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Urban greening solutions such as green roofs help improve residents’ thermal comfort and building insulation. However, not all plants provide the same level of cooling. This is partially due to differences in plant structure and function, including different mechanisms that plants employ to regulate leaf temperature. Ranking of multiple leaf and plant traits involved in the regulation of leaf temperature (and, consequently, plants’ cooling ‘service’) is not well understood. We, therefore, investigated the relative importance of water loss, leaf colour, thickness and extent of pubescence for the regulation of leaf temperature, in the context of species for semi-extensive green roofs. Leaf temperature was measured with an infrared imaging camera in a range of contrasting genotypes within three plant genera (Heuchera, Salvia and Sempervivum). In three glasshouse experiments (each evaluating three or four genotypes of each genus), we varied water availability to the plants and assessed how leaf temperature altered depending on water loss and specific leaf traits. Greatest reductions in leaf temperature were closely associated with higher water loss. Additionally, in non-succulents (Heuchera, Salvia), lighter leaf colour and longer hair length (on pubescent leaves) both contributed to reduced leaf temperature. However, in succulent Sempervivum, colour and pubescence made no significant contribution; leaf thickness and rate of water loss were the key regulating factors. We propose that this can lead to different plant types having significantly different potentials for cooling. We suggest that maintaining transpirational water loss by sustainable irrigation and selecting urban plants with favourable morphological traits are the key to maximising thermal benefits provided by applications such as green roofs.

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Urban trees are instrumental in providing ecosystem services to urban society. However, they are subject to many pressures linked to harsh site conditions created by the urban environment, climate change, attack by pests and diseases and the political and social influences that control how urban trees are perceived and managed. An ongoing provision of services from all trees within an urban area (known as the urban forest) can only be achieved through high resilience to these pressures. This study aims to examine the current state of composition, condition and structure of 12 urban forests across Great Britain, by reviewing data collected from i-Tree Eco city or town-wide surveys and recommending a framework to rate their sustainability. The study shows that urban forests are hardy to local winter temperatures and in good health, but lack in large size trees and tree species diversity. Also, several urban forests offer small percentage canopy cover. The wider application of this framework and the creation of detailed tree strategies could help local authorities to establish management strategies, set goals and monitor progress towards achieving greater sustainability.