Ecological Sustainability of Winter Harvesting in the Duck Mountain Provincial Park, SK: The Effects of Skidder Traffic, Slash Loading, and Cumulative Effects on Soils and Aspen Regeneration

Abstract

In over-mature trembling aspen (Populus tremuloides) forests, like those of the Duck Mountain Provincial Park (DMPP), mechanical harvesting has been shown to be an effective source of disturbance to re-establish a healthy and productive forest. However, harvesting operations can result in a degree of unwanted disturbance that could threaten the success of regeneration. The overall goal of this study was to assess, on a landscape scale, whether winter harvesting of the old growth aspen forests in the park is an ecologically sustainable practice for successful aspen regeneration. Skidder traffic intensity, slash coverage, and vegetation indices were calculated for six harvested blocks using Global Positioning Systems (GPS), Unoccupied Aerial Vehicles (UAVs) and Geographic Information Systems (GIS). Based on this information, soil bulk density and early sucker growth was measured to assess the effects of winter harvesting. Soil bulk density increased significantly following 1-5 skidder passes (1.39 g cm-3) compared to unharvested controls (1.29 g cm-3) but remained relatively constant as skidder traffic continued to increase. In areas of high skidder traffic (51-100 passes) aspen sucker density decreased by approximately 50% while sucker height decreased by over 20 cm compared to areas with less traffic. Soil bulk density, vegetation indices, and slash coverage (up to 60%) showed no relationship with the level of aspen regeneration in harvested blocks. To assess cumulative effects, principal component analysis, principal component regression, and fuzzy logic analysis were used to determine the regeneration suitability across harvested blocks. This analysis indicated that the majority of a harvested block (51-71% of the area) occurred with a rating of low to below average regeneration suitability. On average, low suitability areas had significantly more traffic and slash compared to the other levels of suitability, experiencing 27 more skidder passes and 6% more slash cover compared to high suitability areas. Aspen sucker height, root collar diameter, and dry leaf biomass were also significantly higher in areas with high regeneration suitability compared to areas with low suitability. Therefore, skidder traffic and slash must be properly managed and distributed throughout harvested blocks to ensure the sustainability of future aspen forests in the DMPP.