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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.

Quaking aspen (Populus tremuloides) exhibits striking intraspecific variation in concentrations of phenolic glycosides, compounds that play important roles in mediating interactions with herbivorous insects. This research was conducted to assess the contribution of genetic variation to overall phenotypic variation in aspen chemistry and interactions with gypsy moths (Lymantria dispar) and forest tent caterpillars (Malacosoma disstria). Thirteen aspen clones were propagated from field-collected root material. Insect performance assays, measuring survival, development, growth, and food utilization indices, were conducted with second and/or fourth instars. Leaf samples were assayed for water, nitrogen, total nonstructural carbohydrates, condensed tannins, and phenolic glycosides. Results showed substantial among-clone variation in the performance of both insect species. Chemical analyses revealed significant among-clone variation in all foliar constituents and that variation in allelochemical contents differed more than variation in primary metabolites. Regression analyses indicated that phenolic glycosides were the dominant factor responsible for among-clone variation in insect performance. We also found significant genetic trade-offs between growth and defense among aspen clones. Our results suggest that genetic factors are likely responsible for much of the tremendous phenotypic variation in secondary chemistry exhibited by aspen, and that the genetic structure of aspen populations may play important roles in the evolution of interactions with phytophagous insects.

Mature aspen (genus Populus L..) and maple (genus Acer L.) forests appear to be high-quality breeding habitat for Yellow-bellied Sapsuckers ( Sphyrapicus varius (L., 1766)). To explore some of the mechanisms that influence quality of the breeding habitat, we measured demographic and breeding resources in four forest stands each of 95-year-old quaking aspen ( Populus tremuloides Michx.) and 182-year-old sugar maple ( Acer saccharum Marsh.) in Algonquin Provincial Park, Ontario, from 2007 to 2009. Population density was higher, egg laying earlier, clutches larger, hatching success greater, and per capita and population fledgling production higher in maple stands compared with aspen stands. Longer nest-building and prelaying stages in aspen stands delayed egg laying by 5 days relative to maple stands. The delay in egg laying in aspen stands may have been caused by lower quality sap resources, which then resulted in lower productivity. Adults delivered smaller food loads to nests in aspen stands than in maple stands, which may have increased nestling mortality in aspen stands. Although per capita fledgling production was lower in aspen stands than in maple stands, Yellow-bellied Sapsuckers in both forests were able to replace themselves, suggesting that mature deciduous and mixed-deciduous forests, in general, are high-quality breeding habitat for this sapsucker.

We monitored tree mortality in northern Arizona (USA) mixed-conifer and ponderosa pine (Pinus ponderosa Dougl. ex Laws) forests from 1997 to 2007, a period of severe drought in this area. Mortality was pervasive, occurring on 100 and 98% of 53 mixed-conifer and 60 ponderosa pine plots (1-ha each), respectively. Most mortality was attributable to a suite of forest insects, mediated by drought stress. The number of trees dying from 2002 to 2007 was more than 200% greater than the number dying from 1997 to 2002 in mixed-conifer forest and 74% greater in ponderosa pine forest. Extent of mortality was spatially variable in both forest types. Median cumulative mortality (the ratio of dead to live trees) increased by approximately 53 and 65% in mixed-conifer and ponderosa pine forests, respectively, from 2002 to 2007. Median mortality rates from 2002 to 2007 were approximately 2.0% year-1 in mixed-conifer forest (range = 0-28.5%) and 0.4% year-1 in ponderosa pine forest (range = 0-13.6%). Mortality rates generally were not strongly related to either elevation or stand density. Mortality was nonrandom with respect to tree size classes and species. Proportions of trees dying were greatest in the largest size classes, particularly in mixed-conifer forest, where mortality in the largest size class exceeded 22% from 2002 to 2007. Mortality in mixed-conifer forest was particularly pronounced for quaking aspen (85%) and white fir (28%), the least drought tolerant species present. These results provide an early glimpse of how these forest types are likely to respond to predicted climate changes in the southwestern USA. They suggest that these forests are not resilient to climate change, and that treatments to increase resilience to climate change may be appropriate. Research on causes of spatial heterogeneity in extent of mortality might suggest valuable approaches to aid in increasing resilience.

In this study we developed an individual tree height prediction model for quaking aspen ( Populus tremuloides Michx.) grown in boreal mixedwood forests in Alberta using the nonlinear mixed model (NLMM) approach. We examined the impacts of density, species composition, and top height on aspen height predictions. Statistically significant stand level variables were incorporated into the base height–diameter model to increase the predictive ability and accuracy of the model at both the population and subject-specific levels. Our analyses showed that top height and density impacted height growth, but species composition did not. More importantly, we found that the inclusion of additional variables into the base model, despite improving model fitting statistics on the modelling data, did not improve the model's predictive ability and accuracy when cross-validated and when tested on an independent testing data set. Under the NLMM framework the base model performed as well as or better than the expanded models that contained other stand level variables. This has important theoretical and practical implications because, other than for biological reasons, more accurate local tree height predictions for aspen can be achieved simply by using the base height–diameter model fitted with the NLMM approach without the inclusion of other variables.

The most widely distributed tree in North America, quaking aspen (Populus tremuloides, Michx.), reproduces sexually via seed and clonally via suckers. The size of aspen clones varies geographically, generally smaller in the east and large in the arid Intermountain West. In order to describe clone size and genetic structure of aspen in the southern Cascade Mountains, 864 stems from six sites were assayed at 15 isozyme and 6 microsatellite loci. Although isozymes reveal significantly lower levels of allelic richness (P

Stands of quaking aspen ( Populus tremuloides ) rank among the most biologically diverse plant communities across the intermountain region of western North America. Marked declines of aspen have occurred in recent decades, likely due to a combination of effects from changes in fire regimes, herbivory, climate (e.g. drought), and interspecific competition with conifer species. However, it is poorly understood how the effects of these factors are manifested at a landscape scale over decadal time periods. Analysis of field data combined with topographic information collected across the 500,000 ha Owyhee Plateau in southwestern Idaho revealed that aspen in the area occur in three different biophysical settings; First, aspen stands exist at high altitudes on south-facing slopes where local conifer species are not likely to occur because of limiting temperature or precipitation levels under current climate conditions. In these areas aspen is the potential vegetation type rather than conifers. Second, aspen grow on anomalously wet microsites (e.g. near springs), and third, aspen grow within upland mixed aspen/conifer stands, which are experiencing rapid rates of conifer establishment. Based on a paired t -test ( α = 0.05) we conclude that stands growing on wet microsites show significantly slower successional rates of conifer establishment relative to upland aspen stands. We developed a conceptual state-and-transition model for upland aspen/conifer stands occurring across a range of topographic positions. We then parameterized the model using extensive field data in the vegetation dynamics computer simulation model Vegetation Dynamics Development Tool (VDDT), and examined the current and future aspen distribution under varying fire regimes. Model results indicate that average fire return intervals of 50–70 years are desirable for maintenance of aspen in upland areas where conifers are present. Under the current fire regime in the area many upland aspen/conifer stands will likely be lost within 80–200 years. Thresholds for the effect of conifer encroachment and browsing on aspen regeneration identified through this research are similar to those described by others across the West. We therefore suggest that the results presented for the Owyhee Plateau are likely applicable to semi-arid aspen woodlands across the American West where succession to conifers is a cause of aspen decline.

Concentrated patches of recent trembling aspen (Populus tremuloides) mortality covered 56,091 ha of Colorado forests in 2006. Mortality has progressed rapidly. Area affected increased 58% between 2005 and 2006 on the Mancos-Dolores Ranger District, San Juan National Forest, where it equaled nearly 10% of the aspen cover type. In four stands that were measured twice, incidence of mortality increased from 7–9% in 2002/2003 to 31–60% in 2006. Mortality generally decreased with increasing elevation over the primary elevation range of aspen and occurred on less steep slopes than healthy aspen. Slope-weighted mean aspects of aspen cover type were northern at low elevations and generally southern at high elevations. Relative frequency of mortality was generally highest on southern to western aspects. In 31 stands measured in detail, mortality ranged from 0 to 100% (mean 32%) and was negatively correlated with stand density (P = 0.033). Size of trees affected was strongly correlated with amount of current mortality (P Agents that typically kill mature trees in aspen stands were unimportant in this mortality. Instead, a group of interchangeable, usually secondary agents was most commonly associated with mortality, including Cytospora canker (usually caused by Valsa sordida), aspen bark beetles (Trypophloeus populi and Procryphalus mucronatus), poplar borer (Saperda calcarata), and bronze poplar borer (Agrilus liragus). The rapidity of mortality, mortality agents involved, and probably other causal factors distinguish this phenomenon from the long-term loss of aspen cover usually attributed to successional processes operating in an altered disturbance regime (and often exacerbated by ungulate browsing). Our data are consistent with a hypothesis that (a) predisposing factors include stand maturation, low density, southern aspects and low elevations; (b) a major inciting factor was the recent, acute drought accompanied by high temperatures, and; (c) contributing factors and proximate agents of mortality are the common biotic agents observed. On sites with poor regeneration and weak root systems, clones may die, resulting in the long-term loss of aspen forest cover.

Quaking aspen ( Populus tremuloides ) is commonly believed to be declining throughout western North America. A literature review of recent local-scale aspen studies, however, indicates that regional-scale spatial variability in aspen dynamics is greater than previously recognized and there are areas of successful aspen regeneration and recruitment. This study examines 20th century aspen recruitment and regeneration patterns along a 15 km long lower forest–grassland ecotone in the Centennial Valley, Montana to determine if the Centennial Valley ecotone aspen stands support the observation that there are areas of successful recruitment and regeneration and provide another example of such areas. A total of 525 adult aspen stems were cored and 8924 aspen suckers were counted and mapped in 106 rectangular plots randomly distributed along the ecotone. Aspen-age distribution was constructed at a decadal time scale to examine 20th century recruitment patterns. Sucker density was examined to evaluate aspen regeneration. The tree-age distribution patterns indicated successful aspen recruitment during the 20th century, while the sucker density assessment suggested successful regeneration. The Centennial Valley ecotone aspen stands appear to support the observation that aspen recruitment and regeneration are spatially variable and there are areas of aspen persistence. Learning about the underlying factors that enable successful aspen regeneration and recruitment in such areas can help management and restoration activities at other locales. Recognizing the regional-scale spatial variability in aspen dynamics can inform regional-level policy and management.

Following the extensive 1988 fires in Yellowstone, a mosaic of high-density patches of fallen logs and regenerating lodgepole pine ( Pinus contorta var. latifolia Engelm. ex Wats.) saplings developed in the landscape. Such patches could potentially provide browsing refugia for post-fire aspen ( Populus tremuloides Michx.) seedlings. We asked two primary research questions: (1) Do elk ( Cervus elaphus L.) reduce their use of high-density patches of coarse-wood and pine saplings? and (2) Are the abundance, height, and probability of presence of aspen positively related to the density of coarse wood or pine saplings? We visited 65 sites distributed across density gradients of downed wood and regenerating saplings. At each site, aspen seedlings were counted along five 50 m × 2 m belt transects. The height, basal diameter and presence of browse damage were recorded for each individual. Fallen logs and elk fecal pellet groups were counted along the same transects. Aspen seedlings were heavily browsed throughout the study area and were less likely to be found near high-elevation meadows. Variation in elk pellet densities was not explained by the density of logs at the scale of the transect or the site. The height of aspen seedlings was not related to density of logs, pine saplings or elk fecal pellet groups. However, taller aspen were found at higher elevations and with more open meadow in the landscape. This suggests that later snowmelt and alternative forage may reduce browsing pressure on aspen. Given that some of the sites had densities of pine saplings in excess of 60,000 stems ha −1 and densities of downed logs greater than 2000 logs ha −1 , these results suggest that fire-induced coarse wood and pine saplings will not create broad-scale browsing refugia for aspen in the landscape of the Yellowstone plateau.

The regeneration of Quaking aspen (Populus tremuloides) in the western U.S. in some habitats occurs after severe fire which removes competing conifers and also triggers root suckering of aspen. Consequently, fire exclusion during the 20th century, sometimes in combination with elk browsing, has often been argued to have resulted in a decline in the extent of aspen relative to its pre-20th century extent. However, aspen regeneration is not always dependent on fire and many stands have structures indicative of aspen persistence even in the absence of fire. Thus, to investigate regeneration dynamics of both persistent and seral aspen stands, we reconstructed patterns of stand development in 26 aspen stands in northwestern Colorado. Seral and persistent aspen stands were initially selected based on geographic information system-based analysis of historic vegetation and fire occurrence. Stand-level age structures were determined from 1919 increment cores and size structures from counts and diameters of all juvenile, subcanopy and canopy trees in 40 m × 40 m plots. Stand structures were interpreted to determine modes of tree regeneration and patterns of stand development. As expected, in the eight seral stands aspen regeneration generally depended on coarse-scale, severe disturbance by fire. However, most aspen stands in this study showed signs of aspen self-replacement despite presence of conifers. In the 16 persistent aspen stands showing no conifer invasion, aspen are able to regenerate through a variety of regeneration modes that did not appear to require severe disturbance. Over 70% of the persistent aspen stands sampled did not require coarse-scale disturbance to regenerate. Two (11%) of the persistent stands regenerated continuously through time, and 11 (61%) had aspen cohorts that appeared to develop episodically without being triggered by severe fire or other severe disturbance. In the persistent stands sampled in our study, episodic regeneration not dependent on severe fire is the main mode of stand re-initiation. At many sites in northwestern Colorado aspen regeneration occurs episodically and fits a general stand development model of cohort senescence. According to this model, many of the currently “decadent” or deteriorating aspen stands in western Colorado may be replaced by regenerating stands as the senescent stages experience high levels of canopy tree mortality, thus assuring aspen persistence in the future.

Bats play a variety of ecological roles in forest ecosystems and forest management can impact habitat conditions for forest-dwelling bats. We examined the use and characteristics of roosts selected by reproductive female big brown bats ( Eptesicus fuscus ) in the Channeled Scablands of northeastern Washington. We radio-tracked 14 bats to locate 36 roosts. Bats were found in colonies averaging 27 and these colonies switched roosts about every 3.7 days. Habitat variables were measured for the roost itself and at a 0.1-ha microplot and 78-ha macroplot surrounding each roost. We measured habitat variables at random 0.1-ha microplots in the vicinity of each roost and at general random 0.1-ha microplots and 78-ha macroplots. Of the 36 roosts located, 34 were in natural tree cavities; 28 were in ponderosa pine ( Pinus ponderosa ) and eight in quaking aspen ( Populus tremuloides ). Dead tops of live pine trees were used significantly more as roosts than pine snags. Although there were significantly more roosts in trees >30 cm in diameter and >12 m high than what was available, roosts were not always the tallest tree in the stand. A significantly greater proportion of big brown bat roosts were found in open pine, aspen and mixed-aspen pine forests and less in grasslands and closed pine than expected. Forest management strategies should protect both large diameter snags and existing dead top live trees and maintain natural population levels of biological agents that create dead tops. Restoration of historic open conditions in ponderosa pine and aspen stands will provide improved habitat for big brown bats.

Quaking aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America, but its presence is declining across much of the Western United States. Aspen decline is complex, but results largely from two factors widely divergent in temporal scale: (1) Holocene climatic drying of the region has led to water limitation of aspen seedling recruitment, and (2) anthropogenic fire suppression during the 20th century has allowed shading of aspen clones by fire-intolerant conifers. These processes interact variously and often diffusely, but traditional, binary habitat mapping approaches can only resolve their net effect after complete loss of aspen patches. To provide information for preemptive land management in the Book Cliffs – a biogeographic link between the Utah and Colorado Rocky Mountains and a location experiencing aspen decline typical of the region – we developed a regression-based generalization of niche/habitat analyses that is usable in GIS, is capable of detecting anomalies in cover before complete patch conversion, and can be incorporated smoothly into the decision-making process. We estimated the realized Hutchinsonian niche of quaking aspen to potential evapotranspiration (PET) by nonparametric, likelihood-based regression techniques, projected (continuous) values of aspen's niche expectation and uncertainty geographically, and correlated differences between observed and expected aspen cover to remotely sensed conifer cover. Results confirm the strong constraint of site water demand on aspen cover and suggest that conifer cover decreases aspen cover beneath its expectation given the PET environment. Compared to sites without quaking aspen, our aspen sites have lower PET in every month of the growing season, but the difference increases over the growing season as drought effects become more extreme. Superimposed on this broader environmental constraint, conifer cover displaces aspen cover and shows a positive correlation with model deviance (r = 0.344). Ultimately, the thematic information conserved by our approach allowed us to resolve detailed rasters of management potential and map a modest potential increase of aspen cover – 14.63 ha (0.14%) of the study area, or +2.46% of current aspen cover – within one management cycle.

Stacked propagation is a novel method of growing quaking aspen (Populus tremuloides Michx. [Salicaceae]) and other plants that reproduce from underground stems or root cuttings. Because the mother plant is not damaged, it is particularly well suited for rare plants or those that can't be propagated by normal methods. Our initial trials indicate that hundreds of vigorous plants can be produced by this method in each propagation cycle.

Reports of decreasing quaking aspen (Populus tremuloides) cover in forests of the western USA have caused concern about the long-term persistence of aspen on landscape scales. We assessed changes in overstory aspen dominance on the Uncompahgre Plateau in western Colorado over a 20 year period. We measured stand density, species composition and regeneration in 53 undisturbed, mature pure aspen, pure conifer, and mixed aspen/conifer stands originally inventoried between 1979 and 1983. Ages of overstory and understory trees were used to evaluate long-term change in regeneration and overstory development. While pure aspen stands occupy 16% of the study area, mixed aspen and conifer stands cover 62% of the forested landscape on the Uncompahgre Plateau. Pure aspen stands were self-thinning, but stable over the twenty-year study period, with high amounts of regeneration and without conifer invasion. Mixed stands of aspen and conifer had undergone significant change. In aspen dominated mixed species stands, conifer basal area increased from 10 to 23 m 2 ha 1 in the last 20 years, while aspen basal area decreased. In conifer dominated mixed species stands, conifer basal area increased from 18 to 24 m 2 ha 1 . Most overstory aspen in pure aspen stands were between 80 and 120 years old. Substantial aspen suckering was occurring, but all suckers were

The lipophilic extractives, that is, wood resin, in Populus grandidentata (bigtooth aspen) and Populus tremuloides (quaking aspen) sapwood, heartwood, and knots were analyzed. The amount and composition of the extractives in sapwood were similar in both species. Triglycerides, short‐chain fatty acids, and steryl esters were predominant in sapwood samples. The knots of both species contained similar amounts of extractives as the stemwood, but the composition was quite different. Monoglycerides; long‐chain fatty acids and alcohols, with 22 to 28 carbon atoms; hydroxy fatty acids; and ferulic acid esters of fatty alcohols were the major components in heartwood and knots, but were minor components in the sapwood. This work was carried out within the project “BioExtra” funded by the National Technology Agency of Finland (Tekes), Raisio Group, and UPM. This work is part of the activities at the Abo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2000–2005) selected b...

Snags provide an important resource for a rich assemblage of cavity-nesting birds in the southwestern United States. To expand our knowledge of snag use by cavity-nesting birds in this region, we documented characteristics of snags with and without excavated cavities in mixed-conifer and ponderosa pine (Pinus ponderosa Dougl. ex Laws) forest in north-central Arizona. Snags were sampled in 113 square plots (1 ha each) randomly located within a study area covering approximately 73,000 ha across two National Forests. Density of snags was three times greater in mixed-conifer forest (n = 53 plots) than in ponderosa pine forest (n = 60 plots), but density of snags containing cavities and overall cavity density did not differ between forest types. In both forest types, snags containing cavities were larger in diameter and retained less bark cover than snags without cavities. Most cavities were in ponderosa pine and Gambel oak (Quercus gambelii Nutt.) snags, and most were in snags in advanced decay classes with broken tops. Our results are largely consistent with previous results from ponderosa pine forest, but differ from previous studies that documented heavy use of quaking aspen (Populus tremuloides Michaux) by cavity nesters in mixed-conifer forest. These results support management to protect and recruit large snags well distributed across the landscape. The relatively high use of ponderosa pine and Gambel oak snags in both forest types suggests that recruitment of large pine and oak snags should be emphasized, and previous studies suggest emphasizing aspen recruitment as well. This may require special management efforts in mixed-conifer forest. These species are relatively shade-intolerant seral species in this forest type, and are apparently declining in this forest type due to fire-suppression efforts and resultant patterns of ecological succession.

A hypothesis that the pattern of spatial leaf distribution in forest canopies is numerically related to the exponent of the self-thinning relationship in even-aged monocultures was tested by evaluating the crown fractal dimension of Pinus banksiana (jack pine) and Populus tremuloides (quaking aspen) in Wood Buffalo National Park, Canada. Pure species stands that were considered the most dense for a given mean tree size were measured to establish the empirical self-thinning relationships. The value of the self-thinning exponent was estimated as –1.42 with 95% Confidence Interval (CI) (−1.47, −1.36) for Pinus banksiana, and –1.29 with 95% CI (−1.45, −1.14) for Populus tremuloides. For each species the box dimension of spatial leaf distribution was estimated from unit cylinders described by sequentially lowering in forest canopies, horizontal “flaps” of one of various diameters attached to the top of a height-measuring pole. The box dimension appeared as 1.95 (1.84, 2.06) for Pinus banksiana, and 2.24 (2.05, 2.43) for Populus tremuloides. By assuming that the box dimension is equivalent to the fractal dimension at the inter-population level, the self-thinning exponent was predicted to be –1.53 (−1.62, −1.45) for Pinus banksiana, and –1.33 (−1.45, −1.23) for Populus tremuloides. The empirical exponent was equivalent to that predicted from the box fractal dimension, as judged by the 95% CI of the dimensions. We conclude that spatial patterns of leaf distribution in forest canopies, as being characterized by the box fractal dimension, are closely related to the value of the self-thinning exponent in the dense monocultures of the species we examined.

During a repeat photography study quaking aspen (Populus tremuloides Michx.) was observed invading conifer stands at treeline in the San Juan Mountains of south-western Colorado. Aspen tree core samples were collected from nine plots ranging in elevation from 3192 to 3547 m, and estimated dates of establishment of aspen were grouped into 10-year intervals for analysis. Estimated periods of establishment were compared with century-long climate data records to derive any correlations with aspen invasion. Other disturbance agents, such as fire and livestock grazing were also considered. Quantitative analysis of climate variables suggests that decreased mean spring precipitation and increased mean summer maximum temperature provide optimal conditions for aspen establishment. Episodes of invasion were non-synchronous, but all occurred after 1900, and are likely from seed germination, considered unusual in aspen. Different climate variables explain stand initiation from seed and subsequent peak establishment from vegetative reproduction. Long-term climate records indicate a general warming since the beginning of the 20th century and explain the continued invasion and persistence of aspen at treeline, resulting from asexual reproduction. Short-term climate records identify anomalously cool, moist years that explain rarely observed sexual reproduction in aspen.

Summary 1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34‐36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

1. Competition effects on growth of individual trees were examined for 4 years in aggrading, mixed-clone stands of trembling aspen (Populus tremuloides Michx.) at the Aspen-FACE free-air Co2 and O3 enrichment facility in northern Wisconsin, USA. During each growing season stands received one of four combinations of atmospheric [CO2] (ambient vs~56 Pa) and [O3] (ambient vs~1.5 x ambient). 2. Non-destructive measurements of annual tree growth were compared within and among clones and treatments in relation to an index of competitive status based on the difference between a tree's height and that of its four nearest neighbours. Competitive status strongly influenced tree growth, and the positive growth response to elevated [CO2] was greater for competitively advantaged individuals than for disadvantaged individuals of most clones. 3. The magnitude of O3 effects on growth depended on clone and competitive status: for some clones, negative O3 effects were stronger with competitive advantage while others showed stronger O3 effects with competitive disadvantage. The interactive effects of CO2 and O3 differed among clones, with negative effects of O3 amplified or ameliorated by elevated CO2, depending on clone and competitive status. 4. Treatments modified competitive interactions by affecting the magnitude of growth differences among clones. These modifications did not alter clone rankings of competitive performance, but when CO2 and 03 were both elevated, the differences in competitive performance among clones decreased.

Aim Fire suppression, elk (Cervus elaphus Erxleben) browsing and drought have been suggested as possible explanations for low aspen (Populus tremuloides Michx.) ramet regeneration in elk winter range of the Greater Yellowstone Area (GYA) during the twentieth century. This study analyses the relationship between the rates of aspen regeneration, biophysical factors and human land use since 1830. This approach reveals the importance of indirect human impacts, especially through fire and elk management strategies, on forest structure. Location This study was conducted in and around the winter range of the Jackson Hole elk herd in western Wyoming, USA. Aspen stands in this region represent the ecotone between coniferous forest and sagebrush-grassland vegetation. Methods Age structures of aspen stands were reconstructed from tree-rings in order to determine how variation in drought, elk populations and fire occurrence may have affected aspen ramet regeneration since 1830. The effects of recent prescribed fires and elk browse on aspen regeneration were also evaluated by measuring stem height, browse intensity and age of ramets < 2 m tall in prescribed burns and in unburned areas. Results Aspen ramet regeneration has occurred consistently but at low frequencies since 1830, with three peaks of regeneration: 1860-85, 1915-40 and 1955-90. Periods of frequent ramet regeneration coincided with low to moderate elk populations and aspen regenerated only sporadically when elk populations were high. Based on a comparison of the age structures with a tree-ring reconstruction of Palmer drought severity index (PDSI), observed PDSI and annual precipitation, drought variability appears unrelated to episodes of aspen regeneration. Recent regeneration patterns suggest that fire does not enhance the recruitment of tree-sized aspen stems in the presence of elk browse, although sample sizes were small. Since 1900, elk hunting and spatially clustered elk feeding in the study area has facilitated low but consistent regeneration of aspen ramets in the twentieth century. In contrast, extremely low ramet regeneration has been observed in national parks of the Rocky Mountains, where elk have been managed according to the 'natural regulation' policy (no hunting or feeding) since 1969. Main conclusions Over time, different management strategies have altered the interactions between fire, herbivory and aspen regeneration suggesting that management history and the causes of change in management, should be explicitly included in ecological studies and future management strategies. These results also point to the value of using the spatial and temporal variation in human interactions with ecological systems as a method for understanding ecological relationships.

The effect of soil disturbance on quaking aspen (Populus tremuloides Michx.) sucker density and height growth was measured in mixed aspen and hardwood stands in east central Minnesota, USA. Loggers harvested stands during the growing season resulting in a range of soil disturbance conditions. Soil disturbance and parent stand aspen density accounted for a significant amount of the variation in suckering two full growing seasons after the harvest. Mineral soil exposure did not significantly affect total sucker density, but decreased sucker height growth as measured by the cumulative height of all suckers in a plot (sucker length) and density of suckers more than 1.2 m tall. Both shallow ( 15 cm) soil disturbance caused by machine traffic reduced sucker density and sucker length. Results indicated an effect of machine traffic on suckers in trafficked areas, but not adjacent areas. The long-term effects of reduced height growth in disturbed areas and severely reduced density in trafficked areas are not well understood. Loss in early height growth is likely to have long-term effects on productivity, and it is unlikely that aspen will recolonize rutted areas in time to recover full aspen productivity in trafficked areas in one rotation.

Long-term qualitative observations suggest a marked decline in quaking aspen (Populus tremuloides Michx.) primarily due to advancing succession and fire suppression. This study presents an ecoregional coarse-grid analysis of the current aspen situation using Forest Health Monitoring (FHM) data from Idaho, Wyoming, and Colorado. A unique feature of aspen forests in western North America is regeneration primarily by asexual “suckering” although rare seeding events do occur. The dominant clonal process provides the basis for this analysis. In essence, the remaining aspen stems of previously large clones provide a window to the past and possibly a view of the future. The author uses baseline observations of aspen and associated tree species regeneration, forest size and structure components, stand age, tree damage, and recent disturbance to assess regional aspen conditions. Analysis of stands where aspen is dominant (aspen forest type) and where aspen merely occurs (aspen present) are presented. Basic groupings within the aspen forest type plots were obtained by cluster analysis of 10 FHM variables derived from tree- and plot-level measurements. Stable and unstable aspen forest types were verified using principal component analysis. A further criterion of at least 25% conifer species present was placed on the unstable group to render a more conservative population estimate of instability. The unstable aspen forest types, along with the plots having only the presence of aspen, comprise the dynamic portion of the aspen community in this area. These results support the hypothesis of an aspen decline within the past 100 years. However, additional regional plots and long-term remeasurements should provide a clearer picture of the decline’s extent. Altering current and future management practices may significantly affect the rate of change.

To study the impact of ozone (O3) and O3 plus CO2 on aspen growth, we planted two trembling aspen clones, differing in sensitivity to O3 in the ground in open-top chambers and exposed them to different concentrations of O3 and O3 plus CO, for 98 days. Ozone exposure (58 to 97 microl l(-1)-h. total exposure) decreased growth and modified crown architecture of both aspen clones. Ozone exposure decreased leaf, stem, branch, and root dry weight particularly in the O3 sensitive clone (clone 259). The addition of CO2 (150 microl l(-1) over ambient) to the O3 exposure counteracted the negative impact of O3 only in the O3 tolerant clone (clone 216). Ozone had relatively little effect on allometric ratios such as, shoot/root ratio, leaf weight ratio, or root weight ratio. In both clones, however, O3 decreased the shoot dry weight, shoot length ratio and shoot diameter. This decrease in wood strength caused both current terminals and long shoots to droop and increased the branch angle of termination. These results show that aspen growth is highly sensitive to O3 and that O3 can also significantly affect crown architecture. Aspen plants with drooping terminals and lateral branches would be at a competitive disadvantage in dense stands with limited light.

The Intergovernmental Panel of Climate Change (IPCC) has concluded that the greenhouse gases carbon dioxide (CO2) and tropospheric ozone (O3) are increasing concomitantly globally. Little is known about the effect of these interacting gases on growth, survival, and productivity of forest ecosystems. In this study we assess the effects of three successive years of exposure to combinations of elevated CO2 and O3 on growth responses in a five trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45 N8 9W) and employs free air carbon dioxide and ozone enrichment (FACE) technology. The aspen stand was exposed to a factorial combination of four treatments consisting of elevated CO2 (560 ppm), elevated O3 (episodic exposure-90 m 11 1 hour 1 ), a combination of elevated CO2 and O3, and ambient control in 30 m treatment rings with three replications. Our overall results showed that our three growth parameters including height, diameter and volume were increased by elevated CO2, decreased by elevated O3, and were not significantly different from the ambient control under elevated CO2+O3. However, there were significant clonal differences in the responses; all five clones exhibited increased growth with elevated CO2, one clone showed an increase with elevated O3, and two clones showed an increase over the control with elevated CO2+O3, two clones showed a decrease, and one was not significantly different from the control. Notably, there was a significant increase in current terminal shoot dieback with elevated CO2 during the 1999–2000 dormant season. Dieback was especially prominent in two of the five clones, and was attributed to those clones growing longer into the autumnal season where they were subject to frost. Our results show that elevated O3 negates expected positive growth effects of elevated CO2 in Populus tremuloides in the field, and suggest that future climate model predictions should take into account the offsetting effects of elevated O3 on CO2 enrichment when estimating future growth of trembling aspen stands. # 2001 Published by Elsevier Science Ltd. All rights reserved.

The early colonization of newly created coarse woody material (CWM) by beetles was studied in aspen mixedwood forests at two locations in north-central Alberta. Healthy trembling aspen (Populus tremuloides Michx.) trees, in old (>100 years) and mature (40–80 years) stands, were cut to provide three types of CWM: stumps, bolts on the ground (logs), and bolts suspended above the ground to simulate snags. Over 2 years, 1049 Coleoptera, representing 49 taxa, were collected. Faunal structure differed little between the two locations. Species diversity was higher in old than in mature stands, and higher in stumps and logs than in suspended bolts; however, these "snags" tended to have higher abundance when compared with stumps and logs. Overall beetle abundance and the catch of wood-boring beetles was significantly higher in the first year post-treatment, mainly because of the ambrosia beetle (Trypodendron retusum (LeConte)) and one of its predators, Rhizophagus remotus LeConte; however, beetle diversity was higher in the second year, suggesting that early wood-boring species may "precondition" the wood for a number of succeeding species. The high turnover rate of taxa and spatial or temporal variation in faunal structure suggests that effort focused on habitat classification of CWM will facilitate management to conserve saproxylic faunal diversity.

A generalized logistic model of individual tree mortality was developed for trembling aspen (Populus tremuloides Michx.), white spruce (Picea glauca (Moench) Voss), and lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) in Alberta boreal mixedwood forests based on an empirical data base of permanent sample plots. The model is suitable for observations from unequal remeasurement intervals. The maximum likelihood estimation was used to fit the model, the likelihood ratio test was combined with our understanding of mortality process to select the important variables, and the Hosmer-Lemeshow goodness-of-fit test was conducted to evaluate the fit. The fitted model predicts the survival probability of an individual tree based on the tree diameter at breast height, annual diameter increment, stand basal area, species composition, and site productivity.

Enriched atmospheric CO2 alters the quantity and quality of plant production, but how such effects vary among plant genotypes is poorly known. We evaluated the independent and interactive effects of CO2 and nutrient availability on growth, allocation and phytochemistry of six aspen (Populus tremuloides Michx.) genotypes. One-year-old trees, propagated from root cuttings, were grown in CO2-controlled glasshouses for 64 days, then harvested. Foliage was analyzed for levels of water, nitrogen, starch, phenolic glycosides and condensed tannins. Of seven plant growth/allocation variables measured, four (biomass production, stem growth, relative growth rate and root:shoot ratio) exhibited marginally to highly significant CO2 × genotype interactions. CO2 enrichment stimulated growth of some genotypes more than others, and this interaction was itself influenced by soil nutrient availability. In addition, enriched CO2 increased the magnitude of the among-genotype variance for four of the growth/allocation variables. Of six foliar chemical constituents analyzed, CO2-mediated responses of two (the phenolic glycoside tremulacin and condensed tannins) varied among genotypes. Moreover, enriched CO2 increased the magnitude of among-genotype variance for four of the chemical variables. Given the importance of these growth and chemical characteristics to the biological fitness of aspen, this research suggests that projected atmospheric CO2 increases are likely to alter the genetic structures and evolutionary trajectories of aspen populations.

Quaking aspen (Populus tremuloides Michx.) seedlings were grown in nutrient solutions to investigate their responses to a range of solution Ca levels (10-1000 µM) similar to those found in aspen stands of upper Lakes States. Growth increased significantly with increasing level of solution Ca. The level of solution Ca associated with 90% of maximum elongation (critical level) was 61 µM for shoots and 88 µM for roots. Critical Ca levels for biomass probably were lower. Concentrations of Ca in leaves and roots increased significantly with increasing solution Ca. Elongation of shoots and roots was also strongly and positively related to Ca concentrations in leaves and roots. Critical Ca concentrations (oven-dry mass) for shoot elongation were 0.46% in leaf tissue and 0.12% in root tissue, while critical Ca concentrations for root elongation were 0.54% in leaf tissue and 0.13% in root tissue. Solution Ca may have induced deficiencies of other elements, but direct Ca deficiency was the primary cause of growth reduction. These critical Ca levels in solutions or tissues cannot be used to diagnose Ca deficiency in aspen forests until it is known how other soil and plant factors affect the Ca requirement of aspen.

We investigated the behavioral feeding preference and the chemoreception of leaf polar extracts from trembling aspen, Populus tremuloides, and from sun and shade sugar maple, Acer saccharum, by larvae of the polyphagous forest tent caterpillar, Malacosoma disstria, a defoliator of deciduous forests in the Northern Hemisphere. Three polar extracts were obtained from each tree species: a total extract, a water fraction, and a methanol fraction. M. disstria larvae were allowed ad libitum access to an artificial diet from eclosion to the fifth instar. Two-choice cafeteria tests were performed comparing the mean (±SE) surface area eaten of the total extracts, and the following order of preference was obtained: aspen > sun maple water fraction > total extract. Our findings are discussed in light of previously known information about this insect's performance on these host plants.

We investigated aspen (Populus tremuloides)regeneration in the Gros Ventre River Valley, the National Elk Refuge and a small part of Grand Teton National Park, Wyoming, USA to see if elk (Cervus elaphus) browsing was as damaging as previously thought. We conducted a landscape-scale survey to assess aspen regeneration across gradients of wintering elk concentrations using 68 randomly selected aspen stands in the 1090 km2 study area. Forty-four percent of the stands sampled supported some newer regeneration that had reached the canopy. There were no significant differences of regeneration across elk winter range classification (p=0.25) or distance from feedgrounds (p=0.96). However, a multiple linear regression found that the concentration of elk was one of several important predictors of successful aspen regeneration (p=0.005, R2=0.36). Our results suggest that stand-replacing regeneration occurs across the landscape at a variety of elk densities despite some trends of reduced regeneration under greater elk concentrations. We propose that high spatial and temporal variation and scattered patches of successful aspen regeneration characterize aspen persistence between periods of episodic regeneration and recruitment.

We conducted an analysis of aspen (Populus tremuloides) overstory recruitment on the northern range of Yellowstone National Park (YNP) using information provided in a monograph published by Warren (Warren, E.R., 1926. A study of beaver in the Yaney region of Yellowstone National Park, Roosevelt-Wildl. Ann. 1, 1‐191), increment cores collected from riparian aspen stands in 1998, and an extensive random sample of aspen increment cores collected over YNP’s entire northern range in 1997 and 1998. We summarized aspen size classes reported by Warren and estimated overstory origination dates of the stands he described using a linear regression based on our riparian aspen diameter/age relationship. Applying our regression results to Warren’s diameter measurements, we predicted that the stands measured by Warren contained aspen that originated between approximately 1751 and 1920. The random set of aspen increment cores were used to analyze the age distribution of the current aspen overstory on YNP’s northern range. These increment core data showed that approximately 10% of the current overstory aspen originated before 1871, 85% between 1871 and 1920, and 5% after 1921. Based upon our analysis of the Warren data and our aspen increment cores, we conclude that successful aspen overstory recruitment occurred on the northern range of YNP from the middle to late 1700’s until the 1920’s, after which it essentially ceased. Rocky Mountain Elk (Cervus elaphus) browsing has been identified as significantly impacting aspen overstory recruitment on YNP’s northern range. We hypothesized why elk browsing has a diAerent influence on aspen now than it did historically. We discussed several potential social and ecological factors and hypothesize that a main factor is YNP’s loss of significant predator/prey relationships in the early 1900’s, especially the influence of gray wolves (Canis lupus). We found that aspen overstory recruitment ceased during the same years that wolves, a significant source of elk predation, were removed from YNP. Wolves may positively influence aspen overstory recruitment through a trophic cascades eAect by reducing elk populations, modifying elk movement, and changing elk browsing patterns on aspen. # 2000 Elsevier Science Ltd. All rights reserved.

Summary 1. This study investigated how phytochemical variation among clones of quaking aspen Populus tremuloides, growing in a common habitat, affects the growth and fecundity of a model herbivore. 2. Gypsy moth Lymantria dispar larvae were reared from egg hatch to pupation on 10 aspen clones in the field or on excised foliage in the laboratory. Foliage was collected from each clone, and concentrations of phenolic glycosides, condensed tannins, nitrogen, and water were determined. 3. Herbivore fitness parameters and aspen phytochemical concentrations varied significantly among clones. In both the field and laboratory, larvae reared on clones containing high concentrations of phenolic glycosides exhibited prolonged developmental times and reduced pupal weights and fecundity. Herbivore performance parameters were also related positively to foliar nitrogen concentrations in the laboratory. Food consumption, but neither growth nor reproductive parameters, were related positively to condensed tannin concentrations. 4. In this study, foliar concentrations of phenolic glycosides were implicated as a significant determinant of food quality for gypsy moths, consistent with results of previous laboratory experiments. Additionally, this study documents a case in which host plant variation at a local level influences the performance and possibly the distribution and abundance of an important herbivore.

Direct and interactive effects of CO2 and light on tree phytochemistry and insect fitness parameters were examined through experimental manipulations of plant growth conditions and performance of insect bioassays. Three species of deciduous trees (quaking aspen, Populus tremuloides: paper birch, Betula papyrifera; sugar maple, Acer saccharum) were grown under ambient (387 +/- 8 mu L/L) and elevated (696 +/- 2 mu L/L) levels of atmospheric CO2, with low and high light availability (375 and 855 mu mol x m(-2) x s(-1) at solar noon). Effects on the population and individual performance of a generalist phytophagous insect, the white-marked tussock moth (Orgyia leucostigma) were evaluated. Caterpillars were reared on experimental trees for the duration of the larval stage; and complementary short-term (fourth instar) Feeding trials were conducted with insects fed detached leaves. Phytochemical analyses demonstrated strong effects of both CO2 and light on all foliar nutritional variables (water. starch and nitrogen). For all species. enriched CO2 decreased water content and increased starch content, especially under high light conditions. High CO2 availability reduced levels of foliar nitrogen. but effects were species specific and most pronounced for high light aspen and birch. Analyses of secondary plant compounds revealed that levels of phenolic glycosides (salicortin and tremulacin) in aspen and condensed tannins in birch and maple were positively influenced by levels of both CO2 and light. In contrast, levels of condensed tannins in aspen were primarily affected by light, whereas levels of ellagitannins and gallotannins in maple responded to light and CO2, respectively. The lone-term bioassays showed strong treatment effects on survival, development time, and pupal mass. In general. CO2 effects were pronounced in high light and decreased along the gradient aspen > birch > maple. For larvae reared on high light aspen, enriched CO2 resulted in 62% fewer survivors. with increased development time, and reduced pupal mass. For maple-fed insects, elevated CO2 levels had negative effects on survival and pupal mass in low light. For birch, the only negative CO2 effects were observed in high light, where female larvae showed prolonged development. Fourth instar feeding trials demonstrated that low food conversion efficiency reduced insect performance. Elevated levels of CO2 significantly reduced total consumption, especially by insects on high light aspen and loa: light maple. This research demonstrates that effects of CO2 on phytochemistry and insect performance can be strongly light-dependent, and that plant responses to these two environmental variables differ among species. Overall, increased CO2 availability appeared to increase the defensive capacity of early-successional species primarily under high light conditions, and of late successional species under low light conditions. Due to the interactive effects of tree species, light, CO2, and herbivory, community composition of forests may change in the future. (Less)

Ring widths from 1458 trees from Massachusetts, New York, and New Jersey were measured to determine the influence of gypsy moth (Lymantria dispar L.) defoliation on both hosts (primarily oaks (Quercus spp.)) and nonhosts. Previous year's defoliation negatively influenced radial increment in all oak species. Defoliation also negatively affected radial growth of pitch pine (Pinus rigida Mill.), an intermediate host. There was little or no effect of defoliation on increment of trembling aspen (Populus tremuloides Michx.) and bigtooth aspen (Populus grandidentata Michx.), but both are preferred hosts. Defoliation of host trees in mixed stands resulted in increased increment in the nonhosts tulip-tree (Liriodendron tulipifera L.) and ash (Fraxinus spp.) in the year following defoliation. The effect of defoliation was also evident in the relative production of earlywood and latewood with a pronounced dominance of earlywood production in host trees during the same year as defoliation and often in the following year.

Rates of mass loss of foliar litter of trembling aspen (Populus tremuloides Michx.) were measured for 4 years after a single fertilization with N at 200 kg N/ha or a mix of N, P, K. Ca, Mg, S, and B. Foliar litter from fertilized plots had higher N concentrations than litter from control plots; litter from the nutrient-mix plots had higher concentrations of N, P, and Ca. The average rate of mass loss of aspen foliar litter was 60% after 4 years. The higher concentrations of nutrients in litter from fertilized plots did not affect the rate of decomposition. There was some difference in rates of decomposition of litter incubated in plots of the three treatments. Mass loss was significantly slower in the nutrient-mix plots after 12 and 18 months compared with control plots. This experiment provided no evidence that fertilization of boreal aspen forests will increase rates of litter decomposition.

The effects of different amounts of residual canopy on stand development of quaking aspen (Populus tremuloides Michx.) were examined in a chronosequence of 32 stands spanning 6-10 years since harvest. Residual canopy covers ranged from 0 to 65%, and residual basal areas ranged from 0 to 14.4 m2/ha. Aspen regeneration densities ranged from 7130 to 43 672 stems/ha. Regeneration stem density was affected primarily by residual canopy cover (R2 = 0.27, P = 0.0001) and secondarily by stand age (R2 = 0.09, P = 0.004). Aspen density decreased significantly with increasing residual canopy cover for 7-year-old and 8-year-old regeneration. Residual canopy cover did not significantly affect aspen density in 9-year-old regeneration (R2 = 0.02, P = 0.579) but was negatively related to total height of 9-year-old codominant aspens (R2 = 0.49, P = 0.002). Canopy cover was a more accurate representation of the amount of shade the regeneration received than the density or basal area of residual trees. However, the low value of the coefficient of determination from a multiple-regression model indicates that considerable variation in stem densities and height was unexplained by residual canopy cover, even though it was the best predictor of the variables measured.

The performance of gypsy moths (Lymantria dispar) feeding on quaking aspen (Populus tremuloides) is strongly influenced by host foliar chemistry and susceptibility to a nuclear polyhedrosis virus (LdNPV), but the relationship of susceptibility to chemistry is poorly understood. We investigated the effects of genetic and resource-mediated variation in phytochemistry on viral pathogenicity. Trees were grown in pots in a common garden. Disks were punched from aspen leaves, inoculated with LdNPV and fed to third instars. Additional leaves were analyzed for levels of nitrogen, starch, phenolic glycosides, and condensed tannins. Despite marked variation among trees in levels of phenolic glycosides and tannins, we observed minimal variation in larval susceptibility to LdNPV. Viral pathogenicity was only weakly (inversely) correlated with tannin concentrations in one of two experiments. These results suggest that differential defoliation of aspen by gypsy moths in the field is due to the direct effects of host chemistry on larval performance rather than to the indirect effects of host chemistry on efficacy of this natural enemy.

Elk (Cervus elaphus) populations in Rocky Mountain National Park are higher than at any time in the past century, and heavy browsing by elk may interfere with aspen (Populus tremuloides Michx.) regneration. We used aerial photographs to identify all aspen stands within Rocky Mountain National Park, and all aspen stands within the elk winter range range (defined as 2400 to 2800 m elevation) in three portions of the adjacent Roosevelt National Forest. From this population of aspen stands, we randomly selected 57 stands for evaluation of aspen regeneration. Stands that contained stems younger than 30 years and taller than 2.5 m tall were classified as regenerating suc- cessfully. Only 20% of the aspen stands in Estes Valley contained a cohort of regenerating aspen stems, whereas 45-to- 75% of aspen stands across the larger landscape of the Front Range had regenerating cohorts of aspen. Within the elk winter range of the Roosevelt National Forest, 13 of 17 aspen stands were regenerating. In the elk winter range on the east side of the Park but outside of Estes Valley, 11 of 15 aspen stands were regenerating successfully. Only a few aspen stands exist in the elk winter range on the western side of the Park, and none of the five aspen stands sampled in Kawuneeche Valley had a regenerating cohort. The lack of regeneration in Kawuneeche Valley may result from locally heavy elk use in both winter and summer. In the summer elk range at higher elevations in the Park (2800 to 3200 m), 16 of 23 stands had regenerated. At landscape scales, all locations outside of the heavily impacted Estes Valley averaged about two cohorts/stand that regenerated after the mid-1960s. All stands that lacked a regenerating cohort showed evidence of moderate-to-severe damage from elk browsing of stems. No regenerating stands showed evidence of severe browsing. We conclude that at landscape scales, regeneration within aspen stands is very common across the Front Range, except in local areas of the highest elk use where little regeneration has occurred in the past 30 years. , i i Introduction

Elevated atmospheric CO2 has the potential to change below-ground nutrient cycling and thereby alter the soil-atmosphere exchange of biogenic trace gases. We measured fluxes of CH4 and N2O in trembling aspen (Populus tremuloides Michx.) stands grown in open-top chambers under ambient and twice-ambient CO2 concentrations crossed with ‘high’ and low soil-N conditions.

To develop anamorphic height growth and site index models for trembling aspen (Populus tremuloides Michx.) stands in British Columbia, a total of 33 naturally established, fire-originated, unmanaged, and even-aged stands were located in the Boreal White and Black Spruce zone. The breast-height ages of dominant trees in these stands were older than 50 years. The height growth models developed from conditioned logistic, Chapman–Richards', and conditioned Chapman–Richards' functions were unbiased and provided an equally good fit. Similarly, the site index models constructed by conditioned logistic and Chapman–Richards' functions were unbiased and provided a similar fit, however, the precision of these models for predicting site index from young stands was low. The height growth models developed from aspen stands in Alberta and Ontario were significantly biased in estimating the aspen height growth in British Columbia, which may imply that the height growth pattern of aspen varies with climate. This finding suggest a low portability of height growth models between climatically different regions.

Air-dried leaf litter is universally used in decomposition experiments, but limnological research indicates that air-drying considerably increases the susceptibility of litter to leaching. In an aspen forest in the Rocky Mountains of Alberta, Canada, I compared first-year decomposition of air-dried leaf litter (7–8% moisture) of trembling aspen (Populus tremuloides) and green alder (Alnus crispa) against decomposition of freshly fallen leaf litter that was still moist (57–62% moisture) when placed in the litterbags. Contrary to expectations, mass loss was substantially faster from fresh litter than from air-dried litter of both species, even in the first month in the field. Over the 334-d experiment, fresh alder leaves decayed nearly twice as fast as air-dried leaves; fresh aspen leaves decayed 62% faster than air-dried leaves. Air-drying was estimated to add at least another year to the time required to reach 50% mass loss. Moisture content of fresh leaves was higher than that of air-dried leaves at every sampling time. Fresh aspen litter rapidly accumulated external N during only the first month, while air-dried aspen litter accumulated external N throughout the experiment. N-rich alder leaves released N in the first week in the field, but the loss was greater for fresh leaves than for air-dried leaves and the content difference persisted for the remainder of the experiment. Both mass loss rates and rates of N uptake and release may be substantially underestimated by using air-dried leaves in decomposition experiments.

Elevated concentrations of atmospheric CO2 are likely to interact with other factors affecting plant physiology to alter plant chemical profiles and plant–herbivore interactions. We evaluated the independent and interactive effects of enriched CO2 and artificial defoliation on foliar chemistry of quaking aspen (Populus tremuloides) and sugar maple (Acer saccharum), and the consequences of such changes for short-term performance of the gypsy moth (Lymantria dispar). We grew aspen and maple seedlings in ambient (~360 ppm) and enriched (650 ppm) CO2 environments at the University of Wisconsin Biotron. Seven weeks after budbreak, trees in half of the rooms were subjected to 50% defoliation. Afterwards, foliage was collected for chemical analyses, and feeding trials were conducted with fourth-stadium gypsy moths. Enriched CO2 altered foliar levels of water, nitrogen, carbohydrates, and phenolics, and responses generally differed between the two tree species. Defoliation induced chemical changes only in aspen. We found no significant interactions between CO2 and defoliation for levels of carbon-based defenses (phenolic glycosides and tannins). CO2 treatment altered the performance of larvae fed aspen, but not maple, whereas defoliation had little effect on performance of insects. In general, results from this experimental system do not support the hypothesis that induction of carbon-based chemical defenses, and attendant effects on insects, will be stronger in a CO2-enriched world.

Summary Ramets derived from root sprouts of two quaking aspen (Populus tremuloides) parents were either mechanically wounded or wounded and inoculated with mycelium of Hypoxylon mammatum. Female aspen parents were considered either resistant or susceptible based on a 30-year field trial of their open-pollinated progenies that had 21% and 93% mortality, respectively, caused by H. mammatum. Each stem was wounded twice at the base and in the nonlignified zone. Samples were taken 5 mm above the site of wounding at 7 and 14 days. Histochemicals used were phloroglucinol-HCl and Sudan Black B to detect lignin and suberin, respectively. Thin sections of the resistant aspen ramets showed intensive staining phloroglucinol-HCl, which was localized along both sides of the wound margin. Collenchyma callus was well developed by day 7 and was derived from the phloem tissue internal to the cell walls staining positive for phloroglucinol-HCl, near the vascular cambium. In contrast, the tissues of susceptible aspen ramets were less stained, the stained cells were less scattered and less collenchyma callus had formed than on the resistant ramets. The presence of the fungus increased the amount of staining observed with phloroglucinol-HCl and delayed wound closure in both aspen genotypes. No distinguishing pattern could be identified between the two aspen genotypes with respect to suberin formation. When comparing stem response to phloroglucinol-HCl with respect to the tissue type on the same stem (succulent versus woody), the inoculation of older woody tissue resulted in a more defined host response and thus were better in separating the two aspen genotypes.

Increasing concentrations of atmospheric CO2 will interact with other en- vironmental factors to influence the physiology and ecology of trees. This research evaluated how plant phytochemical responses to enriched atmospheric CO2 are affected by the avail- ability of soil nitrate (NO3-) and how these chemical changes, in turn, alter the performance of a tree-feeding folivore. Seedlings of three deciduous tree species-quaking aspen (Pop- ulus tremuloides), red oak (Quercus rubra), and sugar maple (Acer saccharum)-were grown in ambient (355 FL/L) or elevated (650 FL/L) CO2 in combination with low (1.25 mmol/L) or high (7.5 mmol/L) soil NO3- availability. After 60 d, foliage was analyzed for changes in nutrients and allelochemicals likely to be influenced by the availability of CO2 and NO3-. Penultimate gypsy moth larvae (Lymantria dispar) were reared on foliage (aspen and maple) to determine how performance would be affected by host chemical changes. Using the framework of carbon-nutrient balance (CNB) theory, we tested three hypotheses regarding the impact of CO2 and NO3- availability on plant chemistry and insect performance: (1) nitrogen-based compounds will decrease, and carbon-based compounds will increase in- response to elevated CO2 and/or low NO3-; (2) aspen will exhibit the greatest change in C:N ratios, and maple the least; and (3) phytochemical changes will influence gypsy moth perfor- mance, with larvae fed aspen being affected more than those fed maple. Concentrations of nitrogen and soluble protein decreased, whereas concentrations of starch, condensed tannins, and ellagitannins increased, in response to elevated CO2 and/or low NO3-. Responses of simple carbohydrates and phenolic glycosides were variable, however, suggesting that foliar accumulations of "dynamic metabolites" do not follow the predictions of CNB theory as well as do those of stable end products. With respect to Hypothesis 2, we found that absolute (net) changes in foliar C:N ratios were greatest for aspen and least for oak, whereas relative (proportional) changes were greatest for maple and least for aspen. Thus, Hypothesis 2 was only partially supported by the data. Considering Hypothesis 3, we found that elevated CO2 treatments had little effect on gypsy moth development time, growth rate, or larval mass. Larvae reared on aspen foliage grown under elevated CO2 exhibited increased consumption but decreased conversion efficiencies. Gypsy moth responses to NO3- were strongly host spe- cific: the highest consumption and food digestibility occurred in larvae on high-NO3- aspen, whereas the fastest growth rates occurred in larvae on high-NO3- maple. In short, our results again only partially supported the predicted pattern. They indicate, however, that the magnitude of insect response elicited by resource-mediated shifts in host chemistry will depend on how levels of compounds with specific importance to insect fitness (e.g., phenolic glycosides in aspen) are affected. Overall, we observed relatively few true interactions (i.e., nonadditive) between carbon and nitrogen availability vis a vis foliar chemistry and insect performance. Tree species, however, frequently interacted with CO2 and/or NO3- availability to affect both sets of parameters. These results suggest that the effects of elevated atmospheric CO2 on terrestrial plant communities will not be homogeneous, but will depend on species composition and soil nutrient availability.

This research evaluated the direct and interactive effects of atmospheric CO2 and soil NO3 - availability on growth and biomass partitioning of quaking aspen (Populus tremuloides Michx.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marsh.). In the split split plot experimental design, NO3 - availability (low and high) and tree species were nested in two levels of atmospheric CO2 (ambient, 355 μL/L; elevated, 650 μL/L). Seedlings were grown for 57 days in environmental control rooms. Increased CO2 and NO3 - availability positively and (mostly) independently influenced total growth and relative growth rates. Moderate to weak interactions between CO2 and NO3 - for several growth parameters (e.g., leaf production, shoot length, root collar diameter) in some species indicated an enhanced response to CO2 enrichment under conditions of high NO3 - availability. Interactive effects were most pronounced in aspen. Seedling growth and allocation responses to CO2 and NO3 - were frequently species specific and associated with successional status. For example, proportional increases in growth in response to elevated CO2 were greatest for sugar maple and least for quaking aspen, whereas the converse was true with respect to response to high NO3 - availability. This research indicates that the impact of enriched CO2 atmospheres on forest communities will be influenced by both nutrient availability and unique species characteristics.

We surveyed foliar morphological and chemical variation in 31 clones of Populus tremuloides in northern lower Michigan. Significant variation among clones was exhibited for leaf weight, specific leaf area, nitrogen, starch, phenolic glycosides (salicortin and tremulacin) and condensed tannins. Variation among clones with respect to nitrogen, starch, salicortin, tremulacin and condensed tannin concentrations was 1.7-, 8.0-, 10.3-, 5.9-, and 2.1-fold, respectively. Interactions between aspen and phytophagous insects in this region are likely to be influenced by substantial interclonal variation in secondary chemistry.

Growth of red pine (Pinusresinosa Ait.) and quaking aspen (Populustremuloides Michx.) target trees with varying degrees of competition by neighboring aspen was measured in a 7-year-old clearcut in northeastern Minnesota. Competitive conditions were measured using indices of resource availability (percent open sky) and neighborhood stand density. Three-year diameter growth of target trees was reduced by more than 50% in both species because of competition from neighboring quaking aspen. The last 2 year's height growth was also reduced by approximately 30% for both species. The relationship between growth and percent open sky was convex for red pine and concave for quaking aspen, potentially indicating a greater sensitivity in aspen to any reduction below optimal resource availability. Competition reduced crown size in both species, but self-pruning occurred only in quaking aspen. Needle and leaf size were not affected by competition, but red pine needle density was lower in highly competitive environments. Red pine had a higher specific gravity when growing under competition, partially offsetting the reduction in wood volume.

Most stands of trembling aspen (Populus tremuloides) in northern Yellow- stone National Park appear to have become established between 1870 and 1890, with little regeneration since 1900. There has been controversy throughout this century regarding the relative roles of browsing by elk (Cervus elaphus) and fire suppression in preventing aspen regeneration. Fires in 1988 burned 22% of the northern ungulate winter range in the park, and created an unusual opportunity to investigate interactions between fire, ungulate brows- ing, and aspen regeneration. We tested two hypotheses. (1) The fires would stimulate such prolific sprouting of new aspen stems in burned stands that many stems would escape ungulate browsing and regenerate a canopy of large aspen stems. (2) Browsing pressure would be so intense that it would inhibit aspen canopy regeneration in the burned stands, despite prolific sprouting, but increased forage production in the burned areas would attract elk so that they would not seek out remote aspen stands, and hence, aspen regeneration would occur in unburned aspen stands remote from the burned areas. We sampled aspen sprout density, height, growth form, and browsing intensity in six burned aspen stands, six unburned stands close ( 4 km) from the burned area. Density of sprouts was generally greater in the burned stands than in the unburned stands in spring 1990 (2 yr after the fires), but was approaching the density of unburned stands by fall 1991. There were no significant dif- ferences in browsing intensity (percent of aspen sprouts browsed by ungulates) in 1990 or 1991 among burned, unburned close, or unburned remote stands, nor were there differences in relation to growth form (juvenile vs. adult sprouts). Unbrowsed sprouts generally were lower than the depth of the snowpack, suggesting that elk browsed nearly all sprouts that were accessible.