Understanding urban trees: form, function and vulnerability

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BackgroundMore than half the global population already live in urban areas and that figure will rise to 66% by 2020. Ecosystem service provision in urban areas is therefore becoming increasingly important for people's quality of life and the wider environment, to which urban trees make a disproportionate contribution. These services include urban heat island mitigation, air pollutant filtration, storm water regulation and carbon sequestration, but there is little understanding of how ecosystem service provision varies among tree species. In addition, whilst the majority of urban forest research focuses on the ability of trees to improve the urban environment, less attention is paid to the effects of the urban environment (e.g. increased pollution, urban heat island effect, drought stress) on the trees themselves. Without understanding the resilience of the existing urban forest to these pressures, it is challenging for people planting trees, from both a local authority level down to an individual, to select species suitable for the urban environment, particularly in the face of future threats such as climate change.This PhD will address the three following questions:1) What are the most common urban trees in a typical UK city and how are they distributed throughout the urban fabric?2) What is the relationship between the distribution of common tree species, size and ecosystem service provision?3) Which urban tree species, at specific life-stages, are vulnerable to typical urban stresses?ObjectivesThis project will use Sheffield as a case-study to establish long-term monitoring sites at a city-scale by fitting common urban tree species with in-situ growth monitoring devices. This in-situ monitoring network will be combined with regular fieldwork, to measure physiological variables and key indicators of ecosystem service provision, lab analysis, GIS and remote sensing to explore how urban tree distribution, species and size affects tree function and therefore ecosystem service provision and how these trees are affected by urban stresses. It will address the following objectives:1. Determine the distribution of common urban tree species and sizes within different land-covers at a city-scale.2. Assess the contribution of urban trees to key ecosystem services, including air temperature, pollutant removal and carbon storage in different urban land-covers. 3. Determine the effects of urban stressors, including pollution, heat and drought, on the form and function of tree species across the urban landscape. 4. Co-design with the local authority tree planting guidelines to ensure the resilience of the urban forest in the future.This project will be managed in partnership with Sheffield City Council. Dr Nicola McHugh, a Data Scientist at Sheffield City Council, will co-supervise the PhD. She has particular expertise in GIS based analyses of the urban forest. In addition, the student will work with Jerry Gunton, the Trees and Woodlands Manager. They will provide in-kind support to the student including access to the i-Tree Eco survey data (in-kind value £20,000) and staff time (in-kind value £2,000). Novelty and timelinessThe Governments' 25 Year Environment Plan (published in 2018), aims to plant 1 million urban trees by 2020. This research will better inform decision making on providing 'more and better quality green infrastructure, including trees', and aid development of the national framework of green infrastructure standards proposed in the 25 Year Plan. It will help local authorities to understand how to plant the 'right tree for the right place' in order to both maximise the trees chance of survival and to provide the ecosystem service required at that specific location. Understanding these questions is key to ensuring a sustainable and resilient urban forest into the future.