Introduction The Aspen-FACE experiment was an 11-year study of the effect of elevated CO 2 and ozone (alone and in combination) on the growth of model aspen communities (pure aspen, aspen-birch, and aspen-maple) in the field in northern Wisconsin, USA. Uncertainty remains about how these short-term plot-level responses might play out over broader temporal and spatial scales where climate change, competition, succession, and disturbances interact with tree-level responses. In this study, we used a new physiology-based approach (PnET-Succession v3.1) within the forest landscape model LANDIS-II to extrapolate the FACE results to broader temporal scales (and ultimately to landscape scale) by mechanistically accounting for the globally changing drivers of temperature, precipitation, CO 2 , and ozone. We added novel algorithms to the model to mechanistically simulate the effects of ozone on photosynthesis through ozone-induced impairment of stomatal control (i.e., stomatal sluggishness) and damage of photosynthetic capacity at the chloroplast level. Results We calibrated the model to empirical observations of competitive interactions on the elevated CO 2 and O 3 plots of the Aspen-FACE experiment and successfully validated it on the combined factor plots. We used the validated model to extend the Aspen-FACE experiment for 80 years. When only aspen clones competed, we found that clone 271 always dominated, although the ozone-tolerant clone was co-dominant when ozone was present. Under all treatments, when aspen clone 216 and birch competed, birch was always dominant or co-dominant, and when clone 216 and maple competed, clone 216 was dominant, although maple was able to grow steadily because of its shade tolerance. We also predicted long-term competitive outcomes for novel assemblages of taxa under each treatment and discovered that future composition and dominant taxa depend on treatment, and that short-term trends do not always persist in the long term. Conclusions We identified the strengths and weaknesses of PnET-Succession v3.1 and conclude that it can generate potentially robust predictions of the effects of elevated CO 2 and ozone at landscape scales because of its mechanistically motivated algorithms. These capabilities can be used to project forest dynamics under anticipated future conditions that have no historical analog with which to parameterize less mechanistic models.

The effect of rising atmospheric concentrations of carbon dioxide (CO2) on tree size–density relations has not been thoroughly investigated despite such relations forming the basis of contemporary forest stand stocking metrics. As an initial exploration of the effects of varying CO2 concentrations on size–density relations, we treated model stands of trembling aspen with increased concentration of CO2 (560 ppm) for 11 years in northern Wisconsin and found compelling differences in stand development compared with those in ambient atmosphere. Pure and mixed trembling aspen forests grew faster in elevated atmospheric CO2 and with developmental trajectories of self-thinning communities carrying more stand basal area at equivalent trees per hectare. Comparison of our experimental results with historical stand density data suggests forest developmental characteristics may have changed in response to historical increases in atmospheric CO2; thus, current stocking guides could underestimate maximum stand carrying capacity. Although our results may be constrained to our study species in a controlled experiment, they still suggest that comprehensive reassessments of stand growth dynamics and size–density relations may be needed across suites of species and spatial extents because of global atmospheric CO2 increases that have already occurred.

Forest landscape models (FLM) are increasingly used to project the effects of climate change on forested landscapes, yet most use phenomenological approaches with untested assumptions about future forest dynamics. We used a FLM that relies on first principles to mechanistically simulate growth (LANDIS-II with PnET-Succession) to systematically explore how landscapes composed of tree species with various life history traits respond to individual climate and abiotic drivers. Moderate temperature rise (+3 degrees C) concurrent with rising CO2 concentration increased net photosynthesis of cohorts, but decreased biomass production because of increased maintenance respiration costs. However, an increase of 6 degrees C decreased both photosynthesis and biomass production, regardless of species optimal temperature. Increasing precipitation generally increased photosynthesis and biomass. Reduced cloudiness had a positive effect on photosynthesis and biomass, but much less than the other treatment factors. Our study informs expectations for the outcome of modeling studies that project forest futures under climate change. Published by Elsevier Ltd.

Fundamental drivers of ecosystem processes such as temperature and precipitation are rapidly changing and creating novel environmental conditions. Forest landscape models ( FLM ) are used by managers and policy‐makers to make projections of future ecosystem dynamics under alternative management or policy options, but the links between the fundamental drivers and projected responses are weak and indirect, limiting their reliability for projecting the impacts of climate change. We developed and tested a relatively mechanistic method to simulate the effects of changing precipitation on species competition within the LANDIS ‐ II FLM . Using data from a field precipitation manipulation experiment in a piñon pine ( Pinus edulis ) and juniper ( Juniperus monosperma ) ecosystem in New Mexico ( USA ), we calibrated our model to measurements from ambient control plots and tested predictions under the drought and irrigation treatments against empirical measurements. The model successfully predicted behavior of physiological variables under the treatments. Discrepancies between model output and empirical data occurred when the monthly time step of the model failed to capture the short‐term dynamics of the ecosystem as recorded by instantaneous field measurements. We applied the model to heuristically assess the effect of alternative climate scenarios on the piñon–juniper ecosystem and found that warmer and drier climate reduced productivity and increased the risk of drought‐induced mortality, especially for piñon. We concluded that the direct links between fundamental drivers and growth rates in our model hold great promise to improve our understanding of ecosystem processes under climate change and improve management decisions because of its greater reliance on first principles.

Three young northern temperate forest communities in the north‐central United States were exposed to factorial combinations of elevated carbon dioxide (CO2) and tropospheric ozone (O3) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3. Treatment effects on ecosystem C content resulted primarily from changes in the near‐surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r2 = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m−2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆NPP/∆N) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2. Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long‐term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well‐understood and limit our ability to predict changes in ecosystem C content.

We studied the interactive effects of elevated concentrations of CO2 and O3 on radial growth and wood properties of four trembling aspen (Populus tremuloides Michx.) clones and paper birch (Betula papyrifera Marsh.) saplings. The material for the study was collected from the Aspen FACE (free-air CO2 enrichment) experiment in Rhinelander (WI, USA). Trees had been exposed to four treatments [control, elevated CO2 (560 ppm), elevated O3 (1.5 times ambient) and combined CO2 + O3 ] during growing seasons 1998-2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO2- and O3-exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO2, radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O3 decreased radial growth and the diameters of vessels and fibres. Clone-specific decrease in wood density and cell wall thickness was observed under elevated CO2 . In birch, the treatments had no major impacts on wood anatomy or wood density. Our study indicates that short-term impact studies conducted with young seedlings may not give a realistic view of long-term ecosystem responses.

The Aspen-FACE experiment generated 11 years of empirical data on the effect of CO 2 enrichment and elevated ozone on the growth of field-grown trees (maple, birch and six aspen clones) in northern Wisconsin, but it is not known how these short-term plot-level responses might play out at the landscape scale over multiple decades where competition, succession and disturbances interact with tree-level responses. In this study we used a forest landscape model (LANDIS-II) to scale these site level results to broader temporal and spatial scales. These general principles emerged from the results. (1) The productivity of taxa under future conditions is the primary determinant of short-term taxon dominance. (2) Longer-term, longevity and shade tolerance may supersede productivity as the determinant of importance, depending on the disturbance regime. This result offers hope that, even in the face of atmospheric changes, managers may have some control over future forest composition and carbon sequestration through modification of disturbance regimes. (3) Changes in the abundance of taxa were mostly gradual and none of the taxa were extirpated from the landscape, even under treatments for which they were poorly adapted. This suggests that as atmospheric conditions change, abrupt extirpations are expected to be rare. (4) Similarly, different taxa fared relatively well under different treatments. This suggests that maintaining species and genetic diversity is a prudent forest management strategy in the face of global change. (5) Accounting for spatial processes is important because seed dispersal and establishment may limit the ability of some species to colonize available habitat.

The world's forests are currently exposed to increasing concentrations of carbon dioxide (CO2) and ozone (O3). Both pollutants can potentially exert a selective effect on plant populations. This, in turn, may lead to changes in ecosystem properties, such as carbon sequestration. Here, we report how elevated CO2 and O3 affect the genetic composition of a woody plant population via altered survival. Using data from the Aspen free-air CO2 enrichment (FACE) experiment (in which aspen clones were grown in factorial combinations of CO2 and O3), we develop a hierarchical Bayesian model of survival. We also examine how survival differences between clones could affect pollutant responses in the next generation. Our model predicts that the relative abundance of the tested clones, given equal initial abundance, would shift under either elevated CO2 or O3 as a result of changing survival rates. Survival was strongly affected by between-clone differences in growth responses. Selection could noticeably decrease O3 sensitivity in the next generation, depending on the heritability of growth responses and the distribution of seed production. The response to selection by CO2, however, is likely to be small. Our results suggest that the changing atmospheric composition could shift the genotypic composition and average pollutant responses of tree populations over moderate timescales.

Plant growth responses to rising atmospheric CO2 and O3 vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N. Ascribable to the size‐symmetric nature of belowground competition, we reasoned that differential growth responses to CO2 and O3 should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12‐year‐long forest FACE experiment, we used tracer 15N and whole‐plant N content to assess belowground competitive interactions among five Populus tremuloides genotypes, between a single P. tremuloides genotype and Betula papryrifera, as well as between the same single P. tremuloides genotype and Acer saccharum. Under elevated CO2, the amount of soil N and 15N obtained by the P. tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer 15N under ambient and elevated CO2; however, its acquisition of soil N under elevated CO2 increased by a significant margin when grown in competition with B. papyrifera (+30%) and A. saccharum (+60%). In contrast, elevated O3 had no effect on soil N and 15N acquisition by the P. tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO2, the rank order of N acquisition among P. tremuloides genotypes shifted over time, indicating that growth responses to CO2 change during ontogeny; this was not the case under elevated O3. In the aspen‐birch community, the competitive advantage elevated CO2 initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO2 and O3 will shape the genetic structure and composition of future forests is, in part, contingent on the time‐dependent effects of belowground competition on plant growth response.

We studied the effect of high ozone (O-3) concentration (110-490 nmol mol(-1)) on regenerating aspen (Populus tremuloides) and maple (Acer saccharum) trees at an open-air O-3 pollution experiment near Rhinelander WI USA. This study is the first of its kind to examine the effects of acute O-3 exposure on aspen and maple sprouts after the parent trees, which were grown under elevated O-3 and/or CO2 for 12 years, were harvested. Acute O-3 damage was not uniform within the crowns of aspen suckers; it was most severe in the mature, fully expanded photosynthesizing leaves. Young expanding leaves showed no visible signs of acute O-3 damage contrary to expectations. Stomatal conductance played a primary role in the severity of acute O-3 damage as it directly controlled O-3 uptake. Maple sprouts, which had lower stomatal conductance, smaller stomatal aperture, higher stomatal density and larger leaf surface area, were tolerant of acute O-3 exposure. Moreover, elevated CO2 did not ameliorate the adverse effects of acute O-3 dose on aspen and maple sprouts, in contrast to its ability to counteract the effects of long-term chronic exposure to lower O-3 levels.

The accumulation of anthropogenic CO₂ in the Earth's atmosphere, and hence the rate of climate warming, is sensitive to stimulation of plant growth by higher concentrations of atmospheric CO₂. Here, we synthesise data from a field experiment in which three developing northern forest communities have been exposed to factorial combinations of elevated CO₂ and O₃. Enhanced net primary productivity (NPP) (c. 26% increase) under elevated CO₂ was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of litter decomposition and microbial N release during decay. Despite initial declines in forest productivity under elevated O₃, compensatory growth of O₃ -tolerant individuals resulted in equivalent NPP under ambient and elevated O₃. After a decade, NPP has remained enhanced under elevated CO₂ and has recovered under elevated O₃ by mechanisms that remain un-calibrated or not considered in coupled climate-biogeochemical models simulating interactions between the global C cycle and climate warming.

The effect of elevated CO2 and O3 on apparent quantum yield (f), maximum photosynthesis (Pmax), carboxylation efficiency (Vcmax) and electron transport capacity (Jmax) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O3 tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO2 alone did not affect f or Pmax, and increased Jmax in the O3-sensitive, but not in the O3-tolerant clone. Elevated O3 decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O3 increased through time. Significant interaction effect, whereby the negative impact of elevated O3 was exaggerated by elevated CO2 was seen in Chl, N and Jmax, and occurred in both O3-tolerant and O3-sensitive clones. The clonal differences in the level of CO2 O3 interaction suggest a relationship between photosynthetic acclimation and background O3 concentration. 2009 Elsevier Ltd. All rights reserved.

This study was conducted at the Aspen Free-Air Carbon Dioxide Enrichment (FACE) experimental site, Rhinelander, WI, (USA). Since 1998, 12 experimental rings planted in 1997 underwent four different treatments: control; elevated CO2 (560 ppm); elevated O3 (1.5X ambient) and elevated CO2 (560 ppm) + O3 (1.5X ambient). In our study, foliage samples of Populus tremuloides Michx. were collected in the summer of 2006, 2007 and 2008 from each of three clones; namely, 216 (O3 tolerant), 271 (intermediately O3 tolerant) and 259 (O3 sensitive) growing within the aspen only sector of each ring.