Soil disturbance following four wheel rubber skidder logging on the steep trail in the north mountainous forest of Iran

Reduced soil porosity, water content, forest floor mass and rutting are four common disturbances during timber harvesting and skidding. The present study was conducted to evaluate effects of skid trail slope and traffic frequency on soil disturbance in a temperate mountainous forest. Four levels of traffic (3, 7, 14 and 20 passes of a rubber skidder HSM 904), and three levels of slopes (gentle <10%, moderate 10-20%, steep >20%) were applied in three replicates consequently, 36 plots with 10m long by 4m wide were utilized in the study. All samples were taken from four randomized 4m-transects in each plot across the wheel track perpendicular to the direction of travel with 2m buffer zone between lines to avoid interactions. Results showed that during skidding total porosity, soil water and Forest floor decreased significantly (P <=0.05). Average total porosity on the treatments of >20% slope reduced significantly (P <=0.05) in comparison with those of <20% slope through all level of traffics. The lowest total porosity was identified in treatment with slope >20% and 14 passes by soil compaction. Soil water content was decreased up to 45% after skidding in comparison with undisturbed area. There was a negative correlation between water content and skid trail slope. The forest floor mass decreased on steep trail during skidding in all traffics. The amount of forest floor mass on the treatments under the 7 passes and slopes of >20% treatments (437.6kgha⁻¹) was significantly (P <=0.05) lower than those under the 14 passes and slopes of <10% (841.4kgha⁻¹). Rut depth were recorded in 14 passes and increased with slope (18cm<10%, 27.5cm in 10-20% and 34.5cm in >20%). In 3 passes, increase of skid trail slope from 10% to 20% corresponded to a decrease of 15%, 22%, and 67% in total porosity, water content and forest floor mass, respectively. The results indicated that slope steepness had a strong effect on the soil physical properties. In the present study, the skidder extracted different volume of logs during skidding operation. In order to obtain better interpretation the skidder must carry a fixed load during operation.