Changes in Spatiotemporal Patterns of 20th Century Spruce Budworm Outbreaks in Eastern Canadian Boreal Forests

In scenarios of future climate change, there is a projectedincrease in the occurrence and severity of natural disturbances inboreal forests. Spruce budworm (Choristoneura fumiferana)(SBW) is the main defoliator of conifer trees in the North American boreal forests affecting large areas and causing marked losses of timber supplies. However, the impact and the spatiotemporal patterns of SBW dynamics at the landscape scale over the last century remain poorly known. This is particularly true for northern regions dominated by spruce species. The main goal of this study is to reconstruct SBW outbreaks during the 20th century at the landscape scale and to evaluate changes in the associated spatiotemporal patterns in terms of distribution area, frequency, and severity. We rely on a dendroecological approach from sites within the eastern Canadian boreal forest and draw from a large dataset of almost 4,000 trees across a study area of nearly 800,000 km2. Interpolation and analyses of hotspots determined reductions in tree growth related to insect outbreak periods and identified the spatiotemporal patterns of SBW activity over the last century. The use of an Ordinary Least Squares model including regional temperature and precipitation anomalies allows us to assess the impact of climate variables on growth reductions and to compensate for the lack of non-host trees in northern regions. We identified three insect outbreaks having different spatiotemporal patterns, duration, and severity. The first (1905–1930) affected up to 40% of the studied trees, initially synchronizing from local infestations and then migrating to northern stands. The second outbreak (1935–1965) was the longest and the least severe with only up to 30% of trees affected by SBW activity. The third event (1968–1988) was the shortest, yet it was also the most severe and extensive, affecting nearly up to 50% of trees and 70% of the study area. This most recent event was identified for the first time at the limit of the commercial forest illustrating a northward shift of the SBW distribution area during the 20th century. Overall, this research confirms that insect outbreaks are a complex and dynamic ecological phenomena, which makes the understanding of natural disturbance cycles at multiple scales a major priority especially in the context of future regional climate change

À la recherche de l’aménagement durable en forêt boréale : croissance, mortalité et régénération des pessières noires soumises à différents systèmes sylvicoles

Le Canada est le troisième pays du monde en termes de surface forestière avec 347 millions d'hectares. Jusqu’à aujourd’hui, le traitement sylvicole le plus utilisé a été la coupe totale représentant 93% de la surface récoltée dans la forêt boréale canadienne. Les impacts de cette méthode de coupe sur la simplification des structures du peuplement, la perte de biodiversité et la durabilité de cette forêt sont bien connus. De ce fait, l’aménagement forestier écosystémique propose les coupes partielles comme alternative afin d’intégrer les objectifs écologiques, économiques et sociaux dans la planification sylvicole de façon à atteindre l’aménagement durable de la forêt boréale. Pour la mise en oeuvre de l'aménagement écosystémique, les coupes partielles sont de plus en plus utilisées comme traitement sylvicole pour l'exploitation forestière. Pourtant, il y a encore des modalités de coupes partielles qui ne sont pas suffisamment connues et étudiées dans le contexte de la forêt boréale canadienne, comme les coupes progressives régulières (CPR). La CPR est un système sylvicole classique qui n’a pas de forme adaptée à la forêt boréale et son application est jusqu’à maintenant demeurée marginale. Celle-ci vise la régénération des peuplements équiennes par l’installation de semis sous couvert avant la coupe finale, grâce à l’ouverture progressive du couvert instaurée par les coupes partielles. Ce système sylvicole à une approche prometteuse pour favoriser la croissance des arbres résiduels, car il crée les conditions nécessaires à la maximisation de la production résineuse. En conséquence, le développement de projets de recherche qui visent à étudier les réponses des peuplements soumis aux CPR sont nécessaires afin de trouver des alternatives d’aménagement et d’aborder un des plus grands défis de la foresterie canadienne : le développement durable de la forêt boréale. L’objectif principal de cette thèse de doctorat consiste à «évaluer l'effet à moyen terme des différents traitements de CPR sur la croissance, la mortalité et la régénération comme alternative sylvicole dans le cadre de l’aménagement forestier écosystémique des pessières noires boréales». Pour y répondre, 3 axes d’étude ont été créés pour étudier les réponses des forêts 10 ans après coupe: 1) Croissance des arbres résiduels, 2) Mortalité après coupe et 3) Régénération et croissance des semis. Notre dispositif expérimental est, à plusieurs égards, unique au monde, notamment parce qu’il est le premier concernant la CPR sur les pessières noires, et le premier portant sur des modalités adaptées aux opérations mécanisées. Il a été établi en 2003-2004, dans des peuplements matures et équiennes d’épinette noire au nord du Saguenay et sur la Côte-Nord. Le dispositif a pris en compte deux types de structure de forêts : denses et ouvertes. Il est composé de six blocs d’étude comprenant chacun trois traitements expérimentaux de CPR, une coupe totale, une réserve de semencier et un témoin sans intervention sylvicole. Des parcelles permanentes d’échantillonnage ont été établies avant coupe, ainsi qu’un suivi des sujets d’étude pendant 10 ans suite à l’intervention. Après l'intervention sylvicole des trois traitements de CPR et la réserve de semencier, les arbres résiduels ont enregistré une augmentation de la croissance radiale. Cet accroissement a été encore plus prononcé dans les peuplements jeunes et denses ainsi que dans les arbres localisés en bordure du sentier de récolte. Grâce à cette étude, l’effet bordure sur la croissance a été déterminé pour la première fois dans des peuplements équiennes d’épinette noire. Notre étude dendrochronologique a confirmé que tous les traitements de CPR ont été efficaces en termes de croissance radiale et sont influencés par : l’âge, la position spatiale, le diamètre, le traitement et le temps. Cependant, moins de 50% de la variation de la croissance des arbres après coupe a été expliquée. En conséquence, une autre étude avec une nouvelle méthodologie a été développée pour mieux comprendre la variabilité de la croissance, en utilisant des modèles individuels non linéaires. Cette nouvelle approche a réussi à caractériser la grande hétérogénéité des réponses des arbres en quatre patrons de croissance, lesquels ont été interprétés et décrits selon la théorie écologique en identifiant les facteurs impliqués. Cette méthodologie a permis de mieux comprendre la croissance de l’épinette noire après coupe partielle en expliquant entre 61 et 80% de la variabilité des arbres. L’étude de mortalité de ce projet de recherche est l’une des seules qui possède un gradient de coupe en forêt boréale, variant de 0 à 75% d’intensité de coupe. Les résultats révèlent que, 10 ans après traitement, les CPR étudiées montrent un niveau de mortalité entre 15 et 20% plus élevée que celle du témoin. Ces valeurs sont proches de la mortalité naturelle dans la région d’étude. En conséquence, les CPR ont minimisé les pertes après traitement, en comparaison avec la réserve de semencier qui ont enregistré des valeurs de mortalité d’environ 60% des arbres résiduels. 80% de la mortalité après coupe a été causée par des chablis et l’intensité de coupe a été l’un des facteurs principaux dans ce phénomène écologique. L’une des grandes contributions de cette étude a été la considération des types de mortalité (arbres cassés, renversés et morts debout), car la majorité des études précédentes n’ont pas pris en compte cette différentiation. Cette étude a également mis en évidence que ces types de mortalité sont des processus écologiques différents à considérer lors de future recherches. Ce design expérimental est l'un des rares dispositifs qui permet l’étude de la régénération 10 ans après l’intervention sylvicole dans le cadre de la foresterie québécoise, surtout grâce à l’incorporation des variables lumière et substrat dans la méthodologie. Cette recherche a déterminé que les CPR et la réserve de semencier sont des traitements capables de promouvoir et d’établir des niveaux de densité de régénération adéquats pour garantir la persistance des pessières noires. Nos analyses ont montré que la régénération de l’épinette noire est dépendante en grande majorité du substrat mais pas de la lumière, car le scarifiage a été le lit de germination le plus efficace pour favoriser l’établissement de semis. Cette étude a permis de mieux comprendre le processus de régénération des pessières noires et les facteurs qui conditionnent la croissance et la densité des semis après coupe partielle. La présente thèse démontre que les CPR sont une alternative sylvicole pertinente lors de l’implantation des stratégies d’aménagement forestier écosystémique en pessière noire. Les traitements étudiés ont promu la croissance des arbres résiduels, minimisé les pertes par chablis après coupe, en plus d’avoir favorisé l’établissement de la régénération afin de maintenir les bénéfices écologiques et économiques en forêt boréale. Les contributions de ce doctorat représentent une avancée de la connaissance dans le domaine de la sylviculture boréale, et plus spécifiquement, sur l’évaluation des coupes partielles dans le contexte du développement durable des forêts. Canada possesses the third largest forest cover in the world (347 MHA), and of the forest area that is harvested, 93% of logging is done by clear-cut methods. These cutting methods have serious impacts in terms of the simplification of stand structure, biodiversity, and the sustainability of forest ecosystems. Boreal forest ecosystem management proposes partial cutting as an alternative harvesting approach to better integrate the ecological, economical, and social objectives into silvicultural planning and to develop a sustainable forestry. Over the last 20 years, partial cutting treatments have been increasingly used for forest exploitation due to the implementation of ecosystem management strategies. However, for the Canadian boreal forest, the impacts of partial cutting treatments, for example shelterwoods, remain poorly known. Shelterwood is a traditional silvicultural system that has not yet been adapted to boreal forest conditions. Its application remains marginal, at present. Shelterwood aims is to regenerate even-aged stands, thanks to the gradual opening of the canopy created by the partial cuttings. This silvicultural system is a promising approach for promoting the growth of residual trees as it creates the necessary conditions for maximizing resinous production (more light, less competition, etc.). Thus, research into the response of stands subjected to shelterwood is vital for finding forest management alternatives that ensure the sustainable development of the boreal forest in Canada. The main objective of this PhD project is to determine the effect of different experimental shelterwood treatments on the growth, mortality, and regeneration of trees over the medium-term to evaluate shelterwood as a silvicultural alternative in the context of ecosystem management of black spruce forests". To answer this question, three study axes were created to assess forest responses 10 years after cutting: 1) growth of residual trees; 2) mortality after cutting; 3) regeneration and growth of seedlings. An experimental design was established in 2003–2004 in mature and even-aged black spruce stands in the northern portion of the Saguenay (Quebec) region and along Quebec’s north shore. This design analyzed six study blocks and two types of forest structure, dense and open stands. Each block had six experimental units: three experimental shelterwood treatments, a clear-cut, a seed-tree, and an untreated control. Permanent sampling plots were established prior to cutting and monitoring occurred for 10 years following the intervention. This experimental design is unique in many respects, especially as it is the first for assessing the shelterwood treatments in black spruce forests as well as being the first involving mechanized operations. After shelterwood and seed-tree treatments, the remaining trees showed increased radial growth. This increase was most pronounced in young and dense stands, as well as for trees located along the edge of skidding trails. This study also determined, for the first time, the edge effect on growth in even-aged black spruce stands. Dendrochronological analysis confirmed that all shelterwood treatments were effective promoting radial growth and that tree growth response after cutting was influenced by age, spatial position, diameter, treatment, and time. However, measured variables could only explain less than 50% of the variation in tree growth after cutting. As such, a new methodology using individual nonlinear models was used to better understand this variability. This new approach succeeded in summarizing the heterogeneity of tree response into four growth patterns. By identifying the factors involved, these patterns were then interpreted according to ecological theory. This methodology improved the understanding of the growth of black spruce after partial cutting, explaining between 61 and 80 % of the variability. The study of mortality of this research project is one of the few studies using a silvicultural gradient in the boreal forest, having a harvest intensity varying from 0 to 75 %. Trees in the shelterwood plots had a mortality level 15 to 20 % higher than that of the untreated control, 10 years after treatment. These values are similar to the natural mortality in the study area. Shelterwood techniques reduced post-treatment losses relative to seed-tree plots, which had mortality values of 60% for the residual trees. Most post-treatment mortality (80 %) was caused by windthrow and harvest intensity was one of the main factors related to this phenomenon. One of the major contributions of this study was the consideration of mortality types (broken, reversed, and standing dead) as most previous studies did not take this differentiation into account. This study showed that these types of mortality are affected by different ecological factors, making it necessary to include mortality type in similar future studies. This experimental design used in this thesis is one of a few, in the framework of Quebec forestry, that has studied regeneration 10 years following a silvicultural intervention, in particular incorporating light and substrate variables into the methodology. The study of forest regeneration has determined that shelterwood and seed-tree treatments can favour adequate regeneration density levels in black spruce forests. Black spruce regeneration was largely dependent on the substrate but not on the availability of light, as scarifying was the most effective germination bed for seedling establishment. This study provides a better understanding of the regeneration processes for black spruce stands and the factors that influence the growth and density of seedlings after partial cutting. This PhD thesis demonstrates that shelterwoods offer a silvicultural alternative that can be implemented into ecosystem forest management strategies for black spruce forests. The shelterwood approach promoted the growth of residual trees, minimized windthrow losses, and favoured regeneration thereby maintaining ecological and economic benefits for the boreal forest. The findings of this PhD advance our knowledge in the field, provide tools for forest managers involved in boreal silviculture and, more specifically, permit the evaluation of partial cutting in the context of sustainable forest development

Autoregressive models for time series of random sums of positive variables: Application to tree growth as a function of climate and insect outbreak

We present a broad class of semi-parametric models for time series of random sums of positive variables. Our methodology allows the number of terms inside the sum to be time-varying and is therefore well suited to many examples encountered in the natural sciences. We study the stability properties of the models and provide a valid statistical inference procedure to estimate the model parameters. It is shown that the proposed quasi-maximum likelihood estimator is consistent and asymptotically Gaussian distributed. This work is complemented by simulation results and applied to time series representing growth rates of white spruce (Picea glauca) trees from a few dozen sites in Quebec (Canada). This time series spans 41 years, including one major spruce budworm (Choristoneura fumiferana) outbreak between 1968 and 1991. We found significant growth reductions related to budworm-induced defoliation up to two years post-outbreak. Our results also revealed the positive effects of maximum summer temperature, precipitation, and the climate moisture index on white spruce growth. We also identified the negative effects of the climate moisture index in the spring and the maximum temperature of the previous summer. However, the model's performance on this data set was not improved when the interactions between climate and defoliation on growth were considered. This study represents a major advance in our understanding of budworm-climate-tree interactions and provides a useful tool to project the combined effects of climate and insect defoliation on tree growth in a context of greater frequency and severity of outbreaks coupled with the anticipated increases in temperature

Challenges for the Sustainable Management of the Boreal Forest Under Climate Change

The increasing effects of climate and global change oblige ecosystem-based management to adapt forestry practices to deal with uncertainties. Here we provide an overview to identify the challenges facing the boreal forest under projected future change, including altered natural disturbance regimes, biodiversity loss, increased forest fragmentation, the rapid loss of old-growth forests, and the need to develop novel silvicultural approaches. We specifically address subjects previously lacking from the ecosystem-based management framework, e.g., Indigenous communities, social concerns, ecological restoration, and impacts on aquatic ecosystems. We conclude by providing recommendations for ensuring the successful long-term management of the boreal biome facing climate change

Dynamics of Territorial Occupation by North American Beavers in Canadian Boreal Forests: A Novel Dendroecological Approach

Research Highlights: Our study highlights a new, simple, and effective method for studying the habitat use by beavers in Canadian boreal forests. Information regarding the presence of beaver colonies and their habitat occupation is essential for proper forest management and damage prevention in the boreal forest. Background and Objectives: The North American beaver (Castor canadensis) is a major element of natural disturbance, altering the dynamics and structure of boreal forest landscapes. Beaver-related activities also affect human infrastructure, cause floods, and lead to important monetary losses for forestry industries. Our study aimed to determine the spatiotemporal patterns of beaver occupation of lodges over time. Materials and Methods: Using a dendroecological approach to date browsing activity, we studied the occupation of two lodges per water body for eight water bodies located in the boreal forest of Quebec, Canada. Results: Three sites showed alternating patterns of lodge use (occupation) over time, three sites (37.5%) demonstrated no alternating patterns of use, and two sites (25%) presented unclear patterns of lodge use. Conclusions: Alternating patterns of lodge use can be linked to food depletion and the need to regenerate vegetation around lodges, while non-alternating patterns may be related to fluctuations in water levels, the specific shrub and tree species surrounding the lodges, the size of the beaver territory, and the number of lodges present on a water body

Vulnerability of conifer regeneration to spruce budworm outbreaks in the Eastern Canadian boreal forest

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

Post-cutting Mortality Following Experimental Silvicultural Treatments in Unmanaged Boreal Forest Stands

Partial cutting has been recommended as an alternative harvesting method to ensure the sustainable management of boreal forests. The success of this approach is closely linked to the survival of residual trees as additional losses through mortality could affect post-cutting timber production at harvest. To better quantify post-cutting mortality in previously unmanaged boreal forests, we addressed two main questions: (1) what is the level of mortality 10 years after cutting? and (2) what ecological factors are involved in this phenomenon? Even-aged black spruce [Picea mariana (Mill.) B.S.P.] stands in the Canadian boreal forest were subjected to three experimental shelterwood treatments, a seed-tree treatment and an untreated control. Tree status (live/dead) was recorded prior to cutting and 10 years after cutting. Dead trees were classified as standing dead, overturned or broken. Ten years after experimental seed-tree treatment, 60% of residual trees were dead, compared to 30% for the shelterwood cuttings. Windthrow (overturned and broken trees) represented 80% of residual tree mortality; only the amount of overturning was influenced by treatment. Broken trees were associated with small-diameter trunks, stands having high growth prior to cutting, younger stands or forest plots located near to adjacent cuts (<200 m). Overturning was associated with a high harvesting intensity and large-diameter trees. Standing dead mortality was the most difficult to explain: it was related to untreated plots having suppressed and small-diameter trees. Based on these results, applying intermediate levels of harvest intensity could reduce post-cutting damage. Understanding tree mortality after cutting is essential to reduce economic losses, improve silvicultural planning and stand selection and ensure ultimately the sustainable harvest of North American boreal forests

Forest Carbon Management: a Review of Silvicultural Practices and Management Strategies Across Boreal, Temperate and Tropical Forests

Purpose of Review Carbon sequestration and storage in forest ecosystems is often promoted as a solution for reducing CO2 concentrations in the atmosphere. Yet, our understanding is lacking regarding how forest management strategies affect the net removal of greenhouse gases and contribute to climate change mitigation. Here, we present a review of carbon sequestration and stock dynamics, following three strategies that are widely used in boreal, temperate and tropical forests: extensive forest management, intensive forest management and old-growth forest conservation. Recent Findings Several studies show that specific forest management strategies can improve carbon sequestration capacity and soil carbon storage. Within these studies, the old-growth forest conservation strategy results in greater carbon storage in soils than do extensive and intensive forest management. Intensive forest management enhances forest carbon sequestration capacity through afforestation using fast-growing species, mechanical soil preparation from low to moderate intensity and N fertilization. Extensive forest management is an intermediate compromise regarding carbon sequestration and soil carbon storage, between conservation and intensive forest management strategies. With respect to silvicultural treatments, partial cutting is a practice that increases forest carbon sequestration rates and maintains higher carbon storage in soils compared to clear-cuts. Each silvicultural practice that is discussed in this review showed a similar effect on forest carbon in all biomes, although the magnitude of these effects differs mainly in terms of heterotrophic respiration. To achieve sustainable management and fulfill industrial demand and profitability, specific gaps must be dealt with to improve our scientific knowledge regarding forest carbon sequestration in a climate change context, mainly through the integration of the three aforementioned strategies in a functional zoning approach at the landscape scale. We present a review with promising strategies for guiding sustainable forest management in such a global context

Adjacent-category models for ordinal time series and their application to climate-dependent spruce budworm defoliation dynamics

This work proposes an adjacent-category autoregressive model for time series of ordinal variables. We apply this model to dendrochronological records to study the effect of climate on the intensity of spruce budworm defoliation during outbreaks in two sites in eastern Canada. The model's parameters are estimated using the maximum likelihood approach. We show that this estimator is consistent and asymptotically Gaussian distributed. We also propose a Portemanteau test for goodness-of-fit. Our study shows that the seasonal ranges of maximum daily temperatures in the spring and summer have a significant quadratic effect on defoliation. The study reveals that for both regions, a greater range of summer daily maximum temperatures is associated with lower levels of defoliation up to a threshold estimated at 22.7C (CI of 0-39.7C at 95%) in T\'emiscamingue and 21.8C (CI of 0-54.2C at 95%) for Matawinie. For Matawinie, a greater range in spring daily maximum temperatures increased defoliation, up to a threshold of 32.5C (CI of 0-80.0C). We also present a statistical test to compare the autoregressive parameter values between different fits of the model, which allows us to detect changes in the defoliation dynamics between the study sites in terms of their respective autoregression structures

Land Sparing Can Maintain Bird Diversity in Northeastern Bangladesh

One of humanity's most significant challenges in the process of attaining the established sustainability goals is balancing the growing human demand for food and the need to conserve biodiversity. This challenge requires appropriate land uses that are able to conserve biodiversity while ensuring ample food supply. This study compares bird species diversity and abundance in areas undergoing land sharing and land sparing in northeastern Bangladesh (West Bhanugach Reserved Forest). Birds serve as useful biologic indicators because of their presence within different trophic levels and their well-studied ecology. To survey birds, we selected a total of 66 sampling sites within land-sharing (33) and land-sparing (33) land-use areas. Between May and June 2017, we observed and recorded bird calls within a 50-m radius around each sampling site. We counted 541 individuals from 46 species of birds. The Shannon bird diversity was higher in the land-sparing sites (1.52) than in the land-sharing sites (1.23). We found approximately 30% more bird species (39 vs. 30) and 40% more individuals (318 vs. 223) in the land-sparing areas than land-sharing areas. Three bird species,Arachnothera longirostra,Micropternus brachyurusandCopsychus malabaricus, were significantly associated with the land-sparing sites. This study shows that land sharing negatively affects bird diversity, richness and abundance compared to land-sparing. The use of chemical fertilizers and the lack of food, such as insects, for birds can explain the lower diversity, richness and abundance of birds in the land-sharing areas. Although land sharing is an effective means of producing food, land sparing is the most effective land-use practice for preserving bird diversity in northeastern Bangladesh