Wooded Buffer Strips at Watercourse (One or Both Banks, Continuous or Scattered)
November 18th, 2021Key Concepts
Inclusion of trees into an otherwise grass buffer benefits many functions associated with longer term nutrient retention, ability to mediate groundwater nutrient pathways, habitat, stream shading and stopping spray drift. Primarily native broadleaved trees are placed along watercourses, with potential to vary management system to maximise pollutant uptake and removal, for example short rotation coppice. Threes can increase surface roughness and potentially soil infiltration, depending on soil wetness to reduce pollutant run-off. They can reduce aerial dispersion and deposition of pollutants to watercourse such as pesticides and ammonia, as well as provide shelter. Trees benefit the aquatic habitat as sources of woody debris that can promote the formation of leaky dams and the formation of riffle and pool structures, for providing shade, cooling water temperatures for the benefit of salmonid fish.Improves watercourse hydromorphology. Also they can helps to slow otherwise fast runoff and reduce downstream flood risk. River corridor woodland are habitats in their own right, but can link up wider habitats (e.g. isolated woodland), creating woodland habitat networks and wildlife corridor.
Summary of Evidence on Functions
Trees are found to stabilise river banks, reduce runoff thereby helping to reduce risks of flooding. The tree planting gives shelter for animals, habitat and enhances woodland connectivity. Tree planting in buffers may especially be beneficial when tragetting areas prone to flooding, bank erosion or suffering elevated water temperatures or areas where woodland connectivity is lacking.
Technical Evidence References
Stockan & Cole 2014. Soil and Vegetation Responses to Forested Riparian Buffer Strips. RPC RB 2014/14;
Odoni NA and SN Lane. 'Assessment of the impact of upstream land management measures on flood flows in Pickering using OVERFLOW' Contract report to Forest Research for the Slowing the Flow at Pickering project. Durham University, 2010;
Broadmeadow S and Nisbet TR. 'The effects of riparian forest management on the freshwater environment: a literature review of best management practice' Hydrology and Earth System Sciences, 2004. 8(3): pages 286-305;
Nisbet T, Silgram M, Shah N, Morrow K, Broadmeadow S. 'Woodland for Water: Woodland measures for meeting Water Framework Directive objectives' Forest Research Monograph, 2011. 4: p. 156;
Fortier J et al. Biomass carbon, nitrogen and phosphorus stocks in hybrid poplar buffers, herbaceous buffers and natural woodlots in the riparian zone on agricultural land. Journal of Environmental Management, 2015. 154: pages 333-45;
Sweeney BW and Newbold JD. Streamside Forest Buffer Width Needed to Protect Stream Water Quality, Habitat, and Organisms: A Literature Review. JAWRA Journal of the American Water Resources Association, 2014. 50(3), 560-584;
Hubble TCT et al. The role of riparian trees in maintaining riverbank stability: A review of Australian experience and practice' Ecological Engineering, 2010. 36(3), 292-304;
Main Countries Where This Is Practiced
UK
Challenges and Limitations, Including Site-specific Factors, Planning Requirements and Particular Suitability to Farming System
Need time to establish, with some benefits such as inputs of deadwood being slow to be realised. If placed in a livestock field, these features need to be fenced. Reduces size of productive field and can impact crop yield due to shading, depending on aspect. Flows can be bypassed at depth, although this can be reduced by tree rooting. Potentially needs access for management such as thinning and eventual harvesting. Potentially vulnerable to wind blow. Can increase local flood risk by backing up flood waters and the washout of dead wood. Limits access to watercourses for maintaining flood embankments, water supply and crossings. Potential for excessive shade, requiring management. Potential to deflect watercourse flows, increasing wetness and loss of local land.
Ability to Modify It For Site Requirements
There is much scope to vary and tailor design/structure in line with nature of pollutant, loading and transport pathways. Choice of trees will be varied according to dominant species for the climate and soil conditions. Native species support more biodiversity.
Critical Factors in Establishment and Ongoing Management
Most management comes during establishment of trees: getting rid of grass and competing weeds at tree bases is recommended until canopy closure to help trees become established. Some trees can be planted into a weed suppressing geotextile groundcover for small areas, but most are planted directly with spot herbicide treatment used around the tree for several years. Protection from deer is necessary in most places using a tree gaurd and stake for each tree. Harvesting wood will provide biomass but requires effort and can detract from some habitat and shade benefits for a duration afterwards. Access for harvesting machinery must be done with care not to destroy soil structure or leave preferential run-off paths and may give marginal returns on small areas relative to effort.
Guidance and Other Resources
Forest Research (UK) has guidance on riparian woodland creation management, role of shade in coltrolling water temperatures and practice for clearing conifer plantation from banks accessible via https://www.forestresearch.gov.uk/research/riparian-woodland-and-water-protection/the-effect-of-riparian-woodland-management-on-the-freshwater-environment/;
SEPA have comprehensive poplicy and guidance on managing vegetation close to the water environment at https://www.sepa.org.uk/media/151010/wat_sg_44.pdf;
An interesting case study of woodland planting by conservation volunteers on the Allan water for Natural Flood Management is given at https://blogs.tcv.org.uk/natural-talent/2013/11/27/restoring-native-riparian-woodland/;
A further case study comes from the Forth Rivers Trust at https://forthriverstrust.org/project/river-larig-riparian-woodland-project/