Understanding urban trees: form, function and vulnerability

Urban trees provide critical ecosystem services for the sustainability and resilience of cities. Ecosystem service delivery by urban trees is determined by tree species and physiological function, but urban environments can present challenges to this. However, our understanding of the physiological function of many common urban tree species, and the long-term impact of urban pressures, is limited. This thesis examines urban tree form, function and vulnerability at the leaf, tree and city-level to address these knowledge gaps. Chapter 2 demonstrates that tree species was the dominant control of urban tree physiology, with no impact of pollution or soil sealing, and that hyperspectral remote sensing could reliably predict urban tree leaf traits, reducing dependence on field surveying. In Chapter 3, the impact of a widespread pathogen (Phytophthora plurivora) on the water use, growth rate and cooling capacity of a common urban tree species (Tilia x europaea) was quantified using IoT linked sensors. While these parameters were significantly higher in asymptomatic trees, infected trees with less advanced disease still provided ecosystem benefit, highlighting trade-offs between public safety and tree disease management. Chapter 4 used a novel network of IoT linked sensors across the urban environment to explore water use and growth rate of urban trees. There were significant differences between tree species, but street trees were not water limited, with significantly higher water use than woodland trees. Extreme heat and drought events dramatically reduced this. Chapter 5 demonstrates the potential of radiative transfer modelling and high-resolution satellite imagery to map urban tree leaf chlorophyll content, a key indicator of tree vitality, across a city. Collectively, the results show that increasing urban tree species diversity via strategic species selection could improve the benefits and resilience of urban forests. Remote sensing offers a robust way to monitor urban trees, vital under global environmental change.