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

 

Permanent landscape attributes such as topography (elevation [m]) and microtopography (local variation in elevation [cm]) can increase the risk of cold air drainage down-slopes and in microtopographic depressions, causing important temperature gradients that generate localized growing-season frosts. Since most studies on growing-season frosts are restricted to the northern parts of the boreal forest or to mountainous areas, their negative consequences on tree productivity at the boreal-temperate forest ecotone is often ignored. We quantified the intensity and probability of growing-season frosts at the boreal-temperate forest ecotone in regard to topographic and microtopographic landscape attributes, which were extracted from airborne LiDAR surveys. In situ air temperature was measured for two summers (2016–2017) with 252 temperature loggers installed in two 18-years-old spruce plantations established in both temperate and boreal mixedwood forests. Growing-season frosts were more intense and probable at the boreal mixedwood forest site compared to the temperate forest site. Still, at both sites, when growing-season frost occurred, air temperature could vary by 4 °C along the elevation gradient of 15 m, often reaching sub-zero values at low elevation while reaching above-freezing values at high elevation. The importance of microtopography on the risk of sub-zero temperatures increased where frost events were less likely to occur such as at the temperate forest and at high elevation. Considering that growing-season frosts can considerably reduce tree productivity, the effects of both topography and planting microsite should be considered when determining where to establish plantations in the landscape and to determine suitable frost-free planting microsites within a plantation.

Reforestation in the boreal forest is challenging; trees must survive to large daily temperature variations and to the cold environment. Even if local tree species are adapted to withstand these harsh environmental conditions, spruce plantation failure after artificial regeneration occurs frequently, with important impacts on sustainable forest management. We hypothesized that this regeneration problem is caused by recurrent frost events occurring during the growing season. These events would freeze the terminal bud and the newly formed needles of the planted trees, thus limiting photosynthesis capacity and height growth. Our goal was to identify key permanent physical attributes of the landscape (elevation, slope shape and angular slope) and of microsite conditions (hole vs. mound) that best predict tree height and frost damage to foliage. In summer 2016 and 2017, we sampled tree height of 2,943 white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Mill.] B.S.P.) trees in 66 monoculture plantations aged between 6 and 13 years distributed in the Clay Belt region of Quebec (Canada), and environment prone to frequent growing-season frosts. Using linear and binomial mixed regression models, we analyzed the effects of the physical attributes of the landscape and of microsite conditions on tree height (linear) and on frost damage (binomial). Tree height increased with increasing elevation and when seedlings were planted on mounds compared to planted in holes. The impact of microsite conditions on tree height increased as plantations aged, but the importance of elevation on tree height decreased with age. The probability of frost damage to foliage decreased for trees planted on mounds compared to trees planted in holes and from concave to convex slopes. These relations were most important in young plantations, but trees showing growth problems were still shorter by 2 m, even 13 years after planting. We also observed differences between species: white spruce was significantly more damaged by frost and was smaller compared to black spruce. Therefore, growing-season frosts can cause growth suppression problems in white spruce plantations established in the boreal mixedwood region. Since microsite conditions also play a key role in driving plantation success, mechanical site preparation techniques should not only focus on reducing the competition between the planted trees and the competing vegetation but should also focus on limiting frost damage by planting trees on elevated microsites. Our results will support forestry practices limiting plantation failure in boreal mixedwoods.

Increase in frost damage to trees due to earlier spring dehardening could outweigh the expected increase in forest productivity caused by climate warming. We quantified the impact of growing?season frosts on the performance of three spruce species (white, black, and Norway spruce) and various seed sources with different frost tolerance in two plantations, established on both sides of the eastern Canadian boreal?temperate forest ecotone. The objectives of this study were to determine (a) if spruce species and seed sources planted in sites far from their natural provenance would be less adapted to local site conditions, leading to increased frost damage and reduced height growth; (b) at which height above the ground growing?season frosts ceased to damage apical meristems; and (c) if height growth was best predicted by extreme climatic events (growing?season frosts) or by mean annual or summer temperature. At each site and for all spruce species and seed sources, we cross?sectioned spruce trees at different heights above the ground. Tree rings were cross?dated and screened for frost rings, which were then given a severity score based on cellular damage. Frost severity reduced height growth of all spruce species and provenances at both sites. Height growth of the non?native Norway spruce was the most reduced by frost severity and was the smallest species at both sites. Frost caused the highest growth reduction in white spruce at the boreal mixedwood site and had the least effect on black spruce at both sites. For all spruce species, height growth was affected up to 2 m above the ground. Model selection based on corrected Akaike's information criteria (AICc) identified that minimum temperature in May was by far the best climate variable predicting tree growth (AICc weight = 1), highlighting the importance of considering extreme climatic events, which are likely to increase in the future.

Climate warming-driven early leaf-out is expected to increase forest productivity but concurrently increases leaf exposure to spring frosts, which could reduce forests' net productivity. We hypothesized that due to their damaging effect on buds, spring frosts exert a stronger control on bud phenology than do growing degree-days. We monitored bud flush phenology of three white spruce seed sources (one local seed source from the boreal mixedwood forest and two seed sources from the temperate forest), one black spruce seed source originating from the boreal mixedwood forest and four nonlocal Norway spruce seed sources in 2016 and 2017 in two plantations located on both sides of the temperate-boreal mixedwood forest ecotone in eastern Canada (Quebec). We aimed to determine inter- and intraspecies variations in bud break timing and sensitivity to air temperature and photoperiod. We expected that bud break timing for boreal species and seed sources would be better synchronized with the decrease in frost probability than for nonlocal species and seed sources. We used mixed binomial regressions and AICc model selection to determine the best environmental variables predicting each transition from one stage of bud phenology to the next. At both plantation sites, white spruce bud flush began and ended earlier compared to black and Norway spruce. Buds of all spruce species were sensitive to frost probability for early phenological stages, whereas growing degree-days controlled the remaining stages. Photoperiod sensitivity was higher for white spruce compared to black and Norway spruce and reached its maximum in the temperate forest. At intraspecies level, the two southern white spruce seed sources opened their buds earlier than the local source and were more sensitive to photoperiod, which increased their exposure to spring frosts. Onset of spruce bud flush is driven by spring frosts and photoperiod, but once started, bud phenology responds to temperature. The high photoperiod sensitivity in white spruces could counterbalance climate warming and limit future premature leaf-out, whereas the low photoperiod sensitivity in black spruce should not restrain leaf-out advancement with climate warming. Our results call for adapting the temperature-driven hypotheses of ecophysiological models predicting leaf-out to include spring frost probability.

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

Canada’s boreal forest represents an important contributor of the world’s wood supply industry. However, maintaining or increasing productivity of the boreal forest may be challenging in areas dominated by forested peatlands. Moreover, sustainable management of these forests must also consider other important aspects of the forest ecosystem such as biodiversity and carbon sequestration. To address these concerns, ecosystem-based management has been implemented in some Canadian jurisdictions, such as in regions where a large portion of the boreal forest is dominated by forested peatlands. The objectives of this paper are (1) to summarize our current understanding of how natural disturbances influence stand dynamics and biodiversity in forested peatlands, and (2) to review the main differences between natural and managed forest stands with respect to soil properties, stand productivity, understory plant communities. We also discuss how even-age management regime succeeds or fails to preserve old forests and how this loss affects both forest structure and habitat diversity at the landscape level. We conclude by showing how, in boreal forested peatlands, forest management could contribute to carbon sequestration and mitigate projected climate change.

Wildfires are a major driver of carbon stocks and ecosystem development in Canadian boreal forests, but there is little information on amounts and properties of the charcoal produced. Using data and samples available from a previous study, we determined amounts, depth distribution and chemical properties of visually-determined charcoal (>2 mm) in a boreal chronosequence in the Abitibi region of Quebec, Canada. Sites ranged from 24 to 2,355 years since fire (ysf) and originated from low- and high-severity soil burns (>5 or <5 cm organic horizon unburned, respectively). Two or three pits were sampled at 1-cm depth intervals from 20 jack pine (Pinus banksiana) sites (one low severity and 19 high severity) and 31 black spruce (Picea mariana) sites (12 low severity and 19 high severity). Site-level charcoal stocks ranged from 50 to 5,527 kg ha?1 with high within-site variability and lower stocks for the oldest sites. Depth distributions typically peaked around the organic-mineral interface, but some low-severity sites also had charcoal layers within the organic horizon. Means from 30 charcoal samples were 569 mg g?1 total C, 4.1 mg g?1 total N and 140 C/N (molar), with total C and C/N showing a trend of decline with time since fire, and total N showing an increase. Solid-state 13C CPMAS NMR spectra of nine samples showed high variability among the younger samples, but a trend to higher aromaticity for the older ones. A literature survey focusing on boreal forests similarly showed highly variable stocks and chemical properties of charcoal in organic horizon and upper mineral soil, with reduction of variance and lower stocks after several hundred years. This initial variation was also consistent with reports of highly variable temperatures and duration of charring in wildfires. Adding reports available for char production, and considering that most studies of char stocks and production are limited to the organic horizon (forest floor), suggests that initial production of charred material from boreal wildfires might be around 5–10 tons ha?1.

Fire is considered the major disturbance in boreal forests. Nonetheless, in several areas logging has become the primary driver of forest dynamics. In many areas of the boreal forest, stands may undergo paludification (i.e. the accumulation of thick, poorly decomposed organic layers over the mineral soil) in the prolonged absence of fire, which reduces forest productivity. Whereas high-severity fires (HSF) may restore forest productivity by burning the soil organic layer (SOL), low-severity fires (LSF) mainly burn the soil surface and do not significantly reduce SOL thickness. In the Clay Belt region of eastern Canada, an area prone to paludification, forest stands have historically been harvested by clearcutting (CC), but concerns about the protection of soils and tree regeneration lead to the replacement of CC by careful logging (CL). Whereas CC disturbs the SOL and is thought to favor tree growth, CL has little impact on the SOL. Furthermore, it has been suggested that prescribed burning after clearcut (CCPB) could also be used to control paludification. Using a retrospective approach, this study sought to understand how CC, CL, and CCPB compare to LSF and HSF with respect to soil properties, SOL thickness, vegetation ground cover, tree nutrition, and stand height in paludified black spruce stands of the Clay Belt region. HSF led to significantly taller trees than CL and LSF, but did not differ from CC and CCPB. Foliar N was significantly higher in HSF and CCPB sites relative to CL and LSF, with an intermediate value in CC sites. Ground cover of Rhododendron groenlandicum was significantly lower in HSF and CC sites relative to LSF, with intermediate values in CL and CCPB sites. Sphagnum spp. ground cover was significantly lower in HSF and CCPB sites relative to CL, with intermediate values in CC and LSF sites. High-severity fire sites had a significantly thinner SOL than the four other disturbances. Finally, regression tree analysis showed that SOL thickness represented the best predictor of tree height, whereas segmented regression showed that tree height was negatively correlated to SOL thickness and revealed a cut-off point circa 23 cm, which suggests that tree growth is impeded beyond this threshold. These results support the idea that management strategies intending to regenerate paludified forests should primarily aim at reducing organic layer thickness, either through mechanical disturbance or combustion.

It has been suggested that without sufficient soil disturbance, harvest in boreal forested peatlands may accelerate paludification and reduce forest productivity. The objectives of this study were to compare the effects of harvest methods (clearcutting vs. careful logging) and season (summer vs. winter harvest) on black spruce regeneration and growth in boreal forested peatlands of eastern Canada, and to identify the soil variables that favour tree growth following harvest. Moreover, we sought to determine how stand growth following harvest compared with that observed following fire. The average tree height of summer clearcuts was greater than that of summer carefully logged stands and that of all winter harvested sites. Summer clearcutting also resulted in a higher density of trees > 3 m and > 4 m tall and in a 50% reduction in Rhododendron groenlandicum cover, a species associated with reduced black spruce growth. Height growth of sample trees was related to foliar N and P concentrations, and to soil total N, pH and available Ca and Mg but not to harvest method or season. Our results also indicate that summer clearcutting could produce stand productivity levels comparable to those observed after high-severity soil burns. These results suggest that summer clearcutting could be used to restore forest productivity following harvest in forested peatlands, and offer further support to the idea that sufficient levels of soil disturbance may be required to restore productivity in ecosystems undergoing paludification.

The observed long-term decrease in the regional fire activity of Eastern Canada results in excessive accumulation of organic layer on the forest floor of coniferous forests, which may affect climate–growth relationships in canopy trees. To test this hypothesis, we related tree-ring chronologies of black spruce (Picea mariana (Mill.) B.S.P.) to soil organic layer (SOL) depth at the stand scale in the lowland forests of Quebec’s Clay Belt. Late-winter and early-spring temperatures and temperature at the end of the previous year’s growing season were the major monthly level environmental controls of spruce growth. The effect of SOL on climate–growth relationships was moderate and reversed the association between tree growth and summer aridity from a negative to a positive relationship: trees growing on thin organic layers were thus negatively affected by drought, whereas it was the opposite for sites with deep (>20–30 cm) organic layers. This indicates the development of wetter conditions on sites with thicker SOL. Deep SOL were also associated with an increased frequency of negative growth anomalies (pointer years) in tree-ring chronologies. Our results emphasize the presence of nonlinear growth responses to SOL accumulation, suggesting 20–30 cm as a provisional threshold with respect to the effects of SOL on the climate–growth relationship. Given the current climatic conditions characterized by generally low-fire activity and a trend toward accumulation of SOL, the importance of SOL effects in the black spruce ecosystem is expected to increase in the future. Content Type Journal Article

Ecosystem-based forest management is based on the principle of emulating regional natural disturbance regimes with forest management. An interesting area for a case study of the potential of ecosystem-based forest management is the boreal forest of north-western Que´bec and north-eastern Ontario, where the disturbance regime creates a mosaic of stands with both complex and simple structures. Old-growth stands of this region have multistoried, open structures, thick soil organic layers, and are unproductive, while young post-fire stands established following severe fires that consumed most of the organic soil show dense and even-sized/aged structures and are more productive. Current forest management emulates the effects of low severity fires, which only partially consume the organic layers, and could lead to unproductive even-aged stands. The natural disturbance and forest management regimes differ in such a way that both young productive and old-growth forests could ultimately be under-represented on the landscape under a fully regulated forest management regime. Two major challenges for ecosystem-based forest management of this region are thus to: (1) maintain complex structures associated with old-growth forests, and (2) promote the establishment of productive post-harvest stands, while at the same time maintaining harvested volume. We discuss different silvicultural approaches that offer solutions to these challenges, namely the use of (1) partial harvesting to create or maintain complex structures typical of old-growth stands, and (2) site preparation techniques to emulate severe soil burns and create productive post-harvest stands. A similar approach could be applied to any region where the natural disturbance regime creates a landscape where both even-aged stands established after stand-replacing disturbances and irregular old-growth stands created by smaller scale disturbances are significant.

Abstract: In many northern forest ecosystems, soil organic matter accumulation can lead to paludification and forest productivity losses. Paludification rate is primarily influenced by topography and time elapsed since fire, two factors whose influence is often confounded and whose discrimination would help forest management. This study, which was conducted in the black spruce (Picea mariana (Mill.) BSP) boreal forest of northwestern Quebec (Canada), aimed to (1) quantify the effect of slope and time since fire on paludification rates, (2) determine whether soil organic layer depth could be estimated by surface variables that can potentially be remotely sensed, and (3) relate the degree of paludification to tree productivity. In this study, soil organic layer depth was used as an estimator of the degree of paludification. Slope and postfire age strongly affected paludification dynamics. Young stands growing on steep slopes had thinner organic layers and lower organic matter accumulation rates compared with young stands growing on flat sites. Black spruce basal area and Sphagnum cover were strong predictors of organic layer depth, potentially allowing mapping of paludification degree across the landscape. Tree productivity was negatively related to organic layer depth (R² = 0.57). The equations developed here can be used to quantify forest productivity decline in stands that are undergoing paludification, as well as potential productivity recovery given appropriate site preparation techniques.

Résumé : Dans plusieurs écosystèmes forestiers nordiques, l'accumulation de matière organique peut mener à la paludification des sols et entraîner des pertes de productivité forestière. Le taux de paludification est principalement influencé par la topographie et le temps écoulé depuis le dernier feu, deux facteurs dont l'influence est souvent confondue et dont la séparation aiderait à l'aménagement forestier. Cette étude réalisée dans la pessière noire (Picea mariana (Mill.) BSP) du nord-ouest du Québec (Canada) avait comme objectifs : (1) de quantifier l'effet de la pente et du temps depuis le dernier feu sur le taux de paludification, (2) de déterminer si l'épaisseur de la couche organique pouvait être estimée à partir de deux variables de surface quantifiables au moyen de la télédétection, et (3) d'établir la relation entre le degré de paludification et la productivité des arbres. Dans cette étude, l'épaisseur de la couche organique du sol a été utilisée pour estimer le degré de paludification. La pente et l'âge des peuplements après feu influencent fortement la dynamique de paludification. La couche organique était plus mince et avait un taux d'accumulation plus faible dans les jeunes peuplements établis sur des pentes fortes que dans les peuplements d'âge similaire établis sur terrain plat. La surface terrière en épinette noire et le recouvrement de sphaignes avaient un fort pouvoir de prédiction de l'épaisseur de la couche organique, ce qui pourrait permettre de cartographier le degré de paludification à l'échelle du paysage. Enfin, la productivité des arbres était négativement reliée à l'épaisseur de la couche organique (R² = 0,57). Les équations développées dans ce travail peuvent être utilisées pour quantifier le déclin de productivité forestière dans les peuplements sujets à la paludification, ainsi que le potentiel de récupération de productivité à la suite d'une préparation de terrain appropriée.

Long-term forest productivity decline in boreal forests has been extensively studied in the last decades, yet its causes are still unclear. Soil conditions associated with soil organic matter accumulation are thought to be responsible for site productivity decline. The objectives of this study were to determine if paludification of boreal soils resulted in reduced forest productivity, and to identify changes in the physical and chemical properties of soils associated with reduction in productivity. We used a chronosequence of 23 black spruce stands ranging in postfire age from 50 to 2350 years and calculated three different stand productivity indices, including site index. We assessed changes in forest productivity with time using two complementary approaches: (1) by comparing productivity among the chronosequence stands and (2) by comparing the productivity of successive cohorts of trees within the same stands to determine the influence of time independently of other site factors.

Charcoal stratigraphy indicates that the forest stands differ in their fire history and originated either from high- or low-severity soil burns. Both chronosequence and cohort approaches demonstrate declines in black spruce productivity of 50–80% with increased paludification, particularly during the first centuries after fire. Paludification alters bryophyte abundance and succession, increases soil moisture, reduces soil temperature and nutrient availability, and alters the vertical distribution of roots. Low-severity soil burns significantly accelerate rates of paludification and productivity decline compared with high-severity fires and ultimately reduce nutrient content in black spruce needles. The two combined approaches indicate that paludification can be driven by forest succession only, independently of site factors such as position on slope. This successional paludification contrasts with edaphic paludification, where topography and drainage primarily control the extent and rate of paludification. At the landscape scale, the fire regime (frequency and severity) controls paludification and forest productivity through its effect on soil organic layers. Implications for global carbon budgets and sustainable forestry are discussed. ©2007 ESA

Depuis des décennies, plusieurs ont soutenu que les effets de la coupe forestière et des feux de forêts étaient significativement différents et que ceci aurait des effets significatifs sur les processus des écosystèmes et sur la biodiversité. Toutefois, c’est seulement récemment que des quantités appréciables de données scientifiques ont été amassées à ce propos. Dans ce rapport, nous présentons une revue des similarités et des différences entre les effets écologiques des perturbations associées aux incendies et à la coupe forestière qui ont été observées dans la littérature scientifique. Les comparaisons des effets de ces perturbations sur de nombreux attributs forestiers (les débris ligneux grossiers, les éléments nutritifs du sol, la productivité forestière, la diversité des plantes et la réponse de la faune) sont présentées selon deux échelles spatiales distinctes : celle du peuplement et celle du paysage.

À l’échelle du peuplement, notre revue a révélé des différences significatives entre les coupes forestières et les feux de forêts tôt après la perturbation. D’un point de vue de la structure des peuplements, les forêts après incendie sont caractérisées par un plus grand nombre de chicots et moins de débris ligneux au sol, de même qu’un humus forestier significativement plus mince que celui des sites récoltés. De plus, même si les deux perturbations génèrent une augmentation dans la quantité d’éléments nutritifs extractibles dans le sol, l’intensité de cette augmentation est plus grande après feu qu’après coupe. Également, on observe une augmentation du pH du sol après un incendie contrairement à peu de changement ou à une faible baisse après coupe.

Tôt après la perturbation, les éléments de biodiversité diffèrent significativement dans les sites incendiés comparativement aux sites récoltés. Des communautés différentes de plantes vasculaires et non vasculaires du sous-bois colonisent généralement les sites incendiés et récoltés quoique les différences soient habituellement une question d’abondance plutôt que d’absence/présence des espèces. Comparativement au feu, les assemblages fauniques, que ce soit les mammifères, les invertébrés ou les oiseaux, semblent tous répondre différemment à la récolte. Parmi ces groupes fauniques, les espèces spécifiquement associées aux chicots étaient les plus susceptibles de montrer une réponse différente selon la perturbation (coupe ou feu).

Les espèces d’arbres répondent différemment selon la perturbation (feu ou coupe), la récolte favorisant l’établissement d’espèces feuillues (notamment le peuplier faux-tremble, Populus tremuloides) et d’espèces de conifères qui ne sont pas adaptées aux incendies telles que le sapin baumier (Abies balsamea). Par ailleurs, il y a typiquement plus d’arbres feuillus résiduels suite aux coupes à blanc qu’après incendies. Étant donné ce fait et à cause de la réponse différentielle des espèces d’arbres à ces deux types de perturbations, des dynamiques de succession divergentes, pour ce qui est de la composition des arbres, peuvent être observés entre les peuplements incendiés et récoltés. Même si nous avons observé une certaine variabilité entre les études, la productivité forestière est généralement similaire dans les sites incendiés et récoltés.

Lorsqu’on compare les effets des perturbations associées à la récolte et aux feux de forêts à des échelles temporelles plus longues, notre revue indique que la plupart des attributs forestiers, qui étaient différents tôt après la perturbation, convergeaient quelques décennies après perturbation. Néanmoins, les humus forestiers plus épais observés après la récolte, comparativement à ceux observés après un incendie, semblaient persister plusieurs décennies après la perturbation. De plus, même si les communautés fauniques deviennent plus similaires avec le temps, plus tard dans la succession, certaines espèces présentes dans les peuplements incendiés sont soit significativement moins abondantes ou soit absentes dans les peuplements récoltés d’âge similaire. Enfin, plusieurs études soulignent que même si les effets des perturbations associées aux feux et à la récolte ne diffèrent pas significativement après quelques décennies, il y a certains doutes qui subsistent quant à la capacité des coupes forestières à recréer le large éventail de variabilité naturelle observée à la suite d’un incendie.

Malheureusement, peu de recherches se sont penchées sur la comparaison entre les interventions sylvicoles innovatrices (rétention variable, coupe partielle, etc.) ou des techniques de préparation de terrain (brûlage dirigé, scarifiage, etc.) et ceux des feux de forêts. Toutefois, les quelques études qui comparent les effets des brûlages dirigés et les différents niveaux de rétention indiquent que ces pratiques peuvent atténuer certaines différences observées entre les coupes forestière et le feu tôt après la perturbation. Notre revue indique que comparativement aux feux, la coupe de récupération après feu peut avoir des effets significatifs sur les processus écologiques, les legs biologiques et l’abondance des espèces généralement rencontrées seulement après un incendie. L’enlèvement des arbres morts à la suite d’un incendie peut affecter la régénération des arbres, la composition du sousbois, l’abondance et la distribution du bois mort, l’habitat faunique et les propriétés du sol. Néanmoins, plusieurs de ces effets sont spécifiques au site. Par conséquent, il est nécessaire de déployer des efforts de recherche additionnels afin de soutenir les décisions d’aménagement et l’élaboration des politiques.

À l’échelle du paysage, la principale différence entre les régimes d’incendie et de récolte s’avère la distribution des classes d’âges des peuplements. La proportion des peuplements plus âgés que la période de révolution (habituellement 100 ans) tend vers zéro sous un régime de récoltes complètement normalisé, tandis qu’elle se situe autour de 35% sous un régime d’incendies dont la période de révolution est similaire. Cette différence fondamentale donne lieu à une perte significative des forêts comportant des stages avancés de succession dans les paysages aménagés, ce qui affecte les organismes qui sont essentiellement associés à de tels peuplements. Fait intéressant, puisque la coupe forestière n’est pas en mesure de recréer les conditions généralement retrouvées dans les jeunes peuplements incendiés, les paysages sous l’influence de la récolte seront également caractérisés par une réduction des peuplements en mesure de remplacer le rôle écologique des jeunes peuplements incendiés. Malheureusement, seulement quelques études ont effectué la comparaison empirique des effets des feux de forêts et de la récolte à l’échelle du paysage sous des périodes de révolution similaires. Néanmoins, la recherche démontre que les incendies produisent habituellement des paysages plus hétérogènes que des coupes à blanc, incluant plus d’îlots résiduels. Les incendies possèdent également une forme plus complexes et des bordures qui sont plus graduelles que les coupes à blanc.

En conclusion, notre revue a révélé deux principaux défis auxquels font face les gestionnaires forestiers afin de générer des effets écologiques similaires à ceux produits lors d’un incendie. D’abord, les gestionnaires ont besoin d’améliorer les pratiques forestières dans le but de minimiser les différences observées entre les jeunes peuplements coupés et les jeunes peuplements incendiés, particulièrement en ce qui concerne les débris ligneux grossiers et les conditions au sol. Ensuite, les gestionnaires doivent maintenir au sein des paysages aménagés des peuplements forestiers dont la composition et la structure sont similaires à celles retrouvées dans des peuplements matures et surmatures. De tels peuplements peuvent occuper une portion significative des paysages affectés par le National Council for Air and Stream Improvement feu. Ceci peut impliquer le rallongement de la période de révolution d’une certaine portion des peuplements au sein des paysages aménagés ou alors nécessiter l’application de méthodes de récolte innovatrices qui peuvent recréer la structure et la composition des arbres caractéristiques des stages avancés de succession. Cette revue conclue en identifiant les besoins de recherche future qui sont susceptibles d’aider à faire face à ces défis.

For decades, many have hypothesised that the effects of harvesting and wildfire differed significantly and that this would have significant effects on ecosystem processes and biodiversity. However, it is only recently that an appreciable amount of scientific data has emerged on this topic. In this report, we present our review of the similarities and differences between the ecological effects of fire- and harvesting-induced disturbances that have been noted in the scientific literature. Comparisons of the effects of these disturbances on numerous forest attributes (coarse woody debris, soil nutrients, productivity, plant diversity, wildlife response) are presented at two distinct spatial scales: stand and landscape.

At the stand scale, our review noted significant differences between harvesting and wildfire early after disturbance. Structurally, young post-fire stands are characterized by more snags, less downed woody debris, and significantly thinner forest floors than logged sites. Additionally, while both disturbances generate a pulse of extractable nutrients, the intensity of the pulse is greater after wildfire than clearcut harvesting and an increase in soil pH is observed after fire as opposed to little change or a slight decrease after harvesting.

Early after disturbance, biodiversity elements significantly differ between burned and logged sites. Dissimilar understory vascular and non-vascular communities generally colonize burned and logged sites, although differences are usually a question of abundance rather than species absence/presence. As compared to fire, faunal assemblages, be it mammals, invertebrates or birds, all seem to respond differently to harvesting. Among these faunal groups, species specifically associated with snags were the most likely to show a contrasting response to harvesting- and wildfire-induced disturbances.

Tree species respond differently to fire- and harvesting-induced disturbances, with harvesting favouring the establishment of deciduous species (notably trembling aspen, Populus tremuloides) and of coniferous tree species not adapted to fire such as balsam fir (Abies balsamea). Furthermore, there are commonly more residual deciduous trees in clearcuts than in fires. Because of this and the differential response of tree species to these two types of disturbances, divergent successional patterns with respect to overstory tree species compositions can be observed in burned and harvested stands. While we noted some variability among studies, stand and tree productivity are generally similar in burned and logged sites.

When the effects of harvesting- and wildfire-induced disturbances are compared at longer temporal scales, our review noted that most forest attributes that were reported as dissimilar early after disturbance converged a few decades post-disturbance. Nonetheless, thicker forest floors observed after logging as compared to fire appear to persist numerous decades after disturbance. Additionally, while faunal communities do become less different as time passes, late in succession, some species present in burned stands are either significantly less abundant or absent in similarly aged logged stands. Finally, several studies warn that while the effects of wildfire- and harvesting-induced disturbances do not significantly differ after a few decades, there is some concern about the ability of harvesting-induced disturbances to recreate the full range of natural variability observed during post-fire stand succession.

Unfortunately, little research has compared the effects of alternative silvicultural interventions (partial retention, partial cutting, etc.) or site preparation techniques (controlled burning, scarification, etc.) to the effects of wildfires. However, the few studies comparing the effects of post-logging control burns and different levels of retention indicate that these practices may attenuate some of the differences observed early after disturbance. Our review indicates that as compared to wildfire alone, salvage logging can have significant effects on ecological processes, biological legacies and the abundance of species commonly encountered only after fire. Removal of fire-killed trees can affect tree regeneration, understory composition, the abundance and distribution of dead wood, wildlife habitat, and soil properties. Nonetheless, many of these effects are site-specific; hence, additional investments in research are needed to support management decisions and policy development.

At the landscape scale, the main difference between fire and harvesting regimes is the distribution of stand age classes. The proportion of stands older than the rotation period (usually 100 yrs) tends toward zero under a fully regulated harvesting regime, while it is around 35% under a fire regime of similar rotation period. This fundamental difference results in a significant loss of advanced seral stage forests in managed landscapes, thereby affecting organisms that are primarily associated with such stands. Interestingly, since harvesting-induced disturbances are unable to recreate the conditions commonly found in young burned stands, landscapes under the influence of harvesting will also be characterized by a reduction of stands capable of replacing the ecological role of young burned stands within landscapes. Unfortunately, only a few studies have empirically compared the effects of wildfire and harvesting at the landscape scale under similar rotation periods. Nonetheless, research demonstrates that fires usually produce more heterogeneous landscapes than clearcuts, with more remnant islands. Fires are also more complex in shape, and have edges that are more gradual than clearcuts.

In conclusion, our review reveals two main challenges faced by forest managers in order to generate similar ecological effects as produced by fire. First, managers need to improve management practices in order to minimize the differences observed between young post-harvest stands and young post-fire stands, particularly with respect to coarse woody debris and soil conditions. Second, managers need to maintain some areas with the tree species composition and structural attributes characteristic of over-mature fire-origin stands. Such stands can occupy a significant portion of fire-mediated landscapes. This may necessitate lengthening the rotation period of a certain proportion of stands within managed landscapes or may require the application of alternative harvesting methods that can recreate the structure and tree composition characteristic of advanced seral stage stands. This review concludes by identifying future research needs that might help meet these challenges.

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m)2 y)1) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.

Question and Location: How does soil burn severity and early post-fire tree composition affect long-term understorey vegetation dynamics in the coniferous forests of eastern Canada? Method: Vegetation dynamics were assessed using paleoecological methods and a chronosequence analysis of extant stands. The relation between environmental factors and succession was evaluated using ordination techniques on the chronosequence data. Understorey succession was studied by regression analysis on the chronosequence data and through within-site Markovian transition probabilities between successive 1-cm layers of plant macroremains from soil organic matter profiles. Results: Initial tree composition (Picea mariana and Pinus banksiana) had little effect on understorey composition. Soil burn severity (measured as the thickness of the residual forest floor humus) significantly affected temporal changes in understorey species. Following fires of high severity, stands underwent a gradual paludification with a net increase in Sphagnum and ericaceous shrubs (Ledum groenlandicum), and a decrease in feathermosses. Paludification was accelerated after low severity fires, which led to the dominance of Sphagnum less than 200 years after fire, and of L. groenlandicum shortly after fire. In situ paleo-ecological work confirmed results obtained with the chronosequence analysis. Conclusions: One vegetation gradient related to time after disturbance is insufficient to account for the full complexity of longterm changes in understorey composition following fire. Current forestry practices that protect the forest floor humus may induce a premature paludification.