Forest Tent Caterpillar Defoliation Research Articles

Twenty-year responses of aspen stands to forest tent caterpillar defoliation and overstory dieback in Northeastern Ontario, Canada

Forest tent caterpillar (Malacosoma disstria Hbn.) is a major forest defoliator in North American boreal forests. This pest periodically affects hardwood trees such as trembling aspen (Populus tremuloides Michx.) and balsam poplar (Populus balsamifera L.) across large areas, causing mortality and altering stand attributes and long-term dynamics. In this study, I quantified the responses of aspen stands to forest tent caterpillar defoliation and overstory dieback, including short-term (5 years after dieback) and long-term (20 years later) dynamics. Results indicate that affected stands fully recovered to pre-dieback density but not basal area. Defoliation caused overstory dieback that stimulated understory growth and regeneration but the hardwood regeneration (mostly aspen) did not adequately replace hardwood trees lost to defoliation, resulting in decreased hardwood composition relative to unaffected stands. The high density in affected stands suggested possible further basal area recovery as regeneration and released understory conifers grew into main canopy, following the general trends of boreal mixedwood succession. The results support earlier projections based on residual stand attributes shortly after dieback and reported boreal species growth and mortality rates that indicate forest tent caterpillar defoliation and subsequent overstory dieback would accelerate aspen stand transition to conifer dominance and delay the availability of stands for harvesting by 40–50 years as stands recover.

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.

Growth of Sugar Maple (Acer saccharum Marsh.) after Ice Storm Damage and Forest Tent Caterpillar (Malacosoma disstria Hubner) Defoliation

The January 1998 ice storm broke off tree crowns across a wide geographic area in northeastern North America, and forest tent caterpillar (Malacosoma disstria Hubner) defoliated some of the same stands in 2004–2007. We assessed the post-event growth responses of upper canopy sugar maples (Acersaccharum Marsh.) in previously thinned and recently rehabilitated even-aged northern hardwood stands in New York State, USA. Cores from ice-storm-damaged trees showed an initial radial growth reduction, a recovery after one year, and an increase to or above pre-storm levels after three years. A later forest tent caterpillar defoliation in the same stand caused a second reduction of growth, and another recovery after one year. We observed greater post-storm radial growth on trees released by a post-ice storm rehabilitation treatment than in the untreated control, with growth exceeding pre-storm rates. Cores from another site thinned 38 years earlier and impacted only by the forest tent caterpillar showed a more moderate growth reduction, and a prompt but smaller post-defoliation growth response than among trees affected by both the ice storm and defoliation. Findings reflect the potential for growth of upper canopy sugar maple trees to recover after a single or two closely occurring crown disturbances, and provide guidance to managers who must decide about removing or continuing to manage stands after similar kinds of ice storm damage or defoliation.

Long-Term Responses of Nutrient Budgets to Concurrent Climate-Related Stressors in a Boreal Watershed

The climate of the circumpolar Boreal forest is changing rapidly, resulting in a growing frequency of wildfires and changing precipitation patterns. These climate-related stressors may influence the cycling of nutrients within, and overall ecosystem condition of, Boreal watersheds. However, long-term perspectives of concurrent climate-related impacts on the cycling of nutrients in watersheds are rare. We present multi-decadal terrestrial and lake mass budgets of nitrogen, phosphorus and carbon within a headwater Boreal Shield watershed that was recovering from an extensive wildfire while experiencing measureable increases in annual precipitation. We used these budgets to quantify associations between nutrient retention in each ecosystem and changes in metrics defining landscape recovery after wildfire or precipitation. The terrestrial watershed retained over half of all nitrogen and phosphorus delivered to it by the atmosphere. Strong nutrient retention occurred despite ongoing landscape recovery from wildfire, measurable increases in precipitation, a forest tent caterpillar defoliation and rising atmospheric deposition. A downstream headwater lake was also a strong and consistent sink of nitrogen and phosphorus, highlighting a whole-watershed resistance to environmental disturbances. However, carbon was strongly lost downstream from the terrestrial ecosystem in close and positive association with precipitation, resulting in a darkening of the headwater lake over time with implications for the functioning of its ecosystem. Long-term mass budget monitoring of a Boreal catchment has provided insight into the resistances and dynamic changes within a northern watershed exposed to concurrent wildfire and increasing precipitation conditions.

Effects of forest tent caterpillar defoliation on carbon and water fluxes in a boreal aspen stand

Insect outbreaks can significantly influence carbon (C) and water balances of forests. Forest tent caterpillars (FTC) (Malacosoma disstria Hübner) are one of the most prominent insects found in aspen forests in Canada and have the potential to considerably influence regional C and water fluxes. In the summer of 2016, an FTC infestation occurred in a ca. 100 -year-old trembling aspen (Populus tremuloides) stand in the southern boreal forest where the longterm research site known as Old Aspen (OA) is located. The infestation led to nearly complete defoliation of the canopy during the leafing out period when photosynthesis, and thus C uptake, is progressing towards maximum levels. We used 21 years of eddy-covariance (EC) and climate measurements covering pre-infestation and infestation periods to estimate the impact of the FTC infestation on net ecosystem production (NEP), gross ecosystem production (GEP) and evapotranspiration (E). Defoliation in 2016 reduced annual NEP to −130 g C m−2 y−1 and GEP to 798 g C m−2 y−1, respectively, which were much less than their 20-year means (NEP = 118 ± 53 g C m−2 y−1, GEP = 1057 ± 74 g C m−2 y−1), and resulted in the most negative annual NEP value of the 21 years of measurements at the OA site. NEP for 2016 was even lower than values observed during three drought years (2001–2003). However, FTC infestation caused little effect on annual E. FTC infestation reduced the near-surface remotely-measured greenness index, green chromatic coordinate (GCC), to ∼0.32 on June 10 in comparison to ∼0.40 in other years. The defoliation, observable from space as reductions in normalized difference vegetation index (NDVI) values, also showed a negligible effect on E but a large effect on the C fluxes.

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.

Variable effects of climate on forest growth in relation to climate extremes, disturbance, and forest dynamics.

Changes in the frequency, duration, and severity of climate extremes are forecast to occur under global climate change. The impacts of climate extremes on forest productivity and health remain difficult to predict due to potential interactions with disturbance events and forest dynamics-changes in forest stand composition, density, size and age structure over time. Such interactions may lead to non-linear forest growth responses to climate involving thresholds and lag effects. Understanding how forest dynamics influence growth responses to climate is particularly important given stand structure and composition can be modified through management to increase forest resistance and resilience to climate change. To inform such adaptive management, we develop a hierarchical Bayesian state space model in which climate effects on tree growth are allowed to vary over time and in relation to past climate extremes, disturbance events, and forest dynamics. The model is an important step toward integrating disturbance and forest dynamics into predictions of forest growth responses to climate extremes. We apply the model to a dendrochronology data set from forest stands of varying composition, structure, and development stage in northeastern Minnesota that have experienced extreme climate years and forest tent caterpillar defoliation events. Mean forest growth was most sensitive to water balance variables representing climatic water deficit. Forest growth responses to water deficit were partitioned into responses driven by climatic threshold exceedances and interactions with insect defoliation. Forest growth was both resistant and resilient to climate extremes with the majority of forest growth responses occurring after multiple climatic threshold exceedances across seasons and years. Interactions between climate and disturbance were observed in a subset of years with insect defoliation increasing forest growth sensitivity to water availability. Forest growth was particularly sensitive to climate extremes during periods of high stem density following major regeneration events when average inter-tree competition was high. Results suggest the resistance and resilience of forest growth to climate extremes can be increased through management steps such as thinning to reduce competition during early stages of stand development and small-group selection harvests to maintain forest structures characteristic of older, mature stands.

Classification of annual non-stand replacing boreal forest change in Canada using Landsat time series: a case study in northern Ontario

ABSTRACTStandardized protocols for forest change detection and classification based on Landsat time series data are becoming more common for use in characterizing multi-decadal history or trends in forest dynamics. One such protocol, referred to as Composite-2-Change (C2C), is a highly automated process developed in Canada that is applicable across extensive forest regions and includes change detection and typing based on Best-Available-Pixel image compositing, spectral trend analysis of breakpoints, object-based segmentation, and Random Forest classification. The aim of this article is to assess the classification accuracy of the Composite-2-Change (C2C) protocol in an eastern Canadian boreal forest environment in northern Ontario. Results demonstrated that the Landsat-derived change detection and attribution approach was approximately 90% accurate for stand-replacing forest change (fire, harvesting, roads), and approximately 75% for four non-stand replacing forest changes caused by spruce budworm and forest tent caterpillar defoliation, wetland and forest flooding caused by localized hydrological variations, and one class of multiple/other disturbances. The C2C protocol approach offers unique independent data layers for modelling that can be used to relate and inform on a range of substantive and subtle changes, which in turn can be labelled and tracked, offering otherwise unavailable information on forest dynamics over large areas.

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.

The influence of stand and landscape characteristics on forest tent caterpillar (Malacosoma disstria) defoliation dynamics: the case of the 1999–2002 outbreak in northwestern Quebec

The forest tent caterpillar (Malacosoma disstria Hbn.) is an eruptive forest insect common across North America and an important defoliator of trembling aspen (Populus tremuloides Michx.). Forest stands having suffered severe defoliations by the forest tent caterpillar over multiple years are known to incur reduced tree growth and increased tree mortality. In this study, we developed a predictive model of forest tent caterpillar defoliation dynamics using local and contextual variables expressing forest composition and structure, and their heterogeneity, at different scales. Of all scales considered (500, 1000, 1500, and 2000 m), contextual variables at 1500 m were found to have the greatest effect on defoliation dynamics. At this scale, we found that a greater proportion of preferred host trees in the landscape increased defoliation severity, but duration was modulated by compositional heterogeneity, where persistence was reduced in highly heterogeneous landscapes. Indeed, the likelihood of a single year of defoliation was much greater in highly diverse landscapes than the likelihood of multiple years of defoliation. These findings are consistent with ecological theory. Contrary to the expected result that older trees would be most susceptible, we found that “middle-aged” trees (~50 years) were most likely to be defoliated.

Response of aspen stands to forest tent caterpillar defoliation and subsequent overstory mortality in northeastern Ontario, Canada

Overstory mortality, understory tree recruitment, and vegetation development were assessed in trembling aspen (Populus tremuloides Michx.) stands following two recent episodes of forest tent caterpillar defoliation (Malacosoma disstria Hbn.) in northeastern Ontario. The results suggest that poplar (aspen and balsam poplar (Populus balsamifera L.)) mortality increased with consecutive years of insect defoliation occurring from the mid-1980s to mid-2000s and the proportion of poplars in the overstory, but decreased with improved pre-defoliation tree vigour (DBH increment). The first outbreak, which lasted from the mid-1980s to early 1990s, was more severe in terms of insect defoliation and contributed more to poplar mortality and decline. The decline began in the late 1990s and peaked in early 2000s. Poplar regeneration and understory shrubs responded rapidly to foliage loss to insect defoliation and mortality of overstory poplars. The regenerated poplars were able to maintain their growth under developing shrubs and residual overstory canopy and numbers were sufficient to compensate for the poplar trees lost to insect infestation. The defoliation-induced overstory decline will accelerate the transition of aspen stands to conifer dominance through enhanced conifer recruitment and growth, and reduced hardwood overstory in aspen-dominated stands, while hardwood dominance will persist in pure aspen stands. From a timber supply perspective, the decline caused by forest tent caterpillar defoliation could delay the availability of aspen stands for harvesting by 40–50 years.

The Combined Effects of Forest Tent Caterpillar Defoliation and Timber Stand Improvement Activity on Epicormic Sprouting of Sugar Maple and Black Cherry

Abstract A 2 × 2 factorial analysis of variance design assessed the combined effects of severe defoliation by the forest tent caterpillar, Malacosoma disstria (Hub.), from 1991 through 1993 and concurrent timber stand improvement (TSI) treatments on the occurrence of epicormic sproutingfor both sugar maple (SM), Acer saccharum (Marsh.) and black cherry (BC), Prunus serotina (Ehrh.) in the Tug Hill region of New York State. The potential economic effects of epicormic sprouting by dominant and codominant (DC) SM after defoliation and TSI were statistically significantbut negligible. TSI treatments alone and defoliation alone, however, promoted sprouting in intermediate SM, which may result in future grade reduction. The combination of defoliation and TSI treatment resulted in prolific epicormic sprouting by DC BC. These two disturbances have the potential to cause significant standwide value losses in this species.

Response of trembling aspen to partial cutting and subsequent forest tent caterpillar defoliation in a boreal mixedwood stand in northeastern Ontario, Canada

The growth response and mortality of trembling aspen ( Populus tremuloides Michx.) were monitored 1, 3, 5, and 11 years after partially harvesting an aspen-dominated mature mixedwood stand in northeastern Ontario. Both diameter and height of aspen trees responded positively to harvesting. However, 3 years after harvest, the growth rates of residual aspen were severely suppressed by 3 years of moderate to severe defoliation by forest tent caterpillar ( Malacosoma disstria Hbn.). The diameter growth of surviving trees almost recovered, but due to continuous crown dieback in the 6 years after defoliation ceased, height growth did not. Over the 11 years after harvest, 70% of residual aspen died, with 80% of that mortality occurring in the 6 years after defoliation. Trees in the lower canopy and in the partial cut had higher mortality rates than those in the upper canopy and in the uncut control. Increased mortality at the individual tree level was generally associated with greater foliage loss. Aspen growth and mortality were associated with defoliation levels, particularly for trees in partial cut treatments. Partial cutting and crown class strongly influenced the response of aspen trees to insect defoliation and should be considered when selecting residual trees during harvesting.

The dynamics of forest tent caterpillar outbreaks in Québec, Canada

Historical patterns of forest tent caterpillar defoliation over the period 1938–2002 in the province of Québec, eastern Canada, were analyzed in relation to forest inventory data. The extent of defoliation over time was largely nonstationary and only somewhat periodic; however six major defoliation episodes could be identified. Individual outbreaks tended to span only 36.6% (±13.1% S.E.) of the total area defoliated, suggesting they are frequently terminated before attaining their maximum potential extent. Although outbreaks tended to recur periodically, they were not perfectly synchronized across the province. Two core regions, 14 000 and 20 000 km 2 in size, located in the northwestern, aspen-dominated boreal forest region and the southeastern, maple-dominated mixedwood forest region, were found to exhibit cyclic patterns of defoliation, with periodicities of 9 and 13 years, respectively. These oscillations were characterized by strong second-order negative feedback, suggesting regulation by lagged density-dependent processes. Outbreak cycles in the two core regions were in phase with one another ( r = 0.39) until 1963, when a sudden, large-scale outbreak collapse occurred in the North during the initial phase of the third cycle. Since that time outbreak oscillations have been completely out of phase ( r = −0.16), leading to a persistent wave-like pattern of outbreak spread back and forth between regions along a northwest–southeast axis. Within core regions, cycle amplitude varied in a slow and smooth manner, with the phasing pattern of amplitude modulation differing substantially between regions. Although the timing of population cycle peaks appears to be highly predictable, at least within the core regions, the levels of defoliation experienced during these peaks appears to be unpredictable and may be modulated by factors yet to be identified.

Factors affecting trembling aspen ( Populus tremuloides Michx.) health in the boreal forest of Alberta, Saskatchewan, and Manitoba, Canada

The value of trembling aspen ( Populus tremuloides Michx.) as a resource has increased in Canada in recent years, but key information regarding the health of this species and its interrelationships with biotic and abiotic agents is lacking. A regional study was conducted to (i) assess the general health of trembling aspen trees in the three prairie provinces of Canada, (ii) determine the incidence of biotic and abiotic agents affecting these trees and (iii) determine what factors most influenced the health of trembling aspen trees between forest tent caterpillar ( Malacosoma disstria Hübner) outbreaks. A total of 8296 trembling aspen trees in 85 plots were monitored during 1992–1994. Trembling aspen health was assessed and related to biotic and abiotic damage agents and symptoms. Trembling aspen in the study area were generally healthy: crowns were in good condition, trees had few pests, and mortality rates were low. Confidence intervals for proportion of trees dead less than 2 years are reported by age class. Large aspen tortrix ( Choristoneura conflictana [Wlk.]) and poplar peniophora ( Peniophora polygonia [Pers.:Fr.]) Boud. were the most common pests, occurring on 15 and 13% of live trees, respectively; incidences of other pests were <7%. Significant associations between incidences of pests and several stand characteristics were found. Regression analysis showed tree age, a climate moisture index, number of years of forest tent caterpillar defoliation, and incidence of Armillaria root disease ( Armillaria spp.) accounted for a significant proportion of the variation in trembling aspen health and mortality. Our results highlight the need to consider pests, especially forest tent caterpillar and Armillaria root disease, in the management of trembling aspen or in landscape-level simulations of trembling aspen productivity.