Soil compaction and water balance in skid trails: a three-year analysis of German forest sites

Skid trails in forests experience repeated traffic by heavy machinery, leading to pronounced soil compaction. Wheeling is often visible at the soil surface in the form of deep ruts and lateral soil displacement. These structural changes are not restricted to the topsoil but also affect soil water dynamics down to the subsoil. This study aimed to quantify the effects of skid trails on soil physical properties and water dynamics compared to adjacent areas (within the lane and 50 cm outside the wheel track) and unwheeled forest soil.

Three sites in Lower Saxony, Germany, were investigated: two sites in the Solling region (beech and spruce) and one site near Braunschweig (oak). Soil samples were collected at 15, 30, 45, and 60 cm depth in each plot to determine bulk density and soil water retention characteristics using the simplified evaporation method. Simultaneously, soil moisture was monitored at the same depths, and soil water tension was measured at 30 and 60 cm depth from July 2024 to February 2026.

At the Solling sites, skid trails exhibited the highest compaction, with bulk densities reaching up to 1.62 g cm⁻³ in the topsoil and 1.75 g cm⁻³ in the subsoil. Adjacent areas showed moderate compaction (up to 1.46 g cm⁻³), whereas unwheeled soils remained comparatively loose, ranging from 1.17 g cm⁻³ at 15 cm depth to 1.62 g cm⁻³ at 60 cm depth. Macroporosity in wheeled soils was significantly reduced by up to more than one order of magnitude down to the subsoil, while mesopores were only slightly affected. This effect was less pronounced in adjacent areas.

Soil moisture monitoring at the Solling sites showed that skid trails were consistently wetter throughout the year. During winter months, waterlogging occurred, while in summer skid trails remained moist, whereas unwheeled soils experienced pronounced drying. At the Braunschweig site, differences in soil moisture were less pronounced, with only slightly higher water contents in wheeled soils.

The study highlights the strong local effects of skid trails on soil structure and water balance. Soil compaction leads to significant alterations of the pore size distribution, resulting in reduced hydraulic conductivity and increased soil moisture. These changes limit the potential for soil-friendly machine traffic, emphasizing the need for adapted forest management strategies to mitigate soil compaction in skid trails.