If you take a riverside walk, you might see strips of woodland growing on the banks. This type of woodland is known as ‘riparian’ (ripa means 'bank' in Latin). In many areas, including the Garnock Valley, these riparian habitats have been lost or degraded due to human influences like clearance, grazing, industrialisation, pollution and intensification of agriculture. But riparian woodlands are vital for the health of our rivers.
Riparian woodlands and the health of our rivers
So why are trees so important for rivers? Riparian woodlands create a network of wildlife corridors, allowing species to move more freely through the landscape and adapt to the effect of changing weather and climate. Healthier riverside habitats will also help us manage water and soils more sustainably in the future, tackling problems of erosion and pollution.
Riparian woodland strips can slow water flow, trap sediment and pollutants running off farmland and can offset the effects of flooding, which all lead to higher water quality. Without tree roots to stabilise the riverbank, too much silt enters the river, reducing its clarity and oxygen levels. Bank erosion can also occur, causing ‘over-widening’ – this makes the river shallower, increasing water temperatures, which can be dangerous for fish. Trees are also fundamental for fish life in the river as they provide shade, create habitat for fish in their root systems and are a source of invertebrate food.
The effects of riparian woodland on rivers. Left: river with degraded riparian woodland. Right: river with healthy riparian woodland.
Trees found growing along riverbanks are obviously those which like wet conditions and don't mind some temporary flooding. Species commonly found on riversides in the Garnock Connections landscape include alder (Alnus glutinosa), aspen (Populus tremula) and various types of willow (Salix species).
Did you know...
...alder timber is used to provide the supports for piers?
2000 year old alder timbers have been found still intact at Crannog sites (ancient loch dwellings) within Scottish lochs. There is anecdotal evidence of the presence of crannogs in both Castle Semple and Kilbirnie lochs, and dug out canoes were found nearby.
The old gaelic name for aspen was Eadha which translates as “most buoyant one” - aspen timber was used to make oars and paddles.
A reconstructed crannog on Loch Tay, Perthshire
Trees as straws and umbrellas
Trees use and lose water in two ways.
Firstly, trees act like straws, drawing water from the soil into their roots. This water then travels through the tree to the leaves, where it evaporates. This process is called transpiration.
Secondly, when it rains, tree canopies act like a big umbrella, catching the raindrops. These then evaporate back into the atmosphere.
The combined amount of water which entres the atmopshere from these processes is called evapotranspiration – and it’s vital for flood prevention.
Riparian woodlands return 55% of our annual rainfall to the atmosphere...
We can use annual rainfall levels combined with the species of tree to calculate the total amount of evapotranspiration - rainfall returned to the atmosphere.
The annual rainfall in an area influences the proportion of the rain which will be intercepted by the trees. In the Garnock Valley, the average annual rainfall will be around 1400mm; the trees ‘catch’ around 19% of this, or around 270mm/yr.
The transpiration rate - the amount of water drawn from the soil and released into the atmposhere from the leaves – is between 300 and 350mm/yr for an average tree. But willow and aspen – two of the most common riparian tree species - are very thirsty, and have particularly high transpiration rates, in excess of 500mm/yr!
So, taken together, the total annual evapotranspiration resulting from a riparian woodland will be around 770mm/yr - which equals 55% of the annual rainfall. That's pretty astonishing.
...so it doesn't overfill our rivers and cause flooding
This means that 55% of our annual rainfall will be returned to the atmosphere, instead of causing flooding. As climate change is likely to lead to wetter conditions in the west of Scotland, expanding riparian woodland will be a crucial and effective strategy for flood prevention.
Growing for Garnock: restoring our rivers
Because of the amazing services riparian woodlands provide, Garnock Connections have identified them as a priority for habitat improvement work. Eadha Enterprises' Growing for Garnock Project is helping to create new riparian woodlands across the landscape.
Riparian tree planting by Eadha Enterprises and volunteers along the Garnock near Kilwinning for its Growing for Garnock Project.
Planting trees for a new woodland is only the first stage in creating a habitat, though. New planting often takes place on old farm pasture which has been ‘improved’ for agriculture - but unfortunately, this actually degrades habitat for wildflowers and depletes the range of plant species present. We can enhance woodland creation by underplanting with shrubs and wildflowers, but introducing species associated with woodland is best left until trees mature and create a dense enough canopy to block out the brightest light - they thrive in shady conditions. Eadha Enterprises is growing these plants in its nursery so they can be planted into new riparian woodland sites.
Common Knapweed, Yellow Flag Iris and Valerian
Many of the wildflowers which grow on damp riversides in the Garnock Connections landscape will be flowering at this time of year - and the Bioblitz is a perfect time to go looking for them! You can download our guide to riparian wildflowers here.
Peter Livingstone is the CEO of Eadha Enterprises and Project Lead for Growing for Garnock.
For more information about its Growing for Garnock project contact Peter at peter@eadha.co.uk
You can follow Eadha on Facebook and Twitter (@eadhaaspen)
