Societal Impact Statement

Diversification of urban forests is essential to enhance their resilience to future biotic threats as well as those posed by a changing climate. Arboreta and botanic gardens host a wide range of plant material that can be evaluated to inform tree selection policy. This study demonstrates that plant functional traits, such as the water potential at leaf turgor loss, can be highly instructive when developing evidence‐based recommendations for urban environments. However, if botanic collections are to fulfil a critical role in understanding plant response to environment, they should not be managed solely as visitor attractions but must have scientific objectives at the forefront of management policy.

Summary

• Arboreta and botanic gardens host a multitude of species that can be utilized in research focused on improving diversity within urban forests. Higher tree species diversity will enhance the resilience of urban forests to abiotic and biotic threats and help deliver strategies that foster sustainable communities. Consequently, this study aims to demonstrate the value of botanic collections as a resource for research into tree species selection for more resilient urban landscapes.
• As water stress is a major constraint for trees in urban environments, understanding the drought tolerance of species is essential for urban tree selection. This study evaluates a key functional trait relating to drought tolerance. Using vapor pressure osmometry, the water potential at leaf turgor loss was evaluated for 96 species using plant material from seven botanic collections in North America and Europe.
• Leaf turgor loss contrasted widely in the temperate deciduous trees evaluated and, in summer, ranged from −1.7 MPa to −3.9 MPa. Significant differences in drought tolerance were also apparent across genera and closely related cultivars. Osmotic adjustment was shown to be a major physiological factor driving leaf turgor loss. A meta‐analysis also demonstrated that leaf turgor loss was closely related to a drought‐tolerance scale based on observations of tree performance under drought.
• Arboreta and botanic collections can play a vital role in the evaluation of plant material for urban environments, provided they are curated with scientific objectives at the forefront of management policy and are not managed purely as visitor attractions.

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