In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.     

Carbon and water fluxes in a calcareous grassland under elevated CO2

1. As part of a long-term study of the effects of elevated CO₂ on biodiversity and ecosystem function in a calcareous grassland, we measured ecosystem carbon dioxide and water-vapour fluxes over 24-h periods during the 1994 and 1995 growing seasons. Data were used to derive CO₂ and H₂O gas-exchange response functions to quantum flux density (QFD).
2. The relative increase in net ecosystem CO₂ flux (NEC) owing to CO₂ enrichment increased as QFD rose. Daytime NEC at high QFD under elevated CO₂ increased by 25% to 60%, with the greatest increases in the spring and after mowing in June when above-ground biomass was lowest. There was much less stimulation of NEC in early June and again in October when the canopy was fully developed. Night-time NEC was not significantly altered under elevated CO₂.
3. Short-term reversal of CO₂ concentrations between treatments after two seasons of CO₂ exposure provided evidence for a 50% downward adjustment of NEC expressed per unit above-ground plant dry weight. However, when expressed on a land area basis, this difference disappeared because of a c. 20% increase in above-ground biomass under elevated CO₂.
4. Ecosystem evapotranspiration (ET) was not significantly altered by elevated CO₂ when averaged over all measurement dates and positions. However, ET was reduced 3–18% at high QFD in plots at the top of the slope at our study site. In summary, CO₂ enrichment resulted in a large stimulation of ecosystem CO₂ capture, especially during periods of a large demand of carbon in relationship to its supply, and resulted in a relatively small and variable effect on ecosystem water consumption.     

Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub-oak ecosystem

This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO₂ (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO₂ caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO₂ was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia , while Quercus geminata , the other co-dominant oak, displayed no significant CO₂ response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO₂ interaction was partially masked at the community level by species-specific responses: elevated CO₂ had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO₂ concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO₂ may have been caused by other factors such as nutrient limitation and disturbance.     

The density dependence of plant responses to elevated CO2

1 Stands of the annual Brassica kaber were grown at a range of six densities in both ambient and elevated CO₂ environments, and measurements of shoot growth were made from seedling emergence through to reproduction.
2 Early in stand development (21 days following emergence), CO₂ enhancement (β) for above-ground biomass was highly density-dependent, ranging from 1.41 at the lowest density (20 plants m−2) to 0.59 at the highest density (652 plants m−2).
3 As stands matured and total biomass exceeded a relatively low threshold level (<10.0 g m^{−2; c.}  20% of final yield), the density-dependence of β disappeared. Above this shoot biomass threshold, β-values remained remarkably stable (β = 0.34) across a broad range of stand biomass, independent of a stand's initial density or age.
4 Average stand-level reproductive β-values at a final harvest were very similar to biomass values (β = 0.38) and, as with biomass values at later stages, showed no apparent density-dependence.
5 These results highlight the importance of considering density and the time-course of stand development simultaneously when assessing the potential for CO₂-induced growth enhancements in plants.     

Root competition and elevated CO2: effects on seedling growth in Linum usitatissimum populations and Linum–Silene cretica mixtures

1. Root competition can be an important determinant of the performance of neighbours within plant populations and communities. Because plants often maintain larger root systems and allocate more of their carbon to root systems under elevated atmospheric CO₂ than they do at lower CO₂ concentrations, root–root interactions could play an increasingly important role in determining competitive outcomes among individuals and plant species as global CO₂ concentration continues to rise.
2. We established 12 pure stands of Linum usitatissimum (flax) and 12 mixed stands of Linum and its naturally co-occurring weed species Silene cretica in opaque plastic trays each filled with the same amount of nutrient-rich soil mix. In half of the trays from each of these stand types, vertical waterproof partitions separated the root systems of individual plants from each other to prevent root competition, while in the other half no partitions were present. Half of the trays from all treatments were allowed to grow under low atmospheric CO₂ concentration (320μll^–1) and the other half under elevated CO₂ (600μll^–1), in daylight growth chambers for 30 days from seedling emergence until harvest in mid-June. All trays received equal amounts of water so that soils in the low CO₂ treatment were maintained at field capacity.
3. Our results indicate that under high soil fertilities: (1) intra-specific root–root interactions alone play a relatively insignificant role in determining plant biomass production within pure Linum populations and (2) the impact of an aggressive species ( Silene ) on co-occurring less aggressive species ( Linum ) becomes more severe under elevated CO₂ as a result of amplified interspecific root competition.     

Effects of Free-Air CO2 Enrichment (FACE) on experimental grassland communities

1. Experimental grassland communities (turves) were exposed to elevated (60 Pa) and ambient (35 Pa) CO₂ partial pressures (pCO₂) in a Free-Air Carbon Dioxide Enrichment (FACE) experiment between 30 March 1995 and 4 July 1996. The vegetation was cut once during the experiment prior to the final harvest (harvest 2).
2. No significant treatment effects on total plant biomass at the whole turf level were detected, although biomass was typically about 25% higher under fumigation in year 1 and about 15% higher in year 2.
3. Biomass for two of the six sown species was significantly higher at harvest 2 than at harvest 1. There were no significant differences between individual species' biomass under the two CO₂ treatments at either harvest 1 or 2 or in terms of overall cumulative biomass. However, in four of the five sown species in both years biomass tended to be higher in the fumigated than in the control rings ( Cerastium holosteiodes, Phleum pratense, Plantago lanceolata and Poa trivialis ). In contrast, Lolium perenne showed increased biomass under the control treatment relative to the fumigated treatment in both years. Owing to the high variance both within and between rings for each of the two treatments the statistical power of most, but not all, of the analyses carried out was poor.
4. The relative proportions of each species in the turves under fumigated and control treatments was broadly similar after the first summer, with differences in the second year being mainly owing to the negative response of L. perenne to CO₂ fumigation.     

The significance of differences in the mechanisms of photosynthetic acclimation to light, nitrogen and CO2 for return on investment in leaves

1. We report changes in photosynthetic capacity of leaves developed in varying photon flux density (PFD), nitrogen supply and CO₂ concentration. We determined the relative effect of these environmental factors on photosynthetic capacity per unit leaf volume as well as the volume of tissue per unit leaf area. We calculated resource-use efficiencies from the photosynthetic capacities and measurements of leaf dry mass, carbohydrates and nitrogen content.
2. There were clear differences between the mechanisms of photosynthetic acclimation to PFD, nitrogen supply and CO₂. PFD primarily affected volume of tissue per unit area whereas nitrogen supply primarily affected photosynthetic capacity per unit volume. CO₂ concentration affected both of these parameters and interacted strongly with the PFD and nitrogen treatments.
3. Photosynthetic capacity per unit carbon invested in leaves increased in the low PFD, high nitrogen and low CO₂ treatments. Photosynthetic capacity per unit nitrogen was significantly affected only by nitrogen supply.
4. The responses to low PFD and low nitrogen appear to function to increase the efficiency of utilization of the limiting resource. However, the responses to elevated CO₂ in the high PFD and high nitrogen treatments suggest that high CO₂ can result in a situation where growth is not limited by either carbon or nitrogen supply. Limitation of growth at elevated CO₂ appears to result from internal plant factors that limit utilization of carbohydrates at sinks and/or transport of carbohydrates to sinks.     

Biomass Production and Carbohydrate Content of Arabidopsis thaliana at Atmospheric CO2 Concentrations from 390 to 1680 μl l-1

Abstract: The concentration dependency of the impact of elevated atmospheric CO₂ concentrations on Arabidopsis thaliana L. was studied. Plants were exposed to nearly ambient (390), 560, 810, 1240 and 1680 μl I^-1 CO₂ during the vegetative growth phase for 8 days. Shoot biomass production and dry matter content were increased upon exposure to elevated CO₂. Maximal increase in shoot fresh and dry weight was obtained at 560 μl I^-1 CU₂, which was due to a transient stimulation of the relative growth rate for up to 3 days. The shoot starch content increased with increasing CO₂ concentrations up to two-fold at 1680 μl I^-1 CO₂, whereas the contents of soluble sugars and phenolic compounds were hardly affected by elevated CO₂. The chlorophyll and carotenoid contents were not substantially affected at elevated CO₂ and the chlorophyll a/b ratio remained unaltered. There was no acclimation of photosynthesis at elevated CO₂; the photosynthetic capacity of leaves, which had completely developed at elevated CO₂ was similar to that of leaves developed in ambient air. The possible consequences of an elevated atmospheric CO₂ concentration to Arabidopsis thaliana in its natural habitat is discussed.     

The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration

Abstract. Herbaceous C₃ plants grown in elevated CO₂ show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO₂-enriched herbaceous plants and decrease for CO₂-enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO₂ show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO₂. Cold-adapted plants show little response to elevated levels of CO₂, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO₂-enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.     

Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype

1. Four Lotus corniculatus genotypes differing in cyanoglycoside and condensed tannin concentrations were grown in either low (350 ppm) or high (700 ppm) atmospheric CO₂ environments. Larval performance, consumption and conversion efficiency of Polyommatus icarus feeding on this plant material were measured.
2. Plants grown under elevated CO₂ contained less cyanoglycosides, more condensed tannins and more starch than control plants. However, water concentration, nitrogen and protein as well as nitrogen concentration in relation to carbon concentration did not differ between CO₂ treatments.
3. The four genotypes differed significantly in condensed tannins, cyanoglucoside, leaf water and leaf nitrogen but no genotype–CO₂ interaction was detected, except for total phenolics and condensed tannins in which two plant genotypes showed stronger increases under elevated CO₂ than the other two.
4. Larvae of P . icarus consumed more plant material and used and converted it more efficiently from plants grown at high atmospheric CO₂.
5. Larvae developed significantly faster and were significantly heavier when fed plant material grown under elevated CO₂. The observed difference in mass disappeared in the pupal and adult stages. However, lipid concentration of adults from the elevated CO₂ treatment was marginally significantly higher than of controls.
6. It is concluded that the higher carbohydrate concentration of L . corniculatus plants grown at elevated CO₂ renders leaves more suitable and better digestible to P . icarus . Furthermore, differences in allelochemicals might influence the palatability of L . corniculatus leaves for this specialist on Fabaceae.     

Elevated CO2 and conifer roots: effects on growth, life span and turnover

Elevated CO₂ increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO₂ changes the proportion of C allocated to root biomass, measured as either the root:shoot ratio or the fine root:needle ratio. Elevated CO₂ tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO₂ on conifer fine-root life span, and there is no clear trend. Elevated CO₂ increases the absolute fine-root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO₂ on relative turnover rates (turnover:biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO₂ can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO₂ concentration initially increases biomass, but this stimulation declines with the response to elevated CO₂ because increases in assimilation are not matched by increases in nutrient supply.     

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO₂ and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO₂ on the contribution of fast- ( r -strategists) and slow-growing ( K -strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO₂ treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25–2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C_mic), the maximum specific growth rate (μ), growing microbial biomass (GMB) and lag-period ( t _lag) were estimated by the kinetics of CO₂ emission from bulk soil and aggregates amended with glucose and nutrients. Although C_org and C_mic were unaffected by elevated CO₂, μ values were significantly higher under elevated than ambient CO₂ for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO₂ treatments. Based on changes in μ, GMB and lag period, we conclude that elevated atmospheric CO₂ stimulated the r- selected microorganisms, especially in soil microaggregates. Such an increase in r -selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO₂ environment.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

The influence of elevated CO2 on species phenology, growth and reproduction in a Mediterranean old-field community

We studied the effects on the phenology, growth and reproduction of 19 Mediterranean species, of elevating the atmospheric CO₂ concentration ([CO₂]) to twice-ambient. Intact monoliths were taken from an old-field and put, during a six month growing season, into growth chambers in which external climatic conditions were mimicked and [CO₂] was regulated. Fruit set time was significantly changed in six species under elevated [CO₂] and leaf and branch senescence accelerated in most species. Grasses had fewer leaves and legumes were more branched at peak production under elevated [CO₂] than under ambient. Plant seed number was not significantly changed under elevated [CO₂], whereas the reproductive effort of grasses was significantly depressed. Reproductive and vegetative characteristics showed related responses to [CO₂], as species with enhanced biomass had a hastened fruit set time, a higher number of fruits per plant and a higher reproductive biomass under elevated [CO₂] than under ambient conditions, while species with depressed biomass had a delayed fruit set time, a lower number of fruits per plant and a lower reproductive biomass. Our results also show a high interspecific variability in [CO₂] response, but some trends emerged at the family level: the production of vegetative and reproductive modules were depressed in grasses and slightly stimulated in legumes.     

Influences of soil volume and an elevated CO2 level on growth and CO2 exchange for the Crassulacean acid metabolism plant Opuntia ficus-indica

Effects of the current (38 Pa) and an elevated (74 Pa) CO₂ partial pressure on root and shoot areas, biomass accumulation and daily net CO₂ exchange were determined for Opuntia ficus-indica (L.) Miller, a highly productive Crassulacean acid metabolism species cultivated worldwide. Plants were grown in environmentally controlled rooms for 18 weeks in pots of three soil volumes (2 600, 6 500 and 26 000 cm³), the smallest of which was intended to restrict root growth. For plants in the medium-sized soil volume, basal cladodes tended to be thicker and areas of main and lateral roots tended to be greater as the CO₂ level was doubled. Daughter cladodes tended to be initiated sooner at the current compared with the elevated CO₂ level but total areas were similar by 10 weeks. At 10 weeks, daily net CO₂ uptake for the three soil volumes averaged 24% higher for plants growing under elevated compared with current CO₂ levels, but at 18 weeks only 3% enhancement in uptake occurred. Dry weight gain was enhanced 24% by elevated CO₂ during the first 10 weeks but only 8% over 18 weeks. Increasing the soil volume 10-fold led to a greater stimulation of daily net CO₂ uptake and biomass production than did doubling the CO₂ level. At 18 weeks, root biomass doubled and shoot biomass nearly doubled as the soil volume was increased 10-fold; the effects of soil volume tended to be greater for elevated CO₂. The amount of cladode nitrogen per unit dry weight decreased as the CO₂ level was raised and increased as soil volume increased, the latter suggesting that the effects of soil volume could be due to nitrogen limitations.     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization and phosphorus inflow in 10 herbaceous species of contrasting growth strategies

1. Ten herbaceous species were grown over a 4-month period under ambient (360 μmol mol^–1) and elevated (610 μmol mol^–1) atmospheric CO₂ conditions. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae and given a phosphorus (P) supply which was not immediately available to the plants.
2. Multiple harvests were taken in order to determine whether the effect of elevated CO₂ on mycorrhizal colonization and phosphorus inflow was independent of its effect on plant growth.
3. All species grew faster under elevated CO₂ and carbon partitioning was altered, generally in favour of the shoots. All species responded similarly to elevated CO₂.
4. Elevated CO₂ did not affect the percentage of root length colonized by AM fungi, but the total amount of colonized root length was increased, because the plants were bigger.
5. Elevated CO₂ increased total P content, but had little or no effect on P concentration. At a given age, P inflow was stimulated by elevated CO₂, but when root length was taken into account the CO₂ effect disappeared.
6. In these host species there is no evidence for a direct effect of elevated CO₂ on mycorrhizal functioning, because both internal mycorrhizal colonization and P inflow are unaffected.
7. Future research should concentrate on the potential for carbon flow to the soil via the external mycelial network.     

Response of Mediterranean coralline algae to ocean acidification and elevated temperature

The effects of elevated partial pressure of CO₂ ( p CO₂) and temperature, alone and in combination, on survival, calcification and dissolution were investigated in the crustose coralline alga Lithophyllum cabiochae . Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 °C) and at ambient [ca. 400 parts per million (ppm)] or elevated p CO₂ (ca. 700 ppm). Algal necroses appeared at the end of summer under elevated temperature first at 700 ppm (60% of the thallus surface) and then at 400 ppm (30%). The death of algae was observed only under elevated temperature and was two- to threefold higher under elevated p CO₂. During the first month of the experiment, net calcification was significantly reduced under elevated p CO₂. At the end of the summer period, net calcification decreased by 50% when both temperature and p CO₂ were elevated while no effect was found under elevated temperature and elevated p CO₂ alone. In autumn and winter, net calcification in healthy algae increased with increasing temperature, independently of the p CO₂ level, while necroses and death in the algal population caused a net dissolution at elevated temperature and p CO₂. The dissolution of dead algal thalli was affected by elevated p CO₂, being two- to fourfold higher than under ambient p CO₂. These results suggest that net dissolution is likely to exceed net calcification in L. cabiochae by the end of this century. This could have major consequences in terms of biodiversity and biogeochemistry in coralligenous communities dominated by these algae.     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.     

Coppicing shifts CO2 stimulation of poplar productivity to above-ground pools: a synthesis of leaf to stand level results from the POP/EUROFACE experiment

A poplar short rotation coppice (SRC) grown for the production of bioenergy can combine carbon (C) storage with fossil fuel substitution. Here, we summarize the responses of a poplar ( Populus ) plantation to 6 yr of free air CO₂ enrichment (POP/EUROFACE consisting of two rotation cycles). We show that a poplar plantation growing in nonlimiting light, nutrient and water conditions will significantly increase its productivity in elevated CO₂ concentrations ([CO₂]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO₂]. Sufficient nutrient availability, increased nitrogen use efficiency (NUE) and the large sink capacity of poplars contributed to the sustained increase in C uptake over 6 yr. Additional C taken up in high [CO₂] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO₂] to above-ground pools, as fine root biomass declined and its [CO₂] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO₂] during the 6 yr experiment. However, elevated [CO₂] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO₂] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].