Understanding the value of Southampton’s urban trees (WC, 2017) Forest Research is the Research Agency of the Forestry Commission and is the leading UK organisation engaged in forestry and tree related research. The Agency aims to support and enhance forestry and its role in sustainable development by providing innovative, high quality scientific research, technical support and consultancy services. Treeconomics is a social enterprise, whose mission is to highlight the value of trees and woodlands and to understand how trees improve our towns and cities. Treeconomics works with businesses, communities, research organisations and public bodies to achieve this. i-Tree is a state-of-the-art, peer-reviewed software suite from the USDA Forest Service that provides urban and community forestry analysis and benefits assessment tools, including i-Tree Eco. The Forest Service, Davey Tree Expert Company, National Arbor Day Foundation, Society of Municipal Arborists, International Society of Arboriculture, and Casey Trees have entered into a cooperative partnership to further develop, disseminate and provide technical support for the suite. In association with: Citation This report should be cited as: Mutch, E.M., Doick, K.J., Davies, H.J., Handley, P., Hudson, M.D., Kiss, S., McCulloch, L., Parks, K.E., Rogers, K. and Schreckenberg, K. (2017) Understanding the value of Southampton’s urban trees. Results of the 2016 i-Tree Eco survey. University of Southampton, Forest Research, Treeconomics and Southampton City Council. Southampton. 81pp. ISBN 978-0-85432-004-2. 2 Technical Report | i-Tree Eco survey of Southampton’s urban trees | Nov 2017 Contents Key definitions ............................................................................................................................. 5 Executive summary ...................................................................................................................... 6 Key results ........................................................................................................................................... 7 Key conclusions ................................................................................................................................... 8 Introduction ................................................................................................................................ 9 Natural capital ................................................................................................................................... 11 Southampton studies ........................................................................................................................ 11 What i-Tree can and cannot do ........................................................................................................ 12 Policy context .................................................................................................................................... 12 Opportunities .................................................................................................................................... 14 Links .................................................................................................................................................. 14 Methodology ............................................................................................................................. 16 Description of amenity value ............................................................................................................ 19 Pests and diseases ............................................................................................................................ 19 Habitat provision ............................................................................................................................... 19 Calculations ....................................................................................................................................... 19 Comparison with other UK i-Tree Eco Studies .................................................................................. 20 Results and discussion ................................................................................................................ 21 Canopy cover .................................................................................................................................... 21 Ground cover .................................................................................................................................... 21 Urban forest structure ...................................................................................................................... 22 Species composition ...................................................................................................................... 22 Species diversity ............................................................................................................................ 23 Size class distribution .................................................................................................................... 26 Tree condition ............................................................................................................................... 28 Leaf area and “dominance value” ................................................................................................. 30 Amenity value ................................................................................................................................... 31 Avoided surface water runoff ........................................................................................................... 34 Air pollution removal ........................................................................................................................ 36 Carbon storage .................................................................................................................................. 40 Carbon sequestration ....................................................................................................................... 42 IMD analysis ...................................................................................................................................... 43 3 Technical Report | i-Tree Eco survey of Southampton’s urban trees | Nov 2017 Habitat provision ............................................................................................................................... 46 Risks of pests and disease ................................................................................................................. 49 Conclusions ............................................................................................................................... 53 Main findings .................................................................................................................................... 53 Recommendations ............................................................................................................................ 54 Limitations ........................................................................................................................................ 55 The authors ............................................................................................................................... 56 Acknowledgements.................................................................................................................... 57 References ................................................................................................................................. 58 Appendix I. Detailed methodology ............................................................................................. 64 Field measurements and i-Tree Eco Models ..................................................................................... 64 Monetising carbon and pollutant capture ........................................................................................ 67 CAVAT Analysis.................................................................................................................................. 68 Appendix II. Species dominance list ............................................................................................ 70 Appendix III. Non-traded values for carbon stored in Southampton’s trees .................................. 74 Appendix IV. Pests and diseases ................................................................................................. 77 Glossary..................................................................................................................................... 81 4 Technical Report | i-Tree Eco survey of Southampton’s urban trees | Nov 2017 Key definitions Urban areas: are often defined by the presence of buildings, roads and railways; a centre of commerce, industry and entertainment; a preponderance of concrete and tarmac; atmospheric pollution; and a population which does not engage in agriculture. In England, urban and rural land use is defined by population density: Rural areas: settlements of less than 10,000 people. Urban areas: settlements of 10,000 or more people (RUC, 2011). Southampton: is classified as ‘urban with city and town’ (class 4 of Defra’s 2011 Rural-Urban Classification system) (Defra, 2014), with a population of 254,2751. It covers an area of 14,174 ha (Bibby & Brindley, 2013). For this study, we used an area of 5,019 ha for the core area of Southampton City. Southampton