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The aim of this study was to examine the effects of elevated carbon dioxide [CO2] and ozone [O3] and their interaction on wood chemistry and anatomy of five clones of 3-year-old trembling aspen (Populus tremuloides Michx.). Wood chemistry was studied also on paper birch (Betula papyrifera Marsh.) and sugar maple (Acer saccharum Marsh.) seedling-origin saplings of the same age. Material for the study was collected from the Aspen Free-Air CO2 Enrichment (FACE) experiment in Rhinelander, WI, USA, where the saplings had been exposed to four treatments: control (C; ambient CO2, ambient O3), elevated CO2 (560ppm during daylight hours), elevated O3 (1.5 × ambient during daylight hours) and their combination (CO2 + O3) for three growing seasons (1998–2000). Wood chemistry responses to the elevated CO2 and O3 treatments differed between species. Aspen was most responsive, while maple was the least responsive of the three tree species. Aspen genotype affected the responses of wood chemistry and, to some extent, wood structure to the treatments. The lignin concentration increased under elevated O3 in four clones of aspen and in birch. However, elevated CO2 ameliorated the effect. In two aspen clones, nitrogen in wood samples decreased under combined exposure to CO2 and O2. Soluble sugar concentration in one aspen clone and starch concentration in two clones were increased by elevated CO2. In aspen wood, α-cellulose concentration changed under elevated CO2, decreasing under ambient 03 and slightly increasing under elevated O3. Hemicellulose concentration in birch was decreased by elevated CO2 and increased by elevated O3. In aspen, elevated O3 induced statistically significant reductions in distance from the pith to the bark and vessel lumen diameter, as well as increased wall thickness and wall percentage, and in one clone, decreased fibre lumen diameter. Our results show that juvenile wood properties of broadleaves, depending on species and genotype, were altered by atmospheric gas concentrations predicted for the year 2050 and that CO2 ameliorates some adverse effects of elevated O3 on wood chemistry. [ABSTRACT FROM AUTHOR]

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

Questions Does the increase in disturbance associated with removing harvest residues negatively impact biodiversity and resilience in aspen-dominated forest ecosystems? How do responses of functional diversity measures relate to community recovery and standing biomass? Location Aspen ( Populus tremuloides, Michx.) mixedwood forests in Minnesota and Michigan, USA. Methods Three levels for two factors, organic matter removal and compaction, were fully crossed, resulting in nine experimental treatments that spanned a range of disturbance severity. Each treatment was replicated three times at each of three sites dominated by the same tree species but having different soil textures (clay, silty loam, sandy). Community composition and taxonomic diversity (species richness, species evenness, Shannon diversity index) were quantified based on woody species abundance sampled 5, 10 and 15 yr after disturbance. Community composition response was assessed using non-metric multidimensional scaling. Functional diversity (functional richness, evenness, dispersion and divergence) was also estimated using eight species effect and response traits. Finally, we examined community recovery as well as responses of species and functional diversity to disturbance severity over time using repeated measures ANOVA. Results Two responses indicated a potentially negative impact of whole-tree harvest relative to conventional, stem-only harvest: functional richness on silty loam soils and species evenness on clayey soils. Otherwise, negative impacts were restricted to forest floor removal or increased compaction. Recovery in community composition was reduced by the most severe treatments, particularly forest floor removal, across the study, but the responses of functional and taxonomic diversity varied among sites, with some measures increasing as a result of severe disturbance. Conclusions Maximization of standing biomass may mean a short-term sacrifice in species and functional diversity. Also, examinations of forest management impacts on species and functional diversity and composition should apply multiple metrics and indices to ensure potential impacts are not obscured by the reliance on a single approach. [ABSTRACT FROM AUTHOR]

ABSTRACT Snags (standing dead trees) are important components of forests that provide resources for numerous species of wildlife and contribute to decay dynamics and other ecological processes. Managers charged with managing populations of snags need information about standing rates of snags and factors influencing those rates, yet such data are limited for ponderosa pine ( Pinus ponderosa) and especially mixed-conifer forests in the southwestern United States. We monitored standing rates of snags in 1-ha plots in Arizona mixed-conifer ( n = 53 plots) and ponderosa pine ( n = 60 plots) forests from 1997 through 2012. We used the Burnham live-dead, mark-resight model in Program MARK and multimodel inference to estimate standing rates during 5-year intervals while accounting for imperfect detection. Because snag standing rates may be influenced by plot characteristics, we used plots rather than snags as sampling units and conducted bootstrap analyses (500 iterations per model) to resample plots and estimate standing rates and associated parameters. We modeled standing rates in 3 discrete steps. First, we selected a parsimonious base model from a set of models including snag species, and then we evaluated models created by adding snag and plot covariates to the base model in steps 2 and 3, respectively. Snag standing rates differed among snag species and 5-year sampling intervals. Standing rates were positively related to snag diameter, negatively related to snag height, and were lower for snags with intact tops than for broken-topped snags. Standing rates also were positively related to topographic roughness, elevation, tree density, and an index of northness, and negatively related to slope and relative topographic exposure. Our results provide comparative data on standing rates of multiple species of snags based on a large and spatially extensive sample and rigorous analysis, and quantify the relative importance of several snag and plot characteristics on those rates. They indicate that modeling snag dynamics is complicated by both spatial and temporal variation in standing rates and identify areas where further work is needed to facilitate such modeling. Published 2015. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Aim Compare contemporary with pre-fire-disruption forest structures, assessing the influence of factors that caused ecological change and evaluating remote sites as relatively natural areas. Location Grand Canyon National Park contains the largest never-harvested and long-term ungrazed forest ecosystem in Arizona, providing valuable sites for measuring natural variability. However, anthropogenic disruption of natural fire regimes since Euro-American settlement c. 1880 has led to changes in forest structure. Methods We compared species composition, tree structure, regeneration, and canopy cover on large (135–603 ha) ponderosa pine-dominated study sites: (1) isolated points on the North Rim where some surface fires continued after 1880, (2) a higher-elevation North Rim site where fire has been excluded and (3) a South Rim site, also without recent fire, with a paired Kaibab National Forest site. Forest tree structure prior to fire-regime disruption was reconstructed with dendroecological techniques. Results Before fire exclusion, all sites had relatively low tree density (140–246 trees ha-1 ) dominated by large trees (basal area 9.1–28.5 m2 ha-1 ), primarily ponderosa pine or pine/Gambel oak on the South Rim. Currently all sites are relatively dense (389–955 trees ha-1 , 14.1–41.3 m2 ha-1 ) but patterns of species composition and regeneration differed substantially with fire regime and elevation. Regeneration at continued-fire sites was primarily through sprouting species, Gambel oak and New Mexican locust, forming a shrubby midstorey under a relatively open pine canopy. In contrast, all fire-excluded sites were dense with seed-reproducing conifer species. Main conclusions Comparison of change caused by climate fluctuation, tree cutting, fire exclusion, livestock herbivory, and wildlife herbivory, suggests that fire regime alteration appears to have played the... [ABSTRACT FROM AUTHOR]