Cities nationwide are pursuing ambitious tree canopy cover goals and tree planting initiatives because of the numerous perceived and quantified benefits of urban trees including urban heat island mitigation, stormwater runoff reduction, and beautification (Morgenroth et al., 2016; Doroski et al., 2020). Management priorities for cities include creating diverse urban tree communities to offer protection against pest outbreaks, climate change, and other stressors (Kendal et al., 2014). A common rule of thumb for minimizing risk from tree threats, such as pest outbreaks, is the “10-20-30 rule” in tree diversity selection, which recommends no more than 10% of individuals in a city's forest should be of a single species, 20% from a single genus, and 30% from a single family (Santamour, 1990). This rule is a means to enhance the resilience of urban forests to pest outbreaks and other disturbances by increasing taxonomic and phylogenetic diversity. Having a rule about how to plant trees to maintain species diversity highlights the fact that urban forests contain many trees that are planted (Nowak, 2012) and that interspecific diversity is a recognized goal to prevent catastrophic urban tree loss to invasive pests and diseases. While there are guidelines and research on taxonomic and phylogenetic diversity in cities (Morgenroth et al., 2016), surprisingly little is known about intraspecific genetic diversity of urban tree populations and its effect on urban forest sustainability. In his seminal book, Darwin (1859) describes two types of selection: natural and artificial. Natural selection occurs when genotypes that are well adapted to local environmental conditions reproduce and pass on their alleles, while those genotypes that are maladapted do not. Over time this results in a change in the genetic structure of a population. Artificial selection refers to the human breeding of genotypes with traits that are deemed advantageous and desirable to the people breeding the species and are adapted to “man's [sic] use or fancy” (Darwin, 1859). In contrast to natural selection, the effect of artificial selection is not widely studied, and when it is, examples are typically limited to domestic animals such as dogs, livestock, and other agricultural crops. While there is extensive research on the domestication of crops, breeding of trees is relatively unexplored, and may have significant consequences for the sustainability of urban forests. Most urban planted trees are sourced from local nurseries (Burcham and Lyons, 2013; Conway and Vander Vecht, 2015). The majority of trees sold at nurseries have undergone artificial selection, for example, breeding cultivars for phenotypic traits of interest such as fall leaf color or pest resistance (Santamour, 1990). The ability to easily cultivate plants by asexual means through budding, grafting, cuttings, or other methods is an important component of horticultural economic viability (Santamour, 1990; Sæbø et al., 2003). Many cultivars of urban trees are either clonally propagated or produced through other methods once it is clear that the desired traits are heritable (Santamour, 1990), allowing for some, albeit limited, genetic diversity among them. The horticulture industry's heavy reliance on clonal propagation has potentially reduced genetic diversity of cultivated species relative to natural tree populations that are sexually reproducing, and likely produced a concomitant contraction in the phenotypic and functional trait space. The overall phenotype of a tree affects its performance and survival while simultaneously shaping other ecological communities (e.g., insect herbivores, microbes) and ecosystem processes such as decomposition and nutrient cycling (Whitham et al., 2006). The strong evidence for an “extended phenotype” in a variety of forested ecosystems demonstrates that changes in the distribution of genotypes on the landscape can and does ripple through to affect both community structure and ecosystem function across multiple trophic levels (Whitham et al., 2006; Bangert et al., 2008). According to Nowak (2012), evidence shows that up to 90% of trees in urban forests are planted, so it follows that artificial selection may have important effects on the insect and microbial communities associated with planted urban trees through the selection of particular genotypes and/or phenotypes. This is important because a key ecosystem service of urban trees is to support biodiversity (Morgenroth et al., 2016). While it is recognized that artificial selection and current planting practices may be reducing genetic diversity of urban tree populations, which is a critical component of urban forest diversity (Lohr, 2013), the topic remains understudied. Genetic diversity is important for community and ecosystem properties (Bailey et al., 2009), and populations with lower genetic diversity are less likely to be able to successfully adapt to changing conditions as compared to more genetically diverse populations with a broader range of genetic material for selection to act upon (Jump et al., 2009). Because of the importance of genetic diversity for future adaptability, preservation of genetic diversity within tree populations has been suggested as a conservation tool (Alfaro et al., 2014). In one sense, genetically impoverished planted urban tree populations might not be a problem because if a tree dies it can just be replanted, and planted city trees might not need high genetic diversity given their low evolutionary potential. However, there is reason to think it could be quite important. First, genetically impoverished urban tree populations might be even more susceptible to pests and climate changes than genetically diverse populations, leaving urban tree populations vulnerable to catastrophic loss, as has been observed in the past with previous diseases (Roman et al., 2018). Widespread mortality of planted trees has high consequences for cities, including for local ecological processes, the health and well-being of people living there, and the economic costs of removing and replacing the trees. Second, when cultivars of native species are planted, there could be interbreeding between planted and naturally growing forest trees in urban patches, which may dilute genetic diversity of urban populations or introduce either adaptive or maladaptive alleles. Thus, urban forest patches, where evolutionary processes do take place, might be more vulnerable than previously thought. There are few existing studies on the genetic diversity of urban tree populations. The few that do exist support the idea that more recently planted urban tree populations may have very low diversity. Two studies comparing historically planted trees with current nursery offerings found similar results. In Pittsburgh, Pennsylvania, USA, the genetic diversity of London Plane trees planted in a park ~100 years ago had higher diversity than those being sold in nurseries today—only three clones (Morton and Gruszka, 2008). In the Netherlands and Belgium, urban populations of Tillia species that were planted in the 1700s had higher diversity than what is being sold in present day nurseries—mostly only two clones (Vanden Broeck et al., 2018). Another study of Tillia × europaea cultivars in Europe details the fascinating extent to which human activities control the propagation of urban trees. Hansen et al. (2014) found that the same genotype has been planted for over 250 years, suggesting a single clone is being continually propagated over several human generations at nurseries. Furthermore, these clones of Tillia × europaea are widely planted across Europe (Wolff et al., 2019). The two examples of commonly planted urban trees on different continents detail a similar story, i.e., only a few clones are being offered by modern nurseries. However, this problem is unrecognized by practitioners who assert that enough genetic diversity is being sold at nurseries (Lohr, 2013), suggesting that this problem is currently underrecognized in the horticultural industry. Although tree populations generally have high genetic diversity (Alfaro et al., 2014; Ingvarsson and Dahlberg, 2019), the widespread planting of cultivars may be reducing the genetic diversity of the urban forest. This may result in urban tree populations, which humans depend on for ecosystem services (Morgenroth et al., 2016), being less resilient and unable to adapt to climate change. City trees face many challenges including invasive pests and disease, climatic extremes, and altered soil hydrology and biogeochemistry (Sæbø et al., 2003). Despite the 10-20-30 rule on tree planting, few cities actually realize this goal (Kendal et al., 2014), most likely because certain trees are better at surviving urban conditions resulting in them being frequently planted, as, for example, the red maple in Baltimore (Burghardt et al., 2023). If, as suspected, the genetic diversity within urban tree populations is reduced because of artificial selection, the effects of low diversity may be compounded. There are many potential steps that can be taken to ensure more genetically diverse urban forests going forward. First, it is necessary to characterize the nature of the problem and better understand how diverse urban tree populations are, and if interbreeding between planted cultivars and naturally regenerating trees is in fact diluting the genetic diversity of urban tree populations. Second, there is a need for more interaction among breeders in the horticultural industry, academic researchers, and individuals and organizations planting trees in cities. An exchange of information about concerns and current practices would help illuminate the problem and identify potential solutions that would appease all involved parties. An end goal would be to offer and plant more trees with different genetic backgrounds. Ultimately, diverse urban tree populations and communities will be the most resilient to an uncertain future. M.L.A. conceived of and wrote the paper. I thank Karin Burghardt, J. Morgan Grove, Dexter Locke, Nancy Sonti, and Beatriz Shobe for wonderful conversations about the genetic diversity of urban trees, and the USDA NIFA Award #2020-04369 for funding this work. I also thank two anonymous reviewers for their insightful comments.