Changes to headwater stream morphology, habitats and riparian vegetation recorded 15 years after pre-Forest Practices Code forest clearfelling in upland granite terrain, Tasmania, Australia

Abstract

The effects of clearfelling on headwater wet sclerophyll eucalypt forest streams and their riparian zones were examined in upland granite terrain in north-east Tasmania, Australia. Five first-order, headwater stream catchments, clearfelled and regenerated to native forest in 1985 (REGEN streams) were selected for comparison with five similar ‘control’ stream catchments (CONTROL streams). Channel morphology and stream habitat features were measured in 50 m representative study reaches along each stream. Stream sediments and riparian soils were analysed for particle size, loss on ignition, and carbon and nitrogen content. Vegetation composition and structure were assessed in plots within the riparian (0–10 m) and adjacent near-riparian (10–20 m) forest, adjacent to the study reaches. All catchments were assigned a historical disturbance rating based on aerial photograph and field observations. There were significant differences between the composition and structure of the riparian and near-riparian vegetation of REGEN and CONTROL streams. Differences were less pronounced in the more homogenous riparian vegetation than in the more heterogenous near-riparian vegetation. Species richness was higher in the REGEN near-riparian vegetation, but not in the riparian vegetation. As expected, structural differences between the vegetation of the REGEN and CONTROL streams were still evident within both the riparian and near-riparian zones, 15 years after clearfelling. REGEN streams were overlain by more logs, were more entrenched, and less complex morphologically, with a lower proportion of pools and bars, than CONTROL streams. REGEN streams contained less organic debris, and more variable bankfull widths, exposed boulders and locally variable slopes than CONTROL streams. REGEN streams had coarser sediments with a lower C/N ratio than CONTROL streams. REGEN stream riparian soils contained more total nitrogen than those of CONTROL streams. Impacts of pre-Forest Practices Code clearfell logging and forest regeneration methods on granitic headwater streams are still apparent after 15 years. Observed differences in stream morphology and habitats were significantly correlated with the rating of the harvesting disturbance. We ascribe the differences to direct and indirect effects of harvesting, and deduce that these catchments are still adjusting to that disturbance. Current Code prescriptions will limit impacts of current harvesting in similar terrain, though indirect effects of current harvesting methods on headwater stream character may still be significant.