The ability of trees, woodlands and forests to reduce downstream flooding is increasingly recognised and valued by society, driving a demand for assessments of this important ecosystem service. This study updates a previous evaluation (Broadmeadow et al., 2018) with improved estimates for the volume of flood water potentially removed by woodland or retained by its hydraulic roughness. The impacts of woodlands are compared with two alternative land uses, short grass and bare soil. The scope of the analysis was also extended to include the contribution of conifer woodlands, small woodlands, and trees outside woodlands. A companion report (Fitch et al., 2022) is available that applies the approach to other natural capital types, including woodland, although values are not directly comparable due to methodological differences.  

As before, the assessment focussed on ‘Flood Risk Catchments’ to identify the area of woodland draining to downstream communities impacted by flooding. Water volumes are expressed in m³/ha. The analysis assumes that an equivalent level of flood water storage would have to be provided if the woodland cover was absent and replaced by either managed grassland or bare soil. The “replacement” cost of providing such flood water storage (based on seven reservoirs at an average of £14/m³) was then used as an estimate of the flood alleviation value provided by woodland. Values were estimated for woodland by country across Britain and for the public and private woodland estates.

The natural capital value (over 100 years) of the flood regulation service provided by woodland across Britain, including trees outside woodlands (ToW), was estimated at £25.1 billion (£7,974/ha) compared to bare soil and £12.5 billion (£3,970/ha) compared to grass. Expressing the flood regulation service as an annualised central estimate gave values of £843 million/yr (£268/ha/yr) and £420 million/yr (£133/ha/yr) compared to bare soil and grass, respectively. These values are significantly greater than those generated by the previous assessment, reflecting the improved modelling of woodland water use and the broader range of woodland types in the analysis. The findings highlight the value of trees, woodlands and forests in improving flood resilience and protecting at-risk communities from flooding, a service that is likely to become increasingly important with climate change. The results are informing forest policy and support for tree planting for multiple benefits.

Citation: Broadmeadow, S., Nisbet, T., Valatin, G., Blyth, E., Robinson, E., Fitch, A., & Jones, L. 2023. Revised valuation of flood regulation services of existing forest cover to inform natural capital accounts. Forest Research Report to Forestry Commission, Scottish Forestry, Defra and Welsh Government. Forest Research, Alice Holt Lodge, Farnham, Surrey, 38pp.