Influence of slope on physical soil disturbance due to farm tractor forwarding in a Hyrcanian forest of Northern Iran

Tractor-based systems are the most common type of small-scale timber harvesting equipment in the Hyrcanian forest, where farm tractors equipped with a 2-wheel trailer typically perform forwarding operations for pulpwood and fuelwood. The objective of this study was to evaluate the effects of different frequencies of machine passes on soil bulk density (BD), penetration resistance (PR) and total porosity (TP) in different slope gradient and forwarding direction (i.e., uphill vs. downhill). The study, performed on a brown soil with a silt loam to loamy texture, was designed as a factorial experiment with three slope gradient/forwarding direction classes (0-10% downhill forwarding slope, 10-20% downhill forwarding slope and 0-10% uphill forwarding slope) and four traffic intensity classes: 0 (undisturbed control area), 1-4, 5-10 and >10 ma chine passes. Responses were assessed at four soil profile depth classes (5, 10, 15 and 20 cm). BD and PR significantly increased and TP significantly decreased with increasing machine traffic, with magnitudes of change that were not consistently greater in the upper 5 cm compared to the 20 cm soil profile depth class. Whereas the majority of changes in BD and TP occurred after fewer than 5 passes, large increases in PR occurred even after 10 passes. Changes of BD, PR and TP in response to machine traffic differed significantly among slope gradient/forwarding direction classes. Increases in BD and PR and decreases in TP were largest on the 0-10% uphill forwarding slope, followed by the 10-20% downhill and 0-10% downhill forwarding slope. We conclude that the application of farm tractors in forwarding operations should be limited to gentle slopes and uphill forwarding with these machines should be avoided.