Planting trees can reduce flood risk in some cases, but a high intensity forest land use, such as grazing, could counteract any positive effect of trees, a new study suggests.
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. The Centre for Ecology & Hydrology (CEH) and Lancaster University collaborated on the new research which investigated the influence of forest land use under two different tree species on the ability to mitigate the risks posed by flooding.
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 structure and its ability to absorb water but less is known about the effect of this land use in forests compared to grasslands where it has been well studied.
Researchers investigated the rate that water infiltrated the soil under trees at an experimental agroforestry site in Scotland. They 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.
Researchers found that infiltration rates were between ten and one 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.
The scientists conclude that although tree species with differing characteristics can create large differences in soil hydraulic properties, the influence of land use can mask the influence of trees. They argue that the choice of tree species may be less important than forest land use for mitigating the effects of surface run-off which can lead to flooding.
Using rainfall records the researchers were able to infer that a storm with a probability of occurring at least every two years would be very likely to generate surface runoff in the grazed forest at the field site.
Dr Aidan Keith, a soil ecologist at the Centre for Ecology & Hydrology worked on the study. He said, "The results provide evidence that undisturbed forest has the capacity to not only reduce surface runoff but also to ‘soak up’ runoff generated further up the hillslope."
The scientists say further research is now needed to understand vegetation effects under different land use intensities and how vegetation and land use effects interact. However, they maintain that when considering the influence of forest on soil saturated hydraulic conductivity and, indeed, other soil properties, this study emphasises the importance of differentiating between undisturbed forest as a land use and forest as the vegetation cover.
Lead author Dr Kathy Chandler from Lancaster University said, "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."
Full paper reference: Chandler, K R, Stevens, C J, Binley, A, Keith A M. 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
The study focused on the Glensaugh experimental site in Scotland where the average annual rainfall is 1,168 mm. The site is managed by the James Hutton Institute and is also part of the Environmental Change Network.
CEH recently published a systematic review looking at the role of trees in flood alleviation