Effects of forestry operations on dissolved organic carbon concentrations and export in boreal first‐order streams

Abstract

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.