One of the mantras of working with nature to tackle the effects of climate change is to “slow the flow”. When rain falls, the ideal scenario is that most of it soaks gently into the soil and down into groundwater, after which it is gradually released to the river system by natural springs. This not only helps reduce flood risk but also re-supplies groundwater storage which helps maintain river levels during drier conditions.
But, if the soils are compacted (by heavy machinery, or the hooves of livestock for example), the rain can’t soak in and runs over the surface, often washing out soil particles at the same time. This muddy water quickly runs into the nearest stream and then into the river, creating flood conditions if we are unlucky. Not only that, but the soil particles suspended in the water damage the river, silting up the bed and affecting fish breeding grounds and many other species. With climate change, as the intensity and volume of rainfall events increases, this is likely to happen more and more frequently. Plus, hotter, drier summers mean low river levels are more likely and so our groundwaters will need more regular replenishment.
Slowing the flow aims first to get more water into the ground at the point of the rainfall’s arrival. It’s important to look at a wide range of factors to achieve this, including farming practices as these tend to be the biggest part of the picture in lowland settings. Trees alone are not the solution – but they can play an important part, and they also bring lots of other benefits including carbon storage, capturing farm nutrient run-off, wildlife habitat, air quality, recreation space and building materials.
A woodland is a sheltered environment and the canopy of leaves means rain falls more gently to the ground, reducing erosion. Water also evaporates from those leaves (even in the middle of a rainstorm). And, most importantly, the average woodland floor is highly absorbent to water – up to 67 times more so than a compacted field surface. Why is this?
- It doesn’t usually get driven over by large machines or trampled by livestock.
- It has a dense root system reaching deep into the ground that creates lots of routes (!) for water to travel down.
- And it has a rich humus layer from decaying leaves, again creating a porous, open-celled soil structure.
Woodlands also physically slow the flow of water because they offer a “rougher” surface – about 5 times rougher than a field. The trees, their fallen branches, the shrubs and vegetated woodland floor are all barriers that slow the flow as water runs across the surface. This same roughness can also capture sediment and other pollutants that would otherwise end up in the watercourse. And individual trees on river banks can help prevent bank erosion because of their strong network of roots.
All told, a woodland is a wonderful sponge! One study found that run-off from woodland was 78% less than from adjoining grazed pasture.
So, what does all this mean for the River Culm? We want to encourage landowners throughout the catchment to plant trees – either as woodland blocks, or as restored hedgerows, or even as individual bankside trees, all of which act as buffers that protect the river from many impacts. The Forestry Commission has a programme to support landowners with the costs of planting and, because the whole of the Culm catchment is in a Water Quality priority zone, this means grants are available for planting just 1 hectare of woodland, which can be broken up into smaller blocks (minimum size 0.1 ha). The grant helps pay the initial costs of the trees, stakes and guards, planting, fencing around blocks and gates; it then provides annual payments for the first ten years after planting, as the trees get established. You can find out more about the scheme here, and the Forestry Commission’s local officer (Karl Forte) can advise on the grants available and how to apply. Our own Farm Advisor, Daniel Halford, can look at a whole farm plan with you and how other measures can link in with tree planting to make your land a water sponge rather than a water accelerator! (you can contact Daniel via firstname.lastname@example.org).
The film below is a recording of our “Trees for the Culm” Catchment Adaptation Workshop in January 2021, where Karl and colleagues from the Forestry Commission shared their knowledge and opened it all up for discussion. We were also joined by Rosie Cotgreave of Devon Wildlife Trust talking about the Devon Treescapes Project.