Temporary increases in suspended sediment yields following selective eucalypt forest harvesting

Abstract

Unmitigated forestry operations have the potential to impact upon suspended sediment yields within headwater catchments. Best Management Practices (BMPs) are therefore required to reduce the effects on downstream users and to protect the integrity of aquatic ecosystems. Accordingly, in New South Wales, Australia, harvesting and roading activities on multiple use State forests must comply with Environment Protection Licences (EPLs) that require BMPs to be used to protect the aquatic environment from water pollution. The BMPs include soil conservation measures for the design of bridges, culverts and causeways; appropriate drainage spacings on roads and skid trails; seasonal harvesting restrictions; slope restrictions for harvesting and road construction activities; wet weather restrictions on the use of roads and log landings; and protection of all drainage features, including zero order streams, by the use of filter strips and/or buffer strips from where harvesting is excluded. In this study, conducted between 2001 and 2009 in Kangaroo River State forest, a replicated paired catchment experimental design was utilised to assess the effects of forestry activities on suspended sediment yields within three catchments selectively harvested using EPL-compliant BMPs. We hypothesised that harvesting within three treated catchments would increase suspended sediment yields but that the BMPs would reduce the magnitude and persistence of any measured effects. Harvesting during 2007 resulted in an increase in streamflow equivalent to 25.2 mm y⁻¹ to 46.4 mm y⁻¹ for each 10% of the area harvested in two of the three treated catchments, which is consistent with previous studies worldwide. Mean monthly concentrations of suspended sediment did not change following harvesting; however, the suspended sediment yield of one catchment, I-3, was significantly increased by 25.2% in the immediate post-harvest period. The overall suspended sediment yields remained low with monthly yields ranging from 0 kg ha⁻¹ during cease-to-flow conditions in all catchments to a high of 116.1 kg ha⁻¹ during February 2009 in the I-2 catchment. The measured increase in suspended sediment yield due to selective harvesting in the I-3 catchment was limited to a few post-harvest flow events and had subsided within 12 months of the cessation of harvesting. The BMPs utilised during the harvesting operations, the ridge-top location of roads and log landings, and the high degree of groundcover retained on skid trails and the General Harvest Area in the absence of a regeneration burn contributed to the minimal impacts measured during this study.