Impacts of Wildfire and Salvage Harvesting on Stream Nitrogen across Nine Years of Watershed Research
Abstract:The Southern Rockies Watershed Project (SRWP) was established to document the magnitude and recovery from the severe 2003 Lost Creek wildfire along the eastern slopes of the Rocky Mountains in Alberta, Canada. Hydrology and water quality have been studied in seven instrumented catchments (4-14 km2) encompassing burned, burned and salvage logged, and unburned (reference) conditions since 2004.
Across 9 years, the mean total nitrogen (TN) concentration and yields in the unburned catchments were lower than in the burned catchments and the salvage logged catchments. Differences were most pronounced in the first 3 years (2004-2006) after the fire – TN concentrations were 3.0-times greater in the burned catchments (760.2 μg/l) and 2.2-times greater in the salvaged logged catchments (571.1 μg/l) compared to the unburned catchments (257.1 μg/l). Similarly, the TN yields were 3.8-times greater in the burned catchments (14.0 kg ha/yr) and 2.5-times greater in the salvaged logged catchments (9.1 kg/ha/yr) compared to the unburned catchments (3.7 kg/ha/yr). Nitrate (NO3-) constituted the majority of TN in the unburned catchments (71%) and the burned catchments (64%), but was a minor component of TN in the salvage logged catchments (33%). Across the 9 years of the study, the mean NO3- concentration in the unburned catchments (121.8 μg/l) was lower than in the burned catchments (223.1 μg/l), but slightly higher than in the salvage logged catchments (106.0 μg/l). In the first 3 years after the fire the NO3- concentrations were considerably lower in the unburned catchments (106.0 μg/l) compared to both the burned catchments (432.7 μg/l) and the salvage logged catchments (271.4 μg/l). Trend analysis showed no significant trend in TN and NO3- concentrations and yields in the unburned catchments over the 9 year study, but showed a significant decreasing trend in TN and NO3- in both the burned and salvage logged catchments.