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

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.