Soil damage in forestry from machinery used in thinning operations

Abstract

The potential increase in timber yields achieved through the thinning process in the forest is often reduced by soil compaction caused by the machinery involved in the thinning. This is often compounded by use of unsuitable machinery on difficult sites that are poorly drained, or in waterlogged conditions during wet weather. The aims of this project were to measure the degree of soil compaction and to investigate the impacts of different types of machinery. The parameters used to measure the soil compaction caused were bulk density, soil shear strength, cone penetration resistance and water infiltration rates. Soil chemical analyses were performed to investigate the possible impact on potassium, phosphorus, nitrate, total nitrogen, pH, organic carbon and organic matter levels in the soil. The impact on some microbial populations was also examined. The harvester felling the timber during thinning was found to have little impact on soil compaction in comparison to the forwarders which all cause soil compaction to some extent during timber extraction. On mineral soils it was found that the forwarder with the bigger load capacity, requiring fewer loaded passes over the extraction rack caused less soil compaction and disturbance. On wet soils the use of traction aids such as band tracks appears to reduce the soil damage and improve machine mobility. However when used unnecessarily during dry conditions, the band tracks will themselves cause significant soil compaction. The use of brash mats on the extraction racks from felled tree branches are important to reduce soil compaction and ground disturbance. All the parameters measured indicate lower values for compaction on the extraction racks with brash mats. Brash protects the root mat layer from the direct compactive forces of the passing machineiy and thus reduces potential tree root damage. On wet and slippery soils, brash acts as a good traction aid for the machinery. Soil nutrients do not appear to be directly affected by increased soil compaction. Root damage, reduced water infiltration and soil rutting caused by machinery will affect the uptake of the nutrients by the growing trees.