H. Ro-Poulsen

Interactive effects of drought, elevated CO2 and warming on photosynthetic capacity and photosystem performance in temperate heath plants

Increased temperature, atmospheric CO(2) and change in precipitation patterns affect plant physiological and ecosystem processes. In combination, the interactions between these effects result in complex responses that challenge our current understanding. In a multi-factorial field experiment with elevated CO(2) (CO2, FACE), nighttime warming (T) and periodic drought (D), we investigated photosynthetic capacity and PSII performance in the evergreen dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa in a temperate heath ecosystem. Photosynthetic capacity was evaluated using A/C(i) curves, leaf nitrogen content and chlorophyll-a fluorescence OJIP induction curves. The PSII performance was evaluated via the total performance index PI(total), which integrates the function of antenna, reaction centers, electron transport and end-acceptor reduction according to the OJIP-test. The PSII performance was negatively influenced by high air temperature, low soil water content and high irradiance dose. The experimental treatments of elevated CO(2) and prolonged drought generally down-regulated J(max), V(cmax) and PI(total). Recovery from these depressions was found in the evergreen shrub after rewetting, while post-rewetting up-regulation of these parameters was observed in the grass. Warming effects acted indirectly to improve early season J(max), V(cmax) and PI(total). The responses in the multi-factorial experimental manipulations demonstrated complex interactive effects of T×CO2, D×CO2 and T×D×CO2 on photosynthetic capacity and PSII performance. The impact on the O-J, J-I and I-P phases which determine the response of PI(total) are discussed. The single factor effects on PSII performance and their interactions could be explained by parallel adjustments of V(cmax), J(max) and leaf nitrogen in combination. Despite the highly variable natural environment, the OJIP-test was very robust in detecting the impacts of T, D, CO2 and their interactions. This study demonstrates that future climate will affect fundamental plant physiological processes in a way that is not predictable from single factor treatments. The interaction effects that were observed depended upon both the growth strategy of the species considered, and their ability to adjust during drought and rewetting periods.

Ozone uptake by an evergreen forest canopy : temporal variation and possible mechanisms

Patterns of ozone concentration ([O(3)]), O(3) deposition velocity (v(d)) and O(3) flux (F(c)) over an evergreen forest canopy are shown in relation to measuring method, physiological activity of the trees, and time of year. The gradient and eddy correlation methods were compared and showed similar diel v(d) patterns. Daytime F(c) was correlated with CO(2) and water vapour fluxes, while no correlation between [O(3)] in the range 10-70 ppb (nl l(-1)) and F(c) was seen in this study. F(c) was primarily driven by stomatal conductance, reactions with surfaces, particles and gases, and not by [O(3)]. On a monthly basis, [O(3)] was always highest in the afternoon while v(d) was typically higher in the morning, resulting in an equal F(c) over the day. Night-time F(c) was more than half of the daytime O(3) flux. The data reveal the importance of emissions of nitric oxide and terpenes as O(3) removal factors in evergreen forest dominated by Norway spruce.

Interactive effects of elevated CO2, warming, and drought on photosynthesis of Deschampsia flexuosa in a temperate heath ecosystem

Global change factors affect plant carbon uptake in concert. In order to investigate the response directions and potential interactive effects, and to understand the underlying mechanisms, multifactor experiments are needed. The focus of this study was on the photosynthetic response to elevated CO2 [CO2; free air CO2 enrichment (FACE)], drought (D; water-excluding curtains), and night-time warming (T; infrared-reflective curtains) in a temperate heath. A/Ci curves were measured, allowing analysis of light-saturated net photosynthesis (Pn), light- and CO2-saturated net photosynthesis (Pmax), stomatal conductance (gs), the maximal rate of Rubisco carboxylation (Vcmax), and the maximal rate of ribulose bisphosphate (RuBP) regeneration (Jmax) along with leaf δ13C, and carbon and nitrogen concentration on a monthly basis in the grass Deschampsia flexuosa. Seasonal drought reduced Pn via gs, but severe (experimental) drought decreased Pn via a reduction in photosynthetic capacity (Pmax, Jmax, and Vcmax). The effects were completely reversed by rewetting and stimulated Pn via photosynthetic capacity stimulation. Warming increased early and late season Pn via higher Pmax and Jmax. Elevated CO2 did not decrease gs, but stimulated Pn via increased Ci. The T×CO2 synergistically increased plant carbon uptake via photosynthetic capacity up-regulation in early season and by better access to water after rewetting. The effects of the combination of drought and elevated CO2 depended on soil water availability, with additive effects when the soil water content was low and D×CO2 synergistic stimulation of Pn after rewetting. The photosynthetic responses appeared to be highly influenced by growth pattern. The grass has opportunistic water consumption, and a biphasic growth pattern allowing for leaf dieback at low soil water availability followed by rapid re-growth of active leaves when rewetted and possibly a large resource allocation capability mediated by the rhizome. This growth characteristic allowed for the photosynthetic capacity up-regulations that mediated the T×CO2 and D×CO2 synergistic effects on photosynthesis. These are clearly advantageous characteristics when exposed to climate changes. In conclusion, after 1 year of experimentation, the limitations by low soil water availability and stimulation in early and late season by warming clearly structure and interact with the photosynthetic response to elevated CO2 in this grassland species.

Effects of elevated CO2, warming and drought episodes on plant carbon uptake in a temperate heath ecosystem are controlled by soil water status

The impact of elevated CO₂, periodic drought and warming on photosynthesis and leaf characteristics of the evergreen dwarf shrub Calluna vulgaris in a temperate heath ecosystem was investigated. Photosynthesis was reduced by drought in midsummer and increased by elevated CO₂ throughout the growing season, whereas warming only stimulated photosynthesis early in the year. At the beginning and end of the growing season, a T × CO₂ interaction synergistically stimulated plant carbon uptake in the combination of warming and elevated CO₂. At peak drought, the D × CO₂ interaction antagonistically down-regulated photosynthesis, suggesting a limited ability of elevated CO₂ to counteract the negative effect of drought. The response of photosynthesis in the full factorial combination (TDCO₂) could be explained by the main effect of experimental treatments (T, D, CO₂) and the two-factor interactions (D × CO₂, T × CO₂). The interactive responses in the experimental treatments including elevated CO₂ seemed to be linked to the realized range of treatment variability, for example with negative effects following experimental drought or positive effects following the relatively higher impact of night-time warming during cold periods early and late in the year. Longer-term experiments are needed to evaluate whether photosynthetic down-regulation will dampen the stimulation of photosynthesis under prolonged exposure to elevated CO₂.

Ambient UV-B radiation decreases photosynthesis in high arctic Vaccinium uliginosum

An UV-B-exclusion experiment was established in high arctic Zackenberg, Northeast Greenland, to investigate the possible effects of ambient UV-B on plant performance. During almost a whole growing season, canopy gas exchange and Chl fluorescence were measured on Vaccinium uliginosum (bog blueberry). Leaf area, biomass, carbon, nitrogen and UV-B-absorbing compounds were determined from a late season harvest. Compared with the reduced UV-B treatment, the plants in ambient UV-B were found to have a higher content of UV-B-absorbing compounds, and canopy net photosynthesis was as an average 23% lower during the season. By means of the JIP-test, it was found that the potential of processing light energy through the photosynthetic machinery was slightly reduced in ambient UV-B. This indicates that not only the UV-B effects on PSII may be responsible for some of the observed reduction of photosynthesis but also the effects on other parts of the photosynthetic machinery, e.g. the Calvin cycle, might be important. The 60% reduction of the UV-B irradiance used in this study implies a higher relative change in the UV-B load than many of the supplemental experiments do, but the substantial effect on photosynthesis clearly indicates that V. uliginosum is negatively affected by the current level of UV-B.

Growing-Season Carbon Dioxide Flux in a Dry Subarctic Heath: Responses to Long-term Manipulations

Abstract Carbon dioxide fluxes in a dry subarctic heath were examined after 10 and 11 yr of experimental manipulations of temperature, light, and nutrients. The aim was to investigate how growing season carbon (C) balance was affected by the major climatic factors that are expected to change in the future. Carbon flux was measured in closed chambers as uptake through gross ecosystem production (GP), release through ecosystem respiration (ER), and as net ecosystem production (NEP). Diurnal NEP through a day with clear skies at peak growing season was consistently negative through all treatments the first year of measurement, and day-time NEP varied around zero at eight days across the growing season the second year, implying that a net release of C from the ecosystem to the atmosphere may take place during the growing season. Our results suggest that respiration was the main determinant of C balance, and that variations in light levels and temperature could alter the balance between C uptake and C loss. Fertilization strongly enhanced both ER and GP whereas temperature enhancement changed neither ER nor GP. Shading decreased both ER and GP. After harvest of the aboveground plant biomass, the belowground respiration was 72 to 93% of the ER before harvest. The significant treatment effects on belowground respiration after harvest were similar to the effects on ER before harvest. These results suggest that the ER were mainly from belowground respiration, and that the treatments affected the belowground respiration more than the respiration above ground.

Comparisons of Measured and Modelled Ozone Deposition to Forests in Northern Europe

The performance of a new dry deposition module, developedfor the European-scale mapping and modelling of ozone flux to vegetation, was tested against micrometeorological ozone and water vapour flux measurements. The measurement data are for twoconiferous (Scots pine in Finland, Norway spruce in Denmark) and one deciduous forest (mountain birch in Finland). On average, themodel performs well for the Scots pine forest, if local inputdata are used. The daytime deposition rates are somewhat over-predicted at the Danish site, especially in the afternoon. The mountain birch data indicate that the generic parameterisationof stomatal responses is not very representative of this northernspecies. The module was also tested by using modelled meteorological data that constitute the input for a photochemical transport model.

The shift in plant species composition in a subarctic mountain birch forest floor due to climate change would modify the biogenic volatile organic compound emission profile

Background and aimsMountain birch forests dominate in the Subarctic but little is known of their non-methane biogenic volatile organic compound (BVOC) emissions. The dwarf shrubs Empetrum hermaphroditum, Vaccinium myrtillus and Vaccinium uliginosum co-dominate in the forest floors of these forests. The abundance of these three dwarf shrubs relative to each other could be affected by climate warming expected to increase nutrient availability by accelerating litter decomposition and nutrient mineralization. We 1) compared the BVOC emission profiles of vegetation covers dominated by E. hermaphroditum and V. myrtillus plus V. uliginosum in a subarctic mountain birch forest floor, 2) distinguished the BVOCs emitted from plants and soil and 3) measured how the BVOC emissions from the different vegetation covers differed under darkness.MethodsBVOCs were sampled during two growing seasons using a conventional ecosystem chamber-based method, collected on adsorbent and analyzed with gas chromatography–mass spectrometry.ResultsHigh abundance of E. hermaphroditum increased the sesquiterpene emissions. Soil released fewer different BVOCs than controls (i.e. natural vegetation) but the total emission rates were similar. Darkness did not affect the emissions. Carbon emitted as BVOCs was less than 0.2% of the CO2 exchange.ConclusionsOur results suggest that sesquiterpene emissions from subarctic mountain birch forest floors would be reduced following an increased abundance of V. myrtillus and V. uliginosum with climate change because these species respond rapidly to increased nutrient availability.

Reduction of the Ambient UV-B Radiation in the High-Arctic Increases Fv/Fm in Salix arctica and Vaccinium uliginosum and Reduces Stomatal Conductance and Internal CO2 Concentration in Salix arctica

Abstract This study investigates effects of reducing the ambient UV radiation on gas exchange and chlorophyll fluorescence of two shrub species, Salix arctica and Vaccinium uliginosum, in a high arctic heath in Northeast Greenland in July and August. On two sites films, Mylar and Lexan, were used to reduce UV-B radiation and UV-B + A radiation, respectively. A UV transparent film, Teflon, and no film were used as controls. Field measurements showed that the plants under Teflon, Mylar, and Lexan received approximately 91, 39, and 17% of the ambient UV-B irradiance, respectively. Reduced UV radiation increased maximal photochemical efficiency (Fv/Fm) in both species. The responses varied in significance according to species, sites and time of growing season. Net assimilation (Pn), measured as net CO2 uptake, was not significantly affected. But over the whole growing season stomatal conductance and intercellular CO2concentration were decreased by both UV treatments. The underlying mechanisms for these results are discussed. PAR-irradiance had a negative influence on the absolute values of Fv/Fm. A positive correlation was found between Fv/Fm and Pn measured at ambient CO2-level. It is concluded that it cannot be excluded, that the observed short-term effects could result in long-term negative effects on growth and survival for the investigated species.

Atmospheric composition change

Publisher Summary The coupling between climate change and atmospheric composition results from the basic structure of the Earth atmosphere climate system, and the fundamental processes within it. The composition of the atmosphere is determined by natural and human-related emissions, and the energy that flows into, out of, and within the atmosphere. Atmospheric composition influences climate by regulating the radiation budget. Potentially significant contributions to the climate impact are provided by compounds such as CO2, CH4, O3, particles, and cirrus clouds. For the chemically active gases, processes in the atmosphere are important, with large spatial and temporal variations. The climate–chemistry interactions are therefore characterized by significant regional differences with regions such as South East Asia being a future key region due to significant increases in energy use and pollution emission. Likewise, ship and air traffic represent important sectors because of significant increases in emissions in recent years. The relative contributions to the emissions from various sectors are expected to change significantly over the next few decades due to differences in mitigation options and costs.Chemically active climate compounds are either primary compounds like methane (CH4), removed by oxidation in the atmosphere, or secondary compounds like ozone (O-3), sulfate and organic aerosols, both formed and removed in the atmosphere. Man-induced climate-chemistry interaction is a two-way process: Emissions of pollutants change the atmospheric composition contributing to climate change through the aforementioned climate components, and climate change, through changes in temperature, dynamics, the hydrological cycle, atmospheric stability, and biosphere-atmosphere interactions, affects the atmospheric composition and oxidation processes in the troposphere. Here we present progress in our understanding of processes of importance for climate-chemistry interactions, and their contributions to changes in atmospheric composition and climate forcing. A key factor is the oxidation potential involving compounds like O-3 and the hydroxyl radical (OH). Reported studies represent both current and future changes. Reported results include new estimates of radiative forcing based on extensive model studies of chemically active climate compounds like O-3, and of particles inducing both direct and indirect effects. Through EU projects like ACCENT, QUANTIFY, and the AeroCom project, extensive studies on regional and sector-wise differences in the impact on atmospheric distribution are performed. Studies have shown that land-based emissions have a different effect on climate than ship and aircraft emissions, and different measures are needed to reduce the climate impact. Several areas where climate change can affect the tropospheric oxidation process and the chemical composition are identified. This can take place through enhanced stratospheric-tropospheric exchange of ozone, more frequent periods with stable conditions favoring pollution build up over industrial areas, enhanced temperature induced biogenic emissions, methane releases from permafrost thawing, and enhanced concentration through reduced biospheric uptake. During the last 5-10 years, new observational data have been made available and used for model validation and the study of atmospheric processes. Although there are significant uncertainties in the modeling of composition changes, access to new observational data has improved modeling capability. Emission scenarios for the coming decades have a large uncertainty range, in particular with respect to regional trends, leading to a significant uncertainty range in estimated regional composition changes and climate impact.