Understanding the value of Southampton’s urban trees

Abstract

Urban trees collectively form a forest resource that provides a range of benefits to human populations living in and around them. Termed ecosystem services, these shared benefits provided by the urban forest help to offset many problems associated with increased urban development. Trees improve local air quality, capture and store carbon, reduce flooding and cool urban environments. They provide a home for animals, a space for people to relax or exercise, and can improve social interrelation in communities. These direct benefits to the people who live, work and rest close to Southampton are the focus of this report. Using a well-known assessment and evaluation model – i-Tree Eco v6.0 – the urban forest benefits are herein given a value so that health and wellbeing, and the introduction of biological diversity in an otherwise austere, hard architectural environment, can be appropriately resourced to ensure that the benefits are maintained and where appropriate enhanced. Ecosystem service benefits are directly influenced by the management actions that impact upon the overall structure and vitality of the urban forest resource. Gaining an accurate knowledge of the structure, composition and distribution of trees is a first step to understanding the make-up of the urban forest. Assimilating information from a survey can develop a baseline from which to understand the threats, set goals and monitor progress towards optimising the resource. By measuring the structure of the urban forest, through recording information about the tree species present, their size and condition, the benefits can be determined and the value of these benefits calculated and, in some cases, expressed in monetary terms. By putting a monetary value on these benefits provided by the urban forest, this can increase the profile of the forest, and so help to ensure its value is maintained and improved upon. This was achieved by undertaking an i-Tree Eco v6 survey in summer 2016. The data provides detailed information on the forest’s structure and its composition. It also demonstrates that residents living in Southampton benefit significantly from urban trees: in terms of avoided water runoff, carbon sequestration and the removal of five types of air pollutants we estimate that Southampton’s urban forest provides citizens with ecosystem services worth more than £2.41 million per year. This huge value is still just an underestimate. It excludes the many ecosystem services provided by trees that are not currently assessed by i-Tree Eco, including cooling local air temperatures and reducing noise pollution, and so this value is a conservative estimate of the ecosystem services provided. This study captures a snapshot-in-time. It does not consider how the urban forest has temporally or physically changed over time or the reasons for this change. However, it does start to provide the means to make informed decisions on how the structure and composition of the urban forest of Southampton should change in the future and how to ensure that it is resilient to the effects of a changing climate. This study goes a long way to providing the necessary baseline data required to inform decision making for the future. The study was funded by the University of Southampton Excel internship programme and carried out by Southampton City Council, Treeconomics, Forest Research and students and staff from the University of Southampton.