Combined effects of traffic intensity, skid trail slope, skidder type, and soil moisture content on soil degradation in the Hyrcanian forest of Iran

Skidding operations affect soil physical properties, which may impact soil sustainability and forest productivity. While the relationship among harvest machine traffic, slope gradients, and increased soil damage has been well-investigated, it is still unclear how soil damage due to repeated passes of different skidder types is interacting with soil moisture and slope conditions. We examined dry bulk density (BD), total porosity (TP), and rutting depth (RD) of skid trail soil in an Iranian temperate forest. The study took into consideration a combination of five different traffic intensities (TI) (1, 4, 8, 12, and 15 passes), two levels of slope gradients (SGs) (& LE;20% and > 20%), two rubber-tired skidder types (STs) (Timberjack 450C and TAF E655), and two soil moisture contents (SMC) (18% and 31%). Results showed that changes in BD and TP were mainly related to TI regardless of the ST and the skid trail slope. Regardless of TI, SG, and SMC, the TAF E655 skidder caused a higher dry BD increase and soil porosity reduction than the Timberjack 450C. Furthermore, the higher the SMC, the deeper the ruts at all combinations of TI, SG, and ST. Our findings highlighted that ST, SMC, TI, and SG strongly affect rutting and physical properties of soil.