Trees alongside rivers don’t only help slow the flow of flood waters and reduce diffuse pollution; they can also help with the survival of some of our most iconic fish species.
This is because the shade that riparian trees provide can help keep our rivers and streams cooler even as the summer air temperature increases – a service that is vital for the survival of many freshwater species but notably Atlantic salmon and brown trout.
Heating up and keeping cool
Water temperatures that rise above 22°C for more than seven consecutive days can be lethal for brown trout, and some smaller fresh water streams in Southern England have already recorded temperatures of over 31°C.
Average summer air temperatures are predicted to increase by between 2°C and 4°C by the 2050s and river temperatures are expected to rise by a similar amount – an increase that could make these already vulnerable species even more at risk.
By planting trees, or encouraging natural regeneration, alongside our rivers and streams – especially smaller, headwater streams without much groundwater influence – we can help create a better habitat for fish and invertebrates. Research has shown that shaded reaches can have lower summer water temperatures by an average of 1.5°C than open areas and this figure can be greater on hotter days and where there is a wide area of riparian trees.
But the challenge is pretty big – using LiDAR (Light Detection & Ranging) data the Environment Agency has estimated that only 15 per cent of channels in England and Wales have riparian tree cover and this figure may yet decline as ash dieback and other diseases take their toll.
The impact of climate change
Just as with flooding, this isn’t simply a 'nice to'. We are already seeing the impact of climate change on these species.
For example, the Environment Agency’s recently released Annual Fisheries Report 2014 to 2015 shows that the declared salmon rod catch in England for 2014 was the ‘lowest on record’ and of England’s 42 principal salmon rivers, 38 were assessed as being ‘at risk’ or ‘probably at risk’. None were categorised as ‘not at risk’.
It’s not just an English problem either – the report states that ‘the poor state of Atlantic salmon is not unique to England but is reflected across the UK and across its range’.
Salmon are already coping with the impact of climate change on marine feeding opportunities – marine survival has nearly halved over the last 20 years. So it is critical that their freshwater phase is improved and protected, which is where trees come in!
The Keeping Rivers Cool project
The good news is that there are already some great examples of projects that are delivering increased riparian shade – see Ribble Rivers Trust, Wessex Tree Planting Project, and the River Tyne Keeping Rivers Cool Project for a selection of projects that the Trust is supporting.
But with the publication of a new Guidance Manual to help and support those who might be in a position to plant more riparian trees, we, and our partners in the Keeping Rivers Cool initiative, are hoping to see many more. Do please take a look and let us know if you need any additional help.
Ultimately, planting trees to help salmon and trout may also help improve water quality for all and perhaps even play a part in natural flood risk management. We all need some help adapting to climate change – and trees have a key role to play.