Mixedwood Forest Research Articles

A Holocene Perspective of Vegetation Controls on Seasonal Boreal Wildfire Sizes Using Numerical Paleo-Ecology

Wildland fire is the most important disturbance in boreal forests of eastern North America, shaping composition, structure and spatial arrangement of the flora. Although the long-term evolution of frequency and quantity of biomass burnt in these forest are known relatively well from paleoecological studies, we know little about the evolution of fire sizes. Here, we developed a modelling framework that provides insights into processes and changes involved through time in the historical fire-vegetation-climate environment of the coniferous and mixedwood forests of eastern boreal North America, with a particular attention on the metric of fire size. On the one hand, lacustrine charcoal particles sequestered in sediments from mixedwood forest and coniferous forest regions (MF and CF, respectively) were analyzed with the most advanced statistical treatments to reconstruct changes in biomass burning, fire frequency, and their ratio, the fire size (FS index), over the last 7000 BP. On the other hand, a fire propagation model was used to simulate fire sizes in the past using both a reference landscape, whose MF and CF compositions through time were prescribed using pollen reconstructions, and climate inputs provided by the HadCM3BL-M1 snapshot simulations. Lacustrine charcoals showed that Holocene FS indices have not differed significantly between MF and CF due to high variability in fire frequencies. However, biomass burning from MF has always been lower than from CF, such differences being significant since 5000 BP. Beyond the variability, MF’s mean FS index was lower than CF’s one throughout the Holocene, with slight changes in both forests from 7000 to 1000 BP, and simultaneous increases over the last millennium. The fire model showed that MF fires were systematically smaller than CF ones throughout the Holocene, with larger differences in the past than today. The fire model also highlighted that spring fires in both forest types have always been larger than summer fires over the last 7000 years, such results being in agreement with present-day fire statistics. This study illustrates how fire models, built and used for forecast and firefighting today, can be used under past conditions to enhance our understanding in the fire-vegetation-climate nexus.

Growing-season frost is a better predictor of tree growth than mean annual temperature in boreal mixedwood forest plantations.

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

Application of UAV Photogrammetry with LiDAR Data to Facilitate the Estimation of Tree Locations and DBH Values for High-Value Timber Species in Northern Japanese Mixed-Wood Forests

High-value timber species play an important economic role in forest management. The individual tree information for such species is necessary for practical forest management and for conservation purposes. Digital aerial photogrammetry derived from an unmanned aerial vehicle (UAV-DAP) can provide fine spatial and spectral information, as well as information on the three-dimensional (3D) structure of a forest canopy. Light detection and ranging (LiDAR) data enable area-wide 3D tree mapping and provide accurate forest floor terrain information. In this study, we evaluated the potential use of UAV-DAP and LiDAR data for the estimation of individual tree location and diameter at breast height (DBH) values of large-size high-value timber species in northern Japanese mixed-wood forests. We performed multiresolution segmentation of UAV-DAP orthophotographs to derive individual tree crown. We used object-based image analysis and random forest algorithm to classify the forest canopy into five categories: three high-value timber species, other broadleaf species, and conifer species. The UAV-DAP technique produced overall accuracy values of 73% and 63% for classification of the forest canopy in two forest management sub-compartments. In addition, we estimated individual tree DBH Values of high-value timber species through field survey, LiDAR, and UAV-DAP data. The results indicated that UAV-DAP can predict individual tree DBH Values, with comparable accuracy to DBH prediction using field and LiDAR data. The results of this study are useful for forest managers when searching for high-value timber trees and estimating tree size in large mixed-wood forests and can be applied in single-tree management systems for high-value timber species.

Eight-year ecophysiology and growth dynamics of Picea rubens seedlings planted in harvest gaps of partially cut stands

In order to compensate for regeneration failure of red spruce (Picea rubens) in the managed temperate mixedwood forest, an enrichment planting study was conducted in yellow birch (Betula alleghaniensis)–conifer stands, in Quebec, Canada. The experimental design compared four cutting treatments: an uncut control (26 m2·ha−1 of merchantable basal area [BA], with mean natural canopy gap areas of ≤ 100 m2 each) and three hybrid single-tree and group-selection cuttings corresponding to a light cut (20% removal; residual BA [RBA]: 21 m2·ha−1; mean harvest gap area: 220 m2), a moderate cut (31% removal; RBA: 18 m2·ha−1; gap area: 270 m2), and a heavy cut (42% removal; RBA: 15 m2·ha−1; gap area: 340 m2). In the cuts, large-sized containerized seedlings of red spruce were planted within harvest gaps, while in the control, seedlings were planted systematically, for comparison. Three years later, a mechanical release treatment was applied in half of planted plots to limit competing vegetation. After eight years, seedling survival in the three cuts was equally high (> 80%), and greater than in the control (40%). Morphophysiological performance of seedlings was best in the heavy cut creating the largest gaps. In general, the release treatment enhanced survival in the control and in the light cut with the smallest gap sizes. Release also improved morphophysiological performance (which, however, remained suboptimal) in all cutting treatments, but especially in the heavy cut. Although red spruce is highly shade-tolerant, the low survival and performance in the control suggest that for this species, seedlings grown with traditional forest nursery practices are not adapted to plantation under a closed canopy. Results indicate that enrichment planting in intermediate-size harvest gaps (of about 340 m2 in area) of partially cut stands (of about 40% in BA) combined to mechanical release could be a good strategy to maintain or reintroduce red spruce and promote its morphophysiological performance.

Effect of competition on individual white spruce production in young boreal mixedwood forests

Increasing the production of wood fibre from conifer species such as white spruce (Picea glauca (Moench) Voss) is one of many challenges in the management of boreal mixedwood forests. The effects of various competition measures on relative growth and relative growth rate variables were calculated for individual white spruce subject trees. Correlation analysis was used to explore relationships with competitor structural features, including the ratio of competitor basal area to subject tree basal area (CBAS), the ratio of competitor height to subject tree height (AHCS), and the proportion of softwood (FSW). Regression analysis was used to explore relationships with three distance-dependent competition indices. The ratio of subject tree height to diameter at breast height (DBH) (HDR), crown ratio (CR), and crown relative increment rate (CRIR) were significantly correlated with CBAS and AHCS. HDR, CR, CRIR, and DBH relative growth rate were all statistically significantly related to the competition indices. Results indicated that (i) relative growth and relative growth rate measures successfully captured a range of competition, (ii) crowns of trees with larger diameters used their horizontal growing space more efficiently to produce stemwood, and (iii) the proportion of softwood contributing to competition did not appear to influence subject tree production efficiency. Growth efficiency variables have the potential to improve our understanding of boreal mixedwood dynamics.

Individual tree species identification using Dense Convolutional Network (DenseNet) on multitemporal RGB images from UAV

Tree species identification at the individual tree level is crucial for forest operations and management, yet its automated mapping remains challenging. Emerging technology, such as the high-resolution imagery from unmanned aerial vehicles (UAV) that is now becoming part of every forester’s surveillance kit, can potentially provide a solution to better characterize the tree canopy. To address this need, we have developed an approach based on a deep Convolutional Neural Network (CNN) to classify forest tree species at the individual tree-level that uses high-resolution RGB images acquired from a consumer-grade camera mounted on a UAV platform. This work explores the ability of the Dense Convolutional Network (DenseNet) to classify commonly available economic coniferous tree species in eastern Canada. The network was trained using multitemporal images captured under varying acquisition parameters to include seasonal, temporal, illumination, and angular variability. Validation of this model using distinct images over a mixed-wood forest in Ontario, Canada, showed over 84% classification accuracy in distinguishing five predominant species of coniferous trees. The model remains highly robust even when using images taken during different seasons and times, and with varying illumination and angles.

Probability of Spring Frosts, Not Growing Degree-Days, Drives Onset of Spruce Bud Burst in Plantations at the Boreal-Temperate Forest Ecotone.

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

A topographic moisture index explains understory vegetation response to retention harvesting

To inform biodiversity conservation efforts in managed forest landscapes, we explore if a topographic moisture index (depth-to-water, based on remotely-sensed (LIDAR) data) can provide insight into responses of understory vegetation to retention harvesting in the boreal mixedwood forests of northwestern Alberta, Canada. Sample plots were placed along the depth-to-water moisture gradient in three forest types: coniferous, mixedwood, and deciduous (broadleaf), and in four retention harvesting treatments: unharvested (control), 50% (dispersed green-tree) retention, 20% retention, and clearcut (2% retention). Understory diversity, abundance, and composition were assessed 15 years after harvest.Harvesting affected the relationships between understory variables and the depth-to-water index, with the effects differing between forest types. Coniferous stands showed the most dramatic responses to harvesting, in that most relationships between understory attributes and the depth-to-water index changed due to harvesting. For instance, harvested coniferous stands had higher diversity on wetter sites, rather than on drier sites as was seen in the unharvested stands. In mixedwood stands only the relationship between composition and depth-to-water was affected by harvesting. Broadleaf stands were intermediate; abundance and composition showed a significant depth-to-water by harvesting treatment interaction. Abundance and depth-to-water relationships were weaker in harvested, as compared to unharvested, broadleaf stands. Within each forest type, the effects of harvesting also varied along the depth-to-water gradient. In coniferous and mixedwood forest types, wetter sites were most sensitive to harvesting while in broadleaf stands drier sites were more sensitive. Our study shows that the depth-to-water index can be used to better understand and predict the response of understory vegetation to harvesting and can be useful for guiding the placement of retention.

Forest Inventory and Diversity Attribute Modelling Using Structural and Intensity Metrics from Multi-Spectral Airborne Laser Scanning Data

Airborne laser scanning (ALS) systems tuned to the near-infrared (NIR; 1064 nm) wavelength have become the best available data source for characterizing vegetation structure. Proliferation of multi-spectral ALS (M-ALS) data with lasers tuned at two additional wavelengths (commonly 532 nm; green, and 1550 nm; short-wave infrared (SWIR)) has promoted interest in the benefit of additional wavelengths for forest inventory modelling. In this study, structural and intensity based M-ALS metrics were derived from wavelengths independently and combined to assess their value for modelling forest inventory attributes (Lorey’s height (HL), gross volume (V), and basal area (BA)) and overstorey species diversity (Shannon index (H), Simpson index (D), and species richness (R)) in a diverse mixed-wood forest in Ontario, Canada. The area-based approach (ABA) to forest attribute modelling was used, where structural- and intensity-based metrics were calculated and used as inputs for random forest models. Structural metrics from the SWIR channel (SWIRstruc) were found to be the most accurate for H and R (%RMSE = 14.3 and 14.9), and NIRstruc were most accurate for V (%RMSE = 20.4). The addition of intensity metrics marginally increased the accuracy of HL models for SWIR and combined channels (%RMSE = 7.5). Additionally, a multi-resolution (0.5, 1, 2 m) voxel analysis was performed, where intensity data were used to calculate a suite of spectral indices. Plot-level summaries of spectral indices from each voxel resolution alone, as well as combined with structural metrics from the NIR wavelength, were used as random forest predictors. The addition of structural metrics from the NIR band reduced %RMSE for all models with HL, BA, and V realizing the largest improvements. Intensity metrics were found to be important variables in the 1 m and 2 m voxel models for D and H. Overall, results indicated that structural metrics were the most appropriate. However, the inclusion of intensity metrics, and continued testing of their potential for modelling diversity indices is warranted, given minor improvements when included. Continued analyses using M-ALS intensity metrics and voxel-based indices would help to better understand the value of these data, and their future role in forest management.

Recovery of a boreal ground‐beetle (Coleoptera: Carabidae) fauna 15 years after variable retention harvest

Abstract Retention harvests are preferred over traditional clear‐cuts for sustainable forest management because maintenance and re‐establishment of native forest biodiversity is a priority. However, few studies have examined long‐term responses of biotic assemblages to retention harvest at particular sites. We studied the effects of decreasing initial harvest intensities (clear‐cut, 10%, 20%, 50% and 75% dispersed green‐tree retention) on carabid beetle assemblages relative to assemblage changes in un‐harvested control stands in four successionally ordered cover‐types of boreal mixedwood forest. We also studied temporal effects by comparing assemblages over a 16‐year pre‐ and post‐harvest period, using data collected through monitoring of the EMEND (Ecosystem Management Emulating Natural Disturbance) experiment in NW Alberta, Canada. Retention harvests affected assemblages differently across cover‐types. Assemblages in compartments harvested in the earlier forest successional stages of ‘deciduous’ or ‘deciduous with spruce understorey’ converged towards the pre‐harvest structure of corresponding controls over time. In contrast, beetle assemblages in ‘mixed’ or ‘conifer’ compartments, that represent later successional forest, moved steadily away from their pre‐harvest structures during the first post‐harvest decade. These latter assemblages became strikingly more similar to those under deciduous canopies by 15‐year post‐harvest. Synthesis and applications. Variable retention harvests will promote and maintain biodiversity better than clear‐cutting. Higher retention levels promote faster recovery, but towards fauna typical of early successional forest in all cover‐types. Carabids associated with conifer habitats are less resistant to impact from harvesting than are those from broadleaf deciduous forest. Therefore, conifer‐dominated stands present the most significant management challenge and higher retention levels are required to promote rapid and effective faunal recovery in such late successional stands.

Successional Variation in the Soil Microbial Community in Odaesan National Park, Korea

Succession is defined as variation in ecological communities caused by environmental changes. Environmental succession can be caused by rapid environmental changes, but in many cases, it is slowly caused by climate change or constant low-intensity disturbances. Odaesan National Park is a well-preserved forest located in the Taebaek mountain range in South Korea. The forest in this national park is progressing from a mixed-wood forest to a broad-leaved forest. In this study, microbial community composition was investigated using 454 sequencing of soil samples collected from 13 different locations in Odaesan National Park. We assessed whether microbial communities are affected by changes in environmental factors such as water content (WC), nutrient availability (total carbon (TC) and total nitrogen (TN)) and pH caused by forest succession. WC, TC, TN and pH significantly differed between the successional stages of the forest. The WC, TC and TN of the forest soils tended to increase as succession progressed, while pH tended to decrease. In both successional stages, the bacterial genus Pseudolabrys was the most abundant, followed by Afipia and Bradyrhizobium. In addition, the fungal genus Saitozyma showed the highest abundance in the forest soils. Microbial community composition changed according to forest successional stage and soil properties (WC, TC, TN, and pH). Furthermore, network analysis of both bacterial and fungal taxa revealed strong relationships of the microbial community depending on the soil properties affected by forest succession.

Stand breakdown and surface fuel accumulation due to spruce budworm (Choristoneura fumiferana) defoliation in the boreal mixedwood forest of central Canada

Spruce budworm (Choristoneura fumiferana (Clemens)) defoliation has been shown to increase the likelihood of large forest fires in central Canada. However, the time frame of heightened risk based on the duration of spruce budworm defoliation has not yet been quantified. In this article, we document the extent of stand breakdown and surface fuel accumulation after a period of spruce budworm defoliation that occurred between 1972 and 1976. Data on stand characteristics were derived from previous studies at three different locations in the boreal mixedwood forests of central Canada: Aubinadong (B.J. Stocks. 1987. For. Chron. 63: 8–14), Gogama, and Gowganda in Ontario. Stand breakdown was measured using a series of transects set in plots 7 years following aerially mapped defoliation (1977–1983). Results show that during the 4 years following 5 years of defoliation, crown breakage, a typical symptom of defoliation, increased by nearly 200%, and surface fuel increased by 145% from predisturbance levels. The high correlation between crown breakage and surface fuels linked defoliation to fuel buildup. We begin to solve the challenge of measuring fuel structure over the expansive scale of spruce budworm outbreaks by quantifying the relationship among stand breakdown, time since the end of defoliation, and the duration of defoliation so that the expected fuel structure can be modelled from annual defoliation surveys.

Composition Changes in the Boreal Mixedwood Forest of Western Quebec Since Euro-Canadian Settlement

Colonisation of eastern North America has generated significant modifications in forest composition. In regions highly influenced by human activity, historical ecology can be used to reconstruct pre-settlement forest composition. In this study, we reconstruct the composition of the pre-settlement (1909-1937) forest of a 4,134 km2 sector of the boreal mixedwood forest using early land survey archives. The pre-settlement composition was compared with modern composition using recent eco-forest inventories (1980-2008), and the influence of surficial deposits types on the spatial structuration of compositional changes was assessed. During the pre-settlement period, the landscape was primarily dominated by spruce, which was evenly distributed across surficial deposit types. Trembling aspen, although frequent, rarely dominated stands. In contrast, the present-day landscape is dominated by trembling aspen, notably on clay and till deposits. In general, conifers have undergone a severe reduction in frequency. Spruce and pine forests are today mainly restricted to organic and sandy surficial deposits compared to their distribution during the pre-settlement period. Composition changes observed in western Abitibi are essentially the results of fires and forest harvesting, but surficial deposits have contributed in structuring the current abundance and spatial distribution of the different taxa. In the context of sustainable forest management, considerable effort should be deployed to restore conifer dominance in the region, notably on the fertile deposits that appear particularly susceptible to composition changes.

Distance from the Forest Edge Influences Soil Fungal Communities Colonizing a Reclaimed Soil Borrow Site in Boreal Mixedwood Forest

Soil fungi are important components of boreal forest ecosystems; for example, saprotrophic fungi regulate nutrient cycling, and mycorrhizal species facilitate nutrient uptake by plants. This study aimed to assess soil fungal communities in a reclaimed area and an adjacent natural mixedwood forest and to identify the distribution of taxa available for seedling colonization. Soil fungal microbiomes were assessed along three transects (from 10 m inside the interior of the undisturbed forest to 40 m inside the reclaimed area) and in the roots of small aspen within the natural forest. Using high-throughput deoxyribonucleic acid (DNA) sequencing of internal transcribed spacer amplicons, a total of 2796 unique fungal taxa were detected across fine roots, forest floor, and mineral soils collected along the transects, whereas 166 taxa were detected in the aspen roots from the natural forest. Within the interior of the forest, ectomycorrhizal fungi were more common, whereas in the reclaimed areas, arbuscular mycorrhizae and saprophytes were more common. This survey showed that natural areas of adjacent undisturbed forest can act as a source of ectomycorrhizal fungi for dispersal into reclaimed areas. Notably, soil fungal taxa colonizing the root systems of small aspen included species that are specifically associated with soils from the undisturbed forest (primarily ectomycorrhizae) or the reclaimed clearing (saprotrophs and plant pathogens).

Projected effects of climate change on boreal bird community accentuated by anthropogenic disturbances in western boreal forest, Canada

AbstractAimClimate change is expected to influence boreal bird communities significantly, notably through changes in forest habitat (composition and age structure), in the coming decades. How these changes will accumulate and interact with anthropogenic disturbances remains an open question for most species.LocationNortheastern Alberta, Canada.MethodsWe used the LANDIS‐II forest landscape model to project changes in forest landscapes, and associated bird populations (72 passerine species), according to three climatic scenarios (baseline, RCP 4.5 and RCP 8.5) and three forest harvesting scenarios of differing intensity.ResultsBoth forest harvesting and climate‐related drivers were projected to have large impacts on bird communities in this region. As a result of climate‐induced increases in fire activity as well as decreased conifer productivity, our simulations projected that an important proportion of Alberta's boreal forests would transition to treeless habitat (i.e. grass‐ or shrub‐dominated vegetation) while many conifer‐dominated stands would likely be replaced by broadleaf tree cover. Consequently, the abundance of bird species associated with open and deciduous habitats were projected to increase. With a strong anthropogenic climate‐forcing scenario (RCP 8.5), sharp declines in abundance of coniferous trees were also projected, particularly in mature and old forest stands, triggering major declines for bird species associated with coniferous and mixedwood forest types.Main conclusionsAs the most comprehensive simulation of climate change and harvesting impacts on avian habitats in the North American boreal region to date, our study stresses the importance of considering key habitat characteristics like forest age structure and composition through forest landscape modelling and identifies 18 bird species particularly sensitive to climate change. Our simulations suggest that a change in forest management practices could play an important role in the conservation of boreal bird species vulnerable to climate change. The intensive forest harvesting simulated accelerated declines in bird abundance compared to a “no harvesting” scenario.

Forest Landscape Heterogeneity Increases Shrub Diversity at the Expense of Tree Seedling Diversity in Temperate Mixedwood Forests

Partial disturbances enhance spatial heterogeneity through the diversification of forest structure, which contributes to niche partitioning and consequently to species diversity. However, this heterogeneity–diversity relationship may differ between groups of species, and is potentially modified by biotic interactions at the community level. We propose that shrub diversity will be greater in heterogeneous landscapes, while tree diversity will be lower in those same landscapes, due to the biotic interactions of shrub competition. We conducted field sampling in the balsam fir/yellow birch bioclimatic domain in western Québec, a forested ecosystem disturbed by natural and anthropogenic partial disturbances. We selected 12 forested landscapes (1 km2), four in each of three classes of landscape heterogeneity (heterogeneous, moderately heterogeneous, homogenous). Shrub and tree species regeneration abundance was measured in three different size classes of canopy gaps and the forest understory. Gap proportions were assessed in each landscape using aerial LiDAR data. Tree and shrub alpha-diversity significantly responded to landscape heterogeneity, shrubs being more diverse while tree seedlings were less diverse in heterogeneous landscapes. Heterogeneous landscapes showed highest species accumulation rates for shrubs in medium-sized gaps. For tree seedlings, species accumulation rates were highest in heterogeneous landscapes in the forest understory. Our study thus supports the heterogeneity–diversity relationship with shrubs having higher alpha and beta diversity in heterogeneous landscapes whereas local-scale tree diversity was higher in homogenous landscapes.

Manipulating aspen (Populus tremuloides) seedling size characteristics to improve initial establishment and growth on competitive sites

ABSTRACTTrembling aspen is an important component of the boreal mixedwood forest region and is a key component of numerous ecological functions. As a result, aspen has become a target species for forest restoration after industrial disturbance; however, aspen seedling afforestation research is still in its infancy as seedling and site characteristics and their interactions which drive establishment success in the field are poorly understood. This study examined the growth response of three aspen size classes across six sites differing in competition and soil moisture availability. With competing vegetation present, tall seedlings with lower root-to-shoot ratios had less height growth aboveground and reduced root egress (relative to total root mass) belowground, indicating that light was not the most limiting factor aboveground and that higher root-to-shoot ratios may aid in competition for rooting space. Differences between size classes were less pronounced on sites with no competition or sites with extreme conditions (high competition or low soil moisture availability) during outplanting. Reducing competing vegetation before outplanting improved height growth, particularly in seedlings with high root-to-shoot ratios. For aspen, planting short to medium-sized seedlings with higher root-to-shoot ratios offers a reasonable balance between cost and production of a stress-tolerant seedling.

Assessing tree-related microhabitat retention according to a harvest gradient using tree-defect surveys as proxies in Eastern Canadian mixedwood forests

Tree-related microhabitats (hereafter “TreMs”) play a key role in forest biodiversity. However, harvesting may cause their erosion. In North America, knowledge about TreMs is still lacking but defect surveys are largely available in managed forests. The objectives of our study were: (1) to demonstrate that defect surveys can be a reliable resource to identify TreMs; and, (2)to evaluate the capacity of silvicultural treatments to maintain TreM abundance and diversity according to a harvest gradient.To achieve these objectives, we identified TreMs from a defect survey performed the year a harvest gradient was applied to20 plots, including uncut control, shelterwood treatments removing 50%, 43% and 36% of basal area, and clearcut (4 plots/treatment). The density and composition of TreMs were then compared based on treatments. Overall, 38% of defectsactually corresponded to TreMs, confirming that tree-defects can be used as TreM proxies. Bark loss was the most abundantTreM. While there was practically no TreM in clearcuts, all shelterwood treatments initially maintained TreM diversity anddensity at the same values found in uncut control plots. Shelterwood systems, especially those maintaining a continuouscover, could therefore prove helpful to sustain TreMs and their biodiversity in managed forests.

Dynamics of ground-nest egg depredation by rodents in a mixed-wood forest

Predation is a leading cause of nest failure in birds, which has significant consequences on their population dynamics. Rodents commonly prey on nests in deciduous forest habitats. This has important implications considering how rodent populations fluctuate with changes in resource availability, such as synchronized but intermittent tree-seed production (mast). In this study, we investigated ground-nest egg predation by rodents over 2 years characterized by high and low beech (Fagus grandifolia Ehrh.) seed availability (mast vs. no-mast years). We used artificial nests monitored by motion-sensing cameras on a study site where eastern chipmunk (Tamias striatus (Linnaeus, 1758)) local abundance was known. We placed the artificial nests in areas of high and low chipmunk abundance as proxy of predation risk. Blue Jays (Cyanocitta cristata (Linnaeus, 1758)) depredated the most eggs overall, but rodent population and egg depredation increased in 2018 following the 2017 mast. However, chipmunks were minor predators and their local abundance did not reflect predation risk. Our results highlight the complexity of predation dynamics on ground-nesting birds and the importance of studying them locally and over multiple years.

Postfire deciduous canopies drive patterns in snowshoe hare herbivory of regenerating black spruce

The future of boreal forests in Alaska, United States, will likely consist of more deciduous-dominant stands because larger and more severe fires facilitate the establishment of deciduous species such as trembling aspen (Populus tremuloides Michx.) and Alaska birch (Betula neoalaskana Sarg.). Whether stands transition to a deciduous-dominant system or mixed-wood forest or return to being dominated by black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.) depends on the capacity of regenerating black spruce to grow and produce seed before the next fire. We hypothesized that winter herbivory by snowshoe hares (Lepus americanus Erxleben, 1777) can suppress black spruce under deciduous canopies. We addressed this question by measuring changes in spruce height and herbivory across 54 plots in Interior Alaska that burned 8–88 years ago and related these data to plot-level data collected by the Bonanza Creek Long-Term Ecological Research program. Spruce were more likely browsed at deciduous-dominant sites with dense canopies, and this browsing likely reduced their height growth. Although we found more subtle effects of browsing on height at the individual level, browsing was an important variable in a confirmatory path analysis at the plot level. These observations affirm our broader hypothesis of the selectivity of hare browsing, in that snowshoe hares prefer to browse spruce that are taller and faster growing, effectively “leveling” regenerating seedlings and saplings so that browsed and unbrowsed individuals within a site are the same height.