Everyone knows the role of trees in alleviating carbon dioxide and climate change , but less well known is their ability to clean water and control flooding. The Woodland Trust has just published an interesting new report on the role of woodlands in water management. They say, “Conserving and restoring native woods, and creating new ones, can improve water quality, reduce localised flooding, and may alleviate the effects of larger floods.” They estimate that native woodland planted in the right place has the ability to mitigate flooding and reduce pollution by up to 90%!
Both the quantity of surface water and the cleanliness of rivers, streams and ditches are affected by the land use around them. For instance, around agricultural fields nitrates may be found in the run-off in surrounding ditches and soaking down into the groundwater. Trees take up large amounts of water through their root system and by “catching” rainfall in their leaves and branches, which can help keep river levels down.
Also, tree roots stabilise soil, increasing water absorption and lessening run-off from the surface. Trees and shrubs also act as a physical barrier to flooding. Keeping surrounding soil where it is and reducing erosion reduces the amount of, possibly contaminated, sediment being washed into streams and rivers. Nutrients and pollutants in the soil are also taken up by trees and thereby neutralised. On the less positive side, leaves catch air pollutants on their surface, which are then washed off onto the ground when it rains. This effect is particularly noticeable with conifers.
The full report is available at http://www.woodland-trust.org.uk/pdf/woodswater26_03-08.pdf