Insect herbivory in an intact forest understory under experimental CO2 enrichment

Human-induced increases in atmospheric CO₂ concentration have the potential to alter the chemical composition of plant tissue, and thereby affect the amount of tissue consumed by herbivorous arthropods. At the Duke Forest free-air concentration enrichment (FACE) facility in North Carolina (FACTS–1 research facility), we measured the amount of leaf tissue damaged by insects and other herbivorous arthropods during two growing seasons in a deciduous forest understory continuously exposed to ambient (360 l l^–1) and elevated (~560 µl l^–1) CO₂ conditions. In 1999, there was a significant interaction between CO₂ and species such that winged elm (Ulmus alata) showed lower herbivory in elevated CO₂ plots, whereas red maple (Acer rubra) and sweetgum (Liquidambar styraciflua) did not. In 2000, our results did not achieve statistical significance but the magnitude of the result was consistent with the 1999 results. In 1999 and 2000, we found a decline (10–46%) in community-level herbivory in elevated CO₂ plots driven primarily by reductions in herbivory on elm. The major contribution to total leaf damage was from missing tissue (66% of the damaged tissue), with galls, skeletonized, and discolored tissue making smaller contributions. It is unclear whether the decline in leaf damage is a result of altered insect populations, altered feeding, or a combination. We were not able to quantify insect populations, and our measurements did not resolve an effect of elevated CO₂ on leaf chemical composition (total nitrogen, carbon, C/N, sugars, phenolics, starch). Despite predictions from a large number of single-species studies that herbivory may increase under elevated CO₂, we have found a decrease in herbivory in a naturally established forest understory exposed to a full suite of insect herbivores and their predators.     

Seeing the forest for the trees: long-term exposure to elevated CO2 increases some herbivore densities

The effects of elevated CO₂ on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub‐oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO₂, over an 11‐year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO₂ than ambient CO₂ for each of three species of oaks, Quercus myrtifolia, Quercus chapmanii and Quercus geminata. However, leaf litter production per 0.1143 m² was greater under elevated CO₂ than ambient CO₂ for Q. myrtifolia and Q. chapmanii, and this difference increased over the 11 years of the study. Leaf production of Q. geminata under elevated CO₂ did not increase. Leafminer densities per 0.1143 m² of litterfall for Q. myrtifolia and Q. chapmanii were initially lower under elevated CO₂. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m² of litter fall became higher under elevated CO₂ and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m² were also higher under elevated CO₂ for Q. myrtifolia and Q. chapmanii over the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m² of litter for Q. geminata. These results show three phenomena. First, they show that elevated CO₂ decreases herbivory on all oak species in the Florida scrub‐oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO₂, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO₂ on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m² in elevated CO₂ over time while another oak species did not. Our results point to a future world of elevated CO₂ where, despite lower plant herbivory, some insect herbivores may become more common.     

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.     

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.     

Effects of elevated CO2 on five plant-aphid interactions

We investigated interactions between five species of phloem-feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO₂; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO₂ generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO₂, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO₂ were species-specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO₂ treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO₂. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO₂ and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO₂ in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem-feeders may not be negatively affected by increased CO₂ concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO₂.     

Consequences of elevated temperature and pCO2 on insect folivory at the ecosystem level: perspectives from the fossil record

Paleoecological studies document the net effects of atmospheric and climate change in a natural laboratory over timescales not accessible to laboratory or ecological studies. Insect feeding damage is visible on well‐preserved fossil leaves, and changes in leaf damage through time can be compared to environmental changes. We measured percent leaf area damaged on four fossil leaf assemblages from the Bighorn Basin, Wyoming, that range in age from 56.1 to 52.65 million years (Ma). We also include similar published data from three US sites 49.4 to ~45 Ma in our analyses. Regional climate was subtropical or warmer throughout this period, and the second oldest assemblage (56 Ma) was deposited during the Paleocene–Eocene Thermal Maximum (PETM), a geologically abrupt global warming event caused by massive release of carbon into the atmosphere. Total and leaf‐chewing damage are highest during the PETM, whether considering percent area damaged on the bulk flora, the average of individual host plants, or a single plant host that occurs at multiple sites. Another fossil assemblage in our study, the 52.65 Ma Fifteenmile Creek paleoflora, also lived during a period of globally high temperature and pCO₂, but does not have elevated herbivory. Comparison of these two sites, as well as regression analyses conducted on the entire dataset, demonstrates that, over long timescales, temperature and pCO₂ are uncorrelated with total insect consumption at the ecosystem level. Rather, the most important factor affecting herbivory is the relative abundance of plants with nitrogen‐fixing symbionts. Legumes dominate the PETM site; their prevalence would have decreased nitrogen limitation across the ecosystem, buffering generalist herbivore populations against decreased leaf nutritional quality that commonly occurs at high pCO₂. We hypothesize that nitrogen concentration regulates the opposing effects of elevated temperature and CO₂ on insect abundance and thereby total insect consumption, which has important implications for agricultural practices in today's world of steadily increasing pCO₂.     

Plant Nitrogen Dynamics in Shortgrass Steppe under Elevated Atmospheric Carbon Dioxide

The direct and indirect effects of increasing levels of atmospheric carbon dioxide (CO₂) on plant nitrogen (N) content were studied in a shortgrass steppe ecosystem in northeastern Colorado, USA. Beginning in 1997 nine experimental plots were established: three open-top chambers with ambient CO₂ levels (approximately 365 mol mol^–1), three open-top chambers with twice-ambient CO₂ levels (approximately 720 mol mol^–1), and three unchambered control plots. After 3 years of growing-season CO₂ treatment, the aboveground N concentration of plants grown under elevated atmospheric CO₂ decreased, and the carbon–nitrogen (C:N) ratio increased. At the same time, increased aboveground biomass production under elevated atmospheric CO₂ conditions increased the net transfer of N out of the soil of elevated-CO₂ plots. Aboveground biomass production after simulated herbivory was also greater under elevated CO₂ compared to ambient CO₂. Surprisingly, no significant changes in belowground plant tissue N content were detected in response to elevated CO₂. Measurements of individual species at peak standing phytomass showed significant effects of CO₂ treatment on aboveground plant tissue N concentration and significant differences between species in N concentration, suggesting that changes in species composition under elevated CO₂ will contribute to overall changes in nutrient cycling. Changes in plant N content, driven by changes in aboveground plant N concentration, could have important consequences for biogeochemical cycling rates and the long-term productivity of the shortgrass steppe as atmospheric CO₂ concentrations increase.     

Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?

Though field data for naturally senesced leaf litter are rare, it is commonly assumed that rising atmospheric CO₂ concentrations will reduce leaf litter quality and decomposition rates in terrestrial ecosystems and that this will lead to decreased rates of nutrient cycling and increased carbon sequestration in native ecosystems. We generally found that the quality of␣naturally senesced leaf litter (i.e. concentrations of C, N and lignin; C:N, lignin:N) of a variety of native plant species produced in alpine, temperate and tropical communities maintained at elevated CO₂ (600–680 μl l⁻¹) was not significantly different from that produced in similar communities maintained at current ambient CO₂ concentrations (340–355 μl l⁻¹). When this litter was allowed to decompose in situ in a humid tropical forest in Panama (Cecropia peltata, Elettaria cardamomum, and Ficus benjamina, 130 days exposure) and in a lowland temperate calcareous grassland in Switzerland (Carex flacca and a graminoid species mixture; 261 days exposure), decomposition rates of litter produced under ambient and elevated CO₂ did not differ significantly. The one exception to this pattern occurred in the high alpine sedge, Carex curvula, growing in the Swiss Alps. Decomposition of litter produced in situ under elevated CO₂ was significantly slower than that of litter produced under ambient CO₂ (14% vs. 21% of the initial litter mass had decomposed over a 61-day exposure period, respectively). Overall, our results indicate that relatively little or no change in leaf litter quality can be expected in plant communities growing under soil fertilities common in many native ecosystems as atmospheric CO₂ concentrations continue to rise. Even in situations where small reductions in litter quality do occur, these may not necessarily lead to significantly slower rates of decomposition. Hence in many native species in situ litter decomposition rates, and the time course of decomposition, may remain relatively unaffected by rising CO₂. Received: 12 September 1996 / Accepted: 30 November 1996     

Growth dynamics and population development in an alpine grassland under elevated CO2

Leaf expansion, population dynamics and reproduction under elevated CO₂ were studied for two dominant and four subdominant species in a high alpine grassland (2500 above sea level, Swiss Central Alps). Plots of alpine heath were exposed to 335 l l^-1 and 680 l l^-1 CO₂ in open-top chambers over three growing seasons. Treatments also included natural and moderately improved mineral nutrient supply (40 kg N ha^-1 year^-1 in an NPK fertilizer mix). Seasonal dynamics of leaf expansion, which was studied for the dominant graminoid Carex curvula only, were not affected by elevated CO₂ during two warm seasons or during a cool season. Improved nutrient supply increased both the expansion rate and the duration of leaf growth but elevated CO₂ did not cause any further stimulation. Plant and tiller density (studied in all species) increased under elevated CO₂ in the codominant Leontodon helveticus and the subdominant Trifolium alpinum, remained unchanged in two other minor species Poa alpina and Phyteuma globulariifolium, and decreased in Carex curvula. In Potentilla aurea elevated CO₂ compensated for a natural decline in shoot number. By year 3 the number of fertile shoots in Leontodon and individual seed weight in Carex were slightly increased under elevated CO₂, indicating CO₂ effects on sexual reproduction in these two dominant species. The results suggest that the effects of elevated CO₂ on the population dynamics of the species studied were not general, but species-specific and rather moderate effects. However, the reduction of tiller density in Carex curvula, in contrast to the increases observed in Leontodon helveticus and Trifolium alpinum, indicates that elevated CO₂ may negatively affect the abundance of the species most characteristic of this alpine plant community.     

Impacts of urbanization on insect herbivory and plant defences in oak trees

Systematic comparisons of species interactions in urban versus rural environments can improve our understanding of shifts in ecological processes due to urbanization. However, such studies are relatively uncommon and the mechanisms driving urbanization effects on species interactions (e.g. between plants and insect herbivores) remain elusive. Here we investigated the effects of urbanization on leaf herbivory by insect chewers and miners associated with the English oak Quercus robur by sampling trees in rural and urban areas throughout most of the latitudinal distribution of this species. In performing these comparisons, we also controlled for the size of the urban areas (18 cities) and gathered data on CO₂ emissions. In addition, we assessed whether urbanization affected leaf chemical defences (phenolic compounds) and nutritional traits (phosphorus and nitrogen), and whether such changes correlated with herbivory levels. Urbanization significantly reduced leaf chewer damage but did not affect leaf miners. In addition, we found that leaves from urban locations had lower levels of chemical defences (condensed and hydrolysable tannins) and higher levels of nutrients (nitrogen and phosphorus) compared to leaves in rural locations. The magnitude of urbanization effects on herbivory and leaf defences was not contingent upon city size. Importantly, while the effects of urbanization on chemical defences were associated with CO₂ emissions, changes in leaf chewer damage were not associated with either leaf traits or CO₂ levels. These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here. Overall, our simultaneous assessment of insect herbivory, plant traits and abiotic correlates advances our understanding of the main drivers of urbanization effects on plant–herbivore interactions.     

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.     

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.     

Increase of fast nutrient cycling in grassland microcosms through insect herbivory depends on plant functional composition and species diversity

Nutrient cycling in terrestrial ecosystems is affected by various factors such as plant diversity and insect herbivory. While several studies suggest insect herbivory to depend on plant diversity, their interacting effect on nutrient cycling is unclear. In a greenhouse experiment with grassland microcosms of one to six plant species of two functional groups (grasses and legumes), we tested the influence of plant species richness (diversity) and functional composition on plant community biomass production, insect foliar herbivory, soil microbial biomass, and nutrient concentrations in throughfall. To manipulate herbivory, zero, three or six generalist grasshoppers (Chorthippus parallelus) were added to the plant communities. Increasing plant species richness increased shoot biomass and grasshopper performance, without significantly affecting root biomass or insect herbivory. Plant functional composition affected all of these parameters, e.g. legume communities showed the highest shoot biomass, the lowest grasshopper performance and suffered the least herbivory. Nutrient concentrations (dissolved mineral N, PO₄‐P, SO₄‐S) and pH in throughfall increased with herbivory. PO₄‐P and pH increases were positively affected by plant diversity, especially under high herbivore pressure. Plant functional composition affected several throughfall variables, sometimes fully explaining diversity effects. Increasing plant diversity tended to increase soil microbial biomass, but only under high herbivore pressure. Faeces quantities strongly correlated with changes in pH and PO₄‐P; frass may therefore be an important driver of throughfall pH and a main source of PO₄‐P released from living plants. Our results indicate that insect herbivory may significantly influence fast nutrient cycling processes in natural communities, particularly so in managed grasslands.     

Effects of elevated atmospheric CO<Subscript>2</Subscript> on the nutritional ecology of C<Subscript>3</Subscript> and C<Subscript>4</Subscript> grass-feeding caterpillars

It is plausible that the nutritional quality of C₃ plants will decline more under elevated atmospheric CO₂ than will the nutritional quality of C₄ plants, causing herbivorous insects to increase their feeding on C₃ plants relative to C₄ plants. We tested this hypothesis with a C₃ and C₄ grass and two caterpillar species with different diet breadths. Lolium multiflorum (C₃) and Bouteloua curtipendula (C₄) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changed to a greater extent than did that of B. curtipendula when grown in elevated CO₂; levels of protein (considered growth limiting) declined in the C₃ grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C₃ grass to compensate for its lower nutritional quality when grown in an elevated CO₂ atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C₃ grass than the C₄ grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C₄ grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO₂ levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C₄ grasses would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C₄ grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.     

Potential effects of elevated carbon dioxide on leaf-feeding forest insects

The elevated concentration of atmospheric CO₂ may result in a decline of leaf nutritional quality (especially N) and an increase in some kinds of defensive secondary components (such as phenolics). The changes in the phytochemistry of trees, combined with the effect of elevated CO₂ per se, have a potential negative influence on insect herbivores. Here, we review the effect of elevated CO₂ on the performance of leaffeeding forest insects at individual-level and community-level. The elevated CO₂ per se have little influence on the metabolism of insects. Over half of the tree-insect experimental systems show that the performance of individual insect become poorer under high-CO₂ grown trees; but the others show that the insects have just little or no response to the treatments. The direction and magnitude of the changes in the performance of insects could be mediated by various factors. The effects of treatment are strongly species-dependent. The magnitude of changes in the phytochemistry, the sensitivity and adaptive capacity of insects to the poorer leaf quality, the differences in plant growth conditions and experimental methods, and the mediated effects of other environmental factors (such as soil nutrient availability, light, temperature, O₃) were all closely related to the final performance of insects. However, the larvae’s consumption usually increased under enriched CO₂ treatment, which was widely thought to be a compensatory response to poorer plant quality. The experiments on forest community-level found identically a reduction in herbivory, which was contrary to the results from small-scale experiments. The changes in insect population and the actual response of consumption by leaf-feeding forest insects under CO₂ enrichment remain unclear, and more field-based experiments need to be conducted. __________ Translated from Chinese Journal of Applied Ecology, 2006, 17(4): 720–726 [译自: 应用生态学报]     

Performance of a generalist grasshopper on a C<Subscript>3</Subscript> and a C<Subscript>4</Subscript> grass: compensation for the effects of elevated CO<Subscript>2</Subscript> on plant nutritional quality

The increasing CO₂ concentration in Earths atmosphere is expected to cause a greater decline in the nutritional quality of C₃ than C₄ plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C₃ plants. These hypotheses were tested by growing the grasses Lolium multiflorum C₃) and Bouteloua curtipendula C₄) at ambient (370 ppm) and elevated (740 ppm) CO₂ levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant × CO₂ treatment combinations. As expected, the nutritional quality of the C₃ grass declined to a greater extent than did that of the C₄ grass at elevated CO₂; protein levels declined in the C₃ grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C₃ grass grown under elevated CO₂. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C₃ grass under elevated CO₂. Consumption rates of the C₃ and C₄ grasses were also similar, demonstrating a lack of compensatory feeding on the C₄ grass. We also examined the relative efficiencies of nutrient utilization from a C₃ and C₄ grass by M. sanguinipes to test the basis for the C₄ plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C₄ grass than from the C₃ grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C₃ grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C₃ grass and the better growth performance of M. sanguinipes on this grass at both CO₂ levels, we conclude that C₃ grasses are likely to remain better host plants than C₄ grasses in future CO₂ conditions.     

Effects of elevated CO2 on development and larval food-plant preference in the butterfly Coenonympha pamphilus (Lepidoptera, Satyridae)

The objective of this study was to determine how increasing atmospheric CO₂ change plant tissue quality in four native grassland grass species (Agrostis stolonifera, Anthoxanthum odoratum, Festuca rubra, Poa pratensis) which are all larval food‐plants of Coenonympha pamphilus (Lepidoptera, Satyridae). We assessed the effect of these changes on the performance and larval food‐plant preference of C. pamphilus in a greenhouse experiment. Furthermore, we tested the interactive effects of elevated CO₂ and soil nutritional availability in F. rubra and its effect an larval development of C. pamphilus. In general, elevated CO₂ decreased leaf water concentration, nitrogen concentration and specific leaf area (SLA), while leaf starch concentration was increased in all grass species. A species‐specific reaction to elevated CO₂ was only found for foliar starch concentration. P. pratensis did not increase its starch concentration under elevated CO₂ conditions, whereas the other three species did. Fertilisation, investigated only for F. rubra, increased leaf nitrogen concentration and amplified the CO₂‐induced decrease in leaf nitrogen. Development time of C. pamphilus was on the average prolonged by two days under elevated CO₂ and the prolongation differed from 0.7 to 5.3 days among food‐plant species. Pupal fresh weight differed marginally between CO₂ treatments. Fertilisation of the larval food‐plant F. rubra shortened development time by one day and significantly increased pupal and adult fresh weights. C. pamphilus larvae showed a clear food‐plant preference among grass species at the age of 36 h or older. Additionally, a change of food‐plant preference under elevated CO₂ was found. Larvae at ambient CO₂ preferred Agrostis stolonifera and F. rubra, while under elevated CO₂Anthoxanthum odoratum and P. pratensis were preferred. The present study demonstrates that larval development of C. pamphilus is affected by food‐plant species and CO₂ induced changes in foliar chemistry. Although we found some species‐specific reactions to elevated CO₂ for foliar chemistry, no such CO₂ by species interaction was found for insect development. The change in food‐plant preference of larvae under elevated CO₂ implies potential changes in selection pressure for grass species and might therefore affect evolutionary processes.     

Loblolly pine grown under elevated CO2 affects early instar pine sawfly performance

Seedlings of loblolly pine Pinus taeda (L.), were grown in open-topped field chambers under three CO₂ regimes: ambient, 150 l l^–1 CO₂ above ambient, and 300 l l^–1 CO₂ above ambient. A fourth, non-chambered ambient treatment was included to assess chamber effects. Needles were used in 96 h feeding trials to determine the performance of young, second instar larvae of loblolly pine's principal leaf herbivore, red-headed pine sawfly, Neodiprion lecontei (Fitch). The relative consumption rate of larvae significantly increased on plants grown under elevated CO₂, and needles grown in the highest CO₂ regime were consumed 21% more rapidly than needles grown in ambient CO₂. Both the significant decline in leaf nitrogen content and the substantial increase in leaf starch content contributed to a significant increase in the starch:nitrogen ratio in plants grown in elevated CO₂. Insect consumption rate was negatively related to leaf nitrogen content and positively related to the starch:nitrogen ratio. Of the four volatile leaf monoterpenes measured, only -pinene exhibited a significant CO₂ effect and declined in plants grown in elevated CO₂. Although consumption changed, the relative growth rates of larvae were not different among CO₂ treatments. Despite lower nitrogen consumption rates by larvae feeding on the plants grown in elevated CO₂, nitrogen accumulation rates were the same for all treatments due to a significant increase in nitrogen utilization efficiency. The ability of this insect to respond at an early, potentially susceptible larval stage to poorer food quality and declining levels of a leaf monoterpene suggest that changes in needle quality within pines in future elevated-CO₂ atmospheres may not especially affect young insects and that tree-feeding sawflies may respond in a manner similar to herb-feeding lepidopterans.     

Nitrogen dynamics and growth of seedlings of an N-fixing tree (Gliricidia sepium (Jacq.) Walp.) exposed to elevated atmospheric carbon dioxide

Summary Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO₂ and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO₂ partial pressures of 350 and 650 bar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO₂ when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO₂ enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO₂ enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO₂. In seedlings supplied with soil N, elevated CO₂ increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf ¹⁵N). While CO₂ enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO₂ partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO₂ will depend greatly on plant and soil nutrient status.     

Will chemical defenses become more effective against specialist herbivores under elevated CO2?

Elevated atmospheric CO₂ is known to affect plant–insect herbivore interactions. Elevated CO₂ causes leaf nitrogen to decrease, the ostensible cause of herbivore compensatory feeding. CO₂ may also affect herbivore consumption by altering chemical defenses via changes in plant hormones. We considered the effects of elevated CO₂, in conjunction with soil fertility and damage (simulated herbivory), on glucosinolate concentrations of mustard (Brassica nigra) and collard (B. oleracea var. acephala) and the effects of leaf nitrogen and glucosinolate groups on specialist Pieris rapae consumption. Elevated CO₂ affected B. oleracea but not B. nigra glucosinolates; responses to soil fertility and damage were also species‐specific. Soil fertility and damage also affected B. oleracea glucosinolates differently under elevated CO₂. Glucosinolates did not affect P. rapae consumption at either CO₂ concentration in B. nigra, but had CO₂‐specific effects on consumption in B. oleracea. At ambient CO₂, leaf nitrogen had strong effects on glucosinolate concentrations and P. rapae consumption but only gluconasturtiin was a feeding stimulant. At elevated CO₂, direct effects of leaf nitrogen were weaker, but glucosinolates had stronger effects on consumption. Gluconasturtiin and aliphatic glucosinolates were feeding stimulants and indole glucosinolates were feeding deterrents. These results do not support the compensatory feeding hypothesis as the sole driver of changes in P. rapae consumption under elevated CO₂. Support for hormone‐mediated CO₂ response (HMCR) was mixed; it explained few treatment effects on constitutive or induced glucosinolates, but did explain patterns in SEMs. Further, the novel feeding deterrent effect of indole glucosinolates under elevated CO₂ in B. oleracae underscores the importance of defensive chemistry in CO₂ response. We speculate that P. rapae indole glucosinolate detoxification mechanisms may have been overwhelmed under elevated CO₂ forcing slowed consumption. Specialists may have to contend with hosts with poorer nutritional quality and more effective chemical defenses under elevated CO₂.