1 INTRODUCTION

Plants sustain life on our planet by regulating major biogeochemical cycles (Bolin & Cook, 1983; Melillo, Field, & Moldan, 2003). Plants also provide multiple benefits to people, in the form of goods (e.g. food, raw material) and services (e.g. aesthetic, physiological) (Balick & Cox, 1996; Lohr, 2007). In urban ecosystems, plants contribute significantly to human welfare via their contributions to carbon sequestration, pollutant assimilation, heat mitigation, increased biodiversity, improvement of human health, cultural value, and social integration (e.g. Alvey, 2006; Ballinas & Barradas, 2016; Germann‐Chiari & Seeland, 2004; Nowak, Hoehn, & Crane, 2007; Oliveira, Andrade, & Vaz, 2011; Tzoulas et al., 2007).

Urban forests—a collection of trees that grow within and around human settlements—are unique ecosystems, typically existing in highly fragmented environments, with a distinctive species composition and structure (Williams et al., 2009). Human needs and preferences play a decisive role in determining the composition of urban forests (Gerstenberg & Hofmann, 2016; Sæbø, Benedikz, & Randrup, 2003), which can vary depending on the type of urban setting, its location, and the desired benefits associated with the planting (Song, Tan, Edwards, & Richards, 2017). For instance, cities typically have areas with significantly higher temperatures compared to the surrounding non‐urban areas—the urban heat island effect. In such locations, species with broad canopies and high transpiration rates might be selected to aid with heat mitigation (Ballinas & Barradas, 2016). Although, climate might not necessarily dictate which species are planted in an urban forest, the species’ climate of origin has been identified as a key factor determining species’ survival and performance (Kendal et al., 2018). In addition, many species are able to grow successfully in climatic conditions beyond those of their natural distribution (Booth, 2017; Booth, Nix, Busby, & Hutchinson, 2014).

The unique characteristics of the urban environment may increase the vulnerability of some species to climate change. Given that there are thermal and aridity limits to species distributions (Stuart‐Haëntjens et al., 2018; Woodward & Williams, 1987), predicted changes in climate (means and extremes) may threaten both existing and future urban forests. The impervious surfaces throughout urban areas generally have high heat retention properties and may alter the retention, infiltration, and reuse of water compared to natural substrates (Kumar et al., 2016; Norton et al., 2015). Extremes of low soil moisture availability will be exacerbated by changes in rainfall seasonality and associated increases in the duration and intensity of drought (De Sherbinin, Schiller, & Pulsipher, 2007). Additionally, the urban heat island effect will be aggravated by projected increases in temperature (Corburn, 2009), with expected increases in the magnitude and duration of heatwaves impacting plant and animal species as well as human populations (King & Karoly, 2017; Perkins‐Kirkpatrick & Gibson, 2017). Thus, assessing the relationships between species occurrence and climatic conditions, such as temperature and precipitation, can provide valuable information on tree species’ potential vulnerability to climate change (Busby, 1988).

Extreme climate conditions are likely to affect species performance and, ultimately, alter the composition of urban forests. This issue is important because urban forests typically include species that are geographically and climatically distant from their natural distributions (Kendal et al., 2018). For species that originate from cooler climates, increases in temperature may limit their persistence in warmer cities. However, rising temperatures will make cities in cooler climates more suitable for species that originate from warmer climates (Jenerette et al., 2016; Kendal et al., 2018). Similarly, in areas where trees receive irrigation, plants are unlikely to be constrained by ambient rainfall and will be less vulnerable to reductions in precipitation (Vogt et al., 2017). Despite the role that climatic suitability may play in shaping species choice and performance in urban areas, few comprehensive studies have explored tree species composition of urban forests across wide‐ranging climate zones (but see Jenerette et al., 2016; Ramage, Roman, & Dukes, 2013).

Urban forests require species that can tolerate the climatic conditions likely to occur during the life‐span of the individuals (McPherson, Berry, & Doorn, 2018). However, species selection requires consideration of many underlying complexities, including land use, land ownership, competing priorities, and governmental policy (e.g. biosecurity protocols). Furthermore, selection must consider climate adaptation, disease resistance, and phenotypic plasticity, along with aesthetic and social factors (Roman et al., 2015; Sæbø et al., 2003). Thus, evaluating relationships between existing urban forest species and climate provides a basis for identifying vulnerabilities. Vulnerability is defined here as the potential for an adverse impact of external environmental stressors (e.g. heat and moisture) on trees in urban environments. Vulnerability is influenced by the exposure and sensitivity of a system to climate change, as well as its capacity to adapt to this change (IPCC, 2014).

The aim of this study was to (a) assess the tree species composition of urban forests within 22 of Australia's Significant Urban Areas (SUAs); (b) assess the potential sensitivity of tree species in urban environments to extreme temperature and precipitation conditions; and (c) undertake a categorization of potential vulnerability. Combined, the 22 SUAs capture a wide range of urban climates, from cool and wet to hot and dry. We collated lists of tree species planted in urban areas across Australia, evaluated the breadth of species’ realized climate niches based on native and non‐native occurrence data, and compared these to the extremes of climate currently experienced in each of the 22 SUAs where the species are planted. Species’ vulnerability was assessed by determining whether it is likely to be affected, adversely or beneficially, when exposed to climatic extremes that are experienced in the SUAs where they are planted. The resulting categorization of potential climate vulnerability can be used for future urban planning and species selection.