Forests play a major role in the ecological functioning of the freshwater environment. Forest streams, rivers, lakes, ponds and wetlands all provide habitat for a large range of plant and animal species. Several protected and priority species such as otters, water vole, Atlantic salmon and freshwater pearl mussels are particularly associated with woodlands.
To meet UK Forestry Standard requirements, forest design and management must restore and maintain the natural features, processess and habitat of the freshwater ecology. The water elements of the forest must be considered in the preparation of forest design plans and all forest operations to ensure the freshwater biodiversity is protected and the water resource suitable for a range of purposes.
The following management recommendations for UK forestry are based on an international literature review.
Catchment management plan: management of riparian woodland should be based on a consideration of site sensitivity, intrinsic value and potential. A stragetic plan for the forest should be prepared in consultation with local stakeholders.
Width: the riparian buffer should reflect stream size and the natural dimensions of the functional riparian zone. The recommended widths ranged from 5 m for stream upto 1 m wide to over 2+m for streams >2 m wide. Where the natural riparian zone exceeds these widths, the dimensions of the buffer area should be increased, up to twice the minimum recommended width.
Canopy structure: establish and maintain a variable, predominantly open woodland canopy with half the watercourse open to sunlight and the remainder under dappled shade.
Species choice: the vegetation established within the riparian buffer should be native and appropriate to the location and soil water regime. In the UK the Ecological Site Classification toolkit can be used to identify suitable species.
Establishment method: natural regeneration is considered the best method of establishing native tree and shrub species where an appropriate seed source exists. Where natural regeneration is unsuitable tree planting sites should be prepared by mounding, either by hand or using a lightweight excavator to minimise trafficking. No drainage is permitted within a riparian buffer and very wet sites should be left unplanted. Planting should favour light-foliaged species in irregular small groups to replicate the vegetation structure of a secondary forest.
Canopy maintenance: the level of shade should be assessed regularly to determine the need for canopy management. Areas of dense shade may require selective pruning, thinning, pollarding or coppicing. Thinning or group felling operations within the riparian buffer should be carefully planned to minimise site disturbance.
Dead wood and veteran trees: Dead wood forms an important habitat for many species associated with the shaded and moist conditions of riparian woodland. Veteran trees should be identified, mapped and retained where possible as they often provide important habitats and character to the area. Debris dams are natural features of wooded streams and greatly enhance the aquatic habitat. They should not be removed unless downstream sites are at risk of flooding from debris blockage or they form a significant barrier to migratory fish.
Structural engineering: In severely eroded streams, consideration should be given to reinforcing stream banks through the use of gabions or timber groynes. These will reduce hydraulic scouring of the banks creating deeper stream channels and facilitate the establishment of marginal vegetation.
Forest operations within the riparian zone: A riparian buffer must not be used to store equipment, fuel, oil or chemicals due to the high risk of leakage polluting adjacent watercourses. During harvesting operations on the adjacent land, care should be taken to avoid brash being deposited within the riparian buffer area. Stream crossings should be avoided and temporary culverts created when this is not possible. The drainage system on restock sites should be designed so that drains terminate beyound the edge of the riparian buffer.
Targetted woodland creation can sigficantly improve water quality. Woodland planting along watercourses replaces the agricultural activities in areas that typically form the main source zones for diffuse pollutants. Riparian woodland planting is an effective intervention to buffer the effects of the polluting agricultural activity in the wider catchment.
Quantify the effectivenss of riparian woodland creation as a measure to improve water quality.
Identify options to target effctive woodland creation to improve freshwater ecology in case study catchments
Support national intitiatives to encourage riparian woodland creation
Over 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.
Our research project investigates the role of riparian woodland in controlling stream water temperature through the provision of shade. Observations in recent summers show that stream water temperatures in open streams draining southern England can greatly exceed tolerance levels for salmon and trout. The situation is expected to get worse with average temperatures predicted to rise by between 2 and 5 ºC by the end of this century. The influence of riparian woodland in limiting rises in water terperature is being investigated at several sites in England and Scotland.
Some fish species such as salmonids (salmon and trout) are very sensitive to high water temperature, with possible effects on the timing of spawning, fish growth rates and even survival. Salmonid fish require temperatures of between 5 and 15 ºC for normal growth and temperatures above 24 ºC can be lethal. Temperatures in excess of 31ºC have been recorded in recent summers in small lowland streams in the New Forest, demonstrating that this tolerance limit is already being significantly breached in some small watercourses in southern England.
Riparian woodland features such as, exposed tree roots, fallen stems and large woody debris dams, create structural diversity in the instream habitat and maintain pockets of cool water below undercut river banks and deep pools. These provide important thermal reguia for salmonids.
Riparian woodland canopy may require management to maintain water temperatures within a favourable range for salmonid fish and other sensitive freshwater fauna.
The shade provided by riparian trees can significantly reduce peak summer water temperature. Planting trees on river banks may therefore have an increasingly important role to play in limiting the impact of climate warming on freshwater life. Riparian woodland creation is the key objective of the Keeping Rivers Cool project, which is currently being led by the Woodland Trust.
A joint field study with Southampton University was set up in the New Forest to evaluate the cooling effect of riparian shade.
Twenty sites with variable levels of shade on the Dockens Water and Ober Water have been instrumented to characterise the thermal regime and assess the effects of shading on streamwater temperature and on fish populations, including fish survival, growth rates and behaviour.
The results will help to determine whether thermal stress poses a serious problem in these watercourses and if so, how riparian woodland management could help to protect the freshwater life from future rises in water temperature.
Forestry Commission Practice Guide