Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to considering the heterogeneity of climate change impacts as well as the fact that risks, opportunities, and adaptation capacities might differ regionally. Here we provide the Regional Integrated Assessment of climate change on Quebec's forests, a work that involved several research teams and focused on climate change impacts on Quebec's commercial forests and on potential adaptation solutions. Our work showed that climate change will alter several ecological processes within Quebec's forests. These changes will result in important modifications in forest landscapes. Harvest will cumulate with climate change effects to further alter future forest landscapes, which will also have consequences on wildlife habitats (including woodland caribou habitat), avian biodiversity, carbon budget, and a variety of forest landscape values for Indigenous peoples. The adaptation of the forest sector will be crucial to mitigate the impacts of climate change on forest ecosystem goods and services and improve their resilience. Moving forward, a broad range of adaptation measures, notably through reducing harvest levels, should be explored to help strike a balance among social, ecological, and economic values. We conclude that without climate adaptation, strong negative economic and ecological impacts will likely affect Quebec's forests.

Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to consider the heterogeneity of climate change impacts but also the fact that risks, opportunities and adaptation capacities might differ regionally. Here we provide the Regional Integrated Assessment of climate change on Quebec’s forests, a work that involved several research teams and that focused on climate change impacts on Quebec’s commercial forests and on potential adaptation solutions. Our work showed that climate change will alter several ecological processes within Quebec’s forests. These changes will result in important modifications in forest landscapes. Harvest will cumulate with climate change effects to further alter future forest landscapes which will also have consequences on wildlife habitat (including woodland caribou habitat), avian biodiversity, carbon budget and a variety of forest landscape values for Indigenous peoples. The adaptation of the forest sector, will be crucial to mitigate the impacts of climate change on forest ecosystem goods and services and improve their resilience. Moving forward, a broad range of adaptation measures, notably through reducing harvest levels, should be explored to help strike a balance among social, ecological and economic values. We conclude that without climate adaptation strong negative economical and ecological impacts will likely affect Quebec’s forests.

Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.

 

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

Monitoring of forest response to gradual environmental changes or abrupt disturbances provides insights into how forested ecosystems operate and allows for quantification of forest health. In this chapter, we provide an overview of Smartforests Canada, a national-scale research network consisting of regional investigators who support a wealth of existing and new monitoring sites. The objectives of Smartforests are threefold: (1) establish and coordinate a network of high-precision monitoring plots across a 4400 km gradient of environmental and forest conditions, (2) synthesize the collected multivariate observations to examine the effects of global changes on complex above- and belowground forest dynamics and resilience, and (3) analyze the collected data to guide the development of the next-generation forest growth models and inform policy-makers on best forest management and adaptation strategies. We present the methodological framework implemented in Smartforests to fulfill the aforementioned objectives. We then use an example from a temperate hardwood Smartforests site in Quebec to illustrate our approach for climate-smart forestry. We conclude by discussing how information from the Smartforests network can be integrated with existing data streams, from within Canada and abroad, guiding forest management and the development of climate change adaptation strategies.

Although the spatiotemporal dynamics of spruce budworm outbreaks have been intensively studied, forecasting outbreaks remains challenging. During outbreaks, budworm-linked warblers (Tennessee, Cape May, and bay-breasted warbler) show a strong positive response to increases in spruce budworm, but little is known about the relative timing of these responses.

Purpose of Review

Forest managers have long suggested that forests can be made more resilient to insect pests by reducing the abundance of hosts, yet this has rarely been done. The goal of our paper is to review whether recent scientific evidence supports forest manipulation to decrease vulnerability. To achieve this goal, we first ask if outbreaks of forest insect pests have been more severe in recent decades. Next, we assess the relative importance of climate change and forest management–induced changes in forest composition/structure in driving these changes in severity.

Recent Findings

Forest structure and composition continue to be implicated in pest outbreak severity. Mechanisms, however, remain elusive. Recent research elucidates how forest compositional and structural diversity at neighbourhood, stand, and landscape scales can increase forest resistance to outbreaks. Many recent outbreaks of herbivorous forest insects have been unprecedented in terms of duration and spatial extent. Climate change may be a contributing factor, but forest structure and composition have been clearly identified as contributing to these unprecedented outbreaks.

Summary

Current research supports using silviculture to create pest-resistant forest landscapes. However, the precise mechanisms by which silviculture can increase resistance remains uncertain. Further, humans tend to more often create pest-prone forests due to political, economic, and human resistance to change and a short-sighted risk management perspective that focuses on reactive rather than proactive responses to insect outbreak threats. Future research efforts need to focus on social, political, cultural, and educational mechanisms to motivate implementation of proven ecological solutions if pest-resistant forests are to be favoured by management.

Eastern Canadian boreal forests are mainly influenced by natural wildfires and forest management activities. To evaluate forest dynamics under possible interactions among fire and timber harvest in a future climate warming scenario (RCP 2.6, RCP 4.5 and RCP 8.5) the forest landscape model Landis II was used to simulate the dynamics of the 78000 km2 of boreal forests in eastern Canada. Forest management intensity scenarios were modeled considering the changes in the annual harvested area (0.5%, 1%, and 2%) and the age that conifers and hardwoods can be harvested (50 and 30 years, 70 and 50 years, and 90 and 70 years). The results of the 300-year model projections implied that both forest management intensity and climatic scenarios explained most of the variability in aboveground biomass, aboveground net primary productivity and forest composition. Forest management seems to be the most important factor that modified the landscape in the southern forests because there were scheduled stands with the age and composition required by each harvesting prescription to deal with the annual allowable cut volume. On the contrary, in the northern forests there was a mixed effect of climate change and forest management because many of the areas suitable for harvesting were previously burned limiting the amount of area available for harvesting. Thus, although it is expected an increase in wildfire area burned due to climate change, the intensification of forest management seems to be the most important driver of the increase of hardwoods and mixed stands and the decrease of conifers stands on the mixedwood boreal landscape, mainly in the southern forests. These results suggest that timber supply would be at risk in the Abitibi Plain, therefore, some strategies should be applied to adapt forest management to climate change.

Defoliation can enhance tree water status by reducing canopy transpiration under drought. During long-lasting insect outbreaks however, this effect can be transient as reduced foliage affects not only transpiration but also the entire soil-plant-atmosphere continuum. In this study, we investigated the effects of defoliation and vapor pressure deficit (VPD) on plant and soil water status in balsam fir and black spruce defoliated by spruce budworm, Choristoneura fumiferana (Clemens). We sampled 48 fir trees and 36 spruce trees subjected to differing severities of defoliation. In May–September 2014 and 2015, we monitored the relative shoot water content (RWC) and soil volumetric water content (VWC), and midday shoot water potential (?md, only in 2015). We applied linear mixed models (LMMs) to assess changes in RWC, ?md, and VWC to defoliation and VPD and we ran structural equation models (SEM) to determine the causal relationships between the measured variables in relation to defoliation and VPD. In LMMs models, defoliation and VPD, as individual factors, reduced ?md in both balsam fir and pooled species models but did not affect RWC. Defoliation alone increased VWC in balsam fir and in pooled models. We observed no interaction between VPD and defoliation on tree water status, but significant effect on VWC (in balsam fir and pooled models), indicating that both factors had independent and additive effects on plants but not on soil. However, in SEM models, RWC was negatively correlated to defoliation, suggesting a hydraulic safety margin. Under conditions of multiple-years of natural defoliation during a spruce budworm outbreak, the decrease in ?md reflects the amount of internal water capacitance that could be caused by both a lower ?md due to larval feeding and a negative feedback between defoliation and xylem vulnerability.

Spruce budworm (SBW) outbreaks are a major natural disturbance in boreal forests of eastern North America. During large?scale infestations, aerial spraying of bacterial insecticides is used to suppress local high?density SBW populations. While the primary goal of spraying is the protection of wood volume for later harvest, it should also maintain carbon stored in trees. This study provides the first quantitative analysis of the efficacy of aerial spraying against SBW on carbon dynamics in balsam fir, spruce, and mixed fir–spruce forests. In this study, we used the TRIPLEX?Insect model to simulate carbon dynamics with and without spray applications in 14 sites of the boreal forest located in various regions of Québec. We found that the efficacy of aerial spraying on reducing annual defoliation was greater in the early stage (<5 yr since the outbreak began) of the outbreak than in later (5–10 yr since the outbreak began) stage. Our results showed that more net ecosystem productivity is maintained in balsam fir (the most vulnerable species) than in either spruce or mixed fir–spruce forests following spraying. Also, average losses in aboveground biomass due to the SBW following spraying occurred more slowly than without spraying in balsam fir forests. Our findings suggest that aerial spraying could be used to maintain carbon in conifer forests during SBW disturbances, but that the efficacy of spray programs is affected by host species and stage of the SBW outbreak.

Phenology has become a field of growing importance due to the increasingly apparent impacts of climate change. However, the time-consuming, subjective and tedious nature of traditional human field observations have hindered the development of large-scale phenology networks. Such networks are rare and rely on time-lapse cameras and simplistic color indexes to monitor phenology. To automatize rapid, detailed and repeatable analyzes, we propose an Artificial Intelligence (AI) framework based on machine learning and computer vision techniques. Our approach extracts multiple ecologically-relevant indicators from time-lapse digital photography datasets. The proposed framework consists of three main components: (i) a random forest model to automatically select relevant images based on color information; (ii) a convolutional neural network (CNN) to identify and localize open tree buds; and (iii) a density-based spatial clustering algorithm to cluster open bud detections across the time-series. We tested this framework on a dataset including thousands of black spruce and balsam fir tree images captured using our phenological camera network. The performed experiments showed the efficiency of the proposed approach under challenging perturbation factors, such as significant image noise. Our framework is exceedingly faster and more accurate than human analysts, reducing the time-series processing time from multiple days to under an hour. The proposed methodology is particularly appropriate for large-scale and long-term analyzes of ecological imagery datasets. Our work demonstrates that the use of computer vision and machine learning methods represents a promising direction for the implementation of national, continental, or even global plant phenology networks.

1. As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.
2. We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, nonlinear, responses of stand?level net primary production (NPP) to climate variability (period 1902–2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.
3. Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.
4. Synthesis. Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

1. Considering intraspecific trait variability (ITV) in ecological studies has improved our understanding of species persistence and coexistence. These advances are based on the growing number of leaf ITV studies over local gradients, but logistical constraints have prevented a solid examination of ITV in root traits or at scales reflecting species’ geographic ranges.
2. We compared the magnitude of ITV in above? and below?ground plant organs across three spatial scales (biophysical region, locality and plot). We focused on six understorey species (four herbs and two shrubs) that occur both in disturbed and undisturbed habitats across boreal and temperate Canadian forests. We aimed to document ITV structure over broad ecological and geographical scales by asking: (a) What is the breadth of ITV across species range?scale? (b) What proportion of ITV is captured at different spatial scales, particularly when local scale disturbances are considered? and (c) Is the variance structure consistent between analogous leaf and root traits, and between morphological and chemical traits?
3. Following standardized methods, we sampled 818 populations across 79 forest plots simultaneously, including disturbed and undisturbed stands, spanning four biophysical regions (~5,200 km). Traits measured included specific leaf area (SLA), specific root length (SRL) and leaf and root nutrient concentrations (N, P, K, Mg, Ca). We used variance decomposition techniques to characterize ITV structure across scales.
4. Our results show that an important proportion of ITV occurred at the local scale when sampling included contrasting environmental conditions resulting from local disturbance. A certain proportion of the variability in both leaf and root traits remained unaccounted for by the three sampling scales included in the design (36% on average), with the largest amount for SRL (54%). Substantial differences in magnitude of ITV were found among the six species, and between analogous traits, suggesting that trait distribution was influenced by species strategy and reflects the extent of understorey environment heterogeneity.
5. Even for species with broad geographical distributions, a large proportion of within?species trait variability can be captured by sampling locally across ecological gradients. This has practical implications for sampling design and trait selection for both local studies and continental?scale modelling.

Climate change is predicted to alter relationships between trophic levels by changing the phenology of interacting species. We tested whether synchrony between two critical phenological events, budburst of host species and larval emergence from diapause of eastern spruce budworm, increased at warmer temperatures in the boreal forest in northeastern Canada. Budburst was up to 4.6 ± 0.7 days earlier in balsam fir and up to 2.8 ± 0.8 days earlier in black spruce per degree increase in temperature, in naturally occurring microclimates. Larval emergence from diapause did not exhibit a similar response. Instead, larvae emerged once average ambient temperatures reached 10°C, regardless of differences in microclimate. Phenological synchrony increased with warmer microclimates, tightening the relationship between spruce budworm and its host species. Synchrony increased by up to 4.5 ± 0.7 days for balsam fir and up to 2.8 ± 0.8 days for black spruce per degree increase in temperature. Under a warmer climate, defoliation could potentially begin earlier in the season, in which case, damage on the primary host, balsam fir may increase. Black spruce, which escapes severe herbivory because of a 2?week delay in budburst, would become more suitable as a resource for the spruce budworm. The northern boreal forest could become more vulnerable to outbreaks in the future.

Natural disturbances are fundamental to forest ecosystem dynamics and have been used for two decades to improve forest management, notably in the boreal forest. Initially based on fire regimes, there is now a need to extend the concept to include other types of disturbances as they can greatly contribute to forest dynamics in some regions of the boreal zone. Here we review the main descriptors—that is, the severity, specificity, spatial and temporal descriptors and legacies, of windthrow and spruce bud worm outbreak disturbance regimes in boreal forests—in order to facilitate incorporating them into a natural disturbance-based forest management framework. We also describe the biological legacies that are generated by these disturbances. Temporal and spatial descriptors characterising both disturbance types are generally variable in time and space. This makes them difficult to reproduce in an ecosystem management framework. However, severity and specificity descriptors may provide a template upon which policies for maintaining post harvesting and salvage logging biological legacies can be based. In a context in which management mainly targets mature and old-growth stages, integrating insect and wind disturbances in a management framework is an important goal, as these disturbances contribute to creating heterogeneity in mature and old-growth forest characteristics.

Although advance regeneration abundance and vigor are critical factors determining future forest composition and productivity, very few studies have focused on how they are affected by spruce budworm (SBW) outbreaks even though they affect millions of hectares of boreal forest on a cyclical basis. Post-SBW salvage logging is often used to reduce economic losses but could interact with the outbreak to affect advance regeneration. This study aims to determine the impact of SBW outbreaks and post-outbreak salvage logging on the defoliation of advance regeneration in mixed coniferous stands of northeastern Canada. Specifically, we assessed the effect of regeneration height and species (balsam fir or black spruce), as well as canopy composition, on the defoliation of advance regeneration. We then evaluated the effect of salvage logging on defoliation sustained by advance regeneration and compared it to the one observed in stands only affected by the SBW. Regeneration height and species, canopy composition and salvage logging all significantly affected defoliation and showed multiple interactions. Taller balsam fir seedlings were three times as defoliated as smaller ones, whereas it was 2.3 times for black spruce. Balsam fir seedlings were 15% more defoliated than black spruce. Seedlings of both species located beneath a balsam fir canopy were also more defoliated (>50% defoliation) than seedlings found under black spruce trees (about 26% defoliation). Salvage logging in black spruce-dominated stands resulted in a ?25% increase in defoliation of tall (2.5?m) black spruce regeneration when compared to non-harvested sites. We speculate that this could increase the fir component in spruce-dominated stands, leading to forests that are more susceptible to future SBW outbreaks. To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change occurs or not, particularly in spruce-dominated stands. As a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.

1.As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.

2.We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, non?linear, responses of stand?level net primary production (NPP) to climate variability (period 1902?2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.

3.Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.

4.Synthesis: Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

This article is protected by copyright. All rights reserved.

Climate change is causing northward shifts in species ranges. For mobile species such as insects, this will increase their access to forest ecosystems where in the past their presence and impact was limited. Range expansion and increases in outbreak severity of forest pests have been documented in Europe and North America (Jepsen et al. 2008; Bentz et al. 2010). Temperature-mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies are linked to the long-term effects of range expansion on boreal ecosystems. The degree to which temperate and boreal forest ecosystems are resilient to novel disturbance regimes will have direct consequences on the provisioning of goods and services from these forests and on long-term forest management planning. These concerns were the impetus for the organization of a workshop on climate-induced range shifts in boreal forest pests. Contributions to this special feature are selected papers from the International Union of Forest Research Organizations (IUFRO) workshop held in July 2016 in northeastern Quebec. The workshop was organized around five themes related to the consequences of range shifts of boreal forest insect pests: (1) plant–insect phenology, (2) species range expansions, (3) ecosystem response to changes in disturbance regimes, (4) interactions among disturbances, and (5) forest management and adaptation to change.

Changes in natural and anthropogenic disturbances in the mixedwood boreal forests of northwest Quebec have modified the landscape heterogeneity in terms of its composition and configuration. The aim of this study was to evaluate the heterogeneity (composition and configuration) of 78,000 km2 of mixedwood boreal forest landscape during recent decades (using Landsat imagery from 1985 to 2013) in areas affected by forest management. The classes Conifer, Mixed-Conifer, Mixed-Hardwood, and Hardwood were differentiated with an object-based classification (accuracy >80.2, Kappa coefficient >0.7). Also, 5 metrics (mean area, largest patch index, percentage the landscape of the core area, perimeter-area fractal dimension, and aggregation index) were calculated. This study showed that conifer-dominant cover dominates the mosaic and accounted for one third of the study area. Nevertheless, the conifer-dominant class showed the greatest decrease (reduction of 35% of its initial area at a rate of 1.7% per year). The metrics indicated that forest management in recent years produced a more heterogeneous landscape in 2013 unlike the landscape in 1985. The fire-influenced, old-growth conifer forests that previously dominated the landscape in northwestern Quebec were transformed by forestry practices into a more heterogeneous landscape.

Les régimes des perturbations naturelles et anthropiques dans les forêts boréales mixtes du nord-ouest du Québec ont modifié l'hétérogénéité du paysage en termes de composition et de configuration. L'objectif de cette étude était d'évaluer l'hétérogénéité (composition et configuration) d'un territoire de 78,000 Km2 de la forêt boréale mixte aménagée au cours des dernières décennies en utilisant des images Landsat de 1985 à 2013. Les classes de couvertures forestières Résineux, Résineux mélangés, Feuillus mélangés et Feuillus ont été différenciées à l'aide d'une méthode de classification basée sur l'identification d'objets (précision >80.2, coefficient Kappa >0.7). En outre, cinq paramètres (superficie moyenne, indice de la taille de la plus grande parcelle, pourcentage du paysage de l'aire centrale, dimension fractale entre périmètre et surface et indice d'agrégation) ont été calculés. Les résultats de cette étude ont montré que la classe Résineux, domine la mosaïque et représente un tiers de la zone d'étude. Nonobstant, cette même classe a enregistré la plus forte diminution au cours de la période d'évaluation avec une réduction de 35% de sa superficie initiale au taux de 1,7% par an. Les paramètres indiquent que l'aménagement forestier au cours des dernières années a produit un paysage plus hétérogène en 2013, contrairement au paysage en 1985. Ces résultats indiquent que les forêts anciennes résineuses et influencées par le feu qui dominaient auparavant le paysage dans le nord-ouest du Québec ont été transformées par les pratiques forestières en un paysage plus hétérogène.

Plant neighborhood effects on herbivore damage have been observed in many systems although few studies have assessed the different component effects of the neighborhood (conspecific neighbors vs. heterospecific neighbors) on defoliation. No earlier studies have monitored how temporal scale influences neighborhood effects. We tested hypotheses on resource concentration/dilution and associational effects in the eastern spruce budworm (Choristoneura fumiferana)–forest system over a 10-yr period across different stands including stands dominated by highly vulnerable hosts (balsam fir), stands dominated by a species with low vulnerability (black spruce), and mixed composition stands (fir and spruce). We observed persistent resource concentration effects on the primary host (balsam fir) during the increasing phase of the outbreak in balsam fir-dominated and mixed stands, while both resource dilution and associational susceptibility effects were observed on the secondary host (black spruce) and the strength of associational susceptibility increased with an increase in resource dilution. We did not observe associational effects on the primary host in mixed and secondary host-dominated stands but in stands dominated by the primary host, we observed associational resistance when the resource was highly depleted. Therefore, the complexity of neighborhood effects suggests that future studies should consider the separate effects of conspecific and heterospecific neighbors, as well as changes through time in order to predict herbivore damage in different systems, and provide better preventive and reactive strategies to manage herbivore outbreaks.

Context

Forest management alters patterns generated by natural disturbances, particularly in ecosystems with infrequent fires. Management effects can differ according to spatial scale and affect ecological processes.

Objectives

To assess the effect of 80 years of forest management at both the landscape and burn/harvest scales on forest age, composition, density, spatial pattern and heterogeneity.

Methods

Forest inventory maps and satellite images were used to compare two contiguous landscapes, respectively managed and unmanaged, of the eastern boreal forest of Canada, in a region with infrequent fires. Burns and harvests occurring from 1920–1950 were also compared.

Results

In addition to reducing the proportion of old-growth stands in the landscape, forest management changed forest composition at both scales, favouring the late-successional species balsam fir. Landscape metrics indicated that old-growth forests and spruce-dominated ones were more fragmented, less connected, and confined to smaller patches in the managed landscape than in the unmanaged one. Forest management increased heterogeneity at the landscape scale, but decreased it at the burn/harvest scale. Logging had a homogenizing effect at the burn/harvest scale by attenuating the effect of the physical environment on forest density.

Conclusions

This study provides knowledge to help reduce effects of forest management at both scales. In this forest region with low fire recurrence, the goal should be to manage for greater forest heterogeneity at the burn/harvest scale whereas, at the landscape scale, restoration strategies should aim to create large contiguous patches of coniferous forests to increase spatial continuity as these were reduced by past management activities.

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

Location

Eastern black spruce–moss forest, Quebec, Canada.

Methods

A total of 92 plots were sampled, each with a radius of 11.28 m; 49 of these plots were salvaged while 43 were unsalvaged. Regeneration density, plant diversity and seedbeds were characterized. We tested the effect of microtopography and windthrow severity on species richness and Shannon diversity index for salvaged and unsalvaged windthrows using a mixed model. Partial redundancy analysis (RDA) determined which environmental and stand characteristics were most important in explaining differences in plant species and forest floor types among the treatments. The effects of treatments (salvaged and unsalvaged windthrows), microtopography attributes, windthrow severity and regeneration species on seedling and sapling abundance were tested using a linear mixed model.

Results

Salvaged windthrow, with a large proportion of skid trails, dead mosses and Sphagnum, had a lower degree of seedbed heterogeneity. Also, some understorey species present in the unsalvaged ecosystem were absent from the salvaged windthrow. Sphagnum and other moss species were clearly associated with the unsalvaged treatment. White birches were positively associated with mound microtopography in the unsalvaged windthrow.

Conclusion

From an ecosystem-based forest management perspective, natural post-windthrow understorey conditions and microsite heterogeneity can be in part maintained in salvaged cut blocks by incorporating retention patches that include downed and standing dead wood and living trees of diverse sizes. These steps should favour plant regeneration and augment diversity for salvage logging after wind disturbance.

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

Question: In the boreal forest of eastern Canada, how does forest vegetation change in the sustained absence of fire?

Location: Eastern boreal forest in Quebec's North Shore region, Canada (49°30'–50°00'N; 67°30'–68°35'W).

Methods: Aerial photos from three different periods (1930, 1965 and 1987) were used to characterize changes in vegetation composition in 23 scenes of 200 ha. Time since fire, presence of secondary disturbances and data on soil and topographic variables were obtained. Ordination and clustering techniques were used to define compositional trajectories of change over the 57-yr period. These trajectories were further grouped into pathways based on compositional changes, time since fire and preferential deposit-drainage types.

Results: Among the 26 compositional trajectories, three successional pathways were distinguished. Two start post-fire succession with a dominance of intolerant hardwood. In one of these, this is followed by an increase in Abies balsamea, while in the second the importance of Picea mariana increases with time. In the third pathway P. mariana is an important component from the outset. In this pathway, we observed modest fluctuation in the relative dominance of P. mariana and A. balsamea and variation in stand structure.

Conclusion: The boreal forest vegetation of Eastern Canada is diverse and dynamic even in the absence of fire, notably under the influence of partial disturbances. Such disturbances can be associated with changes in composition or stand structure. The development of management strategies aimed at maintaining stand diversity by emulating a broader variety of partial and secondary disturbances should be encouraged.

Emulating natural disturbances in managed forests has been suggested as a potential solution to maintain habitat conditions similar to those observed in old-growth forests. We examined the gap attributes and disturbance history of old-growth Picea abies-dominated stands in the northern boreal vegetation zone of the Pallas-Yllästunturi National Park in northwestern Finland to evaluate the influence of gaps on forest dynamics and the temporal patterns of gap creation. Six stands located at two sites were sampled along 400-m-long linear transects so that all intersected gaps were measured and dated. The average proportion of the forest area in the gaps was 43.1% ± 7.5%. An average gap size was estimated to be 221 m² ± 198 m², whereas the median gap size was 170.2 m². While only 20% of the gaps were smaller than 100 m², nearly 85% of them were smaller than 300 m². Gap creation was constant with no distinct peaks from 1965 to 2005. Thus, forest dynamics were driven by continuous small-scale disturbances and were characterized by quasi-equilibrium structure. However, the results of the growth release analysis indicated that more severe disturbance(s) may have occurred almost two centuries ago. Emulating this type of forest dynamics would imply selective or group harvesting of trees as the predominant methods, but larger-scale, more intensive cuttings could also be carried out periodically.

In unmanaged boreal forests standing dead trees are an important component of stand structure, but knowledge on their quantities and dynamics is limited. We characterized the populations of standing dead trees, and modeled their decay-class dynamics in the northeastern boreal old-growth forests of Quebec, Canada. Using 40 m x 400 m plots, we sampled five Picea mariana-dominated, five mixed P. mariana-Abies balsamea, and five A. balsamea-dominated stands. We classified dead trees into five decay classes, extracted sample disks, and crossdated the year of death of 190 trees. Mean times since death in each decay class were used to construct a matrix model for transition dynamics between the classes. Standing dead trees were abundant in all stands, but with large between-stand variation (density, 89.4-229.4 trees ha^-1; volume, 8.3-49.2 m³ ha^-1). On average, dead trees represented 21.4% of the number of all standing trees in P. mariana-dominated stands, 34.0% in mixed P. mariana-A. balsamea stands, and 33.7% in A. balsamea-dominated stands. Modeling indicated lower transition rates between decay classes for P. mariana than for A. balsamea. Half-life as standing dead trees was 35-40 years for P. mariana and 30-35 years for A. balsamea. Our results showed that standing dead trees are an important and long-lasting structural component of the stands studied. To retain stand structural complexity and the associated species diversity in managed forests of this region, strategies for managing standing dead trees need to be developed.

Abstract:

Spatial patterns, rates, and temporal variation of standing-tree mortality were studied in unmanaged boreal old-growth forests of northeastern Quebec. The study was carried out by sampling living and dead trees within 15 transects (400 m long, 40 m wide). The transects lay in stands that were classified according to their species composition in three types: dominated by black spruce, Picea mariana (Mill.) BSP; mixed P. mariana and balsam fir, Abies balsamea (L.) Mill.; and dominated by A. balsamea. Spatial patterns were analysed using Ripley's K function. The year of death was cross-dated using 190 sample discs extracted from dead standing A. balsamea and P. mariana to assess the rates and temporal variation of mortality. The spatial patterns of standing dead trees in P. mariana stands were predominantly clustered. The spatial patterns of large dead trees (>19 cm diameter at breast height (1.3 m height; DBH)) in mixed and A. balsamea-dominated stands were mainly random, with few stands showing clustered patterns. Small dead trees (9–19 cm DBH) in these stands were generally more clustered than larger trees. Tree mortality varied from year to year, though some mortality was observed in all the studied stand types for almost every year. Standing trees that had recently died accounted for 62%, 48%, and 51% of overall mortality in P. mariana-dominated, mixed, and A. balsamea-dominated stands, respectively. The results of this study indicate that mortality of standing trees outside of episodic mortality events (such as insect outbreaks) is an important process in the creation of structural complexity and habitat diversity in these stands.

Résumé

La distribution spatiale, le taux et la variation temporelle de la mortalité des arbres sur pied ont été étudiés dans des forêts boréales anciennes et non aménagées du nord-est du Québec. L'étude a été réalisée en échantillonnant les arbres vivants et morts le long de 15 transects (400 m de longueur et 40 m de largeur). Les transects ont été établis dans des peuplements regroupés en trois types selon leur composition en espèces : peuplements dominés par Picea mariana (Mill.) BSP, peuplements mixtes de P. mariana et d'Abies balsamea (L.) Mill. et peuplements dominés par A. balsamea. La distribution spatiale a été analysée à l'aide de la fonction K de Ripley. L'année de la mort des arbres a été déterminée par recoupement à partir d'un échantillon de 190 disques de bois prélevés sur des arbres morts sur pied de A. balsamea et de P. mariana de façon à estimer le taux et la variation temporelle de la mortalité. Les arbres morts sur pied dans les peuplements de P. mariana étaient, dans la plupart des cas, regroupés. Les gros arbres morts (diamètre >19 cm à hauteur de poitrine (1,3 m; DHP)) dans les peuplements mixtes et les peuplements dominés par A. balsamea étaient distribués de façon aléatoire dans la plupart des cas et regroupés dans quelques peuplements. Dans ces peuplements, les petits arbres morts (DHP de 9 à 19 cm) étaient généralement plus regroupés que les gros arbres. La mortalité des arbres a varié d'une année à l'autre. Toutefois, des arbres morts ont été observés presque à chaque année dans tous les types de peuplement étudiés. Les arbres sur pied morts récemment correspondaient respectivement à 62, 48 et 51 % de la mortalité totale dans les peuplements de P. mariana, les peuplements mixtes et les peuplements d'A. balsamea. Nos résultats indiquent qu'à l'exception des événements épisodiques de mortalité, tels que les épidémies d'insectes, la mortalité des arbres sur pied est un processus important pour assurer la complexité structurale et la diversité des habitats dans ces peuplements. ©2007 NRC Canada

Questions: How do gap abundance and the spatial pattern of trees and snags change throughout stand development in Picea mariana forests? Does spatial pattern differ among site types and structural components of a forest?

Location: Boreal forests dominated by Picea mariana, northern Quebec and Ontario, Canada.

Methods: Data on the abundance, characteristics and spatial location of trees, snags and gaps were collected along 200 m transects at 91 sites along a chronosequence. Spatial analyses included 3TLQV, NLV and autocorrelation analysis. Nonparametric analyses were used to analyse trends with time and differences among structural components and site types.

Results: Gaps became more abundant, numerous and more evenly distributed with time. At distances of 1-4 m, tree cover, sapling density and snag density became more heterogeneous with time. Tree cover appeared to be more uniform for the 10- 33 m interval, although this was not significant. Patch size and variance at 1 m were greater for overstorey than for understorey tree cover. Snags were less spatially variable than trees at 1 m, but more so at intermediate distances (4 - 8 m). Few significant differences were found among site types.

Conclusions: During stand development in P. mariana forest, gaps formed by tree mortality are filled in slowly due to poor regeneration and growth, leading to greater gap abundance and clumping of trees and snags at fine scales. At broader scales, patchy regeneration is followed by homogenization of forest stands as trees become smaller with low productivity due to paludification.

The influence of stand age and site conditions on the structure of coniferous stands was studied in the boreal forest of Québec North Shore, a region with a low fire recurrence. Stand diameter diversity was measured in 2202 forest inventory plots in black spruce (Picea mariana), balsam fir (Abies balsamea) and mixed stands, using the Shannon-Wiener diversity index. A relative productivity index was developed, based on the relationship between height and age of dominant trees. A stepwise regression analysis indicated that this productivity index best explains stand structure variation in all composition types, while stand age seems to influence structure more at the beginning of stand development. The results suggest that productive stands become uneven-sized earlier than unproductive stands and also maintain a greater diameter diversity. These contrasting structural dynamics may be explained by (i) a higher growth rate in richer stands that likely induces earlier senescence and thus an earlier passage to an uneven-sized structure, and (ii) a restricted maximum tree diameter in poor stands caused by a scarcity of resources, which in turn reduces the diameter diversity of these stands, even after their breakup time.

L’influence de l’âge et des conditions de site sur la structure des peuplements résineux a été étudiée dans la forêt boréale de la Côte-Nord du Québec, une région à faible récurrence des feux. La diversité diamétrale des peuplements a été mesurée à l’aide de l’indice de diversité de Shannon-Wiener dans 2202 placettes d’inventaire forestier incluant des pessières (Picea mariana), des sapinières (Abies balsamea) et des peuplements mixtes. Un indice de productivité relative a été développé à partir de la relation âge-hauteur des arbres dominants. Une analyse de régression pas-à-pas a révélé que cet indice de productivité expliquait le mieux la variation de la structure des peuplements, et ce pour tous les types de composition, alors que l’âge du peuplement semblait influencer la structure davantage au début du développement. Nos résultats suggèrent que les peuplements productifs deviennent irréguliers plus hâtivement que les peuplements peu productifs et maintiennent une diversité diamétrale plus élevée. Ces dynamiques contrastées peuvent être expliquées (i) par un taux de croissance supérieur chez les peuplements les plus riches, entraînant vraisemblablement une sénescence plus hâtive et donc un passage plus hâtif à une structure irrégulière, et (ii) par un diamètre maximum des arbres qui est restreint par la rareté des ressources chez les peuplements pauvres, restreignant leur diversité diamétrale, même après l’âge de bris.

We investigated stand development along a chronosequence on organic, clay and sand sites in black spruce boreal forest in northwestern Quebec, Canada. Our objectiveswere: (1) to describe trends and stages of structural development following fire; (2) to compare trends and stages of development both in isolation from and in conjunction with species replacement. We tested the hypothesis that although trends in structural development are similar among site types, productivity and composition affect the timing of developmental stages. Data on live trees, snags and logs were collected at 91 sites. Trends with time since fire were analyzed using segmented piecewise linear regression. On organic sites, tree basal area and density increased continuously with time since fire, while deadwood abundance decreased and then increased. Live tree basal area, tree density and deadwood abundance generally followed expected S-, N- and U-shaped trends, respectively, on clay sites, but often with decreases in later stages due to paludification. Fewer trends were significant on sand sites, although tree basal area decreased likely due to a change in species composition. Older forests on all site types weremore structurally diverse. To estimate the timing of the stages of structural development, we introduce a newanalysis technique which uses the breakpoints of the piecewise regressions.On organic sites, only three stages of stand development were evident, whereas a four-stage stand development model was appropriate for both clay and sand sites.We found that local conditions affected not only the timing of developmental stages, but also the number of stages and the trends themselves.We attributed these differences to changes in species composition and productivity. We refine the theory of structural development by representing patterns in both live and deadwood as two-stage trends with two possible outcomes for each stage. Our new method of determining the timing of the developmental stages using empirical data can be used to develop management practices that emulate structural development in order to conserve biodiversity on a landscape scale. © 2005 Elsevier B.V. All rights reserved.

The northeastern boreal forest of Quebec is characterized by a humid climate. Consequently, fires are less frequent and small-scale disturbances play an important role in forest dynamics. Natural mortality and nonfire disturbances such as insect outbreaks and windthrow lead to gap-driven processes. Changes in structure and species composition can result from gap dynamics. The objectives of this study were to characterize gaps and examine patterns of species replacement in gaps in old conifer stands. Line intersect sampling was used to sample stands dominated by balsam fir (Abies balsamea (L.) Mill.) and (or) black spruce (Picea mariana (Mill.) BSP). Results show that 54% of the forest was in expanded gaps and that canopy gaps are relatively small, since 87% of them were smaller than 100 m². The majority (94%) of the openings were caused by the mortality of less than 10 gap makers. Replacement probabilities show self-replacement of A. balsamea in Abies stands and of P. mariana in Picea stands. However, in Abies-Picea stands, there seems to be a reciprocal replacement of the two species. These results provide knowledge of the disturbance dynamics of the region as a basis for development of silvicultural practices that preserve the structural components of older forest stands.

La forêt boréale du Nord-Est du Québec est caractérisée par un climat humide, de sorte que les feux y sont moins fréquents. La dynamique forestière est alors contrôlée par des perturbations secondaires (vent et insectes) ainsi que par la mortalité naturelle des arbres engendrant ainsi une dynamique de trouées. Des changements dans la structure et la composition des peuplements peuvent alors survenir. Les objectifs de la présente étude sont d'examiner les caractéristiques des trouées ainsi que les patrons de remplacement des espèces à l'intérieur des trouées de vieux peuplements résineux. Des transects ont été établis dans des peuplements de sapin baumier (Abies balsamea (L.) Mill.) ou d'épinette noire (Picea mariana (Mill.) BSP). La proportion de peuplements qui se présentent sous forme de trouée étendue atteint 54 % et 87 % des trouées de canopée ont une taille inférieure à 100 m². La majorité (94 %) des ouvertures est créée par la mortalité de moins de 10 individus. Les probabilités de remplacement indiquent qu'A. balsamea et P. mariana se succèdent à eux-mêmes dans les peuplements d'A. balsamea et de P. mariana respectivement mais qu'il y a un remplacement réciproque de ces espèces dans les peuplements d'Abies–Picea. Une meilleure compréhension de la dynamique naturelle permettra de développer des pratiques sylvicoles qui préserveront la structure des vieux peuplements.©2004 NRC Canada

Old-growth black spruce (Picea mariana) boreal forest in the Clay Belt region of Ontario and Quebec is an open forest with a low canopy, quite different from what many consider to be "old growth". Here, we provide an overview of the characteristics of old-growth black spruce forest for three different site types on organic, clay, and coarse deposits. Our objectives were (1) to identify the extent of older forests; (2) to describe the structure, composition, and diversity in different age classes; and (3) to identify key processes in old-growth black spruce forest. We sampled canopy composition, deadwood abundance, understorey composition, and nonvascular plant species in 91 forest stands along a chronosequence that extended from 20 to more than 250 years after fire. We used a peak in tree basal area, which occurred at 100 years on clay and coarse sites and at 200 years on organic sites, as a process-based means of defining the start of old-growth forest. Old-growth forests are extensive in the Clay Belt, covering 30–50% of the forested landscape. Black spruce was dominant on all organic sites, and in all older stands. Although there were fewer understorey species and none exclusive to old-growth, these forests were structurally diverse and had greater abundance of Sphagnum, epiphytic lichens, and ericaceous species. Paludification, a process characteristic of old-growth forest stands on clay deposits in this region, causes decreases in tree and deadwood abundance. Old-growth black spruce forests, therefore, lack the large trees and snags that are characteristic of other old-growth forests. Small-scale disturbances such as spruce budworm and windthrow are common, creating numerous gaps. Landscape and stand level management strategies could minimize structural changes caused by harvesting, but unmanaged forest in all stages of development must be preserved in order to conserve all the attributes of old-growth black spruce forest.

Natural dynamics in the boreal forest is influenced by disturbances. Fire recurrence affects community development and landscape diversity. Forest development was studied in the northeastern boreal forest of Quebec. The objective was to describe succession following fire and to assess the factors related to the changes in forest composition and structure. The study area is located in northeastern Quebec, 50 km north of Baie-Comeau. We used the forest inventory data gathered by the Ministère des Ressources naturelles du Québec (MRNQ). In circular plots of 400 m² the diameter at breast height (DBH) of all stems of tree species greater than 10 cm was recorded and in 40 m² subplots, stems smaller than 10 cm were measured. A total of 380 plots were sampled in an area of 6000 km². The fire history reconstruction was done based on historical maps, old aerial photographs and field sampling. A time-since-fire class, a deposit type, slope, slope aspect and altitude were attributed to each plot. Each plot was also described according to species richness and size structure characteristics. Traces of recent disturbance were also recorded in each plot. Changes in forest composition were described using ordination analyses (NMDS and CCA) and correlated with the explanatory variables. Two successional pathways were observed in the area and characterized by the early dominance of intolerant hardwood species or Picea mariana. With time elapsed since the last fire, composition converged towards either Picea mariana, Abies balsamea or a mixture of both species and the size structure of the coniferous dominated stands got more irregular. The environmental conditions varied between stands and explained part of the variability in composition. Their effect tended to decrease with increasing time elapsed since fire, as canopy composition was getting more similar. Gaps may be important to control forest dynamics in old successional communities.

In order to describe and compare the post-fire succession patterns of the two ecological regions (mixed-wood and coniferous ecoregions) of northwestern Quebec, 260 forest stands were sampled with the point-centred plot method. The mixed-wood ecological region belongs to the Abies balsamea-Betula papyrifera bioclimatic domain whereas the coniferous ecological region belongs to the Picea mariana-moss bioclimatic domain. In each plot, tree composition was described, surficial deposits and drainage were recorded, and fire history was reconstructed using standard dendro-ecological methods. Ordination techniques (Correspondence Analysis and Canonical Correspondence Analysis) were used to describe the successional patterns of forest vegetation and to correlate them with the explanatory variables. The results showed the importance of surficial deposits, the time since fire and the ecoregion in explaining the variation of stand composition. Abies balsamea tends to increase in importance with an increase in time since fire, and this trend is more pronounced in the mixed-wood region. Even when controlling both for surficial deposits and time since fire, differences in successional trends were observed between the two ecoregions. As all the species are present in both ecoregions and as they are all observed further north, our results suggest that both the landscape configuration and fire regime parameters such as fire size and fire intensity are important factors involved in these differences.

Small-scale canopy openings are being increasingly recognized for their importance in boreal forest stand development. Yet more work is necessary to understand their effects on seedling growth. This study investigated the effect of different degrees of canopy opening (all trees cut, conifers cut, conifers girdled and control quadrats) in different stand types on Abies balsamea seedling recruitment, growth and architecture. The lack of a treatment effect on seedling establishment suggests that gaps primarily affect advance regeneration. In the first year after treatment the seedlings in the cut blocks (both conifer cut and all trees cut) responded with an increase in height growth. Changes in the leader to lateral branch ratio were also significant. Continued architectural change in terms of number of branches produced did not occur until after two years had passed. Although not significantly different from the control, increases can be observed in all measurements for the girdled treatment. It is therefore concluded that the growth response of advance regeneration is more important following canopy opening than new seedling recruitment and that seedling performance is greatest where degree of opening is greatest.

Le sud de la forêt boréale se caractérise par un régime de perturbations complexe où l’interaction entre les feux et les épidémies de la tordeuse des bourgeons de l’épinette influence les processus successionnels selon différentes échelles spatiales et temporelles. Les feux en forêt boréale, qui sont de forte intensité et qui couvrent de vastes superficies, vont initier la succession secondaire. Par contre les trouées, qui résultent d’épidémies de la tordeuse des bourgeons de l’épinette, vont affecter la dynamique locale des communautés de sous-bois. Ces perturbations locales permettent entre autres à certaines espèces de sous-bois d’augmenter leur abondance pour ainsi se maintenir plus longtemps le long du gradient successionnel après feu. On observe aussi des augmentations de la diversité des communautés de sous-bois suite au passage de la tordeuse. Bien que ces phénomènes s’observent localement, on peut supposer qu’ils aient des répercussions à des échelles spatiale et temporelle plus grandes. La compréhension des processus successionnels dans cette région de la forêt boréale se doit d’inclure l’ensemble du régime des perturbations. Cette étude vise donc à mieux comprendre l’impact des perturbations locales sur la dynamique des communautés du sous-bois dans le sud de la forêt boréale, à différentes échelles spatiales et temporelles. Ceci dans le but éventuel de développer des outils prédictifs qui pourront conduire à un meilleur aménagement des forêts exploitées.

Dans la première partie de la thèse nous documentons la résistance des communautés de sous-bois en réponse à la création de trouées expérimentales. Les objectifs étaient de décrire les changements dans la composition des communautés de sous-bois après l’ouverture de la strate forestière en relation au statut successionnel des sites et d’évaluer les facteurs qui peuvent être responsables d’une plus forte résistance des communautés face à cette perturbation. Des ouvertures expérimentales de 400m² ont été effectuées dans six peuplements le long d’un gradient successionnel après feu. A l’intérieur de chacune de ces trouées le recouvrement des strates de sous-bois a été mesuré avant et jusqu’à quatre ans après la création des trouées. La résistance a été mesurée en termes de changements de la composition en espèces. Les résultats ont montré que les jeunes communautés changent moins que les plus vieilles. La richesse et la diversité avant coupe, à l’échelle du quadrat, ont positivement été corrélées à la résistance face à la création des trouées. Les quadrats les plus riches contenaient des espèces qui ont rapidement réagi à la perturbation en augmentant leur abondance, pour ainsi empêcher les espèces envahissantes de dominer. Les mécanismes qui permettent une plus forte résistance du sous-bois face à l’ouverture de la strate forestière pourraient être reliés au régime des perturbations particulier (combinaison des deux types) qui caractérise le sud de la forêt boréale.

Au chapitre III un modèle prédictif, qui intègre la combinaison des deux principaux types de perturbations qui caractérisent le sud de la forêt boréale, a été développé dans le but d’analyser les changements de la diversité des plantes de sous-bois en fonction d’un changement du régime des perturbations, aux niveaux de l’habitat et du paysage. Au total, six espèces réparties à l’intérieur de trois groupes, définis en fonction des stratégies vitales des espèces, sont inclus dans le modèle. Le choix des espèces de même que leur distribution à l’intérieur des groupes a été déterminé de sorte à refléter les patrons de diversité et d’abondance observés le long du gradient successionnel après feu, dans le sud de la forêt boréale. La croissance de chacune des espèces a été simulée dans quatre sites différents, dont l’ensemble constitue le paysage. Des perturbations (feux et épidémies de la tordeuse des bourgeons de l’épinette) peuvent survenir indépendamment dans chacun des sites. La diversité du sous-bois est analysée à l’échelle de l’habitat et du paysage selon différentes fréquences de perturbation. Avec seulement le feu comme élément de perturbation, les résultats montrent que la diversité du paysage est maximale selon un cycle de feu qui varie entre 75 et 100 ans. La courbe de diversité selon le cycle de feu correspond à l’hypothèse des perturbations intermédiaires de Connell. L’ajout de la tordeuse des bourgeons de l’épinette au système contribue à augmenter la diversité à l’échelle du paysage. A l’échelle de l’habitat, le changement de la fréquence des feux a un effet important sur les patrons de diversité pour les jeunes communautés successionnelles après feu. L’effet de la tordeuse a des répercussions sur la diversité qui s’observent tout le long du gradient successionnel.

Le quatrième chapitre traite de la dynamique des communautés de sous-bois soumises à des ouvertures expérimentales de la strate forestière qui imitent des épidémies de la tordeuse des bourgeons de l’épinette. L’objectif principal de cette étude est de mieux comprendre la réponse immédiate des plantes du sous-bois suite à l’ouverture de la strate forestière, et ce selon le statut successionnel des sites. Un autre objectif de l’étude est d’évaluer les facteurs abiotiques qui sont reliés à ces changements. Différents degrés d’ouverture dans des quadrats de 100m² ont été effectués dans quatre peuplements d’âge successionnel différent et répétés trois fois dans chaque site. Les changements à court terme (jusqu’à 2 ans après création des ouvertures) du recouvrement et de la densité des plantes de sous-bois dans des placettes permanentes de 1m² , ainsi que des conditions abiotiques (lumière, température et humidité du sol, minéralisation de l’azote) ont été mesurés. Des ANOVA ont testé l’effet de l’âge des peuplements et de l’intensité des traitements sur ces variables. Les résultats montrent que la strate arbustive n’a pas été affectée significativement par la application des traitements ni par l’âge. Par contre, la strate herbacée a fortement réagi à la création des ouvertures, tant au niveau des changements en recouvrement qu’en densité. Le statut successionnel des sites a aussi influencé la réaction des plantes herbacées. La lumière et la température du sol ont été influencées significativement par les traitements et l’âge des peuplements après feu. La lumière apparaît comme le principal facteur associé aux changements de recouvrement des herbacées hautes. Le changement en recouvrement des herbacées qui poussent prostrées au sol a été associé significativement au pourcentage d’humidité du sol. Ces résultats confirment l’importance des perturbations locales sur la dynamique des communautés de sous-bois dans le sud de la forêt boréale. Des ouvertures de la strate forestière de 100m² et moins apparaissent suffisantes pour entraîner des changements importants de l’abondance des espèces herbacées et des conditions abiotiques.

Cette thèse démontre l’importance du rôle des perturbations secondaires sur la dynamique des communautés de sous-bois au sud de la forêt boréale. Les trouées qui résultent des épidémies de la tordeuse des bourgeons de l’épinette semblent associées à la stabilité des communautés de sous-bois et étroitement liées à leur diversité, tant à l’échelle de l’habitat que du paysage. © 1997 UQAM tous droits réservés.

1 We investigated the response of boreal understorey communities to gap formation in order to evaluate the relationship between community stability and changes in community composition. We described the early changes in understorey species composition after gap formation and assessed some of the factors responsible for higher resistance to disturbance.

2 At six sites, in the southern boreal forest of north-western Quebec, representing a successional gradient after fire, trees were cut and removed from an area of 400 m². In each experimental gap, we recorded the cover of the understorey species in four 4 m² permanent quadrats from before to 4 years after gap formation. We estimated resistance to disturbance in terms of changes in species composition and abundance.

3 Younger communities changed less than older ones after disturbance, Sites with more species changed less in composition after gap formation than less rich sites. Richer quadrats contained species that could react quickly to disturbance by increasing their cover and preventing invaders from dominating the understorey.

4 Stronger resistance of understorey species communities to gap formation can persist in older sites because of the particular disturbance regime. Gaps created by the spruce budworm in older sites dominated by balsam fir increase microsite heterogeneity and diversity by allowing the co-existence of early and late successional species.