Soil Compaction and Porosity Changes Caused During the Operation of Timberjack 450C Skidder in Northern Iran

Skidding by means of heavy forestry machinery can affect soil physical properties. We assessed the effects of ground based skidding on soil bulk density and total porosity under the Iranian mountainous forest conditions. Treatments included a combination of four levels of traffic intensity (1, 3, 6, and 15 passes) of a Timberjack 450C rubber skidder and two levels of slope (<20% and >20%). The bulk density was highest in samples taken in the wheel tracks and between them, and decreased towards both ends of the track (0.5 to 4 m). The results showed that bulk density increased with traffic frequency, while total porosity decreased. Average soil bulk density ranged from 0.96 g cm (after one machine pass and slope <20%) up to 1.41 g cm (after 15 machine passes and slope >20%) on the skid trail, while the respective value was 0.7 g cm for the undisturbed area. On compacted soil, total porosity at the 0–10 cm depth decreased by 37% compared with non-compacted soil. The results showed that slope steepness had a strong effect on the soil disturbance, with the critical value for bulk density occurring after 15 machine passes at slope <20% and six machines passes at slope >20%. The impacts of soil compaction could be evidenced in a distance of up to 2 m from the end of the skidding trail. The latter finding suggests that special interest in the form of managerial measures should be taken during the skidding operations in an effort to minimize the adverse effects of ground based skidding on the physical properties of the soil.