Growth of Eastern Cottonwoods (Populus deltoides) in elevated [CO2] stimulates stand-level respiration and rhizodeposition of carbohydrates, accelerates soil nutrient depletion, yet stimulates above- and belowground biomass production

We took advantage of the distinctive system‐level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO₂] on carbon flux dynamics, above‐ and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO₂] in carbon and N‐replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO₂ exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO₂ exchange under different scales in space and time. Elevated [CO₂] significantly stimulated whole‐system maximum net CO₂ influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4‐year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO₂] treatments. After 2 years of growth at elevated [CO₂], early season stand respiration was decoupled from CO₂ influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO₂] treatments. Soil respiration was stimulated by elevated [CO₂] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate‐induced respiration in vitro. Elevated [CO₂] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of above‐ and belowground biomass production by elevated [CO₂] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.     

Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

Simulated mechanisms of soil N feedback on the forest CO2 response

An improved understanding of the response of forest ecosystems to elevated levels of CO₂ in the atmosphere is crucial because atmospheric CO₂ concentration continues to increase at an accelerating rate and forests are an important sink in the global carbon cycle. Several CO₂‐enrichment experiments have now been running for more than 10 years, with highly variable short‐term results after the first decade. Responses to rising [CO₂] over the next few decades will depend on several plant and ecosystem feedbacks that are inadequately understood. In this study, we conduct a sensitivity analysis, within the context of the simulated CO₂ response, using a new version of the G'DAY ecosystem model, with an improved decomposition submodel, applied to a nitrogen‐limited Norway spruce forest site in the north of Sweden. The new decomposition model incorporates important modifications to soil processes, including some that constitute negative feedbacks on an ecosystem's growth response to elevated [CO₂]. The sensitivity analysis reveals key parameters and processes that are important for the simulated CO₂ response on the short term and others that are more important on the long term. A process that has a strong impact on the short‐term response is a change in decomposer composition, potentially in response to altered litter quality. Parameters that become increasingly important in the long term are carbon allocation to root exudates that are directly or indirectly associated with atmospheric N₂ fixation, and the rate of humification of soil organic matter. We identify factors intrinsic to species and site (microbes and resources) and ecosystem nutrient supply that determine the duration of the enhanced simulated growth response to elevated [CO₂].     

Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO₂ altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO₂, and initial soil mineralization rates of 45 and 348 μg N g^?1 d^?1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO₂ and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO₂, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO₂–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO₂, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.     

Elevated CO2 impacts ectomycorrhiza-mediated forest soil carbon flow: fungal biomass production,respiration and exudation

Large quantities of carbon are exchanged between terrestrial ecosystems and the atmosphere, and extensive research efforts are made to understand carbon cycling and the impact of elevated atmospheric CO₂ levels. The response of soils to increased carbon availability is largely driven by root associated ectomycorrhizal fungi in forest ecosystems, since they partition host derived carbon belowground. In this review I examine how CO₂ enrichment affects ectomycorrhizal fungal biomass production, exudation, respiration, soil carbon fluxes, and other soil microbes, and the importance of the fungal species in these responses. I briefly discuss the significance of CO₂ alterations in the mycorrhizal symbiosis in the context of consequences for carbon sequestration, and present research priorities.     

Leaf senescence and decline of end-of-season gas exchange in five temperate deciduous tree species grown in elevated CO2 concentrations

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO₂ concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO₂] generally had smaller canopies than their current [CO₂]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO₂] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO₂] counterparts by midway through the senescent period. CO₂ enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO₂]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO₂] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO₂ enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO₂] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO₂-enriched forests of the future.     

Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3

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.     

Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

The dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO₂ and nitrogen (N) fertilization are important aspects of long‐term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular‐level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0–15 cm) from the Duke forest free air CO₂ enrichment (FACE) experiment. We measured microbial responses to elevated CO₂ and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO₂ were observed at the molecular‐level by two degradation parameters of plant‐derived steroids and cutin‐derived compounds. The ratios of fungal to bacterial PLFAs and Gram‐negative to Gram‐positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin‐derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. ¹H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf‐derived alkyl structures with both elevated CO₂ and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO₂ and N fertilization, which led to the enrichment of plant‐derived recalcitrant structures (such as alkyl carbon) in the soil.     

Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure

Increased levels of atmospheric CO₂ may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO₂ influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L^–1) and elevated (700 μL L^–1) atmospheric CO₂. For both individually and competitively grown paper birch seedlings, there was a greater CO₂ growth enhancement for seedlings watered less frequently than for well-watered seedlings. This differential change in CO₂ responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO₂ only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO₂ reduced this sensitivity. Additionally, elevated CO₂ improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO₂ may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO₂ levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.     

Soil respiration in northern forests exposed to elevated atmospheric carbon dioxide and ozone

The aspen free-air CO₂ and O₃ enrichment (FACTS II–FACE) study in Rhinelander, Wisconsin, USA, is designed to understand the mechanisms by which young northern deciduous forest ecosystems respond to elevated atmospheric carbon dioxide (CO₂) and elevated tropospheric ozone (O₃) in a replicated, factorial, field experiment. Soil respiration is the second largest flux of carbon (C) in these ecosystems, and the objective of this study was to understand how soil respiration responded to the experimental treatments as these fast-growing stands of pure aspen and birch + aspen approached maximum leaf area. Rates of soil respiration were typically lowest in the elevated O₃ treatment. Elevated CO₂ significantly stimulated soil respiration (8–26%) compared to the control treatment in both community types over all three growing seasons. In years 6–7 of the experiment, the greatest rates of soil respiration occurred in the interaction treatment (CO₂ + O₃), and rates of soil respiration were 15–25% greater in this treatment than in the elevated CO₂ treatment, depending on year and community type. Two of the treatments, elevated CO₂ and elevated CO₂ + O₃, were fumigated with ¹³C-depleted CO₂, and in these two treatments we used standard isotope mixing models to understand the proportions of new and old C in soil respiration. During the peak of the growing season, C fixed since the initiation of the experiment in 1998 (new C) accounted for 60–80% of total soil respiration. The isotope measurements independently confirmed that more new C was respired from the interaction treatment compared to the elevated CO₂ treatment. A period of low soil moisture late in the 2003 growing season resulted in soil respiration with an isotopic signature 4–6‰ enriched in ¹³C compared to sample dates when the percentage soil moisture was higher. In 2004, an extended period of low soil moisture during August and early September, punctuated by a significant rainfall event, resulted in soil respiration that was temporarily 4–6‰ more depleted in ¹³C. Up to 50% of the Earth’s forests will see elevated concentrations of both CO₂ and O₃ in the coming decades and these interacting atmospheric trace gases stimulated soil respiration in this study.     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.     

Long-term Effects of Free Air CO2 Enrichment (FACE) on Soil Respiration

Emissions of CO₂ from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO₂ to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO₂ concentration affects soil CO₂ dynamics over 7 years of continuous enrichment. Soil respiration, soil CO₂ concentrations, and the isotopic signature of soil CO₂ were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO₂ efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO₂ enrichment effect on soil CO₂ efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO₂ concentrations and depletion in the δ¹³C of soil CO₂ at all but the shallowest soil depths.     

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short‐term direct and long‐term indirect effects of elevated CO₂ on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol^?1 atmospheric CO₂ using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol^?1 increase in CO₂ reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO₂ concentration under which the trees were grown. Growth under elevated CO₂ did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q₁₀, R₀). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO₂ concentration, although leaf construction respiration decreased at elevated CO₂ in both species for leaves at the top of the canopy. We conclude that elevated CO₂ has little effect on leaf tissue respiration, and that the influence of elevated CO₂ on plant respiratory carbon flux is primarily through increased biomass.     

The effects of elevated atmospheric CO2 on the amount and depth distribution of plant water uptake in a California annual grassland

Soil moisture profiles can affect species composition and ecosystem processes, but the effects of increased concentrations of atmospheric carbon dioxide ([CO₂]) on the vertical distribution of plant water uptake have not been studied. Because plant species composition affects soil moisture profiles, and is likely to shift under elevated [CO₂], it is also important to test whether the indirect effects of [CO₂] on soil water content may depend on species composition. We examined the effects of elevated [CO₂] and species composition on soil moisture profiles in an annual grassland of California. We grew monocultures and a mixture of Avena barbata and Hemizonia congesta– the dominant species of two phenological groups – in microcosms exposed to ambient (～370 μmol mol^?1) and elevated (～700 μmol mol^?1) [CO₂]. Both species increased intrinsic and yield‐based water use efficiency under elevated [CO₂], but soil moisture increased only in communities with A. barbata, the dominant early‐season annual grass. In A. barbata monocultures, the [CO₂] treatment did not affect the depth distribution of soil water loss. In contrast to communities with A. barbata, monocultures of H. congesta, a late‐season annual forb, did not conserve water under elevated [CO₂], reflecting the increased growth of these plants. In late spring, elevated [CO₂] also increased the efficiency of deep roots in H. congesta monocultures. Under ambient [CO₂], roots below 60 cm accounted for 22% of total root biomass and were associated with 9% of total water loss, whereas in elevated [CO₂], 16% of total belowground biomass was associated with 34% of total water loss. Both soil moisture and isotope data showed that H. congesta monocultures grown under elevated [CO₂] began extracting water from deep soils 2 weeks earlier than plants in ambient [CO₂].     

Carbon fluxes in the rhizosphere of sweet chestnut seedlings (Castanea sativa) grown under two atmospheric CO2 concentrations: 14C partitioning after pulse labelling

Partitioning of ¹⁴C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO₂] environments during two vegetative cycles. The seedlings were exposed to ¹⁴CO₂ atmosphere in both high and low [CO₂] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to ¹⁴CO₂, the plants were harvested, their dry mass and the radioactivity were evaluated. ¹⁴C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO₂]. When measuring total respiration, i.e. CO₂ released from the root/soil system, it is difficult to separate CO₂ originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO₂ concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO₂] stayed equally distributed between these increased root activities. ei]H Lambers     

Plant responses to atmospheric carbon dioxide enrichment: interactions with some soil and atmospheric conditions

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration. Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels. Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation. Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.     

Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem

We investigated the effects of elevated atmospheric CO₂ concentrations (ambient + 200 ppm) on fine root production and soil carbon dynamics in a loblolly pine (Pinus taeda) forest subject to free‐air CO₂ enrichment (FACE) near Durham, NC (USA). Live fine root mass (LFR) showed less seasonal variation than dead fine root mass (DFR), which was correlated with seasonal changes in soil moisture and soil temperature. LFR mass increased significantly (by 86%) in the elevated CO₂ treatment, with an increment of 37 g(dry weight) m^?2 above the control plots after two years of CO₂ fumigation. There was no long‐term increment in DFR associated with elevated CO₂, but significant seasonal accumulations of DFR mass occurred during the summer of the second year of fumigation. Overall, root net primary production (RNPP) was not significantly different, but annual carbon inputs were 21.7 gC m^?2 y^?1 (68%) higher in the elevated CO₂ treatment compared to controls. Specific root respiration was not altered by the CO₂ treatment during most of the year; however, it was significantly higher by 21% and 13% in September 1997 and May 1998, respectively, in elevated CO₂. We did not find statistically significant differences in the C/N ratio of the root tissue, root decomposition or phosphatase activity in soil and roots associated with the treatment. Our data show that the early response of a loblolly pine forest ecosystem subject to CO₂ enrichment is an increase in its fine root population and a trend towards higher total RNPP after two years of CO₂ fumigation.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

System-level adjustments to elevated CO2 in model spruce ecosystems

Atmospheric carbon dioxide enrichment and increasing nitrogen deposition are often predicted to increase forest productivity based on currently available data for isolated forest tree seedlings or their leaves. However, it is highly uncertain whether such seedling responses will scale to the stand level. Therefore, we studied the effects of increasing CO₂ (280, 420 and 560 μL L^-1) and increasing rates of wet N deposition (0, 30 and 90 kg ha^-1 y^-1) on whole stands of 4-year-old spruce trees (Picea abies). One tree from each of six clones, together with two herbaceous understory species, were established in each of nine 0.7 m² model ecosystems in nutrient poor forest soil and grown in a simulated montane climate for two years. Shoot level light-saturated net photosynthesis measured at growth CO₂ concentrations increased with increasing CO₂, as well as with increasing N deposition. However, predawn shoot respiration was unaffected by treatments. When measured at a common CO₂ concentration of 420 μL L^-1 37% down-regulation of photosynthesis was observed in plants grown at 560 μL CO₂ L^-1. Length growth of shoots and stem diameter were not affected by CO₂ or N deposition. Bud burst was delayed, leaf area index (LAI) was lower, needle litter fall increased and soil CO₂ efflux increased with increasing CO₂. N deposition had no effect on these traits. At the ecosystem level the rate of net CO₂ exchange was not significantly different between CO₂ and N treatments. Most of the responses to CO₂ studied here were nonlinear with the most significant differences between 280 and 420 μL CO₂ L^-1 and relatively small changes between 420 and 560 μL CO₂ L^-1. Our results suggest that the lack of above-ground growth responses to elevated CO₂ is due to the combined effects of physiological down-regulation of photosynthesis at the leaf level, allometric adjustment at the canopy level (reduced LAI), and increasing strength of below-ground carbon sinks. The non-linearity of treatment effects further suggests that major responses of coniferous forests to atmospheric CO₂ enrichment might already be under way and that future responses may be comparatively smaller.     

No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO2 enrichment

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100‐year‐old mixed deciduous forest. The use of ¹³C‐depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ～550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂‐exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three‐way interaction). During wet periods, soil water savings under CO₂‐enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was ≤40% and concurrent soil temperature was high (>15 °C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.