Effects of clear-cutting on annual and seasonal runoff from a boreal forest catchment in eastern Finland

To examine the effects of clear-cutting on annual and seasonal runoff from a boreal headwater catchment, a paired-catchment study was conducted in Norway spruce- (Picea abies (L.) Karsten) and Scots pine- (Pinus sylvestris L.) dominated forest catchments in eastern Finland. Long-term hydrological data were obtained for 5 years before and 18 years after clear-cutting, during which the clear-cut area was ploughed in the 4th year and planted with Scots pine seedlings in the 5th year. Calibration equations for annual and seasonal runoff were developed between the paired (i.e. treatment and control) catchments during the pre-treatment period and after clear-cutting, the treatment effect (TE) was calculated as the difference between the measured and predicted runoff in the treatment catchment. The annual TE was highest in the 1st year after the clear-cutting and tended to decrease with time, gradually diminishing from the 8th year after the clear-cutting. The seasonal TE had different temporal variations from the annual TE. The spring TE continued to be positive until the end of the observation period. On the other hand, the summer TE continued to be negative from the 8th year to the end of the observation period. The autumn TE was positive until the 6th year, but subsequently became negative. The winter TE was almost zero throughout the observation period. The decreasing trend in annual TE therefore resulted from the fact that the persistent positive spring TEs were offset by the negative summer and autumn TEs from the 8th year. These results demonstrate that seasonal TEs, i.e. spring and summer TEs, more clearly represent the persistence of changes in runoff after clear-cutting than does the annual TE. We thus conclude that the effects of clear-cutting on runoff continued for at least 18 years in our catchment. Our study emphasizes the importance of investigating seasonal runoff together with annual runoff to enable a better understanding of clear-cutting effects on runoff from forest catchments.