Approximately eight billion people are living on Earth today with more than half (55%, ∼4.2 billion) living in cities—a proportion predicted to increase to 70% (∼6.6. billion) by 2050. As the human population grows, urban residents will face increasingly extreme temperatures under future climate change, which will affect human well-being, health, and mortality. However, nature-based solutions offer promising strategies to mitigate these impacts. Here, we analyst future projections of the maximum temperature of the warmest month, as a proxy for extreme heat exposure across 5646 cities in 218 countries. We show that by mid-century, this climate metric is projected to increase by an average of +1.7 °C (± 0.5 °C), with the largest increases (∼4 °C) projected to occur in mid-to-high latitude cities of Europe, North America, and Australia. We highlight the urgent need to adopt nature-based solutions to mitigate projected increases in urban heat and contribute to net-zero CO _2 emissions goals.

Urban ecosystems are complex and dynamic, shaped by feedback loops between social and ecological components. However, urban ecology requires tools to unravel this complexity. Social-ecological networks (SENs) offer a conceptual and analytical framework by integrating network theory to understand the relationships between and within social-ecological systems. Here, we integrate perspectives from urban ecology and SEN research to introduce SENs as a promising, yet underexplored, framework for advancing urban ecology research. With an example from Melbourne, Australia, we demonstrate how SENs can advance our understanding of urban biodiversity conservation. Lastly, we propose nine key themes for future urban biodiversity research that will benefit from exploration through an SEN approach. By adopting and further developing the SEN framework for urban ecology, researchers can gain structural and relational insights into urban social-ecological systems. Importantly, an SEN framework may not only bridge the inter- but also transdisciplinary gap between research and practice.

Urban tree cover (UTC) is a commonly used metric in policy and management activities, including urban forest resources assessment, equity and distribution, and ecosystem services modelling. Despite the well-established benefits associated with urban tree canopy, declining tree cover has catalysed many cities into setting UTC targets. In this short communication, we used an assessment of UTC targets set by 57 cities worldwide to discuss the merits and drawbacks of setting UTC targets and to inform recommendations for setting effective UTC targets. We found that UTC targets range in ambition, varying between 4 % and 50 %. To meet these targets, cities would have to increase their current UTC by between 0.47 and 23.3 percentage points within stated timelines of between 3 and 51 years. We found that cities with lower current UTC set ambitious targets, requiring relatively large annual increases in canopy cover. Moreover, cities in xeric or dry biomes set lower targets (< 20 %) than cities in temperate or tropical biomes (> 25 %). We found that setting UTC targets can provide a range of benefits, but achieving a UTC target at the expense of other indicators of urban forest structure and quality poses risks. We reflect on pathways to set specific, measurable, achievable, resourced, and time-bound UTC targets, while acknowledging the associated issues. This exploration of UTC targets will help ensure that UTC remains a useful metric for urban forest management and planning.

Cities are particularly vulnerable to climate change, which continues to drive rising air temperatures. The morphology of a city also influences local climate through diverse surface properties and configurations, leading to diverse responses to warming trends. We examined the impact of climate change on Santiago, Chile, a valley city with a semi-arid climate. Near-surface air temperature and rainfall data from Chile's Bureau of Meteorology over the past 63 years were collected from three different local climate zones (LCZs) in a peri-urban, urban park, and residential areas. The research employed regression analysis, a seasonal time series model, and standardised anomalies to assess air temperature and rainfall trends. Results show that the peri-urban area has experienced the highest rate of warming and the greatest decline in rainfall, alongside a doubling of the warming rate in recent years. Discrepancies in maximum and minimum temperature trends resulted in varying daily temperature ranges (DTR) across LCZs. The peri-urban area also displayed significant intra-annual variability in air temperature and rainfall, leading to trend variations over the years in contrast to the other LCZs. Our findings underscore the accentuated impact of climate change in the peri-urban area due to its permeable bare soil surfaces with an increase of 0.36 degrees C per decade of warming, compared to the areas characterised by impermeable surfaces and well-irrigated green spaces of 0.25 degrees C per decade on average. It highlights the importance of analysing urban air temperatures through LCZ classifications, challenging the conventional urban-rural temperature dichotomy that underpins urban heat island assessments.

There are increasing concerns over the human heat stress and burn risks of using artificial turf in private outdoor spaces such as backyards. This study aimed to compare the microclimate, human heat stress, surface energy fluxes, and human skin burn risk of artificial turf as compared to natural turf in a backyard environment in summer in Melbourne, Australia. The outdoor experiment consisted of nine identical plots (6 x 6 m) with three treatments (each treatment had three replicates): artificial turf, unirrigated natural turf, and irrigated natural turf (4 mm d-1). These three treatments were measured continuously for 51 days during summer. The artificial turf had significantly greater daytime (10:00-16:59 local time) mean air temperature (28.8 degrees C) than the unirrigated (28.2 degrees C) and irrigated (28.0 degrees C) natural turf. The differences in daytime mean radiant temperature among the treatments were too small to make a noticeable difference in human heat stress, measured in universal thermal climate index and wet-bulb globe temperature. The artificial turf had significantly greater daytime mean turf surface temperature (49.9 degrees C) than the unirrigated (31.7 degrees C) and irrigated (29.9 degrees C) natural turf. The turf surface temperature of the artificial turf exceeded the 48 degrees C skin burn threshold for almost four hours of the day when averaged over the 51 days of measurement. The lower albedo of artificial turf (0.09) increased the absorption of incoming shortwave radiation, compared to natural turf (0.19). The lower specific heat capacity of artificial turf also led to its greater surface temperature.

As cities heat up and expand in area and population, urban forests offer a nature-based solution to enhance liveability and reduce rising temperatures in cities. However, urban forests are vulnerable to climate change and face costly establishment and maintenance challenges. Here we explore four key ecological and socioeconomic barriers to achieving resilient urban forests: species selection, tree supply, tree life cycle (establishment and maintenance, including irrigation) and community engagement. We discuss how integrating traditional urban forestry practices with emerging technology offers a holistic approach to creating resilient, sustainable urban forests that can adapt to climate change while meeting community needs. Forests help to sustain cities and all who live in them. This Review considers barriers to sustaining resilient forests and suggests paths for overcoming them.

The ways people think about urban nature affect how people engage with and support nature-based solutions for climate change adaptation in cities. While geographical and socio-demographic characteristics are known to influence people’s thoughts about urban nature, there is little knowledge on how these perceptions can shift over time, especially in response to major events that disrupt the human-nature relationship (such as hurricanes, wildfires, and pandemics). Considering urban trees are a key nature-based solution in cities, this study explores the shift in people’s perceptions about urban trees before and after the COVID-19 pandemic lockdowns. We also assessed how urban context and socio-demographics influenced this shift. Using Melbourne, Australia, as a case study, we delivered an online panel survey based on validated psychometrics about urban trees in summer 2020 (pre-COVID-19 lockdowns) and again in summer 2023 (post-COVID-19 lockdowns). The survey helped us explore temporal changes related to values and beliefs associated with urban forests and trees. Our results showed a change in two perceptions, with a 2% decrease in the importance of urban trees for nature (p = 0.02, r = 0.04) and a 4.3% increase in negative beliefs about trees (p < 0.01, r = 0.08) in 2023, compared to 2020. These shifts were greatest in outer urban areas. Furthermore, we observed that most socio-demographic groups rated the importance of the natural values lower and rated negative beliefs about urban trees higher in 2023, compared to 2020. While previous studies have found people had a more positive connection to urban nature during COVID-19 lockdowns, our study highlights that perceptions of urban trees may shift over time, which can lead to future changes in community support and engagement with urban forest management.

With the bulk of the global population now living in cities, creating a cool, green refuge through extensive urban forests is a priority. However, we are concerned that tree species currently growing in our cities may not tolerate future climates. Esperon-Rodriguez et al. (2024) recently presented an estimate of ‘climate risk’ for a given tree species in a given location using a climate safety margin, based on the difference between the current climate of that city and the realized climate niche of that tree species globally. We attempted to validate this method by relating safety margin estimates with hydraulic vulnerability, a key plant functional trait linked to tree species drought tolerance. However, we found no relationship and therefore caution against the use of climate-based, safety margin methods of assessing urban tree species suitability or climate risk without further context. To develop a robust method of validation, we suggest greater focus on establishing urban forest inventory and tree health data in future climate analogue cities.

The benefits associated with urban forests have led municipalities to set ambitious canopy cover targets to be achieved over the next few decades. Identifying tree species and planting strategies that can achieve these targets is crucial. We applied species-specific tree crown growth models for 20 species commonly planted in Melbourne, Australia to simulate canopy cover increase in newly developed residential suburb over 30 years (2025-2055). Tree species selection and planting strategies were simulated under high and low rainfall. The default strategy represented current tree species selection and planting practices. The two alternative strategies i) occupied all available street planting spaces prioritizing the planting of large crown species at maturity, and ii) occupied all available street planting spaces, maximizing the number of trees planted. Both the default strategy and maximising the number of trees planted achieved 11-15 % canopy cover after 30 years. Prioritising planting trees with large crowns at maturity achieved 16-22 % canopy cover after 30 years. Low rainfall reduced canopy cover in all scenarios by 4-6 %. Increasing the number of species with a large crown at maturity will likely achieve higher canopy cover. However, canopy cover targets will be difficult to achieve unless tree planting on private property increases.

This record contains the microclimate and soil moisture data from a field experiment that investigated the impacts of irrigation scheduling on urban green space irrigation in Melbourne, Australia.

Wildflower meadows are a low-maintenance landscape treatment that can improve urban biodiversity and achieve conservation outcomes, especially when designed to use plants from threatened grassy ecosystems. Cost-effective approaches to create meadows include direct seeding onto mined sand substrates that are placed onto site soils to supress weed competition and enhance sown plant establishment. However, waste subsoils diverted from landfill could provide a more sustainable alternative. This study compares a mined sand with a clay subsoil to understand the relative differences in sown plant establishment and root growth for a range of south-east Australian grassland species. Germination, seedling emergence and root development were assessed for six species sown in an 80 mm deep cap of two low nutrient substrates (sand and clay subsoil) overlying a simulated site soil. Rhizoboxes were used to assess the rate at which plant roots could access soils beneath capping substrates. Sand and clay subsoil supported the establishment of the six sown species. Five species had significantly greater total root length, leaf area and aboveground biomass when sown in recycled subsoil, compared with sand. Edaphic conditions affect the establishment of native grassland species in wildflower meadows. Compared to sand, recycled clay subsoils provide a more sustainable alternative for establishing designed wildflower meadows and can support rapid root and plant growth in south-east Australian grassland species.

Nature-based solutions are informed by how communities think about nature. However, research on how urban communities think about urban nature is seldom carried out across urban contexts. In doing so it can be useful to select specific aspects of urban nature, such as urban forests and urban trees. Our study responds to these needs by measuring the cognitive constructs of values, beliefs, and attitudes towards urban forests and modelling their relationships using a representative survey of >3400 residents living across two different urban contexts: Toronto, Canada, and Melbourne, Australia. Means difference, generalized linear regression, and structural equation analyses, were used to test how values, beliefs, and attitudes differed between metropolitan areas, and how they related to other cognitive constructs, social-ecological context, and demographic factors. We found that resident values and beliefs (more abstract and general constructs) about urban trees were similar across metropolitan areas, but some attitudes (more specific and variable constructs) were different between metropolitan areas, including residents' level of trust in how municipalities manage urban forests and their level of satisfaction with trees and their management. Female residents, and residents who had higher levels of nature relatedness and subjective wellbeing, valued urban forests more. Values, beliefs, and knowledge of trees were significant drivers of resident satisfaction with trees and their management. We discuss implications for urban nature policies.

As keystone structures in urban ecosystems, trees are critical to addressing many of the current livability, health, and environmental challenges facing cities. Every day, trees are removed from urban landscapes as part of routine management. These tree removals are an opportunity for implementing manipulative experiments to directly measure the social and ecological functions of trees. Here we review the kinds of tree removals that commonly occur in cities, assess the relevant opportunities that arise for research–practice partnerships, and discuss the challenges posed when implementing experiments of this nature. We argue that experimental studies on the routine removal of urban trees will improve and expand the mechanistic understanding of how trees support biodiversity and human well‐being in cities beyond current knowledge, which is largely based on correlative studies. Finally, we highlight the opportunity for experiments to be co‐designed by scientists and urban land managers, and how “learning while doing” can generate tangible research impacts and improve urban forest decision making.

• Urban tree canopy cover is a promising solution for mitigating heat island. • Data-driven guidance on tree selection and planting locations is still limited. • Four research priorities are proposed, requiring a collaborative research effort. • Cross-climate morphological and physiological characteristics are desired. • Integration with atmospheric boundary layer models is suggested.

Many cities are developing ambitious future canopy cover targets in recognition of urban trees' numerous benefits. Selecting fast-growing and climate-adapted tree species is important to help achieve these targets. The assessment of growth performance of tree species is challenging, but essential for selecting species that grow in different environments. Often, remote sensing is used to measure canopy cover change at a landscape or neighbourhood scale in urban forests, but rarely at individual tree or species scales. In this study, we developed a novel spatial analysis method combining remotely sensed canopy cover mapping with georeferenced urban tree inventory data to identify individual tree crowns and measure crown expansion rates. We developed species-specific models of crown expansion rates for 20 most common street tree species growing in two rainfall zones (478 to 665mm). Predicted crown areas at 10 years after planting ranged from 6.6 m2 to 43.7 m2. The species showed four different crown expansion responses at 10 years after planting: Fast and consistent growth; Fast but sensitive growth; Slow and consistent growth; Slow and sensitive growth. This study demonstrates a simple, but robust method to delineateindividual tree crowns that can be used to develop species-specific crown expansion models. It also shows the importance of developing species specific crown expansion models in different rainfall zones, as some tree species were clearly sensitive to rainfall differences. Using this spatial analysis method, urban forest managers can make informed decisions regarding tree species selection, considering rainfall zone specific environmental growth conditions, as well as space constraints and water availability.

Naturalistic plantings, such as meadow-style plantings, can improve the quality of urban green spaces through aesthetic, biodiversity and low maintenance features. Species selection for, and maintenance of naturalistic plantings are key to their success. While herbaceous and grassy meadows can be mowed, naturalistic plantings with woody plants require more intense maintenance to remove biomass and promote resprouting. We aim to understand woody plant responses to diverse disturbance regimes to potentially inform the selection and management of woody species in urban plantings. We conducted a quantitative systematic literature review of 72 papers and investigated what main external (climate, disturbance regime) and internal (buds, life stage, storage reserves) factors influence the resprouting response of woody plants. We found resprouting literature is geographically widespread for woody plants, but studies are skewed towards Temperate climates in USA and Australia, with a focus on high severity and high frequency fire disturbance. Resprouting response was mostly defined as a continuous response to disturbance dependent on disturbance regime, climate and plant traits. Maintenance and management of naturalistic woody plantings, through hard pruning techniques such as coppicing, may be informed by analogous high severity and high frequency disturbance studies. However, the literature on woody plant resprouting has several knowledge gaps for lower severity and lower frequency disturbance regimes and in more arid climates. Future research should evaluate the response of naturalistic woody plantings to disturbance in specific urban contexts.

Wildflower meadows support biodiversity in urban environments, whilst providing low-maintenance, amenity landscapes. By moving from international plant palettes to those that include species from threatened plant communities in meadows, we can reintroduce native species to urban landscapes and help achieve conservation objectives. Adapting horticultural techniques from Europe, we determined if such approaches could support the germination and establishment of grasses and wildflowers from critically endangered plant communities in Melbourne, Australia. Working in an urban park, we sowed seed of 27 species on soil capped with two depths of sand (10 and 80 mm) and site soil without sand to determine the impact of sand on weed emergence, slug grazing and growth of sown species. We quantified weed biomass and the time spent weeding unsown species from the emerging meadow. We also tested if covering the sites with a jute mesh enhanced sown species establishment. Twelve months after sowing, the percentage cover of sown species did not differ significantly among treatments. However, the cover of forb species was greater on plots without jute, whereas the cover of grasses was greater on plots with jute. Forb density and species richness were highest on 80 mm sand treatments without jute and lowest on 80 mm sand with jute. We attribute this to greater competition from grasses and higher slug abundance in the presence of jute, leading to the loss of forb species that are more palatable to slugs. Greater sand depths significantly reduced the time to weed and the biomass of weed species removed from the emerging meadow. Synthesis and applications. Using low-nutrient substrates to cap and bury the weed seed bank, irrigation and direct seeding, we demonstrate it is possible to return plants from threatened grassy ecosystems to urban sites, creating a dense and species-rich native understorey within 1 year whilst reducing labour requirements during meadow establishment. Using low-nutrient substrates to cap and bury the weed seed bank, irrigation and direct seeding, we demonstrate it is possible to return plants from threatened grassy ecosystems to urban sites, creating a dense and species-rich native understorey within 1 year whilst reducing labour requirements during meadow establishment.image