Combined effects of soil texture and machine operating trail gradient on changes in forest soil physical properties during ground-based skidding

Wood extraction by heavy machinery has always been associated with soil disturbance in mountain forests, and the degree of soil degradation is influenced by several factors, including site and soil characteristics, soil moisture, type of equipment used, and number of machine passes. The effects of ground-based skidding operations on the physical properties of soils with different texture were evaluated at different levels of traffic frequency and trail gradient at two sites in an Iranian temperate forest. The treatments included combinations of three different traffic frequencies (3, 8, and 14 passes of a rubber-tired cable skidder), three levels of trail gradient (< 10%, 10%–20%, and > 20%) and two soil texture classes, clay loam (Site 1) and sandy loam (Site 2). The average gravimetric soil moisture at the time of skidding was 23% (Site 1) and 20% (Site 2). The average dry bulk density and total porosity of the undisturbed soil (control) were 0.71 g cm^–3 and 73.3% at Site 1 (clay loam) and 0.86 g cm^–3 and 59.1% at Site 2 (sandy loam), respectively. At site 1 (fine-textured soil), rutting began after three passes of the skidder, whereas at site 2 (coarse-textured soil), rutting occurred only after eight passes. Independent of the traffic frequency and trail gradient, machine impact on the fine-textured soil caused greater increases in bulk density and rut depth compared to that on the coarse-textured soil. After three skidder passes and independent from trail gradients, dry bulk density at Site 1 increased by 54.8% compared to that of the undisturbed control, and the increase was 45.5% at Site 2. Therefore, medium to fine-textured soils are more susceptible to compaction than coarse-textured soils. Such soils, especially when moist, should be protected using brush mats created from harvesting residues during the forest processing phase.