Search

In recent decades, climate change has disrupted forest functioning by promoting large-scale mortality events, declines in productivity and reduced regeneration. Understanding the temporal dynamics and spatial extent of these changes is critical given the essential ecosystem services provided by forests. As the most widespread tree species in North America, quaking aspen (Populus tremuloides) is well suited for studying the dynamics of tree populations during a period of unprecedented climate change. Synthesizing continent-wide data, we show that mortality rates of mature aspen stems have increased over the past two-to-three decades, while relative gains in aspen basal area have decreased during the same period. Patterns were pervasive across multiple stand size classes and composition types in western North America biomes, suggesting that trends in demographic rates were not simply a reflection of stand development and succession. Our review of the literature revealed that increased aspen mortality and reduced growth rates were most often associated with hotter, drier conditions, whereas reduced recruitment was most often associated with herbivory. Furthermore, interactions between climate and competition, as well as climate and insect herbivory, had important, context-dependent effects on mortality and growth, respectively. Our analyses of aspen across its entire geographic range indicate that this important tree species is experiencing substantial increases in mortality and decreases in population growth rates across multiple biomes. If such trends are not accompanied by increased recruitment, we expect that the reduced dominance of aspen in forests will lead to major declines in the many essential ecosystem services it provides.

Oystershell scale (OSS; Lepidosaphes ulmi) is an emerging invasive insect that poses a serious threat to conservation of quaking aspen (Populus tremuloides) in the southwestern US. Although OSS has been an urban pest in the US since the 1700s, it has recently spread into natural aspen stands in northern Arizona, where outbreaks are causing dieback and mortality. We quantified the ongoing outbreak of OSS at two scales: (1) local severity at two sites and (2) regional distribution across northern Arizona. Our regional survey indicated that OSS is widespread in lower elevation aspen stands and is particularly pervasive in ungulate exclosures. Advanced regeneration had the highest levels of infestation and mortality, which is concerning because this size class is an underrepresented component of aspen stands in northern Arizona. If OSS continues to spread and outbreaks result in dieback and mortality like we observed, then aspen in the southwestern US, and perhaps beyond, will be threatened. Three interacting factors contribute to OSS’s potential as a high-impact invasive insect that could spread rapidly: (1) its hypothesized role as a sleeper species, (2) potential interactions between OSS and climate change, and (3) the species’ polyphagous nature. Invasive pests like OSS pose an imminent threat to native tree species and, therefore, represent an immediate research and monitoring priority. We conclude with recommendations for future research and monitoring in order to understand OSS’s biology in natural aspen stands, quantify impacts, limit future spread, and mitigate mortality and loss of aspen and other host species.

Populus tremuloides is the widest‐ranging tree species in North America and an ecologically important component of mesic forest ecosystems displaced by the Pleistocene glaciations. Using phylogeographic analyses of genome‐wide SNPs (34,796 SNPs, 183 individuals) and ecological niche modeling, we inferred population structure, ploidy levels, admixture, and Pleistocene range dynamics of P. tremuloides, and tested several historical biogeographical hypotheses. We found three genetic lineages located mainly in coastal–Cascades (cluster 1), east‐slope Cascades–Sierra Nevadas–Northern Rockies (cluster 2), and U.S. Rocky Mountains through southern Canadian (cluster 3) regions of the P. tremuloides range, with tree graph relationships of the form ((cluster 1, cluster 2), cluster 3). Populations consisted mainly of diploids (86%) but also small numbers of triploids (12%) and tetraploids (1%), and ploidy did not adversely affect our genetic inferences. The main vector of admixture was from cluster 3 into cluster 2, with the admixture zone trending northwest through the Rocky Mountains along a recognized phenotypic cline (Utah to Idaho). Clusters 1 and 2 provided strong support for the “stable‐edge hypothesis” that unglaciated southwestern populations persisted in situ since the last glaciation. By contrast, despite a lack of clinal genetic variation, cluster 3 exhibited “trailing‐edge” dynamics from niche suitability predictions signifying complete northward postglacial expansion. Results were also consistent with the “inland dispersal hypothesis” predicting postglacial assembly of Pacific Northwestern forest ecosystems, but rejected the hypothesis that Pacific‐coastal populations were colonized during outburst flooding from glacial Lake Missoula. Overall, congruent patterns between our phylogeographic and ecological niche modeling results and fossil pollen data demonstrate complex mixtures of stable‐edge, refugial locations, and postglacial expansion within P. tremuloides. These findings confirm and refine previous genetic studies, while strongly supporting a distinct Pacific‐coastal genetic lineage of quaking aspen.

Spatial interactions between trees influence forest community succession. The objective of this study was to investigate how shifts in forest composition and proximity between tree species affect stand development over time in mixed forest systems. At six locations across the Fishlake National Forest, Utah, USA, in stands where facilitation has been documented previously, tree-ring samples were collected from aspen and subalpine fir trees. Basal area increment was calculated to characterize the effects of the proximity of overstory trees on multidecadal growth responses of aspen and subalpine fir in aspen-dominant and mixed aspen–conifer stands. Subalpine fir seedlings were established next to aspen (within 10 cm) when aspen was between 15 and 120 years old with a mean age of 60 years. Aspen and subalpine fir growth rates were reduced with increasing conifer abundance. Aspen trees growing next to a proximate subalpine fir tree had slower growth rates over time than aspen trees growing independently. Growth rates of subalpine fir in aspen-dominated stands were similar when growing independently or near aspen trees. However, subalpine fir in conifer-dominated stands maintained higher growth rates when growing next to an aspen tree than when growing independently. The data suggest that as stand competition increases with conifer abundance, the proximity of overstory trees increases competitive exclusion of aspen while having a beneficial growth effect on subalpine fir. These results underscore the importance of maintaining natural fire regimes in forest systems that keep competitive interactions in balance.

Species responses to climate change depend on environment, genetics, and interactions among these factors. Intraspecific cytotype (ploidy level) variation is a common type of genetic variation in many species. However, the importance of intraspecific cytotype variation in determining demography across environments is poorly known. We studied the tree species quaking aspen (Populus tremuloides), which occurs in diploid and triploid cytotypes. This widespread species is experiencing contractions in its western range, which could potentially be linked to cytotype- dependent drought tolerance. We found that interactions between cytotype and environment drive mortality and recruitment across 503 plots in Colorado. Triploids were more vulnerable to mortality relative to diploids and had reduced recruitment on more drought-prone and disturbed plots relative to diploids. Furthermore, there was substantial genotype-dependent variation in demography. Thus, cytotype and genotype variation are associated with decline in this foundation species. Future assessment of demographic responses under climate change will require knowledge of how genetic and environmental mosaics interact to determine species' ecophysiology and demography.

1) Ploidy level in plants may influence ecological functioning, demography and response to climate change. However, measuring ploidy level typically requires intensive cell or molecular methods. 2) We map ploidy level variation in quaking aspen, a dominant North American tree species that can be diploid or triploid and that grows in spatially extensive clones. We identify the predictors and spatial scale of ploidy level variation using a combination of genetic and ground‐based and airborne remote sensing methods. 3) We show that ground‐based leaf spectra and airborne canopy spectra can both classify aspen by ploidy level with a precision‐recall harmonic mean of 0.75–0.95 and Cohen's kappa of c. 0.6–0.9. Ground‐based bark spectra cannot classify ploidy level better than chance. We also found that diploids are more common on higher elevation and steeper sites in a network of forest plots in Colorado, and that ploidy level distribution varies at subkilometer spatial scales. 4) Synthesis. Our proof‐of‐concept study shows that remote sensing of ploidy level could become feasible in this tree species. Mapping ploidy level across landscapes could provide insights into the genetic basis of species' responses to climate change.

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

The Northern Great Plains contains a diverse group of vegetative communities, primarily dominated by grassland communities.Precipitation declines along an east-west gradient, ranging from 27.4 inches at Detroit Lakes, Minnesota to 12.4 inches at Miles City, Montana, and productivity follows a similar decline.Precipitation falls primarily during the growing season, which combined with the lower mean annual temperature results in productive, high-quality, coolseason dominated grasslands.Although the region is primarily dominated by areas of tallgrass, midgrass, and shortgrass prairie, there are outcrops of limber (Pinus flexilis) and ponderosa pine (P. ponderosa) along the Little Missouri River and stands of quaking aspen (Populus tremuloides) in the Turtle Mountains.Besides climate and soils, fire, drought, and grazing have also contributed to the rich diversity of communities in the region.Recent invasions of perennial cool-season grasses are threatening historic plant communities; whether these invasions can be reversed and altered environmental services restored are the primary questions facing grassland managers.

The root systems of forest ecosystem are both important and understudied. Roots function as the critical organs of nutrient uptake and anchor tree stems against wind throw and aboveground disturbance. Roots are also an important terrestrial reservoir of carbon and play an important role in global climate. Conversion or disturbance of forest lands can result in an emission of carbon dioxide to the atmosphere, and so quantifying the amount of carbon in root biomass is of interest for forest scientists and the climate policy their research informs.This dissertation contributes to the science of root biomass measurement and modeling, providing techniques and approaches to improve those estimates, and models for leaf area that can be used to estimate root biomass. It focuses on techniques to improve measurement of root biomass using ground penetrating radar, an emergent method to measure root biomass. First, I investigate what above-ground metrics might best be used to model and predict root biomass and distribution and I develop new models relating basal area to leaf area for quaking aspen (Populus tremuloides) in the Sierra Nevada. Second, I investigate root biomass and distribution in coast redwood (Sequoia sempervirens), and how water acquisition may drive the allocation to roots in redwoods.Ground penetrating radar is shown to have considerable utility for the measurement of coarse root biomass. Above ground biomass is shown to be an excellent predictor of coarse root biomass in Sierran ecosystems. Leaf area, which is predicted by pipe model theory to have a tight relationship with fine root biomass, is shown to have some predictive power for root biomass, and my model represents an improvement over other reported metrics. Finally, I conclude that global root biomass estimates have not been based on reliable or unbiased data to date and I contribute A plan to correct this gap in future global estimates.

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.

Across the northern hemisphere, six species of aspen (Populus spp.) play a disproportionately important role in promoting biodiversity, sequestering carbon, limiting forest disturbances, and providing other ecosystem services. These species are illustrative of efforts to move beyond single-species conservation because they facilitate hundreds of plants and animals worldwide. This review is intended to place aspen in a global conservation context by focusing on the many scientific advances taking place in such biologically diverse systems. In this manner, aspen may serve as a model for other widespread keystone systems where science-based practice may have world implications for biodiversity conservation. In many regions, aspen can maintain canopy dominance for decades to centuries as the sole major broadleaf trees in forested landscapes otherwise dominated by conifers. Aspen ecosystems are valued for many reasons, but here we highlight their potential as key contributors to regional and global biodiversity. We present global trends in research priorities, strengths, and weaknesses based on, 1) a qualitative survey, 2) a systematic literature analysis, and 3) regional syntheses of leading research topics. These regional syntheses explore important aspen uses, threats, and research priorities with the ultimate intent of research sharing focused on sound conservation practice. In all regions, we found that aspen enhance biodiversity, facilitate rapid (re)colonization in natural and damaged settings (e.g., abandoned mines), and provide adaptability in changing environments. Common threats to aspen ecosystems in many, but not all, regions include effects of herbivory, land clearing, logging practices favoring conifer species, and projected climate warming. We also highlight regional research gaps that emerged from the three survey approaches above. We believe multi-scale research is needed that examines disturbance processes in the context of dynamic climates where ecological, physiological, and genetic variability will ultimately determine widespread aspen sustainability. Based on this global review of aspen research, we argue for the advancement of the “mega-conservation” strategy, centered on the idea of sustaining a set of common keystone communities (aspen) that support wide arrays of obligate species. This approach contrasts with conventional preservation which focuses limited resources on individual species residing in narrow niches. Keywords: Populus tremula, P. tremuloides, P. davidiana, P. adenopoda, Biodiversity, Forests, World conservation, Literature review

Populus xsmithii is an uncommon hybrid of quaking aspen (Populus tremuloides) and bigtooth aspen (Populus grandidentata). Like its parents, Populus xsmithii is an early successional member of boreal forest communities, dependent on disturbance events that clear areas of competitive stems and spur an increase in clonal suckering. In recent years, aspen dieback has been noted across much of the western United States, a condition characterized by mortality of older stems and a lack of recruitment of suckers to maturity. In the Niobrara River Valley of Cherry County, Nebraska, USA, a disjunct population of Pleistocene relict Populus xsmithii has been targeted for management via clearing of competitive conifer species and establishment of fenced refugia to protect suckers from herbivory. The stands currently contain abundant suckers, which occur in three types of sites: the fenced refugia created by managers, the open habitat cleared of other species in the stands, and the woodpiles left by said clearing. This study assessed the growth and vigor of these aspen suckers over a nine-month period (summer 2013-spring 2014) and compared the effects of different site treatments. We found that aspen suckers in the open areas were significantly shorter, had smaller basal diameter, and had higher damage scores than those in the two protected site types (fenced and woodpile). Because this population is on the margin of the distribution for aspen, evaluating the effectiveness of management techniques will provide valuable information for those who seek to ensure the survival of this aspen population and others.

Throughout much of the 20th century, the heights of young quaking aspen (Populus tremuloides) in Yellowstone National Park’s northern ungulate winter range were suppressed due to intensive herbivory by Rocky Mountain elk (Cervus elaphus). However, following the 1995–96 reintroduction of gray wolves (Canis lupus), completing the park’s large predator guild, young aspen in various portions of the northern range began to increase in height. From 1999 to 2015, a 17-yr period of declining elk densities in the northern range, browsing rates declined and young aspen heights increased once elk densities dropped below ∼4 elk/km2. The inverse relationship between browsing rate and young aspen height, a relationship linking elk and plants, was consistent with a re-established trophic cascade. Within the Glen Creek study area (8.3 km2), decreased browsing and increased heights of young aspen were associated, at least in part, with two hypothesized small-scale predation risk factors (i.e., escape impediment, view impediment). However, the young aspen height increases did not occur in the Mammoth study area (6.0 km2) and heights there remained short. With high levels of human activity at the Mammoth townsite, wolf activity near the townsite remained low, an example of “human shielding,” thereby allowing elk browsing to continue the suppression of young aspen. Overall, results indicated that Yellowstone’s contemporary large predator guild, by altering elk behavior and density at several spatial scales, has not only contributed to a relatively widespread pattern of increased young aspen heights across much of the park’s northern range, but also greater spatial variation in those heights.

Populus tremuloides Michx. is an iconic tree of the mountains of the western United States. In Utah, it very commonly suffers from white trunk rot caused by Phellinus tremulae (Bondartsev) Bondartsev & P.N.Borisov. The incidence of this disease is affected by tree and site characteristics, but the magnitude of these effects appears to be site-dependent. To minimize harvest wood loss in Utah, we determined the locally important factors that explain the wide variation in the incidence of sporocarps of Phellinus tremulae. To avoid confounding of factors, we utilized a multiple regression approach. We found that while the incidence of Phellinus tremulae sporocarps on quaking aspen was always low at high elevations, it was variable at low elevations. Our logistic regression model indicated that variation in the incidence of sporocarps at low elevations was attributable, in part, to variation in aspect, slope, environmental stress, and tree age and size. Based on these results, we recommend that harvesting at elevations below 2500 m be confined to younger trees or to sites on relatively steep, north-facing slopes. Because of site-dependency, the same general method could be used to establish harvesting criteria in other regions.

Ecosystem resilience to climate change is contingent on post-disturbance plant regeneration. Sparse gymnosperm regeneration has been documented in subalpine forests following recent wildfires and compounded disturbances, both of which are increasing. In the US Intermountain West, this may cause a shift to non-forest in some areas, but other forests may demonstrate adaptive resilience through increased quaking aspen (Populus tremuloides Michx.) dominance. However, this potential depends on ill-defined constraints of aspen sexual regeneration under current climate. We created an ensemble of species distribution models for aspen seedling distribution following severe wildfire to define constraints on establishment. We recorded P. tremuloides seedling locations across a post-fire, post-blowdown landscape. We used 3 algorithms (Mahalanobis Typicalities, Multilayer Perceptron Artificial Neural Network, and MaxEnt) to create spatial distribution models for aspen seedlings and to define constraints. Each model performed with high accuracy and was incorporated into an ensemble model, which performed with the highest overall accuracy of all the models. Populus tremuloides seedling distribution is constrained primarily by proximity to unburned aspen forest and annual temperature ranges, and secondarily by light availability, summer precipitation, and fire severity. Based on model predictions and validation data, P. tremuloides seedling regeneration is viable throughout 54% of the post-fire landscape, 97% of which was previously conifer-dominated. Aspen are less susceptible to many climatically-sensitive disturbances (e.g. fire, beetle outbreak, wind disturbance), thus, aspen expansion represents an important adaptation to climate change. Continued aspen expansion into post-disturbance landscapes through sexual reproduction at the level suggested by these results would represent an important adaptation to climate change and would confer adaptive forest resilience by maintaining forest cover, but would also alter future disturbance regimes, biodiversity, and ecosystem services.

On the Ground • Spatial data is valuable to researchers for locating studies that occur in a particular area of interest, or one with similar attributes. • Without a standard in publishing protocol, spatial data largely goes unreported, or is difficult to find without searching the publication. • Assigning location data and displaying points on a public web map makes locating publications based on spatial location possible.

The American quaking aspen (Populus tremuloides Michx.) and its close relative, the Eurasian quaking aspen (Populus tremula L.), cover a realm that is perhaps the most expansive of all tree species in the world. In North America, sudden aspen decline (SAD) is a growing concern that marks the rapid decline of quaking aspen trees leading to mortality at the stand and landscape scale. Research suggests that drought and water stress are the primary causes of SAD. Predisposing factors (age, structure, and landscape position), as well as associated stressors (i.e., pests and pathogens), have been linked to mortality in affected stands. The conflation of multiple interacting factors across the aspen&rsquo, s broad geographic range in North America has produced significant debate over the classification of SAD as a disease and the proper management of affected stands. Interestingly, no such effects have been reported for the Eurasian aspen. We here review and synthesize the growing body of literature for North America and suggest that SAD is a novel decline disease resulting from multiple inciting and interacting factors related to climate, land-use history, and successional dynamics. We suggest that the range of aspen observed at the onset of the 21st Century was bolstered by a wet period in western North America that coincided with widespread regional cutting and clearing of late-successional forests for timber and grazing. No comparable land-use history, successional status, or age-class structure is apparent or linked for Eurasian forests. Eurasian aspen is either absent or young in managed forests, or old and decadent in parks in Fenno-Scandinavia, or it grows more intimately with a more diverse mixture of tree species that have arisen from a longer period of frequent timber cutting in Russia. Based on these insights we provide recommendations for practical management techniques that can promote stand resilience and recovery across a range of stand conditions in North America. Managers should attempt to identify SAD-prone stands using the presence of predisposing conditions and focus treatments such as coppice or prescribed fire on stands with suitable topographies, elevations, and climates. We conclude that SAD will persist throughout the coming decades, given the enormity of past cutting history, fire exclusion, and current changes in climate until a more active restoration agenda is implemented.

On the Ground Quaking aspen is widely regarded as a key resource for humans, livestock, and wildlife with these values often competing with each other, leading to overuse of aspen in some locations and declines. Wereview trends in aspen science and management, particularly in Utah. Historically, research conducted here holds a prestigious place in international aspen circles. We highlight recent studies continuing the tradition to keep rangeland managers informed of important developments, focusing on aspen functional types, historical cover change and climate warming, ungulate herbivory, and disturbance interactions.

Recent concern regarding the potential decline of quaking aspen (Populus tremuloides Michx.) forests in the western United States has sparked concern over whether the species can be effectively regenerated. Using a retrospective approach, we quantified the response of regenerating aspen stems to an ordinary set of silvicultural treatments conducted over approximately the past decade in southern Utah, USA. A suite of variables describing stand structure and composition, stand vigor, physiographic factors, herbivore pressure, and treatment types were measured to predict the possible controls, as well as their relative importance, on aspen regeneration. Results suggested that aspen regeneration was most strongly related to browsing pressure, site preparation technique, and the presence of advance reproduction before treatment, which is a novel finding. Secondary predictors included elevation, site index, and overstory conditions, which are generally characteristics of stand vigor. Management recommendations based on our results should recognize the strong primary control that browsing pressure exerts on regeneration. First, the height of advance reproduction is inherently dependent on antecedent herbivory and also indicative of present browsing and should be assessed before treatment. Second, the most effective site preparation techniques, namely broadcast burning and browsing reduction, will directly reduce browsing pressure, assuming ungulate populations are not too large. Any management targeting timely and effective aspen regeneration should incorporate monitoring and (or) controlling browsing pressure, both before and after treatment.

Summary 1.Overbrowsing by ungulates is a major cause of poor aspen stand regeneration across North America and Eurasia. In general, factors driving ungulate browser preferences include concentrations of plant secondary compounds and the nutritional composition (non-structural carbohydrates, protein, and minerals) of foliage. While each of these phytochemical factors has been shown to independently influence ungulate preference, the relative impact of each factor is unknown, as no study to date has examined them simultaneously. 2.Plant fitness depends not only on the capacity of plants to resist browsing, but also on their capacity to tolerate browsing once it has occurred. Little is known of aspen tolerance to browsing, which inflicts a different form of damage than insect herbivory. 3.We employed multiple aspen genotypes, replicate trees of which were subjected to different soil nutrient treatments, to investigate: 1) the effects of aspen genotype, nutrient treatment, and genotype x nutrient interactions on susceptibility to browsing by white-tailed deer, 2) the phytochemical basis for the patterns observed in (1), and 3) the effects of genotype, soil nutrients, and their interaction on short-term tolerance to deer browsing. 4.Aspen genotypes varied markedly in their vernal susceptibility to deer browsing. Genetic variation in early season levels of non-structural carbohydrates (sugars), protein, and multiple macro- and trace minerals had the strongest influence on tree susceptibility to browsing. In contrast, levels of phytochemical defenses had minimal effects, although the range of levels expressed in this study was small. Soil nutrient availability did not significantly influence deer preference. 5.The extent of browsing affected post-browse tolerance across genotypes. Soil nutrient treatment had little differential effect on tolerance, and, for the most part, genotypes did not display differential tolerance to browsing, regardless of which soil nutrient treatment they experienced. 6.Synthesis: Genetic variation for susceptibility to browsing indicates that ungulate browsers have the potential to be agents of selection in aspen populations. In contrast with previous studies in aspen highlighting the importance of phytochemical defenses in shaping preferences of browsing mammals, our results indicate that the nutritional composition of foliage (sugars, protein, and mineral concentrations) can have sizable effects on preference. The observed lack of influence of soil nutrient availability on tree browsing tolerance contrasts with predictions of the limiting resource model, the prevailing model for plant tolerance. This article is protected by copyright. All rights reserved.

Quaking aspen ( Populus tremuloides ) is the most widespread tree species in North America and has supported a unique ecosystem for tens of thousands of years, yet is currently threatened by dramatic loss and possible local extinctions. While multiple factors such as climate change and fire suppression are thought to contribute to aspen’s decline, increased browsing by elk ( Cervus elaphus ), which have experienced dramatic population increases in the last ∼80 years, may severely inhibit aspen growth and regeneration. Fires are known to favor aspen recovery, but in the last several decades the spatial scale and intensity of wildfires has greatly increased, with poorly understood ramifications for aspen growth. Here, focusing on the 2000 Cerro Grande fire in central New Mexico – one of the earliest fires described as a “mega-fire” - we use three methods to examine the impact of elk browsing on aspen regeneration after a mega-fire. First, we use an exclosure experiment to show that aspen growing in the absence of elk were 3× taller than trees growing in the presence of elk. Further, aspen that were both protected from elk and experienced burning were 8.5× taller than unburned trees growing in the presence of elk, suggesting that the combination of release from herbivores and stimulation from fire creates the largest aspen growth rates. Second, using surveys at the landscape level, we found a correlation between elk browsing intensity and aspen height, such that where elk browsing was highest, aspen were shortest. This relationship between elk browsing intensity and aspen height was stronger in burned ( r = −0.53) compared to unburned ( r = −0.24) areas. Third, in conjunction with the landscape-level surveys, we identified possible natural refugia, microsites containing downed logs, shrubs etc. that may inhibit elk browsing by physically blocking aspen from elk or by impeding elk’s ability to move through the forest patch. We did not find any consistent patterns between refuge elements and aspen size or canopy cover suggesting that natural refugia are not aiding in aspen recruitment and that all young aspen were susceptible to browsing. In much of their normal range, aspen are not growing to large size classes, which threatens the future of this iconic species and calls into question the ability of ecosystems to recover from mega-fires. Our results highlight the importance of considering multiple interacting factors (i.e. fire and increased elk browsing) when considering aspen management and regeneration.

Expansive vistas in mountain systems make scenic viewscapes – the visible portions of a landscape with which people form a connection – essential providers of cultural ecosystem services (CES). Like the dynamic systems they encapsulate, mountain viewscapes are subject to change, but the CES they provide are rarely considered from future or dynamic perspectives. Here we forecasted change in CES, using climatic shifts of a culturally valuable tree species, quaking aspen (Populus tremuloides), along scenic byways as an example of how viewscapes change through time. We simulated future aspen distributions in the Colorado Rocky Mountains through 2120 under three climate change scenarios and computed change in aspen visibility in 32,949 viewscapes. Across each scenario, the total area of aspen and its visibility from byways declined, but visible declines were 1.5–3.1 fold greater than declines in the study area overall. Differences between visible and total aspen peaked in mid-elevations (2000–3000 m) where aspen is most abundant. In contrast, aspen is forecasted to increase and become disproportionately more visible from scenic byways at lower elevations. Mismatch between total and visible declines in aspen highlights opportunities for tighter connections between landscape planning and ecological research for more comprehensive understanding of future changes in CES.

Quaking aspen (Populus tremuloides) forests throughout the western United States have experienced significant mortality in recent decades, which has been influenced by climate variability, especially drought. In the western portion of its range, where most precipitation arrives during winter as snowfall and summers are dry, snowdrifts that persist into the growing season provide soil moisture recharge that sustain aspen groves. Aspen groves are important locations of biodiversity within these landscapes. There is growing concern that reduced snowpack due to climate change may reduce the long-term persistence and productivity of aspen communities in these regions. In this study, we evaluated the potential for climate change and drought to reduce or eliminate isolated aspen communities in southwestern Idaho. We used a landscape simulation model integrated with inputs from an empirically derived biogeochemical model of growth, and a species distribution model of regeneration to forecast how changes in climate, declining snowpack, and competition with a conifer species is likely to affect aspen occupancy over the next 85-years. We found that simulated reductions in snowpack depth (and associated increases in climatic water deficit) caused a reduction in aspen persistence; aspen occupancy was reduced under all high emissions climate scenarios. Douglas-fir (Pseudotsuga menziesii) occupancy also declined under all future climates. Aspen regeneration declined over the course of all simulations, with an ensemble ratio of mortality/establishment increasing over the course of both low and high emissions climate scenarios. Climate-induced mortality of aspen clones increased in frequency under all climate scenarios and, under the most severe emissions scenarios, contributed to a substantial decline of aspen cover. Our research suggests that snowbanks will become an important determinant of long-term persistence of aspen under changing climate in the region.

European aspen (Populus tremula L.) is a keystone species in boreal forests that are dominated by coniferous tree species. Both living and dead aspen trees contribute significantly to the species diversity of forest landscapes. Thus, spatial and temporal continuity of aspen is a prerequisite for the long-term persistence of viable populations of numerous aspen-associated species. In this review, we collate existing knowledge on the ecological role of European aspen, assess the knowledge needs for aspen occurrence patterns and dynamics in boreal forests and discuss the potential of different remote sensing techniques in mapping aspen at various spatiotemporal scales. The role of aspen as a key ecological feature has received significant attention, and studies have recognised the negative effects of modern forest management methods and heavy browsing on aspen occurrence and regeneration. However, the spatial knowledge of occurrence, abundance and temporal dynamics of aspen is scarce and incomprehensive. The remote sensing studies reviewed here highlight particularly the potential of three-dimensional data derived from airborne laser scanning or photogrammetric point clouds and airborne imaging spectroscopy in mapping European aspen, quaking aspen (Populus tremuloides Michx.) and other Populus species. In addition to tree species discrimination, these methods can provide information on biophysical, biochemical properties and even genetic diversity of aspen trees. Major obstacles in aspen detection using remote sensing are the low proportion and scattered occurrence of European aspen in boreal forests and the overlap of spectral and/or structural properties of European aspen and quaking aspen with some other tree species. Furthermore, the suitability of remote sensing data for aspen mapping and monitoring depends on the geographical coverage of data, the availability of multitemporal data and the costs of data acquisition. Our review highlights that integration of ecological knowledge with spatiotemporal information acquired by remote sensing is key to understanding the current and future distribution patterns of aspen-related biodiversity.

Altered climate and changing fire regimes are synergistically impacting forest communities globally, resulting in deviations from historical norms and creation of novel successional dynamics. These changes are particularly important when considering the stability of a keystone species such as quaking aspen (Populus tremuloides Michx.), which contributes critical ecosystem services across its broad North American range. As a relatively drought intolerant species, projected changes of altered precipitation timing, amount, and type (e.g. snow or rain) may influence aspen response to fire, especially in moisture-limited and winter precipitation-dominated portions of its range. Aspen is generally considered an early-seral species that benefits from fire, but increases in fire activity across much of the western United States could affect the species in unpredictable ways. This study examined post-fire aspen stands across a regional climate gradient spanning from the north-central Great Basin to the northeastern portion of the Greater Yellowstone Ecosystem (USA). We investigated the influence of seasonal precipitation and temperature variables, snowpack, and site conditions (e.g. browsing levels, topography) on density of post-fire aspen regeneration (i.e. all small trees ha−1) and recruitment (i.e. small trees ≥2 m tall ha−1) across 15 fires that occurred between 2000 and 2009. The range of post-fire regeneration (2500–71,600 small trees ha−1) and recruitment (0–32,500 small trees ≥2 m ha−1) densities varied widely across plots. Linear mixed effects models demonstrated that both response variables increased primarily with early winter (Oct-Dec) precipitation during the ‘fire-regen period’ (i.e., fire year and five years after fire) relative to the 30-year mean. The 30-year mean of early winter precipitation and fire-regen period snowpack were also positively related to recruitment densities. Both response variables decreased with higher shrub cover, highlighting the importance of considering shrub competition in post-fire environments. Regeneration and recruitment densities were negatively related to proportion browsed aspen leaders and animal pellet densities (no./m2), respectively, indicating the influence of ungulate browsing even at the relatively low levels observed across sites. A post-hoc exploratory analysis suggests that deviation in early winter precipitation during the fire-regen period (relative to 30-year means) varied among sites along directional gradients, emphasizing the need to consider multiple spatiotemporal scales when investigating climate effects on post-fire successional dynamics. We discuss our findings in terms of dynamic management and conservation strategies in light of changing fire regimes and climate conditions.

Quaking or trembling aspen (Populus tremuloides Michx.) is a foundational tree species, which is native, common, and broadly distributed in North America. The ecology of aspen has been extensively studied throughout its range, but both research and forest management practices have focused primarily on its ability to regenerate asexually via root suckering. The seed-based reproductive ecology of aspen has received comparatively little attention, and information on the underlying processes, mechanisms, and requirements of seed regeneration tends to be scattered, somewhat anecdotal, or based only on localized research efforts. Here we review and explore some of the variables that influence the sexual reproduction and early establishment of aspen. We focus this review on western North America, where trembling aspen plays a dominant ecological role and may be disproportionately impacted by climate change. This synthesis presents existing information and identifies critical knowledge gaps in our understanding of seed –based aspen regeneration, in particular as it relates to flowering and seed production, as well as germination, first year growth, and survival of aspen seedlings. This information is discussed further in the context of aspen ecology and its application in both passive and active management approaches to aspen seedling regeneration and restoration.

Climatic conditions exert important control over the growth, productivity, and distribution of forests, and characterizing these relationships is essential for understanding how forest ecosystems will respond to climate change. We used dendrochronological methods to develop climate–growth relationships for two dominant species, Populus tremuloides (quaking aspen) and Pinus resinosa (red pine), in the upper Great Lakes region to understand how climate and water availability influence annual forest productivity. Trees were sampled along a topographic gradient at the Marcell Experimental Forest (Minnesota, USA) to assess growth response to variations in temperature and different water availability metrics (precipitation, potential evapotranspiration (PET), cumulative moisture index (CMI), and soil water storage). Climatic variables were able to explain 33–58% of the variation in annual growth (as measured by ring-width increment) for quaking aspen and 37–74% of the variation for red pine. Climate–growth relationships were influenced by topography for quaking aspen but not for red pine. Annual ring growth for quaking aspen decreased with June CMI on ridges, decreased with temperature in the November prior to the growing season on sideslopes, and decreased with June PET on toeslopes. Red pine growth increased with increasing July PET across all topographic positions. These results indicate the sensitivity of both quaking aspen and red pine to local climate and show several vulnerabilities of these species to shifts in water supply and temperature because of climate change.

Two ends of the fire regime spectrum are a frequent low-intensity fire regime and an infrequent high-intensity fire regime, but intermediate fire regimes combine high- and low-severity fire over space and time. We used fire-scar and tree-age data to reconstruct fire regime attributes of mixed-conifer and aspen forests in the North Rim area of Grand Canyon National Park, with a goal of estimating patch sizes of historical high-severity fire and comparing them with modern patch sizes. We used three methods based on (i) aspen groves, (ii) even-aged stands, and (iii) inverse distance weighting, to estimate occurrence and patch sizes of historical high-severity fire. Evidence of high-severity fire was common in the 1800s, and high-severity fire years were associated with drought. High-severity fire patch sizes likely ranged from 10−1 to 102 ha. However, the forest is quite young, and we cannot rule out a historical large high-severity fire that could have reinitiated much of the 1400 ha study area. Fire scars, which are indicative of low-severity fire, were also common. Historical fire was likely heterogeneous across the landscape. Maintaining heterogeneity of fire severity, size, and frequency would promote heterogeneity of forest structure and composition and resilience to future disturbances.

We analyzed a series of increment cores collected from 260 adult dominant or co-dominant quaking aspen (Populus tremuloides Michx.) trees from national forests across Colorado and southern Wyoming in 2009 and 2010. Half of the cores were collected from trees in stands with a high amount of crown dieback, and half were from lightly damaged stands. We define the level of stand damage based on stand survey data in which lightly damaged stands had average crown dieback of 16% and heavily damaged stands averaged 41%. Upon analysis, two-thirds of the cores collected did not exhibit radial growth correlated with region-wide patterns (e.g., climate) and were classified as having a low cohesive response. The site variable most predictive of whether a stand exhibited high cohesive response or low cohesive response was site elevation, followed by aspect, slope, and canopy closure. Sites with high cohesive response stands were more likely to have aspen bark beetle damage, white rot, and Cryptosphaeria canker. We did not detect relationships between tree growth and summer precipitation from 1900–2008, but there was a relationship between growth and annual precipitation. A growth model included maximum May and July temperatures, as well as the current and previous year’s annual precipitation.

Mixed-severity fire regimes may be the most extensive yet poorly understood fire regimes of western North America. Understanding their long-term spatiotemporal dynamics is central to debates regarding altered fire regimes and the need for restoration in the context of changing climate and nearly a century of active fire suppression. However, the complexity of fire patterns and forest stand and landscape structures characteristic of mixed-severity regimes poses a substantial challenge to understanding their long-term dynamics. In this study, we develop analysis methods aimed at understanding the fire-driven forest dynamics of mixed-severity systems and apply them in the San Juan Mountains of southwestern Colorado. We sampled fire scars, stand structure, and >4300 tree ages across two 1340-ha landscapes (Williams Creek and Squaretop Mountain) that span the environmental gradient of montane mixed-conifer and aspen forests. New approaches were applied to identify pulses of tree recruitment, evaluate climate and fire as potential drivers of synchronous recruitment pulses, and combine fire scar and recruitment data to reconstruct fires. The reconstructions provided detailed fire history for each stand, which in turn was used to develop a fire-severity metric, compare fire frequency and severity by forest type, and develop a simulation procedure to evaluate the degree to which tree regeneration depended on fire by species within each forest type. Twenty fires were reconstructed since 1685 at Williams Creek and 13 fires since 1748 at Squaretop Mountain. Patterns of fire severity varied within each fire and over successive events, including high-severity patches of hundreds of hectares in both study areas. Dry mixed-conifer forests experienced relatively short fire intervals (mean 21 years) and low fire severity, and regeneration of the main conifer species was largely dependent on open conditions sustained over successive fires. Moist mixed-conifer forests experienced longer fire intervals (mean 32 years) and a broader range of severities, and fire-caused canopy openings were important for initiating pulses of tree recruitment. Most (83%) aspen stands included two or more post-fire cohorts. The methods presented here can be adapted to other mixed-severity systems to better understand their long-term spatial and temporal dynamics and develop restoration priorities.

The bark of quaking aspen (Populus tremuloides), a clonal hardwood that is dominant in many Rocky Mountain forests, varies from white to orange to green among both ramets and populations. The proximate causes of this color variation remain controversial. We collected 72 samples of aspen bark from 11 locations in Boulder County, Colorado, and used microscopy, dissection, and thin layer chromatography to assess the structural and biochemical basis for the color gradient. Our study shows that the cork cells closest to the cork cambium of the aspen were consistently orange and conglutinated. Outward from this layer, the cork cells transitioned from orange to white; they aged and simultaneously lost cohesion. Green bark was visible due to thinnest cork layers, which revealed subcortical chlorenchyma tissue, the only bark tissue in which photosynthetic pigments were detected. Tests for photosynthetic pigmentation in the cork were negative. Comparison to standards indicated that b-carotene is not the pigment responsible for the orange hue of the cambium. We conclude that the powdery substance found on the tree surface is composed of bark cells, and their color variation seen on the aspen bark is attributed to the cell's pigment content and thickness, although the molecule responsible for cork cell coloration remains unidentified.

In the southwestern United States, populations of quaking aspen (Populus tremuloides Michx.) are experiencing widespread mortality. Although environmental factors contributing to mortality have been well characterized, less is known about how genotype and particularly ploidy level affect susceptibility. We used five microsatellite markers to infer the ploidy level of 212 aspen stems in Kaibab National Forest, Arizona. Many multilocus genotypes showed three alleles at one or more loci, suggestive of frequent triploidy among our samples. Sites populated with putative triploids had higher mortality. In addition, heterozygosity was positively associated with mortality and crown dieback. Our results suggest that triploidy is a predisposing factor for aspen mortality in Kaibab National Forest.

Depending on how disease impacts tree exposure to risk, both the prevalence of disease and disease effects on survival may contribute to patterns of mortality risk across a species' range. Disease may accelerate tree species' declines in response to global change factors, such as drought, biotic interactions, such as competition, or functional traits, such as allometry. To assess the role of disease in mediating mortality risk in quaking aspen (Populus tremuloides), we developed hierarchical Bayesian models for both disease prevalence in live aspen stems and the resulting survival rates of healthy and diseased aspen near the species' southern range limit using 5088 individual trees on 281 United States Forest Service Forest Inventory and Analysis plots in the southwestern United States. We found that disease prevalence depended primarily on tree size, tree allometry, and spatial variation in precipitation, while mortality depended on tree size, allometry, competition, spatial variation in summer temperature, and both temporal and spatial variation in summer precipitation. Disease prevalence was highest in large trees with low slenderness found on dry sites. For healthy trees, mortality decreased with diameter, slenderness, and temporal variation in summer precipitation, but increased with competition and spatial variation in summer temperature. Mortality of diseased trees decreased with diameter and aspen relative basal area and increased with mean summer temperature and precipitation. Disease infection increased aspen mortality, especially in trees of intermediate size and trees on plots at climatic extremes (i.e., cool, wet and warm, dry climates). By examining variation in disease prevalence, mortality of healthy trees, and mortality of diseased trees, we showed that the role of disease in aspen tree mortality depended on the scale of inference. For variation among individuals in diameter, disease tended to expose intermediate-size trees experiencing moderate risk to greater risk. For spatial variation in summer temperature, disease exposed lower risk populations to greater mortality probabilities, but the magnitude of this exposure depended on summer precipitation. Furthermore, the importance of diameter and slenderness in mediating responses to climate supports the increasing emphasis on trait variation in studies of ecological responses to global change.

By impacting performance of individual offspring, oviposition site choice can have a large influence on female fitness; however, a female’s fitness is also impacted by her own survival and future reproductive potential. Factors influencing oviposition site selection include a female’s own predation risk and nutritional needs, as well as the performance of offspring. Trade-offs may occur when oviposition sites differ in their favorability for these fitness-related functions. Adults of the leaf-mining moth, Phyllocnistis populiella, forage at the extrafloral nectaries (EFNs) of quaking aspen, Populus tremuloides. Females are synovigenic and likely rely on adult nutrition for egg production; however, proximity to EFNs may be associated with reduced offspring survival. In a controlled experiment, when given the choice of oviposition on either a leaf with or without EFNs, P. populiella preferred leaves lacking EFNs. Seven years of field survey data revealed significantly higher oviposition and lower egg predation on leaves lacking EFNs. Aspen shoots with a higher proportion of leaves expressing EFNs experienced higher oviposition but no difference in egg predation. At the leaf level, eggs were overdispersed when at low to moderate densities, likely to decrease interference competition. Females mated multiply, and the acquisition of spermatophores through repeated matings may decrease P. populiella’s reliance on EF nectar for egg production. P. populiella appears to balance the trade-off between resource rich and high offspring performance sites by ovipositing in neighborhoods with a high proportion of leaves bearing EFNs, while preferring leaves lacking EFNs which experience lower egg predation.

Changing aspen distribution in response to climate change and fire is a major focus of biodiversity conservation, yet little is known about the potential response of aspen to these two driving forces along topoclimatic gradients. This study is set to evaluate how aspen distribution might shift in response to different climate-fire scenarios in a semi-arid montane landscape, and quantify the influence of fire regime along topoclimatic gradients. We used a novel integration of a forest landscape succession and disturbance model (LANDIS-II) with a fine-scale climatic water deficit approach to simulate dynamics of aspen and associated conifer and shrub species over the next 150 years under various climate-fire scenarios. Simulations suggest that many aspen stands could persist without fire for centuries under current climate conditions. However, a simulated 2–5 °C increase in temperature caused a substantial reduction of aspen coverage at lower elevations and a modest increase at upper elevations, leading to an overall reduction of aspen range at the landscape level. Increasing fire activity may favor aspen increase at its upper elevation limits adjacent to coniferous forest, but may also favor reduction of aspen at lower elevation limits adjacent to xeric shrubland. Our study highlights the importance of incorporating fine-scale terrain effects on climatic water deficit and ecohydrology when modeling species distribution response to climate change. This modeling study suggests that climate mitigation and adaptation strategies that use fire would benefit from consideration of spatial context at landscape scales.

Food acquisition and predation avoidance are key drivers of herbivore behaviour. We investigated the interaction of top-down (predator) and bottom-up (food, fire, thermal) effects by measuring the relationship between wolf (Canis lupus L., 1758) predation risk perceived by elk (Cervus elaphus L., 1758) and elk landscape use. We conducted fecal pellet and wolf scat surveys in three valleys with three wolf population levels (Saint Mary: low; Waterton: moderate; North Fork: high). In the North Fork, 90% of quaking aspen (Populus tremuloides Michx.) stands burned recently; the other valleys had no fire. We created predictive models of elk pellet density that incorporated bottom-up and top-down variables. All valleys had a high elk pellet density (≥10 per 100 m2). Wolf scat density was similar where there was no fire, but one order of magnitude greater in burned areas. Elk pellet density was lower in the North Fork, a predation-related response. In all valleys, site-specific elk density declined as impediments to detecting or escaping wolves increased, and elk avoided aspen, except for North Fork unburned areas. Models that best predicted elk density contained bottom-up and top-down effects. At local scales, high predation risk negatively influence elk occurrence, suggesting that even with minimal wolf exposure elk avoid risky sites.

The persistence of quaking aspen (Populus tremuloides Michx.) is of significant importance to land managers in the Rocky Mountain region. Fire suppression in the past century has been implicated as a mechanism influencing aspen population dynamics, as aspen are generally considered an early seral disturbance adapted species. The heterogeneity of aspen community assemblages and fire regimes makes it difficult to discern what the result of fire suppression has been at large spatial and temporal scales. Decision makers should investigate the questions at hand at the stand level in their study location to best determine the mechanisms at play, as well as consider future potential changes to the system.

High rates of triploidy have recently been described in quaking aspen (Populus tremuloides Michx.) of the Intermountain West, raising questions about the contributions of triploidy to stand persistence and dynamics. In this study, we investigated cytotype differences between diploid and triploid aspen clones using dendrochronological techniques. We used tree-ring data collected from stems within an aspen stand near Fish Lake, Utah, to test for differences in stem age, population structure, growth, and response to climate. This stand contains the well-known Pando clone, which is purported to be the largest organism documented on earth. Our results show that triploid aspen stems grew more rapidly than diploids, and that this difference was most pronounced early in stand development. Growth response to climate varied little between triploids and diploids, where wide rings were associated with cool, moist years, and narrow rings were associated with above-average growing season temperatures. Stand development processes and inherent genetic differences are mechanisms possibly controlling the observed differences in aspen ring width between triploids and diploids. Regardless of the mechanism, the results have specific management implications. Conventional regeneration methods involving coppicing and the associated intermediate treatments will promote asexually reproducing triploids, leading to static or reduced genetic diversity. Enhanced genetic diversity will be favored by management actions that explicitly account for (i) the potential existence of multiple cytotypes within a stand and (ii) the observed differences in growth rates between diploid and triploid individuals.

Climate- and management-induced changes in tree species distributions are raising questions regarding tree species-specific effects on soil organic carbon (SOC) storage and stability. Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, but fire exclusion often promotes the succession to conifer dominated forests. Aspen in the Western US have been found to store more SOC in the mineral soil than nearby conifers, but we do not yet fully understand the source of this differential SOC accumulation. We measured total SOC storage (0–50 cm), characterized stable and labile SOC pools, and quantified above- and belowground litter inputs and dissolved organic carbon (DOC) fluxes during snowmelt in plots located in N and S Utah, to elucidate the role of foliage vs. root detritus in SOC storage and stabilization in both ecosystems. While leaf litterfall was twice as high under aspen as under conifers, input of litter-derived DOC with snowmelt water was consistently higher under conifers. Fine root (

Summary Resource selection specialization may increase vulnerability of populations to environmental change. One environmental change that may negatively impact some populations is the broad decline of quaking aspen Populus tremuloides, a preferred nest tree of cavity-nesting organisms who are commonly limited by nest-site availability. However, the long-term consequences of this habitat change for cavity-nesting bird populations are poorly studied. I counted densities of woody plants and eight cavity-nesting bird species over 29 years in 15 high-elevation riparian drainages in Arizona, USA. I also studied nest-tree use and specialization over time based on 4946 nests across species. Aspen suffered a severe decline in availability over time, while understorey woody plants and canopy deciduous trees also declined. The decline of plants resulted from increased elk Cervus canadensis browsing linked to declining snowfall. Woodpeckers exhibited very high specialization (>95% of nests) on aspen for nesting, and densities of all six species declined with aspen over time. Mountain chickadees Poecile gambeli and house wrens Troglodytes aedon exhibited increasingly less specialization on aspen. Chickadees strongly increased in density over time, despite a relatively high specialization on aspen. House wren densities declined moderately over time, but nest-box addition experiments demonstrated that nest-site availability was not limiting their population. House wren densities increased with understorey vegetation recovery in elk exclosures via increased generality of nest-site use, demonstrating that the decline in understorey vegetation on the broader landscape was the cause of their population decline. Synthesis and applications. Management should target species that specialize in resource selection on a declining resource. Species with greater resource selection generalization can reduce population impacts of environmental change. Resource generalization can allow a species like the wren to take advantage of habitat refuges, such as those provided by the elk exclosures. Yet, resource generalization cannot offset the negative impacts of broad-scale declines in habitat quality on the landscape, as demonstrated by the general decline of wrens. Ultimately, aspen is an important habitat for biodiversity, and land management programmes that protect and aid recovery of aspen habitats may be critical.

Current understanding of aspen fire ecology in western North America includes the paradoxical characterization that aspen-dominated stands, although often regenerated following fire, are “fire-proof”. We tested this idea by predicting potential fire behavior across a gradient of aspen dominance in northern Utah using the Forest Vegetation Simulator and the Fire and Fuels Extension. The wind speeds necessary for crowning (crown-to-crown fire spread) and torching (surface to crown fire spread) were evaluated to test the hypothesis that predicted fire behavior is influenced by the proportion of aspen in the stand. Results showed a strong effect of species composition on crowning, but only under moderate fire weather, where aspen-dominated stands were unlikely to crown or torch. Although rarely observed in actual fires, conifer-dominated stands were likely to crown but not to torch, an example of “hysteresis” in crown fire behavior. Results support the hypothesis that potential crown fire behavior varies across a gradient of aspen dominance and fire weather, where it was likely under extreme and severe fire weather, and unlikely under moderate and high fire weather. Furthermore, the “fire-proof” nature of aspen stands broke down across the gradient of aspen dominance and fire weather.

Prescribed fire is commonly used for restoration, but the effects of reintroducing fire following a century of fire exclusion are unknown in many ecosystems. We assessed the effects of three prescribed fires, native ungulate browsing, and conifer competition on quaking aspen (Populus tremuloides Michx.) regeneration in four small groves (0.5 ha to 3.0 ha) in Lassen Volcanic National Park, California, USA, over an 11 yr period. The effects of fire on aspen regeneration density and height were variable within and among sites. Post-fire aspen regeneration density generally decreased with greater conifer basal area (rs = −0.73), but there was a wide range of aspen regeneration densities (4000 to 36 667 stems ha−1) at transects with no live conifers post-fire. The height of aspen regeneration increased as a function of increasing years-since-fire (1 yr to 11 yr), but heavy browsing by mule deer (Odocoileus hemionus Rafinesque) may alter future growth trajectories. Median percent of aspen regeneration browsed was high in burned (91 %) and unburned (81 %) transects. Only 7 % (282 stems ha−1 to 333 stems ha−1) of post-fire aspen regeneration in 11-year old burns exceeded the height necessary to escape mule deer browsing (150 cm). Browsing may also be altering aspen growth form, such that multi-stemmed aspen regeneration was positively associated with proportion of aspen regeneration browsed. These four case studies indicate that the effects of prescribed fires on quaking aspen in the southern Cascade Range of northern California were highly variable and, when coupled with biotic factors (such as deer browsing and competing vegetation) and varying fire severity, fire may either benefit or hasten the decline of small aspen groves.