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

Abstract:

Forest 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 (A_critical) 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 A_critical. We suggest that the estimates of A_critical could be used in sensitive river networks to provide harvesting-thresholds to be included in forest planning that includes considerations of the negative impact of clear-cutting on water quality.