conifer‐dominated Wilderness area. Yet, we also observed asynchronous outbreak dynamics across the study area, such that correlations with slower‐changing forest landscape variables varied starkly among outbreak pulses. Consequently, the strength of relationship between landscape variables and FTC outbreak patterns varied strongly through time—a result that may explain why short‐term studies yield conclusions that are at odds with one another. Our results speak to the importance of long time series, contrasting landscape structure, use of multivariate methods, and controlling for climatic variation when investigating the effects of forest landscape structure on the cyclic‐eruptive spatial dynamics for forest defoliators.

A Continuous Model for Oscillating Outbreaks of the Population of a Phytophagous Moth, the Tent Caterpillar, Malacosoma disstria (Lepidoptera, Lasiocampidae)

Outbreaks of individual species populations are important phenomena in many aspects and are not alike in terms of the theory of multispecies community dynamics. Outbreaks of insect populations develop more quickly with long-lasting effects experienced by the forest industry. These events are considered as extreme unbalanced and transient processes. The mechanisms of the development and subsidence of insect outbreaks differ in different taxonomic groups of pests. The duration and occurrence of repeated outbreaks of psyllids and forest moths, which affect deciduous or coniferous forests in the same region, are different. Computational simulation is needed for understanding the dynamics of insect outbreaks. For the mathematical description of the outbreaks of forest tent caterpillar, in addition to the threshold version of the development of the insect outbreak, it is interesting to modify continuous computational models for the analysis of fluctuation dynamics. In this paper, we simulate the dynamics of spontaneously damping oscillations under a specific scenario during a population outbreak using a continuous model with delayed regulation and nonlinear counteraction by the biotic environment. The scenario described by the new phenomenological equation, which consists of a series of maxima of different sizes and final attenuation of peaks near balance, occurs for the pest tent caterpillar, Malacosoma disstria, which affects deciduous forests in North America leading to large-scale defoliation. The new scenario is qualitatively different from our model of the threshold development and subsidence of outbreaks of the psyllid Cardiaspina albitextura in Australia.

Density-dependent processes fluctuate over 50years in an ecotone forest.

Spatial patterns can inform us of forest recruitment, mortality, and tree interactions through time and disturbance. Specifically, successional trajectories of self-thinning and heterospecific negative density dependence can be interpreted from the spatial arrangement of forest stems. We conducted a 50-year spatial analysis of a forest undergoing succession at the ecotone of the southwestern Canadian boreal forest. The forest progressed from early to late sere and experienced repeated severe droughts, forest tent caterpillar outbreaks (Malacosoma disstria), as well as the outbreak of bark beetles. Cumulatively, the forest lost 70% of stems due to natural succession and a combination of disturbance events. Here, we describe spatial patterns displaying signals of successional self-thinning, responses to disturbance, and changes in patterns of density dependence across 50years. Forest succession and disturbance events resulted in fluctuating patterns of density-dependent mortality and recruitment that persisted into late seral stages. The combined effects of conspecific and heterospecific density-dependent effects on mortality and recruitment resulted in near-spatial equilibrium over the study period. However, the strength and direction of these demographic and spatial processes varied in response with time and disturbance severity. The outbreak of forest tent caterpillar, pronounced drought, and bark beetles combined to reduce stand aggregation and promote a spatial equilibrium. Density-dependent processes of competition and facilitation changed in strength and direction with succession of the plot and in combination with disturbance. Together these results reinforce the importance of successional stage and disturbance to spatial patterns.

Decline of an ecotone forest: 50 years of demography in the southern boreal forest

AbstractVariation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type.

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Drought and surface‐level solar radiation predict the severity of outbreaks of a widespread defoliating insect

AbstractDefoliating insects are a major factor impacting tree growth in temperate and boreal forests, but the effects of climate change on the severity and frequency of outbreaks of these insects are not well understood. Dendrochronological reconstruction of forest tent caterpillar (Malacosoma disstria) outbreaks on trembling aspen (Populus tremuloides) in southwestern Manitoba, Canada, from 1851 to 2010 was used to examine the effects of changes in temperature, incident surface‐level solar radiation, and drought severity on the cyclicity, frequency, and severity of outbreaks. Outbreak severity was determined by comparing the tree‐ring widths of trembling aspen with those of white spruce (Picea glauca), which is not a forest tent caterpillar host. Wavelet analysis suggested the cyclicity of outbreaks increased after 1930. From 1930 to 2010, the dominant period length of outbreak cycles steadily increased over time from three to seven years, coinciding with long‐term declines in drought severity. The strength of forest tent caterpillar suppression of trembling aspen growth in a given year was inversely related to incident solar radiation and drought severity during late spring (the larval feeding period) of the same year. The effect of late spring drought on the impact of forest tent caterpillar on ring growth differed from that of summer drought. There was a nonlinear relationship between growth suppression in a given year and summer drought severity in the previous year; slightly above‐average drought severity was associated with reduced impacts of the defoliator, whereas severe drought was associated with intense growth suppression. Strong suppression of aspen growth observed during outbreaks from 1983 to 2010 may be partly explained by observed long‐term trends of decreasing atmospheric transmittance of solar radiation (a phenomenon termed “global dimming”) and increasing soil moisture availability during late spring. However, as the severity and frequency of summer droughts are expected to increase over the 21st century, the nonlinear relationship between summer drought severity and growth suppression the following year suggests a future in which summer droughts precede severe outbreaks that threaten the health of boreal forest hardwood trees.

Effects of partial harvesting on species and structural diversity in aspen-dominated boreal mixedwood stands

Abstract Partial harvesting is thought to result in more structurally diverse forest stands, providing a promising alternative to clearcutting and helping balance timber production and biodiversity conservation. Evidence for this hypothesis is based mostly on understory vegetation communities, with little information available on how partial harvesting affects tree species and structural diversity. In this study, we examined overstory and understory tree species and structural diversity of boreal mixedwoods over 20 years in stands that were unharvested, partially harvested with 40% canopy removal, and clearcut. As expected, clearcutting reduced overstory tree species and structural diversity relative to that of unharvested stands; partial harvesting, however, did not affect neighbourhood (area of 4 trees including one subject tree and 3 neighbours) or stand diversity until after a forest tent caterpillar outbreak that occurred during years 3–5 post-harvesting. The insect-induced trembling aspen (Populus tremuloides Michx.) mortality was higher in the partially harvested than in the unharvested areas decreasing structural diversity at both neighbourhood and stand scales. In comparison, within-stand variations (horizontal complexity or spatial heterogeneity) in neighbourhood diversity values (Shannon species diversity, Shannon structural diversity by 2-m height class, and structural diversity by height variation) increased following the insect outbreak. The post-harvesting temporal dynamics of overstory diversity appeared time dependent and related to harvest removal, forest tent caterpillar defoliation, and growth of released balsam fir (Abies balsamea (L.) Mill.) and aspen regeneration. In all treatments, understory diversity was decreased by clearcutting, which stimulated profuse aspen suckering, and abundant balsam fir regeneration. We conclude that partial harvesting with 40% canopy removal did not significantly change tree species or structural diversity of boreal mixedwood stands in northeastern Ontario, Canada. Changes in overstory diversity following a forest tent caterpillar outbreak, however, suggest that partial harvesting that removes more of the canopy and reduces individual tree dominance may increase species diversity and within-stand structural variation but reduce structural diversity in the overstory.

A Forest Tent Caterpillar Outbreak Increased Resource Levels and Seedling Growth in a Northern Hardwood Forest.

In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008–2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species Fraxinus americana and Prunus serotina, and the shade tolerant species Ostrya virginiana. The strong growth response of F. americana and P. serotina suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests.

Root connections affect radial growth of balsam poplar trees

Connected root systems reduced the negative impact of August temperatures and insect outbreak on growth. Root connections between trees can be an ecological advantage of clonal plant species in environments with unevenly distributed resources. We investigated the effects of root connectivity in stands of balsam poplar in Quebec (Canada). We evaluated differences in growth response between groups of trees with and without root connections through climate-growth analyses, comparison of the growth dynamics, and analysis of growth response to a severe forest tent caterpillar (FTC) outbreak. Current May temperature had a positive influence on radial growth of both connected and non-connected trees. Growth of non-connected trees was negatively affected by August temperatures (r = −0.3) while connected trees did not reveal a significant relationship for that month. A mixed effect ANOVA showed a significant difference (F 1, 25 = 5.59, p = 0.02) in growth responses to FTC outbreak between connected and non-connected trees. Connected trees grew on average 16 % better than unconnected trees during the outbreak, with bootstrapped 95 % confidence range from 2.28 to 31.36 %. The study suggests a sharing of resources through root connections, affecting radial growth of connected balsam poplar trees under both average and extreme environmental conditions.

Competitive balance among tree species altered by forest tent caterpillar defoliation

In northern hardwood forests with a closed canopy, tree growth is largely driven by tree size and competition with neighboring trees. Canopy defoliation during insect outbreaks temporarily increases light availability and may decrease light competition among neighboring trees. Defoliation typically reduces tree growth, but canopy defoliation could also stimulate growth of trees that maintain some leaf area. We compared the growth responses of mature trees of non-host species Acer rubrum, and the three host species Acer saccharum, Quercus rubra and Tilia americana, to defoliation by forest tent caterpillar (Malacosoma disstria, Lasiocampidae), a native forest insect with a ∼10-year outbreak periodicity, in a northern hardwood forest in northern lower Michigan.We compared tree growth during the forest tent caterpillar outbreak in 2009, and in the following year, to the average growth before the outbreak in relation to the defoliation level of both the focal tree and its neighbors. Growth responses were related to focal tree defoliation, but we found little direct evidence for effects of neighborhood defoliation; relatively uniform defoliation within sites may have obscured detection of neighborhood effects. Growth of T.americana increased substantially in the year following defoliation, despite the growth reduction during the outbreak in 2009. T.americana rapidly produced new leaves after peak defoliation in 2009, which could explain increased growth in 2010. A.rubrum, which was not defoliated, decreased growth slightly during the outbreak. In 2010, however, one year after peak defoliation, A. rubrum growth was similar to pre-outbreak growth. The two other defoliated species, A. saccharum and Q. rubra, decreased growth in both 2009 and 2010. These strongly species-specific growth responses to canopy defoliation might result in changes in tree species’ interactions and shifts in species advantages over time.

Site factors contribute to aspen decline and stand vulnerability following a forest tent caterpillar outbreak in the Canadian Clay Belt

Following the recent forest tent caterpillar (Malacosoma disstria Hbn.; FTC) outbreaks in the Canadian Clay Belt, several trembling aspen (Populus tremuloides Michx.) stands that have expressed variable mortality levels did not regenerate and were invaded by speckled alder (Alnus rugosa (DuRoi.) Sprengel.). The present study has been initiated to identify the causes of aspen dieback and of suckering inhibition, and to test silvicultural treatments to reinitiate stands having high stocking levels. A total of 84 plots were established in 2009 along a gradient of tree mortality based on the residual aspen live basal area (ALBA) in stands that had a high crown closure prior the last FTC outbreak. Treatments included winter and early summer harvest prior to the 2010 growing season, with or without alder removal in the winter of 2011. Results indicate that alder removal is not needed to regenerate the stands, that apical dominance of the residual trees played an important role in the inhibition of suckering but that ALBA was the main factor leading to higher sucker densities, suggesting that the root system was affected in the most degraded stands. A clear threshold of 11m2ha−1 ALBA has been evidenced above which stands can be regenerated. Below this threshold, suckering is too low to regenerate to productive stands. Moreover, this study is the first to show a strong and positive association between aspen mortality, humus layer thickness and water table height. Aspen was more vulnerable to FTC defoliation on sites combining a high water table and a thick humus layer. Because the basal area increment of residual trees has been lower since the 1950s on those sites, aspen dieback following the last FTC outbreak appears to be a symptom of a slower, long-term process of aspen decline. From a management viewpoint, it is suggested to convert these degraded sites to conifer stands.

Growth and mortality of trembling aspen (Populus tremuloides) in response to artificial defoliation

To simulate the effects of forest tent caterpillar (FTC) defoliation on trembling aspen growth and mortality, an artificial defoliation experiment was performed over three years in young aspen stands of northwestern Quebec. Defoliation plots of 15 × 15 m were established on three sites, together with associated control stands of pure trembling aspen. In 2007, root collar diameters were measured and positions of all trees were mapped prior defoliation. Severe FTC defoliation was simulated for three successive years (2007–2009) by manually removing all leaves from all but 7–10% of the trees present in the defoliation plots. Yearly surveys of growth and mortality were conducted until 2010 to evaluate defoliation effects on defoliated as well as surrounding undefoliated trees. In absence of other factors, growth and mortality of trembling aspen decreased and increased, respectively, after defoliation. Our study further revealed that small diameter trees died after one year of artificial defoliation, while larger-diameter trees died after repeated defoliations. Distributions of tree mortality tended to be aggregated at small scales (<5 m), corroborating gap patterns observed in mature stands following FTC outbreaks. This experiment revealed that trembling aspen mortality can be directly attributed solely to defoliation. Repeated defoliations during FTC outbreaks have the potential to profoundly modify stand productivity and structure by reducing tree growth and increasing tree mortality in the absence of predisposing factors.

Effects of a forest tent caterpillar outbreak on the dynamics of mixedwood boreal forests of eastern Canada

In boreal mixedwood stands dominated by trembling aspen (Populus tremuloides), forest tent caterpillar (Malacosoma disstria, FTC) outbreaks are recurrent events whose effects on stand dynamics are poorly documented. To describe and characterize the effects of FTC outbreaks, we assessed canopy opening, gap size, and understory tree recruitment in 12 stands dominated by trembling aspen that had experienced different levels of defoliation (in terms of severity and duration) during the last outbreak in northwestern Quebec (1999–2002). The study showed a significant increase in canopy opening and gap size with defoliation intensity. Furthermore, the proportion of large gaps and aspen mortality increased with defoliation intensity. Balsam fir (Abies balsamea) regeneration benefited from changes in canopy structure caused by the FTC, while aspen did not. Forest succession in mixedwood stands that had been defoliated for 1 y was not profoundly affected, while multiple years of defoliation likely caused more rapid canopy transition from aspen to fir. By creating a variety of gaps, FTC outbreaks modify stand structure in ways that differ from succession to coniferous dominance controlled by single-stem exclusion.

The dynamics of forest tent caterpillar outbreaks across east‐central Canada

An analysis of forest tent caterpillar Malacosoma disstria defoliation records from Ontario and Quebec over the period 1938–2002 indicates that outbreaks recur periodically and somewhat synchronously among regions of the two provinces. Cluster analysis revealed that the most strongly periodic, large‐scale, synchronized fluctuations occurred within three regions: northwestern Ontario, eastern Ontario/western Quebec and southeastern Quebec. Defoliation in the vast surrounding hinterlands tended to be infrequent and sporadic, loosely tracking defoliation in the core outbreak regions. One small cluster in northeastern Ontario stood out as anomalous, as a result of an increasing trend in the duration of periodic defoliation episodes, marked by an unprecedented double‐wave of defoliation that persisted from 1992 to 1999. This is the precise area where, in the early 2000s, trembling aspen Populus tremuloides stems were mapped as being in a state of decline of unprecedented severity and extent. Our results suggest forest tent caterpillar has the potential to cause significant impacts on forest health and, hence, carbon budgets in east‐central Canada and that the forest tent caterpillar deserves more attention as a model system of forest insect disturbance ecology.

Gap dynamics in aspen stands of the Clay Belt of northwestern Quebec following a forest tent caterpillar outbreak

Forest tent caterpillar (FTC; Malacosoma disstria Hübner) outbreaks represent an important natural disturbance in broadleaf-dominated stands; however, their effects on forest gap dynamics are not well understood. To describe such effects on canopy gaps and tree recruitment patterns, we investigated 20 trembling aspen (Populus tremuloides Michx.) dominated stands defoliated severely over 0 to 3 years during the last outbreak (1998–2003) in the northwestern Clay Belt of Quebec. Results show that canopy opening more than tripled (12.3%–43.7%) from 0 to 3 years of severe defoliation, and mean gap size was more than 12 times greater (7.2–87.5 m2) over the same gradient. Regeneration patterns suggest that aspen recruitment is not sufficient to completely restore closed canopies in stands defoliated 0, 1, and 2 years, whereas it should be sufficient in stands defoliated 3 years, where large gaps allow trembling aspen establishment. Our results clearly indicate that FTC outbreaks represent an important factor of gap formation in trembling aspen stands. At the stand level, gaps create uneven stand structures, and at the landscape level, FTC defoliation duration creates a large range of even to uneven stand structures.