Soil protection with logging residues during mechanised harvesting of native forests in Tasmania: A preliminary study

In Tasmania, Australia, mechanised forest harvesting operations often use ‘cording’ to limit soil disturbance. This means logs are laid across the snig tracks, perpendicular to the flow of traffic, providing a rigid barrier between the machinery and the ground surface. At the conclusion of harvesting, the cording material is lifted using an excavator and burnt on site. This latter procedure enhances soil nutrition through the ‘ash-bed’ effect, whilst increasing the area of seedbed available for forest regeneration. To date, the efficacy of cording in protecting soil values remains largely untested. The shallow and stony nature of many forest soils, and the presence of logging residues and cording material along snig tracks, make the investigation of soil physical properties difficult. In response, this study was undertaken to assess the effectiveness of cording by using forest regeneration (density of seedlings and seedling height) as an indicator of productivity along former snig tracks protected with cording. Available seedbed (%) and seedling form, including the incidence of browsing and disease, were recorded and key soil chemical properties (0–10 cm soil depth: organic carbon, total nitrogen and total phosphorus), were measured. In the absence of physical soil testing, the theoretical distribution and attenuation of machine loads by the cording are discussed. Forest regeneration along the former snig tracks was consistently inferior in quantity and quality compared to that of the untrafficked control areas. This was attributed largely to: (a) a reduction in the available seedbed (%) due to incomplete combustion of the cording material and retention of logging residues, both on the ground surface and mixed with the upper soil layers, and (b) poor growing conditions—the combined effects of lowered organic carbon, nitrogen and phosphorus concentrations along the former snig tracks. Operational recommendations based on this study include: (a) avoiding trafficking of bare ground during the construction of cording, (b) beginning with a layer of small—diameter material to help spread machine loads more evenly and (c) where appropriate, leaving permanent snig tracks.