Impacts of off-road traffic on soil physical properties of forest clear-cuts: X-ray and laboratory analysis

Due to the great year-round demand for forest products, off-road forestry traffic occurs even when the ground is susceptible to soil compaction and rutting. We investigated the impacts of repeated passes with a laden forwarder (34 Mg) on the soil physical properties of two clear-cuts on stony till soils in northern Sweden. Core samples (n = 71) were collected from the top 5 cm of mineral soil in and beside wheel tracks, after six passes with the forwarder. Soil physical properties were quantified using classical soil physical analyses and X-ray tomography. The hydraulic conductivity was 70% lower in the wheel tracks than in the soil beside. The X-ray image analysis indicated that this was due to the smaller total volume and lower connectivity of structural pores (φ > 60 µm). Total porosity was 24% and 12% lower in the tracks at the two sites respectively, and mean bulk density was 1.39 g cm−3 in the tracks, compared to 1.13 g cm−3 beside them. To conclude, traffic changed the soil physical properties in a way that may lead to longer periods of high water content in the wheel tracks, increased risk of surface runoff and insufficient aeration for optimal seedling growth.