Below-ground responses of silver birch trees exposed to elevated CO2 and O3 levels during three growing seasons

Field‐growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO₂ and O₃ in open‐top chambers for three consecutive growing seasons (1999–2001). At the beginning of the OTC experiment, all trees were 7 years old. We studied the single and interaction effects of CO₂ and O₃ on silver birch below‐ground carbon pools (i.e. effects on fine roots and mycorrhizas, soil microbial communities and sporocarp production) and also assessed whether there are any clonal differences in these below‐ground CO₂ and O₃ responses. The total mycorrhizal infection level of both clones was stimulated by elevated CO₂ alone and elevated O₃ alone, but not when elevated CO₂ was used in fumigation in combination with elevated O₃. In both clones, elevated CO₂ affected negatively light brown/orange mycorrhizas, while its effect on other mycorrhizal morphotypes was negligible. Elevated O₃, instead, clearly decreased the proportions of black and liver‐brown mycorrhizas and increased that of light brown/orange mycorrhizas. Elevated O₃ had a tendency to decrease standing fine root mass and sporocarp production as well, both of these O₃ effects mainly manifesting in clone 4 trees. CO₂ and O₃ treatment effects on soil microbial community composition (PLFA, 2‐ and 3‐OH‐FA profiles) were negligible, but quantitative PLFA data showed that in 2001 the PLFA fungi : bacteria‐ratio of clone 80 trees was marginally increased because of elevated CO₂ treatments. This study shows that O₃ effects were most clearly visible at the mycorrhizal root level and that some clonal differences in CO₂ and O₃ responses were observable in the below‐ground carbon pools. In conclusion, the present data suggests that CO₂ effects were minor, whereas increasing tropospheric O₃ levels can be an important stress factor in northern birch forests, as they might alter mycorrhizal morphotype assemblages, mycorrhizal infection rates and sporocarp production.     

Wood properties of two silver birch clones exposed to elevated CO2 and O3

Effects of elevated carbon dioxide (CO₂) and ozone (O₃) on wood properties of two initially 7‐year‐old silver birch (Betula pendula Roth) clones were studied after a fumigation during three growing seasons. Forty trees, representing two fast‐growing clones (4 and 80), were exposed in open‐top chambers to the following treatments: outside control, chamber control, 2 × ambient [CO₂], 2 × ambient [O₃] and 2 × ambient [CO₂]+2 × ambient [O₃]. After the 3‐year exposure, the trees were felled and wood properties were analyzed. The treatments affected both stem wood structure and chemistry. Elevated [CO₂] increased annual ring width, and concentrations of extractives and starch, and decreased concentrations of cellulose and gravimetric lignin. Elevated O₃ decreased vessel percentage and increased cell wall percentage in clone 80. In vessel percentage, elevated CO₂ ameliorated the O₃‐induced decrease. In clone 4, elevated O₃ decreased nitrogen concentration of wood. The two clones had different wood properties. In clone 4, the concentrations of extractives, starch, soluble sugars and nitrogen were greater than in clone 80, while in clone 80 the concentrations of cellulose and acid‐soluble lignin were higher. Clone 4 also had slightly longer fibres, greater vessel lumen diameter and vessel percentage than clone 80, while in clone 80 cell wall percentage was greater. Our results show that wood properties of young silver birch trees were altered under elevated CO₂ in both clones, whereas the effects of O₃ depended on clone.     

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O₃) regimes, two concentrations of CO₂, and a combination of O₃ and CO₂ treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O₃ concentrations led to a significant decline in the CO₂ compensation point (Г^*), maximum RuP₂-saturated rate of carboxylation (V_cmax), maximum rate of electron transport (J_max), maximum stomatal conductance (g_smax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (?g_s/?D_v) in both shoot-age classes. However, the effect of elevated O₃ concentrations on the respiration rate in light (R_d) was dependent on shoot age. Elevated CO₂(700 μmol mol^?1) significantly decreased J_max and g_smax but increased R_d in 1-year-old shoots and the ?g_s/?D_v in both shoot-age classes. The interactive effects of O₃ and CO₂ on some key parameters (e.g. V_cmax and J_max) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO₂. As a consequence, the injury induced by O₃ was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O₃ concentration, reduced O₃ concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O₃ concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.     

Response of small birch plants (Betula pendula Roth.) to elevated CO2 and nitrogen supply

Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol^-1) or elevated (700 μmol mol^-1) concentrations of CO₂. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO₂ concentration at sub-optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO₂. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N-concentration. At all N-supply rates, elevated CO₂ resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO₂. Specific leaf area (and leaf area ratio) was less at higher CO₂ and at lower rates of N-supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO₂. At sub-optimal N-supply, the higher net assimilation rate at elevated CO₂ was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N-supply. In the treatment with lowest N-supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N-supplies was greater at elevated CO₂. These responses of the root system to lower N-supply and elevated CO₂ may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO₂. The advantage of maintaining steady-state nutrition in small plants while investigating the effects of elevated CO₂ on growth is emphasized.     

Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3

Litter decay dynamics of paper birch (Betula papyrifera) were assessed at the Aspen free‐air CO₂ enrichment (FACE) facility in northern Wisconsin, USA. Leaf litter was decomposed for 12 months under factorial combinations of 360 vs. 560 μL CO₂ L^?1, crossed with 36 vs. 55 nL O₃ L^?1. To differentiate between substrate quality and environment effects, litterbags were placed in their Native Plots of origin or transplanted into the other treatments. CO₂ enrichment, regardless of O₃ concentration, produced poorer quality litter (high C/N, lignin/N and condensed tannins) than did ambient CO₂ (low C/N, lignin/N and condensed tannins). Substrate quality differences were reflected in the mass loss rates (k‐values), which were high for litter generated under ambient CO₂ (0.887 year^?1) and low for litter generated under elevated CO₂ (0.674 year^?1). The rate‐retarding effects of CO₂ enrichment were neither alleviated nor exacerbated by O₃ exposure. Decay rates varied, however, depending on whether litter was placed back into its plot of origin or transplanted to Common Gardens. The results of this study are species specific, but they have important implications for understanding the processes regulating storage of fixed C and the release of CO₂ from northern forest ecosystems.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf 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 (l_s) 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 l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a 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 C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

The influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Young Scots pine trees naturally established at a pine heath were exposed to two concentrations of CO₂ (ambient and doubled ambient) and two O₃ regimes (ambient and doubled ambient) and their combination in open-top field chambers during growing seasons 1994, 1995 and 1996 (late May to 15 September). Filtered ozone treatment and chamberless control trees were also included in the treatment comparisons. Root ingrowth cores were inserted to the undisturbed soil below the branch projection of each tree at the beginning of the fumigation period in 1994 and were harvested at the end of the fumigation periods in 1995 and 1996. Root biomasses were determined from different soil layers in the ingrowth cores, and the infection levels of different mycorrhizal types were calculated. Elevated O₃ and CO₂ did not have significant effects on the biomass production of Scots pine coarse (Ø > 2 mm) or fine roots (Ø < 2 mm) and roots of grasses and dwarf shrubs. Elevated O₃ caused a transient stimulation, observable in 1995, in the proportion of tuber-like mycorrhizas, total mycorrhizas and total short roots but this stimulation disappeared during the last study year. Elevated CO₂ did not enhance carbon allocation to root growth or mycorrhiza formation, although a diminishing trend in the mycorrhiza formation was observed. In the combination treatment increased CO₂ inhibited the transient stimulating effect of ozone, and a significant increase of old mycorrhizas was observed. Our conclusion is that doubled CO₂ is not able to increase carbon allocation to growth of fine roots or mycorrhizas in nutrient poor forest sites and realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.     

Growth responses of yellow-poplar(Liriodendron tulipifera L.) exposed to 5 years of O3 aloneor combined with elevated CO2

Field‐grown yellow‐poplar (Liriodendron tulipifera L.) werefumigated from May to October in 1992–96 within open‐topchambers to determine the impact of ozone (O₃) aloneor combined with elevated carbon dioxide (CO₂) on saplinggrowth. Treatments were replicated three times and included: charcoal‐filteredair (CF); 1 × ambient ozone (1 × O₃);1·5 × ambient ozone (1·5 × O₃);1·5 × ambient ozone plus 350 p.p.m.carbon dioxide (1·5 × O₃ + CO₂)(target of 700 p.p.m. CO₂); and open‐air chamberlessplot (OA). After five seasons, the total cumulative O₃ exposure (SUM₀₀ = sumof hourly O₃ concentrations during the study) rangedfrom 145 (CF) to 861 (1·5 × O₃) p.p.m. × h (partsper million hour). Ozone had no statistically significant effecton yellow‐poplar growth or biomass, even though total root biomasswas reduced by 13% in the 1·5 × O₃‐exposedsaplings relative to CF controls. Although exposure to 1·5 × O₃ + CO₂ hada stimulatory effect on yearly basal area growth increment aftertwo seasons, significant increases in shoot and root biomass (～ 60% increaserelative to all others) were not detected until the fifth season.After five seasons, the yearly basal area growth increment of saplingsexposed to 1·5 × O₃ + CO₂‐air increasedby 41% relative to all others. Based on this multi‐yearstudy, it appears that chronic O₃ effects on yellow‐poplargrowth are limited and slow to manifest, and are consistent withprevious studies that show yellow‐poplar growth is not highly responsiveto O₃ exposure. In addition, these results show thatenriched CO₂ may ameliorate the negative effects of elevatedO₃ on yellow‐poplar shoot growth and root biomass underfield conditions.     

Effects of elevated atmospheric CO2 on net ecosystem CO2 exchange of a scrub–oak ecosystem

We report the results of a 2‐year study of effects of the elevated (current ambient plus 350 μmol CO₂ mol^?1) atmospheric CO₂ concentration (C_a) on net ecosystem CO₂ exchange (NEE) of a scrub–oak ecosystem. The measurements were made in open‐top chambers (OTCs) modified to function as open gas‐exchange systems. The OTCs enclosed samples of the ecosystem (ca. 10 m² surface area) that had regenerated after a fire, 5 years before, in either current ambient or elevated C_a. Throughout the study, elevated C_a increased maximum NEE (NEE_max) and the apparent quantum yield of the NEE (φ_NEE) during the photoperiod. The magnitude of the stimulation of NEE_max, expressed per unit ground area, was seasonal, rising from 50% in the winter to 180% in the summer. The key to this stimulation was effects of elevated C_a, and their interaction with the seasonal changes in the environment, on ecosystem leaf area index, photosynthesis and respiration. The separation of these factors was difficult. When expressed per unit leaf area the stimulation of the NEE_max ranged from 7% to 60%, with the increase being dependent on increasing soil water content (W_soil). At night, the CO₂ effluxes from the ecosystem (NEE_night) were on an average 39% higher in elevated C_a. However, the increase varied between 6% and 64%, and had no clear seasonality. The partitioning of NEE_night into its belowground (R_below) and aboveground (R_above) components was carried out in the winter only. A 35% and 27% stimulation of NEE_night in December 1999 and 2000, respectively, was largely due to a 26% and 28% stimulation of R_below in the respective periods, because R_below constituted ca. 87% of NEE_night. The 37% and 42% stimulation of R_above in December 1999 and 2000, respectively, was less than the 65% and 80% stimulation of the aboveground biomass by elevated C_a at these times. An increase in the relative amount of the aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species Quercus myrtifolia in elevated C_a, was responsible for this effect. Throughout this study, elevated C_a had a greater effect on carbon uptake than on carbon loss, in terms of both the absolute flux and relative stimulation. Consequently, for this scrub–oak ecosystem carbon sequestration was greater in the elevated C_a during this 2‐year study period.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Acclimation of photosynthesis and respiration to elevated atmospheric CO2 in two Scrub Oaks

Abstract For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf‐Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol^?1) C_a, in open‐top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 µmol mol^?1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a—the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP‐saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.     

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3- enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open‐top chambers over 3 years (age 7–9 years). The treatments were increased CO₂ concentration (+CO₂, 72 Pa), increased O₃ concentration (+O₃, 2 × ambient O₃ with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO₂ + O₃). Thirty‐seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer‐operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b₆f, Rubisco specificity and K_m (CO₂) were not influenced by treatments. Net photosynthetic rate responded to +CO₂ with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO₂ in clones K1659 and V5952, respectively. The elevated O₃ caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O₃ had less effect in the presence of +CO₂ than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O₃ treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.     

Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth) and the feeding behaviour of the weevil Phyllobius maculicornis

Abstract 1 The effect of elevated CO₂ and temperature on the foliar chemistry Betula pendula Roth and the feeding performance of polyphagous weevils Phyllobius maculicornis Germ. was studied. Birch seedlings were grown during one growing season in chamber‐less field conditions and in closed‐top chambers exposed to four different treatments: ambient CO₂ (350 p.p.m) and temperature, elevated atmospheric CO₂ (700 p.p.m) and ambient temperature, elevated temperature +3 °C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. 2 In leaves under CO₂ enrichment, the concentration of nitrogen and some flavonol glycosides significantly decreased, whereas the concentration of total phenolics, condensed tannins and (+)‐catechin significantly increased. The total concentration of cinnamoylquinic acids was significantly increased by CO₂ and decreased by temperature. The concentration of salidroside increased under elevated temperature. 3 Weevil‐feeding experiments were carried out in a five‐choice arrangement, one leaf from each of the five treatments (chamber‐less field controls and four different treatments in chambers) being placed in random order in a plastic box. The weevils preferred the leaves grown under elevated CO₂, which had low nitrogen, high phenolics and the highest condensed tannin concentrations. Whether the reason for this trend is due to the stimulating effect of condensed tannins and/or a change in other secondary compounds, remains unknown. The weevils did not show any obviously different response in feeding performance to temperature and the combination of elevated CO₂ and temperature.     

Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3

Increases in atmospheric CO₂ and tropospheric O₃ may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free‐air CO₂–O₃ enrichment experiment, plant growth has been stimulated by elevated CO₂ resulting in greater substrate input to soil; elevated O₃ has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO₂, and that CO₂ effects would be dampened by O₃. We found that elevated CO₂ did not alter gross N transformation rates. Elevated O₃ significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO₂ and O₃: (i) gross N mineralization was greater under elevated CO₂ (1.0 mg N kg^?1 day^?1) than in the presence of both CO₂ and O₃ (0.5 mg N kg^?1 day^?1) and (ii) gross NH₄⁺ immobilization was also greater under elevated CO₂ (0.8 mg N kg^?1 day^?1) than under CO₂ plus O₃ (0.4 mg N kg^?1 day^?1). We used a laboratory ¹⁵N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO₂ led to greater recovery of NH₄⁺‐¹⁵N in microbial biomass and corresponding lower recovery in the extractable NO₃^? pool. Elevated CO₂ resulted in a substantial increase in NO₃^?‐¹⁵N recovery in soil organic matter. We observed no O₃ main effect and no CO₂ by O₃ interaction effect on ¹⁵N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO₂ has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre‐existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity.     

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.     

Root exudation (net efflux of amino acids) may increase rhizodeposition under elevated CO2

Increases in atmospheric CO₂ concentration ([CO₂]) can lead to global climate change and theoretically could enhance carbon (C) deposition in soil, but data on this complex issue are contradictory. One approach for clarifying the diverse forces influencing plant‐derived C in the rhizosphere involves defining how elevated [CO₂] alters the fundamental process of C transfer from plant roots to the soil. We examine here how a step increase in [CO₂] affects the innate influx and efflux components of root exudation in axenic plants, as one foundation for understanding how climate change may affect rhizodeposition. Increasing [CO₂] from 425 to 850 μmol mol^?1 during short‐term trials enhanced shoot and root dry weight (P<0.01) of annual rye grass (Lolium multiflorum Lam.) and medic (Medicago truncatula L.) but had no effect on growth of maize (Zea mays L.). Root amino‐acid flux in the same plants changed only in maize, which increased the efflux rate (nmol g root fresh weight^?1 h^?1) of six amino acids (arginine, alanine, proline, tyrosine, lysine and leucine) significantly (P<0.05) under elevated [CO₂]. None of the three plant species altered the steady‐state concentration of 16 amino acids released into a hydroponic solution with changing [CO₂], apparently because amino‐acid influx rates, measured at 2.5 μm , consistently exceeded efflux rates. Indeed, plants recovered amino acids at rates 94–374% higher than they were lost from roots regardless of [CO₂]. These results indicate that, in theory, any effect of [CO₂] doubling on amino‐acid efflux can be offset by innately higher rates of influx. In practice, however, higher rates of amino‐acid cycling (i.e., efflux+influx) for each root segment (in C₄ maize) or from more root tissue (in the two C₃ species) should increase root exudation by plants exposed to elevated [CO₂] as additional amino acids would be adsorbed to soil particles or be taken up by soil microorganisms.