The effect of elevated CO2 and temperature on the secondary chemistry of Betula pendula seedlings

The effect of elevated atmospheric CO₂ and temperature on resource allocation and secondary chemistry of white birch (Betula pendula Roth) under a non-limiting nutrient and water supply was investigated. Birch seedlings were grown in closed-top chambers exposed to ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm), elevated temperature (2°C above ambient) and a combination of elevated CO₂ and temperature for one growing season. Elevated CO₂ significantly increased the total biomass of the seedlings. The combined effect of the elevated CO₂ and temperature treatments further increased the total biomass, but not significantly. The content of nitrogen and water decreased, while some secondary compounds (such as condensed tannins and flavonol glycosides) increased in leaves subjected to CO₂ enrichment. Elevated temperature increased the concentration of total flavone aglycones and decreased that of total HPLC-phenolics in the leaves, due to the decrease in individual flavonol glycosides, cinnamoylquinic acids and (+)-catechin. There were no significant interactive effects between CO₂ and temperature in the phenolic concentrations of the leaves and in the stems, while the number of resin droplets in the top part of the stems showed significant interaction. This clearly implies that carbon allocation into secondary metabolites in the leaves and stems differ under enhanced CO₂ and temperature, and the combined effect of CO₂ and temperature on the herbivore resistance of birches, is lower than that of CO₂ and temperature alone.     

Combination treatment of elevated UVB radiation,CO2 and temperature has little effect on silver birch (Betula pendula) growth and phytochemistry

Elevations of carbon dioxide, temperature and ultraviolet‐B (UBV) radiation in the growth environment may have a high impact on the accumulation of carbon in plants, and the different factors may work in opposite directions or induce additive effects. To detect the changes in the growth and phytochemistry of silver birch (Betula pendula) seedlings, six genotypes were exposed to combinations of ambient or elevated levels of CO₂, temperature and UVB radiation in top‐closed chambers for 7 weeks. The genotypes were relatively similar in their responses, and no significant interactive effects of three‐level climate factors on the measured parameters were observed. Elevated UVB had no effect on growth, nor did it alter plant responses to CO₂ and/or temperature in combined treatments. Growth in all plant parts increased under elevated CO₂, and height and stem biomass increased under elevated temperature. Increased carbon distribution to biomass did not reduce its allocation to phytochemicals: condensed tannins, most flavonols and phenolic acids accumulated under elevated CO₂ and elevated UVB, but this effect disappeared under elevated temperature. Leaf nitrogen content decreased under elevated CO₂. We conclude that, as a result of high genetic variability in phytochemicals, B. pendula seedlings have potential to adapt to the tested environmental changes. The induction in protective flavonoids under UVB radiation together with the positive impact of elevated CO₂ and temperature mitigates possible UVB stress effects, and thus atmospheric CO₂ concentration and temperature are the climate change factors that will dictate the establishment and success of birch at higher altitudes in the future.     

Elevated CO2 alters birch resistance to Lagomorpha herbivores

We studied the three‐way interaction of elevated CO₂, nitrogen (N), and temperature (T), and the two‐way interaction of elevated CO₂ and early‐season defoliation on the secondary chemistry and resistance of Eurasian silver birch (Betula pendula) and North American paper birch (B. papyrifera) against the Eurasian hare (Lepus timidus) and the North American eastern cottontail rabbit (Sylvilagus floridanus), respectively. Elevated CO₂ decreased the palatability of winter‐dormant silver and paper birch stems to both hares and rabbits, respectively. But the effect on hares was only apparent at intermediate levels of N fertilization. Elevated T had no effect on palatability. The effects of elevated CO₂, N, and T on levels of silver birch bark phenolics and terpenoids were dominated by two‐way interactions between N and CO₂, and N and T. Generally, however, N amendments elicited a parabolic response in carbon partitioning to most biosynthetic classes of silver birch phenolics (i.e. highest concentrations occurring at intermediate N). CO₂ elevation was most enhancing at highest levels of N. On the other hand, T increases, more often than not, elicited reductions in phenolics, but especially so at the highest N level. In the case of B. papyrifera, elevated CO₂ increased carbon partitioning to Folin‐Denis stem and branch phenolics and condensed tannins. Early‐season defoliation, on the other hand, had no effect on phenolics and tannins but lowered both N and energy levels of branches. Elevated CO₂ substantially ameliorated the negative effects of severe defoliation on tree growth. These results support the hypothesis that continuing anthropogenic alterations of the atmosphere may trigger significant changes in plant phenotypic resistance to mammalian herbivores owing to an increasing net carbon balance between the highly vagile supply and demand capacities of plant carbon sources and sinks.     

Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure

Increased levels of atmospheric CO₂ may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO₂ influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L^–1) and elevated (700 μL L^–1) atmospheric CO₂. For both individually and competitively grown paper birch seedlings, there was a greater CO₂ growth enhancement for seedlings watered less frequently than for well-watered seedlings. This differential change in CO₂ responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO₂ only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO₂ reduced this sensitivity. Additionally, elevated CO₂ improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO₂ may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO₂ levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.     

Wood properties of two silver birch clones exposed to elevated CO2 and O3

Effects of elevated carbon dioxide (CO₂) and ozone (O₃) on wood properties of two initially 7‐year‐old silver birch (Betula pendula Roth) clones were studied after a fumigation during three growing seasons. Forty trees, representing two fast‐growing clones (4 and 80), were exposed in open‐top chambers to the following treatments: outside control, chamber control, 2 × ambient [CO₂], 2 × ambient [O₃] and 2 × ambient [CO₂]+2 × ambient [O₃]. After the 3‐year exposure, the trees were felled and wood properties were analyzed. The treatments affected both stem wood structure and chemistry. Elevated [CO₂] increased annual ring width, and concentrations of extractives and starch, and decreased concentrations of cellulose and gravimetric lignin. Elevated O₃ decreased vessel percentage and increased cell wall percentage in clone 80. In vessel percentage, elevated CO₂ ameliorated the O₃‐induced decrease. In clone 4, elevated O₃ decreased nitrogen concentration of wood. The two clones had different wood properties. In clone 4, the concentrations of extractives, starch, soluble sugars and nitrogen were greater than in clone 80, while in clone 80 the concentrations of cellulose and acid‐soluble lignin were higher. Clone 4 also had slightly longer fibres, greater vessel lumen diameter and vessel percentage than clone 80, while in clone 80 cell wall percentage was greater. Our results show that wood properties of young silver birch trees were altered under elevated CO₂ in both clones, whereas the effects of O₃ depended on clone.     

Seasonal patterns of photosynthesis in Douglas fir seedlings during the third and fourth year of exposure to elevated CO2 and temperature

The interactive effects of elevated atmospheric CO₂ and temperature on seasonal patterns of photosynthesis in Douglas fir (Psuedotsuga menziesii (Mirb.) Franco) seedlings were examined. Seedlings were grown in sunlit chambers controlled to track either ambient (~400 p.p.m.) CO₂ or ambient +200 p.p.m. CO₂, and either ambient temperature or ambient +4 °C. Light‐saturated net photosynthetic rates were measured approximately monthly over a 21 month period. Elevated CO₂ increased net photosynthetic rates by an average of 21% across temperature treatments during both the 1996 hydrologic year, the third year of exposure, and the 1997 hydrologic year. Elevated mean annual temperature increased net photosynthetic rates by an average of 33% across CO₂ treatments during both years. Seasonal temperature changes also affected net photosynthetic rates. Across treatments, net photosynthetic rates were highest in the spring and autumn, and lowest in July, August and December–January. Seasonal increases in temperature were not correlated with increases in the relative photosynthetic response to elevated CO₂. Seasonal shifts in the photosynthetic temperature optimum reduced temperature effects on the relative response to elevated CO₂. These results suggest that the effects of elevated CO₂ on net photosynthetic rates in Douglas fir are largely independent of temperature.     

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.     

Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]

An important question with respect to plant performance in future climatic scenarios is whether the offspring of mature trees that have experienced lifelong exposure to elevated [CO₂] show altered physiological responses to elevated [CO₂] compared with those originating from current ambient CO₂ concentrations. To investigate this question, acorns were collected from two seed sources, denoted as ‘control’ and ‘spring’, from Quercus ilex mother trees grown at ambient (36 Pa) and at about twice ambient CO₂ concentrations, respectively, close to a natural CO₂ spring, Laiatico, central Italy. The seedlings were raised for 8 months under controlled conditions at ambient and elevated [CO₂] in a reciprocal experimental design and were used for the determination of biomass, photosynthesis and foliar carbohydrate concentrations, as well as the accumulation of structural biomass and lignin during leaf maturation. Under ambient [CO₂], biomass and foliar carbon acquisition in control progeny were not significantly different from spring progeny. However, under elevated [CO₂], spring seedlings showed less CO₂ acclimation than control seedlings but no significant differences in non‐structural carbohydrate concentrations and structural biomass per unit leaf dry mass. Developmental lignin accumulation in leaves was delayed under elevated [CO₂] compared with ambient [CO₂], but only in control progeny. Under elevated [CO₂], whole‐plant biomass, leaf area and stem diameter were significantly increased in Quercus ilex seedlings from both seed sources but with a higher stimulation of above‐ground biomass in spring than in control seedlings and a higher stimulation of below‐ground biomass in control seedlings. These results indicate that life history and/or progeny may determine the species‐specific CO₂ response and suggest that positive CO₂ acclimation is possible.     

Low soil temperature inhibits the stimulatory effect of elevated [CO2] on height and biomass accumulation of white birch seedlings grown under three non‐limiting phosphorus conditions

White birch (Betula papyrifera Marsh.) seedlings were exposed to ambient or doubled ambient carbon dioxide concentration ([CO₂]), three soil temperatures (T_soil) (low, intermediate, high), and three phosphorus (P) regimes (low, medium, high) in environment‐controlled greenhouses. Height (H), root‐collar diameter (RCD), biomass, and leaf phosphorus concentration (leaf P) were determined four months after initiation of treatments. The low T_soil reduced H, RCD, shoot biomass, root biomass and total seedling biomass whereas the high‐P level and the [CO₂] elevation increased all the growth and biomass parameters. Elevated [CO₂] significantly reduced leaf P. There were significant two‐factor interactions suggesting that the effect of elevated [CO₂] on (1) H, total biomass, biomass of plant components, and leaf P was dependent on T_soil, (2) total biomass was contingent on P regime. For instance, the positive response of H and total biomass to elevated [CO₂] was limited to seedlings raised under the intermediate and high T_soil, respectively. In addition, [CO₂] elevation increased total biomass only at the high‐P regime but not at the low‐ or medium‐P level where the effect of [CO₂] was statistically insignificant. No significant main effect of treatment or interaction was observed for root to shoot biomass ratio.     

Elevated CO2 and temperature alter nitrogen allocation in Douglas-fir

The effects of elevated CO₂ and temperature on principal carbon constituents (PCC) and C and N allocation between needle, woody (stem and branches) and root tissue of Pseudotsuga menziesii Mirb. Franco seedlings were determined. The seedlings were grown in sun‐lit controlled‐environment chambers that contained a native soil. Chambers were controlled to reproduce ambient or ambient +180 ppm CO₂ and either ambient temperature or ambient +3.5 °C for 4 years. There were no significant CO₂ × temperature interactions; consequently the data are presented for the CO₂ and temperature effects. At the final harvest, elevated CO₂ decreased the nonpolar fraction of the PCC and increased the polar fraction and amount of sugars in the needles. In contrast, elevated temperature increased the nonpolar fraction of the PCC and decreased sugars in needles. There were no CO₂ or temperature effects on the PCC fractions in the woody tissue or root tissue. Elevated CO₂ and temperature had no significant effects on the C content of any of the plant tissues or fractions. In contrast, the foliar N content declined under elevated CO₂ and increased under elevated temperature; there were no significant effects in other tissues. The changes in the foliar N concentrations were in the cellulose and lignin fractions, the fractions, which contain protein, and are the consequences of changes in N allocation under the treatments. These results indicate reallocation of N among plant organs to optimize C assimilation, which is mediated via changes in the selectivity of Rubisco and carbohydrate modulation of gene expression.     

Xeromorphy increases in shoots of Pseudotsuga menziesii (Mirb.) Franco seedlings with exposure to elevated temperature but not elevated CO2

Seedling structure influences tree structure and function, ultimately determining the potential productivity of trees and their competitiveness for resources. We investigated changes in shoot structure for seedlings of Pseudotsuga menziesii (Douglas-fir) grown under climate change scenarios of ambient or elevated CO₂ (+180 mol mol^–1) plus ambient or elevated temperature (+3.5°C), for 4 years in outdoor, sunlit chambers. Mass allocation and allometry were measured for buds, leaves, branches, and stems, and anatomy was evaluated for leaves and stems. Seedlings became more xeromorphic with elevated temperature: allocation of total mass to branches over stems and leaves increased, sapwood area to height ratio increased, number of growing points relative to seedling size increased, and stem and branch length and mass decreased for sections initiated during the three full CO₂ and temperature seasons. Neither stem nor leaf anatomy was affected by elevated temperature. Elevated CO₂ increased specific mass of leaves, but had few other effects on mass allocation, allometry, or anatomy for any shoot organ. There were no CO₂ × temperature interactions for any important parameter. Thus, under realistic simulated field environmental conditions representative for in at least some P. menziesii forests (i.e., OR, USA, forests with limited soil nitrogen and summer soil moisture), elevated temperature, but not elevated CO₂, may affect seedling shoot structure and, hence, function.     

Interactive effects of elevated CO2 and drought stress on leaf water potential and growth in Caragana intermedia

We studied the responses of leaf water potential (Ψ_w), morphology, biomass accumulation and allocation, and canopy productivity index (CPI) to the combined effects of elevated CO₂ and drought stress in Caragana intermedia seedlings. Seedlings were grown at two CO₂ concentrations (350 and 700 μmol mol⁻¹) interacted with three water regimes (60–70%, 45–55%, and 30–40% of field capacity of soil). Elevated CO₂ significantly increased Ψ_w, decreased specific leaf area (SLA) and leaf area ratio (LAR) of drought-stressed seedlings, and increased tree height, basal diameter, shoot biomass, root biomass as well as total biomass under the all the three water regimes. Growth responses to elevated CO₂ were greater in well-watered seedlings than in drought-stressed seedlings. CPI was significantly increased by elevated CO₂, and the increase in CPI became stronger as the level of drought stress increased. There were significant interactions between elevated CO₂ and drought stress on leaf water potential, basal diameter, leaf area, and biomass accumulation. Our results suggest that elevated CO₂ may enhance drought avoidance and improved water relations, thus weakening the effect of drought stress on growth of C. intermedia seedings.     

The influence of elevated O3 and CO2 concentrations on secondary metabolites of Scots pine (Pinus sylvestris L. ) seedlings

Terpene, resin acid and total phenolic concentrations in five‐year‐old Scots pine (Pinus sylvestris L.) seedlings were analysed after exposure to ambient and realistically elevated (2 × ambient) O₃ and CO₂ concentrations and their combination in open‐top chambers during two growing seasons. Under O₃ exposure, limonene concentration in needles and isopimaric concentration in stems decreased significantly. As a response to elevated CO₂, α‐pinene and total phenolic concentrations in needles increased significantly, while bornyl acetate concentration in needles and palustric + levopimaric and neoabietic acid concentrations in stems decreased significantly. Some terpenes and resin acids were found at lower concentrations in the combined O₃ and CO₂ treatment than in O₃ exposure or elevated CO₂. A negative chamber effect was found: seedlings growing inside the chambers with ambient air had significantly lower concentrations of some terpenes and resin acids than seedlings growing outside the chambers. There was a lot of between‐tree variation in terpene and resin acid concentrations, which is typical of open‐pollinated populations. The results of this study suggest that, at least in short‐term experiments, Scots pine secondary metabolites are relatively insensitive to climate change factors. Total phenolics in the needles were the most responsive group showing about 25% increase in elevated CO₂, and O₃ exposure did not mitigate this CO₂ effect. Terpenes and resin acids were less responsive, although some individual compounds showed notable responses, e.g. α‐pinene in needles, which increased about 50% in response to elevated CO₂. As a consequence, although there were only slight effects on total pools of needle secondary metabolites, considerable O₃ and CO₂ effects on certain individual compounds might have ecological significance via trophic amplification, e.g. in decomposing processes of needle litter.     

Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth) and the feeding behaviour of the weevil Phyllobius maculicornis

Abstract 1 The effect of elevated CO₂ and temperature on the foliar chemistry Betula pendula Roth and the feeding performance of polyphagous weevils Phyllobius maculicornis Germ. was studied. Birch seedlings were grown during one growing season in chamber‐less field conditions and in closed‐top chambers exposed to four different treatments: ambient CO₂ (350 p.p.m) and temperature, elevated atmospheric CO₂ (700 p.p.m) and ambient temperature, elevated temperature +3 °C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. 2 In leaves under CO₂ enrichment, the concentration of nitrogen and some flavonol glycosides significantly decreased, whereas the concentration of total phenolics, condensed tannins and (+)‐catechin significantly increased. The total concentration of cinnamoylquinic acids was significantly increased by CO₂ and decreased by temperature. The concentration of salidroside increased under elevated temperature. 3 Weevil‐feeding experiments were carried out in a five‐choice arrangement, one leaf from each of the five treatments (chamber‐less field controls and four different treatments in chambers) being placed in random order in a plastic box. The weevils preferred the leaves grown under elevated CO₂, which had low nitrogen, high phenolics and the highest condensed tannin concentrations. Whether the reason for this trend is due to the stimulating effect of condensed tannins and/or a change in other secondary compounds, remains unknown. The weevils did not show any obviously different response in feeding performance to temperature and the combination of elevated CO₂ and temperature.     

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

We investigated how light and CO₂ levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO₂. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO₂ than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO₂ Elevated CO₂ prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO₂. Elevated CO₂ did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO₂ and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/C_i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO₂. Absolute levels of chlorophyll were affected by the interaction of CO₂ and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/C_i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO₂ accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.     

Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2

To determine the long-term impact of elevated CO₂ on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO₂ levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO₂ from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0–100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO₂ plots than ambient. Peak above-ground biomass was greater on elevated CO₂ plots than ambient CO₂ plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO₂ than under ambient CO₂ in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO₂ plots than ambient CO₂ plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO₂ plots and in ambient CO₂ plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO₂. Greater biomass production under elevated CO₂ in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO₂ concentrations in the future.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.     

Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO₂ plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO₂ partial pressures were maintained at ambient CO₂ (Amb) and elevated CO₂ (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO₂ under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO₂ produced 90% more biomass after four growing seasons than did trees grown in ambient CO₂. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO₂. Although there was not a sustained effect of elevated CO₂ on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO₂ as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO₂ on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO₂, but these differences were greater in the summer (60–130% increase) than in the winter (14–44% increase), reflecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO₂ will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO₂ compared with ambient CO₂. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then these trees could be a large sink for fossil carbon emitted to the atmosphere and produce a negative feedback on atmospheric CO₂.     

Effects of elevated CO2 on the nutrient compositions and enzymes activities of Nilaparvata lugens nymphs fed on rice plants

Elevated CO 2 may reduce the tolerance of Nilaparvata lugen (N. lugens) to adverse environmental factors through the biological and physiological degeneration of N. lugens. In an artificial climate box, under 375 and 750 μL L 1 CO 2 levels, the rice stems nutrient content, the nutrient content and enzyme activities of N. lugens nymph fed on rice seedlings exposed to ambient and elevated CO 2 were studied. The results showed that rice stems had significantly higher protein and total amino acid levels under ambient than elevated CO 2 levels. Nymphs had significantly higher protein levels in the ambient CO 2 treatment, while their glucose levels were significantly lower under ambient CO 2 conditions. Significantly higher trypsin activity was observed in nymphs grown in elevated CO 2 . Significantly lower activities of the protective enzymes total superoxide dismutase and catalase were observed in the nymphs under ambient CO 2 . Meanwhile, the activity of the detoxification enzyme glutathione S-transferase was significantly higher in the ambient CO 2 treatment. Measuring how energy and resources were allocated to enzymes in N. lugens nymphs under elevated CO 2 conditions can provide a more meaningful evaluation of their metabolic tolerances to adverse climatic conditions.