The Role of Urban Tree Areas for Biodiversity Conservation in Degraded Urban Landscapes

Abstract

Urban tree diversity plays a crucial role in enhancing the resilience of cities by contributing to ecosystem services such as mitigating the effects of land degradation, combating urban heat islands, improving air quality, and fostering biodiversity habitats. A diverse tree population enhances resilience to vulnerabilities related to climatic stress, disease, and habitat loss by promoting stability, adaptability, and efficiency within the ecosystem. Little is known about urban tree diversity in Serbia; therefore, this study examines the diversity of tree species in the City of Niš, Serbia, to assess its implications for urban resilience and biodiversity preservation in the context of land-use change. Using the Shannon Diversity Index, we quantify species richness and evenness across both central and suburban zones of the city. The results are benchmarked against similar indices in five other European cities to assess how patterns of urban tree distribution vary under different urbanisation pressures. The study reveals that tree diversity is markedly lower in the city centre than in peripheral areas, highlighting spatial inequalities in green infrastructure that may accelerate biodiversity loss due to compact urban development. These findings demonstrate how urban expansion and infrastructure density contribute to ecological fragmentation, potentially leading to long-term effects on ecosystem services. This study emphasises the strategic importance of integrating greenery diversity into urban and landscape planning, particularly in rapidly growing urban centres in Southeastern Europe. This research contributes to the existing body of literature, providing a deeper understanding of the interdependencies between urban tree diversity, land degradation, and biodiversity loss, offering data-driven insights. This enables urban planners, landscape architects, and policy advisors to make informed decisions about street tree diversity and green city infrastructure, contributing to the development of sustainable cities.

1. Introduction

Interest in urban biodiversity has notably increased among researchers since the 1990s [1,2]. Accelerated urbanisation, alongside economic, political, and cultural transformations, has led to the endangerment of plant species in urban areas, particularly within city centres [3]. In contemporary development contexts, public awareness of the significance of urban greenery is growing [4]. To improve quality of life, it is essential to foster a positive relationship between residents and their environment [5,6,7]. Further reinforcing this point, Savard et al. [7] argue that “enhancement of biodiversity in urban ecosystems can have a positive impact on the quality of life and education of urban dwellers and thus facilitate the preservation of biodiversity in natural ecosystems.” The institutional approach is vital in guiding the transition towards new models of sustainable development, as highlighted by Andjelković et al. [8]. It highlights the need for effective governance through comprehensive urban greenery and biodiversity policies, as we find in Germany, France, the Netherlands, Sweden, the United Kingdom, and Finland in Europe. For Serbia, the city of Belgrade has begun developing strategies to increase green spaces, promote parks, and enhance urban biodiversity [9].

Yet, land degradation, frequently connected to the sealing of soil resulting from infrastructure projects, the disappearance of plant life, and the fragmentation of ecosystems in urbanized regions, has become a significant challenge facing cities across the world [10,11]. In this context, the composition and distribution of tree species become vital indicators not only of biodiversity health but also of how vulnerable urban landscapes are to land degradation processes.

Little is known about urban biodiversity in the mid-sized city of Niš, Serbia. This study provides valuable insights into understanding ecosystem health and enables a comparison of tree biodiversity in Niš with that of Amsterdam, Bologna, Cambridge, Oslo, and Paris using the Shannon Diversity Index [12]. Although the analysed cities are very different in size, population density, available resources and governmental systems, the analysis remains meaningful and applicable precisely because of the valuable insights and transferable knowledge it can generate for urban biodiversity management and policy-making. Moreover, the implications for Niš’s urban resilience are also discussed, since smaller or less-resourced cities, such as Niš, can benefit from lessons learned and best practices identified in larger or more affluent cities like Amsterdam or Paris, by adapting their successful approaches to local circumstances.

By employing the Shannon Diversity Index, this study quantified both species richness and evenness within the urban centre and adjacent urban areas, providing a robust informational foundation for understanding tree diversity patterns. The results of this research include a detailed analysis of biodiversity distribution across various European cities. This study contributes to the ongoing discourse in urban resilience, offering insights that can inform strategies for sustainable urban forestry and biodiversity conservation in the City of Niš.

The following hypotheses were tested in this study:

• The City of Niš has a similar diversity of street trees in the city centre and surrounding areas as other European cities (higher dominance of specific tree species in the centre compared with a lower dominance of specific tree species in the surrounding areas), and
• Tree diversity is higher in areas outside the city centre than within the city centre in Niš.

2. Urban Biodiversity and Urban Planning

Norton et al. [6] highlight the growing importance of biodiversity conservation in a rapidly urbanising world. Hasa et al. [13] highlight that “human activities have significantly altered natural habitats, leading to biodiversity loss and ecosystem degradation”. Urban areas often lack sufficient green spaces, such as parks, gardens, and forests, which are critical for supporting biodiversity. The existing green spaces are often isolated from each other, creating ‘islands’ that cannot sustain large or diverse populations of species [14] (Institute for Sustainable Studies, 2024).

In response to these challenges, ideas regarding biophilic planning [15] have guided and shaped urban and regional planning strategies and practices over time, reflecting a growing recognition of the need to incorporate green spaces into urban planning and design effectively to address the multifaceted demands of urban environments [16]. Central to this discussion is the concept of green infrastructure, which has become a vital part of the path towards urban sustainability [17]. Green infrastructure offers multiple ecosystem services that enhance urban environments and support human health. As a result, planners and managers focus on understanding and enhancing the multifunctionality of green infrastructure [18,19].

To foster sustainable cities, adaptation strategies must incorporate regenerative principles into urban planning, enabling cities to mitigate the impacts of climate change effectively. This connection emphasises the necessity of considering urban biodiversity as a fundamental component of sustainable urban frameworks that promote resilience and adaptability in the face of environmental challenges [20,21,22,23].

It requires collaborative efforts among various stakeholders, including ecologists, urban planners and designers, policymakers, and the local community [24]. This multidisciplinary approach ensures that the unique ecological characteristics of each urban area are preserved and enhanced, providing tailored solutions to specific environmental challenges.

While the biodiversity in urban environments is a crucial component of overall resilience to environmental challenges, trees are crucial for preventing the deterioration of these environments in the face of global change and biological challenges over decades and even centuries [25]. A relatively small yet significant component of urban forests, street trees play a vital role in enhancing urban biodiversity [26]. Understanding urban tree diversity and the patterns of spatial variation across cities can provide essential insights for local practitioners aiming to target tree diversity efforts more effectively [27].

The significance of preserving trees is evident in their multifaceted benefits. Some studies [25], according to [28], show that the well-being of trees is reflected in the following aspects:

• Provisioning (food, fresh water, medicines, raw materials) [25];
• Cultural (aesthetic inspiration, mental and physical health, recreation, spirituality, tourism) [29];
• Regulating (air quality and local climate, carbon storage, extreme event protection, soil improvement, waste treatment, biological pest control, pollination) [28];
• Supporting (wildlife habitat and diversity, genetic diversity) [22,28].

Trees play a crucial role in enhancing air quality by absorbing pollutants and producing oxygen. By providing shade, they mitigate the urban heat island effect, which can lead to reduced cooling costs for buildings and improved thermal comfort during periods of elevated temperatures. Moreover, a diverse array of tree species is better equipped to adapt to changing climatic conditions. As temperatures increase, heat-tolerant species may become more dominant, while those less suited to such changes may decline. The maintenance of multiple species within urban forests fosters greater adaptability to climate variability.

The root systems of trees also play a vital role in hydrological processes, facilitating the absorption of rainwater and thereby reducing soil erosion and flood risks. Furthermore, trees provide essential habitats for various species, including birds, insects, and other wildlife, thereby contributing significantly to urban biodiversity. Although urbanisation typically results in habitat loss and biological homogenization, recent studies indicate that cities can maintain considerable species richness due to specific conditions that human activities can create [30,31]. “It requires a holistic approach that takes into account the complex interactions between trees and their urban environment, including the surrounding infrastructure, land use, and climate” [32]

The presence of diverse tree species enhances urban biodiversity, which is integral to maintaining ecological balance [33]. Urban greenery that incorporates a variety of species demonstrates increased resilience against pests, diseases, and climatic extremes [25,34]. Different species exhibit varied responses to environmental stressors; consequently, if one species declines due to disease or climate change, others may thrive, thus ensuring stable canopy cover and the ongoing provision of critical ecosystem services. This functional diversity supports a broader range of ecosystem services, contributing to overall ecological resilience. The diversity of tree species in urban areas is crucial, as integrating tree species diversity into urban forest management can help foster resilient tree populations [35].

Research conducted by Huang, Xiang and Shurong [36] shows that “results throw light on the importance of protecting both species and functional group richness to maintain temporal stability in the context of global changes.” They claim that stability is a fundamental property of ecosystems. In the conditions of changing the external environment that affects the ecosystem, many theorists and researchers are engaged in investigating the impact of these changes on the stability of biodiversity and ecosystems. Some of these studies “have demonstrated that communities with higher species richness tend to exhibit greater temporal stability” [36]. The research results showed “that a renewed focus on the mechanisms by which populations can be stabilized (and destabilized) by diversity is needed” [37]. Selecting and planting tree species that are well-adapted to evolving climatic conditions is important for the long-term management of urban environments [38].

The interplay between tree diversity and urban resilience is a critical focus area within the fields of urban ecology and environmental planning [25,37]. This encompasses the ability to manage the impacts of climate change, respond to natural disasters, and address persistent urban challenges such as pollution and habitat degradation.

The lack of diversity is certainly not the only cause of ecosystem degradation. While it can significantly contribute to deterioration, other factors such as urbanization (especially residential construction, streets, boulevards), pollution, urban pressure, climate change, and habitat destruction also play a crucial role. (Figure 1) These factors often act synergistically. Urbanisation leads to habitat fragmentation, which makes it difficult for tree diversity to flourish. Fragmentation can result in aesthetic preferences, where only certain tree species are favoured, leading to reduced diversity. Additionally, urbanisation contributes to biodiversity loss, particularly due to invasive species, which further diminishes tree diversity. This reduced tree diversity, combined with climate change, results in threatened tree health. A lack of urban resilience is often correlated with low species diversity [39,40,41]. If only a few species are predominantly present in a city, it suggests a reduced ability to withstand ecological changes, diseases, or invasive species. A diverse range of species can protect the ecosystem from shocks, so relying on a limited number of species indicates vulnerability [39,40,41].

If the city is mostly populated by only a few resilient or invasive species, it may indicate that the ecosystem is degraded and does not support a diversity of native or sensitive species that contribute to biodiversity. Some studies indicate that “rapidly changing climate is weakening the resilience of forest ecosystems through vitality loss of major native tree species, which reduces the ability of forests to deliver ecosystem services” [42].

The increase in the number of dry days due to climate change points to the need for finding solutions for the survival of species and the preservation of their diversity. It is noted that “even a small increase in diversity from monocultures to two- to six-species mixtures already delivers substantial benefits” [43].

If a city lacks diversity, it may be more susceptible to disturbances, leading to greater vulnerability and less resilience. A varied spectrum of species increases the chances that certain organisms will be able to adapt to changing ecological conditions. Without diversity, the ecosystem may have fewer options for adjustment, making it less capable of effectively responding to ecological stresses. The lack of species diversity can signal that the urban ecosystem is degraded, which may hinder its ability to provide essential services (such as air and water purification, temperature regulation, etc.). This degradation can further erode resilience.

It is important to keep in mind that biodiversity supports social and economic resilience. Diverse ecosystems can offer recreational opportunities, enhance property values, and support local economies (for example, through tourism). A lack of biodiversity can limit these benefits, reducing the overall resilience of the community.

Understanding species diversity helps shape conservation strategies by identifying key species and habitats that need protection, as well as understanding threats to biodiversity. It is observed, particularly in countries at a developmental level like Serbia, that there is a low level of attention given to the importance of biodiversity and the environment in general. State investments are minimal, and the management of these systems is insufficient. Serbia, like the City of Niš, is rich in natural resources, but ensuring their quality and management so that they endure for future generations poses a challenge. According to the World Economic Forum, Serbia ranks 86th in the world for the indicator -Total Known Species among 119 analyzed countries. This indicator shows that in 2024, Serbia recorded only 1.596 species, while Brazil, the country at the top of this list, recorded as many as 15.876 different species [44]. This situation raises concerns and highlights the need for wider-ranging research on biodiversity in Serbia, emphasizing the importance and multidisciplinary nature of biodiversity research to elevate awareness among the academic community, policymakers, urban planners, and individuals alike.

The assessment and monitoring of biodiversity utilizing quantitative indicators, such as biodiversity indices, can be part of a broader strategy to achieve ecological and management goals. However, implementation of index-driven approaches is useful, but not sufficient on its own. A comprehensive understanding of biodiversity should integrate quantitative indices with qualitative assessments, ecological research, and community engagement.

3. Methods and Data

Urban biodiversity can be analysed at various analytical levels. “Tree diversity metrics and benchmarks provide concrete figures that are relatively standardised and comparable, which can prove useful for calibrating planting and management targets, as well as carrying out broader diversity analyses across multiple locales” [27]. To analyse tree diversity, we employed a quantitative approach using the Shannon Diversity Index. Tree diversity, as well as species diversity, is typically assessed using two key components: species richness and species evenness. Richness refers to the number of different species found in a specific area, whereas evenness reflects how evenly individuals are distributed among those species. Numerous indices have been developed to quantify the species diversity. Their main purpose is to provide a numerical representation of biological variability, enabling comparisons across different spatial or temporal scales. Diversity indices are generally categorized into simple and composite types. Whereas simple indices are limited to capturing a single component of diversity, composite indices, such as the Shannon Diversity index, integrate both species richness and evenness into a single metric. These indices are advantageous due to their ease of calculation and long-standing use.

3.1. The Shannon Diversity Index

The Shannon-Weiner Diversity Index, originally proposed by Claude Shannon in 1948, also known as the Shannon Diversity Index, serves as a quantitative measure of species diversity within a community [45]. This index is among the most frequently used metrics for quantifying species diversity. Using the Shannon Diversity Index (H) we calculate the proportion of species (p) relative to the total number of species (pi). This is multiplied by the logarithm of this proportion (ln (pi)) and multiplied by −1 [45].

H = −Σpi * ln (pi)

The value of the Shannon Diversity Index ranges from 0 to infinity, where a value of 0 signifies no diversity (indicative of all elements in the system belonging to a single category); conversely, higher values indicate greater diversity or uncertainty within the system.

3.2. Site Description and Data Collection

The City of Niš spans an area of 596.78 km² and is primarily defined by its forested areas, which dominate the administrative territory [46]. However, the forest belt is located around the edge of the city. The benefits that the forest area provides, in terms of creating better environmental quality (air, temperature, humidity, and biodiversity), are not enjoyed in the central area of the administrative territory of the City of Niš.

Figure 2 shows the concentration of trees in the city center and outside of the center. Trees are marked as black points. In the city center, fragmentation of the trees can be observed, while outside of the center, trees are more concentrated in the space.

All street trees in the City of Niš’s inventory are divided into five zones: Niška Banja, Bulevar Nemanjića, Park Čair, Plava zona, and Železnička stanica. Keeping in mind that the administrative territory of the City of Niš encompasses not only urban areas, but also rural areas, this study examines tree diversity in both areas. The urban area covers trees in the urban part of the administrative territory of the City of Niš (zones: Bulevar Nemanjića, and Park Čair), while the surrounding area covers the territory of the zones of Niška Banja, Plava zona and Železnička stanica.

Given the goal of this paper to conduct a comparative analysis of urban tree diversity, we utilised data collected by researchers Galle, Halpern, Nitoslawski, Duarte, Ratti, and Pilla [27] across eight cities internationally. In their study, the authors mapped street tree diversity using data sourced from OpenTrees.org [27]. However, the database lacks information on Serbian cities, necessitating the use of alternative data sources.

For this research, we utilised data from Public Utility Company Mediana Niš, which includes comprehensive information about tree types and the location of trees in the city of Niš, Serbia. Collaboration between the Faculty of Economics in Niš and local policymakers, along with public companies, has made significant data on the city’s tree composition publicly available for both scientific and public use. The data can be accessed through the project website CROSS-REIS [47]. This data set from 2024 contains the number, location, and species of trees for the city of Niš in Serbia.

In a next step, we have used data obtained from the research of Galle et al. [27] only for European cities. The city centre area is defined for each of the chosen cities (Amsterdam, Bologna, Cambridge, Oslo, and Paris) (

Table 1. Definition of city centre area.

City	City Center Definition
Amsterdam	Administrative boundary “centrum” (center)
Bologna	Area within old city walls
Cambridge	Historic preservation district
Oslo	Administrative boundary “centrum” (center)
Paris	“Arrondissements” (districts) 0 to 7
Niš	Zones: Bulevar Nemanjića and Park Čair

Source: [27,48].

).

The analysis of the tree diversity in the City of Niš included 30 trees species in the city centre and 38 species outside of the center. Some of the tree species covered in the analysis are: Acer campestre, Acer negundo, Acer platanoides, Acer pseudoplatanus, Betula verrucosa, Castanea sativa, Catalpa nana, Cedrus atlantica, Celtis australis, Celtis occidentalis, Fraxinus excelsior, Fraxinus var.globosa, Juglans regia, Katalpa nana, Liquidambar globosa, Platanus acerifolia, Populus canadensis, Populus tremula, Prunus pissardii, Prunus cerasifera, Tilia spp. [35].

4. Results

In the analysis of tree diversity we used data about the total number of street trees outside the city centre (4.590) than in the city centre (3.998). High number of trees outside of the city centre suggests that the urban area located in the centre may face challenges regarding green space density. Factors contributing to this difference could include limited space for planting trees in densely developed urban areas and competition with infrastructure.

The data presented in

Table 2. Diversity of street trees within and outside the Niš city centre.

City	Area	Stem Count	Most Abundant Species (%)	Name of the Species
Niš	Centre	3998	22.3 18.1 7.4 7.4 5.5 4.8	Platanus acerifolia Thuja spp. Tilia spp. Betula verrucosa Acer saccharinum Aesculus hypocastanum
	Outside	4590	21.6 11.4 11.1 6.2 5.4 5.1	Tilia spp. Platanus accerifolia Fraxinus var.globosa Acer pseudoplatanus Celtis occidentalis Acer saccharinum

Source: Author’s calculation based on [47].

outline the differences in street tree diversity between the city centre and the surrounding areas in Niš. It includes specific metrics such as stem count and the percentage of the most abundant species.

The most abundant species in the city centre is Platanus acerifolia, accounting for 22.3% of the tree population. This species is known for its ability to tolerate urban conditions, including pollution and confined root spaces [49]. Its prevalence indicates a strategy of selecting trees that can withstand the stresses of urban environments, but it also raises questions about biodiversity. A reliance on a single species can lead to vulnerabilities. If pests or diseases target Platanus acerifolia, the city’s tree canopy could suffer significantly

In contrast, the most abundant species outside the city centre is Tilia spp., which comprises 21.6% of the tree population. Tilia spp. are also well-adapted to urban conditions and provide numerous ecosystem services, including shade, carbon sequestration, and aesthetic value [50]. Their presence outside the city centre suggests a potentially more diverse tree population in these areas, which may include various other species.

The following species were abundant outside of the city. Tilia spp. include several species, among which the most numerous are: Tilia cordata and Tilia platyphyllos. Besides Tilia spp., the next abundant species outside of the city are also: Platanus acerifolia, Acer pseudoplatanus, Celtis occidentalis and Fraxinus var. globose. In the central zone of the city, along with Platanus acerifolia, the dominant species include Tilia spp., Betula verrucosa, Aesculus hypocastanum and Acer saccharinum.

5. Discussion and Recommendation

The observed disparities in tree diversity between urban cores and peripheral areas in Niš reveal critical patterns of spatial degradation that echo broader land-use dynamics across rapidly urbanizing regions. In the densely built city centre, the concentration of a few resilient species such as Platanus acerifolia reflects an adaptive response to environmental stressors such as pollution, soil compaction, and limited rooting space. However, this functional adaptation often comes at the cost of ecological homogenization, reducing the landscape’s capacity to support diverse flora and fauna. Such patterns exemplify a key mechanism of land degradation in urban areas, namely, the transformation of biologically diverse habitats into simplified and fragmented green structures. In contrast, peripheral areas with slightly greater species richness suggest lower levels of anthropogenic disturbance and more favorable conditions for biodiversity preservation. This contrast highlights the importance of monitoring urban tree diversity as a proxy for ecological health in cities, and as a measure of resistance to ongoing land degradation that drives biodiversity loss.

The observed disparity in tree diversity between the city centre and peripheral zones of Niš reflects deeper ecological and urban planning dynamics. In the compact urban core, infrastructure density and soil sealing limit rooting space, alter hydrological regimes, and increase heat and pollution stress, favouring a small set of resilient, stress-tolerant species such as Platanus acerifolia. This homogenisation is reinforced by historical planting practices that prioritised aesthetic uniformity and maintenance efficiency over biodiversity, a trend noted in other European cities as well [27]. Furthermore, central green spaces are more fragmented and isolated, reducing habitat connectivity and impeding species dispersal, a phenomenon well-documented as a driver of urban biodiversity loss [6]. By contrast, peripheral areas, with lower building density, larger contiguous green patches, and often more flexible land-use histories, provide more heterogeneous microhabitats that support higher species richness and evenness. The lower diversity in the city centre is therefore not solely a product of current management, but also the cumulative outcome of historical urbanisation patterns, green space governance frameworks, and limited integration of biodiversity criteria into zoning and infrastructure development. Addressing these factors through targeted green space expansion, species diversification, and ecological corridor creation could help mitigate ecological fragmentation and strengthen urban resilience [28].

A comparison of street tree diversity in the City of Niš with other European cities is presented in

Table 3. Street tree diversity in the City of Niš and European cities.

City	Area	Stem Count	Most Abundant Species (%)	Name of the Species
Niš	Centre	3998	22	Platanus acerifolia
	Outside	4590	21	Tilia spp.
Amsterdam	Centre	7828	31	Ulmus hollandica
	Outside	156,240	8	Platanus acerifolia
Bologna	Centre	3053	17	Platanus acerifolia
	Outside	65,863	12	Celtis australis
Cambridge	Centre	1928	14	Gleditsia triacanthos
	Outside	16,503	10	Acer platanoides
Oslo	Centre	828	56	Tilia spp.
	Outside	13,735	15	Tilia spp.
Paris	Centre	13,228	36	Platanus acerifolia
	Outside	90,638	29	Platanus acerifolia

Source: [27,47].

.

The table summarises key metrics regarding the street tree populations in various European cities, illustrating stem counts, the most abundant species, and their respective percentages within both urban centres and surrounding areas. The cities compared include Niš, Amsterdam, Bologna, Cambridge, Oslo, and Paris.

The stem counts vary significantly among the cities. Amsterdam has the highest number of street trees in the city centre (7828) and an extraordinary 156,240 trees outside the centre, indicating a robust investment in urban greening initiatives. Niš, with a count of 3998 trees in the centre and 4590 trees outside, reflects lower overall tree density, suggesting potential opportunities for enhancing urban canopy cover.

Oslo stands out, with Tilia spp. being the most abundant species at 56%, signifying a strategic choice of species that thrive in urban environments. In comparison, Platanus acerifolia is prevalent in several other cities, including Niš, Bologna, and Paris, indicating its common use in urban landscaping due to its resilience and ability to cope with urban stressors. However, it is vital to consider the biodiversity implications of relying heavily on a singular species in urban areas.

Interestingly, the area surrounding Amsterdam has a significant number of trees. However, their most abundant species is only Platanus acerifolia at 8%. This highlights that there is a high diversity of species. Bologna shows a more favourable species diversity, with Celtis australis being the second most abundant tree species outside the city centre (12%), suggesting a healthier biodiversity scenario. Cambridge has a relatively simple tree composition in both areas, with Gleditsia triacanthos and Acer platanoides indicating a potential area for improvement in increasing diversity.

Despite significant differences in geographic, climatic, and socio-economic contexts, cities share common urban challenges such as air pollution, urban heat islands, climate change resilience, and the need for biodiversity conservation. Comparing these cities can highlight universal patterns, challenges, and effective strategies and helps cities like Niš identify the magnitude of potential improvements and benchmarking targets.

The reliance on a small number of species, such as Platanus acerifolia and Tilia spp., raises concerns regarding the long-term resilience of urban forests. Diverse ecosystems are better equipped to handle environmental stressors, pests, and diseases, reducing vulnerability. Cities like Oslo, which exhibit a high percentage of a single species in the centre, may face risks if that species faces a biological threat.

The data illustrate the need for informed urban planning that emphasises biodiversity. “This has intensified the need for green infrastructure solutions that leverage native flora adapted to local climatic stresses, particularly drought” [51]. Cities should focus on creating more diverse urban forests by incorporating a wider range of native and adaptable species to improve resilience, ecological stability, and the range of ecosystem services provided to urban populations. The observed differences in species abundance and structure between urban centres and surrounding areas underscore the potential benefits of expanding green spaces and diversifying tree species in urban environments. The structure of tree diversity in the City of Niš is similar to that in other European cities; thus the first hypothesis is confirmed.

Figure 3 shows the calculated Shannon Diversity Index (H) values for different European cities, delineating the indices for both city centres and areas outside the city centres. The Shannon Diversity Index values show a clear distinction between city centres and surrounding areas across all examined cities. Niš has notably lower values, with a Shannon Index of 1.37 in the centre and 2.80 outside. This suggests relatively low species richness and evenness in the urban forest within the city centre. The higher index outside the city centre indicates a more diverse tree population in the surrounding areas, where a greater number of species or a more balanced distribution may be present. Thus, hypothesis 2, that tree diversity is higher in areas outside the centre than within the city centre areas in the City of Niš, is confirmed.

Amsterdam had the highest values, with a Shannon Index of 3.08 in the centre and 4.18 outside. This reflects a robust ecosystem with both a rich variety of species and an even distribution, emphasising the effectiveness of urban greening and biodiversity initiatives. Cambridge follows closely, with values of 3.41 in the centre and 3.56 outside, indicating a similarly rich ecology. Bologna also demonstrates considerable diversity, with a Shannon Index of 3.07 in the centre and 3.84 outside, reinforcing the trend of higher diversity in peripheral areas.

Oslo and Paris have intermediate values, highlighting the diversity challenges urban centres face. Oslo’s index is particularly low in the city centre (2.00), whereas its surrounding area (3.61) suggests that promoting diverse vegetation in city centres remains a critical challenge. Paris also has a noticeable difference in tree diversity between its urban centre (2.61) and surrounding area (3.13), indicating an opportunity to enhance biodiversity within the central urban environment.

The comparative analysis across Niš and five European cities—Amsterdam, Bologna, Cambridge, Oslo, and Paris—reveals a consistent pattern of declining biodiversity in urban cores relative to surrounding areas, suggesting a widespread urban land degradation phenomenon driven by infrastructure density and anthropogenic pressures. While cities like Amsterdam and Bologna exhibit higher overall diversity indices, even these more affluent and green-policy-driven contexts demonstrate the ecological cost of central urban densification. Niš, with one of the lowest Shannon Diversity Index values in its centre, exemplifies the compounded effects of limited greening investments, species redundancy, and historical urban development constraints in Eastern and Southeastern Europe. The contrast between tree diversity in urban and peripheral areas underscores a critical implication: land degradation in city centres—through soil sealing, reduced habitat variety, and uniform tree planting schemes—is not just a spatial planning issue but a driver of long-term biodiversity loss. This suggests that resilience strategies must be adapted to context, ensuring both biodiversity preservation and equitable ecosystem services across spatial gradients in cities.

The lower Shannon Diversity Index values in city centres across the board suggest a prevailing trend in urban environments where species diversity may be compromised due to infrastructural limitations and the urban heat island effect. Urban planners and city stakeholders should take these findings into account when designing green spaces to ensure a more diverse and resilient urban forest. Strategies could include introducing a wider variety of native species and enhancing connectivity between green spaces to provide habitats for urban wildlife.

By implementing some improvements, urban environments can work towards enhancing biodiversity, which not only supports ecological health but also provides significant social, aesthetic, and recreational benefits to city residents. To combat the lack of biodiversity, urban planners should prioritize the development and enhancement of green spaces, such as parks, gardens, and community forests. This can help to create habitats for various species and improve connectivity between these spaces, reducing isolation and encouraging diverse populations. Proposing the creation of green infrastructure that supports multiple ecosystem services and biodiversity could include green roofs, rain gardens, and urban woodlands, which provide environments that can sustain a variety of species while offering benefits such as water management and temperature regulation. Identifying and restoring degraded natural habitats within the urban area to promote the resurgence of native species can contribute to a healthier and more resilient ecosystem. The survival of native populations is of great importance. If further diversity of species occurs at the expense of the native ones, it can negatively impact the ecological functions of the entire ecosystem. Engaging local communities in biodiversity conservation efforts through education and participatory programs could include tree-planting initiatives, creating community gardens, or citizen science projects that involve residents in monitoring and supporting local biodiversity.

In the City of Niš, there is habitat fragmentation (Figure 2), which, even with the existing diversity of species, may indicate that populations will not be sustainable in the long term. An important task for the management overseeing urban greenery is to connect green infrastructures, make green corridors and ensure the movement of species, gene flow, and thereby influence the overall health of the ecosystem. Mapping trees in this way can open a discussion about green corridors as pathways suitable for natural ecosystems, enabling the free movement of plants and animals between them. Providing habitats to connect allows for the migration and dispersion of species, as well as the preservation of biodiversity. This is one of the suggested actions that urban planners and policymakers could take.

The existing diversity is certainly affected by urban pressures such as pollution, noise, or habitat destruction. Although there is species diversity, the quality of habitats may be low, which suggests that the ecological integrity of the urban environment is at risk [52].

Integrating biodiversity criteria into zoning regulations by the public utility company is vital for enhancing tree diversity and promoting the overall ecological health of the city. This integration ensures that urban planning and land-use decisions prioritize the preservation and improvement of local tree populations. This recommendation can contribute to creation a framework that encourages the planting and protection of a variety of tree species, thereby fostering resilient urban forests.

The resilience of overall biodiversity to environmental challenges can be illustrated through the example of the City of Niš, where species such as Betula verrucosa and Picea abies are declining over time, indicating their vulnerability to emerging living conditions. Conversely, the persistence of certain species, such as Platanus acerifolia, demonstrates their resilience and survival in altered conditions within the new ecosystem. This data is not publicly available. It was obtained through an interview with the management of the Public Utility Company Mediana Niš, which manages the trees in the city.

The findings underscore the need for urban planners to focus on diversifying tree species in both urban and rural areas. The enhancement of green spaces in the city centre through the introduction of diverse species could mitigate the adverse effects of urbanisation, promote biodiversity, and improve air quality. Additionally, there is a need for strategic planning of urban forests that focuses not only on increasing the number of trees but also on increasing the diversity of species, especially in urban centres, in order to strengthen resilience to climate change and other stressors.

6. Conclusions

The diversity of trees in urban areas plays an important role in improving urban resilience and sustainability. Integrating ecological principles into urban planning through tree diversity significantly contributes to a city’s ability to adapt and survive in the face of environmental change.

This paper uses the Shannon diversity index to highlight differences in diversity of trees between the centre and periphery of Niš, which is in line with patterns observed in other European cities. Findings from Niš illustrate that lower diversity of trees in the city center makes it more susceptible to ecological vulnerabilities, while higher diversity of trees in peripheral areas provides protection against habitat loss and promotes ecological stability. The findings of this research indicate that the choice of tree species to be planted in urban areas is not a matter of indifference. There is a crucial need for careful selection of tree species to guide urban biodiversity towards adaptation to new living conditions within the ecosystem.

Key insights gained by comparing Niš with cities such as Amsterdam, Bologna and Paris highlight the strategic imperative for urban planners to prioritize biodiversity. By promoting different types of trees, cities not only strengthen themselves against the adverse effects of climate change and urbanization, but also improve the quality of life of citizens through improved air quality, temperature regulation and aesthetic value.

As urban expansion and infrastructure development continue to present challenges, it is essential that cities like Niš adopt biodiversity-oriented planning strategies. By embedding biodiversity targets into urban greening and land-use policies, local authorities can mitigate the effects of soil degradation, enhance ecological connectivity, and improve long-term urban resilience.

Further studies could explore the ecological impacts of the current species distribution in Niš, including considerations of the ecological roles played by Platanus acerifolia and Tilia spp. and how the presence of other tree species can contribute to urban biodiversity. A particularly interesting observation in the paper is that Platanus acerifolia is the species most present in the central zone of Niš, outside of the center in Amsterdam, in the center of Bologna, and in Paris, both in the center and outside the city center. Is the reason for the greatest presence of this species that it is more resistant to new living conditions or is it the result of planted patterns? A possible answer is that its relative share is growing in the total population of trees in cities due to the gradual death of other trees that are not resistant to current environmental conditions. This could be the subject of future research and identification of patterns for biodiversity criteria for urban tree development.