Impacts of forest biomass operations on forest hydrologic and soil erosion processes

Abstract

The use of biomass from forests by humans has been practiced since the dawn of time. Such practices have resulted in severe watershed degradation historically and in today's world. As modern society seeks alternative energy sources, they are once again turning to forests, but now use mechanized harvesting systems to remove the biomass and transport it to processing or utilization sites. This study investigated the impacts of different mechanized biomass operations on sediment displacement, soil hydraulic conductivity, and canopy interception to determine if severe watershed impacts associated with biomass utilization in the northwest U.S.A. may occur as seen elsewhere. Three field sites were selected that used different biomass operation methods: the Payette National Forest (NF) in Idaho using a forwarder system, the Colville NF in Washington State using a tractor-skidder system, and the Flathead NF in Montana using a skyline system. A total of 47 silt fences (5 m × 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 ground-based operations was lower than the control plots, but unlikely to cause significant changes in runoff on any of the plots except the Payette forwarder 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. Ground cover increased on the disturbed plots to levels observed on undisturbed plots within two years. The only significant increase in bulk density was observed on the forwarder trails at the Payette NF site where the density averaged 1.41 on the trails, compared to the densities of 1.0 elsewhere. The loss of canopy observed on the three biomass studies led to a complementary study on the effects of canopy loss on snow accumulation and melt on the Priest River Experimental Forest in Northern Idaho. The study found that the canopy only intercepted an average of 7 mm of snow water equivalent during three winter months, but average annual interception of precipitation was 114 mm, indicating that interception of rainfall is greater than snow. Overall, these results suggest that there were no direct detrimental hydrologic impacts due to biomass operations except on the compacted, and sometimes pulverized forwarder trail conditions observed in the Payette NF. The impact of the canopy on rainfall interception is less than the water yield differences due to clear cutting on nearby forested watersheds that were attributed to differences in evapotranspiration.