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Forest grazing counteracts the effectiveness of trees to reduce flood risk

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October 10, 2017 Lancaster University read full ScienceDaily article here

As the frequency and severity of flooding becomes an increasing problem, land managers are turning to natural flood management measures, such as tree planting, to reduce the risk. When rainfall exceeds the rate at which water can enter the soil it flows rapidly over the land’s surface into streams and rivers. Trees can help to reduce the risk of surface runoff by increasing the number of large pores in the soil through which water can drain more easily. Land use, such as grazing, also affects the soil’s ability to absorb water; however, while the effect of land use on surface runoff has been well studied in grasslands, little is known about the effect of land use in forests.

…Researchers found that infiltration rates were between ten and a hundred times higher under trees, when the forested area remained relatively undisturbed, compared with adjacent pasture. Where sheep were allowed to graze under the trees there was no observable difference from the pasture.

They also compared forest types — conifer forest planted with Scots Pine and broadleaved forest planted with sycamore — and found that infiltration rates were significantly higher under Scots Pine than under sycamore, but only when the forest was ungrazed…

…”Tree planting can make an important contribution to flood risk management, but forest buffer zones, with restricted access, strategically placed to intercept surface runoff before it reaches the stream may be more effective than larger scale planting when the forested areas are used for other purposes.”…

K.R.Chandler, C.J.Stevens, A.Binley, A.M.Keith. Influence of tree species and forest land use on soil hydraulic conductivity and implications for surface runoff generation. Geoderma, 2017 DOI: 10.1016/j.geoderma.2017.08.011

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