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.     

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.     

Responses of trembling aspen (Populus tremuloides) phytochemistry and aspen blotch leafminer (Phyllonorycter tremuloidiella) performance to elevated levels of atmospheric CO2 and O3

1 This research was conducted at the Aspen FACE (Free Air CO₂ Enrichment) site located in northern Wisconsin, U.S.A. where trembling aspen (Populus tremuloides Michaux) trees were exposed to one of four atmospheric treatments: elevated carbon dioxide (CO₂; 560 µL/L), elevated ozone (O₃; ambient × 1.5), elevated CO₂ and O₃, or ambient air. We evaluated the effects of these fumigants on aspen foliar quality and the performance of aspen blotch leafminer (Phyllonorycter tremuloidiella Braun). 2 CO₂ and O₃ each affected foliar quality, with the major changes consisting of an 11% reduction in nitrogen under elevated CO₂ and a 20% reduction in tremulacin under elevated O₃. In the CO₂ + O₃ treatment, nitrogen levels were reduced by 15% and CO₂ ameliorated the O₃‐mediated reduction in tremulacin levels. 3 Phyllonorycter tremuloidiella were allowed to colonize trees naturally. Elevated CO₂ and O₃ reduced colonization rates by 42 and 49% relative to ambient CO₂ and O₃, respectively. The only effect of fumigation treatments on larval performance occurred under elevated O₃, where male development time and larval consumption increased by 8 and 28%, respectively, over insects reared under ambient O₃. 4 These data demonstrate that the individual and combined effects of CO₂ and O₃ can alter aspen foliar chemistry and that these alterations in foliar chemistry produce little to no change in larval performance. However, both CO₂ and O₃ greatly reduced oviposition. In order to ascertain the full effects of CO₂ and O₃ on insect performance, future studies should address both population‐ and individual‐level characteristics.     

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open-field experiment, 50-year-old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO₂ and NO₂ (10–15 nl I^?1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (F_v/F_M) was assessed in current and one-year-old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O₂ evolution and chlorophyll fluorescence were made in current-year needles at 20°C. In general, the F_v/F_M ratio as well as the gross rate of O₂ evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one-year-old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower F_v/F_M values than corresponding needles of control trees. Similar differences in F_v/F_M correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of F_v/F_M in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in F_v/F_M between the treatments were not significant during autumn and early winter. The gross rate of O₂ evolution at high irradiances was significantly lower in current-year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current-year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.     

Elevated CO2 alters birch resistance to Lagomorpha herbivores

We studied the three‐way interaction of elevated CO₂, nitrogen (N), and temperature (T), and the two‐way interaction of elevated CO₂ and early‐season defoliation on the secondary chemistry and resistance of Eurasian silver birch (Betula pendula) and North American paper birch (B. papyrifera) against the Eurasian hare (Lepus timidus) and the North American eastern cottontail rabbit (Sylvilagus floridanus), respectively. Elevated CO₂ decreased the palatability of winter‐dormant silver and paper birch stems to both hares and rabbits, respectively. But the effect on hares was only apparent at intermediate levels of N fertilization. Elevated T had no effect on palatability. The effects of elevated CO₂, N, and T on levels of silver birch bark phenolics and terpenoids were dominated by two‐way interactions between N and CO₂, and N and T. Generally, however, N amendments elicited a parabolic response in carbon partitioning to most biosynthetic classes of silver birch phenolics (i.e. highest concentrations occurring at intermediate N). CO₂ elevation was most enhancing at highest levels of N. On the other hand, T increases, more often than not, elicited reductions in phenolics, but especially so at the highest N level. In the case of B. papyrifera, elevated CO₂ increased carbon partitioning to Folin‐Denis stem and branch phenolics and condensed tannins. Early‐season defoliation, on the other hand, had no effect on phenolics and tannins but lowered both N and energy levels of branches. Elevated CO₂ substantially ameliorated the negative effects of severe defoliation on tree growth. These results support the hypothesis that continuing anthropogenic alterations of the atmosphere may trigger significant changes in plant phenotypic resistance to mammalian herbivores owing to an increasing net carbon balance between the highly vagile supply and demand capacities of plant carbon sources and sinks.     

Carbon partitioning to mobile and structural fractions in poplar wood under elevated CO2 (EUROFACE) and N fertilization

To determine whether globally increasing atmospheric carbon dioxide (CO₂) concentrations can affect carbon partitioning between nonstructural and structural carbon pools in agroforestry plantations, Populus nigra was grown in ambient air (about 370 μmol mol^?1 CO₂) and in air with elevated CO₂ concentrations (about 550 μmol mol^?1 CO₂) using free‐air CO₂ enrichment (FACE) technology. FACE was maintained for 5 years. After three growing seasons, the plantation was coppiced and one half of each experimental plot was fertilized with nitrogen. Carbon concentrations and stocks were measured in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Although FACE, N fertilization and season had significant tissue‐specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of FACE and N fertilization was on cell wall biomass production, resulting in about 30% increased above ground stocks of both mobile and immobile carbon pools compared with fertilized trees under ambient CO₂. Relative C partitioning between mobile and immobile C pools was not significantly affected by FACE or N fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C‐homeostasis under changing environmental conditions and illustrate that nonstructural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric [CO₂] in fertilized agroforestry systems. Thus, structural biomass production on abandoned agricultural land may contribute to achieving the goals of the Kyoto protocol.     

Interaction between CO2 enrichment and salinity stress in the C4 non-halophyte Andropogon glomeratus (Walter) BSP

Abstract Increasing atmospheric CO₂ may result in alleviation of salinity stress in salt-sensitive plants. In order to assess the effect of enriched CO₂ on salinity stress in Andropogon glomeratus, a C₄ non-halophyte found in the higher regions of salt marshes, plants were grown at 350, 500, and 650 cm³ m^?3 CO₂ with 0 or 100 mol m^?3 NaCl watering treatments. Increases in leaf area and biomass with increasing CO₂ were measured in salt-stressed plants, while decreases in these same parameters were measured in non-salt-stressed plants. Tillering increased substantially with increasing CO₂ in salt-stressed plants, resulting in the increased biomass. Six weeks following initiation of treatments, there was no difference in photosynthesis on a leaf area basis with increasing CO₂ in salt-stressed plants, although short-term increases probably occurred. Stomatal conductance decreased with increasing CO₂ in salt-stressed plants, resulting in higher water-use efficiency, and may have improved the diurnal water status of the plants. Concentrations of Na⁺ and Cl^? were higher in salt stressed-plants while the converse was found for K ⁺. There were no differences in leaf ion content between CO₂ treatments in the salt-stressed plants. Decreases in photosynthesis in salt-stressed plants occurred primarily as a result of decreased internal (non-stomatal) conductance.     

The response of soil CO2 flux to changes in atmospheric CO2, nitrogen supply and plant diversity

We measured soil CO₂ flux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmospheric carbon dioxide (CO₂) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO₂ and N fertilization increased soil CO₂ flux by 0.57 µmol m^?2 s^?1 (13% increase) and 0.37 µmol m^?2 s^?1 (8% increase) above average control soil CO₂ flux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO₂ flux by 0.23, 0.41 and 1.09 µmol m^?2 s^?1 (5%, 8% and 21% decreases), respectively. There were no statistically significant pairwise interactions among the three treatments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO₂ increased soil CO₂ flux most when soil moisture was low and soils were warm. Effects on soil CO₂ flux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no significant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correlated over time; neither were correlated with the CO₂ effect. Models of soil CO₂ flux will need to incorporate ecosystem CO₂ and N availability, as well as ecosystem plant diversity, and incorporate different environmental factors when determining the magnitude of the CO₂, N and diversity effects on soil CO₂ flux.     

An in vivo analysis of the effect of SO2 fumigation on photosynthesis in Zea mays.

Fumigation of leaves with SO₂ can reduce the capacity for photosynthetic CO₂ uptake even in the absence of visible symptoms of damage. In vitro studies suggest that this invisible injury to intact leaves could be affected by damage to each of the main stages in the photosynthetic process. Reduced stomatal apertures may also reduce photosynthesis following SO₂ fumigation. The responses of CO₂ uptake by leaves to intercellular CO₂ concentration and to absorbed light provide information for quantitative separation of the in vivo contribution of the different stages of photosynthesis to reduction in overall rate. This study uses these techniques to examine the basis of reduction in CO₂ uptake in Zea mays cv. LG11 leaves following short-term fumigation with SO₂. Fumigation with 33 μmol m^–3 SO₂ for 30 min reduced light saturated CO₂ uptake by about one-third. An even greater reduction in light limited CO₂ uptake was observed and with no significant change in light absorptance this was attributed to a reduced quantum yield of photosynthesis. The light saturated CO₂ uptake rate and the stomatal conductance decreased in parallel. However, the relationship of CO₂ uptake to the intercellular CO₂ concentration suggested that the reduced stomatal conductance did not account for the reduced rate of CO₂ uptake following fumigation. Both the initial slope and plateau of this relationship were significantly reduced, suggesting that both carboxylation efficiency and capacity for regeneration of CO₂ acceptor were diminished by SO₂ fumigation. The operating intercellular CO₂ concentration indicated that both processes were co-limiting, before and after fumigation. The time required for induction of photosynthetic CO₂ uptake on illumination was approximately doubled following SO₂ fumigation, showing that fumigation impairs the ability of the photosynthetic apparatus to adapt to fluctuations in light level.     

Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3

Elevated atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations have both been shown to affect plant tissue quality, which in turn could affect litter decomposition and carbon (C) and nutrient cycling. In order to evaluate effects of climate change on litter chemistry, needle litter was collected from Scots pine (Pinus sylvestris L.) saplings exposed to elevated CO₂ or O₃ concentration and their combination over three growing seasons in open‐top chambers. The decomposition of needle litter was followed for 19 months in a pine forest. During decomposition, needle samples for secondary compound analysis were collected and the mass loss of needles was followed. Main nutrients and total phenolics were analysed from litter in the beginning and at the end of the experiment. After 19‐month decomposition, the accumulated mass loss was about 34%; however, no significant differences were found in decomposition rates of needle litter between various treatments. Concentrations of total monoterpenes were about 4%, total resin acids 21% and total phenolics 14% of the initial concentrations in litter after 19‐month decomposition. In the beginning of litter decomposition, concentrations of individual monoterpenes –α‐pinene and β‐pinene – were significantly higher in needle litter grown under elevated CO₂. However, concentrations of total monoterpenes during the whole decomposition period were not significantly affected by CO₂ or O₃ treatments. Concentrations of some individual and total resin acids were higher in needle litter grown under elevated CO₂ or O₃ than under ambient air. There were no significant differences in concentrations of total phenolics as well as nitrogen (N) and the main nutrient concentrations between treatments during decomposition. High concentrations of monoterpenes and resin acids in needles might slightly delay C recycling in forest soils. It is concluded that elevated CO₂ and O₃ concentrations do not have remarkable impacts on litter decomposition processes in Scots pine forests.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

The effects of elevated CO2 and temperature on the resistance of winter-dormant birch seedlings (Betula pendula) to hares and voles

The effects of elevated CO₂ and temperature on the resource allocation pattern and resistance against mammalian herbivores of silver birch (Betula pendula Roth) were studied. Birch seedlings were grown through two growing seasons in closed‐top chambers exposed to four different treatments: ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm) and ambient temperature, elevated temperature (+3°C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. After winter hardening of the seedlings, the growth of the seedlings was measured and the concentration of secondary compounds such as phenolics and papyriferic acid determined. The top parts of the stem were fed to hares, and the basal parts of the same stems were offered to voles. Elevated CO₂ increased the height and basal diameter of the shoots, shoot biomass and total biomass of the seedlings but did not have any effect on secondary chemistry. Elevated temperature increased the height and shoot biomass, but did not have a significant effect on the total biomass of the seedlings. Elevated temperature decreased the concentration of condensed tannins and their precursor, (+)‐catechin, in the top part of the stems, but only the concentration of (+)‐catechin in the basal part of the stems. There were no significant interactive effects between CO₂ and temperature on phenolics in the stems, while the concentration of papyriferic acid showed significant interaction in the top part of the stems. This indicates high accumulation of papyriferic acid in ambient CO₂ under increased temperature. Consequently, elevated temperature increased the resistance of birch against hares, but did not affect the resistance of the basal parts of the same birches to voles. Our results indicate that the predicted climatic change will not necessarily lead to increased browsing damage by the mountain hare and the field vole to silver birch.     

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.     

The effect of open-air fumigation with SO2 and O3 on carbohydrate metabolism in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies)

The carbohydrate metabolism of the needles of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) has been examined in trees that were exposed to SO₂, and O₃, in an open-air fumigation experiment located in the Liphook forest in southern England. Two-year-old seedlings were planted in 1985 in seven experimental plots. Five plots received fumigation treatments of SO₂, O₃ or a combination of these gases to give a 2 × 3 factorial design with one additional ambient plot Fumigation with SO₂, occurred from May 1987 to December 1990 and O₃, fumigation occurred from March to December 1988, May to December 1989 and February to December 1990. Five samples of needles for investigation of carbohydrate metabolism were taken between February and July 1989. The concentrations of soluble carbohydrates (including sucrose and hexoses) were greatly reduced in the needles taken from Scots pine growing in the treated plots, and were also reduced, but to a lesser extent, in the needles taken from Norway spruce. Little variation in the concentration of starch in the needles of either species was detected. The activities of the two final enzymes of sucrose synthesis, sucrose phosphate synthase and sucrose 6-phos-phate phosphatase, were greatly reduced in the needles of Scots pine and were also reduced, but to a lesser extent, in the needles of Norway spruce in the fumigated plots. These reductions could be correlated with decreases in rates of photosynthetic CO₂ assimilation determined by independent groups of researchers working on the Liphook site.     

N2 fixation by Acacia species increases under elevated atmospheric CO2

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.     

Interacting effects of CO2 concentration, temperature and nitrogen supply on the photosynthesis and composition of winter wheat leaves

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO₂ concentrations (350 and 700 μmol mol⁻¹), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha⁻¹ and 87 kg ha⁻¹). Leaves grown at 700 μmol mol⁻¹ CO₂ had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO₂ concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long-term exposure to elevated CO₂ concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO₂ concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO₂ concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.     

Soil CO2 efflux of two silver birch clones exposed to elevated CO2 and O3 levels during three growing seasons

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.     

Oceanic sinks for atmospheric CO2

There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO₂ levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO₂, while during glacial–interglacial transitions, it is a source of CO₂ to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO₂ between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO₂. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO₃ in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO₂. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO₂ produced during the industrial revolution. In large part this is due to CO₂ solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO₂ inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO₂ burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.     

Elevated CO2 effects on decomposition processes in a grazed grassland

The effects of elevated atmospheric CO₂ (475 μL L^?1) on in situ decomposition of plant litter and animal faecal material were studied over 2 years in a free air CO₂ enrichment (FACE) facility. The pasture was grazed by sheep and contained a mixture of C₃ and C₄ grasses, legumes and forbs. There was no effect of elevated CO₂ on decomposition within plant species but marked differences between species with faster decomposition in dicots; a group that increased in abundance at elevated CO₂. Decomposition of mixed herbage root material occurred at a similar rate to that of leaf litter suggesting that any CO₂‐induced increase in carbon allocation to roots would not reduce rates of decomposition. Sheep faeces resulting from a ‘high‐CO₂ diet’ decomposed significantly slower during summer but not during winter. The overall outcome of these experiments were explored using scenarios that took account of changes in botanical composition, allocation to roots and the presence of herbivores. In the absence of herbivores, elevated CO₂ led to a 15% increase in the rate of mass loss and an 18% increase in the rate of nitrogen (N) release. In the presence of herbivores, these effects were partially removed (11% increase in rate of mass loss and 9% decrease in N release rate) because of the recycling occurring through the animals in the form of faeces.     

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.