Defoliation reduces soil biota – and modifies stimulating effects of elevated CO2

To understand the responses to external disturbance such as defoliation and possible feedback mechanisms at global change in terrestrial ecosystems, it is necessary to examine the extent and nature of effects on aboveground–belowground interactions. We studied a temperate heathland system subjected to experimental climate and atmospheric factors based on prognoses for year 2075 and further exposed to defoliation. By defoliating plants, we were able to study how global change modifies the interactions of the plant–soil system. Shoot production, root biomass, microbial biomass, and nematode abundance were assessed in the rhizosphere of manually defoliated patches of Deschampsia flexuosa in June in a full‐factorial FACE experiment with the treatments: increased atmospheric CO₂, increased nighttime temperatures, summer droughts, and all of their combinations. We found a negative effect of defoliation on microbial biomass that was not apparently affected by global change. The negative effect of defoliation cascades through to soil nematodes as dependent on CO₂ and drought. At ambient CO₂, drought and defoliation each reduced nematodes. In contrast, at elevated CO₂, a combination of drought and defoliation was needed to reduce nematodes. We found positive effects of CO₂ on root density and microbial biomass. Defoliation affected soil biota negatively, whereas elevated CO₂ stimulated the plant–soil system. This effect seen in June is contrasted by the effects seen in September at the same site. Late season defoliation increased activity and biomass of soil biota and more so at elevated CO₂. Based on soil biota responses, plants defoliated in active growth therefore conserve resources, whereas defoliation after termination of growth results in release of resources. This result challenges the idea that plants via exudation of organic carbon stimulate their rhizosphere biota when in apparent need of nutrients for growth.     

Belowground heathland responses after 2 years of combined warming, elevated CO2 and summer drought

Terrestrial ecosystems are exposed to atmospheric and climatic changes including increases in atmospheric CO₂ concentration, temperature and alterations of precipitation patterns, which are predicted to continue with consequences for ecosystem services and functioning in the future. In a field scale experiment on temperate heathland, manipulation of precipitation and temperature was performed with retractable curtains, and atmospheric CO₂ concentration was increased by FACE. The combination of elevated CO₂ and warming was expected to affect belowground processes additively, through increased belowground sequestration of labile carbohydrates due to elevated CO₂ in combination with temperature increased process rates. Together, these changes might increase microbial activity and availability of plant nutrients. Two years after the start of the experiment, belowground processes responded significantly to the treatments. In the combined temperature and CO₂ treatment the dissolved organic nitrogen concentration decreased and the ammonium concentration increased, but this release of nutrients was not mirrored by plant parameters. Microbial biomass carbon and microbial enrichment with ¹³C and ¹⁵N (1 year after ¹³C ₂ ¹⁵ N-glycine was injected into the soil) increased in warmed plots and in elevated CO₂ plots, but not when these treatments were combined. Furthermore, drought led to an increase in Calluna biomass and total plant nitrogen pool. The full combination of warming, elevated CO₂ and periodic drought did not unambiguously express the ecosystem responses of single factors additively, which complicates predictions of ecosystem responses to multifactor climate change.     

Effects of elevated CO2, warming and precipitation change on plant growth,photosynthesis and peroxidation in dominant species from North China grassland

Warming, watering and elevated atmospheric CO₂-concentration effects have been extensively studied separately; however, their combined impact on plants is not well understood. In the current research, we examined plant growth and physiological responses of three dominant species from the Eurasian Steppe with different functional traits to a combination of elevated CO₂, high temperature, and four simulated precipitation patterns. Elevated CO₂ stimulated plant growth by 10.8–41.7 % for a C₃ leguminous shrub, Caragana microphylla, and by 33.2–52.3 % for a C₃ grass, Stipa grandis, across all temperature and watering treatments. Elevated CO₂, however, did not affect plant biomass of a C₄ grass, Cleistogenes squarrosa, under normal or increased precipitation, whereas a 20.0–69.7 % stimulation of growth occurred with elevated CO₂ under drought conditions. Plant growth was enhanced in the C₃ shrub and the C₄ grass by warming under normal precipitation, but declined drastically with severe drought. The effects of elevated CO₂ on leaf traits, biomass allocation and photosynthetic potential were remarkably species-dependent. Suppression of photosynthetic activity, and enhancement of cell peroxidation by a combination of warming and severe drought, were partly alleviated by elevated CO₂. The relationships between plant functional traits and physiological activities and their responses to climate change were discussed. The present results suggested that the response to CO₂ enrichment may strongly depend on the response of specific species under varying patterns of precipitation, with or without warming, highlighting that individual species and multifactor dependencies must be considered in a projection of terrestrial ecosystem response to climatic change.     

Reduced N cycling in response to elevated CO2, warming,and drought in a Danish heathland: Synthesizing results of the CLIMAITE project after two years of treatments

Field‐scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO₂), warming, and extended summer drought in all combinations. Here, we report on the short‐term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO₂ significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co‐dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO₂ had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH₄⁺ consumption, gross mineralization, potential nitrification, denitrification and N₂O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO₂, as well as drought‐induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO₂ concentration.     

Rising sea level,temperature, and precipitation impact plant and ecosystem responses to elevated CO2 on a Chesapeake Bay wetland: review of a 28‐year study

An ongoing field study of the effects of elevated atmospheric CO₂ on a brackish wetland on Chesapeake Bay, started in 1987, is unique as the longest continually running investigation of the effects of elevated CO₂ on an ecosystem. Since the beginning of the study, atmospheric CO₂ increased 18%, sea level rose 20 cm, and growing season temperature varied with approximately the same range as predicted for global warming in the 21st century. This review looks back at this study for clues about how the effects of rising sea level, temperature, and precipitation interact with high atmospheric CO₂ to alter the physiology of C3 and C4 photosynthetic species, carbon assimilation, evapotranspiration, plant and ecosystem nitrogen, and distribution of plant communities in this brackish wetland. Rising sea level caused a shift to higher elevations in the Scirpus olneyi C3 populations on the wetland, displacing the Spartina patens C4 populations. Elevated CO₂ stimulated carbon assimilation in the Scirpus C3 species measured by increased shoot and root density and biomass, net ecosystem production, dissolved organic and inorganic carbon, and methane production. But elevated CO₂ also decreased biomass of the grass, S. patens C4. The elevated CO₂ treatment reduced tissue nitrogen concentration in shoots, roots, and total canopy nitrogen, which was associated with reduced ecosystem respiration. Net ecosystem production was mediated by precipitation through soil salinity: high salinity reduced the CO₂ effect on net ecosystem production, which was zero in years of severe drought. The elevated CO₂ stimulation of shoot density in the Scirpus C3 species was sustained throughout the 28 years of the study. Results from this study suggest that rising CO₂ can add substantial amounts of carbon to ecosystems through stimulation of carbon assimilation, increased root exudates to supply nitrogen fixation, reduced dark respiration, and improved water and nitrogen use efficiency.     

Soil microorganisms respond to five years of climate change manipulations and elevated atmospheric CO2 in a temperate heath ecosystem

Background and aims

Soil microbial responses to global change can affect organic matter turnover and nutrient cycling thereby altering the overall ecosystem functioning. In a large-scale experiment, we investigated the impact of 5 years of climate change and elevated atmospheric CO₂ on soil microorganisms and nutrient availability in a temperate heathland.

Methods

The future climate was simulated by increased soil temperature (+0.3 °C), extended pre-summer drought (excluding 5–8 % of the annual precipitation) and elevated CO₂ (+130 ppm) in a factorial design. Soil organic matter and nutrient pools were analysed and linked to microbial measures by quantitative PCR of bacteria and fungi, chloroform fumigation extraction, and substrate-induced respiration to assess their impact of climate change on nutrient availability.

Results

Warming resulted in higher measures of fungi and bacteria, of microbial biomass and of microbial growth potential, however, this did not reduce the availability of nitrogen or phosphorus in the soil. Elevated CO₂ did not directly affect the microbial measures or nutrient pools, whereas drought shifted the microbial community towards a higher fungal dominance.

Conclusions

Although we were not able to show strong interactive effects of the global change factors, warming and drought changed both nutrient availability and microbial community composition in the heathland soil, which could alter the ecosystem carbon and nutrient flow in the long-term.     

Plant nutrient mobilization in temperate heathland responds to elevated CO2, temperature and drought

Temperate terrestrial ecosystems are currently exposed to increased atmospheric CO₂ and progressive climatic changes with increased temperature and periodical drought. We here present results from a field experiment, where the effects of these three main climate change related factors are investigated solely and in all combinations at a temperate heathland. Significant responses were found in the top soils below the two dominant species (Calluna vulgaris and Deschampsia flexuosa). During winter incubation, microbial immobilization of N and ammonification rate decreased in response to warming in Deschampsia soil, and microbial immobilization of N and P decreased in warmed Calluna soil. Warming tended to increase microbial N and P in Calluna but not in Deschampsia soil in fall, and more microbial C was accumulated under drought in Calluna soil. The effects of warming were often counteracted or erased when combined with CO₂ and drought. Below Deschampsia, the net nitrification rate decreased in response to drought and, while phosphorus availability and microbial P immobilization decreased, but nitrification increased in response to elevated CO₂. Furthermore, leaf litter decomposition of both species decreased in response to drought. These complex changes in availability and release of nutrients from soil organic matter turnover and mineralization in response to elevated CO₂ and climate change may influence the future plant carbon sequestration and species composition at temperate heathlands.     

Soil respiration is stimulated by elevated CO2 and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE)

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (R_S) in a Danish Calluna‐Deschampsia‐heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO₂ (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average R_S, observed in the control over 3 years of measurements (1.7 μmol CO₂ m^?2 sec^?1), increased 38% under elevated CO₂, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased R_S by 14%, while elevated soil temperature did not affect R_S overall. A significant interaction between elevated temperature and drought resulted in further reduction of R_S when these treatments were combined. A detailed analysis of short‐term R_S dynamics associated with drought periods showed that R_S was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of R_S to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts R_S from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in R_S. The model predicted annual sums of R_S in 2006 and 2007, in the control, were 672 and 719 g C m^?2 y^?1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m^?2 y^?1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.     

No cumulative effect of 10 years of elevated [CO2] on perennial plant biomass components in the Mojave Desert

Elevated atmospheric CO₂ concentrations ([CO₂]) generally increase primary production of terrestrial ecosystems. Production responses to elevated [CO₂] may be particularly large in deserts, but information on their long‐term response is unknown. We evaluated the cumulative effects of elevated [CO₂] on primary production at the Nevada Desert FACE (free‐air carbon dioxide enrichment) Facility. Aboveground and belowground perennial plant biomass was harvested in an intact Mojave Desert ecosystem at the end of a 10‐year elevated [CO₂] experiment. We measured community standing biomass, biomass allocation, canopy cover, leaf area index (LAI), carbon and nitrogen content, and isotopic composition of plant tissues for five to eight dominant species. We provide the first long‐term results of elevated [CO₂] on biomass components of a desert ecosystem and offer information on understudied Mojave Desert species. In contrast to initial expectations, 10 years of elevated [CO₂] had no significant effect on standing biomass, biomass allocation, canopy cover, and C : N ratios of above‐ and belowground components. However, elevated [CO₂] increased short‐term responses, including leaf water‐use efficiency (WUE) as measured by carbon isotope discrimination and increased plot‐level LAI. Standing biomass, biomass allocation, canopy cover, and C : N ratios of above‐ and belowground pools significantly differed among dominant species, but responses to elevated [CO₂] did not vary among species, photosynthetic pathway (C₃ vs. C₄), or growth form (drought‐deciduous shrub vs. evergreen shrub vs. grass). Thus, even though previous and current results occasionally show increased leaf‐level photosynthetic rates, WUE, LAI, and plant growth under elevated [CO₂] during the 10‐year experiment, most responses were in wet years and did not lead to sustained increases in community biomass. We presume that the lack of sustained biomass responses to elevated [CO₂] is explained by inter‐annual differences in water availability. Therefore, the high frequency of low precipitation years may constrain cumulative biomass responses to elevated [CO₂] in desert environments.     

Interactive Effects of Elevated CO2, Drought, and Warming on Plants

Adverse climate change attributed to elevated atmospheric carbon dioxide concentration (CO₂) and increased temperature components of global warming has been a central issue affecting economic and social development. Climate change, particularly global warming, imposes a severe impact on the terrestrial ecosystem. Elevated CO₂, drought, and high temperature have been extensively documented individually; however, relatively little is known about how plants respond to the interaction of these factors. To summarize current knowledge on the response of plants to global change factors, we focus on the interactive effects of CO₂ enrichment, warming, and drought on plant growth, carbon allocation, and photosynthesis. Stimulation due to elevated CO₂ might be suppressed under other negative climatic/environmental stresses such as drought, high temperature, and their combination. However, elevated CO₂ could alleviate deleterious effects of moderate drought via reducing stomatal conductance, altering leaf surface, and regulating gene expression. High CO₂ levels and rising temperatures may result in opposite responses in plant water use efficiency. Stimulation of plant growth due to elevated CO₂ for C₃ species occurs regardless of water conditions, but only under a water deficit for C₄ species. The positive effect of elevated CO₂ on C₄ species is derived mainly from the improved water status. Plant adaptive or maladaptive responses to multivariate environments are interactive; thus, researchers need to explore the ecological underpinnings involved in such responses to the multiple factors involved in climate change.     

Responses of fungal root colonization,plant cover and leaf nutrients to long‐term exposure to elevated atmospheric CO2 and warming in a subarctic birch forest understory

Responses of the mycorrhizal fungal community in terrestrial ecosystems to global change factors are not well understood. However, virtually all land plants form symbiotic associations with mycorrhizal fungi, with approximately 20% of the plants' net primary production transported down to the fungal symbionts. In this study, we investigated how ericoid mycorrhiza (ErM), fine endophytes (FE) and dark septate endophytes (DSE) in roots responded to elevated atmospheric CO₂ concentrations and warming in the dwarf shrub understory of a birch forest in the subarctic region of northern Sweden. To place the belowground results into an ecosystem context we also investigated how plant cover and nutrient concentrations in leaves responded to elevated atmospheric CO₂ concentrations and warming. The ErM colonization in ericaceous dwarf shrubs increased under elevated atmospheric CO₂ concentrations, but did not respond to warming following 6 years of treatment. This suggests that the higher ErM colonization under elevated CO₂ might be due to increased transport of carbon belowground to acquire limiting resources such as N, which was diluted in leaves of ericaceous plants under enhanced CO₂. The elevated CO₂ did not affect total plant cover but the plant cover was increased under warming, which might be due to increased N availability in soil. FE colonization in grass roots decreased under enhanced CO₂ and under warming, which might be due to increased root growth, to which the FE fungi could not keep up, resulting in proportionally lower colonization. However, no responses in aboveground cover of Deschampsia flexuosa were seen. DSE hyphal colonization in grass roots significantly increased under warmer conditions, but did not respond to elevated CO₂. This complex set of responses by mycorrhizal and other root‐associated fungi to global change factors of all the fungal types studied could have broad implications for plant community structure and biogeochemistry of subarctic ecosystems.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

Interactive effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of reduced sulphur in young pedunculate oak trees (Quercus robur L.)

Pedunculate oak (Quercus robur L.) was germinated and grown under nutrient non-limiting conditions for a total of 10–15 weeks at ambient CO₂ concentration and 1100 μmol mol^–1 CO₂ either in the presence or the absence of the mycorrhizal fungus Laccaria laccata. Half of the oak trees of these treatments were exposed to drought during final growth by suspending the water supply for 21 d. Mycorrhization and elevated atmospheric CO₂ each enhanced total plant biomass per tree. Whereas additional biomass accumulation of trees grown under elevated CO₂ was mainly attributed to increased growth of lateral roots, mycorrhization promoted shoot growth. Water deficiency reduced biomass accumulation without affecting relative water content, but this effect was more pronounced in mycorrhizal as compared to non-mycorrhizal trees. Elevated CO₂ partially prevented the development of drought stress, as indicated by leaf water potential, but did not counteract the negative effects of water deficiency on growth during the time studied. Enhanced biomass accumulation requires adaption in protein synthesis and, as a consequence, enhanced allocation of reduced sulphur produced in the leaves to growing tissues. Therefore, allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH, the main long-distance transport form of reduced sulphur, to mature oak leaves. Export of radiolabel proceeded almost exclusively in basipetal direction to the roots. The rate of export of radioactivity out of the fed leaves was significantly enhanced under elevated CO₂, irrespective of mycorrhization. A higher proportion of the exported GSH was transported to the roots than to basipetal stem sections under elevated CO₂ as compared to ambient CO₂. Mycorrhization did not affect ³⁵S export out of the fed leaves, but the distribution of radiolabel between stem and roots was altered in preference of the stem. Trees exposed to drought did not show appreciable export of the ³⁵S radioactivity fed to the leaves when grown under ambient CO₂. Apparently, drought inhibited basipetal transport of reduced sulphur at the level of phloem loading and/or phloem transport. Elevated CO₂ seemed to counteract this effect of drought stress to some extent, since higher leaf water potentials and improved ³⁵S export out of the fed leaves was observed in oak trees exposed to drought and elevated CO₂ as compared to trees exposed to drought and ambient CO₂.     

Elevated CO<Subscript>2</Subscript> Concentration and Drought Stress Exert Opposite Effects on Plant Biomass,Nitrogen, and Phosphorus Allocation in <Emphasis Type="Italic">Bothriochloa ischaemum</Emphasis>

Increased concentrations of atmospheric carbon dioxide (CO₂) and drought stress have greatly influenced plant growth, the status of nitrogen (N) and phosphorus (P), and N:P ratios. We identified the plant biomass, N and P distributional patterns, and N:P stoichiometry of a grass species on the Loess Plateau in China under elevated CO₂ concentration and drought stress conditions. Bothriochloa ischaemum, a C4 perennial herbaceous grass, was grown in pots at CO₂ concentrations of 400 (ambient) and 800 (elevated) μmol mol^?1 and at 60 ± 5 and 40 ± 5 % of field capacity. The elevated CO₂ concentration significantly increased plant total biomass, N concentration, N and P content, allocation of biomass to roots, and allocation of N to shoots, and increased the N:P ratios of whole plants and the shoots, especially under well-watered conditions. Drought stress significantly decreased plant biomass and plant N and P content, especially under elevated CO₂. Drought stress decreased the N:P ratios, but was only significant in the roots under ambient CO₂. Drought stress may attenuate the stimulation of plant growth and N and P acquisition by CO₂ enrichment, and projected elevated CO₂ concentrations may partially offset the negative effects of increased drought by increasing the assimilation of N and P.     

Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide

Elevated atmospheric CO₂ increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO₂on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO₂ levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO₂ in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO₂ concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO₂ on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO₂ on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO₂ enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO₂, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.     

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

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

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.     

Effects of Warming, Summer Drought, and CO2 Enrichment on Aboveground Biomass Production, Flowering Phenology, and Community Structure in an Upland Grassland Ecosystem

Future climate scenarios predict simultaneous changes in environmental conditions, but the impacts of multiple climate change drivers on ecosystem structure and function remain unclear. We used a novel experimental approach to examine the responses of an upland grassland ecosystem to the 2080 climate scenario predicted for the study area (3.5°C temperature increase, 20% reduction in summer precipitation, atmospheric CO₂ levels of 600 ppm) over three growing seasons. We also assessed whether patterns of grassland response to a combination of climate change treatments could be forecast by ecosystem responses to single climate change drivers. Effects of climate change on aboveground production showed considerable seasonal and interannual variation; April biomass increased in response to both warming and the simultaneous application of warming, summer drought, and CO₂ enrichment, whereas October biomass responses were either non-significant or negative depending on the year. Negative impacts of summer drought on production were only observed in combination with a below-average rainfall regime, and showed lagged effects on spring biomass. Elevated CO₂ had no significant effect on aboveground biomass during this study. Both warming and the 2080 climate change scenario were associated with a significant advance in flowering time for the dominant grass species studied. However, flowering phenology showed no significant response to either summer drought or elevated CO₂. Species diversity and equitability showed no response to climate change treatments throughout this study. Overall, our data suggest that single-factor warming experiments may provide valuable information for projections of future ecosystem changes in cool temperate grasslands.     

Genetic variation in germination, growth, and survivorship of red maple in response to subambient through elevated atmospheric CO2

Genetic variation in plant response to atmospheric carbon dioxide (CO₂) may have influenced paleo‐vegetation dynamics and could determine how future elevated CO₂ drives plant evolution and ecosystem productivity. We established how levels of relatedness – the maternal family, population, and provenance – affect variation in the CO₂ response of a species. This 2‐year growth chamber experiment focused on the germination, growth, biomass allocation, and survivorship responses of Acer rubrum to four concentrations of CO₂: 180, 270, 360, and 600 μL L^?1– representing Pleistocene through potential future conditions. We found that all levels of relatedness interacted with CO₂ to contribute to variation in response. Germination responses to CO₂ varied among families and populations, growth responses depended on families and regions of origin, and survivorship responses to CO₂ were particularly affected by regional identities. Differences among geographic regions accounted for 23% of the variation in biomass response to CO₂. If seeds produced under subambient CO₂ conditions behave similarly, our results suggest that A. rubrum may have experienced strong selection on seedling survivorship at Pleistocene CO₂ levels. Further, this species may evolve in response to globally rising CO₂ so as to increase productivity above that experimentally observed today. Species responses to future atmospheric CO₂ and the accompanying biotic effects on the global carbon cycle will vary among families, populations, and provenances.     

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].