Scaling of increased dissolved organic carbon inputs by forest clear-cutting – What arrives downstream?

• Final-felling increases DOC concentrations in downstream rivers.
• Concentration increases dependent on the percentage clear-cut area.
• Lakes and ponds may attenuate the downstream effects.
• Significant increases occur if more than 11–25% are final-felled.
• Threshold values could be used to minimize the negative effects.

SummaryForest clear-cutting has been found to significantly increase concentrations of dissolved organic carbon (DOC) in boreal first-order streams. Here, we address the questions of (1) how the additional inputs of DOC by upstream forest harvesting affect downstream locations within a stream network and (2) what catchment area has to be harvested to cause a significant downstream increase in DOC concentration. We combined the use of primary data from a paired-catchment experiment, clear-cut history of a nested stream network derived from satellite images with a mixing-model approach in order to quantify the importance of upstream clear-cuts on two downstream sites with different catchment sizes. Modeled [DOC] agreed well with the measured concentrations in the smaller, 8.7 km2 catchment located above a larger wetland area, but discrepancies occurred for the larger 22.9 km2 catchment located downstream of the wetland. Estimates of the critical area (Acritical) needed to be harvested to cause a significant impact on downstream DOC concentrations was quantified to be 11% for p < 0.05 and 23–25% for p < 0.001. Our results suggests that (i) increased DOC concentrations induced by forest harvesting affect downstream sites and (ii) additional DOC inputs by harvests have a significant impact on stream water quality, if harvests exceed Acritical. We suggest that the estimates of Acritical could be used in sensitive river networks to provide harvesting-thresholds. The latter could be implemented into forest planning that includes considerations of the negative impact of clear-cutting on water quality.