The impact of conifer clearance from the banks of upland streams

Photo showing conifer clearance from banks of a streamOver the last 20 years, the Forestry Commission has been gradually clearing back conifer plantations from streamsides to create riparian buffers.

A collaborative project was set up in the mid-1980’s to guide and inform the practice in upland forests in the UK. Three sites were established in Scotland and Wales to assess the impact of forest clearance on stream water chemistry and/or aquatic invertebrate and fish populations. These sites were recently revisited some 10 to 15 years after the original clearance to establish the medium term effects on the freshwater environment.

The results suggest that bankside clearance can improve the quality of riparian and aquatic habitat with increases in invertebrate abundance, diversity and fish stocks. However, bankside clearance will have a minimal effect on biological recovery in acidified catchments until chemical recovery is well underway.

Key findings from the final report (see right) are given below.

Riparian habitat quality

  • Cleared streams tended to have the best quality of habitat. Uncleared sections had bare, eroding stream banks, while open pasture sections frequently had damaged banks due to overgrazing and trampling.
  • Cleared streams with relic broadleaf trees and shrubs or where broadleaf trees were replanted after felling had the highest habitat scores.
  • Bankside clearance had little effect on the cover of marginal and in-stream vegetation.
  • Cleared streams with relic broadleaved trees tended to have more woody debris dams, which helped to create structural diversity in the instream habitat.
  • Natural regeneration was very slow where there was an inadequate seed source or grazing by sheep. Light to moderate shading by the canopies of relic broadleaves controlled grass growth and increased the floric diversity.

Macroinvertebrate abundance and diversity

  • There was little evidence that bankside clearance affects the diversity of benthic invertebrates. The main effect was to increase invertebrate abundance, but this appears to have been limited to certain acid sensitive species, which were absent from acidic streams.
  • Acidic moorland and ‘young’ cleared streams may have a lower invertebrate abundance than uncleared conifer streams due to the lack of allochthonous input.

Fish

  • Bankside clearance has the potential to increase numbers of fry, parr and adult trout where other factors such as acidity and access are not limiting. Increases are thought to be mainly due to improved carrying capacity through better cover and food.
  • The development of overhanging vegetation, undercut banks and pools is particularly beneficial in providing shelter for older fish. In time, heavy shading may reverse these benefits unless controlled by active management.
  • Coarse woody debris dams can provide a serious barrier to fish movement where they become sealed by tree stumps, brash and silt. The condition of dams needs to be assessed after harvesting and cleared or cleaned where required, otherwise they will prevent upstream recolonisation.