Effect of the longitudinal slope of trail and number of mule passes on forest soil compaction during wood transportation

Animal trails are the primary wood hauling trails from stump location to the log depot. These trails are created by animal passes. Incomplete access forest roads, low timber volume per hectare and environmental pressure in unstable and steep terrain cause to use mule logging system in some regions of northern forests of Iran. The aim of this research was to investigate the effects of the longitudinal slope of trail used by mule for wood extraction and number of mule passes on soil compaction and porosity. In addition, the optimum soil moisture was determined for scheduling mule logging in forest to reduce site damage.
A mule trail with downward direction was selected in Saad Abad forestry plan. Trail was classifies into 0-10, 10-20 and >20% with respect to longitudinal slope and classified into 2, 4, 8 and 12 passes with respect to number of mule passes. In each class the soil bulk density or compaction, porosity percentage and soil moisture at mule spoor were sampled using metal cylinder and then laboratory measurements. Experiments were done based on factorial design with two main factors of mule passes in four levels and longitudinal slope in three levels. Totally, 12 treatments plus control treatment was performed in SAS software.
Findings showed that soil bilk density increased and soil porosity decreased with increasing the longitudinal slope of trail. Moreover, soil bulk density increase rapidly with increasing mule passes. In 8th pass soil reach to its maximum bulk density and then increasing passes hadn’t significant effect of soil compaction. Interaction effects of passes and longitudinal slopes on soil moisture were significant at probability level of 99%. R2 values showed that variations in most models could describe more than 80% of variation in relationship of soil bulk density and soil moisture. The value of optimum moisture for reducing soil compaction by mule was 35-40%.
It was concluded that in steep slopes and high number of passes the rate of soil compaction was more, therefore before wood extraction in such conditions it is necessary to determine the optimum moisture of soil to schedule logging operation and reduce damage to soil.