Long-term response in nutrient load from commercial forest management operations in a mountainous watershed

• Published in: Forest ecology and management 2021-08, Vol.494, p.119312, Article 119312
• Main Authors: Deval, C., Brooks, E.S., Gravelle, J.A., Link, T.E., Dobre, M., Elliot, W.J.
• Format: Article
• Language: English
• Subjects: Before-After-Control-Impact-Paired Series (BACIPS); Forest Management; Nitrogen; Nutrient Loads; Phosphorus; Timber Harvest
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2021.119312

•Long term monitoring of water quality impacts from commercial timber harvest practices.•Data were analyzed using a before-after control-impact paired series (BACIPS) design.•Significant increases in nitrate concentration and load were observed.•Impacts on particulate-bound pollutants were largely insignificant. An increased emphasis on fuel management strategies to mitigate wildfire risks is raising the awareness and need for comprehensive forest management strategies that satisfy long-term water quantity and water quality needs. Forest management activities can alter the soil nutrient pools and affect the timing and magnitude of stream water quantity and quality. We investigated the effect of contemporary forest management activities, including clear-cutting and thinning, on water yield and stream nitrogen and phosphorus dynamics in a quarter-century-long (1992–2016) paired and nested watershed study in the interior Pacific Northwest, US. Monthly water samples were collected and analyzed for total Kjeldhal nitrogen (TKN), total available nitrogen (TAN), nitrate + nitrite (NO3 + NO2), total phosphorus (TP), and orthophosphate (OP) concentrations throughout the study period. Five years of pre-disturbance data, 4 years of post-road construction, 6 years of Phase I (PH-I) experimental post-harvest, and 9 years of Phase II (PH-II) operational harvest data were analyzed using a before-after, control-impact paired series (BACIPS) study design. We found statistically significant increases in stream NO3 + NO2 loading from the paired and nested watersheds following timber harvest treatments. In the case of OP, any increase in nutrient load was attributed to increases in streamflow, as OP concentrations remained near minimum detectable concentrations. Streamflow increased the greatest following clear-cut practices (33.4% at W1 during PH-I) with the largest response in stream NO3 + NO2 concentration (up to 0.33 mg-N L−1 at W1). In-stream NO3 + NO2 and OP concentrations were lower in the downstream cumulative watersheds, which was likely due to dilution and nutrient assimilation effects. Interestingly, the NO3 + NO2 concentration, streamflow, and loads of NO3 + NO2 and OP from the undisturbed control watershed also increased. This increase was, however, smaller than the harvested watersheds and likely driven by climate variability or subtle forest succession changes. In summary, we found that contemporary forest management activities increased stream NO3 + NO2 concen