Challenges for tree officers to enhance the provision of regulating ecosystem services from urban forests

Unveiling the complex networks of urban tree diversity research: A global perspective

Ecosystem services offered by urban forests must be proactively managed to remain diverse and sustainable. Recent research findings deserve a systematic synthesis to elucidate inherent knowledge structures and dynamics. This study focused on the urban tree diversity theme from 2000 to 2022. Web of Science Core Collection database provided bibliometric details on academic publications. The data-driven quantitative analysis explored research quantities, emphasis, trends, patterns, linkages, and impacts by countries, institutions, authors, journals, and citations. Publications and research topics have expanded continually, with accelerated growth in recent years. Research activities, outputs and interactions demonstrated conspicuous spatial clustering. A few countries, institutions and researchers generated a notable proportion of publications. Their scholarly contributions were visualized in knowledge graphs as complex networks of nodes and inter-node links. Keyword analysis generated a network to indicate research hotspots and frontiers to steer and prioritize future studies. Recent findings affirmed that cities can harbor substantial tree diversity due to enhanced habitat heterogeneity and successful species adaptation. Aligning tree traits with environmental conditions and management objectives can improve benefits. Urbanization can filter tree traits to shape community assemblages through stressors: habitat degradation, fragmentation and loss, in conjunction with pollution, climate change, and introduced species. Diversity preservation strategies include protecting remnant natural vegetation, connecting green spaces, and restoring complex canopy geometry and biomass structure. The emerging frontiers are marked by modeling future species distributions, leveraging technologies like remote sensing, linking ecology with human values, and committing to community-based stewardship. Management can be upgraded by interdisciplinary perspectives integrating ecological science and social engagement. The findings highlight the need for biodiversity enrichment anchored by native species, trait-matched assemblages, adaptive policies, and community participation to create livable-green cities. This review synthesizes key advances in urban tree ecology and biodiversity research to inform the planning and stewardship of resilient urban forests.

Effect of air quality improvement by urban parks on mitigating PM2.5 and its associated heavy metals: A mobile-monitoring field study

Field mobile monitoring of PM_2.5, equipped with a highly accurate device, was performed for two types of urban parks in Taiwan. Measurements were taken in the morning and evening rush hours, on certain weekdays and weekends, every month over a year. We designed six calculation schemes of the rate of PM_2.5 mitigation by urban parks to comprehensively compare the average and maximum mitigation effects in relation to the vegetation barriers. The mitigation rate, from the lowest (2.51%) to the highest (35.57%) depended on the calculation schemes. The Taipei Botanical Garden (TBG) with a dense, multilevel forest has a stable PM_2.5 mitigation effect and strong ability to improve air quality inside the park under severe PM_2.5 pollution. In contrast, Zhonghe No.4 Park (ZHP), an open park with mostly a single-storied stand, has variable PM_2.5 mitigation effect, leading to either quick dissipation or accumulation of PM_2.5 inside the park. Furthermore, the dry deposition of PM and the associated heavy metals were investigated using camphor trees as bioaccumulators. Dry deposition flux of the leaf-deposited PM_2.5 exhibited similar results in ZHP; whereas, noticeable higher results were observed inside TBG. In addition, most of the PM_2.5 deposition flux from field estimations were similar to those in i-Tree Eco when considering the loss of mass due to the dissolution through water filtration, indicating that i-Tree Eco may be reliable to model the removal of PM_2.5 in the parks in Taiwan. Moreover, we examined nine heavy metals' content in the deposited PM, and most of the detectable elements were significantly higher outside both parks, demonstrating the mitigation effects of urban parks in reducing not only the PM_2.5 concentration but also the toxicity of the pollutant. This study provides direct evidence of the important ecosystem services, namely air quality improvement and biomonitoring effect, derived from urban parks.

Evaluating the Forest Ecosystem through a Semi-Autonomous Quadruped Robot and a Hexacopter UAV

Accurate and timely monitoring is imperative to the resilience of forests for economic growth and climate regulation. In the UK, forest management depends on citizen science to perform tedious and time-consuming data collection tasks. In this study, an unmanned aerial vehicle (UAV) equipped with a light sensor and positioning capabilities is deployed to perform aerial surveying and to observe a series of forest health indicators (FHIs) which are inaccessible from the ground. However, many FHIs such as burrows and deadwood can only be observed from under the tree canopy. Hence, we take the initiative of employing a quadruped robot with an integrated camera as well as an external sensing platform (ESP) equipped with light and infrared cameras, computing, communication and power modules to observe these FHIs from the ground. The forest-monitoring time can be extended by reducing computation and conserving energy. Therefore, we analysed different versions of the YOLO object-detection algorithm in terms of accuracy, deployment and usability by the EXP to accomplish an extensive low-latency detection. In addition, we constructed a series of new datasets to train the YOLOv5x and YOLOv5s for recognising FHIs. Our results reveal that YOLOv5s is lightweight and easy to train for FHI detection while performing close to real-time, cost-effective and autonomous forest monitoring.

Microclimatic and Environmental Improvement in a Mediterranean City through the Regeneration of an Area with Nature-Based Solutions: A Case Study

Dense urban areas are facing relevant issues related to their high vulnerability to the impacts of climate change and ecosystem health. The study presents a case study of a regeneration project with Nature-based Solutions in the city of Genoa (Italy) and, more specifically, in a neighbourhood characterised by relevant health and well-being issues. The performances of three design scenarios for a city hotspot, including plant species selected with a systemic approach and light pavements, are analysed in terms of improved microclimate by means of the ENVI-met software V4.4.5. The results show different benefits on the microclimate compared to the current state depending on the different scenarios: A UTCI decrease from 4.1 °C to 5.4 °C, a reduction of mean radiant temperature from 12.3 °C to 17.3 °C, a relative humidity increase from 3.8% to 5.6%, and a progressive decrease in wind speed are detected in a directly proportional way to the gradual increase in greenery inside the scenarios. In reverse, better results for air temperatures are detected for the scenario with less greening (Δt = 1.8 °C). The study relies on the re-parametrisation of plant species characteristics in the ENVI-met database to reach a high level of accuracy.

Greening the city: Thriving for biodiversity and sustainability

Urban population and urbanisation are increasing rapidly, mainly in developing countries, usually at the expense of green and blue areas. This trend will decrease the ecosystems' capacity to supply ecosystem services (ES) and threaten human wellbeing. Therefore, it is key to establish greening policies in urbanising areas, which are essential to improve the liveability of cities. Restoring and developing green and blue infrastructures using nature-based solutions is vital to improving urban biodiversity and urban ecosystems. Healthy urban ecosystems have a high capacity to supply regulating (e.g., air, noise, climate and water regulation), provisioning (e.g., food, medicinal plants, biomass) and cultural (e.g., recreation, landscape aesthetics, social cohesion) ES. This multifunctionality can provide diverse environmental, social and economic benefits to urban residents, hence contributing to the sustainability of urban areas. However, urban green and blue areas are also associated with ecosystem disservices (e.g., plant allergies or poisoning, emission of biogenic volatile organic compounds, unpleasant smells), tradeoffs (e.g., increased water consumption, wildfire risk, associated management costs) and implementation barriers (e.g., political motivation, lack of knowledge, time and workload). Overall, the SI published 8 articles from different parts of the world, such as China, the USA, Italy or Spain, focused on important aspects of greening the city (e.g., green roofs, green walls, green infrastructures, sustainable mobility).

State of the Art and Latest Advances in Exploring Business Models for Nature-Based Solutions

Nature-based solutions (NBS) offer multiple solutions to urban challenges simultaneously, but realising funding for NBS remains a challenge. When the concept of NBS for societal challenges was first defined by the EC in 2017, financing was recognised as one of the major challenges to its mainstreaming. The complexity of NBS finance has its origin in the multiple benefits/stakeholders involved, which obscures the argument for both public and private sector investment. Since 2017, subsequent waves of EU research- and innovation-funded projects have substantially contributed to the knowledge base of funding and business models for NBS, particularly in the urban context. Collaborating and sharing knowledge through an EU Task Force, this first set of EU projects laid important knowledge foundations, reviewing existing literature, and compiling empirical evidence of different financing approaches and the business models that underpinned them. The second set of EU innovation actions advanced this knowledge base, developing and testing new implementation models, business model tools, and approaches. This paper presents the findings of these projects from a business model perspective to improve our understanding of the value propositions of NBS to support their mainstreaming.

Nature-based solutions as tools for air phytoremediation: A review of the current knowledge and gaps

Monitoring of air quality and the application of strategies for its improvement are perceived as key areas for reducing environmental pollution. The research on Nature Based Solutions for the mitigation of pollutant concentrations in the air has increasingly developed in the last twenty years. The purpose of this review is to evaluate whether the current knowledge about Nature-Based Solutions provides a quantitative answer of the real benefits of air phytoremediation. To address this question, the literature on air phytoremediation over the last twenty years was analyzed. Altogether, 52 variables were selected, grouped into six categories, to briefly characterize the contents, methodology and outcome of the peer-reviewed articles. Altogether, 413 plant species found in the analyzed studies were recorded. The results show the trends about the most studied pollutants and on the methodologies mostly applied, in relation to the study outcomes. The analysis demonstrated that particulate matter (PM_x) was the most frequently examined pollutant, most studies on NBS are based on experiments with exposure chambers, and scaling up the results with models has been limited. Although effective reductions in pollutant concentrations have been shown in the majority of studies, there is a strong fragmentation of the approaches, most studies have looked at a single pollutant and detailed information for model parameterization is only available for a few species. Thus, the review highlights that studies of Nature Based Solutions in air phytoremediation require unification of methodologies, and should consider a broader range of pollutants and plant organisms useful for mitigating the impacts of air pollutants in indoor and outdoor human environments.

Mapping the Research Landscape of Nature-Based Solutions in Urbanism

Nature-based solutions (NBS) are an innovative concept that mimics the processes of natural ecosystems, popularized principally in the European Union. With a substantial body of literature amassed since the term’s inception in 2015, there is a need to systematically review existing literature to identify overarching gaps and trends, according to disciplinary focus, geographic scope, and key themes, and direct future research inquiry and policy recommendations. This review consists of bibliometric analysis and thematic analysis for NBS studies in urbanism. NBS studies were found to relate strongly with other concepts of ‘Ecosystem Services’, ‘Green Infrastructure’, ‘Climate Change’, and ‘Risk management and Resilience’, which align with four major thematic goals set by the European Commission. Within NBS scholarship, various sub-themes have emerged, namely, ‘Greening’, ‘Urban Development’, ‘Water’, ‘Wellbeing’, and ‘Governance’. Furthermore, we illustrate that the amount and thematic focus of NBS research have been unevenly distributed worldwide. Analysis of emerging trends shows a recent increase in topics, such as adaptive governance of NBS, and the incorporation of social justice in sustainability transitions. Based on an assessment of extant NBS literature, we offer some recommendations for the future direction of the research fields.

Beyond 'trees are good': Disservices, management costs, and tradeoffs in urban forestry

The provision of ecosystem services is a prominent rationale for urban greening, and there is a prevailing mantra that 'trees are good'. However, understanding how urban trees contribute to sustainability must also consider disservices. In this perspective article, we discuss recent research on ecosystem disservices of urban trees, including infrastructure conflicts, health and safety impacts, aesthetic issues, and environmentally detrimental consequences, as well as management costs related to ecological disturbances and risk management. We also discuss tradeoffs regarding species selection and local conservation concerns, as well as the central role of human perception in the interpretation of ecosystem services and disservices, particularly the uncritical assertion that 'everybody loves trees'. Urban forestry decision-making that fails to account for disservices can have unintended negative consequences for communities. Further research is needed regarding life cycle assessments, stakeholder decision-making, return-on-investment, and framings of services and disservices in urban forestry.

Cultivating our urban forest future: a value-chain perspective

For cities to grow their urban forest canopy the formula appears rather straightforward: the right trees, plus the right conditions, plus the right care equals success. These simplified “tree chain of custody” steps, however, represent activities within a complex value-chain in Canada. Given that there is heightened demand for urban tree planting as natural climate solutions become the norm, how can we prepare the value-chain to meet these demands? To answer this question, we outline the pathways by which trees presently go from nurseries into urban and peri-urban areas. Delineating the actors, roles, and present barriers to success exposes the complexity of the process and relationships in the value-chain, as there are distinct phases with multiple actor groups involved who influence, and are influenced, by one another. We explore the issues that pose prominent challenges to, as well as opportunities for, the value-chain. Emergent themes include communication, forecasting demand and timing, underpricing and undervaluing tree establishment, lack of awareness on the importance of soils, juvenile tree health, species selection, and gaps in evidence-based decision support tools. The touchstones of science and innovation, collaboration, and knowledge mobilization are pertinent for the value-chain in Canada to draw upon to navigate the future.

Increasing Green Infrastructure in Cities: Impact on Ambient Temperature, Air Quality and Heat-Related Mortality and Morbidity

Urban vegetation provides undeniable benefits to urban climate, health, thermal comfort and environmental quality of cities and represents one of the most considered urban heat mitigation measures. Despite the plethora of available scientific information, very little is known about the holistic and global impact of a potential increase of urban green infrastructure (GI) on urban climate, environmental quality and health, and their synergies and trade-offs. There is a need to evaluate globally the extent to which additional GI provides benefits and quantify the problems arising from the deployment of additional greenery in cities which are usually overlooked or neglected. The present paper has reviewed and analysed 55 fully evaluated scenarios and case studies investigating the impact of additional GI on urban temperature, air pollution and health for 39 cities. Statistically significant correlations between the percentage increase of the urban GI and the peak daily and night ambient temperatures are obtained. The average maximum peak daily and night-time temperature drop may not exceed 1.8 and 2.3 °C respectively, even for a maximum GI fraction. In parallel, a statistically significant correlation between the peak daily temperature decrease caused by higher GI fractions and heat-related mortality is found. When the peak daily temperature drops by 0.1 °C, then the percentage of heat-related mortality decreases on average by 3.0% The impact of additional urban GI on the concentration of urban pollutants is analysed, and the main parameters contributing to decrease or increase of the pollutants’ concentration are presented.

How Urban Forest Managers Evaluate Management and Governance Challenges in Their Decision-Making

Decisions about urban forests are critical to urban liveability and resilience. This study aimed to evaluate the range of positions held by urban forest managers from local governments in the state of Victoria, Australia, regarding the management and governance challenges that affect their decision-making. This study was based on a Q-method approach, a procedure that allows researchers to evaluate the range of positions that exist about a topic in a structured manner based on the experiences of a wide group of people. We created statements on a wide range of urban forest management and governance challenges and asked urban forest managers to rate their level of agreement with these statements via an online survey. Managers generally agreed about the challenges posed by urban development and climate change for implementing local government policies on urban forest protection and expansion. However, there were divergent views about how effective solutions based on increasing operational capacities, such as increasing budgets and personnel, could address these challenges. For some managers, it was more effective to improve critical governance challenges, such as inter-departmental and inter-municipal coordination, community engagement, and addressing the culture of risk aversion in local governments. Urban forest regional strategies aimed at coordinating management and governance issues across cities should build on existing consensus on development and environmental threats and address critical management and governance issues not solely related to local government operational capacity.

Knowledge, attitudes, intentions, and behavior related to green infrastructure for flood management: A systematic literature review

Green infrastructure (GI), which mimics natural hydrological systems, is a promising solution for flood management at the intersection of urban built infrastructure and natural systems. However, it has not yet achieved widespread uptake, due in part to insufficient understanding of human dimensions of the broader socio-ecological-technical system. We therefore conducted a multidisciplinary systematic literature review to synthesize research on people's existing knowledge about flood risk and GI, and how that shapes their attitudes and motivation to adopt new solutions. We systematically screened 21,207 studies on GI for flood management; 85 met our inclusion criteria. We qualitatively analyzed these studies to extract results on knowledge, attitudes, intentions, and behavior relating to GI for flood management. Overall, knowledge of GI was low across the 44 studies in which it was evaluated. Seventy studies assessed attitudes about GI, including the functional, aesthetic, health and safety, recreational, conservation, financial, and cultural value of GI, albeit their measurement was inconsistent. Willingness to implement or pay for GI varied considerably across 55 studies in which it was measured. Twenty studies measured and documented behavior relating to GI use, and these found low rates of adoption. Few studies systematically assessed the role of demographic, socio-economic, or geographic characteristics that could influence individuals' knowledge, attitudes, intentions or behavior, and thereby the success of GI programs. We recommend that researchers should more systematically capture data on human dimensions of GI (i.e. knowledge, attitudes, intentions, and behavior) across diverse settings to improve program design and uptake, especially among vulnerable populations. Greater attention to the social component of the socio-ecological-technical system will help ensure that GI programs are equitable, inclusive, and sustainable.

The Importance of Multi-Scale Temporal and Spatial Management for Cemetery Trees in Malmö, Sweden

Research Highlights: A large body of research highlighted the important contributions that urban forests make to cities and their inhabitants. However, our urban forests face threats from issues such as rapid urbanization, climate change, and the spread of pests and diseases. As such, proactive and effective management is necessary to ensure their long-term sustainability. Given the multiple spatial and temporal scales on which threats can arise, effective management needs to account for these scales and adjust accordingly. The degree to which this currently happens is unclear. Background and Objectives: The aim of this study was to determine the role of multi-scale management in urban forestry, using cemeteries in Malmö, Sweden as a case study. Cemeteries can provide extensive tree canopy but are not readily considered in urban forest management. We sought to determine (1) the threats to the current cemetery tree populations, (2) the extent of multi-scale cemetery tree management, (3) whether tree management plans promote multi-scale management, and (4) how cemetery tree management can be improved. Materials and Methods: Malmö cemetery tree inventories were analyzed with respect to size class and species diversity. Existing cemetery tree management plans were examined to determine the spatial and temporal scales of their recommendations. Interviews were conducted with cemetery managers to determine management priorities and actions. Results: We found that cemetery tree populations in Malmö suffer from a lack of age class and species diversity. Management tends to occur on short time scales and efforts focus mainly on addressing individual trees, although some consideration is given to large-scale species diversification. The management plans previously created for these cemetery trees make recommendations for age class and species diversification but are yet to be used extensively by cemetery managers. Conclusions: The long-term stability of Malmö’s cemetery tree populations is threatened by a lack of species and age diversity. Current management efforts emphasize addressing small-scale issues. Although there is a desire to improve species diversity, this can cause conflict with existing cultural values.

Assessment and Mapping Green Areas Ecosystem Services and Socio-Demographic Characteristics in Turin Neighborhoods (Italy)

The ecosystem services (ES) and human well-being are keywords that guide the Italian strategy on urban greening. The development of ES priorities linked to specific land uses help to guide the drafting of management plans. The aim of the research was to assess and map green areas ecosystem services and socio-demographic characteristics in Turin neighborhoods in order to identify where to improve the provision of ecosystem services and the socio-demographic conditions. The Preliminary Assessment Method (PAM) was used for the assessment of provision and regulating services based on land use. The Species-specific Air Quality index (S-AQI) was used to assess the regulating services provided by trees. Three socio-demographic characteristics were analyzed at the neighborhood level—age index, housing density, and % of economically assisted citizens. PAM results show that Turin provides more ecosystem services in peripheral areas of the city. Trees with high S-AQI values represent 21% of the censed trees. Not recommended trees are 18%. The neighborhoods with higher S-AQI values are not always characterized by a higher number of trees/km2 or species richness. Results show that the northern part of the city is characterized by higher values of ES and socio-demographic conditions than the central-southern part. This aspect is related to the conspicuous presence of agricultural land uses and water bodies, together with the presence of tree species with a high S-AQI values and high or medium socio-demographic conditions. 57% of the neighborhoods present low results for both aspects. Actions to improve the quality of green spaces in those neighborhoods could have great effects on liveability. Future management and planning strategies for increasing citizens’ well-being through urban greening should consider the proposed approach.

Management and Social Problems Linked to the Human Use of European Urban and Suburban Forests

This review regards the management and social problems in European urban and suburban forests linked to their maintenance and human use. They can be divided into major categories: forest management problems (e.g., the low priority of urban forestry, various or diffused urban forest management, lack of management plans or lack of sufficient funds); the social reception of forest works and forests (e.g., emotional reactions to total clear-cutting, negative evaluation of logging traces, negative evaluation of poor tourist infrastructure, specific expectations concerning a model forest: e.g., tall, of low density, mixed, old); and relations between forest users (problems related to e.g., crowding, fast-moving people, the presence of dogs, littering, thefts or noisy behaviour). Here, special attention is paid to problems and negative interactions, as they are challenges to forest management, as well as to the development of plans, strategies, and policies, both in relation to existent forests and those planned in various parts of Europe. Taking into account the feelings and expectations of forest users concerning forests, forest works/management, and infrastructure, as well as their attitude to other forest users, may reduce conflicts concerning various kinds of forest perception and use, and (with the support of societal education) may help to increase the sense of social responsibility for the “shared” forests. The presented findings are expected to be practical and useful for the management of urban and suburban forests, regardless of the location, as a type of checklist of possible problems, that may prove to be important and up-to-date in a particular location.

Pressures and Threats to Nature Related to Human Activities in European Urban and Suburban Forests

This review regards the pressures and threats linked with the human use of European urban and suburban forests. They can be divided into the following major categories: urban development, fragmentation, and isolation of forests; human pressures on soil and vegetation (e.g., changes in vegetation due to trampling, environmental and especially air pollution); human pressures on animals (e.g., wildlife losses due to collisions, frequent presence of dogs accompanying the visitors); and other threats and damages (e.g., littering and acts of vandalism). The directions of negative relations between people and forests shown in this review draw attention to the high complexity of the discussed issues. Awareness of this complexity (when planning and implementing forest management) can limit or counteract conflicts arising from the use of urban and suburban forests by people. This is of particular importance in the era of progressing urbanization and the evolution of human needs regarding the use of forests.

Urban agglomeration (Guangzhou-Foshan-Zhaoqing) ecosystem management under uncertainty: A factorial fuzzy chance-constrained programming method

In this study, a factorial fuzzy chance-constrained programming (FFCP) method is developed for planning urban agglomeration ecosystem under uncertainty. FFCP cannot only address uncertainties presented as possibility distributions and random variables, but also quantitatively evaluate the individual and the interactive effects of multiple eco-environmental factors on urban agglomeration ecosystem. The FFCP method is applied to planning the Guangzhou-Foshan-Zhaoqing (GFZ) urban agglomeration, one of the most important economic circles of the Pearl River Delta region. With rapid industrialization, urbanization and population growth, a number of eco-environmental issues (e.g., water contamination, air pollution, and ecological deterioration) are becoming more and more serious. Results reveal that (i) the main environmental problem of Guangzhou is water pollution due to excessive chemical oxygen demand (COD) discharge (with a contribution of 53.1%), (ii) Foshan would suffer air pollution due to sulfur dioxide (SO₂) emitted from industrial processing and production, and (iii) solid waste disposal would continue to be challenge faced by Zhaoqing. Results also disclose that multiple uncertainties in system components have sound effects on the urban agglomeration ecosystem management as well as eco-environmental constraints have individual and/or joint effects on the system benefit. Different probability levels would lead to changed land-use patterns. Decisions at a lower violation level would lead to an increased reliability in fulfilling ecosystem requirement but with a lower system benefit. The findings can help decision makers to gain insights into tradeoff between economic development and eco-environmental protection as well as generate synergetic development strategies of urban agglomeration in an economic-effective and ecological-harmonious way.

Discriminating Urban Forest Types from Sentinel-2A Image Data through Linear Spectral Mixture Analysis: A Case Study of Xuzhou, East China

Urban forests are an important component of the urban ecosystem. Urban forest types are a key piece of information required for monitoring the condition of an urban ecosystem. In this study, we propose an urban forest type discrimination method based on linear spectral mixture analysis (LSMA) and a support vector machine (SVM) in the case study of Xuzhou, east China. From 10-m Sentinel-2A imagery data, three different vegetation endmembers, namely broadleaved forest, coniferous forest, and low vegetation, and their abundances were extracted through LSMA. Using a combination of image spectra, topography, texture, and vegetation abundances, four SVM classification models were performed and compared to investigate the impact of these features on classification accuracy. With a particular interest in the role that vegetation abundances play in classification, we also compared SVM and other classifiers, i.e., random forest (RF), artificial neural network (ANN), and quick unbiased efficient statistical tree (QUEST). Results indicate that (1) the LSMA method can derive accurate vegetation abundances from Sentinel-2A image data, and the root-mean-square error (RMSE) was 0.019; (2) the classification accuracies of the four SVM models were improved after adding topographic features, textural features, and vegetation abundances one after the other; (3) the SVM produced higher classification accuracies than the other three classifiers when identical classification features were used; and (4) vegetation endmember abundances improved classification accuracy regardless of which classifier was used. It is concluded that Sentinel-2A image data has a strong capability to discriminate urban forest types in spectrally heterogeneous urban areas, and that vegetation abundances derived from LSMA can enhance such discrimination.

Should we see urban trees as effective solutions to reduce increasing ozone levels in cities?

Outdoor air pollution is considered as the most serious environmental problem for human health, associated with some million deaths worldwide per year. Cities have to cope with the challenges due to poor air quality impacting human health and citizen well-being. According to an analysis in the framework of this study, the annual mean concentrations of tropospheric ozone (O₃) have been increasing by on average 0.16 ppb year^-1 in cities across the globe over the time period 1995-2014. Green urban infrastructure can improve air quality by removing O₃. To efficiently reduce O₃ in cities, it is important to define suitable urban forest management, including proper species selection, with focus on the removal ability of O₃ and other air pollutants, biogenic emission rates, allergenic effects and maintenance requirements. This study reanalyzes the literature to i) quantify O₃ removal by urban vegetation categorized into trees/shrubs and green roofs; ii) rank 95 urban plant species based on the ability to maximize air quality and minimize disservices, and iii) provide novel insights on the management of urban green spaces to maximize urban air quality. Trees showed higher O₃ removal capacity (3.4 g m^-2 year^-1 on average) than green roofs (2.9 g m^-2 year^-1 as average removal rate), with lower installation and maintenance costs (around 10 times). To overcome present gaps and uncertainties, a novel Species-specific Air Quality Index (S-AQI) of suitability to air quality improvement is proposed for tree/shrub species. We recommend city planners to select species with an S-AQI>8, i.e. with high O₃ removal capacity, O₃-tolerant, resistant to pests and diseases, tolerant to drought and non-allergenic (e.g. Acer sp., Carpinus sp., Larix decidua, Prunus sp.). Green roofs can be used to supplement urban trees in improving air quality in cities. Urban vegetation, as a cost-effective and nature-based approach, aids in meeting clean air standards and should be taken into account by policy-makers.

Analyzing urban ecosystem variation in the City of Dongguan: A stepwise cluster modeling approach

In this study, a stepwise cluster modeling approach (SCMA) is developed for analyzing urban ecosystem variation via Normalized Difference Vegetation Index (NDVI). NDVI is an indicator of vegetation growth and coverage and useful in reflecting urban ecosystem. SCMA is established on a cluster tree that can characterize the complex relationship between independent and dependent variables. SCMA is applied to the City of Dongguan for simulating the urban NDVI and identifying associated drivers of human activity, topography and meteorology without specific functions. Results show that SCMA performances better than conventional statistical methods, illustrating the ability of SCMA in capturing the complex and nonlinear features of urban ecosystem. Results disclose that human activities play negative effects on NDVI due to the destruction of green space for pursuing more space for buildings. NDVI reduces gradually from the south part to the north part of Dongguan due to increased gross domestic product and population density, indicating that the ecosystem in Dongguan is better in the south part. NDVI in the northeast part (dominated by agriculture) is sensitive to the growth of economy and population. More attention should be paid to this part for sustainable development, such as increasing afforestation, planting grass and constructing parks. Precipitation has a positive effect on NDVI due to the promotion of soil moisture that is beneficial to plants' growth. Awareness of these complexities is helpful for sustainable development of urban ecosystem.

Nature-based solutions for resilient landscapes and cities

Nature-based solutions (NBS) are increasingly applied to guide the design of resilient landscapes and cities to enable them to reach economic development goals with beneficial outcomes for the environment and society. The NBS concept is closely related to other concepts including sustainability, resilience, ecosystem services, coupled human and environment, and green (blue) infrastructure; however, NBS represent a more efficient and cost-effective approach to development than traditional approaches. The European Commission is actively engaged in investing in NBS as a driver in developing ecosystem services-based approaches throughout Europe and the world. The pool of knowledge and expertise presented in this Special Issue of Environmental Research highlights the applications of NBS as 'living' and adaptable tools to boost the capacity of landscapes and cities to face today's critical environmental, economic and societal challenges. Based on the literature and papers of this Special Issue, we propose five specific challenges for the future of NBS.