Impact of Multiple Ecological Stressors on a Sub-Arctic Ecosystem: No Interaction Between Extreme Winter Warming Events, Nitrogen Addition and Grazing
Bokhorst, Stef; Berg, atty P.; Edvinsen, Guri K.; Ellers, Jacintha; Heitman, Amber; Jaakola, Laura; Mæhre, Hanne K.; Phoenix, Gareth K.; Tømmervik, Hans; Bjerke, Jarle W.
Peer reviewed, Journal article
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Date
2018Metadata
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- Scientific publications [1437]
Original version
10.3389/fpls.2018.01787Abstract
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 (CN) 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
CN 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 ofWWevents, and eicosadienoic
acid (C20:2) was lower when plants were exposed to the combination of WW and CN.
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 CN 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 thereforebe even stronger. In addition, CN 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.