Abstract
High species diversity is argued to be the most important requisite for a resilient urban forest. In spite of this, there are many cities in the northern hemisphere that have very limited species diversity within their tree population. Consequently, there is an immense risk to urban canopy cover, if these over-used species succumb to serious pests or pathogens. Recognition of this should motivate the use of less commonly used species. Analysis of plant traits, such as the leaf water potential at turgor loss (ΨP0), can provide useful insights into a species’ capacity to grow in warm and dry urban environments. Therefore, the aim of this study was to evaluate ΨP0 of 45 tree species, the majority of which are rare in urban environments. To help evaluate the potential for using ΨP0 data to support future decision-making, a survey of professionals engaged with establishing trees in urban environments was also used to assess the relationship between the measured ΨP0 and the perceived drought tolerance of selected species. This study demonstrates that ΨP0 gives strong evidence for a species’ capacity to tolerate dry growing conditions and is a trait that varies substantially across species. Furthermore, ΨP0 was shown to closely relate to the experience of professionals involved in establishing trees in urban environments, thus providing evidence of its practical significance. Use of plant traits, such as ΨP0, should, therefore, give those specifying trees confidence to recommend non-traditional species for challenging urban environments.
Highlights
The benefits of trees in urban environments are increasingly being recognised
It is important to be strategic in the selection of trees so that new plantings can develop into mature trees, as their contribution to ecosystem services is improved with increasing tree size (Gómez-Muñoz et al 2010; Hirons and Sjöman 2018)
With the exception of Pyrus calleryana, which is quite widely planted in the UK and North America, tree inventory data suggests that they are all rare as urban trees in temperate regions of the northern hemisphere (Bourne and Conway 2014; Cowett and Bassuk 2014; Sjöman et al 2012; Yang et al 2012; Raupp et al 2006)
Summary
Looking forward, the ecosystem services provided by the urban forest will be vital to urban communities (Benedict and McMahon 2006) They include provisioning services (e.g. fuel), regulating services (e.g. storm water management, urban heat island mitigation, air pollution regulation), cultural services (e.g. recreation, physical and mental health benefits), and supporting services (e.g. wildlife habitats) (Akbari et al 2001; Costanza et al 1997; Gill et al 2007; Grahn and Stigsdotter 2003; Morgenroth et al 2016; Tyrväinen et al 2005; Xiao and McPherson 2002). Similar ‘million tree programs’ have been launched in other parts of the world, such as Shanghai (Shanghai roots and shoots 2007) Within such planting programs, it is important to be strategic in the selection of trees so that new plantings can develop into mature trees, as their contribution to ecosystem services is improved with increasing tree size (Gómez-Muñoz et al 2010; Hirons and Sjöman 2018). This requires that species are carefully matched to their planting site and that their vulnerability to pests and pathogens is minimised
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