Evaluating the effects of riparian restoration on a temperate river-system using standardized habitat survey. (Special Issue: Recent developments in classification, assessment and management strategies for freshwater habitats in Britain and Europe.)

摘要

The restoration of degraded riparian zones to improve a range of functions is attracting increasing interest, but there are still questions about (i) how effectively restoration changes riparian or channel conditions; (ii) whether riparian management offsets the effects of wider catchment pressures; and (iii) whether these effects can be detected quantitatively. A catchment-scale experiment was used to assess the effects of riparian restoration on riparian and channel conditions in the Welsh River Wye. In a hierarchically designed survey, variations in river habitat character were assessed among tributaries where riparian zones were recently managed for restoration (n=9 streams), unmanaged controls (n=12), intensively grazed pastures (n=3) and coniferous plantation (n=3). Management between 1997 and 2003, largely involving coppicing, was designed to exclude grazing through fencing in order to enable vegetation development while creating salmonid refuges. River habitat character was assessed using the UK 'River Habitat Survey' (RHS) method, with habitat variation quantified using Principal Components Analysis. Stream habitats varied significantly among treatment categories. Streams draining plantation conifer had 'harder' channel features, while those draining intensively grazed pasture were characterized by finer substrata and more active channels than elsewhere. Riparian management reduced livestock trampling (=poaching) and increased algal cover relative to controls. Coppicing and riparian fencing successfully excluded grazing on banks while increasing in-stream vegetation cover, but did not affect substrata, flow-types and channel features. These data show that RHS can detect habitat variation among streams in contrasting riparian land-use, revealing some apparently significant effects of recent restoration. We advocate longer-term investigations at reach to catchment scales to assess longer-term effects on channel and flow character, and to appraise fully the extent to which local riparian management can offset impairments at a catchment or larger scale, such as altered run-off regimes, sediment delivery and climate change.