Cumulative and discrete effects of forest harvest and drainage on the hydrological regime and nutrient dynamics in boreal catchments

In boreal landscapes, forest management has the potential to become a major driver of surface water quality due to the large proportion of actively-used land areas and the intensity of forestry operations. In Fennoscandia, forest management is comprised of different operations during a single rotation, where final harvest by clear cutting and subsequent ditch cleaning to restore drainage capacity are among the most influential on water quality. Here, we analyzed the single and combined effect of these forest management operations on the concentrations and exports of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (PO 4 ) in boreal Sweden. We measured groundwater table level, stream discharge, and water chemistry data continuously following experimental clear cutting and ditch cleaning applied to a historically drained forest using a before-after-control-impact (BACI) design. We used linear mixed models to test whether DOC, DON, DIN and PO 4 concentrations were affected after each individual forest management operation, and further analyzed the response of the cumulative operations. We found that after clear cutting, concentrations of organic and inorganic nutrients increased significantly. However, for catchments with ditch cleaning after clear cutting, concentrations of organic nutrients in surface water decreased to pre-disturbance levels; inorganic nutrient concentrations also decreased but less strongly than organic counterparts. Despite this effect, catchments with ditch cleaning after clear cutting still showed an increase in overall organic and inorganic nutrient exports when compared to the reference catchments and the pre-treatment period. Nevertheless, catchments without ditch cleaning showed an even higher increase in both concentration and exports of most solutes. Overall, our results suggest changes in C, N and P exports due to forest management, along with the large spatial extent of this activity, could promote biogeochemical shifts and trigger water quality deterioration in boreal streams. • We present a catchment-scale BACI experiment evaluating how forest clear-cutting and ditch cleaning affect C, N, and P exports. • Clear cutting increases C, N and P concentration in outlet water, while ditch cleaning mitigates these impacts. • Overall, forest management operations increases C, N and P exports from catchments. • Intensive forestry operations might disrupt the natural seasonal stoichiometry of C, N and P supplied to aquatic ecosystems.