Impact of forest site preparation on soil functions in a temperate alluvial forest

Alluvial forests in the temperate zone of Europe are frequently changing drastically in their hydrological regime, vegetation composition and structure, and disturbance dynamics. Causes are river regulations, historic land use, recent forest management, and introduced species such as Solidago canadensis agg. or the pathogenic fungus Hymenoscyphus fraxineus causing ash dieback. Climate change increases the scale of these changes. As a result, like in the present case study, pre-mature stands dominated by Pica abies or Fraxinus excelsior have to be clearcut. In order to achieve a tree species composition which is adapted to the altered site conditions and still economically desirable, tree planting in a larger scale is inevitable. To control competing vegetation (e.g., Solidago, Clematis, Rubus…), site preparation (mulching and tilling of planting strips) was deemed to be necessary but is discussed controversially. Effects of site preparation on indicators for soil functions were compared for the two dominating soil types, Fluvisols and Rendzic Leptosols, using a chronosequence approach. The following key results were obtained: (1) Soil type has a significant effect upon most indicators. (2) Areas treated ≥5 years ago have a significantly higher bulk density (and thus, despite partly decreased C-concentration, higher C-stocks) in the 20 cm topsoil indicating compaction. (3) Tilling strips significantly impact anecic earthworm abundance (+) compared to areas only mulched. (4) Effects of site preparation on organic C concentration (-), C/N-ratio (-), ratio of microbial to organic carbon (+), abundance of anecic earthworms (+) and hydraulic conductivity estimated from pedotransfer functions (-) were mainly significant for Rendzic Leptosols. This may reflect the mobilization of accumulated forest floor, which was present in mature spruce stands on Rendzic Leptosols but not on Fluvisols. (5) The ground vegetation shows an expected response to clearing (increased cover of light demanding species including Solidago canadensis agg.). Effects of site preparation could not be separated from clearing effects. (6) Nitrate concentrations in seepage are below drinking water standards and show no clear treatment effect, though highest values were found in declining spruce stands on Leptosols.