Impact of Multiple Ecological Stressors on a Sub-Arctic Ecosystem: No Interaction Between Extreme Winter Warming Events, Nitrogen Addition and Grazing

Stef Bokhorst*, Matty P. Berg, Guro K. Edvinsen, Jacintha Ellers, Amber Heitman, Laura Jaakola, Hanne K. Maehre, Gareth K. Phoenix, Hans Tommervik, Jarle W. Bjerke

*Corresponding author for this work

    Research output: Contribution to JournalArticleAcademicpeer-review


    Climate change is one of many ongoing human-induced environmental changes, but few studies consider interactive effects between multiple anthropogenic disturbances. In coastal sub-arctic heathland, we quantified the impact of a factorial design simulating extreme winter warming (WW) events (7 days at 6–7°C) combined with episodic summer nitrogen (+N) depositions (5 kg N ha-1) on plant winter physiology, plant community composition and ecosystem CO2 fluxes of an Empetrum nigrum dominated heathland during 3 consecutive years in northern Norway. We expected that the +N would exacerbate any stress effects caused by the WW treatment. During WW events, ecosystem respiration doubled, leaf respiration declined (-58%), efficiency of Photosystem II (Fv/Fm) increased (between 26 and 88%), while cell membrane fatty acids showed strong compositional changes as a result of the warming and freezing. In particular, longer fatty acid chains increased as a result of WW events, and eicosadienoic acid (C20:2) was lower when plants were exposed to the combination of WW and +N. A larval outbreak of geometrid moths (Epirrita autumnata and Operophtera brumata) following the first WW led to a near-complete leaf defoliation of the dominant dwarf shrubs E. nigrum (-87%) and Vaccinium myrtillus (-81%) across all experimental plots. Leaf emergence timing, plant biomass or composition, NDVI and growing season ecosystem CO2 fluxes were unresponsive to the WW and +N treatments. The limited plant community response reflected the relative mild winter freezing temperatures (-6.6°C to -11.8°C) recorded after the WW events, and that the grazing pressure probably overshadowed any potential treatment effects. The grazing pressure and WW both induce damage to the evergreen shrubs and their combination should therefore be even stronger. In addition, +N could have exacerbated the impact of both extreme events, but the ecosystem responses did not support this. Therefore, our results indicate that these sub-arctic Empetrum-dominated ecosystems are highly resilient and that their responses may be limited to the event with the strongest impact.

    Original languageEnglish
    Article number1787
    Pages (from-to)1-13
    Number of pages13
    JournalFrontiers in Plant Science
    Publication statusPublished - 30 Nov 2018


    We would like to thank all people who provided invaluable assistance during the field and lab work conducted for this research: Manuel Ballesteros, Dick Bokhorst, Ellen Elverland, Vigdis Frivoll, Trond Vidar Johnsen, Marit Jørgensen, Kristian Laustsen, Leidulf Lund, Geraldine Mabille, Kjell Nilsen, Ilona Peltoniemi, Martina Schwartzmüller, Laura Stendardi, Leif Einar Støvern, Rachael Treharne, Grzegorz Wierzbinski, and Matthias Zielke. We thank the two reviewers for their constructive comments. Funding. This work was financed by a grant from the Research Council of Norway (Grant No. 225006) to JB and SB, and by financial support from the Flagship Programme “Effects of climate change on terrestrial ecosystems, landscapes, society and indigenous peoples” of FRAM – High North Research Centre for Climate and the Environment (Grant No. 362206), and the Norwegian Institute for Nature Research.

    FundersFunder number
    Norges forskningsråd225006


      • CO fluxes
      • Cryptogam
      • Fatty acids
      • Frost
      • Geometrid moth
      • Herbivory
      • Multiple stress
      • Snow


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