Aspen Stands Research Articles

Growth–defense trade-offs shape population genetic composition in an iconic forest tree species

All organisms experience fundamental conflicts between divergent metabolic processes. In plants, a pivotal conflict occurs between allocation to growth, which accelerates resource acquisition, and to defense, which protects existing tissue against herbivory. Trade-offs between growth and defense traits are not universally observed, and a central prediction of plant evolutionary ecology is that context-dependence of these trade-offs contributes to the maintenance of intraspecific variation in defense [Züst and Agrawal, Annu. Rev. Plant Biol., 68, 513-534 (2017)]. This prediction has rarely been tested, however, and the evolutionary consequences of growth-defense trade-offs in different environments are poorly understood, especially in long-lived species [Cipollini et al., Annual Plant Reviews (Wiley, 2014), pp. 263-307]. Here we show that intraspecific trait trade-offs, even when fixed across divergent environments, interact with competition to drive natural selection of tree genotypes corresponding to their growth-defense phenotypes. Our results show that a functional trait trade-off, when coupled with environmental variation, causes real-time divergence in the genetic architecture of tree populations in an experimental setting. Specifically, competitive selection for faster growth resulted in dominance by fast-growing tree genotypes that were poorly defended against natural enemies. This outcome is a signature example of eco-evolutionary dynamics: Competitive interactions affected microevolutionary trajectories on a timescale relevant to subsequent ecological interactions [Brunner et al., Funct. Ecol. 33, 7-12 (2019)]. Eco-evolutionary drivers of tree growth and defense are thus critical to stand-level trait variation, which structures communities and ecosystems over expansive spatiotemporal scales.

Relationships between Overstory and Understory Components of Young Natural and Reconstructed Boreal Aspen Stands

Relationships between Overstory and Understory Components of Young Natural and Reconstructed Boreal Aspen Stands

Availability and Structure of Coarse Woody Debris in Hemiboreal Mature to Old-Growth Aspen Stands and Its Implications for Forest Carbon Pool

European aspen deadwood is extensively studied as a habitat for saproxylic species, while less is known of its dynamics and role in carbon sequestration. We studied unmanaged mature (41–60 years), moderately overmature (61–80 years), overmature (81–100 years), and old-growth (101–140 years) and managed mature and moderately overmature aspen stands on fertile mineral soils. In unmanaged stands, marginal mean CWD volume was from 67.3 ± 12.1 m3 ha−1 in moderately overmature to 92.4 ± 5.1 m3 ha−1 in old-growth stands, with corresponding marginal mean CWD carbon pool 8.2 ± 1.6 t ha−1 and 12.5 ± 0.7 t ha−1 (all p > 0.05), respectively. High CWD volume was present in most stands, by at least two-thirds of plots comprising more than 20 m3 ha−1, and about half of CWD was larger than 30 cm in diameter. Changes in CWD species composition toward a higher proportion of deciduous deadwood in old-growth stands, together with a high volume of recently dead trees, suggest early senescence of the dominant aspen cohort.

Applications of Integrating Wildlife Habitat Suitability and Habitat Potential Models

ABSTRACTAssessment of habitat suitability provides natural resource managers with insights on the quality and spatial distribution of habitat for wildlife species. However, habitat suitability models only provide information on current habitat parameters, and do not consider changes in habitat due to forest succession and disturbances. Habitat potential models have been developed by identifying habitat types and their successional trajectories to provide insights on how landscapes change with time. We developed habitat suitability index (HSI) models and habitat potential models for elk (Cervus elaphus nelsoni) on public and private lands within the Michigan elk range (1,220 km2) in northeastern lower Michigan, USA. Our objective was to demonstrate how integration of habitat suitability and habitat potential models can provide spatiotemporal insights on wildlife habitat. When constructing public‐lands models, we used state forest compartment‐inventory data to identify cover types important to elk, and assigned suitability values (0 = low, 1 = high) to each cover type for elk life requisites (i.e., spring food, winter food, winter thermal cover). Additionally, we modified suitability values based on stand conditions acquired from state forest inventory records (e.g., stand size, percent canopy closure, age of aspen [Populus spp.]). For our private‐lands models, we used satellite imagery to classify cover types and assigned suitability values to cover types for each elk life requisite, and modified values based on percent canopy closure for winter thermal cover. Elk habitat potential was modeled by delineating habitat types by overlaying digital spatial data layers (soils, land‐type associations, vegetation) and identifying successional trajectories using habitat classification guides and literature. We assigned suitability values to each habitat type for life requisites at early to late successional stages. The highest suitability value of each habitat type's successional stage determined the habitat potential for each habitat type. Our winter thermal cover HSI model indicated several large areas (5–13 km2) of high suitability (i.e., lowland conifers) in the southern third of our study area. Our winter food HSI model indicated a heterogeneous arrangement of high suitability areas (hardwoods, upland conifers, aspen) throughout our study area. Our spring food HSI model indicated few areas of high suitability (openings) primarily on private lands. Our habitat potential models indicated high potential for each elk life requisite across the elk range. Comparisons between current elk habitat suitability and habitat potential identify key areas where managers can maximize management efforts for elk in Michigan. Areas determined to have high habitat potential (e.g., mature aspen stands) may become focus areas if they currently have low habitat suitability. Conversely, managers can avoid committing resources to areas with low habitat potential. Integrating habitat suitability and potential models provides insights on how wildlife habitats may respond to forest succession, land management practices, and disturbances, and ultimately help natural resource professionals prioritize and meet a diversity of wildlife management goals. © 2021 The Wildlife Society.

76-year decline and recovery of aspen mediated by contrasting fire regimes: Long-unburned, infrequent and frequent mixed-severity wildfire.

Quaking aspen (Populus tremuloides) is a valued, minor component on northeastern California landscapes. It provides a wide range of ecosystem services and has been in decline throughout the region for the last century. This decline may be explained partially by the lack of fire on the landscape due to heavier fire suppression, as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. However, there is little known about how aspen stand area changes in response to overlapping fire. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 52-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos and satellite imagery to delineate aspen stands and assess conifer cover percent. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We observed these same stands within the first period of fire exclusion for comparison (i.e., 1941–1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting expansion of aspen stand area. Stands with less edge:area ratio also expanded in area more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater loss of area and changes in location (movement) than in the return of fire period, highlighting their vulnerability to loss via succession to conifers in the absence of disturbances that provide adequate growing space for aspen over time.

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.

Effects of Thinning on Dynamics and Drought Resistance of Aspen-White Spruce Mixtures: Results From Two Study Sites in Saskatchewan

Drought stress associated with warm temperatures is causing increased mortality and reduced growth of trees in drier portions of the boreal forests of western Canada with both warming and drought expected to increase over the coming decades. While thinning is often shown to reduce drought stress, there is little information on its effects in stands comprised of mixtures of trembling aspen and white spruce that are common in the forests of this region. This study examined effects of pre-commercial thinning on aspen and spruce growth, response to drought stress, and stand dynamics for two study sites located in Saskatchewan, Canada. In unthinned plots aspen densities declined from initial densities of 40,000 to 200,000 trees ha−1 to 2,639 trees ha−1 at age 26. Twenty-one years after thinning (i.e., at age 26) diameter of aspen and spruce had increased, with largest trees being found at the lowest aspen densities (200 aspen ha−1 for aspen and 0 aspen ha−1 for spruce). Aspen density affected average height but not height of aspen top height trees. Spruce height decreased significantly with increasing aspen density. Crown width and live crown ratio of both aspen and spruce declined with increasing aspen density. Data from cores collected from aspen and spruce indicate significant positive effects of tree size (basal area at beginning of the year), and CMI (Climate Moisture Index) on basal area increment of both aspen and spruce while increasing aspen basal area (m2ha−1) had negative effects on aspen and spruce growth. Increasing tree size (basal area) and aspen competition (basal area ha−1) both had negative effects on drought resistance and resilience for both aspen and spruce. Yield projections provided by the Mixedwood Growth Model (MGM) suggest that the mixture of 1,500 aspen ha−1 and 1,000 spruce ha−1 has the potential to provide a 23% increase in total mean annual increment (MAI) with a single harvest at age 100, compared to unthinned aspen stands, but with a 44% reduction in spruce MAI compared to pure spruce stands. Thinning of aspen to densities below 4,000 trees ha−1 at age 5 resulted in reduced aspen yields but increased spruce yields.

Root-Plate Characteristics of Common Aspen in Hemiboreal Forests of Latvia: A Case Study

Climate change will cause winds to strengthen and storms to become more frequent in Northern Europe. Windstorms reduce the financial value of forests by bending, breaking, or uprooting trees, and wind-thrown trees cause additional economic losses. The resistance of trees to wind damage depends on tree species, tree- and stand-scale parameters, and root-soil plate characteristics such as root-plate size, weight, and rooting depth. The root-soil plate is a complex structure whose mechanical strength is dependent on root-plate width and depth, as the root system provides root attachment with soil and structural support. In Latvia, the common aspen (Populus tremula L.) root system has been studied to develop a belowground biomass model, because information about root system characteristics in relation to tree wind resistance is scarce. The aim of this study was to assess the root-plate dimensions of common aspen stands on fertile mineral soil (luvisol). Study material was collected in the central region of Latvia, where pure mature (41–60 years old) common aspen stands were randomly selected, and dominant trees within the stand were chosen. In total, ten sample trees from ten stands were uprooted. The diameter at breast height (DBH) and tree height (H) were measured for each sample tree, and their roots were excavated, divided into groups, washed, measured, and weighed. The highest naturally moist biomass values were observed for coarse roots, and fine root biomass was significantly lower compared to other root groups. All root group biomass values had a strong correlation with the tree DBH. The obtained results show that there is a close, negative relationship between the relative distance from the stem and the relative root-plate depth distribution.

Ріст і продуктивність порослевих осичників у свіжих і вологих грудах рівнинної частини України

За результатами аналізу повидільної бази даних ВО «Укрдержліспроект» змодельовано хід росту модальних порослевих осикових насаджень свіжого та вологого груду за природними зонами. Встановлено, що модальні порослеві осичники у грудових типах лісорослинних умов ростуть за І класом бонітету, а їхній запас у 70-річному віці сягає 345 м3·га-1 у Поліссі, 320 м3·га-1 у Лісостепу та 270 м3·га-1 у Степу. Підтверджено, що деревостани свіжих і вологих грудів є продуктивнішими, ніж деревостани у відповідних гігротопах сугрудових типів лісорослинних умов. Визначено різницю між запасами деревостанів сугрудів і грудів. В умовах Східного Полісся вона становить 1–2 %, в умовах Лісостепу збільшується до 11–15 %, а у Степу становить 3–9 %. Визначено вік кількісної стиглості осикових насаджень, який в обох типах лісорослинних умов становить 25–30 років.

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.

Understory responses to restoration in aspen‐conifer forests around the Lake Tahoe Basin: residual stand attributes predict recovery

The removal of conifers from aspen (Populus tremuloides) stands is being undertaken throughout the western United States to restore aspen for local‐ and landscape‐level biodiversity. Current practices include mechanically removing conifers or hand thinning, piling, and burning cut conifers in and adjacent to aspen‐conifer stands. To evaluate the effectiveness of restoration treatments, we examined tree regeneration and herbaceous vegetation cover in thinned, thinned and pile burned, and non‐thinned control stands. Growth rates of small conifer saplings threatening to outcompete and replace aspen were also measured. Two to four years after pile burning, herbaceous vegetation cover within the footprint of burned piles (i.e. burn scars) was 35–73% of that in adjacent areas. Aspen was more likely to regenerate inside burn scars where fewer surrounding trees were true firs. Conifer seedlings were more likely to regenerate in burn scars where more of the surrounding trees were conifers (pine or fir) as opposed to aspen. Fir saplings had much slower growth than did aspen saplings. Overall, our findings show that restoration treatments are promoting desirable outcomes such as enhancing aspen regeneration but that follow‐up treatments will be needed to remove numerous conifer seedlings becoming established after restoration activities. Eliminating conifers, while they are small, growing slowly, and contributing little to fuel loads may be an economical way to prolong restoration treatment effectiveness.

Contrasting Root System Structure and Belowground Interactions between Black Spruce (Picea mariana (Mill.) B.S.P) and Trembling Aspen (Populus tremuloides Michx) in Boreal Mixedwoods of Eastern Canada

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0–15 cm, and bottom 15–30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce’s more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer.

Моделювання перебігу росту за середньою висотою модальних деревостанів твердолистяних деревних видів України

In Ukrainian Polissia, forestry practices often deviate from reforestation and afforestation projects due to the intensive distribution and rapid growth of some tree species. In addition, there is an intensive process of afforestation of former agricultural lands by stands of Scots pine (Pinus sylvestris L.), silver birch (Betula pendula L.), black alder (Alnus glutinosa L.), and common aspen (Populus tremula L.). Based on the analysis of the current regulatory-reference tools, the absence of growth tables of mixed modal birch stands was determined and the necessity of creating reference materials using a unified system for forest mensuration of softwood deciduous stands was determined. Statistically significant differences in the growth of monocultures and mixed stands, seed and vegetative origin birch stands by major biometric indicators were established. The difference in the growth of mixed birch and pine-birch stands was found. According to the results of mathema tic modelling stands biometrical parameters, tables of the course of growth of modal mixed birch stands of Ukrainian Polissia have been developed. Other representatives of such stands can be Black alder and Common aspen. Mathematic models were tested for compliance with the biological features of the growth of stands. Thus, a system of models of the growth of modal birch stands was developed, which reflects the dynamics of the main biometric indicators, including average height, average diameter, basal area, growing stock volume and total productivity. Compiled yield tables for modal birch stands, reflecting the specific features of the growth of the studied forests, can be the basic regulatory-reference tools for their inventory. In addition, tables of growth modal stands are the basis for the solution of many other organizational, technical, and planned forest management calculations.

Coppicing improves the growth response of short-rotation hybrid aspen to elevated atmospheric humidity

Aspens are fast-growing clonal trees with a wide circumboreal distribution range, suitable for the production of pulp and bioenergy. The adaptability of aspen short-rotation coppice systems to climate change has rarely been investigated. For a large part of aspens’ northern range, climate models predict an increase in precipitation and, consequently, in atmospheric humidity. Our aim was to clarify the long-term effect of elevated air humidity on vegetative reproduction capacity and dynamics of above-ground growth and size structure in aspen stands. We analysed tree growth data from two consecutive 6-year rotations (a planted and a coppice generation) in experimental short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) stands in the Free Air Humidity Manipulation (FAHM) experiment in Estonia. In three plots, mean relative air humidity was elevated by 7% and three plots were controls. Across two rotation periods, the humidification effect on tree height and/or stem basal area increment was year-dependent (p < 0.001): negative in 4 years, positive also in 4 years and non-significant in 3 years. Mean basal area of humidified (11.6 ± 0.8 cm2) and control trees (15.0 ± 1.0 cm2) differed significantly (p = 0.035) at the end of the first but not the second rotation period (9.3 ± 0.9 cm2 and 9.3 ± 1.2 cm2, respectively). Average growth differences levelled out already in the beginning of the second rotation, suggesting that some root-level acclimation must have taken place. The annual size-growth relationships (SGR) indicated a more size-symmetric growth in humidified (SGR = 1.00 ± 0.05) and a size-asymmetric growth (SGR = 1.12 ± 0.04) in control stands, implying a greater role of root-competition in humidified stands. In humidified stands, the growth of re-sprouting trees was more strongly determined by parent tree size, indicating a stronger carry-over of size hierarchy. The tree height diversity fluctuated more in control stands, where mortality was higher, especially after dry years. To summarise, short- and long-term responses of hybrid aspen to elevated air humidity varied, emphasizing the importance of long-term climate manipulations with trees. Generally, hybrid aspen short-rotation coppice forests showed promising acclimation capacity with future more humid climate predicted for northern latitudes.

The influence of sexual dimorphism on the phenotypical properties of poplar

Abstract The influence of sexual dimorphism on commercial-biological traits of the most widespread poplar of the world Populus tremula L. in the Central forest-steppe of the European territory of Russia was studied. The analysis was made of the distribution of male and female clones in general, the effects of humidity, the richness of the soil, and surrounding vegetation on their productive capacity as well as core rot damage. The study presents survey results of 620 aspen plots spread out over an area of 823 hectares as well as more detailed data obtained from 3 pairs of test plots in which the indicators of male and female aspen stands were compared under identical or similar conditions. The study showed that the area of male aspen in the studied region is 4.5 times larger than that of females. The former prevail in drier and less fertile conditions while females gravitate towards more favorable growing conditions. The comparison of the productive capacity of male and female clones of the same age class, growing in identical or similar growing conditions showed that the height, diameter, and volume of the trunks were higher in male trees. Female aspen trees were less affected by core rot in the best conditions of growth, and male ones were more resistant at the worse ones.

Climate and disturbance influence self-sustaining stand dynamics of aspen (Populus tremuloides) near its range margin.

Species that are primarily seral may form stable (self-sustaining) communities under certain disturbance regimes or environmental conditions, yet such populations may also be particularly vulnerable to ecological change. Aspen (Populus spp.) are generally considered seral throughout the Northern Hemisphere, including P.tremuloides, the most widely distributed tree species in North America. Recent declines in aspen populations have occurred, especially along drought-sensitive margins of its range and where fire exclusion and herbivory have promoted community transition. However, aspen also forms stable stands, and examination of the mechanisms that influence persistence can offer conservation insights, especially where populations are vulnerable to changing climate or altered disturbance dynamics. We sampled tree age and stand characteristics of isolated aspen forests in the arid Great Basin (USA) to determine if (1) aspen communities are more fire-dependent and seral or fire-independent and stable; (2) ungulate browsing inhibits aspen stability; and (3) temporal patterns of vegetative reproduction (i.e., ramet establishment or "suckering") are correlated with climate. Aspen size and age class densities strongly fit negative exponential distributions, whether grouped geographically or by functional type, suggesting landscape-scale persistence. Continuous age distributions and high proportions of recruitment-sized to overstory trees suggest stability at stand scales, with exceptions including stands with higher browsing pressure. Few stands had evidence of fire, and relationships between dead tree size and variability in live tree size suggest a lack of fire dependency. Several 5-yr averaged climate variables and one sea surface temperature index were correlated with aspen ramet establishment densities over time, with strongest relationships occurring ~5yr prior to establishment year, often followed by inverse relationships ~1yr after. Indeed, aspen establishment density for a recent 41-yr period was reliably reconstructed using antecedent climate conditions derived from a single drought index. Temporally synchronized aspen ramet establishment across the study region may be due to climate-driven storage of nonstructural carbohydrate reserves in clonal root systems later used for regeneration. Complex regeneration dynamics of these self-sustaining aspen stands, especially sensitivity to climate variability, suggest they may serve as harbingers of ecological change in the arid Great Basin and in other aspen populations near their range margin.

Stable or seral? Fire-driven alternative states in aspen forests of western North America

As important centres for biological diversity, aspen forests are essential to the function and aesthetics of montane ecosystems in western North America. Aspen stands are maintained by a nuanced relationship with wildfire, although in recent decades aspen mortality has increased. The need to understand the baseline environmental conditions that favour aspen is clear; however, long-term fire history reconstructions are rare due to the scarcity of natural archives in dry montane settings. Here, we analyse a high-resolution lake sediment record from southwestern, Utah, USA to quantify the compositional and burning conditions that promote stable (or seral) aspen forests. Our results show that aspen presence is negatively correlated with subalpine fir and that severe fires tend to promote persistent and diverse aspen ecosystems over centennial timescales. This information improves our understanding of aspen disturbance ecology and identifies the circumstances where critical transitions in montane forests may occur.

The relative importance of biotic and abiotic factors influencing aspen recruitment in Arizona

Poor recruitment in some quaking aspen (Populus tremuloides) stands has led to debate over which factors play the largest role in aspen forest persistence. Understanding the relative importance of the many relevant factors over a large landscape could inform management strategies regarding aspen recruitment by focusing efforts on the most important factors. Therefore, between 2011 and 2016 we collected data on 29 biotic and abiotic factors thought to affect aspen recruitment from 92 randomly-selected aspen stands growing along the southwestern limit of its distribution in Arizona, USA. We assessed the condition of selected aspen stands by quantifying the number of recently recruited aspen stems (saplings >2 m tall and <5 cm dbh) in each sampling plot. We used negative binomial regression to estimate the relationship between aspen recruitment and the measured covariates. We fit a balanced set of models, calculated AIC weights for those models, and summed the weights of the models containing each covariate as a measure of covariate relative importance. Six covariates had relative importance values that were significantly greater than random: fire severity in 2011, conifer encroachment, rust presence, fire severity in 2015, blight presence, and the standardized precipitation-evapotranspiration index. There were no significant differences in the ranking of these six covariates by relative importance. Although we estimated that cattle and elk had a significant negative impact on aspen recruitment, these factors were relatively unimportant. This seemingly counter-intuitive result arose because many sites lacked ungulates, but still failed to recruit aspen, indicating that other factors were more important for aspen recruitment. Our results indicate that conifer removal and increased fire activity could be among the most effective management tools to help promote aspen recruitment.

Thermal environments within aspen (Populus tremuloides) tree cavities during summer: Implications for breeding and roosting cavity users

Summer temperature patterns within tree cavities might influence occupancy of cavities by different animals such as birds and bats, and furthermore, cavity temperatures can influence processes such as embryonic development or the development of young. Our study aimed to identify the environmental variables influencing cavity temperatures during summer (June–July) in quaking aspen (Populus tremuloides) in southeastern Idaho. We collected temperature data between June 22 and July 27, 2015 using iButtons distributed across 30 tree cavities in 30 aspen stands in the Craters of the Moon National Monument, Idaho. We used every third day of data to ensure temporal independence amongst readings from the same cavity. We used a multi-model selection framework to examine how environmental characteristics might influence cavity thermal environments, and we modeled how environmental variables, tree, and cavity characteristics might influence daily maximum cavity temperatures. Cavity temperatures ranged as low as 1 °C to as high as 46 °C. Approximately 13% of the cavities experienced temperatures above 40 °C for at least an hour, and these temperatures are noted to be lethal to developing avian embryos. The two top competing models explaining the daily maximum cavity temperatures included tree diameter at cavity height, canopy cover, cavity orientation, and daily maximum ambient temperature. Daily maximum cavity temperatures were significantly associated with daily maximum ambient temperatures and canopy cover; warmer cavities were positively associated with warmer ambient temperatures and negatively associated with canopy cover. Because cavities in aspen are an important resource for multiple species, understanding the factors that influence the microclimate of tree cavities can have broad implications for cavity using species in the context of changing climates.

Ungulate Herbivory Is Correlated with High Aspen Suckering Density but Reductions in Aspen Growth Rates and Recruitment

Aspen is a foundation species that provides vital habitat for hundreds of plant and animal species. However, changing ungulate herbivory regimes may be altering recruitment success and resilience of aspen forests. The objective of this observational study was to quantify the impacts of ungulate herbivory on aspen recruitment potential. We sampled 36 aspen dominant stands on the Fishlake National Forest in Utah for browse of apical meristems, suckering density (< 180 cm in height), and recruitment density (≥ 180 cm). Our analysis indicated that for each 10% of apical meristems browsed, annual vertical and radial growth rates and recruitment density were reduced by 1.2 cm, 0.02 mm, and 17 ha−1, respectively. In contrast, aspen suckering density increased by 1 230 suckers ha−1, with each 10% browse of apical meristems. This suggests that ungulates contribute to aspen suckers persisting in the understory for longer periods and may alter recruitment success of aspen stands. Aspen that were under high ungulate browse pressure were associated with lower growth rates and persisted for twice as long in the understory. For example, a 5-yr-old sucker was predicted to be 115 cm without browse of apical meristems and 60 cm with 100% browse of apical meristems. Further, aspens of the same height with 100% browse of apical meristems were nearly 4 yr older on average than aspens with no evidence of browse of apical meristems. Approximately 34% of aspen suckers had persisted in the understory < 100 cm for ≥ 6 yr, and 7% of suckers had persisted in the understory for ≥ 10 yr. Our results suggest that high rates of meristem removal are correlated with increased aspen suckering yet reduced aspen growth and recruitment.