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Soil disturbance caused by different skidding methods in mountainous forests of Northern Iran

Harvest traffic with heavy equipment causes damage to forest soils. Whereas increased soil damage has been reported with increasing harvest equipment traffic and on increasing slope gradients, it is unclear how much soil damage is caused by different types of harvesting equipment. We visually assessed 16 categories of soil surface disturbances and measured soil bulk density, porosity, and rut depth on skid trails following increasing harvest traffic intensities by two rubber-tired skidders, a crawler skidder, and a mule. Regardless of the skidding type, the most severe soil disturbances were observed where equipment traffic was most frequent and on steeper slope gradients. The different skidding equipment resulted in different proportions of disturbed soil areas that ranged from 61.5% of harvested area for mule hauling to 70.3% for skidding with a crawler skidder, 76.6% for skidding with a Timberjack 450C skidder, and 87.1% for skidding with a TAF E655. Shallow disturbances, slash cover, and apparent compaction (i.e. a deep soil disturbance) were the most common types of machine-induced soil disturbance. Mule hauling did not cause any deep soil disturbance. Results showed that bulk density increased with traffic intensity regardless of the skidding method used and the slope of the skid and mule trails. Porosity decreased in the surface layer (0–10 cm) with increasing number of passes and the impact increased with increasing slope. Rutting was not observed when skidding with a crawler skidder or hauling with a mule. The results indicated that equipment type, traffic intensity, and slope gradient had strong effects on the physical properties of the soil.