Elevated CO<Subscript>2</Subscript>, litter chemistry,and decomposition: a synthesis

The results of published and unpublished experiments investigating the impacts of elevated [CO₂] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO₂] have an impact on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO₂] compared to that in ambient [CO₂]. This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO₂ enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO₂-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO₂] showed any consistent pattern or differences from litter grown in ambient [CO₂]. The effects of [CO₂] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO₂] are small when compared to other potential impacts of elevated [CO₂] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO₂-enrichment experiments are outlined.     

Effects of elevated carbon dioxide on green leaf tissue and leaf litter quality in an intact Mojave Desert ecosystem

The effects of elevated carbon dioxide (CO₂) on plant litter are critical determinants of ecosystem feedback to changing atmospheric CO₂ concentrations. We measured concentrations of nitrogen (N) and carbon (C) and calculated C : N ratios of green leaves of two desert perennial shrubs, and the same quality parameters plus lignin and cellulose content of leaf litter from four shrub species exposed to elevated CO₂ (FACE technology; Hendrey & Kimball, 1994 ) for 3 years in an intact Mojave Desert ecosystem. Shrubs tested were Larrea tridentata, Lycium pallidum, Lycium andersonii and Ambrosia dumosa. We calculated resorption efficiency from green tissue and leaf litter N data and measured lignin and cellulose content in litter in the last year study. Green leaves of L. tridentata grown under elevated CO₂ had significantly lower N concentrations and higher C : N ratios than shrubs grown in ambient conditions in 1999 (P < 0.05). Lycium pallidum green leaves grown under elevated CO₂ had significantly lower N concentrations and higher C : N ratios than shrubs grown under ambient conditions in 2000 (P < 0.05). There was no CO₂ effect on C content of either species. We found no effect of CO₂ on N or C content, C : N ratios, or lignin or cellulose concentrations in leaf litter of L. tridentata, L. pallidum, L. andersonii, or A. dumosa. There was no significant effect of CO₂ on estimates of shrub resorption efficiency. There was a seasonal effect on green tissue and litter tissue quality for L. tridentata, with lower tissue N content in summer than in spring or winter months. These data suggest that any productivity increases with elevated CO₂ in desert ecosystems may not be limited by lower leaf litter quality and that resorption efficiency calculations are best performed on an individual leaf basis.     

Decomposition and nitrogen release from leaves of three hardwood species grown under elevated O3 and/or CO2

Elevated concentrations of O₃ and CO₂ have both been shown to affect structure, nutrient status, and deposition of secondary metabolites in leaves of forest trees. While such studies have produced robust models of the effects of such air pollutants on tree ecophysiology and growth, few have considered the potential for broader, ecosystem-level effects after these chemically and structurally altered leaves fall as leaf litter and decay. To determine the effects of elevated O₃ and/or CO₂ on the subsequent decomposition and nutrient release from the leaves grown in such altered atmospheres, we grew seedlings of three widespread North American forest trees, black cherry (Prunus serotina) (BC), sugar maple (Acer saccharum) (SM), and yellow-poplar (Liriodendron tulipifera) (YP) for two growing seasons in charcoal-filtered air (CF-air=approximately 25% ambient O₃), ambient O₃ (1X) or twice-ambient O₃ (2X) in outdoor open-top chambers. We then assayed the loss of mass and N from the litter derived from those seedlings through one year litterbag incubations in the forest floor of a neighboring forest stand. Mass loss followed linear functions and was not affected by the O₃ regime in which the leaves were grown. Instantaneous decay rates (i.e. k values) averaged SM:–0.707 y^-1, BC:–0.613 y^-1, and YP:–0.859 y^-1. N loss from ambient (1X) O₃-grown SM leaves was significantly greater than from CF-air leaves: N loss from BC leaves did not differ among treatments. Significantly less N was released from CF-air-grown YP leaves than from 1X or 2X O₃-treated leaves. YP leaves from plants grown in pots at 2X O₃ and 350 ppm supplemental CO₂ in indoor pollutant fumigation chambers (CSTRs or Continuously Stirred Tank Reactors) loss 40% as much mass and 27% as much N over one year as did leaves from YP grown in CF-air or 2X O₃. Thus, for leaves from plants grown in pots in controlled environment fumigation chambers, the concentrations of both O₃ and CO₂ can affect N release from litter incubated in the field whereas mass loss rate was affected only by CO₂. Because both mass loss and N release from leaves grown at elevated CO₂ were reduced significantly (at least for yellow-poplar), forests exposed to elevated CO₂ may have significantly reduced N turnover rates, thereby resulting in increased N limitation of tree growth, especially in forests which are already N-limited.     

Vascular Plant Responses to Elevated CO2 in a Temperate Lowland Sphagnum Peatland

Vascular plant responses to experimental enrichment with atmospheric carbon dioxide (CO₂), using MINIFACE technology, were studied in a Dutch lowland peatland dominated by Sphagnum and Phragmites for 3 years. We hypothesized that vascular plant carbon would accumulate in this peatland in response to CO₂ enrichment owing to increased productivity of the predominant species and poorer quality (higher C/N ratios) and consequently lower decomposability of the leaf litter of these species. Carbon isotope signatures demonstrated that the extra 180 ppmv CO₂ in enriched plots had been incorporated into vegetation biomass accordingly. However, on the CO₂ sequestration side of the ecosystem carbon budget, there were neither any significant responses of total aboveground abundance of vascular plants, nor of any of the individual species. On the CO₂ release side of the carbon budget (decomposition pathway), litter quantity did not differ between ambient and CO₂ treatments, while the changes in litter quality (N and P concentration, C/N and C/P ratio) were marginal and inconsistent. It appeared therefore that the afterlife effects of significant CO₂-induced changes in green-leaf chemistry (lower N and P concentrations, higher C/N and C/P) were partly offset by greater resorption of mobile carbohydrates from green leaves during senescence in CO₂-enriched plants. The decomposability of leaf litters of three predominant species from ambient and CO₂-enriched plots, as measured in a laboratory litter respiration assay, showed no differences. The relatively short time period, environmental spatial heterogeneity and small plot sizes might explain part of the lack of CO₂ response. When our results are combined with those from other Sphagnum peatland studies, the common pattern emerges that the vascular vegetation in these ecosystems is genuinely resistant to CO₂-induced change. On decadal time-scales, water management and its effects on peatland hydrology, N deposition from anthropogenic sources and land management regimes that arrest the early successional phase (mowing, tree and shrub removal), may have a greater impact on the vascular plant species composition, carbon balance and functioning of lowland Sphagnum–Phragmites reedlands than increasing CO₂ concentrations in the atmosphere.     

Elevated CO2 increases the long-term decomposition rate of Quercus myrtifolia leaf litter

Decomposition of Quercus myrtifolia leaf litter in a Florida scrub oak community was followed for 3 years in two separate experiments. In the first experiment, we examined the effects CO₂ and herbivore damage on litter quality and subsequent decomposition. Undamaged, chewed and mined litter generated under ambient and elevated (ambient+350 ppm V) CO₂ was allowed to decompose under ambient conditions for 3 years. Initial litter chemistry indicated that CO₂ levels had minor effects on litter quality. Litter damaged by leaf miners had higher initial concentrations of condensed tannins and nitrogen (N) and lower concentrations of hemicellulose and C : N ratios compared with undamaged and chewed litter. Despite variation in litter quality associated with CO₂, herbivory, and their interaction, there was no subsequent effect on rates of decomposition under ambient atmospheric conditions. In the second experiment, we examined the effects of source (ambient and elevated) of litter and decomposition site (ambient and elevated) on litter decomposition and N dynamics. Litter was not separated by damage type. The litter from both elevated and ambient CO₂ was then decomposed in both elevated and ambient CO₂ chambers. Initial litter chemistry indicated that concentrations of carbon (C), hemicellulose, and lignin were higher in litter from elevated than ambient CO₂ chambers. Despite differences in C and fiber concentrations, litter from ambient and elevated CO₂ decomposed at comparable rates. However, the atmosphere in which the decomposition took place resulted in significant differences in rates of decomposition. Litter decomposing under elevated CO₂ decomposed more rapidly than litter under ambient CO₂, and exhibited higher rates of mineral N accumulation. The results suggest that the atmospheric conditions during the decomposition process have a greater impact on rates of decomposition and N cycling than do the atmospheric conditions under which the foliage was produced.     

Development of gypsy moth larvae feeding on red maple saplings at elevated CO<Subscript>2</Subscript> and temperature

Predicted increases in atmospheric CO₂ and global mean temperature may alter important plant-insect associations due to the direct effects of temperature on insect development and the indirect effects of elevated temperature and CO₂ enrichment on phytochemicals important for insect success. We investigated the effects of CO₂ and temperature on the interaction between gypsy moth (Lymantria dispar L.) larvae and red maple (Acer rubrum L.) saplings by bagging first instar larvae within open-top chambers at four CO₂/temperature treatments: (1) ambient temperature, ambient CO₂, (2) ambient temperature, elevated CO₂ (+300 l l^-1 CO₂), (3) elevated temperature (+3.5°C), ambient CO₂, and (4) elevated temperature, elevated CO₂. Larvae were reared to pupation and leaf samples taken biweekly to determine levels of total N, water, non-structural carbohydrates, and an estimate of defensive phenolic compounds in three age classes of foliage: (1) immature, (2) mid-mature and (3) mature. Elevated growth temperature marginally reduced (P <0.1) leaf N and significantly reduced (P <0.05) leaf water across CO₂ treatments in mature leaves, whereas leaves grown at elevated CO₂ concentration had a significant decrease in leaf N and a significant increase in the ratio of starch:N and total non-structural carbohydrates:N. Leaf N and water decreased and starch:N and total non-structural carbohydrates:N ratios increased as leaves aged. Phenolics were unaffected by CO₂ or temperature treatment. There were no interactive effects of CO₂ and temperature on any phytochemical measure. Gypsy moth larvae reached pupation earlier at the elevated temperature (female =8 days, P <0.07; male =7.5 days, P <0.03), whereas mortality and pupal fresh weight of insects were unrelated to either CO₂, temperature or their interaction. Our data show that CO₂ or temperature-induced alterations in leaf constituents had no effect on insect performance; instead, the long-term exposure to a 3.5°C increase in temperature shortened insect development but had no effect on pupal weight. It appears that in some tree-herbivorous insect systems the direct effects of an increased global mean temperature may have greater consequences for altering plant-insect interactions than the indirect effects of an increased temperature or CO₂ concentration on leaf constituents.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Does elevated atmospheric carbon dioxide affect internal nitrogen allocation in the temperate trees Alnus glutinosa and Pinus sylvestris?

Nitrogen‐fixing plant species growing in elevated atmospheric carbon dioxide concentration ([CO₂]) should be able to maintain a high nutrient supply and thus grow better than other species. This could in turn engender changes in internal storage of nitrogen (N) and remobilisation during periods of growth. In order to investigate this one‐year‐old‐seedlings of Alnus glutinosa (L.) Gaertn and Pinus sylvestris (L.) were exposed to ambient [CO₂] (350 µ mol mol ^{? 1}) and elevated [CO₂] (700 µ mol mol ^{? 1}) in open top chambers (OTCs). This constituted a main comparison between a nitrogen‐fixing tree and a nonfixer, but also between an evergreen and a deciduous species. The trees were supplied with a full nutrient solution and in July 1994, the trees were given a pulse of ¹⁵N‐labelled fertiliser. The allocation of labelled N to different tissues (root, leaves, shoots) was followed from September 1994 to June 1995. While N allocation in P. sylvestris (Scots pine) showed no response to elevated [CO₂], A. glutinosa (common alder) responded in several ways. During the main nutrient uptake period of June–August, trees grown in elevated [CO₂] had a higher percentage of N derived from labelled fertiliser than trees grown in ambient [CO₂]. Remobilisation of labelled N for spring growth was significantly higher in A. glutinosa grown in elevated [CO₂] (9.09% contribution in ambient vs. 29.93% in elevated [CO₂] leaves). Exposure to elevated [CO₂] increased N allocation to shoots in the winter of 1994–1995 (12.66 mg in ambient vs. 43.42 mg in elevated 1993 shoots; 4.81 mg in ambient vs. 40.00 mg in elevated 1994 shoots). Subsequently significantly more labelled N was found in new leaves in April 1995. These significant increases in movement of labelled N between tissues could not be explained by associated increases in tissue biomass, and there was a significant shift in C‐biomass allocation away from the leaves towards the shoots (all above‐ground material except leaves) in A. glutinosa. This experiment provides the first evidence that not only are shifts in C allocation affected by elevated [CO₂], but also internal N resource utilisation in an N₂‐fixing tree.     

Effects of elevated CO2 and nitrogen fertilization pretreatments on decomposition on tallgrass prairie leaf litter

Standing dead and green foliage litter was collected in early November 1990 from Andropogon gerardii (C₄), Sorghastrum nutans (C₄), and Poa pratensis (C₃) plants that were grown in large open-top chambers under ambient or twice ambient CO₂ and with or without nitrogen fertilization (45 kg N ha⁻¹). The litter was placed in mesh bags on the soil surface of pristine prairie adjacent to the growth treatment plots and allowed to decay under natural conditions. Litter bags were retrieved at fixed intervals and litter was analyzed for mass loss, carbon chemistry, and total Kjeldahl nitrogen and phosphorus. The results indicate that growth treatments had a relatively minor effect on the initial chemical composition of the litter and its subsequent rate of decay or chemical composition. This suggests that a large indirect effect of CO₂ on surface litter decomposition in the tallgrass prairie would not occur by way of changes in chemistry of leaf litter. However, there was a large difference in characteristics of leaf litter decomposition among the species. Poa leaf litter had a different initial chemistry and decayed more rapidly than C₄ grasses. We conclude that an indirect effect of CO₂ on decomposition and nutrient cycling could occur if CO₂ induces changes in the relative aboveground biomass of the prairie species.     

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

We tested the hypothesis that elevated CO₂ would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO₂. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO₂ and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g⁻¹ day⁻¹ net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO₂ resulted in significantly higher maximum leaf photosynthesis (A _max) at both soil N levels. A _max was higher at high N than at low N soil in elevated, but not ambient CO₂. Photosynthetic N use efficiency was higher at elevated than ambient CO₂ in both soil types. Elevated CO₂ resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO₂ and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V _cmax/J _max) than at ambient CO₂ and/or low N availability. From the top to the bottom of the tree crowns, V _cmax/J _max increased in ambient CO₂, but it decreased in elevated CO₂ indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO₂. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A _leaf for each segment. This analysis indicated that increased A _leaf at elevated CO₂ did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO₂. Received: 12 August 1996 / Accepted: 12 November 1996     

Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment

The long‐term effects of elevated (ambient plus 350 μmol mol^?1) atmospheric CO₂ concentration (C_a) on the leaf senescence of Quercus myrtifolia Willd was studied in a scrub‐oak community during the transition from autumn (December 1997) to spring (April 1998). Plants were grown in large open‐top chambers at the Smithsonian CO₂ Research Site, Merritt Island Wildlife Refuge, Cape Canaveral, Florida. Chlorophyll (a + b) concentration, Rubisco activity and N concentration decreased by 75%, 82%, and 52%, respectively, from December (1997) to April (1998) in the leaves grown at ambient C_a. In contrast, the leaves of plants grown at elevated C_a showed no significant decrease in chlorophyll (a + b) concentration or Rubisco activity, and only a 25% reduction in nitrogen. These results indicate that leaf senescence was delayed during this period at elevated C_a. Delayed leaf senescence in elevated C_a had important consequences for leaf photosynthesis. In elevated C_a the net photosynthetic rate of leaves that flushed in Spring 1997 (last year's leaves) and were 13 months old was not different from fully‐expanded leaves that flushed in 1998, and were approximately 1 month old (current year's leaves). In ambient C_a the net photosynthetic rate of last year's leaves was 54% lower than for current year's leaves. When leaves were fully senesced, nitrogen concentration decreased to about 40% of the concentration in non‐senesced leaves, in both CO₂ treatments. In April, net photosynthesis was 97% greater in leaves grown in elevated C_a than in those grown at ambient. During the period when elevated C_a delayed leaf senescence, more leaves operating at higher photosynthetic rate would allow the ecosystem dominated by Q. myrtifolia to gain more carbon at elevated C_a than at ambient C_a.     

Effect of CO2 enrichment on non-structural carbohydrates in leaves, stems and ears of spring wheat

Field-grown spring wheat (Triticum aestivum L. cv. Dragon) was exposed to ambient and elevated CO₂ concentrations (1.5 and 2 times ambient) in open-top chambers. Contents of non-structural carbohydrates were analysed enzymatically in leaves, stems and ears six times during the growing season. The impact of elevated CO₂ on wheat carbohydrates was non-significant in most harvests. However, differences in the carbohydrate contents due to elevated CO₂ were found in all plant compartments. Before anthesis, at growth stage (GS) 30 (the stem is 1 cm to the shoot apex), the plants grown in elevated CO₂ contained significantly more water soluble carbohydrates (WSC), fructans, starch and total non-structural carbohydrates (TNC) in the leaves in comparison with the plants grown in ambient CO₂. It is hypothesised that the plants from the treatments with elevated CO₂ were sink-limited at GS30. After anthesis, the leaf WSC and TNC contents of the plants from elevated CO₂ started to decline earlier than those of the plants from ambient CO₂. This may indicate that the leaves of plants grown in the chambers with elevated CO₂ senesced earlier. Elevated CO₂ accelerated grain development: 2 weeks after anthesis, the plants grown in elevated CO₂ contained significantly more starch and significantly less fructans in the ears compared to the plants grown in ambient CO₂. Elevated CO₂ had no effect on ear starch and TNC contents at the final harvest. Increasing the CO₂ concentration from 360 to 520 μmol mol^?1 had a larger effect on wheat non-structural carbohydrates than the further increase from 520 to 680 μmol mol^?1. The results are discussed in relation to the effects of elevated CO₂ on yield and yield components.     

Photosynthetic responses to elevated CO2and O3 in field-grown potato(Solanum tuberosum)

Potato plants (Solanum tuberosum L. cv. Bintje) were grown to maturity in open-top chambers under three carbon dioxide (CO₂; ambient and 24 h d⁻¹ seasonal mean concentrations of 550 and 680 μmol mol⁻¹) and two ozone levels (O₃; ambient and an 8 h d⁻¹ seasonal mean of 50 nmol mol⁻¹). Chlorophyll content, photosynthetic characteristics, and stomatal responses were determined to test the hypothesis that elevated atmospheric CO₂ may alleviate the damaging influence of O₃ by reducing uptake by the leaves. Elevated O₃ had no detectable effect on photosynthetic characteristics, leaf conductance, or chlorophyll content, but did reduce SPAD values for leaf 15, the youngest leaf examined. Elevated CO₂ also reduced SPAD values for leaf 15, but not for older leaves; destructive analysis confirmed that chlorophyll content was decreased. Leaf conductance was generally reduced by elevated CO₂, and declined with time in the youngest leaves examined, as did assimilation rate (A). A generally increased under elevated CO₂, particularly in the older leaves during the latter stages of the season, thereby increasing instantaneous transpiration efficiency. Exposure to elevated CO₂ and/or O₃ had no detectable effect on dark-adapted fluorescence, although the values decreased with time. Analysis of the relationships between assimilation rate and intercellular CO₂ concentration and photosynthetically active photon flux density showed there was initially little treatment effect on CO₂-saturated assimilation rates for leaf 15. However, the values for plants grown under 550 μmol mol⁻¹ CO₂ were subsequently greater than in the ambient and 680 μmol mol⁻¹ treatments, although the beneficial influence of the former treatment declined sharply towards the end of the season. Light-saturated assimilation was consistently greater under elevated CO₂, but decreased with time in all treatments. The values decreased sharply when leaves grown under elevated CO₂ were measured under ambient CO₂, but increased when leaves grown under ambient CO₂ were examined under elevated CO₂. The results obtained indicate that, although elevated CO₂ initially increased assimilation and growth, these beneficial effects were not necessarily sustained to maturity as a result of photosynthetic acclimation and the induction of earlier senescence.     

Growth, light interception and yield responses of spring wheat (Triticum aestivum L.) grown under elevated CO2 and O3 in open-top chambers

Spring wheat cv. Minaret was grown to maturity under three carbon dioxide (CO₂) and two ozone (O₃) concentrations in open-top chambers (OTC). Green leaf area index (LAI) was increased by elevated CO₂ under ambient O₃ conditions as a direct result of increases in tillering, rather than individual leaf areas. Yellow LAI was also greater in the 550 and 680 μmol mol^–1 CO₂ treatments than in the chambered ambient control; individual leaves on the main shoot senesced more rapidly under 550 μmol mol^–1 CO₂, but senescence was delayed at 680 μmol mol^–1 CO₂. Fractional light interception (f) during the vegetative period was up to 26% greater under 680 μmol mol^–1 CO₂ than in the control treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during canopy development, plants grown under elevated CO₂ were taller at anthesis and stem and ear biomass were 27 and 16% greater than in control plants. At maturity, yield was 30% greater in the 680 μmol mol^–1 CO₂ treatment, due to a combination of increases in the number of ears per m^–2, grain number per ear and individual grain weight (IGW). Exposure to a seasonal mean (7 h d^–1) of 84 nmol mol^–1 O₃ under ambient CO₂ decreased green LAI and increased yellow LAI, thereby reducing both f and accumulated intercepted radiation by ≈ 16%. Individual leaves senesced completely 7–28 days earlier than in control plants. At anthesis, the plants were shorter than controls and exhibited reductions in stem and ear biomass of 15 and 23%. Grain yield at maturity was decreased by 30% due to a combination of reductions in ear number m^–2, the numbers of grains per spikelet and per ear and IGW. The presence of elevated CO₂ reduced the rate of O₃-induced leaf senescence and resulted in the maintenance of a higher green LAI during vegetative growth under ambient CO₂ conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding elevated CO₂ treatments in the absence of elevated O₃. Thus, although the presence of elevated CO₂ reduced the damaging impact of ozone on radiation interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be susceptible to O₃-induced injury during anthesis irrespective of the atmospheric CO₂ concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grain filling are discussed.     

Enriched atmospheric CO2 and defoliation: effects on tree chemistry and insect performance

We examined the effects of CO₂ and defoliation on tree chemistry and performance of the forest tent caterpillar, Malacosoma disstria. Quaking aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees were grown in open-top chambers under ambient or elevated concentrations of CO₂. During the second year of growth, half of the trees were exposed to free-feeding forest tent caterpillars, while the remaining trees served as nondefoliated controls. Foliage was collected weekly for phytochemical analysis. Insect performance was evaluated on foliage from each of the treatments. At the sampling date coincident with insect bioassays, levels of foliar nitrogen and starch were lower and higher, respectively, in high CO₂ foliage, and this trend persisted throughout the study. CO₂-mediated increases in secondary compounds were observed for condensed tannins in aspen and gallotannins in maple. Defoliation reduced levels of water and nitrogen in aspen but had no effect on primary metabolites in maple. Similarly, defoliation induced accumulations of secondary compounds in aspen but not in maple. Larvae fed foliage from the enriched CO₂ or defoliated treatments exhibited reduced growth and food processing efficiencies, relative to larvae on ambient CO₂ or nondefoliated diets, but the patterns were host species-specific. Overall, CO₂ and defoliation appeared to exert independent effects on foliar chemistry and forest tent caterpillar performance.     

The effects of elevated [CO2] on the C:N and C:P mass ratios of plant tissues

The influence of elevated CO₂ concentration ([CO₂]) during plant growth on the carbon:nutrient ratios of tissues depends in part on the time and space scales considered. Most evidence relates to individual plants examined over weeks to just a few years. The C:N ratio of live tissues is found to increase, decrease or remain the same under elevated [CO₂]. On average it increases by about 15% under a doubled [CO₂]. A testable hypothesis is proposed to explain why it increases in some situations and decreases in others. It includes the notion that only in the intermediate range of N-availability will C:N of live tissues increase under elevated [CO₂]. Five hypotheses to explain the mechanism of such increase in C:N are discussed; none of these options explains all the published results. Where elevated [CO₂] did increase the C:N of green leaves, that response was not necessarily expressed as a higher C:N of senesced leaves. An hypothesis is explored to explain the observed range in the degree of propogation of a CO₂ effect on live tissues through to the litter derived from them. Data on C:P ratios under elevated [CO₂] are sparse and also variable. They do not yet suggest a generalising-hypothesis of responses. Although, unlike for C:N, there is no theoretical expectation that C:P of plants would increase under elevated [CO₂], the average trend in the data is of such an increase. The processes determining the C:P response to elevated [CO₂] seem to be largely independent of those for C:N. Research to advance the topic should be structured to examine the components of the hypotheses to explain effects on C:N. This involves experiments in which plants are grown over the full range of N and of P availability from extreme limitation to beyond saturation. Measurements need to: distinguish structural from non-structural dry matter; organic from inorganic forms of the nutrient in the tissues; involve all parts of the plant to evaluate nutrient and C allocation changes with treatments; determine resorption factors during tissue senescence; and be made with cognisance of the temporal and spatial aspects of the phenomena involved. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO2 enrichment predisposes foliage of a eucalypt to freezing injury and reduces spring growth

Seedlings of Eucalyptus pauciflora, were grown in open-top chambers fumigated with ambient and elevated [CO₂], and were divided into two populations using 10% light transmittance screens. The aim was to separate the effects of timing of light interception, temperature and [CO₂] on plant growth. The orientation of the screens exposed plants to a similar total irradiance, but incident during either cold mornings (east-facing) or warm afternoons (west-facing). Following the first autumn freezing event elevated CO₂-grown plants had 10 times more necrotic leaf area than ambient CO₂ plants. West-facing plants had significantly greater (25% more) leaf damage and lower photochemical efficiency (F_v/F_m) in comparison with east-facing plants. Following a late spring freezing event east-facing elevated CO₂ plants suffered a greater sustained loss in F_v/F_m than west-facing elevated CO₂- and ambient CO₂-grown plants. Stomatal conductance was lower under elevated CO₂ than ambient CO₂ except during late spring, with the highest leaf temperatures occurring in west-facing plants under elevated CO₂. These higher leaf temperatures apparently interfered with cold acclimation thereby enhancing frost damage and reducing the ability to take advantage of optimal growing conditions under elevated CO₂.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

Long-term effects of elevated atmospheric CO2 on below-ground biomass and transformations to soil organic matter in grassland

We determined the effects of elevated [CO₂] on the quantity and quality of below-ground biomass and several soil organic matter pools at the conclusion of an eight-year CO₂ enrichment experiment on native tallgrass prairie. Plots in open-top chambers were exposed continuously to ambient and twice-ambient [CO₂] from early April through late October of each year. Soil was sampled to a depth of 30 cm beneath and next to the crowns of C4 grasses in these plots and in unchambered plots. Elevated [CO₂] increased the standing crops of rhizomes (87%), coarse roots (46%), and fibrous roots (40%) but had no effect on root litter (mostly fine root fragments and sloughed cortex material >500 μm). Soil C and N stocks also increased under elevated [CO₂], with accumulations in the silt/clay fraction over twice that of particulate organic matter (POM; >53 μm). The mostly root-like, light POM (density ≤1.8 Mg m^-3) appeared to turn over more rapidly, while the more amorphous and rendered heavy POM (density >1.8 Mg m^-3) accumulated under elevated [CO₂]. Overall, rhizome and root C:N ratios were not greatly affected by CO₂ enrichment. However, elevated [CO₂] increased the C:N ratios of root litter and POM in the surface 5 cm and induced a small but significant increase in the C:N ratio of the silt/clay fraction to a depth of 15 cm. Our data suggest that 8 years of CO₂ enrichment may have affected elements of the N cycle (including mineralization, immobilization, and asymbiotic fixation) but that any changes in N dynamics were insufficient to prevent significant plant growth responses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Does elevated atmospheric CO2 concentrations affect wood decomposition?

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric [CO₂] have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech (Fagus sylvatica L.) twigs grown under ambient and elevated [CO₂] in open top chambers. Elevated [CO₂] significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient [CO₂]. The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated [CO₂]. However, the elevated [CO₂] treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the [CO₂] treatment, despite initial changes in N and lignin concentrations between the ambient and elevated [CO₂] beech wood. This revised version was published online in June 2006 with corrections to the Cover Date.