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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.

To investigate the extent and causes of recent quaking aspen (Populus tremuloides) recruitment in northern Yellowstone National Park, we measured browsing intensity and height of young aspen in 87 randomly selected aspen stands in 2012, and compared our results to similar data collected in 1997-1998. We also examined the relationship between aspen recovery and the distribution of Rocky Mountain elk (Cervus elaphus) and bison (Bison bison) on the Yellowstone northern ungulate winter range, using ungulate fecal pile densities and annual elk count data. In 1998, 90% of young aspen were browsed and none were taller-than 200 cm, the height at which aspen begin to escape from elk browsing. In 2012, only 37% in the east and 63% in the west portions of the winter range were browsed, and 65% of stands in the east had young aspen taller than 200 cm. Heights of young aspen were inversely related to browsing intensity, with the least browsing and greatest heights in the eastern portion of the range, corresponding with recent changes in elk density and distribution. In contrast with historical elk distribution (1930s-1990s), the greatest densities of elk recently (2006-2012) have been north of the park boundary (approximately 5 elk/km2), and in the western part of the range (2-4 elk/km2), with relatively few elk in the eastern portion of the range (

After a large crown fire in Arizona, we examined the direct and indirect effects of fire and herbivory (and their interaction) on the regeneration of aspen (Populus tremuloides) and arthropod species richness and abundance. We used elk exclosures covering 150 ha and other experiments to examine these interactions. Several major patterns emerged. First, in the absence of elk, there is a positive relationship between burn severity and the regeneration of aspens via asexual reproduction. Specifically, aboveground biomass of aspen resprouts was 10 times greater at sites of high burn severity than in sites of intermediate burn severity, and there was virtually no aspen regeneration without fire. Second, elk selectively browsed aspen ramets in high-severity burn sites two times more intensely than aspen ramets in intermediate-severity burn sites, largely negating the enhanced regeneration that would have otherwise occurred, thus resulting in three times greater regeneration in intermediate burn sites than in high burn sites. Third, fire and elk browsing had opposing impacts on an arthropod community composed of 33 taxa from 11 orders and 21 families. Fire severity alone showed no effect on arthropod richness and abundance; however, intermediate-severity fire and moderate levels of elk browsing resulted in 30% greater richness and almost 40% greater abundance. In contrast, high-severity fire and high levels of elk browsing resulted in 69% lower arthropod richness and 72% lower abundance. Fourth, the interaction of fire intensity and selective elk browsing resulted in four arthropod community types where the overall mosaic produced the greatest diversity. Our study demonstrates that patterns can completely reverse depending on the factors involved. This argues against a reductionist perspective and argues for studies incorporating greater complexity. At the very least, we need to be aware of such biases and consider how they may alter important decisions that affect basic ecological theory and management practices.

Because forest stand structure, age, and productivity can mediate the impacts of climate on quaking aspen (Populus tremuloides) mortality, ignoring stand-scale factors limits inference on the drivers of recent sudden aspen decline. Using the proportion of aspen trees that were dead as an index of recent mortality at 841 forest inventory plots, we examined the relationship of this mortality index to forest structure and climate in the Rocky Mountains and Intermountain Western United States. We found that forest structure explained most of the patterns in mortality indices, but that variation in growing-season vapor pressure deficit and winter precipitation over the last 20 years was important. Mortality index sensitivity to precipitation was highest in forests where aspen exhibited high densities, relative basal areas, quadratic mean diameters, and productivities, whereas sensitivity to vapor pressure deficit was highest in young forest stands. These results indicate that the effects of drought on mortality may be mediated by forest stand development, competition with encroaching conifers, and physiological vulnerabilities of large trees to drought. By examining mortality index responses to both forest structure and climate, we show that forest succession cannot be ignored in studies attempting to understand the causes and consequences of sudden aspen decline.

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.

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.

Although rising levels of atmospheric carbon dioxide are expected to directly affect forest ecosystems, little is known of how specific ecological interactions will be modified. This research evaluated the effects of enriched CO2 on the productivity and phytochemistry of forest trees and performance of associated insects. Our experimental system consisted of three tree species (quaking aspen (Populus tre- muloides), red oak (Quercus rubra), sugar maple (Acer saccharum)) that span a range from fast to slow growing, and two species of leaf-feeding insects (gypsy moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria)). Carbon-nutrient balance theory provided a framework for tests of three hypoth- eses; in response to enriched CO2: (1) relative increases in tree growth rates will be greatest for aspen and least for maple, (2) relative decreases in protein and increases in carbon-based compounds will be greatest for aspen and least for maple, and (3) relative reductions in performance will be greatest for insects fed aspen and least for insects fed maple. We grew 1-yr-old seedlings for 60 d under ambient (385 ? 5 AL/ L) or elevated (642 ? 2 pL/L) CO2 regimes at the University of Wisconsin Biotron. After 50 d, we conducted feeding trials with penultimate-instar gypsy moth and forest tent caterpillars. After 60 d, a second set of trees was harvested and partitioned into root, stem, and leaf tissues. We subsequently analyzed leaf material for a variety of compounds known to affect performance of insect herbivores. In terms of actual dry-matter production, aspen responded the most to enriched CO2 atmospheres whereas maple responded the least. Proportional growth increases (relative to ambient plants), however, were highest for oak and least for maple. Effects of elevated CO2 on biomass allocation patterns differed among the three species; root-to-shoot ratios increased in aspen, decreased in oak, and did not change in maple. Enriched CO2 altered concentrations of primary and secondary metabolites in leaves, but the magnitude and direction of effects were species-specific. Aspen showed the largest change in storage carbon compounds (starch), whereas maple experienced the largest change in defensive carbon compounds (con- densed and hydrolyzable tannins). Consumption rates of insects fed high-CO2 aspen increased dramatically, but growth rates declined. The two species of insects differed in response to oak and maple grown under enriched CO2. Gypsy moths grew better on high-CO2 oak, whereas forest tent caterpillars were unaffected; tent caterpillars tended to grow less on high-CO2 maple, whereas gypsy moths were unaffected. Changes in insect performance parameters were related to changes in foliar chemistry. Responses of plants and insects agreed with some, but not all, of the predictions of carbon-nutrient balance theory. This study illustrates that tree productivity and chemistry, and the performance of associated insects, will change under CO2 atmospheres predicted for the next century. Changes in higher level ecological processes, such as community structure and nutrient cycling, are also implicated.

Although quaking aspen (Populus tremuloides Michx.) bark appears greener at higher elevations in the Sangre de Cristo mountain range of New Mexico, chlorophyll concentrations are inversely correlated (p = .001) with altitude. The higher concentration of chlorophyll at lower elevations is effectively masked by a white bloom of dead periderm cells which is also characteristic of exposed site trees. At higher elevations the bloom is yellow—brown and more translucent, thus allowing the chlorophyll layers beneath to show through. Therefore, trees at higher elevations appear greener even though they have lower cortical chlorophyll concentrations. The altitudinal gradient in bark reflectance characteristics may be of adaptive value. If cortical photosynthesis is more important at higher elevations, the translucent bloom would be advantageous in making more light available for photosynthesis. Increased light absorption might also be important in raising bark temperatures for physiological activity during the winter months. At lower elevations where air temperatures are higher, the highly reflective bloom may protected the trees from high bark temperatures and sunscald.

The distribution and cycling of N, P, K, Ca, Mg, S, Fe, and Zn were examined over a 2—yr period in an aspen—mixed—hardwood—Spodosol ecosystem in order to determine the roles of overstory aspen and understory sugar maple in nutrient retention. Stand biomass and net primary production (NPP) were 198 Mg/ha and 12.5 Mg°ha—1°yr—1, respectively. Overstory quaking aspen (Populus tremuloides) comprised 60% of the biomass and 50% of the NPP, and understory sugar maple (Acer saccharum) comprised 24 and 25% of these values, respectively. The total soil N pool and extractable soil P and Mg pools were larger than the respective vegetation nutrient pools. Nutrient cycling within this ecosystem is characterized by large uptake rates, retention of between 40 and 60% of nutrient uptake in perennial tissues, and leaching losses of

Leaf resistance to water vapor flow was determined relative to leaf—water potential for white ash, quaking aspen, and bigtooth aspen. Ash leaves were found to close their stomata at approximately —20 bars and bigtooth aspen leaves at approximately —30 bars, whereas quaking aspen maintained low leaf resistance to —60 bars. This lack of control of water loss is suggested as a possible reason for the apparent moist site preference for quaking aspen in the study region. Bigtooth aspen may be able to survive in more xeric habitats than ash because of the latter's high leaf—water potential for stomatal closure. See full-text article at JSTOR

Extensive forests of quaking aspen (Populus tremuloides Michx.), which occur widely at intermediate elevations in the Intermountain and Rocky Mountain West, constitute an important resource, both esthetically and economically. The aspen type is esteemed among sightseers, picnickers, and campers because of its beauty. Aspen forests protect the soil of an important snow-collecting belt on western mountain watersheds. Although aspen is not one of the important timber species, aspen wood products are in considerable and increasing demand. Finally, the type supports undergrowth capable of furnishing a great amount of cover and forage for wildlife and livestock. On aspen range in good condition one may wade waist deep through a rich mixture of many plants, including species of Heracleum, Mertensia, Delphinium, Osmorphiza, Agastache, Erigeron, Rudbeckia, Senecio, Thalictrum, Agropyron, Bromus, Elymus, and Carex. Most aspen range in the Intermountain region is depleted from prolonged overgrazing, however, and palatable species that were formerly abundant are likely to be scarce or absent. Curiously, much less vegetation is produced in openings, as a rule, than beneath the aspen canopy.3 Usually the vegetation of openings is the sparser and shorter, and it tends to include a smaller proportion of desirable forage species (Houston 1954). It may, indeed, include undesirable species such as the annual Madia glomerata Hook., that are absent under the aspens. We have personally observed such differences in production and species composition on aspen ranges in Utah, Nevada, southern Idaho, western Wyoming, and western Colorado. These observations have included a great variety of sites with respect to soil and exposure, and many variations in character of vegetation. Why should such a difference in ground-cover production exist? Is it a product of those factors. responsible initially for the existence of openings in an aspen forest? Is it caused by differences in microclimate under and away from the aspen canopy? Is it a result of heavier grazing in openings than under the aspen? Or is it the result of interactions between some or all of these factors? Whatever the cause, this widely observed difference seems anomalous, because it would appearthat demands upon the environment by the aspen trees themselves might be expected to infringe on the needs of plants of the shrub and herb layers. Logically, therefore, being free of such competi-tion, the openings should produce more vegetation, not less, than equal areas under aspens. Such a relation has been described by Moinat (1956) in the Quercus gambelii type in southwestern Colorado, a zone somewhat warmer and drier than the aspen-fir zone. Moinat shows that production ofthe field layer in grassy parks is markedly higher than in scrub-oak thickets. Many trenching experiments (reviewed byKorstian and Coile 1938) have demonstrated the adverse effects of root competition by forest overstory on growth of native ground vegetation or artificially planted trees. Shirley (1945), in a study carefully designed to test the relativeeffects of shade and root competition of aspen anct jack pine on tree seedlings and planting stock in Minnesota, demonstrated a root-competitive depression of conifer growth, although not of sur-vival. Shirley's comment is that, "In these studies, however intense the root competition of the overwood, its effect on survival was more than offset by the benefits of the shade, provided this did not reduce the light intensity below 20 percent." Weknow of no experiments of this kind involvingwestern aspen except for Pearson's (1914) comparisons of survival of planted Douglas-fir under aspen and in openings. These showed better sur-vival under aspen.

ike miany other mammals, beavers thrive in sub-climax vegetation. Specifically, they often flourish where fire has been followed by appearance of Quaking Aspen, Populus tremuloides. This tree is the one most commonly felled for food andl building mnaterial by beaver and its distribbution is remarkably similar to that of the b)eaver. Where beavers do occur in the absence of aspen, they usually depend on willow. Various species of Salix are found even more widely distributed than aspen, and very few beavers maintain themselves outside its range although they may temporarily subsist on such unlikely forage as sagebrush or cornstalks. As a matter of fact willow is ofteni abundant in conjunction with aspeni, and West and Rasmusseni (1947) feel that willow, llot aspen, is the key plant in beaver habitat. The primary purpose of this study has been to analyze and interpret patterns of use of varyinig supplies of willow and aspen at 3 beaver colonies in the northern Sierra Nevada. Several investigationis similar to the present oone lhave been most helpful and interesting for purposes of comparison. Notable are those of Aldous (1938) in Minnesota, Bradt (1938) in Miclhigan, Gese and Shadle (1943) and Tevis (1950) in New York, Haas (1943) in Utah, Haseltine (1948) anid Hodgdon and Hunit (1953) in Maine, and MacDonald (1956) in Colorado.

Three tracer solutions applied to 46 aspen (Populus tremuloides) clones in northern Utah revealed as many as 43 stems connected on a common root system. The maximum lateral distance between a donor and a receptor tree was 16.9 m. The number of stems connected by roots was not related to the percentage of sand, gravel, or clay in the soil, or to bulk soil density or elevation of site. See full-text article at JSTOR