How do trees help mitigate flood risk?

Guest blog by Christina Hooley at Treesponsiblity

Slow The Flow has worked closely with Treesponsibility for over 5 years through The Source Partnership, a group of organisations who collectively work with nature-based solutions to reduce flood risk.  Tree planting is key in this quest and so we asked Christina Hooley who is the Coordinator at Treesponsibility to write why trees are so important to reducing flood risk. 

Treesponsibility has been around for 23 years so hopefully, some of you will have heard of us. 

Treesponsibility is an unincorporated community group founded in March 1998 to educate about the need for action on climate. Trees draw down and sequester carbon dioxide from the atmosphere. The idea was to offer the opportunity to plant a tree, and that through taking this positive step, people would then be inspired to take further steps towards reducing their carbon footprint.

Over the next couple of years, the aims of the group broadened to embrace adaptation to climate change. However quickly or not the world moves towards carbon neutrality, the world is warming. Some of the effects are unavoidable and are already being felt in our valley and across the globe.

In June 2000, Treesponsibility published a book – The Seeds of Change. The newly printed copies of which were sitting on a pallet at the printers when, on the 4th   of June, heavy persistent rain caused the Calder to burst its banks resulting in serious flooding in the towns nesting in the valley bottoms. The house in Hebden Bridge that I lived in at the time was flooded. The rising flood waters having entered the printer’s premises, stopped just below where the newly printed copies of Treesponsibility’s book were stacked awaiting distribution.

 I always think of this flood as the first climate change flood. Though it was not widely thought of as such at the time. The climate change scenario for our valley is for more frequent and intense heavy rainfall events leading to flooding. Treesponsibility’s founder, the wonderful Penny Eastwood, aka Dongria Khond, who sadly passed away early this year, was very much aware of this predicted scenario. She was also aware that tree planting had a role to play in mitigating flood risk.

Penny/Dongria, always on the ball, proposed that Treesponsibility launch a new project ‘After the flood the Forest’.  A 3-hectare site at Midgelden Bank on the Bacup road was acquired. The land was a steep 45-degree slope and seriously eroded. Much of its surface was bare shale. 3,000 trees were planted. It took a while for these to establish due to the conditions on the site. However, it was a good site strategically for flood mitigation. 

Midgelden Bank 10 years on

How do trees help mitigate flood risk?

  • The first is interception. Trees interrupt the flow of storm water down the steep valley sides, and when in leaf they will intercept up to 30% of rainfall which will then evaporate away without ever reaching the ground. 
  • 70% of the substance of a tree is water. They take up water through their roots thus leaving more capacity for the soil to store water. During the growing season they are constantly drawing up water through their root systems as water is transpired through the leaves.
  • As the trees grow and go through the cycle of coming into leaf through to autumn and winter, the falling leaves gradually accumulate and form new soil. This gives great capacity for absorption and storage of storm water. 
  • The roots of the trees, provided they are deep rooted trees like our native oaks and birches stabilise the soil preventing erosion. Unfortunately, many of the trees standing on the steep valley sides are beech trees, and whilst beautiful, they are shallow rooted and more likely to exacerbate the risk of land slippage than prevent it. Their dense canopies shade out the ground cover plants leaving the soil bare and vulnerable to being washed away into the water courses. 
  • Finally, and perhaps most importantly, the roots of the trees penetrate compacted topsoil and impermeable clay sub soils creating a system of pores allowing water to make its way down into the ground rather than running off. 

Reference – 

A 2020 study showed that trees increase the penetration of storm water into the soil after only 15 years from being planted. This summer, an A’level geography student contacted Treesponsibility asking if he could carry out an experiment on a site, we had planted to test the theory. I was delighted to be able to take him to a site planted with 1,600 trees in 2006. He was able to measure the infiltration rate in the woodland and that of the adjacent pasture. He told me at the time that the results on the ground proved his hypothesis. He is busy working up a report on this data and will be publishing this very soon. 

The right tree in the right place.

I want to talk a little bit here about woodland management. This is often a question of, ‘the right tree in the right place’. In the section above on how trees mitigate flood risk, I explained that deep rooted trees help to stabilise soil and that the beech trees that thrive here in our valley are shallow rooted and dense of canopy and that both factors increase the risk of erosion. You will be aware that woodland management is being carried out in Hardcastle Craggs by the National Trust and in other locations by Calderdale Council. This involves the selective removal of beech, sycamore, and conifer. These species are not native to our valley. They were planted in the 19th century either for visual amenity, as in the case of beech, or as resource for the textile industry as in the case of sycamore.

Does it matter if a tree is not native? Well, yes it does. Beech and sycamore have done such a good job of reproducing themselves that they have out competed our native species and these are suffering a decline. Native species like oak will support more than 2,000 species of animal, plant and fungi in complex relationships that have evolved over millennia. Beech and sycamore will support only two or three hundred species. The dominance of beech and sycamore is rapidly reducing the biodiversity of our valley. 

The loss of soils to erosion not only increases the risk of flooding, it also releases stored carbon to the atmosphere as carbon dioxide. Soil particles in the waterways contaminate fish spawning grounds destabilising aquatic ecosystems. Reference –

The vulnerable beech trees gradually losing their purchase in eroding soils also pose a risk to us as we enjoy an invigorating walk in the woods.

The managed trees will either be left as standing or as lying deadwood where they will continue to provide habitat and sequester carbon. Thousands of native trees are being planted to replace the trees that are being removed.

Treesponsibility doesn’t own any land, we are entirely reliant on landowners coming forward to offer land for tree planting. And over the years there has been a steady trickle of landowners coming forward enabling us to meet our target of 10,000 trees a year. Some years we’ve been able to plant 20,000 trees. In the planting season 2019/2020, we were able to plant 24,000 trees on one site alone as well as the trees planted on smaller sites

We have some special landscapes in Calderdale that are doing a very good job of sequestering Carbon and absorbing storm water as they are and planting trees in these areas would be counterproductive. 

These areas are also providing habitats for increasingly rare and threatened wildlife. There are deep peat soils that, whilst they remain intact, sequester more carbon than would the equivalent area of woodland. Peat lands support wading birds such as curlew and lapwing. We have unimproved pastures rich in meadow species that support pollinating insects. We have rare and beautiful wax cap fungi that indicate a healthy soil with a stable system of pores that allow the infiltration of water. So, if you offer land for tree planting and then find it is not appropriate for that, do not despair. Be proud to know that you are the guardian and protector of a very precious habitat.

What can YOU do? 

Luckily now, there is plenty of funding for tree planting under the various nature for climate and woodlands for water schemes.  

If you would like some trees planted on your land, get in touch and we will arrange a site visit to walk the land and discuss options with you. We will draw up a plan and once approved by you, the plan will be put forward for funding. It could be up to a year before we can plant the trees as all the sites need to be surveyed.

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