Changes in mesofauna abundance in peat soils drained for forestry

Soil mesofauna community structure was studied in a drainage-succession continuum of peatland sites supporting Scots pine ( Pinus sylvestris L.) dominated stands in southern Finland. The numbers of Enchytraeidae, Collembola, Oribatida, Mesostigmata and Prostigmata in an 8-cm-deep surface peat layer were compared among a pristine undrained pine mire site, comparable sites drained for forestry 12, 26 and 60 years earlier, and a 42-year-old drained site re-wetted two years earlier. Drainage and the consequent water-level draw-down clearly increased the numbers of all soil animals studied, the numbers showing highly significant correlation with the water-table level of the sites. On the older drained sites, the populations were generally about ten times higher than on the undrained site; for Collembola, almost 100 times higher. After re-wetting, the numbers dropped abruptly, falling between those of the undrained and the youngest drained site. Oribatida were the most frequently found animals on the sites studied: on the undrained site almost 60% of the total number were these mites, the relative proportion somewhat decreasing along the drainage succession. The proportion of Enchytraeidae also decreased slightly after drainage, whereas that of Collembola clearly increased. Most of the soil fauna was found close to the soil surface. In general, >80% of Collembola, Oribatida, Mesostigmata and Prostigmata, and >60% of Enchytraeidae were found in the topmost 4-cm layer. In the course of the drainage succession, the soil fauna community structure became more similar to that of upland sites with similar tree-stand growth potential.