Long-term fluctuation of the soil fauna (Collembola and Oribatida) at groundwater-near sites in an alder wood

Collembola and oribatid mites were investigated at four sites along a cross section in an alder wood at a lake margin (northern Germany) over a period of 7 years. Monthly samples of the litter and the mineral layer were taken. Additionally, fluctuations of groundwater level, soil moisture, precipitation and soil temperature at 2 cm depth were measured. The alder wood was characterised by a depression between the lake margin and the foot of a hill slope, where waterlogged periods occurred. Groundwater level was the main environmental factor influencing the composition of collembolan and oribatid mite assemblages. Climatic factors, e.g. July temperature and July precipitation were also found to be significant factors, but with a much lower influence. Collembola predominantly showed higher variation in time than in space indicating that more collembolan species migrate within the investigated cross section or react with higher abundance fluctuations on the groundwater level changes, while Oribatida had a higher part of space variation, indicating that migration potential is lower and the environmental gradient is of higher influence on the distribution. Reaction time of soil fauna species on the groundwater fluctuation varies between 1 and 12 months. Retreat of Collembola from the waterlogged situation was between 4 and 6 months in the litter layer and 3 months in the mineral layer. Recolonisation of the waterlogged site lasted approximately 12 months. A positive reaction by precipitation was observed in four collembolan species that was 1 month in the edaphic species and between 9–10 months in the larger litter dwelling species. Only few oribatid mites reacted on the waterlogged situation.