The purpose of this study was to quantify the effects of clear-cutting and site preparation on dissolved organic carbon (DOC) concentrations and export in four boreal headwater streams in northern Sweden. The data set included intensive stream water monitoring from 2 years of pretreatment conditions (2004–2005), a 2 year post-clear-cut period (2006–2007), and a 2 year period after site preparation (2008–2009). To investigate differences in [DOC], an analysis of variance on ranks was performed on the data sets. Clear-cutting increased the median DOC concentrations significantly from 15.9 to 20.4 mg L−1, which represents a net increase (treatment versus control) of 3.0 mg L−1 in the 2006–2007 period. Site preparation had an even more profound effect on DOC levels; an increase from 20.4 to 27.6 mg L−1was found in the site-prepared catchments, whereas the control sites increased slightly from 17.4 to 21.4 mg L−1during the wetter years of 2008–2009. Riverine C fluxes increased significantly by 100% after clear-cutting and by 79% after site preparation (92% and 195%, respectively, if compared to pretreatment conditions). When comparing these yearly C fluxes (183 kg C ha−1 yr−1after clear-cutting; 280 kg C ha−1 yr−1after site preparation) to the net ecosystem exchange (NEE) of a forest in the region, the DOC flux represented 10% of NEE before harvest, increased to 18% after the clear-cut, and increased to 28% after site preparation. These results underline the large impact of forestry operations on stream water quality as well as DOC exports leaving managed boreal forests.
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