Planting native trees can help battle to control flooding

Andrew St Ledger of The Woodland League looks at the benefits that planting native trees can bring in the battle to control waterways prone to flooding

Native woodlands served to control water levels in flood-prone areas until their destruction by poor forestry policy.

WE CAN all agree that we are in desperate need of wise flood-prevention measures that work.

This is a long-running problem — to quote the Chinese emperor Yu, 3,160 years ago whose ancient proverb reminds us, “To protect your rivers, first protect your mountains”.

Within only a few weeks of the conclusion of the Paris Climate Conference — COP 21, many of our lowland towns have experienced extensive flooding. The cause may lie partly with climate change, but one solution to the flooding is much nearer home: In our uplands, where the wrong species of tree and forestry model have been chosen for decades. What is needed on appropriate selected sites in the uplands are native, deep-rooted trees, planted with a view to flood alleviation, soil protection, carbon uptake, small farm viability, and biodiversity — rather than quick profit for the few, which those downstream end up paying for.

Native woodlands in uplands reduce the effects of flooding. This is proven by recent scientific research such as the Pontbren Project, by Bangor University in Wales. They examined the management of upland sites by a group of farmers. It established that soil under mixed native trees absorbs water 67 times faster than under grass: Native trees have such deep roots that they provide channels to send the water much further underground. The soil under native trees acts as a sponge — a reservoir — which sucks in water, then releases it slowly.

The Pontbren Project in Wales is a farmer-led approach to sustainable land management in upland sites comparable to Ireland. It started in 1997, when three disillusioned sheep-farmers decided to farm sustainably. They were joined by seven others farming a total of 1,000 hectares. To date they have planted 120,000 trees, 5% of the area. They established their own tree nurseries to supply the locally-adapted trees for their sites reducing the cost, since they receive no subsidies for this important work.

They reinstated 26.5 km of hedgerows and created 12 ponds covering 5.4 acres of ground: Restored natural wetland sites which act as reservoirs. They cut and chip wood to make bedding for their animals, saving money, as they no longer pay for straw. The research found, if all the farmers in the catchment followed their example, flooding downstream would reduce by 29%. Full reforestation would reduce the flooding by 50% or more.

During the 1600s Ireland’s uplands were stripped of their native forest cover. Since then, overgrazing by sheep, cattle, deer, and poor forestry policy have ensured the native forest has never recovered. By 1900 tree cover stood at about 1%. A non-native industrial forestry model was then started, one that was continued by the Irish Free State. It targeted the poorer-quality uplands, as good land was needed for agriculture. The policy has continued into the present day.

With regard to increasing tree cover on the island of Ireland, the emphasis now must be on planting native species, not only in the Shannon and Lee river basins but also in the degraded uplands. This is where the clear felling of trees using heavy machines compacts soil, not allowing water to soak and vertical forest drains cause water to speed up after heavy rain, carrying soil and debris silting up the rivers, streams, and underground waterways. All of which contribute to flooding.

There are now calls for dredging which will not solve the problem: The silting will occur again since the major cause of the problem is in the uplands. It will just speed up the water flow and cause more damage further downstream. We need to slow the water down in the uplands first, and then manage the water in restored river systems, allowing them to meander again, with natural floodplains to allow the soil to soak up the water. Many countries which dredged their rivers are now rectifying their mistake.

Scrub and hedgerow removal in recent times has removed more native trees reducing the soil’s ability to soak up water adding to the flooding problem. Recent forestry inventory data from (NFI 2012) under land use type change, 2006-2012, shows an approximate loss of 2,356 hectares per year since 2006. These deforestation statistics, when placed beside low planting figures, break down to show net afforestation (new area under trees) figures of approx 3,279 ha per year: a dismal record needing attention.

The national forestry research body, Coford (Council for Forest Research and Development), published a paper in 2007 recommending for acid sensitive areas, the planting of sessile oak, the deep-rooting climax tree of the hillside; downy birch, the soil improver which raises the pH of acid soils; and rowan.

Research in 2003 by woodland botanists Daniel Kelly and others, investigated restoring native oak woods on former Sitka spruce sites, found that sessile oak, birch with holly, and rowan were their chosen species to “mend the hills and repair the streams, their pH, and their aquatic and insect life”.

In 2013, the Irish forest policy review group report stated: “There is good evidence that riparian forests and forests in flood plains reduce flow velocity, enhance out of bank flows, and increase water storage on flood plains, resulting in an overall smaller downstream flood event.”

This catastrophic flooding scenario is upon us now. Ministers one and all say they have no answers — all the more reason why we need to take seriously the idea of restoring native woodlands in our vulnerable uplands. We must now take heed of the Welsh and Irish scientific research informing us that this is the first port of call towards finding solutions. This is in order to reap the known multiple benefits that will accrue immediately and long into the future. The Welsh research found that planted trees were showing positive effects on water retention after two years.

It is now time for the powers-that-be to remove their blinkers and make an honest reappraisal of our flawed forest policy while their attention is drawn to this major flooding problem.

Andrew St Ledger is the chairman of the Woodland League: Dedicated to Restoring the Relationship between People and their Native Woodlands. See www.woodlandleague.org.

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