How extreme is an extreme climatic event to a subarctic peatland springtail community?

Extreme climate events are increasing in frequency and duration and may directly impact belowground foodwebs and the activities of component soil organisms. The soil invertebrate community, which includes keystone decomposers, might respond to these newly induced soil microclimate conditions by shifts in density, species composition, spatial patterning and/or functional traits. To test if and how short-term extreme climatic conditions alter the structure, the vertical stratification and the community weighted trait means of the springtail (Collembola) community in sub-arctic peatbogs, we experimentally subjected Sphagnum peat cores in a field setting to factorial treatments of elevated temperature and episodically increased moisture content. The large precipitation peaks did not affect the springtail community, but an average soil temperature increase of 4°C halved its density in the shallower peat layers, mainly caused by the reduced dominance of Folsomia quadrioculata. A hypothesized net downward shift of the surface-dwelling springtail community, however, was not observed. We observed species-specific responses to warming but the overall community composition in subsequent organic layers was not significantly altered. Although the effects of an extreme warming event on density, species composition and vertical stratification pattern seemed subtle, functional trait analysis revealed directional community responses, i.e. an overall increase of soil-dwelling species due to warming, even though warming did not alter layer-specific community weighted trait means. We suggest that subtle changes in moisture conditions, due to increased evapotranspiration, have decreased typically surface-dwelling species relative to soil-dwelling species. The extent to which this directional change in the community is maintained after an extreme event, and its costs for the community's resilience to multiple sequential extreme events will consequently determine its longer-term effects on the community and on ecosystem functioning.