Gradual increase in the intensity of machine traffic in timber harvesting: effects on soil composition and functionality

This study describes the effects of gradually increasing the intensity of machine traffic in timber harvesting of Pinus taeda L. in a cut-to-length system on the severity of compaction and changes in functionality of a subtropical Oxisol in southern Brazil. The study was conducted in an 11-year-old P. taeda stand. The composition (bulk density, BD; degree-of-compactness, DC; total porosity, TP; macroporosity, Mac; microporosity, Mic) and functionality (saturated hydraulic conductivity, Ks; mechanical resistance to penetration, PR) of the soil were evaluated at 0.00–0.10 m, 0.10–0.20 m, 0.20–0.30 m, and 0.30–0.40 m. The traffic intensities were pine stand before harvesting (PSF), after one pass of harvester followed by one (HV-FW1), three (HV-FW3), six (HV-FW6), and eight (HV-FW8) complete passes of the forwarder. The surface soil layer was more susceptible to compaction due to the change in BD and DC, but the pores were affected by machine traffic at depth (to at least 0.30 m). HV-FW1 provided the greatest changes in composition and functional properties compared to PSF. BD and DC at 0.00–0.10 m increased by approximately 20% in HV-FW1. Mac decreased and Mic increased in all layers, and the traffic effect decreased with increasing soil depth (to 0.30 m) and with increasing traffic intensity. Ks decreased in all layers after HV-FW1. An average decrease of 0.06 m3 m−3 in Mac in HV-FW1 provided a decrease in Ks of approximately 270 mm h−1 compared to PSF. Only the PR reached values considered limiting for plant growth (PR > 2.0 MPa). A gradual increase in the intensity of machine traffic affects soil properties, although the proportion of this effect generally decreased with increasing traffic intensity. Analysis of soil composition properties alone is not sufficient to assess the effects of traffic intensity on soil degradation because functional properties are more sensitive to the effects of machine traffic intensity.