Assessment of the impact of ground-based skidding on soil physical properties: initial effect and medium-term recovery

Soil profile disturbance and compaction can have a medium-term detrimental impact on soil physical properties. We investigated the recovery of physical properties of disturbed soils on four abandoned skid trails in a deciduous mountain forest in northern Iran. The trails ranged from immediately after skidding operations (IAS), through 5 years and 10 years, to 20 years since forest harvesting. For each recovery period, mean values for soil bulk density, total porosity, macroporosity, and rut depth were assessed for three levels of traffic intensity and two levels of slope gradients, as well as within the general harvesting area and compared to those in untrafficked control areas. Over the 20-year recovery period, skid trails with lower traffic intensity on gentle slopes exhibited mean values that were 16.1% (bulk density), and 3.5 cm (rut depth) greater and 9.5% (total porosity) and 32.7% (macroporosity) lower compared to undisturbed areas; on steep trails with higher traffic intensity, values were 43.1% (bulk density), and 13 cm (rut depth) greater and 30.3% (total porosity) and 64.5% (macroporosity) lower, respectively. During this period (over the 20-year recovery period), surface soil compaction showed recovery in the general harvesting area. These results raise serious concerns about medium-term resilience to traffic on forest soils and indicate the necessity to properly retire skid trails to avoid further soil degradation as trails age.