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iForest -Biogeosciences and Forestry
IntroductionThe entry of forestry machineries into forest has resulted in some cases in levels of soil disturbance that may have adverse effects on forest ecosystems and subsequently on long-term tree growth (Miller & Anderson 2002, Landsberg et al. 2003, Ilstedta et al. 2004, Murphy et al. 2004. Numerous studies have documented changes in physical soil properties following harvesting machine traffic, such as increased soil compaction, soil displacement, rutting and soil profile disturbances (Adams & Froehlich 1984, Wronski & Murphy 1994, Rab 1996, Rohand et al. 2004, Eliasson 2005, Susnjar et al. 2006, Eliasson & Wasterlund 2007, Horn et al. 2007, Wang et al. 2007, Zenner et al. 2007, Ampoorter et al. 2010, 2012, Picchio et al. 2012. For instance, soil compaction, in which soil pores (especially macropores) are destroyed and surface aggregates are broken down (Ampoorter et al. 2010), depend on the soil texture and moisture at the time of harvest (McNabb et al. 2001, Bock & Van Rees 2002, but also on the number of machines passing on a specific point. Generally, soil compaction occurs in the first ten passes of a vehicle, but most of the compaction occurs in the first three trips; subsequent passes generally have little additional effect (Gayoso & Iroume 1991, Eliasson 2005, Eliasson & Wasterlund 2007, Ampoorter et al. 2010. So it would be appropriate to concentrate machine traffic on carefully designed skid trails in such a way to minimize the affected area and leave most trees undisturbed (Zenner & Berger 2008, Ampoorter et al. 2010.It is unclear, however, to what extent the slope gradient (i.e., longitudinal and transversal slope) of the skid trail and the direction of machine traffic (i.e., uphill vs. downhill) could modify the impacts of vehicle traffic on the soil. Whereas some studies have failed to document an effect of slope on bulk density following harvesting traffic (e.g., Sidle & Drlica 1981, Jamshidi et al. 2008, others have shown that soil disturbances increase with slope gradient (Ezzati et al. 2012), possibly because machines slip more and remain in a given place for a longer time on steep terrain, causing soil pulling and dragging (Gayoso & Iroume 1991). Thus, steepness may have an even stronger effect on soil disturbance than that of the soil moisture (Krag et al. 1986), but possibly less so than that of the traffic intensity (Ezzati et al. 2012).Soil disturbance is also strongly affected by the type of harvesting equipment used. Small-scale harvesting technology, such as farm tractors or better small skidder or other specific machines (such as ATV), is thought to result in lower levels of residual stand and soil damage because the equipment is typically smaller and lighter than regular skidders (Updegraff & Blinn 2000, Savelli et al. 2010, Spinelli & Magagnotti 2012. A recent study dealing with soil compaction has found that the highest relative increase in bulk density was associated with the use of bulldozers, the lowest with the use of a forwarder, a...
