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  • 1. Abbott, Benjamin W.
    et al.
    Jones, Jeremy B.
    Schuur, Edward A. G.
    Chapin, F. Stuart, III
    Bowden, William B.
    Bret-Harte, M. Syndonia
    Epstein, Howard E.
    Flannigan, Michael D.
    Harms, Tamara K.
    Hollingsworth, Teresa N.
    Mack, Michelle C.
    McGuire, A. David
    Natali, Susan M.
    Rocha, Adrian V.
    Tank, Suzanne E.
    Turetsky, Merritt R.
    Vonk, Jorien E.
    Wickland, Kimberly P.
    Aiken, George R.
    Alexander, Heather D.
    Amon, Rainer M. W.
    Benscoter, Brian W.
    Bergeron, Yves
    Bishop, Kevin
    Blarquez, Olivier
    Bond-Lamberty, Ben
    Breen, Amy L.
    Buffam, Ishi
    Cai, Yihua
    Carcaillet, Christopher
    Carey, Sean K.
    Chen, Jing M.
    Chen, Han Y. H.
    Christensen, Torben R.
    Cooper, Lee W.
    Cornelissen, J. Hans C.
    de Groot, William J.
    DeLuca, Thomas H.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Fetcher, Ned
    Finlay, Jacques C.
    Forbes, Bruce C.
    French, Nancy H. F.
    Gauthier, Sylvie
    Girardin, Martin P.
    Goetz, Scott J.
    Goldammer, Johann G.
    Gough, Laura
    Grogan, Paul
    Guo, Laodong
    Higuera, Philip E.
    Hinzman, Larry
    Hu, Feng Sheng
    Hugelius, Gustaf
    Jafarov, Elchin E.
    Jandt, Randi
    Johnstone, Jill F.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kasischke, Eric S.
    Kattner, Gerhard
    Kelly, Ryan
    Keuper, Frida
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kling, George W.
    Kortelainen, Pirkko
    Kouki, Jari
    Kuhry, Peter
    Laudon, Hjalmar
    Laurion, Isabelle
    Macdonald, Robie W.
    Mann, Paul J.
    Martikainen, Pertti J.
    McClelland, James W.
    Molau, Ulf
    Oberbauer, Steven F.
    Olefeldt, David
    Pare, David
    Parisien, Marc-Andre
    Payette, Serge
    Peng, Changhui
    Pokrovsky, Oleg S.
    Rastetter, Edward B.
    Raymond, Peter A.
    Raynolds, Martha K.
    Rein, Guillermo
    Reynolds, James F.
    Robards, Martin
    Rogers, Brendan M.
    Schaedel, Christina
    Schaefer, Kevin
    Schmidt, Inger K.
    Shvidenko, Anatoly
    Sky, Jasper
    Spencer, Robert G. M.
    Starr, Gregory
    Striegl, Robert G.
    Teisserenc, Roman
    Tranvik, Lars J.
    Virtanen, Tarmo
    Welker, Jeffrey M.
    Zimov, Sergei
    Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment2016In: Environmental Research Letters, ISSN 1748-9326, E-ISSN 1748-9326, Vol. 11, no 3, article id 034014Article in journal (Refereed)
    Abstract [en]

    As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%-85% of permafrost carbon release can still be avoided if human emissions are actively reduced.

  • 2. Aerts, R.
    et al.
    Callaghan, T. V.
    Dorrepaal, E.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Systems Ecology, Department of Ecological Science, VU University Amsterdam, Amsterdam, The Netherlands.
    van Logtestijn, R. S. P.
    Cornelissen, J. H. C.
    Seasonal climate manipulations have only minor effects on litter decomposition rates and N dynamics but strong effects on litter P dynamics of sub-arctic bog species2012In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 170, no 3, p. 809-819Article in journal (Refereed)
    Abstract [en]

    Litter decomposition and nutrient mineralization in high-latitude peatlands are constrained by low temperatures. So far, little is known about the effects of seasonal components of climate change (higher spring and summer temperatures, more snow which leads to higher winter soil temperatures) on these processes. In a 4-year field experiment, we manipulated these seasonal components in a sub-arctic bog and studied the effects on the decomposition and N and P dynamics of leaf litter of Calamagrostis lapponica, Betula nana, and Rubus chamaemorus, incubated both in a common ambient environment and in the treatment plots. Mass loss in the controls increased in the order Calamagrostis < Betula < Rubus. After 4 years, overall mass loss in the climate-treatment plots was 10 % higher compared to the ambient incubation environment. Litter chemistry showed within each incubation environment only a few and species-specific responses. Compared to the interspecific differences, they resulted in only moderate climate treatment effects on mass loss and these differed among seasons and species. Neither N nor P mineralization in the litter were affected by the incubation environment. Remarkably, for all species, no net N mineralization had occurred in any of the treatments during 4 years. Species differed in P-release patterns, and summer warming strongly stimulated P release for all species. Thus, moderate changes in summer temperatures and/or winter snow addition have limited effects on litter decomposition rates and N dynamics, but summer warming does stimulate litter P release. As a result, N-limitation of plant growth in this sub-arctic bog may be sustained or even further promoted.

  • 3.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Defoliation of a grass is mediated by the positive effect of dung deposition, moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment2019In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 128, no 10, p. 1515-1524Article in journal (Refereed)
    Abstract [en]

    Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing-tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily-grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long-lasting effects on plant productivity and ecosystem functioning after grazing has ceased.

  • 4.
    Barthelemy, Hélène
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Defoliation, soil grazing legacy, dung and moss cover influence growth and nutrient uptake of the common grass species, Festuca ovinaManuscript (preprint) (Other academic)
    Abstract [en]

    Herbivores can strongly influence plant growth directly through defoliation and the return of nutrients in the form of dung and urine but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. The relative importance of these driving mechanisms of plant response to herbivory are still poorly understood. In a common garden experiment, we studied the aboveground and belowground responses of Festuca ovina, a grazing tolerant grass common in arctic secondary grassland, to defoliation, reindeer dung addition, changes in soil microclimate induced by the presence or the absence of a moss cover, and soil grazing legacy. Defoliation strongly reduced shoot and root growth and plant nutrient uptake. Plants did thus not compensate for the tissue lost due to defoliation, even at a higher nutrient availability. By contrast, defoliation enhanced plant N concentration and decreased plant C to N ratio. Soil from heavily grazed sites and dung addition increased plant production, plant N concentrations and nutrient uptake, although the effects of dung addition were only small. Mosses had a strong negative effect of root biomass and reduced plant compensatory growth after defoliation. Interestingly mosses also had facilitative effects on aboveground plant growth in absence of defoliation and on plant nutrient uptake and N concentrations. Although plants suffered severely from defoliation, they were also strongly favoured by the increased nutrient availability associated with herbivory. After two years, plants produced as much biomass when all positive and negative effects of herbivores were considered (defoliation, soil communities and nutrient availability under heavily grazing, dung addition and no moss cover) as in the ungrazed conditions (no defoliation, soil communities and nutrient availability under lightly grazing, no dung addition, a thick moss cover). This study indicates that graminoids can tolerate high densities of herbivores, although it suffer from defoliation directly, and suggests that changes in plant quality following defoliation and grazing-induced changes in soil processes are two key mechanisms through which herbivores can control plant productivity in arctic secondary grasslands. Plant tolerance to herbivory will depends on how herbivores utilise a pasture area and on the balance between the positive and the negative effects of grazing on plant growth.

  • 5.
    Blume-Werry, Gesche
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Experimental Plant Ecology, Institute of Botany and Landscape Ecology, Greifswald University, Germany.
    Milbau, Ann
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Research Institute for Nature and Forest INBO, Brussels, Belgium.
    Teuber, Laurenz M.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Experimental Plant Ecology, Institute of Botany and Landscape Ecology, Greifswald, Germany.
    Johansson, Margareta
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dwelling in the deep – strongly increased root growth and rooting depth enhance plant interactions with thawing permafrost soil2019In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 223, no 3, p. 1328-1339Article in journal (Refereed)
    Abstract [en]

    Climate‐warming‐induced permafrost thaw exposes large amounts of carbon and nitrogen in soil at considerable depths, below the seasonally thawing active layer. The extent to which plant roots can reach and interact with these hitherto detached, deep carbon and nitrogen stores remains unknown.

    We aimed to quantify how permafrost thaw affects root dynamics across soil depths and plant functional types compared with above‐ground abundance, and potential consequences for plant–soil interactions.

    A decade of experimental permafrost thaw strongly increased total root length and growth in the active layer, and deep roots invaded the newly thawed permafrost underneath. Root litter input to soil across all depths was 10 times greater with permafrost thaw. Root growth timing was unaffected by experimental permafrost thaw but peaked later in deeper soil, reflecting the seasonally receding thaw front. Deep‐rooting species could sequester 15N added at the base of the ambient active layer in October, which was after root growth had ceased.

    Deep soil organic matter that has long been locked up in permafrost is thus no longer detached from plant processes upon thaw. Whether via nutrient uptake, carbon storage, or rhizosphere priming, plant root interactions with thawing permafrost soils may feed back on our climate both positively and negatively.

  • 6.
    Blume-Werry, Gesche
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Milbau, Ann
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Teuber, Laurenz M.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Margareta, Johansson
    Lund University.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dwelling in the deep – permafrost thawing strongly increases plant root growth and root litter inputManuscript (preprint) (Other academic)
    Abstract [en]

    Plant roots play a key role in ecosystem carbon and nutrient cycling. Climate warming induced thawing of permafrost exposes large amounts of carbon and nitrogen at greater soil depths that hitherto have been detached from plant influences. Whether plant roots can reach and interact with these carbon and nitrogen sources upon permafrost thaw remains unknown. Here, we use a long-term permafrost thaw experiment and a short-term deep fertilization experiment in northern Sweden to assess changes in vegetation composition and root dynamics (deep nitrogen uptake, root depth distribution, root growth and phenology, root mortality and litter input) related to permafrost thaw, both in active layer and in newly thawed permafrost. We show that Eriophorum vaginatum and Rubus chamaemorus, both relatively deep-rooting species, can take up nitrogen released at depth of permafrost thaw, despite the late release time in autumn when plant activity is expected to have ceased. Also, root dynamics changed drastically after a decade of experimental permafrost thaw. Total root length, root growth and root litter input all strongly increased, not only in the active layer but also in the newly thawed permafrost, and the timing of root growth was related to the seasonality of soil thaw. These responses were driven by Eriophorum vaginatum, which differed greatly in root dynamics compared to the other species and thus worked as an ecosystem engineer. This study demonstrates that soil organic matter currently locked-up at depth in permafrost is no longer detached from plant processes upon thaw. Given the pivotal role that roots have in the carbon cycle and the importance of the large carbon stocks in arctic soils, the changes observed here have the potential to feedback onto the global climate system.

  • 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 University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Variable temperature effects of Open Top Chambers at polar and alpine sites explained by irradiance and snow depth2013In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, no 1, p. 64-74Article, review/survey (Refereed)
    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. De Long, Jonathan R.
    et al.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kardol, Paul
    Nilsson, Marie-Charlotte
    Teuber, Laurenz M.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wardle, David A.
    Contrasting Responses of Soil Microbial and Nematode Communities to Warming and Plant Functional Group Removal Across a Post-fire Boreal Forest Successional Gradient2016In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 19, no 2, p. 339-355Article in journal (Refereed)
    Abstract [en]

    Global warming is causing increases in surface temperatures and has the potential to influence the structure of soil microbial and faunal communities. However, little is known about how warming interacts with other ecosystem drivers, such as plant functional groups or changes associated with succession, to affect the soil community and thereby alter ecosystem functioning. We investigated how experimental warming and the removal of plant functional groups along a post-fire boreal forest successional gradient impacted soil microbial and nematode communities. Our results showed that warming altered soil microbial communities and favored bacterial-based microbial communities, but these effects were mediated by mosses and shrubs, and often varied with successional stage. Meanwhile, the nematode community was generally unaffected by warming and was positively affected by the presence of mosses and shrubs, with these effects mostly independent of successional stage. These results highlight that different groups of soil organisms may respond dissimilarly to interactions between warming and changes to plant functional groups, with likely consequences for ecosystem functioning that may vary with successional stage. Due to the ubiquitous presence of shrubs and mosses in boreal forests, the effects observed in this study are likely to be significant over a large proportion of the terrestrial land surface. Our results demonstrate that it is crucial to consider interactive effects between warming, plant functional groups, and successional stage when predicting soil community responses to global climate change in forested ecosystems.

  • 9. De Long, Jonathan R.
    et al.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kardol, Paul
    Nilsson, Marie-Charlotte
    Teuber, Laurenz M.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wardle, David A.
    Understory plant functional groups and litter species identity are stronger drivers of litter decomposition than warming along a boreal forest post-fire successional gradient2016In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 98, p. 159-170Article in journal (Refereed)
    Abstract [en]

    Increasing surface temperatures due to climate change have the potential to alter plant litter mass loss and nutrient release during decomposition. However, a great deal of uncertainty remains concerning how ecosystem functioning may be affected by interactions between warming and other drivers, such as plant functional group composition and environmental context. In this study, we investigated how vascular plant litter decomposition and nutrient release were affected by experimental warming, moss removal and shrub removal along a post-fire boreal forest successional gradient. Our results show that litter decomposition and nutrient loss were primarily driven by understory plant functional group removal. The removal of mosses generally reduced litter mass loss and increased litter phosphorus (P) loss, while shrub removal typically increased litter mass loss and in one litter species reduced immobilization of P. Litter nitrogen (N) loss was unaffected by plant functional group removal. Warming interacted with successional stage and species identity of the litter decomposed, but these effects were uncommon and generally weak. As climate change advances, moss cover is expected to decrease, while shrub cover is expected to increase. Taken together with our results, this suggests that lower moss cover will decrease leaf litter decomposition rates and increase P release from litter, while increasing shrub cover will decrease decomposition rates and may reduce P release from litter. Our results demonstrate that in the short term, the direct effects of warming and successional stage will play a relatively minor role in driving litter decomposition processes in the boreal forest. In the long term, as the climate warms, temperature and its indirect effects via changes in the understory vegetation will play an important role in driving litter decomposition, thereby potentially altering C storage and nutrient cycling. 

  • 10. Elmendorf, Sarah C.
    et al.
    Henry, Gregory H. R.
    Hollister, Robert D.
    Bjork, Robert G.
    Boulanger-Lapointe, Noemie
    Cooper, Elisabeth J.
    Cornelissen, Johannes H. C.
    Day, Thomas A.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Elumeeva, Tatiana G.
    Gill, Mike
    Gould, William A.
    Harte, John
    Hik, David S.
    Hofgaard, Annika
    Johnson, David R.
    Johnstone, Jill F.
    Jonsdottir, Ingibjorg Svala
    Jorgenson, Janet C.
    Klanderud, Kari
    Klein, Julia A.
    Koh, Saewan
    Kudo, Gaku
    Lara, Mark
    Levesque, Esther
    Magnusson, Borgthor
    May, Jeremy L.
    Mercado-Diaz, Joel A.
    Michelsen, Anders
    Molau, Ulf
    Myers-Smith, Isla H.
    Oberbauer, Steven F.
    Onipchenko, Vladimir G.
    Rixen, Christian
    Schmidt, Niels Martin
    Shaver, Gaius R.
    Spasojevic, Marko J.
    Porhallsdottir, Pora Ellen
    Tolvanen, Anne
    Troxler, Tiffany
    Tweedie, Craig E.
    Villareal, Sandra
    Wahren, Carl-Henrik
    Walker, Xanthe
    Webber, Patrick J.
    Welker, Jeffrey M.
    Wipf, Sonja
    Plot-scale evidence of tundra vegetation change and links to recent summer warming2012In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 2, no 6, p. 453-457Article in journal (Refereed)
    Abstract [en]

    Temperature is increasing at unprecedented rates across most of the tundra biome(1). Remote-sensing data indicate that contemporary climate warming has already resulted in increased productivity over much of the Arctic(2,3), but plot-based evidence for vegetation transformation is not widespread. We analysed change in tundra vegetation surveyed between 1980 and 2010 in 158 plant communities spread across 46 locations. We found biome-wide trends of increased height of the plant canopy and maximum observed plant height for most vascular growth forms; increased abundance of litter; increased abundance of evergreen, low-growing and tall shrubs; and decreased abundance of bare ground. Intersite comparisons indicated an association between the degree of summer warming and change in vascular plant abundance, with shrubs, forbs and rushes increasing with warming. However, the association was dependent on the climate zone, the moisture regime and the presence of permafrost. Our data provide plot-scale evidence linking changes in vascular plant abundance to local summer warming in widely dispersed tundra locations across the globe.

  • 11.
    Gavazov, Konstantin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Swiss Federal Institute for Forest, Snowand Landscape Research, WSL SiteLausanne, Lausanne, Switzerland; Laboratory of Ecological Systems ECOS,School of Architecture, Civil and Environmental Engine ering ENAC, EcolePolytechnique Fédérale de Lausanne EPFL,Lausanne, Switzerland.
    Albrecht, Remy
    Buttler, Alexandre
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    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 change2018In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 24, no 9, p. 3911-3921Article in journal (Refereed)
    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-C-14) 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.

  • 12. Granath, Gustaf
    et al.
    Rydin, Håkan
    Baltzer, Jennifer L.
    Bengtsson, Fia
    Boncek, Nicholas
    Bragazza, Luca
    Bu, Zhao-Jun
    Caporn, Simon J. M.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Galanina, Olga
    Galka, Mariusz
    Ganeva, Anna
    Gillikin, David P.
    Goia, Irina
    Goncharova, Nadezhda
    Hájek, Michal
    Haraguchi, Akira
    Harris, Lorna I.
    Humphreys, Elyn
    Jiroušek, Martin
    Kajukalo, Katarzyna
    Karofeld, Edgar
    Koronatova, Natalia G.
    Kosykh, Natalia P.
    Lamentowicz, Mariusz
    Lapshina, Elena
    Limpens, Juul
    Linkosalmi, Maiju
    Ma, Jin-Ze
    Mauritz, Marguerite
    Munir, Tariq M.
    Natali, Susan M.
    Natcheva, Rayna
    Noskova, Maria
    Payne, Richard J.
    Pilkington, Kyle
    Robinson, Sean
    Robroek, Bjorn J. M.
    Rochefort, Line
    Singer, David
    Stenøien, Hans K.
    Tuittila, Eeva-Stiina
    Vellak, Kai
    Verheyden, Anouk
    MichaelWaddington, James
    Rice, Steven K.
    Environmental and taxonomic controls of carbon and oxygen stable isotope composition in Sphagnum across broad climatic and geographic ranges2018In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 15, no 16, p. 5189-5202Article in journal (Refereed)
    Abstract [en]

    Rain-fed peatlands are dominated by peat mosses (Sphagnum sp.), which for their growth depend on nutrients, water and CO2 uptake from the atmosphere. As the isotopic composition of carbon (C-12(,)13) and oxygen (O-16(,)18) of these Sphagnum mosses are affected by environmental conditions, Sphagnum tissue accumulated in peat constitutes a potential long-term archive that can be used for climate reconstruction. However, there is inadequate understanding of how isotope values are influenced by environmental conditions, which restricts their current use as environmental and palaeoenvironmental indicators. Here we tested (i) to what extent C and O isotopic variation in living tissue of Sphagnum is speciesspecific and associated with local hydrological gradients, climatic gradients (evapotranspiration, temperature, precipitation) and elevation; (ii) whether the C isotopic signature can be a proxy for net primary productivity (NPP) of Sphagnum; and (iii) to what extent Sphagnum tissue delta O-18 tracks the delta O-18 isotope signature of precipitation. In total, we analysed 337 samples from 93 sites across North America and Eurasia us ing two important peat-forming Sphagnum species (S. magellanicum, S. fuscum) common to the Holarctic realm. There were differences in delta C-13 values between species. For S. magellanicum delta C-13 decreased with increasing height above the water table (HWT, R-2 = 17 %) and was positively correlated to productivity (R-2 = 7 %). Together these two variables explained 46 % of the between-site variation in delta C-13 values. For S. fuscum, productivity was the only significant predictor of delta C-13 but had low explanatory power (total R-2 = 6 %). For delta O-18 values, approximately 90 % of the variation was found between sites. Globally modelled annual delta O-18 values in precipitation explained 69 % of the between-site variation in tissue delta O-18. S. magellanicum showed lower delta O-18 enrichment than S. fuscum (-0.83 %0 lower). Elevation and climatic variables were weak predictors of tissue delta O-18 values after controlling for delta O-18 values of the precipitation. To summarize, our study provides evidence for (a) good predictability of tissue delta O-18 values from modelled annual delta O-18 values in precipitation, and (b) the possibility of relating tissue delta C-13 values to HWT and NPP, but this appears to be species-dependent. These results suggest that isotope composition can be used on a large scale for climatic reconstructions but that such models should be species-specific.

  • 13. 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
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Decadal warming causes a consistent and persistent shift from heterotrophic to autotrophic respiration in contrasting permafrost ecosystems2015In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, no 12, p. 4508-4519Article in journal (Refereed)
    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 (<5years 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.

  • 14.
    Jansson, Johan
    et al.
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Business Administration.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Personal Norms for Dealing with Climate Change: Results from a Survey Using Moral Foundations Theory2015In: Sustainable Development, ISSN 0968-0802, E-ISSN 1099-1719, Vol. 23, no 6, p. 381-395Article in journal (Refereed)
    Abstract [en]

    Climate change has become one of the main issues in environmental and sustainability discussions during the last decade. Acting to reduce climate change can be viewed as a prosocial behavior, and previous research has found that personal norms are important in explaining these types of behavior, together with other attitudinal factors. In this study we use Moral Foundations Theory (MFT) to explore the antecedents of personal climate change norms together with three attitudinal factors: problem awareness, social norms and adherence to the New Ecological Paradigm. Analyzing data from a nationwide survey (N = 1086) conducted in Sweden, we find that the moral foundations concerning harm and fairness are positively associated with personal climate change norms, whereas authority has a negative relation. However, the moral foundations from MFT contribute less in explaining personal climate change norms compared with the attitudinal factors included in the study. Theoretical and empirical implications are discussed.

  • 15. Keuper, Frida
    et al.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Van Bodegom, Peter M.
    Aerts, Rien
    Van Logtestijn, Richard S.P.
    Callaghan, Terry V.
    Cornelissen, Johannes H . C .
    A race for space?: How Sphagnum fuscumstabilizes vegetation composition during long-termclimate manipulations2011In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 17, no 6, p. 2162-2171Article in journal (Refereed)
    Abstract [en]

    Strong climate warming is predicted at higher latitudes this century, with potentially major consequences forproductivity and carbon sequestration. Although northern peatlands contain one-third of the world’s soil organiccarbon, little is known about the long-term responses to experimental climate change of vascular plant communities inthese Sphagnum-dominated ecosystems.We aimed to see how long-term experimental climate manipulations, relevantto different predicted future climate scenarios, affect total vascular plant abundance and species composition whenthe community is dominated by mosses. During 8 years, we investigated how the vascular plant community of aSphagnum fuscum-dominated subarctic peat bog responded to six experimental climate regimes, including factorialcombinations of summer as well as spring warming and a thicker snow cover. Vascular plant species composition inour peat bog was more stable than is typically observed in (sub)arctic experiments: neither changes in total vascularplant abundance, nor in individual species abundances, Shannon’s diversity or evenness were found in response tothe climate manipulations. For three key species (Empetrum hermaphroditum, Betula nana and S. fuscum) we alsomeasured whether the treatments had a sustained effect on plant length growth responses and how these responsesinteracted. Contrasting with the stability at the community level, both key shrubs and the peatmoss showed sustainedpositive growth responses at the plant level to the climate treatments. However, a higher percentage of mossencroachedE. hermaphroditum shoots and a lack of change in B. nana net shrub height indicated encroachment byS. fuscum, resulting in long-term stability of the vascular community composition: in a warmer world, vascular speciesof subarctic peat bogs appear to just keep pace with growing Sphagnum in their race for space. Our findings contributeto general ecological theory by demonstrating that community resistance to environmental changes does notnecessarily mean inertia in vegetation response.

  • 16.
    Keuper, Frida
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Systems Ecology, Department of Ecological Science, VU University Amsterdam, Amsterdam, The Netherlands ; UR1158 AgroImpact, INRA, Barenton-Bugny, France.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Systems Ecology, Department of Ecological Science, VU University Amsterdam, Amsterdam, The Netherlands.
    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 species2017In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, no 10, p. 4257-4266Article in journal (Refereed)
    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 (N-15-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.

  • 17.
    Keuper, Frida
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Parmentier, Frans-Jan W.
    Lund Univ, Dept Phys Geog & Ecosyst Sci, Lund, Sweden .
    Blok, Daan
    Univ Copenhagen, Ctr Permafrost Dynam Greenland CENPERM, Copenhagen, Denmark .
    van Bodegom, Peter M.
    Vrije Univ Amsterdam, Dept Ecol Sci, Fac Earth & Life Sci, Amsterdam, Netherlands.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    van Hal, Jürgen R.
    Vrije Univ Amsterdam, Dept Ecol Sci, Fac Earth & Life Sci, Amsterdam, Netherlands.
    van Logtestijn, Richard S. P.
    Vrije Univ Amsterdam, Dept Ecol Sci, Fac Earth & Life Sci, Amsterdam, Netherlands.
    Aerts, Rien
    Vrije Univ Amsterdam, Dept Ecol Sci, Fac Earth & Life Sci, Amsterdam, Netherlands.
    Tundra in the rain: Differential vegetation responses to three years of experimentally doubled summer precipitation in Siberian shrub and Swedish bog tundra2012In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 41, no Suppl. 3, p. 269-280Article in journal (Refereed)
    Abstract [en]

    Precipitation amounts and patterns at high latitude sites have been predicted to change as a result of global climatic changes. We addressed vegetation responses to three years of experimentally increased summer precipitation in two previously unaddressed tundra types: Betula nana-dominated shrub tundra (northeast Siberia) and a dry Sphagnum fuscum-dominated bog (northern Sweden). Positive responses to approximately doubled ambient precipitation (an increase of 200 mm year(-1)) were observed at the Siberian site, for B. nana (30 % larger length increments), Salix pulchra (leaf size and length increments) and Arctagrostis latifolia (leaf size and specific leaf area), but none were observed at the Swedish site. Total biomass production did not increase at either of the study sites. This study corroborates studies in other tundra vegetation types and shows that despite regional differences at the plant level, total tundra plant productivity is, at least at the short or medium term, largely irresponsive to experimentally increased summer precipitation.

  • 18.
    Keuper, Frida
    et al.
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    van Bodegom, Peter M.
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Weedon, James T.
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    van Hal, Jurgen
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    van Logtestijn, Richard S. P.
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    Aerts, Rien
    Vrije Univ Amsterdam, Dept Ecol Sci, NL-1081 HV Amsterdam, Netherlands .
    A frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands2012In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 18, no 6, p. 1998-2007Article in journal (Refereed)
    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 (010cm below the thawfront) to those taken from a current rooting zone layer (515cm 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 degrees C (over 120days, 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 (273mgNm-2 and 1348mgNm-2 per growing season for near-surface permafrost at 0.5 degrees C and 11 degrees C respectively, compared to -30mgNm-2 for current rooting zone soil at 11 degrees 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.

  • 19.
    Krab, Eveline J
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Monteux, Sylvain
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Weedon, James T.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Plant expansion drives bacteria and collembola communities under winter climate change in frost-affected tundra2019In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 138, article id UNSP 107569Article in journal (Refereed)
    Abstract [en]

    At high latitudes, winter warming facilitates vegetation expansion into barren frost-affected soils. The interplay of changes in winter climate and plant presence may alter soil functioning via effects on decomposers. Responses of decomposer soil fauna and microorganisms to such changes likely differ from each other, since their life histories, dispersal mechanisms and microhabitats vary greatly.

    We investigated the relative impacts of short-term winter warming and increases in plant cover on bacteria and collembola community composition in cryoturbated, non-sorted circle tundra. By covering non-sorted circles with insulating gardening fibre cloth (fleeces) or using stone walls accumulating snow, we imposed two climate-change scenarios: snow accumulation increased autumn-to-late winter soil temperatures (−1 cm) by 1.4 °C, while fleeces warmed soils during that period by 1 °C and increased spring temperatures by 1.1 °C. Summer bacteria and collembola communities were sampled from within-circle locations differing in vegetation abundance and soil properties.

    Two years of winter warming had no effects on either decomposer community. Instead, their community compositions were strongly determined by sampling location: communities in barren circle centres were distinct from those in vegetated outer rims, while communities in sparsely vegetated patches of circle centres were intermediate. Diversity patterns indicate that collembola communities are tightly linked to plant presence while bacteria communities correlated with soil properties.

    Our results thus suggest that direct effects of short-term winter warming are likely to be minimal, but that vegetation encroachment on barren cryoturbated ground will affect decomposer community composition substantially. At decadal timescales, collembola community changes may follow relatively fast after warming-driven plant establishment into barren areas, whereas bacteria communities may take longer to respond. If shifts in decomposer community composition are indicative for changes in their activity, vegetation overgrowth will likely have much stronger effects on soil functioning in frost-affected tundra than short-term winter warming.

  • 20.
    Krab, Eveline J.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Soil and Environment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Roennefarth, Jonas
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Experimental Plant Ecology, Institute of Botany and Landscape Ecology, Greifswald University, Greifswald, Germany.
    Becher, Marina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Blume-Werry, Gesche
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Experimental Plant Ecology, Institute of Botany and Landscape Ecology, Greifswald University, Greifswald, Germany.
    Keuper, Frida
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. INRA, AgroImpact UR1158, Barenton Bugny, France.
    Klaminder, Jonatan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kreyling, Juergen
    Makoto, Kobayashi
    Milbau, Ann
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biodiversity and Natural Environment, Research Institute for Nature and Forest - INBO, Brussels, Belgium.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Winter warming effects on tundra shrub performance are species-specific and dependent on spring conditions2018In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 106, no 2, p. 599-612Article in journal (Refereed)
    Abstract [en]

    Climate change-driven increases in winter temperatures positively affect conditions for shrub growth in arctic tundra by decreasing plant frost damage and stimulation of nutrient availability. However, the extent to which shrubs may benefit from these conditions may be strongly dependent on the following spring climate. Species-specific differences in phenology and spring frost sensitivity likely affect shrub growth responses to warming. Additionally, effects of changes in winter and spring climate may differ over small spatial scales, as shrub growth may be dependent on natural variation in snow cover, shrub density and cryoturbation. We investigated the effects of winter warming and altered spring climate on growing-season performance of three common and widespread shrub species in cryoturbated non-sorted circle arctic tundra. By insulating sparsely vegetated non-sorted circles and parts of the surrounding heath with additional snow or gardening fleeces, we created two climate change scenarios: snow addition increased soil temperatures in autumn and winter and delayed snowmelt timing without increasing spring temperatures, whereas fleeces increased soil temperature similarly in autumn and winter, but created warmer spring conditions without altering snowmelt timing. Winter warming affected shrub performance, but the direction and magnitude were species-specific and dependent on spring conditions. Spring warming advanced, and later snowmelt delayed canopy green-up. The fleece treatment did not affect shoot growth and biomass in any shrub species despite decreasing leaf frost damage in Empetrum nigrum. Snow addition decreased frost damage and stimulated growth of Vaccinium vitis-idaea by c. 50%, while decreasing Betula nana growth (p < .1). All of these effects were consistent the mostly barren circles and surrounding heath. Synthesis. In cryoturbated arctic tundra, growth of Vaccinium vitis-idaea may substantially increase when a thicker snow cover delays snowmelt, whereas in longer term, warmer winters and springs may favour E. nigrum instead. This may affect shrub community composition and cover, with potentially far-reaching effects on arctic ecosystem functioning via its effects on cryoturbation, carbon cycling and trophic cascading. Our results highlight the importance of disentangling effects of winter and spring climate change timing and nature, as spring conditions are a crucial factor in determining the impact of winter warming on plant performance.

  • 21. Kuhry, P
    et al.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hugelius, G
    Schuur, EAG
    Tarnocai, C
    Potential remobilization of belowground permafrost carbon under future global warming2010In: Permafrost and Periglacial Processes, ISSN 1045-6740, E-ISSN 1099-1530, Vol. 21, no 2, p. 208-214Article in journal (Refereed)
    Abstract [en]

    Research on permafrost carbon has dramatically increased in the past few years. A new estimate of 1672 Pg C of belowground organic carbon in the northern circumpolar permafrost region more than doubles the previous value and highlights the potential role of permafrost carbon in the Earth System. Uncertainties in this new estimate remain due to relatively few available pedon data for certain geographic sectors and the deeper cryoturbated soil horizons, and the large polygon size in the soil maps used for upscaling. The large permafrost carbon pool is not equally distributed across the landscape: peat deposits, cryoturbated soils and the loess-like deposits of the yedoma complex contain disproportionately large amounts of soil organic matter, often exhibiting a low degree of decomposition. Recent findings in Alaska and northern Sweden provide strong evidence that the deeper soil carbon in permafrost terrain is starting to be released, supporting previous reports from Siberia. The permafrost carbon pool is not yet fully integrated in climate and ecosystem models and an important objective should be to define typical pedons appropriate for model setups. The thawing permafrost carbon feedback needs to be included in model projections of future climate change. Copyright © 2010 John Wiley & Sons, Ltd.

  • 22.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Terrestrial Ecology Section, Department of Biology, University of Copenhagen, Denmark.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Global drivers of tree seedling establishment at alpine treelines in a changing climate2018In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 32, no 7, p. 1666-1680Article, review/survey (Refereed)
    Abstract [en]

    1. Alpine and Arctic treeline expansion depends on establishment of tree seedlings beyond the current treeline, which is expected to occur with climate warming. However, treelines often fail to respond to higher temperatures, and it is therefore likely that other environmental factors are important for seedling establishment.

    2. We aimed to analyse our current understanding of how temperature and a range of other environmental drivers affect tree seedling establishment at the alpine and Arctic treelines world-wide and to assess the relative importance of temperature compared with other factors and how they interact.

    3. We collected 366 observations from 76 experimental and observational papers for a qualitative analysis of the role of a wide range of environmental factors on tree seed germination, tree seedling growth, survival and natural occurrence. For a subset of these studies, where the experimental design allowed, we conducted formal meta-analyses to reveal if there were global drivers for different seedling life traits.

    4. The analyses showed that a wide range of abiotic and biotic factors affected tree seedling establishment besides from temperature, including water, snow, nutrients, light and surrounding vegetation. The meta-analyses showed that different seedling life stages do not respond similarly to environmental factors. For example, temperature had positive effects on growth, while tree seedling survival and germination showed mixed responses to warming. Further, warming was as often as not the strongest factor controlling tree seedling establishment, when compared to with one of five other environmental factors. Moreover, warming effects often depended on other factors such as moisture or the presence of surrounding vegetation.

    5. Our results suggest that population dynamics of trees at the alpine and Arctic treeline is responsive to environmental changes and show that there is a clear need for multifactorial studies if we want to fully understand and predict the interplay between warming and other environmental factors and their effect on tree seedling establishment across current treelines.

  • 23.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    The struggle for life at the alpine tree line - Environmental factors acting on tree seedling establishmentManuscript (preprint) (Other academic)
  • 24.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Marie-Charlotte
    Wardle, David
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bryophyte traits explain climate-warming effects on tree seedling establishment2017In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 105, no 2, p. 496-506Article in journal (Refereed)
    Abstract [en]

    Above the alpine tree line, bryophytes cover much of the tundra soil surface in dense, often monospecific carpets. Therefore, when climate warming enables tree seedling establishment above the tree line, interaction with the bryophyte layer is inevitable. Bryophytes are known to modify their environment in various ways. However, little is known about to which extent and by which mechanisms bryophytes affect the response of tree seedlings to climate warming.

    We aimed to assess and understand the importance of bryophyte species identity and traits for tree seedling performance at tree line temperatures and their response to warmer conditions. Seedlings of two common, tree line-forming tree species (Betula pubescens and Pinus sylvestris) were planted into intact cushions of eight common tundra bryophyte species and bryophyte-free soil and grown for 18 weeks at current (7·0 °C) and near-future (30–50 years; 9·2 °C) tree line average growing-season temperatures. Seedling performance (biomass increase and N-uptake) was measured and related to bryophyte species identity and traits indicative of their impact on the environment.

    Tree seedlings performed equally well or better in the presence of bryophytes than in bryophyte-free soil, which contrasts to their usually negative effects in milder climates. In addition, seedling performance and their response to higher temperatures depended on bryophyte species and seedlings of both species grew largest in the pan-boreal and subarctic bryophyte Hylocomium splendens. However, B. pubescens seedlings showed much stronger responses to higher temperatures when grown in bryophytes than in bryophyte-free soil, while the opposite was true for P. sylvestris seedlings. For B. pubescens, but not for P. sylvestris, available organic nitrogen of the bryophyte species was the trait that best predicted seedling responses to higher temperatures, likely because these seedlings had increased N-demands.

    Synthesis. Climatically driven changes in bryophyte species distribution may not only have knock-on effects on vascular plant establishment, but temperature effects on seedling performance are themselves moderated by bryophytes in a species-specific way. Bryophyte traits can serve as a useful tool for understanding and predicting these complex interactions.

  • 25.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Teuber, Laurenz
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Krab, Eveline
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Michelsen, Anders
    Nilsson, Marie-Charlotte
    Wardle, David
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Mosses mediate effects of warmer and wetter conditions on tree seedlings at the alpine tree lineManuscript (preprint) (Other academic)
  • 26.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wardle, David A.
    Nilsson, Marie-Charlotte
    Teuber, Laurenz M.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    The role of bryophytes for tree seedling responses to winter climate change: Implications for the stress gradient hypothesis2018In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 106, no 3, p. 1142-1155Article in journal (Refereed)
    Abstract [en]

    When tree seedlings establish beyond the current tree line due to climate warming, they encounter existing vegetation, such as bryophytes that often dominate in arctic and alpine tundra. The stress gradient hypothesis (SGH) predicts that plant interactions in tundra become increasingly negative as climate warms and conditions become less harsh. However, for seedlings, climate warming might not result in lower winter stress, if insulating snow cover is reduced. We aimed to understand if bryophytes facilitate seedling survival in a changing winter climate and if these effects of bryophytes on tree seedlings comply with the SGH along elevational gradients under contrasting snow conditions. In the Swedish subarctic, we transplanted intact bryophyte cores covered by each of three bryophyte species and bryophyte-free control soil from above the tree line to two field common garden sites, representing current and future tree line air temperature conditions (i.e. current tree line elevation and a lower, warmer, elevation below the tree line). We planted seedlings of Betula pubescens and Pinus sylvestris into these cores and subjected them to experimental manipulation of snow cover during one winter. In agreement with the SGH, milder conditions caused by increased snow cover enhanced the generally negative or neutral effects of bryophytes on seedlings immediately after winter. Furthermore, survival of P. sylvestris seedlings after one full year was higher at lower elevation, especially when snow cover was thinner. However, in contrast with the SGH, impacts of bryophytes on over-winter survival of seedlings did not differ between elevations, and impacts on survival of B. pubescens seedlings after 1year was more negative at lower elevation. Bryophyte species differed in their effect on seedling survival after winter, but these differences were not related to their insulating capacity.Synthesis. Our study demonstrates that interactions from bryophytes can modify the impacts of winter climate change on tree seedlings, and vice versa. These responses do not always comply with SGH, but could ultimately have consequences for large-scale ecological processes such as tree line shifts. These new insights need to be taken into account in predictions of plant species responses to climate change.

  • 27.
    Lett, Signe
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wardle, David
    Nilsson, Marie-Charlotte
    Teuber, Laurenz
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    The impact of bryophytes in mediating tree seedling responses to winter stress: implications for the stress gradient hypothesisManuscript (preprint) (Other academic)
  • 28.
    Monteux, Sylvain
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Weedon, James T.
    Blume-Werry, Gesche
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gavazov, Konstantin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Federal Institute for Forest, Snow and Landscape Research WSL, Lausanne, Switzerland.
    Jassey, Vincent E. J.
    Johansson, Margareta
    Keuper, Frida
    Olid, Carolina
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Long-term in situ permafrost thaw effects on bacterial communities and potential aerobic respiration2018In: The ISME Journal, ISSN 1751-7362, E-ISSN 1751-7370, Vol. 12, no 9, p. 2129-2141Article in journal (Refereed)
    Abstract [en]

    The decomposition of large stocks of soil organic carbon in thawing permafrost might depend on more than climate change-induced temperature increases: indirect effects of thawing via altered bacterial community structure (BCS) or rooting patterns are largely unexplored. We used a 10-year in situ permafrost thaw experiment and aerobic incubations to investigate alterations in BCS and potential respiration at different depths, and the extent to which they are related with each other and with root density. Active layer and permafrost BCS strongly differed, and the BCS in formerly frozen soils (below the natural thawfront) converged under induced deep thaw to strongly resemble the active layer BCS, possibly as a result of colonization by overlying microorganisms. Overall, respiration rates decreased with depth and soils showed lower potential respiration when subjected to deeper thaw, which we attributed to gradual labile carbon pool depletion. Despite deeper rooting under induced deep thaw, root density measurements did not improve soil chemistry-based models of potential respiration. However, BCS explained an additional unique portion of variation in respiration, particularly when accounting for differences in organic matter content. Our results suggest that by measuring bacterial community composition, we can improve both our understanding and the modeling of the permafrost carbon feedback.

  • 29. Ramirez, Kelly S.
    et al.
    Knight, Christopher G.
    de Hollander, Mattias
    Brearley, Francis Q.
    Constantinides, Bede
    Cotton, Anne
    Creer, Si
    Crowther, Thomas W.
    Davison, John
    Delgado-Baquerizo, Manuel
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Elliott, David R.
    Fox, Graeme
    Griffiths, Robert I.
    Hale, Chris
    Hartman, Kyle
    Houlden, Ashley
    Jones, David L.
    Krab, Eveline J.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Maestre, Fernando T.
    McGuire, Krista L.
    Monteux, Sylvain
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Orr, Caroline H.
    van der Putten, Wim H.
    Roberts, Ian S.
    Robinson, David A.
    Rocca, Jennifer D.
    Rowntree, Jennifer
    Schlaeppi, Klaus
    Shepherd, Matthew
    Singh, Brajesh K.
    Straathof, Angela L.
    Bhatnagar, Jennifer M.
    Thion, Cecile
    van der Heijden, Marcel G. A.
    de Vries, Franciska T.
    Detecting macroecological patterns in bacterial communities across independent studies of global soils2018In: Nature Microbiology, E-ISSN 2058-5276, Vol. 3, no 2, p. 189-196Article in journal (Refereed)
    Abstract [en]

    The emergence of high-throughput DNA sequencing methods provides unprecedented opportunities to further unravel bacterial biodiversity and its worldwide role from human health to ecosystem functioning. However, despite the abundance of sequencing studies, combining data from multiple individual studies to address macroecological questions of bacterial diversity remains methodically challenging and plagued with biases. Here, using a machine-learning approach that accounts for differences among studies and complex interactions among taxa, we merge 30 independent bacterial data sets comprising 1,998 soil samples from 21 countries. Whereas previous meta-analysis efforts have focused on bacterial diversity measures or abundances of major taxa, we show that disparate amplicon sequence data can be combined at the taxonomy-based level to assess bacterial community structure. We find that rarer taxa are more important for structuring soil communities than abundant taxa, and that these rarer taxa are better predictors of community structure than environmental factors, which are often confounded across studies. We conclude that combining data from independent studies can be used to explore bacterial community dynamics, identify potential 'indicator' taxa with an important role in structuring communities, and propose hypotheses on the factors that shape bacterial biogeography that have been overlooked in the past.

  • 30.
    Väisänen, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University, Arctic Research Centre at Umeå University.
    Gavazov, Konstantin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University, Arctic Research Centre at Umeå University.
    Krab, Eveline J
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University, Arctic Research Centre at Umeå University. Department of Soil and Environment, Swedish University of Agricultural Sciences, Lennart Hjelms väg 9, SE-750 07 Uppsala, Sweden.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University, Arctic Research Centre at Umeå University.
    The Legacy Effects of Winter Climate on Microbial Functioning After Snowmelt in a Subarctic Tundra2019In: Microbial Ecology, ISSN 0095-3628, E-ISSN 1432-184X, Vol. 77, no 1, p. 186-190Article in journal (Refereed)
    Abstract [en]

    Warming-induced increases in microbial CO2 release in northern tundra may positively feedback to climate change. However, shifts in microbial extracellular enzyme activities (EEAs) may alter the impacts of warming over the longer term. We investigated the in situ effects of 3years of winter warming in combination with the in vitro effects of a rapid warming (6days) on microbial CO2 release and EEAs in a subarctic tundra heath after snowmelt in spring. Winter warming did not change microbial CO2 release at ambient (10 degrees C) or at rapidly increased temperatures, i.e., a warm spell (18 degrees C) but induced changes (P<0.1) in the Q(10) of microbial respiration and an oxidative EEA. Thus, although warmer winters may induce legacy effects in microbial temperature acclimation, we found no evidence for changes in potential carbon mineralization after spring thaw.

  • 31.
    Väisänen, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Arctic Centre, University of Lapland, Rovaniemi, Finland.
    Krab, Eveline J.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Carbon dynamics at frost-patterned tundra driven by long-term vegetation change rather than by short-term non-growing season warming2017In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 136, no 1, p. 103-117Article in journal (Refereed)
    Abstract [en]

    Frost-patterned grounds, such as mostly barren frost boils surrounded by denser vegetation, are typical habitat mosaics in tundra. Plant and microbial processes in these habitats may be susceptible to short-term warming outside the growing season, while the areal cover of barren frost boils has decreased during the past decades due to climate warming-induced shrub expansion. The relative importance of such short-term and long-term climate impacts on carbon (C) dynamics remains unknown. We measured ecosystem CO2 uptake and release (in the field), microbial respiration (in the laboratory), as well as microbial biomass N and soil extractable N in frost boils and the directly adjacent heath in late spring and late summer. These habitats had been experimentally warmed with insulating fleeces from late September until late May for three consecutive years, which allowed us to investigate the direct short-term effects of warming and longer-term, indirect climate effects via vegetation establishment into frost boils. Non-growing season warming increased C uptake at the frost boils in late spring and decreased it in late summer, while the timing and direction of responses was opposite for the heath. Experimental warming had no effects on microbial or ecosystem C release or soil N at either of the habitats. However, C cycling was manifold higher at the heath compared to the frost boils, likely because of a higher SOM stock in the soil. Short-term climate change can thus directly alter ecosystem C uptake at frost-patterned grounds but will most likely not affect microbial C release. We conclude that the C dynamics at frost-patterned grounds under a changing climate depend most strongly on the potential of vegetation to encroach into frost boils in the long-term.

  • 32. Weston, David J.
    et al.
    Turetsky, Merritt R.
    Johnson, Matthew G.
    Granath, Gustaf
    Lindo, Zoë
    Belyea, Lisa R.
    Rice, Steven K.
    Hanson, David T.
    Engelhardt, Katharina A. M.
    Schmutz, Jeremy
    Dorrepaal, Ellen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Euskirchen, Eugenie S.
    Stenoien, Hans K.
    Szovenyi, Peter
    Jackson, Michelle
    Piatkowski, Bryan T.
    Muchero, Wellington
    Norby, Richard J.
    Kostka, Joel E.
    Glass, Jennifer B.
    Rydin, Hakan
    Limpens, Juul
    Tuittila, Eeva-Stiina
    Ullrich, Kristian K.
    Carrell, Alyssa
    Benscoter, Brian W.
    Chen, Jin-Gui
    Oke, Tobi A.
    Nilsson, Mats B.
    Ranjan, Priya
    Jacobson, Daniel
    Lilleskov, Erik A.
    Clymo, R. S.
    Shaw, A. Jonathan
    The Sphagnome Project: enabling ecological and evolutionary insights through a genus-level sequencing project2018In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 217, no 1, p. 16-25Article in journal (Other academic)
    Abstract [en]

    Considerable progress has been made in ecological and evolutionary genetics with studies demonstrating how genes underlying plant and microbial traits can influence adaptation and even 'extend' to influence community structure and ecosystem level processes. Progress in this area is limited to model systems with deep genetic and genomic resources that often have negligible ecological impact or interest. Thus, important linkages between genetic adaptations and their consequences at organismal and ecological scales are often lacking. Here we introduce the Sphagnome Project, which incorporates genomics into a long-running history of Sphagnum research that has documented unparalleled contributions to peatland ecology, carbon sequestration, biogeochemistry, microbiome research, niche construction, and ecosystem engineering. The Sphagnome Project encompasses a genus-level sequencing effort that represents a new type of model system driven not only by genetic tractability, but by ecologically relevant questions and hypotheses.

1 - 32 of 32
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