Wildfire and Salvage Logging Impacts on Stream Water Nitrogen in Southern Alberta's Rocky Mountains

Increased size and frequency of wildfires in North America has been linked to changing climate over the past 2-3 decades, raising concerns over impacts of wildfire on downstream water quality. In 2003, the Lost Creek wildfire burned more than 21,000 ha in the highest water yielding area of the Rocky Mountain region of southwestern Alberta (Crowsnest Pass). The objective of this study was to examine initial effects of the fire and post-fire salvage logging on concentrations, yield, and total export of several nitrogen (N) species, and to explore initial recovery of these effects within the first four years after the fire. Streams draining burned and post-fire salvage logged watersheds produced much higher concentrations of total nitrogen (TN), total dissolved nitrogen (TDN), and nitrate (NO3-) compared to reference streams in the first two years following the fire (p < 0.001). This resulted in average nutrient yields that were considerably greater for TN (6-fold), TDN (6-fold), and NO3- (9-fold) in burned streams than in reference streams. Salvage logging produced generally similar effects on the concentrations and yields for most N species. The temporal trend for TN, TDN, and NO3- in stream water from burned watersheds was a rapid decline in mean watershed exports over the four seasons after the fire to levels similar to those of the reference watersheds. However, exports of TN were still elevated in the fourth post-fire year in watersheds impacted by the additional disturbance of salvage logging. The effects of the burn were most noticeable (i.e., produced the greatest N concentrations, yields, and exports) during or following higher discharge periods (snowmelt freshet and storm flows) (p < 0.001). Small differences were still evident during base-flow periods, emphasizing the importance of groundwater and subsurface contributions to the headwater streams in this study.