Effects of forest harvesting and woody debris removal on two Northland streams, New Zealand

Abstract

The short‐term effects of Pinus radiata forest harvesting to the stream edge followed by stream‐cleaning (removal of woody debris from the stream channel), on instream light levels, stream temperature, dissolved oxygen concentrations, and aquatic invertebrates were assessed in streams draining partly (25% clear‐cut) and totally (100% clear‐cut) harvested catchments, compared with nearby indigenous forest and mature pine plantation reference sites. There were marked increases in in‐stream light levels and water temperatures following forest harvest and stream‐cleaning at both sites. In‐stream light levels increased from 8–13% to 60–90% and maximum monthly water temperatures increased on average by 5.6°C in the partly harvested and 3.6°C in the fully harvested catchment. Dissolved oxygen levels decreased at both sites shortly after harvest (94%‐71% saturation in the partly harvested catchment; 72%‐37% saturation in the totally harvested catchment), increasing to 75% and 81%, respectively, 1 year later. Although aquatic invertebrate mean density and taxa richness increased at both sites following harvest, the relative abundance of sensitive mayfly, caddisfly, and stonefly species decreased and community composition changed to one dominated by Chironomidae (midges) or Mollusca. Impacts relative to pre‐harvest conditions were not as marked in the totally harvested catchment, possibly because of pre‐existing elevated stream temperatures and high levels of sand and silt. Any downstream protection provided by the forested headwaters of the partly harvested catchment was soon lost after the stream entered the clear‐cut area, although these forested headwaters may provide a potential source of aquatic invertebrates for re‐colonisation in the future as water quality and habitat recover. Our results suggest that: (1) pre‐existing constraints on habitat quality can influence the magnitude of harvesting impacts; and (2) length of stream edge harvested may be a better indicator of impact on some aspects of stream ecology, such as lighting, stream temperature, dissolved oxygen, and aquatic invertebrate community composition, than percentage of catchment harvested. This study also highlights the importance of considering the hydro‐logical and landscape context for mitigating harvesting and wood management impacts on stream ecosystems.