Soil disturbances are unavoidable in ground-based skidding operations and it is well-known that the extent and severity of disturbances increase with increasing traffic frequency of harvesting equipment and soil conditions (e.g. soil moisture at the time of harvesting). It is unclear, however, how different traffic intensities and soil moisture contents affect the extent, size and intensity of damage inflicted upon residual trees and how these factors might further interact with the layout of skid trails. Skid trail layout in this study is concerned with the horizontal course of the trail that encompasses straight sections and curves of different deflection angles (i.e. narrower and wider curves). Due to expected rolling of logs and vehicle off-tracking in curves, we hypothesised that damage to residual trees would be greater in curves than on straight sections of skid trails and that damage would be greatest in the narrowest curves. We further hypothesised that the relationship between tree damage and curvature would hold regardless of traffic intensity and soil condition. To test these hypotheses, we conducted a survey of the residual stand damage caused by ground-based skidding operations for different trail curvatures and traffic intensities at two Hyrcanian forest sites in northern Iran with different soil conditions. The studied treatment combinations included three curvatures of the skid trail (i.e. narrow curve with a deflection angle of 60°–75°, wide curve with a deflection angle of 110°–130°, no curve (straight trail)), and three traffic frequencies (low, medium and high traffic intensity) of a rubber-tyred cable skidder operated on a clay soil with 18% soil moisture content (Site 1) and a clay-loam with 33% soil moisture content (Site 2). The percentage of damaged trees varied between 45% (Site 1) and 68% (Site 2). The results showed that the number of damaged trees as well as the distance from the skid trail edge to which trees were damaged increased significantly at both sites with increasing traffic frequency, increasing curvature (i.e. narrowing curve) and their interaction. Further, location, size and intensity of damage to residual trees varied significantly with curvature, traffic intensity and soil condition. At each traffic frequency, narrow curves exhibited more damaged trees and damaged trees were found at greater distances to the edge of the skid trail than in wide curves. The average damage area was 86 cm 2 and 125 cm 2 in Sites 1 and 2, respectively. The significant effect of trail curvature on the location, size and intensity of damage of residual trees has major implications for the design of a skid trail system capable of preserving residual tree quality.
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