Kristian Rost Albert

Abstract: The predicted increase in summer temperature might change the plant growth dynamics and biomass production of arctic ecosystems and their feedback to climate change. Nevertheless, up to date, no information is available on the impact of warming on stem secondary growth and aboveground relative growth rate (RGR) of shrubs and whole-community net primary production (NPP) in the High Arctic. Here, we addressed these knowledge gaps by investigating the impact of 7 years of artificial warming (open greenhouses) on two heath communities dominated by the dwarf-shrubs Cassiope tetragona and Salix arctica in North-East Greenland. Annual ring width was responsive to warming for both species. However, the response was small for Salix (with non-significant variation of secondary RGR) and large for Cassiope (with doubling of secondary RGR), particularly for old Cassiope stem segments formed before the treatment began. Similarly, apical RGR of Salix was not responsive to warming, whereas apical RGR of Cassiope increased by 83%. For Cassiope, the increase in RGR was associated with an increase in branching and photosynthetic uptake but not in net ecosystem production because of the concurrent increase in ecosystem respiration. Whole-community NPP and standing biomass showed similar patterns as individual RGR but the larger data variability prevented the detection of any significant effect. However, stem secondary growth was a significant component of NPP (20-25%) and it should not be neglected in future studies on plant growth, biomass production and carbon cycle in the High Arctic.

Abstract: Climatic change factors in concert are likely to induce unpredictable interactive impacts on plant carbon uptake and water consumption and multi-factor experimentation serves as a tool to gain knowledge into such processes. In the second year of experimentation, we investigated the combined impact of elevated CO2 (CO2) drought (D) and nighttime warming (T) on photosynthesis and leaf characteristics of the evergreen dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa in a temperate heath ecosystem. Elevated CO2 did not induce down-regulation of Jmax or Vcmax, but increased net photosynthesis via higher intercellular CO2. Drought reduced the soil water content, xylem plant water potential, transpiration and net photosynthesis. The responses seen in the combinations were mainly the result of antagonistic effects, but these were strongly influenced by the drought. Antagonistic D�CO2 and T�D�CO2 interactions influenced the response of net photosynthesis, transpiration and plant water potential in Calluna and antagonistic T�D�CO2 interaction on photosynthesis were observed in Deschampsia. The level of photosynthesis in the full combination of warming, drought and elevated CO2 did not differ from control in Calluna and only small non-significantly photosynthetic stimulation was observed in Deschampsia. The antagonistic effects of D�CO2 and T�D�CO2 were strongly influenced by drought and revealed the impact of D were only dampened to a small degree, but in a nonlinear way, when combined with warming and elevated CO2. This exemplifies the unpredictability of interaction effects. The strong dampening of photosynthesis in the full combination of warming, drought and elevated CO2, clearly point that future elevated CO2 stimulation are strongly counteracted by warming and drought in this temperate heath ecosystem and that this counteraction is controlled by water availability.

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Abstract: In a multi-factor climate change experiment, we tested effects of three independent global change drivers on insect herbivore performance. We found that most drivers adversely affected herbivore performance. The most surprising result of our study, however, was that the number of global change drivers (0, 1, 2 or 3) additively affected herbivore performance.

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Abstract: Full recovery of the ozone layer is not expected for several decades and consequently, the incoming level of solar ultraviolet-B (UV-B) will only slowly be reduced. Therefore to investigate the structural and photosynthetic responses to changes in solar UV-B we conducted a 5-year UV-B exclusion study in high arctic Greenland. During the growing season, the gas exchange (H2O and CO2) and chlorophyll-a fluorescence were measured in Vaccinium uliginosum. The leaf dry weight, carbon, nitrogen, stable carbon isotope ratio, chlorophyll and carotenoid content were determined from a late season harvest. The net photosynthesis per leaf area was on average 22% higher in 61% reduced UV-B treatment across the season, but per ground area photosynthesis was unchanged. The leaf level increase in photosynthesis was accompanied by increased leaf nitrogen, higher stomatal conductance and Fv/Fm. There was no change in total leaf biomass, but reduction in total leaf area caused a pronounced reduction of specific leaf area and leaf area index in reduced UV-B. This demonstrates the structural changes to counterbalance the reduced plant carbon uptake seen per leaf area in ambient UV-B as the resulting plant carbon uptake per ground area was not affected. Thus, our understanding of long-term responses to UV-B reduction must take into account both leaf level processes as well as structural changes to understand the apparent robustness of plant carbon uptake per ground area. In this perspective, V. uliginosum seems able to adjust plant carbon uptake to the present amount of solar UV-B radiation in the High Arctic.

Abstract: The global burden of carbon monoxide, CO, is rather uncertain. In this paper we address the potential of UV-induced CO emission by terrestrial surfaces. Real-time measurements of [CO] were made with a cavity enhanced laser connected in closed loop to either an ecosystem chamber or a leaf scale chamber. Sand and leaves of all examined plant species exhibited emission of CO in response to artificial UV-radiation and the UV-component of natural solar radiation. The UV-induced rate of CO emission exhibited a rather low dependence on temperature, indicating an abiotic process. The emission of CO in response to the UV-component of natural solar radiation was also evident at the ecosystem scale. When scaled to the global level, the UV-induced emission of CO by the major types of terrestrial surfaces, living leaves and soil (here represented by sand), amounts up to 28 Tg yr-1. This source has till now not been accounted for by IPCC, but is equivalent to 14-56% of the 50-200 Tg yr-1 from sources currently accounted for (IPCC 2001). In addition to this are other known sources that ought to be considered. The hitherto unaccounted for terrestrial sources of CO amounts up to 207 Tg yr-1, almost two-thirds of the latest estimated global CO burden of 360 Tg yr-1 (IPCC 2001).

Abstract: The long-term and diurnal responses of photosystem II (PSII) performance to near-ambient UV-B radiation were investigated in High Arctic Betula nana. We conducted an UV exclusion experiment with five replicated blocks consisting of open control (no filter), photosynthetic active radiation and UV-B transparent filter control (Teflon), UV-B-absorbing filter (Mylar) and UV-AB-absorbing filter (Lexan). Ethylenediurea (EDU), a chemical normally used to protect plants against ozone injury, was sprayed on the leaves both in the field and in an additional laboratory study to investigate if EDU mitigated the effects of UV-B. Chlorophyll-a fluorescence induction curves were used for analysis of OJIP test parameters. Near-ambient UV-B radiation reduced across season maximum quantum yield (TRo/ABS = Fv/Fm), approximated number of active PSII reaction center (RC/ABS) and the performance index (PIABS), despite improved leaf screening against UV-B with higher content of UV-B-absorbing compounds and a lower specific leaf area. EDU application counteracted the negative impact of UV-B on TRo/ABS, RC/ABS and PIABS. This indicates that the mechanisms behind UV-B and ozone damage share some common features. The midday depression was present in all treatments, but TRo/ABS and PIABS were persistently lower in near-ambient UV-B compared to UV-B reduction. The recovery phase was particularly impaired in near-ambient UV-B and interactive effects between treatment � hour raised TRo/ABS, RC/ABS and PIABS higher in reduced UV-B compared to near-ambient UV-B. This demonstrates current solar UV-B to reduce the PSII performance both on a daily as well as a seasonal basis in this High Arctic species.

Abstract: Future changes in precipitation patterns affect plant carbon uptake but the outcome depends much on individual plant species responses. We investigated the leaf level photosynthetic performance, leaf C, N and δ13C along with vegetation cover and biomass in the dwarf shrub Calluna vulgaris and the grass species Deschampsia flexuosa in a temperate heath during a season with dry and rewetting conditions. We demonstrate that Deschampsia have adopted a strategy where shorter lived leaves gain a higher photosynthetic uptake via a higher capacity, whereas longer lived Calluna leaves gain lower instantaneous photosynthetic uptake, but over time a higher accumulated photosynthetic uptake. The higher level of grass photosynthetic capacity was governed by higher Jmax, Vcmax, Pmax which sustained in the non-vilting leaves during very dry conditions. During periods with high soil water content higher leaf nitrogen content was observed and the grass photosynthetic capacity was stimulated. In contrast, the evergreen dwarf shrub demonstrated a strategy with pronounced tolerance to endure water shortage as it preserved the investment in lower level photosynthetic capacity at low soil water content and maintained plant cover and aboveground biomass through dry periods. This tolerance strategy may not be advantageous if a dry period becomes too long. Such longer dry periods may be of transient importance in the grass in which the opportunistic grass growth strategy are assumed to induce a strong regrowth when rewetted. Therefore the different growth strategies and their close relation to duration of dry periods and rewetting add to complexity, when the yearly accumulated plant carbon uptake between the co-existing species has to be predicted.

Abstract: 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 1 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.

Abstract: In temperate regions, climate change is predicted to increase annual mean temperature and intensify the duration and frequency of summer droughts, which together with elevated atmospheric carbon dioxide (CO2) concentrations, may affect the exchange of nitrous oxide (N2O) and methane (CH4) between terrestrial ecosystems and the atmosphere. We report results from the CLIMAITE experiment, where the effects of these three climate change parameters were investigated solely and in all combinations in a temperate heathland. Field measurements of N2O and CH4 fluxes took place 1e2 years after the climate change manipulations were initiated. The soil was generally a net sink for atmospheric CH4. Elevated temperature (T) increased the CH4 uptake by on average 10 mg C m2 h1, corresponding to a rise in the uptake rate of about 20%. However, during winter elevated CO2 (CO2) reduced the CH4 uptake, which outweighed the positive effect of warming when analyzed across the study period. Emissions of N2O were generally low (<10 mg N m2 h1). As single experimental factors, elevated CO2, temperature and summer drought (D) had no major effect on the N2O fluxes, but the combination of CO2 and warming (TCO2) stimulated N2O emission, whereas the N2O emission ceased when CO2 was combined with drought (DCO2). We suggest that these N2O responses are related to increased rhizodeposition under elevated CO2 combined with increased and reduced nitrogen turnover rates caused by warming and drought, respectively. The N2O flux in the multifactor treatment TDCO2 was not different from the ambient control treatment. Overall, our study suggests that in the future, CH4 uptake may increase slightly, while N2O emission will remain unchanged in temperate ecosystems on well-aerated soils. However, we propose that continued exposure to altered climate could potentially change the greenhouse gas flux pattern in the investigated heathland.

Abstract: 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 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, e.g. with negative effects following experimental drought or positive effects following the relatively higher impact of nighttime 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.

Abstract: 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. We focused on photosynthetic response to elevated CO2 (CO2, FACE), drought (D, water excluding curtains) and night time warming (T, IR-reflective curtains) in a temperate heath. We measured A/Ci curves allowing analysis of light saturated net photosynthesis (Pn), light and CO2 saturated net photosynthesis (Pmax), stomatal conductance (gs), maximal rate of Rubisco carboxylation (Vcmax) and RuBP regeneration (Jmax) along with leaf δ13C, carbon and nitrogen concentration on monthly basis in the grass Deschampsia flexuosa. Seasonal drought reduced Pn via gs, but severe (experimental) drought decreased Pn via reduction in photosynthetic capacity (Pmax, Jmax, 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 in the combination of drought and elevated CO2 depended on soil water availability, with additive effects when SWC 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, bi-phasic 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. This is clearly advantageous characteristics when exposed to climatic changes. In conclusion, after one year of experimentation the limitations by low soil water availability and stimulation in early and late season by warming clearly structures and interact with the photosynthetic response to elevated CO2 in this grassland species.

Abstract: Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74°N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0° ([`]level site') and 45° ([`]sloping site'), while at a third site the leaves were fixed in an angle of 45° to homogenize the irradiance dose ([`]fixed leaf angle site'). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing compounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45o position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (Jmax) and the maximum carboxylation rate of Rubisco (Vcmax), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is an significant stress factor for plants in high Arctic Greenland.

Abstract: Increased temperature, atmospheric CO2 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 CO2 (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/Ci curves, leaf nitrogen content and chlorophyll a fluorescence OJIP induction curves. The PSII performance was evaluated via the total performance index PItotal, which integrates the function of antenna, reaction centers, electron transport and end-acceptor reduction according to the OJIP-test. The observed PSII performance was negatively influenced by high air temperature, low soil water content and high irradiance dose. The experimental treatments of elevated CO2 and prolonged drought, in general, down-regulated Jmax, Vcmax and PItotal. 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 Jmax, Vcmax and PItotal. 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 PItotal are discussed. The single factor effects on PSII performance, and their interactions, could be explained by parallel adjustments of Vcmax, Jmax 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.

Abstract: Agriculture Ecosystems & Environment 136: 3-4. pp199-208. Conventional cropping systems rely on targeted short-term fertility management, whereas organic systems depend, in part, on long-term increase in soil fertility as determined by crop rotation and management. Such differences influence soil nitrogen (N) cycling and availability through the year. The main objective of this study was to compare nitrous oxide (N2O) emissions from soil under winter wheat (Triticum aestivum L.) within three organic and one conventional cropping system that differed in type of fertilizer, presence of catch crops and proportion of N2- fixing crops. The study was replicated in two identical long-term crop rotation experiments on sandy loam soils under different climatic conditions in Denmark (Flakkebjerg�eastern Denmark and Foulum�western Denmark). The conventional rotation received 165�170 kg N ha-1 in the form of NH4NO3, while the organic rotations received 100�110 kg N ha-1 as pig slurry. For at least 11 months, as from September 2007, static chambers were used to measure N2O emissions at least twice every calendar month. Mean daily N2O emissions across the year ranged from 172 to 438 μg N m-2 d-1 at Flakkebjerg, and from 173 to 250 μg N m-2 d-1 at Foulum. A multiple linear regression analysis showed inter-seasonal variations in emissions (P<0.001), but annual N2O emissions from organic and conventional systems were not significantly different despite the lower N input in organic rotations. The annual emissions ranged from 54 to 137 mg N m-2, which corresponded to 0.5�0.8% of the N applied in manure or mineral fertilizer. Selected soil attributes were monitored to support the interpretation of N2O emission patterns. A second multiple linear regression analysis with potential drivers of N2O emissions showed a negative response to soil temperature (P=0.008) and percent water filled pore space (WFPS) (P=0.052) at Foulum. However, there were positive interactions of both factors with NO3-N, i.e. high N2O emissions occurred during periods when high soil nitrate levels coincided with high soil temperature (P=0.016) or high soil water content (P=0.056). A positive effect (P=0.03) of soil temperature was identified at Flakkebjerg, but the number of soil samplings was limited. Effects of cropping system on N2O emissions were not observed.

Abstract: Long-term responses of ambient solar ultraviolet (UV) radiation were investigated on Salix arctica and Vaccinium uliginosum in a High Arctic heath ecosystem in Zackenberg, northeast Greenland. Over a period of six years, UV exclusion was conducted in the growing season by means of filters: 60% UV-B reduction, 90% UV-B + UV-A reduction, UV transparent filter control, and an open control without filter. Plant responses were evaluated using specific leaf area, leaf content of UV-B absorbing compounds and PSII performance parameters derived from chlorophyll-a fluorescence induction curves. Based on the JIP-test, we calculated the total performance index PItotal, which includes the integrating antennae, the PSII reaction center, intersystem electron transport and reduction of PSI end acceptors-dependent parameters. In both species, UV exclusion significantly decreased the content of UV-B-absorbing compounds. Salix increased its specific leaf area, while Vaccinium decreased it. UV exclusion increased the PItotal in both species during all six years of experimentation. This response was governed by a significantly decreased RC/ABS, a marginally non-significant increased ETo/TRo and a significantly increased TRo/ABS = FV/FM and REo/ETo. These results demonstrate the current level of ambient UV-B to decrease PSII performance significantly in these High Arctic plants. It appears that the two plant species both have improved their UV-screening capacity, but through different strategies, although this did not sufficiently prevent negative effects of the ambient UV radiation. We argue the decreased PSII performance to be part of a response decreasing plant carbon uptake. We speculate the negative effects on PSII performance mediated by ambient UV irradiance to be present in years where warming induces early snowmelt, exposing the vegetation to high spring UV-B, and to be present in the future to the degree the ozone layer is not fully recovered.

Abstract: As soil�atmosphere fluxes of greenhouse gases are characterized by high temporal fluctuations, frequent measurements in the range of hours to days need to be deployed, resulting in high analytical costs. We have therefore developed a new low-cost system that combines high-frequency automated sampling with low-frequency chemical analysis. The System for Inert Gas Monitoring by Accumulation (SIGMA) is suited particularly for stand-alone observations in remote locations. The SIGMA is connected to an automated chamber with headspace sampling several times per day. Air samples are aggregated in sampling bags, which reduces the number of subsequent laboratory analyses and allows calculation of average flux rates over extended sampling periods. The SIGMA was tested under field conditions and compared with a conventional autochamber system, where flux rates were measured several times per day. Sample air fillings of the SIGMA sampling bags varied less than 5% between bags and diverged <1 to 6% of the pre-set values, justifying the assumption about proportional mixing of chamber headspace samples. When the SIGMA and conventional autochambers were compared simultaneously over the same patch of grassland in Denmark, the estimates agreed within ±12% in the assessment of average N2O fluxes. When deployed to discrete autochambers situated a few metres apart and under various environmental conditions in Denmark and the UK, we found no consistent difference between the two measuring systems. For a N-fertilized cropland in Denmark the SIGMA N2O and CO2 fluxes were 28 and 23% less, respectively than those recorded with the conventional autochamber, while for a N-fertilized grassland in the UK the SIGMA N2O flux was 47% more than that recorded with the conventional autochamber and 6% less than that recorded with a manual chamber system. Such differences are within the range of natural spatial variability in trace gas fluxes for these ecosystems and indicate the potential for further application of the SIGMA approach as a cost-effective technique to estimate long-term trace gas fluxes.

Abstract: Arctic, Antarctic, and Alpine Research, Vol. 41, No. 2, 2009, pp. 164�173. The Arctic is extremely vulnerable to projected climate change, and global warming may result in major community reorganizations. The aim of this study was a thorough investigation of plant biomass production throughout an entire growing season in five different high arctic vegetation types: Cassiope, Dryas, and Salix heath, grassland, and fen. The main focus was on the gross ecosystem production (GEP), and the biotic and abiotic factors which may influence GEP. Photosynthesis, aboveground biomass, and carbon, nitrogen, and chlorophyll content were measured weekly during nine weeks. There were large differences in seasonal growth and production within and among vegetation types. Mosses contributed considerably to the total C and N pool in grassland, fen, and Salix heath. Fen, which had the highest pool of leaf N, leaf chlorophyll, and moss N, was the most productive vegetation type in terms of GEP, despite the lowest total biomass. Across vegetation types, leaf biomass, leaf N, and moss N pool size influenced GEP. Within most vegetation types GEP correlated with leaf N, in correspondence with the notion that N may limit plant production in many high arctic ecosystems. The timing of the peaks in C and N pools in leaves did not coincide with that in the mosses and in woody tissues. This emphasizes the importance of sampling throughout the growing season, when using field data from the Arctic to estimate plant biomasses and modeling C and N fluxes and pool sizes.

Abstract: Physiologia Plantarum 133: 2. pp 199-210 . 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 UVB. 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.

Abstract: Functional Ecology 22: 1. 185-195 1. Recent findings indicate that the interactions among CO2, temperature and water can be substantial, and that the combined effects on the biological systems of several factors may not be predicted from experiments with one or a few factors. Therefore realistic multifactorial experiments involving a larger set of main factors are needed. 2. We describe a new Danish climate change-related field scale experiment, CLIMAITE, in a heath/ grassland ecosystem. CLIMAITE is a full factorial combination of elevated CO2, elevated temperature and prolonged summer drought. The manipulations are intended to mimic anticipated major environmental changes at the site by year 2075 as closely as possible. The impacts on ecosystem processes and functioning (at ecophysiological levels, through responses by individuals and communities to ecosystem-level responses) are investigated simultaneously. 3. The increase of [CO2] closely corresponds with the scenarios for year 2075, while the warming treatment is at the lower end of the predictions and seems to be the most difficult treatment to increase without unwanted side effects on the other variables. The drought treatment follows predictions of increased frequency of drought periods in summer. The combination of the treatments does not create new unwanted side effects on the treatments relative to the treatments alone.

Abstract: ADVANCES IN ECOLOGICAL RESEARCH 40: 421-440. Depletion of the ozone layer and the consequent increase in solar ultraviolet�B (UV�B) radiation may impact living conditions for arctic plants significantly. In order to evaluate how the prevailing UV�B fluxes aVect the heath ecosystem at Zackenberg (74300N, 20300W) and other high�arctic regions, manipulation experiments with various set�ups have been performed. Activation of plant defence mechanisms by production of UV�B�absorbing compounds was significant in ambient UV�B in comparison to a filter treatment reducing the UV�B radiation. Despite the UV�B screening response, ambient UV�B was demonstrated to decrease photosynthesis and shift carbon allocation from shoots to roots. Moreover, ambient UV�B increased plant stress with detrimental eVects on electron processing in the photosynthetic apparatus. Plant responses did not lead to clear changes in the amount of fungal root symbionts (mycorrhiza) or in the biomass of microbes in the soil of the root zone. However, the composition of the soil microbial community was diVerent in the soils under ambient and reduced UV�radiation after three treatment years. These results provide new insight into the negative impact of current UV�B fluxes on high�arctic vegetation. They supplement previous investigations from the Arctic focussing on other variables like growth and so on, which have reported no or minor plant responses to UV�B, and the presented synthesis clearly indicates that UV�B radiation is an important factor aVecting plant life at high�arctic Zackenberg. However, long�time experiments are needed in order to see whether the observed changes are transient or whether they accumulate over years. Such experiments are especially important for valid determination of below�ground responses, which potentially lead to feedbacks on the ecosystem functioning.

Abstract: ALBERT K.R., RO-POULSEN H., MIKKELSEN T.N., BREDAHL L. & HAAKANSSON K.B. 2004. Effects of reducing the ambient UV-B radiation in the high arctic on Salix arctica and Vaccin-ium uliginosum. - Phyton 45 (4): (41)-(49). Effects of reducing the ambient UV-B radiation on gas exchange and chlorophyll fluores-cence of two dwarf shrub species, Salix arctica and Vaccinium uliginosum, was studied in a high arctic heath in North East Greenland during two growing seasons. Films (Mylar, transmitting λ > 320 nm, and Lexan, transmitting λ > 400 nm) were used to reduce UV-B radiation and UV-B+A respectively. A UV transparent film (Teflon, transmitting λ > 280 nm) and no film were used as controls. Field measurements showed that the plants under Teflon, Mylar and Lexan received app. 91%, 39% and 17% of the ambient UV-B irradiance, respectively. UV radiation decreased the maximal photochemical efficiency (Fv/Fm) and other fast fluorescence transient derived parameters in both species, despite an increased level of leaf flavonoid content. The responses varied in signifi-cance according to species and site. The relation of these effects to a significantly decreased stomatal conductance (gs) and intercellular CO2 concentration (Ci) pointed to respiration as an im-portant factor in the interpretation of the observed unaffected net CO2 assimilation (Pn) in UV- re-duced treatments. It is concluded that the studied species have not fully acclimatized to the level of ambient UV-B radiation, and that ambient UV-B level is an important stress factor for the investi-gated plants in High Arctic.

Abstract: Physiologia Plantarum 124: 2. 208-226 . A UV-B exclusion-experiment was conducted in the high Arctic Zackenberg, NE Greenland, in which Salix arctica leaves during most of the growing season were fixed perpendicular to the solar zenith angle, thereby receiving maximal solar radiation. Covered with Teflon and Mylar foil, the leaves received approximately 90 and 40% of the ambient UV-B irradiance, respectively. The effects were examined through recordings of chlorophyll a fluorescence transients, determination of biomass and analysis of total carbon and nitrogen content and amount of soluble flavonoids in the leaves. The processing of light was analysed by means of the chlorophyll a fluorescence transient, using the so-called JIP test, as evolved by Reto J. Strasser and his coworkers. Reduction of the UV-B irradiance caused a rise in many of the fluorescence parameters during July, but not in August (late season). Thus increases in the efficiency that an absorbed photon will be trapped by the PSII reaction centre with the resultant reduction of QA to QA� (ET0/ABS 5 FV/FM) and the efficiency that an electron residing on QA� will enter the intersystem electron transport chain (ET0/TR0) were observed in reduced UV-B. Moreover, estimated per cross-section of leaf sample, the number of active PSII reaction centres (RC/ CSM) and electron transport rate (ETM/CSM) and all performance indexes (PIABS, PICSo and PICSm) were increased in reduced UV-B. The total soluble flavonoid content was highest in ambient UV-B. The treatment effects on fluorescence parameters that were directly measured (e.g. F0 and FM) and those that were derived (e.g. quantum efficiencies, parameters per PSII reaction centres and per cross-section of leaf sample) are discussed in relation to one another, in relation to daily and seasonal variation, and from the perspective of evaluating the relative importance of UV-B of donor and acceptor side capacity in Photosystem II. In conclusion, the experimental set-up and non-invasive measurements proved to be a sensitive method to screen for effects of UV-B stress.