Elevated CO2 levels and herbivore damage alter host plant preferences

Interactions between the moth Spodoptera littoralis and two of its host plants, alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) were examined, using plants grown under ambient (350 ppm) and elevated (700 ppm) CO₂ conditions. To determine strength and effects of herbivore‐induced responses assays were performed with both undamaged (control) and herbivore damaged plants. CO₂ and damage effects on larval host plant preferences were determined through dual‐choice bioassays. In addition, larvae were reared from hatching to pupation on experimental foliage to examine effects on larval growth and development. When undamaged plants were used S. littoralis larvae in consumed more cotton than alfalfa, and CO₂ enrichment caused a reduction in the preference for cotton. With damaged plants larvae consumed equal amounts of the two plant species (ambient CO₂ conditions), but CO₂ enrichment strongly shifted preferences towards cotton, which was then consumed three times more than alfalfa. Complementary assays showed that elevated CO₂ levels had no effect on the herbivore‐induced responses of cotton, whereas those of alfalfa were significantly increased. Larval growth was highest for larvae fed undamaged cotton irrespectively of CO₂ level, and lowest for larvae on damaged alfalfa from the high CO₂ treatment. Development time increased on damaged cotton irrespectively of CO₂ treatment, and on damaged alfalfa in the elevated CO₂ treatment. These results demonstrate that elevated CO₂ levels can cause insect herbivores to alter host plant preferences, and that effects on herbivore‐induced responses may be a key mechanism behind these processes. Furthermore, since the insects were shown to avoid foliage that reduced their physiological performance, our data suggest that behavioural host plant shifts result in partial escape from negative consequences of feeding on high CO₂ foliage. Thus, CO₂ enrichment can alter both physiology and behaviour of important insect herbivores, which in turn may to impact plant biodiversity.     

Effects of elevated pCO2 on epilithic and endolithic metabolism of reef carbonates

We investigated effects of elevated partial pressure of CO₂ (pCO₂) on the metabolism of epilithic and endolithic phototrophic communities that colonized experimental coral blocks. Blocks of the massive coral Porites lobata were exposed to colonization by epilithic and endolithic organisms at an oceanic site in Kaneohe Bay (Hawaii) for 6 months, and then were transported to laboratory tanks. A bubbling system was used to maintain two treatments for 3 months, one at ambient pCO₂ (400 ppm) and the second at elevated pCO₂ (750 ppm). Net photosynthetic rates of epilithic communities in the high pCO₂ treatment, dominated by encrusting coralline algae, decreased by 35% while respiration rates remained constant. In contrast, metabolism of endolithic phototrophs, comprised of cyanobacteria and algae, was not significantly affected by the elevated pCO₂ even though endoliths contributed about 63% to block production.     

Responses of multiple generations of Gastrophysa viridula, feeding on Rumex obtusifolius, to elevated CO2

Rumex obtusifolius plants and three generations of the tri-voltine leaf beetle Gastrophysa viridula were simultaneously exposed to elevated CO₂ (600 ppm) to determine its effect on plant quality and insect performance. This exposure resulted in a reduction in leaf nitrogen, an increase in the C/N ratio and lower concentrations of oxalate in the leaves than in ambient air (350 ppm). Despite these changes in food quality, the effect of elevated CO₂ on larvae of Gastrophysa viridula over three generations was minimal. However, the effect of CO₂ did differ slightly between the generations of the insect. For the first generation, the results obtained were different from many of the published results in that elevated CO₂ had no measurable effects on performance, except that third instar larvae showed compensatory feeding. Food quality, including leaf nitrogen content, declined over time in material grown in both ambient and elevated CO₂. The results obtained for the second generation were similar to the first except that first instar larvae showed reduced relative growth rate in elevated CO₂. Development time from hatching to pupation decreased over each generation, probably as a result of increasing temperatures. Measurements of adult performance showed that fecundity at the end of the second generation was reduced relative to the first, in line with the reduction in food quality. In addition at the end of the second generation, but not at the end of the first generation, adult females in elevated CO₂ laid 30% fewer eggs per day and the eggs laid were 15% lighter than those in ambient conditions. These lighter eggs, coupled with no effect of elevated CO₂ on growth during the third generation, meant that the larvae were consistently smaller in elevated CO₂ during this generation. These results offer further insights into the effect that elevated CO₂ will have on insect herbivores and provide a more detailed basis for population predictions.     

Aphid and host‐plant genotype × genotype interactions under elevated CO2

1. Elevated CO₂ can alter plant physiology and morphology, and these changes are expected to impact diet quality for insect herbivores. While the plastic responses of insect herbivores have been well studied, less is known about the propensity of insects to adapt to such changes. Genetic variation in insect responses to elevated CO₂ and genetic interactions between insects and their host plants may exist and provide the necessary raw material for adaptation. 2. We used clonal lines of Rhopalosiphum padi (L.) aphids to examine genotype‐specific responses to elevated CO₂. We used the host plant Schedonorus arundinaceus (tall fescue; Schreb), which is capable of asexual reproduction, to investigate host plant genotype‐specific effects and possible host plant‐by‐insect genotype interactions. The abundance and density of three R. padi genotypes on three tall fescue genotypes under three concentrations of CO₂ (ambient, 700, and 1000 ppm) in a controlled greenhouse environment were examined. 3. Aphid abundance decreased in the 700 ppm CO₂ concentration, but increased in the 1000 ppm concentration relative to ambient. The effect of CO₂ on aphid density was dependent on host plant genotype; the density of aphids in high CO₂ decreased for two plant genotypes but was unchanged in one. No interaction between aphid genotype and elevated CO₂ was found, nor did we find significant genotype‐by‐genotype interactions. 4. This study suggests that the density of R. padi aphids feeding on tall fescue may decrease under elevated CO₂ for some plant genotypes. The likely impact of genotype‐specific responses on future changes in the genetic structure of plant and insect populations is discussed.     

Effects of elevated CO2 and temperature on plant growth and herbivore defensive chemistry

Concentration of atmospheric CO₂ and temperature have both been rising for the last three decades. In this century, the temperature has been predicted to rise by 2–5 °C and the CO₂ concentration to double. These changes may affect the primary and secondary metabolism of plants and thus have implications for other trophic levels. However, the biotic interactions in changing climate conditions are poorly known. In this study, two questions were addressed: (i) How will climate change affect growth and the amounts of secondary compounds in flexible plant species? and (ii) How will this affect herbivores living on this species. Four clones of the dark‐leaved willow (Salix myrsinifolia (Salisb.)) seedlings were grown in closed‐top chambers with two controlled factors: concentration of atmospheric CO₂ and temperature (T). There were four combinations of these factors, each combination replicated four times (total of 16 chambers): (i) Control CO₂ (350 ppm) and control T, (ii) Elevated CO₂ (700 ppm) and control T, (iii) Control CO₂ and elevated T (2 °C), and (iv) Elevated CO₂ and elevated T. Stem growth and aerial biomass of the plants were determined; and the leaf phenolics, nitrogen and water concentrations were analysed. In addition the growth rate of larvae and feeding preference of adults of a specialist herbivore, the chrysomelid beetle Phratora vitellinae (L.), on the treated willow leaves were measured. Elevated temperature and CO₂ concentration increased the stem biomass and elevated CO₂ increased leaf biomass and total aerial biomass of the willows. Patterns of biomass allocation were different in different temperature treatments. At elevated temperature there was less branch and leaf material in relation to stems than at the control temperature. Moreover, patterns of biomass allocation differed among clones. CO₂ enhancement increased the specific leaf weight (SLW) and reduced both water and nitrogen content of the leaves, however, leaf area was unaffected by the treatments. Carbon dioxide (CO₂) and T enhancement reduced the concentrations of several phenolic compounds in the leaves. Phenolic compounds, nutrients, and water in the leaves might be diluted partly due to increased carbon allocation to different structures (e.g. thickening of cell wall and increase of trichomes, etc.). In some cases plant clones showed specific responses to treatments. The CO₂ enhancement reduced the relative growth rate (RGR) of the beetle larvae, and in contrast, temperature elevation increased it. Adult beetles did not clearly discriminate between willow leaves grown in different T and CO₂ environments, but tended to eat more leaf material from chambers with doubled CO₂ concentration. At elevated CO₂ adult beetles may need to eat more leaf material in order to reproduce, which may in turn prolong the life cycles, increasing the risk of being eaten and possibly affecting ability to overwinter successfully. Overall, climate change may significantly modify the dynamic interaction between willow and beetle populations.     

Metabolite analysis of the effects of elevated CO2 and nitrogen fertilization on the association between tall fescue (Schedonorus arundinaceus) and its fungal symbiont Neotyphodium coenophialum

Atmospheric CO₂ is expected to increase to between 550 ppm and 1000 ppm in the next century. CO₂‐induced changes in plant physiology can have ecosystem‐wide implications and may alter plant‐plant, plant‐herbivore and plant‐symbiont interactions. We examined the effects of three concentrations of CO₂ (390, 800 and 1000 ppm) and two concentrations of nitrogen fertilizer (0.004 g N/week versus 0.2 g N/week) on the physiological response of Neotyphodium fungal endophyte‐infected and uninfected tall fescue plants. We used quantitative PCR to estimate the concentration of endophyte under altered CO₂ and N conditions. We found that elevated CO₂ increased the concentration of water‐soluble carbohydrates and decreased the concentration of plant total amino acids in plants. Fungal‐derived alkaloids decreased in response to elevated CO₂ and increased in response to nitrogen fertilization. Endophyte concentration (expressed as the number of copies of an endophyte‐specific gene per total genomic DNA) increased under elevated CO₂ and nitrogen fertilization. The correlation between endophyte concentration and alkaloid production observed at ambient conditions was not observed under elevated CO₂. These results suggest that nutrient exchange dynamics important for maintaining the symbiotic relationship between fungal endophytes and their grass hosts may be altered by changes in environmental variables such as CO₂ and nitrogen fertilization.     

Towards an understanding of how phloem amino acid composition shapes elevated CO2‐induced changes in aphid population dynamics

1. The performance of foliage feeders tends to decrease under elevated CO₂, but the responses of phloem‐feeding insects have been much more equivocal. As phloem tissues are less accessible than whole‐plant tissues, much less is known about how phloem composition is altered under elevated CO₂ and the mechanisms driving changes in aphid performance. 2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO₂ were examined. We used aphid stylectomy to sample pure phloem from plants in CO₂‐controlled conditions and high‐performance liquid chromatography to analyse phloem samples for amino acid concentrations. 3. Aphid abundance significantly increased by 127% under elevated CO₂. Consequently, plant biomass decreased under elevated CO₂ in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase (r_m) was significantly higher under elevated CO₂; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO₂. 4. These results indicate that CO₂ may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO₂. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO₂.     

Elevated CO2 reduces disease incidence and severity of a red maple fungal pathogen via changes in host physiology and leaf chemistry

Atmospheric CO₂ concentrations are predicted to double within the next century. Despite this trend, the extent and mechanisms through which elevated CO₂ affects plant diseases remain uncertain. In this study, we assessed how elevated CO₂ affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum growing in the understory at the Duke Forest free‐air CO₂ enrichment experiment in Durham, North Carolina. Surveys of A. rubrum saplings in the 6th, 7th, and 8th years of the CO₂ exposure revealed that elevated CO₂ significantly reduced disease incidence, with 22%, 27%, and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three consecutive years, respectively. Elevated CO₂ also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced 35%, 50%, and 10% in 2002, 2003, and 2004, respectively). To assess the mechanisms underlying these changes, we combined leaf structural, physiological and chemical analyses with growth chamber studies of P. minima growth and host infection. In vitro exponential growth rates of P. minima were enhanced by 17% under elevated CO₂, discounting the possibility that disease reductions were because of direct negative effects of elevated CO₂ on fungal performance. Scanning electron micrographs (SEM) verified that conidia germ tubes of P. minima infect A. rubrum leaves by entering through the stomata. While stomatal size and density were unchanged, stomatal conductance was reduced by 21–36% under elevated CO₂, providing smaller openings for infecting germ tubes. Reduced disease severity under elevated CO₂ was likely due to altered leaf chemistry and reduced nutritive quality; elevated CO₂ reduced leaf N by 20% and increased the C : N ratio by 20%, total phenolics by 15%, and tannins by 14% (P<0.05 for each factor). The potential dual mechanism we describe here of reduced stomatal opening and altered leaf chemistry that results in reduced disease incidence and severity under elevated CO₂ may be prevalent in many plant pathosystems where the pathogen targets the stomata.     

Effects of elevated CO2 on the chrysanthemum leaf-miner, Chromatomyia syngenesiae: a greenhouse study

Although feeding behaviour of Chromatomyia syngenesiae on plants grown in elevated CO₂ (ambient + 200ppm) was unaffected, leaf-miner development was slower in elevated compared to ambient CO₂ atmospheres. Pupal weight was lower at high CO₂ and correlated with the area of leaf mined; no such correlation existed in ambient CO₂. There appears to be no compensatory feeding by the leaf-miner for the reduced food quality of plants growing in elevated CO₂. The implications of these findings are discussed.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Biodiversity effects of elevated CO2 in species-rich model communities from the semi-arid Negev of Israel

Species‐specific responses to atmospheric CO₂ enrichment may affect biodiversity, which in turn may alter ecosystem functioning. Here we have explored biodiversity effects in model assemblages of semi‐arid grassland of the northern Negev, Israel, at 280 ppm (pre‐industrial era), 440 ppm (early 21st century) and 600 ppm CO₂ (mid to late 21st century). Thirty‐two mostly annual species were grown together in large containers (ca 400 kg each) on native soil and under a dynamic simulation of the Negev winter climate. CO₂ enrichment increased concentrations of total non‐structural carbohydrates and C/N ratios, and reduced specific leaf area and nitrogen concentrations in leaves of all species. In contrast to these uniform CO₂ effects on leaf quality, biomass and reproductive output remained unchanged in most species, and varied greatly among the few responsive ones (?80 to +145%). Biomass was significantly increased at elevated CO₂ in Onobrychis crista‐galli (one of the six legume species) and was reduced in Biscutella didyma (Brassicaceae). Seed yield increased in three out of six legumes and in the root hemiparasite Parentucellia flaviflora, and decreased in the grass Aegilops peregrina. Fruit dry matter tended to be reduced in two Brassicaceae. Onobrychis, the largest and most responsive species present, was the most ‘mesic’ legume, and might have profited most from the higher soil moisture induced by CO₂ enrichment. The significant CO₂ response of only 5–6 out of 32 species, in particular their altered seed yield, suggests a potential shift in biodiversity. In a future CO₂‐enriched atmosphere, ‘mesic’ legumes and root hemiparasites might be favored, while some Brassicaceae and grasses might decline. As indicated by significant 280‐ vs 440‐ppm differences, reductions in leaf nitrogen concentration of grasses and legumes are likely to be under way right now, and may negatively affect grazers. Altered seed yields were more pronounced between 440 and 600 ppm, suggesting that these changes could intensify as the atmospheric CO₂ concentration continues to rise.     

The effect of elevated CO2 concentration and soil pH on the relationship between plant growth and rhizosphere denitrification potential

The effect of CO₂ concentration on plant growth and the size of the rhizosphere denitrifier population was investigated for ryegrass grown at 3 different soil pH values (pH 4.3, 5.9 and 7.0). Soil microcosms were planted with ryegrass and maintained under constant growth conditions at either ambient (450ppm) or elevated (720ppm) CO₂ concentration. At harvest, the rhizosphere soil was collected and subjected to a potential denitrification assay to provide an estimate of the size of the denitrifier population present. Ryegrass dry matter production varied across the pH range studied and contrary to other studies, elevated CO₂ concentration did not consistently increase growth. Plant growth was reduced by ≈ 35% and 23% at pH 4.3 and pH 5.9, respectively, under elevated CO₂ concentration. At pH 7.0, however, plant growth was increased by ≈ 45% under elevated CO₂. Potential denitrification rates within the rhizosphere followed a similar pattern to plant growth in the different treatments, suggesting that plant growth and the size of denitrifier population within the rhizosphere are coupled. This study investigates the relationship between plant growth and rhizosphere denitrification potential, thereby providing an estimate of the size of the denitrifier population under increased CO₂ concentration and soil pH.     

Potassium limits potential growth of bog vegetation under elevated atmospheric CO2 and N deposition

The free air carbon dioxide enrichment (FACE) and N deposition experiments on four ombrotrophic bogs in Finland, Sweden, the Netherlands and Switzerland, revealed that after three years of treatment: (1) elevated atmospheric CO₂ concentration had no significant effect on the biomass growth of Sphagnum and vascular species; and (2) increased N deposition reduced Sphagnum growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum, while vascular plant biomass growth was not affected. This paper focuses on water chemistry, plant nutrient content, and litter decomposition rates. Potassium limitation, or low supply of K and P, may have prevented a significant increase of Sphagnum growth under elevated CO₂ and N deposition. Vascular plant growth under elevated CO₂ and N deposition was also limited by K, or by K in combination with P or N (N in CO₂ experiment). Elevated CO₂ and N deposition had no effect on decomposition rates of Sphagnum and vascular plant litter. Aside from a possible effect of N deposition on light competition between species, we expect that elevated atmospheric CO₂ and N deposition concentrations will not affect Sphagnum and vascular plant growth in bogs of north‐west Europe due to K‐, or K in combination with N‐ or P‐, limited growth. For the same reason we expect no effect of elevated CO₂ and N deposition on litter decomposition. Net primary production of raised ombrotrophic bogs that are at or close to steady state, is regulated by input of nutrients through atmospheric deposition. Therefore, we hypothesize that the expected increase of plant growth under elevated CO₂ and N deposition is diminished by current levels of K (and to some extent P and N) in atmospheric deposition.     

Differential effects of elevated CO2 on acorn density, weight, germination, and predation among three oak species in a scrub-oak forest

Much research on the effects of elevated CO₂ on forest trees has focused on quantitative changes in photosynthesis, secondary chemistry, and plant biomass. However, plant fitness responses to rising CO₂ should also include quantitative measures of reproduction, since most forest systems are recruitment limited. Until now, it has proved very difficult to grow forest trees to sexual maturity in a CO₂‐enriched environment. This paper is the first of its kind to address the effects of elevated CO₂ on the reproduction of hardwood trees in a natural forest. Beginning in 1996, scrub‐oak vegetation, predominantly three species of scrub‐oaks, Quercus myrtifolia, Q. chapmanii, and Q. geminata, were grown inside eight chambers with elevated CO₂ (704 parts per million (ppm)) and eight with ambient CO₂ (379 ppm) at Kennedy Space Center, Florida. In elevated CO₂, acorn production increased significantly for the dominant species Q. myrtifolia and for Q. chapmanii, but it did not increase for the subdominant, Q. geminata. Acorn weight, germination rate, and predation by weevils were unaffected by CO₂. Thus, recruitment of some forest tree species into the Florida scrub‐oak community is likely to be accelerated in an atmosphere of increased CO₂. However, because the acceleration of recruitment differs among species, over the long term, Q. myrtifolia and Q. chapmanii will be favored over Q. geminata and this is likely to change patterns of species diversity.     

Inhibition of whole plant respiration by elevated CO2 as modified by growth temperature

Plants of alfalfa (Medicago sativa) and orchard grass (Dactylus glomerata) were grown in controlled environment chambers at two CO₂ concentrations (350 and 700 μmol mol^-1) and 4 constant day/night growth temperatures of 15, 20, 25 and 30°C for 50–90 days to determine changes in growth and whole plant CO₂ efflux (dark respiration). To facilitate comparisons with other studies, respiration data were expressed on the basis of leaf area, dry weight and protein. Growth at elevated CO₂ increased total plant biomass at all temperatures relative to ambient CO₂, but the relative enhancement declined (P≤0.05) as temperature increased. Whole plant respiration (R_d) at elevated CO₂ declined at 15 and 20°C in D. glomerata on an area, weight or protein basis and in M. sativa on a weight or protein basis when compared to ambient CO₂. Separation of R_d into respiration required for growth (R_g) and maintenance (R_m) showed a significant effect of elevated CO₂ on both components. R_m was reduced in both species but only at lower temperatures (15°C in M. sativa and 15 and 20°C in D. glomerata). The effect on R_m could not be accounted for by protein content in either species. R_g was also reduced with elevated CO₂; however no particular effect of temperature was observed, i. e. R_g was reduced at 20, 25 and 30°C in M. sativa and at 15 and 25°C in D. glomerata. For the two perennial species used in the present study, the data suggest that both R_g and R_m can be reduced by anticipated increases in atmospheric CO₂; however, CO₂ inhibition of total plant respiration may decline as a function of increasing temperature     

Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean

Plants grown at elevated CO₂ often acclimate such that their photosynthetic capacities are reduced relative to ambient CO₂-grown plants. Reductions in synthesis of photosynthetic enzymes could result either from reduced photosynthetic gene expression or from reduced availability of nitrogen-containing substrates for enzyme synthesis. Increased carbohydrate concentrations resulting from increased photosynthetic carbon fixation at elevated CO₂ concentrations have been suggested to reduce the expression of photosynthetic genes. However, recent studies have also suggested that nitrogen uptake may be depressed by elevated CO₂, or at least that it is not increased enough to keep pace with increased carbohydrate production. This response could induce a nitrogen limitation in elevated-CO₂ plants that might account for the reduction in photosynthetic enzyme synthesis. If CO₂ acclimation were a response to limited nitrogen uptake, the effects of elevated CO₂ and limiting nitrogen supply on photosynthesis and nitrogen allocation should be similar. To test this hypothesis we grew non-nodulating soybeans at two levels each of nitrogen and CO₂ concentration and measured leaf nitrogen contents, photosynthetic capacities and Rubisco contents. Both low nitrogen and elevated CO₂ reduced nitrogen as a percentage of total leaf dry mass but only low nitrogen supply produced significant decreases in nitrogen as a percentage of leaf structural dry mass. The primary effect of elevated CO₂ was to increase non-structural carbohydrate storage rather than to decrease nitrogen content. Both low nitrogen supply and elevated CO₂ also decreased carboxylation capacity (V_cmax) and Rubisco content per unit leaf area. However, when V_cmax and Rubisco content were expressed per unit nitrogen, low nitrogen supply generally caused them to increase whereas elevated CO₂ generally caused them to decrease. Finally, elevated CO₂ significantly increased the ratio of RuBP regeneration capacity to V_cmax whereas neither nitrogen supply nor plant age had a significant effect on this parameter. We conclude that reductions in photosynthetic enzyme synthesis in elevated CO₂ appear not to result from limited nitrogen supply but instead may result from feedback inhibition by increased carbohydrate contents.     

Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling

The response of temperate forest ecosystems to elevated atmospheric CO₂ concentrations is important because these ecosystems represent a significant component of the global carbon cycle. Two important but not well understood processes which elevated CO₂ may substantially alter in these systems are regeneration and nitrogen cycling. If elevated CO₂ leads to changes in species composition in regenerating forest communities then the structure and function of these ecosystems may be affected. In most temperate forests, nitrogen appears to be a limiting nutrient. If elevated CO₂ leads to reductions in nitrogen cycling through increased sequestration of nitrogen in plant biomass or reductions in mineralization rates, long-term forest productivity may be constrained. To study these processes, we established mesocosms of regenerating forest communities in controlled environments maintained at either ambient (375 ppm) or elevated (700 ppm) CO₂ concentrations. Mesocosms were constructed from intact monoliths of organic forest soil. We maintained these mesocosms for 2 years without any external inputs of nitrogen and allowed the plants naturally present as seeds and rhizomes to regenerate. We used ¹⁵N pool dilution techniques to quantify nitrogen fluxes within the mesocosms at the end of the 2 years. Elevated atmospheric CO₂ concentration significantly affected a number of plant and soil processes in the experimental regenerating forest mesocosms. These changes included increases in total plant biomass production, plant C/N ratios, ectomycorrhizal colonization of tree fine roots, changes in tree fine root architecture, and decreases in plant NH₄ ⁺ uptake rates, gross NH₄ ⁺ mineralization rates, and gross NH₄ ⁺ consumption rates. In addition, there was a shift in the relative biomass contribution of the two dominant regenerating tree species; the proportion of total biomass contributed by white birch (Betula papyrifera) decreased and the proportion of total biomass contributed by yellow birch (B. alleghaniensis) increased. However, elevated CO₂ had no significant effect on the total amount of nitrogen in plant and soil microbial biomass. In this study we observed a suite of effects due to elevated CO₂, some of which could lead to increases in potential long term growth responses to elevated CO₂, other to decreases. The reduced plant NH₄ ⁺ uptake rates we observed are consistent with reduced NH₄ ⁺ availability due to reduced gross mineralization rates. Reduced NH₄ ⁺ mineralization rates are consistent with the increases in C/N ratios we observed for leaf and fine root material. Together, these data suggest the positive increases in plant root architectural parameters and mycorrhizal colonization may not be as important as the potential negative effects of reduced nitrogen availability through decreased decomposition rates in a future atmosphere with elevated CO₂. Received: 10 January 1997 / Accepted: 25 July 1997     

Growth and nitrogen uptake in an experimental community of annuals exposed to elevated atmospheric CO2

Rising levels of atmospheric CO₂ may alter patterns of plant biomass production. These changes will be dependent on the ability of plants to acquire sufficient nutrients to maintain enhanced growth. Species-specific differences in responsiveness to CO₂ may lead to changes in plant community composition and biodiversity. Differences in species-level growth responses to CO₂ may be, in a large part, driven by differences in the ability to acquire nutrients. To understand the mechanisms of how elevated CO₂ leads to changes in community-level productivity, we need to study the growth responses and patterns of nutrient acquisition for each of the species that comprise the community. In this paper, we present a study of how elevated CO₂ affects community-level and species-level patterns of nitrogen uptake and biomass production. As an experimental system we use experimental communities of 11 co-occurring annuals common to disturbed seasonal grasslands in south-western U.S.A. We established experimental communities with approximately even numbers of each species in three different atmospheric CO₂ concentrations (375, 550, and 700 ppm). We maintained these communities for 1, 1.5, and 2 months at which times we applied a ¹⁵N tracer (¹⁵NH₄¹⁵NO₃) to quantify the nitrogen uptake and then measured plant biomass, nitrogen content, and nitrogen uptake rates for the entire communities as well as for each species. Overall, community-level responses to elevated CO₂ were consistent with the majority of other studies of individual- and multispecies assemblages, where elevated CO₂ leads to enhanced biomass production early on, but this enhancement declines through time. In contrast, the responses of the individual species within the communities was highly variable, showing the full range of responses from positive to negative. Due to the large variation in size between the different species, community-level responses were generally determined by the responses of only one or a few species. Thus, while several of the smaller species showed trends of increased biomass and nitrogen uptake in elevated CO₂ at the end of the experiment, community-level patterns showed a decrease in these parameters due to the significant reduction in biomass and nitrogen content in the single largest species. The relationship between enhancement of nitrogen uptake and biomass production in elevated CO₂ was highly significant for both 550 ppm and 700 ppm CO₂. This relationship strongly suggests that the ability of plants to increase nitrogen uptake (through changes in physiology, morphology, architecture, or mycorrhizal symbionts) may be an important determinant of which species in a community will be able to respond to increased CO₂ levels with increased biomass production. The fact that the most dominant species within the community showed reduced enhancement and the smaller species showed increased enhancement suggest that through time, elevated CO₂ may lead to significant changes in community composition. At the community level, nitrogen uptake rates relative to plant nitrogen content were invariable between the three different CO₂ levels at each harvest. This was in contrast to significant reductions in total plant nitrogen uptake and nitrogen uptake relative to total plant biomass. These patterns support the hypothesis that plant nitrogen uptake is largely regulated by physiological activity, assuming that physiological activity is controlled by nitrogen content and thus protein and enzyme content.     

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.     

In deep shade, elevated CO2 increases the vigor of tropical climbing plants

Climbing plants have profound influences on tropical forest dynamics and may take particular advantage from atmospheric CO₂ enrichment, thus potentially enhancing tree turnover. Here we test the effect of a four‐step CO₂‐enrichment on growth of three typical Yucatan (Mexico) climbers, across two low photon flux densities, representing typical understory situations. In pairs of two, species of Gonolobus (Asclepiadaceae), Ceratophytum (Bignoniaceae) and Thinouia (Sapindaceae) were grown on Yucatan forest soil in growth cabinets, which simulated the diurnal climate variation. Biomass increased non‐linearly in response to CO₂ enrichment from 280 (preindustrial) to 420 ppm and 560 ppm, but then (700 ppm) leveled off. The relative effect of CO₂‐enrichment between the two lower (280–420 ppm) CO₂ concentrations was 63% at low light (LL == 42 µmol m^2?2 s^2?1), compared to 37% at high light (HL = 87 µmol m^2?2 s^2?1). This overall response of species pairs was the combined effect of linear and non‐linear responses of the individual species across CO₂ treatments. Plant biomass was 61% larger in HL compared to LL. The species‐specific response depended on the neighbor, a species grew with h, irrespective of plant size. Stem length increased, but not consistently across species and light conditions. Specific stem length (SSL, length per dry mass) declined non‐linearly in all three species as CO₂ concentration increased (more pronounced at LL than at HL). SLA (leaf area per unit leaf dry mass) became lower as CO₂ concentration increased (more pronounced in HL). Enhanced vigor of climbers under elevated CO₂ as documented here may accelerate tropical forest dynamics and lead to greater abundance of early succesional tree species. This could reduce forest carbon stocking in the long run.