Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Continuing enrichment of atmospheric CO₂ may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO₂ enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C₄‐dominated grassland communities on contrasting soils along a CO₂ concentration gradient spanning pre‐industrial to expected mid‐21st century levels (250–500 μl/L). CO₂ enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO₂ enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R² = .34–.74), a measure of species abundance. CO₂ enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species‐specific controls on flowering in two general forms: (1) CO₂ effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO₂ effect. Understanding the heterogeneity in species responses to CO₂ enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO₂ effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.     

Post‐fire resprouting responses of native and exotic grasses from Cumberland Plain Woodland (Sydney,Australia) under elevated carbon dioxide

This study investigated the effect of elevated CO₂ on the post‐fire resprouting response of a grassland system of perennial grass species of Cumberland Plain Woodland. Plants were grown in mixtures in natural soil in mesocosms, each containing three exotic grasses (Nassella neesiana, Chloris gayana, Eragrostis curvula) and three native grasses (Themeda australis, Microlaena stipoides, Chloris ventricosa) under elevated (700 ppm) and ambient (385 ppm) CO₂ conditions. Resprouting response after fire at the community‐ and species‐level was assessed. There was no difference in community‐level biomass between CO₂ treatments; however, exotic species made up a larger proportion of the community biomass under all treatments. There were species‐level responses to elevated CO₂ but no significant interactions found between CO₂ and burning or plant status. Two exotic grasses (N. neesiana and E. curvula, a C₃ and a C₄ species respectively), and one native grass (M. stipoides, a C₃ species) significantly increased in biomass, and a native C₄ grass (C. ventricosa) significantly decreased in biomass under elevated CO₂. These results suggest that although overall productivity of this community may not change with increases in CO₂ and fire frequency, the community composition may alter due to differential species responses.     

Increased soil moisture content increases plant N uptake and the abundance of 15N in plant biomass

The natural abundance of ¹⁵N in plant biomass has been used to infer how N dynamics change with elevated atmospheric CO₂ and changing water availability. However, it remains unclear if atmospheric CO₂ effects on plant biomass ¹⁵N are driven by CO₂-induced changes in soil moisture. We tested whether ¹⁵N abundance (expressed as δ¹⁵N) in plant biomass would increase with increasing soil moisture content at two atmospheric CO₂ levels. In a greenhouse experiment we grew sunflower (Helianthus annuus) at ambient and elevated CO₂ (760 ppm) with three soil moisture levels maintained at 45, 65, and 85% of field capacity, thereby eliminating potential CO₂-induced soil moisture effects. The δ¹⁵N value of total plant biomass increased significantly with increased soil moisture content at both CO₂ levels, possibly due to increased uptake of ¹⁵N-rich organic N. Although not adequately replicated, plant biomass δ¹⁵N was lower under elevated than under ambient CO₂ after adjusting for plant N uptake effects. Thus, increases in soil moisture can increase plant biomass δ¹⁵N, while elevated CO₂ can decrease plant biomass δ¹⁵N other than by modifying soil moisture.     

Interactive effects of diversity, nutrients and elevated CO2 on experimental plant communities

The effects of species richness and elevated CO₂ on community productivity under altered nutrient levels were studied in experimental herbaceous communities composed of species from the Midwestern United States annual community, which consists of three functional groups C₃, C₄ and N‐fixers. Aboveground and belowground biomass were measured at flowering stage and at the end of the experiment when fruits of most plants were ripe. At the low nutrient level, species richness did not have a significant effect on community productivity. However, at the high nutrient level, the community biomass decreased with decreasing species richness at both ambient and elevated CO₂ in the first harvest, and at elevated CO₂ in the second harvest. At low nutrient level, CO₂ slightly increased community biomass at medium and high species richness. At high nutrient level, CO₂ significantly increased community biomass in all species‐richness treatments in the first harvest, but a significant response was observed only in the high richness treatment in the second harvest. At the functional group level, biomass of C₃ responded positively to CO₂, and C₄ responded very negatively to CO₂. The N‐fixers responded positively to CO₂ at low and medium species richness, but negatively at high species richness, showing a CO₂×richness interaction. CO₂ increased species evenness in the communities, depending on nutrient level. Species varied in the responses of light‐saturated net photosynthesis (P_max) to elevated CO₂, even within functional groups. Our findings suggest that (1) the relationship between productivity and species diversity was dependent on nutrient levels. (2) Species diversity enhances responses of communities to elevated CO₂. (3) Harvest time can affect the results of diversity‐productivity experiments. (4) Responses of C₃, C₄, and N‐fixers to elevated CO₂ in communities did not follow the prediction based on functional groups or plants grown individually, rather it depended on species richness.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

Interactions between C3 and C4 salt marsh plant species during four years of exposure to elevated atmospheric CO2

Elevated atmospheric CO2 is known to stimulate photosynthesis and growth of plants with the C₃ pathway but less of plants with the C₄ pathway. An increase in the CO₂ concentration can therefore be expected to change the competitive interactions between C₃ and C₄ species. The effect of long term exposure to elevated CO₂ (ambient CO₂ concentration +340 µmol CO₂ mol^-1) on a salt marsh vegetation with both C₃ and C₄ species was investigated. Elevated CO₂ increased the biomass of the C₃ sedgeScirpus olneyi growing in a pure stand, while the biomass of the C₄ grassSpartina patens in a monospecific community was not affected. In the mixed C₃/C₄ community the C₃ sedge showed a very large relative increase in biomass in elevated CO₂ while the biomass of the C₄ species declined.The C₄ grassSpartina patens dominated the higher areas of the salt marsh, while the C₃ sedgeScirpus olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate elevations.Scirpus growth may have been restricted by drought and salt stress at the higher elevations, whileSpartina growth at the lower elevations may be affected by the higher frequency of flooding. Elevated CO₂ may affect the species distribution in the salt marsh if it allowsScirpus to grow at higher elevations where it in turn may affect the growth ofSpartina.     

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

Atmospheric CO₂ enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO₂ enrichment, with an emphasis on water relations. Increasing CO₂ led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO₂ enrichment were relatively greater in dry years. In contrast, CO₂-induced aboveground biomass increases in the Texas C₃/C₄ grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO₂ in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO₂-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO₂ were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO₂ on plant and soil water relations may contribute substantially to experimentally induced CO₂-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO₂ effects on photosynthesis and those indirect CO₂ effects via soil moisture as documented here.     

Moisture Effects on Temperature Sensitivity of CO<Subscript>2</Subscript> Exchange in a Subarctic Heath Ecosystem

Carbon fluxes between natural ecosystems and the atmosphere have received increased attention in recent years due to the impact they have on climate. In order to investigate independently how soil moisture and temperature control carbon fluxes into and out of a dry subarctic dwarf shrub dominated heath, monoliths containing soil and plants were incubated at three different moisture levels and subjected to four different temperature levels between 7 and 20 °C. Ecosystem CO₂ exchange was monitored continuously day and night during the 16 to 18 days that each of three experiments lasted. Additionally, the carbon allocation pattern of the plants was investigated by labelling monoliths with ¹⁴CO₂ followed by harvest of above and below ground plant parts. The results revealed that the three different soil moisture levels caused distinctly differing levels of CO₂ fluxes. Also, both carbon fixation calculated as gross ecosystem production (GEP) and carbon release measured as ecosystem respiration (ER) increased with increasing temperatures, with ER increasing faster than GEP. Hence, short term carbon loss from the ecosystem accelerated with raised temperatures. Temperature sensitivity of the ecosystem was dependent on the soil moisture level, shown by differing Q10 values of both GEP and ER at different soil moisture levels. It is therefore highly important to take soil moisture levels into consideration when evaluating responses of ecosystem carbon balance to changes in temperature. The greatest C fixation took place via the two most dominant species of the ecosystem, Vaccinium uliginosum and Empetrum hermaphroditum, with the former being responsible for the different size of C fixation at the three moisture levels.     

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO₂ enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO₂ stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C₄ grass species were grown for 5 years along a field CO₂ gradient (250–500 μL L^?1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO₂ enrichment increased community ANPP by 0–117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO₂‐induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP‐CO₂ regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30–60% of the CO₂‐mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP‐CO₂ response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C₄ grass species. By favoring a mesic C₄ tall grass, CO₂ enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO₂‐stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.     

Biosynthesis of plant phenolic compounds in elevated atmospheric CO2

Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO₂) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO₂, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO₂ on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO₂ in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO₂ in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO₂ over the three generations. When the species were grown alone in pots, elevated CO₂ increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO₂ in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO₂ level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.     

CO2‐caused change in plant species composition rivals the shift in vegetation between mid‐grass and tallgrass prairies

Atmospheric CO₂ enrichment usually changes the relative contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which change is mediated differ among ecosystems. We measured changes in the contributions of C₃ perennial forbs and C₄ grasses to aboveground biomass production of tallgrass prairie assemblages grown along a field CO₂ gradient (250–500 μmol mol^?1) in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season mean of precipitation for the area. We predicted that CO₂ enrichment would increase the forb contribution to community production, and favor tall‐grasses over mid‐grasses by increasing soil water content and reducing the frequency with which soil water fell below a limitation threshold. CO₂ enrichment favored forbs over grasses on only one of three soil types, a Mollisol. The grass fraction of production increased dramatically across the CO₂ gradient on all soils. Contribution of the tall‐grass Sorghastrum nutans to production increased at elevated CO₂ on the two most coarse‐textured of the soils studied, a clay Mollisol and sandy Alfisol. The CO₂‐caused increase in Sorghastrum was accompanied by an offsetting decline in production of the mid‐grass Bouteloua curtipendula. Increased CO₂ favored the tall‐grass over mid‐grass by increasing soil water content and apparently intensifying competition for light or other resources (Mollisol) or reducing the frequency with which soil water dipped below threshold levels (Alfisol). An increase in CO₂ of 250 μmol mol^?1 above the pre‐industrial level thus led to a shift in the relative production of established species that is similar in magnitude to differences observed between mid‐grass and tallgrass prairies along a precipitation gradient in the central USA. By reducing water limitation to plants, atmospheric CO₂ enrichment may alter the composition and even structure of grassland vegetation.     

The response of plants to elevated CO2

Tree saplings, two groups of three species from each of two deciduous tree communities, were grown in competition at three CO₂ concentrations and two light levels. After one growing season, biomass was measured to assess the effect of CO₂ on community structure, and nitrogen and phosphorus concentrations were measured for leaves, stems, and roots of all trees. Gas-exchange measurements were made on the same species grown under the same CO₂ concentrations.Photosynthetic capacity (rate of photosynthesis at saturating CO₂ and light) tended to decline as CO₂ concentration increased, but differences were not statistically significant. Stomatal conductance declined significantly as CO₂ increased. Nitrogen and phosphorus concentrations generally declined as CO₂ increased, but there were some unexpected patterns in roots and stems. CO₂ concentration did not significantly affect the overall growth of either community after one season, but the relative biomass of each species changed in a complex way, depending on CO₂ light level, and community.     

Elevated atmospheric CO2 increases microbial growth rates in soil: results of three CO2 enrichment experiments

Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO₂ can cause a shift in the structure and activity of the microbial community responsible for the turnover of organic matter in soil. We investigated the long‐term effect of elevated CO₂ in the atmosphere on microbial biomass and specific growth rates in root‐free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (μ) were determined using the substrate‐induced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher μ were observed under elevated vs. ambient CO₂, depending on site, plant species and N fertilization. The μ‐values increased linearly with atmospheric CO₂ concentration at all three sites. The effect of elevated CO₂ on rhizosphere microorganisms was plant dependent and increased for: Brassica napus=Triticum aestivum<Beta vulgaris<Populus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The μ‐value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO₂ was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO₂. We conclude that the more abundant available organics released by roots at elevated CO₂ altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast‐growing r‐selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO₂ concentration.     

CO<Subscript>2</Subscript> enrichment increases element concentrations in grass mixtures by changing species abundances

Atmospheric carbon dioxide (CO₂) enrichment may increase plant growth more than the uptake of chemical elements from soil. Increased CO₂ also may alter element levels in biomass from multi-species vegetation by changing plant species abundances. We measured concentrations of ten elements in aboveground tissues of three C₄ grasses that had been exposed for 2–3 growing seasons to a continuous gradient in CO₂ from 250 to 500 μmol mol⁻¹. The grasses, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans, are competitive dominants in assemblages of tallgrass prairie vegetation growing on each of three soil types along a field CO₂ gradient in central Texas, USA. Our objective was to determine whether CO₂ influences element concentrations in grass mixtures by changing concentrations in individual species or shifting species abundances. Increased CO₂ had little effect on element concentrations in grasses compared to differences observed among grass species and soils. Increasing CO₂ from the pre-Industrial to elevated levels reduced the phosphorus concentration in grasses grown on a clay and sandy loam soil. Concentrations of most other elements did not respond to CO₂ treatment. Cover of the mid-grass Bouteloua declined at higher CO₂ levels as cover of the taller grass Sorghastrum increased. Concentrations of several elements were lower in Bouteloua than Sorghastrum; hence, this exchange of species at higher CO₂ increased element concentrations in grass assemblages. Potential consequences include an improvement in the nutritional quality of plants for herbivores. Results highlight the underappreciated impact that CO₂ enrichment may have on ecosystem functioning by changing plant composition.     

The response of plants to elevated CO2

Summary Communities, consisting of six co-occurring, disturbed site annuals, were subjected to CO₂ unenriched (300 ppm) and to CO₂ enriched (450 and 600 ppm) atmospheres at different levels of light and nutrient availability. In general, total community production increased with CO₂ enrichment to 450 ppm, but a further increase in CO₂ to 600 ppm had little or no effect. The response of community production to CO₂ level was not affected by nutrient availability but was affected by light level.Of the six species, four display C₃ metabolism. The proportion of total community production contributed by these species increased as a result of CO₂ enrichment, and was dependent upon both light and nutrient availability. The relative success of some species, particularly in terms of reproduction (total seed biomass), was significantly altered by CO₂ concentration depending on the level of nutrients. There were not only changes in reproductive success (seed biomass) and shoot biomass but also changes in the proportion of biomass allocated to seed.These experiments demonstrate that CO₂ enrichment does affect annual plant communities both in terms of productivity and species composition and that the affect of CO₂ on such system may depend upon other resources such as light and nutrients.     

Elevated atmospheric CO2 stimulates aboveground biomass in a fire-regenerated scrub-oak ecosystem

The effect of elevated atmospheric CO₂ concentration (C_a) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire‐regenerated scrub‐oak ecosystem. After burning a stand of scrub‐oak vegetation, re‐growing plants were exposed to either current ambient (379 µL L^?1 CO₂) or elevated (704 µL L^?1 CO₂) C_a in 16 open‐top chambers over a four‐year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated C_a significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated C_a had no effect on shoot density. The relative effect of elevated C_a on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated C_a was species specific: elevated C_a significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO₂ has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species‐specific growth responses could, in the long term, alter the composition of the scrub‐oak community.     

Effects of elevated atmospheric CO2 concentration on C and N pools and rhizosphere processes in a Florida scrub oak community

The effect of elevated atmospheric CO₂ concentration (C_a) on soil carbon and nitrogen accumulation and soil microbial biomass and activity in a native Florida scrub oak community was studied. The plant community, dominated by Quercus myrtifolia Willd. and Q. geminata Small, was exposed for 2 years to elevated C_a in open‐top chambers. Buried subsoil bags were retrieved after 1 year of exposure to elevated C_a. In addition, soil cores were taken twice from the chambers within two weeks in July 1998 (the first after a long dry spell and the second after 25 mm of rainfall) and divided into rhizosphere and bulk soil. Soil organic matter accumulation (excluding roots) into the buried subsoil bags was lower in elevated than in ambient C_a. Concentrations of soluble carbon and ninhydrin‐reactive nitrogen (N_ninh) in the rhizosphere soil were reduced by elevated C_a for the first sampling date and unaffected for the second sampling date. Microbial activity, measured as fluorescein diacetate (FDA) hydrolysis, decreased in elevated C_a for the first sampling date. Microbial biomass carbon and nitrogen in the bulk soil were unaffected by elevated C_a. There was no effect of elevated C_a on bacterial numbers in the rhizosphere.     

Elevated CO2 differentially alters the responses of coocurring birch and maple seedlings to a moisture gradient

Summary To determine the effects of elevated CO₂ and soil moisture status on growth and niche characteristics of birch and maple seedlings, gray birch (Betula populifolia) and red maple (Acer rubrum) were experimentally raised along a soil moisture gradient ranging from extreme drought to flooded conditions at both ambient and elevated atmospheric CO₂ levels. The magnitude of growth enhancement due to CO₂ was largely contingent on soil moisture conditions, but differently so for maple than for birch seedlings. Red maple showed greatest CO₂ enhancements under moderately moist soil conditions, whereas gray birch showed greatest enhancements under moderately dry soil conditions. Additionally, CO₂ had a relatively greater ameliorating effect in flooded conditions for red maple than for gray birch, whereas the reverse pattern was true for these species under extreme drought conditions. For both species, elevated CO₂ resulted in a reduction in niche breadths on the moisture gradient; 5% for gray birch and 23% for red maple. Species niche overlap (proportional overall) was also lower at elevated CO₂ (0.98 to: 0.88: 11%). This study highlights the utility of of experiments crossing CO₂ levels with gradients of other resources as effective tools for elucidating the potential consequences of elevated CO₂ on species distributions and potential interactions in natural communities.     

Biomass and compositional changes occur in chalk grassland turves exposed to elevated CO2 for two seasons in FACE

Artificial turves composed of 7 chalk grassland species (Festuca ovina L.; Briza media L.; Bromopsis erecta (Hudson) Fourr.; Plantago media L.; Sanguisorba minor Scop.; Anthyllis vulneraria L. and Lotus corniculatus L.) were grown from seed and exposed to two seasons of elevated (600 μmol mol^–1) and ambient (340 μmol mol^–1) CO₂ concentrations in free air CO₂ enrichment (ETH-FACE, Zurich). The turves were clipped regularly to a height of 5 cm and assessed for above ground biomass production and relative abundance based on accumulated clipped dry biomass as well as by point quadrat recording. Below ground biomass production was assessed with root in-growth bags during the second season of growth. Increases in total biomass (> 30%) were noted in elevated CO₂, but the differences did not become significant until the second season of growth. Individual species’ biomass varied in response to elevated CO₂, with significant increases in biomass in elevated CO₂ turves for both legume species, and no significant CO₂ effect on S. minor or P. media. An initial positive CO₂ effect on biomass of combined grass species was reversed by the end of the experiment with less biomass and a significantly smaller proportion of total biomass present in elevated CO₂, which was attributed primarily to changes in proportion of F. ovina. Species relative abundance was significantly affected by elevated CO₂ in the final 4 of the 6 clip events, with the legume species increasing in proportion at the expense of the other species, particularly the grasses. Root length and dry weight were both significantly increased in elevated CO₂ (77% and 89%, respectively), and these increases were greater than increases in shoot biomass (36%) from the same period. Species responses to elevated CO₂, within the model community, were not consistent with predictions made from data on individual species, leading to the conclusion that responses to elevated CO₂, at the community level, and species within the community level, are the result of direct physiological effects and indirect competitive effects. These conclusions are discussed with respect to the ecological responses of natural communities, and the chalk grassland community in particular, to elevated CO₂.     

The effects of elevated atmospheric CO2 on the amount and depth distribution of plant water uptake in a California annual grassland

Soil moisture profiles can affect species composition and ecosystem processes, but the effects of increased concentrations of atmospheric carbon dioxide ([CO₂]) on the vertical distribution of plant water uptake have not been studied. Because plant species composition affects soil moisture profiles, and is likely to shift under elevated [CO₂], it is also important to test whether the indirect effects of [CO₂] on soil water content may depend on species composition. We examined the effects of elevated [CO₂] and species composition on soil moisture profiles in an annual grassland of California. We grew monocultures and a mixture of Avena barbata and Hemizonia congesta– the dominant species of two phenological groups – in microcosms exposed to ambient (～370 μmol mol^?1) and elevated (～700 μmol mol^?1) [CO₂]. Both species increased intrinsic and yield‐based water use efficiency under elevated [CO₂], but soil moisture increased only in communities with A. barbata, the dominant early‐season annual grass. In A. barbata monocultures, the [CO₂] treatment did not affect the depth distribution of soil water loss. In contrast to communities with A. barbata, monocultures of H. congesta, a late‐season annual forb, did not conserve water under elevated [CO₂], reflecting the increased growth of these plants. In late spring, elevated [CO₂] also increased the efficiency of deep roots in H. congesta monocultures. Under ambient [CO₂], roots below 60 cm accounted for 22% of total root biomass and were associated with 9% of total water loss, whereas in elevated [CO₂], 16% of total belowground biomass was associated with 34% of total water loss. Both soil moisture and isotope data showed that H. congesta monocultures grown under elevated [CO₂] began extracting water from deep soils 2 weeks earlier than plants in ambient [CO₂].