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The effect of climate change on forest dynamics is likely to increase in importance in the forthcoming decades. For this reason, it is essential to predict the extent to which changes in temperature, precipitation, and atmospheric CO2 might affect the development of forest ecosystems and successional pathways. The gap model ZELIG-CFS was used to simulate the potential long-term effects of climate change on species-specific annual change in mean basal area and stand density under two scenarios of representative concentration pathways (RCP), 4.5 and 8.5, for the boreal forest region of Ontario, Canada, where mean temperature, precipitation, and atmospheric CO2 are expected to increase. Forest ecosystems in this boreal region included pure and mixed stands of black spruce (Picea mariana [Mill.] B.S.P.), paper birch (Betula papyrifera Marsh.), balsam fir (Abies balsamea [L.] Mill.), jack pine (Pinus banksiana Lamb.), trembling aspen (Populus tremuloides Michx.), white spruce (Picea glauca [Moench] Voss), northern white cedar (Thuja occidentalis L.), American larch (Larix laricina [Du Roi] K. Koch), and balsam poplar (Populus balsamifera L.). Simulation results under climate change generally predicted a decline in the basal area and stand density for black spruce, balsam fir, jack pine, and white spruce, but an increase for paper birch, trembling aspen, American larch, and balsam poplar. However, the extent of change differed regionally among species. Forest composition is expected to change over the long term. Simulation results indicated that shade-intolerant deciduous and conifer species will increase their dominance over the 100-year time horizon. This transition toward the increasing presence of deciduous forests is likely explained by more favorable temperature conditions for their growth and development. [ABSTRACT FROM AUTHOR]

Increased fire severity in boreal forests of Interior Alaska is shifting forest canopy composition from black spruce ( Picea mariana ) to deciduous species, including trembling aspen ( Populus tremuloides ) and Alaska paper birch ( Betula neoalaskana ). Because deciduous trees are less flammable than black spruce, the dominant disturbance regime in deciduous forests could move away from fire to one of gap disturbances. In this study, we quantified forest gap characteristics and vegetation within eight mature (62–119-yr-old) deciduous stands in Interior Alaska. Canopy gaps were generally small (true gap area <50 m 2 ), formed by the mortality of 4–16 gap makers (which were always deciduous trees), and occupied ∼17–29% of the forest except in the oldest stand, where gap fraction exceeded 45%, and in one anomalous 84-yr old stand, where gaps were absent. Canopy openness increased linearly with gap area, but density of both deciduous and evergreen tree recruits was generally low and insufficient to create future stands with densities similar to those currently found in mature stands across the landscape. Canopy openness was instead correlated with decreased leaf litter cover and increased cover of moss, lichen, and evergreen shrubs. Given the low recruitment of trees with canopy gaps and the decreased probability of fire, deciduous stands will likely transition to non-forested areas or low density stands once overstory trees reach maturity and die. This could have numerous implications for ecosystem function, including carbon (C), water, and energy balance, and potential feedbacks to future fire occurrence and regional climate. [ABSTRACT FROM AUTHOR]

1. Boreal forest soils contain large stocks of soil carbon (C) that may be sensitive to changes in climate and disturbance. Destablization of boreal forest soil C through changes in C inputs, belowground C pools and/or wildfire could feedback to accelerate rising atmospheric CO2 concentration. Additionally, increasing frequency of severe fires may be changing the dominant forest types and reshaping aboveground C stocks. 2. Although controls on ecosystem C pools have received considerable attention, many studies have been limited to locations near the road system, leading to uncertainty in current and future C stocks across boreal Alaska. Here, we leveraged 545 randomly selected and spatially balanced Forest Inventory and Analysis (FIA) plots across ~13.5 million hectares in interior Alaska to examine the factors governing soil and live tree C pools. 3. Forest type mediated the effects of mean summer air temperature and the probability of near-surface permafrost on soil C. Meanwhile, forest type, stand age and aspect were the primary drivers of live tree C. Overall, plots with a known history of wildfire during the past 70 years did not have significantly different soil C stocks than plots without a known history of fire, likely due to the historical predominance of low severity fires. 4. Where wildfire likely initiated a transition to deciduous trees (19% of plots), live tree and soil C pools were reduced by 16% and 20%, respectively. Ecosystem C likely recovered over time, as maturing deciduous stands rapidly gained C in live trees. Deciduous stands without a known fire had comparatively very large live tree C stocks, suggesting a significant change in the distribution of ecosystem C following severe fire. 5. Synthesis. Our results highlight the nuanced interactions among wildfire, landscape position and forest type that will play important roles in shaping future boreal ecosystem C stocks. [ABSTRACT FROM AUTHOR]

As the vegetation in the Arctic changes, tundra ecosystems along the southern border of the Arctic are becoming greener and gradually giving way to boreal ecosystems. This change is affecting local populations, wildlife, energy exchange processes between environmental compartments, and the carbon cycle. To understand the progression and the implications of this change in vegetation, satellite measurements and surface models can be employed. However, in situ observational data are required to validate these measurements and models. This paper presents observational data from two nearby sites in the forest–tundra ecotone in the Tasiapik Valley near Umiujaq in Northern Quebec, Canada. One site is on a mixture of lichen and shrub tundra. The associated data set comprises 9 years of meteorological, soil and snow data as well as 3 years of eddy covariance data. The other site, 850 m away, features vegetation consisting mostly of tall shrubs and black spruce. For that location, 6 years of meteorological, soil and snow data are available. In addition to the data from the automated stations, profiles of snow density and specific surface area were collected during field campaigns. The data are available at 10.1594/PANGAEA.964743 (Domine et al., 2024). [ABSTRACT FROM AUTHOR]

Many disturbances are shifting in severity, frequency, and extent due to changing climate and human activities. Altered disturbance regimes can trigger shifts in ecosystem state where recovery to the pre-disturbance ecosystem is uncertain. In the western North American boreal forest, the intensification of wildfire can cause canopy dominance to switch from black spruce (Picea mariana) to deciduous trees such as Alaska paper birch (Betula neoalaskana) and trembling aspen (Populus tremuloides). Understanding the key mechanisms that determine the resilience and stability of these alternative community types is required for the prediction of future forest dynamics. Here, we assess patterns of post-fire tree recovery across a pre-fire gradient of spruce- to deciduous-dominated forests in Interior Alaska and quantify compositional and environmental thresholds that support the resilience of alternative canopy types. We found post-fire organic soil depth of stands on a recovery trajectory to deciduous dominance (7.3 ± 5.5 cm) were similar regardless of pre-fire composition and significantly shallower than spruce (14.9 ± 9.0 cm) or mixed trajectories (10.4 ± 5.9 cm). Deciduous-dominated stands were highly resilient to fire, as 100% remained deciduous-dominated post-fire. Even when deciduous trees only accounted for a small proportion (12%) of the pre-fire stand, deciduous trees often became dominant after wildfire. We conclude that the establishment of deciduous bud banks and seed sources creates a strong hysteresis in stand recovery that reinforces the resilience of deciduous-dominated boreal forests to wildfire. Accounting for the resilience of this alternative stable state to wildfire suggests that shifts from spruce to deciduous dominance caused by shifting wildfire will have long-term effects on future structure and function of boreal forests and vegetation feedbacks to climate change. [ABSTRACT FROM AUTHOR]

In boreal forests in Canada, broadleaf stands are characterized by generally well-drained soils and a humus-rich layer. In contrast, spruce-moss stands are often characterized by more poorly drained soils and acidic humus layer. However, presence of these two forest types in various degrees of mixture in stands can be beneficial to spruce seedlings productivity. It was hypothesized that leaf litter and humus from pure spruces-moss stands, pure broadleaf stands, and mixed stand may influence Black spruce (Picea mariana (Mill.) BSP) seedling growth and development differently. A greenhouse experiment was carried out to evaluate the effect of different leaf litter and different humus on spruces seedlings. Our results suggest better development for seedlings grown in humus from mixed stands and pure broadleaf stands compared to humus from pure B. spruce or standard forest nursery substrate. Furthermore, leaf litter from broadleaf trees species, such as species Speckle alder (Alnus rugosa (Du Roi) R.T. Clausen), T. aspen (Populus tremuloides Michx), Willows (Salix spp.) and Paper birch (Betula papyrifera Marsh.), has shown distinct results in the growth and development of B. spruce seedlings in greenhouse. Furthermore, promotion of mixed stand can increase B. spruce productivity by improving the physicochemical composition of the forest floor. [ABSTRACT FROM AUTHOR]

In the boreal forest of Alaska, increased fire severity associated with climate change is expanding deciduous forest cover in areas previously dominated by black spruce ( Picea mariana). Needle-leaf conifer and broad-leaf deciduous species are commonly associated with differences in tree growth, carbon (C) and nutrient cycling, and C accumulation in soils. Although this suggests that changes in tree species composition in Alaska could impact C and nutrient pools and fluxes, few studies have measured these linkages. We quantified C, nitrogen, phosphorus, and base cation pools and fluxes in three stands of black spruce and Alaska paper birch ( Betula neoalaskana) that established following a single fire event in 1958. Paper birch consistently displayed characteristics of more rapid C and nutrient cycling, including greater aboveground net primary productivity, higher live foliage and litter nutrient concentrations, and larger ammonium and nitrate pools in the soil organic layer (SOL). Ecosystem C stocks (aboveground + SOL + 0-10 cm mineral soil) were similar for the two species; however, in black spruce, 78% of measured C was found in soil pools, primarily in the SOL, whereas aboveground biomass dominated ecosystem C pools in birch forest. Radiocarbon analysis indicated that approximately one-quarter of the black spruce SOL C accumulated prior to the 1958 fire, whereas no pre-fire C was observed in birch soils. Our findings suggest that tree species exert a strong influence over C and nutrient cycling in boreal forest and forest compositional shifts may have long-term implications for ecosystem C and nutrient dynamics. [ABSTRACT FROM AUTHOR]

Black spruce is widely used for lumber production in Eastern Canada, and it has the potential to replace trembling aspen and paper birch for oriented strand board (OSB) manufacturing. This study evaluated the bending modulus of elasticity (MOE) and modulus of rupture (MOR), the internal bond (IB), and the thickness swelling (TS) of OSB panels made from black spruce and trembling aspen strands and how they were affected by strand geometry. All the panels met the CSA O437 (1993) standard for class O-2 properties except for the TS. The strand thickness had a significantly negative effect on the bending properties but a significantly positive effect on the IB and TS properties. The strand length had a significantly positive effect on the parallel bending properties but a significantly negative effect on the perpendicular bending properties and the IB, except for the TS. The OSB panels made from aspen obtained better bending properties, while the IB and TS properties were lower than those of the OSB black spruce panels. The results indicate that black spruce strands obtained from the Eastern Canadian softwood lumber industry are suitable for OSB production, but more work is required to reduce the TS. [ABSTRACT FROM AUTHOR]

This study aims to investigate the feasibility of producing OSB from softwood species used in the Eastern Canadian softwood lumber industry in the context of the overcapacity of softwood chips traditionally produced for the pulp and paper industry. Balsam fir, black spruce, and jack pine logs were used to make 15 mm thick OSB panels with a target density of 600 kg/m3. The panels were manufactured at a temperature of 210 °C during a pressing cycle of 300 s. Strands with different thicknesses were used to obtain a constant specific surface of 6.7 m2/kg for the three species. The bending modulus of the rupture and modulus of elasticity, internal bond, and thickness swelling of the OSB panels were determined and compared to the CSA standard requirements. The species significantly affected the physical and mechanical properties of OSB. The bending properties of OSB decreased with an increase in the species wood density. The internal bond strength of OSB increased with the increase in species wood density. The panels made from softwood species showed physical and mechanical properties exceeding the standard requirements, except for high thickness swelling. The combination of softwood and aspen strands significantly improved the thickness swelling of softwood-based OSB. The reduction in the surface layer density could also be explored to reduce the thickness swelling of the OSB made from the softwoods considered in this study. [ABSTRACT FROM AUTHOR]

As the vegetation in the Arctic changes, tundra ecosystems along the southern border of the Arctic are becoming greener and gradually giving way to boreal ecosystems. This change is affecting local populations, wildlife, energy exchange processes between environmental compartments, and the carbon cycle. To understand the progression and the implications of this change in vegetation, satellite measurements and surface models can be employed. However, in situ observational data are required to validate these measurements and models. This paper presents observational data from two nearby sites in the forest-tundra ecotone in the Tasiapik Valley near Umiujaq in Northern Quebec, Canada. One site is on a mixture of lichen and shrub tundra. The associated data set comprises 9 years of meteorological, soil and snow data as well as 3 years of eddy covariance data. The other site, 850 m away, features vegetation consisting mostly of tall shrubs and black spruce. For that location, 6 years of meteorological, soil and snow data are available. In addition to the data from the automated stations, profiles for snow density and specific surface area were collected during field campaigns. The data are available at https://doi.org/10.1594/PANGAEA.946538 (Lackner et al., 2022b). [ABSTRACT FROM AUTHOR]

Increasing rates of short‐interval disturbances have the potential to rapidly transform ecosystems via shifts in post‐disturbance regeneration. While research has explored compound events in multiple biomes, we know little regarding how local site conditions interact with short‐interval disturbances to influence post‐disturbance regeneration. Furthermore, questions remain regarding the consequences of continued high frequency events: What happens when emerging new communities are themselves subject to short‐interval disturbances? To investigate effects of ongoing short‐interval fires on regeneration, we examined post‐fire forest regeneration in two locations in interior Alaska. We established 50 plots across a mosaic of fire histories (one, two, or three fires in <70 yr) in an upland and lowland site in interior Alaska. To investigate how shifts in community driven by short‐interval fires differ according to local site conditions, we quantified abundance, proportion, and density of conifer and deciduous regeneration in a drier upland site and a wetter lowland site. Both sites were dominated by black spruce prior to burning. In the drier upland site, black spruce (Picea mariana) presence declined sharply after two fires, while paper birch (Betula neoalaskana) became increasingly abundant with each additional fire. In the wetter lowland site, less organic soil was consumed by fire and presence of black spruce persisted through an initial single reburn (two fires), indicating local topography may temporarily buffer reburning impacts. However, after three burns, conifers were effectively eliminated in both upland and lowland stands. Deciduous regeneration differed with site: Birch dominated in upland plots, while willow (Salix spp.) and aspen (Populus tremuloides) dominated in lowlands. These results offer strong empirical evidence of the divergence of boreal successional trajectories from previous historic norms. Furthermore, results from this study demonstrate shifts in post‐fire succession in forested ecosystems continue to accumulate with additional short‐interval disturbance events, overwhelming the interactive effects of local site conditions. [ABSTRACT FROM AUTHOR]

In this paper, we summarize Ontario's Long-term Soil Productivity (LTSP) experience focusing on our efforts to forge lasting research partnerships, highlight the approaches we feel were effective in getting emerging science into forest policy within an adaptive management (AM) framework, and describe the future direction of Ontario's LTSP program as new policy issues are emerging as part of the continuous AM cycle. Fourteen installations were established on nutrient-poor, conifer-dominated sites, considered to be the most sensitive to increased biomass removals. From 1993 to 1995, all sites were clearcut-harvested, with replicated (three reps per site) biomass removal treatments that included: stem only, full-tree, and full-tree + forest floor removal. Routine (every 5 years) measurements have been carried out to track changes in soil carbon and nutrient levels, as well as stand- and individual-tree growth and development metrics and foliar nutrition. The published results from Ontario's LTSP program, in combination with the North American-wide LTSP synthesis outputs, have suggested that these nutrient-poor, conifer-dominated sites are less sensitive to biomass (nutrient) removals than previously thought. The evidence provided through peer-reviewed publications, conference and workshop presentations, and field tours was substantive and led to a review and revision of the full-tree logging direction within Ontario's guidelines. We conclude with a set of recommendations (lessons learned) for the successful delivery of any new long-term, interdisciplinary research projects examining the sustainability of forest-management practices. [ABSTRACT FROM AUTHOR]

Plant–soil feedbacks can maintain or reinforce alternative states within ecological systems. In Alaskan boreal forests, changes in fire characteristics have stimulated the replacement of needle-leaf black spruce (Picea mariana) by broadleaf deciduous trees. Feather mosses have strong associations with forest type: They dominate black spruce forest understories and are uncommon in broadleaf stands, with consequences for nutrient cycling and carbon storage. Here we test a long-standing hypothesis that broadleaf litter directly excludes mosses with a field experiment in broadleaf paper birch (Betula neoalaskana) and black spruce stands. We established 30 plots (15 each in birch and spruce dominated areas) with three Hylocomium splendens transplants treated with one of three treatments in each plot (ambient leaf litter deposition, birch leaf litter exclusion or addition), and 30 natural H. splendens areas. We measured moss growth and reproductive potential over 3 years. A 1-year experiment assessed leaf leachate and physical structure impacts on moss growth. Moss shoot growth in natural patches was larger in spruce than in birch stands (24.8 vs. 17.3 mg) and H. splendens made large contributions to ecosystem productivity in spruce stands. In both stand types, we observed a 40% reduction in moss biomass between litter addition and exclusion treatments and litter additions decreased sporophyte production. We found no difference in growth for mosses treated for 1 year with leaf leachates or physical litter structures. Leaf litter effects appear strong enough to exclude mosses from broadleaf forests, providing experimental support for hypothesized plant–soil interactions that may stabilize alternate forest types. [ABSTRACT FROM AUTHOR]

Introduction: In eastern Canada, the boreal forest is associated with an important latitudinal shift in forest composition and structure, which occurs in the transition between the mixed southern boreal forest and the coniferous northern boreal forest. Along this transition, upland mixedwood stands with large deciduous trees (important for cavity-dependent vertebrate species) are gradually replaced by forests with smaller conifer trees, primarily black spruce (Picea mariana). Concomitantly, the availability of lowland forests flooded by the American beaver (Castor canadensis), which can provide adequate conditions for tree-cavity users, is also decreasing. Methods: We hypothesized that this latitudinal gradient would bring important changes in the functional diversity and network structure of vertebrate cavity- using communities. Along this latitudinal gradient we used a nest web approach to analyze the structure and robustness of networks of cavity users in upland forests and in lowland forests flooded by beavers. Results: Despite their low availability in the northern forest region, we found that mixedwood stands persisted throughout the boreal forest in being the main drivers of nest webs network structure of upland forests whereas old black spruce stands contribution was low. In lowland forests, beaver ponds harbored nest webs with a rich and complex structure in both forest regions. Species removal simulations revealed that across our latitudinal gradient upland and lowland forest nest webs responded differently. In upland forests, the removal of trembling aspen and the Pileated Woodpecker (Dryocopus pileatus) caused the highest proportions of secondary extinctions, showing low robustness of nest webs given that these two species were highly connected to the other species. Contrastingly, nest webs in beaver ponds were more robust mainly because excavator species used a higher diversity of tree species despite the removal of the Northern Flicker (Colaptes auratus) which induced numerous secondary extinctions. The Pileated Woodpecker remained the pivotal species across the two forest regions in upland forests whereas the Northern Flicker became the main large cavity provider in beaver ponds across the latitudinal gradient. Discussion: We discuss how mixedwood forests and beaver ponds, which are key habitat types for the cavity-using vertebrate community across our latitudinal gradient, should be maintained and protected in landscapes under industrial timber harvesting. [ABSTRACT FROM AUTHOR]

Forest fires and logging drive the structure of boreal forest landscapes. According to recent studies, stand origin is a key driver of the variation in wood properties in black spruce (Picea mariana (Mill.) BSP.), although the underlying mechanisms remain to be elucidated. By comparing post-cut, post-fire and old-growth forests, this study aimed to better distinguish the effects of stand structure from those of the seed or layer origin of the trees on wood properties. We conducted comparative analyses based on ecological characteristics of the sites and static bending tests of small, defect-free wood specimens. Black spruce stands with a regular structure established after logging or fire exhibited higher stiffness at a given cambial age than old-growth forests with irregular structures, as well as lower wood density in the first 40 rings near the pith. However, the bending strength was comparable in all three types of forests studied. Differences in wood stiffness among stand types appeared to be driven more by stand structure than by the seed or layer origin of the stems. [ABSTRACT FROM AUTHOR]

Mixed plantations are garnering increased attention due to their potential to provide a broader array of benefits compared to monocultures. Although numerous studies have indicated promising complementarity between black spruce (Picea mariana) and tamarack (Larix laricina), few have delved into individual tree growth interactions to thoroughly assess early growth complementarity. We sampled 119 planted black spruce and their immediate environment to quantify and qualify any differences between two conditions in young plantations: mixed tamarack ("mixtures") and black spruce monocultures ("monocultures") within young plantations. We investigated the effect of neighbouring under four perspectives: tree competition, microenvironment, foliar nutrients, and soil nutrients. Our results showed increased values for black spruce foliar nitrogen total concentration, soil pH, and canopy closure in mixtures compared to monocultures. Furthermore, black spruce stem volume was increased by 38.1% in mixture compared to monoculture. Black spruce stem volume was negatively affected (86% decrease) by the combined effect of shrubs and non-crop trees under high competition pressure, despite the plantation being mechanically released in 2017. Collectively, our results suggest that black spruce growing in mixtures holds a greater growth potential than black spruce in monocultures. [ABSTRACT FROM AUTHOR]

Individual tree diameter at breast height (DBH) distribution is an important information for forest management planning. Forest managers obtain the DBH data either by field measurements or estimations using predictive models. However, probability distribution models are still lacking or need improvement. Therefore, we aimed to construct and fit diameter distribution models that reflect forest structure and composition change. We evaluated gamma, log-normal, and Weibull probability distribution functions (PDFs) for two commercially important tree species, black spruce (Picea mariana (Mill) B.S.P.) and jack pine (Pinus banksiana Lamb), grown in natural stands across Ontario, Canada. We modelled the parameters of the distributions as a function of stand-level variables for these species. We used DBH data from 735 permanent sample plots. Our results showed that all three evaluated PDFs reflected observed DBH distribution. We demonstrated that the moment-based recovered parameters could represent the maximum likelihood-estimated parameters precisely, and parameters of the PDFs can be modelled as a function of stand-level dynamic covariates. The models unbiasedly predicted the PDF parameters DBH means and DBH classes. The R2 of the model fit ranged between 0.35 and 0.98 for the predicted parameters and 0.90 and 0.97 for the predicted DBH. [ABSTRACT FROM AUTHOR]

European forests are facing ongoing climate change, and certain tree species are being critically impacted. The Norway spruce (Picea abies (L.) Karst.) is one of the most sensitive species to climate fluctuations, a fact manifesting itself through massive dieback resulting in a lack of high-quality timber and timber market destabilization. Therefore, the possibility of wood substitution with non-native spruce species, namely, black spruce (Picea mariana (Mill.) Britt., Sterns, et Poggenburg), Serbian spruce (Picea omorika (Pančić) Purk.), and blue spruce (Picea pungens Engelm.), under the specific conditions of forest reclamations with great potential for future afforestation was tested. Wood density, modulus of rupture, and modulus of elasticity were used to evaluate wood quality in comparison with native Norway spruce. The results confirmed that only the Serbian spruce reached the quality of Norway spruce and even exceeded it in terms of wood density (P. omorika 525 kg·m−3 vs. P. abies 517 kg·m−3) and exhibited comparable parameters with regard to other properties. The density of the other species was significantly lower for blue spruce (476 kg·m−3) and black spruce (468 kg·m−3). A similar trend was found for other wood parameters, which confirmed that Norway spruce quality was nearly comparable with that of Serbian spruce. On the other hand, black spruce and blue spruce did not match the quality of Norway spruce. The within-stem variability of the properties tested was low for all the spruce species examined. In conclusion, the Serbian spruce showed great potential for future usage in forest management and is one of the possible methods of Norway spruce replacement in times of unprecedented forest disturbances under the effects of global climate change. [ABSTRACT FROM AUTHOR]

Black spruce is the most common tree species in Canada, dominating treed peatlands where they are usually stunted. We used 495 destructively sampled trees from 56 plots to develop allometric models of aboveground biomass by components (stem, branches, and needles) for small (<5 m tall) black spruce from peatlands in the Taiga Plains and Boreal Plains Ecozones of Western Canada, for which there were no specific models available of biomass by components. We used leave-one-plot-out cross-validation to assess transferability and compare our models with existing national and ecozone-specific equations. Our models predicted total tree biomass with better accuracy and less biased estimates than the national model (relative RMSE: 30% versus 35% national; relative bias: +1% versus –12% national). Similar results were obtained in other external datasets. Existing ecozone equations performed worse than either our models or the national ones. When we applied the models at the plot level to predict aboveground biomass density (Mg·ha−1), our models outperformed the national model again (relative RMSE: 15.9% versus 18.6% national, relative bias: +3.5% versus –13.6% national). These results indicate that at least for peatlands of Western Canada, our models provide better aboveground biomass estimates for small black spruce trees than existing models. [ABSTRACT FROM AUTHOR]

Carbon release from thawing permafrost soils could significantly exacerbate global warming as the active-layer deepens, exposing more carbon to decay. Plant community and soil properties provide a major control on this by influencing the maximum depth of thaw each summer (active-layer thickness; ALT), but a quantitative understanding of the relative importance of plant and soil characteristics, and their interactions in determine ALTs, is currently lacking. To address this, we undertook an extensive survey of multiple vegetation and edaphic characteristics and ALTs across multiple plots in four field sites within boreal forest in the discontinuous permafrost zone ( NWT, Canada). Our sites included mature black spruce, burned black spruce and paper birch, allowing us to determine vegetation and edaphic drivers that emerge as the most important and broadly applicable across these key vegetation and disturbance gradients, as well as providing insight into site-specific differences. Across sites, the most important vegetation characteristics limiting thaw (shallower ALTs) were tree leaf area index ( LAI), moss layer thickness and understory LAI in that order. Thicker soil organic layers also reduced ALTs, though were less influential than moss thickness. Surface moisture (0-6 cm) promoted increased ALTs, whereas deeper soil moisture (11-16 cm) acted to modify the impact of the vegetation, in particular increasing the importance of understory or tree canopy shading in reducing thaw. These direct and indirect effects of moisture indicate that future changes in precipitation and evapotranspiration may have large influences on ALTs. Our work also suggests that forest fires cause greater ALTs by simultaneously decreasing multiple ecosystem characteristics which otherwise protect permafrost. Given that vegetation and edaphic characteristics have such clear and large influences on ALTs, our data provide a key benchmark against which to evaluate process models used to predict future impacts of climate warming on permafrost degradation and subsequent feedback to climate. [ABSTRACT FROM AUTHOR]

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Climate change is rapidly altering weather patterns, resulting in shifts in climatic zones. The survival of trees in specific locations depends on their functional traits. Local populations exhibit trait adaptations that ensure their survival and accomplishment of growth and reproduction processes during the growing season. Studying these traits offers valuable insights into species responses to present and future environmental conditions, aiding the implementation of measures to ensure forest resilience and productivity. This study investigates the variability in functional traits among five black spruce (Picea mariana (Mill.) B.S.P.) provenances originating from a latitudinal gradient along the boreal forest, and planted in a common garden in Quebec, Canada. We examined differences in bud phenology, growth performance, lifetime first reproduction, and the impact of a late-frost event on tree growth and phenological adjustments. The findings revealed that trees from northern sites exhibit earlier budbreak, lower growth increments, and reach reproductive maturity earlier than those from southern sites. Late-frost damage affected growth performance, but no phenological adjustment was observed in the successive year. Local adaptation in the functional traits may lead to maladaptation of black spruce under future climate conditions or serve as a potent evolutionary force promoting rapid adaptation under changing environmental conditions. [ABSTRACT FROM AUTHOR]

The cutting speed (CS) and feed per knife (FK) are among the most important variables affecting chip size produced by chipper-canters. Nine groups of black spruce logs were processed at three CS (20, 25, and 30 m/s) and three FK (19, 25, and 32 mm). Each log was processed under frozen (−13°C) and unfrozen (19°C) conditions. Chip size was assessed by thickness and by width/length. Chip size increased as CS decreased and FK increased. Frozen logs produced thinner chips and higher proportions of small chips. The weighted mean chip thickness (WCT) increased as the FK increased and CS decreased. The highest accepts proportion by thickness was obtained at 19 mm FK and 20 m/s CS, while the highest width/length accepts were produced at 32 mm FK and 20 m/s CS. Grain angle and knot proportion were the most significative covariates for chip size. Regressions showed that FK, CS, knot proportion, grain angle, and taper were the best predictors for WCT, explaining 86% and 81% of the WCT variations for frozen and unfrozen logs, respectively. Therefore, a combined evaluation of cutting parameters and raw material is essential to predict WTC, reduce chip size variations, and thus improve chip quality. [ABSTRACT FROM AUTHOR]

The effects of wood species on the performance of the strander-canting process were studied. Logs of balsam fir (Abies balsamea (L.) Mill.), black spruce (Picea mariana (Mill.) B.S.P.), and jack pine (Pinus banksiana Lamb) were processed under two temperature conditions (-13.3 °C and 22.3 °C). The cutting and feed speeds, rake angle, cutting width, and strand thickness were kept constant. The strander-canting process was evaluated by the strand dimensions and yield, as well as by the energy requirements. The results showed that wood species significantly affected the proportions of strands and fines, maximum power, mean energy consumption, and specific cutting energy when processing the logs under frozen conditions. In unfrozen conditions, wood species only affected the strand width and the maximum power. Unfrozen logs produced higher proportions of strands and a lower volume of pin chips and fines than frozen logs. The maximum power, mean energy consumption, and specific cutting energy were, on average, 2 to 4 times higher for the processing frozen logs than for unfrozen logs. [ABSTRACT FROM AUTHOR]

Balsam fir (Abies balsamea) and black spruce (Picea mariana) forests are the main conifer forest types in the North American boreal zone. The coexistence of the two species as well as their respective canopy dominance in distinct stands raises questions about the long-term evolution from one forest type to the other in relation to environmental factors including climate and stand disturbance. We tested the hypothesis that repetitive fire events promote the succession of balsam fir forest to black spruce forest and vice versa. Postfire chronosequences of one black spruce (BSP) and one balsam fir (BFI) sites were reconstructed based on the botanical composition and 14C-dated soil macrocharcoals. The results support the hypothesis of a successional dynamics. The BSP site has been affected by fires for the last 7600 years, whereas the BFI site, after having been impacted by several fires during the first half of the Holocene, evolved in a fire-free environment for the last 4400 years. Periods of fire activity facilitated the dominance of black spruce forests. The cessation of fires around 4400 cal. years BP on BFI site marks the beginning of the transition from black spruce to balsam fir stands. This succession is a long process, due to the ability of black spruce to regenerate by layering in the absence of fire. The resulting balsam fir stands are ancient and precarious ecosystems, since fire generally leads to the return of black spruce. The increase in balsam fir to the detriment of black spruce in boreal forests is a response to a decrease in fire frequency. [ABSTRACT FROM AUTHOR]

In the face of increasing temperature at high latitudes, ecosystem respiration (RE) is a key to determining sink and source dynamics of a boreal forest. In this paper, we analyzed four-year RE data obtained in an open black spruce forest—a typical boreal forest ecosystem with permafrost in Interior Alaska. RE measured as nighttime CO 2 effluxes for both the ecosystem and the forest floor were clearly situated along the exponential temperature-dependent curve, except for the data obtained in extremely dry conditions in mid-summer. More than 93 % of RE data measured at nighttime with high vapor pressure deficit (VPD > 400 Pa) were lower than the values predicted from the temperature-dependent curve. Consequently, the year 2013 (with an unusually dry summer) had a 15 % lower amount of annual RE than that could be expected from the temperature-dependent curve without considering the effect of high VPD. The suppression of RE under dry conditions was also related to decreases in soil moisture and net ecosystem productivity. Finally, assuming daytime RE could be extrapolated from the temperature-dependent curves, annual daytime RE estimated with the effect of high VPD was decreased by up to 62% from RE estimated without the VPD effect. The down-regulation of RE presented in this study postulates a possible negative feedback for the carbon budget of boreal forests in response to climate warming. [ABSTRACT FROM AUTHOR]

Fifteen stems of jack pine (Pinus banksianaLamb.) of 3 commercial thinned plots (control, moderate, and intensive thinning) and 15 stems of black spruce (Picea mariana(Mill.) B.S.P), both coming from the Abitibi-Témiscamingue region – Canada, were cross-cut into three 2.4 m length sections: bottom, middle, and top logs. Logs were processed with a chipper-canter at three cutting widths (12.7, 19.1, and 25.4 mm), producing chips and a three-faced cant. The middle section of the cant was used to evaluate surface quality across the grain on each face. Roughness and waviness parameters and depth of torn grain were recorded. Knot characteristics were assessed in the three cant faces. Poorer surface quality was found in the lower part compared to the upper part of the cant for both species. At larger cutting widths, jack pine logs coming from a natural stand showed lower surface quality compared to logs from thinned stands. Black spruce waviness increased with the cutting width and stem height. These results were attributed to the increase of forces and vibration when cutting at larger cutting widths, which was worsened by the presence of bigger, more numerous knots at the control plot and in the top logs. Black spruce had deeper torn grain compared to jack pine. Their differences in knot characteristics resulted in a maximum torn grain depth favored by the presence of more knots rather than by bigger knot size. Other knot characteristics, such as the position of the knots in the cant face, the insertion angle of the branch and the distribution of the knots around the log, should be studied to better understand the relationship between torn grain formation and knottiness. [ABSTRACT FROM PUBLISHER]

Resilience of plant communities to disturbance is supported by multiple mechanisms, including ecological legacies affecting propagule availability, species' environmental tolerances, and biotic interactions. Understanding the relative importance of these mechanisms for plant community resilience supports predictions of where and how resilience will be altered with disturbance. We tested mechanisms underlying resilience of forests dominated by black spruce (Picea mariana) to fire disturbance across a heterogeneous forest landscape in the Northwest Territories, Canada. We combined surveys of naturally regenerating seedlings at 219 burned plots with experimental manipulations of ecological legacies via seed addition of four tree species and vertebrate exclosures to limit granivory and herbivory at 30 plots varying in moisture and fire severity. Black spruce recovery was greatest where it dominated pre-fire, at wet sites with deep residual soil organic layers, and fire conditions of low soil or canopy combustion and longer return intervals. Experimental addition of seed indicated all species were seed-limited, emphasizing the importance of propagule legacies. Black spruce and birch (Betula papyrifera) recruitment were enhanced with vertebrate exclusion. Our combination of observational and experimental studies demonstrates black spruce is vulnerable to effects of increased fire activity that erode ecological legacies. Moreover, black spruce relies on wet areas with deep soil organic layers where other species are less competitive. However, other species can colonize these areas if enough seed is available or soil moisture is altered by climate change. Testing mechanisms underlying species' resilience to disturbance aids predictions of where vegetation will transform with effects of climate change. [ABSTRACT FROM AUTHOR]

Ericaceous shrubs often interfere with the growth of black spruce seedlings on regenerating forest sites in Eastern Canada. Mechanical site preparation such as scarification may improve this situation, but it is uncertain whether this is solely due to a reduction in direct competition from the shrubs, or also from a sustained improvement in nutritional site quality. We sampled experimental plots in two boreal climate regions (i.e. warmer-drier Abitibi vs. cooler-wetter Côte-Nord) where scarification, performed 18 years earlier, had increased the growth of black spruce relative to non-scarified plots. Trees of scarified plots had closed the canopy more than trees of non-scarified plots in Côte-Nord, but not in Abitibi. Total ground cover of ericaceous shrubs was lower in scarified plots at both sites, the main species being Kalmia angustifolia (i.e. Kalmia) in Abitibi and Rhododendron groenlandicum (i.e. Labrador tea) in Côte-Nord. Scarified plots at both sites had significantly shorter current-year ericaceous rhizomes than non-scarified plots, but the difference between treatments was significantly greater in Côte-Nord than in Abitibi. In Côte-Nord, ericaceous shrubs on scarified plots had a lower specific rhizome mass, higher specific leaf area, lower tannin and higher N concentrations in leaves and litter, and lower N use efficiency than on non-scarified plots. By comparison, scarification in Abitibi affected only one foliar property, namely a reduction in the C:N ratio of Kalmia leaf litter. Forest floor N mineralization rates and black spruce needle N concentrations were higher in scarified than non-scarified plots across both sites. Taken collectively, results suggest that mechanical scarification on ericaceous shrub-dominated cutovers can reduce competitive traits of boreal ericaceous shrubs and improve nutritional site quality, especially in cooler-wetter climates. Highlights Mechanical scarification increased growth rates of black spruce saplings. Scarified plots showed less above- and below-ground growth of ericaceous shrubs. Foliar and rhizomatous traits suggested higher soil fertility on scarified plots. Soil N mineralization and spruce needle N were higher on scarified plots. Positive effects of scarification can be higher in wetter cooler maritime climate. [ABSTRACT FROM AUTHOR]

Herbivores affect plants via direct consumption, but also indirectly via changes in the vegetation. These indirect effects may only manifest after decades of exposure and, as a result, their impact is rarely accounted for. To better understand the direct and indirect impacts of a large herbivore, moose (Alces alces), on forests, we measured tree growth in areas that were both subjected to and excluded from herbivory for over 80 years. Growth data were gathered from five tree species, ranging from low to high palatability. We found that at small sizes, Betula papyrifera, a preferred species, benefited from herbivore exclusion. However, larger individuals grew more when exposed to herbivory, a response we attribute to lower competition in heavily browsed conditions. Populus tremuloides, a highly preferred tree, did not show any differences between levels of herbivory. Abies balsamea, a preferred winter browsed, was only marginally affected by browsing at smaller sizes. The two non-preferred species, Picea glauca and Picea mariana, did not show differential growth between herbivory levels. We conclude that herbivores can impact forests through both direct and indirect effects, that these effects are size specific, and that effects vary among species in ways not always predicted by consumption patterns. [ABSTRACT FROM AUTHOR]

Climate change has increased the occurrence, severity, and impact of disturbances on forested ecosystems worldwide, resulting in a need to identify factors that contribute to an ecosystem's resilience or capacity to recover from disturbance. Forest resilience to disturbance may decline with climate change if mature trees are able to persist under stressful environmental conditions that do not permit successful recruitment and survival after a disturbance. In this study, we used the change in proportional representation of black spruce pre- to post-fire as a surrogate for resilience. We explored links between patterns of resilience and tree ring signals of drought stress across topographic moisture gradients within the boreal forest. We sampled 72 recently (2004) burned stands of black spruce in interior Alaska (USA); the relative dominance of black spruce after fire ranged from almost no change (high resilience) to a 90% decrease (low resilience). Variance partitioning analysis indicated that resilience was related to site environmental characteristics and climate-growth responses, with no unique contribution of pre-fire stand composition. The largest shifts in post-fire species composition occurred in sites that experienced the compounding effects of pre-fire drought stress and shallow post-fire organic layer thickness. These sites were generally located at warmer and drier landscape positions, suggesting they are less resilient to disturbance than sites in cool and moist locations. Climate-growth responses can provide an estimate of stand environmental stress to climate change and as such are a valuable tool for predicting landscape variations in forest ecosystem resilience. [ABSTRACT FROM AUTHOR]

Ecogeographic variation in black spruce clear wood properties was investigated for the two main vegetation types of the managed boreal forest of the province of Quebec, Canada. In total, 409 co-dominant and dominant trees from 82 mature stands were sampled. Basic wood density, modulus of elasticity and microfibril angle were measured using SilviScan. Mature fiber length was determined using a high-resolution Fiber Quality Analyzer. Wood growing in pure black spruce stands had longer mature fibers, a significantly denser wood with better mechanical characteristics than the wood growing in mixed stands with balsam fir. All wood properties were clearly influenced by radial growth and species composition. Given the limited number of sample plots for mapping purposes, a two-stage modeling approach was assessed to predict stand-level estimates of black spruce clear wood properties. This scaling-up method, based on field measurements and ring data from 3350 inventory plots, has improved the performance of all models. Stand-level models explained 47%, 57%, 63% and 63% of variance in wood density, modulus of elasticity, microfibril angle and mature fiber length respectively with estimated root mean square errors of 8.9 kg/m 3 , 0.52 GPa, 0.60° and 0.06 mm. An east-west gradient in black spruce clear wood properties was revealed as possibly the by-product of the change in relative proportions of both studied vegetation types across the study area. The results indicate that the black spruce wood from western regions of the managed boreal forest has a better potential for producing pulp and paper, lumber or engineered products due to its longer mature fibers and higher mechanical properties. [ABSTRACT FROM AUTHOR]

Although commonly cited as being particularly sensitive to biomass/nutrient removals via full-tree harvesting (i.e., entire aboveground portion of the tree is removed and processed at roadside), there have been no long-term experimental trials installed or reported on to date that specifically target shallow-soiled (<20 cm) sites common in strongly glaciated boreal landscapes. This study reports on ten year results examining the effects (i.e., regeneration potential, tree growth, soil N availability, and foliar nutrition) of operational full-tree harvesting compared to operational stem only harvesting (i.e., individual stems delimbed and topped at the stump) on replicated, shallow-soiled sites with contrasting soil textures (i.e., sand versus loam) and dominant commercial tree species (i.e., jack pine – Pinus banksiana Lamb. versus black spruce – Picea mariana (Mill.) B.S.P.). The operational harvests at the seven study sites were all conducted during the fall prior to freeze up. In terms of tree regeneration metrics, the harvest treatments only had a significant effect on free-to-grow (>1 m) crop tree (jack pine or black spruce) density. Despite the removal of cone-bearing branch material to roadside, the FT harvest treatment blocks had significantly higher densities compared to the SO blocks likely the result of increased mineral soil exposure, improved seedbed receptivity, and warmer soil surface temperatures during the growing season. Initially (5th year), jack pine height growth was better on the FT treatment blocks, but by year 10 this significant effect on total height was gone and the 5-year growth increment was better on the SO treatments. In contrast, black spruce height growth was better on the SO blocks in both year 5 and 10, although the 5-year increment was not significantly different between harvest treatments. Although soil N pools were higher in the SO treatments in year 10, the difference was only reflected in jack pine foliar N, suggesting black spruce may be less sensitive/responsive to declines in soil N pools. The 10-year results would suggest that no additional biomass retention restrictions are necessary for full-tree harvesting operations that are targeting traditional wood products (sawlogs and/or pulp & paper) on these shallow-soiled sites. However, some of the noted shifts in some of the measured parameters between year 5 and 10 would suggest longer-term monitoring of these sites will be necessary to provide more conclusive empirical evidence as to the long-term effects of full-tree harvesting on shallow soils within the boreal forest region. [ABSTRACT FROM AUTHOR]

Aim: Altitudinal and latitudinal treeline ecotones have not consistently responded to climate warming in the direction and/or magnitude predicted by climate alone, suggesting that non‐climatic mechanisms (e.g. biotic interactions) also mediate treeline range dynamics. Through a collaborative research approach, we assessed environmental conditions associated with pre‐dispersal insect cone granivory and how this biotic interaction may govern the reproductive potential, and therefore range dynamics, of spruce‐dominated treelines across northern Canada. Location: In all, 10 boreal forest treelines, tundra and alpine, from Yukon to Newfoundland and Labrador, Canada. Taxa: White spruce (Picea glauca [Moench] Voss), Black spruce (Picea mariana [Mill.] B.S.P.), Strobilomyia spp., Megastigmus spp. Methods: Treeline sites were assessed for presence and magnitude of pre‐dispersal seed granivory by insects along with viable seed availability. We quantified stand density metrics, organic layer depth and understorey vegetation composition at each location and, subsequently, incorporated those variables into generalized linear mixed models to establish predictors of granivory magnitude and viability of available seed. Results: Insect granivory was widespread across sites; however, site‐specific patterns of granivory were associated with increased moss cover and decreased shrub cover and stand density. While all black spruce‐dominated sites exhibited seed viability rates > 50%, the number of seeds produced per cone varied, suggesting that within‐site abiotic conditions and biotic interaction pressures limit successful colonization of novel environments in advance of seed dispersal. Main Conclusions: The modelled relationships between granivory, seed viability and environmental conditions represent an essential step towards generalizing how and when biotic interactions across subarctic treelines influence boreal tree range dynamics before seed dispersal. Connections between granivory magnitude and site‐level treeline characteristics (e.g. stand density, understorey vegetation) will provide a more comprehensive understanding of treeline range dynamics under continued climate change. [ABSTRACT FROM AUTHOR]

Forest trees are the world's most important renewable natural resources in terms of their dominance among other biomasses and the diversity of molecules that they produce. Forest tree extractives include terpenes and polyphenols, widely recognized for their biological activity. These molecules are found in forest by-products, such as bark, buds, leaves, and knots, commonly ignored in forestry decisions. The present literature review focuses on in vitro experimental bioactivity from the phytochemicals of Myrianthus arboreus, Acer rubrum, and Picea mariana forest resources and by-products with potential for further nutraceutical, cosmeceutical, and pharmaceutical development. Although these forest extracts function as antioxidants in vitro and may act on signaling pathways involved in diabetes, psoriasis, inflammation, and skin aging, much still remains to be investigated before using them as therapeutic candidates, cosmetics, or functional foods. Traditional forest management systems focused on wood must evolve towards a holistic approach, allowing the use of these extractives for developing new value-added products. [ABSTRACT FROM AUTHOR]