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Data collected over a 30-year period from an experiment replicated across 21 locations in western Canada are used to explore the effects of precommercial thinning of trembling aspen to a range of densities in combination with three initial white spruce densities on tree growth and stand dynamics. Increasing differentiation amongst the 15 treatments was observed with age after thinning for both spruce and aspen responses. Spruce height and diameter declined with increasing aspen density. At age 10 spruce diameter with no aspen was 1.5× that of spruce in unthinned while it was 2.6× that of spruce in unthinned at age 28. Following thinning aspen DBH and crown width declined with increasing density of retained aspen while slenderness and height to crown base of aspen increased. Thinning to 1500 aspen·ha−1 resulted in aspen DBH that was 22% larger relative to the unthinned at age 28. Spruce volume and stemwood biomass at age 90, estimated using the Mixedwood Growth Model (MGM21), declined with increasing initial aspen density, with the complete removal of aspen resulting in nearly double the spruce volume of unthinned plots. However, total stand volume, total stand biomass, and stemwood carbon at age 90 are predicted to be largest in mixed stands with aspen densities of 1500 stems·ha−1 or higher. [ABSTRACT FROM AUTHOR]

Boreal mixed forests of trembling aspen (Populus tremuloides) and white spruce (Picea glauca) can provide higher outputs of many ecosystem goods and services, combined with resilience for changing environmental conditions. In this study, we examine the growth of white spruce and aspen over a range of stand compositions created by spot and broadcast treatments of broadleaves using manual and chemical means, aspen spacing, and untreated control. Twenty-two growing seasons postharvest, treatment responses created a range of broadleaf densities and spatial arrangements, reflected in significant differences in heights and diameters of the spruce mixedwood component. At this early seral stage, treatments have generated diversity in both tree species composition and stand structure. Modeling of the stand structures created by the different treatments provided outcomes ranging from pure conifer to broadleaf dominated when assessed by merchantable volumes. Broadleaf manipulations changed the relative proportion of spruce and broadleaf species in a mixture, achieving a variety of potential landscape and stand-level goals. Treatment differences were often not statistically significant until at least 16 years after stand initiation. Monitoring the achievement of mixedwood management goals will require longer time periods than currently employed for broadleaf or conifer monocultures. [ABSTRACT FROM AUTHOR]

Herbivores shape vegetative communities via numerous mechanisms, including browse. We used vegetation monitoring data from Isle Royale National Park to examine woody species change across a nine‐year interval, coinciding with herbivore escalation. Here, moose and snowshoe hare are the dominant herbivores, while the gray wolf is the apex predator. Our initial sampling period (2010) followed six years of low moose abundance, while our second sampling event (2019) followed a nine‐year escalation in moose density. We tested for change in both saplings and shrubs and compared diameter size distributions of common tree species in three island sections. We found a decline in large saplings of sugar maple, a species limited to the west section of the island. We also saw declines in small saplings of sugar maple, paper birch, and trembling aspen. For some species, including black spruce and white spruce, taxa that are unpalatable to moose, diameter distributions were proportionally larger (i.e., indicative of fewer small individuals) in 2019 than in 2010. In contrast, diameter distributions of black ash in the central section and trembling aspen in the central and east sections of the island were proportionally smaller (indicative of more small individuals) in 2019 than in 2010. We found that ~50% of common shrub taxa were more abundant during the second sampling event, while none declined. Our work here brings to light several unanticipated results. While we demonstrated partial recovery of trembling aspen, likely originating during the earlier period of low moose abundance, our results suggest further changes may be attributable to other island herbivores. The response of black ash, a hydric species, may be stemming from a sixfold increase in active beaver sites, while that of spruce may be attributable to a historic spike in snowshoe hare density. We currently have only a limited understanding of the impacts of the island's lesser studied herbivores on overstory regeneration. Further research on these linkages can inform decisions addressing controls on forest structure. [ABSTRACT FROM AUTHOR]

Purpose: Most plants interact with soil biota that positively or negatively affect seedling performance. These plant-soil feedbacks (PSFs) can strongly affect recruitment, potentially for years after death. We tested whether PSFs persisted following death for Populus tremuloides Michx. (aspen) and if these effects were environment dependent using soils collected from live and dead aspen and heterospecific Picea glauca (Moench) Voss in 24 stands across two ecosystems. Methods: We conducted a greenhouse experiment using soils from 24 aspen stands. At each stand, we collected soil from three trees of each tree type and used live and sterilized versions of these soils to inoculate aspen seedlings. We then recorded mortality and growth of the seedlings over three months. Results: Live inocula reduced aspen survival and growth relative to sterilized inocula, suggesting that pathogens drive PSF. Plant responses to live and dead aspen inocula were correlated across environments; however, responses to aspen and heterospecific Picea inocula were uncorrelated, suggesting that specialized pathogens may drive PSF. Conclusions: Pathogen-driven negative PSFs can persist for multiple years irrespective of the environment, potentially limiting the regeneration of aspen stands following dieback. Persistent PSFs thus have potential to cause lagged effects on population and community dynamics. [ABSTRACT FROM AUTHOR]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Broadcast motor-manual or manual brushing treatments applied to control aspen (Populus tremuloides Michx.) in young spruce (Picea glauca (Moench) Voss) plantations often result in increases in the number of aspen stems and the amount of aspen competition. In this study data collected at three locations is used to examine the potential effectiveness of spot brushing treatments, applied over radii ranging between 1.0 and 2.5 m around individual spruce, to aspen that are two, four or six years old. Results indicate that spot manual or motor-manual treatments result in reductions in the number and size of aspen stump sprouts compared to the untreated control. However, when aspen size and vigour are reduced due to site or other factors, as observed for blocks at one location, post-treatment aspen densities may still be high (e.g., above 5000 stems ha−1). Spot treatments applied when aspen regeneration was two years old were observed to be less effective than treatments applied at ages 4 or 6. At two study locations where control was effective, spot brushing treatments significantly increased spruce diameters after treatment compared to untreated. Broadcast or spot treatment using basal bark application of triclopyr ester (Release®) herbicide at one of the study sites resulted in similar increases in spruce diameter to those observed for motor-manual treatment. [ABSTRACT FROM AUTHOR]

Understory protection harvesting is a form of partial cutting that can be used in aspen (Populus tremuloides Michx.)-dominated stands that have understories of white spruce (Picea glauca (Moench) Voss). This practice involves removing 75% to 85% of the merchantable aspen while minimizing damage to the advance spruce regeneration, in addition to leaving 15% to 25% of the aspen standing to reduce potential windthrow of the spruce understory. In this paper, we summarize results from 18 stands measured 10 to 12 years after understory protection harvest. Diameter growth of spruce increased during the first five years after harvest while height growth increased during the second five-year period (5 to 10 or 7 to 12 years after release). Consistent with other studies, mortality rates of aspen trees ≥7.1 cm DBH (diameter breast height, 1.3 m) averaged 45.0% over the 10–12 year period following harvesting. Spruce mortality averaged 27.5% over the same 10–12 year period. Substantial aspen regeneration was evident across most harvested blocks, with aspen sapling densities 10–12 years from harvest being higher in removal (14,637 stems⋅ha−1) than in buffer areas (6686 stems⋅ha−1) and in extraction trails (7654 stems⋅ha−1). Spruce sapling (>1.3 m height and <4 cm DBH) densities averaged 1140 stems⋅ha−1 in removal areas at ages 10–12, with these trees likely being present as seedlings at the time of harvest. Mixedwood Growth Model projections indicate merchantable volumes averaging 168 m3⋅ha−1 (conifer) and 106 m3⋅ha−1 (deciduous) 70 years from harvest, resulting in MAI (mean annual increment) for this period averaging 2.0 m3⋅ha−1⋅y−1 with MAI for a full 150-year rotation of approximately 2.5 m3⋅ha−1⋅y−1. [ABSTRACT FROM AUTHOR]

In the face of anticipated climate change, growth models require modifications to effectively model likely future growth and survival of trees and stands. To support incorporation of climate effects in the Mixedwood Growth Model (MGM), we developed climate-sensitive mixed effects models for lodgepole pine (Pinus contorta Douglas ex Loudon), jack pine (Pinus banksiana Lamb.), trembling aspen (Populus tremuloides Michx.), balsam poplar (Populus balsamifera L.) and white spruce (Picea glauca (Moench) Voss), and examined how tree size, intra- and interspecific competition and climate will influence individual-tree diameter growth. Remeasurement data from permanent growth and yield sample plots from across the western Canadian boreal forest were used for this study. Based on the model that we developed, tree size, competition and climate had varying effects on growth, among tree species. Although intraspecific competition had a stronger negative effect on growth for deciduous and white spruce trees, interspecific competition had a stronger negative effect on the growth of pine. Deciduous trees growing in stands having longer frost-free period experienced an increase in growth but a negative effect of frost-free period was observed for conifers. In addition, higher moisture was found to increase tree growth, but the influence of available moisture on growth was dependent on competition. Overall, these results suggest that, under climate warming, between-species differences in growth responses will contribute to our understanding of the stand dynamics in the western boreal region of Canada. Climate sensitive growth models developed in this study will be useful in the recalibration of MGM and other individual-tree models, and predictions will contribute to better-informed decisions about silvicultural treatments for these economically important boreal tree species. [ABSTRACT FROM AUTHOR]

Numerous recent studies have argued that moisture limitation is leading to growth declines and mortality of black spruce (Picea mariana) and white spruce (Picea glauca) in the North American boreal forest. A parallel line of research suggests that increasingly common severe wildfires are altering successional pathways and leading to long‐term replacement of spruce forests with those dominated by paper birch (Betula papyrifera, Betula neoalaskana) and trembling aspen (Populus tremuloides). When both conifers and deciduous trees establish after fire, this biome shift hypothesis implicitly assumes that deciduous species will outcompete the conifers, owing to their more rapid vertical growth and because they might be less sensitive to warm and dry conditions.We established a research site in a white spruce‐paper birch forest on an east‐facing slope in interior Alaska and tested the hypothesis that Alaska paper birch are better adapted to warm and dry conditions than white spruce. Over 6 years (2013–2018), we made hourly measurements of microclimate and xylem sap flux of both species. We also collected increment cores and conducted climate‐growth analyses for both species.During our 6‐year study, growing seasons with low volumetric soil water content (SWC) were those that followed shallow winter snowpacks and had limited summer rainfall, not necessarily those with warm air temperature. Both species were sensitive to moisture limitation. The tree‐ring data revealed significant positive effects of cumulative water year precipitation on radial growth, with a steeper slope for paper birch than for white spruce. Radial growth of both species was also positively related to mean water year air temperature. Sap flux density declined progressively for white spruce over the range of observed SWC and abruptly for paper birch when SWC fell below ~15%.Synthesis. Our results show that, while paper birch might be less sensitive to mild drought than white spruce, it may be more sensitive to severe drought, raising questions about the ability of paper birch to outcompete co‐occurring white spruce in a drier climate. [ABSTRACT FROM AUTHOR]

In Fig 2, the label "r2 = 0.36, P < 0.0001" refers to Panel A, and the label "r2 = 0.28, P < 0.0001" refers to panel B. Please see the correct Fig 2 here. Mean aspen DBH vs. percentage of trees that were aspen (= relative aspen density) across stands where aspen grew with white spruce, black spruce, Alaskan paper birch, or a combination (A) and aspen basal area vs. mean aspen DBH (B) across study sites. [Extracted from the article]

Planting white spruce ( Picea glauca (Moench) Voss) under established aspen ( Populus tremuloides Michx.) stands has substantial potential for regenerating mixedwood ecosystems in the western Canadian boreal forest. The presence of an aspen overstory serves to ameliorate frost and winter injury problems and suppresses understory vegetation that may compete with white spruce. Under future climatic regimes with more frequent and severe drought episodes, underplanting may be a cost-effective strategy for lowering the risk of mortality in mixedwood regeneration. We examine the growth of white spruce during the first 18 years after being planted beneath a 39-year-old stand of trembling aspen. Treatments included thinning from over 6000 stems·ha−1 to 3000, 2000, and 1000 stems·ha−1 and fertilization. Initial stimulation of understory vegetation by fertilization had no measureable effect on spruce heights or diameters at year 18. Aspen thinning treatments did not have a significant effect on spruce height growth rates after spruce crowns had emerged above the understory shrub layer due to rapid aspen basal area increases after thinning. Small, but significant, increases for spruce height and diameter were present in the 1000 and 2000 stem·ha−1 aspen thinnings. A much wider range of aspen stand conditions may be suitable for planting spruce to create mixedwood ecosystems than has been previously considered. [ABSTRACT FROM AUTHOR]

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Aspen ( Populus tremuloides Michx.) and white spruce ( Picea glauca (Moench) Voss) are archetypal species of the boreal forest of western Canada. With the many new and varied environmental pressures (e.g., fire, harvesting, climate change) on, this boreal forest biome, we were interested in understanding the importance of potential changes in organic matter inputs to its soil microbial community structure and function. Double -labeled ( 15 N and 13 C) aspen leaf litter was utilized as an amendment to the forest floor of two stands, an aspen and a spruce, in a 16-month paired field experiment, where amended mesocosms were compared to control mesocosms with no litter added. Nitrogen mineralization from the aspen leaf litter occurred within both stand types, as evidenced by higher δ 15 N values in roots and live aboveground vegetation. In term of carbon fluxes, stable isotope 13 C analysis of the forest floor microbial phospholipid fatty acid (PLFAs) indicated a slight initial enrichment within all amended mesocosms, likely due to leaching of 13 C enriched solubles from the labeled aspen litter. Within the aspen stand, continued 13 C enrichment of most PLFAs demonstrated that incorporation of the litter-derived carbon by a wide range of microorganisms was maintained through both growing seasons. Response of the spruce microbial community differed from that of aspen. Its overall structural composition did not alter in response to aspen litter addition; yet, δ 13 C values of several PLFA biomarkers decreased at the end of the first growing season, suggesting that priming stimulated mineralization of a 13 C-depleted pool. These results indicate that while nitrogen mineralization was maintained, the deep forest floor carbon stocks of spruce stands may be vulnerable to increased decomposition rates with changing organic matter input. [ABSTRACT FROM AUTHOR]

Intimate mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are a key feature of western Canadian boreal forests. These mixtures have the potential to produce high yields of merchantable fibre and provide numerous ecological services. Achievement of this potential has been difficult, and often expensive, to realize as a regeneration goal in managed forests. We report 21 to 25 year results of managed mixtures on two study sites where the white spruce was planted, and the density of aspen natural regeneration manipulated within five years of the stand initiation disturbance. On both sites, white spruce mortality did not increase with increasing aspen density. While height and diameter growth of white spruce declined with increasing aspen density, the effect was not entirely consistent across the two sites. Abrasion from aspen branches was the most common source of damage to spruce crowns. Mixed stands had greater merchantable volume production than pure spruce stands based on model projections. Application of aspen harvest at year 60, while protecting the spruce component for a second harvest entry at year 90, was projected to optimize combined yield for the mixedwood stands. [ABSTRACT FROM AUTHOR]

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Numerous models are available in northeastern North America to estimate aboveground tree biomass, yet most have focused on trees ≥12.5 cm diameter, and these models are often poor predictors of small tree biomass (<12.5 cm diameter). Additionally, models available to estimate small tree biomass often lack independent evaluation with field data. We developed a new set of additive biomass component (foliage, branch, and bole) models for five naturally regenerated hardwood species, four hybrid poplar (Populus sp.) clones and white spruce (Picea glauca (Moench.), from trees in eastern Maine. Biomass measurements were used to evaluate national and regional models for naturally regenerated species, and species specific models for planted stock. Results showed that the regional and national models provided similar fits for total biomass across all species. Moreover, the small tree biomass models currently used to predict woody biomass substantially underestimated biomass from 19.0% to 36.6% for all of the naturally regenerated hardwood species at our site. This substantial underestimation of small tree biomass may have contributed to the recent 34% decrease in estimated sapling woody biomass, and 15% decrease in aboveground biomass of all living trees in Maine, where nearly one-quarter of the 70,000 km2 forestlands are dominated by saplings. This problem may exist in other regions of the United States if substantial proportions of forestlands are dominated by small trees. Further model evaluation is warranted to assess the performance of the current models. [ABSTRACT FROM AUTHOR]

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• Short lodgepole pine trees were more likely to be dead when surrounded by tall trees. • Trembling aspen was more likely to be alive when surrounded by tall neighbours. • White spruce trees were more likely to be dead when surrounded conspecific trees. • Tree mortality can be studied by using unmanned aerial vehicles. Advances in unmanned aerial vehicle (UAV) technology have opened new opportunities for measuring canopy mortality processes through high-resolution aerial imagery. Unlike traditional plot-based sampling, UAVs can easily survey areas several hectares in size that encompass hundreds of canopy trees. The resulting imagery provides extensive data on the number and locations of dead trees (snags), which can be related to neighbourhood conditions and environmental factors that vary among stands. Here, we used a UAV to measure the relative proportions of live and dead canopy trees in stands varying in soil moisture across a mixed forest landscape. Crowns of live trees and snags were first segmented and classified to species from high-resolution canopy imagery. We then modeled whether crowns of each species were alive or dead as a function of tree height, neighbourhood competition, soil moisture, and the relative abundance of conspecifics. Short lodgepole pine (Pinus contorta var. latifolia) trees were more likely to be dead when surrounded by a tall canopy in sites with greater soil moisture, reflecting effects of competition on this shade-intolerant species. Trembling aspen (Populus tremuloides) was more likely to be alive when surrounded by tall neighbours, perhaps because this clonal species can benefit from facilitation through root connections with neighbouring aspen stems. There was a higher frequency of white spruce (Picea glauca) snags in sites where it had a high relative abundance, suggesting strong effects of intraspecific competition for this species. Soil moisture did not appear to have a direct effect on the snag frequency of any of these species, despite pronounced niche partitioning along an elevation-driven moisture gradient. Our models explained 39% and 33% of the variation in white spruce and lodgepole pine snag frequency, respectively, but did not have much predictive power for trembling aspen or total snag frequency. Our results reflect the important role of competition in determining tree mortality, but also indicate that stochastic or unexplained processes account for considerable variation in snag frequency among stands. As UAV technology becomes more widely used by ecologists, it may enable a better understanding of how biotic and abiotic processes produce local variation in canopy mortality. [ABSTRACT FROM AUTHOR]

We evaluated the Mixedwood Growth Model (MGM) at a whole model scale for pure and mixed species stands of aspen and white spruce in the western boreal forest. MGM is an individual tree-based, distance-independent growth model, designed to evaluate growth and yield implications relating to the management of white spruce, black spruce, aspen, lodgepole pine, and mixedwood stands in Alberta, British Columbia, Saskatchewan, and Manitoba. Our validation compared stand-level model predictions against re-measured data (volume, basal area, diameter at breast height (DBH), average and top height and density) from permanent sample plots using combined analysis of residual plots, bias statistics, efficiency and an innovative application of the equivalence test. For state variables, the model effectively simulated juvenile and mature stages of stand development for both pure and mixed species stands of aspen and white spruce in Alberta. MGM overestimates increment in older stands likely due to age-related pathology and weather-related stand damage. We identified underestimates of deciduous density and volume in Saskatchewan. MGM performs well for increment in postharvest stands less than 30 years of age. These results illustrate the comprehensive application of validation metrics to evaluate a complex model, and provide support for the use of MGM in management planning. [ABSTRACT FROM AUTHOR]

Mixedwood forests of white spruce ( Picea glauca (Moench) Voss) and trembling aspen ( Populus tremuloides Michx.) may possess ecological advantages over monospecific white spruce stands. Belowground competition may be reduced through vertically stratified roots; facilitation of growth may occur in upper soil layers through nutrient-rich trembling aspen litterfall. These effects may incentivize white spruce to preferentially exploit upper soil layers in mixedwood stands, resulting in wider root systems. This research contrasted white spruce fine root (diameter <2 mm) distributions in organic layers of white spruce and mixedwood stands. Research occurred at the Fallingsnow Ecosystem Project site in northwestern Ontario. Eighteen plots represented mixedwood and pure white spruce stands. Trees were mapped; foliage and root samples were collected. Roots were separated by species and scanned to determine length. Simple sequence repeat DNA profiles were determined for all white spruce trees and for 45 white spruce root fragments per plot. Root and tree DNA profiles were matched; corresponding distances were calculated. Most (80%) root fragments were within 3.2 m of tree stems. Root prevalence decreased rapidly with distance. Organic layer pH was significantly less acidic in mixedwood plots, but only in one block. A subtle significant trend towards wider root distributions occurred in mixedwood stands. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the combined effect of trembling aspen competition and climate on white spruce growth using data from a long term study with matching treatments spread across the boreal mixedwood forests of Alberta and Saskatchewan (Canada). Results indicate that competition (i.e., aspen basal area), initial size of the tree and mean annual temperature can account for 88% of the year to year variation in spruce volume growth for these six locations. Based on the model that we developed, spruce growth, in the absence of competition, is estimated to increase by up to 17% compared with an increase in mean annual temperature from 2°C to 3.3°C, while, at high levels of competition (aspen basal area=27m2 ha−1) spruce growth increases by only 8%. Moreover, effects of aspen on spruce growth increase more than proportionally as temperature increases. This outcome indicates that abundant aspen competition limits the spruce responses to rising temperature, presumably due to competition for light and potentially increased competition for soil resources. Results also show that competition and climate effects vary between locations, indicating that spruce growth is strongly influenced by local factors such as micro-climate, topography, and soil properties. [Copyright &y& Elsevier]

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1. Variable retention harvest is an increasingly popular management alternative to clear-cutting in boreal forest ecosystems. This harvest system retains a portion of the mature trees to maintain structural elements of the forest and preserve biodiversity. Residual trees must, however, survive in the post-harvest environment to realize these benefits. Here, we investigate which tree and stand characteristics are associated with high survival to aid foresters in designing harvesting prescriptions. 2. We conducted an operational-scale experiment broadly representative of western North American boreal forests. Treatments were four overstorey compositions (ranging from deciduous to conifer dominated) and five rates of canopy retention (10%, 20%, 50%, 75% and a 100% control). Factors affecting mortality were analysed for the leading tree species (aspen Populus tremuloides Michx. and white spruce Picea glauca (Moench) Voss 5 and 10 years after harvest. 3. Mortality of residual aspen was highest in 10% and 20% retention treatments with c.30% mortality after 5 years and c. 50% after 10 years. At year 5, most of the dead trees remained upright as snags (87%), but by year 10, c.40% of dead trees had fallen. By contrast, mortality was much lower in the 50% and 75% retention treatments, and similar to that of the unharvested control. Results for white spruce mortality were similar, except the shallow-rooted spruce that was more susceptible to be blown down. 4. Regression tree analysis was used to identify individual tree attributes that predict 5- and 10-year survival. Mortality was higher for trees with a low percentage of live crown and high height/diameter ratio, when trees were very tall or were close to machine corridors. Smaller residual spruce trees from deciduous-dominated stands had the lowest mortality rates. 5. Synthesis and application. Where live trees are critical elements of wildlife habitat, forest managers should maintain ≥50% in retention harvest systems. At lower levels of retention (≤20%), mortality can be reduced if foresters select retention trees that are stout, possess large live crowns and are small to intermediate in size. These factors can be used to help design partial harvest systems for effective biodiversity conservation strategies. [ABSTRACT FROM AUTHOR]

Abstract: The effects of 4years of simulated nitrogen (N) and sulfur (S) depositions on gross N transformations in a boreal forest soil in the Athabasca oil sands region (AOSR) in Alberta, Canada, were investigated using the 15N pool dilution method. Gross NH4 + transformation rates in the organic layer tended to decline (P <0.10, marginal statistical significance, same below) in the order of control (CK, i.e., no N or S addition), +N (30kgNha−1 yr−1), +S (30kgSha−1 yr−1), and +NS treatments, with an opposite trend in the mineral soil. Gross NH4 + immobilization rates were generally higher than gross N mineralization rates across the treatments, suggesting that the studied soil still had potential for microbial immobilization of NH4 +, even after 4years of elevated levels of simulated N and S depositions. For both soil layers, N addition tended to increase (P <0.10) the gross nitrification and NO3 − immobilization rates. In contrast, S addition reduced (P <0.001) and increased (P <0.001) gross nitrification as well as tended (P <0.10) to reduce and increase gross NO3 − immobilization rates in the organic and mineral soils, respectively. Gross nitrification and gross NO3 − immobilization rates were tightly coupled in both soil layers. The combination of rapid NH4 + cycling, negligible net nitrification rates and the small NO3 − pool size after 4years of elevated N and S depositions observed here suggest that the risk of NO3 − leaching would be low in the studied boreal forest soil, consistent with N leaching measurements in other concurrent studies at the site that are reported elsewhere. [Copyright &y& Elsevier]

Managing boreal mixed stands of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca [Moench] Voss) is more likely to sustain a diversity of values and has the potential to increase productivity at both the site and landscape levels compared with pure broadleaf or conifer management. In this study, we examine growth of white spruce and aspen after 11 growing seasons over a range of aspen densities created by spot and broadcast treatment of broadleaves using manual and chemical means, aspen spacing, and an untreated control. Results indicate that survival and growth of both spruce and aspen were similar across the range of treatments. Spruce groundline diameter was greater, and height to groundline diameter ratio was lower, for the treatments in which aspen was chemically controlled or uniformly spaced compared with the control. Light measurements at the individual tree level suggested that increased light availability improved white spruce diameter growth. Spruce height growth did not vary by treatment. The status of these experimental mixedwoods was compared with current conifer and mixedwood regeneration evaluations, as well as the preharvest composition of the original stand. After 11 growing seasons, growth of aspen and white spruce indicated that opportunities exist to further modify aspen density to enhance treatment longevity and effectiveness to produce a greater range of boreal mixedwood stand types. [ABSTRACT FROM AUTHOR]

Vertical and horizontal patterns of light within gaps and expansion of trees into gaps were examined in young stands dominated by trembling aspen (Populus tremuloides Michx.) in northeastern British Columbia and northwestern Alberta. Lateral growth of branches of aspen edge trees averaged 15.2 cm/year and was similar to crown expansion rates reported for edge trees in eastern North America. Branches growing into gaps were significantly longer than those growing away from gaps and are likely to lead to gap shrinkage and reductions in light within gaps. However, height growth of edge trees was not significantly different from that of trees within the surrounding stands. Models describing relationships between diffuse transmittance and locations within gaps and surrounding stand height were developed in this paper. Sky view angle (SVA) was found to be effective for predicting transmittance to different locations within gaps. When SVA values exceed 1.2 (at the B.C. sites), white spruce (Picea glauca (Moench) Voss) will receive <40% of full sunlight, which is less than optimal for spruce height growth. [ABSTRACT FROM AUTHOR]

In mixedwood boreal forests of western Canada, stands classified as “pure deciduous” by forest inventories sometimes contain a few large white spruce (Picea glauca (Moench) Voss) trees among or emerging from the canopy. These trees are important as regeneration seed sources and for habitat structure. Neither their abundance nor the characteristics of stands in which they occur have previously been quantified. Of 275 “pure aspen” stands in northeastern Alberta, 19.6% contained at least one such spruce detectable in an aerial photograph. These trees were found in stands across the range of sampled canopy heights, densities, age classes, and stand sizes and were often present in the interior of stands, not just on the perimeter. The frequency of 3 ha cells containing at least one spruce was related to (i) stand shape and size, (ii) amount of mature white spruce in adjacent forest, (iii) canopy height, (iv) stand age, and (v) stand density. We conclude that such trees are relatively abundant and widely distributed within the boreal mixedwood forests of Alberta. There is presently no provision to maintain this landscape element within managed forests. More information is needed to determine if or how they should be considered in forest management planning. [ABSTRACT FROM AUTHOR]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Establishing white spruce (Picea glauca (Moench) Voss) by planting it under established aspen (Populus tremuloides Michx.), stands has substantial potential as a technique for regenerating boreal mixedwood stands. The presence of an aspen overstory serves to ameliorate frost and winter injury problems and suppresses understory vegetation that may compete with white spruce. In this study we examine the growth of white spruce during the first 10 years after being planted underneath a 39-year-old stand of trembling aspen following thinning and fertilization. Results indicate successful establishment and reasonable growth rates of white spruce planted under thinned and unthinned aspen stands, even with aspen basal area of 51m2 ha−1. Thinning of overstory aspen to 1000 or 2000stemsha−1 did not increase light reaching seedlings, but did result in improvements in light above the shrub layer and in diameter and height growth of the underplanted seedlings. However, these increases in growth of underplanted spruce may not justify the expense of thinnings. Fertilization of these stands prior to planting had no effect on spruce growth. Growth of spruce underplanted at this site near Fort Nelson was similar to that at two other stands near Dawson Creek, B.C. Information on treatment effects on light, beam fraction, air temperature, and soil temperature are presented. Measurements of light obtained using LAI-2000 and hemispherical photography were compared with those obtained from hourly PPFD measurements. Results indicate that while both methods provide estimates of transmittance that are highly correlated with those provided by actual light measurements, they provide substantial underestimates of actual light levels. [Copyright &y& Elsevier]

Environmental concerns and economic pressures on forest ecosystems have led to the development of sustainable forest management practices. As a consequence, forest managers must evaluate the long-term effects of their management decisions on potential forest successional pathways. As changes in forest ecosystems occur very slowly, simulation models are logical and efficient tools to predict the patterns of forest growth and succession. However, as models are an imperfect representation of reality, it is desirable to evaluate them with historical long-term forest data. Using remeasured tree and stand data from three data sets from two ecoregions in northern Ontario, the succession gap model ZELIG-CFS was evaluated for mixed boreal forests composed of black spruce (Picea mariana [Mill.] B.S.P.), balsam fir (Abies balsamea [L.] Mill.), jack pine (Pinus banksiana L.), white spruce (Picea glauca [Moench] Voss), trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.), northern white cedar (Thuja occidentalis L.), American larch (Larix laricina [Du Roi] K. Koch), and balsam poplar (Populus balsamefera L.). The comparison of observed and predicted basal areas and stand densities indicated that ZELIG-CFS predicted the dynamics of most species consistently for periods varying between 5 and 57 simulation years. The patterns of forest succession observed in this study support gap phase dynamics at the plot scale and shade-tolerance complementarity hypotheses at the regional scale. [ABSTRACT FROM AUTHOR]

Abstract: In 1992, the Western Boreal Growth and Yield Association (WESBOGY) began a long-term study to evaluate the dynamics of regenerated aspen (Populus tremuloides Michx.)—white spruce (Picea glauca (Moench) Voss) mixedwood stands following manipulation of aspen to a range of densities. In this study six levels of aspen (0, 200, 500, 1500, 4000stemsha−1 and natural) and three levels of spruce (0, 500 and 1000stemsha−1) densities have been created. Data from four locations demonstrate substantial variation in initial aspen densities following clearcutting of aspen dominated stands. After 9 years densities begin to converge with the highest rates of mortality associated with high starting densities. A model was developed that shows a significant relationship between the proportion of trees surviving to the end of a year and the density at the beginning of the year. Size-density relationships based on quadratic mean root collar diameter, mean tree volume and mean tree height are presented. Three to four years following spacing of aspen to densities ranging from 200 to 4000stemsha−1 there were no significant effects of density on aspen size. In addition, spacing of the aspen had no significant effect on spruce height at year 9 (3–4 years after spacing), but spruce root collar diameter (RCD) was significantly smaller in the unspaced compared to the spaced plots. The ratio of height to root collar diameter (HDR) for white spruce showed a significant and clear response to aspen density and increased with increasing aspen density. [Copyright &y& Elsevier]

The influence of microenvironmental conditions on the growth performance (i.e., diameter and height growth) of White Spruce [Picea glauca (Moench) Voss] seedlings was examined within three contrasting habitats: White Spruce tree islands, open prairies and Trembling Aspen (Populus tremuloides Michx.) groves. These habitats exist within a disjunct occurrence of White Spruce at its southern limit of distribution in three mixed-grass prairie preserves in the Spruce Woods Provincial Park within the forest-prairie ecotone of southwestern Manitoba. Microenvironmental measurements (i.e., light, temperature, relative humidity, soil moisture) were obtained on 10 sites in each of the three habitats and growth characteristics of 60 White Spruce seedlings were measured in each of three habitats. Higher light and soil temperature conditions occurred within the open prairie. In contrast, lower light and soil temperature conditions occurred under the tree canopy of aspen groves and the northern aspect of spruce islands, which moderated the effect of the dry regional climate. Height growth did not vary significantly among the three habitats. The greater diameter growth and decreased slenderness of White Spruce seedlings in the open versus the shaded habitats appears to be a result of increased photosynthesis at higher light intensity and may also represent a morphological adaptation to withstand the effect of increased wind exposure. The increased slenderness of White Spruce in the shaded habitats appears to be a morphological adaptation of increasing carbon allocation towards height growth and thus maximizing effective competition for tight. [ABSTRACT FROM AUTHOR]

Trembling aspen (Populus tremuloides Michx.) density and growth were assessed 9-12 years after stand establishment to determine whether mechanical site preparation (MSP) affects crop tree quality. Study sites were either treated with disc trenching or ripper ploughing and planted with white spruce (Picea glauca (Moench) Voss) seedlings immediately after harvest (treated) or were undisturbed since harvest (control). Stands were surveyed during the summer of 2002 with standard regeneration survey plots. Results show that aspen stem density and height were lower in MSP-treated areas relative to untreated areas. Diameter growth rates were unaffected by treatment, however the percentage of stem discolouration was higher in untreated control stands compared to site prepared areas. The results of this study indicate that there are no long-term benefits to carry out MSP for aspen promotion. However, as MSP does not appear to seriously harm the aspen crop, we suggest that this treatment can still be used on sites where aspen densities may be low without treatment (e.g., sites with extremely low soil temperatures, poor soil aeration, or vigorous competitive vegetation) or where a mixture of aspen and planted spruce are desired. [ABSTRACT FROM AUTHOR]

Abstract: With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce (Picea glauca; SPRUCE) or by trembling aspen (Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites. [Copyright &y& Elsevier]

We examined jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) BSP), and white spruce (Picea glauca (Moench) Voss) seed germination and seedling recruitment in aspen (Populus tremuloides Michx.) and conifer mixedwood stands following the 1999 Black River fire in southeastern Manitoba, Canada. Three postfire seedbed types were tested: scorched (surface litter only partially consumed), lightly burned (surface litter consumed with little or no duff consumption), and severely burned (complete consumption of litter and duff exposing mineral soil). Seeds were sown in 1999, 2000, and 2001, and each cohort was monitored for 3 years. In 1999, severely burned seedbeds had poor germination, while scorched seedbeds had the highest germination. The reverse was true in 2001. After the first growing season, continued survival of seedlings was greater on severely burned seedbeds for all three cohorts. However, better survival on severely burned seedbeds was not sufficient to overcome poor germination in 1999 and 2000. When using artificial seeding to promote conifer regeneration, we recommend a delay of one full year after a severe spring fire for jack pine and two full years for black spruce and white spruce on boreal aspen and conifer mixedwood sites. [ABSTRACT FROM AUTHOR]

Abstract: In the Canadian boreal mixedwood region, there is currently increasing management desire to foster and release understory trees. Consequently, there is also an increased interest in modeling and predicting understory light levels. In this study, species-specific crown openness is defined as the fraction of sky that can be seen through the crown of an individual tree of a given species. Species-specific crown openness is of general interest for understanding light transmission through forest canopies, and is an essential part of the light submodel in SORTIE. The main objective of this study was to investigate whether regional differences in mean species-specific crown openness exist for aspen (Populus tremuloides Michx) and white spruce (Picea glauca (Moench) Voss) in western boreal Canada. To ensure a robust comparison of regional mean species-specific crown openness, we initially investigated the underlying assumption that crown openness is unaffected by dbh and angle of view. In our data, both aspen and spruce crown openness was found to be independent of angle of view. Crown openness was also independent of dbh in aspen, while weak indications of a correlation between crown openness and dbh was found for white spruce. However, this relationship has little actual effect on crown openness and its effect on predicted understory light level is judged to be small. We found significant regional differences in mean crown openness for both aspen and spruce. However, these regional differences are small and are likely to have relatively little effect on understory light levels predicted with SORTIE. The results from this study indicate that, although previous estimates of aspen and spruce crown openness in western boreal Canada varied greatly, this is more likely the result of different methodologies than actual differences in crown openness. [Copyright &y& Elsevier]

Abstract: Effective utilization of a patch or cluster approach to growing aspen-white spruce mixtures in the boreal forest requires an understanding of how the faster growing aspen patches influence microclimate and spruce growth in adjacent spruce patches. In this paper we examine, how young (11–13 year old) aspen (Populus tremuloides Michx) patches influence microclimatic conditions in adjacent openings and how these are reflected in the growth of white spruce (Picea glauca [Moench] Voss) on three boreal mixedwood sites in west-central Alberta, Canada. Light levels increased continuously across the aspen stand boundary and reached maximum levels within the opening, while soil moisture was highest near the edge of the opening and then decreased with distance into the aspen stand or into the adjacent opening north of the aspen stand. Light levels were reduced over a greater distance when moving north from aspen stand edges compared to openings located south of the young aspen. These young aspen stands provided growing season frost protection within one tree length from the edge. The growth of white spruce was positively related to initial crown size and either light levels or distance from the edge. Stem volume growth was best predicted by initial tree size and the light levels at the midcrown of seedlings. [Copyright &y& Elsevier]

Alterations in the chemical properties of the forest floor following clear-cut harvesting may have implications for forest productivity in boreal stands. We used proximate analysis, carbon-13 (13C) isotopic determination, and cross-polarization, magic-angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy to examine differences in the characteristics of the forest floors from uncut stands and clear-cut stands dominated by white spruce (Picea glauca (Moench) Voss; SPRUCE) and trembling aspen (Populus tremuloides Michx.; ASPEN) in northern Alberta. Proximate analysis revealed no difference in the chemical properties of forest floors from clear-cut and uncut stands in either stand type, but the acid-insoluble residue of forest floors from clear-cut ASPEN stands was enriched in 13C compared with those from uncut ASPEN stands. CPMAS 13C NMR spectroscopy revealed that forest floors from clearcuts were enriched in total aromatic C, particularly in ASPEN stands, and depleted in phenolic C, particularly in SPRUCE stands. These patterns indicate that forest floors from the clearcuts have become more humified, which may reflect stand-type differences in the amount of labile C available to the forest-floor microbial community and reductions in above- and below-ground inputs to the forest floor following clear-cutting in both stand types. Changes in the chemical properties of forest floors from clear-cut SPRUCE and ASPEN stands could exacerbate C limitation in these soils and alter patterns of nutrient cycling. [ABSTRACT FROM AUTHOR]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We examined the growth of white spruce planted underneath a 33-year-old stand of trembling aspen following thinning and fertilization. Thinning the stand to 2000 stems·ha–1 resulted in increased light reaching spruce seedlings and significant increases in height and diameter growth of seedlings over that observed in either the unspaced or 3000 stems·ha–1 treatments. Fertilization (applied 3 years prior to planting) stimulated development of understory vegetation and did not benefit the planted spruce. While growth was improved by thinning, the benefits of the thinning treatment to establishment of white spruce appeared to be small, and amounted to increasing seedling height from 62 cm (unthinned) to 73 cm (thinned to 1000 or 2000 stems·ha–1) at the end of 5 years. Results indicated that when stands are thinned for the purpose of accelerating growth rates of underplanted spruce, residual basal areas above 25 m2·ha–1 should be retained to suppress growth of understory vegetation. Comparison of height at age 5 for seedlings planted under untended stands at Fort Nelson with three sites near Dawson Creek indicated no differences between locations, when height at the time of planting was included as a covariate. [ABSTRACT FROM AUTHOR]

Meta-analysis was used to summarize the research results on the growth response of understory white spruce to release from overstory aspen from different studies available from published and unpublished sources. The data were screened for the suitability for meta-analysis. Treatment effect sizes were calculated using response ratio from mean cumulative increments of released and control trees since release in height, diameter, and volume and modeled using a polynomial mixed effect regression procedure. Predictor variables include linear, quadratic, and cubic components of three independent variables — initial tree height, number of years after release, and residual basal area at release — and their linear interactions. Models with a reasonable predictive power were developed for height, diameter, and volume response, but no significant model was identified for survival. The models developed in this study can be applied to predict the growth response of understory white spruce to release, based on the growth of unreleased control trees, initial tree height, residual basal area at release, and time since release. The individual tree prediction can be easily scaled up to stand level if residual tree density and distribution is known. [ABSTRACT FROM AUTHOR]

Temporal, spatial, and structural patterns of adult trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) mortality were studied in intact 150-year-old stands in the southwestern boreal forest of Quebec. For both species, mortality decreases (number of dead trees/total number of trees) with distance from the lake edge until 100–150 m, from which point it slightly increases. Strong peaks in mortality were found for 40- to 60-year-old aspen mainly between 1974 and 1992. Such mortality in relatively young aspen is likely related to competition for light from the dominant canopy trees. Also, the recruitment of this young aspen cohort is presumably the result of a stand breakup that occurred when the initial aspen-dominated stand was between 90 and 110 years old. For spruce, strong peaks in mortality were found in 110- to 150-year-old trees and they occurred mainly after 1980. No clear explanation could be found for these peaks, but we suggest that they may be related to senescence or weakening of the trees following the last spruce budworm outbreak. Suppressed and codominant aspen had a much higher mortality ratio than spruce in the same height class, while more surprisingly, no difference in mortality rate was found between dominant trees of the two species. Most spruce trees were found as standing dead, which leads us to reject the hypothesis that windthrow is an important cause of mortality for spruce in our forests. [ABSTRACT FROM AUTHOR]

Low soil temperatures, common during the growing season in northern forests, have the potential to impede plant growth. In this study, water uptake, water relations, and growth characteristics were examined in aspen (Populus tremuloides) and white spruce (Picea glauca) seedlings that were inoculated with ectomycorrhizal fungi and grown at 20°C daytime air temperatures and low soil temperatures of 4°C and 8°C. Mycorrhizal associations had little effect on root and shoot biomass at both soil temperatures. Root hydraulic conductance (K[sub r] ) was higher in both mycorrhizal plant species compared to nonmycorrhizal plants, but there was no soil temperature effect on K[sub r] in either species. Mycorrhizae also increased shoot water potential (Ψ[sub w] ) in Populus tremuloides but had no effect on Ψ[sub w] in Picea glauca. The increases in K[sub r] and Ψ[sub w] were not reflected by changes in stomatal conductance (g[sub s] ) and transpiration rates (E), suggesting that the reduction of water flow in seedlings exposed to low soil temperature was not likely the factor limiting g[sub s] in both plant species.Key words: boreal forest, root hydraulic conductance, root growth, stomatal conductance, water uptake.Les basses températures du sol prévalant au cours de la saison de croissance dans les forêts nordiques, peuvent ralentir la croissance des plantes. Les auteurs ont examiné l'absorption de l'eau, les relations hydriques et les caractéristiques de croissance de plantules du peuplier faux-tremble (Populus tremuloides) et de l'épinette blanche (Picea glauca) préalablement inoculées avec des champignons ectomycorhiziens et cultivées à des températures diurnes de l'air de 20°C, et des températures basses du sol à 4°C et 8°C. L'association mycorhizienne a peu d'effet sur les biomasses racinaires et caulinaires, aux deux températures du sol. La conductance hydraulique (K[sub r] ) est plus élevée chez les deux espèces mycorhiziennes comparativement à des plantes nonmycorhiziennes, mais il n'y a pas d'effet de la température du sol chez aucune des deux espèces. Les mycorhizes augmentent également le potentiel hydrique de la tige (Ψ[sub w] ) chez le Populus tremuloides mais est sans effet sur le Ψ[sub w] du Picea glauca. Les augmentations du K[sub r] et du Ψ[sub w] ne se traduisent pas par des changements de la conductance stomatale (g[sub s] ) et des taux de transpiration (E), ce qui suggère que la réduction du transit de l'eau chez les plantules exposées à une basse température du sol n'est vraisemblablement pas le facteur limitant le g[sub s] chez ces deux plantes.Mots clés : forêt boréale, conductance hydraulique, croissance racinaire, conductance stomatale, absorption de l'eau.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Focuses on the use of generalized logistic model of individual tree mortality for trembling aspen, white spruce and lodgepole pine. Determination on the survival probability of an individual tree; Stand characteristics of trees; Estimation of species-specific site productivity index.

Discusses the treatment of a clear-cut aspen stand, near Prince Albert, Saskatchewan, to encourage the establishment of mixed aspen-white spruce forest. Site preparation methods used, such as the use of a v-blade, disc trencher and Bracke mounder, as well as a drum chopper in combination with the disc trencher and Bracke; Production of moderate height and biomass increases and low mortality rates; Reduction of competing aspen and shrub growth.

A few studies in young mixedwood stands demonstrate that precommercial thinning of aspen at early ages can improve the growth of spruce and increase stand resilience to drought. However, information on tree and stand responses to thinning in older mixedwood stands is lacking. To address this need, a study was initiated in 2008 in Alberta, Canada in 14 boreal mixedwood stands (seven each at ages 17 and 22). This study investigated growth responses following thinning of aspen to five densities (0, 1000, 2500, 5000 stems ha−1 and unthinned (control)). Measurements were collected in the year of establishment, and three and eight years later. Mortality of aspen in the unthinned plots was greater than in the thinned plots which were not significantly different amongst each other. Eight years following treatment, aspen diameter was positively influenced by thinning, while there was no effect on aspen height. The density of aspen had no significant effect on the survival of planted spruce. Spruce height and diameter growth increased with both aspen thinning intensity and time since treatment. Differentiation among treatments in spruce diameter growth was evident three years from treatment, while differentiation in height was not significant until eight years following treatment. Yield projections using two growth models (Mixedwood Growth Model (MGM) and Growth and Yield Projection System (GYPSY)) were initialized using data from the year eight re-measurements. Results indicate that heavy precommercial aspen thinning (to ~1000 aspen crop trees ha−1) can result in an increase in conifer merchantable volume without reducing aspen volume at the time of harvest. However, light to moderate thinning (to ~2500 aspen stems ha−1 or higher), is unlikely to result in gains in either deciduous or conifer merchantable harvest volume over those of unthinned stands. [ABSTRACT FROM AUTHOR]

The boreal forest is the single largest terrestrial store of carbon on Earth, and approximately 25% of these carbon stocks are in the forest floor. Climate change is expected to alter boreal vegetation, and aspen-dominated stands will replace conifers in Western Canada. We investigated how these vegetation shifts could affect the composition and function of soil microbial communities, using forest floor samples from the Ecosystem Management Emulating Natural Disturbance (EMEND) project in northwestern Alberta, Canada. Soil microbial communities were surveyed in rhizosphere and bulk forest floor of 17-year-old spruce clear-cuts where aspen was naturally regenerating, mature stands of aspen and spruce, and 17-year-old clear-cuts of aspen. Phospholipid fatty acid (PLFA) analysis was used to characterize microbial community composition and multiple substrate induced respiration (MSIR) to quantify microbial community functional capacity. Carbon source utilization by microorganisms was investigated through natural abundance isotope analysis of individual PLFAs. Rhizosphere samples had a significantly higher proportion of fungi and a higher gram-negative to gram-positive bacteria ratio compared to bulk soil. Fungi and gram-negative bacteria biomarkers in the rhizosphere showed 13C depletion compared to bulk forest floor, indicating that they had assimilated more newly-photosynthesized carbon than bulk forest floor microbes. Aspen trees exerted a greater influence over their rhizospheres than spruce trees in terms of microbial community composition and functional capacity, and aspen rhizospheres showed the highest basal respiration. In less than two decades, aspen regeneration in former spruce stands shifted the composition of microbial communities towards that of native aspen stands, with the rhizosphere microbiome responding more quickly than bulk forest floor. This study suggests that predicted vegetation shifts in the boreal have the potential to cause more immediate and profound changes in the rhizosphere, and emphasizes the need to include the rhizosphere in future studies. • Clear differences existed between rhizosphere and bulk soil microbial communities. • Natural abundance 13C PLFA patterns differed between rhizosphere and bulk soil. • Vegetation shift caused changes in microbial communities in less than two decades. • Rhizosphere microbes responded more readily to the vegetation shift. [ABSTRACT FROM AUTHOR]

In boreal mixedwood forests, aspen (Populus tremuloides) and white spruce (Picea glauca) commonly grow in mixture. These species may avoid competition through differential shade tolerance, physical separation of canopies, phenological differences, successional separation, and differences in soil resource utilization. Aspen may also be able to positively affect the growth of white spruce by improving litter decomposition and nutrient cycling rates, controlling grass and shrub competition, ameliorating environmental extremes, and reducing pest attack. These positive relationships likely make mixed-species stands more productive than pure stands of the same species. The evidence regarding the productivity of pure versus mixed aspen/white spruce stands in natural unmanaged forests is examined in this paper. [ABSTRACT FROM AUTHOR]