3.1 Temporal distribution of publications

From 2000 to 2022, research outputs on urban tree diversity grew considerably, increasing by 20-fold from just 9 in 2000 to 207 in 2022 (Figure 3). Some 96% of the publications were journal articles reporting original empirical findings. In the early 2000–2005 period, articles were the only publication type, with no reviews or other categories. This rather monolithic makeup reflects the nascent state of the field, focused on the initial analysis of tree diversity in various cities through data collection and field surveys. The first review appeared in 2005 as the literature began to accumulate. Reviews gradually became more common over time, constituting 3.5% of the outputs by 2022, with 58 reviews published over 23 years. These synthesized insights signaled the field's progression from isolated studies to a more connected and consolidated knowledge body. Other publication types, like reports, first emerged in 2000 but remained scarce and never exceeded 1% in any given year.

The publication trends expressed three distinct developmental phases (Figure 3). The formative 2000–2007 period had a low output of <15 papers per year, all basic diversity descriptions. Moderate growth occurred in 2008–2013, exceeding 20 publications per year by 2008 and reaching 74 by 2013, and reviews and reports began to appear sporadically. Notable expansion happened after 2013, with annual output more than doubled from 83 in 2014 to 213 in 2021. Reviews also became more frequent (Morgenroth et al., 2016), constituting up to 4% of publications by 2021, although articles remained dominate at 95% of the papers. This phase indicated urban tree diversity emerging as a major research focus, with original studies on threats, management strategies, social perspectives, and projections (Roebuck et al., 2022) and reviews synthesizing the burgeoning knowledge base (Nitoslawski et al., 2016).

Overall, the literature has prioritized empirical studies, providing essential data on patterns, drivers, and implications of urban forest diversity. Meanwhile, reviews have gradually increased, offering topical syntheses to complement and synergize insights from hundreds of articles. This ever-growing publication corpus reflects rising scientific and policy interest in understanding and protecting urban biodiversity in the face of pressing urban challenges.

3.2 Notable bibliometric features

3.2.2 Main research contributions by country and region

The top 10 countries collectively accounted for over 80% of all outputs, indicating the concentration of research capacity in developed Western nations and emerging economies like China and Brazil. The USA led global scholarship on urban tree diversity, accounting for 22.92% of publications from 2000 to 2022 (Table 2). With 383 articles and over 10,000 citations, US researchers made fundamental contributions across biodiversity assessments, social analyses, and policy guidance (Nitoslawski et al., 2016). China ranked second with 15.08% of papers, reflecting rapid urbanization pressures on its urban forest ecosystems (Jim, 2013). Brazil, Australia, Canada, Germany, the UK, Italy, France, and Poland joined the top 10 producing nations, each contributing 4%–7% of the literature. Notably, the contributions of Australia and Canada highlighted urban forest priorities for these highly urbanized countries (Davies et al., 2017; Hartigan et al., 2021). Though the USA led in volume and influence, urban forest scholarship has spanned several continents, with some nations making disproportionate contributions relative to the global share of urban forests.

TABLE 2. The top 10 countries reckoned by the number of publications on urban tree diversity from 2000 to 2022.

Country	Publication	Percent	Total citation	Mean citation	h-indexa
The USA	383	22.92	10,722	27.99	53
China	252	15.08	4239	16.82	33
Brazil	129	7.72	1505	11.67	21
Australia	110	6.58	3554	32.31	32
Canada	110	6.58	3069	27.90	32
Germany	109	6.52	3307	30.34	33
The UK	89	5.33	3569	40.10	31
Italy	82	4.91	2358	28.76	26
France	74	4.43	2503	33.82	30
Poland	63	3.77	1235	19.60	19

• ^a The h-index measures the productivity and citation impact of the publications of a scholar or the research group. It is defined as the maximum number of h papers a scholar or research group has published that have been cited at least h times (Hirsch, 2005).

Analysis of co-authorship patterns revealed five distinct clusters of country collaboration in urban tree diversity research from 2000 to 2022 (Figure 4). The red cluster indicated regional research alliances on Mediterranean urban forests. The green cluster reflected cooperation in investigating tropical and developing city landscapes. The blue cluster embodied the core Anglosphere countries. The yellow cluster showed the pattern signified research partnerships across Asia's rapidly growing urban regions. Finally, the Purple cluster demonstrated an expansive reach beyond geographical proximity. Meanwhile, several alliance patterns could be observed. First, proximity facilitated collaboration, as evidenced by Western Europe, Anglosphere, and Asian clusters. Second, shared language and academic systems promoted partnerships like the UK's ties with Europe, Asia, and its former colonies. Third, research capacity appeared crucial—productive entities like the USA, China, and Brazil anchored collaborations with smaller countries. Fourth, shared concerns of climate, biome, and urbanization pressures induced partnerships, such as tropical Africa with Western Europe. Targeted networking initiatives connecting these clusters could enhance idea exchange and capacity building. Overall, more extensive cross-cluster engagement could further advance the field.

3.2.3 Major research institutions

The Chinese Academy of Sciences was the top contributing institution, with 54 publications, reflecting China's recent rapid expansion in urban ecology research (Table 3). The USDA Forest Service ranked second with 41 papers, underscoring the established strength of US scholarship propped by its universities and extensive government-agency research. The Swedish University of Agricultural Sciences ranked third with 31 articles, highlighting Sweden's prominence in the field relative to its population size and city count (Morgenroth et al., 2016; Sjoman et al., 2019). The University of Florida and the University of Melbourne rounded out the top 5 with over 25 papers each, demonstrating robust output from these leaders in tropical and temperate urban forestry. The National Autonomous University of Mexico, the University of Hong Kong, the University of British Columbia, the University of the Chinese Academy of Sciences, and Beijing Forestry University completed the list, collectively generating over 200 publications (Jim, 2013; Zambrano et al., 2019). However, the mean citations and h-indices were often higher for Western institutions versus emerging economy universities. Urban tree diversity scholarship will likely continue to grow in developing countries in the coming decades with the anticipated expansion of research capacity.

TABLE 3. The top 10 institutions reckoned by the number of publications on urban tree diversity from 2000 to 2022.

Institution	Publication	Percent	Total citation	Mean citation	h-Indexa	Country
Chinese Academy of Sciences	54	3.23	1257	23.28	22	China
United States Forest Service	41	2.45	1547	37.73	23	The USA
Swedish University of Agricultural Sciences	31	1.86	1134	36.58	18	Sweden
University of Florida	28	1.68	579	20.68	14	The USA
University of Melbourne	26	1.56	1415	54.42	16	Australia
National Autonomous University of Mexico	25	1.50	424	16.96	12	Mexico
University of Hong Kong	25	1.50	706	28.24	17	China
University of British Columbia	24	1.44	793	33.04	9	Canada
University of the Chinese Academy of Sciences	22	1.32	320	14.55	12	China
Beijing Forestry University	21	1.26	400	19.05	11	China

• ^a Refer to Table 2 for the meaning of the h-index.

Analysis of co-authorship networks disclosed five distinct collaborative clusters among institutions with not less than seven joint publications on urban tree diversity from 2000 to 2022 (Figure 5). The red cluster centered on partnerships between the US institutions, pointing to robust intra-national teamwork. The green cluster linked the University of British Columbia with universities across the Pacific in Hong Kong and mainland China, reflecting pan-Pacific international cooperation. The purple cluster contained European institutions suggesting regional alliances. The yellow cluster joined highlighted long-range transcontinental collaboration. Finally, the pink cluster showed extensive co-publishing among major Chinese institutions.

3.2.5 Key authors

C.Y. Jim from the Education University of Hong Kong (previously affiliated with the University of Hong Kong) published the highest number of articles (27) on urban tree diversity as the leading author (Table 5). Other prolific authors include MacGregor-Fors from the University of Helsinki (13 articles), Sjöman from the Gothenburg Botanical Garden (12 articles), and Duinker from the Dalhousie University (11 articles). The top authors came from diverse research institutions worldwide, including North America, Europe, China, and Australia. Most were affiliated with universities that include studies on environmental science, forestry, horticulture, and urban planning, such as the University of Melbourne and the Czech University of Life Sciences. The top publishing authors generally have h-indexes ranging from 8 to 14, indicating their research influence. The articles from these productive authors have significantly contributed to the literature on urban tree diversity. More cross-country and cross-disciplinary collaborations could further enhance the productivity and impact of global urban tree diversity research.

TABLE 5. The top authors with more than 10 publications on urban tree diversity from 2000 to 2022.

Author	Publication	Institution	Country	h-Indexa
Jim, C Y	27	Education University of Hong Kong and University of Hong Kong	China	14
MacGregor-Fors, Ian	13	University of Helsinki	Finland	13
Sjöman, Henrik	12	Gothenburg Botanical Garden	Sweden	10
Duinker, Peter N.	11	Dalhousie University	Canada	8
Avolio, Meghan L	11	Johns Hopkins University	USA	9
Tryjanowski, Piotr	11	Poznan University of Life Sciences	Poland	8
Escobedo, Francisco J	10	USDA Forest Service	USA	8
Morelli, Federico	10	Czech University of Life Sciences	Czechia	9
Ordóñez, Camilo	10	University of Melbourne	Australia	8
Williams, Nicholas S G	10	University of Melbourne	Australia	9
Kendal, Dave	10	University of Tasmania	Australia	8

• ^a The h-index here specifically refers to an author's influence only in the field of urban tree diversity rather than the influence of all publications of an author.

The co-citation analysis identified four major clusters of influential authors in urban tree diversity research from 2000 to 2022 (Figure 6). The red cluster contained authors focusing on urban ecology and conservation, like Anderson, Clergeau, Savard, and Moller (Clergeau et al., 2006; Rega-Brodsky et al., 2022). The green cluster represented leading researchers on urban forestry and green infrastructure, including Jim, Nowak, McPherson, Kendal, and Escobedo (Escobedo et al., 2019; Jim et al., 2018). The blue cluster covered seminal community and landscape ecology authors like McDonnell, Pickett, Magurran, and Hobbs (McDonnell & Hahs, 2008; Zhou et al., 2017). Finally, the yellow cluster comprised key scientists studying biological invasions and plant diversity like Kowarik, Pysek, Knapp, and La Sorte (Kowarik et al., 2020; Pyšek et al., 2004). The analysis reflected the multidisciplinary nature of urban tree diversity research spanning urban ecology, forestry, invasion biology, community ecology, and landscape ecology. While some prominent cross-citations existed between clusters, the four groups also denoted distinct intellectual silos. More interdisciplinary collaboration and co-citation linkages between the clusters could benefit future urban tree diversity research. The co-citation patterns illustrated the field's evolution over 20 years through its pioneering members and their scholarly connections.

3.3 Principal research hotspots

From the knowledge graph presented in Figure 7, four principal groups of research hotspots could be identified:

(1) Urban tree diversity research coalesced around several interconnected hotspots. They included forest structure and composition, ecosystem services, biodiversity patterns, environmental drivers, and sustainable management (Figure 7, red cluster). Studies of urban forest vegetation quantified tree communities across cities to catalog species diversity while revealing differences based on climate, disturbance history, socioeconomics, and biogeography (Esperon-Rodriguez et al., 2020; Jim, 2013; Salmond et al., 2016). Assessing ecosystem services became a major research priority, with studies valuing the contributions of urban forests to air pollution removal, carbon sequestration, stormwater mitigation, and physical and mental health (Roebuck et al., 2022; Roy et al., 2012; Salmond et al., 2016). Biodiversity research examined how urbanization impacts species diversity and community tree assemblages, understory plants, birds, insects, and soil microbiota (Livesley et al., 2016; Sandström et al., 2006). Environmental drivers such as land use change, climate change, invasive species, pollution, and urban heat islands were investigated for their effects on urban forest diversity (Bourne & Conway, 2014; Morabito et al., 2021; Nitoslawski et al., 2016; Threlfall et al., 2022).

Sustainable urban forest management demonstrated a growing focus, with efforts to balance competing priorities of vegetation density, species diversity, public safety, esthetics, and resilience (Jim, 2013; Kowarik et al., 2020). To inform management, studies modeled future urbanization scenarios, landscape connectivity, species distributions, and implications of climate change (Roy et al., 2012; Stas et al., 2020). Research on the human dimension evaluated public perceptions, behaviors, and governance related to urban trees. Synthesizing findings across disciplines, scales, and methods enabled a holistic understanding of linkages between urban tree diversity and human well-being (Morgenroth et al., 2016; Zambrano et al., 2019). With urbanization accelerating worldwide, transdisciplinary collaborations, comparative studies across cities, and linking science with policy and practice would be critical to translating urban tree diversity research into effective management solutions for resilient, sustainable, and biodiverse cities globally.

(2) Understanding patterns, dynamics, and drivers of tree diversity presented a major research priority for urban ecosystems worldwide (Figure 7, green cluster). Studies analyzed alpha and beta diversities to quantify variations in community composition between sites and regions (Bourne & Conway, 2014; Ricotta et al., 2012). Biodiversity mapping projects documented population distributions, revealing citywide gradients and hotspots (Esperon-Rodriguez et al., 2020; McDonnell & Hahs, 2008; Wang et al., 2020). Examining temporal dynamics provided insights into colonization, extinction, and turnover as impacted by urbanization. Demographic research tracked tree growth, recruitment, and mortality, with findings often indicating reduced regeneration (Haase & Hellwig, 2022; Raupp et al., 2006; Smith et al., 2019). Investigating population dynamics involved mapping size and age structures to identify sustainable cycles versus impending loss. Fragmentation was studied as a driver of diversity, with habitat patches supporting distinct communities separated by barriers (Mavimbela et al., 2018). Connectivity research assessed dispersal limitations and landscape connectivity to guide restoration (Bierwagen, 2007).

Studies also considered trait diversity involving tolerances, growth rates, and functions (Conway & Vander Vecht, 2015; Haase & Hellwig, 2022; Roebuck et al., 2022). Disturbances altered competitive hierarchies between native and invasive trees, impacting diversity (Johnson & Handel, 2016; Wang et al., 2020). Interactions between urbanization drivers such as heat islands and impervious surface and tree diversity were examined (Morabito et al., 2021). Changing environmental conditions due to land use and climate change and implications for biodiversity were active research frontiers (Bourne & Conway, 2014; Burley et al., 2019). Findings were applied to assessing ecosystem services, modeling vegetation shifts, predicting future distributions, and managing diversity. Comparative urban diversity studies along latitudinal and environmental gradients provided the spatial context to understand macro-scale patterns (Esperon-Rodriguez et al., 2020; Wang et al., 2020). Integrating diversity patterns, processes, and projections enabled deeper ecological understanding and informed global sustainable urban forest planning and management.

(3) Biodiversity conservation in urban ecosystems demonstrated an advancing frontier, with studies quantifying tree diversity through species inventories and richness assessments across metropolitan landscapes (Figure 7, blue cluster). The research characterized urbanization impacts on community composition, often finding reduced native tree cover and shifts toward non-native and ornamental species (Gregg et al., 2003; Ruas et al., 2022). Land use analyses associated higher diversity with parks, gardens, and riparian areas, contrasting with lower diversity in heavily paved areas (Bourne & Conway, 2014; Nitoslawski et al., 2016). Examining urbanization as a gradient facilitated comparisons of tree communities along rural-suburban-urban transects and between cities differing in density and sprawl (Wang et al., 2020). Landscape ecology approaches assessed habitat patches, fragmentation, and connectivity to understand compositional patterns (Norton et al., 2016). Remote sensing, field surveys, and citizen science datasets were combined to map and monitor communities at broader scales (Ozkan et al., 2016; Robinson et al., 2020). Network analysis quantified biodiversity distribution across green space networks. Comparative studies along latitudinal, climatic, and biogeographic gradients provided the regional context while revealing global patterns of biotic homogenization and diversity filtering in cities (Aronson et al., 2016).

Experimental studies in gardens and field sites provided a process-based understanding of diversity drivers (Gaston et al., 2005; Mathieu et al., 2007). Biodiversity projections incorporated urbanization scenarios and environmental changes. Multiple facets were analyzed, from species composition to functional to phylogenetic diversity, to support conservation planning and prioritization (Pyšek et al., 2004; Ricotta et al., 2012; Stas et al., 2020). The findings informed practical measures to increase native species establishment in new and existing habitat patches, restore connectivity through corridors, optimize site and species selection in greening projects, and balance various facets of biodiversity effects on human well-being. Mainstreaming biodiversity into urban planning and policy indicated an increasing emphasis on improving the urban milieu to benefit nature and people (Carroll et al., 2022). Synthesis of conservation science with governance, social values, and design principles shaped diverse, resilient urban tree communities yielding critical ecosystem services.

(4) Urban trees provided critical ecosystem services, with improving air quality an emerging priority (Figure 7, yellow cluster). The research characterized plant traits and functions related to pollution removal, investigating leaf and canopy properties that could enhance air filtration and absorption (Esperon-Rodriguez et al., 2020; Grote et al., 2016). Assessments were conducted to quantify pollution removal rates for common tree species, with databases developed to inform selection (Grote et al., 2016). Functional diversity studies involved categorizing trees based on traits to maximize service delivery, such as particulate capture and carbon sequestration. Green space networks with diverse functional traits could increase cumulative services.

Leaf and soil chemical analyses detected bioindicators and monitored pollution impacts on vegetation health (Azzazy, 2020; Molnár et al., 2020). Comparative studies found pollution tolerance thresholds across species (Shahrukh et al., 2023). Experimental exposure trials determined damage symptoms and mechanisms (Haase & Hellwig, 2022). Chemical, morphological, and genetic adaptations to pollutants indicated active research areas (Banerjee et al., 2022). Air pollution effects on growth, reproduction, and mortality rates were quantified. Interactions between pollutants and other stressors like drought and pests were studied (Azzazy, 2020). Investigating environmental justice dimensions involved mapping pollution removal across neighborhoods, with findings often highlighting inequities (Pope et al., 2016). Modeling future air quality and health benefits of urban tree planting initiatives helped prioritize communities (Locosselli & Buckeridge, 2023; Roy et al., 2012).

Assessing cost-effectiveness demonstrated value compared to engineering solutions (O'Sullivan et al., 2017). Developing low-maintenance trees suited for harsh urban environments indicated a practical goal for the management (Sæbø et al., 2005; Santamour Jr, 2004). Synthesizing research on pollution mitigation, vegetation sensitivity, and adaptive capacities could guide the selection of hardy, high-functioning species for air quality improvement. Mainstreaming findings into urban forest planning and policy could ensure environmental quality as an integral greening goal. Multifunctional urban tree diversity could be leveraged through strategic planting and management to enhance public health and livability in changing urban environments (Singh et al., 2020).

3.4 Notable research frontiers

The bibliometric analysis provided hints to identify the notable research frontiers:

(1) Understanding and modeling future distributions, traits, and ecosystem services of urban trees under climate change was a critical and increasingly nurtured research frontier (Burley et al., 2019). As climate change could lead to warming temperatures, altered precipitation patterns, and more frequent extreme weather events globally, researchers focused on predicting such impacts on urban forests (Pretzsch et al., 2017). The studies applied climate projection and species distribution models to forecast changes in tree growth, mortality, reproduction, phenology, physiology, and habitat ranges under different emission scenarios (Gregg et al., 2003; Smith et al., 2019). The findings could enable more accurate predictions of future urban vegetation states and ecosystem services provisioning.

Specific research questions involved whether warming minimum temperatures could expand southern tree species range but increase the mortality of northern species adapted to colder climates (Brune, 2016). The resulting phenology changes were examined, including earlier spring budburst, delayed autumn senescence, longer growing seasons, and mismatches with pollinators or herbivores (Esperon-Rodriguez et al., 2020). Plasticity in drought tolerance traits and acclimation abilities were evaluated to determine resilient versus vulnerable species (Haase & Hellwig, 2022). Population genomic approaches assessed the capacity for adaptation (Housset et al., 2018).

Experiments evaluated precipitation and temperature trends to quantify climate change effects directly (Ettinger et al., 2019). Climate velocity models projected migration rates in response to shifts in isotherms poleward and upward in elevation (Holsinger et al., 2019). The results identified dispersal lags and extinction risks in fragmented urban landscapes. The findings were incorporated into process-based models projecting growth, mortality, regeneration, and biomass under varying climate scenarios (Kowarik, 2023; O'Sullivan et al., 2017). Outputs were applied to assess future ecosystem services like carbon storage, air pollution removal, and stormwater mitigation (Davies et al., 2017; Salmond et al., 2016). Comparative studies along urban climate gradients provided analogs for future conditions. Such research could support climate-adaptive planning and management to foster resilient, biodiverse, and functional urban forests.

(2) Research was conducted to quantify the relationships between urban tree diversity, human health, and environmental justice. As cities grappled with issues of inequality, studies queried how tree canopy cover, species composition, and access to nature correlated with health outcomes across neighborhoods (Threlfall et al., 2022). Some researchers employed spatial analysis and statistical models to evaluate how the canopy cover in residential areas could relate to outdoor physical activities, the prevalence of chronic diseases, cardiovascular mortality, mental health, social cohesion, and crime rates (Livesley et al., 2016; Salmond et al., 2016). The ecosystem services of trees in providing shade, cooling, and desirable spaces were quantitatively connected to public health (Elderbrock et al., 2020; Salmond et al., 2016). Comparisons across socioeconomic gradients examined distributional equity in exposure to heat islands and air pollution relative to tree cover (Fan & Sengupta, 2022). Field measurements and satellite imagery were used to map canopy cover, land surface temperatures, and high-resolution vegetation indices across cities (Mathieu et al., 2007).

Controlled experiments studied how interacting with diverse vegetation versus artificial environments could affect stress, cognition, and emotions (Gidlow et al., 2016). Biodiversity experiments manipulated species richness and assessed benefits to concentration, impulse control, and emotional well-being (Locosselli & Buckeridge, 2023). The findings expounded on urban nature's positive, direct and indirect impacts on public health (Singh et al., 2020). Environmental justice research focused on tree canopy deficiencies of low-income and ethnic minority communities (Wolch et al., 2014). Urban heat mapping found many underserved neighborhoods disproportionately hot with less mitigation by green space (Calderón-Argelich et al., 2021; Fan & Sengupta, 2022).

Community-based participatory research engaged residents in assessing needs and priorities. Inventory data revealed the patterns and magnitude of inequitable tree maintenance across districts. As expanding canopy cover was linked to health improvement, researchers recommended prioritizing greening initiatives and tree planting in marginalized communities to attain equity and justice (Pope et al., 2016). Citizen science apps empowered residents to map their neighborhoods and advocate for change. The fast-growing research on urban greening, citizen health, and justice is critical for guiding policies, planning, and interventions to foster greener, cooler, healthier, and more equitable cities.

(3) Sustainable urban forest management strategies were developed to optimize city trees' multiple ecological, social, and economic functions. Urban forests provided diverse services, from air purification to esthetic beauty (Duinker et al., 2015), but maximizing one benefit can compromise others (O'Sullivan et al., 2017). Multicriteria decision analyses and scenario planning tools could balance competing objectives (Bizikova & Krcmar, 2015). Specifically, the research used analytical hierarchy processes to weigh and rank priorities for attributes like biodiversity, carbon sequestration, stormwater mitigation, air cooling, noise reduction, property values, public safety, and cost-effectiveness (Aronson et al., 2016; Clergeau et al., 2006).

Advanced simulation models were applied to test different planting regimes, maintenance schedules, and climate adaptation plans to quantify trade-offs between goals like native diversity versus resilience (Pauleit et al., 2005; Pretzsch et al., 2017). Optimization algorithms identified management plans to maximize multivariate benefits within budget constraints (Song et al., 2020). However, pure optimization could lead to monocultures like London plane trees that could maximize services at the expense of reduced biodiversity (Haase & Hellwig, 2022; Sæbø et al., 2005). Scenario planning developed quantitative models representing different management paradigms to facilitate stakeholder deliberation and value-focused priorities.

It was recommended to integrate adaptive planning cycles to monitor indicators and periodically reassess urban forest structures and functions (Escobedo et al., 2019; Hartigan et al., 2021). Climate change could bring species suitability shifts and societal values changes, demanding flexible yet systemic management (Holsinger et al., 2019; Housset et al., 2018; Ruas et al., 2022). Adding equity considerations like environmental justice and equal access to nature required dynamic reallocation of resources over time and space.

Ultimately, researchers applied decision science, simulation, spatial analysis, and participatory tools to develop integrated, sustainable urban forest plans (Hartigan et al., 2021). Rather than siloed with reactive management over-optimizing a service at the expense of others, cities could benefit from proactive strategies to maximize multifunctionality, flexibility, and community involvement (Hawthorne et al., 2015). Comprehensive, value-focused urban forest management could offer the key to equitably and sustainably accessing nature's diverse benefits and improving human health, well-being, and quality of life.

(4) Studies were conducted to leverage new technologies like remote sensing, drones, mobile apps, and simulation models to enhance research and monitoring over large urban areas. High-resolution satellite imagery, aerial LiDAR scans, and hyperspectral drones were applied to map tree canopy structure, species composition, and functions at sub-meter resolution across entire cities (Berland & Lange, 2017). Cloud computing enabled the processing of terabytes of geospatial urban forest data into biodiversity indices, microclimate maps, and environmental models (Le & Bakaeva, 2023; Li et al., 2022). Researchers harnessed crowd-sourced citizen science through cell phone apps, empowering residents to map, identify, and collect data on neighborhood trees (Hawthorne et al., 2015; Robinson et al., 2020). Municipal tree inventories combined with ground-level observations could scale up via modeling to citywide mappings of species distributions, diversity patterns, and vulnerabilities (Mathieu et al., 2007).

As urban tree databases grew globally, big data analytics effectively identified socioeconomic, planning policy, and biogeographic drivers of diversity (Carroll et al., 2022; Kendal et al., 2014; Ruas et al., 2022; Sage, 2020). Simulation models manipulated virtual urban forests to predict the impacts of future scenarios like invasive pests, climate change, or planting regimes on biodiversity, risks, and ecosystem services. Digital twins merged real-time sensor data with simulations to improve model accuracy. Researchers could survey urban forests at unprecedented scope and resolution by combining publicly available satellite imagery with crowd-sourced citizen science data (Robinson et al., 2020).

Cloud computing made large-scale urban tree data accessible for grassroots discovery and planetary awareness (Hartling et al., 2021). As digital maps and tools engaged communities in research and stewardship, a vision emerged where every tree in every city is sustainably managed using technologies scaling local monitoring to global knowledge sharing (Berland & Lange, 2017). Advanced technologies created better and more opportunities to understand urban tree diversity, drivers, and services with revolutionary breadth and depth.

Additional research priorities in the urban tree diversity domain can be found beyond the emerging frontiers. For instance, studying propagation, establishment, and trait plasticity can guide climate-resilience efforts by identifying species and cultivars capable of acclimating or adapting to rapid urban environmental change through assisted migration (Esperon-Rodriguez et al., 2020; Housset et al., 2018). Examining gene flow and evolutionary dynamics between urban and rural tree populations can help understand urbanization as a driver of divergent natural selection and ecotypic variation (Miles et al., 2019). Developing best practices to maximize diversity and ecosystem services in green infrastructure installations, like green roofs and bioretention cells, is needed to improve multifunctionality (Wooster et al., 2022). Finally, modeling trade-offs and synergies between tree diversity, density, and services can help inform optimal, balanced planting goals and species selection (Kendal et al., 2014; O'Sullivan et al., 2017). As urban forests expand worldwide, research can aim at enlightening resilience, evolution, eco-technology performance, and planning for multibenefit systems. Targeted studies in these areas can allow sustainable management to keep pace with global urbanization and expectations.