Trees and forests hold significant importance in urban environments, by being essential elements of urban green infrastructure and nature-based solutions (NBS) and providing plenty of ecosystem services as well as carrying various meanings for urban residents. A growing number of studies reported on the vital role of urban trees in enhancing human well-being, adapting to and mitigating climate change and supporting biodiversity (Knapp and Dushkova 2023; Kong et al. 2021). Additionally, the studies pointed out various benefits provided by trees such as recreational opportunities, food sources, aesthetic appeal, carbon storage, etc. (Haase et al. 2014; Haase and Hellwig 2022). As a result, there has been a growing emphasis on urban greening initiatives worldwide, with efforts to expand tree canopy cover in towns and cities. Nevertheless, the capacity of urban trees to provide these benefits is progressively jeopardized by a rise in disturbances of various origins, such as tree pests, storms, droughts, and fires.

Climate change, continuous urbanization processes along with land use conflicts, technogenic pollution, inappropriate planning and management often worsen these disruptions by weakening trees and rendering them more susceptible to harm. Our initial studies (see Andersson et al. 2021; Dushkova and Haase 2020; Haase and Hellwig 2022; Haase 2021; Haase et al. 2014, 2018; Knapp and Dushkova 2023; Otto et al. 2024; Pauleit et al. 2019; Wolff et al. 2023) were focused on urban trees and their ecological roles, services and functions in a changing climate. Also several examples of designing resilient green infrastructure by applying both biodiversity and co-creation approaches were demonstrated and discussed. Nevertheless, there is a need for a deeper understanding of the ambivalent relationship between humans and nature in regard to urban trees. Based mostly on the analysis of the cases from Central and Eastern Europe (with some examples from Northern America and Asia), this commentary is aimed at unveiling this ambivalence using the research framework below (Fig. 1).

Fig. 1
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Research framework applied to this study (Source authors)

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First, it demonstrates and analyses the variety of ecological functions and ecosystem services provided by urban trees, their values and significance for both people and nature as well as outlines the ambivalence in human-nature relations. Ambivalence here is defined through simultaneous and contradictory attitudes, perceptions, or feelings, namely attraction and repulsion, towards an object or a matter. Then, it investigates how this apparent ambivalence towards trees in cities can be explained since it is a key to future development and interventions, as it allows for divergent opinions and events on the ground, as well as divergent needs of humans and nature (in cities). Finally, it explores what are the implications for urban biodiversity and trees as NBS.

The significance of urban trees, their roles, and ecological values and services provided by them

Few other representatives of urban nature are the subject of such intense debate and controversy as urban trees (Haase and Hellwig 2020). This explicitly refers to forest trees, park trees, street trees and those that grow successively on large brownfield sites (Fig. 2a). One reason for this is that trees are inherently large and highly visible elements of the urban landscape and can live to an age similar to, and in some cases well beyond, that of humans, even in cities (Fig. 2b). Another reason is that trees and their large crowns supply a range of ecosystem services such as air cooling and purification, recreation, improvement of health and well-being, etc. (Fig. 2c), which can be used directly or indirectly (Haase et al. 2014). Additionally, trees provide diverse habitats for many species of animals, fungi, and lichens (Fig. 2d), and are thus quite comparable to a residential building, as they emit life impulses into the urban space that people can see, hear or perceive, which can be defined as traits (Andersson et al. 2021).

Fig. 2
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Successions on brownfields (Leipzig) (a); an oak planted at the Tverskaya str. (Moscow) in 1814 (b); tree as a place to escape and refresh during the heat in summer (Leipzig) (c); (d) tree as a habitat (d) (Photos: authors)

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Trees make a decisive contribution to the biodiversity of urban ecosystems, be it through the diversity of the tree taxa themselves, but also through the diversity of species that live on and in the tree (Allen et al. 2010; Przewoźna et al. 2022). In most cities around the world, the inventories of urban trees are made up of native species, i.e. locally occurring species, and unintendedly immigrant or introduced species. Concering Central Europe, for example, some of them can be described as invasive, such as Acer negundo, Ailanthus altissima, Fraxinus pennsylvanica, Pinus strobus, Populus canadensis, Prunus serotina, Quercus rubra, and, omnipresent, Robinia psuedoacacia. From a scientific perspective, all species are invasive if they are not native to an area and reproduce and spread there (Kowarik 2010). This is an aspect that is not easy to evaluate due to the constantly changing climate in many regions of the world during the Holocene and the relatively young history of cities on the planet, e.g. 3,000 to 4,000 years (Haase et al. 2018). Today’s tree populations in large cities are a product of the evolution of both the environment and biological life on the one hand and the development of human society (in and of cities) on the other.

Trees have always been a central element in the design of cities, be it in central squares, courtyards or even gardens (Pauleit et al. 2019) (Fig. 3). Since the 19th century, when city walls were razed, trees have played a major role as structural components of large recreational parks, first in the Global North and later also in the Global South. In Central Europe, the study on trees and the ecosystem services provided by them as well as experiences with peoples’ participation in greening practices is one of the oldest and the most advanced in the world.

People orientate themselves by trees when navigating through the city (Otto et al. 2024).In addition to their importance as an element of the green infrastructure, trees also became essential components of the city’s general furniture (e.g. using trees for placing site orientation and traffic signs).

Fig. 3
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Functional and aesthetic role of trees in urban design and adaptation strategies: street trees to provide shade and a more comfortable pedestrian environment (Lido, Venice) (a); trees as focal point of landscape design (Charlottenburg Palace, Berlin (b) (Photo: authors)

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Integrated within the existing urban green infrastructure (UGI) framework, urban trees have been extensively studied in the context of NBS (Escobedo et al. 2019; Kong et al. 2021), which can encompass a range of current urban environmental, socio-political and ecological challenges, especially through the active use of participatory processes (Dushkova and Haase 2020; Knapp and Dushkova 2023).

Oak, mountain ash, black poplar and all other urban trees serve as a basis for life for many birds, insects, and mammals (Pauleit et al. 2019). A habitat tree is a standing tree that supports at least one microhabitat defined as a very small, delimited habitat that is used (often only temporarily) by many different, sometimes highly specific animal, plant, lichen, and fungal species. Moreover, species diversity is also high on the bark and under the tree. The best-known inhabitants of these tree species are probably bark beetles, but there are also spiders, mites and other predators that feed on the insect larvae (Guilland et al. 2018). Moreover, universally popular cave-nesting woodpeckers, owls, martens, and dormice nest inside the tree trunk. Various smaller bird species have their homes in the leaves. Wild animals in the city and on the outskirts such as wild boars, foxes and deer seek protection in the thickets of trees (Magle et al. 2012). Even what falls from trees serves as shelter or food for many animals. Of all the native tree species, the most insect species live in and on the oak tree and its subspecies. Oak tree like hardly any other tree species, at least in Central Europe, has a large number of fungal companions that can be observed from seedling to tree stump.

Ambivalence: why and how it relates to urban trees?

On the other hand, cities are first and foremost places for people, they are the centres of most human societies and places where the global gross domestic product is created. No other structure has a comparable density of people and transport as a city does. In terms of temperatures, cities are among the hottest places on our planet (Lin et al. 2021). No wonder that the term "urban heat island" has been established here. Cities have highly sealed surfaces, which are often characterized by a well-developed underground system (Haase 2014). These underground systems represent the absolute obstacle for trees, tree landscapes and forests, and vice versa (Fig. 4a) (at least partially). As we learn every day from the regional and local press, large and old trees, as excellent habitats, are obstacles to the construction of new housing (https://www.change.org/p/kahlschlag-am-mauerweg). At the same time, trees can be replanted in the front- and backyards of the new apartment blocks (Fig. 4b). These young trees, however, will take years, if not decades, to become comparable habitats to those that are removed . Just this small facet of the human-nature relationship in regard to trees demonstrates that this relationship is indeed highly ambivalent.

Fig. 4
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Tree planting and further greening as measures of a Leipzig citizens’ initiative against increasing parking, traffic, and the deficit of green spaces (a); a young tree on the front yard of the newly created residence area in Leipzig (b) (Photos: D. Dushkova)

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This ambivalence will be addressed below, as it may be a key to better understanding the nature of human-tree relationships in our cities, and thus a key to future development and intervention, as it allows for divergent opinions and events on the ground, as well as divergent needs of humans and nature (in cities). Ambivalence in regard to urban trees is illustrated in Fig. 5 and explained below.

Fig. 5
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Ambivalence in human-tree relationships in the cities (Source authors)

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We need space for basic human services in the city of the 21st century. This primarily affects residential and administrative buildings and transport infrastructure, communication infrastructure, both above ground and underground. This grey infrastructure currently hardly allows tree growth; moreover, development plans usually limit trees to the adjacent green areas, so-called accompanying green (Haase and Hellwig 2020). Cities produce high amounts of air and noise pollution, which negatively impacts both human health and nature. The aforementioned construction of buildings and roads constantly causes increased temperatures and accelerates air pollution-induced cardiovascular- and asthma-related inflammable issues (Kabisch et al. 2021). Cities and their grey infrastructure disconnect urban dwellers from nature and the trees in it. And again that’s when ambivalence comes into play: On the one hand, distances to greenery increase and accessibility decreases, on the other, people tend to spend more time indoors and less time engaging with natural surroundings (Xue et al. 2004). Therefore, underlying socio-economic disparities catalyzed nature in such a way as well as caused tree alienation in cities.

At the same time, climate change means hotter days and nights in cities, which has a huge impact on people, especially in aging societies (Lin et al. 2020). We are absolutely dependent on mechanisms and structures that can cool down temperatures (Andersson et al. 2020). We need urban nature, and urban trees in particular, to survive in times of climate change. There is also a necessity for the diversity of urban tree nature and associated habitats to enable trees to survive under conditions of increasing climate change and urbanization (Fig. 6) and resist tree pests.

Fig. 6
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Trees and a lawn suffering from the heat and drought in mid-summer/July, Leipzig (Photos: D. Dushkova)

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Unveiling urban tree ambivalence: implications for biodiversity and NBS

Worldwide, urban planners are well aware of the great importance of urban trees. For this purpose, a variety of supportive strategies and measures to maintain and increase the urban tree population has been developed and implemented. To mention some of the most prominent of them, there is the “Million Trees NYC initiative”, New York City, USA, launched in 2007 and aimed to plant and care for one million new trees across all five boroughs of New York City. The project sought to improve air quality, reduce energy consumption, and enhance overall quality of life. Other examples refer to the “Melbourne Urban Forest Strategy” and “Greening Melbourne” in Australia which include various tree planting and urban greening projects, aiming to increase green spaces, improve air quality, and enhance the overall liveability of the city. In Japan, the "Green Belt Project Tokyo” develops a network of green spaces, parks, and tree-lined areas across the city to improve air quality, reduce heat island effects, and provide recreational spaces for residents. Paris has undertaken various projects to increase green spaces and enhance biodiversity. One notable initiative is the “Reinventing Paris” project, which includes plans for the creation of more urban green spaces and sustainable development. Under the framework of “London’s Urban Forest”, the city has been working on increasing its urban forest through initiatives such as the “London Urban Forest Plan”. Its aim is to enhance the city’s green infrastructure, provide environmental benefits, and improve human well-being (https://www.treesforcities.org/our-work/urban-trees). The "Trees for London program" also contributes to the Mayor’s initiative to plant thousands of new trees over the next few years. Increasing London’s tree cover will help to cool the city and provide valuable shade, as well as help to prevent flooding. This tree-planting initiative is in line with the Mayor’s vision to transform London into a greener, more liveable city by ensuring that all residents have access to green and leafy outdoor spaces. Since 2016, the planting of the 500,000th tree funded through this program has been marked, demonstrating a major effort and action to make the city greener and more resilient to the impacts of climate change. Additionally, it encouraged citizen engagement in greening strategies and improved biodiversity.

Last but not least, the city of Leipzig has been running the “Tree Strong City” campaign since 1997, which is recognized as the long-term, forward-looking planning and management instrument for climate-sensitive urban development. This initiative encourages donations and sponsorships for trees in public green spaces (Fig. 7). Since the campaign’s inception, more than 700,000 euros have been raised and more than 8000 trees have been planted. Anyone can become a tree sponsor with a donation of €250 or more. In 2012 alone, almost €84,000 was donated. Studies have shown that there is often a close relationship between the sponsoring household and the chosen tree, which creates a sense of place (Baumpaten 2020; Dushkova and Haase 2020). In particular, the tree sponsorship concept allows different stakeholders in urban society to be actively involved in the design and management of their city (e.g. financing the tree through a donation, as well as caring for it, watering it and reporting any damage). This is facilitated by the fact that tree sponsorship builds on the tradition of planting trees on personal occasions, with a dedication on a plaque on the tree. It also transfers the knowledge in the community about different tree species and thus about biodiversity, as the species name of the tree appears on the plaque (Fig. 7). As different tree species are planted in different neighborhoods and sometimes even in the same street, the initiative contributes to the diversification of the street tree population at different spatial levels and thus to the resilience of the city as a whole. In contrast, there have been increasing press and blog reports of illegal tree removal in Leipzig, both in residential areas and in public spaces.

Fig. 7
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Initiative “Tree-strong City”: implemented trees in the Lene-Vogt Park (a) at a revitalised brownfield in Leipzig and an example of steles with dedication text (b) (Photos: D. Dushkova)

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How can this apparent ambivalence towards trees in cities be explained, and what are the implications for biodiversity and trees as NBS? It must be clear that it only makes sense if both sides, that of human society and that of the tree (ecosystems), are linked but not played off against each other. Competing claims should be negotiated, and this is where the ambivalence described above can be explicitly exploited. But how?

Firstly, the city of the future should not only develop but also live and implement the acceptance that a sustainable human-nature relationship with urban trees can only be achieved if space for people and nature is available and preserved. There is already a whole range of spatial instruments in planning that would allow this acceptance of different and also diverging (minimum) space requirements to be realized. In particular, those instruments include distancing (Haase 2021), zoning, and other approaches from landscape research. Among them are land sparing (a form of land use in which a sustainable intensification and increase in productivity have endeavoured on the part of the human-used area of a region to relieve or protect other areas or natural habitats) and land sharing (which involves a patchwork of low-intensity land use incorporating natural features rather than keeping intense land use and wilderness separate). It is important that people and (tree-based) nature are both given rights for distancing and zoning, which in turn will become enforceable at some point (in the sense of a right of nature).

Secondly, we also need to select climate-adapted tree species and their varieties, origins and genotypes that can cope with the combination of warm and dry conditions as well as other urban stressors. Even if this might mean less tree diversity in parts of the city or no habitat trees for longer time periods, it will t guarantee air cooling for city dwellers. However, this cooling potential must be limited to the inhabited areas and non-inhabited areas such as administrative, commercial, educational, or brownfield sites that should not be optimised but instead be able to contain local species and hybrids, even at the risk of not surviving longer droughts. An optimization of street or park trees with regard to the cooling function based on a large LAI (leaf area index) or the physical resistance of the tree species in dry conditions could mean a decrease in the number of species (e.g. supporting species with large one-sided green leaf area per unit ground area such as beech, trumpet tree, oak, maple or chestnut) meanwhile stabilizing or even increasing the ecological output function (ecosystem service). The situation is similar with trees, which contribute significantly to the pollen distribution that pose a health risk for ever larger groups of the urban population (Legg and Kabisch 2024). This shows that design of green infrastructure towards a specific goal such as drought resilience can lead to certain conflicts. In particular, biological invasions, i.e. the risk that some of the alien species will overgrow and spread strongly, might result in certain impairments of native flora and fauna as well as infrastructure or human health (Döhren and Haase 2022; Knapp and Dushkova 2023; Pyšek et al. 2020). Considering the above mentioned, decisions in favour of low-allergy-risk tree species must be prioritized over tree species diversity, but only in the large public recreational parks, which have the best accessibility. Other green spaces, including parks, should then be as diverse as possible with different tree species, including pollen-rich ones, to consider both human health and tree species diversity.

Third, the simultaneous pursuit of connectivity ideas (Wolff et al. 2023) coupled with the deliberate isolation of species-rich areas (Vega et al. 2021) would also be a consistent application of the ambivalent goals within spatial development and nature conservation (species diversity enhancement). Connectivity supports different species than the ‘island’ isolation strategy. Both strategies can be implemented in the city in very different ways, either allowing major construction projects and land use or not doing so. Thus, neither the goals of society nor the development of nature are mutually exclusive.

Fourthly, the non-use of urban space is in itself ambivalent in regard to ever-increasing demands for its use. In planning law, laissez-faire exists only in theory, if at all. But it can be an active strategy for urban wastelands, where secondary or ruderal nature is consciously accepted to give nature time and space to adapt to new climatic conditions. Moreover, it will enrich the spectrum and amount of ecosystem services provided by urban ecosystems, and serve as a template for planners afterward because spontaneous urban nature is in itself the best NBS (Ignatieva et al. 2020). Trees have an important role to play here, as we often find hybrid tree species, such as hybrid poplars, that survive without additional irrigation, remediation or melioration, and thus may appear to be the answer to tree life under climate change-induced drought (at least it is very much relevant to the European context). For example, recent studies have recognised preserving brownfield sites as an innovative approach to urban green infrastructure design, highlighting their contribution (particularly those spontaneously colonised by shrub and tree species) to supporting nationally and internationally important biodiversity (Nash 2017; Rink and Schmidt 2021).

Finally, there is ambivalence in the whole urban society and in the diversity of urban citizens. This inherently contradictory pattern can also be harnessed for sustainable coexistence between people and nature, in our case trees. Co-creation has become an increasingly well-researched approach to collaboration within the blue-green infrastructure and NBS in recent years (DeLosRíos-White et al. 2020; Frantzeskaki 2019; Langemeyer and Baró 2021; Morello et al. 2018).  It is defined as a collaborative approach to engagement that enables various groups of stakeholders (including end-users) to participate in designing and building more inclusive and sustainable pathways and mechanisms for change (Dushkova and Kuhlicke 2024; Morello et al. 2018). By allowing to co-design and co-implement with people and not only for them, co-creation promotes direct contact between people and nature, i.e. in relation to urban trees, planting campaigns, tree sponsorships, watering campaigns (such as “Giess den Kietz in Berlin” https://www.giessdenkiez.de or ”Leipzig gießt” https://giessdeinviertel.codeforleipzig.de/) and also reports of tree damage. Despite the promoted idea of citizen science, there is a big potential for co-creation to be actively used and more integrated into expert planning (Dushkova and Haase 2020, 2023). However, co-creation is not conflict-free, but structured conflict can be productive moments of learning together and co-producing new knowledge. The same applies to crowdfunding as a distributive strategy for promoting urban greening and other sustainable practices. Crowdfunding is a method of collecting donations, loans or investments (repayment according to the financial results of the project with, in some cases, participation in governance) from a large number of people online, usually through a dedicated digital/web platform (Brimont 2017). As studies show (Adhami et al. 2019; Brimont 2017; Sturiale et al. 2023), crowdfunding serves not only as a financing tool but also as a means to promote and communicate as well as to mobilize and engage local stakeholders and citizens in significant projects initiated by public authorities. Nevertheless, it’s essential to recognize that crowdfunding is not a panacea. The extent of citizens’ contributions to ecological transition can impact crowdfunding (both positively and negatively) through their active involvement in project governance (Brimont 2017).

Conflict research is not yet part of urban planning, but in many contexts it would ideally complement the joint efforts of ecologists, gardeners, architects, urban planners and engineers in designing and implementing biodiversity-oriented NBS in cities. Again, by involving a wide range of stakeholders, co-creation can play a crucial role in addressing (potential) conflicts that may arise during planning, by overcoming scepticism and increasing motivation (and even empowering) to act, co-producing knowledge and building social capacity (Zingraff-Hamed et al. 2020, Dushkova and Haase 2023; Dushkova and Kuhlicke 2024).

The EU Nature Restoration Law, currently in discussion, is aimed at the long-term recovery of Europe’s heavily destroyed natural areas and the animals that live there and only implicitly addresses cities. This is a missed opportunity, as the law is also dedicated to combating the illegal timber trade and illegal deforestation and is quite significant for urban areas, as this commentary on the ambivalence of our human-nature relationship clearly shows. It is therefore necessary to take a fresh, systematic, and comprehensive perspective on how nature and ecosystem conservation law can more honestly and broadly address divergent urban human-nature systems. This should consider conflicting goals and ideas by embracing ambivalence and promoting science and practices of tree planting and management towards urban sustainability and resilience.