The Effect of Riparian Forest Management on the Freshwater Environment
The Forestry Commission's Forests & Water Guidelines recommend that riparian zones should be managed in an integrated way so that the aquatic environment and distinctive riparian habitats are protected or enhanced. This is thought best achieved by maintaining open or partially wooded conditions, with about half the length of a stream exposed to direct sunlight. Where existing conifer crops provide dense shade, it is advocated that these should be cleared back from streamsides and replaced with predominantly broadleaved trees and shrubs. The overall aim is to create an ecologically rich riparian buffer area that will help protect the aquatic zone from disturbance by silvicultural operations on the adjacent land.
Much of this guidance, however, is founded on principal rather than experimental study. In particular, there is a lack of knowledge about how the width of the buffer, the structure of the vegetation and the mix of species affect the main functions of the riparian buffer, which are considered to be: sediment removal and erosion control, protection of water quality, moderation of shade and water temperature, maintenance of habitat structural diversity and ecological integrity, and improvement of landscape quality. Information on these aspects is urgently required to guide the ongoing major restructuring of upland conifer forests.
This report addresses this need through a review of the international literature on the effects of riparian forest management on the freshwater environment, and by revisiting earlier experimental bankside clearance treatments in the UK. It concludes by summarising best management practice for riparian bufferareas. The key findings were:
- Bankside clearance improves riparian habitat quality, with cleared streams having the potential to form attractive riparian woodland with a diverse broadleaf canopy and vigorous herbaceous ground cover. Habitat quality can exceed that of open pasture or moorland and appears to be relatively unaffected by riparian buffer width in the range 10-30 m. Successful establishment depends on a reliable seed source or direct planting, the removal of conifer brash, controlled grazing and the clearance of reseeded conifer crops. The presence of relic broadleaves or native conifers aids woodland regeneration and the formation of coarse woody debris dams. Long term continuity of management is important to ensure that the potential benefits to the aquatic zone are realised.
- There is little evidence that bankside clearance affects the diversity of benthic invertebrates. The main effect is to increase invertebrate abundance, but this appears to be limited to certain acid sensitive species, which are 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.
- 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.
- Bankside clearance is likely to have a minimal effect on biological recovery in acidified catchments until chemical recovery is well underway.
Riparian habitat, conifer bankside clearance, riparian zone width, riparian zone structure, riparian zone management.
Copies of this report are available from the Foundation, price £35.00 less 20% to FWR members