Aspen Regeneration Research Articles

Relative potential for stand persistence of riparian and upland aspen stands of a semi-arid montane landscape of the Southern Rocky Mountains

Several studies have predicted a loss of quaking aspen (Populus tremuloides) from many western landscapes, but other studies have suggested that aspen persistence is driven by local site factors. Increased frequency of acute drought has been implicated as an important factor driving overstory mortality and reduced regeneration densities in the region. We investigated the relationship between aspen regeneration and site moisture availability potential using ecosystem type as a proxy. We hypothesized that aspen stands growing along perennial-flowing streams would support higher aspen regeneration densities than upland aspen stands. We compared stand structure, groundcover composition, and regeneration densities of nine riparian aspen stands with nine paired upland aspen stands in the Caribou-Targhee National Forest. Aspen regeneration densities were significantly higher in the riparian aspen stands (845. 3 + 318.7 stems ha-1) compared to the upland aspen stands (249.1 + 74.1 stems ha-1) for regeneration shorter than one meter (p = 0.0391). Riparian stands also exhibited significantly higher forb (p< 0.001) and graminoid (p < 0.001) cover compared to upland aspen stands, suggesting that riparian sites provided higher site moisture availability. We suggest that riparian areas may provide refugia for aspen in the future considering projections of increased incidence of acute drought.

Mixing trembling aspen and white spruce increases the understory vegetation cover and improves soil properties but effects vary with broadleaf density

Despite the importance of species mixtures to ecosystem-based forest management, density effect of broadleaf trees in the mixture on understory vegetation and soil properties are poorly understood. In this study, we examine the effects of trembling aspen [aspen] (Populus tremuloides Michx.) -white spruce [ spruce] (Picea glauca (Moench) Vos) mixtures that vary in the proportion of aspen on understory vegetation cover, soil nutrient supply rates, and soil properties. Data were collected from an established study located in Alberta, Canada at age 20 and included the following treatments: 1. Natural regeneration of pure aspen; 2. Retaining only 1600 planted spruce ha−1 by controlling aspen and understory vegetation; 3. Retaining only 1600 planted spruce ha−1 by controlling understory vegetation; 4–7. Retaining only 400 planted spruce ha−1 along with unthinned aspen, 2000, 1200, and 800 aspen ha−1. In mixed stands, competition control around the spruce was done for its establishment, and subsequent thinning was done to control the density of aspen. The findings of our analysis showed that mixed stands had higher total vascular plant cover, potential soil N supply rates, soil pH, and lowered C: N ratio compared to monocultures of spruce or aspen. The treatment (mixed stands) with unthinned aspen along with spruce increased the above-mentioned variables most compared to other mixtures. Moreover, total forb cover was higher in the mixture with the highest density of aspen, but grass cover was higher in the lowest density of aspen. Our findings hint that the effect of mixing tree species on understory vegetation cover may be influenced by the density of broadleaf tree in the mixture. Further studies need to be conducted to reach a concrete conclusion. Regardless, this result might be useful for forest managers to take a decision for ecosystem-based forest management.

Sustainability and drivers of Populus tremuloides regeneration and recruitment near the southwestern edge of its range

Abstract Quaking aspen (Populus tremuloides Michx.) ecosystems are highly valued in the southwestern United States because of the ecological, economic, and aesthetic benefits they provide. Aspen has experienced extensive mortality in recent decades, and there is evidence that many areas in Arizona, United States lack adequate recruitment to replace dying overstory trees. Maintaining sustainable levels of regeneration and recruitment (i.e. juveniles) is critical for promoting aspen ecosystem resilience and adaptive capacity, but questions remain about which factors currently limit juvenile aspen and which strategies are appropriate for managing aspen in an increasingly uncertain future. To fill these critical knowledge gaps, we sampled aspen populations across Arizona and collected data representing a suite of biotic and abiotic factors that potentially influence juvenile aspen. Specifically, we addressed two questions: (i) Is aspen sustainably regenerating and recruiting in Arizona? and (2) Which biotic and abiotic factors significantly influence aspen regeneration and recruitment? We found that many aspen populations in Arizona lack sustainable levels of juvenile aspen, and the status of recruitment was especially dire, with 40% of study plots lacking a single recruiting stem. Aspen regeneration was less abundant on warmer sites than cooler ones, highlighting the threat that a rapidly warming climate poses to aspen sustainability. Aspen recruitment was significantly more abundant in areas with recent fire than in areas without fire, and recruitment had a strong positive relationship with fire severity. The most important limiting factors for aspen recruitment were ungulate browse, especially by introduced Rocky Mountain elk (Cervus canadensis nelsoni), and the invasive insect, oystershell scale (Lepidosaphes ulmi). We conclude with a discussion of how management can promote sustainability of aspen populations by addressing the array of threats that aspen faces, such as a warming climate, chronic ungulate browse, and outbreaks of oystershell scale.

Pile burning after conifer removal from aspen stands affects tree mortality, regeneration, and understory recovery

Quaking aspen (Populus tremuloides Michx.) stands throughout the western United States provide valuable ecosystem services but can be lost via succession from aspen to conifer. Forest managers are cutting conifers, but disposal of cut wood can be challenging in remote or sensitive areas. Piling and burning is being tested within aspen stands but ecosystem responses to this treatment are understudied. We assessed aspen tree mortality, tree regeneration, and understory vegetation after forest restoration thinning followed by pile burning in seven aspen-conifer stands around Lake Tahoe, California and Nevada, USA. Pile burning was conducted after cut wood had dried (1.5–7.5 years post cutting). Pile burning was more likely to kill aspen trees closer to piles regardless of tree size or pile size. Aspen regenerated rapidly inside the footprint of some burned piles, more often inside piles located in close proximity to other aspen, presumably by suckering from lateral roots beneath burned piles. Similarly, areas where higher densities of conifers were regenerating naturally were indicative of a greater likelihood for conifer regeneration inside pile footprints. Understory vegetation varied within and among study sites, and had similar vegetation cover and species richness inside pile footprints to vegetation in the vicinity of each pile. Overall, understory vegetation had mostly recovered after 2.5–8.5 years since pile burning. Thinning followed by burning of hand piles and smaller machine piles appears to be effective at promoting regeneration of aspen without lasting impact on understory vegetation. However, without further disturbance such as continued cutting and piling or use of prescribed fire, we caution that succession to conifer appears to continue, albeit slowly, via seedling regeneration.

Early natural tree regeneration after wildfire in a Central European Scots pine forest: Forest management, fire severity and distance matters

In the last years, several wildfires destroyed large areas in Central European forest stands and the risk for fires is predicted to increase in the future. However, little is known about post-fire succession including the effect of forest management for this region. We studied recolonization patterns of natural regeneration of the pioneer tree species Betula pendula Roth (silver birch), Pinus sylvestris L. (Scots pine), Populus tremula L. (European aspen) and Salix caprea L. (goat willow) on 200 plots in a large, 334 ha burned pine forest stand two years after a severe wildfire. Natural regeneration in general was high, reaching higher densities than artificial regeneration on most plots. The influence of fire severity, post-fire forest management (complete and partial salvage logging and soil disturbance like skidder traffic, ploughing and raking) and distance to the next potential seed source on the number of tree regeneration was tested with multiple linear regression. High fire severities decreased abundance of pine, aspen and willow. The regeneration of all four investigated tree species was influenced by forest management. High birch regeneration was predicted for completely salvage logged plots in combination with skidder traffic or early ploughing and plots without any forest management. Pine regeneration was predicted to be highest on completely salvage logged and sites without any salvage logging. Forest management which includes partial salvage logging showed mostly lower predicted regeneration of birch, pine and aspen as probably an effect of timing. Willow regeneration was promoted by forest management which includes late ploughing. Regeneration of birch and pine decreased with increasing distance to the next potential seed tree, while aspen and willow sapling densities were not affected by distance due to their excellent dispersal by wind. For birch and especially pine, close seed trees are crucial for sufficient natural regeneration. Recolonization of burned areas is a highly dynamic process dependent on several factors. However, natural regeneration provides a fast, cost-effective way of reforestation which can be facilitated by suitable silvicultural treatments. We recommend to keep survived trees as green islands on site to provide close seed sources. To avoid seedling disturbance, forest management activities should be complied according to life cycles of naturally established tree seedlings. We also suggest using the benefits of deciduous pioneer trees in forestry in order to establish more diverse, less fire-prone forests after wildfires in the long term.

Building Ecosystem Resilience and Adaptive Capacity: a Systematic Review of Aspen Ecology and Management in the Southwest

AbstractQuaking aspen (Populus tremuloides Michx.) has high conservation value on the southwestern edge of its range, which extends from the southwestern United States (i.e., Arizona, New Mexico, and Texas) to central Mexico. This value is driven by aspen’s ecological importance, positive impact on local economies, and aesthetic and cultural values. Generally, the scant aspen populations that remain in the Southwest lack resilience and adaptive capacity, and managers are unsure how best to maintain the species in an uncertain future. This systematic review seeks to address that need by reviewing existing literature from the Southwest on which biotic and abiotic factors influence aspen forest dynamics and by synthesizing that literature with a discussion of how management can promote aspen ecosystem resilience and adaptive capacity. We found that fire and silvicultural treatments promote aspen regeneration, but chronic ungulate browse inhibits recruitment. Moreover, drought is a driver of overstory mortality and has a negative influence on recruitment. In the second half of this review, we propose three management objectives for increasing aspen resilience and adaptive capacity: (1) promote diversity in age structure, (2) mitigate ungulate impacts, and (3) enhance complexity. We consider how various management strategies could meet these objectives and highlight potential threats to aspen forest health and resilience.

Competitive interactions under current climate allow temperate tree species to grow and survive in boreal mixedwood forest

With climate change, climatic optima are shifting poleward more rapidly than tree migration processes, resulting in a mismatch between species distributions and bioclimatic envelopes. Temperate hardwood tree species may take advantage of the release of climate constraints and forest management to migrate into the boreal forest. Here, we use the SORTIE‐ND forest simulation model to determine the potential for the persistence of three temperate species (sugar maple, red maple and yellow birch) when introduced at seedling stage in typical balsam fir–paper birch (BF–PB) bioclimatic domain stands of eastern Canada, quantifying the consequences on the native species composition. SORTIE‐ND is a spatially explicit, individual‐based forest stand model that simulates tree growth, regeneration and mortality. We performed a novel parameterization of the SORTIE‐ND tree growth equation allowing for the inclusion of climate modifiers on tree growth. After validating our model with data from permanent forest inventory plots, we modeled the dynamics of unharvested stands at different successional stages, as well as post‐harvest stands, after the addition of sugar maple, red maple and yellow birch seedlings at different densities. Our results show that current BF–PB domain climate conditions do not limit growth and survival of temperate species in boreal stands. Of the temperate species introduced, sugar maple had the lowest ability to grow and survive by the end of the simulation. Species assemblages of host stands were impacted by the presence of temperate species when the addition of seedlings was above 5000 temperate seedlings per hectare at the beginning of the simulation. For stands that were recently clear cut, temperate seedlings were unable to grow due to intense competition from aspen regeneration. Our results suggest that both current climate and competitive interactions between temperate species and boreal species should not impede the ability of temperate species to grow and survive in the BF–PB domain.

Can Spot Motor-Manual Brushing Treatments Be Effective for Controlling Aspen and Increasing Spruce Growth in Regenerating Mixedwood Stands?

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.

Ungulate herbivores as drivers of aspen recruitment and understory composition throughout arid montane landscapes

AbstractHerbivory by wild and domestic ungulates can influence tree recruitment and understory forest communities throughout the world. Herbivore‐driven declines in tree recruitment have been observed for quaking aspen (Populus tremuloides), a foundation species whose health and management is recognized as a critical priority throughout much of its range. Livestock fencing is commonly used to promote aspen regeneration, but its effectiveness is rarely assessed, especially across large spatial scales. Using a livestock‐reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of climate change, conifer encroachment, and herbivory. We found that livestock fencing only reduced the intensity of herbivore browsing on aspen when wild ungulate abundance was low and did not increase stem densities of aspen recruits. Contrary to expectations, wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within fenced, but not unfenced, aspen stands, and when the abundance of these herbivores was high, browsing intensity in fenced stands exceeded that in unfenced stands. The density of aspen recruits decreased with browsing intensity on juvenile aspen and with the density of both adult aspen and conifers, suggesting that both herbivory and intra‐ and interspecific competition are important drivers of recruitment. Fire history was also an important driver of recruitment, with stands that burned 10–20 years ago having the greatest density of aspen recruits. Finally, in the stand understory, we found that livestock fencing decreased forb cover, increased shrub species richness, and increased the cover of exotic annual grasses, a group dominated by Bromus tectorum. This latter finding suggests that livestock fencing may not be appropriate in areas where controlling the spread of this invader is a priority. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates, is mediated by competition with neighboring trees and fire history, and will require management actions beyond livestock fencing, as this approach does not control browsing by wild ungulates.

Trembling Aspen Stand Response 15 Years after Windthrow, Salvage Harvesting, and Forest Renewal

Windthrow is a common occurrence in boreal forests, affecting wood supply and presenting regeneration challenges for forest managers. Salvage harvesting is often conducted afterwards to extract valuable wood and improve access for forest renewal activities. Research efforts, however, are generally limited to the effects of windthrow and salvage harvesting in the first few years following disturbance. In 2006, a catastrophic wind event occurred in a trembling aspen (Populus tremuloides Michx.) forest in northeastern Ontario. A field study was established with a range of silvicultural treatments from leaving after windthrow (W) to forest renewal treatments including windrow site preparation, planting, and herbicide release (WSPR). While the results of first 5-year assessment have been reported, the objective of the current study was to reassess treatment effects at 15 years post-disturbance, a stage of stand development that is more indicative of future forest conditions. Compared to the results of the earlier assessment, the 15-year assessment indicated that standing dead wood (snags) declined, whereas coarse downed wood did not change substantially over time. Post-disturbance salvage harvesting improved aspen regeneration in terms of density, stocking, and growth, but substantially reduced the proportion of conifers relative to windthrow only. Thus, salvage harvesting helped to sustain aspen composition, compared to other treatment options that increased proportions of pine and spruce trees from W to windthrow and salvage harvesting combined with windrow site preparation and planting (WSP), and from WSP to WSPR.

Emulating Succession of Boreal Mixedwood Forests in Alberta Using Understory Protection Harvesting

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.

Comment mettre la génomique forestière et la génomique de la conservation au service des communautés autochtones?

Les projets d’aménagement durable et de conservation (ADC) des ressources naturelles en collaboration avec les peuples autochtones ayant recours à une approche en génomique sont en émergence. Les informations et applications issues de la génomique peuvent leur être utiles particulièrement dans un contexte de changements climatiques. Toutefois, le défi de transposer ces applications dans la pratique et de les mettre au service des communautés demeure. Nous présentons ici une revue de littérature exploratoire qui aborde (1) les utilités démontrées de la génomique dans les projets d’ADC impliquant les peuples autochtones, (2) certains enjeux qui peuvent limiter l’adoption des applications de la génomique et (3) le travail collaboratif entre chercheurs et communautés autochtones dans les études analysées. Les utilités démontrées identifiées ont été essentiellement de nature socioécologique. La nature complémentaire des savoirs autochtones et des savoirs scientifiques en génomique a été reconnue comme une opportunité qui devrait être développée davantage pour relever les défis actuels, tels que les changements climatiques. En ce qui concerne l’adoption de cette technologie en ADC dans la pratique, en plus de faire face à des enjeux similaires à d’autres utilisateurs finaux, l’intégration des besoins, des valeurs traditionnelles et des connaissances des communautés autochtones dans les projets de génomique représente également un défi dans un contexte de décolonisation de la recherche en génomique. Finalement, la collaboration communauté-chercheur a été identifiée comme un élément clé pour favoriser la réussite de la transposition de la génomique en ADC.

Twenty-six years of aspen regeneration under varying light conditions in a boreal mixedwood forest

Density, height, and diameter at breast height of trembling aspen (Populus tremuloides Michx.) sucker regeneration were assessed over a 26-year period in openings created by harvesting in a 40-year-old aspen stand in northeastern Ontario (Canada). The opening types were 9- and 18-m diameter circles, 9- and 18- _ 150-m east-west strips, and a 100- _ 150-m clearcut, representing a range of light conditions. Density, height, and diameter at breast height of aspen regeneration were significantly affected by opening type, location relative to opening, and time since harvest. By year 26, aspen densities in circular openings declined to 0, despite high initial recruitment, and trembling aspen heights were significantly lower in the 9-m strips than in the 18-m strips or the clearcut. Year 26 aspen density, volume, and basal area increased with increasing initial light intensity, with the highest rate of increase between 80 and 100% light levels. Understory vegetation cover was largely unaffected by opening size; however, substantial understory aspen regeneration occurred in the smaller openings. Results support the traditional view that aspen is best managed under the clearcut silvicultural system, and >80% full light is recommended for adequate long-term aspen regeneration.

Low-density aspen seedling establishment is widespread following recent wildfires in the western United States.

Sexual regeneration is increasingly recognized as an important regeneration pathway for aspen in the western United States, a region previously thought to be too dry for seedling establishment except for during unusually wet periods. Because of this historical assumption, information on aspen seedling establishment and factors influencing its occurrence is limited and frequently anecdotal. We conducted a systematic field survey of 15 recent fire footprints that burned in 2018 in the western United States to quantify how common aspen seedling establishment is following fire and to identify factors associated with establishment. We found aspen seedling establishment in 12 of 15 (80%) of fire footprints surveyed, although densities were mostly low. Establishment probability was positively associated with mean annual precipitation and negatively associated with seed-source distance and the density of asexual aspen regeneration. Our results suggest that aspen seedling establishment may be a widespread, if often low-density, feature in postdisturbance areas. Even in low numbers, aspen seedlings may play a disproportionately large role in aspen regeneration ecology, providing adaptive capacity and facilitating local range expansion.

Rhizosphere response to predicted vegetation shifts in boreal forest floors

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.

Modelling the influence of different harvesting methods on forest dynamics in the boreal mixedwoods of western Quebec, Canada

Forest management aims to better understand effects of natural disturbance regimes on forest dynamics and use this knowledge to formulate guidelines in forest planning, thereby narrowing gaps between managed and unmanaged forest landscapes. Using forest simulators to reconstruct forest dynamics in relation to ecosystem processes, including disturbances, could help forest managers to better understand harvesting effects on forest dynamics. Using SORTIE-ND, a spatially explicit forest simulator, we generated stand dynamics for 100 years following simulated clear-cut and partial harvests (dispersed vs aggregated, with 30% and 60% basal area removal). Based on the hardwood: conifer basal area ratio, we grouped post-fire stands into three stand types corresponding to natural post-fire succession (deciduous, mixed deciduous, and mixed coniferous) and assessed long-term effects of clear-cutting and partial harvesting on each. Our results suggest that spatial configurations of harvested and residual trees had a greater effect on stand dynamics than did tree removal intensity. Following dispersed partial harvesting, both deciduous and mixed deciduous stands had species composition and structure similar to unharvested stands of the next successional stage. In these same stand types, aggregated harvests and clear-cutting favoured increased regeneration and basal area increments of aspen, which set succession back to aspen dominance, as has been observed after wildfire. Dispersed partial harvests (both 30% and 60%) and 30% aggregated cuts, in mixed coniferous stands, maintained recruitment and dominance of conifers to levels comparable with unmanaged stands. Clear-cutting in all stand types greatly modified stand compositional and structural attributes, and, when conducted in stands where aspen was abundant, performed as a stand-replacing disturbance, setting succession back to early developmental stages, i.e., to aspen dominance. We conclude that partial harvesting, which emulates gap dynamics similar to undisturbed stands, can maintain natural stand dynamics.

Natural Regeneration Following Partial and Clear-Cut Harvesting in Mature Aspen-Jack Pine Stands in Eastern Canada

Over the last three decades, the ecological basis for the generalized use of even-aged silviculture in boreal forests has been increasingly challenged. In boreal mixed-wood landscapes, the diminishing proportion of conifers, to the benefit of intolerant hardwoods, has been a primary concern, coupled with the general rarefication of old-growth conifer-dominated stands. In this context, partial cutting, extended rotations and forest renewal techniques that eliminate or reduce regenerating hardwoods have been proposed as means of regaining greater conifer cover. As a result, experimentation and industrial application of various forms of both variable retention and partial harvesting are occurring across the commercial Canadian boreal forest. In this study, we compared the effects of two harvesting intensities, clearcutting and low-intensity partial cutting (removal of 25–31% of tree basal area), on hardwood and conifer regeneration levels 7–19 years following treatments in aspen (Populus tremuloides)-dominated stands and verified whether regeneration differences existed between micro-sites on and off machinery trails. The abundance of aspen regeneration increased with percent basal area removal and was positively correlated to the abundance of mature aspen prior to harvesting. The abundance of fir (Abies balsamea) regeneration after partial cutting was similar to controls and higher than after clear-cutting and was positively correlated with ground cover of mixed litter (i.e., mixture of needles and leaves) and negatively correlated with ground cover of broadleaf litter. These results suggest that it is possible in boreal mixed-woods to control aspen abundance and promote or maintain conifer regeneration through silvicultural treatments that limit canopy opening and promote mixed forest floor litter.

Canopy structure and below-canopy temperatures interact to shape seedling response to disturbance in a Rocky Mountain subalpine forest

We examined the effects of two recent, high-severity disturbances on seed dispersal and conifer seedling establishment in a subalpine spruce-fir forest in the San Juan Mountains, Colorado. Our study area had undergone high forest mortality from a spruce beetle (Dendroctonus rufipennis) outbreak beginning ca. 2004, and a portion of the study area was additionally burned by the West Fork Complex wildfire in 2013. The aim of this study was to determine how seedling re-establishment patterns vary with seed availability, temperature, overstory composition, and understory cover across a topographically heterogeneous landscape with varying disturbance impacts. We established thirty study sites in the burned area and sixty sites in the unburned, beetle-affected area in order to capture a gradient of beetle outbreak severity. We assessed post-disturbance regeneration by conducting seedling counts in the summer of 2018, five years after the fire and more than a decade since the onset of the beetle outbreak. We determined seedling establishment after disturbances by using terminal bud scar counts to estimate seedling age. We measured Engelmann spruce (Picea engelmannii) seed dispersal and below-canopy temperatures over three years (2017–2019) using in situ seed traps and Logtag® temperature sensors at 2 m height. We assessed relationships between (1) seed availability and live overstory remaining after the spruce beetle outbreak, and (2) seedling abundance and all explanatory variables using hierarchical Bayesian models. Conifer seed and seedling counts were zero in most of the burned area, though there was abundant aspen regeneration at lower elevations. In unburned, beetle-affected forests, we found that the degree of spruce overstory mortality did not strongly affect seed availability but showed a strong effect on spruce seedling densities. Conifer seedling densities were also influenced by below-canopy temperatures, aspect, and understory litter and shrub cover. These results indicate that the West Fork Complex fire has potentially resulted in a long-term loss of conifer forest throughout much of the burn area where distance to a seed source is >100 m. High-severity spruce beetle outbreaks have also limited regeneration, although seedlings are still present in the majority of beetle-killed sites. Future patterns of re-establishment will also be strongly influenced by topography and future warming trends.

Early Regeneration Dynamics of Pure Black Spruce and Aspen Forests after Wildfire in Boreal Alberta, Canada

Research Highlights: Black spruce (Picea mariana Mill.) and trembling aspen (Populus tremuloides Michx.) both regenerated vigorously after wildfire. However, pure semi-upland black spruce stands are at increasing risk of changing successional trajectories, due to greater aspen recruitment. Background and Objectives: Black spruce and aspen are found across the boreal forest with black spruce dominating lowlands and aspen being common in uplands. Both species are well adapted to wildfire with black spruce holding an aerial seedbank while aspen reproduce rapidly via root suckering. In the summer of 2016, the Horse River wildfire burned 589,617 hectares of northern Alberta’s boreal forest. Methods: We assessed early regeneration dynamics of both pure aspen and pure black spruce forests. For black spruce, 12 plots were established in both bog and semi-upland habitats to assess seedling regeneration and seedbed availability. For aspen, 12 plots were established in each of the low, moderate, and high burn severities, as well as 5 unburned plots. Results: Post-fire black spruce regeneration densities did not differ between bog and semi-upland habitats, but were positively correlated with forb cover and charred organic matter seedbeds. Aspen regeneration within pure black sprue stands was substantial, particularly in semi-upland habitats, indicating a potential shift in successional trajectory. Fire severity did not significantly affect aspen regeneration in pure aspen stands, but regeneration density in all severity types was >90,000 stems ha−1. Aspen regeneration densities were negatively related to post-fire forb and shrub cover, likely due to competition and cooler soil temperature.

Assessment of residual slash coverage using UAVs and implications for aspen regeneration

Proper redistribution of residual slash following harvesting is crucial for ensuring successful regeneration and continued health in trembling aspen (Populus tremuloides) forests. As traditional methods of measuring residual slash are a strenuous and tedious process, the objective of this study was to develop a new, faster, and more detailed method to assess residual slash distribution for entire harvested blocks. This study also aimed to assess the influence residual slash coverage had on the success of aspen regeneration 1 year after winter harvesting. Using high-resolution UAV imagery and maximum likelihood supervised image classification, residual slash was differentiated from the underlying forest floor. Overall, classification accuracy ranged between 85% and 96% with the highest accuracy occurring when aerial imagery was collected at the beginning of the second spring following winter harvesting. Slash distribution was quite consistent across harvested blocks, with 92% of harvested blocks experiencing <33% coverage. There was no relationship between the level of aspen regeneration following 1 year of growth and percentage slash coverage up to 60%. No vegetation plots occurred in areas with >60% slash coverage; therefore, it is unknown whether aspen regeneration will be affected in areas with higher slash coverage.