Effects of traffic intensity and travel speed on forest soil disturbance at different soil moisture conditions

ABSTRACT Machine traffic during wood harvesting causes damage to forest soils. Whereas increased soil damage has been associated with increased traffic frequency and soil water content, the impact of travel speed on soil disturbance, still remains unclear. We examined the effects of traffic frequency, travel speed, and soil water content on the dry bulk density, total porosity and rut depth of skidding trail soil in an Iranian temperate forest. The studied treatments included combinations of five traffic frequencies (1, 3, 7, 10, and 15 passes) of a rubber-tired skidder, three levels of travel speed (1, 3, and 5 m s−1) and two levels of soil water content (18% and 31%). The impact on soil properties was greatest during the initial skidder passes. At higher water content, only one skidder pass was required to cause substantial increases in soil bulk density compared to the control plots, at all travel speed levels. Regardless of traffic frequency and water content, a lower travel speed caused substantially greater increases in dry bulk density and greater decreases in soil porosity than did a higher travel speed. Furthermore, higher soil water content resulted in deeper ruts at all combinations of traffic intensity and travel speed. In conclusion, fewer skidder passes at a slower travel speed were required to achieve substantial soil disturbance than was the case at a faster travel speed, possibly due to the higher amount of vibration, and consequently compaction exerted during a machine pass at a slower speed.