Glossy buckthorn (Frangula alnus Miller) is an invasive alien plant species (IAPS) rapidly expanding in North America but is largely understudied compared to the common buckthorn (Rhamnus cathartica). Our study investigated the effects of a 27-year-old F. alnus invasion on native understory plant and saprophagous macroarthropod communities in a wet deciduous woodland in Southern Québec, Canada. We hypothesized a decreased taxonomic diversity and a change in community composition of both indicator taxa with increasing F. alnus density. The understory plant and saprophagous macroarthropod communities were characterized, respectively, through vegetation surveys and pitfall trapping across a density gradient of 43 plots invaded by F. alnus. Our results demonstrated that F. alnus did not exert a strong influence on species community composition, although the homogenization of understory plant communities was observed. Despite several decades of F. alnus invasion at our study site, the consequences on the selected indicator taxa were overall relatively small, suggesting that the magnitude of effects is variable. We suggest that further investigation at a larger scale should be performed to evaluate the effect of F. alnus on a broad diversity of indicators and understand any context dependency.

Abstract In forested ecosystems, shrubs must succeed in persisting in low-light environments, while simultaneously having the ability to rapidly expand and occupy newly created canopy openings, yet little is known about the traits that make this possible. We hypothesize that shrub species that are abundant in the understory exhibit a specific set of functional traits that define their ability to persist during unfavorable periods and to rapidly exploit newly created habitats. We tested this by comparing field-measured functional traits such as biomass allocation, leaf display, crown morphology, and leaf traits, across individual size classes and two gap-forest environments of five shrub species. We observed significant differences in traits between species, size classes, and gap-forest environments. These differences were primarily related to biomass allocation traits, followed by leaf display, crown morphology, and leaf traits. Abundant shrubs like mountain maple (Acer spicatum) and hazelnut (Corylus cornuta) invested significantly more biomass in roots, had a larger total leaf area, and displayed leaves in a more efficient manner to intercept light. The high investment in root biomass can be interpreted as shrubs exploiting the persistence and colonization strategy through resprouting. Permanent sub-canopy status likely explains the importance of efficient leaf display, wherein abundant shrubs had a large leaf area with minimal support structures.

The stand-level risk of both defoliation and mortality due to the spruce budworm (Choristoneura fumiferana, Clem., SBW) is mainly linked to the quality of the resource such that stands dominated by the primary host species, balsam fir (Abies balsamea [L.] Mill.), are considered the most vulnerable. At the landscape scale, insect dispersal suggests that both configuration and resource availability should influence the onset and subsequent mortality of an outbreak. However, outbreak risk at this scale is yet to be quantified for the SBW, one of the most studied forest pest defoliators in the world. We aim to determine the risk of both SBW defoliation onset and resultant tree mortality at a landscape scale due to factors that have been associated with SBW defoliation at the stand scale. We used Cox proportional hazard models to quantify the relative risk of different landscape configuration and species composition patterns on defoliation onset and risk of mortality of the current SBW outbreak in Eastern Canada. In particular, we measured the risk associated with primary host proportion, relative amounts of secondary host and hardwood trees to fir trees, the structure of balsam fir in the landscape, and the structure of the forest overall. We show that defoliation onset is more related to the configuration of the landscape than to the species composition while the risk of ensuing mortality increases as the proportion of the primary host increases. Defoliation onset risk is increased by a more complex configuration of fir patches in the landscape. Mortality risk, however, is unaffected by fir configuration but is reduced where there is more black spruce (Picea mariana [Mill.] B.S.P.) or a decrease of balsam fir in the landscape. The usefulness of survival analysis for assessing risk for insect outbreaks has been overlooked and can quantify the effect of relevant factors to guide management and mitigation strategies. The results of our analysis suggest that forest management strategies should avoid clear cuts and the development of fir monocultures and instead maintain or increase the proportion of black spruce and improve forest connectivity across large forested landscapes to delay defoliation onset and reduce SBW-related mortality.

Natural disturbances drive forest dynamics and biodiversity at different spatial and temporal scales. Forests in the boreal biome are shaped by several types of disturbance, including fire, windthrow, and insect outbreaks, that vary in frequency, extent, severity, and specificity. In managed forests, disturbances also affect the amount and quality of available timber. Ecosystem management uses information on disturbance regimes as a guide to finding a balance between ecological, economic, and social viewpoints. In this chapter, we review current knowledge on disturbance regimes in boreal forests and discuss some implications for managing the impact and risk of disturbances in the context of forest ecosystem management and restoration.

As global climate conditions continue to change, disturbance regimes and environmental drivers will continue to shift, impacting global vegetation dynamics. Following a period of vegetation greening, there has been a progressive increase in remotely sensed vegetation browning globally. Given the many societal benefits that forests provide, it is critical that we understand vegetation dynamic alterations. Here, we review associative drivers, impacts, and feedbacks, revealing the complexity of browning. Concomitant increases in browning include the weakening of ecosystem services and functions and alterations to vegetation structure and species composition, as well as the development of potential positive climate change feedbacks. Also discussed are the current challenges in browning detection and understanding associated impacts and feedbacks. Finally, we outline recommended strategies.

Despite new knowledge in recent years, our understanding of the phenology of wood formation for various species growing in different environments remains limited. To enhance our knowledge of the tree growth dynamics of boreal tree species, we investigated the average seasonal, monthly, daily, and diel patterns of tree growth and water status from 11 years of observations with the 15 min and 1.5 µm resolved stem radial size variation data of 12 balsam fir (Abies balsamea (L.) Mill.) trees growing in a cold and humid boreal environment. Growth only occurred above an air temperature threshold of 9–10 °C, and the maximal growth rate over the year (23–24 June) was synchronous with the maximal day length (20–21 June) and not with the maximal air temperature, which occurred on average about 2 weeks later (4–5 July). Tree growth was mostly restricted by air temperature and solar radiation under these cold and wet boreal conditions, but our results also highlight a turgor-driven growth mechanism. Diel dynamics reveal that tree growth is minimal during the day when the stem dehydrates, and higher past midnight when the stem is fully rehydrated. This pattern suggests that carbon assimilation through photosynthesis occurs primarily during the day, while energy production and carbon allocation to woody tissues occur primarily at night via cellular respiration. Overall, our results show that the temporal patterns of the growth and water status of balsam fir growing in cold and humid boreal environments are controlled by a set of environmental factors that influence various physiological processes and mechanisms, many of which still need to be documented.

Increased greenhouse gas emissions are causing unprecedented climate change, which is, in turn, altering emissions and removals (referring to the oxidation of atmospheric CH4 by methanotrophs within the soil) of the atmospheric CH4 in terrestrial ecosystems. In the global CH4 budget, wetlands are the dominant natural source and upland soils are the primary biological sink. However, it is unclear whether and how the soil CH4 exchanges across terrestrial ecosystems and the atmosphere will be affected by warming and changes in precipitation patterns. Here, we synthesize 762 observations of in situ soil CH4 flux data based on the chamber method from the past three decades related to temperature and precipitation changes across major terrestrial ecosystems worldwide. Our meta-analysis reveals that warming (average warming of +2°C) promotes upland soil CH4 uptake and wetland soil CH4 emission. Decreased precipitation (ranging from −100% to −7% of local mean annual precipitation) stimulates upland soil CH4 uptake. Increased precipitation (ranging from +4% to +94% of local mean annual precipitation) accelerates the upland soil CH4 emission. By 2100, under the shared socioeconomic pathway with a high radiative forcing level (SSP585), CH4 emissions from global terrestrial ecosystems will increase by 22.8 ± 3.6 Tg CH4 yr−1 as an additional CH4 source, which may be mainly attributed to the increase in precipitation over 30°N latitudes. Our meta-analysis strongly suggests that future climate change would weaken the natural buffering ability of terrestrial ecosystems on CH4 fluxes and thus contributes to a positive feedback spiral.

Sapwood characteristics, such as sapwood area as well as thermal and hydraulic conductivity, are linked to species-specific hydraulic function and resource allocation to water transport tissues (xylem). These characteristics are often unknown and thus a major source of uncertainty in sap flow data processing and transpiration estimates because bulk rather than species-specific values are usually applied. Here, we analyzed the sapwood characteristics of fifteen common tree species in eastern North America from different taxonomic (i.e., angiosperms and gymnosperms) and xylem porosity groups (i.e., tracheid-bearing, diffuse- or ring-porous species) and we assessed how uncertainties in sapwood characteristics involved in sap flow calculations are propagated in tree water use estimates. We quantified their sapwood area changes with stem diameter (allometric scaling) and thermal conductivity. We combined these measurements with species-specific values of wood density and hydraulic conductivity found in the literature and assessed the role of wood anatomy in orchestrating their covariation. Using an example sap flow dataset from tree species with different xylem porosity, we assessed the sensitivity of tree water use estimates to sapwood characteristics and their interactions. Angiosperms (ring- and diffuse-porous species), with specialized vessels for water transport, showed a steeper relationship (scaling) between tree stem diameter and sapwood area in comparison to gymnosperms (tracheid-bearing species). Gymnosperms (angiosperms) were characterized by lower (higher) wood density and higher (lower) sapwood moisture content, resulting in non-significant differences in sapwood thermal conductivity between taxonomic and xylem porosity groups. Clustering of species sapwood characteristics based on taxonomic or xylem porosity groups and constraining these parameters could facilitate more accurate sap flow calculations and tree water use estimates. When combined with an increasing number of sap flow observations, these findings should improve tree- and landscape-level transpiration estimates, leading to more robust partitioning of terrestrial water fluxes.

Soil temperature (Tsoil) and soil water (θsoil) are fundamental variables that have an essential role in many processes in forest ecosystems, as well as influencing the tree species distribution and forest composition over time. We tested the Canadian Land Surface Scheme (CLASS) capacity to simulate Tsoil and θsoil in the boreal forest using a sixteen-year data set of daily measurements. Sensitivity analyses were also carried out to evaluate the impact of the thickness of organic layer (TOL), soil texture (percentage sand and clay: PS and PC), drainage parameter (DP), and water freezing point (FP) on simulated Tsoil and θsoil. Finally, the model was also calibrated with a combination of model parameters. CLASS well simulated Tsoil while its performance for θsoil varied by soil horizon and season. In winter particularly, soil liquid water was greatly underestimated because simulated Tsoil was below 0 °C. Nevertheless, simulated θsoil seasonal variation corresponded well with observations. Based on sensitivity analyses, TOL had an important effect on both Tsoil and θsoil. Although PS, PC and DP had almost no effect on Tsoil, their effects on θsoil were substantial. Tsoil increased throughout the year and θsoil increased during the winter with decreasing FP, yet the match between modelled θsoil and observations was not substantially improved. In general, Tsoil was well simulated by CLASS, except for the freezing during winter. Model calibration improves greatly both simulated Tsoil and θsoil, especially during winter in all soil layers. However, despite the model calibration, CLASS still requires improvement for modelling Tsoil and θsoil, hence emphasizing the need to review the equations governing these variables in CLASS.

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.

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.

Aims

In the boreal mixedwood, fire initiates forest succession; however, over time other disturbances such as insect outbreaks cause pulses of mortality and opportunities for recruitment of shade?intolerant species.

Questions

What are the respective roles of time since fire and insect outbreaks in driving directional, vs cyclical and retrogressive succession? Do assessments from direct measurements and chronosequence approaches converge? We hypothesise that the chronosequence approach will accurately characterise large?scale compositional patterns especially in younger stands and that direct measurements will better describe small?scale, non?directional changes in succession.

Location

Boreal mixed wood of northwestern Quebec (79°1? W, 48°30? N).

Methods

We sampled 469 plots over an 18?year interval (in 1991 and 2009) in mixed hardwood/conifer forests to observe in situ the changes in stands having originated from seven fires covering a 249?year chronosequence.

Results

The combination of the remeasurement and chronosequence analyses validates predictions of forest succession. Our results indicate that time since last fire is the dominant factor explaining forest succession for the first 150 years after fire and globally at the scale of the entire forest, although smaller?scale disturbances such as spruce budworm outbreaks can change the course of forest succession, especially at stand scales.

Conclusion

While time since fire is the dominant driver of forest succession in younger forests, secondary disturbances, such as spruce budworm outbreaks, can advance or reverse forest succession in older forests. This study also serves as a validation of the overall patterns described in spatial chronosequence approaches at the landscape level in fire?dominated systems but highlights that local succession may deviate from the overall pattern especially in older forests or in forests affected by non?stand re?initiating intermediate disturbances.

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.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.

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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.

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. 

Due to the unprecedented loss of old-growth forests to harvesting throughout circumboreal regions an understanding of similarities and differences in old-growth dynamics is needed to design effective restoration, management and conservation efforts. This paper reviews concepts, prevalence and variability of old-growth forests across landscapes, and evaluates different stand scale dynamics at the old-growth stage across the circumboreal zone. Old-growth historically dominated many boreal forest landscapes in both Eurasia and North America. Throughout much of North America, and to some extent in western Siberia, the natural prevalence and development of old-growth forests are regulated by the occurrence of stand-replacing fires. In eastern North America and Siberia, insect outbreaks may, however, be more important. Insect outbreaks as well as recurrent non-stand replacing surface fires and windthrows, when occurring at the old-growth stage, often form stands characterized by several tree age-class cohorts. This multi age-class forest development type is common in Europe and eastern Siberia but its prevalence and importance in boreal North-America is not well documented. Similarities in successional dynamics across the circumboreal region are found in the development of mono-dominant even-aged stands, the replacement of shade intolerant tree species by shade tolerant species, as well as in all-aged stands driven by small-scale gap dynamics. The message to land managers is that the focus should not only be on setting aside remaining old-growth forests or in restoring static old-growth attributes, but also in emulating natural disturbances and successional dynamics at landscape and regional scales to maintain natural variability in old-growth attributes through time.

L’objectif de cette recherche, réalisée en forêt mélangée au Témiscamingue, était de comparer une forêt dynami-sée par des perturbations naturelles partielles avec deux types de coupe partielle : une coupe de succession (CS) et une coupe avec protection des petites tiges marchandes (CPPTM). Les aspects suivants étaient étudiés : 1) la diversité structurelle des peuplements résiduels; 2) la croissance des arbres résiduels; 3) la composition de la régénération et 4) la croissance des gaules de sapin baumier. Les résultats démontrent que la CS et la CPPTM maintiennent une diversité structurelle et une densité d’espèces tolérantes à l’ombre semblables à celles que nous avons mesurées en forêt naturelle partiellement perturbée. À la suite de la CS et de la CPPTM, les es-pèces étudiées, à l’exception de l’épinette noire, avaient un accroissement radial important, et même supérieur à celui mesuré en forêt naturelle. En conclusion, les deux traitements sylvicoles évalués présentent un intérêt po-tentiel pour l’industrie forestière, car ces deux coupes permettent de maintenir une diversité structurelle et une composition en espèces qui équivalent à celles de la forêt dynamisée par des perturbations naturelles partielles.

Les conditions lumineuses jouent un rôle important dans la dynamique forestière et sont directement affectée par les interventions sylvicoles. L'objectif de cette étude était de déterminer comment les conditions lumineuses du sous-bois d'un peuplement boréal mixte sont affectées par des variations dans l'intensité et le patron spatial de récolte. À l'aide du modèle de simulation SORTIE-ND, nous avons simulé neuf traitements de coupe partielle en combinant trois taux de prélèvement (30 %, 45 % et 60 % de la surface terrière) et trois patrons de récolte (uniforme, par bandes étroites et par trouées). Nos résultats démontrent que la récolte n'augmente pas nécessai-rement la transmission de la lumière en sous-étage d'une façon proportionnelle au taux de prélèvement, le patron spatial de récolte jouant un rôle déterminant. De façon générale, on observe que, pour un même niveau de prélè-vement, un patron de récolte plus agrégé génère des conditions lumineuses plus élevées.

In the context of partial harvesting, adequately managing post-harvest light conditions are essential to obtain a desired composition of tree species regeneration. The objective of this study was to determine how varying the intensity and spatial pattern of harvest would affect understory light conditions in boreal mixedwood stands of northwestern Quebec using the spatially explicit SORTIE-ND light model. The model was evaluated based on comparisons of observed and predicted light levels in both mapped and un-mapped plots. In mapped plots, reasonably accurate predictions of the overall variation in light levels were obtained, but predictions tended to lack spatial precision. In un-mapped plots, SORTIE-ND accurately predicted stand-level mean GLI (Gap Light Index) under a range of harvest intensities. The model was then used to simulate nine silvicultural treatments based on combinations of three intensities of overstory removal (30%, 45% and 60% of basal area) and three harvest patterns (uniform, narrow strips, large gaps). Simulations showed that increasing overstory removal had less impact on light conditions with uniform harvests, and a more marked effect with more aggregated harvest patterns. Whatever the harvest intensity, uniform cuts almost never created high light conditions (GLI > 50%). Gap cuts, on the other hand, resulted in up to 40% of microsites receiving GLI > 50%. Our results suggest that either a 30% strip or gap cut or a 45–60% uniform partial harvest could be used to accelerate the transition from an aspen dominated composition to a mixedwood stand because both types of cut generate the greatest proportion of moderately low light levels (e.g., 15–40% GLI). These light levels tend to favour an accelerated growth response among shade-tolerant conifers, while preventing excessive recruitment of shade-intolerant species. A better understanding of how spatial patterns of harvest interact with tree removal intensity to affect understory light conditions can provide opportunities for designing silvicultural prescriptions that are tailored to species’ traits and better suited to meet a variety of management objectives.

1. Mixed-wood boreal forests are often considered to undergo directional succession from shade-intolerant to shade-tolerant species. It is thus expected that overstorey gaps should lead to the recruitment of shade-tolerant conifers into the canopy in all stand development stages and that the recruitment of shade-intolerant hardwoods would be minimal except in the largest gaps.
2. We analysed short-term gap dynamics over a large 6-km2 spatial area of mixed-wood boreal forest across a gradient of stands in different developmental stages with different times of origin since fire (expressed as stand 'age') that were affected differentially by the last spruce budworm (SBW) outbreak. Structural measurements of the canopy from lidar data were combined with spectral classification of broad species groups to characterize the gap disturbance regime and to evaluate the effect of gap openings on forest dynamics.
3. Estimated annual gap opening rates increased from 0.16% for 84-year-old stands to 0.88% for 248-year-old stands. Trees on gap peripheries in all stands were more vulnerable to mortality than interior canopy trees.
4. Due to recovery from the last SBW outbreak 16 years previously, gap closure rates were higher than opening rates, ranging from 0.44% to 2.05% annually, but did not show any relationship with stand age. There was, however, a continuing legacy of the last SBW outbreak in old-conifer stands in terms of a continued high mortality of conifers. In all stands, the majority of the openings were filled from below, although a smaller but significant proportion filled from lateral growth of gap edge trees.
5. Synthesis. The forest response to moderate- to small-scale disturbances in old-growth boreal forest counters the earlier assumption that the transition from one forest state to the next is slow and directional with time since the last fire. Overall, a small 6% increase in hardwoods was observed over 5 years, largely due to regeneration in-filling of hardwoods in gaps instead of successional transition to more shade-tolerant conifers. Gaps are vital for hardwood maintenance while transition to softwoods can occur without perceived gap-formation as overstorey trees die, releasing understorey trees.

Black spruce forests growing on clay soils in northwestern Quebec change structure from dense even-aged stands to open uneven-aged stands such that almost all forests older than 200 years have an open canopy. These forests become unproductive over time because they are prone to paludification. The main goal of our study was to document the transition between dense and open stands in terms of gap dynamics, with a focus on tree regeneration. Our objective was to determine whether forests remain open due to a lack of regeneration, a lack of growth or both. Nine stands along a 50–250-year-old time since fire gradient were sampled with the line intersect sampling method. Gap fraction increased with stand age and reached a maximum of 77% in the oldest site. In old-growth stands, gaps were interconnected due to the low density of these forests. Most of the gap makers were found with broken stems. Regeneration was dominated by black spruce layers and was relatively abundant (1.71 stems/m²). However, the majority of gap fillers were smaller than 1 m in height in stands of all ages. Instead of a lack of regeneration, the opening of the forests is due to a lack of growth associated with cold and wet organic deposits. Partial harvesting could be implemented on the most productive sites, while management techniques including soil disturbances will be required on low productivity sites to recreate good growth conditions.

Forest management has been criticised in the last 20 years for its negative impact on the native species, structures and functions of the forest. Of many possible alternatives proposed to minimize these effects, the functional zoning (or TRIAD) approach is gaining popularity in North America. The goal of this approach is to minimize the negative environmental impacts of forestry while maintain timber supply by dividing the forest into three broad land-use zones: (1) conservation, (2) ecosystem management, and (3) wood production. In this study, we used a spatially explicit landscape model to simulate the effects of fire and six different forest management scenarios on a boreal mixedwood forest management unit in central Quebec. The management scenarios examined included the current practices scenario, a scenario proposed by the provincial government, and four TRIAD scenarios varying in the amount of forest allocated to each of the three zones. For each scenario, we examined the harvest volume, percentage old-growth forest or old forest managed to favour old-growth attributes, and effective mesh size of forest patches by 20-year age classes. With more area set aside for conservation and high-retention partial cut harvesting techniques designed to maintain the attributes of old-growth stands, all TRIAD scenarios resulted in higher percentages of stands with old-growth attributes than the current practices scenario and the government proposed scenario, and two of the four TRIAD scenarios also resulted in higher harvest volume over the long term. All forest management scenarios resulted in significantly lower effective mesh size than the fire-only scenario, but this difference was not as pronounced for the four TRIAD scenarios as for the current practice and government proposed scenarios. We conclude that the TRIAD approach has the potential to minimize some of the negative impacts of forestry on the landscape, while maintaining timber supply over the long term.

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.

The TRIAD approach to forest management involves dividing the forest into 3 zones, each with its own management objectives, but with the overall goal of increasing the ecological and economic sustainability of the forest. For the past 5 years, we have been experimenting with TRIAD zoning in central Quebec, incorporating social interests into the original concept of TRIAD management. Results generally indicate that this approach is economically viable, socially acceptable, and preferable ecologically in this area. Although much remains to be done, thus far the consensus among the various project participants is that this approach may be a good fit for the public forest of Canada.

Dans le cadre d’un aménagement forestier TRIADE, la forêt est divisée en trois zones ayant chacune ses propres objectifs d’aménagement. L’objectif global est cependant toujours l’atteinte d’une gestion durable au niveau écologique et économique. Depuis 5 ans, nous expérimentons avec le concept de zonage TRIADE au centre du Québec, incorporant les intérêts sociaux dans le concept original de la TRIADE. Nos résultats indiquent que cette approche est viable au niveau économique, acceptable au niveau social et préférable au niveau écologique. Il nous reste beaucoup à faire, mais le consensus général des nombreux participants est que cette approche pourrait nous aider à mieux gérer les forêts publiques du Canada.

Jusqu’à récemment, la dynamique naturelle des peuplements mixtes a été largement ignorée, ce qui a mené à la transformation de plusieurs peuplements mixtes d’Amérique du Nord en peuplements dominés par des conifères ou des feuillus. Le but de cette étude était d’examiner la dynamique des trouées dans le couvert de peuplements mixtes composés de sapin baumier (Abies balsamea (L.) Mill.) et de bouleau jaune (Betula alleghaniensis Britt.) pour améliorer la compréhension des mécanismes potentiels de coexistence entre les espèces. La proportion des trouées dans les 12 peuplements étudiés se situait entre 9 % et 30 % de la superficie totale des peuplements alors que les trouées couvraient de 20 à 2 100 m². La mortalité du sapin baumier était la cause principale de la formation des trouées. Le sapin baumier et l’érable à épis (Acer spicatum Lamb.) dominaient les strates de régénération formées respectivement par des arbres et des arbustes. La compétition exercée par les arbustes ralentit le repeuplement naturel des trouées par des espèces arborescentes. Nos résultats indiquent que le bouleau jaune est l’espèce la plus abondante dans les trouées de plus de 800 m² alors que le sapin baumier domine les trouées de moins de 200 m². Des modèles de transition ont indiqué que la longévité supérieure du bouleau jaune par rapport à celle du sapin baumier assure son maintien dans la strate dominante. La coexistence des espèces dominantes est donc le résultat de l’utilisation différente des ressources disponibles en fonction du temps et de l’espace. L’aménagement forestier devrait maintenir la variabilité de l’âge et de la taille de la récolte parce l’utilisation d’une seule taille de trouée ou d’un seul âge de récolte mènera à un déséquilibre dans la proportion des espèces par rapport à ce qui est observé dans les peuplements naturels. © 2007 NRC tous droits réservés.

Abstract

In order to receive forest certification and to respond to societal desires,many forest companies are attempting to demonstrate that their forest activities are «sustainable». The main objective of this paper is to qualitatively evaluate the ways in which forestry-related provincial regulations in the three provinces (Alberta, Ontario and Quebec) that contain most of the Canadian boreal forest help forest companies achieve certification with respect to ecological criteria. In the process of continually evolving towards sustainable forest management,we evaluate how these provincial regulations governing forest operations can be helpful in maintaining three criteria: biodiversity, the aquatic environment and soils. This study shows that the regulations evaluated have varied in their approach and thus have different strengths that must be underlined: (1) Ontario’s approach is the strongest in terms of biodiversity, (2) Alberta and Ontario provide measures to abandon roads after harvesting, (3) Quebec provides the greatest specific measures for protecting waterways and aquatic species, (4) Alberta shows the greatest consideration for maintaining the most soil properties and functions. Better links between different regulations are necessary in all jurisdictions. The continual improvement of Canadian forest rules is often slow and advances at a different pace depending on regulators but it should be supported in all provinces.

Résumé

Afin d’obtenir une certification forestière et de répondre aux besoins de la société, les compagnies doivent démontrer que leurs activités forestières sont «durables». L’objectif principal de cet article est d’évaluer de quelles façons les normes forestières provinciales des ministères des ressources naturelles aident les compagnies forestières à obtenir une certification environnementale.Nous avons évalué de façon qualitative les lois encadrant les opérations forestières en forêt boréale, la zone forestière dominante au Canada. Nous mettons l’emphase sur trois provinces (Alberta, Ontario et Québec) qui couvrent la plus grande proportion de la zone boréale au pays. Dans une démarche d’amélioration continue, nous évaluons comment ces normes peuvent aider à maintenir trois critères : la biodiversité, le milieu aquatique et les sols. Cette étude montre que ces règlements présentent des différences dans leurs approches et des forces qui doivent être soulignées : (1) l’approche de l’Ontario est la plus forte en termes de biodiversité, (2) l’Alberta et l’Ontario proposent des mesures pour abandonner les routes suite aux activités de récolte, (3) le Québec offre les meilleures mesures spécifiques pour un nombre de milieux humides et d’espèces aquatiques, (4) l’Alberta présente la meilleure réflexion sur le maintien du plus grand nombre de propriétés et fonctions des sols. De meilleurs liens entre les différentes lois sont nécessaires bien qu’il faille appuyer l’amélioration continue des règlements forestiers au Canada.

Résumé :Au cours de la dernière décennie, un intérêt grandissant pour le développement d'approches d'aménagement basées sur notre compréhension de la dynamique historique des perturbations naturelles s'est manifesté. Ces approches reposent sur l'idée qu'un aménagement favorisant une composition des paysages et une structure des peuplements similaires à celles créées dans les forêts passées devrait aussi maintenir la diversité biologique et les fonctions écologiques essentielles de ces mêmes paysages et peuplements. Dans les paysages contrôlés par les feux, cette approche est possible seulement si les fréquences de feux actuelles et futures sont suffisamment faibles lorsque comparées aux fréquences pré-industrielles, cela afin de permettre de substituer le feu par la coupe forestière. Nous évaluons cette possibilité en comparant les fréquences de feux actuelles et futures aux fréquences historiques à partir d'études réalisées dans la forêt commerciale québécoise. Les fréquences actuelles et futures des feux, simulées en utilisant deux scénarios de concentration de CO2 (2× et 3× la concentration actuelle), sont plus faibles que les fréquences passées pour la majorité du territoire, suggérant que l'aménagement forestier pourrait potentiellement être utilisé afin de recréer la structure d'âge de la forêt soumise à un régime de feux sévères. Les aménagements équiennes actuels tendent toutefois à réduire la variabilité naturelle du système: par exemple, un aménagement équienne amputera, à terme, la structure d'âge de la forêt naturelle éliminant ainsi les forêts surannées et anciennes du paysage. Le développement de techniques de sylviculture permettant le maintien d'un spectre de compositions et structures forestières à différentes échelles de paysage est une des avenues proposées afin de maintenir cette variabilité. ©2006 NRC Canada

Published literature was used to determine both fire cycle and mean time since last fire of forests across the Canadian boreal forest. Based on the mean time since fire of the stands, the percentage of forest which could be managed to reproduce the fire controlled age structure conditions found for each Canadian region studied was determined. This report provides forest managers with a tool that can be used to help achieve sustainable forest management and the conservation of biodiversity.

We studied mortality caused by the last spruce budworm (Choristoneura fumiferana Clem.) outbreak (1972-1984) and the subsequent regeneration processes in the mixed conifer-hardwood forest zone of the Temiscamingue region, western Quebec. Three kinds of late successional stands, undisturbed by human activities and representative of forest conditions on different sites. Were studied using permanent plots, dendrochronology, and field data. The tree species affected directly by the spruce budworm, balsam fir (Abies balsamea L.), white spruce (Picea glauca Wench.), and black spruce (Picea mariana Mill.) suffered high overall mortality rates during the 1972-1996 period covered by the permanent plot measurements (97.6. 63.5. and 59.6%. Respectively). The mortality of balsam fir and white spruce were positively correlated with pre-outbreak balsam fir basal area in the plots, as was the mortality of a non-host species, white birch (Betula papyrifera Marsh.). Mortality of host and non-host species following the outbreak might have been due in part to modified environmental conditions following canopy disruption. Particularly on imperfectly drained sites that were dominated by balsam fir before the outbreak. This is also supported by the fact that radial increments of surviving white spruce trees remained significantly, lower after the outbreak in balsam fir-dominated stands compared with other stand types. The outbreak has also favoured the emergence of an abundant sapling layer mostly dominated by balsam fir. Due to important fluctuations in the shape of canopy gaps over the years. The correlations between sapling abundance and actual canopy openings are low for all sapling species. In the present-day mixed species stands. Density, and species composition of the sapling layer seem to depend more on the presence of suitable micro-sites than canopy openings. These results suggest that the last outbreak gave rise to a gradient of stand regeneration patterns, depending on species composition before the outbreak and host and non-host mortality following it, ranging from overall stand mortality and replacement in balsam fir-dominated stands, dual-cohort structures in mixed boreal stands where mortality was partial and quasi-gap dynamics in mixed hardwood stands dominated by yellow birch (Betula alleghaniensis Britton). © 2005 Elsevier B.V. All rights reserved.

We studied community development and mortality among tree species at multiple spatial scales in a 236-year mixedwood chronosequence at Lake Duparquet, Quebec. Spatial relationships, species associations, and understory–overstory replacement patterns were studied using spatial statistics, patch indices, G tests, and transition matrices. Results of these analyses showed that shade-intolerant and shade-tolerant tree communities formed increasingly fine-grained patches in midsuccession. These fine-grained associations appear to be partly due to suppressed stems being released into a slowly thinning canopy and partly due to new recruitment of shade-tolerant conifers. In 1-ha plots, tree species richness and evenness peaked in 25-, 100-, and 400-m² subplots during midsuccession. Forest cover type diversity also peaked in midsuccessional landscapes assessed using aerial photographs. The oldest 1-ha plot was dominated by monospecific patches of eastern white cedar (Thuja occidentalis L.), which was the most likely replacement for live and dead trees and the most abundant species in the numerous subplots that were occupied by trees smaller than 8 cm DBH. In spite of this dominance, long-lived paper birch (Betula papyrifera Marsh.) and residual fir (Abies balsamea (L.) Mill.) seedlings may maintain the mixedwood character of this stand into the future. The implications of our findings for stand management and conservation are discussed.

Les auteurs ont étudié le développement et la mortalité de communautés d'arbres en peuplements mixtes à plusieurs échelles spatiales dans une chronoséquence de 236 ans au lac Duparquet, Québec. Les relations spatiales, les associations d'espèces et les patrons de remplacement entre les espèces du couvert dominant et celles du sous-bois ont été étudiés en utilisant des statistiques spatiales, des indices d'agglomération, des tests de G et des matrices de transition. Les résultats de ces analyses montrent que les communautés d'arbres intolérants et tolérants à l'ombre forment un nombre croissant d'agglomérations de petites tailles à l'étape du milieu de la succession. La formation de ces associations de petites tailles semble partiellement attribuable, d'une part, aux tiges opprimées qui sont libérées à l'intérieur d'un couvert qui s'éclaircit lentement et, d'autre part, au recrutement de conifères tolérants à l'ombre. Dans des parcelles de 1 ha, la richesse en espèces et l'équitabilité ont culminé dans des sous-parcelles de 25, 100 et 400 m² de peuplements parvenus à l'étape du milieu de la succession. La diversité du type de couvert forestier estimée à partir de photographies aériennes a aussi culminé sur des territoires parvenus à l'étape de milieu de succession. La plus vieille placette de 1 ha était dominée par des agglomérations monospécifiques de thuya occidental (Thuja occidentalis L.), l'espèce la plus susceptible de remplacer les arbres morts et vivants et l'espèce la plus abondante dans les nombreuses sous-parcelles occupées par des arbres dont le DHP est inférieur à 8 cm. Malgré cette dominance, les espèces à forte longévité que sont le bouleau à papier (Betula papyrifera Marsh.) et les semis résiduels de sapin baumier (Abies balsamea (L.) Mill.) peuvent maintenir la mixité d'espèces de ce type de peuplement dans le futur. Les implications de nos résultats sont discutées en regard de l'aménagement et de la conservation des forêts. ©2005 NRC Canada

Canopy gaps play an important role in the dynamics of old-growth forests, although it is not well known how gap dynamics differ among regions. To further our understanding of natural gap dynamics in mixedwood forests, this study compares mixed stands located in eastern (Gaspésie region) and western (Témiscamingue region) Quebec. We tested whether the gap fraction in mixedwood stands was similar in these two regions. Data from field transects were used to characterize current canopy gaps, and aerial photos were used to contrast gap characteristics before and after the most recent spruce budworm (Choristoneura fumiferana Clem.) outbreak, which occurred from 1973 to 1991 in Gaspésie and from 1972 to 1984 in Témiscamingue. The current gap fraction was found to differ between the two regions: it varied from 2% to 48% with an average of 25.6% for the Gaspésie region and from 24% to 52% with an average of 36.6% for the Témiscamingue region. While the last spruce budworm outbreak coincided with a significant increase in canopy openings in the Témiscamingue region (p = 0.047), no such effect was observed in Gaspésie. These results suggest that the temporal pattern of small-scale disturbances can vary among regions, even when similar forest types are compared.

On sait que les trouées de canopée sont importantes pour la dynamique des peuplements de fin de succession, mais nous disposons de peu d'informations sur la variabilité inter-régionale. Dans le but d'améliorer nos connaissances sur la création des trouées en forêt mixte, cette étude compare des peuplements mélangés de l'est (Gaspésie) et l'ouest (Témiscamingue) du Québec. Nous avons vérifié si le pourcentage d'ouverture de la forêt, pour un même type de forêt, est similaire dans ces deux régions. Des données provenant de transects établis sur le terrain on permis de caractériser les ouvertures actuelles, et des photographies aériennes ont été utilisées pour comparer la proportion de trouées avant et après la dernière épidémie de tordeuse des bourgeons de l'épinette (Choristoneura fumiferana Clem.), qui a eu lieu entre 1973 et 1991 en Gaspésie et de 1972 à 1984 au Témiscamingue. Les résultats obtenus indiquent que le degré d'ouverture de la canopée diffère entre les deux régions : en Gaspésie, le pourcentage d'ouverture de la forêt varie de 2 à 48 % avec une moyenne de 25,6 % alors qu'au Témiscamingue il varie de 24 à 52 % avec une moyenne de 36,6 %. Alors que la dernière épidémie de tordeuse des bourgeons de l'épinette a coïncidé avec une augmentation significative des ouvertures de canopée dans le Témiscamingue (p = 0,047), un tel effet n'a pas été observé en Gaspésie. Ces résultats suggèrent que le taux de création de petites ouvertures peut varier entre les régions, même lorsque des types forestiers similaires sont comparés. ©2005 NRC Canada

Fire history was reconstructed for a 2500-km² area at the interface between the boreal coniferous and northern hardwood forests of southwestern Quebec. The fire cycle, the time required for an area equal to the study site to burn once over, was described using a random sampling strategy that included dendrochronological techniques in conjunction with provincial and national government archival data. Physiographic elements were not found to spatially influence fire frequency; however, human land-use patterns were observed to significantly affect the fire frequency. A temporal shift in fire frequency was also detected, which coincided with the period of Euro-Canadian colonization and known extreme dry years for the study site. Additionally, a fire-free period was identified in the most recent times that could be associated with fire suppression and climate change. The estimated cycles (approx. 188–314 years) for the southeastern section of the study area were thought to better represent the natural cycles for this transition zone as a result of less anthropogenic influence. The importance of gap-type dynamics becomes evident with the increased presence of old-growth forest, given the derived fire cycle estimations for the region. Even-aged management with short rotations, consequently, is questioned because fire cycle estimations suggest more complex harvest systems using an ecosystem management approach.

L'historique des feux a été reconstruit pour une superficie de 2500 km² à l'interface entre la forêt boréale et la forêt feuillue septentrionale du sud-ouest du Québec. Le cycle de feu (nombre d'années requises pour que soit brûlée une superficie équivalente au territoire à l'étude) a été décrit avec un dispositif d'échantillonnage aléatoire utilizant la dendrochronologie conjointement avec les archives des gouvernements provincial et fédéral. Les éléments physiographiques n'ont pas eu d'influence spatiale sur la fréquence des feux. Cependant, les patrons d'utilization des terres associés à la colonization ont significativement affecté la fréquence des feux. Un changement temporel dans la fréquence des feux a été aussi détecté. Il est synchrone avec la période de colonization euro-canadienne, ainsi qu'avec des années de sécheresse extrême pour le site d'étude. De plus, l'absence de feu notée depuis 1950 pourrait être expliquée par l'effet cumulé de la suppression du feu et des changements climatiques. Les cycles estimés (approx. 188 à 314 ans) pour la section sud-est du site d'étude ayant subi une moins grande influence anthropique représenteraient mieux les cycles naturels du territoire étudié. Avec la présence d'une proportion plus élevée de forêt ancienne, les perturbations secondaires par trouées deviennent sont importantes étant donné les estimations du cycle de feu dans la région. Par conséquent, les régimes d'aménagement équienne à rotation courte sont à remettre en question pour ce secteur. Comme le suggère les estimations du cycle de feu, des pratiques sylvicoles plus diversifiées et l'utilization d'une approche de gestion écosystémique seraient plus appropriées. ©2005 NRC Canada

En examinant l'histoire de la conservation au Québec, on se rend compte que le population a généralement mis l'accent sur la protection des éléments considérées beaux ou exceptionnels. Leur motivation vient du désir de maintenir ce qu'ils estiment comme unique ou différent au sein d'un paysage.

Temporal, spatial, and structural patterns of adult trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) mortality were studied in intact 150-year-old stands in the southwestern boreal forest of Quebec. For both species, mortality decreases (number of dead trees/total number of trees) with distance from the lake edge until 100–150 m, from which point it slightly increases. Strong peaks in mortality were found for 40- to 60-year-old aspen mainly between 1974 and 1992. Such mortality in relatively young aspen is likely related to competition for light from the dominant canopy trees. Also, the recruitment of this young aspen cohort is presumably the result of a stand breakup that occurred when the initial aspen-dominated stand was between 90 and 110 years old. For spruce, strong peaks in mortality were found in 110- to 150-year-old trees and they occurred mainly after 1980. No clear explanation could be found for these peaks, but we suggest that they may be related to senescence or weakening of the trees following the last spruce budworm outbreak. Suppressed and codominant aspen had a much higher mortality ratio than spruce in the same height class, while more surprisingly, no difference in mortality rate was found between dominant trees of the two species. Most spruce trees were found as standing dead, which leads us to reject the hypothesis that windthrow is an important cause of mortality for spruce in our forests.

Les patrons de mortalité temporel, spatial et structurel ont été établis pour le peuplier faux-tremble (Populus tremuloides Michx.) et l'épinette blanche (Picea glauca (Moench) Voss) adultes dans des forêts de 150 ans situées dans le sud-ouest de la forêt boréale du Québec. Chez les deux espèces, on observe une diminution de la mortalité (nombre d'arbres morts / nombre d'arbes total) du bord du lac jusqu'à 100–150 m et une faible augmentation par la suite. La mortalité était importante chez les peupliers de 40 à 60 ans et s'était produite entre les années 1974 et 1992. Cette mortalité chez les jeunes peupliers est probablement reliée à la compétition pour la lumière venant des arbres dominants. Le recrutement de cette jeune cohorte de peuplier est présumé être le résultat d'une ouverture assez soudaine du peuplement mature de peuplier entre les années 90 et 110 ans. Chez l'épinette blanche, une mortalité importante fut observée pour les arbres âgés entre 110 et 150 ans et s'est produite surtout après 1980. Nous croyons que cette mortalité est induite par la sénescence des arbres, ou en raison de leur affaiblissement suite à l'épidémie de la tordeuse des bourgeons de l'épinette. Les peupliers supprimés et codominants avaient un taux de mortalité beaucoup plus élevé que les épinettes, mais il n'y avait pas de différence entre les arbres dominants des deux espèces. La plupart des épinettes étaient morts debout, ce qui rejette l'hypothèse voulant que le vent soit une cause importante de mortalité chez l'épinette.©2004 NRC Canada

We propose a simple method that uses aerial photographs to characterize the impacts of a spruce budworm (Choristoneura fumiferana (Clem.)) outbreak on canopy structure. Using aerial photographs taken at the beginning (1972) and after (1994) a major spruce budworm outbreak (1970–1987), we evaluated the change in canopy openness that occurred during the period of the outbreak, in five compositionally different stands. We compared canopy openness evaluated by photointerpretation with two independent field techniques and found a high degree of similarity between methods. Interpretation of the 1972 photographs (prior to the outbreak) shows that regardless of composition, four of our five analysed stands had about the same degree of mean canopy openness (17%–20%). Following the outbreak, openness increased in all stands except for the hardwood-dominated stand. The highest increase in openness (from 18% to 45%) occurred in the stand with the highest conifer content. Thematic maps and spatial analysis techniques were used to describe canopy openness distribution. Openness was low and uniformly distributed before the outbreak, whereas after the outbreak, the various degrees of openness had a patchy distribution in most stands. Furthermore, patch size increased with conifer content. Using the amount of increase in canopy openness and its specific distribution within stands, we propose guidelines for the development of silvicultural practices that mimic spruce budworm disturbances in boreal mixedwoods.

Cette étude propose une méthode simple qui utilise l'interprétation de photos aériennes pour caractériser les impacts d'une épidémie de la tordeuse des bourgeons de l'épinette (Choristoneura fumiferana (Clem.)) sur la structure du couvert forestier. En utilisant des photos prises au début (1972) et après (1994) l'épidémie majeure qui a frappé notre secteur de 1970 à 1987, nous avons évalué les changements dans le degré d'ouverture du couvert forestier qui se sont produits dans cinq peuplements de composition différente. La méthode d'évaluation du degré d'ouverture par photo-interprétation a été comparée à deux méthodes d'estimation utilisées sur le terrain. L'interprétation des photos prises au début de l'épidémie montre qu'indépendamment de la composition, quatre des cinq peuplements étudiés avaient approximativement le même pourcentage d'ouverture moyen (17–20 %). L'épidémie a entraîné une augmentation du degré d'ouverture dans tous les peuplements à l'exception du peuplement dominé par les feuillus. Sur les photos prises après l'épidémie, l'ouverture augmente en fonction de la proportion de conifères. Le peuplement le plus affecté a atteint un degré d'ouverture de 45 % en 1994. L'analyse spatiale de nos résultats révèle que les degrés d'ou verture variables trouvés après l'épidémie sont distribués en îlots alors qu'ils étaient faibles et distribués uniformément avant l'épidémie. La taille de ces îlots augmente avec l'âge des peuplements. Ces résultats nous permettent de proposer un scénario préliminaire de pratiques sylvicoles basées sur les impacts causés par une épidémie de la tordeuse des bourgeons de l'épinette en forêt boréale mixte.©2004 NRC Canada

Old-growth forests have been identified as a potentially important stage of stand development for maintaining biodiversity in the landscape, yet they have also been targeted by the forest industry in their drive to regulate the forest. We will attempt to propose a definition of old growth, applicable throughout the North American boreal forest, that takes into account the dynamic nature of forest development and that could be useful for management and conservation purposes. We define the start of the old-growth stage as occurring when the initial post-disturbance cohort begins dying off, concurrent with understorey stem recruitment into the canopy. We propose that species longevity and the regional fire cycle can be used to assess the extent of this phase in different regions. Using published data on fire history, we show that the amount of old growth expected to occur in western and central Canada is less than in eastern Canada, where most stands (in area) escape fire for periods longer than that necessary to incur substantial mortality of the initial cohort. At the stand level, we show that the old-growth stage is characterized by small-scale disturbances that engender gap dynamics. Until recently, this process had not been studied in the boreal forest. The old-growth index we present suggests that the relationship between time since the last major disturbance and old-growth status varies most in areas that have not been disturbed for long periods. Both management and conservation strategies have to take into account that old-growth forests are dynamic. To be effective, reserves should contain all stages of development and should be sufficiently large to encompass rare but large disturbances. The abundance of old growth in many boreal regions of North America also suggests that forest management strategies other than even-aged, fully regulated systems have to be developed.

The development of a coherent system of criteria and indicators (C&I) requires collaboration and communication among scientists, government, the public, certifying organizations, and the forest industry. It also demands the integration of knowledge from many fields of study, which is foreign to the disciplinary nature of most forestry research. There needs to be greater effort to link groups of indicators and to favour those that are assimilative in nature. Modelling tools adapted to a multi-disciplinary approach and collaborative development will help to integrate knowledge from various fields and institutions. Specific challenges for implementation of C&I have been identified, including: leadership and vision in the evolution towards sustainable forest management (SFM); linking of grass-roots and higher level CI initiatives; streamlining and co-ordinating different certification initiatives and agencies; technology transfer; and collaboration among disciplines.

Sustainable forest management has replaced sustained yield as the new management strategy for most countries and forest companies. This concept has generated a lot of interest and discussion, and a great deal of effort is being made to modify current forestry practices to be sustainable. In this paper, we argue that the still somewhat vague concept of sustainable forest management calls for a substantial modification in our way of thinking about and practising forestry. To move toward that goal, we recognize important social and economic challenges to sustainable management and suggest nine essential notions: 1) manage the forest ecosystem as a whole and not in parts nor only for the crop species; 2) conserve a significant proportion of the boreal forest (i.e,, at least 12%); 3) practice intensive forestry on a small portion of the land to recover the fibre lost from notions 1 and 2; 4) strive for innovation in thinking and acting; 5) foster research and development to support notion 4; 6) balance regional needs with that of the global community; 7) encourage public participation; 8) consider the impact of substantial change in climate over the next 100 years (or next rotation); and 9) substitute regulations that are adaptive for those that are restrictive, An example of the kind of silviculture that could be used in ecosystem management for the black spruce forest is also discussed.

Gap dynamics theory suggests that spatial and temporal patterns of species recruitment within canopy gaps may be due to differences in shade tolerance. Although this theory has been developed in low latitude forests, we suggest that it is also applicable in boreal forests. This study investigates temporal and spatial patterns of seedling and sapling regeneration in canopy,gaps caused by spruce budworm. Tree seedlings, saplings, and shrubs were mapped, measured, and aged in five gaps in a 234-year-old conifer stand in the southern part of the boreal forest. Shade-tolerant fir and cedar seedlings and saplings were found in the southern part of the gaps, whereas intolerant aspen root suckers were associated with the northern part of the gaps. Within gaps, fir seedling and sapling density at the quadrat scale was negatively related to mountain maple cover. A temporal partitioning of seedlings and saplings into pre- and post-gap formation also showed a gradient of species establishment related to shade tolerance. The zone of high tight in the northern part of large gaps may permit the maintenance of shade-intolerant species in these forests. However, the small area of this zone and the slowness of gap formation will limit shade-intolerant species abundance.

Stand mapping and vertical and oblique tree crown projections were used to study gap characteristics and gap effects on the regeneration and stand development of Larix gmelini. The hypothesis is that waves of advance Larix regeneration are recruited into the canopy layer following the creation of canopy openings. In old-growth Larix forests of the northern Da Xingan Ling Mts., at 52 degrees N, obliquely projected gaps (OPG) begin at a distance of 60 - 80 % of the canopy tree height from the southernmost stems bordering the gap and thus the OPGs may extend beyond the northern boundaries of the vertically projected gaps (VPG). Changes in the environment and resource availability in the OPG result in increased Larix sapling survival. Due to a greater incidence of light, 10 - 30 yr old OPG saplings were more abundant than saplings in either a near-oblique projected gap (NOPG) or in the shadow of obliquely projected crowns (SOPC). The survival of saplings more than 30-yr old was highest when they were found in the OPG of one canopy opening and the VPG of another. This means that, following recruitment into an OPG, saplings then require the space found in a VPG to permit growth into the canopy. Thus, various-sized gaps contribute to the survival of different aged saplings by increasing the complexity of stand structure. Although individuals may regenerate in an OPG, successful recruitment into the canopy requires the available growing space of a VPG. This research suggests that shade intolerant Larix gmelini can maintain its canopy dominance without fire via gap regeneration.

This study identifies patterns in the gap disturbance regime along a successional gradient in the southern boreal forest and uses this information to investigate canopy composition changes. Gaps were characterized in hardwood, mixed-forest, and conifer stands surrounding Lake Duparquet in northwestern Quebec. From 39 to 80 gaps were evaluated along transects established in each of these stands. The abundance of gap makers and gap fillers and total regeneration was evaluated by species, as well as the size of each gap encountered along the transects. The percentage of the forest in canopy gap was calculated directly from the proportion of the transect in gap and by using gap area and line-intercept techniques. Changes in composition were evaluated from gap-maker and gap-filler distributions and by using transition matrices based on species mortality and regeneration in canopy gaps.

The percentage of the forest in canopy gap ranges from 7.1% in a 50-yr-old forest dominated primarily by aspen to 40.4% in a 234-yr-old fir-dominated forest. Gap events are due to individual or small-group tree mortality in the early successional forest but become species-specific events controlled by spruce budworm outbreaks in the later stages of succession. Due to the high latitude, direct light only reaches the forest floor in the very largest gaps of the conifer-dominated stands. However, these gaps form slowly as budworm-caused mortality occurs over a number of years, whereas in aspen-dominated stands gaps are formed quickly by the snapping of tree stems. Balsam fir is the most abundant gap-filling species; however, its abundance is negatively correlated to gap size in all stand types. Markovian transition matrices suggest that in the young aspen-dominated forests small gaps lead to species replacment by more shade-tolerant conifers but that in the oldest forests the larger gaps will result in maintenance of the intolerant species and an increase in the abundance of cedar.

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.

Les épidémies de la tordeuse de bourgeons de l’épinette (TBE) constituent la principale perturbation naturelle entre deux feux successifs dans les forêts boréales de l’est du Canada. Des intervalles plus longs entre les feux et l’utilisation de la régénération préétablie pour régénérer les forêts exploitées favorisent une augmentation de l’importance du sapin baumier l’hôte de la TBE. Un effet croissantdes épidémies de la TBE sur la dynamique de nos forêts est ainsi attendu.

Suite à la mortalité des arbres adultes, le recrutement des semis vers les strates supérieures assure la fermeture des trouées. La connaissance de la réponse de la régénération aux trouées permettrait donc une meilleure compréhension du rôle de la TBE dans le développement de la forêt. En Abitibi, le lieu de l’étude, dû au régime de feu, les forêts issuent des feux possèdent des peuplements d’âges et de compositions très variables, ce qui leur confèrent différents niveaux de vulnérabilité à la TBE. Il devient donc nécessaire de caractériser l’effet des épidémies de la TBE et la réponse de la régénération préétablie selon les différentes stades successionels.

Ainsi, le premier chapitre de cette thèse vise à déterminer les facteurs écologiques qui influencent l’abondance de la régénération préétablie selon les différents stades de développement de la forêt. Les résultats indiquent que les facteurs abiotiques sont faiblement corrélés à la densité des semis de la plupart des espèces. Par contre, les facteurs biotiques tels que la surface terrière des semenciers, le recouvrement arbustif et la présence des trouées sont liés à l’abondance de la régénération coniférienne. De plus, les semis de sapin baumier et d’épinette blanche sont plus abondants dans les peuplements mixtes d’âge intermédiaire que dans les vieux peuplements conifériens. La quantité de semis d’espèces intolérantes à l’ombre demeure quant à elle faible mais constante à travers les stades successionels. Seul l’abondance du thuya augmente avec la durée du temps après feu. Dans les vieux peuplements conifériens, les trouées causées par la TBE sont grandes, ce qui pourrait permettre le maintien du recrutement des espèces intolérantes à l’ombre.

Le deuxième chapitre a pour objet la caractérisation des régimes de perturbations par les trouées et son influence sur la succession. Un échantillonnage des trouées montre que l’abondance et la superficie de ces dernières sont proportionelles à l’abondance des adultes de sapin baumier ainsi qu’au temps écoulé depuis le dernier feu. Une importante régénération du sapin baumier est aussi associée à des trouées de petites dimension. La modélisation de la succession grâce à des matrices de transition suggère que le sapin remplacera le tremble dans les petites trouées de la forêt de feuillus, mais que dans les grandes trouées des vieilles forêts conifériennes, les feuillus réussiront à se maintenir et que le thuya deviendra plus abondant.

Une étude portant sur la réponse du sapin baumier à des trouées expérimentales est presentées dans le troisième chapitre. Une série de trouées expérimentales ont donc été créés afin d’évaluer l’effet du degré d’ouverture sur la réponse des semis. Quatre traitements présentant différents degrés d’ouvertures de la canopée (témoin, annelage des conifères, coupe de conifères, et coupe totale) de 10m x 10m ont été répliqués dans quatre forêts d’âge et de composition différents. Aucun des traitements n'a eu un effet significatif sur le recrutement de nouveaux semis de sapin. Ceci suggère que l’impact des trouées se répercute surtout au niveau de la régénération préétablie. Même un an après les traitements «coupe conifère» et «coupe totale», les semis ont enregistré une hausse significative de leur accroissement en hauteur et une modification importante de leur architecture. En raison du délai entre le traitement « annelage » et la formation de la trouée, la réponse des semis était plus lente que celle suite aux coupes. La vitesse de la formation des trouées après une épidémie devrait donc aussi avoir un effet sur la réponse des semis.

Dans le chapitre quatre nous présentons les résultats d’une étude des patrons spatiaux et temporels de recrutement à l’intérieur de trouées formées durant la dernière épidémie de TBE dans la forêt la plus affectée. Les résultats montrent que les semis des espèces hôtes pour la TBE s’installent non seulement avant l’épidémie mais aussi durant et après celle-ci. Ce phénomène est probablement dû à la présence de quelques arbres qui survivent à l’attaque de la TBE. Le bouleau s’installe durant la formation de la trouée et le tremble surtout après une ouverture complète. Spatialement, la probabilité de trouver le tremble est plus élevée à la lumière directe au nord de la trouée, tandis que le sapin se retrouve dans l’ombre, au sud de la trouée. L’espèce compétitrice la plus importante, l’érable à épis, limite cependant l’abondance du sapin. En effet, le nombre de semis de sapin diminue avec la proportion d’érable à épis dont le recouvrement est plus important dans les grandes trouées.

L’ensemble des résultats suggère que les grandes trouées formées suite aux épidémies de la TBE sont importantes pour le maintien des espèces intolérantes à l’ombre dans les vieilles forêts. Ces espèces, en particulier le tremble, sont associées avec la région nord de la trouée, où la lumière directe se retrouve. Cependant, à cause du faible angle d’incidence de la lumière et de la formation lente des trouées causées par la TBE, les périodes et les zones de lumière directe sont limitées. C’est en partie pourquoi le sapin, dû à sa tolérance à l’ombre, demeure l’espèce la plus abondante. Seul le thuya semble être capable de déplacer le sapin. Ceci s’explique par sa tolérance à l’ombre mais aussi par sa longevité et par le fait que cette espèce n’est pas un hôte de la TBE. Nos résultats confirment que le thuya augmente en abondance et en proportion dans les vieux peuplements. L’interaction avec les arbustes, surtout l’érable à épis, affecte aussi l’abondance des différentes espèces. © 1997 UQAM tous droits réservés.

The goal of this study was to describe white spruce (Picea glauca (Moench) Voss) and balsam fir (Abies balsamea (L.) Mill.) recolonization in a forest destroyed in 1923 by an intense fire. Regeneration was evaluated considering both ecological site factors and distance to a seed source. Sampling was conducted along four transects crossing this stand. Each transect began in an unburned zone or at the edge of the intact forest and progressed towards the centre of the 1923 bum taking into account the two types of soil parent materials. Demographic variables of these two species were assessed by dating, using dendrochronology, at least 10 individuals found within each of the quadrats. The area and distance of the quadrats were determined as a function of the changes observed in stand structure and composition. As shown by its age distribution, fir recruitment was low during the first 30 years of succession but increased to higher levels thereafter. Colonization by white spruce is, on the other hand, characterized by two waves of recruitment, the first peak occurring during the initial phase of stand establishment and the second smaller peak occurring later during succession. The two factors found to have the greatest predictive power in determining the regeneration density of these two conifer species are the distance to the preserved zone, and hence to a seed source, and the type of soil parent material, as it determines the receptivity of the germination bed after fire. White spruce benefitted from its ability to recolonize more rapidly than balsam fir by long distance reinvasion of the open site in the initial stage of succession. Balsam fir, on the other hand, recolonizes the site by advancing waves of regeneration in which groups of individuals are established at relatively short distances from parent trees. Although slower to recolonize, balsam fir demonstrates a greater ability for continued long-term recolonization. With respect to the ability to predict the pattern of natural regeneration abundance following fire, the results of this study demonstrate the importance of spatial distribution of seed sources and quality of germination beds as it varies with superficial deposit type. Therefore, these two components should be considered in models predicting forest succession following fire. ©1997 NRC Canada