| Summary: | Extreme weather events can have negative impacts on species survival and community structure when surpassinglethal thresholds. Extreme winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonablywarm air (2–10 °C for 2–14 days), but returning to cold winter climate exposes the ecosystem to lowertemperatures by the loss of insulating snow. Soil animals, which play an integral part in soil processes, may bevery susceptible to such events depending on the intensity of soil warming and low temperatures following theseevents. We simulated week-long extreme winter warming events – using infrared heating lamps, alone or with soilwarming cables – for two consecutive years in a sub-Arctic dwarf shrub heathland. Minimum temperatures werelower and freeze-thaw cycles were 2–11 times more frequent in treatment plots compared with control plots. Followingthe 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 smallersoil dwelling (eu-edaphic) Collembola species dominance to larger litter dwelling (hemi-edaphic) species dominancein 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.
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