Independent, Interactive, and Species-Specific Responses of Leaf Litter Decomposition to Elevated CO2 and O3 in a Northern Hardwood Forest

The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO₂-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions with pollutants like O₃. We evaluated the independent and interactive effects of elevated CO₂ (560 μl l⁻¹) and O₃ (55 nl l l⁻¹) on leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO₂ enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO₂, +O₃, and +CO₂ + O₃ FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO₂ decayed more slowly than litter produced under ambient CO₂, and this effect was exacerbated by elevated O₃. Similarly, birch litter produced under elevated CO₂ also decayed more slowly than litter produced under ambient CO₂. In contrast to results for aspen, however, elevated O₃ accelerated birch decay under ambient CO₂, but decelerated decay under enriched CO₂. Changes in decomposition rates (k-values) were due to CO₂- and O₃-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO₂ will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O₃.     

A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology

Quantitative integration of the literature on the effect of elevated CO₂ on woody plants is important to aid our understanding of forest health in coming decades and to better predict terrestrial feedbacks on the global carbon cycle. We used meta-analytic methods to summarize and interpret more than 500 reports of effects of elevated CO₂ on woody plant biomass accumulation and partitioning, gas exchange, and leaf nitrogen and starch content. The CO₂ effect size metric we used was the log-transformed ratio of elevated compared to ambient response means weighted by the inverse of the variance of the log ratio. Variation in effect size among studies was partitioned according to the presence of interacting stress factors, length of CO₂ exposure, functional group status, pot size, and type of CO₂ exposure facility. Both total biomass (W _T) and net CO₂ assimilation (A) increased significantly at about twice ambient CO₂, regardless of growth conditions. Low soil nutrient availability reduced the CO₂ stimulation of W _T by half, from +31% under optimal conditions to +16%, while low light increased the response to +52%. We found no significant shifts in biomass allocation under high CO₂. Interacting stress factors had no effect on the magnitude of responses of A to CO₂, although plants grown in growth chambers had significantly lower responses (+19%) than those grown in greenhouses or in open-top chambers (+54%). We found no consistent evidence for photosynthetic acclimation to CO₂ enrichment except in trees grown in pots <0.5 l (−36%) and no significant CO₂ effect on stomatal conductance. Both leaf dark respiration and leaf nitrogen were significantly reduced under elevated CO₂ (−18% and −16% respectively, data expressed on a leaf mass basis), while leaf starch content increased significantly except in low nutrient grown gymnosperms. Our results provide robust, statistically defensible estimates of elevated CO₂ effect sizes against which new results may be compared or for use in forest and climate model parameterization. Received: 16 May 1997 / Accepted: 9 September 1997     

Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

Effects of elevated atmospheric CO2 and tropospheric O3 on nutrient dynamics: decomposition of leaf litter in trembling aspen and paper birch communities

Atmospheric changes could strongly influence how terrestrial ecosystems function by altering nutrient cycling. We examined how the dynamics of nutrient release from leaf litter responded to two important atmospheric changes: rising atmospheric CO₂ and tropospheric O₃. We evaluated the independent and combined effects of these gases on foliar litter nutrient dynamics in aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh)/aspen communities at the Aspen FACE Project in Rhinelander, WI. Naturally senesced leaf litter was incubated in litter bags in the field for 735 days. Decomposing litter was sampled six times during incubation and was analyzed for carbon, and both macro (N, P, K, S, Ca, and Mg) and micro (Mn, B, Zn and Cu) nutrient concentrations. Elevated CO₂ significantly decreased the initial litter concentrations of N (−10.7%) and B (−14.4%), and increased the concentrations of K (+23.7%) and P (+19.7%), with no change in the other elements. Elevated O₃ significantly decreased the initial litter concentrations of P (−11.2%), S (−8.1%), Ca (−12.1%), and Zn (−19.5%), with no change in the other elements. Pairing concentration data with litterfall data, we estimated that elevated CO₂ significantly increased the fluxes to soil of all nutrients: N (+12.5%), P (+61.0%), K (+67.1%), S (+28.0%), and Mg (+40.7%), Ca (+44.0%), Cu (+38.9%), Mn (+62.8%), and Zn (+33.1%). Elevated O₃ had the opposite effect: N (−22.4%), P (−25.4%), K (−27.2%), S (−23.6%), Ca (−27.6%), Mg (−21.7%), B (−16.2%), Cu (−20.8%), and Zn (−31.6%). The relative release rates of the nine elements during the incubation was: K ≥ P ≥ mass ≥ Mg ≥ B ≥ Ca ≥ S ≥ N ≥ Mn ≥ Cu ≥ Zn. Atmospheric changes had little effect on nutrient release rates, except for decreasing Ca and B release under elevated CO₂ and decreasing N and Ca release under elevated O₃. We conclude that elevated CO₂ and elevated O₃ will alter nutrient cycling more through effects on litter production, rather than litter nutrient concentrations or release rates.     

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     

Elevated CO<Subscript>2</Subscript> ameliorated oxidative stress induced by elevated O<Subscript>3</Subscript> in <Emphasis Type="Italic">Quercus mongolica</Emphasis>

Using open top chambers, the effects of elevated O₃ (80 nmol mol⁻¹) and elevated CO₂ (700 μmol mol⁻¹), alone and in combination, were studied on young trees of Quercus mongolica. The results showed that elevated O₃ increased malondialdehyde content and decreased photosynthetic rate after 45 days of exposure, and prolonged exposure (105 days) induced significant increase in electrolyte leakage and reduction of chlorophyll content. All these changes were alleviated by elevated CO₂, indicating that oxidative stress on cell membrane and photosynthesis was ameliorated. After 45 days of exposure, elevated O₃ stimulated activities of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11), but the stimulation was dampened under elevated CO₂ exposure. Furthermore, ascorbate (AsA) and total phenolics contents were not higher in the combined gas treatment than those in elevated O₃ treatment. It indicates that the protective effect of elevated CO₂ against O₃ stress was achieved hardly by enhancing ROS scavenging ability after 45 days of exposure. After 105 days of exposure, elevated O₃ significantly decreased activities of SOD, catalase (CAT, EC 1.11.1.6) and APX and AsA content. Elevated CO₂ suppressed the O₃-induced decrease, which could ameliorate the oxidative stress in some extent. In addition, elevated CO₂ increased total phenolics content in the leaves both under ambient O₃ and elevated O₃ exposure, which might contribute to the protection against O₃-induced oxidative stress as well.     

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition

Ecosystem-level experiments on the effects of atmospheric CO₂ enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m² of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO₂ concentrations (280, 420, 560 μl l⁻¹) and three different rates of wet N deposition (0, 30, 90 kg ha⁻¹ year⁻¹) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO₂ concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO₂ leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO₂. In contrast to CO₂ enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO₂ ×  N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO₂ l⁻¹, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO₂ largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO₂ (−18% at 420 and −31% at 560 compared to 280 μl CO₂ l⁻¹). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha⁻¹ year⁻¹). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO₂ enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO₂ suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO₂ may indicate CO₂-induced reductions of soil N availability. Received: 11 May 1997 / Accepted: 4 August 1997     

Down-regulation of Rubisco activity under combined increases of CO<Subscript>2</Subscript> and temperature minimized by changes in Rubisco k<Subscript>cat</Subscript> in wheat

Increases in growth temperature have been observed to affect photosynthesis differently under long-term exposure to ambient- and twice ambient-air CO₂ concentrations. This study investigates the causes of this interaction in wheat (Triticum aestivum L.) grown in the field over two consecutive years under temperature gradient chambers in ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂ concentrations and at ambient or ambient +4°C temperatures, with either a low or a high nitrogen supply. The photosynthesis-internal CO₂ response curves and the activity, activation state, k_cat and amount of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) were measured, as well as the soluble protein concentration in flag leaves at ear emergence and 8–15 days after anthesis. A high nitrogen supply increased V_cmax, the Rubisco amount and activity and soluble protein contents, but did not significantly change the Rubisco k_cat. Both elevated CO₂ and above ambient temperatures had negative effects on V_cmax and Rubisco activity, but at elevated CO₂, an increase in temperature did not decrease V_cmax or Rubisco activity in relation to ambient temperature. The amounts of Rubisco and soluble protein decreased with elevated CO₂ and temperature. The negative impact of elevated CO₂ on Rubisco properties was somewhat counteracted at elevated temperatures by an increase in k_cat. This effect can diminish the detrimental effects on photosynthesis of combined increases of CO₂ and temperature.     

Effects of elevated CO2 and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland

We investigated the effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) and phosphorus supply (1 g P m⁻² year⁻¹ vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenhouse. Aboveground community dry mass remained almost unaffected by elevated CO₂ in the first year (+6%, n.s.), but was significantly stimulated by CO₂ enrichment in year two (+26%, P<0.01). Among functional groups, only graminoids contributed significantly to this increase. The effect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in year one, +9% in year two, 0.1 ≥P > 0.05). Belowground biomass and stubble after two seasons were not different in elevated CO₂ and when P was added. The small initial increase in aboveground community biomass under elevated CO₂ is explained by the fact that some species, in particular Carex flacca, responded very positively right from the beginning, while others, especially the dominant Bromus erectus, responded negatively to CO₂ enrichment. Shifts in community composition towards more responsive species explain the much larger CO₂ response in the second year. These shifts, i.e., a decline in xerophytic elements (B. erectus) and an increase in mesophytic grasses and legumes occurred independently of treatments in all monoliths but were accelerated significantly by elevated CO₂. The difference in average biomass production at elevated compared to ambient CO₂ was higher when P was supplied (at the community level the CO₂ response was enhanced from 20% to 33% when P was added, in graminoids from 17% to 27%, in legumes from 4% to 60%, and in C. flacca from 120% to 298% by year two). Based on observations in this and similar studies, we suggest that interactions between CO₂ concentration, species presence, and nutrient availability will govern community responses to elevated CO₂. Received: 12 July 1997 / Accepted: 28 March 1998     

Growth and photosynthetic response of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings to ozone and/or elevated carbon dioxide

We investigated the effects of ozone (O₃) and/or elevated CO₂ concentration ([CO₂]) on growth and photosynthetic traits of Fagus crenata seedlings. Two-year-old seedlings were grown in four experimental treatments comprising two O₃ treatments (charcoal-filtered air and 100 nmol mol⁻¹ O₃; 6 h/day, 3 days/week) in combination with two CO₂ treatments (350 and 700 μmol mol⁻¹) for 18 weeks in environmental control growth chambers. The four treatments were designated as control, elevated O₃, elevated CO₂, and elevated CO₂ + O₃. Dry matter growth of the seedlings was greater in elevated CO₂ + O₃ than in elevated CO₂. In elevated CO₂ + O₃, a marked increase of second-flush leaves, considered a compensative response to O₃, was observed. The net photosynthetic rate of first-flush leaves in elevated CO₂ + O₃ increased earlier and was maintained for a longer period of time than that in elevated CO₂. Because emergence of second-flush leaves of F. crenata is greatly affected by the amount of assimilation products of first-flush leaves in current year, we consider that an early increase in the net photosynthetic rate of first-flush leaves contributed to the marked increase in second-flush leaf emergence under elevated CO₂ + O₃. These results imply that we must account for changes in compensative capacity with respect to not only morphological traits but also phenological traits and physiological functions such as photosynthesis when evaluating effects of O₃ on F. crenata under elevated [CO₂].     

Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone

Atmospheric change may affect plant phenolic compounds, which play an important part in plant survival. Therefore, we studied the impacts of CO₂ and O₃ on the accumulation of 27 phenolic compounds in the short‐shoot leaves of two European silver birch (Betula pendula Roth) clones (clones 4 and 80). Seven‐year‐old soil‐grown trees were exposed in open‐top chambers over three growing seasons to ambient and twice ambient CO₂ and O₃ concentrations singly and in combination in central Finland. Elevated CO₂ increased the concentration of the phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%) by increasing their accumulation in the leaves of the silver birch trees, but decreased the flavone aglycons (?7%) by growth dilution. Elevated O₃ increased the concentration of 3,4′‐dihydroxypropiophenone 3‐β‐d ‐glucoside (+22%), chlorogenic acid (+19%) and flavone aglycons (+4%) by inducing their accumulation possibly as a response to increased oxidative stress in the leaf cells. Nevertheless, this induction of antioxidant phenolic compounds did not seem to protect the birch leaves from detrimental O₃ effects on leaf weight and area, but may have even exacerbated them. On the other hand, elevated CO₂ did seem to protect the leaves from elevated O₃ because all the O₃‐derived effects on the leaf phenolics and traits were prevented by elevated CO₂. The effects of the chamber and elevated CO₂ on some compounds changed over time in response to the changes in the leaf traits, which implies that the trees were acclimatizing to the altered environmental conditions. Although the two clones used possessed different composition and concentrations of phenolic compounds, which could be related to their different latitudinal origin and physiological characteristics, they responded similarly to the treatments. However, in some cases the variation in phenolic concentrations caused by genotype or chamber environment was much larger than the changes caused by either elevated CO₂ or O₃.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Shrub establishment under experimental global changes in a California grassland

Accelerating invasion of grasslands by woody species is a widespread global phenomenon. The native shrub Baccharis pilularis has recently increased in abundance in some California grasslands, with large local community and ecosystem effects. I investigated potential contributions of (1) future global climate and atmospheric changes and (2) variation in moisture and nutrient availability to increased Baccharis germination and early establishment rates. I examined responses of Baccharis seeds and seedlings to simulated warming (+ 1−2 °C) and elevated CO₂ (+ 300 ppm) in a 2-year field experiment. Warming and CO₂ treatments were applied at ambient and increased water and nitrogen levels chosen to simulate future increases in precipitation (+ 50%) and N deposition (+ 7 gN m⁻² y⁻¹). Elevated CO₂ and water addition each increased or accelerated germination. Herbivory strongly reduced seedling populations during the winter wet season; drought further reduced seedling survival in the spring. Overall Baccharis survivorship was extremely low (<0.1%) across all treatments, complicating the interpretation of global change effects.     

Effects of elevated CO2 and O3 on leaf damage and insect abundance in a soybean agroecosystem

By altering myriad aspects of leaf chemistry, increasing concentrations of CO₂ and O₃ in the atmosphere derived from human activities may fundamentally alter the relationships between insect herbivores and plants. Because exposure to elevated CO₂ can alter the nutritional value of leaves, some herbivores may increase consumption rates to compensate. The effects of O₃ on leaf nutritional quality are less clear; however, increased senescence may also reduce leaf quality for insect herbivores. Additionally, changes in secondary chemistry and the microclimate of leaves may render plants more susceptible to herbivory in elevated CO₂ and O₃. Damage to soybean (Glycine max L.) leaves and the size and composition of the insect community in the plant canopy were examined in large intact plots exposed to elevated CO₂ (~550 μmol mol⁻¹) and elevated O₃ (1.2*ambient) in a fully factorial design with a Soybean Free Air Concentration Enrichment system (SoyFACE). Leaf area removed by folivorous insects was estimated by digital photography and insect surveys were conducted during two consecutive growing seasons, 2003 and 2004. Elevated CO₂ alone and in combination with O₃ increased the number of insects and the amount of leaf area removed by insect herbivores across feeding guilds. Exposure to elevated CO₂ significantly increased the number of western corn rootworm (Diabrotica virgifera) adults (foliage chewer) and soybean aphids (Aphis glycines; phloem feeder). No consistent effect of elevated O₃ on herbivory or insect population size was detected. Increased loss of leaf area to herbivores was associated with increased carbon-to-nitrogen ratio and leaf surface temperature. Soybean aphids are invasive pests in North America and new to this ecosystem. Higher concentrations of CO₂ in the atmosphere may increase herbivory in the soybean agroecosystem, particularly by recently introduced insect herbivores. Handling editor: Gary Felton.     

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].     

Ozone and density affect the response of biomass and seed yield to elevated CO2 in rice

Tropospheric O₃ reduces growth and yield of many crop species, whereas CO₂ ameliorates the negative effects of O₃. Thus, in a combined elevated CO₂ and O₃ atmosphere, seed yield is at least restored to that of charcoal‐filtered (CF) air at ambient CO₂. The CO₂‐induced yield increase in CF air is highly variable, suggesting other potential resource limitations. To understand such variability in response, we tested that (1) competition for resources precludes some of the CO₂ enhancement on biomass and yield; and (2) O₃ reduces competition in elevated CO₂. We grew rice (Oryza sativa L.) at five densities in CF and O₃‐fumigated (+O3) air at ambient (A) and elevated [CO₂] (+CO2) in 1997 and 1998. O₃ reduced biomass by 25% and seed yield by 13–20% in A, but had little effect in +CO2. A competition model of biomass and yield response to density based on resource availability without competition showed that fewer resources were used for biomass in +O3 than in CF (average 53% vs. 70%) in A, while in +CO2 85% of resources were used for biomass regardless of O₃ suggesting greater depletion of resources. The enhanced biomass response to CO₂ with O₃ is consistent with a 22% greater CO₂ enhancement ratio [mass in +CO2 air/mass in A air; enhancement ratio (ER)] in +O3 than in CF air. For seed yield, few resources were used (average 17% and 25% for CF in 1997 and 1998, respectively), and ER was 13% greater in +O3. With competition the rate of change of individual plant biomass to density was not affected by +CO2 in CF air in 1997 but was increased 19% with more nutrients in 1998, indicating resource limitations with +CO2. The rate of change of individual plant yield to density was reduced with CO₂ in 1997 and unchanged in 1998 showing a different response to resource limitation for reproductive biomass. The resource use in +O3‐A suggested that increased density and soil fertility might compensate for pollutant damage. Although ambient [O₃] can modulate the response to elevated CO₂, resource limitation precludes the CO₂ fertilization impact and both factors need consideration for better management and forecasts of future productivity.     

Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide

The effects of elevated CO₂ concentrations on the photochemistry, biochemistry and physiology of C₄ photosynthesis were studied in maize (Zea mays L.). Plants were grown at ambient (350 μL L⁻¹) or ca. 3 times ambient (1100 μL L⁻¹) CO₂ levels under high light conditions in a greenhouse for 30 d. Relative to plants grown at ambient CO₂ levels, plants grown under elevated CO₂ accumulated ca. 20% more biomass and 23% more leaf area. When measured at the CO₂ concentration of growth, mature leaves of high-CO₂-grown plants had higher light-saturated rates of photosynthesis (ca. 15%), lower stomatal conductance (71%), higher water-use efficiency (225%) and higher dark respiration rates (100%). High-CO₂-grown plants had lower carboxylation efficiencies (23%), measured under limiting CO₂, and lower leaf protein contents (22%). Activities of a number of C₃ and C₄ cycle enzymes decreased on a leaf-area basis in the high-CO₂-grown plants by 5–30%, with NADP-malate dehydrogenase exhibiting the greatest decrease. In contrast, activities of fructose 1,6-bisphosphatase and ADP-glucose pyrophosphorylase increased significantly under elevated CO₂ condition (8% and 36%, respectively). These data show that the C₄ plant maize may benefit from elevated CO₂ through acclimation in the capacities of certain photosynthetic enzymes. The increased capacity to synthesize sucrose and starch, and to utilize these end-products of photosynthesis to produce extra energy by respiration, may contribute to the enhanced growth of maize under elevated CO₂. Received: 30 April 1999 / Accepted: 17 June 1999     

A 3-year exposure to CO2 and O3 induced minor changes in soil N cycling in a meadow ecosystem

We investigated the individual and interactive effects of moderately elevated CO₂ (ambient air + 100 ppm) and/or O₃ (40–50 ppb) on soil N cycling and microbial biomass N in a 3-year open-top chamber experiment conducted in meadow mesocosms. The results show that elevated O₃ decreased the concentrations of mineral N and NH₄⁺-N in the mesocosm soil in the last growing season (2004). Total N, NO₃⁻-N, microbial biomass N, decomposition rate, potential nitrification and denitrification were not affected by elevated O₃ and/or CO₂. It is thus concluded that the proposed future ambient O₃ and CO₂ levels, such as used in this experiment, may not induce major changes in the below-ground N processes in N-poor northern European hay meadow ecosystems.