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Author(s): Kevin E. Percy (corresponding author) [1]; Caroline S. Awmack [2]; Richard L. Lindroth [2]; Mark E. Kubiske [3]; Brian J. Kopper [2]; J. G. Isebrands [4]; Kurt S. Pregitzer [...]

Author(s): Kevin E. Percy; Caroline S. Awmack; Richard L. Lindroth; Mark E. Kubiske; Brian J. Kopper; J. G. Isebrands; Kurt S. Pregitzer; George R. Hendrey; Richard E. Dickson; Donald R. [...]

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 ( r 2 = 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. [ABSTRACT FROM AUTHOR]

We collected soil samples from 27 study sites across North Central United States to compare the soil carbon of short rotation poplar plantations to adjacent agricultural crops and woodlots. Soil organic carbon (SOC) ranged from 20 to more than 160 Mg/ha across the sampled sites. Lowest SOC levels were found in uplands and highest levels in riparian soils. We attributed differences in bulk density and SOC among cover types to the inclusion of woodlot soils in the analysis. Paired comparison found few differences between poplar and agricultural crops. Sites with significant comparisons varied in magnitude and direction. Relatively greater SOC was often observed in poplar when native soil carbon was low, but there were important exceptions. Woodlots consistently contained greater SOC than the other crops, especially at depth. We observed little difference between paired poplar and switchgrass, both promising bioenergy crops. There was no evidence of changes in poplar SOC relative to adjacent agricultural soils when considered for stand ages up to 12 years. Highly variable native SOC levels and subtle changes over time make verification of soil carbon sequestration among land cover types difficult. In addition to soil carbon storage potential, it is therefore important to consider opportunities offered by long-term sequestration of carbon in solid wood products and carbon-offset through production of bioenergy crops. Furthermore, short rotation poplars and switchgrass offer additional carbon sequestration and other environmental benefits such as soil erosion control, runoff abatement, and wildlife habitat improvement. [ABSTRACT FROM AUTHOR]

Summary 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 × 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 O3 were both elevated, the differences in competitive performance among clones decreased. [ABSTRACT FROM AUTHOR]

Human activity causes increasing background concentrations of the greenhouse gases CO2 and O3. Increased levels of CO2 can be found in all terrestrial ecosystems. Damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100 (ref. 3). Although individual effects of CO2 and O3 on vegetation have been widely investigated, very little is known about their interaction, and long-term studies on mature trees and higher trophic levels are extremely rare. Here we present evidence from the most widely distributed North American tree species, Populus tremuloides, showing that CO2 and O3, singly and in combination, affected productivity, physical and chemical leaf defences and, because of changes in plant quality, insect and disease populations. Our data show that feedbacks to plant growth from changes induced by CO2 and O3 in plant quality and pest performance are likely. Assessments of global change effects on forest ecosystems must therefore consider the interacting effects of CO2 and O3 on plant performance, as well as the implications of increased pest activity. [ABSTRACT FROM AUTHOR]

Abstract Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO2 enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO2, respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud-free days (PPFDday,sat) around the lowermost branches was 16·8 ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO2 canopies, respectively. Elevated CO2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFDday,sat throughout the canopies, and elevated CO2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO2, depending upon canopy position. Elevated CO2 increased Pssat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO2. Consequently, estimated daily integrated Ps on cloud-free days (Psday,sat) was stimulated by elevated CO2 only in the upper canopy. Psday,sat of the lowermost branches was actually lower with elevated CO2 because of the darker light environment. The lack of CO2 stimulation at the mid- and lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Psday,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO2 had no effect on the optimization of canopy N allocation. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 may stimulate future forest productivity, possibly increasing carbon storage in terrestrial ecosystems, but how tropospheric ozone will modify this response is unknown. Because of the importance of fine roots to the belowground C cycle, we monitored fine-root biomass and associated C fluxes in regenerating stands of trembling aspen, and mixed stands of trembling aspen and paper birch at FACTS-II, the Aspen FACE project in Rhinelander, Wisconsin. Free-air CO2 enrichment (FACE) was used to elevate concentrations of CO2 (average enrichment concentration 535 µl l–1) and O3 (53 nl l–1) in developing forest stands in 1998 and 1999. Soil respiration, soil pCO2, and dissolved organic carbon in soil solution (DOC) were monitored biweekly. Soil respiration was measured with a portable infrared gas analyzer. Soil pCO2 and DOC samples were collected from soil gas wells and tension lysimeters, respectively, at depths of 15, 30, and 125 cm. Fine-root biomass averaged 263 g m–2 in control plots and increased 96% under elevated CO2. The increased root biomass was accompanied by a 39% increase in soil respiration and a 27% increase in soil pCO2. Both soil respiration and pCO2 exhibited a strong seasonal signal, which was positively correlated with soil temperature. DOC concentrations in soil solution averaged ~12 mg l–1 in surface horizons, declined with depth, and were little affected by the treatments. A simplified belowground C budget for the site indicated that native soil organic matter still dominated the system, and that soil respiration was by far the largest flux. Ozone decreased the above responses to elevated CO2, but effects were rarely statistically significant. We conclude that regenerating stands of northern hardwoods have the potential for substantially greater C input to soil due to greater fine-root production under elevated CO2. Greater fine-root biomass will be accompanied by greater soil C efflux as soil respiration, but leaching losses of C will probably be unaffected. [ABSTRACT FROM AUTHOR]

ABSTRACTLeaf gas exchange parameters and the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO2 and/or O3. The plants were exposed either to ambient air (control), elevated CO2 (560 p.p.m.) elevated O3 (55 p.p.b.) or a mixture of elevated CO2 and O3 in a free air CO2 enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light-saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO2 and decreased by O3 at both control and elevated CO2. The relative stomatal limitation to photosynthesis (ls) was in both clones about 10% under control and elevated O3. Exposure to elevated CO2 + O3 in both clones and to elevated CO2 in clone 259, decreased ls even further – to about 5%. The corresponding changes in Rubisco content and the stability of Ci/Ca ratio suggest that the changes in photosynthesis in response to elevated CO2 and O3 were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O3 tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable Ci under the given treatment, that indicates close coupling between stomatal and mesophyll processes. [ABSTRACT FROM AUTHOR]

Seasonal patterns of [14C]‐labeled photosynthate distribution within two intensively cultured Populus clones with contrasting phenology (P. tristis × P. balsamifera cv. ‘Tristis no. 1′; P. × euramericana cv. Eugenei) were investigated during the establishment year. During active shoot elongation upper mature leaves exported 14C acropetally to the expanding leaves and elongating internodes, and basipetally to the stem. Little 14C was exported to lower mature leaves or lateral branches. At budset the 14C export pattern shifted dramatically in the basipetal direction, i.e., to the lower stem, hardwood cutting, and roots. The timing of budset was the primary factor determining the differences between the clones, except that in all cases Tristis exported more 14C to the roots than Eugenei. After budset lower mature leaves had a similar export pattern to upper leaves, but the quantity of 14C exported to the roots was slightly higher. The results confirm the importance of autumn foliage for root growth in poplar. Clonal differences in seasonal patterns of photosynthate distribution offer potential for the poplar breeder seeking to match a clone's growth pattern with the specific growing season of the site. [ABSTRACT FROM AUTHOR]

Variable internal plant nutrient content may confound plant response to environmental stress. Plant nutrient content may be controlled with relative addition rate techniques in solution culture. However, because raising large numbers of plants in flowing solution culture is difficult, we investigated the feasibility of raising plants in soil mix using relative fertilizer additions. Aspen (Populus tremuloides Michx.) clones (216, 259 and 271) planted in pots containing a peat, sand and vermiculite (2:1:1, v/v/v) soil mix were grown with exponentially increasing fertilizer concentrations and harvested periodically to assess growth. Addition rate treatments ranged from 0.01 to 0.05 day−1. The lag phase of growth, in which plants adjusted to the fertilizer regime, lasted 40 days after which plants entered the experimental period characterized by constant relative growth rates equivalent to applied fertilizer addition rates. Total plant nutrient concentration was (1) unique for each addition rate, (2) linearly related to addition rate and growth rate, and (3) tended to increase at the highest, and decrease at the lowest addition rates. Regardless, the plants appeared to have attained steady‐state conditions. Allocation of carbon to roots increased with lower addition rate treatments and was not dependent upon ontogeny. There were no treatment differences in growth response among aspen clones. Yet there were treatment differences in leaf chlorophyll and photosynthesis within the clones. For the 0.05 day−1 addition rate treatment, chlorophyll, leaf N concentration and photosynthetic rate were strongly correlated with one another, were at a maximum in recently mature leaves, and rapidly declined with leaf age. The rate of decline in these leaf characteristics was slowest in clone 271, consistent with the leaf longevity stress response reported elsewhere. Plant responses from these relative fertilizer addition trials in soil mix agree closely with those run in hydroponics, indicating that steady‐state nutrition can be achieved with a technically simple experimental assemblage. [ABSTRACT FROM AUTHOR]

We studied root growth and respiration of potted plants and field-grown aspen trees (Populus tremuloides Michx.) exposed to ambient or twice-ambient ozone. Root dry weight of potted plants decreased up to 45% after 12 weeks of ozone treatment, and root system respiration decreased by 27%. The ozone-induced decrease in root system respiration of potted plants was more closely correlated with decreased root dry weight than with specific root respiration, suggesting that aspen root metabolism was less affected by ozone than root growth. We used minirhizotrons to study the appearance and disappearance of roots in the field. Length of live roots of field-grown trees increased rapidly early in the season and peaked by midseason in association with a decrease in root production and an increase in root disappearance. In the twice-ambient ozone treatment, live root lengths were 17% less than those of controls, but the effect was not statistically significant. Seasonal soil CO2 efflux of field-grown trees decreased significantly in the ozone treatments, but because differences in live root length were not significant and root dry weights were not available, the effect on CO2 efflux could not be attributed directly to decreased root growth. [ABSTRACT FROM AUTHOR]

Ethylene losses from the gas phase of various container-seal systems were studied to develop acceptable methods for containing ethylene during experiments. Ethylene at an initial amount of 104 μl l-1 was stored in glass vials at near atmospheric pressure for 20 h at 25-27°C and at 35% relative humidity external to the vials. Crimped serum vials sealed with saturated (NH4)2SO4 solution, neoprene rubber septa, nitrile rubber (Hycar) septa, butyl rubber septa, and brown translucent silicone rubber septa lost ethylene at the rate of 1.8. 10.2, 16.2, 16.5, and 40.2 nl m-2s-1, respectively, over the 20-h period. Screw-capped reaction vials sealed with white silicone rubber septa lost ethylene at the rate of 30.2 nl -2s-1. The (NH4)2SO4 solution was utilized as a seal by inverting a vial so that the salt solution covered the internal surface of the vial septum. Saturated (NH4)2SO4 solution is an effective seal. Silicone rubber should be avoided as a seal in systems for containing ethylene. Ethylene production values in the literature may be underestimates where silicone rubber seals have been used. [ABSTRACT FROM AUTHOR]

Abstract We investigated the interaction of elevated CO2 and/or (Ozone) O3 on the occurrence and severity of aspen leaf rust (Melampsora medusae Thuem. f. sp. tremuloidae ) on trembling aspen (Populus tremuloides Michx.). Furthermore, we examined the role of changes in leaf surface properties induced by elevated CO2 and/or O3 in this host–pathogen interaction. Three- to five-fold increases in levels of rust infection index were found in 2 consecutive years following growing-season-long exposures with either O3 alone or CO2 + O3 depending on aspen clone. Examination of leaf surface properties (wax appearance, wax amount, wax chemical composition, leaf surface and wettability) suggested significant effects by O3 and CO2 + O3 . We conclude that elevated O3 is altering aspen leaf surfaces in such a way that it is likely predisposing the plants to increased infection by aspen leaf rust. [ABSTRACT FROM AUTHOR]

Predicting ozone-induced reduction of carbon sequestration of forests under elevated tropospheric ozone concentrations requires robust mechanistic leaf-level models, scaled up to whole tree and stand level.As ozone effects depend on genotype, the ability to predict these effects on forest carbon cycling via competitive response between genotypes will also be required. This study tests a process-based model that predicts the relative effects of ozone on the photosynthetic rate and growth of an ozone-sensitive aspen clone, as a first step in simulating the competitive response of genotypes to atmospheric and climate change. The resulting composite model simulated the relative aboveground growth response of ozone-sensitive aspen clone 259 exposed tosquare wave variation in ozone concentration. This included a greater effect on stem diameter than on stem height, earlier leaf abscission, and reduced stem and leaf dry matter production at the end of the growing season. Further development of the model to reduce predictiveuncertainty is discussed. [ABSTRACT FROM AUTHOR]

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 afive trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45 deg. N 89 deg. W) 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 Mu1 1-1 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 byelevated 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 withelevated 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 wh [ABSTRACT FROM AUTHOR]

To determine whether elevated CO2 reduces or exacerbates the detrimental effects of O3 on aspen (Populus tremuloides Michx.), aspen clones 216 and 271 (O3 tolerant), and 259(O3 sensitive) were exposed to ambient levels of CO2 and O3 or elevated levels of CO2, O3, or CO2+O3 in the FACTS II (Aspen FACE)experiment, and physiological and molecular responses were measured and compared. Clone 259, the most O3-sensitive clone, showed the greatest amount of visible foliar symptoms as well as significant decreases in chlorophyll, carotenoid, starch, and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) concentrations and transcription levels for the Rubisco small subunit. Generally, the constitutive (basic) transcript levels for phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) and the average antioxidant activities were lower for the ozone sensitive clone 259 as compared to the more tolerant 216 and 271 clones. A significant decrease in chlorophyll a, band total (a+b) concentrations in CO2, O3, andCO2+O3 plants was observed for all clones. Carotenoid concentrations were also significantly lower in all clones; however, CHS transcript levels were not significantly affected, suggesting a possible degradation of carotenoid pigments in O3-stressed plants. Antioxidant activities and PAL and 1-aminocyclopropane-1-carboxylic acid (ACC)-oxidase transcript levels showed a general increase in all O3 treated clones, while remaining low in CO2 and CO2+O3 plants (although not all differences were significant). Our results suggest that the ascorbate-glutathione and phenylpropanoid pathways were activated under ozone stress and suppressed during exposure to elevated CO2.Although CO2+O3 treatment resulted in a slightreduc [ABSTRACT FROM AUTHOR]

Atmospheric chemical composition affects foliar chemical composition, which in turn influences the dynamics of both herbivory and decomposition in ecosystems. We assessed the independent and interactive effects of CO2 and O3 fumigation on foliar chemistry of quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera) at a Free-Air CO2 Enrichment (FACE) facility in northern Wisconsin. Leaf samples were collected at five time periods during a single growing season, and analyzed for nitrogen, starch and condensed tannin concentrations, nitrogen resorption efficiencies (NREs), and C:N ratios. Enriched CO2 reduced foliar nitrogen concentrations in aspen and birch; O3 only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with O3 exposure in birch, and thisdecline was ameliorated by enriched CO2. C:N ratios of abscised leaves increased in response to enriched CO2 in both tree species. O3 did not significantly alter C:N ratios in aspen, although values tended to be higher in +CO2+O3 leaves. For birch, O3 decreased C:N ratios underambient CO2 and increased C:N ratios under elevated CO2. Thus, under the combined pollutants, the C:N ratios of both aspen and birch leaves were elevated above the averaged responses to the individual and independent trace gas treatments. Starch concentrations were largely unresponsive to CO2 and O3treatments in aspen, but increased in response to elevated CO2 in birch. Levels of condensed tannins were negligibly affected by CO2 and O3 treatments in aspen, but increased in response to enriched CO2 in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO2 are likely to impact herbivory and decomposition [ABSTRACT FROM AUTHOR]

Seasonal patterns of photosynthates export were followed on 1- and 2-year-old trees of four Populus clones, belonging to the species Populus trichocarpa Torr. & Gray, Populus deltoides Bartr., and their interspecific hybrids grown in the field under short rotation. Differenttypes of branches (including sylleptic and proleptic) and cohorts ofleaves on the main stem were exposed to 14CO2 at monthly and bimonthly intervals in the first and second growing season. Patterns of photosynthates export were influenced by phenology and differed markedly among clones. Differences of translocation patterns were also found among various portions of the crown. Sylleptic branches exported carbon mainly to the lower stem and the roots, whereas main stem leaves, before bud set, contributed much to the height growth of the tree. Sylleptic branches also had greater translocation efficiency than proleptic, contributing more, on a per unit mass basis, to the growth of the tree. Within branches of the same order and among branches of different order, export of assimilates followed patterns similar to those found in the main stem. There was little exportof assimilates between adjacent branches of the same order or between branches and main stem leaves. Clones 11-11 and 1-12, which had thehighest number of sylleptic branches, were also the most productive clones, respectively, for the hybrids and the parental species. [ABSTRACT FROM AUTHOR]

Abstract unavailable [ABSTRACT FROM AUTHOR]