Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

Stomata of trees growing in CO2-enriched air show reduced sensitivity to vapour pressure deficit and drought

Stomatal conductance ( g _s) and photosynthetic rate ( A ) were measured in young beech ( Fagus sylvatica ), chestnut ( Castanea sativa ) and oak ( Quercus robur ) growing in ambient or CO₂-enriched air. In oak, g _s was consistently reduced in elevated CO₂. However, in beech and chestnut, the stomata of trees growing in elevated CO₂ failed to close normally in response to increased leaf-to-air vapour pressure deficit (LAVPD). Consequently, while g _s was reduced in elevated CO₂ on days with low LAVPD, on warm sunny days (with correspondingly high LAVPD) g _s was unchanged or even slightly higher in elevated CO₂. Furthermore, during drought, g _s of beech and chestnut was unresponsive to [CO₂], over a wide range of ambient LAVPD, whereas in oak g _s was reduced by an average of 50% in elevated CO₂. Stimulation of A by elevated CO₂ in beech and chestnut was restricted to days with high irradiance, and was greatest in beech during drought. Hence, most of the additional carbon gain in elevated CO₂ was made at the expense of water economy, at precisely those times (drought, high evaporative demand) when water conservation was most important. Such effects could have serious consequences for drought tolerance, growth and, ultimately, survival as atmospheric [CO₂] increases.     

Does soil nitrogen influence growth,water transport and survival of snow gum (Eucalyptus pauciflora Sieber ex Sprengel.) under CO2 enrichment?

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370  µ L L⁻¹) and elevated (700  µ L L⁻¹) atmospheric [CO₂] in open-top chambers (OTCs) in the field and temperature-controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO₂-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root–shoot junction in CO₂-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated [CO₂] (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated [CO₂] were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15–40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric [CO₂], particularly during water stress.     

Elevated atmospheric CO2 alters wood production, wood quality and wood strength of Scots pine (Pinus sylvestris L) after three years of enrichment

Scots pine ( Pinus sylvestris L.) trees were grown in open top chambers for three years under ambient and elevated CO₂ concentrations. The trees were aged 3 y at the beginning of the CO₂ exposure, and the effects of the treatment on total stem volume, stem wood biomass, wood quality and wood anatomy were examined at the end of the exposure. The elevated CO₂ treatment lead to a 49% and 38% increase in stem biomass and stem wood volume, respectively. However, no significant effects of the elevated CO₂ treatment on wood density were observed, neither when green wood density was estimated from stem biomass and stem volume, nor when oven-dry wood density was measured on small wood samples. Under elevated CO₂ significantly wider growth rings were observed. The effect of elevated CO₂ on growth ring width was primarily the result of an increase in earlywood width. Wood compression strength decreased under elevated CO₂ conditions, which could be explained by significantly larger tracheids and the increased earlywood band, that has thinner walls and larger cavities. A significant decrease of the number of resin canals in the third growth ring was observed under the elevated treatment; this might indicate that trees produced and contained less resin, which has implications for disease and pest resistance. So, although wood volume yield in Scots pine increased significantly with elevated CO₂ after three years of treatment, wood density remained unchanged, while wood strength decreased. Whilst wood volume and stem biomass production may increase in this major boreal forest tree species, wood quality and resin production might decrease under future elevated CO₂ conditions.     

Stem wood properties of mature Norway spruce after 3 years of continuous exposure to elevated [CO2] and temperature

The objective of the study was to investigate the interactive effects of elevated atmospheric carbon dioxide concentration, [CO₂], and temperature on the wood properties of mature field-grown Norway spruce ( Picea abies (L.) Karst.) trees. Material for the study was obtained from an experiment in Flakaliden, northern Sweden, where trees were grown for 3 years in whole-tree chambers at ambient (365 μmol mol⁻¹) or elevated [CO₂] (700 μmol mol⁻¹) and ambient or elevated air temperature (ambient +5.6 °C in winter and ambient +2.8 °C in summer). Elevated temperature affected both wood chemical composition and structure, but had no effect on stem radial growth. Elevated temperature decreased the concentrations of acetone-soluble extractives and soluble sugars, while mean and earlywood (EW) cell wall thickness and wood density were increased. Elevated [CO₂] had no effect on stem wood chemistry or radial growth. In wood structure, elevated [CO₂] decreased EW cell wall thickness and increased tracheid radial diameter in latewood (LW). Some significant interactions between elevated [CO₂] and temperature were found in the anatomical and physical properties of stem wood (e.g. microfibril angle, and LW cell wall thickness and density). Our results show that the wood material properties of mature Norway spruce were altered under exposure to elevated [CO₂] and temperature, although stem radial growth was not affected by the treatments.     

Wood composition and energy content in a poplar short rotation plantation on fertilized agricultural land in a future CO2 atmosphere

To study the influence of elevated CO₂ and nitrogen (N) fertilization on wood properties and energy, Populus × euramericana trees were exposed to ambient CO₂ (about 370 μmol mol⁻¹ CO₂) or elevated CO₂ (about 550 μmol mol⁻¹ CO₂) using Free Air CO₂ Enrichment (FACE) technology in combination with two N levels. Elevated CO₂ was maintained for 5 years. After three growing seasons, the plantation was coppiced, one half of each experimental plot was fertilized and secondary sprouts were harvested after two growing seasons. Fourier transform infrared (FT-IR) spectra of wood revealed significant effects of both elevated CO₂ and N fertilization on wood chemistry, in particular, significant increases in lignin and decreases in N content. These results were corroborated by chemical analysis. Neither elevated CO₂ nor N fertilization affected the calorific value of wood, which was 19.3 MJ kg⁻¹. N fertilization enhanced the energy production per land area by 16–69% because of higher aboveground woody biomass production than on nonfertilized land. Estimates indicate that high yielding poplar short rotation cultivation may significantly contribute as an alternative feedstock for energy production.     

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO₂ and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO₂ on the contribution of fast- ( r -strategists) and slow-growing ( K -strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO₂ treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25–2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C_mic), the maximum specific growth rate (μ), growing microbial biomass (GMB) and lag-period ( t _lag) were estimated by the kinetics of CO₂ emission from bulk soil and aggregates amended with glucose and nutrients. Although C_org and C_mic were unaffected by elevated CO₂, μ values were significantly higher under elevated than ambient CO₂ for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO₂ treatments. Based on changes in μ, GMB and lag period, we conclude that elevated atmospheric CO₂ stimulated the r- selected microorganisms, especially in soil microaggregates. Such an increase in r -selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO₂ environment.     

CO2-mediated changes in aspen chemistry: effects on gypsy moth performance and susceptibility to virus

We investigated the effects of long-term CO₂ enrichment on foliar chemistry of quaking aspen ( Populus tremuloides ) and the consequences of chemical changes for performance of the gypsy moth ( Lymantria dispar ) and susceptibility of the gypsy moth to a nucleopolyhedrosis virus (NPV). Foliage was collected from outdoor open-top chambers and fed to insects in a quarantine rearing facility. Under enriched CO₂, levels of leaf nitrogen declined marginally, levels of starch and phenolic glycosides did not change, and levels of condensed tannins increased. Long-term bioassays revealed reduced growth (especially females), prolonged development and increased consumption in larvae fed high-CO₂ foliage but no significant differences in final pupal weights or female fecundity. Short-term bioassays showed weaker, and sex-specific, effects of CO₂ treatment on larval performance. Correlation analyses revealed strong, negative associations between insect performance and phenolic glycoside concentrations, independent of CO₂ treatment. Larval susceptibility to NPV did not differ between CO₂ treatments, suggesting that effects of this natural enemy on gypsy moths are buffered from CO₂-induced changes in foliar chemistry. Our results emphasize that the impact of enriched CO₂ on plant–insect interactions will be determined not only by how concentrations of plant compounds are altered, but also by the relevance of particular compounds for insect fitness. This work also underscores the need for studies of genetic variation in plant responses to enriched CO₂ and long-term population-level responses of insects to CO₂-induced changes in host quality.     

Elevated CO2 and nitrogen influence exudation of soluble organic compounds by ectomycorrhizal root systems

Root and mycelial exudation contributes significantly to soil carbon (C) fluxes, and is likely to be altered by an elevated atmospheric carbon dioxide (CO₂) concentration and nitrogen (N) deposition. We quantified soluble, low-molecular-weight (LMW) organic compounds exuded by ectomycorrhizal plants grown under ambient (360 p.p.m.) or elevated (710 p.p.m.) CO₂ concentrations and with different N sources. Scots pine seedlings, colonized by one of five different ectomycorrhizal or nonmycorrhizal fungi, received 70 μM N, either as NH₄Cl or as alanine, in a liquid growth medium. Exudation of LMW organic acids (LMWOAs), dissolved monosaccharides and total dissolved organic carbon were determined. Both N and CO₂ had a significant impact on exudation, especially of LMWOAs. Exudation of LMWOAs was negatively affected by inorganic N and decreased by 30–85% compared with the organic N treatment, irrespective of the CO₂ treatment. Elevated CO₂ had a clear impact on the production of individual LMWOAs, although with very contrasting effects depending on which N source was supplied.     

Response of multiple generations of cotton bollworm Helicoverpa armigera Hübner, feeding on spring wheat, to elevated CO2

Abstract:  The growth, development and consumption of three successive generations of cotton bollworm, Helicoverpa armigera (Hübner), reared on milky grains of spring wheat grown under elevated CO₂ (double-ambient vs. ambient) in open-top chambers (OTCs) were examined. Decreases in protein, total amino acid, water and nitrogen content, and increases in total non-structure carbohydrates (TNCs) and ratio of TNC : nitrogen were found in wheat milky grains grown under elevated CO₂ conditions. Changes in quality of wheat grains affected the growth, development and food utilization of H. armigera . Significantly longer larval lifespan for the third generation and lower pupal weight for all generations were observed in cotton bollworm fed on milky grains of spring wheat grown under elevated CO₂ conditions. Bollworm fecundity was significantly decreased for the second and third generations under elevated CO₂ levels. The consumption, frass per larva and relative consumption rate significantly increased in elevated CO₂ compared with ambient CO₂ conditions. However, the potential population consumption was significant reduced by elevated CO₂ in the second and third generations. The results of this study indicate that elevated CO₂ levels adversely affect grain quality, resulting in consistently increased consumption per larva for a longer period to produce less fecund bollworm through generations, suggesting that net damage of cotton bollworm on wheat will be less under elevated atmospheric CO₂ levels because increased consumption is offset by slower development and reduced fecundity.     

Responses of tree sap-feeding herbivores to elevated CO2

Five species of sap-feeding homoptera were studied on Fagus sylvatica and Acer pseudoplatanus and exposed to elevated concentrations of carbon dioxide (600 μL L^–1). The concentration of total soluble amino acids in foliage of F. sylvatica was unaffected by growing saplings in elevated atmospheric CO₂ concentrations. Although experiments on individual aphids indicated poorer performance of Phyllaphis fagi (fewer, smaller nymphs produced), resultant populations did not differ from those in ambient (350 μL L^–1) conditions. The area of beech foliage stippled by the leafhopper Fagocyba cruenta was similar at ambient and elevated CO₂ concentrations. The concentration of total amino acids and that of serine of A. pseudoplatanus foliage were significantly lower at elevated CO₂ concentrations. However, the relative growth rates of two aphid species Drepanosiphum platanoidis and Periphyllus testudinaceus and one leafhopper Ossiannilssonola callosa were not significantly different in elevated CO₂. No evidence was found that, under the conditions of these experiments, populations of aphids and leafhoppers will change as concentrations of CO₂ increase.     

Monoterpene levels in needles of Douglas fir exposed to elevated CO2 and temperature

Monoterpene levels in current year needles of Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) seedlings were measured at the end of 4 years of exposure to ambient or elevated CO₂ (+179 µmol mol⁻¹), and ambient or elevated temperature (+0.3.5^C). Eleven monoterpenes were identified and quantified using gas chromatography/flame ionization detector/mass spectroscopy, with eight of these compounds regularly occurring in all trees examined. Elevated CO₂ exposure significantly reduced the levels for four of the eight main compounds in needles. Total monoterpene production was reduced by 52% ( P  < 0.05). Elevated temperature also reduced monoterpene levels ( P  < 0.07). The combination of elevated temperature and elevated CO₂ resulted in a 64% reduction in total monoterpenes compared with needles on ambient temperature trees. Two-way anova showed no significant temperature-CO₂ interaction. It is hypothesized that seasonal reductions in needle monoterpene pools under elevated CO₂ and temperature conditions may be due to a combination of competing carbon sinks, including increased carbon flux through the roots.     

Architectural and physiological mechanisms of reduced size inequality in CO2-enriched stands of common ragweed (Ambrosia artemisiifolia)

Testing whether and how subordinate individuals differ from dominants in the utilization of enriched CO₂ atmospheres is important for understanding future stand and community structure. We hypothesized that subordinate and dominant Ambrosia artemisiifolia L. (Asteraceae) (common ragweed) plants growing in dense stands would not equally acquire or utilize carbon gains from CO₂-enrichment, and that the resulting disproportionate growth gains to subordinates would reduce size inequalities in competing stands. We grew experimental stands of A. artemisiifolia in either ambient (360 μL L⁻¹) or twice ambient (720 μL L⁻¹) levels of atmospheric CO₂. We compared the relative growth, photosynthetic capacity, and architecture of subordinate and dominant plants in each treatment, and assessed size inequalities using the stand-level coefficient of variation (CV). In elevated CO₂, plants grew larger, but subordinate plants shifted more mass to upper stem allocation than dominants. Dominant plants demonstrated reduced leaf-level photosynthetic gains in elevated CO₂ compared with subordinate plants. Reduced CVs in plant size reflected smaller proportional growth gains by dominants over subordinates in elevated vs. ambient stands. We conclude that differences in the architectural and physiological responses of subordinate and dominant ragweed plants reduce competition and allow subordinate plants to catch up to dominants in elevated CO₂ conditions.     

Elevated CO2 concentration, nitrogen use, and seed production in annual plants

Elevated atmospheric CO₂ concentration ([CO₂]) stimulates seed mass production in many species, but the extent of stimulation shows large variation among species. We examined (1) whether seed production is enhanced more in species with lower seed nitrogen concentrations, and (2) whether seed production is enhanced by elevated [CO₂] when the plant uses more N for seed production. We grew 11 annuals in open top chambers that have different [CO₂] conditions (ambient: 370 μmol mol⁻¹, elevated: 700 μmol mol⁻¹). Elevated [CO₂] significantly increased seed production in six out of 11 species with a large interspecific variation (0.84–2.12, elevated/ambient [CO₂]). Seed nitrogen concentration was not correlated with the enhancement of seed production by elevated [CO₂]. The enhancement of seed production was strongly correlated with the enhancement of seed nitrogen per plant caused by increased N acquisition during the reproductive period. In particular, legume species tended to acquire more N and produced more seeds at elevated [CO₂] than non-nitrogen fixing species. Elevated [CO₂] little affected seed [N] in all species. We conclude that seed production is limited primarily by nitrogen availability and will be enhanced by elevated [CO₂] only when the plant is able to increase nitrogen acquisition.     

Mitigation of adverse effects of rising CO2 on a planktonic herbivore by mixed algal diets

Putative future increase in atmospheric CO₂ is expected to adversely affect herbivore growth due to decrease in contents of key nutrients such as nitrogen and phosphorus (P) relative to carbon in primary producers including plant and algal species. However, as many herbivores are polyphagous and as the response of primary producers to elevated CO₂ is highly species-specific, effects of elevated CO₂ on herbivore growth may differ between feeding conditions with monospecific and multiproducer diets. To examine this possibility, we performed CO₂ manipulation experiments under a P-limited condition with a planktonic herbivore, Daphnia , and three algal species, Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria). Semibatch cultures with single algal species (monocultures) and multiple algal species (mixed cultures) were grown at ambient (360 ppm) and high CO₂ levels (2000 ppm) that were within the natural range in lakes. Both in the mono- and mixed cultures, algal steady state abundance increased but algal P : C and N : C ratios decreased when they were grown at high CO₂. As expected, Daphnia fed monospecific algae cultured at high CO₂ had decreased growth rates despite increased algal abundance. However, when fed mixed algae cultured at high CO₂, especially consisting of diatoms and cyanobacteria or the three algal species, Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets. These results imply that algal diets composed of multiple species can mitigate the adverse effects of elevated CO₂ on herbivore performance, although the magnitude of this mitigation depends on the composition of algal species involved in the diets.     

The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide

A recognized invasive weed, Canada thistle ( Cirsium arvense L. Scop.) was grown at ambient and pre-ambient concentrations of atmospheric carbon dioxide [CO₂] (373 and 287 μmol mol⁻¹, respectively) at three levels of supplemental nitrogen (N) (3, 6 and 14.5 m M ) from seeding until flowering [77 days after sowing (DAS)]. The primary objective of the study was to determine if N supply limited the potential photosynthetic and growth response of this species to the increase in atmospheric [CO₂] which occurred during the 20th century (i.e. approximately 290 to 370 μmol mol⁻¹ CO₂). Leaf photosynthesis increased both as a function of growth [CO₂] and N supply during the first 46 DAS. Although by 46 DAS photosynthetic acclimation was observed relative to a common measurement CO₂ concentration, there was no interaction with N supply. Both [CO₂] and N increased biomass, relative growth rates and leaf area whereas root : shoot ratio was increased by CO₂ and decreased by increasing N; however, N supply did not effect the relative response to [CO₂] for any measured vegetative parameter up to 77 DAS. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated [CO₂] for all levels of supplemental N, but nitrogen use efficiency (NUE) did not differ as a function of [CO₂]. Overall, these data suggest that any potential response to increased atmospheric [CO₂] in recent decades for this noxious weedy species was probably not limited by nitrogen supply.     

Elevated atmospheric CO2 does not affect per se the preference for symbiotic nitrogen as opposed to mineral nitrogen of Trifolium repens L.

The objective of this investigation was to examine the effect of an elevated atmospheric CO₂ partial pressure ( p CO₂) on the N-sink strength and performance of symbiotic N₂ fixation in Trifolium repens L. cv. Milkanova. After initial growth under ambient p CO₂ in a nitrogen-free nutrient solution, T. repens in the exponential growth stage was exposed to ambient and elevated p CO₂ (35 and 60 Pa) and two levels of mineral N (N-free and 7·5 mol m^–3 N) for 36 d in single pots filled with silica sand in growth chambers. Elevated p CO₂ evoked a significant increase in biomass production from day 12 after the start of CO₂ enrichment. For plants supplied with 7·5 mol m^–3 N, the relative contribution of symbiotically fixed N (%N_sym) as opposed to N assimilated from mineral sources (¹⁵N-isotope-dilution method), dropped to 40%. However, in the presence of this high level of mineral N, %N_sym was unaffected by atmospheric p CO₂ over the entire experimental period. In plants fully dependent on N₂ fixation, the increase in N yield reflects a stimulation of symbiotic N₂ fixation that was the result of the formation of more nodules rather than of higher specific N₂ fixation. These results are discussed with regard to physiological processes governing symbiotic N₂ fixation and to the response of symbiotic N₂ fixation to elevated p CO₂ in field-grown T. repens .     

Effects of elevated CO2-grown loblolly pine needles on the growth, consumption, development, and pupal weight of red-headed pine sawfly larvae reared within open-topped chambers

Seedlings of loblolly pine, Pinus taeda , were grown in open-topped chambers under four levels of CO₂: two ambient and two elevated. Larvae of the red-headed pine sawfly, Neodiprion lecontei , were reared from early instar to pupation, primarily on branches within chambers. Larval growth and mortality were assessed and leaf phytochemistry samples of immature and mature leaves collected weekly. Mature leaves grown under elevated CO₂ had significant reductions in leaf nitrogen and increases in non-structural carbohydrate contents, resulting in foliage being a poorer food source for larvae, i.e. higher carbohydrate:nitrogen ratio. Nutritional constituents of immature needles were unaffected by seedling CO₂ treatment. Volatile mono- and sesquiterpenes were unrelated to plant CO₂ treatments for either leaf age class. Larval consumption of immature needles significantly increased on seedlings grown under CO₂ enrichment, while mature needle consumption was not different between the treatments. The average weight gain per larva significantly declined in late instar larvae consuming elevated CO₂-grown needles. In spite of this reduced growth, neither the days to pupation nor pupal weights were different among the CO₂ treatments. This study suggests that enriched CO₂-induced alterations in pine needle phytochemistry can affect red-headed pine sawfly performance. However, compensatory measures by larvae, such as choosing to consume more nutritious immature needles, apparently helps offset enriched CO₂-induced reductions in the leaf quality of mature needles.     

Are legume‐feeding herbivores buffered against direct effects of elevated carbon dioxide on host plants? A test with the sulfur butterfly,Colias philodice

When grown under elevated atmospheric carbon dioxide (CO₂), leaf nitrogen content decreases less for legumes than for nonlegume C₃ plants. Given that elevated CO₂ adversely affects insect herbivores primarily through dilution of plant nitrogen, it is reasonable to expect that legume-feeding herbivores will be relatively buffered against CO₂-induced reduction in performance. However, despite their ecological and economic importance, very few studies have addressed the effects of elevated CO₂ on legume-feeding herbivores. Unlike the responses of the vast majority of nonlegume C₃ plants, when the legumes Trifolium pratense and Melilotus alba were grown under elevated (742 ppm) CO₂, leaf nitrogen and carbon contents and C : N ratios did not change. For Colias philodice larvae fed T. pratense , elevated CO₂ had little or no effect on consumption, digestion, or conversion of whole food or nitrogen and, consequently, no effect on growth rate, instar duration, or pupal weight. For larvae fed M. alba , elevated CO₂ had little or no effect on consumption of whole food or nitrogen, increased digestion but decreased conversion of both and, consequently, had no effect on growth rate, instar duration or pupal weight. These results suggest that, relative to herbivores of nonlegume C₃ plants, legume-feeding herbivores will be less affected as atmospheric CO₂ continues to rise.