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.

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

The capacity of individual species to tolerate environmental changes is a major driver of ecosystem resilience. This subject has been a hot topic in boreal forests due to the greater effect of climate change anticipated at northern latitudes. However, to date it has been mainly examined for trees, with comparatively little emphasis on other species groups. In this study, bryophytes were sampled in the boreal forest of Québec (Canada) using a stratified sampling design allowing us to separate the effect of dominant canopy composition (two types: coniferous and mixedwood) and of the spatial climatic gradient (relatively warm and relatively cold sites) on bryophyte community structure (species richness and composition). The results indicated that bryophyte composition was different between coniferous and mixedwood stands. The impacts of spatial variation in climate were more visible in mixedwood than coniferous stands, indicating an interaction between stand type and climate. Specifically, bryophytes from mixedwood stands, and particularly specialist communities associated with deadwood and live tree bases, were climate?sensitive and represent a potential target group to investigate the effects of climate change. In contrast, bryophyte communities from coniferous stands are stand structure?sensitive, with little response to climate, presumably partly because coniferous canopies buffer climatic variations. Since bryophyte community sensitivity to regional variations in climatic conditions depends on dominant canopy characteristics and microhabitat availability, we suggest that adapted forest management practices could play a role in attenuating climate change impacts on these organisms.

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

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

Les principaux facteurs qui déterminent la composition des communautés de lichens sont encore mal connus dans les écosystèmes boréaux nordiques. Cette étude compare les effets des caractéristiques (hauteur, couvert forestier et âge) des peuplements forestiers sur les communautés de lichens épiphytes fruticuleux et terricoles à travers une vaste région située à la limite ente les forêts boréales à couvert fermé et les forêts nordiques ouvertes dans la province de Québec (Canada). Le jeu de données est composé de 875 placettes réparties sur un territoire de 242?000 km2 qui s’étend entre les extrémités est et ouest de la province. La biomasse des lichens épiphytes fruticuleux (Alectoria, Bryoria, Evernia et Usnea) a été évaluée à l’échelle de la branche, de l’arbre et de la placette; le recouvrement de lichens terricoles (Cladonia spp.) a été évalué à l’échelle de la placette. Les résultats montrent que les lichens épiphytes et terricoles réagissent de façon significative mais différente aux variations des caractéristiques de la forêt. À l’échelle de la placette, la biomasse des lichens épiphytes était la plus élevée dans les plus vieux peuplements (>100 ans), et la plus faible dans les peuplements avec un faible couvert forestier (<25%) ou dans les peuplements dominés par des arbres de petite taille (<7 m). Par contre, le recouvrement de lichens terricoles était le plus élevé dans les peuplements dominés par des arbres de petite taille (<7 m) ou de taille intermédiaire (7–12 m) et le plus faible dans les peuplements avec un couvert forestier relativement important (>40 %) ou dans les peuplements d’âge intermédiaire (60–100 ans). La composition en espèces des communautés épiphytes a également été étudiée et certaines espèces ou genres étaient étroitement associés aux peuplements plus vieux (Alectoria sarmentosa (Ach.) Ach., Bryoria spp.), ou à des régions particulières le long du gradient est-ouest d’environ 1500 km (Evernia mesomorpha Nyl. à l’ouest, Bryoria spp. au centre et A. sarmentosa à l’est). En termes de conservation, ces résultats indiquent que les communautés de lichens épiphytes sont potentiellement sensibles à l’exploitation préférentielle des vieux peuplements. Les communautés de lichens épiphytes et terricoles sont aussi potentiellement sensibles aux effets appréhendés des changements climatiques tels que l’augmentation de la fréquence des feux ou l’augmentation de la croissance de la forêt.

In boreal forests, partial cutting is increasingly proposed as a suitable alternative to the widespread use of clearcutting in order to conciliate forest management with habitat conservation for epiphytic species. We compared the growth of two epiphytic lichen species, Bryoria nadvornikiana and Evernia mesomorpha, in old forest stands recently treated by partial cutting and untreated controls, located in black spruce boreal forests of western Québec. Lichen growth rates were measured over a period of two years from transplants of the two species, and several environmental variables (e.g., canopy openness, thallus temperature, and thallus wetness) were also measured directly at the sampling sites. Despite important within-treatment variation in growth rates among transplants, we observed reduced growth rates in partial cuts for both species. Canopy openness measurements of more than 40% resulted in negative growth rates for B. nadvornikiana, a species typically associated with relatively closed canopies, and canopy openness over 70% resulted in negative growth rates for E. mesomorpha, a species that tends to be associated with open canopies. This negative growth response contrasts with what is generally reported in the literature about the effect of canopy opening creation on epiphytic lichen growth. As a function of the environmental parameters that were measured on site, we suggest that a reduction in the duration of hydration periods, and an increased risk of thallus fragmentation in partially cut stands, especially for B. nadvornikiana, could explain this result. Because this negative effect could be more likely to occur during dry periods, future trends in the response of epiphytic lichens to the creation of canopy openings could be influenced by climate change. This study suggest that even if partial cuts can be a good alternative to clearcutting for the conservation of epiphytic lichen species, they are more likely to succeed if dense clumps of residual trees (canopy cover > 70%) are retained in the treated stands.

The northern Témiscamingue region (western Québec) sustained regional-scale pulses of natural disturbances during the 1850–2000 period, such as severe fires during the 1908–1926 period, two severe spruce budworm outbreaks that occurred in 1909–1918 and 1974–1984, and two birch dieback episodes around 1940 and 1980. These disturbances produced synchronous fluctuations in forest characteristics over large spatial scales. In this paper, we review possible responses of flora and fauna to pulsed large-scale disturbance events and speculate on whether they should be emulated to reduce the impacts of forest management on non-timber resources. The importance of large-scale disturbance pulses for biodiversity and forest ecosystem integrity is potentially great, but this aspect has been poorly investigated by previous research, and thus there is little information available to guide forest management. Large-scale, synchronous disturbances could be emulated by clustering harvesting activities in time, for example by creating “harvest pulses” of 10–20 years, separated by periods of 50–100 years or so with low harvest rates. A potential disadvantage of this strategy is that when our capacity to predict future natural disturbances is low, there is a higher probability of accidentally taking the forest ecosystem outside of the range of natural variability compared with a status quo forest management scenario. From a socio-economic perspective, another potential disadvantage is in creating irregular wood flows to the forest transformation industry. Nonetheless, in a context where the forest has been over-disturbed in the recent past, a forest management strategy involving fluctuating harvest rates could provide the means for faster ecosystem recovery compared with a status quo strategy. We recommend that the potential importance of disturbance pulses for boreal and sub-boreal ecosystems be more thoroughly investigated by future research to inform management and conservation policies. © Elsevier B.V. All rights reserved.

In order to assess the long-term spatiotemporal influence of the spruce budworm in sub-boreal mixedwood forests, we studied the effect of three successive outbreaks in a region of western Quebec, Canada. We used dendrochronology to detect past outbreaks in three areas (111–185 ha), based on the recruitment age of balsam fir (Abies balsamea) and on growth patterns of white spruce (Picea glauca), the two main host species of this defoliating insect. We also used a series of aerial photographs taken between 1935 and 2003 to evaluate overstory mortality and post-outbreak succession patterns in these same areas. Individual outbreaks had a spatially homogenous impact on host species throughout the region, but successive outbreaks differed in intensity: the two outbreaks around 1910 and 1980 caused widespread mortality in the overstory, but an outbreak around 1945 had little impact, probably because the forest mosaic had not yet recuperated from the 1910 outbreak. No clear outbreak was detected in the later part of the 19th century. In portions of the study areas where the 1910 outbreak had a major impact, between 36% and 50% of the stands were reoccupied by balsam fir stands in the period up to the 1980 outbreak (cyclic succession), the rest being at least partly replaced by nonhost species such as Betula spp. Changes in forest composition after the 1910 outbreak were mostly associated with upper-slope positions in all study areas. The 1980 outbreak also had a higher impact than earlier outbreaks in lower-slope positions dominated by black spruce (Picea mariana)–balsam fir mixtures. These results suggest that, at the regional scale, the abundance of mature or over-mature balsam fir stands does not determine the outbreak cycle. When an outbreak occurs, however, its impact will be strongly constrained by forest characteristics such as stand composition and structure, which are themselves influenced by previous disturbances and slope position.

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

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

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

Depuis quelques années, l'étude des perturbations naturelles comme les feux et les épidémies d'insectes fait l'objet d'un intérêt croissant. Il est important de connaître ces régimes de perturbations puisqu'ils sont les principaux facteurs dynamiques dans les écosystèmes forestiers créant des habitats nécessaires au maintien de la biodiversité. Dans les forêts mixtes du nord du Témiscamingue, où le cycle des feux est relativement long, les peuplements de fin de succession occupent une proportion importante de la mosaïque forestière. Ces peuplements, non affectés par les feux depuis au moins 150 ans, sont généralement dominés par des espèces comme le sapin baumier (Abies balsamea), les bouleaux (Betula spp.), les érables (Acer spp.) et épinettes (Picea spp.). Cette thèse a pour objectif premier d'examiner l'effet des épidémies successives de tordeuse des bourgeons de l'épinette (Choristoneura fumiferana) ayant sévi dans ces peuplements de fin de succession lors des 100 dernières années. Ces connaissances permettront de mieux comprendre le fonctionnement des écosystèmes forestiers mixtes, et d'adapter les pratiques d'aménagement forestier en conséquence.

L'analyse de placettes-échantillon permanentes, installées en 1972 puis remesurées en 1979 et 1996, indique que la dernière épidémie de tordeuse des bourgeons de l'épinette (1972-1984) a eu un effet dévastateur dans les peuplements dominés par le sapin baumier, la principale espèce hôte. En effet, 97.6 % des sapins baumiers de plus de 9 cm DHP sont morts suite à cette épidémie, une proportion qui s'élève à 63.5 % et 59.6 % pour les deux autres espèces potentiellement affectées, soit l'épinette blanche (Picea glauca) et l'épinette noire (Picea mariana) respectivement. Cette épidémie semble également responsable d'une importante mortalité indirecte, notamment chez le bouleau blanc, une espèce non-hôte. Cette mortalité, essentiellement postépidémique, est probablement due au choc physiologique faisant suite aux modifications environnementales (e.g. lumière, température) créées par la mortalité directe, ayant favorisé les dépérissements (mort-en-cime) ou des chablis dans les peuplements résiduels.

L'analyse combinée des cernes de croissances de l'épinette blanche (dendrochronologie) et d'anciennes photographies aériennes (1935-2003) révèle par ailleurs qu'une épidémie ayant eu lieu lors des années 1938-1948 n'a pratiquement pas causé de mortalité, alors que l'épidémie précédente (1909-1920) a eu un effet majeur, un peu comme celle de 1974-1984. Aucune épidémie majeure ne semble avoir eu lieu durant le 19e siècle, sauf peut-être vers 1830-40. La mortalité causée par les épidémies successives paraît donc très variable. Par ailleurs, les modèles de succession postépidémique montrent aussi une certaine variabilité d'une épidémie à l'autre. Ainsi, environ 50% des sites sévèrement affectés par l'épidémie de 1909-1920, autrefois présumément dominés par le sapin, ont été envahis par d'autres espèces suite à l'épidémie (surtout le bouleau blanc [Betula papyrifera]). À l'opposé, les peuplements sévèrement affectés par la dernière épidémie semblent très peu envahis par les espèces non-hôtes. En outre, l'envahissement ayant fait suite à l'épidémie de 1910 est significativement associé aux positions élevées sur le gradient topographique, suggérant que sur ces hauts de pente les sols sont relativement secs, et donc moins propices pour la germination et la survie d'une banque de semis de sapin baumier apte à remplacer le peuplement tué.

Les structures d'âge détaillées des peuplements vierges près du lac Sandeau indiquent que les épidémies successives créent des conditions favorables pour le maintien à long terme de peuplements mixtes hétérogènes, particulièrement sur les stations bien drainées. En créant d'importantes ouvertures dans le couvert forestier suite à la mortalité du sapin baumier à tous les 60-80 ans (comme en 1980, en 1910, et peut-être en 1830), les épidémies permettent un recrutement important d'espèces tolérantes et intolérantes à l'ombre. Alors que sur les bas de pente on observe généralement une ou deux cohortes, sur les hauts de pente bien drainés trois ou quatre cohortes peuvent co-exister dans une même parcelle, probablement grâce à la présence d'espèces qui vivent longtemps comme le thuya (Thuja occidentalis) ou le bouleau jaune (Betula alleghaniensis).

Nos résultats sur l'effet des épidémies de tordeuse des bourgeons de l'épinette et ceux d'études précédentes sur les feux montrent que dans cette partie de la zone de forêt mixte ou sub-boréale, les perturbations naturelles d'intensité variable dans le temps ont produit une mosaïque forestière plutôt irrégulière. Par exemple, les peuplements originaires des grands feux et épidémies des années 1900-1930 sont surreprésentés par rapport aux autres classes d’âge. L'abondance de certains habitats pour la faune, par exemple l'abondance de bois mort ou encore la quantité de jeunes peuplements en régénération, a fortement fluctué au fil du 20e siècle en réponse à ces perturbations naturelles. Cependant, la plupart des modèles d'aménagement actuellement en vigueur ont pour conséquence de régulariser cette structure forestière et d’en estomper la variabilité. La régularisation des structures d'âges par l'aménagement forestier pourrait altérer le fonctionnement des écosystèmes, une question qui a cependant fait l'objet de peu de recherches jusqu'à maintenant. Une approche préventive d'aménagement forestier pourrait consister à concentrer dans le temps et dans l'espace les coupes visant à émuler ces perturbations. Nous suggérons qu'une telle solution, associée à une planification forestière rigoureuse et d'autres mesures inspirées de l'approche par filtre brut, permettrait de maintenir une structure forestière hétérogène. Avant d'adopter des recommandations plus contraignantes sur le plan socio-économique, il serait impératif d'approfondir nos connaissances sur les liens potentiels entre les perturbations naturelles irrégulières dans l'espace et dans le temps, l'hétérogénéité forestière et la diversité biologique en forêt mixte. © 2005 UQAM tous droits réservés.