Review of advances in biological CO<Subscript>2</Subscript> mitigation technology

CO₂ fixation by microalgae has emerged as a promising option for CO₂ mitigation. Intensive research work has been carried out to develop a feasible system for removing CO₂ from industrial exhaust gases. However, there are still several challenging points to overcome in order to make the process more practical. In this paper, recent research activities on three key technologies of biological CO₂ fixation, an identification of a suitable algal strain, development of high efficient photobioreactor and utilization of algal cells produced, are described. Finally the barriers, progress, and prospects of commercially developing a biological CO₂ fixation process are summarized.     

Photosynthetic CO<Subscript>2</Subscript> uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO<Subscript>2</Subscript>

Do short-term fluctuations in CO₂ concentrations at elevated CO₂ levels affect net CO₂ uptake rates of plants? When exposed to 600 μl CO₂ l^?1, net CO₂ uptake rates in shoots or leaves of seedlings of two tropical C₃ tree species, teak (Tectona grandis L. f.) and barrigon [Pseudobombax septenatum (Jacq.) Dug.], increased by 28 and 52% respectively. In the presence of oscillations with half-cycles of 20 s, amplitude of ca. 170 μl CO₂ l^?1 and mean of 600 μl CO₂ l^?1, the stimulation in net CO₂ uptake by the two species was reduced to 19 and 36%, respectively, i.e. the CO₂ stimulation in photosynthesis associated with a change in exposure from 370 to 600 μl CO₂ l^?1 was reduced by a third in both species. Similar reductions in CO₂-stimulated net CO₂ uptake were observed in T. grandis exposed to 40-s oscillations. Rates of CO₂ efflux in the dark by whole shoots of T. grandis decreased by 4.8% upon exposure of plants grown at 370 μl CO₂ l^?1 to 600 μl CO₂ l^?1. The potential implications of the observations on CO₂ oscillations and dark respiration are discussed in the context of free-air CO₂ enrichment (FACE) systems in which short-term fluctuations of CO₂ concentration are a common feature.     

CO<Subscript>2</Subscript> bio-mitigation using microalgae

Microalgae are a group of unicellular or simple multicellular photosynthetic microorganisms that can fix CO(2) efficiently from different sources, including the atmosphere, industrial exhaust gases, and soluble carbonate salts. Combination of CO(2) fixation, biofuel production, and wastewater treatment may provide a very promising alternative to current CO(2) mitigation strategies.     

Activity of the photosynthetic apparatus at periodic elevation of CO<Subscript>2</Subscript> concentration

We studied the influence of prolonged (a few weeks) and short-term (a few hours) periodical elevation of the ambient CO₂ concentration ([C_a]) on the photosynthetic apparatus and carbohydrate content in the third leaf of three-week-old cucumber (Cucumis sativus L.) plants. On the basis of experimental data and subsequent modeling, we revealed the limiting processes in the photosynthetic apparatus functioning: Rubisco activity, the rate of ribulose bisphosphate (CO₂ acceptor) regeneration, the rate of triose phosphate utilization in the Calvin cycle, and the influence of stomata on the photosynthesis rate. An increase in soluble carbohydrate content and a decrease in starch accumulation at a short-term [C_a] elevation indicate an important role of carbohydrate accumulation and their partition between organs in the regulation of the photosynthesis. We concluded that periodic [C_a] elevation can be used to improve plant productivity.     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

Elevated CO<Subscript>2</Subscript> reduces sap flux in mature deciduous forest trees

We enriched in CO₂ the canopy of 14 broad-leaved trees in a species-rich, ca. 30-m-tall forest in NW Switzerland to test whether elevated CO₂ reduces water use in mature forest trees. Measurements of sap flux density (J_S) were made prior to CO₂ enrichment (summer 2000) and throughout the first whole growing season of CO₂ exposure (2001) using the constant heat-flow technique. The short-term responses of sap flux to brief (1.5–3 h) interruptions of CO₂ enrichment were also examined. There were no significant a priori differences in morphological and physiological traits between trees which were later exposed to elevated CO₂ (n=14) and trees later used as controls (n=19). Over the entire growing season, CO₂ enrichment resulted in an average 10.7% reduction in mean daily J_S across all species compared to control trees. Responses were most pronounced in Carpinus, Acer, Prunus and Tilia, smaller in Quercus and close to zero in Fagus trees. The J_S of treated trees significantly increased by 7% upon transient exposure to ambient CO₂ concentrations at noon. Hence, responses of the different species were, in the short term, similar in magnitude to those observed over the whole season (though opposite because of the reversed treatment). The reductions in mean J_S of CO₂-enriched trees were high (22%) under conditions of low evaporative demand (vapour pressure deficit, VPD <5 hPa) and small (2%) when mean daily VPD was greater than 10 hPa. During a relatively dry period, the effect of elevated CO₂ on J_S even appeared to be reversed. These results suggest that daily water savings by CO₂-enriched trees may have accumulated to a significantly improved water status by the time when control trees were short of soil moisture. Our data indicate that the magnitude of CO₂ effects on stand transpiration will depend on rainfall regimes and the relative abundance of the different species, being more pronounced under humid conditions and in stands dominated by species such as Carpinus and negligible in mono-specific Fagus forests.     

Rise in CO<Subscript>2</Subscript> affects soil water transport through repellency

Several studies have shown improved soil stability under elevated atmospheric CO₂ caused by increased plant and microbial biomass. These studies have not quantified the mechanisms responsible for soil stabilisation or the effect on water relations. The objective of this study was to assess changes in water repellency under elevated CO₂. We hypothesised that increased plant biomass will drive an increase in water repellency, either directly or through secondary microbial processes. Barley plants were grown at ambient (360 ppm) and elevated (720 ppm) CO₂ concentrations in controlled chambers. Each plant was grown in a separate tube of 1.2 m length constructed from 22 mm depth × 47 mm width plastic conduit trunk and packed with sieved arable soil to 55% porosity. After 10 weeks growth the soil was dried at 40°C before measuring water sorptivity, ethanol sorptivity and repellency at many depths with a 0.14 mm radius microinfiltrometer. This provided a microscale measure of the capacity of soil to rewet after severe drying. At testing roots extended throughout the depth of the soil in the tube. The depth of the measurement had no effect on sorptivity or repellency. A rise in CO₂ resulted in a decrease in water sorptivity from 1.13 ± 0.06 (s.e) mm s^−1/2 to 1.00 ± 0.05 mm s^−1/2 (P < 0.05) and an increase in water repellency from 1.80 ± 0.09 to 2.07 ± 0.08 (P < 0.05). Ethanol sorptivity was not affected by CO₂ concentration, suggesting a similar pore structure. Repellency was therefore the primary cause of decreased water sorptivity. The implications will be both positive and negative, with repellency potentially increasing soil stability but also causing patchier wetting of the root-zone.     

Subsurface CO<Subscript>2</Subscript> Dynamics in Temperate Beech and Spruce Forest Stands

Rates of soil respiration (CO₂ effluxes), subsurface pore gas CO₂/O₂ concentrations, soil temperature and soil water content were measured for 15 months in two temperate and contrasting Danish forest ecosystems: beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.). Soil CO₂ effluxes showed a distinct seasonal trend in the range of 0.48–3.3 μmol CO₂ m⁻² s⁻¹ for beech and 0.50–2.92 μmol CO₂ m⁻² s⁻¹ for spruce and were well-correlated with near-surface soil temperatures. The soil organic C-stock (upper 1 m including the O-horizon) was higher in the spruce stand (184±23 Mg C ha⁻¹) compared to the beech stand (93±19 Mg C ha⁻¹) and resulted in a faster turnover time as calculated by mass/flux in soil beneath the beech stand (28 years) compared to spruce stand (60 years). Observed soil CO₂ concentrations and effluxes were simulated using a Fickian diffusion-reaction model based on vertical CO₂ production rates and soil diffusivity. Temporal trends were simulated on the basis of observed trends in the distribution of soil water, temperature, and live roots as well as temperature and water content sensitivity functions. These functions were established based on controlled laboratory incubation experiments. The model was successfully validated against observed soil CO₂ effluxes and concentrations and revealed that temporal trends generally could be linked to variations in subsurface CO₂ production rates and diffusion over time and with depths. However, periods with exceptionally high CO₂ effluxes (> 20 μmol CO₂ m⁻² s⁻¹) were noted in March 2000 in relation to drying after heavy rain and after the removal of snow from collars. Both cases were considered non-steady state and could not be simulated.     

An estimation of CO<Subscript>2</Subscript> fixation capacity in mangrove forest using two methods of CO<Subscript>2</Subscript> gas exchange and growth curve analysis

In many coastal areas of South-East Asia, attempts have been made to revive coastal ecosystem by initiating projects that encourage planting of mangrove trees. Compared to the terrestrial trees, mangrove trees possess a higher carbon fixation capacity. It becomes a very significant option for clean development mechanism (CDM) program. However, a reliable method to estimate CO₂ fixation capacity of mangrove trees has not been established. Acknowledging the above fact, we decided to set up an estimation method for the CDM program, using gas exchange analysis to estimate mangrove productivity, we put into consideration the net CO₂ fixation of reforested Kandelia candel (5-, 10-, and 15-year-old stand). This was estimated by gas exchange analysis and growth curve analysis. In growth curve analysis, we drew a growth curve of a single stand using data of above- and below-ground biomass. In the gas exchange analysis, we calculated CO₂ fixation capacity by (1) measuring respiration rate of each organ of stand and calculating respiratory CO₂ emission from above- to below-ground biomass. (2) Measuring the single-leaf photosynthetic rate in response to light intensity and calculating the photosynthetic CO₂ absorption. (3) We also developed a model for the diurnal changes in temperature, and monthly averages based on one-day estimation of CO₂ absorption and emission, which we corrected by this model in order to estimate the net CO₂ fixation capacity in response to temperature. Comparing the biomass accumulation of the two methods constructed for the same forest, the above-ground biomass accumulation of 10-year-old forest (34.3 ton ha⁻¹ yr⁻¹) estimated by gas exchange analysis was closely compared to those of growth curve analysis (26.6 ton ha⁻¹ yr⁻¹), suggesting that the gas exchange analysis was capable of estimating mangrove productivity. The validity of the estimated CO₂ fixation capacity by the gas exchange analysis and the growth curve analysis was also discussed.     

Effect of elevated CO<Subscript>2</Subscript> levels on leaf starch,nitrogen and photosynthesis of plants growing at three natural CO<Subscript>2</Subscript> springs in Japan

Plant communities around natural CO₂ springs have been exposed to elevated CO₂ levels over many generations and give us a unique opportunity to investigate the effects of long-term elevated CO₂ levels on wild plants. We searched for natural CO₂ springs in cool temperate climate regions in Japan and found three springs that were suitable for studying long-term responses of plants to elevated levels of CO₂: Ryuzin-numa, Yuno-kawa and Nyuu. At these CO₂ springs, the surrounding air was at high CO₂ concentration with no toxic gas emissions throughout the growth season, and there was natural vegetation around the springs. At each site, high-CO₂ (HC) and low-CO₂ (LC) plots were established, and three dominant species at the shrub layers were used for physiological analyses. Although the microenvironments were different among the springs, dicotyledonous species growing at the HC plots tended to have more starch and less nitrogen per unit dry mass in the leaves than those growing at the LC plots. In contrast, monocotyledonous species growing in the HC and LC plots had similar starch and nitrogen concentrations. Photosynthetic rates at the mean growth CO₂ concentration were higher in HC plants than LC plants, but photosynthetic rates at a common CO₂ concentration were lower in HC plants. Efficiency of water and nitrogen use of leaves at growth CO₂ concentration was greatly increased in HC plants. These results suggest that natural plants growing in elevated CO₂ levels under cool temperate climate conditions have down-regulated their photosynthetic capacity but that they increased photosynthetic rates and resource use efficiencies due to the direct effect of elevated CO₂ concentration.     

Reducing CO<Subscript>2</Subscript> Emissions from Domestic Travel: Exploring the Social and Health Impacts

The importance and meaning of social and recreational travel for a diverse group of Auckland residents is explored in this article. Study participants identified a range of social and health benefits, including maintaining social connections with family and friends, opportunities to participate in physical activity, and reducing stress. However, many of these trips are by car. New Zealand has one of the highest rates of private car ownership internationally, low-density urban development, and a poor public transport infrastructure. Social and recreational trips make up a sizeable proportion of domestic travel and are contributing to New Zealand’s increasing rate of CO₂ emissions. There is an obvious need to address the negative ecological impacts of human activity. Our findings suggest that alongside strategies to reduce CO₂ emissions, it also is important to introduce measures to maintain the benefits from social and recreational travel. Suggestions are made for further areas of research.     

Elevated CO<Subscript>2</Subscript> Effects on Peatland Plant Community Carbon Dynamics and DOC Production

Northern peatlands are important stores of carbon and reservoirs of biodiversity that are vulnerable to global change. However, the carbon dynamics of individual peatland plant species is poorly understood, despite the potential for rising atmospheric CO₂ to affect the vegetation’s contribution to overall ecosystem carbon function. Here, we examined the effects of 3 years exposure to elevated CO₂ (eCO₂) on (a) peatland plant community composition and biomass, and (b) plant carbon dynamics and the production of dissolved organic carbon (DOC) using a ¹³CO₂ pulse–chase approach. Results showed that under eCO₂, Sphagnum spp. cover declined by 39% (P < 0.05) and Juncus effusus L. cover increased by 40% (P < 0.001). There was a concurrent increase in above- and belowground plant biomass of 115% (P < 0.01) and 96% (P < 0.01), respectively. Vascular species assimilated and turned over more ¹³CO₂-derived carbon than Sphagnum spp. (49% greater turnover of assimilated ¹³C in J. effusus and F. ovina L. leaf tissues compared with Sphagnum, P < 0.01). Elevated CO₂ also produced a 66% rise in DOC concentrations (P < 0.001) and an order of magnitude more ‘new’ exudate ¹³DOC than control samples (24 h after ¹³CO₂ pulse-labelling 2.5 ± 0.5 and 0.2 ± 0.1% in eCO₂ and control leachate, respectively, P < 0.05). We attribute the observed increase in DOC concentrations under eCO₂ to the switch from predominantly Sphagnum spp. to vascular species (namely J. effusus), leading to enhanced exudation and decomposition (litter and peat). The potential for reduced peatland carbon accretion, increased DOC exports and positive feedback to climate change are discussed.     

Effects of elevated CO<Subscript>2</Subscript> and/or O<Subscript>3</Subscript> on hormone IAA in needles of Chinese pine

This study examined the effects of season-long exposure of Chinese pine (Pinus tabulaeformis) to elevated carbon dioxide (CO₂) and/or ozone (O₃) on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in needles. Trees grown in open-top chambers (OTC) were exposed to control (ambient O₃, 55 nmol mol⁻¹ + ambient CO₂, 350 μmol mol⁻¹, CK), elevated CO₂ (ambient O₃ + high CO₂, 700 μmol mol⁻¹, EC) and elevated O₃ (high O₃, 80 ± 8 nmol mol⁻¹ + ambient CO₂, EO) OTCs from 1 June to 30 September. Plants grown in elevated CO₂ OTC had a growth increase of axial shoot and needle length, compared to control, by 20% and 10% respectively, while the growth in elevated O₃ OTC was 43% and 7% less respectively, than control. An increase in IAA content and POD activity and decrease in IAAO activity were observed in trees exposed to elevated CO₂ concentration compared with control. Elevated O₃ decreased IAA content and had no significant effect on IAAO activity, but significantly increased POD activity. When trees pre-exposed to elevated CO₂ were transferred to elevated O₃ (EC–EO) or trees pre-exposed to elevated O₃ were transferred to elevated CO₂ (EO–EC), IAA content was lower while IAAO activity was higher than that transferred to CK (EC–CK or EO–CK), the change in IAA content was also related to IAAO activity. The results indicated that IAAO and POD activities in Chinese pine needles may be affected by the changes in the atmospheric environment, resulting in the change of IAA metabolism which in turn may cause changes in Chinese pine’s growth. An erratum to this article can be found at     

Seasonal CO<Subscript>2</Subscript>-exchange variations of temperate semi-desert grassland in Hungary

CO₂ exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO₂ uptake and release, soil respiration rate (R _s), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO₂ uptake and release were –3.240 and 1.903 mol m^–2 s^–1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum R _s were 1.470 and 0.226 mol(CO₂) m^–2 s^–1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than R _s, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type.     

Protective Effect of Enzymatic Extracts from Microalgae Against DNA Damage Induced by H<Subscript>2</Subscript>O<Subscript>2</Subscript>

The enzymatic extracts from seven species of microalgae (Pediastrum duplex, Dactylococcopsis fascicularis, Halochlorococcum porphyrae, Oltmannsiellopsis unicellularis, Achnanthes longipes, Navicula sp. and Amphora coffeaeformis) collected from three habitats (freshwater, tidal pool, and coastal benthic) at Jeju Island in Korea were investigated for their antioxidant activity. Of the extracts tested, the AMG 300 L (an exo 1, 4-α-d-glucosidase) extract of P. duplex, the Viscozyme extract of Navicula sp., and the Celluclast extract of A. longipes provided the most potential as antioxidants. Meanwhile, the Termamyl extract of P. duplex in an H₂O₂ scavenging assay exhibited an approximate 60% scavenging effect. In this study, we report that the DNA damage inhibitory effects of P. duplex (Termamyl extract) and D. fascicularis (Kojizyme extract) were nearly 80% and 69% respectively at a concentration of 100 μg/ml. Thus, it is suggested that the microalgae tested in this study yield promising DNA damage inhibitory properties on mouse lymphoma L 5178 cells that are treated with H₂O₂. Therefore, microalgae such as P. duplex may be an excellent source of naturally occurring antioxidant compounds with potent DNA damage inhibition potential.     

Report on non-temperature related variations in CO<Subscript>2</Subscript> efflux rates from young tree stems in the dormant season

Respiration rates are reported to increase exponentially with temperature. Respiration rates of woody tissues are commonly measured as CO₂ efflux rates () from that tissue. However, this paper describes clear variations in stem that were not related to temperature for the case of a young beech (Fagus sylvatica L.) and oak (Quercus robur L.) tree during the dormant season. The CO₂ concentration ([CO₂]) in the xylem of the beech tree showed similar temperature-independent variations. The trees were grown in a growth chamber in which radiation patterns and temperature were kept constant. was measured with an IRGA connected to cuvettes surrounding a stem segment. Xylem [CO₂] was measured in situ using a CO₂ microelectrode. Depressions in and [CO₂] occurred during the light period, despite equal temperatures in the light and dark period. Explanations found in literature for discrepancies in the exponential relationship between temperature and are the influence of (1) sap flow or (2) decreased cell water content. However, (1) the variations were observed in the dormant season, when no sap flow was observed yet, and (2) reduced cell water content was not likely to be apparent as differences in stem transpiration rates between the dark and light period were not significant. Hence, previously formulated theories failed to explain our results. This work therefore provides a new ground for discussion on other possible causes of daytime depressions in . One might be the refixation of respired CO₂ by corticular photosynthesis in the stem parts adjacent to the stem segment enclosed by the cuvette.     

Elevated CO<Subscript>2</Subscript> mitigates the adverse effects of drought on daytime net ecosystem CO<Subscript>2</Subscript> exchange and photosynthesis in a Florida scrub-oak ecosystem

Drought is a normal, recurrent feature of climate. In order to understand the potential effect of increasing atmospheric CO₂ concentration (C _a) on ecosystems, it is essential to determine the combined effects of drought and elevated C _a (EC) under field conditions. A severe drought occurred in Central Florida in 1998 when precipitation was 88 % less than the average between 1984 and 2002. We determined daytime net ecosystem CO₂ exchange (NEE) before, during, and after the drought in the Florida scrub-oak ecosystem exposed to doubled C _a in open-top chamber since May 1996. We measured diurnal leaf net photosynthetic rate (P _N) of Quercus myrtifolia Willd, the dominant species, during and after the drought. Drought caused a midday depression in NEE and P _N at ambient CO₂ concentration (AC) and EC. EC mitigated the midday depression in NEE by about 60 % compared to AC and the effect of EC on leaf P _N was similar to its effect on NEE. Growth in EC lowered the sensitivity of NEE to air vapor pressure deficit under drought. Thus EC would help the scrub-oak ecosystem to survive the consequences of the effects of rising atmospheric CO₂ on climate change, including increased frequency of drought, while simultaneously sequestering more anthropogenic carbon.     

Leaf dynamics of a deciduous forest canopy: no response to elevated CO<Subscript>2</Subscript>

Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO₂ concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO₂]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO₂ (542 ppm) in a free-air CO₂ enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO₂ treatment. LAI throughout the 1999–2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO₂] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO₂ enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO₂] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO₂] on APAR, and the observed increase in net primary productivity in elevated [CO₂] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ^–1. The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO₂-enriched atmosphere and that increases in LUE and productivity in elevated [CO₂] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO₂ enrichment of the atmosphere.     

Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO<Subscript>2</Subscript>, O<Subscript>3</Subscript>, and plant genotype

This study examined the effects of carbon dioxide (CO₂)-, ozone (O₃)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO₂ Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO₂ and O₃, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO₂ decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O₃ decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO₂, O₃, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO₂ had little effect on forest tent caterpillar performance under ambient O₃, but reduced performance (for insects on one aspen genotype) under elevated O₃. Conversely, elevated O₃ improved forest tent caterpillar performance under ambient, but not elevated, CO₂. Parasitoid larval survivorship decreased under elevated O₃, depending upon levels of CO₂ and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.