Forestry impact on water quality: a landscape perspective on dissolved organic carbon

Sammanfattning: Dissolved organic carbon (DOC) is a fundamental variable defining boreal stream ecosystems. In this thesis the impact of forestry practices that are commonly performed in the boreal regions of Scandinavia for stream water quality were evaluated. The thesis is based on combining the use of primary data from the Balsjö paired catchment experiment in northern Sweden with various modeling approaches. Final-felling strongly increased DOC concentrations in boreal first-order streams during the first four years after harvest. Median concentrations increased by 3.0 mg/L after clear-cutting and 6.2 mg/L after site preparation with concentrations being 5-24 mg/L higher in the clear-cut than in the reference catchment during summer storms. Clear-cutting also increased the riverine carbon (C) export significantly from 95 kg C ha⁻¹ yr⁻¹ to 183 kg C ha⁻¹ yr⁻¹ and to 280 kg C ha⁻¹ yr⁻¹ during pre-treatment, clear-cut and site-preparation periods, respectively. This export represents an important part of the C-balance of a forest in the region. Hydrological effects of clear-cutting included increased snow accumulation by 29 mm (27%) and a modified spring snowmelt. However, the largest effect on the water balance (~189 mm = 31%) was found during summer, when stream runoff was increased due to reduction in evapotranspiration. The drivers of the increased DOC concentrations were identified as changing flow-pathways in riparian soils activating more surficial, DOC rich soil layers, as well as increased soil temperatures that enhanced the DOC availability in riparian soils and therefore increased DOC mobilization from clear-cuts during the summer. In a final step, the impact of these increased, clear-cut induced DOC inputs into a larger scale boreal stream network were investigated by using a mixing model approach. DOC inputs were transferred to downstream sites, which resulted in increases in DOC concentrations at these locations. Further, the modeling approach showed that increases in DOC concentrations can be statistically detected, if the total area harvested within the stream network exceeds threshold values of 11% (p>0.05) and 23-25% (p<0.001) of the catchment area. Thus, this thesis suggests that threshold values for the maximum percentage of harvested area within a river basin should be implemented into forest planning for boreal catchments that are sensitive to changes in DOC concentrations.