Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Fungal biomass production in response to elevated atmospheric CO2 in a Glomus mosseae–Prunus cerasifera model system

Biomass and length of intraradical and extraradical mycorrhizal mycelium under ambient (aCO₂) and elevated (eCO₂ ) atmospheric CO₂ was investigated using a non-destructive in vivo experimental model system. Time-course experiments allowed measurements of intact extraradical mycelium spreading from mycorrhizal roots of Prunus cerasifera micropropagated plants inoculated with the arbuscular mycorrhizal fungus Glomus mosseae, in controlled environmental chambers. The length of extraradical mycelium was significantly increased at the highest CO₂ concentration, ranging from 10.7 to 20.3 m at aCO₂ and eCO₂, respectively. The biochemical determination of mycelial glucosamine content allowed the evaluation of intraradical and extraradical fungal biomass, which were 2 and 3 times larger at eCO₂ than at aCO₂. Present data show that Glomus mosseae responds to increases of CO₂ concentrations producing larger mycorrhizal networks which may potentially represent carbon sink agents in soil ecosystems.     

Response of soil biota to elevated atmospheric CO2 in poplar model systems

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO₂ would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO₂ in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO₂. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO₂. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO₂. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO₂ treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO₂, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO₂ was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose. Received: 18 March 1997 / Accepted: 5 August 1997     

Does carbon partitioning in ectomycorrhizal pine seedlings under elevated CO2 vary with fungal species?

Enhanced soil respiration in response to elevated atmospheric CO₂ has been demonstrated, and ectomycorrhizal (ECM) fungi are of particular interest since they partition host-derived photoassimilates belowground. Although a strong response of ECM fungi to elevated CO₂ has been shown, little is still known about the functional diversity among species. We studied carbon (C) partitioning in mycorrhizal Scots pine seedlings in response to short-term CO₂ enrichment, using seven ECM species with different ecological strategies. Mycorrhizal associations were synthesised and seedlings grown in large Petri dishes containing peat:vermiculite and nutrient solution for 10–15 weeks, after which half of the microcosms were exposed to elevated CO₂ treatment (710 ppm) for 15 days and the other half were kept in ambient CO₂ treatment. Partitioning of C was quantified by pulse labelling the seedlings with ¹⁴CO₂ and examining the distribution of labelled assimilates in shoot, root and extraradical mycelial compartments by destructive harvest and liquid scintillation counting. Fungal biomass was determined with PLFA analysis. The respiratory loss of ¹⁴CO₂ was on average greater in the elevated CO₂ treatment for most species compared to the ambient CO₂ treatment. More label was retrieved in the shoots in the ambient CO₂ treatment compared to elevated CO₂ (significant for P. involutus and P. fallax). Greater amounts of label were found in the extraradical mycelial compartment in all species (except P. involutus) in elevated CO₂ compared to ambient CO₂ (significant for L. bicolor, P. byssinum, P. fallax and R. roseolus). Fungal biomass production increased significantly with elevated CO₂ for two species (H. velutipes and A. muscaria); three species (P. fallax, P. involutus and R. roseolus) showed a similar but non-significant trend, whereas L. bicolor and P. byssinum produced less biomass in elevated CO₂ compared to ambient CO₂. When ¹⁴C in the mycelial compartment and respiration was expressed per unit fungal PLFA the difference between CO₂ treatments disappeared. We demonstrated that different ECM fungal isolates respond differently in C partitioning in response to CO₂ enrichment. These results suggest that under certain growth conditions, when nutrients are not limiting, ECM fungi respond rapidly to increasing C-availability through changed biomass production and respiration.     

Production, turnover and mycorrhizal colonization of root systems of three Populus species grown under elevated CO2 (POPFACE)

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO₂ treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO₂ increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO₂. Response of the mycorrhizal community to elevated CO₂ was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO₂ conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long‐term C storage in the soil.     

Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO2 enrichment

Prunella vulgaris was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO₂ (ambient, 350 μl l⁻¹, and elevated, 600 μl l⁻¹) to test whether a plants response to elevated CO₂ is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots, we also tested whether elevated CO₂ affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO₂; the biomass of the root system, for example, increased by a factor of 2. The colonization of AMF inside the root remained constant, indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO₂ was up to 5 times that at ambient CO₂, indicating that elevated CO₂ promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO₂ but this resulted in either an increase or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO₂ would result in increased P acquistion by the plant. To test this we supplied phosphorus, in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO₂ when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however, play a significant role in the movement of fixed carbon to the soil and increase soil aggregation. Received: 28 March 1998 / Accepted: 27 August 1998     

Ectomycorrhizal fungi associated with Pinus sylvestris seedlings respond differently to increased carbon and nitrogen availability: implications for ecosystem responses to global change

The ectomycorrhizal (ECM) symbiosis can cause both positive and negative feedback with trees under elevated CO₂. Positive feedback arises if the additional carbon (C) increases both nutrient uptake by the fungus and nutrient transfer to the plant, whereas negative feedback results from increased nutrient uptake and immobilization by the fungus and reduced transfer to the plant. Because species of ECM fungi differ in their C and nitrogen (N) demand, understanding fungal species‐specific responses to variation in C and N supply is essential to predict impacts of global change. We investigated fungal species‐specific responses of ECM Scots pine (Pinus sylvestris) seedlings under ambient and elevated CO₂ (350 or 700 μL L⁻¹ CO₂) and under low and high mineral N availability. Each seedling was associated with one of the following ECM species: Hebeloma cylindrosporum, Laccaria bicolor and Suillus bovinus. The experiment lasted 103 days. During the final 27 days, seedlings were labeled with ¹⁴CO₂ and ¹⁵N. Most plant and fungal parameters were significantly affected by fungal species, CO₂ level and N supply. Interactions between fungal species and CO₂ were also regularly significant. At low N availability, elevated CO₂ had the smallest impact on the photosynthetic performance of seedlings inoculated with H. cylindrosporum and the largest impact on seedlings with S. bovinus. At ambient CO₂, increasing N supply had the smallest impact on seedlings inoculated with S. bovinus and the largest on seedlings inoculated with H. cylindrosporum. At low N availability, extraradical hyphal length increased after doubling CO₂ level, but this was significant only for L. bicolor. At ambient CO₂, increasing N levels reduced hyphal length for both H. cylindrosporum and S. bovinus, but not for L. bicolor. We discuss the potential interplay of two major elements of global change, elevated CO₂ and increased N availability, and their effects on plant growth. We conclude that increased N supply potentially relieves mycorrhiza‐induced progressive N limitation under elevated CO₂.     

Nitrogen input mediates the effect of free-air CO2 enrichment on mycorrhizal fungal abundance

Plots containing Lolium perenne L., Trifolium repens L. or a mixture of both plant species were exposed to elevated atmospheric CO₂ (eCO₂) for 10 consecutive seasons using free‐air CO₂ enrichment technology at ETH Zürich, Switzerland. The CO₂ treatment was crossed with a two‐level nitrogen (N) fertilization treatment. In the tenth year, soil samples were collected on three occasions through the growing season to assess the impact of eCO₂ and N fertilization on mycorrhizal fungal abundance. Soil moisture content, which varied with harvest date, was linked to the vegetation type and was higher under eCO₂. Root weight density was affected by vegetation type: lower for clover, higher for grass. Root weight density was stimulated by eCO₂ and decreased by high N fertilization. The percent root length colonized by mycorrhizal fungi was lowest in the clover plots and highest in the grass plots. High N significantly decreased root length colonized. There was no overall effect of eCO₂ on root length colonized; however, there was a significant eCO₂× N interaction: eCO₂ increased root length colonized at high N, but decreased root length colonized at low N. Extraradical mycorrhizal hyphal density was linked to soil moisture content. Extraradical mycorrhizal hyphal density was not affected by eCO₂ or high N individually, but as for root length colonized, there was a significant eCO₂× N interaction: eCO₂ increased extraradical mycorrhizal hyphal density at low N but not at high N. These environmental effects on root colonization and external mycorrhizal hyphae were independent of soil moisture content and root weight density. This field study demonstrated a significant mediating effect of N fertilization on the response of arbuscular mycorrhizal fungi to eCO₂ irrespective of any change in root biomass.     

Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2]

We hypothesized that greater photosynthate supply at elevated [CO₂] could compensate for increased below-ground C demands of arbuscular mycorrhizas. Therefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and mycorrhizas at elevated atmospheric [CO₂]. Half of the seedlings of sour orange (C. aurantium L.) and ‘Ridge Pineapple’ sweet orange (C. sinensis L. Osbeck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants were grown at ambient or 2X ambient [CO₂] in unshaded greenhouses for 11 weeks and fertilized daily with nutrient solution either without added P or with 2 mM P in a low-P soil. High P supply reduced AM colonization whereas elevated [CO₂] counteracted the depressive effect of P on intraradical colonization and vesicle development. Seedlings grown at either elevated [CO₂], high P or with G. intraradices had greater growth, net assimilation of CO₂ (A _CO2) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decreased by elevated [CO₂], P, and AM colonization. Mycorrhizal seedlings had higher leaf-P status but lower leaf N and K concentrations than nonmycorrhizal seedlings which was due to growth dilution effects. Starch in fibrous roots was increased by elevated [CO₂] but reduced by G. intraradices, especially at low-P supply. In fibrous roots, elevated [CO₂] had no effect on C/N, but AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or A _CO2. Mycorrhizas did not increase plant growth at ambient [CO₂]. At elevated [CO₂], however, mycorrhizas stimulated growth at both P levels in sour orange, the more mycorrhiza-dependent species, but only at low-P in sweet orange, the less dependent species. At low-P and elevated [CO₂], colonization by the AM fungus increased A _CO2 in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starch level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO₂] availability to mycorrhizal plants increased CO₂ assimilation, growth and nutrient uptake over that of NM plants especially in sour orange under P limitation. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Mycelial production, spread and root colonisation by the ectomycorrhizal fungi Hebeloma crustuliniforme and Paxillus involutus under elevated atmospheric CO2

Effects of elevated atmospheric carbon dioxide (CO₂) levels on the production and spread of ectomycorrhizal fungal mycelium from colonised Scots pine roots were investigated. Pinus sylvestris (L.) Karst. seedlings inoculated with either Hebeloma crustuliniforme (Bull:Fr.) Quél. or Paxillus involutus (Fr.) Fr. were grown at either ambient (350 ppm) or elevated (700 ppm) levels of CO₂. Mycelial production was measured after 6 weeks in pots, and mycelial spread from inoculated seedlings was studied after 4 months growth in perlite in shallow boxes containing uncolonised bait seedlings. Plant and fungal biomass were analysed, as well as carbon and nitrogen content of seedling shoots. Mycelial biomass production by H. crustuliniforme was significantly greater under elevated CO₂ (up to a 3-fold increase was observed). Significantly lower concentrations and total amounts of N were found in plants exposed to elevated CO₂.     

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

Competition for nitrogen between Pinus sylvestris and ectomycorrhizal fungi generates potential for negative feedback under elevated CO2

We investigated fungal species-specific responses of ectomycorrhizal (ECM) Scots pine (Pinus sylvestris) seedlings on growth and nutrient acquisition together with mycelial development under ambient and elevated CO₂. Each seedling was associated with one of the following ECM species: Hebeloma cylindrosporum, Laccaria bicolor, Suillus bovinus, S. luteus, Piloderma croceum, Paxillus involutus, Boletus badius, or non-mycorrhizal, under ambient, and elevated CO₂ (350 or 700 μl l⁻¹ CO₂); each treatment contained six replicates. The trial lasted 156 days. During the final 28 days, the seedlings were labeled with ¹⁴CO₂. We measured hyphal length, plant biomass, ¹⁴C allocation, and plant nitrogen and phosphorus concentration. Almost all parameters were significantly affected by fungal species and/or CO₂. There were very few significant interactions. Elevated CO₂ decreased shoot-to-root ratio, most strongly so in species with the largest extraradical mycelium. Under elevated CO₂, ECM root growth increased significantly more than hyphal growth. Extraradical hyphal length was significantly negatively correlated with shoot biomass, shoot N content, and total plant N uptake. Root dry weight was significantly negatively correlated with root N and P concentration. Fungal sink strength for N strongly affected plant growth through N immobilization. Mycorrhizal fungal-induced progressive nitrogen limitation (PNL) has the potential to generate negative feedback with plant growth under elevated CO₂. Responsible Editor: Herbert Johannes Kronzucker     

The effects of elevated CO2 and temperature on the resistance of winter-dormant birch seedlings (Betula pendula) to hares and voles

The effects of elevated CO₂ and temperature on the resource allocation pattern and resistance against mammalian herbivores of silver birch (Betula pendula Roth) were studied. Birch seedlings were grown through two growing seasons in closed‐top chambers exposed to four different treatments: ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm) and ambient temperature, elevated temperature (+3°C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. After winter hardening of the seedlings, the growth of the seedlings was measured and the concentration of secondary compounds such as phenolics and papyriferic acid determined. The top parts of the stem were fed to hares, and the basal parts of the same stems were offered to voles. Elevated CO₂ increased the height and basal diameter of the shoots, shoot biomass and total biomass of the seedlings but did not have any effect on secondary chemistry. Elevated temperature increased the height and shoot biomass, but did not have a significant effect on the total biomass of the seedlings. Elevated temperature decreased the concentration of condensed tannins and their precursor, (+)‐catechin, in the top part of the stems, but only the concentration of (+)‐catechin in the basal part of the stems. There were no significant interactive effects between CO₂ and temperature on phenolics in the stems, while the concentration of papyriferic acid showed significant interaction in the top part of the stems. This indicates high accumulation of papyriferic acid in ambient CO₂ under increased temperature. Consequently, elevated temperature increased the resistance of birch against hares, but did not affect the resistance of the basal parts of the same birches to voles. Our results indicate that the predicted climatic change will not necessarily lead to increased browsing damage by the mountain hare and the field vole to silver birch.     

Impact of arbuscular mycorrhizal fungi (AMF) and atmospheric CO2 concentration on the biomass production and partitioning in the forage legume alfalfa

The influence of mycorrhizal symbiosis, atmospheric CO₂ concentration and the interaction between both factors on biomass production and partitioning were assessed in nodulated alfalfa (Medicago sativa L.) associated or not with arbuscular mycorrhizal fungi (AMF) and grown in greenhouse at either ambient (392 μmol?mol^?1) or elevated (700 μmol?mol^?1) CO₂ air concentrations. Measurements were performed at three stages of the vegetative period of plants. Shoot and root biomass achieved by plants at the end of their vegetative period were highly correlated to the photosynthetic rates reached at earlier stages, and there was a significant relationship between CO₂ exchange rates and total nodule biomass per plant. In non-mycorrhizal alfalfa, the production of leaves, stems and nodules biomass significantly increased when plants had been exposed to elevated CO₂ concentration in the atmosphere for 4 weeks. Regardless CO₂ concentration at which alfalfa were cultivated, mycorrhizal symbiosis improved photosynthetic rates and growth of alfalfa at early stages of the vegetative period and then photosynthesis decreased, which suggests that AMF shortened the vegetative period of the host plants. At final stages of the vegetative period, AMF enhanced both area and biomass of leaves as well as the leaves to stems ratio when alfalfa plants were cultivated at ambient CO₂. The interaction of AMF with elevated CO₂ improved root biomass and slightly increased the leaves to stems ratio at the end of the vegetative growth. Therefore, AMF may favor both the forage quality of alfalfa when grown at ambient CO₂ and its perennity for next cutting regrowth cycle when grown under elevated CO₂. Nevertheless, this hypothesis needs to be checked under natural conditions in field.     

Poa annua shows inter-generational differences in response to elevated CO2

Inter-generational effects on the growth of Poa annua (L.) in ambient and elevated atmospheric CO₂ conditions (350 and 550 μl l^–1, respectively) were studied in two different experiments. Both experiments showed similar results. In a greenhouse experiment growth, measured as the numbers of tillers produced per week, was compared for plants grown from first and second generation seeds. Second generation seeds were obtained from plants grown for one whole generation in either ambient or elevated atmospheric CO₂ (‘ambient’ and ‘elevated’ seeds, respectively). First generation plants and second generation ‘ambient’ plants did not respond to elevated CO₂. Second generation ‘elevated’ plants produced significantly more tillers in elevated CO₂. In the second experiment model terrestrial ecosystems growing in the Ecotron and which included Poa annua were used. Above-ground biomass after one and two generations of growth were compared. At the end of Generation 1 no difference was found in biomass production while at the end of Generation 2 biomass increased in elevated CO₂ by 50%. The implications for climate change research are discussed.     

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.     

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.     

Nitrogen and phosphorus dynamics of a tallgrass prairie ecosystem exposed to elevated carbon dioxide

A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO₂ concentrations in open-top chambers and compared to unchambered ambient CO₂ during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii (C₄), A. scoparius (C₄), Sorghastrum nutans (C₄) and Poa pratensis (C₃). Nitrogen and phosphorus concentrations in A. gerardii, P. pratensis and dicotyledonous herbs above ground biomass were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, N and P concentrations in peak biomass were estimated by an early August harvest. N and P concentrations in root production as a function of treatment were estimated using root ingrowth bags that remained in place throughout the growing season. Total N and P in above- and belowground biomass were calculated as products of concentration and peak biomass by species groups. N concentration in A. gerardii and dicotyledonous herb aboveground biomass was lower and total N higher in elevated CO₂ plots than in ambient CO₂ plots. N concentration in P. pratensis aboveground biomass was lower in elevated CO₂ plots than in ambient, but total N did not differ among treatments in 2 out of 3 years. In 1990, N concentration in root ingrowth bag biomass was lower and total N greater in elevated CO₂ than in ambient CO₂ plots. Root ingrowth bag biomass N concentration did not differ among treatments in 1991, but total N was greater in elevated CO₂ plots than in ambient CO₂ plots. P concentration was lower under elevated CO₂ compared to ambient in 1989, but did not differ substantially among treatments in 1990 or 1991. In all years, total P in aboveground A. gerardii and root ingrowth bag biomass was greater under elevated CO₂ than ambient. P concentration and total P in P. pratensis was similar among treatments.