Urbanization is an unstoppable force reshaping landscapes and ecosystems worldwide, driving profound and often irreversible changes to biodiversity. One of the most discussed outcomes of this process is biotic homogenization—the phenomenon where distinct biological communities across different urban areas become increasingly similar over time. However, recent research published in Nature Cities challenges the universality and simplicity of this narrative, providing a nuanced perspective on how urbanization influences plant diversity, specifically focusing on urban tree assemblages across the globe.
The study, led by Yang, Jin, Liu, and colleagues, delves deeply into the compositional dissimilarity—or conversely, similarity—among 39 highly diverse urban tree communities spanning multiple continents. Instead of merely assuming that urban areas foster species that make cities more alike biologically, the researchers applied innovative methodological controls that account for potential biases inherent in previous work. By controlling for differences in regional species pools, this team aimed to achieve a far more precise understanding of how urbanization shapes global tree biodiversity patterns.
Key to their approach was the acknowledgment that regional species availability varies greatly and influences the species composition seen in any urban assemblage. Ignoring this confounding factor has led some older studies to overstate homogenization effects, especially when comparing urban floras across diverse geographic settings. By adjusting for these baseline pool differences, the team was able to isolate the genuine impact of urbanization itself on tree species distribution across cities.
Their findings are both surprising and revealing. Contrary to the prevailing expectation of global-scale biotic homogenization in urban environments, the researchers discovered no uniform global pattern of homogenization or differentiation. Rather, urban tree assemblages exhibit scale-dependent dynamics. Cities separated by large geographical distances tended to show homogenization—meaning they shared a relatively similar set of tree species. By contrast, cities that were geographically closer demonstrated higher differentiation, often featuring more unique or dissimilar species assemblages.
One of the most striking aspects of this study is the role played by nonnative tree species. These alien species emerged as a critical driver of spatial patterns in compositional similarity and dissimilarity. When examining broad spatial scales, the presence of the same nonnative species across distant cities reinforces homogenization, making far-flung urban assemblages biologically alike. Paradoxically, at short distances, cities often share different sets of nonnative species, intensifying species turnover and leading to greater differentiation rather than uniformity.
This dual pattern—homogenization at global scales contrasted with differentiation locally—invites a rethink of prior assumptions about urban biodiversity dynamics. It suggests that urbanization is not a monolithic process driving a simple convergence of biotic communities but a complex interplay of ecological and sociocultural factors that vary with scale. The movement and deliberate planting of nonnative species, along with differing local environmental and urban policy contexts, seem to shape uniquely variegated urban floras.
From a technical perspective, the researchers employed analytical methods that integrated spatial ecology, biogeography, and community ecology principles. By using metrics that quantify beta diversity (a measure of compositional dissimilarity among communities) and statistically accommodating differences in regional species pools, they ensured that ecological inferences were robust and representative. This rigorous methodological framework counters previous criticisms of urban biodiversity studies, which often conflated local species introductions with broader global trends.
The implications for conservation and urban planning are profound. Urban trees are recognized not only for their ecological value but also for enhancing human well-being, providing ecosystem services, and mitigating climate change impacts. Understanding that biotic homogenization is not a universal outcome shifts the focus towards preserving or even promoting urban tree diversity at appropriate spatial scales. This can inform strategies that prevent the loss of locally unique species assemblages and encourage diversity in tree planting initiatives.
Moreover, the complex role of nonnative species highlighted by the study demands nuanced policy responses. While nonnatives sometimes contribute positively by filling ecological niches or enhancing urban resilience, their unpredictable effects on homogenization or differentiation call for careful management. Urban foresters and policymakers should evaluate species introductions not only from an aesthetic or utility viewpoint but also with an eye to long-term biodiversity patterns.
Another consideration arising from the research is the catalytic influence of human preferences and socioeconomic factors on urban tree assemblages. Urban landscapes are often shaped by cultural values, horticultural trends, and economic capabilities, which drive species selection in different cities. These human dimensions intersect with ecological processes to create multifaceted spatial biodiversity mosaics, underscoring the importance of interdisciplinary approaches in urban ecology.
The findings also suggest exciting new research directions. Future studies might explore how temporal aspects of urbanization affect tree diversification or homogenization, as urban environments continue to evolve. Longitudinal data tracking tree species turnover over decades could reveal whether current patterns represent transient states or more permanent ecological configurations.
In addition, integrating genomic studies with ecological surveys could illuminate evolutionary responses of native and nonnative urban tree populations to urban stressors. This would deepen understanding of how urban habitats influence tree resilience and adaptation, potentially informing breeding and conservation programs.
The research further raises the question of how other taxonomic groups respond to urbanization at different scales. Are similar scale-dependent patterns of homogenization and differentiation observed in urban bird, insect, or microbial communities? Cross-taxon comparisons could reveal universal principles or taxa-specific dynamics of urban biodiversity.
Finally, the study underscores the importance of global collaboration and data sharing among urban ecologists. Analyzing urban tree assemblages worldwide is an ambitious endeavor requiring harmonized methodologies and open data infrastructures. Such collaborations enhance the ability to detect broad-scale patterns while respecting regional ecological idiosyncrasies, ultimately advancing the science and practice of urban biodiversity conservation.
In summary, the work by Yang, Jin, Liu, and colleagues critically revises the narrative around urbanization and biodiversity by unpacking the scale-dependent roles of nonnative species in shaping tree communities globally. It challenges simplistic notions of universal biotic homogenization and highlights a far more intricate mosaic of diversification and convergence shaped by geography, species composition, and human influence. This advance not only enriches scientific understanding but also equips urban planners and conservationists with insights necessary to cultivate vibrant, diverse, and resilient urban forests in an increasingly urbanized world.
Subject of Research: Patterns of species homogenization and differentiation in urban tree assemblages globally, with a focus on the role of nonnative species and spatial scale.
Article Title: Homogenization and differentiation of urban tree assemblages globally.
Article References:
Yang, X., Jin, J., Liu, X. et al. Homogenization and differentiation of urban tree assemblages globally. Nat Cities (2026). https://doi.org/10.1038/s44284-026-00393-4
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s44284-026-00393-4
Tags: biodiversity changes in metropolitan areasbiotic homogenization in citiescompositional dissimilarity of urban floracross-continental urban ecology studiesglobal urban tree diversity patternsmethodological advances in urban ecologynature cities research on treesregional species pool effectsspecies assemblages in global citiesurban biodiversity conservation challengesurban tree species compositionurbanization impact on biodiversity
