The role of brush mats in mitigating machine-induced soil disturbances: An assessment using absolute and relative soil bulk density and penetration resistance

Forest soils often exhibit low bearing capacities and as a result are often incapable of withstanding high axle loads. In New Brunswick, Canada, five different brush amounts (0, 5, 10, 15, and 20 kg·m ^–2 ) were applied as brush mats on machine operating trails during a cut-to-length harvesting operation in a softwood stand to analyze soil disturbance as a result of off-road forest harvesting machine traffic. Soil absolute and relative bulk density and soil penetration resistance measurements were completed below the varying brush mats both before and after forwarding. The mean differences between pre-and post-impact absolute soil dry bulk density values recorded on track areas were 0.24 g·cm ^–3 for 5–20 kg·m ^–2 of brush and 0.33 g·cm ^–3 for 0 kg·m ^–2 of brush. On average, 40.5%, 17.9%, 14.3%, 15.5%, and 3.6% of all post-forwarding measurements exceeded the threshold for growth-impeding soil bulk density (80% standard Proctor density) for 0, 5, 10, 15 and 20 kg·m ^–2 of brush, respectively. Soil penetration values >3.0 MPa represented 23.7%, 15.0%, 9.4%, 4.6%, and 0.7% of all post-forwarding test plots with 0, 5, 10, 15, and 20 kg·m ^–2 of brush, respectively. The results suggest that softwood brush mats of 10 to 20 kg·m ^–2 placed on machine operating trails play a considerable role in reducing forwarder-induced soil compaction and penetration resistance.