The boreal forest holds the world's largest soil carbon (C) reservoir. A large portion of it is contained in a thick organic layer originating from the slow decay of bryophytes. Because a thick organic layer slows down tree growth, reduces forest productivity, and thereby reduces the potential wood supply, silvicultural treatments that aim to maintain or restore forest productivity after harvesting often involve mechanical site preparation. However, while these treatments can increase growth and C storage in trees, they can also lead to accelerated decomposition of the soil organic matter, reducing C storage. In this study, we assessed the nine-years effect of two silvicultural treatments on soil C dynamics in forested peatlands of northwestern Quebec, compared to unharvested controls: (1) cut with protection of regeneration and soils (CPRS; low soil disturbance, also called careful logging around advanced growth (CLAAG)), (2) CPRS followed by mechanical site preparation (CPRS + MSP, plowing; severe soil disturbance). The mass loss rate of three bryophytes (Pleurozium schreberi, Sphagnum capillifolium, and Sphagnum fuscum) was measured over two growing seasons together with soil organic carbon (SOC) stocks. We also studied the different effects of temperature, water table level, depth, and type of soil layer on mosses decomposition.We observed a significant influence of silvicultural treatments, bryophyte species, and soil layer type (fibric, mesic, humic and mineral) on bryophyte mass loss, which was higher in the CPRS + MSP treatment (21.6 ± 0.13 % standard error) than in control sites (9.5 ± 0.21 %); CPRS alone resulted in an intermediate mass loss of 11.6 ± 0.23 %, for Sphagnum mosses. Bryophyte mass loss was significantly higher in fibric than humic layer. SOC stocks in the uppermost organic soil layer (fibric) were lower in the CPRS + MSP group than in the control group, while the CPRS group was intermediate; however, differences were not statistically significant for the other soil layer and for total SOC. We conclude that while CPRS + MSP accelerates Sphagnum moss decomposition in the topsoil layer, it has limited impact on total soil C stocks that are detectable with stock change methods.

Sustainable forest management relies on a diversity of harvesting practices to conserve the variety of stand structures and compositions found in natural forests. Extensive use of clearcuts can homogenize stand structure by removing all the canopy, damage forest soils and destroy older downed woody debris already present within stands. In contrast partial cuts maintain some standing overstory and conserve certain biological legacies like large live trees as well as standing and downed dead wood, and thus should better conserve biodiversity. However, given the large number of species potentially affected by harvest operations, balancing harvesting intensity and conservation of biodiversity requires a clear understanding of habitat requirements of resident organisms and species conservation post-harvest. We examined the impacts of increasing intensities of stem removal (33%, 40%, 66% and 100%) 20 years after harvest on bryophyte communities in three stand types common in the succession sequences of Eastern boreal mixedwood forests that were dominated by trembling aspen, mixedwood and conifers. While many generalist and common forest species were shared among all harvesting levels and forest types, more specialized species like liverworts differed strongly between uncut and clearcut stands. However, liverworts and other specialists differed less between uncut and partial cut stands. Bryophyte species in hardwood dominated stands tolerated more fluctuations in environmental conditions than species in mixed stands. We were unable to find habitat specialists typically associated with coniferous stands and may be related to prior outbreaks of spruce budworm that occurred 30 years ago. Our results highlight how harvest intensity and forest stand type interact to affect bryophytes and how intact stands may be required to conserve many species that are sensitive to any degree of harvesting.

Wetlands and predation in boreal ecosystems play essential roles throughout the breeding season for bird assemblages. We found a positive association of beaver activity and a negative influence of American red squirrels (Tamiasciurus hudsonicus) on bird assemblages. We used a multispecies hierarchical model to investigate whether bird communities differ between two major wetland habitats in boreal Canada: beaver ponds and peatland ponds. In addition to including variables such as forest cover and latitude, we adopted a structural equation model approach to estimate the occupancy of American red squirrels and its potential influence on bird communities. Using automated recording stations deployed at 50 ponds, we detected 96 bird species in 2018 and 2019. Bird species were grouped into four taxonomic guilds according to their habitat successional requirements: early successional species, late successional species, generalists, and wetland species. Beaver ponds harbored higher species richness, a pattern driven primarily by early successional species. The occupancy of almost a quarter of the species was lower in the presence of red squirrels. Late successional species responded positively to the cover of forest surrounding the pond. Our results highlight the value of considering acoustic data of red squirrels to quantify habitat quality in boreal forests. We conclude that beaver activity shapes bird assemblages through modification of their habitat, and that some bird guilds are associated negatively with the presence of American red squirrels.

Les milieux humides et la prédation dans les écosystèmes boréaux jouent des rôles essentiels tout au long de la saison de reproduction pour les assemblages d’oiseaux. Nous avons trouvé une association positive entre les assemblages d’oiseaux et l’activité des castors, mais une association négative entre les assemblages d’oiseaux et la présence des écureuils roux d’Amérique (Tamiasciurus hudsonicus). Nous avons utilisé un modèle hiérarchique multi-espèces pour étudier si les communautés d’oiseaux diffèrent entre deux importants types de milieux humides du Canada boréal: les étangs de castors et les étangs de tourbières. Au-delà de l’inclusion de variables telles que le couvert forestier et la latitude, nous avons adopté une approche de modèle d’équation structurelle pour estimer l’occupation de l’écureuil roux d’Amérique et son influence potentielle sur les communautés d’oiseaux. En utilisant des stations d’enregistrement automatisées déployées sur 50 étangs, nous avons détecté 96 espèces d’oiseaux en 2018 et 2019. Les espèces des oiseaux ont été regroupées en quatre guildes taxonomiques en fonction de leurs exigences en matière de succession de l’habitat: espèces de début de succession, espèces de fin de succession, généralistes et espèces des milieux humides. Les étangs de castors abritaient une plus grande richesse d’espèces et une composition d’espèces différente de celle des étangs de tourbières, une tendance principalement liée aux espèces de début de succession. L’occupation de près d’un quart des espèces était plus faible en présence de l’écureuil roux. Les espèces de fin de succession ont répondu positivement à la couverture de la forêt entourant l’étang. Nos résultats soulignent l’intérêt de considérer les données acoustiques de l’écureuil roux pour quantifier la qualité de l’habitat dans les forêts boréales. Nous concluons que l’activité des castors façonne les assemblages d’oiseaux en modifiant leur habitat et que certaines guildes d’oiseaux sont associées négativement à la présence de l’écureuil roux d’Amérique.

Understanding biodiversity patterns and its environmental drivers is crucial to meet conservation targets and develop effective monitoring tools. Inconspicuous species such as lichens require special attention since they are ecologically important but sensitive species that are often overlooked in conservation planning. Remote sensing (RS) can be particularly beneficial for these species as in combination with modelling techniques it allows planners to assess and better understand biodiversity patterns. This study aims to model the lichen α-diversity (species richness) and β-diversity (species turnover) biodiversity components using high resolution RS variables across a subarctic region in Northern Quebec (∼190.25 km2). Two sensors, one commercial (WorldView-3, WV3) and another freely accessible (Sentinel-2, S2), at different resolutions (1.2 m and 10 m, respectively) were tested separately to develop our variables and feed the models. Lichens were sampled in 45 plots across different habitat types, ranging from forested habitats (coniferous, deciduous) to wetlands (bogs, fens) and rocky outcrops. Two sets of uncorrelated variables (Red and NIR; EVI2) from each sensor were parallelly used to build the α- and β-diversity models (8 models in total) through Poisson regressions and generalized dissimilarity modelling (GDM), respectively. Red and NIR variables were useful for modeling the two biodiversity components at both resolutions, providing information on stand canopy closure and structure, respectively. EVI2, especially from WV3, was only informative for assessing β-diversity, providing similar information than Red. Poisson models explained up to 32 % of the variation in lichen α-diversity, with Red, NIR and EVI2, either from WV3 or S2, showing negative relationships with lichen richness. GDMs described well the relationship between β-diversity and spectral dissimilarity (R2 from 0.25 to 0.30), except for the S2 EVI2 model (R2 = 0.07), confirming that more spectrally and thus environmentally different areas tend to harbor different lichen communities. While WV3 often outperformed the S2 sensor, the latter still provides a powerful tool for the study of lichens and their conservation. This study contributes to improve our knowledge and to inform on the use of RS to understand biodiversity patterns of inconspicuous species, which we consider to be an essential step to enhance their representation in conservation planning.

Rare habitats result from the geochemistry, history, or topography of the environment. They shelter regionally rare species (i.e., those with only a few mentions) and are threatened by global changes and human activities. On the clay plains of western Québec, the scattered highest hills emerged from the proglacial lake Ojibway (10,200–8200 BP) as islands and were potentially colonised 2000 years before their present surroundings. This study aims to determine if the effect of this earlier colonisation is still visible today in comparison with nearby formerly submerged hills. To do so, vascular plants and bryophytes were sampled for all habitats (i.e., old-growth forest, wet rock, and dry rock) at matched pairs of formerly emerged and submerged hills with known disturbance histories. Hourly temperature and relative humidity were recorded the top and bottom of each hill for one year. We found that former islands housed richer bryophyte communities than formerly submerged hills while the opposite was true for vascular plants. Old-growth forest was the richest habitat. Although hilltop temperatures are higher than those of the plains, the former islands that made up the higher hills were colder and wetter than the smaller, formerly submerged hills. This did not affect the diversity of bryophytes, but it did have an impact on the diversity of most vascular plant life forms. We believe that the rugged higher hills create more microclimatic heterogeneity and are fire and climate refugia. Disturbances explains a higher vascular plant diversity on lower hills. We conclude that no former island effect exists as such, but rather a complex climate and fire refugia resulting of a more rugged topography in former islands that allowed old-growth dependent species to accumulate through millennia in the right habitat structure. These results give new perspectives to discriminate apparently common habitats.

Natural and anthropogenic causes have produced changes in tree dominance from coniferous to broadleaf deciduous forests, generating shifts in litter inputs and plant understory composition. The impact of changes in canopy-associated factors on belowground microbial communities remain poorly understood. The objective of this study was to better understand how abiotic and biotic factors in black spruce and trembling aspen forests shape soil microbial community structure.

Global change is shifting ecosystem type relative abundance in boreal forests, while the green energy transition results in increased mining activities around the globe. The interaction and consequent effects of these two trends on biodiversity have not been examined in depth. Bryophytes species can be used as indicators to measure these effects because they play key ecological roles in boreal forests. We identified and evaluated the interaction between ecosystem type (i.e., coniferous, deciduous, mixed forest and open canopy) and mining on microhabitat scale bryophyte diversity and composition in 1-km landscapes surrounding six mine sites at different stages of the mining lifecycle in the Canadian boreal forest. Irrespective of microhabitat type, the combined effects of ecosystem type and mining stage were interactive on bryophytes. Bryophyte richness and community composition were negatively affected by offsite effects of mines in only deciduous and mixed forests. The interacted effects on bryophyte richness mainly occurred on the ground r microhabitats. We also found that deciduous, mixed forests (coniferous forest as a reference) and mines had a negative impact on the abundance of feather mosses and sphagna. Furthermore, indicator species were identified for areas affected by mines (Pohlia nutans and Dicranum polysetum) and for control areas (Sphagnum angustifolium and Plagiomnium cuspidatum). Our results suggest the predicted ecosystem shifts with global changes, from coniferous to deciduous forests, could potentially increase the effects of mining on forest ecosystem resistance through the changes in bryophyte community structure. Adding microhabitats (i.e., adding coarse woody debris) near mine sites is a potential strategy to maintain species richness. Collectively, these findings advance our understanding of how mining affects biodiversity and highlight the importance of considering mine offsite landscapes in future environmental evaluations of development projects in the context of global changes.

Sustainable management of forest ecosystems requires the use of reliable and easy to implement biodiversity and naturalness indicators. Tree-related microhabitats (TreMs) can fulfill these roles as they harbor specialized species that directly or indirectly depend on them, and are generally more abundant and diverse in natural forests or forests unmanaged for several decades. The TreM concept is however still recent, implying the existence of many knowledge gaps that can challenge its robustness and applicability. To evaluate the current state of knowledge on TreMs, we conducted a systematic review followed by a bibliometric analysis of the literature identified. A total of 101 articles constituted the final corpus. Most of the articles (60.3%) were published in 2017 or after. TreM research presented a marked lack of geographical representativity, as the vast majority (68.3%) of the articles studied French, German or Italian forests. The main themes addressed by the literature were the value of TreMs as biodiversity indicators, the impact of forest management on TreMs and the factors at the tree- and stand-scales favoring TreMs occurrence. Old-growth and unmanaged forests played a key role as a “natural” forest reference for these previous themes, as TreMs were often much more abundant and diverse compared to managed forests. Arthropods were the main phylum studied for the theme of TreMs as biodiversity indicators. Other more diverse themes were identified, such as restoration, remote sensing, climate change and economy and there was a lack of research related to the social sciences. Overall, current research on TreMs has focused on assessing its robustness as an indicator of biodiversity and naturalness at the stand scale. The important geographical gap identified underscores the importance of expanding the use of the TreMs in other forest ecosystems of the world. The notable efforts made in recent years to standardize TreM studies are an important step in this direction. The novelty of the TreM concept can partially explain the thematic knowledge gaps. Our results nevertheless stress the high potential of TreMs for multidisciplinary research, and we discuss the benefits of expanding the use of TreMs on a larger spatial scale.

Intraspecific trait variability (ITV) provides the material for species' adaptation to environmental changes. To advance our understanding of how ITV can contribute to species' adaptation to a wide range of environmental conditions, we studied five widespread understorey forest species exposed to both continental-scale climate gradients, and local soil and disturbance gradients. We investigated the environmental drivers of between-site leaf and root trait variation, and tested whether higher between-site ITV was associated with increased trait sensitivity to environmental variation (i.e. environmental fit). We measured morphological (specific leaf area: SLA, specific root length: SRL) and chemical traits (Leaf and Root N, P, K, Mg, Ca) of five forest understorey vascular plant species at 78 sites across Canada. A total of 261 species-by-site combinations spanning ~4300 km were sampled, capturing important abiotic and biotic environmental gradients (neighbourhood composition, canopy structure, soil conditions, climate). We used multivariate and univariate linear mixed models to identify drivers of ITV and test the association of between-site ITV with environmental fit. Between-site ITV of leaf traits was primarily driven by canopy structure and climate. Comparatively, environmental drivers explained only a small proportion of variability in root traits: these relationships were trait specific and included soil conditions (Root P), canopy structure (Root N) and neighbourhood composition (SRL, Root K). Between-site ITV was associated with increased environmental fit only for a minority of traits, primarily in response to climate (SLA, Leaf N, SRL). Synthesis. By studying how ITV is structured along environmental gradients among species adapted to a wide range of conditions, we can begin to understand how individual species might respond to environmental change. Our results show that generalisable trait–environment relationships occur primarily aboveground, and only accounted for a small proportion of variability. For our group of species with broad ecological niches, variability in traits was only rarely associated with higher environmental fit, and primarily along climatic gradients. These results point to promising research avenues on the various ways in which trait variation can affect species' performance along different environmental gradients.

Even-aged silviculture based on short-rotation clearcuts had severely altered boreal forests. Silvicultural alternatives (e.g. continuous cover or retention forestry) have the potential to restore and protect the habitats and functions of boreal forests. These alternatives are however often restricted to structurally complex old-growth forest, which are particularly threatened by anthropogenic disturbances. Increasing the use of alternatives to even-aged silviculture in early-successional stands could help recruit more structurally complex forests, with characteristics closer to the old-growth. In this article, we therefore evaluate the potential for silvicultural alternatives to even-aged management in boreal forests that burned less than a century ago.

We analysed 1085 field plots in a 243,000 km² area situated in the boreal forest of eastern Canada. These plots burned 30–100 years before the survey and had not been subjected to previous or subsequent anthropogenic disturbance; they hence represent young primary forests. The main patterns of tree diameter distribution variation within the plots were identified using k-means clustering. Stand structure, tree species composition and environmental variables that most explained the differences among the clusters were identified with a random forest model, and then compared using Kruskal–Wallis and Fisher's exact tests.

The majority (>75%) of the plots presented an irregular structure of stem diameters (i.e. non-normally distributed, with many small diameter trees). The understorey was generally dominated by black spruce (Picea mariana [Mill.] BSP), a shade-tolerant species. Irregular structures were observed in both forests of high and low productivity, implying that different processes (e.g. early regeneration, variable tree growth) can lead to observed early irregular structure. Regular structures were generally characterized by a higher productivity and abundance in hardwood species compared to the irregular structures.

Synthesis and applications. Many boreal forests of eastern Canada progress towards an irregular structure in the decades following the last stand-replacing fire. A substantial part of these early-successional forests may be suitable for alternatives to even-aged silviculture that better maintains habitats and functions of preindustrial boreal forests.

Aim

Detection of rare species is limited by their intrinsic nature and by the constraints associated with traditional field surveys. Remote sensing (RS) provides a powerful alternative to traditional detection methods through the increasing availability of RS products. Here, we assess the capacity of RS at high and medium resolution to detect rare plants with direct and indirect approaches, and how the performance of RS can be influenced by the characteristics of species.

Methods

An extensive literature review was conducted to synthesize the use of RS to detect or predict rare plant occurrence at high and medium resolution (<30 m and 30–300 m, respectively). The concept of “rarity” was based on Rabinowitz's rare species classification. The literature review was performed in Scopus for the period 1990–2020.

Results

While direct detection is often limited, it is possible with high and very high spatial resolution data for rare plants with distinctive traits. RS is also able to capture biophysical conditions driving rare plant distributions, which can indirectly provide accurate predictions for them. Both approaches have the potential to discover new populations of rare plants. RS can also feed SAMs of rare plants, which combined with SDMs can provide a valuable approach for rare plant detection. While direct detection is limited by the space occupied by a species within its habitat and its morphological, phenological and physiological characteristics, the predictive performance of RS-based SDMs (indirect detection) can be influenced by habitat size, habitat specificity and phenological features of rare plants. Similarly, model predictive performance can be influenced by the rarity form of the target species according to the rarity classification criteria.

Main conclusions.

With this synthesis, the strong potential of RS for the purposes of detection and prediction of rare plant has been highlighted, with practical applications for conservation and management.

Aim

Impacts of longitudinal gradients on species distributions have received less attention than latitudinal gradients. In Canada, precipitation varies longitudinally, with direct effects on plant growth and fire regimes. Despite the geographical extent of Canadian boreal forests, vascular plant diversity is relatively limited, with just under 300 species. Understorey communities comprise most of this diversity and play key roles in forests, affecting succession, nutrient cycling and wildlife habitat. Our objective was to evaluate the relative impacts of local and regional environmental conditions on vascular plant community composition and diversity in the boreal forest.

Location

Sampling took place in 33 trembling aspen (Populus tremuloides) stands along a transcontinental gradient in precipitation and fire regimes in boreal Canada.

Methods

We measured community composition and vegetation diversity indices of the vascular understorey in 400-m2 plots and measured environmental variables at the local (drainage, nutrient and light availability, site heterogeneity) and regional (annual temperature and precipitation, fire weather index, landscape heterogeneity) scales. Multivariate analyses and hierarchical model selection were used to analyze patterns.

Results

Species composition varied longitudinally, with western and eastern species pools. Western indicator species tended to be fire-adapted pioneer species, while indicator species at eastern sites were fire avoiders and late-successional species. Species richness and intra-site beta diversity seemed primarily driven by local variables and did not vary across the country. Species evenness, however, was slightly higher in the western region and decreased under higher precipitation, colder temperatures, and higher landscape-level heterogeneity.

Conclusion

Our results suggested that even for similar canopy composition environmental variables play a key role in the establishment of plant communities and structure local plant assemblages by selecting or eliminating species from the regional pool, which was also controlled by fire regimes.

Aim

The main objective of this study was to evaluate the influence of leachates from three typical boreal forest ground layers on young tree growth and to explore the linkages between the chemical composition of the leachates, tree growth, the allocation between belowground and aboveground parts, and ectomycorrhizal colonization.

Methods

An original 6-month greenhouse experiment was set up to investigate (i) the effects of lichen (Cladonia spp.) and feather moss (Pleurozium schreberii [Brid.] Mitt.) leachates on jack pine (Pinus banksiana Lamb.) growth and (ii) the effects of feather moss and Sphagnum spp. leachates on black spruce (Picea mariana [Mill.] B.S.P.) growth.

Results

Belowground growth and root allocation was reduced by lichen leachates in 2-year-old pine seedlings, while the impact was significant on both below- and aboveground growth in 6-month-old pine seedlings. A substance having the same migration time as usnic acid was detected in the lichen leachates by high-performance liquid chromatography. Sphagnum spp. and feather moss leachates were not found to have any effect on black spruce seedling growth, despite a higher supply of dissolved inorganic N in the feather moss leachates compared to the leachates of Sphagnum spp. and the control.

Conclusions

These results demonstrate that ground layer composition plays a crucial role in shaping the plant community in boreal ecosystems by influencing the chemical composition of the soil solution. They suggest that chemical interference may be another mechanism by which lichens promote the self-perpetuation of open woodlands in the closed-crown boreal forest.

Both low- and moderate-severity secondary disturbances are drivers of eastern Canadian boreal old-growth forests dynamics. Moderate-severity disturbances reflect mainly spruce budworm outbreaks. Low-severity disturbances are produced by both spruce budworm outbreaks and random events such as windthrow. Each level of disturbance severity has a specific impact on stand dynamics, and both shape the diversity of boreal old-growth forests.

Context

A regular succession of low-severity disturbances is seen as determining the dynamics of the old-growth stage (gap dynamics); however, recent studies suggest that moderate-severity secondary disturbances also play an important role in the dynamics of eastern Canadian boreal forests.

Aims

This study aims to determine if eastern Canadian boreal old-growth forests are driven by a combination of low- and moderate-severity secondary disturbances.

Methods

We reconstructed the 200-year disturbance history of 20 boreal old-growth stands using dendrochronological analysis. We discriminated low- from moderate-severity disturbances based on their respective influence on mean stand growth.

Results

The secondary disturbance regime of eastern Canadian boreal old-growth forests varies highly over time, reflected by disturbance peaks in the chronological record. Most peaks occurred during spruce budworm outbreaks related to both low- and moderate-severity disturbances. Between each peak, low-severity disturbances dominate. Each level of disturbance severity has specific consequences for stand dynamics.

Conclusion

Both low and moderate secondary disturbances are drivers of forest dynamics in eastern Canadian boreal old-growth stands and shape the structural diversity of these stands. The complexity of these dynamics should be recognized in management planning to ensure the efficiency of old-growth forest conservation policies.

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

Question

Changes in species richness and composition in plant communities as a result of edge creation are well documented in anthropogenically modified landscapes, but what happens after natural disturbance? We investigate the responses of boreal bryophytes to edge creation in a post-wildfire residual forest patch system.

Location

Boreal black spruce–feather moss forest, western Québec, Canada.

Methods

Bryophyte community was sampled in 50-m2 plots: 117 plots in 39 undisturbed forest cores (control) and 108 plots in 30 residual forest patches from wildfire, divided into 48 core plots, 30 north-facing edge plots and 30 south-facing edge plots. Temporal, severity, spatial and structural characteristics of the stands were also recorded to explain bryophyte community composition.

Results

Residual edges and cores were more species-rich than undisturbed cores, particularly north-facing edges, but community composition differed between undisturbed and residual cores. Indicator species of undisturbed cores and residual edges differed in their species traits. Spatial variables explained variations in composition, with a larger difference between core and edge communities in patches over 3 ha compared to smaller patches.

Conclusion

We observed community composition change in response to natural edge creation. While edge influence is reduced in large patches, the bryophyte community remains distinct from undisturbed cores. This community change is a ‘control’ in studies of boreal bryophyte response to anthropogenic edge creation, and suggests that we should consider the natural range of variability. Finally, this study emphasizes that forest remnants do not substitute for continuous and undisturbed forests.

The boreal zone of Canada is of increasing interest nationally and internationally for the natural resources it contains, but also as one of the last generally undeveloped regions of the world. It is also predicted to experience some of the fastest warming over the next decades, owing to climate change.

With this increased interest in the boreal zone, research has evolved from a focus on understanding and mitigating anthropogenic disturbances to include restoration of forest and peatland ecosystems after extreme disturbances. These disturbances that remove nearly all biological legacies of the previous ecosystem could force the sites into an alternate stable state, which may not provide the same desired ecosystem services. Consequently, research needs to be undertaken to ensure that ecosystems can be restored to the original or desired end states. However, the nature of these end states and how to determine when they are reached remain to be defined.

In this context, the symposium “Ecological impacts and restoration of industrial sites: roles of bryophytes and graminoid vascular plants” at the 2015 Botany conference in Edmonton, Alberta, Canada, featured research dealing with habitat restoration. This special issue consists of a commentary integrating the research in a theoretical context (Fenton) and four articles based on presentations from the symposium (Vitt et al., Hartsock et al., Rochefort et al., and Guittonny-Larchevêque et al.).

Vitt et al. and Hartsock et al. follow a reconstructed fen watershed, examining plant composition and net nitrogen mineralization to determine its similarity to natural ecosystems of a similar type. Rochefort et al. also deal with fen ecosystems, and examine ways to restore a peat-generating system on a bog exploited for horticultural peat down to the fen layer. In the article by Guittonny-Larchevêque et al., we move to a terrestrial ecosystem and examine the potential role of graminoids in stabilizing anthropogenic soils generated by mine tailings. Together these articles showcase different types of research programs being undertaken in Canada on ecosystem restoration.

Bryophytes and pteridophytes are important contributors to ecosystem services in boreal regions. Abiotic agents are considered their main dispersers, but recent studies suggest that biotic agents including invertebrates, birds and large mammals might also be efficient dispersal agents. Dispersal of cryptogams by ground-dwelling small mammals is often assumed to occur, but has yet to be demonstrated. In this study, we present the first evidence of boreal cryptogam species being dispersed by ground-dwelling small mammals. In 2013 and 2014, we recorded bryophyte cover and fern presence in 35 sites in black spruce forest. We also collected diaspores by brushing the fur of 99 ground-dwelling small mammals live-trapped in the same sites. Diaspores were then germinated on nutrient agar for six months. Viable diaspores of five bryophyte species and one fern species were successfully grown. No association was found between the cryptogam community sampled on site and the diaspore community grown on artificial substrate. Unlike abiotic agents that randomly disperse cryptogams, small mammals are more likely to transport diaspores to suitable substrates where microhabitat requirements for germination are met. Our results highlight the need to consider a broad spectrum of dispersal agents when focusing on the community dynamics of cryptogams.

Les cryptogames (bryophytes et ptéridophytes) représentent une composante fondamentale des régions boréales, et leur dispersion à moyenne et longue distances dépend principalement d’agents abiotiques. Des études récentes suggèrent pourtant l’importance d’agents biotiques tels que les invertébrés, les oiseaux et les grands mammifères comme vecteur de dispersion. La dispersion des cryptogames par les micromammifères est régulièrement admise mais n’a encore jamais été formellement examinée. Dans cette étude, nous présentons la première preuve tangible du transport des cryptogames boréaux par des micromammifères. Au cours des étés 2013 et 2014, nous avons estimé le couvert des bryophytes et la présence des fougères (communauté in situ) dans 35 sites en pessière noire à mousses. Nous avons brossé le pelage de 99 micromammifères de cinq espèces différentes capturés vivants dans les mêmes sites. Le matériel végétal collecté a été mis à germer durant six mois sur un gel nutritif d’agar. Nous avons observé la germination de cinq espèces de bryophytes et d’une espèce de fougère. Nous n’avons trouvé aucune association entre la communauté de cryptogames in situ présente dans les sites et la communauté germée sur un substrat artificiel. Contrairement à la dispersion aléatoire par des agents abiotiques, la dispersion par les micromammifères est plus susceptible de transporter les diaspores vers des substrats propices à leur germination. Nos résultats soulignent l’importance de considérer une vaste gamme d’agents de dispersion lors de l’étude des dynamiques des communautés de cryptogames.

Le biome boréal canadien est une mosaïque de forêts et de tourbières. Ces écosystèmes se sont développés dynamiquement, affectés périodiquement par des événements perturbateurs d’une étendue spatiale significative et de sévérités variables, réduisant la biomasse des écosystèmes. Les mêmes types d’écosystèmes se régénèrent typiquement à partir des systèmes biologiques antérieurs. Cependant, il y a de plus en plus d’inquiétude quant à l’impact de ces perturbations anthropiques différentes, particulièrement les effets cumulatifs des perturbations dont les changements climatiques, qui ouvrent la porte à des déplacements vers des états stables alternatifs. Une stratégie favorisant la régulation des perturbations anthropiques est la « hiérarchie des mesures d’atténuation » lors de projets de développement, où les impacts sur les écosystèmes sont évités, atténués, réhabilités ou compensés. Cette approche pratique n’est pas encore intégrée dans la théorie de la perturbation et de la résilience. L’auteur développe ici une vision intégrée de la hiérarchie d’attenuation et de la théorie de la résilience et de la perturbation, en contexte boréal, en utilisant des services écosystémiques pour mesurer l’état de l’écosystème dans un processus à deux étapes qui modélise d’abord la perte de la fonction de l’écosystème et qui intègre ensuite la hiérarchie des mesures d’atténuation et la théorie de la résilience. L’application de ce modèle est discutée dans un contexte d’études de la réhabilitation après différents types de catastrophes anthropiques. Ces études, dont certaines sont publiées dans ce numéro spécial, soulignent le rôle important des bryophytes et des plantes de sous-étage pour établir des cibles de réhabilitation et développer des critères et des indicateurs de succès.

Canada’s boreal biome is a mosaic of forests and peatlands. These ecosystems have developed dynamically, periodically affected by disturbance events of significant spatial extent and variable severity, reducing ecosystem biomass. The same ecosystem types typically regenerate from biological legacies. However, concern is growing about the impact of these different anthropogenic disturbances, particularly compound disturbances including climate change, which open the door to shifts to alternate stable states. One strategy promoted to regulate anthropogenic disturbance is the “mitigation hierarchy” for development projects, where impacts on ecosystems are avoided, mitigated, restored, or compensated. This practical approach is not yet integrated into disturbance and resilience theory. Here, I develop an integrated view of the mitigation hierarchy, as well as resilience and disturbance theory, in a boreal context using ecosystem services to measure ecosystem state in a two-step process that first models loss of ecosystem function and then integrates the mitigation hierarchy and resilience theory. The application of this model is discussed in the context of restoration studies after different types of catastrophic anthropogenic disturbance. These studies, some of which are published in this special issue, highlight the important role of bryophytes and understory plants in setting restoration targets and developing criteria and indicators of success.

Old-growth forests have sparked significant interest over the last twenty years and definitions have evolved from structure based to process based, acknowledging the diversity of forests that could be considered old growth. However studies frequently group all forests over a certain age into a single type, negating the dynamic processes that create old growth. In this study we examine a 2350-year chronosequence in boreal black spruce forests in northwestern Quebec to determine whether continued community change can be observed in the bryophyte layer. Bryophytes dominate the understory of boreal forests and influence ecosystem functioning, particularly in paludified forests where production exceeds decomposition in the organic layer. Community composition and richness changed throughout the chronosequence with no evidence of a steady state associated with an old-growth phase. In contrast the bryophyte community continued to evolve with multiple phases being evident. These results suggest that old-growth forests on the Clay Belt of northwestern Quebec and northeastern Ontario, Canada, should be regarded as part of the continuous gradient in forest development rather than a single state. This complicates conservation of these forests as multiple phases should be considered when planning forest reserves.

Context

Following forest harvest, mechanical site preparation (MSP) is commonly used to regenerate harvested sites. In boreal forested peatlands, however, the effectiveness of MSP to regenerate harvested sites is likely to be hampered by thick organic layers.

Aim

We sought to determine the capability of different MSP techniques to improve growth conditions of planted black spruce seedlings in boreal forested peatlands where closed-crown productive forests could revert to unproductive forested peatlands in the absence of severe soil disturbance.

Methods

The effects of disc scarification, mounding and patch scarification on soil chemistry and seedling growth were contrasted.

Results

Seedlings of site-prepared plots were 15% taller than those of untreated ones, irrespective of the MSP technique used, likely owing to the greater abundance of exposed mineral soil and mesic substrates created. Mounding and patch scarification were able to expose mineral soil over a greater proportion (>25% vs. <10%) of the treated area compared with disc scarification and control, whereas the combined surface area of exposed mineral soil and mesic substrates was higher in every MSP treatments relative to the control (>57% vs. 41%, respectively). Individual seedling growth was influenced by substrate type and drainage. Seedlings planted in moderately and well-drained mesic substrates and mineral soil were 25% taller than those planted in poorly drained fibric substrates.

Conclusion

All three MSP techniques were effective because they succeeded in creating high-quality microsites despite thick organic layers.

Abstract: In the eastern boreal forest of Canada long fire cycles allow for a significant portion of stands to become old growth. Theseold-growth boreal forest stands are subjected to secondary disturbances that create uneven structure, which supports avariety of types of organisms. In order to maintain the proportion of stands with uneven structure on the landscape, partialcuts have been suggested as a management technique that could create or maintain these uneven structures. This studycompares the effects of partial and low-retention harvests and un-harvested control on understory plants in four sites fiveyears after harvest. The relative abundance of species was examined by a habitat group. While richness did not vary amongtreatments, increasing severity of harvest favoured pioneer species that prefer disturbed mineral soil and high light levels.Sites with thicker organic layers that were harvested in the winter were significantly less impacted by both partial and lowretentionharvest. Canopy opening alone had little impact in the three most northern sites. As old-growth black spruceforests are open by nature, this suggests that soil perturbation is a bigger driver of community change after harvest thanproportion of canopy removed for this forest type.

Résumé: Dans la forêt boréale de l’Est du Canada, les longs cycles de feu permettent à une portion importante des peuplementsd’évoluer en vieille forêt. Ces peuplements sont soumis à des perturbations secondaires qui entraînent la création de structuresinéquiennes servant de support à une diversité d’organisme. Afin de maintenir, dans le paysage, une proportion depeuplement ayant une structure inéquienne, il est envisagé d’utiliser les coupes partielles en tant qu’outil sylvicole dansl’aménagement forestier pour créer ou maintenir des peuplements ayant des structures proches de celles que l’on retrouvedans les vielles forêts. Cette étude compare l’effet des coupes partielles et des coupes à faible rétention sur les plantes desous-bois dans quatre dispositifs cinq ans après la récolte. L’abondance relative des espèces a été examinée par groupe d’habitat.Alors que la richesse ne variait pas parmi les traitements, l’augmentation de la sévérité de la récolte a favorisé lesespèces pionnières qui préfèrent les sols perturbés et la lumière. Les sites, où la couche de matière organique est la plusépaisse et dans lesquels la récolte a été réalisée en hiver, ont été moins affectés par les coupes partielles et de faible rétention.L’ouverture de la canopée a eu peu d’impact dans les trois sites les plus nordiques. Les vieilles forêts d’épinette noireétant ouvertes naturellement, ceci suggère que les perturbations du sol ont un rôle plus important sur les changementsdans les communautés suite à la récolte que la proportion de la canopée récoltée dans ce type de forêt.

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 slow decomposition rate of boreal forest floor bryophytes contributes both to maintaining high soil C reserves as well as affecting conditions for tree growth by maintaining excessively high soil water content, cooling the soil and slowing nutrient cycles. In this study, mass loss of three bryophyte species (Pleurozium schreberi, Sphagnum capillifolium, S. fuscum) was measured in unharvested, partial cut and low-retention cut forest blocks. Mesh decomposition bags containing the three species and wood sticks were placed at two depths in colonies of either P. schreberi or S. capillifolium (environment) in the three harvest treatments and retrieved after two growing seasons. Mass loss was primarily related to substrate type (P. schreberi > S. capillifolium > wood sticks > S. fuscum) and secondarily to depth. Harvest treatment and environment (P. schreberi or S. capillifolium) only weakly affected sphagna mass loss. The weak effect of harvest treatment suggests that conditions created by low retention cuts do not to stimulate decomposition in this system and are not important enough to stimulate carbon loss, or to counteract paludification. On the other hand, the strong effect of bryophyte type indicates that conditions affecting bryophyte colonization and succession are of great importance in driving carbon and nutrient cycles.

Résumé : Afin de préserver les sols et la régénération durant la récolte, la circulation de la machinerie forestière est fréquemment restreinte à des sentiers définis. Toutefois, cette pratique est remise en question dans les peuplements d’épinette noire (Picea mariana (Mill.) BSP) susceptibles à la paludification où de sévères perturbations des sols causées par le feu sont aptes à améliorer la productivité des peuplements. L’objectif de cette étude était de déterminer comment la perturbation des sols causée lors de la coupe totale et la coupe de protection affectent la régénération de l’épinette noire 8 ans après la récolte, et comment cet effet est conditionné par le type de sol. Alors que la densité de régénération de l’épinette noire était supérieure après la coupe de protection, le coefficient de distribution n’était pas influencé par la méthode de récolte. Les peuplements d’épinette régénérés après coupe totale étaient plus hauts en dépit d’un potentiel de dommage plus grand à la régénération préétablie. Comparée à la coupe de protection, la coupe totale a aussi résulté en une réduction de la couverture au sol de la sphaigne et des éricacées. La densité et le coefficient de distribution de l’épinette noire étaient supérieurs sur les sols organiques et subhydriques et inférieurs sur les sols mésiques. Aucune interaction significative n’a été observée entre la méthode de récolte et le type de sol, indiquant qu’une plus grande hauteur des peuplements et un coefficient de distribution d’épinette adéquat après coupe totale pourrait être applicable à toute la gamme de types de sol considérée dans cette étude. Ces résultats suggèrent qu’un niveau de perturbation des sols suffisant lors des opérations forestières est nécessaire, particulièrement dans les écosystèmes où des processus autogéniques réduisant la productivité s’opèrent.

Abstract:

Boreal forest bryophyte communities are made up of distinct colonies of feathermosses that cover the forest floor. In some black spruce (Picea mariana (Mill.) BSP) boreal forests, Sphagnum spp. establish colonies on the forest floor 30–40 years after the feathermosses, and ultimately expand to dominate the community. The mechanisms that permit the Sphagnum spp. to establish and expand are unknown. The objectives of this study were to examine the establishment and expansion substrates of Sphagnum spp., and the conditions correlated with colony expansion. Forty colonies, in six stands, of Sphagnum capillifolium (Ehrh.) Hedw. were dissected to determine their substrates, and the environmental conditions in which all colonies present were growing were measured. Coarse woody debris was the dominant establishment and early expansion substrate for Sphagnum capillifolium colonies. With age as the control factor, large colonies showed a significant partial correlation with canopy openness, and there were fewer individuals per cm3 in large colonies than there were in small colonies. These results suggest that Sphagnum establishment in these communities is dependent on the presence of coarse woody debris, and expansion is linked to the stand break-up, which would allow an increase in light intensity, and rainfall to reach the colony. Consequently the community change represented by Sphagnum establishment and expansion is initially governed by a stochastic process and ultimately by habitat availability and species competition.

Résumé:

Dans les forêts boréales, les communautés de bryophytes sont composées d’agglomérations de mousses hypnacées qui couvrent le sol. Dans les pessières, (Picea mariana (Mill.) BSP) les sphaignes établissent des colonies sur le sol, 30 à 40 ans après les mousses hypnacées, et réussissent ultimement à dominer la communauté. Les mécanismes qui permettent cet établissement et étalement ne sont pas encore compris. Les objectifs de cette étude sont d’examiner les substrats d’établissement et d’expansion des sphaignes, ainsi que les conditions environnementales qui permettent leur étalement. Pour ce faire, 40 colonies de Sphagnum capillifolium (Ehrh.) Hedw., réparties sur six sites ont été disséquées afin de déterminer leurs substrats d’établissement, et les conditions environnementales présentes autour de toutes les colonies ont été mesurées. Le bois mort était le substrat dominant pour l’établissement et l’étalement des colonies. Quand l’âge des peuplements est contrôlé dans une corrélation partielle, les grandes colonies étaient corrélées positivement avec l’ouverture de la couronne, et il y avait moins d’individus par cm3 dans les grandes colonies versus les petites colonies. Ces résultats suggèrent que l’établissement des sphaignes dépend de la présence de bois mort, et que l’étalement est corrélé avec l’ouverture du peuplement, phénomène qui permettrait à plus de lumière et de pluie d’atteindre les colonies. Par conséquent, l’établissement et l’expansion des colonies, qui représentent un changement dans la communauté, seraient gouvernées initialement par des processus stochastiques, et ultimement par la disponibilité d’habitat et la compétition.   © 2007 NRC Canada

Question: What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests?

Location: The Clay-Belt of Québec and Ontario, Canada.

Methods: The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro-climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire.

Results: Across the chronosequence, feathermosses were replaced by shade and desiccation tolerant slower growing hummock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light availability and the movement of the water table into the forest floor.

Conclusions: As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landscape level variables such as carbon balance.

Des études dans une chronoséquence de peuplements d’épinettes noires ont pu déterminer que l’accumulation avec le temps de matériel organique sur le sol minéral est causée par le climat régional, ainsi que la présence de sphaignes et de plantes éricacées. De plus, il a été démontré que la remontée de la nappe phréatique n’est pas une cause de l’accumulation de la matière organique mais une conséquence de cette dernière. Malgré des hypothèses émises il y a plus de 50 ans, la sphaigne n’a pas joué de rôle direct dans la remontée de la nappe phréatique mais plutôt un rôle indirect via sa contribution à l’accumulation de la matière organique.

Les études portant sur le phénomène de l’entourbement lui-même ont illustré l’importance de la communauté des bryophytes sur le fonctionnement de l’écosystème. La communauté de bryophytes passe d’une communauté dominée par les mousses hypnacées, à une communauté dominée par des sphaignes formant des hummocks, à une communauté dominée par les sphaignes de milieux humides. Les changements en lumière et en humidité disponible associés à l’entourbement ont été identifiés comme étant les facteurs qui causent les changements de communautés lors de la succession. La quantité de lumière disponible pour les bryophytes augmente avec l’âge du peuplement à cause de l’ouverture du couvert forestier suite à la sénescence par pied d’arbre. Le taux d’humidité disponible augmente quant à lui avec l’accumulation de la matière organique et la remontée éventuelle de la nappe phréatique. Puisque les mousses et les sphaignes sont en partie responsables de l’accumulation de la matière organique, ces résultats suggèrent que la succession dans la communauté de bryophytes s’appuie sur le modèle de facilitation.

Malgré que les changements de composition aient été expliqués par les changements dans l’environnement, l’établissement initial des sphaignes dans les jeunes peuplements n’était pas expliqué par des limitations d’habitat. La disponibilité des spores n’explique pas le manque de sphaignes dans les jeunes peuplements. Mais, les colonies de sphaignes se sont établies à plus de 70% du temps sur le bois mort, ce qui potentiellement expliquerait une restriction dans le temps et l’espace de la colonisation. Ensemble, ces résultats nous permettent de voir que les changements dans la communauté de sphaignes sont influencés plutôt par des processus stochastiques (la colonisation) au début de la succession et par des processus dirigé par l’habitat et les processus de compétition à la fin de la succession.

Ces études mettent l’emphase sur deux lacunes dans l’aménagement forestier de peuplements et de paysages entourbés. Les forêts entourbées et les jeunes forêts non entourbées contiennent des communautés de bryophytes riches et distinctes. La coupe avec la protection des sols et de la régénération (CPRS), qui est la norme dans la forêt boréale au Québec, ne recrée pas la structure des jeunes forêts quand c’est appliqué dans les vieilles forêts. Également, la CPRS ne récréer pas la structure des vieilles forêts entourbées et le manque de ces deux types d’habitat dans le futur pourrait mettre les espèces associées avec ces stades en périls. Les coupes partielles ont le potentiel de recréer les communautés de bryophytes associées avec les vieilles forêts mais jusqu’à maintenant, uniquement les sphaignes ont été examinées, et cela à court terme.

En conclusion, une meilleure connaissance et compréhension de la communauté des bryophytes dans les forêts d’épinette noire sur les dépôts fins nous permettra de formuler une meilleure stratégie d’aménagement qui prend en compte la biodiversité des bryophytes de ces forêts extraordinaires et leur productivité.

In the present study we investigated the genetic structure and genetic diversity of Pinus heldreichii populations in Bulgaria using chloroplast microsatellite markers and terpene analysis. We were interested in addressing the following questions: (1) can population structuring in Bosnian pine be detected via chloroplast microsatellite markers; (2) are there differences in population differentiation as determined by terpenes and microsatellites; and (3) how are the patterns of size variant frequencies and geographical distances related. Four provenances were chosen throughout the species' range in Bulgaria. Following DNA extraction, chloroplast microsatellite (cpSSR) loci were surveyed using 6 primer pairs. Between 2 and 5 size variants were identified at each locus. A total of 16 size variants at the 6 loci were identified, 4 occurring at low frequencies. They were combined in 21 different haplotypes including 11 that were unique. AMOVA analysis revealed that 18.25% of the variation was found among populations, while 81.75% was expressed within populations. The cpSSR analysis divided Bosnian pine populations into two groups, the first represented by populations B, C and D located in the south and north-western part of the Pirin and Slavianka mountains, while the second group, represented by population A, is located in the north-eastern Pirin mountain. Terpene analysis revealed that on average, 59% of the monoterpene pool in P. heldreichii is accounted for by limonene (range 36-48%) followed by a-pinene (range 16-17%). The presence of two distinct groups (Pop-A, Pop-D and Pop-B, Pop-C) is more consistent with physical distances between populations. No significant correlation between genetic distance determined by chloroplast microsatellites analysis and chemotype distance determined by terpenes was observed. Our results suggest that the structural pattern of genetic diversity of cpDNA in Bosnian pine populations is the consequence of historical and biogeographical processes. © 2004 Elsevier Ltd. All rights reserved.

Over time boreal black spruce forests on fine-textured soils in western Quebec, Canada develop very thick forest floors composed of poorly decomposed litter created by the tree and understory layers. These paludified soils are typically waterlogged and cold, and in this fire-mediated landscape, are at least partially consumed by stand replacing fires, which facilitates the establishment of the next generation of trees. Within a context of ecosystem-based management, forest harvest should mimic the dual effects of high severity fire on tree and forest floor biomass. This study was designed to investigate potential factors of forest floor thickness in order to determine the impact of removing only a tree layer, and to suggest strategies to limit paludification in this important forestry region. Forest floor thickness, fire severity, basal area, canopy closure, cover of Sphagnum spp. and ericaceous spp. were measured in black spruce stands across a chronosequence from 50 to 350 years after fire. Fire severity was determined to be a key factor in determining forest floor thickness by path analysis. After high severity fires forest floor thickness was primarily dependant on stand age, but was also positively influenced by Sphagntan spp. cover and negatively influenced by the presence of trembling aspen (Populus tremuloides). These results suggest that forest interventions that do no remove the organic layer may be mimicking low severity fires and promoting poor tree growth and regeneration. Forest floor thickness may be limited by avoiding interventions that open the canopy and may promote the presence of Sphagnum spp. and ericaceous spp., and or by practicing mixed silviculture of trembling aspen and black spruce. However, a balance needs to be maintained between the application of these techniques and the preservation of paludified forests in the landscape. (c) 2005 Elsevier B.V. All rights reserved. © 2005 Elsevier B.V. All rights reserved.