Soil compaction in final harvest operations using medium-sized cable skidders: results of an experimental study with controlled payloads

Introduction Ground-based timber extraction is one of the most important harvesting methods used around the world, but it also causes negative effects on forest soils, to a degree that depends on machine mass, soil condition, and operational decisions. Skidding, in particular, may cause compaction in the deeper soil layers and soil mixing at the surface due to the way timber is extracted. Although several studies have examined soil compaction following timber extraction, few have been conducted under controlled measurement and comparison conditions; in addition, the heterogeneity of soil conditions makes it unlikely that existing results can be applied to the wide variety of conditions encountered in regular forest operations. Methods In this study, a controlled experiment was set up to evaluate the effect of successive machine passes on soil compaction, measured as resistance to penetration, specifically accounting for loads typical to selective, final harvests. Measurements of resistance to penetration were taken at 10 control points and then at 10 points located on the wheel footprints after the first, third, fifth, and tenth loaded machine passes, while the machine payload remained constant and there was no ground slope. Variation in resistance to penetration from 0 to 80 cm depth was visually assessed, and the data were partitioned for comparison into 5 cm soil layers from the surface downward. Results and discussion While the first three passes caused significant but small changes in the upper soil layer, more important changes were found after the fifth and tenth machine passes. Five passes caused an increase in resistance to penetration of about 1.0 MPa, particularly in the first 15 cm of soil. After 10 machine passes, resistance to penetration increased further, particularly in the soil layer between 5 and 15 cm depth, although not statistically significant, increases in mean resistance to penetration were also found below the first 15–20 cm of soil. Under the specific conditions of this study (flat terrain, luvisol, full payload, autumn soil moisture), three loaded machine passes did not appear to cause persistent severe soil compaction. This finding should not be generalized without accounting for differences in soil moisture, terrain slope, and machine type. The results of this study are useful for planning, since the number of passes over the same skid trail may be controlled through a well-developed trail network and careful operational planning so as to avoid irreversible soil compaction.