CO2 and O3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria)

Elevated levels of CO₂ and O₃ affect plant growth and phytochemistry, which in turn can alter physiological performance of associated herbivores. Little is known, however, about how generalist insect herbivores respond behaviorally to CO₂‐ and O₃‐mediated changes in their host plants. This research examined the effects of elevated CO₂ and O₃ levels on host plant preferences and consumption of forest tent caterpillar (FTC, Malacosoma disstria Hbn.) larvae. Dual choice feeding assays were performed with foliage from birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259). Trees were grown at the Aspen Free Air CO₂ Enrichment (FACE) facility near Rhinelander, WI, USA, and had been exposed to ambient or elevated concentrations of CO₂ and/or O₃. Levels of nutritional and secondary compounds were quantified through phytochemical analyses. The results showed that elevated O₃ levels increased FTC larval preferences for birch compared with aspen, whereas elevated CO₂ levels had the opposite effect. In assays with the two aspen genotypes, addition of both CO₂ and O₃ caused a shift in feeding preferences from genotype 259 to genotype 216. Consumption was unaffected by experimental treatments in assays comparing aspen and birch, but were increased for larvae given high O₃ foliage in the aspen genotype assays. Elevated levels of CO₂ and O₃ altered tree phytochemistry, but did not explain shifts in feeding preferences. The results demonstrate that increased levels of CO₂ and O₃ can alter insect host plant preferences both between and within tree species. Also, consequences of altered host quality (e.g., compensatory consumption) may be buffered by partial host shifts in situations when alternative plant species are available. Environmentally induced changes in host plant preferences may have the potential to alter the distribution of herbivory across plant genotypes and species, as well as competitive interactions among them.     

Atmospheric change alters performance of an invasive forest insect

Atmospheric change and species invasions are arguably two of the most important factors affecting the long‐term sustainability of natural ecosystems. We examined the independent and interactive effects of atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) on the foliar quality of two host species and performance of an invasive folivorous insect. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under all combinations of ambient and elevated CO₂ and O₃. We measured the effects of elevated CO₂ and O₃ on aspen and birch phytochemistry and on the survivorship, development time, growth, and fecundity of the gypsy moth (Lymantria dispar). Elevated CO₂ had little effect on, whereas elevated O₃ altered, the composite phytochemical profiles of aspen and birch. Nutritional quality in aspen and birch leaves was marginally affected by elevated CO₂ and reduced by elevated O₃. Both gases increased concentrations of phenolic and structural compounds in aspen and birch. Elevated CO₂ offset reduced foliar quality under elevated O₃, but only in aspen, and to a greater extent later than earlier in spring. Elevated CO₂ generally had beneficial effects on, while elevated O₃ detrimentally affected, gypsy moth performance. Elevated CO₂ ameliorated most of the reductions in gypsy moth performance under elevated O₃. Our findings suggest that atmospheric change can alter foliar quality in gypsy moth hosts sufficiently to influence gypsy moth performance, but that these responses will depend on interactions among CO₂, O₃, and tree species. Our findings also contrast with those of earlier studies at Aspen FACE, indicating that foliar quality responses to environmental change are likely influenced by tree stand age and longevity of exposure to pollutants to the extent that they affect plant‐herbivore interactions differently over decadal time spans.     

Effects of elevated CO2 and cutting frequency on plant community structure in a temperate grassland

Monoliths of a fertile, N limited, C3 grassland community were subjected (or not) to an atmospheric CO₂ enrichment (600 µmol mol^‐‐1) using a Mini‐FACE system, from August 1998 to June 2001 and were subjected to two contrasting cutting frequencies (3 and 6 cuts per year). We report here the effects of the CO₂ and cutting frequency factors on the plant community structure and its diversity. Species‐specific responses to elevated CO₂ and cutting frequency were observed, which resulted in significant changes in the botanical composition of the grassland monoliths. Elevated CO₂ significantly increased the proportion of dicotyledones (forbs + legumes) and reduced that of the monocotyledones (grasses). Management differentiated this response as elevated CO₂ increased the proportion of forbs when infrequently and of legumes when frequently defoliated. However, among the two dominant forbs species only one was significantly enhanced by elevated CO₂. Moreover, not all grass species responded negatively to high CO₂. At a low cutting frequency, the observed decline under ambient CO₂ in species diversity (Shannon‐Weaver index) and in forb species number was partly alleviated by elevated CO₂. This experiment shows that the botanical composition of temperate grasslands is likely to be affected by the current rise (+ 0.5% per year) in the atmospheric CO₂ concentration, and that grassland management guidelines may need to be adapted to a future high CO₂ world.     

Performance of the invasive weevil Polydrusus sericeus is influenced by atmospheric CO2 and host species

• 1 Natural forest systems constitute a major portion of the world's land area, and are subject to the potentially negative effects of both global climate change and invasion by exotic insects. A suite of invasive weevils has become established in the northern hardwood forests of North America. How these insects will respond to increasing CO₂ or O₃ is unknown.
• 2 The present study examined the effects of elevated atmospheric CO₂ and O₃ on the invasive weevil Polydrusus sericeus Schaller at the Aspen Free Air CO₂ Enrichment (FACE) site near Rhinelander, Wisconsin. A performance assay was conducted in the laboratory during the summer of 2007 using mated pairs of P. sericeus fed a combination of aspen, birch and maple foliage. We recorded leaf area consumption, oviposition and adult longevity. We also conducted visual abundance surveys in the field from 2004 to 2007 on aspen and birch at Aspen FACE.
• 3 Elevated CO₂, but not O₃, significantly affected P. sericeus performance. Female, but not male, longevity was reduced under elevated CO₂. Polydrusus sericeus also produced fewer eggs under elevated CO₂ conditions compared with ambient conditions. Adult P. sericeus strongly preferred birch over both aspen and maple, regardless of fumigation treatment.
• 4 The effects of elevated CO₂ on P. sericeus populations at Aspen FACE were minimal, and varied among years and host tree species. Polydrusus sericeus abundance was significantly greater on birch than aspen. Over the long term, elevated CO₂ may reduce adult female longevity and fecundity of P. sericeus. Further studies are needed to evaluate how this information may scale to ecosystem impacts.

     

ABA and BAP improve the accumulation of carbohydrates and alter carbon allocation in potato plants at elevated CO2

Elevated CO₂ interactions with other factors affects the plant performance. Regarding the differences between cultivars in response to CO₂ concentrations, identifying the cultivars that better respond to such conditions would maximize their potential benefits. Increasing the ability of plants to benefit more from elevated CO₂ levels alleviates the adverse effects of photoassimilate accumulation on photosynthesis and increases the productivity of plants. Despite its agronomic importance, there is no information about the interactive effects of elevated CO₂ concentration and plant growth regulators (PGRs) on potato (Solanum tuberosum L.) plants. Hence, the physiological response and source-sink relationship of potato plants (cvs. Agria and Fontane) to combined application of CO₂ levels (400 vs. 800 µmol mol⁻¹) and plant growth regulators (PGR) [6-benzylaminopurine (BAP) + Abscisic acid (ABA)] were evaluated under a controlled environment. The results revealed a variation between the potato cultivars in response to a combination of PGRs and CO₂ levels. Cultivars were different in leaf chlorophyll content; Agria had higher chlorophyll a, b, and total chlorophyll content by 23, 43, and 23%, respectively, compared with Fontane. The net photosynthetic rate was doubled at the elevated compared with the ambient CO₂. In Agria, the ratio of leaf intercellular to ambient air CO₂ concentrations [C_i:C_a] was declined in elevated-CO₂-grown plants, which indicated the stomata would become more conservative at higher CO₂ levels. On the other hand, the increased C_i:C_a in Fontane showed a stomatal acclimation to higher CO₂ concentration. The higher leaf dark respiration of the elevated CO₂-grown and BAP + ABA-treated plants was associated with a higher leaf soluble carbohydrates and starch content. Elevated CO₂ and BAP + ABA shifted the dry matter partitioning to the belowground more than the above-media organs. The lower leaf soluble carbohydrate content and greater tuber yield in Fontane might indicate a more efficient photoassimilate translocation than Agria. The results highlighted positive synergic effects of the combined BAP + ABA and elevated CO₂ on tuber yield and productivity of the potato plants.     

Selective grazing modifies previously anticipated responses of plant community composition to elevated CO2 in a temperate grassland

Our limited understanding of terrestrial ecosystem responses to elevated CO₂ is a major constraint on predicting the impacts of climate change. A change in botanical composition has been identified as a key factor in the CO₂ response with profound implications for ecosystem services such as plant production and soil carbon storage. In temperate grasslands, there is a strong consensus that elevated CO₂ will result in a greater physiological stimulus to growth in legumes and to a lesser extent forbs, compared with C3 grasses, and the presumption this will lead in turn to a greater proportion of these functional groups in the plant community. However, this view is based on data mainly collected in experiments of three or less years in duration and not in experiments where defoliation has been by grazing animals. Grazing is, however, the most common management of grasslands and known in itself to influence botanical composition. In a long‐term Free Air Carbon Dioxide Enrichment (FACE) experiment in a temperate grassland managed with grazing animals (sheep), we found the response to elevated CO₂ in plant community composition in the first 5 years was consistent with the expectation of increased proportions of legumes and forbs. However, in the longer term, these differences diminished so that the proportions of grasses, legumes and forbs were the same under both ambient and elevated CO₂. Analysis of vegetation before and after each grazing event showed there was a sustained disproportionately greater removal (‘apparent selection’) of legumes and forbs by the grazing animals. This bias in removal was greater under elevated CO₂ than ambient CO₂. This is consistent with sustained faster growth rates of legumes and forbs under elevated CO₂ being countered by selective defoliation, and so leading to little difference in community composition.     

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

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

Atmospheric change alters frass quality of forest canopy herbivores

This study examined the independent and interactive effects of elevated atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) on the foliar and litter chemistry of two deciduous tree species and the frass chemistry of four lepidopteran folivores. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown under elevated levels of CO₂ and/or O₃ at the Aspen FACE research site in northern WI, USA. We measured the effects of CO₂ and O₃ on nitrogen, carbon to nitrogen (C:N), and condensed tannin levels in aspen and birch leaves and senescent litter and also in the frass of folivores fed aspen or birch green leaves. Overall, the effects of elevated CO₂ on foliar chemistry were less pronounced than those of elevated O₃, and aspen responded more strongly than birch. While the effects of elevated CO₂ and O₃ on foliar chemistry were generally reflected in frass chemistry, the magnitude of the response varied among insect species. Insect frass had higher nitrogen and condensed tannin levels and lower C:N ratios than did litter, although the magnitude of this response varied among fumigation treatments and insect species. Our findings demonstrate that the quality of insect-mediated organic deposition can be indirectly affected by atmospheric change, through altered foliar quality. Our findings also suggest that the quality of frass deposited on the forest floor via herbivory will be strongly affected by herbivore community composition.     

Atmospheric change alters foliar quality of host trees and performance of two outbreak insect species

This study examined the independent and interactive effects of elevated carbon dioxide (CO₂) and ozone (O₃) on the foliar quality of two deciduous trees species and the performance of two outbreak herbivore species. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under four combinations of ambient and elevated CO₂ and O₃. We measured the effects of elevated CO₂ and O₃ on aspen and birch phytochemistry and on gypsy moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria) performance. Elevated CO₂ nominally affected foliar quality for both tree species. Elevated O₃ negatively affected aspen foliar quality, but only marginally influenced birch foliar quality. Elevated CO₂ slightly improved herbivore performance, while elevated O₃ decreased herbivore performance, and both responses were stronger on aspen than birch. Interestingly, elevated CO₂ largely offset decreased herbivore performance under elevated O₃. Nitrogen, lignin, and C:N were identified as having strong influences on herbivore performance when larvae were fed aspen, but no significant relationships were observed for insects fed birch. Our results support the notion that herbivore performance can be affected by atmospheric change through altered foliar quality, but how herbivores will respond will depend on interactions among CO₂, O₃, and tree species. An emergent finding from this study is that tree age and longevity of exposure to pollutants may influence the effects of elevated CO₂ and O₃ on plant–herbivore interactions, highlighting the need to continue long-term atmospheric change research.     

Aphid individual performance may not predict population responses to elevated CO2 or O3

Changes in atmospheric composition affect plant quality and herbivore performance. We used the Aspen Free Air CO₂ Enrichment (FACE) facility to investigate the impacts of elevated carbon dioxide (CO₂) and ozone (O₃) on the performance of the aphid Cepegillettea betulaefoliae Granovsky feeding on paper birch (Betula papyrifera Marsh.). In Year 1, we simultaneously measured individual performance and population growth rates, and in Year 2 we surveyed natural aphid, predator and parasitoid populations throughout the growing season. Aphid growth and development (relative growth rate (RGR), development time, adult weight, embryo number and the birth weight of newborn nymphs) were unaffected by CO₂ and O₃. Aphid fecundity decreased on trees grown at elevated CO₂, O₃ and CO₂+O₃. Neither nymphal performance nor adult size were reliable indicators of future fecundity at elevated CO₂ and/or O₃. Aphid populations protected from natural enemies were unaffected by elevated CO₂, but increased significantly at elevated O₃. Individual fecundity in elevated CO₂ and O₃ atmospheres did not predict population growth rates, probably because of changes in the strength of intraspecific competition or the ability of the aphids to induce nutrient sinks. Natural aphid, predator and parasitoids populations (Year 2) showed few significant responses to CO₂ and O₃, although CO₂ and O₃ did affect the timing of aphid and natural enemy peak abundance. Elevated CO₂ and O₃ affected aphid and natural enemy populations independently: no CO₂× O₃ interactions were observed. We conclude that: (1) aphid individual performance did not predict population responses to CO₂ and O₃ and (2) elevated CO₂ and O₃ atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities.     

Interacting effects of elevated CO2, nutrient availability and plant species on a generalist invertebrate herbivore

By affecting plant growth and phytochemistry elevated CO₂ may have indirect effects on the performance of herbivores. These effects show considerable variability across studies and may depend on nutrient availability, the carbon/nutrient‐balance in plant tissues and the secondary metabolism of plants. We studied the responses to elevated CO₂ and different nutrient availability of 12 herbaceous plant species differing in their investment into secondary compounds. Caterpillars of the generalist herbivore Spodoptera littoralis were reared on the leaves produced and their consumption and growth rates analysed. Elevated CO₂ resulted in a similar increase of biomass in all plant species, whereas the positive effect of fertilization varied among plant species. Specific leaf weight was influenced by elevated CO₂, but the effect depended on nutrient level and identity of plant species. Elevated CO₂ increased the C/N ratio of the leaves of most species. Caterpillars consumed more leaf material when plants were grown under elevated CO₂ and low nutrients. This indicates compensatory feeding due to lower tissue quality. However, the effects of elevated CO₂, nutrient availability and plant species identity on leaf consumption interacted. Both the effects of CO₂ and nutrient availability on the relative growth rate of the herbivore depended on the plant species. The feeding rate of S. littoralis on plant species that do not produce nitrogen‐containing secondary compounds (NCSC) was higher under low nutrient availability. In contrast, in plants producing NCSC nutrient availability had no effect on the feeding rate. This suggests that compensatory feeding in response to low nutrient contents may not be possible if plants produce NCSC. We conclude that elevated CO₂ causes species‐specific changes in the quality of plant tissues and consequently in changes in the preferences of herbivores for plant species. This could result in changes in plant community composition.     

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.     

Elevated CO2 and O3 Alter Soil Nitrogen Transformations beneath Trembling Aspen, Paper Birch, and Sugar Maple

Nitrogen cycling in northern temperate forest ecosystems could change under increasing atmospheric CO₂ and tropospheric O₃ as a result of quantitative and qualitative changes in plant litter production. At the Aspen Free Air CO₂–O₃ Enrichment (FACE) experiment, we previously found that greater substrate inputs to soil under elevated CO₂ did not alter gross N transformation rates in the first 3 years of the experiment. We hypothesized that greater litter production under elevated CO₂ would eventually cause greater gross N transformation rates and that CO₂ effects would be nullified by elevated O₃. Following our original study, we continued measurement of gross N transformation rates for an additional four years. From 1999 to 2003, gross N mineralization doubled, N immobilization increased 4-fold, but changes in microbial biomass N and soil total N were not detected. We observed year-to-year variation in N transformation rates, which peaked during a period of foliar insect damage. Elevated CO₂ caused equivalent increases in gross rates of N mineralization (+34%) and NH ₄ ⁺ immobilization (+36%). These results indicate greater rates of N turnover under elevated CO₂, but do not indicate a negative feedback between elevated CO₂ and soil N availability. Elevated O₃ decreased gross N mineralization (−16%) and had no effect on NH ₄ ⁺ immobilization, indicating reduced N availability under elevated O₃. The effects of CO₂ and O₃ on N mineralization rates were mainly related to changes in litter production, whereas effects on N immobilization were likely influenced by changes in litter chemistry and production. Our findings also indicate that concomitant increases in atmospheric CO₂ and O₃ could lead to a negative feedback on N availability.     

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.     

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.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

How do aphids respond to elevated CO2?

The performance of herbivore insects is determined directly by the quality of host plants. Elevated CO₂ induced a decline in foliar nitrogen, which reduced the growth of chewing insects. Phloem-sucking insects (i.e. aphid), however, had species-specific responses to elevated CO₂ and were the only feeding guild to respond positively to elevated CO₂. Although many studies attempt to illuminate the interaction between aphids and plants under elevated CO₂, few studies can explain why some aphids are more successful than other chewing insects in elevated CO₂. Elevated CO₂ leads to a re-allocation of the carbon and nitrogen resources in plant tissue, which increases the thickness of the microscopic structures of leaves, reduces amino acids content of leaf phloem sap and increases the secondary metabolites. Considering the complexity of aphid–plant interactions, it is difficult and unreasonable to predict the general response of aphids to elevated CO₂ using a single plant component. Instead, it is more likely that aphids are able to overcome the disadvantages of the indirect effects of elevated CO₂ by reducing developmental times and increasing fecundity under elevated CO₂ conditions. Our results provide several clues to why some aphids are successful in elevated CO₂ conditions. We review recent studies of the effects of elevated CO₂ on aphids and discuss the effects of elevated CO₂ on aphid performance on crops using cotton and cereal aphids as examples.     

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.     

Projected Carbon Dioxide to Increase Grass Pollen and Allergen Exposure Despite Higher Ozone Levels

One expected effect of climate change on human health is increasing allergic and asthmatic symptoms through changes in pollen biology. Allergic diseases have a large impact on human health globally, with 10–30% of the population affected by allergic rhinitis and more than 300 million affected by asthma. Pollen from grass species, which are highly allergenic and occur worldwide, elicits allergic responses in 20% of the general population and 40% of atopic individuals. Here we examine the effects of elevated levels of two greenhouse gases, carbon dioxide (CO₂), a growth and reproductive stimulator of plants, and ozone (O₃), a repressor, on pollen and allergen production in Timothy grass (Phleum pratense L.). We conducted a fully factorial experiment in which plants were grown at ambient and/or elevated levels of O₃ and CO₂, to simulate present and projected levels of both gases and their potential interactive effects. We captured and counted pollen from flowers in each treatment and assayed for concentrations of the allergen protein, Phl p 5. We found that elevated levels of CO₂ increased the amount of grass pollen produced by ∼50% per flower, regardless of O₃ levels. Elevated O₃ significantly reduced the Phl p 5 content of the pollen but the net effect of rising pollen numbers with elevated CO₂ indicate increased allergen exposure under elevated levels of both greenhouse gases. Using quantitative estimates of increased pollen production and number of flowering plants per treatment, we estimated that airborne grass pollen concentrations will increase in the future up to ∼200%. Due to the widespread existence of grasses and the particular importance of P. pratense in eliciting allergic responses, our findings provide evidence for significant impacts on human health worldwide as a result of future climate change.