Impacts of Forest Biomass Operations on Soil Erosion

Abstract. This study investigated the impacts of different biomass operations on sediment displacement. Three field sites were selected that used different biomass operation methods: the Payette National Forest (NF) using a forwarder system, the Colville NF using a tractor-skidder system, and the Flathead NF using a skyline system. A total of 47 silt fence plots (5 m x 10 m) were installed to monitor soil movement from biomass operations: 28 silt fences on disturbed plots and 19 on control plots. No soil loss was observed except on the forwarder trails in the Payette NF where animal disturbances were responsible for some soil movement into the silt fences. Infiltration rates measured using a constant head permeameter showed that the saturated hydraulic conductivity on the disturbed plots for the forwarder and tractor-skidder systems was lower than the control plots, but unlikely to cause significant changes in runoff on any of the plots except the Payette trails, where the saturated hydraulic conductivity values averaged only 5.3 mm h^-1, compared to greater than 20 mm h^-1 on all other observations. The only significant increase in bulk density was observed on the forwarder trails at the Payette NF site where the density averaged 1.41 g cm^-3 on the trails, compared to the densities of 1.0 g cm^-3 elsewhere. Ground cover increased on the disturbed plots to levels observed on undisturbed plots within four years. These results suggest that there were no direct detrimental soil and water impacts due to biomass operations except on the compacted forwarder trail observed in the Payette NF.