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  • 1. Aerts, R
    et al.
    Cornelissen, J H C
    Dorrepaal, E
    van Logtestijn, R S P
    Callaghan, T V
    Effects of experimentally imposed climate scenarios on flowering phenology and flower production of subarctic bog species2004Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 10, nr 9, s. 1599-1609Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate scenarios for high-latitude areas predict not only increased summer temperatures, but also larger variation in snowfall and winter temperatures. By using open-top chambers, we experimentally manipulated both summer temperatures and winter and spring snow accumulations and temperatures independently in a blanket bog in subarctic Sweden, yielding six climate scenarios. We studied the effects of these scenarios on flowering phenology and flower production of Andromeda polifolia (woody evergreen) and Rubus chamaemorus (perennial herb) during 2 years. The second year of our study (2002) was characterized by unusually high spring and early summer temperatures. Our winter manipulations led to consistent increases in winter snow cover. As a result, average and minimum air and soil temperatures in the high snow cover treatments were higher than in the winter ambient treatments, whereas temperature fluctuations were smaller. Spring warming resulted in higher average, minimum, and maximum soil temperatures. Summer warming led to higher air and soil temperatures in mid-summer (June-July), but not in late summer (August-September). The unusually high temperatures in 2002 advanced the median flowering date by 2 weeks for both species in all treatments. Superimposed on this effect, we found that for both Andromeda and Rubus, all our climate treatments (except summer warming for Rubus) advanced flowering by 1-4 days. The total flower production of both species showed a more or less similar response: flower production in the warm year 2002 exceeded that in 2001 by far. However, in both species flower production was only stimulated by the spring-warming treatments. Our results show that the reproductive ecology of both species is very responsive to climate change but this response is very dependent on specific climate events, especially those that occur in winter and spring. This suggests that high-latitude climate change experiments should focus more on winter and spring events than has been the case so far.

  • 2.
    Bergström, Ann-Kristin
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Faithfull Mathisen, Carolyn
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Karlsson, Daniel
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Karlsson, Jan
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Nitrogen deposition and warming  – effects on phytoplankton nutrient limitation in subarctic lakes2013Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, nr 8, s. 2557-2568Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aim of this study was to predict the combined effects of enhanced nitrogen (N) deposition and warming on phytoplankton development in high latitude and mountain lakes. Consequently, we assessed, in a series of enclosureexperiments, how lake water nutrient stoichiometry and phytoplankton nutrient limitation varied over the growingseason in 11 lakes situated along an altitudinal/climate gradient with low N-deposition (<1 kg N ha1yr1) in northern subarctic Sweden. Short-term bioassay experiments with N- and P-additions revealed that phytoplankton inhigh-alpine lakes were more prone to P-limitation, and with decreasing altitude became increasingly N- andNP-colimited. Nutrient limitation was additionally most obvious in midsummer. There was also a strong positivecorrelation between phytoplankton growth and water temperature in the bioassays. Although excess nutrients wereavailable in spring and autumn, on these occasions growth was likely constrained by low water temperatures. Theseresults imply that enhanced N-deposition over the Swedish mountain areas will, with the exception of high-alpinelakes, enhance biomass and drive phytoplankton from N- to P-limitation. However, if not accompanied by warming,N-input from deposition will stimulate limited phytoplankton growth due to low water temperatures during largeparts of the growing season. Direct effects of warming, allowing increased metabolic rates and an extension of thegrowing season, seem equally crucial to synergistically enhance phytoplankton development in these lakes.

  • 3. Bergström, Ann-Kristin
    et al.
    Karlsson, Jan
    Light and nutrient control phytoplankton biomass responses to global change in northern lakes2019Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 25, nr 6, s. 2021-2029Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.

  • 4. BOKHORST, S.
    et al.
    BJERKE, J. W.
    BOWLES, F. W.
    MELILLO, J.
    CALLAGHAN, T. V.
    PHOENIX, G. K.
    Impacts of extreme winter warming in the sub-Arctic: growing season responses of dwarf shrub heathland2008Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 14, nr 11, s. 2603-2612Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change scenarios predict an increased frequency of extreme climatic events. In Arctic regions, one of the most profound of these are extreme and sudden winter warming events in which temperatures increase rapidly to above freezing, often causing snow melt across whole landscapes and exposure of ecosystems to warm temperatures. Following warming, vegetation and soils no longer insulated below snow are then exposed to rapidly returning extreme cold. Using a new experimental facility established in sub-Arctic dwarf shrub heathland in northern Sweden, we simulated an extreme winter warming event in the field and report findings on growth, phenology and reproduction during the subsequent growing season. A 1-week long extreme winter warming event was simulated in early March using infrared heating lamps run with or without soil warming cables. Both single short events delayed bud development of Vaccinium myrtillus by up to 3 weeks in the following spring (June) and reduced flower production by more than 80%: this also led to a near-complete elimination of berry production in mid-summer. Empetrum hermaphroditum also showed delayed bud development. In contrast, Vaccinium vitis-idaea showed no delay in bud development, but instead appeared to produce a greater number of actively growing vegetative buds within plots warmed by heating lamps only. Again, there was evidence of reduced flowering and berry production in this species. While bud break was delayed, growing season measurements of growth and photosynthesis did not reveal a differential response in the warmed plants for any of the species. These results demonstrate that a single, short, extreme winter warming event can have considerable impact on bud production, phenology and reproductive effort of dominant plant species within sub-Arctic dwarf shrub heathland. Furthermore, large interspecific differences in sensitivity are seen. These findings are of considerable concern, because they suggest that repeated events may potentially impact on the biodiversity and productivity of these systems should these extreme events increase in frequency as a result of global change. Although climate change may lengthen the growing season by earlier spring snow melt, there is a profound danger for these high-latitude ecosystems if extreme, short-lived warming in winter exposes plants to initial warm temperatures, but then extreme cold for the rest of the winter. Work is ongoing to determine the longer term and wider impacts of these events.

  • 5. BOKHORST, S.
    et al.
    BJERKE, J. W.
    STREET, L. E.
    CALLAGHAN, T. V.
    PHOENIX, G. K.
    Impacts of multiple extreme winter warming events on sub-Arctic heathland: phenology, reproduction, growth, and CO2 flux responses2011Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 17, nr 9, s. 2817-2830Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Extreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short‐lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2–10 °C for 2–14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week‐long extreme winter warming events – using infrared heating lamps and soil warming cables – for 3 consecutive years in a sub‐Arctic heathland dominated by the dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis‐idaea (both evergreen) and Vaccinium myrtillus (deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week for E. hermaphroditum and V. myrtillus, and berry production reduced by 11–75% and 52–95% for E. hermaphroditum and V. myrtillus, respectively. Greater shoot mortality occurred in E. hermaphroditum (up to 52%), V. vitis‐idaea (51%), and V. myrtillus (80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long‐term summer warming simulations and the ‘greening’ seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.

  • 6. Bokhorst, S.
    et al.
    Phoenix, G. K.
    Bjerke, J. W.
    Callaghan, T. V.
    Huyer-Brugman, F.
    Berg, M.P
    Extreme winter warming events more negatively impact small rather than large soil fauna: shift in community composition explained by traits not taxa2012Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Global Change Biology, Vol. 18, nr 3, s. 1152-1162Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Extreme weather events can have negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (2–10 °C for 2–14 days), but returning to cold winter climate exposes the ecosystem to lower temperatures by the loss of insulating snow. Soil animals, which play an integral part in soil processes, may be very susceptible to such events depending on the intensity of soil warming and low temperatures following these events. We simulated week‐long extreme winter warming events – using infrared heating lamps, alone or with soil warming cables – for two consecutive years in a sub‐Arctic dwarf shrub heathland. Minimum temperatures were lower and freeze‐thaw cycles were 2–11 times more frequent in treatment plots compared with control plots. Following the second event, Acari populations decreased by 39%; primarily driven by declines of Prostigmata (69%) and the Mesostigmatic nymphs (74%). A community‐weighted vertical stratification shift occurred from smaller soil dwelling (eu‐edaphic) Collembola species dominance to larger litter dwelling (hemi‐edaphic) species dominance in the canopy‐with‐soil warming plots compared with controls. The most susceptible groups to these winter warming events were the smallest individuals (Prostigmata and eu‐edaphic Collembola). This was not apparent from abundance data at the Collembola taxon level, indicating that life forms and species traits play a major role in community assembly following extreme events. The observed shift in soil community can cascade down to the micro‐flora affecting plant productivity and mineralization rates. Short‐term extreme weather events have the potential to shift community composition through trait composition with potentially large consequences for ecosystem development.

  • 7. Bokhorst, Stef
    et al.
    Huiskes, Ad
    Aerts, Rien
    Convey, Peter
    Cooper, Elisabeth J
    Dalen, Linda
    Erschbamer, Brigitta
    Gudmundsson, Jon
    Hofgaard, Annika
    Hollister, Robert D
    Johnstone, Jill
    Jonsdottir, Ingibjorg S
    Lebouvier, Marc
    Van De Vijver, Bart
    Wahren, Carl-Henrik
    Dorrepaal, Ellen
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Variable temperature effects of Open Top Chambers at polar and alpine sites explained by irradiance and snow depth2013Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, nr 1, s. 64-74Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Environmental manipulation studies are integral to determining biological consequences of climate warming. Open Top Chambers (OTCs) have been widely used to assess summer warming effects on terrestrial biota, with their effects during other seasons normally being given less attention even though chambers are often deployed year-round. In addition, their effects on temperature extremes and freeze-thaw events are poorly documented. To provide robust documentation of the microclimatic influences of OTCs throughout the year, we analysed temperature data from 20 studies distributed across polar and alpine regions. The effects of OTCs on mean temperature showed a large range (-0.9 to 2.1 degrees C) throughout the year, but did not differ significantly between studies. Increases in mean monthly and diurnal temperature were strongly related (R-2 = 0.70) with irradiance, indicating that PAR can be used to predict the mean warming effect of OTCs. Deeper snow trapped in OTCs also induced higher temperatures at soil/vegetation level. OTC-induced changes in the frequency of freeze-thaw events included an increase in autumn and decreases in spring and summer. Frequency of high-temperature events in OTCs increased in spring, summer and autumn compared with non-manipulated control plots. Frequency of low-temperature events was reduced by deeper snow accumulation and higher mean temperatures. The strong interactions identified between aspects of ambient environmental conditions and effects of OTCs suggest that a detailed knowledge of snow depth, temperature and irradiance levels enables us to predict how OTCs will modify the microclimate at a particular site and season. Such predictive power allows a better mechanistic understanding of observed biotic response to experimental warming studies and for more informed design of future experiments. However, a need remains to quantify OTC effects on water availability and wind speed (affecting, for example, drying rates and water stress) in combination with microclimate measurements at organism level.

  • 8. Bokhorst, Stef
    et al.
    Phoenix, Gareth K.
    Berg, Matty P.
    Callaghan, Terry V.
    Kirby-Lambert, Christopher
    Bjerke, Jarle W.
    Climatic and biotic extreme events moderate long-term responses of above- and belowground sub-Arctic heathland communities to climate change2015Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, nr 11, s. 4063-4075Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change impacts are not uniform across the Arctic region because interacting factors causes large variations in local ecosystem change. Extreme climatic events and population cycles of herbivores occur simultaneously against a background of gradual climate warming trends and can redirect ecosystem change along routes that are difficult to predict. Here, we present the results from sub-Arctic heath vegetation and its belowground micro-arthropod community in response to the two main drivers of vegetation damage in this region: extreme winter warming events and subsequent outbreaks of the defoliating autumnal moth caterpillar (Epirrita autumnata). Evergreen dwarf shrub biomass decreased (30%) following extreme winter warming events and again by moth caterpillar grazing. Deciduous shrubs that were previously exposed to an extreme winter warming event were not affected by the moth caterpillar grazing, while those that were not exposed to warming events (control plots) showed reduced (23%) biomass from grazing. Cryptogam cover increased irrespective of grazing or winter warming events. Micro-arthropods declined (46%) following winter warming but did not respond to changes in plant community. Extreme winter warming and caterpillar grazing suppressed the CO2 fluxes of the ecosystem. Evergreen dwarf shrubs are disadvantaged in a future sub-Arctic with more stochastic climatic and biotic events. Given that summer warming may further benefit deciduous over evergreen shrubs, event and trend climate change may both act against evergreen shrubs and the ecosystem functions they provide. This is of particular concern given that Arctic heath vegetation is typically dominated by evergreen shrubs. Other components of the vegetation showed variable responses to abiotic and biotic events, and their interaction indicates that sub-Arctic vegetation response to multiple pressures is not easy to predict from single-factor responses. Therefore, while biotic and climatic events may have clear impacts, more work is needed to understand their net effect on Arctic ecosystems.

  • 9. Creed, Irena F.
    et al.
    Bergström, Ann-Kristin
    Trick, Charles G.
    Grimm, Nancy B.
    Hessen, Dag O.
    Karlsson, Jan
    Kidd, Karen A.
    Kritzberg, Emma
    McKnight, Diane M.
    Freeman, Erika C.
    Senar, Oscar E.
    Andersson, Agneta
    Ask, Jenny
    Berggren, Martin
    Cherif, Mehdi
    Giesler, Reiner
    Hotchkiss, Erin R.
    Kortelainen, Pirkko
    Palta, Monica M.
    Vrede, Tobias
    Weyhenmeyer, Gesa A.
    Global change-driven effects on dissolved organic matter composition: Implications for food webs of northern lakes2018Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 24, nr 8, s. 3692-3714Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial–aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)—the messenger between terrestrial and lake ecosystems—with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

  • 10. De FRENNE, P I E T E R
    et al.
    BRUNET, JÖRG
    SHEVTSOVA, ANNA
    KOLB, ANNETTE
    GRAAE, BENTE J
    CHABRERIE, OLIVIER
    COUSINS, SARA AO
    DECOCQ, GUILLAUME
    De SCHRIJVER, A N
    DIEKMANN, MARTIN
    GRUWEZ, ROBERT
    HEINKEN, THILO
    HERMY, MARTIN
    NILSSON, CHRISTER
    STANTON, SHARON
    TACK, WESLEY
    WILLAERT, JUSTIN
    VERHEYEN, KRIS
    Temperature effects on forest herbs assessed by warming and transplant experiments along a latitudinal gradient2011Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 17, nr 10, s. 3240-3253Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Abstract Slow-colonizing forest understorey plants are probably not able to rapidly adjust their distribution range following large-scale climate change. Therefore, the acclimation potential to climate change within their actual occupied habitats will likely be key for their short- and long-term persistence. We combined transplant experiments along a latitudinal gradient with open-top chambers to assess the effects of temperature on phenology, growth and reproductive performance of multiple populations of slow-colonizing understorey plants, using the spring flowering geophytic forb Anemone nemorosa and the early summer flowering grass Milium effusum as study species. In both species, emergence time and start of flowering clearly advanced with increasing temperatures. Vegetative growth (plant height, aboveground biomass) and reproductive success (seed mass, seed germination and germinable seed output) of A. nemorosa benefited from higher temperatures. Climate warming may thus increase future competitive ability and colonization rates of this species. Apart from the effects on phenology, growth and reproductive performance of M. effusum generally decreased when transplanted southwards (e.g., plant size and number of individuals decreased towards the south) and was probably more limited by light availability in the south. Specific leaf area of both species increased when transplanted southwards, but decreased with open-top chamber installation in A. nemorosa. In general, individuals of both species transplanted at the home site performed best, suggesting local adaptation. We conclude that contrasting understorey plants may display divergent plasticity in response to changing temperatures which may alter future understorey community dynamics.

  • 11. Dorrepaal, E
    et al.
    Aerts, R
    Cornelissen, J H C
    Callaghan, T V
    van Logtestijn, R S P
    Summer warming and increased winter snow cover affect Sphagnum fuscum growth, structure and production in a sub-arctic bog2004Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 10, nr 1, s. 93-104Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sphagnum mosses form a major component of northern peatlands, which are expected to experience substantially higher increases in temperature and winter precipitation than the global average. Sphagnum may play an important role in the responses of the global carbon cycle to climate change. We investigated the responses of summer length growth, carpet structure and production in Sphagnum fuscum to experimentally induced changes in climate in a sub-arctic bog. Thereto, we used open-top chambers (OTCs) to create six climate scenarios including changes in summer temperatures, and changes in winter snow cover and spring temperatures. In winter, the OTCs doubled the snow thickness, resulting in 0.5-2.8degreesC higher average air temperatures. Spring air temperatures in OTCs increased by 1.0degreesC. Summer warming had a maximum effect of 0.9degreesC, while vapor pressure deficit was not affected. The climate manipulations had strong effects on S. fuscum. Summer warming enhanced the length increment by 42-62%, whereas bulk density decreased. This resulted in a trend (P<0.10) of enhanced biomass production. Winter snow addition enhanced dry matter production by 33%, despite the fact that the length growth and bulk density did not change significantly. The addition of spring warming to snow addition alone did not significantly enhance this effect, but we may have missed part of the early spring growth. There were no interactions between the manipulations in summer and those in winter/spring, indicating that the effects were additive. Summer warming may in the long term negatively affect productivity through the adverse effects of changes in Sphagnum structure on moisture holding and transporting capacity. Moreover, the strong length growth enhancement may affect interactions with other mosses and vascular plants. Because winter snow addition enhanced the production of S. fuscum without affecting its structure, it may increase the carbon balance of northern peatlands.

  • 12. Gavazov, Konstantin
    et al.
    Albrecht, Remy
    Buttler, Alexandre
    Dorrepaal, Ellen
    Garnett, Mark H.
    Gogo, Sebastien
    Hagedorn, Frank
    Mills, Robert T. E.
    Robroek, Bjorn J. M.
    Bragazza, Luca
    Vascular plant-mediated controls on atmospheric carbon assimilation and peat carbon decomposition under climate change2018Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 24, nr 9, s. 3911-3921Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change can alter peatland plant community composition by promoting the growth of vascular plants. How such vegetation change affects peatland carbon dynamics remains, however, unclear. In order to assess the effect of vegetation change on carbon uptake and release, we performed a vascular plant-removal experiment in two Sphagnum-dominated peatlands that represent contrasting stages of natural vegetation succession along a climatic gradient. Periodic measurements of net ecosystem CO2 exchange revealed that vascular plants play a crucial role in assuring the potential for net carbon uptake, particularly with a warmer climate. The presence of vascular plants, however, also increased ecosystem respiration, and by using the seasonal variation of respired CO2 radiocarbon (bomb-14C) signature we demonstrate an enhanced heterotrophic decomposition of peat carbon due to rhizosphere priming. The observed rhizosphere priming of peat carbon decomposition was matched by more advanced humification of dissolved organic matter, which remained apparent beyond the plant growing season. Our results underline the relevance of rhizosphere priming in peatlands, especially when assessing the future carbon sink function of peatlands undergoing a shift in vegetation community composition in association with climate change.

  • 13. Ghirardo, Andrea
    et al.
    Lindstein, Frida
    Koch, Kerstin
    Buegger, Franz
    Schloter, Michael
    Albert, Andreas
    Michelsen, Anders
    Winkler, J. Barbro
    Schnitzler, Jörg-Peter
    Rinnan, Riikka
    Origin of volatile organic compound emissions from subarctic tundra under global warming2020Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Warming occurs in the Arctic twice as fast as the global average, which in turn leads to a large enhancement in terpenoid emissions from vegetation. Volatile terpenoids are the main class of biogenic volatile organic compounds (VOCs) that play crucial roles in atmospheric chemistry and climate. However, the biochemical mechanisms behind the temperature‐dependent increase in VOC emissions from subarctic ecosystems are largely unexplored. Using 13CO2‐labeling, we studied the origin of VOCs and the carbon (C) allocation under global warming in the soil–plant–atmosphere system of contrasting subarctic heath tundra vegetation communities characterized by dwarf shrubs of the genera Salix or Betula. The projected temperature rise of the subarctic summer by 5°C was realistically simulated in sophisticated climate chambers. VOC emissions strongly depended on the plant species composition of the heath tundra. Warming caused increased VOC emissions and significant changes in the pattern of volatiles toward more reactive hydrocarbons. The 13C was incorporated to varying degrees in different monoterpene and sesquiterpene isomers. We found that de novo monoterpene biosynthesis contributed to 40%–44% (Salix) and 60%–68% (Betula) of total monoterpene emissions under the current climate, and that warming increased the contribution to 50%–58% (Salix) and 87%–95% (Betula). Analyses of above‐ and belowground 12/13C showed shifts of C allocation in the plant–soil systems and negative effects of warming on C sequestration by lowering net ecosystem exchange of CO2 and increasing C loss as VOCs. This comprehensive analysis provides the scientific basis for mechanistically understanding the processes controlling terpenoid emissions, required for modeling VOC emissions from terrestrial ecosystems and predicting the future chemistry of the arctic atmosphere. By changing the chemical composition and loads of VOCs into the atmosphere, the current data indicate that global warming in the Arctic may have implications for regional and global climate and for the delicate tundra ecosystems.

  • 14. Grogan, P
    et al.
    Jonasson, S
    Temperature and substrate controls on intra-annual variation in ecosystem respiration in two subarctic vegetation types2005Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 11, nr 3, s. 465-475Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Arctic ecosystems are important in the context of climate change because they are expected to undergo the most rapid temperature increases, and could provide a globally significant release of CO2 to the atmosphere from their extensive bulk soil organic carbon reserves. Understanding the relative contributions of bulk soil organic matter and plant-associated carbon pools to ecosystem respiration is critical to predicting the response of arctic ecosystem net carbon balance to climate change. In this study, we determined the variation in ecosystem respiration rates from birch forest understory and heath tundra vegetation types in northern Sweden through a full annual cycle. We used a plant biomass removal treatment to differentiate bulk soil organic matter respiration from total ecosystem respiration in each vegetation type. Plant-associated and bulk soil organic matter carbon pools each contributed significantly to ecosystem respiration during most phases of winter and summer in the two vegetation types. Ecosystem respiration rates through the year did not differ significantly between vegetation types despite substantial differences in biomass pools, soil depth and temperature regime. Most (76-92%) of the intra-annual variation in ecosystem respiration rates from these two common mesic subarctic ecosystems was explained using a first-order exponential equation relating respiration to substrate chemical quality and soil temperature. Removal of plants and their current year’s litter significantly reduced the sensitivity of ecosystem respiration to intra-annual variations in soil temperature for both vegetation types, indicating that respiration derived from recent plant carbon fixation was more temperature sensitive than respiration from bulk soil organic matter carbon stores. Accurate assessment of the potential for positive feedbacks from high-latitude ecosystems to CO2-induced climate change will require the development of ecosystem-level physiological models of net carbon exchange that differentiate the responses of major C pools, that account for effects of vegetation type, and that integrate over summer and winter seasons.

  • 15. Grogan, Paul
    et al.
    Jonasson, Sven
    Ecosystem CO2 production during winter in a Swedish subarctic region: the relative importance of climate and vegetation type2006Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 12, nr 8, s. 1479-1495Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    General circulation models consistently predict that regional warming will be most rapid in the Arctic, that this warming will be predominantly in the winter season, and that it will often be accompanied by increasing snowfall. Paradoxically, despite the strong cold season emphasis in these predictions, we know relatively little about the plot and landscape-level controls on tundra biogeochemical cycling in wintertime as compared to summertime. We investigated the relative influence of vegetation type and climate on CO2 production rates and total wintertime CO2 release in the Scandinavian subarctic. Ecosystem respiration rates and a wide range of associated environmental and substrate pool size variables were measured in the two most common vegetation types of the region (birch understorey and heath tundra) at four paired sites along a 50 km transect through a strong snow depth gradient in northern Sweden. Both climate and vegetation type were strong interactive controls on ecosystem CO2 production rates during winter. Of all variables tested, soil temperature explained by far the largest amount of variation in respiration rates (41-75%). Our results indicate that vegetation type only exerted an influence on respiration when snow depth was below a certain threshold (similar to 1 m). Thus, tall vegetation that enhanced snow accumulation within that threshold resulted in more effective thermal insulation from severe air temperatures, thereby significantly increasing respiratory activity. At the end of winter, within several days of snowmelt, gross ecosystem photosynthesis rates were of a similar magnitude to ecosystem respiration, resulting in significant net carbon gain in some instances. Finally, climate and vegetation type were also strong interactive controls on total wintertime respiration, suggesting that spatial variations in maximum snowdepth may be a primary determinant of regional patterns of wintertime CO2 release. Together, our results have important implications for predictions of how the distribution of tundra vegetation types and the carbon balances of arctic ecosystems will respond to climate change during winter because they indicate a threshold (similar to 1 m) above which there would be little effect of increased snow accumulation on wintertime biogeochemical cycling.

  • 16. Hicks Pries, Caitlin E.
    et al.
    van Logtestijn, Richard S. P.
    Schuur, Edward A. G.
    Natali, Susan M.
    Cornelissen, Johannes H. C.
    Aerts, Rien
    Dorrepaal, Ellen
    Decadal warming causes a consistent and persistent shift from heterotrophic to autotrophic respiration in contrasting permafrost ecosystems2015Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, nr 12, s. 4508-4519Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Soil carbon in permafrost ecosystems has the potential to become a major positive feedback to climate change if permafrost thaw increases heterotrophic decomposition. However, warming can also stimulate autotrophic production leading to increased ecosystem carbon storage—a negative climate change feedback. Few studies partitioning ecosystem respiration examine decadal warming effects or compare responses among ecosystems. Here, we first examined how 11 years of warming during different seasons affected autotrophic and heterotrophic respiration in a bryophyte-dominated peatland in Abisko, Sweden. We used natural abundance radiocarbon to partition ecosystem respiration into autotrophic respiration, associated with production, and heterotrophic decomposition. Summertime warming decreased the age of carbon respired by the ecosystem due to increased proportional contributions from autotrophic and young soil respiration and decreased proportional contributions from old soil. Summertime warming’s large effect was due to not only warmer air temperatures during the growing season, but also to warmer deep soils year-round. Second, we compared ecosystem respiration responses between two contrasting ecosystems, the Abisko peatland and a tussock-dominated tundra in Healy, Alaska. Each ecosystem had two different timescales of warming (<5 years and over a decade). Despite the Abisko peatland having greater ecosystem respiration and larger contributions from heterotrophic respiration than the Healy tundra, both systems responded consistently to short- and long-term warming with increased respiration, increased autotrophic contributions to ecosystem respiration, and increased ratios of autotrophic to heterotrophic respiration. We did not detect an increase in old soil carbon losses with warming at either site. If increased autotrophic respiration is balanced by increased primary production, as is the case in the Healy tundra, warming will not cause these ecosystems to become growing season carbon sources. Warming instead causes a persistent shift from heterotrophic to more autotrophic control of the growing season carbon cycle in these carbon-rich permafrost ecosystems.

  • 17.
    Johansson, T.
    et al.
    Lund Univ.
    Malmer, N.
    Lund Univ.
    Crill, Patrick
    Stockholms universitet, Institutionen för geologi och geokemi.
    Friborg, T.
    Copenhagen Univ.
    Åkerman, J.A.
    Lund Univ.
    Mastepanov, M.
    Lund Univ.
    Christensen, T.R.
    Lund Univ.
    Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net radiative forcing.2006Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 12, nr 12, s. 2352-2369Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Thawing permafrost in the sub-Arctic has implications for the physical stability and biological dynamics of peatland ecosystems. This study provides an analysis of how permafrost thawing and subsequent vegetation changes in a sub-Arctic Swedish mire have changed the net exchange of greenhouse gases, carbon dioxide (CO<sub>2</sub>) and CH<sub>4</sub> over the past three decades. Images of the mire (ca. 17 ha) and surroundings taken with film sensitive in the visible and the near infrared portion of the spectrum, [i.e. colour infrared (CIR) aerial photographs from 1970 and 2000] were used. The results show that during this period the area covered by hummock vegetation decreased by more than 11% and became replaced by wet-growing plant communities. The overall net uptake of C in the vegetation and the release of C by heterotrophic respiration might have increased resulting in increases in both the growing season atmospheric CO<sub>2</sub> sink function with about 16% and the CH<sub>4</sub> emissions with 22%. Calculating the flux as CO<sub>2</sub> equivalents show that the mire in 2000 has a 47% greater radiative forcing on the atmosphere using a 100-year time horizon. Northern peatlands in areas with thawing sporadic or discontinuous permafrost are likely to act as larger greenhouse gas sources over the growing season today than a few decades ago because of increased CH<sub>4</sub> emissions.

  • 18.
    Karlsson, Jan
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Jonsson, Anders
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Jansson, Mats
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Productivity of high-latitude lakes: climate effect inferred from altitude gradient2005Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 11, nr 5, s. 710-715Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change is predicted to be dramatic at high latitudes. Still, climate impact on high latitude lake ecosystems is poorly understood. We studied 15 subarctic lakes located in a climate gradient comprising an air temperature difference of about 6&DEG; C. We show that lake water productivity varied by one order of magnitude along the temperature gradient. This variation was mainly caused by variations in the length of the ice-free period and, more importantly, in the supply of organic carbon and inorganic nutrients, which followed differences in terrestrial vegetation cover along the gradient. The results imply that warming will have rapid effects on the productivity of high latitude lakes, by prolongation of ice-free periods. However, a more pronounced consequence will be a delayed stimulation of the productivity following upon changes of the lakes terrestrial surroundings and subsequent increasing input of elements that stimulate the production of lake biota.

  • 19. KATTGE, J.
    et al.
    DIAZ, S.
    LAVOREL, S.
    PRENTICE, I. C.
    LEADLEY, P.
    B֖NISCH, G.
    GARNIER, E.
    WESTOBY, M.
    REICH, P. B.
    WRIGHT, I. J.
    CORNELISSEN, J. H. C.
    VIOLLE, C.
    HARRISON, S. P.
    Van BODEGOM, P. M.
    REICHSTEIN, M.
    ENQUIST, B. J.
    SOUDZILOVSKAIA, N. A.
    ACKERLY, D. D.
    ANAND, M.
    ATKIN, O.
    BAHN, M.
    BAKER, T. R.
    BALDOCCHI, D.
    BEKKER, R.
    BLANCO, C. C.
    BLONDER, B.
    BOND, W. J.
    BRADSTOCK, R.
    BUNKER, D. E.
    CASANOVES, F.
    CAVENDER-BARES, J.
    CHAMBERS, J. Q.
    CHAPIN III, F. S.
    CHAVE, J.
    COOMES, D.
    CORNWELL, W. K.
    CRAINE, J. M.
    DOBRIN, B. H.
    DUARTE, L.
    DURKA, W.
    ELSER, J.
    ESSER, G.
    ESTIARTE, M.
    FAGAN, W. F.
    FANG, J.
    FERNÁNDEZ-MÉNDEZ, F.
    FIDELIS, A.
    FINEGAN, B.
    FLORES, O.
    FORD, H.
    FRANK, D.
    FRESCHET, G. T.
    FYLLAS, N. M.
    GALLAGHER, R. V.
    GREEN, W. A.
    GUTIERREZ, A. G.
    HICKLER, T.
    HIGGINS, S. I.
    HODGSON, J. G.
    JALILI, A.
    JANSEN, S.
    JOLY, C. A.
    KERKHOFF, A. J.
    KIRKUP, D.
    KITAJIMA, K.
    KLEYER, M.
    KLOTZ, S.
    KNOPS, J. M. H.
    KRAMER, K.
    KÜHN, I.
    KUROKAWA, H.
    LAUGHLIN, D.
    LEE, T. D.
    LEISHMAN, M.
    LENS, F.
    LENZ, T.
    LEWIS, S. L.
    LLOYD, J.
    LLUSIÀ, J.
    LOUAULT, F.
    MA, S.
    MAHECHA, M. D.
    MANNING, P.
    MASSAD, T.
    MEDLYN, B. E.
    MESSIER, J.
    MOLES, A. T.
    MÜLLER, S. C.
    NADROWSKI, K.
    NAEEM, S.
    NIINEMETS, Ü.
    NÖLLERT, S.
    NÜSKE, A.
    OGAYA, R.
    OLEKSYN, J.
    ONIPCHENKO, V. G.
    ONODA, Y.
    ORDOÑEZ, J.
    OVERBECK, G.
    OZINGA, W. A.
    PATIÑO, S.
    PAULA, S.
    PAUSAS, J. G.
    PEÑUELAS, J.
    PHILLIPS, O. L.
    PILLAR, V.
    POORTER, H.
    POORTER, L.
    POSCHLOD, P.
    PRINZING, A.
    PROULX, R.
    RAMMIG, A.
    REINSCH, S.
    REU, B.
    SACK, L.
    SALGADO-NEGRET, B.
    SARDANS, J.
    SHIODERA, S.
    SHIPLEY, B.
    SIEFERT, A.
    SOSINSKI, E.
    SOUSSANA, J.-F.
    SWAINE, E.
    SWENSON, N.
    THOMPSON, K.
    THORNTON, P.
    WALDRAM, M.
    WEIHER, E.
    WHITE, M.
    WHITE, S.
    WRIGHT, S. J.
    YGUEL, B.
    ZAEHLE, S.
    ZANNE, A. E.
    WIRTH, C.
    TRY – a global database of plant traits2011Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Global Change Biology, Vol. 17, nr 9, s. 2905-2935Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Abstract Plant traits – the morphological, anatomical, physiological, biochemical and phenological characteristics of plants and their organs – determine how primary producers respond to environmental factors, affect other trophic levels, influence ecosystem processes and services and provide a link from species richness to ecosystem functional diversity. Trait data thus represent the raw material for a wide range of research from evolutionary biology, community and functional ecology to biogeography. Here we present the global database initiative named TRY, which has united a wide range of the plant trait research community worldwide and gained an unprecedented buy-in of trait data: so far 93 trait databases have been contributed. The data repository currently contains almost three million trait entries for 69 €ƒ000 out of the world’s 300 €ƒ000 plant species, with a focus on 52 groups of traits characterizing the vegetative and regeneration stages of the plant life cycle, including growth, dispersal, establishment and persistence. A first data analysis shows that most plant traits are approximately log-normally distributed, with widely differing ranges of variation across traits. Most trait variation is between species (interspecific), but significant intraspecific variation is also documented, up to 40% of the overall variation. Plant functional types (PFTs), as commonly used in vegetation models, capture a substantial fraction of the observed variation – but for several traits most variation occurs within PFTs, up to 75% of the overall variation. In the context of vegetation models these traits would better be represented by state variables rather than fixed parameter values. The improved availability of plant trait data in the unified global database is expected to support a paradigm shift from species to trait-based ecology, offer new opportunities for synthetic plant trait research and enable a more realistic and empirically grounded representation of terrestrial vegetation in Earth system models.

  • 20. Keuper, Frida
    et al.
    Dorrepaal, Ellen
    van Bodegom, Peter M.
    van Logtestijn, Richard
    Venhuizen, Gemma
    van Hal, Jurgen
    Aerts, Rien
    Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep-rooting subarctic peatland species2017Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, nr 10, s. 4257-4266Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate warming increases nitrogen (N) mineralization in superficial soil layers (the dominant rooting zone) of subarctic peatlands. Thawing and subsequent mineralization of permafrost increases plant-available N around the thaw-front. Because plant production in these peatlands is N-limited, such changes may substantially affect net primary production and species composition. We aimed to identify the potential impact of increased N-availability due to permafrost thawing on subarctic peatland plant production and species performance, relative to the impact of increased N-availability in superficial organic layers. Therefore, we investigated whether plant roots are present at the thaw-front (45 cm depth) and whether N-uptake (15N-tracer) at the thaw-front occurs during maximum thaw-depth, coinciding with the end of the growing season. Moreover, we performed a unique 3-year belowground fertilization experiment with fully factorial combinations of deep- (thaw-front) and shallow-fertilization (10 cm depth) and controls. We found that certain species are present with roots at the thaw-front (Rubus chamaemorus) and have the capacity (R. chamaemorus, Eriophorum vaginatum) for N-uptake from the thaw-front between autumn and spring when aboveground tissue is largely senescent. In response to 3-year shallow-belowground fertilization (S) both shallow- (Empetrum hermaphroditum) and deep-rooting species increased aboveground biomass and N-content, but only deep-rooting species responded positively to enhanced nutrient supply at the thaw-front (D). Moreover, the effects of shallow-fertilization and thaw-front fertilization on aboveground biomass production of the deep-rooting species were similar in magnitude (S: 71%; D: 111% increase compared to control) and additive (S + D: 181% increase). Our results show that plant-available N released from thawing permafrost can form a thus far overlooked additional N-source for deep-rooting subarctic plant species and increase their biomass production beyond the already established impact of warming-driven enhanced shallow N-mineralization. This may result in shifts in plant community composition and may partially counteract the increased carbon losses from thawing permafrost.

  • 21. Keuper, Frida
    et al.
    van Bodegom, Peter M.
    Dorrepaal, Ellen
    Weedon, James T.
    van Hal, Jurgen
    van Logtestijn, Richard S. P.
    Aerts, Rien
    A frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands2012Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Global Change Biology, Vol. 18, nr 6, s. 1998-2007Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many of the world's northern peatlands are underlain by rapidly thawing permafrost. Because plant production in these peatlands is often nitrogen (N)‐limited, a release of N stored in permafrost may stimulate net primary production or change species composition if it is plant‐available. In this study, we aimed to quantify plant‐available N in thawing permafrost soils of subarctic peatlands. We compared plant‐available N‐pools and ‐fluxes in near‐surface permafrost (0–10 cm below the thawfront) to those taken from a current rooting zone layer (5–15 cm depth) across five representative peatlands in subarctic Sweden. A range of complementary methods was used: extractions of inorganic and organic N, inorganic and organic N‐release measurements at 0.5 and 11 °C (over 120 days, relevant to different thaw‐development scenarios) and a bioassay with Poa alpina test plants. All extraction methods, across all peatlands, consistently showed up to seven times more plant‐available N in near‐surface permafrost soil compared to the current rooting zone layer. These results were supported by the bioassay experiment, with an eightfold larger plant N‐uptake from permafrost soil than from other N‐sources such as current rooting zone soil or fresh litter substrates. Moreover, net mineralization rates were much higher in permafrost soils compared to soils from the current rooting zone layer (273 mg N m−2 and 1348 mg N m−2 per growing season for near‐surface permafrost at 0.5 °C and 11 °C respectively, compared to −30 mg N m−2 for current rooting zone soil at 11 °C). Hence, our results demonstrate that near‐surface permafrost soil of subarctic peatlands can release a biologically relevant amount of plant available nitrogen, both directly upon thawing as well as over the course of a growing season through continued microbial mineralization of organically bound N. Given the nitrogen‐limited nature of northern peatlands, this release may have impacts on both plant productivity and species composition.

  • 22. KOKFELT, U.
    et al.
    ROSÉN, P.
    SCHONING, K.
    CHRISTENSEN, T. R.
    FÖRSTER, J.
    KARLSSON, J.
    REUSS, N.
    RUNDGREN, M.
    CALLAGHAN, T. V.
    JONASSON, C.
    HAMMARLUND, D.
    Ecosystem responses to increased precipitation and permafrost decay in subarctic Sweden inferred from peat and lake sediments2009Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 15, nr 7, s. 1652-1663Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake‐water, and changes in δ13C and C, N and δ15N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake‐water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June–September (JJAS) precipitation and that the increase in lake‐water TOC concentration reflects an increase in total annual precipitation. A significant depletion in 13C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake‐sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.

  • 23. Lang, Simone I.
    et al.
    Cornelissen, Johannes H. C.
    Shaver, Gaius R.
    Ahrens, Matthias
    Callaghan, Terry V.
    Molau, Ulf
    Ter Braak, Cajo J. F.
    Hölzer, Adam
    Aerts, Rien
    Arctic warming on two continents has consistent negative effects on lichen diversity and mixed effects on bryophyte diversity2012Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Global Change Biology, Vol. 18, nr 3, s. 1096-1107Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the Arctic and Subarctic, despite the well‐known importance of lichens and bryophytes to the functioning and climate feedbacks of northern ecosystems. We investigated changes in diversity and abundance of lichens and bryophytes within long‐term (9–16 years) warming experiments and along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra. In both Sweden and Alaska, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Bryophytes were less sensitive to experimental warming than lichens, but depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming in terms of both abundance and diversity. Temperature, on both continents, was the main driver of species composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in Alaska, temperature together with soil ammonium availability were the most important factors influencing species composition. Overall, dwarf shrub abundance (deciduous and evergreen) was positively related to warming but so were the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi; the majority of other cryptogams showed a negative relationship to warming. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity and abundance, especially within lichens, is likely to decrease under arctic climate warming. Given the many ecosystem processes affected by cryptogams in high latitudes (e.g. carbon sequestration, N2‐fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.

  • 24. Larsen, Klaus Steenberg
    et al.
    Ibrom, Andreas
    Jonasson, Sven
    Michelsen, Anders
    Beier, Claus
    Significance of cold-season respiration and photosynthesis in a subarctic heath ecosystem in Northern Sweden2007Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 13, nr 7, s. 1498-1508Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially, increased frequency of freeze-thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R-2-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO2 flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO2 uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 +/- 12g Cm-2 to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem.

  • 25. Lenoir, Jonathan
    et al.
    Graae, Bente Jessen
    Aarrestad, Per Arild
    Alsos, Inger Greve
    Armbruster, W. Scott
    Austrheim, Gunnar
    Bergendorff, Claes
    Birks, H. John B.
    Brathen, Kari Anne
    Brunet, Jorg
    Bruun, Hans Henrik
    Dahlberg, Carl Johan
    Stockholms universitet, Botaniska institutionen.
    Decocq, Guillaume
    Diekmann, Martin
    Dynesius, Mats
    Ejrnaes, Rasmus
    Grytnes, John-Arvid
    Hylander, Kristoffer
    Stockholms universitet, Botaniska institutionen.
    Klanderud, Kari
    Luoto, Miska
    Milbau, Ann
    Moora, Mari
    Nygaard, Bettina
    Odland, Arvid
    Ravolainen, Virve Tuulia
    Reinhardt, Stefanie
    Sandvik, Sylvi Marlen
    Schei, Fride Hoistad
    Speed, James David Mervyn
    Tveraabak, Liv Unn
    Vandvik, Vigdis
    Velle, Liv Guri
    Virtanen, Risto
    Zobel, Martin
    Svenning, Jens-Christian
    Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe2013Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, nr 5, s. 1470-1481Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent studies from mountainous areas of small spatial extent (<2500km2) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m2 units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km2 units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km2 units. Ellenberg temperature indicator values in combination with plant assemblages explained 4672% of variation in LmT and 9296% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km2 units peaked at 6065 degrees N and increased with terrain roughness, averaging 1.97 degrees C (SD=0.84 degrees C) and 2.68 degrees C (SD=1.26 degrees C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km2 units was, on average, 1.8 times greater (0.32 degrees Ckm1) than spatial turnover in growing-season GiT (0.18 degrees Ckm1). We conclude that thermal variability within 1-km2 units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

  • 26. Malmer, N
    et al.
    Johansson, T
    Olsrud, M
    Christensen, T R
    Vegetation, climatic changes and net carbon sequestration in a North-Scandinavian subarctic mire over 30 years2005Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 11, nr 11, s. 1895-1909Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study deals with changes in the plant cover and its net carbon sequestration over 30 years on a subarctic Sphagnum-mire with permafrost near Abisko, northernmost Sweden, in relation to climatic variations during the same period. Aerial colour infrared images from 1970 and 2000 were compared to reveal changes in surface structure and vegetation over the whole mire, while the plant populations were studied within a smaller, mainly ombrotrophic part. The results demonstrated two processes, namely (1) that wet sites dominated by graminoids expanded while hummock sites dominated by dwarf shrubs receded, and (2) that on the hummocks lichens expanded while evergreen dwarf shrubs and mosses decreased, both processes creating an instability in the surface structure. A successive degradation of the permafrost is the likely reason for the increase in wet areas, while the changes in the hummock vegetation might have resulted from higher spring temperatures giving rise to an intensified snow melt, exposing the vegetation to frost drought. Because of the vegetation changes, the annual litter input of carbon to the mire has increased slightly, by 4 g m(-2) a(-1) (7.3%), over these years while an increased erosion has resulted in a loss of 40-80 Mg carbon or 7-17 g m(-2) a(-1) for the entire mire over the same period. As the recalcitrant proportion of the litter has decreased, the decay rate in the acrotelm might be expected to increase in the future.

  • 27. Myrstener, Maria
    et al.
    Rocher-Ros, Gerard
    Burrows, Ryan M.
    Bergström, Ann-Kristin
    Giesler, Reiner
    Sponseller, Ryan A.
    Persistent nitrogen limitation of stream biofilm communities along climate gradients in the arctic2018Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 24, nr 8, s. 3680-3691Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change is rapidly reshaping arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll-a accumulation. For unamended biofilms, activity and biomass accrual were not driven by any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition did: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.This article is protected by copyright. All rights reserved.

  • 28. Olefeldt, David
    et al.
    Roulet, Nigel T.
    Permafrost conditions in peatlands regulate magnitude, timing, and chemical composition of catchment dissolved organic carbon export2014Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 20, nr 10, s. 3122-3136Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m−2) dominated by the snow melt period ( 70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m−2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and 10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.

  • 29. Olefeldt, David
    et al.
    Turetsky, Merritt R.
    Crill, Patrick M.
    McGuire, A. David
    Environmental and physical controls on northern terrestrial methane emissions across permafrost zones2013Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, nr 2, s. 589-603Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Methane (CH4) emissions from the northern high-latitude region represent potentially significant biogeochemical feedbacks to the climate system. We compiled a database of growing-season CH4 emissions from terrestrial ecosystems located across permafrost zones, including 303 sites described in 65 studies. Data on environmental and physical variables, including permafrost conditions, were used to assess controls on CH4 emissions. Water table position, soil temperature, and vegetation composition strongly influenced emissions and had interacting effects. Sites with a dense sedge cover had higher emissions than other sites at comparable water table positions, and this was an effect that was more pronounced at low soil temperatures. Sensitivity analysis suggested that CH4 emissions from ecosystems where the water table on average is at or above the soil surface (wet tundra, fen underlain by permafrost, and littoral ecosystems) are more sensitive to variability in soil temperature than drier ecosystems (palsa dry tundra, bog, and fen), whereas the latter ecosystems conversely are relatively more sensitive to changes of the water table position. Sites with near-surface permafrost had lower CH4 fluxes than sites without permafrost at comparable water table positions, a difference that was explained by lower soil temperatures. Neither the active layer depth nor the organic soil layer depth was related to CH4 emissions. Permafrost thaw in lowland regions is often associated with increased soil moisture, higher soil temperatures, and increased sedge cover. In our database, lowland thermokarst sites generally had higher emissions than adjacent sites with intact permafrost, but emissions from thermokarst sites were not statistically higher than emissions from permafrost-free sites with comparable environmental conditions. Overall, these results suggest that future changes to terrestrial high-latitude CH4 emissions will be more proximately related to changes in moisture, soil temperature, and vegetation composition than to increased availability of organic matter following permafrost thaw.

  • 30. Olsrud, Maria
    et al.
    Carlsson, Bengt
    Svensson, Brita M.
    Uppsala universitet, Ekologisk botanik.
    Michelsen, Anders
    Melillo, Jerry M.
    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 understory2010Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 16, nr 6, s. 1820-1829Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 31. Parker, Thomas C.
    et al.
    Subke, Jens-Arne
    Wookey, Philip A.
    Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline2015Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, nr 5, s. 2070-2081Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space-for-time’ approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg) is significantly lower at shrub (2.98 ± 0.48 kg m−2) and forest (2.04 ± 0.25 kg m−2) plots than at heath plots (7.03 ± 0.79 kg m−2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.

  • 32. Reid, Philip C.
    et al.
    Johns, David G.
    Edwards, Martin
    Starr, Michel
    Poulin, Michel
    Snoeijs, Pauli
    A biological consequence of reducing Arctic ice cover: arrival of the Pacific diatom Neodenticula seminae in the North Atlantic for the first time in 800,000 years2007Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 13, nr 9, s. 1910-1921Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Continuous Plankton Recorder survey has monitored plankton in the Northwest Atlantic at monthly intervals since 1962, with an interegnum between 1978 and 1990. In May 1999, large numbers of the Pacific diatom Neodenticula seminae were found in Continuous Plankton Recorder (CPR) samples in the Labrador Sea as the first record in the North Atlantic for more than 800 000 years. The event coincided with modifications in Arctic hydrography and circulation, increased flows of Pacific water into the Northwest Atlantic and in the previous year the exceptional occurrence of extensive ice-free water to the North of Canada. These observations indicate that N. seminae was carried in a pulse of Pacific water in 1998/early 1999 via the Canadian Arctic Archipelago and/or Fram Strait. The species occurred previously in the North Atlantic during the Pleistocene from similar to 1.2 to similar to 0.8 Ma as recorded in deep sea sediment cores. The reappearance of N. seminae in the North Atlantic is an indicator of the scale and speed of changes that are taking place in the Arctic and North Atlantic oceans as a consequence of regional climate warming. Because of the unusual nature of the event it appears that a threshold has been passed, marking a change in the circulation between the North Pacific and North Atlantic Oceans via the Arctic. Trans-Arctic migrations from the Pacific into the Atlantic are likely to occur increasingly over the next 100 years as Arctic ice continues to melt affecting Atlantic biodiversity and the biological pump with consequent feedbacks to the carbon cycle.

  • 33. Rinnan, Riikka
    et al.
    Michelsen, Anders
    Baath, Erland
    Jonasson, Sven
    Fifteen years of climate change manipulations alter soil microbial communities in a subarctic heath ecosystem2007Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 13, nr 1, s. 28-39Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Soil microbial biomass in arctic heaths has been shown to be largely unaffected by treatments simulating climate change with temperature, nutrient and light manipulations. Here, we demonstrate that more than 10 years is needed for development of significant responses, and that changes in microbial biomass are accompanied with strong alterations in microbial community composition. In contrast to slight or nonsignificant responses after 5, 6 and 10 treatment years, 15 years of inorganic NPK fertilizer addition to a subarctic heath had strong effects on the microbial community and, as observed for the first time, warming and shading also led to significant responses, often in opposite direction to the fertilization responses. The effects were clearer in the top 5 cm soil than at the 5-10 cm depth. Fertilization increased microbial biomass C and more than doubled microbial biomass P compared to the non-fertilized plots. However, it only increased microbial biomass N at the 5-10 cm depth. Fertilization increased fungal biomass and the relative abundance of phospholipid fatty acid (PLFA) markers of gram-positive bacteria. Warming and shading decreased the relative abundance of fungal PLFAs, and shading also altered the composition of the bacterial community. The long time lag in responses may be associated with indirect effects of the gradual changes in the plant biomass and community composition. The contrasting responses to warming and fertilization treatments show that results from fertilizer addition may not be similar to the effects of increased nutrient mineralization and availability following climatic warming.

  • 34.
    Rocher-Ros, Gerard
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Sponseller, Ryan A.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Bergström, Ann-Kristin
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Myrstener, Maria
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Giesler, Reiner
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Stream metabolism controls diel patterns and evasion of CO2 in Arctic streams2019Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO2 evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO2 production and evasion from streams remains poorly understood. In this study, we measured O-2 and CO2 continuously in streams draining tundra-dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO2 evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO2 concentrations and evasion at multiple timescales. Photosynthesis caused CO2 concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low-turbulence streams. Diel patterns in CO2 concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO2 evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO2 evasion, suggesting that some stream reaches serve as net sources of CO2, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO2 evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes.

  • 35. Rousk, Kathrin
    et al.
    Michelsen, Anders
    Ecosystem nitrogen fixation throughout the snow-free period in subarctic tundra: effects of willow and birch litter addition and warming2017Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, nr 4, s. 1552-1563Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nitrogen (N) fixation in moss-associated cyanobacteria is one of the main sources of available N for N-limited ecosystems such as subarctic tundra. Yet, N2 fixation in mosses is strongly influenced by soil moisture and temperature. Thus, temporal scaling up of low-frequency in situ measurements to several weeks, months or even the entire growing season without taking into account changes in abiotic conditions cannot capture the variation in moss-associated N2 fixation. We therefore aimed to estimate moss-associated N2 fixation throughout the snow-free period in subarctic tundra in field experiments simulating climate change: willow (Salix myrsinifolia) and birch (Betula pubescens spp. tortuosa) litter addition, and warming. To achieve this, we established relationships between measured in situ N2 fixation rates and soil moisture and soil temperature and used high-resolution measurements of soil moisture and soil temperature (hourly from May to October) to model N2 fixation. The modelled N2 fixation rates were highest in the warmed (2.8 ± 0.3 kg N ha−1) and birch litter addition plots (2.8 ± 0.2 kg N ha−1), and lowest in the plots receiving willow litter (1.6 ± 0.2 kg N ha−1). The control plots had intermediate rates (2.2 ± 0.2 kg N ha−1). Further, N2 fixation was highest during the summer in the warmed plots, but was lowest in the litter addition plots during the same period. The temperature and moisture dependence of N2 fixation was different between the climate change treatments, indicating a shift in the N2 fixer community. Our findings, using a combined empirical and modelling approach, suggest that a longer snow-free period and increased temperatures in a future climate will likely lead to higher N2 fixation rates in mosses. Yet, the consequences of increased litter fall on moss-associated N2 fixation due to shrub expansion in the Arctic will depend on the shrub species’ litter traits.

  • 36. Rousk, Kathrin
    et al.
    Michelsen, Anders
    Rousk, Johannes
    Microbial control of soil organic matter mineralization responses to labile carbon in subarctic climate change treatments2016Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 22, nr 12, s. 4150-4161Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Half the global soil carbon (C) is held in high-latitude systems. Climate change will expose these to warming and a shift towards plant communities with more labile C input. Labile C can also increase the rate of loss of native soil organic matter (SOM); a phenomenon termed ‘priming’. We investigated how warming (+1.1 °C over ambient using open top chambers) and litter addition (90 g m−2 yr−1) treatments in the subarctic influenced the susceptibility of SOM mineralization to priming, and its microbial underpinnings. Labile C appeared to inhibit the mineralization of C from SOM by up to 60% within hours. In contrast, the mineralization of N from SOM was stimulated by up to 300%. These responses occurred rapidly and were unrelated to microbial successional dynamics, suggesting catabolic responses. Considered separately, the labile C inhibited C mineralization is compatible with previously reported findings termed ‘preferential substrate utilization’ or ‘negative apparent priming’, while the stimulated N mineralization responses echo recent reports of ‘real priming’ of SOM mineralization. However, C and N mineralization responses derived from the same SOM source must be interpreted together: This suggested that the microbial SOM-use decreased in magnitude and shifted to components richer in N. This finding highlights that only considering SOM in terms of C may be simplistic, and will not capture all changes in SOM decomposition. The selective mining for N increased in climate change treatments with higher fungal dominance. In conclusion, labile C appeared to trigger catabolic responses of the resident microbial community that shifted the SOM mining to N-rich components; an effect that increased with higher fungal dominance. Extrapolating from these findings, the predicted shrub expansion in the subarctic could result in an altered microbial use of SOM, selectively mining it for N-rich components, and leading to a reduced total SOM-use.

  • 37. Saurer, Matthias
    et al.
    Spahni, Renato
    Frank, David C.
    Joos, Fortunat
    Leuenberger, Markus
    Loader, Neil J.
    McCarroll, Danny
    Gagen, Mary
    Poulter, Ben
    Siegwolf, Rolf T.W.
    Andreu-Hayles, Laia
    Boettger, Tatjana
    Dorado Liñán, Isabel
    Fairchild, Ian J.
    Friedrich, Michael
    Gutierrez, Emilia
    Haupt, Marika
    Hilasvuori, Emmi
    Heinrich, Ingo
    Helle, Gerd
    Grudd, Håkan
    Jalkanen, Risto
    Levanič, Tom
    Linderholm, Hans W.
    Robertson, Iain
    Sonninen, Eloni
    Treydte, Kerstin
    Waterhouse, John S.
    Woodley, Ewan J.
    Wynn, Peter M.
    Young, Giles H.F.
    Spatial variability and temporal trends in water-use efficiency of European forests2014Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 20, nr 12, s. 3700-3712Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The increasing carbon dioxide (CO2) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land–atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation–climate feedbacks are currently still poorly constrained by observational data.

  • 38. SCHERRER, DANIEL
    et al.
    K֖RNER, CHRISTIAN
    Infra-red thermometry of alpine landscapes challenges climatic warming projections2010Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 16, nr 9, s. 2602-2613Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Rough mountain terrain offers climatic conditions (niches) to plants and animals poorly represented by conventional climate station data. However, the extent to which actual temperatures deviate from those of the freely circulating atmosphere had never been assessed at a landscape level. Here, we quantify thermal life conditions across topographically rich mountain terrain by using a combination of thermal (IR) imagery of surface temperature with data from a large number of miniature data loggers buried at 3 cm soil depth. The data obtained from six alpine (Alps) and arctic‐alpine slopes (Norway, Sweden, Svalbard) evidence persistent root zone temperatures of 2–4 K above air temperature during summer. Surface temperatures show strong positive (2–9 K) and negative (3–8 K) deviations from air temperature on bright days and clear nights, respectively. As to be expected, south oriented slopes are warmer than west and north slopes but microclimatic variation on clear sky days was strong within all slopes, with 8.4±2.5 K (mean±SD) surface temperature differences persisting over several hours per day along horizontal (i.e., equal elevation) transects. Life conditions of alpine organisms are thus strongly decoupled from conditions in the free atmosphere and cannot reliably be inferred from climate station data in both, temperate and arctic latitudes. Microtopography can mimic temperature differences of large elevational (or latitudinal) gradients over very short horizontal distances. This is important in the context of climate change because it shows that species do not necessarily need to climb several hundred meters in elevation to escape the warmth. Quite often, few meters of horizontal shift will do. For plants unable to, or too slow to adapt to a warmer climate, thermal microhabitat mosaics offer both refuge habitats as well as stepping stones as atmospheric temperatures rise.

  • 39. Sjogersten, S
    et al.
    Turner, B L
    Mahieu, N
    Condron, L M
    Wookey, P A
    Soil organic matter biochemistry and potential susceptibility to climatic change across the forest-tundra ecotone in the Fennoscandian mountains2003Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 9, nr 5, s. 759-772Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We studied soil organic carbon (C) chemistry at the mountain birch forest-tundra ecotone in three regions of the Fennoscandian mountain range with comparable vegetation cover but contrasting degrees of continentality and latitude. The aim of the study was to identify functional compound classes and their relationships to decomposition and spatial variation across the ecotone and latitudinal gradient. Solid-state (13) C nuclear magnetic resonance (CPMAS (13) C NMR) was used to identify seven functional groups of soil organic C: alkyls , N-alkyls , O-alkyls , acetals , aromatics , phenolics and carboxyls . N-alkyls , O-alkyls and acetals are generally considered labile substrates for a large number of saprotrophic fungi and bacteria, whilst phenolics and aromatics are mainly decomposed by lignolytic organisms and contribute to the formation of soil organic matter together with aliphatic alkyls and carboxyls . All soils contained a similar proportional distribution of functional groups, although relatively high amounts of N-alkyls , O-alkyls and acetals were present in comparison to earlier published studies, suggesting that large amounts of soil C were potentially vulnerable to microbial degradation. Soil organic matter composition was different at the most southerly site (Dovrefjell, Norway), compared with the two more northerly sites (Abisko, Sweden, and Joatka, Norway), with higher concentrations of aromatics and phenolics , as well as pronounced differences in alkyl concentrations between forest and tundra soils. Clear differences between mountain birch forest and tundra heath soil was noted, with generally higher concentrations of labile carbon present in tundra soils. We conclude that, although mesic soils around the forest-tundra ecotone in Fennoscandia are a potential source of C to the atmosphere in a changing environment, the response is likely to vary between comparable ecosystems in relation to latitude and continentality as well as soil properties especially soil nitrogen content and pH.

  • 40. Sloan, Victoria L.
    et al.
    Fletcher, Benjamin J.
    Press, Malcolm C.
    Williams, Mathew
    Phoenix, Gareth K.
    Leaf and fine root carbon stocks and turnover are coupled across Arctic ecosystems2013Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, nr 12, s. 3668-3676Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Estimates of vegetation carbon pools and their turnover rates are central to understanding and modelling ecosystem responses to climate change and their feedbacks to climate. In the Arctic, a region containing globally important stores of soil carbon, and where the most rapid climate change is expected over the coming century, plant communities have on average sixfold more biomass below ground than above ground, but knowledge of the root carbon pool sizes and turnover rates is limited. Here, we show that across eight plant communities, there is a significant positive relationship between leaf and fine root turnover rates (r2 = 0.68, P < 0.05), and that the turnover rates of both leaf (r2 = 0.63, P < 0.05) and fine root (r2 = 0.55, P < 0.05) pools are strongly correlated with leaf area index (LAI, leaf area per unit ground area). This coupling of root and leaf dynamics supports the theory of a whole-plant economics spectrum. We also show that the size of the fine root carbon pool initially increases linearly with increasing LAI, and then levels off at LAI = 1 m2 m−2, suggesting a functional balance between investment in leaves and fine roots at the whole community scale. These ecological relationships not only demonstrate close links between above and below-ground plant carbon dynamics but also allow plant carbon pool sizes and their turnover rates to be predicted from the single readily quantifiable (and remotely sensed) parameter of LAI, including the possibility of estimating root data from satellites.

  • 41. SORENSEN, PERNILLE LAERKEDAL
    et al.
    MICHELSEN, ANDERS
    Long-term warming and litter addition affects nitrogen fixation in a subarctic heath2011Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Global Change Biology, Vol. 17, nr 1, s. 528-537Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Abstract Nitrogen (N) availability is the main constraint on primary production in most Arctic ecosystems, with microbial fixation of atmospheric N as the primary source of N input. However, there are only few reports on N fixation rates in relation to climate change in the Arctic. In order to investigate the effects of anticipated global climate change on N fixation rates in a subarctic moist heath, a field experiment was carried out in Northern Sweden. Warming was induced by plastic tents, and in order to simulate the effects of future increased tree cover, birch litter was added each fall for 9 years before the measurements. We analyzed N fixation rates on both whole-ecosystem level and specifically on two moss species: Sphagnum warnstorfii and Hylocomium splendens. The whole-ecosystem N fixation of the warmed plots almost tripled compared with the control plots. However, in the Sphagnum and Hylocomium mosses we observed either no change or occasionally even a decrease in N fixation after warming. Both measured on whole-ecosystem level and on the two moss species separately, litter addition increased N fixation rates. The results suggest that warming will lead to a general increased ecosystem N input, but also that the N fixation associated to some moss species is likely to decrease. Hence, this study shows that the scale of measurements is crucial when investigating on ecosystem responses to manipulations.

  • 42. Subke, Jens-Arne
    et al.
    Inglima, Ilaria
    Cotrufo, M. Francesca
    Trends and methodological impacts in soil CO2 efflux partitioning: A metaanalytical review2006Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 12, nr 6, s. 921-943Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Partitioning soil carbon dioxide (CO2) efflux (R-S) into autotrophic (R-A; including plant roots and closely associated organisms) and heterotrophic (R-H) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the R-H/R-S ratio for increasing annual R-S fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P < 0.05) R-H/R-S ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the R-H/R-S ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of R-S under which they had been measured was considered, with the exception of studies estimating R-H by means of root mass regressions against R-S, which resulted in consistently lower R-H/R-S estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect R-H/R-S ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO2 efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself. Partitioning soil carbon dioxide (CO2) efflux (R-S) into autotrophic (R-A; including plant roots and closely associated organisms) and heterotrophic (R-H) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the R-H/R-S ratio for increasing annual R-S fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P < 0.05) R-H/R-S ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the R-H/R-S ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of R-S under which they had been measured was considered, with the exception of studie estimating R-H by means of root mass regressions against R-S, which resulted in consistently lower R-H/R-S estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect R-H/R-S ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO2 efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself.

  • 43. T. Weedon, James
    et al.
    A. Kowalchuk, George
    Aerts, Rien
    van Hal, Jurgen
    van Logtestijn, Richard
    Taş, Neslihan
    F. M. Röling, Wilfred
    M. van Bodegom, Peter
    Summer warming accelerates sub-arctic peatland nitrogen cycling without changing enzyme pools or microbial community structure2012Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 18, nr 1, s. 138-150Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The balance of primary production and decomposition in northern peatlands may shift due to climate change, with potential feedbacks to atmospheric CO2 concentrations. Nitrogen availability will modulate this shift, but little is known about the drivers of soil nitrogen dynamics in these environments. We used a long-term (9 years) open top chamber (OTC) experiment in an ombrotrophic Sphagnum peat bog in sub-arctic Sweden, to test for the interactive effects of spring warming, summer warming and winter snow addition on soil nitrogen fluxes, potential activities of nitrogen cycle enzymes, and soil microbial community composition. These simultaneous measurements allowed us to identify the level of organization at which climate change impacts are apparent, an important requirement for developing truly mechanistic understanding. Organic-N pools and fluxes were an order of magnitude higher than inorganic-N pools and fluxes. Summer warming approximately doubled fluxes of soil organic nitrogen and ammonia over the growing season. Such a large increase under 1 °C warming is unlikely to be due to kinetic effects, and we propose that it is linked to an observed seasonal decrease in microbial biomass, suggesting that N flux is driven by a substantial late-season dieback of microbes. This change in N cycle dynamics was not reflected in any of the measured potential peptidase activities. Moreover, the soil microbial community structure was apparently stable across treatments, suggesting a non-specific microbial dieback. Our results show that in these widespread peat bogs, where many plant species are capable of organic-N uptake, organic soil N dynamics are quantitatively far more important than the commonly studied inorganic-N dynamics. Understanding of climate change effects on organic soil N cycling in this system will be advanced by closer investigation of the seasonal dynamics of the microbial biomass and the input of substrates that maintain it.

  • 44. Treat, Claire C.
    et al.
    Natali, Susan M.
    Ernakovich, Jessica
    Iversen, Colleen M.
    Lupascu, Massimo
    McGuire, Anthony David
    Norby, Richard J.
    Roy Chowdhury, Taniya
    Richter, Andreas
    Šantrůčková, Hana
    Schädel, Christina
    Schuur, Edward A. G.
    Sloan, Victoria L.
    Turetsky, Merritt R.
    Waldrop, Mark P.
    A pan-Arctic synthesis of CH4 and CO2 production from anoxic soil incubations2015Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, nr 7, s. 2787-2803Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH4) and carbon dioxide (CO2) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large-scale controls of anaerobic CO2 and CH4 production and compare the relative importance of landscape-level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH4 production per gram soil carbon from organic soils than mineral soils. Maximum CH4 production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH4 production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH4 and median anaerobic CO2 production decreased with depth, while CO2:CH4 production increased with depth. Maximum CH4 production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH4 production in permafrost ecosystems and suggests the need for longer-term anaerobic incubations to fully capture CH4 dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO2 and CH4 production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw.

  • 45. Valolahti, Hanna
    et al.
    Kivimäenpää, Minna
    Faubert, Patrick
    Michelsen, Anders
    Rinnan, Riikka
    Climate change-induced vegetation change as a driver of increased subarctic biogenic volatile organic compound emissions2015Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, nr 9, s. 3478-3488Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Emissions of biogenic volatile organic compounds (BVOCs) have been earlier shown to be highly temperature sensitive in subarctic ecosystems. As these ecosystems experience rapidly advancing pronounced climate warming, we aimed to investigate how warming affects the BVOC emissions in the long term (up to 13 treatment years). We also aimed to assess whether the increased litterfall resulting from the vegetation changes in the warming subarctic would affect the emissions. The study was conducted in a field experiment with factorial open-top chamber warming and annual litter addition treatments on subarctic heath in Abisko, northern Sweden. After 11 and 13 treatment years, BVOCs were sampled from plant communities in the experimental plots using a push–pull enclosure technique and collection into adsorbent cartridges during the growing season and analyzed with gas chromatography–mass spectrometry. Plant species coverage in the plots was analyzed by the point intercept method. Warming by 2 °C caused a 2-fold increase in monoterpene and 5-fold increase in sesquiterpene emissions, averaged over all measurements. When the momentary effect of temperature was diminished by standardization of emissions to a fixed temperature, warming still had a significant effect suggesting that emissions were also indirectly increased. This indirect increase appeared to result from increased plant coverage and changes in vegetation composition. The litter addition treatment also caused significant increases in the emission rates of some BVOC groups, especially when combined with warming. The combined treatment had both the largest vegetation changes and the highest BVOC emissions. The increased emissions under litter addition were probably a result of a changed vegetation composition due to alleviated nutrient limitation and stimulated microbial production of BVOCs. We suggest that the changes in the subarctic vegetation composition induced by climate warming will be the major factor indirectly affecting the BVOC emission potentials and composition.

  • 46. van Wijk, M T
    et al.
    Clemmensen, K E
    Shaver, G R
    Williams, M
    Callaghan, T V
    Chapin, F S
    Cornelissen, J H C
    Gough, L
    Hobbie, S E
    Jonasson, S
    Lee, J A
    Michelsen, A
    Press, M C
    Richardson, S J
    Rueth, H
    Long-term ecosystem level experiments at Toolik Lake, Alaska, and at Abisko, Northern Sweden: generalizations and differences in ecosystem and plant type responses to global change2004Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 10, nr 1, s. 105-123Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Long-term ecosystem-level experiments, in which the environment is manipulated in a controlled manner, are important tools to predict the responses of ecosystem functioning and composition to future global change. We present the results of a meta-analysis performed on the results of long-term ecosystem-level experiments near Toolik Lake, Alaska, and Abisko, Sweden. We quantified aboveground biomass responses of different arctic and subarctic ecosystems to experimental fertilization, warming and shading. We not only analysed the general patterns but also the differences in responsiveness between sites and regions. Aboveground plant biomass showed a broad similarity of responses in both locations, and also showed some important differences. In both locations, aboveground plant biomass, particularly the biomass of deciduous and graminoid plants, responded most strongly to nutrient addition. The biomass of mosses and lichens decreased in both locations as the biomass of vascular plants increased. An important difference between the two regions was the smaller positive aboveground biomass response of deciduous shrubs in Abisko as compared with Toolik Lake. Whereas in Toolik Lake Betula nana increased its dominance and replaced many of the other plant types, in Abisko all vascular plant types increased in abundance without major shifts in relative abundance. The differences between the responses of the dominant vegetation types of the Toolik Lake region, i.e. tussock tundra systems, and that of the Abisko region, i.e. heath systems, may have important implications for ecosystem development under expected patterns of global change. However, there were also large site-specific differences within each region. Several potential mechanistic explanations for the differences between sites and regions are discussed. The response patterns show the need for analyses of joint data sets from many regions and sites, in order to uncover common responses to changes in climate across large arctic regions from regional or local responses.

  • 47. Zacher, Katharina
    et al.
    Wulff, Angela
    Molis, Markus
    Hanelt, Dieter
    Wiencke, Christian
    Ultraviolet radiation and consumer effects on a field-grown intertidal macroalgal assemblage in Antarctica2007Inngår i: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 13, nr 6, s. 1201-1215Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ultraviolet radiation (UVR) research on marine macroalgae has hithero focussed on physiological effects at the organism level, while little is known on the impact of UV radiation on macroalgal assemblages and even less on interactive effects with other community drivers, e.g. consumers. Field experiments on macrobenthos are scarce, particularly in the Antarctic region. Therefore, the effects of UVR and consumers (mainly limpets were excluded) on early successional stages of a hard bottom macroalgal community on King George Island, Antarctica, were studied. In a two-factorial design experimental units [(1) ambient radiation, 280-700 nm; (2) ambient minus UVB, 320-700 nm and (3) ambient minus UVR, 400-700 nm vs. consumer-no consumer] were installed between November 2004 and March 2005 (n = 4 plus controls). Dry mass, species richness, diversity and composition of macroalgal assemblages developing on ceramic tiles were followed. Consumers significantly suppressed green algal recruits and total algal biomass but increased macroalgal richness and diversity. Both UVA and UVB radiation negatively affected macroalgal succession. UVR decreased the density of Monostroma hariotii germlings in the first 10 weeks of the experiment, whereas the density of red algal recruits was significantly depressed by UVR at the end of the study. After 106 days macroalgal diversity was significantly higher in UV depleted than in UV-exposed assemblages. Furthermore, species richness was significantly lower in the UV treatments and species composition differed significantly between the UV-depleted and the UV-exposed treatment. Marine macroalgae are very important primary producers in coastal ecosystems, serving as food for herbivores and as habitat for many organisms. Both, UVR and consumers significantly shape macroalgal succession in the Antarctic intertidal. Consumers, particularly limpets can mediate negative effects of ambient UVR on richness and diversity till a certain level. UVB radiation in general and an increase of this short wavelength due to stratospheric ozone depletion in particular may have the potential to affect the zonation, composition and diversity of Antarctic intertidal seaweeds altering trophic interactions in this system.

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