Relating upstream forest management to stream ecosystem condition in middle catchment reaches in Tasmania

Abstract

We investigated the relationship between the extent of Clearfell, Burn and Sow (CBS) harvesting and conversion to plantation in catchments upstream of mid-catchment study reaches on instream biota and habitat. Study sites were situated in fourth order stream reaches with no adjacent forestry activity, so that impacts represented the accumulated effect of forestry activity in the upstream catchment. In-channel fine sediments increased and the proportion of aquatic insect taxa decreased with increasing percentage land area under CBS (PCBS), with substantive changes observed above 40% of catchment area under CBS. Two different diagnostic attributes for macroinvertebrate taxa (SIGNAL scoring and Flow Exposure Groups) showed different relationships with PCBS, indicating that reductions in flow may also partially drive the macroinvertebrate response to increasing PCBS. Relationships between % area under unsealed roads and PCBS on downstream macroinvertebrate fauna suggested that roading was a substantive co-contributor to the overall impact of upstream CBS operations. An increase of 1% of the upstream catchment area under unsealed roads was associated with a respective decrease of richness by 4–8 EPT species in study reach riffles. For study reaches with upstream catchments containing hardwood (Eucalyptus nitens) plantation operations only, there was also a significant increase in unsealed roading and instream fine sediment with increasing area of plantation, though the increases were considerably less pronounced than for CBS operations occupying similar relative catchment areas. There were no significant relationships between area of plantation and macroinvertebrate community variables, presumably reflecting the lower levels of sediment deposition we observed downstream in plantation-dominated catchments. Plantation dominated catchments, with lower roading density and lower erodibility basaltic soils, had smaller instream impacts relative to CBS-dominated catchments, which are generally characterised by a more prolonged and dispersed disturbance regime including intensive regeneration burns.