Climate change is having complex impacts on the boreal forest, modulating both tree growth limiting factors and fire regime. However, these aspects are usually projected independently when estimating climate change effect on the boreal forest. Using a combination of three different methods, our goal is to assess the combined impact of changes in growth and fire regime due to climate change on the timber supply at the transitions from closed to open boreal coniferous forests in Québec, Canada. To identify the areas that are likely to be the most sensitive to climate change, we projected climate-induced impacts on growth and fire activity at three different time periods: 2011–2040 RCP 8.5 for low growth change and minimum fire activity, 2071–2100 RCP 4.5 for moderate growth change and medium fire activity, and 2071–2100 RCP 8.5 for high growth change and maximum fire activity. Our study shows the importance of incorporating fire in strategic forest management planning especially in a context of climate change. Under the most extreme scenarios, the negative impact of fire activity on productive area and total volume mostly offsets the positive effects of climate change via improved tree growth.

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.

1. Major environmental changes affect the health and capacity of ecosystems to sustain Indigenous people's well-being in boreal landscapes. Collaboration between Indigenous communities and researchers could help assessing and mitigating the consequences of environmental changes.
2. We used Driver Pressure State Impact (DPSI) conceptual models to compare the perspectives of Indigenous and scientific communities on environmental changes in boreal landscapes of Quebec, Canada.
3. The Indigenous DPSI model emerged from interviews with local land-use experts from two Indigenous communities. The scientific model was informed by the publication topics of expert researchers.
4. We compared the Indigenous and scientific models and exposed convergences and divergences between perspectives. Forestry was identified as a major driver of change in both models. Most issues related to mining, hydro-power and forest road development were specific to the Indigenous model. Climate change and wildfires were of greater interest in the scientific model.
5. Convergences between the perspectives of Indigenous and scientific communities are conducive to collaborative research. Divergences could be addressed through reciprocal knowledge transfer activities, which would lead to research that better aligns with the concerns and needs of Indigenous communities.

The reproductive ecology of the semi-serotinous species black spruce (Picea mariana (Mill.) BSP) in northern boreal forests remains poorly understood. There is a general lack of data on cone/seed production and viability as a function of biotic tree-level characteristics and abiotic variables. No studies currently exist to quantify these differences over a large gradient in temperature, elevation, and precipitation. Extensive physical, ecological, dendrometric, and reproductive data were collected from young to very old black spruce stands in northern Quebec. ANOVA and general linear mixed models were used to examine interannual cone production, and the relative importance of the biotic and abiotic explanatory factors in determining total cone production; length of the cone-bearing zone; filled seeds per cone; proportion of filled seeds; and seed viability. The results illustrate that the reproductive ecology of black spruce in northern cold forests is mainly explained by biotic variables such as age and diameter at breast height, and by abiotic variables related to temperature such as elevation, length of the growing season, and growing degree-days. Black spruce exhibits a lower reproductive potential in northern cold forests, making it possibly less resilient to increased fire frequency, particularly in unproductive and very young or very old stands.

Warning: This article contains terms, descriptions, and opinions used for historical context that may be culturally sensitive for some readers.Background: Understanding drivers of boreal forest dynamics supports adaptation strategies in the context of climate change.Aims: We aimed to understand how burn rates varied since the early 1700s in North American boreal forests.Methods: We used 16 fire-history study sites distributed across such forests and investigated variation in burn rates for the historical period spanning 1700-1990. These were benchmarked against recent burn rates estimated for the modern period spanning 1980-2020 using various data sources.Key results: Burn rates during the historical period for most sites showed a declining trend, particularly during the early to mid 1900s. Compared to the historical period, the modern period showed less variable and lower burn rates across sites. Mean burn rates during the modern period presented divergent trends among eastern versus northwestern sites, with increasing trends in mean burn rates in most northwestern North American sites.Conclusions: The synchronicity of trends suggests that large spatial patterns of atmospheric conditions drove burn rates in addition to regional changes in land use like fire exclusion and suppression.Implications: Low burn rates in eastern Canadian boreal forests may continue unless climate change overrides the capacity to suppress fire.

Context
Wildfires play a crucial role in maintaining ecological and societal functions of North American boreal forests. Because of their contagious way of spreading, using statistical methods dealing with spatial autocorrelation has become a major challenge in fire studies analyzing how environmental factors affect their spatial variability.

Objectives
We aimed to demonstrate the performance of a spatially explicit method accounting for spatial autocorrelation in burn rates modelling, and to use this method to determine the relative contribution of climate, physical environment and vegetation to the spatial variability of burn rates between 1972 and 2015.

Methods
Using a 482,000 km² territory located in the coniferous boreal forest of eastern Canada, we built and compared burn rates models with and without accounting for spatial autocorrelation. The relative contribution of climate, physical environment and vegetation to the burn rates variability was identified with variance partitioning.

Results
Accounting for spatial autocorrelation improved the models' performance by a factor of 1.5. Our method allowed the unadulterated extraction of the contribution of climate, physical environment and vegetation to the spatial variability of burn rates. This contribution was similar for the three groups of factors. The spatial autocorrelation extent was linked to the fire size distribution.

Conclusions
Accounting for spatial autocorrelation can highly improve models and avoids biased results and misinterpretation. Considering climate, physical environment and vegetation altogether is essential, especially when attempting to predict future area burned. In addition to the direct effect of climate, changes in vegetation could have important impacts on future burn rates.

Background

Cumulative impacts of wildfires and forest harvesting can cause shifts from closed-crown forest to open woodland in boreal ecosystems. To lower the probability of occurrence of such catastrophic regime shifts, forest logging must decrease when fire frequency increases, so that the combined disturbance rate does not exceed the Holocene maximum. Knowing how climate warming will affect fire regimes is thus crucial to sustainably manage the forest. This study aimed to provide a guide to determine sustainable forest harvesting levels, by reconstructing the Holocene fire history at the northern limit of commercial forestry in Quebec using charcoal particles preserved in lake sediments.

Methods

Sediment cores were sampled from four lakes located close to the northern limit of commercial forestry in Quebec. The cores were sliced into consecutive 0.5 cm thick subsamples from which 1 cm3 was extracted to count and measure charcoal particles larger than 150 microns. Age-depth models were obtained for each core based on accelerator mass spectroscopy (AMS) radiocarbon dates. Holocene fire histories were reconstructed by combining charcoal counts and age-depth models to obtain charcoal accumulation rates and, after statistical treatment, long-term trends in fire occurrence (expressed as number of fires per 1000 years).

Results

Fire occurrence varied between the four studied sites, but fires generally occurred more often during warm and dry periods of the Holocene, especially during the Holocene Thermal Maximum (7000–3500 cal. BP), when fire occurrence was twice as high as at present.

Conclusions

The current fire regime in the study area is still within the natural range of variability observed over the Holocene. However, climatic conditions comparable to the Holocene Thermal Maximum could be reached within the next few decades, thus substantially reducing the amount of wood available to the forest industry.

Abstract

Questions

High moisture levels and low occurrences of wildfires have contributed during recent millennia to the accumulation of thick layers of organic soil and to a succession into open black spruce (Picea mariana)–Sphagnum-dominated forests in the Clay Belt boreal landscapes of eastern North America. In these forests, the anticipated increase in drought frequency with climate change could lead to a shift in forest structure and composition via increased fire disturbance. Here, we quantify the expected changes in fire behaviour, biomass burning and vegetation composition in the Clay Belt forest of North America that could arise under climate change over the next century.

Location

A managed forest unit in the Clay Belt boreal forest of eastern North America.

Methods

The impact of a changing climate from 1971 to 2100 on fire regime characteristics (i.e. rate of spread, fuel consumption, fire intensity, type of fire and depth of burn) and vegetation dynamics (mortality and recruitment) was investigated using the Canadian Fire Effects Model (CanFIRE). Vegetation dynamics were governed by the fire danger and behaviour that affect tree mortality and post-fire recruitment of species, and by long-term successional pathways that are driven by post-fire recruitment and forest age. An ensemble of two climate models forced by three scenarios of greenhouse gas emissions was used to drive CanFIRE simulations.

Results

Results from multiple scenarios suggested that fire danger will increase significantly during the 21st century in the Clay Belt forest. The burn rate was projected to change from 4.2% decade−1 during 1971–2000 to 18.6% decade−1 during 2071–2100. Stand mortality, fire intensity and areas affected by crown fires were also projected to increase. A shift in forest composition did not occur over the simulation period across most of our fire regime scenarios. Dominance of open black spruce–Sphagnum forests was projected to remain in future landscapes.

Conclusions

Moist and cool conditions in these forests prevent high depth of burn and contribute to the ecological resistance of these forests to increasing fire danger.

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.

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.

Boreal forest management in Eastern Canada has caused depletion and fragmentation of oldgrowth ecosystems, with growing impacts on the associated biodiversity. To mitigate impacts of management while maintaining timber supplies, ecosystem management aims to narrow the gap between natural and managed landscapes. Our study describes the fire history and associated natural old-growth forest proportions and distribution of a 5000 km2 area located in the black spruce-feather moss forest of central Quebec. We reconstructed a stand-origin map using archival data, aerial photos and dendrochronology. According to survival analysis (Cox hazard model), the mean fire cycle length was 247 years for the 1734–2009 period. Age-class distribution modelling showed that old-growth forests were present on an average of 55% of the landscape over the last 275 years. The mean fire size was 10 113 ha, while most of the burned area was attributable to fires larger than 10 000 ha, leading to old-growth agglomerations of hundreds of square kilometres. In regards to our findings, we propose ecosystem management targets and strategies to preserve forest diversity and resilience.

Temporal patterns of tree mortality and snag dynamics after fire were investigated over 10 years in a permanent plot design established immediately after a wildfire in an eastern boreal forest landscape of north-western Quebec, Canada. Post-fire tree mortality, snag persistence, tree fall patterns and variables influencing snag dynamics were assessed in deciduous, mixed and coniferous stands that experienced low- and moderate-severity fires. Temporal patterns of tree mortality for the three species revealed that mortality was delayed through time. Most post-fire tree mortality occurred within 2 years following fire but continued until the end of the 10-year observation period. Jack pine was the most persistent snag species, followed by trembling aspen and black spruce. Factors influencing the persistence of snags were multi-scaled and generally species-specific. Fire severity was the only common factor influencing snag persistence among all species, with snags located in severely-burned stands being less susceptible to falling. Trembling aspen snag persistence increased with basal area and diameter. Salvage logging in the vicinity affected black spruce. Fall patterns also differed among species. Bypasses of the snag stage (i.e. when a living tree falls directly to the forest floor) as well as uprooting of snags were common.

Spatial variations in the fire cycle of a large territory (190 000 km²) located in the boreal forest of eastern Canada were assessed using random sampling points. Our main objective was to determine if regions characterised by a large proportion of dry surficial deposit–drainage (SDD) burn more frequently than regions with a smaller proportion. Through a regionalisation of the landscape units, we analysed the effects of SDD on spatial variations of the fire cycle. A discriminant analysis involving the SDD and other physical variables (precipitation, temperature, aridity index, water bodies, elevation and slope) made it possible to identify a combination of variables characterising each region. A considerable variation in fire cycle was observed among the different SDD types (from 144 to 425 years) and between regions (from 90 to 715 years). Through the discriminant analysis, this study suggests that a combination of possible climatic top-down (precipitation R² = 0.727, aridity index R² = 0.663 and temperature R² = 0.574) and bottom-up factors (xeric undifferentiated till R² = 0.819 and humid undifferentiated till R² = 0.691) could explain this variation at the regional scale. Implications of those results for forest protection against fire and regional development are briefly discussed.

In this study, we characterised the composition and configuration of post-fire residual habitats belonging to two physiographic zones of the black spruce–moss domain in western Quebec. Thirty-three large fires (2000–52 000 ha) were selected and extracted on classified Landsat satellite imagery. The results show that a minimum of 2% and a maximum of 22% of burned areas escaped fire, with an overall average of 10.4%. The many forest patches that partially or entirely escaped fire formed residual habitats (RHs). It was found that although the area of RHs follows a linear relationship with fire size, their proportion appears relatively constant. Spatial analyses showed that the fires could be separated into two groups depending on the physiographic zones (East-Canadian Shield v. West-Clay Belt Lowlands). Fires in the west zone generate less RHs and appear to be associated with more extreme weather conditions. In most cases there was no association with water or wetlands; in some fires the presence of RHs is associated with the proximity of water bodies. The failure to find an association between RHs and wetlands suggests that this type of environment is part of the fuel. Coniferous woodland with moss appears particularly overrepresented within RHs. Our results suggest that the local and regional physiographic conditions strongly influence the creation of RHs; therefore, it is important to consider those differences when applying ecosystem-based management.

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.

Résumé : Les différences entre les paysages de forêt boréale engendrés par les régimes de perturbations naturelles et ceux qui sont engendrés par la coupe sont importantes et de mieux en mieux documentées. Afin de poursuivre les opérations de récolte tout en maintenant la biodiversité et les autres services rendus par l’écosystème, les politiques gouvernementales et les processus de certification prônent l’utilisation de pratiques qui conservent les caractéristiques du paysage à l’intérieur d’écarts correspondant à la variation naturelle de ces caractéristiques. Un inconvénient majeur associé à la mise en œuvre d’une telle stratégie est l’absence d’une information spatiale et temporelle complète au sujet de ces caractéristiques du paysage, telles que la proportion de vieux peuplements qui agissent vraisemblablement comme brut grossier pour la conservation s’ils demeurent suffisamment représentatifs des conditions naturelles. L’objectif de cette étude consistait à quantifier la proportion de vieux peuplements dans un très vaste paysage en combinant des bribes de connaissance provenant de deux sources différentes, c.-à-d. un inventaire forestier provincial et les reconstitutions de l’historique des feux à l’aide d’un réseau de croyances bayésien. Cette étude a été réalisée dans un paysage de 6,5 Mha situé dans la ceinture d’argile de la province d’Ontario, au Canada. Les résultats indiquent que 72,4 % de ce territoire est occupé par des peuplements qui n’ont subi aucun feu au cours des 150 dernières années. La discussion porte sur les implications pour l’aménagement et le potentiel pour de futurs travaux de recherche.

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

Abstract:

Several concepts are at the basis of forest ecosystem management, but a relative consensus exists around the idea of a forest management approach that is based on natural disturbances and forest dynamics. This type of approach aims to reproduce the main attributes of natural landscapes in order to maintain ecosystems within their natural range of variability and avoid creating an environment to which species are not adapted. By comparing attributes associated with natural fire regimes and current forest management, we were able to identify four major differences for the black spruce forest of the Clay Belt. The maintenance of older forests, the spatial extent of cutover areas, the maintenance of residuals within cutovers and disturbance severity on soils are major issues that should be addressed. Silvicultural strategies that mitigate differences between natural and managed forests are briefly discussed.

Résumé:

Plusieurs approches conceptuelles sont à l’origine de l’aménagement écosystémique, mais un certain consensus semble exister actuellement sur le fait qu’il doit viser à reproduire des paysages naturels, i.e., des territoires aménagés qui conservent les principaux attributs des forêts naturelles. En créant des paysages avec des attributs forestiers semblables à ceux des paysages naturels, on vise à maintenir l’écosystème à l’intérieur des limites de variabilité naturelle. Ainsi, les espèces ne risquent pas de se retrouver dans un environnement auquel elles n’ont jamais été confrontées historiquement. En comparant le régime naturel des feux et son impact sur la dynamique forestière avec l’aménagement forestier actuel nous avons pu identifier certains écarts importants. Le maintien de forêts surannées, la dispersion spatiale des aires de coupes, la préservation de forêts résiduelles dans les aires de coupes et la sévérité des perturbations au sol constituent des enjeux importants à prendre en considération dans l’aménagement écosystémique des forêts de la ceinture d’argile. Des stratégies sylvicoles permettant de décroître l’écart entre l’aménagement et la dynamique naturelle sont brièvement discutées. ©2007 NRC Canada

Abstract

It is now recognized that in the Canadian boreal forest, timber harvesting activities have replaced wildfires as the main stand-replacing disturbance. Differences in landscape patterns derived from these two sources of disturbance have, however, raised concerns that the way forest harvesting has been dispersed is potentially shifting patterns away from the natural range. In the context of natural disturbance-based management, we used a spatially explicit model designed to capture general fire regimes in order to quantify temporal variability associated with regenerating areas (burnt areas of 25 years or younger), and to develop strategic objectives for harvest agglomeration sizes and dispersion.We first evaluated temporal variability in the proportion of stands younger than 100 years (assumed to be even-aged stands) for various fire regimes (seven fire cycles: 50 to 400 years, and three mean fires sizes: 3000, 15 000 and 60 000 ha). Secondly, we quantified the size distribution and dispersion of regenerating areas for each fire regime. As expected by theoretical fire frequencies and size distributions, the importance of even-aged stands at the forest management unit level was found to decrease with longer fire cycles. However, the temporal variability associated with these proportions is shown to increase with mean fire size. It was also observed that the size distribution and dispersion of regenerating areas was primarily influenced by mean fire size. Based on these observations, natural disturbance-based management objectives were formulated, providing guidelines on harvest agglomeration size and dispersion.

Résumé

Le rajeunissement de la forêt boréale canadienne est maintenant d’avantage lié aux activités forestières qu’à l’action des feux de forêt. Les différences majeures dans les patrons spatiaux créés par ces régimes de perturbations suscitent donc des inquiétudes quant aux pratiques forestières actuelles. Dans le cadre d’une approche d’aménagement écosystémique qui s’inspire des perturbations naturelles, nous proposons d’analyser la variation temporelle des patrons d’aire en régénération créés par le feu (brûlis de 25 ans ou moins) et d‘élaborer des objectifs stratégiques d’espacement et de taille des chantiers de coupe. Les variations dans la proportion de forêt équienne (peuplements de moins de 100 ans) ont d’abord été évaluées et comparées à des valeurs attendues pour plusieurs régimes de feu (sept cycles de feux : 50 à 400 ans, et trois tailles moyennes : 3000, 15 000 et 60 000 ha). Ensuite, la variabilité temporelle liée à la distribution de taille et à la dispersion des aires en régénération a été évaluée. Telle qu’attendue la proportion de forêt de moins de 100 ans est inversement proportionnelle à la longueur du cycle de feu. Cependant, la variabilité temporelle associée à cette proportion serait davantage influencée par la taille moyenne des feux. La taille moyenne des feux serait aussi le principal agent qui influence la taille et l’espacement des aires en régénération. Suite à ces observations nous proposons des lignes directrices d’aménagement qui précisent les proportions de forêt aménagée de façon équienne, leur distribution quant à la superficie des chantiers de coupe et leur dispersion ou espacement minimal.

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.

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.

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.

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

Old-growth forests make up a substantial proportion of the forest mosaic in the Clay Belt region of Ontario and Quebec, Canada, despite fire cycles that are presumed to be relatively short. Two hypotheses have been suggested as explanations for this phenomenon: (1) the old-growth forests in question are located on sites that are protected from fire or (2) the fire hazard is just as great there as elsewhere, and that part of the mosaic is simply the tail of the distribution, having been spared from fire merely by chance. The tree-ring method has proven inadequate as a means of determining the date of the most recent fire in these old-growth forests, as the time that has elapsed since that date probably exceeds the age of the oldest trees. Accordingly, a paleoecological study was conducted with a view to determining the date of the last fire in these forests. Charcoal horizons were located and radiocarbon dated in six old-growth forests. The possibility that these forests have never burned at all is ruled out by the fact that macroscopic charcoal fragments were found at all sites. The proximity of potential firebreaks has a significant influence in the survival model, suggesting fire-cycle heterogeneity throughout the landscape. However, the proportion of old-growth forests observed is in agreement with what would be expected assuming that fire hazard is independent of stand age. Old-growth stands could thus be incorporated into natural disturbance based management, although the great variability of the intervals between catastrophic disturbances should be carefully considered.

Les forêts anciennes constituent une proportion importante de la mosaïque forestière de la ceinture d'argile (Ontario et Québec, Canada) en dépit de cycles de feu que l'on présume relativement courts. Deux hypothèses ont été avancées pour expliquer ce phénomène : (1) Les forêts anciennes sont localisées sur des sites protégés des feux ou (2) le risque d'incendie y est équivalent et cette portion de la mosaïque ne constitue que la queue de la distribution, épargnée par le feu simplement par hasard. La dendrochronologie s'est avérée inadéquate pour dater le dernier feu dans ces forêts anciennes puisque le temps écoulé depuis le dernier feu est probablement supérieur à l'âge des plus vieux arbres. Une étude paléoécologique a donc été effectuée dans le but de dater le dernier feu dans ces forêts. Les horizons contenant du charbon de bois ont été localisés et datés au carbone 14 dans six forêts anciennes. Le fait que des fragments de charbon macroscopiques aient été retrouvés dans tous les sites indique que ces forêts ont bel et bien été affectées par les feux dans le passé. La proximité de coupe-feu potentiels accroît la durée du cycle des feux. Toutefois, la proportion de forêts anciennes observée indique que le risque d'incendie est indépendant du temps écoulé depuis le dernier feu. Les forêts anciennes pourraient donc être intégrées à des aménagements forestiers inspirés par la dynamique des perturbations naturelles. Toutefois, la grande variabilité de l'intervalle entre deux perturbations catastrophiques devrait être soigneusement considérée. ©2005 NRC Canada

Over the past decades, there has been an increasing interest in the development of forest management approaches that are based on an understanding of historical natural disturbance dynamics. The rationale for such an approach is that management to favor landscape compositions and stand structures similar to those of natural ecosystems should also maintain biological diversity and essential ecological functions. In fire-dominated landscapes, this approach is possible only if current and future fire frequencies are sufficiently low, comparing to pre-industrial fire frequency, that we can substitute fire by forest management. We address this question by comparing current and future fire frequency to historical reconstruction of fire frequency from studies realized in the Canadian boreal forest. Current and simulated future fire frequencies using 2 and 3 x CO2 scenarios are lower than the historical fire frequency for many sites, suggesting that forest management could potentially be used to recreate the forest age structure of fire-controlled pre-industrial landscapes. There are however, important limitations to the current even-age management.

The forest fire regime was characterized for the boreal forest of western Quebec using the provincial government's digital databases (1945-1998). Lightning- and human-caused fires account for 71% and 29% of the total area burned, respectively. With regard to ignition sources, lightning was responsible for 38% of the fires while humans were the ignition agent for 62% of fires. The fire regime parameters (burn rate, fire occurrence, and size) were subjected to a stepwise regression analysis on the basis of regional landscape units. Models indicate that climatic factors, particularly summer precipitation and maximum temperatures, play a primary role in forest fire dynamics, regardless of the ignition source. Fire occurrence models were the most predictable with R-2 values of 0.79 and 0.60 for lightning fires and human-caused fires, respectively. Models of burned areas reached an R-2 value of 0.63 for lightning but only 0.22 for human-caused fires; on the other hand, the fire-size model for human-caused fires showed an R-2 value of 0.57 but only 0.24 for lightning fires. In the case of human-induced fires, the density of the road network and sand deposits were important in fire occurrence and burned areas models. Once characterized, landscape units tend to group together naturally, forming extensive areas in which the fire regime is relatively homogeneous. The results of the regionalization based on lightning fire regimes are discussed from the standpoint of sustainable forest management.

Fire history was reconstructed for an area of 15 000 km(2) located in the transition zone between the mixed and coniferous forests in Quebec's southern boreal forest. We used aerial photographs, archives, and dendroecological data (315 sites) to reconstruct a stand initiation map for the area. The cumulative distribution of burnt area in relation to time since fire suggests that the fire frequency has decreased drastically since the end of the Little Ice Age (about 1850) in the entire region. However, a large part of the area was burned between 1910 and 1920 during intensive colonization and when the climate was very conducive to fire. For the period 1920-1945, large fires have mainly been concentrated in the more populated southern area, while few fires have been observed in the virgin coniferous forest in the north. Despite slight differences between the south and the north, fire cycles or the average number of years since fire are not significantly different. Since 1945, there have been far more fires in the south, but the mean fire size was smaller than in the-north. These results suggest that the transition between the mixed and coniferous forests observed in the southern boreal forest cannot be explained by a difference in fire frequency, at least during the last 300 years. As climatic factors and species potential distribution did not vary significantly from south to north, we suggest that the transition from mixedwood to coniferous forests is mainly controlled by fire size and severity. Smaller and less severe fires would favor species associated with the mixedwood forests as many need survivors to reinvade burnt areas. The abundance of deciduous species in mixedwood forests, together with the presence of more lakes that can act as firebreaks, may contribute to decreases in fire size and severity. The transition between the two vegetation zones could be related to the initial setting following the vegetation invasion of the area during the Holocene. In this context, the limit of vegetation zones in systems controlled by disturbance regimes such as fires may not have reached a balance with current climatic conditions. Historical legacies and strong positive feedback between disturbance regimes and composition may filter and delay the responses to changes in climate.

The objective of this paper is to discuss approaches and issues related to modelling stand dynamics for multi-cohort forest management in eastern Canadian boreal forests. In these forests, the interval between wildfires can be rather long, and the development of natural forest stands may include the establishment, growth and mortality of several cohorts of trees. Later cohorts are characterised by increasing structural complexity, including spatial heterogeneity and irregular tree size distribution. A multi-cohort forest management framework has been proposed to maintain this complexity, and associated biodiversity, on the landscape. Multi-cohort forest management planning requires forecasts of the development of stands with complex structure in response to silvicultural treatment and to natural disturbance, but current stand dynamics models in the region are applicable mainly to even-aged mono-specific stands. Possible modelling approaches for complex stands include i) the adaptation of current whole-stand growth and yield models, ii) distance-independent, empirically-derived individual-tree models, such as the USDA Forest Service Forest Vegetation Simulator, and iii) distance-dependent, empirically-derived or process-oriented individual-tree models. We conclude that individual-tree models are needed because observational data for fitting whole-stand models are not available for the full array of silvicultural treatments and natural disturbances encompassed by multi-cohort forest management. Predictive accuracy is a concern with individual-tree models, and the incorporation of coarse-scale constraints into these models is a promising means to control error.

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

Most studies of postfire tree recruitment have occurred in severely burned portions, despite the fact that partial burning is common. In this study we examined regeneration following a 1997 fire in the boreal forest of Quebec. A model of postfire recruitment was elaborated using parameters such as the proportion of trees killed (severity), the proportions of postfire seedbed types and their associated juvenile survivorship, the available seed supply, the available bud supply (for Populus tremuloides Michx.), and the granivory rate. All three species had peak recruitment in the first or second summer, and the recruitment episode was essentially finished after the third year. Mineral soil and surviving Sphagnum were the best seedbeds for both conifer species. Seedbed frequency was essentially independent of crown fire severity except for surviving Sphagnum, which was concentrated primarily where severity was light. Conifer fecundity was much lower in the lightly burned stands, a result we attribute to a higher granivory rate. The fecundity (seedlings/basal area for the conifers or suckers/basal area for Populus) in the severe sites was typical of the few other North American studies of postfire recruitment, where the published data permit us to make the comparison.

La plupart des études sur le recrutement après feu ont été réalisées dans des zones sévèrement brûlées, malgré le fait qu'il y ait fréquemment des zones légèrement brûlées. Dans cette étude, nous avons examiné la régénération qui s'est établie après un feu survenu en 1997 dans la forêt boréale québécoise. Nous avons élaboré un modèle de recrutement après feu à l'aide de paramètres tels que la proportion d'arbres tués (sévérité), la proportion de types de lits de germination après feu et la survie des jeunes tiges associées à ces lits de germination, le stock de graines disponibles, le stock de bourgeons disponibles (pour le Populus tremuloides Michx.) et le taux de prédation des graines. Les trois espèces ont connu un pic de recrutement durant le premier et le second été, et l'épisode de recrutement était à toute fin pratique terminé après la troisième année. Le sol minéral et la sphaigne qui avait survécu étaient les meilleurs lits de germination pour les deux espèces de conifères. La fréquence des lits de germination était essentiellement indépendante de la sévérité du feu de cime, à l'exception de la sphaigne qui avait survécu, laquelle était principalement concentrée aux endroits où la sévérité du feu était légère. La fécondité des conifères était beaucoup plus faible dans les peuplements légèrement brûlés; un résultat que nous attribuons à un plus haut taux de prédation des graines. La fécondité (nombre de semis/surface terrière pour les conifères ou nombre de drageons/surface terrière pour les peupliers) dans les sites sévèrement brûlés était typique de celle observée dans les quelques autres études nord-américaines sur le recrutement après feu dont la publication des résultats nous permet d'établir une comparaison.©2004 NRC Canada

The fire history of two adjacent regions of the boreal forest, one characterized by fogging (Ontario-510,000 ha) and the other by small scale agricultural activities (Quebec-140,000 ha), was studied before and after these regions were opened up to settlement in 1916. From a review of provincial forest fire records and the assessment of the age of fire-initiated forest stands, it appears that large but rare fires occurred during the presettlement period on both sides of the border. After 1916, due to slash and burn activities, the agricultural region (Que) had proportionally about twice the burned areas and ten times more fires than the forestry region (Ont). Despite differences in population density, road network, and land use, fire size class occurrence did not differ between landscapes overtime. However,the occurrence of fires larger than 100 ha, considering three development phases (1916-1939; 1940-1969; 1970-1998), decreased in both regions from settlement to the present, particularly during the late phase (1970-1998) in the agricultural region. An analysis of fluctuations in the Canadian forest Fire Weather Index system (FWI), a rating of fire danger severity, showed major climatic stresses at the beginning of the century (1916-1924), followed by a decrease in the occurrence of extreme FWI values. Combined with the impact of climate, which affected the annual area burned and the number of large fires in both landscapes, the results suggest that the landscape fragmentation, the increase in the percentage of deciduous trees overtime and/or effective fire detection by residents led to a decrease in the number of fires larger than 100 ha on the agricultural side for the late phase (1970-1998).

In Canada, there are still extensive tracts of boreal forest consisting of stands that have resulted from natural disturbances. The country's forests are a mosaic made up to a large extent of old-growth forest that is beyond commercial harvesting age, especially in the boreal forest of eastern Canada. As areas of boreal forest under management steadily expand, as demand for forest products continues to grow and as rotation periods become shorter in response to silvicultural practices, the forest cover will inevitably become younger, causing changes to the structure and composition of the mosaic of forest stands that will affect the aspect of entire landscapes. These changes may have an adverse impact on biological diversity. Forest birds are one group of living organisms that may respond quickly to the advent of younger forest landscapes, thereby acting as a biological indicator. In this paper, we discuss some of the problems that birds face as a result of the truncation of the age-class distribution of managed forest landscapes in eastern Canada's coniferous boreal forest, using data obtained from our research in the Clay Belt region of Quebec and Ontario. More specifically, we look at how birds respond to changes in forest structure and composition in terms of time since natural disturbances, and to variation in dead trees availability. We then consider the impact of the prospective rejuvenation of the forest cover in managed forest landscapes, and possible solutions aimed at mitigating that impact through new management strategies based on the maintenance of forest ecosystem diversity. The ability of these new management strategies to maintain the ecological integrity of bird communities provides an indication of their potential as tools for contributing to the maintenance of biological diversity in a broader sense.

Au Canada, la forêt boréale comporte encore des superficies importantes de peuplements issus de perturbations naturelles. Une forte proportion de la mosaïque forestière est constituée de vieilles forêts qui dépassent l’âge commercial d’exploitation forestière, notam-ment dans la forêt boréale de l’est du Canada. Avec l’importance grandissante des territoires aménagés en forêt boréale, les besoins toujours croissants pour la matière ligneuse et le recours à une sylviculture réduisant les périodes de rotation, des changements dans la structure et la composition des mosaïques de peuplements forestiers liés au rajeunissement du couvert forestier sont à anticiper à l’échelle des paysages. Ces changements peuvent causer des préjudices à la diversité biologique. L’avifaune représente l’un des groupes d’organismes qui peut répondre rapidement au rajeunissement des paysages forestiers et servir ainsi d’indicateur biologique. Dans cet article, nous exposons certains des problèmes auxquels est confrontée l’avifaune face au rajeunissement anticipé des mosaïques forestières aménagées de la forêt boréale résineuse de l’est du Canada à partir de nos travaux réalisés dans la région de la ceinture d’argile du Québec et de l’Ontario. Plus spécifiquement, nous examinons la réponse des oiseaux aux changements de structure et de composition des forêts en fonction du temps depuis les dernières perturbations dans des mosaïques forestières naturelles, ainsi qu’en rapport avec la varia-tion de la densité d’arbres morts disponibles dans ces écosystèmes. Nous discutons ensuite de l’effet du rajeunissement du couvert foresti-er qui est anticipé dans les mosaïques forestières aménagées de ces écosystèmes et des solutions envisageables pour atténuer cet effet au moyen des nouvelles stratégies d’aménagement fondées sur le maintien de la diversité des écosystèmes forestiers. La capacité de ces nouvelles stratégies d’aménagement à maintenir l’intégrité écologique des communautés d’oiseaux forestiers donne une indica-tion de leur potentiel à contribuer plus largement au maintien de la diversité biologique.

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.

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.

Fire history and forest dynamics were reconstructed for a 3800-km² territory located in the south-central boreal forest of Quebec. Fire cycle was characterized using a random sampling strategy combined with archival data on fires that had occurred since 1923 on private land owned by Smurfit-Stone. Bioclimatic subdomain, land use, surficial deposit, and mean distance from a firebreak did not affect the fire cycle. Fire cycles have been longer since the end of the Little Ice Age (similar to1850). Warming after the Little Ice Age seems to have triggered a change in fire frequency. Forest dynamics were characterized by transition matrices for changes in dominant canopy composition from 344 permanent sampling plots. These permanent plots were sampled approximately every 15 years over the preceding 40 years. We observed two distinct patterns of replacement: (i) deciduous and mixed stands were replaced by balsam fir (Abies balsamifera (L.) Mill.) (and, to a lesser extent, by black spruce (Picea mariana (Mill.) BSP)) and (ii) jack pine (Pinus banksiana Lamb.) was replaced by black spruce. Analyses confirm that species replacement occurs in the eastern boreal forest of Canada when the fire-return interval is long enough and that the substrate plays an important role along with other disturbances, such as insect outbreaks. Our results also suggest that the proportion of old-growth forests (>100 years old) in the landscape should increase as a result of the lengthening of the fire cycle. More and more stands are likely to experience species replacement. From the standpoint of sustainable forest management, this perspective calls into question the widespread use of clear-cutting in the boreal forest. Regional context must be taken into account in forest management if the conservation of biodiversity and ecosystem integrity are serious objectives. Economically and ecologically sound silvicultural scenarios that emulate natural processes are discussed.©2002 NRC Canada

Forest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. Silvicultural Forest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. SilviculturalForest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. Silvicultural trials have been established to evaluate stand-level responses to treatments and operational aspects of the approach. © 2002 Elsevier B.V. All rights reserved.

Fire reconstruction and forest inventory maps provided an opportunity to study changes in stand-level characteristics following fire using a data set comprised of all forest stands of fire origin in an area of over 10000 km². We assigned the date of the most recent fire occurrence to over 31000 forest stands in an ecoforestry database. We categorized stands on different substrates into age classes to investigate differences in canopy composition, cover and height, and incidence of secondary disturbance. Stands with over 75% Picea mariana (Mill.) BSP dominated all age classes on organic sites. On other substrates, there was a change in canopy composition from deciduous stands and stands dominated by Pinus banksiana Lamb. to Picea mariana stands after about 100 yr. This transition was later for xeric sites. After a peak in canopy cover and height at about 100 yr, there was a decrease in the area occupied by stands with dense, tall canopies. Structural development was slower on less productive sites. There was little incidence of spruce budworm outbreaks. Partial disturbance by windthrow coincided with canopy break-up at 3 00 yr, but appeared to have little effect on overall canopy structure in later stages. Structural diversity was independent of compositional diversity; on organic sites, stands with similar composition had different canopy structure. Diversity of stands with different composition and structure was greatest in the first 150 yr following fire. Maintaining stands in different stages of structural development on the landscape would serve to maintain regional biodiversity.

The combination of certain features of fire disturbance, notably fire frequency, size and severity, may be used to characterize the disturbance regime in any region of the boreal forest. As some consequences of fire resemble the effects of industrial forest harvesting, conventional forest management is often considered as a disturbance that has effects similar to those of natural disturbances. Although the analogy between forest management and are disturbance in boreal ecosystems has some merit, it is important to recognise that it also has its limitations. Short fire cycles generally described for boreal ecosystems do not appear to be universal rather, important spatial and temporal variations have been observed in Canada. These variations in the fire cycle have an important influence on forest composition and structure at the landscape and regional levels. Size and severity of fires also show a large range of variability. In regions where the natural matrix of the boreal forest remains relatively intact, maintenance of this natural variability should be targeted by forest managers concerned with biodiversity conservation. Current forest management tends to reduce this variability: for example, fully regulated, even-aged management will tend to truncate the natural forest age distribution and eliminate over-mature and old-growth forests from the landscape. We suggest that the development of strategic-level forest management planning approaches and silvicultural techniques designed to maintain a spectrum of forest compositions and structures at different scales in the landscape is one avenue to maintain this variability. Although we use the boreal forest of Quebec for our examples, it is possible to apply the approach to those portions of the boreal forest where the fire regime favours the development of even-aged stands in burns.

We sampled 22 black spruce (Picea mariana) - feathermoss (Pleurozium schreberi) sites (80 to >200 years) to describe and assess the diversity of bryophyte and lichen communities as a function of time since fire and site characteristics. Old growth had no more species than younger forests. We think that this result might be explained by the phenomenon of paludification, which is a major process in this region. Axis 1 of a nonmetric multidimensional scaling ordination (NMS) of terricolous species was interpreted as a paludification gradient. Mature forests were characterized by Pleurozium schreberi, Ptilium crista-castrensis, Polytrichum commune, and Dicranum polysetum, and older sites by a greater abundance of Sphagnum. Axis 1 of epiphytic species ordination (NMS) was interpreted as a gradient of time since the last fire. Abundance of Tuckermannopsis americana, Hypogymnia physodes, and Bryoria furcellata was greater in mature forests. In contrast, Mycoblastus sanguinarius, Bryoria trichodes, and Usnea spp. were more abundant in older forests. The abundance of epiphytic lichens increased with tree age, whereas their richness was higher in sites where trembling aspen (Populus tremuloides) and jack pine (Pinus banksiana) were present. Since species composition varied with time since the last fire, it is important to preserve the diversity of successional stages at the landscape level and the structural diversity at the stand level to maintain the bryophyte and lichen communities.©2002 NRC Canada

Fire reconstruction and forest inventory maps provided an opportunity to study changes in stand-level characteristics following fi re using a data set comprised of all forest stands of fi re origin in an area of over 10 000 km 2 . We assigned the date of the most recent fi re occurrence to over 31 000 forest stands in an ecoforestry database. We categorized stands on different substrates into age classes to investigate differences in canopy composition, cover and height, and incidence of secondary disturbance. Stands with over 75% Picea mariana (Mill.) BSP dominated all age classes on organic sites. On other substrates, there was a change in canopy composition from deciduous stands and stands dominated by Pinus banksiana Lamb. to Picea mariana stands after about 100 yr. This transition was later for xeric sites. After a peak in canopy cover and height at about 100 yr, there was a decrease in the area occupied by stands with dense, tall canopies. Structural development was slower on less productive sites. There was little incidence of spruce budworm outbreaks. Partial disturbance by windthrow coincided with canopy break-up at 100 yr, but appeared to have little effect on overall canopy structure in later stages. Structural diversity was independent of compositional diversity; on organic sites, stands with similar composition had differ-ent canopy structure. Diversity of stands with different composition and structure was greatest in the fi rst 150 yr following fi re. Maintaining stands in different stages of structural development on the landscape would serve to maintain regional biodiversity.

Given that fire is the most important disturbance of the boreal forest, climatically induced changes in fire frequency (i.e., area burnt per year) can have important consequences on the resulting forest mosaic age-class distribution and composition. Using archives and dendroecological data we reconstructed the fire frequency in four large sectors along a transect from eastern Ontario to central Quebec. Results showed a dramatic decrease in fire frequency that began in the mid-19th century and has been accentuated during the 20th century. Although all areas showed a similar temporal decrease in area burned, we observed a gradual increase in fire frequency from the west to Abitibi east, followed by a slight decrease in central Quebec. The global warming that has been occurring since the end of the Little Ice Age (similar to 1850) may have created a climate less prone to large forest fires in the eastern boreal forest of North America. This interpretation is corroborated by predictions of a decrease in forest fires for that region of the boreal forest in the future. A longer fire cycle (i.e., the time needed to burn an area equivalent to the study area) has important consequences for sustainable forest management of the boreal forest of eastern Canada. When considering the important proportion of overmature and old-growth stands in the landscape resulting from the elongation of the fire cycles, it becomes difficult to justify clear-cutting practices over all the entire area as well as short rotations as a means to emulate natural disturbances. Alternative practices involving the uses of variable proportion of clear, partial, and selective cutting are discussed. ©2001 NRC Canada

1 Studies on the variability of natural fire regimes are needed to understand plant responses in a changing environment. Since vegetation changes might follow or trigger changes in fire frequency, climate models suggest that changes in water balance will accompany current global warming, and the response of fire regimes to Holocene hydro-climate changes and vegetation switches may thus serve as a useful analogue for current change. 2 We present high-resolution charcoal records from laminated cores from three small kettle lakes located in mixed-boreal and coniferous-boreal forest. Comparison with some pollen diagrams from the lakes is used to evaluate the role of the local vegetation in the fire history. Fire frequency was reconstructed by measuring the separation of peaks after detrending the charcoal accumulation rate from any background. 3 Several distinct periods of fire regime were detected with fire intervals. Between c. 7000-3000 cal. Year BP, fire intervals were double those in the last 2000 years. Fire frequency changed 1000 years earlier in the coniferous-boreal forest than in the mixed-boreal forest to the south. The absence of changes in combustibility species in the pollen data that could explain the fire frequency transition suggests that the vegetation does not control the long-term fire regime in the boreal forest. 4 Climate appears to be the main process triggering fire. The increased frequency may be the result of more frequent drought due to the increasing influence of cool dry westerly Pacific air-masses from mid to late Holocene, and thus of conditions conducive to ignition and fire spread. In east Canada, this change matches other long-term climate proxies and suggests that a switch in atmospheric circulation 2-3000 years ago triggered a less stable climate with more dry summers. Future warming is moreover likely to reduce fire frequency. 1 Studies on the variability of natural fire regimes are needed to understand plant responses in a changing environment. Since vegetation changes might follow or trigger changes in fire frequency, climate models suggest that changes in water balance will accompany current global warming, and the response of fire regimes to Holocene hydro-climate changes and vegetation switches may thus serve as a useful analogue for current change. 2 We present high-resolution charcoal records from laminated cores from three small kettle lakes located in mixed-boreal and coniferous-boreal forest. Comparison with some pollen diagrams from the lakes is used to evaluate the role of the local vegetation in the fire history. Fire frequency was reconstructed by measuring the separation of peaks after detrending the charcoal accumulation rate from any background. 3 Several distinct periods of fire regime were detected with fire intervals. Between c. 7000-3000 cal. Year BP, fire intervals were double those in the last 2000 years. Fire frequency changed 1000 years earlier in the coniferous-boreal forest than in the mixed-boreal forest to the south. The absence of changes in combustibility species in the pollen data that could explain the fire frequency transition suggests that the vegetation does not control the long-term fire regime in the boreal forest. 4 Climate appears to be the main process triggering fire. The increased frequency may be the result of more frequent drought due to the increasing influence of cool dry westerly Pacific air-masses from mid to late Holocene, and thus of conditions conducive to ignition and fire spread. In east Canada, this change matches other long-term climate proxies and suggests that a switch in atmospheric circulation 2-3000 years ago triggered a less stable climate with more dry summers. Future warming is moreover likely to reduce fire frequency. 1 Studies on the variability of natural fire regimes are needed to understand plant responses in a changing environment. Since vegetation changes might follow or trigger changes in fire frequency, climate models suggest that changes in water balance will accompany current global warming, and the response of fire regimes to Holocene hydro-climate changes and vegetation switches may thus serve as a useful analogue for current change. 2 We present high-resolution charcoal records from laminated cores from three small kettle lakes located in mixed-boreal and coniferous-boreal forest. Comparison with some pollen diagrams from the lakes is used to evaluate the role of the local vegetation in the fire history. Fire frequency was reconstructed by measuring the separation of peaks after detrending the charcoal accumulation rate from any background. 3 Several distinct periods of fire regime were detected with fire intervals. Between c. 7000-3000 cal. Year BP, fire intervals were double those in the last 2000 years. Fire frequency changed 1000 years earlier in the coniferous-boreal forest than in the mixed-boreal forest to the south. The absence of changes in combustibility species in the pollen data that could explain the fire frequency transition suggests that the vegetation does not control the long-term fire regime in the boreal forest. 4 Climate appears to be the main process triggering fire. The increased frequency may be the result of more frequent drought due to the increasing influence of cool dry westerly Pacific air-masses from mid to late Holocene, and thus of conditions conducive to ignition and fire spread. In east Canada, this change matches other long-term climate proxies and suggests that a switch in atmospheric circulation 2-3000 years ago triggered a less stable climate with more dry summers. Future warming is moreover likely to reduce fire frequency.

Within the context of studying the ecological impacts of wildland fires in the boreal forest, a spatial analysis of a major wildfire was conducted. The fire covered nearly 500 km² in the north-western part of Quebec's boreal forest in the summer of 1995. The spatial distribution of different fire impacts on the forest canopy was obtained using timber damage assessment maps. Fire impacts varied throughout the burned area, ranging from areas where trees had completely burned crowns (43%) to remaining patches of trees with green foliage (3%). The effects of local stand and site factors on crown fire, as assessed by the fire impacts, were evaluated using geographic information systems. Despite the large extent and high intensity of the wildfire created by extreme fire weather conditions, stepwise logistic regression and analysis by log-linear models indicated that variations in surface material, stand composition, and estimated stand age played a role in the presence or absence of crowning at the stand level. However, it appears that height and density of stand, as well as topography, did not have a significant influence. Our study presents the variability of fire impacts and its implications, and it provides a better understanding of the relationships between landscape components and fire crowning.

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.

Many studies have shown that certain species of bryophytes and lichens require old-growth forests for their survival. The objective of this study is to evaluate the composition and diversity of epiphytic lichen and bryophyte communities on trembling aspen (Populus tremuloides), as a function of the time elapsed since stand initiation. The study was carried out in the forests surrounding Lake Duparquet, in the southwestern part of Quebec's boreal forest. Stands representing different post-fire successional stages, corresponding to 278, 125, 79, and 51 yr since the last fire were selected. For each stand age, 10 trees from four different sites were sampled. A total of 75 species of non-vascular plants were found, including 34 species of mosses, seven species of liverworts, and 34 species of lichens. More species were observed in the 278-yr old stand, which also contained a greater number of exclusive species. In a correspondence analysis performed on species presence/absence data, axes one and two were correlated with distance from the closet unburnt area and time since fire. For each tree, species richness, diversity, and cover indices were calculated and compared for the different stand ages. These indices tended to be higher in the 79-yr old stand and lower in the 51-yr old stand. By using mixed log-liner models, we found that the frequency of occurrence of some species was linked to the time since fire, and others species to tree age. Old-growth forests are important to bryophytes and lichens since they have a greater number of species and some species are found exclusively or more frequently in old-growth forests. Furthermore, each stand age has some species associated with it. Therefore, forest management practices should be adopted to maintain all successional stages present in the natural forest landscape in order to preserve the diversity of non-vascular plants.

Although the concept of forest ecosystem management based on natural disturbance has generated a great deal of interest, few concrete examples exist of FEM principles being put into application. Silvicultural practices that emulate natural disturbances are proposed with examples from the principal vegetation zones of Quebec. With the exception of the large-scale use of careful logging to protect advanced regeneration in ecosystems generally controlled by fire, stand-level silvicultural practices currently used are reasonably similar to natural disturbances, although important differences exist. In contrast, at the forest-level, even-aged management as is currently practised rarely permits adequate reproduction of the variety of age classes, stand types, and structural components normally found in the boreal forest. A model that allows an even-aged management approach inspired by natural dynamics is proposed.

Although the concept of forest ecosystem management based on natural disturbance has generated a great deal of interest, few concrete examples exist of FEM principles being put into application. Silvicultural practices that emulate natural disturbances are proposed with examples from the principal vegetation zones of Quebec. With the exception of the large-scale use of careful logging to protect advanced regeneration in ecosystems generally controlled by fire, stand-level silvicultural practices currently used are reasonably similar to natural disturbances, although important differences exist. In contrast, at the forest-level, even-aged management as is currently practised rarely permits adequate reproduction of the variety of age classes, stand types, and structural components normally found in the boreal forest. A model that allows an even-aged management approach inspired by natural dynamics is proposed.

Because some consequences of fire resemble the effects of industrial forest harvesting, forest management is often considered as a disturbance having effects similar to those of natural disturbances. Although the analogy between forest management and fire disturbance in boreal ecosystems has some merit, it is important to recognize that it has limitations. First, normal forest rotations truncate the natural forest stand age distribution and eliminate over-mature forests from the landscape. Second, in the boreal mixedwoods, natural forest dynamics following fire may involve a gradual replacement of stands of intolerant broadleaf species by mixedwood and then softwood stands, whereas current silvicultural practices promote successive rotations of similarly composed stands. Third, the large fluctuations observed in fire frequency during the Holocene limit the use of a single fire cycle to characterize natural fire regimes. Short fire cycles generally described for boreal ecosystems do not appear to be universal; rather, shifts between short and long fire cycles have been observed. These shifts imply important changes in forest composition at the landscape and regional levels. All of these factors create a natural variability in forest composition that should be maintained by forest managers concerned with the conservation of biodiversity. One avenue is to develop silvicultural techniques that maintain a spectrum of forest compositions over the landscape.

1Serotiny, the capacity to retain seed in the plant canopy, has evolved in many species under the selective pressure of fires. The effect of disturbance type (lethal or nonlethal fire), time-since-fire and different fire regimes on the serotiny of jack pine (Pinus banksiana), was evaluated in populations from two adjacent landscapes in the southern part of the Canadian boreal forest. The island landscape (Lake Duparquet) has a complex fire regime of small fires of variable intensity, whereas the adjacent mainland has a fire regime characterized by large intense fires.

2Twenty-four jack pine populations (11 island and 13 mainland) on xeric sites were sampled for the degree of serotiny of trees, Fire history and age structure were reconstructed for each population using the fire scar method. For each tree, recruitment was categorized as after a lethal fire, after a nonlethal fire or in the absence of fire.

3Likelihood chi-square tests were used to investigate the variation in serotiny at individual, population and landscape levels.

4At the individual level, the results support our prediction that the occurrence of lethal fires favours trees with high serotiny while low serotiny trees are favoured by other types of disturbances.

5At the population level, the frequency of low serotiny trees increases with time since stand initiation, as a result of higher establishment opportunities after disturbances other than lethal fires. The proportion of low serotiny trees also increases with the occurrence of nonlethal fires.

6 Significant differences were found between the two landscapes. On the mainland, serotinous trees were more abundant, whereas on the islands where nonlethal fires were recorded, low serotiny trees were more frequent. These results support the hypothesis that fire imposes differential selective pressures on serotiny in jack pine.

In natural boreal forests, disturbances such as fire and variation in surficial deposits create a mosaic of forest stands with different species composition and age. At the landscape level, this variety of stands can be considered as the natural mosaic diversity. In this paper, we describe a model that can be used to estimate the natural diversity level of landscapes. We sampled 624 stands for tree species composition and surficial deposits in eight stand-age classes corresponding to eight fire episodes in the region of Lake Duparquet, Abitibi, Quebec at the southern fringe of the Boreal Forest. For six surficial deposit types, stand composition data were used to define equations for vegetation changes with time for a chronosequence of 230 years for four forest types. Using Van Wagner's (1978) model of age class distribution of stands, the proportion of each forest type for several lengths of fire cycle were defined. Finally, for real landscapes (ecological districts) of the ecological region of the "Basses-Terres d'Amos", the proportion of forest types were weighted by the proportion of each surficial deposit type using ecological map information. Examples of the possible uses of the model for management purposes, such as biodiversity conservation and comparisons of different landscapes in terms of diversity and sensitivity to fire regime changes, are discussed.

L'importance des feux de forêt, tant sur la dynamique des communautés végétales et des populations d'espèces que sur les adaptations des organismes à ces perturbations, est de plus en plus reconnue. Toutefois, peu d'études ont tenté de démontrer l'effet de différents régimes des feux, à l'échelle régionale, sur des caractères génétiques ou adaptatifs, au sein des populations d'une même espèce. Dans le cadre de cette étude, deux unités de paysage adjacentes ont été sélectionnées dans le sud de la forêt boréale du Québec, afin de déterminer l'effet de l'insularité et celui de deux régimes des feux distincts sur la structure génétique et sur le sérotinisme de populations de pin gris (Pinus banksiana Lamb.), à une échelle régionale. L'unité de paysage insulaire, le lac Duparquet (Abitibi), constituée de nombreuses îles, possède un régime de perturbations complexe comportant des feux d'intensité variable, qui affectent de faibles surfaces. L'unité terrestre, adjacente au lac, est affectée par un régime de feux destructeurs de grandes superficies, caractéristique de la forêt boréale.

Pour définir l'effet de l'insularité et des régimes des feux sur la structure et la variabilité génétiques, deux populations insulaires et deux populations terrestres ont été comparées au moyen de marqueurs isoenzymatiques. En moyenne, pour les 22 loci analysés, des taux de polymorphisme et d'hétérozygotie, respectivement de 60,0% et de 17,1%, ont été observés. La différenciation entre les populations est faible (Fst = 0,018) et aucune différence significative entre les populations pour les cinq paramètres de variabilité génétique estimés n'est décelée. Le flux génique important semble être responsable de la faible différenciation observée entre les populations des deux unités de paysage.

L'effet sélectif des régimes des feux différentiels a été évalué sur le sérotinisme des individus puisque la capacité des pins gris à produire des cônes sérotineux est considérée comme une adaptation majeure aux feux de forêts. Le sérotinisme de plusieurs centaines d'individus de pin gris a été déterminé dans vingt-cinq populations localisées dans les deux unités de paysage. Dans un premier temps, la taille et l'âge requis pour développer le caractère sérotineux ont été déterminés. La majorité des individus ne possèdent pas de cônes sérotineux avant d'avoir atteint 7 cm de diamètre à hauteur de poitrine (DHP) et la proportion de cônes sérotineux a tendance à augmenter annuellement jusqu'à ce que les arbres aient atteint une taille de 10 cm de DHP. La proportion de cônes sérotineux chez les cônes âgés de 4 années ou plus est inférieure d'environ 10% à celle des cônes plus jeunes. Cette proportion est également plus faible pour les cônes situés sur les branches inférieures à une hauteur de 2,6 m comparativement à celle des cônes sur les branches plus élevées. Ce même résultat est observé pour des cônes soumis à une température contrôlée de 41°C, indiquant que l'environnement thermique naturel auquel les cônes sont soumis n'est pas le seul facteur impliqué dans leur ouverture sur les branches les plus basses. Les résultats permettent de suggérer que l'absence de cône sérotineux chez les pins gris de faible taille est liée à la juvénilité des individus.

La majorité des pins gris échantillonnés sont fortement sérotineux. Cependant, environ 30% des arbres sont non sérotineux ou mixtes. Les individus non sérotineux et mixtes sont plus abondants lors d'établissement en absence de feu, au détriment des arbres sérotineux qui sont favorisés, quant à eux, lors d'établissement après un feu létal. L'âge de la forêt agit également sur la répartition des individus dans les classes de sérotinisme à l'intérieur des populations. Les individus non sérotineux sont avantagés, d'une part, dans les forêts dont l'initiation est relativement récente et, d'autre part, dans les forêts âgées de plus de 200 ans. La répartition des arbres dans les classes de sérotinisme diffère significativement entre les deux unités de paysage. L'intensité de la perturbation apparaît être un facteur important, favorisant les individus sérotineux lorsque les feux sont létaux et avantageant les individus non sérotineux, lorsqu'ils sont non létaux. Les résultats suggèrent que les régimes des feux distincts ont exercé des effets sélectifs à long terme suffisants pour différencier les populations sur le caractère adaptatif qu'est le sérotinisme et ce, en dépit du flux génique important entre les populations. © 1991 UdeM tous droits réservés.