Trees are a key component of a healthy river and an important factor in creating resilience in the ecosystem to floods, droughts and pollution. They are important for rivers because:
- Trees provide shelter for aquatic animals. For example, mayflies need trees to rest and to metamorphose into the final breeding stage of their lifecycle; trout use overhanging branches and tree roots to hide from predators.
- Trees provide leaves and woody material to the river – this is a primary input of nutrients and food for detritus shredding organisms such as Gammarus, which are important food for trout and birds such as dippers.
- Trees provide shade during periods of low flows and high temperature, reducing water temperatures and helping to maintain oxygen in the water. Too much shade can be a problem in suppressing growth of aquatic plants, but a mix of dappled light and shade is ideal.
- Fallen trees have a major role in creating a dynamic river. A fallen tree can trap sediment, create scour pools and clean gravels.
- Trees create an important buffer zone, reducing the amount of run-off that enters the river directly during periods of heavy rain. This run-off carries sediment and potentially also pollutants.
- Tree roots stabilise river banks and can reduce the rate of bank erosion.
This blog post by one of our Conservation Officers, Jonny Grey, describes the importance of trees to rivers.
The Woodland Trust has produced two videos based on research carried out on the link between riparian woodland, water & water quality (below).
How trees and fenced banks help reduce flood risk, erosion and siltation
The video below illustrates how mangroves protect the shore from erosion by absorbing the energy of waves. The same principle applies to the effect of tree roots and trees and branches in the river — they absorb the energy of fast flowing and turbulent water and reduce erosion.
The Pontbren project in Wales is an excellent illustration of how planting shelter belts of deciduous trees in the uplands has multiple benefits for farmers and livestock and flood risk management. Click here for a summary of the project and here for a presentation with notes, created by David Jenkins of Coed Cymru and presented by Vaughan Lewis at the the 2015 Oxford Farming Conference.
This paper, based on research in the Lake District, found that: ‘semi-natural broadleaf woodlands reduce specific peak discharge by 23%–60% and peak run-off coefficients by 30%–60% compared with pasture. Response to storm events took 14–50% longer in woodland compared to pasture. These differences in flood response are partly explained by more permeable woodland soils, 11–20 times greater than pasture soil.’
The Wild Trout Trust has created some guidance on managing habitat, including bankside trees.
Click here to download the ‘Managing Trees’ habitat management sheet. Click here to see all the habitat management sheets and other WTT publications.
Click here to download the ‘Trees’ section of the Chalk Stream Habitat Manual.
These papers by the Woodland Trust & Forestry Commission outline best practice, case studies and evidence for the beneficial effects of sensitively carried out planting schemes:
- Woodland Trust Case Study: Derwent, Click here.
- Forest Research: Woodland for Water. Click here.
- Woodland Trust: Managing the Drought. Click here
- Woodland Trust: Planting Trees to Protect Water. The role of trees and woods on farms in protecting water quality. Click here
- Woodland Trust: Holding back the waters. Click here.