It is National Tree Week, and we are celebrating a relationship that makes our rivers healthier, can reduce flood risk and support fish and insect populations. Riverside trees are also an important tool to future-proof our rivers against climate change and protect the temperature-sensitive species that thrive in our waters.
So, how do trees and rivers work together?
In the river
Perhaps counterintuitively we believe, fish live in trees; and that dead wood is actually full of life. Young fish will often shoal together around submerged branches and roots in the river. They shelter amongst the slower flows and are better protected from predators, such as pike or goosander. Some fish will forage for insects around submerged wood, too.
Trees submerged in water can also kickstart natural processes and support a more diverse tapestry of river life. In fact, many of our interventions use woody material. In some instances, we simply hinge a bankside tree into the watercourse, or use them to slow the flow of water during intense rainfall events.
The debate on the removal of fallen trees from rivers is a lively one, and finding the balance can be difficult. Trent Rivers Trust opposes a blanket approach to remove fallen trees and woody debris from a watercourse, due to the huge benefits their presence has for wildlife and natural processes. We advocate for a more considered decision-making. Any situational assessment ought to give weight to the ecological benefits of woody debris in rivers, as well as how wood may impact infrastructure or increase flood risk in certain situations.
In the riparian zone
Riparian zones or riverbanks are an essential corridor for much of our aquatic wildlife and a healthy, thriving watercourse is often linked to a diverse riverbank. A mosaic of trees along our rivers, can support wildlife and natural processes on many levels.
Temperature control
Many of the species that live in our rivers are “cold-water species”. They thrive in our cool rivers. Shade from trees helps keep the water cool, and provides vital refuge to fish, such as salmon, during hot days. Trees can cool water by up to 6 degrees. In a warming climate, their contribution to keeping rivers cool enough for our freshwater species to survive is vital.
Varied habitat
At Trent Rivers Trust, we use the word ‘habitat mosaic’ to describe a key component of a healthy river ecosystem. Sunny and shaded spots in the riparian zone create important niches for plants, invertebrates, fish and birds.
Trees planted in the right places introduce valuable leaf litter to the freshwater ecosystem and some invertebrates, such as caddisflies, even build full-body armour from leaves to protect themselves against predators.
In the catchment
For healthy rivers, we need healthy catchments. Beyond the riverbank, we work across the landscape, using trees to reduce soil runoff, decrease flood risk and to create more diverse habitat.
Tree planting in the wider catchment can have a positive impact on aquatic wildlife. Their roots help bind the soil together and reduce the amount of mobile sediment. More dense vegetation also adds to surface roughness which traps sediment before it runs into the watercourse. If this sediment reaches the watercourse it can settle on gravels at the detriment of many species. Fish, such as salmon and trout, lay eggs in clean gravels, but if they become covered in sediment the eggs are starved of oxygen and die.
To help reduce the impact of sediment and keep soil where it benefits landowners, Trent Rivers Trust advise on a range of approaches to catchment restoration and management, including tree planting. We’re working collaboratively with landowners and can support with grant applications.
The right trees in the right places
While rivers benefit from trees, tree planting in the wrong place can cause more harm than good. Floodplain meadows, moorlands and wetlands, for example, create valuable niches and are home to a vast range of species. A considered and holistic approach is required to create an overall increase in biodiversity.
At Trent Rivers Trust, we’re using the catchment-based approach. Looking at the whole of the catchment, we identify issues and develop plans to address these. Trees often play an important role in defining these priorities.
Our work areas focusing on river restoration, working with nature and land use and water each support the importance of the right trees in the right places within a diverse and resilient catchment, as part of our hands-on approach to healthier catchments.
