Multi-Decadal Response of Stream Water Quality to Commercial Forest Management Operations in a Mountainous Watershed

Concerns pertaining to the commercial forest management and consequent nutrient losses exist. Forest management activities can potentially alter the soil nutrient stores and the stream water quality. We investigated the impact of contemporary forest management activities on the stream nitrogen and phosphorus dynamics in a two-and-a-half decade-long (1992-2016) paired and nested watershed study in the Pacific Northwest. Monthly grab 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 calibration data, 4 years of post-road construction, 6 years of post-harvest and 9 years of post-harvest data (phase-II) were analyzed using the Before-After-Control-Impact-Paired (BACIP) design. We found statistically significant (α <0.05) increase in NO3+NO2 and OP concentrations following timber harvest. Downstream cumulative watersheds displayed relatively smaller increase in the stream NO3+NO2 and OP concentrations likely due to the dilution and attenuation effects. NO3+NO2 and OP exports also increased from all watersheds following the treatments. More interestingly the undisturbed watershed displayed similar increasing trends in concentrations and export of NO3+NO2 and OP. This increase was, however, smaller in magnitude compared to the disturbed watersheds. In summary, we found that the contemporary forest management activities altered the stream water quality by significantly increasing the stream NO3+NO2 and OP concentrations as well as export from the paired and nested watershed.