Extreme winter warming events more negatively impact small rather than large soil fauna: shift in community composition explained by traits not taxa S. BOKHORST* † , G. K. PHOENIX*, J. W. BJERKE ‡ , T. V. CALLAGHAN* § , F. HUYER-BRUGMAN ¶ and M. P. BERG ¶ *Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK, †Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, S901-83 Umea ˚, Sweden, ‡FRAM – High North Research Centre on Climate and the Environment, Norwegian Institute for Nature Research (NINA), NO-9296 Tromsø, Norway, §Royal Swedish Academy of Sciences, PO Box 50005, 104 05 Stockholm, Sweden, ¶Department of Ecological Science, section Animal Ecology, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Abstract 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 unsea- sonably 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. Fol- lowing 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 domi- nance 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 eco- system development. Keywords: Acari, Arctic, climate change, Collembola, community weighted mean, extreme events, freeze-thaw, soil arthropods, traits, warming experiment, winter Received 26 July 2011; revised version received 21 September 2011 and accepted 21 September 2011 Introduction Extreme weather events can have strong negative impacts on species survival and community develop- ment if they surpass lethal thresholds (Gaines & Denny, 1993). The impacts of extreme weather events, especially during winter, are not well understood for Arctic ecosystems, unfortunately so, as extreme weather is likely to increase in frequency in the near future due to climate warming in this region of the Earth (ACIA 2005; Christensen, 2007). Perhaps the most profound of extreme winter climatic changes in the Arctic are extreme winter warming events. During these events, temperatures increase rapidly to well above freezing and may remain so for typically a week long period (Phoenix & Lee, 2004). Such warming events can result in near complete snow melt across large regions, and also frequently observed are ice lay- ers in the snow or on the soil surface (Coulson, 2000; Johansson, 2011). Return of freezing temperatures can also be rapid, leaving ecosystems – unprotected from loss of the insulating snow cover – exposed to sudden extreme cold. Exposure to extreme cold damages vege- tation (Robinson, 1998) as also shown in the simulation experiments that were run in sub-Arctic Abisko (Bok- horst, 2008) leading to extensive shoot mortality, Correspondence: S. Bokhorst and M. P. Berg, tel. + 46 090 786 8614, fax + 090 786 8166, e-mail: matty.berg@falw.vu.nl; stef.bokhorst@slu.se 1152 © 2011 Blackwell Publishing Ltd Global Change Biology (2012) 18, 1152–1162, doi: 10.1111/j.1365-2486.2011.02565.x