The marginal influence of forest tree species on soils physicochemical properties. A small-scale approach in a granitic mountainous watershed

Forest tree species (FTS) influence on soils remains difficult to disentangle from environmental variability under “natural forest” conditions. Here we investigated the influence of 4 FTS (European beech, silver fir, Norway spruce and mixed beech-fir) on soil physicochemical properties in a geologically homogeneous (granitic) acidic mountainous watershed. To integrate landscape organization, each FTS was investigated along a ridge-slope-lowland gradient. Soil parameters were investigated across the three upper soil horizons. Edaphic variability was primarily structured by soil horizon differentiation and podzolization-related processes. Topography exerted a broader confounding influence on soil parameters than FTS, especially on physical properties except for aggregation, which showed clearer species-related patterns (macro-aggregation: beech ≈ fir > spruce > mixed). FTS-influence remained moderate and mainly expressed in the surface horizon. Chemically richest soils occurred under beech, whereas the most acidified and podzolized solum were found under pure fir, with acidity mitigation under mixed stands all together leading to the following acidification / podzolization trend: fir > mixed > spruce > beech. These results suggest that at such small environmental spatial scale, on granitic coarse textured base poor soils: conventional soil bulk properties are primarily controlled by geological, soil-landscape organization and pedogenetic processes. Tree species effects have more subtle influences on soil properties and mainly in surface horizons. Detecting such low-magnitude contributions therefore requires spatially reduced experimental designs and careful consideration of parameter interrelations. Overall, our results support the use of natural forest studies as complementary to common garden experiments for assessing tree species effects across realistic environmental conditions.