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. 2025 Oct 14;138(1):120–133. doi: 10.1093/aob/mcaf238

Genetic diversity in urban forests and their potential role in conserving the world’s threatened tree species

Natalie L R Love 1,✉, Kieran Althaus 2,3, Manuel Esperon-Rodriguez 4,5, Alessandro Ossola 6,7, Jason Johns 8, Camille Pawlak 9,10, Boris Igić 11, Matt Ritter 12,13, Jenn Yost 14,15
PMCID: PMC13409150  PMID: 41091786

Abstract

Background

Plant species diversity is threatened worldwide, and urgent conservation action is needed. Emerging evidence demonstrates that urban forests, which are ubiquitous in cities globally, are repositories of threatened tree species diversity. Urban forests consist of billions of individual trees, suggesting that they may serve an important role as reservoirs of genetic diversity for ex situ tree species conservation. However, for most urban populations of tree species, little is known about their genetic diversity. How genetically diverse are urban tree populations? What proportion of a species’ genetic diversity is captured in urban populations? How does genetic diversity of urban tree populations vary among species and cities? These questions remain relatively unexplored, undermining our understanding of the conservation value of urban forests and our ability to predict how these critical ecosystems will respond to climate change.

Scope

This perspective explores the importance of genetic diversity in urban forests and reviews the current knowledge on this topic. We also discuss the potential role of urban forests in tree species conservation and propose utilizing urban forests as part of the broader ex situ collections to enhance conservation efforts.

Conclusions

By considering urban forests as part of ex situ conservation collections, we can simultaneously advance conservation and urban forestry goals. Urban greening strategies that prioritize planting a genetically diverse assemblage of tree species can help overcome some of the major challenges to ex situ conservation such as limited planting space. At the same time, this strategy could increase urban forest resilience to disturbances such as pests, diseases, climate change or other stressors. This conservation strategy would build upon the important work done by botanic gardens while also engaging them with multiple stakeholders, including municipal urban foresters, nurseries, universities and the public.

Keywords: Ex situ collections, intraspecific genetic diversity, metacollection, nature-based solution, urban forest resilience, urban greening, urban tree, biodiversity conservation

INTRODUCTION

An estimated 40 % of the world’s vascular plant species are threatened with extinction, and urgent, innovative conservation actions are needed to protect them (Nic Lughadha et al., 2020; IUCN, 2024). While urbanization is a major threat to species diversity (McKinny, 2002), cities have also emerged as unexpected repositories of plant species diversity despite covering only 3 % of the Earth’s land surface (Liu et al., 2014; Ives et al., 2016; Aronson et al., 2017; Ossola et al., 2020; Ismail et al., 2021; Love et al., 2022). Many threatened species are found in remnant or novel ecosystems within cities (Ives et al., 2016; Planchuelo et al., 2019) as well as cultivated in public greenspaces and private gardens (Ossola et al., 2020; Ismail et al., 2021; Offord and Zimmer, 2024). Indeed, some plant species that are extinct in the wild now, exclusively, occur as cultivated species in public and private gardens (Ismail et al., 2021). Thus, urban areas represent a promising, and potentially overlooked, place to safeguard biodiversity and represent a conservation approach that can be employed in tandem with other strategies. Trees in particular need protection, with more than one-third of the world’s species at risk (Guo et al., 2022; Boonman et al., 2024; IUCN, 2024). Given the charismatic role trees play in cities as well as efforts to expand urban forests, there is a potential synergy between conservation and greening initiatives.

Urban forests, defined as all trees in and around densely populated human settlements that are cultivated in municipal parks, along streets in the public right of way and residential backyards, serve an important role in enhancing city livability by providing essential ecosystem services (Turner-Skoff and Cavender, 2019). Because of their well-documented ability to decrease air and land surface temperatures, urban forests are critical nature-based solutions for combating the effects of climate change and the urban heat island (Shishegar, 2014; He et al., 2024). Consequently, there are major efforts underway to increase canopy cover and improve the climate resilience of urban forests (Endreny, 2018; Esperon-Rodriguez et al., 2022a, 2022b). For example, nine of the 12 largest cities in the USA have collectively pledged to plant more than 20 million urban trees (McPherson and Young, 2010).

Cultivating species-rich urban forests is often an explicit urban forest management strategy to meet resilience goals (Santamour, 1990; Muller and Bornstein, 2010; Sjöman et al., 2018). This strategy has resulted in high tree species richness in many urban landscapes (Kendal et al., 2014; Ossola et al., 2020; Love et al., 2022; McCoy et al., 2022; but see Paquette et al., 2021 and Hilbert et al., 2023 for examples and drivers of low urban tree species diversity). Urban forests also harbour species considered threatened by extinction (i.e. those ranked as vulnerable, endangered or critically endangered on the International Union for Conservation of Nature, or IUCN, Red List; IUCN 2024) or extinct in the wild (Fig. 1). This includes at least 479 threatened or extinct in the wild species currently grown in at least 960 cities in 36 countries (Ossola et al., 2020; Love et al., 2022). For example, ginkgo (Ginkgo biloba), an endangered species native to China, is planted in over 200 cities globally, making it one of the most widely distributed urban tree species in the world (Ossola et al., 2020; IUCN Red List Category: Endangered; Sun, 1998). Some threatened tree species are also highly abundant with large populations in urban forests (Fig. 2). Torrey pine (Pinus torreyana) is critically endangered with only 5000 individuals in its native range in California, USA (Farjon, 2013; Hamilton et al., 2017). Yet, there are at least 13 000 individuals inventoried in urban forests from 128 cities (Love et al., 2022; Fig. 2A). With billions of trees growing in cities worldwide (Endreny, 2018), urban forests may serve as significant conservation sites outside species’ natural habitats (i.e. ex situ conservation sites).

Fig. 1.

For image description, please refer to the figure legend and surrounding text.

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Threatened tree species richness in cities. The number of species identified as threatened by extinction or extinct in the wild by the IUCN (IUCN 2024) grown in urban forests in 960 cities. Each data point represents a city where urban trees have been inventoried. Urban tree species inventories are developed by cities to facilitate management of trees in the public right-of-way (e.g. street and park trees). Data are from compiled inventory data, including Global Urban Tree Inventory (GUTI; Ossola et al., 2020) and the California Urban Forest Inventory (CUFI; Love et al., 2022).

Fig. 2.

For image description, please refer to the figure legend and surrounding text.

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Threatened tree species abundance in California, USA. The California Urban Forest Inventory data (CUFI; Love et al., 2022) show that thousands of individuals representing threatened tree species are currently planted in urban forests across the state (California). (A) Torrey pine (Pinus torreyana) and (B) island oak (Quercus tomentella) represent species native to the state of California while (C) sugar gum (Eucalyptus cladocalyx) is an introduced species native to the state of South Australia, Australia. Each point represents an inventoried city and is sized and coloured by the number of individual trees present. County boundaries are indicated with black lines within the state boundary. The native range of each species is indicated in maps in the top right corner, and the extent of the native range for A and B are indicated on the map with a black box.

Understanding the genetic diversity of urban tree populations (i.e. the variety of alleles and genotypes present in the population) is critical for assessing their conservation value and for urban forest management (Griffith et al., 2020; Hoban et al., 2022). Even in species-rich urban forests, low genetic diversity could reduce the resilience of urban populations to disturbances such as climate change or pest and disease outbreaks (Reusch et al., 2005; Hajjar et al., 2008; Kettenring et al., 2014). These disturbances can jeopardize important ecosystem services provided by trees. The same principles that suggest species richness increases urban forest resilience also apply to genetic diversity, which can also help buffer populations against stressors (Reusch et al., 2005; Kettenring et al., 2014). Yet, it is rarely considered in urban management and many fundamental questions remain unaddressed (Lohr et al., 2016). How genetically diverse are urban tree populations? Are urban tree populations more or less genetically diverse than local wild populations? What proportion of a species’ genetic diversity is captured in urban populations? How does genetic diversity of urban tree populations vary among species and cities? Addressing these questions is critical for understanding urban forest conservation value, which reflects their potential to act as reservoirs for species and genetic diversity and their role in ex situ conservation of threatened tree species.

This perspective explores the importance of genetic diversity in urban landscapes and evaluates the current knowledge on this topic through a comprehensive literature review. We also consider the potential role of urban landscapes in conserving the genetic diversity of plants. While concepts addressed in this perspective apply broadly to all plant species, here we specifically explore the role of urban forests in tree species conservation and propose utilizing urban forests as part of the broader ex situ collections to enhance conservation efforts (Griffith et al., 2019). By integrating diverse genetic backgrounds into future greening efforts, we can advance both urban forestry and tree conservation goals simultaneously. This strategy could enhance urban forest resilience and also expand the planting area available to grow tree species ex situ by utilizing urban areas. This approach will build upon important conservation work implemented by botanic gardens and arboreta (henceforth referred to together as botanic gardens) as part of a multi-pronged approach to biodiversity conservation.

GENETIC DIVERSITY IN URBAN FORESTS

The significance of genetic diversity for urban forests

It is widely recognized in urban forestry that species diversity (i.e. the variety and abundance of different species within a geographical area) enhances resilience by spreading the risk of loss or health decline across multiple species (Kendal et al., 2014; Paquette et al., 2021; Kendal 2023). Many cities aim to achieve the well-recognized 10–20–30 goal for urban tree diversity – no more than 10 % of one species, 20 % of one genus or 30 % of one family – to enhance forest resilience and maintain the provisioning of services (Santamour, 1990; Kendal et al., 2014; Truchy et al., 2015). Strategic species selection can also boost trait functional diversity, further enhancing resilience (Sjöman et al., 2018; Paquette et al., 2021). For example, urban forests with greater functional diversity (i.e. the variation and distribution of traits within or among species that influence how organisms interact with their environment) are expected to be more resilient because some species, owing to the various trait combinations they possess, are more likely to respond differently to changing conditions, thus buffering urban forests to a wider range of stressors (Paquette et al., 2021). This includes incorporating species with complementary functional traits, such as nitrogen fixation, drought tolerance, and resistance to key pests and diseases.

Intraspecific genetic diversity is also a key component of biodiversity, fundamental to the resilience of populations and ecosystems (Frankham et al., 2010, 2011, 2017, 2019; Hoban et al., 2020a, 2022). Yet, there is much less recognition for its critical importance in the resilience of urban forest health and function (but see Sjöman et al., 2024 and Pooler et al., 2024). By the same principles as species diversity, genetic diversity can enhance urban forest resilience by increasing the likelihood that some genotypes will respond more favourably to changing conditions (Fig. 3; Reusch et al., 2005; Hajjar et al., 2008; Oliver et al., 2015). The positive relationship between genetic diversity and ecosystem function and resilience has been demonstrated in various systems, including natural (Reusch et al., 2005; Kettenring et al., 2014) and cultivated ecosystems (Hajjar et al., 2008). The underlying mechanisms driving these relationships, such as an increased probability that some genotypes will survive disturbances (i.e. the sampling effect), increased range in functional traits and increased facilitative or complementary interactions, could also act in urban forests (Hughes et al., 2008; Kettenring et al., 2014; Avolio, 2023; Blumstein et al., 2025). Genetically diverse populations may also have a greater diversity of trophic interactions, which could indirectly promote ecosystem function and enhance provisioning of ecosystem services and resilience (Reusch et al., 2005; Crutsinger et al., 2006; Hajjar et al., 2008; Kettenring et al., 2014; Oliver et al., 2015; Valencia-Cuevas et al., 2018; Ingvarsson and Dahlberg, 2019). Given these possibilities, limited knowledge on patterns of genetic diversity in urban forests hinders our understanding of the broader impacts on both resilience and ecosystem function in cities.

Fig. 3.

For image description, please refer to the figure legend and surrounding text.

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High species and genetic diversity can confer similar ecosystem-level benefits (shown in green) for urban forests. However, the benefits of planting genetically and species-rich urban forests may come at a trade-off with predictability of tree traits and maintenance (shown in orange). The figure depicts three hypothetical cities with urban forests that differ in both species and genetic diversity, from a clonal monoculture at the bottom to a fully diversified polyculture at the top. Each tree represents an urban population of a species where genetic diversity among individuals in that population varies from low to high. Different coloured DNA strands indicate different genotypes. Increasing species and genetic diversity from low (i.e. monoculture in City 3) to high (i.e. polyculture in City 1) is depicted by the black arrow.

While increasing genetic diversity is generally considered an asset, the impact of introducing novel genotypes, especially from wild populations, into the urban forest should be explored (Frankham et al., 2011, 2017, 2019). Urban landscapes can be challenging places to grow trees because of the uniquely stressful conditions (e.g. pollution, urban heat island, compact soils, limited soil and water volume, deicing salt, and vandalism) and specific landscape architecture needs for tree form, life span, size and aesthetics (Sæbø et al., 2005). Thus, predictability is important when cultivating trees for urban settings, especially given the expense of planting and maintaining them (Sieghardt et al., 2005; Vogt et al., 2015). For this reason, clonally propagated cultivars and named hybrids, collectively with low genetic diversity, are often utilized for their predictable performance and traits (Santamour, 1990; Lohr et al., 2016; Vanden Broeck et al., 2018; Andrianjara et al., 2021). In contrast, trees in wild populations experience selection for different stressors, and some alleles that are advantageous in wild populations may be deleterious in urban environments and vice versa (Miles et al., 2019; Charmantier et al., 2024). Randomly sampling seed stock from wild populations and planting those individuals in urban populations for the sake of increasing genetic diversity may result in trees that may not fit the needs of urban settings.

This presents a tension between efforts to increase genetic diversity and efforts to maintain the consistent and predictable urban landscapes needed to support long-term management and budgeting (Fig. 3; Vogt et al., 2015). Clonal cultivars may be beneficial for maintenance and planning in some areas, such as along streets where uniformity is desired for aesthetic or logistical reasons. However, populations of urban trees dominated by cultivars probably possess little genetic variation in resistance (depending on the number of cultivars), potentially making these populations vulnerable to pests, diseases or other stressors (Lohr et al., 2016; Pooler et al., 2024). In contrast, genetically diverse plantings may increase urban forest resilience and conservation value but at the cost of uniformity and predictability (Fig. 3). For example, seed planted populations could exhibit greater genetic variation in resistance to outbreaks or disturbance (Sniezko and Liu, 2023). Whether risks of large-scale clonal plantings outweigh the benefits of predictable urban tree growth remains unknown. However, dedicated efforts to increase genetic diversity in urban tree populations could buffer risks and provide a level of redundancy in addition to species diversity. Urban settings such as parks, large street wells or backyards where needs for uniformity and predictability are lower offer suitable opportunities to promote genetically diverse plantings. The principle of ‘right tree, right place’ can guide these efforts, ensuring that diverse tree species are selected based on their suitability for specific urban environments, thereby enhancing resilience and biodiversity. This could help safeguard canopy cover and ecosystem services for people living in cities.

Current knowledge and patterns of genetic diversity in urban forests

Understanding the current state of genetic diversity in urban forests is crucial for developing climate-resilient urban forestry strategies and for assessing the current conservation value of urban forests (Lohr et al., 2016; Hoban et al., 2020b; Avolio, 2023). Here, we provide a comprehensive review of the existing literature on the genetic diversity of cultivated urban tree populations, covering all studies up to March 2025. We used the following combination of keywords – ‘genetic diversity’ and ‘urban tree’ or ‘urban forest’ – to ensure that all relevant research was identified and included in this perspective (see Table 1 for details). This search and subsequent filtering steps uncovered seven relevant studies (Table 1). The relatively few studies found demonstrates that little is known about the current patterns of genetic diversity in urban forests. Moreover, they show contrasting patterns, suggesting that diversity trends are variable among urban tree species, limiting our ability to generalize patterns at large scales. In the sections below, we discuss these studies as well as additional mechanisms that may lead to low or high genetic diversity in urban tree populations. We also discuss the specific consequences of low versus high genetic diversity for both urban forestry and tree conservation.

Table 1.

Literature reviewed to assess the current state of knowledge on genetic diversity in cultivated urban forests. Our literature review was conducted using Web of Science, Scopus and Google Scholar, and included all studies published up to March 2025. We used the following combination of keywords to search each source: ‘genetic diversity’ and ‘urban tree’ or ‘urban forest’ to ensure that all relevant research was identified and included in this perspective. Using these terms, we found 36 papers using Web of Science and 41 papers using Scopus. We also reviewed the top 50 results in Google Scholar. We focused our literature review on original research papers that empirically assessed intraspecific genetic diversity among individuals in urban tree populations, and thus we excluded papers that (1) were not original research (e.g. review and perspectives), (2) assessed genetic diversity (GD) of wild tree populations in urban areas that naturally reproduce and (3) assessed organisms in urban forest ecosystems other than trees or large woody shrubs (e.g. insects or mammals). Using these criteria, we retained seven studies, which are summarized below.

Citation Focal species/genus No. of taxa studied Method Metric of genetic diversity (GD) Comparison to GD of wild populations? Geographical scale of urban populations
Andrianjara et al., 2021 Tilia 5 Microsatellite markers Examined genetic groups to assign clonal identity No City (Paris, France)
Blumstein et al., 2025 Red oak (Quercus rubra) 1 Low-coverage whole genome sequencing Used single nucleotide polymorphisms (SNPs) to assess genetic similarity with K-means clustering and population structure using pairwise FST Yes City (Cambridge, MA, USA)
LaBonte et al., 2017 Kentucky yellowwood (Cladrastis kentukea, Fabaceae) 1 Nuclear microsatellite markers Assess allelic richness, heterozygosity, and pairwise FST between urban and natural populations Yes Regional (multiple states in the Midwest, USA)
Morton and Gruszka, 2008 London plane tree (Platanus ×acerifolia) 1 AFLP Number of polymorphic alleles No City (Pittsburgh, PA, USA)
Rimlinger et al., 2021 African plum tree (Dacryodes edulis, Burseraceae) 1 Nuclear microsatellite markers Number of alleles, number of effective alleles, rarefied allelic richness, expected heterozygosity, observed heterozygosity, inbreeding coefficient, inbreeding coefficient corrected for null alleles Yes Regional (Yaoundé, Cameroon and surrounding areas)
Roberts et al., 2007 Jervis Bay grevillea (Grevillea macleayana, Proteaceae) 1 Microsatellite markers Number of alleles, expected heterozygosity, number of private alleles, population differentiation (FST) Yes City (Jervis Bay, NSW, Australia)
Vanden Broeck et al., 2018 European lime [Tilia ×europaea (syn. Tilia ×vulgaris)] 1 Microsatellite markers and AFLP Examined genetic groups to assign clonal identity No Regional (Northwestern Europe)
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Low genetic diversity in urban tree populations

Several processes related to tree cultivation may generate low levels of genetic diversity in urban tree populations. First, a reliance on asexual propagation of urban trees results in genetically homogeneous populations (Lohr et al., 2016; Pooler et al., 2024). Studies of widely planted hybrid cultivars (i.e. Caucasian linden or lime: Tilia ×euchlora, European linden or lime: Tilia ×europaea, and London plane tree: Platanus ×hispanica) found widespread planting of single clones as well as low estimates of genetic diversity (Morton and Gruszka, 2008; Vanden Broeck et al., 2018; Andrianjara et al., 2021). In addition, these studies found lower genetic diversity in modern relative to historical plantings of both Platanus ×hispanica (Morton and Gruszka, 2008) and Tilia ×europaea (Vanden Broeck et al., 2018). These patterns suggest an increasing reliance on a few, clonal genotypes, further decreasing population genetic diversity as old trees are replaced by new clones.

Different seed sourcing practices may also affect genetic diversity in urban populations. If new seed stock is sourced from sexually reproducing urban trees, then the repeated reintroduction of closely related offspring into the urban population may increase population inbreeding over time (e.g. as inbreeding coefficient, F, approaches 1; Frankham et al., 2017). Inbred individuals (those with high inbreeding coefficients) are typically less fit and less resilient to a wide range of biotic and abiotic stressors – a phenomenon called ‘inbreeding depression’ (Charlesworth and Willis, 2009; Charlesworth and Charlesworth, 2010; Angeloni et al., 2011). In long-lived populations of wild trees, this often results in mortality due to heat and drought stress (Nickolas et al., 2019). With the frequency of extreme climate events increasing (Senevirante et al., 2021), die-off or poor health resulting from inbreeding may pose a critical risk in the future. However, the scale of inbreeding in urban tree populations or its role in urban tree function and performance remains unexplored despite the potential consequences.

Likewise, limited sampling from wild populations for horticultural use (i.e. through one or a few sampling events) can ineffectively capture the genetic diversity of a species. For example, Blumstein et al. (2025) found that seed-grown, urban populations of red oak (Quercus rubra) sourced from nurseries in Cambridge, MA, USA, had low genetic diversity and represented only a small portion of the regional wild genetic diversity present in rural populations. By assessing phenology in rural versus urban landscapes, they showed that limited sampling had biased leaf out dates of the urban populations. Their work demonstrates that limited sampling from wild populations can constrain the functional trait space of urban tree populations, which can have downstream effects on ecosystem function (Avolio, 2023). In the case of Q. rubra, reduced genetic diversity in nursery stock probably reflects a bottleneck caused by a limited number of mother trees being sampled for seed propagation (Clark et al., 2023; Hargrave and Harper, 2024; Pooler et al., 2024). Sourcing adequate seed is a major challenge for commercial suppliers, and these challenges could ultimately manifest in reduced genetic diversity in urban landscapes (Clark et al., 2023; Hargrave and Harper, 2024). However, the extent to which these practices drive broader patterns of genetic diversity at the landscape level remains largely unexplored.

Limited initial sampling from wild populations may also have the secondary consequence of sampling from low-diversity populations, perhaps a result from previous demographic bottlenecks or selection events. This may be especially true for rare or endangered tree species, many of which already have both small population sizes and geographical ranges (Frankham et al., 2017, 2019). For example, the paperbark maple (Acer griseum) is an endangered tree native to China with very limited source material introduced into the horticultural trade. Bachtell et al. (2019) found that most cultivated A. griseum in the USA are derived from one seed collection event in 1901. Legal, practical and economic constraints often limit initial collection efforts, which can present challenges for collecting genetically diverse material for horticultural use (Sjöman et al., 2024; Pooler et al., 2024).

High genetic diversity in urban tree populations

For species grown primarily from seed, genetic diversity could be higher in urban populations relative to local wild populations (LaBonte et al., 2017; Rimlinger et al., 2021). This can be the result of seed collection from multiple wild populations and movement of plant material by humans via the horticultural trade. This effect may be particularly pronounced if seeds are sourced from populations exhibiting high genetic structure or subdivision (i.e. high fixation index: FST; Frankham et al., 2017, 2019). The subsequent mixing of genetic material from wild sources in urban settings can inadvertently lead to a more diverse genetic pool compared to individual source populations.

Regionally, native species might be more likely to exhibit this pattern because collection from multiple populations for horticultural use may be more common. LaBonte et al. (2017) demonstrated this within urban populations of the Kentucky yellowwood (Cladrastis kentukea), a common ornamental tree that has a highly fragmented native range in the southeastern USA. They compared genetic diversity of urban populations in six US cities to wild populations scattered throughout the region and demonstrated that, together, the urban populations had higher genetic diversity than any of the individual wild populations. In this case, high genetic diversity in cities relative to any single wild population suggests that Kentucky yellowwoods in cultivation represent a mixture of genotypes, probably sourced from multiple wild populations and distributed regionally.

In Yaoundé, Cameroon, Rimlinger et al. (2021) used high-density sampling of individual trees (>800 individuals) and found that local seed exchange networks among community members concentrated genetic diversity of the African plum tree (Dacryodes edulis), an important indigenous food source, in the city from distant areas across the region. In a similar study, Roberts et al. (2007) found that an urban population of Jervis Bay grevillea (Grevillea macleayana), a large Australian shrub, had higher genetic diversity than nearby wild populations. In fact, remnant urban populations and planted individuals shared about 69 % of alleles not otherwise found in nearby wild populations.

These examples suggest that human-mediated movement of genotypes, either by the horticultural or seed trade or by local community members, can lead to inadvertent concentration of genetic diversity in some urban plant species. Such human-mediated actions are similar to assisted gene flow via augmentation of individuals into a population, a genetics-based approach to wild species conservation (Frankham et al., 2017, 2019). This approach involves the intentional introduction of individuals or gametes between populations with the goal of maintaining or increasing genetic diversity to improve population resilience (Frankham et al., 2017, 2019). Although not intentional in urban forests, this human-mediated gene flow can potentially generate reservoirs of genetic diversity, especially for species with highly abundant urban tree populations, but these patterns remain largely unstudied.

Variability in genetic diversity among tree species in urban landscapes is probably influenced not only by differences in horticultural practices (e.g. seed versus clonal propagation and seed-sourcing practices) but also by species-specific characteristics such as mating system (self-compatibility versus self-incompatibility), flower type (perfect, monoecious or dioecious), and pollination mechanism (wind versus insect pollination; Hamrick and Godt, 1996; Frankham et al., 2010, 2017, 2019). Understanding how species-specific characteristics and horticultural practices together mediate levels of genetic diversity in urban landscapes will help us better generalize these patterns.

THE ROLE OF URBAN FORESTS IN EX SITU CONSERVATION OF THREATENED TREE SPECIES

The current conservation value of urban forests

The current value of urban forests for conserving genetic diversity can be assessed by measuring the genetic diversity of urban tree populations and evaluating to what degree this represents the diversity across a species’ wild geographical range (e.g. percentage allelic capture; Griffith et al., 2019, 2020; Hoban et al., 2020a, 2020b). These values can be compared across different species and cities. However, the conservation value of clonally propagated cultivars or species is uncertain. For example, the green and white ash (Fraxinus pennsylvanica and F. americana) are critically endangered tree species native to the USA (Jerome et al., 2017; Westwood et al., 2017), yet are ubiquitous in urban forests globally (Ossola et al., 2020). Despite the millions of individuals in urban forests, the widespread planting of clonal cultivars during the past 80 years could have resulted in low genetic diversity and small effective population sizes (Ne) of urban populations (Flower et al., 2018; Ingvarsson and Dahlberg, 2019; Abhainn et al., 2024). However, to date, no work has assessed genetic diversity in urban populations of these two Fraxinus species. If horticultural practices, such as clonal cultivation or limited introduction of wild material, have resulted in low genetic diversity in urban relative to wild populations, this would imply that urban populations do not serve as reservoirs of genetic diversity for broader species conservation purposes.

In contrast to clonal cultivars, urban populations of tree species planted primarily from seed may currently harbour greater levels of genetic diversity (Roberts et al., 2007; LaBonte et al., 2017; Rimlinger et al., 2021). To assess the degree to which urban genetic diversity is valuable for species conservation, future work should focus on assessing percentage allelic capture in urban relative to wild populations (Griffith et al., 2019, 2020; Hoban et al., 2020a). For example, the island oak (Quercus tomentella) is an endangered species native to California, USA (Beckman and Jerome, 2017). This species is well represented in urban forest inventories by 748 individuals in 52 cities with many more planted that are not included in inventories (Fig. 2B). Yet, fewer than 75 individuals exist in the combined network of living collections among all botanic gardens globally (Griffith et al., 2019). Understanding the proportion of the total genetic diversity captured in urban populations relative to botanic gardens and wild populations can provide vital insights into whether and to what degree urban forests currently contribute to the broader conservation collection (Hoban et al., 2022). These data will be valuable for conservation efforts and can also inform urban foresters of the provenance and cultivar identity of trees, potentially helping predict responses to climate change (Tischenko et al., 2024). Collecting the necessary samples from both wild populations of threatened species as well as from urban populations could be challenging, but developing collaborations and coordinating efforts between municipal urban foresters, public park departments, universities and botanic gardens can make this work possible.

Old heritage trees in cities, such as giant sequoias (Sequoiadendron giganteum; IUCN Red List Category: Endangered; Schmid and Farjon, 2013) planted in Europe, may serve as reservoirs of genetic diversity by harbouring rare alleles lost in declining wild populations (Gardner et al., 2019; Ismail et al., 2021). These trees, isolated from their native populations, could possess unique genetic diversity. Similarly, trees that have survived disease outbreaks (i.e. ‘lingering’ trees), such as those impacted by emerald ash borer, white pine blister rust, chestnut blight or Dutch elm disease, may possess valuable resistance alleles, making them crucial for resistance breeding programmes dedicated to conserving these threatened species (Pike et al., 2021; Sniezko and Liu, 2023).

Urban forests also represent critical refugia for species listed as extinct in the wild. Most of these species are currently maintained by only a small number of botanic gardens and seed banks (Smith et al., 2023). There are at least six species listed as extinct in the wild (pride of Burma: Amherstia nobilis; Angel’s trumpets: Brugmansia arborea, B. sanguinea, B. suaveolens, B. versicolor; Franklin tree: Franklinia alatamaha; and toromiro: Sophora toromiro; IUCN 2024) that are currently planted in urban forests in at least 19 cities in nine countries (Ossola et al., 2020). Dedicated efforts to assess the genetic diversity of these urban populations can reveal whether they harbour unique genetic diversity not present in botanic gardens. If so, these urban populations could be important components of breeding programmes designed to maximize diversity and limit inbreeding (Foster et al., 2022).

However, regardless of genetic diversity, species-rich urban forests can still have significant ecosystem conservation value. They offer unique suites of functional traits, potential connectivity (i.e. the ability to serve as links or stepping stones between larger natural areas or other urban green spaces) and educational opportunities, and support urban trophic interactions (Ossola et al., 2020; Wood and Esaian, 2020; Paquette et al., 2021). Moreover, even genetically limited urban populations could harbour rare alleles important for species’ adaptation. However, measuring and potentially enhancing genetic diversity remains crucial for long-term viability and adaptability of urban forests.

Exploring the future role of urban forests for ex situ conservation

The conservation of species outside their native habitats (i.e. ex situ conservation) includes seed and pollen banking as well as living collections (Mounce et al., 2017; Griffith et al., 2019; Wolkis et al., 2024). These resources are managed by multiple stakeholders, including botanic gardens, government organizations (e.g. USDA National Plant Germplasm System and Canada’s National Tree Seed Centre) and others. Collectively, they serve an important role in conserving genetic diversity across a species’ geographical range. This combined network of collections is considered the metacollection, and it has many advantages, including diluting the risk of loss and reducing maintenance costs for institutions (Griffith et al., 2019, 2020).

However, planting space and maintenance requirements limit living collections of ex situ specimens, especially for collections of large plants like trees (Cavender et al., 2015). Recent evidence also suggests that living collections have reached their taxonomic and accession capacity (Cano et al., 2025). Moreover, ex situ collections of many species have not yet achieved genetic diversity targets and do not adequately capture the genetic diversity found in wild populations (Cavender et al., 2015; Hoban et al., 2020b; Diaz-Martin et al., 2023). This emerging picture underscores the urgent need for innovative strategies that can overcome these limitations and enhance the role of living collections in ex situ species conservation.

We assert that by broadening the interpretation of ex situ conservation, there is high potential for urban forests to be utilized to overcome these ex situ conservation challenges by serving as an extension of the metacollection. Thus, intentionally introducing genetic diversity into urban forests can help reach the conservation goals of ex situ collections broadly, including maintaining species’ adaptive potential (Cavender et al., 2015; Frankham et al., 2019; Hoban et al., 2020b; Diaz-Martin et al., 2023). This can complement existing strategies (e.g. maintaining living specimens, seed and pollen banking, and habitat conservation) and can support biodiversity strategies and action plans (Schaal and Leverich, 2004; Frankham et al., 2017, 2019; Hoban et al., 2024). If coordinated regionally, or even globally, this strategy could safeguard species’ genetic diversity by distributing genetic material among cities, thereby disseminating the risk of loss due to disturbances. Over time, this strategy can also increase the demographic depth of ex situ metacollections, further strengthening their resilience (Griffith et al., 2019, 2020; Offord and Zimmer, 2024).

Urban forest planning and management often occur at the local (e.g. city) scale, making it ideal for implementing ex situ conservation strategies. We propose a collaborative approach involving two main steps: (1) selecting target threatened tree species based on factors such as climatic suitability, urban tolerance, invasiveness risk, and susceptibility to pests and diseases (Sjöman et al., 2024); and (2) assessing, enhancing and monitoring genetic diversity in urban populations (Fig. 4). Initial assessments should establish baselines for genetic diversity in urban populations and compare them to wild populations and broader metacollections to evaluate conservation value. Metrics of within-species genetic diversity used to assess urban forests should align with those used for conservation efforts and biodiversity monitoring (e.g. allelic richness and heterozygosity; Hoban et al., 2022). This will facilitate use of genetic data for broader conservation monitoring efforts.

Fig. 4.

For image description, please refer to the figure legend and surrounding text.

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A two-step, collaborative approach to utilizing urban forests for ex situ conservation of threatened tree species at the local (e.g. city) scale. The two main steps include (1) selecting target threatened tree species and (2) assessing, enhancing and monitoring genetic diversity in urban populations. For a given species, three sources (i.e. reservoirs) of genetic diversity are depicted, including wild populations (yellow), existing ex situ metacollections (grey) and urban populations (green). Ex situ metacollections can include material from botanic gardens, seed banks and living germplasm collections. Overlap indicates the potential for shared alleles among sources.

Once baselines are established, efforts can focus on increasing genetic diversity by sourcing novel genotypes from wild populations, metacollections or other urban populations. One approach is to source local genotypes that may be well-suited to current regional climatic conditions (Jensen et al., 2025). Another approach is to source individuals from wild populations in climates that match a city’s future projections (Sjöman et al., 2024). These genotypes may have drought and/or heat tolerance traits that will allow them to persist as climate changes, further promoting resilience (Hannus et al., 2021). These approaches could be beneficial not only for threatened species but also for common urban tree species.

After implementing these strategies, monitoring systems should track genetic diversity, survival rates and growth performance to assess the effectiveness of conservation measures. These systems can help validate actions by measuring whether urban forests meet biodiversity conservation targets and contribute to ecological resilience over time. Developing robust conservation programmes that integrate genetic data into urban forestry practices requires collaboration among researchers, nurseries and urban foresters to ensure the long-term viability of urban tree populations in the face of environmental change (Frankham et al., 2017, 2019; Hoban et al., 2024).

Initiatives that start with species or genera already familiar to urban foresters, park managers, community residents and tree growers may have the highest chance of success. Many threatened gymnosperm species already commonly planted in urban forests are also high conservation priorities (e.g. ginkgo: G. biloba, giant sequoia: S. giganteum, coast redwood: Sequoia sempervirens, dawn redwood: Metasequoia glyptostroboides, Araucaria trees: Araucaria spp., and Wollemi pine: Wollemia nobilis; Forest et al., 2018; Ossola et al., 2020; Gumbs et al., 2023; IUCN 2024; Box 1). Oaks are also culturally and ecologically important urban trees in many cities, yet 26 % of species are threatened and their seeds cannot be effectively seed banked, making living ex situ collections crucial (Cavender et al., 2015; Carrero et al., 2020). Given their familiarity to urban stakeholders and their myriad of available genomic resources, these taxa present an accessible opportunity for increasing genetic diversity in urban forests (Tischenko et al., 2024).

Box 1. Ex situ conservation of the critically endangered Wollemi pine.

For image description, please refer to the figure legend and surrounding text.

Community engagement in conserving habitats and charismatic species has been a cornerstone of conservation biology for decades (Maschinski et al., 2012; Blackmore et al., 2013). The growing public awareness of biodiversity and species conservation can be utilized to support community-centred conservation programmes for threatened tree species and to raise awareness for conservation initiatives (Armitage et al., 2020; Ismail et al., 2021). Initiatives such as ‘adopt-a-tree’ programmes can engage the public in ex situ conservation by utilizing private property for planting and monitoring trees. This may be especially successful for species that are locally native. An approach that focuses on regionally native threatened tree species can harness movements that promote horticultural use of native species while also minimizing phytosanitary concerns and invasion risk. Community-centred conservation approaches not only fosters community involvement but also provides valuable data for urban forestry and conservation. For example, home gardeners have contributed significantly to the conservation of the Wollemi pine (W. nobilis) by cultivating it in private gardens, which aids in maintaining genetic diversity (Offord and Zimmer, 2024).

Considerations and challenges

Understanding the challenges and developing strategies to overcome them is critical to implementing conservation programmes that target the genetic diversity of threatened species in urban forests. Within cities, urban forests are managed by many stakeholders, including municipalities, parks departments, contractors and community residents, which could make implementing conservation projects at scale challenging and expensive (Aronson et al., 2017). For example, it will be important to consider who should care for these trees, how to best track conservation-focused plantings in urban forests (Godbout et al., 2018), how to mitigate phytosanitary concerns (Leal et al., 2010), risks of deleterious gene flow or hybridization with native populations (Frankham et al., 2011; Roe et al., 2014; Zaya et al., 2021; Abhainn et al., 2024), and dissuade tree growers from trademarking threatened species. Another challenge is the substantial financial investment required to monitor genetic diversity across multiple cities. Without dedicated resources, long-term genetic monitoring may be difficult to sustain, even where the scientific and conservation value is clear.

There are also legal and practical challenges to working with threatened tree species, especially with respect to moving genetic resources internationally (Sjöman et al., 2024; Pooler et al., 2024). For this reason, engaging with botanic gardens will be critical. Botanic gardens often cultivate and conserve plant species from around the world, giving them extensive experience in facilitating plant material exchange. Their global focus and expertise make them well positioned to coordinate conservation efforts and navigate the necessary protocols for international plant exchange and conservation. This includes developing agreements that ensure the fair and equitable sharing of benefits arising from exchange of genetic resources. Developing these agreements is a core objective of the Convention on Biological Diversity, which guides diversity conservation programmes globally (i.e. the Nagoya Protocol on access to genetic resources and the fair and equitable sharing of benefits; Secretariat of the Convention on Biological Diversity, 2011).

Conservation through horticulture requires coordination with nurseries, many of which need to grow trees that will sell. Lack of diversity in nursery stock poses a significant barrier to this effort (Clark et al., 2023; Hilbert et al., 2023; Hargrave and Harper, 2024). However, working directly with specialty growers, non-profit nurseries (e.g. those established by non-government tree planting organizations) or seed collectors provides a promising pathway toward increasing genetic diversity in urban forests. These organizations often specialize in sourcing genetically diverse or locally native material, making them less susceptible to market forces (Elton et al., 2020). Engaging these stakeholders is crucial for enhancing urban forest resilience by ensuring access to genetically diverse planting material.

There are also important ecological considerations to address when selecting threatened species for urban conservation programmes. For example, it is crucial to rigorously assess the invasiveness risk, as some species that are rare or threatened in their native habitats have become invasive when introduced to new regions (Staude et al., 2025). Careful evaluation of ecological compatibility, ongoing monitoring, and adherence to local and international guidelines for non-native species introductions are essential to minimize such risks and ensure that conservation efforts do not inadvertently lead to negative ecological impacts.

While our vision for utilizing urban forests for ex situ conservation is ambitious, it builds upon existing urban forestry practices, infrastructure, trade and economies of scale. We acknowledge that sourcing, growing, planting and maintaining threatened tree species in urban landscapes may incur additional costs and require specialized care (Vogt et al., 2015). However, many of the challenges discussed here can be addressed with economic and financial instruments (e.g. grant funding, tax deductions, subsidies, buying schemes) from government agencies, non-government organizations or dedicated conservation organizations. Moreover, the potential loss of large tree populations due to low genetic diversity would also result in substantial direct and indirect costs, and thus investment in programmes to improve it could mitigate some of these losses. By integrating conservation goals into existing urban forestry programmes and engaging in collaboration, this approach can be both cost-effective and logistically feasible, offering a valuable complement to traditional ex situ conservation efforts. Urban forestry is inherently a transdisciplinary field that has a long history of successful collaboration among stakeholders and can support the effective implementation of future conservation initiatives.

CONCLUSIONS

Global urban greening efforts provide a unique opportunity to utilize urban forests for ex situ conservation of threatened tree species. However, current urban tree populations often suffer from low genetic diversity due to factors such as clonal propagation and limited material from wild populations. To enhance population resilience and conservation value, strategies should focus on increasing genetic diversity through collaborative efforts with nurseries, cities and conservation scientists. Planting a genetically diverse assemblage of threatened species can help overcome some of the major challenges to ex situ conservation of trees by expanding available planting space, contributing to metacollection genetic diversity targets, and providing additional resources to maximize the depth and breadth of the biodiversity conserved, allowing urban forests to become vital reservoirs of genetic diversity for the persistence of species in the face of global climate change. Simultaneously, these dedicated efforts to improve the genetic diversity of urban tree populations may also increase urban forest resilience, allowing for long-term persistence of functional communities and ecosystems. Importantly, the large scale of urban tree planting and greening initiatives provides the practical capacity to implement this strategy. These actions will create meaningful synergy between global greening efforts and plant conservation genetics goals. In this way, utilizing urban forests represents an underappreciated but promising strategy to advance plant conservation missions.

Acknowledgements

We thank Nick LaBonte, Henrik Sjöman and Tom Freeth for helpful discussions on genetic diversity in urban tree populations and their insights on working with international collaborators. We are grateful for Jeremy Foster’s help with downloading PlantSearch data. Dall-E 3 was used to generate the silhouette image of the Wollemi pine in Box 1. M.E.R. received funding from the Research Theme Program from Western Sydney University. A.O. is supported by the Hatch Project CA-D-PLS-2735-H.

Contributor Information

Natalie L R Love, Negaunee Institute for Plant Conservation Science and Action, Chicago Botanic Garden, Glencoe, IL 60022, USA.

Kieran Althaus, Center for Tree Science, Morton Arboretum, Lisle, IL 60532, USA; Committee on Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA.

Manuel Esperon-Rodriguez, Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW 2751, Australia; School of Science, Western Sydney University, Penrith, NSW 2751, Australia.

Alessandro Ossola, Department of Plant Sciences, University of California Davis, Davis, CA 95616, USA; School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Richmond, VIC 3010, Australia.

Jason Johns, Urban Forest Institute, San Luis Obispo, CA 93401, USA.

Camille Pawlak, Urban Forest Institute, San Luis Obispo, CA 93401, USA; Department of Geography, University of California, Los Angeles, Los Angeles, CA 90095, USA.

Boris Igić, Department of Biological Sciences, University of Illinois, Chicago, Chicago, IL 60607, USA.

Matt Ritter, Urban Forest Institute, San Luis Obispo, CA 93401, USA; Biological Sciences Department, Cal Poly, San Luis Obispo, CA 93407, USA.

Jenn Yost, Urban Forest Institute, San Luis Obispo, CA 93401, USA; Biological Sciences Department, Cal Poly, San Luis Obispo, CA 93407, USA.

REFERENCES

  1. Abhainn  EA, Shirley  DL, Stanley  RK, Scarpato  T, Koch  JL, Romero-Severson  J. 2024. Gene flow from Fraxinus cultivars into natural stands of Fraxinus pennsylvanica occurs range-wide, is regionally extensive, and is associated with a loss of allele richness. PLoS One  19: e0294829. doi: 10.1371/journal.pone.0294829 [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Andrianjara  I, Bordenave-Jacquemin  M, Roy  V, et al.  2021. Urban tree management: diversity of Tilia genus in streets and parks of Paris based on morphological and genetic characteristics. Urban Forestry & Urban Greening  66: 127382. doi: 10.1016/j.ufug.2021.127382 [DOI] [Google Scholar]
  3. Angeloni  F, Ouborg  NJ, Leimu  R. 2011. Meta-analysis on the association of population size and life history with inbreeding depression in plants. Biological Conservation  144: 35–43. doi: 10.1016/j.biocon.2010.08.016 [DOI] [Google Scholar]
  4. Armitage  D, Mbatha  P, Muhl  E, Rice  W, Sowman  M. 2020. Governance principles for community-centered conservation in the post-2020 global biodiversity framework. Conservation Science and Practice  2: e160. doi: 10.1111/csp2.160 [DOI] [Google Scholar]
  5. Aronson  MF, Lepczyk  CA, Evans  KL, et al.  2017. Biodiversity in the city: key challenges for urban green space management. Frontiers in Ecology and the Environment  15: 189–196. doi: 10.1002/fee.1480 [DOI] [Google Scholar]
  6. Avolio  ML. 2023. The unexplored effects of artificial selection on urban tree populations. American Journal of Botany  110: e16187. doi: 10.1002/ajb2.16187 [DOI] [PubMed] [Google Scholar]
  7. Bachtell  KR, Aiello  AS, Dosman  MS, Wang  K. 2019. Paperbark Maple (Acer griseum) conservation project. IPPS  69: 87–93. https://ipps.org/wpcontent/uploads/2025/03/5bER-Bachtell-2019.pdf [Google Scholar]
  8. Beckman  E, Jerome  D. 2017. Quercus tomentella. The IUCN Red List of Threatened Species 2017: e.T30959A2799049.   10.2305/IUCN.UK.2017-2.RLTS.T30959A2799049.en (25 March 2025, date last accessed). [DOI]
  9. BGCI . 2024. Plantsearch. Richmond, UK: Botanic Gardens Conservation International. [Google Scholar]
  10. Blackmore  E, Underhill  R, McQuilkin  J, Leach  R, Holmes  T. 2013. Common cause for nature: a practical guide to values and frames in conservation. Machynlleth, UK: Public Interest Research Centre. [Google Scholar]
  11. Blumstein  M, Webster  S, Hopkins  R, Basler  D, Yun  J, Des Marais  DL. 2025. Genomics highlight an underestimation of phenology sensitivity to the urban heat island effect. Proceedings of the National Academy of Sciences United States of America  122: e2408564122. doi: 10.1073/pnas.2408564122 [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Boonman  CCF, Serra-Diaz  JM, Hoeks  S, et al.  2024. More than 17,000 tree species are at risk from rapid global change. Nature Communications  15: 166. doi: 10.1038/s41467-023-44321-9 [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Cano  Á, Powell  J, Aiello  AS, et al.  2025. Insights from a century of data reveal global trends in ex situ living plant collections. Nature Ecology & Evolution  9: 214–224. doi: 10.1038/s41559-024-02633-z [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Carrero  C, Jerome  D, Beckman  E, et al.  2020. The red list of oaks 2020. Lisle, IL: The Morton Arboretum. [Google Scholar]
  15. Cavender  N, Westwood  M, Bechtoldt  C, et al.  2015. Strengthening the conservation value of ex situ tree collections. Oryx  49: 416–424. doi: 10.1017/S0030605314000866 [DOI] [Google Scholar]
  16. Charlesworth  B, Charlesworth  D. 2010. The evolution of breeding systems, sex ratios, and life histories. In: Elements of evolutionary genetics. Greenwood Village, CO: Roberts, 444–520. [Google Scholar]
  17. Charlesworth  D, Willis  JH. 2009. The genetics of inbreeding depression. Nature Reviews: Genetics  10: 783–796. doi: 10.1038/nrg2664 [DOI] [PubMed] [Google Scholar]
  18. Charmantier  A, Burkhard  T, Gervais  L, Perrier  C, Schulte-Hostedde  AI, Thompson  MJ. 2024. How does urbanization affect natural selection?  Functional Ecology  38: 2522–2536. doi: 10.1111/1365-2435.14667 [DOI] [Google Scholar]
  19. Clark  PW, D’Amato  AW, Palik  BJ, et al.  2023. A lack of ecological diversity in forest nurseries limits the achievement of tree-planting objectives in response to global change. BioScience  73: 575–586. doi: 10.1093/biosci/biad049 [DOI] [Google Scholar]
  20. Crutsinger  GM, Collins  MD, Fordyce  JA, Gompert  Z, Nice  CC, Sanders  NJ. 2006. Plant genotypic diversity predicts community structure and governs an ecosystem process. Science  313: 966–968. doi: 10.1126/science.1128326 [DOI] [PubMed] [Google Scholar]
  21. Diaz-Martin  Z, Fant  J, Havens  K, Cinea  W, Tucker Lima  JM, Griffith  MP. 2023. Current management practices do not adequately safeguard endangered plant species in conservation collections. Biological Conservation  280: 109955. doi: 10.1016/j.biocon.2023.109955 [DOI] [Google Scholar]
  22. Elton  AJ, Weil  B  Harper  R. 2020. The Worcester tree initiative: a community organization at the center of reclaiming an urban forest. The Citizen Forester  243: 1–4. https://www.mass.gov/doc/citizen-forester-october-2020/download [Google Scholar]
  23. Endreny  TA. 2018. Strategically growing the urban forest will improve our world. Nature Communications  9: 1160. doi: 10.1038/s41467-018-03622-0 [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Esperon-Rodriguez  M, Ordoñez  C, van Doorn  NS, Hirons  A, Messier  C. 2022a. Using climate analogues and vulnerability metrics to inform urban tree species selection in a changing climate: the case for Canadian cities. Landscape and Urban Planning  228: 104578. doi: 10.1016/j.landurbplan.2022.104578 [DOI] [Google Scholar]
  25. Esperon-Rodriguez  M, Tjoelker  MG, Lenoir  J, et al.  2022b. Climate change increases global risk to urban forests. Nature Climate Change  12: 950–955. doi: 10.1038/s41558-022-01465-8 [DOI] [Google Scholar]
  26. Farjon  A. 2013. Pinus torreyana. The IUCN Red List of Threatened Species 2013: e.T42424A2979186. 10.2305/IUCN.UK.2013-1.RLTS.T42424A2979186.en (24 March 2025, date last accessed). [DOI]
  27. Flower  CE, Fant  JB, Hoban  S, et al.  2018. Optimizing conservation strategies for a threatened tree Species: in situ conservation of white ash (Fraxinus americana L.) genetic diversity through insecticide treatment. Forests  9: 202. doi: 10.3390/f9040202 [DOI] [Google Scholar]
  28. Forest  F, Moat  J, Baloch  E, et al.  2018. Gymnosperms on the EDGE. Scientific Reports  8: 6053. doi: 10.1038/s41598-018-24365-4 [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Foster  JA, Walsh  SK, Havens  K, Kramer  AT, Fant  JB. 2022. Supporting long-term sustainability of ex situ collections using a pedigree-based population management approach. Applications in Plant Sciences  10: e11491. doi: 10.1002/aps3.11491 [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Frankham  R, Ballou  JD, Briscoe  DA. 2010. Introduction to conservation genetics. Cambridge, UK: Cambridge University Press. [Google Scholar]
  31. Frankham  R, Ballou  JD, Eldridge  MDB, et al.  2011. Predicting the probability of outbreeding depression. Conservation Biology  25: 465–475. doi: 10.1111/j.1523-1739.2011.01662.x [DOI] [PubMed] [Google Scholar]
  32. Frankham  R, Ballou  JD, Ralls  K, et al.  2017. Genetic management of fragmented animal and plant populations. Oxford, UK: Oxford University Press. [Google Scholar]
  33. Frankham  R, Ballou  JD, Ralls  K, et al.  2019. A practical guide for genetic management of fragmented animal and plant populations. Oxford, UK: Oxford University Press. [Google Scholar]
  34. Gardner  M, Christian  T, Hinchliffe  W, Cubey  R. 2019. Conservation hedges—modern-day arks. Sibbaldia: The Journal of Botanic Garden Horticulture  17: 71–100. doi: 10.24823/Sibbaldia.2019.268 [DOI] [Google Scholar]
  35. Godbout  J, Bomal  C, Farr  K, Williamson  M, Isabel  N. 2018. Genomic tools for traceability: opportunities, challenges and perspectives for the Canadian forestry sector. The Forestry Chronicle  94: 75–87. doi: 10.5558/tfc2018-010 [DOI] [Google Scholar]
  36. Griffith  P, Beckman  E, Callicrate  T, et al.  2019. Toward the metacollection: coordinating conservation collections to safeguard plant diversity. San Marino, USA: Botanic Gardens Conservation International-US. [Google Scholar]
  37. Griffith  MP, Clase  T, Toribio  P, et al.  2020. Can a botanic garden metacollection better conserve wild plant diversity? A case study comparing pooled collections with an ideal sampling model. International Journal of Plant Sciences  181: 485–496. doi: 10.1086/707729 [DOI] [Google Scholar]
  38. Gumbs  R, Gray  CL, Böhm  M, et al.  2023. The EDGE2 protocol: advancing the prioritisation of evolutionarily distinct and globally endangered species for practical conservation action. PLOS Biology  21: e3001991. doi: 10.1371/journal.pbio.3001991 [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Guo  W-Y, Serra-Diaz  JM, Schrodt  F, et al.  2022. High exposure of global tree diversity to human pressure. Proceedings of the National Academy of Sciences Unites States of America  119: e2026733119. doi: 10.1073/pnas.2026733119 [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Hajjar  R, Jarvis  DI, Gemmill-Herren  B. 2008. The utility of crop genetic diversity in maintaining ecosystem services. Agriculture, Ecosystems & Environment  123: 261–270. doi: 10.1016/j.agee.2007.08.003 [DOI] [Google Scholar]
  41. Hamilton  JA, Royauté  R, Wright  JW, Hodgskiss  P, Ledig  FT. 2017. Genetic conservation and management of the California endemic, Torrey pine (Pinus torreyana Parry): implications of genetic rescue in a genetically depauperate species. Ecology and Evolution  7: 7370–7381. doi: 10.1002/ece3.3306 [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Hamrick  JL, Godt  MJW. 1996. Effects of life history traits on genetic diversity in plant species. Philosophical Transactions of the Royal Society of London: Series B, Biological Sciences  351: 1291–1298. doi: 10.1098/rstb.1996.0112 [DOI] [Google Scholar]
  43. Hannus  S, Hirons  A, Baxter  T, McAllister  HA, Wiström  B, Sjöman  H. 2021. Intraspecific drought tolerance of Betula pendula genotypes: an evaluation using leaf turgor loss in a botanical collection. Trees  35: 569–581. doi: 10.1007/s00468-020-02059-7 [DOI] [Google Scholar]
  44. Hargrave  R, Harper  R. 2024. Understanding the relationship between tree supply and diversity in the urban environment. Arborist News  33: 34–37. [Google Scholar]
  45. He  T, Hu  Y, Guo  A, et al.  2024. Quantifying the impact of urban trees on land surface temperature in global cities. ISPRS Journal of Photogrammetry and Remote Sensing  210: 69–79. doi: 10.1016/j.isprsjprs.2024.03.007 [DOI] [Google Scholar]
  46. Hilbert  DR, Koeser  AK, Andreu  MG, et al.  2023. Conceptualizing the human drivers of low tree diversity in planted urban landscapes. Ambio  52: 1532–1542. doi: 10.1007/s13280-023-01876-7 [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Hoban  S, Archer  FI, Bertola  LD, et al.  2022. Global genetic diversity status and trends: towards a suite of Essential Biodiversity Variables (EBVs) for genetic composition. Biological Reviews  97: 1511–1538. doi: 10.1111/brv.12852 [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Hoban  S, Bruford  M, D’Urban Jackson  J, et al.  2020a. Genetic diversity targets and indicators in the CBD post-2020 Global Biodiversity Framework must be improved. Biological Conservation  248: 108654. doi: 10.1016/j.biocon.2020.108654 [DOI] [Google Scholar]
  49. Hoban  S, Callicrate  T, Clark  J, et al.  2020b. Taxonomic similarity does not predict necessary sample size for ex situ conservation: a comparison among five genera. Proceedings of the Royal Society of London: Series B, Biological Sciences  287: 20200102. doi: 10.1098/rspb.2020.0102 [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Hoban  S, Hvilsom  C, Aissi  A, et al.  2024. How can biodiversity strategy and action plans incorporate genetic diversity and align with global commitments?  BioScience  75: 47–60. doi. 10.1093/biosci/biae106 [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Hughes  AR, Inouye  BD, Johnson  MTJ, Underwood  N, Vellend  M. 2008. Ecological consequences of genetic diversity. Ecology Letters  11: 609–623. doi: 10.1111/j.1461-0248.2008.01179.x [DOI] [PubMed] [Google Scholar]
  52. Ingvarsson  PK, Dahlberg  H. 2019. The effects of clonal forestry on genetic diversity in wild and domesticated stands of forest trees. Scandinavian Journal of Forest Research  34: 370–379. doi: 10.1080/02827581.2018.1469665 [DOI] [Google Scholar]
  53. Ismail  SA, Pouteau  R, Van Kleunen  M, Maurel  N, Kueffer  C. 2021. Horticultural plant use as a so-far neglected pillar of ex situ conservation. Conservation Letters  14: e12825. doi: 10.1111/conl.12825 [DOI] [Google Scholar]
  54. IUCN . The IUCN Red List of Threatened Species. 2024. Gland: IUCN.
  55. Ives  CD, Lentini  PE, Threlfall  CG, et al.  2016. Cities are hotspots for threatened species. Global Ecology and Biogeography  25: 117–126. doi: 10.1111/geb.12404 [DOI] [Google Scholar]
  56. Jensen  JK, Hedblom  M, Persson  AS. 2025. Evidence-based greening: a missing piece in biodiversity conservation. Trends in Ecology & Evolution  40: 523–526. doi: 10.1016/j.tree.2025.04.007 [DOI] [PubMed] [Google Scholar]
  57. Jerome  D, Westwood  M, Oldfield  S, Romero-Severson, J. 2017. Fraxinus americana. The IUCN Red List of Threatened Species 2017: e.T61918430A61918432. 10.2305/IUCN.UK.2017-2.RLTS.T61918430A61918432.en (25 March 2025, date last accessed). [DOI]
  58. Kendal  D. 2023. Correspondence: enrich urban biodiversity for sustainable cities. Nature  621: 691. doi: 10.1038/d41586-023-03020-7 [DOI] [PubMed] [Google Scholar]
  59. Kendal  D, Dobbs  C, Lohr  VI. 2014. Global patterns of diversity in the urban forest: is there evidence to support the 10/20/30 rule?  Urban Forestry & Urban Greening  13: 411–417. doi: 10.1016/j.ufug.2014.04.004 [DOI] [Google Scholar]
  60. Kettenring  KM, Mercer  KL, Reinhardt Adams  C, Hines  J. 2014. Application of genetic diversity–ecosystem function research to ecological restoration. Journal of Applied Ecology  51: 339–348. doi: 10.1111/1365-2664.12202 [DOI] [Google Scholar]
  61. LaBonte  N, Tonos  J, Hartel  C, Woeste  KE. 2017. Genetic diversity and differentiation of yellowwood [Cladrastis kentukea (Dum.Cours.) Rudd] growing in the wild and in planted populations outside the natural range. New Forests  48: 263–274. doi: 10.1007/s11056-017-9566-8 [DOI] [Google Scholar]
  62. Leal  I, Allen  E, Humble  L, Sela  S, Uzunovic  A. 2010. Phytosanitary risks associated with the global movement of forest products: a commodity-based approach. British Columbia: Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. [Google Scholar]
  63. Liu  Z, He  C, Zhou  Y, Wu  J. 2014. How much of the world’s land has been urbanized, really? A hierarchical framework for avoiding confusion. Landscape Ecology  29: 763–771. doi: 10.1007/s10980-014-0034-y [DOI] [Google Scholar]
  64. Lohr  VI, Kendal  D, Dobbs  C. 2016. Urban trees worldwide have low species and genetic diversity, posing high risks of tree loss as stresses from climate change increase. Acta Horticulturae  1108: 263–270. doi: 10.17660/ActaHortic.2016.1108.34 [DOI] [Google Scholar]
  65. Love  NLR, Nguyen  V, Pawlak  C, et al.  2022. Diversity and structure in California’s urban forest: what over six million data points tell us about one of the world’s largest urban forests. Urban Forestry & Urban Greening  74: 127679. doi: 10.1016/j.ufug.2022.127679 [DOI] [Google Scholar]
  66. Maschinski  J, Wright  SJ, Lewis  C. 2012. The critical role of the public: plant conservation through volunteer and community outreach projects. In: Maschinski  J, Haskins  K. eds. Plant reintroduction in a changing climate: promises and perils. Washington, DC, USA: Island Press, 53–69. [Google Scholar]
  67. McCoy  DE, Goulet-Scott  B, Meng  W, et al.  2022. Species clustering, climate effects, and introduced species in 5 million city trees across 63 US cities. Elife  11: e77891. doi: 10.7554/eLife.77891 [DOI] [PMC free article] [PubMed] [Google Scholar]
  68. McKinny  ML. 2002. Urbanization, biodiversity, and conservation. BioScience  52: 883–890. doi: 10.1641/0006-3568(2002)052[0883:UBAC]2.0.CO;2 [DOI] [Google Scholar]
  69. McPherson  GE, Young  R. 2010. Understanding the challenges of municipal tree planting. Arborist News  19: 60–62. https://research.fs.usda.gov/treesearch/38107 [Google Scholar]
  70. Miles  LS, Rivkin  LR, Johnson  MTJ, Munshi-South  J, Verrelli  BC. 2019. Gene flow and genetic drift in urban environments. Molecular Ecology  28: 4138–4151. doi: 10.1111/mec.15221 [DOI] [PubMed] [Google Scholar]
  71. Morton  CM, Gruszka  P. 2008. AFLP assessment of genetic variability in old vs. New London plane trees (Platanus ϫ acerfolia). Journal of Horticultural Science & Biotechnology  83: 532–537. doi: 10.1080/14620316.2008.11512418 [DOI] [Google Scholar]
  72. Mounce  R, Smith  P, Brockington  S. 2017. Ex situ conservation of plant diversity in the world’s botanic gardens. Nature Plants  3: 795–802. doi: 10.1038/s41477-017-0019-3 [DOI] [PubMed] [Google Scholar]
  73. Muller  RN, Bornstein  C. 2010. Maintaining the diversity of California’s municipal forests. Arboriculture & Urban Forestry  36: 18–27. doi: 10.48044/jauf.2010.003 [DOI] [Google Scholar]
  74. Nickolas  H, Harrison  PA, Tilyard  P, Vaillancourt  RE, Potts  BM. 2019. Inbreeding depression and differential maladaptation shape the fitness trajectory of two co-occurring Eucalyptus species. Annals of Forest Science  76: 10. doi: 10.1007/s13595-018-0796-5 [DOI] [Google Scholar]
  75. Nic Lughadha  E, Bachman  SP, Leão  TCC, et al.  2020. Extinction risk and threats to plants and fungi. Plants People Planet  2: 389–408. doi: 10.1002/ppp3.10146 [DOI] [Google Scholar]
  76. Offord  CA, Zimmer  HC. 2024. Home gardens contribute to conservation of the critically endangered Wollemi pine: evaluation of a botanic garden-led horticultural release programme. Plants, People, Planet  6: 116–127. doi: 10.1002/ppp3.10410 [DOI] [Google Scholar]
  77. Oliver  TH, Heard  MS, Isaac  NJB, et al.  2015. Biodiversity and resilience of ecosystem functions. Trends in Ecology & Evolution  30: 673–684. doi: 10.1016/j.tree.2015.08.009 [DOI] [PubMed] [Google Scholar]
  78. Ossola  A, Hoeppner  MJ, Burley  HM, Gallagher  RV, Beaumont  LJ, Leishman  MR. 2020. The global urban tree inventory: a database of the diverse tree flora that inhabits the world’s cities. Global Ecology and Biogeography  29: 1907–1914. doi: 10.1111/geb.13169 [DOI] [Google Scholar]
  79. Paquette  A, Sousa-Silva  R, Maure  F, Cameron  E, Belluau  M, Messier  C. 2021. Praise for diversity: a functional approach to reduce risks in urban forests. Urban Forestry & Urban Greening  62: 127157. doi: 10.1016/j.ufug.2021.127157 [DOI] [Google Scholar]
  80. Pike  CC, Koch  J, Nelson  CD. 2021. Breeding for resistance to tree pests: successes, challenges, and a guide to the future. Journal of Forestry  119: 96–105. doi: 10.1093/jofore/fvaa049 [DOI] [Google Scholar]
  81. Planchuelo  G, Von Der Lippe  M, Kowarik  I. 2019. Untangling the role of urban ecosystems as habitats for endangered plant species. Landscape and Urban Planning  189: 320–334. doi: 10.1016/j.landurbplan.2019.05.007 [DOI] [Google Scholar]
  82. Pooler  M, Contreras  RC, Criley  RA, et al.  2024. Seeing the forest for the trees: threats, vulnerabilities, and opportunities for woody landscape plant genetic resources. HortScience  59: 1497–1504. doi: 10.21273/HORTSCI18010-24 [DOI] [Google Scholar]
  83. Reusch  TBH, Ehlers  A, Hämmerli  A, Worm  B. 2005. Ecosystem recovery after climatic extremes enhanced by genotypic diversity. Proceedings of the National Academy of Sciences United States of America  102: 2826–2831. doi: 10.1073/pnas.0500008102 [DOI] [PMC free article] [PubMed] [Google Scholar]
  84. Rimlinger  A, Avana  M-L, Awono  A, et al.  2021. Trees and their seed networks: the social dynamics of urban fruit trees and implications for genetic diversity. PLoS One  16: e0243017. doi: 10.1371/journal.pone.0243017 [DOI] [PMC free article] [PubMed] [Google Scholar]
  85. Roberts  DG, Ayre  DJ, Whelan  RJ. 2007. Urban plants as genetic reservoirs or threats to the integrity of bushland plant populations. Conservation Biology  21: 842–852. doi: 10.1111/j.1523-1739.2007.00691.x [DOI] [PubMed] [Google Scholar]
  86. Roe  AD, MacQuarrie  CJK, Gros-Louis  M, et al.  2014. Fitness dynamics within a poplar hybrid zone: II. Impact of exotic sex on native poplars in an urban jungle. Ecology and Evolution  4: 1876–1889. doi: 10.1002/ece3.1028 [DOI] [PMC free article] [PubMed] [Google Scholar]
  87. Santamour  FS. 1990. Trees for urban planting: diversity, uniformity, and common sense. In: Proceedings of the seventh conference of the metropolitan tree improvement alliance. Lisle, IL: Metria, 57–65. [Google Scholar]
  88. Sæbø  A, Borzan  Ž, Ducatillion  C, et al.  2005. The selection of plant materials for street trees, park trees and urban woodland. In: Konijnendijk  C, Nilsson  K, Randrup  T, Schipperijn  J. eds. Urban forests and trees. Berlin: Springer, 257–280. [Google Scholar]
  89. Schaal  B, Leverich  W. 2004. Population genetic issues in ex situ plant conservation. In: Guerrant  EO, Havens  K, Maunder  M. eds. Ex situ plant conservation: supporting species survival in the wild. Washington, DC, USA: Island Press, 267–285. [Google Scholar]
  90. Schmid  R, Farjon  A. 2013. Sequoiadendron giganteum. The IUCN Red List of Threatened Species 2013: e.T34023A2840676. 10.2305/IUCN.UK.2013-1.RLTS.T34023A2840676.en (25 March 2025, date last accessed). [DOI]
  91. Secretariat of the Convention on Biological Diversity . 2011. Nagoya Protocol on access to genetic resources and the fair and equitable sharing of benefits arising from their utilization to the convention on biological diversity. The Nagoya Protocol on access and benefit sharing of genetic resources.
  92. Senevirante  SI, Zhang  X, Adnan  M, et al.  2021. Weather and climate extreme events in a changing climate. In: Masson–Delmotte  VP, Zhai  P, Pirani  A, et al. eds. Climate change 2021: the physical science basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press, 1513–1766. [Google Scholar]
  93. Shishegar  N. 2014. The impact of green areas on mitigating urban heat island effect. The International Journal of Environmental Sustainability  9: 119–130. doi: 10.18848/2325-1077/CGP/v09i01/55081 [DOI] [Google Scholar]
  94. Sieghardt  M, Mursch-Radlgruber  E, Paoletti  E, et al.  2005. The abiotic urban environment: impact of urban growing conditions on urban vegetation in. In: Konijnendijk  C, Nilsson  K, Randrup  T, Schipperijn  J. eds. Urban forests and trees. Berlin: Springer, 281–323. [Google Scholar]
  95. Sjöman  H, Hirons  AD, Bassuk  NL. 2018. Improving confidence in tree species selection for challenging urban sites: a role for leaf turgor loss. Urban Ecosystems  21: 1171–1188. doi: 10.1007/s11252-018-0791-5 [DOI] [Google Scholar]
  96. Sjöman  H, Watkins  H, Kelly  LJ, et al.  2024. Resilient trees for urban environments: the importance of intraspecific variation. Plants, People, Planet  6: 1180–1189. doi: 10.1002/ppp3.10518 [DOI] [Google Scholar]
  97. Smith  D, Abeli  T, Bruns  EB, et al.  2023. Extinct in the wild: The precarious state of Earth's most threatened group of species. Science  379: eadd2889. doi: 10.1126/science.add2889 [DOI] [PubMed] [Google Scholar]
  98. Sniezko  RA, Liu  JJ. 2023. Prospects for developing durable resistance in populations of forest trees. New Forests  54: 751–767. doi: 10.1007/s11056-021-09898-3 [DOI] [Google Scholar]
  99. Staude  IR, Grenié  M, Thomas  CD, et al.  2025. Many non-native plant species are threatened in parts of their native range. New Phytologist  247: 1579–1583. doi: 10.1111/nph.70193 [DOI] [PMC free article] [PubMed] [Google Scholar]
  100. Sun  W. 1998. Ginkgo biloba. The IUCN Red List of Threatened Species 1998: e.T32353A9700472. 10.2305/IUCN.UK.1998.RLTS.T32353A9700472.en (24 March 2025, date last accessed). [DOI]
  101. Tischenko  L, Williams  M, DeMerchant  I, et al.  2024. Oak genomics for nature-based solutions: shaping future forests and resilient landscapes in Canada. Tree Genetics & Genomes  20: 15. doi: 10.1007/s11295-024-01645-x [DOI] [Google Scholar]
  102. Truchy  A, Angeler  DG, Sponseller  RA, Johnson  RK, McKie  BG. 2015. Linking biodiversity, ecosystem functioning and services, and ecological resilience. Advances in Ecological Research  53: 55–96. doi: 10.1016/bs.aecr.2015.09.004 [DOI] [Google Scholar]
  103. Turner-Skoff  JB, Cavender  N. 2019. The benefits of trees for livable and sustainable communities. Plants, People, Planet  1: 323–335. doi: 10.1002/ppp3.39 [DOI] [Google Scholar]
  104. Valencia-Cuevas  L, Mussali-Galante  P, Cano-Santana  Z, Pujade-Villar  J, Equihua-Martínez  A, Tovar-Sánchez  E. 2018. Genetic variation in foundation species governs the dynamics of trophic interactions. Current Zoology  64: 13–22. doi: 10.1093/cz/zox015 [DOI] [PMC free article] [PubMed] [Google Scholar]
  105. Vanden Broeck  A, Cox  K, Melosik  I, Maes  B, Smets  K. 2018. Genetic diversity loss and homogenization in urban trees: the case of Tilia × europaea in Belgium and The Netherlands. Biodiversity and Conservation  27: 3777–3792. doi: 10.1007/s10531-018-1628-5 [DOI] [Google Scholar]
  106. Vogt  J, Hauer  R, Fischer  B. 2015. The costs of maintaining and not maintaining the urban forest: a review of the urban forestry and arboriculture literature. Arboriculture & Urban Forestry  41, 293–323. doi: 10.48044/jauf.2015.027 [DOI] [Google Scholar]
  107. Westwood  M, Oldfield  S, Jerome  D, Romero-Severson  J. 2017. Fraxinus pennsylvanica. The IUCN Red List of Threatened Species 2017: e.T61918934A61919002. 10.2305/IUCN.UK.2017-2.RLTS.T61918934A61919002.en (25 March 2025, date last accessed). [DOI]
  108. Wolkis  D, Eltringham  C, Fant  J, et al.  2024. Pollen banking is a critical need for conserving plant diversity. Nature Plants  10: 1270–1271. doi: 10.1038/s41477-024-01757-1 [DOI] [PubMed] [Google Scholar]
  109. Wood  EM, Esaian  S. 2020. The importance of street trees to urban avifauna. Ecological Applications  30: e02149. doi: 10.1002/eap.2149 [DOI] [PMC free article] [PubMed] [Google Scholar]
  110. Zaya  DN, Leicht-Young  SA, Pavlovic  NB, Ashley  MV. 2021. Heterospecific pollination by an invasive congener threatens the native American bittersweet, Celastrus scandens. PLoS One  16: e0248635. doi: 10.1371/journal.pone.0248635 [DOI] [PMC free article] [PubMed] [Google Scholar]

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