A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

Mitigation potential of urban greening during heatwaves and stormwater events: a modeling study for Karlsruhe, Germany

Climate change is increasing the frequency and intensity of urban heat islands and stormwater flooding. In order to mitigate these threats cities are turning toward green infrastructure to restore the hydrologic cycle in a way that increases the ecosystem services provided by trees. Strategically designed green infrastructure can mitigate runoff volume by rainfall interception through tree canopies and redirect impervious runoff into bioswales that promote infiltration. In addition, urban greens mitigate extreme heat via evapotranspiration and shading. Here we applied the i-Tree HydroPlus model to the German city of Karlsruhe and its twenty-seven districts with varying initial conditions of tree cover to analyze the potential for both runoff and heat mitigation during dry and wet periods throughout a 5-year period. After analyzing initial tree cover and drainage conditions, we used the model to simulate a green infrastructure scenario for each district with restored hydrology and tree cover at 30%. Regarding trade-offs between runoff and heat mitigation, the results confirm that dry soils before storm events lead to greater runoff reduction by 10%, and wet soils prior to heatwaves resulted in a greater evaporative cooling. Compared to current conditions, the green infrastructure scenarios resulted in decreasing the number of extreme heat hours (Heat Index > 31 °C) per year on average by 64.5%, and to reduce runoff in average by 58% across all city districts. Thus, our simulation results show that investing into a greener infrastructure, has positive impacts on microclimate and hydrology. Finally, we discuss synergies and trade-offs of the investigated management options as well as the transferability of results to other cities.

Urban tree drought stress: Sap flow measurements, model validation, and water management simulations

Urban street trees face increasing drought stress due to climate change and continuous urban development, making effective water management strategies essential. This study monitored the transpiration and soil moisture dynamics of five urban Tilia cordata trees in Berlin over two consecutive years to understand their transpiration responses under varying urban conditions. The collected data were used to validate the URbanTRee model, which was then applied to simulate different passive irrigation scenarios (system-to-catchment ratios ≤ 1:3) and assess their effectiveness in mitigating drought stress. The URbanTRee model successfully captured seasonal variations in transpiration and soil moisture, identifying all major drought stress periods in 2022, although underestimations were observed towards the end of the season. At the hourly scale, the model reasonably depicted reductions in transpiration during shaded hours on clear-sky days (measured by 55-66 %; modelled by 35-60 %), but overestimations of modelled ETa during hours with partial shading or air temperatures above 30 °C suggest room for improvement. The scenario analysis further demonstrated that, depending on catchment type and tree water demand, system-to-catchment ratios of 1:1-1:2 can substantially decrease, but not fully eliminate drought stress for young urban trees in dry years. These findings highlight the importance of considering site-specific conditions and the limitations of passive irrigation when planning sustainable water management strategies for young urban trees.

Impact of urban green spaces on air quality: A study of PM10 reduction across diverse climates

Urban areas face high particulate matter (PM10) levels, increasing the risk of respiratory and cardiovascular diseases. Green spaces can significantly reduce PM10 concentration, as shown at various scales, from boroughs to whole cities. However, long-term monitoring is needed to understand the specific mechanisms and cumulative impact of green spaces on air quality to changing pollution levels. We investigated the influence of neighbourhood green space percentage, climatic variables, and population density on PM10 deposition during the vegetation period across eight cities in contrasting climate zones over 20 years (2000-2020). We used a correlation matrix, generalized additive model, one-way ANOVA, and Tukey HSD test to analyze the impact of these factors on PM10 deposition rates, assess the role of green space percentage in reducing it, and identify significant differences in PM10 parameters at different proximities to emission sources. Cities with higher population density in warmer, drier climates had higher PM levels, since land surface temperature and wind pressure positively correlated with PM10 deposition, while relative humidity showed a negative correlation. The study found significantly higher PM10 concentrations in industrial areas (36.25 μg/m³) than in roadside areas (25.73 μg/m³) and parks (20.17 μg/m³) (p < 0.01). This highlights the need for targeted interventions in different zones. The study found a complex relationship between green space percentage and PM10 deposition rate onto plant surfaces. Our model suggests that at least 27% of green spaces as land cover can significantly reduce the particulate matter flux, although the minimum threshold can vary depending on the specific urban contexts. The study focused on the proportionate cover of green spaces; still, further investigation including quantitative aspects of urban surface forms, and traffic emissions can comprehend the climatic context and determine the optimal extent of green space required for strategic planning toward future urban sustainability initiatives.

Estimation of plant pollution removal capacity based on intensive air quality measurements

This study investigated the role of present vegetation in improving air quality in Bucharest (Romania) by analyzing six years of air quality data (PM10 and NO2) from multiple monitoring stations. The target value for human health protection is regularly exceeded for PM10 and not for NO2 over time. Road traffic has substantially contributed (over 70%) to ambient PM10 and NO2 levels. The results showed high seasonal variations in pollutant concentrations, with a pronounced effect of vegetation in reducing PM10 and NO2 levels. Indeed, air quality improvements of 7% for PM10 and 25% for NO2 during the growing season were reported. By using Principal Component Analysis and pollution data subtraction methodology, we have disentangled the impact of vegetation on air pollution and observed distinct annual patterns, particularly higher differences in PM10 and NO2 concentrations during the warm season. Despite limitations such as a lack of full tree inventory for Bucharest and a limited number of monitoring stations, the study highlighted the efficiency of urban vegetation to mitigate air pollution.

Synergistic control of urban heat island and urban pollution island effects using green infrastructure

Urban heat island (UHI) and urban pollution island (UPI) effects are two major challenges that affect the liveability and sustainability of cities under the circumstance of climate change. However, existing studies mostly addressed them separately. Urban green infrastructure offers nature-based solutions to alleviate urban heat, enhance air quality and promote sustainability. This review paper provides a comprehensive synthesis of the roles of urban green spaces, street trees, street hedges, green roofs and vertical greenery in mitigating UHI and UPI effects. These types of green infrastructure can promote the thermal environment and air quality, but also potentially lead to conflicting impacts. Medium-sized urban green spaces are recommended for heat mitigation because they can provide a balance between cooling efficiency and magnitude. Conversely, street trees pose a complex challenge since they can provide cooling through shading and evapotranspiration while hindering pollutant dispersion due to reduced air ventilation. Integrated research that considers simultaneous UHI and UPI mitigation using green infrastructure, their interaction with building features, and the urban geographical environment is crucial to inform urban planning and maximize the benefits of green infrastructure installations.

Effects of tree species and planting forms on the thermal comfort of campsites in hot and humid areas of China

Camping has become a popular outdoor activity in China. However, the long and scorching summers in China's hot and humid regions pose challenges for campsites in maintaining thermal comfort. Therefore, we explored the impact of tree species and planting methods on the thermal comfort of urban campsites in hot and humid areas using the ENVI-met model to simulate the conditions of the study area. The reliability of the model was validated by comparing the simulated values of air temperature (Ta) and relative humidity (RH) with field measurements. We conducted an in-depth analysis of common trees in hot and humid areas and analyzed the effects of five tree species and four tree planting forms on the microclimate of campsites in such areas, using the physiological equivalent temperature (PET) as the evaluation index of thermal comfort. The results indicated that: (1) trees with larger crown widths were most effective in improving outdoor thermal comfort. The ability of trees to regulate microclimate was more influenced by crown width than by leaf area index (LAI), and (2) trees planted in patches provided the highest level of thermal comfort, whereas single trees provided the lowest. However, relying solely on tree planting made it difficult to significantly reduce outdoor heat stress. Therefore, other methods such as increasing ventilation or mist spray should be adopted to modify camping area. This study provides a reference for the planting design of outdoor campsites in hot and humid regions of China.

A comparative analysis of urban forests for storm-water management

Large-scale urban growth has modified the hydrological cycle of our cities, causing greater and faster runoff. Urban forests (UF), i.e. the stock of trees and shrubs, can substantially reduce runoff; still, how climate, tree functional types influence rainfall partitioning into uptake and runoff is mostly unknown. We analyzed 92 published studies to investigate: interception (I), transpiration (T), soil infiltration (IR) and the subsequent reduction in runoff. Trees showed the best runoff protection compared to other land uses. Within functional types, conifers provided better protection on an annual scale through higher I and T but broadleaved species provided better IR. Regarding tree traits, leaf area index (LAI) showed a positive influence for both I and T. For every unit of LAI increment, additional 5% rainfall partition through T (3%) and I (2%) can be predicted. Overall, runoff was significantly lower under mixed species stands. Increase of conifer stock to 30% in climate zones with significant winter precipitation and to 20% in areas of no dry season can reduce runoff to an additional 4%. The study presented an overview of UF potential to partition rainfall, which might help to select species and land uses in different climate zones for better storm-water management.

Biodiversity in Urban Green Space: A Bibliometric Review on the Current Research Field and Its Prospects

Understanding the development process of urban green space and biodiversity conservation strategies in urban green space is vital for sustainable urban development. However, a systematic review of the urban green space biodiversity research is still lacking. We have retrieved 3806 articles in WOS core journals and carried out the bibliometrics analysis through the three related search terms: urban, green space, and biodiversity. We found that: (1) the year 2009 was a changing point, and the number of articles have increased exponentially since 2009. The United States, China, Europe, and Australia are closely linked, and four research centers have formed; (2) all studies can be classified into three research themes: "Pattern of Urban Green Biodiversity", "Ecological Function of Urban Green Biodiversity", and "Sustainability of Urban Green Biodiversity"; (3) based on the evolution of keywords, this field is divided into the budding stage (1998-2012) and the development stage (2012-2021). The keywords in the budding stage focus on the diversity of different species, and the keywords in the development stage focus on the ecosystem services, biodiversity protection, and residents' satisfaction; (4) the future research focus may be in three aspects: studies on green space in the less urbanized area and urban-rural ecotone, the regulation mechanism and cultural services of urban green space, and the rational layout and management of urban green space. This study hopes to provide a reference for future research on urban green space biodiversity and promote the sustainable development of urban green space.

Conifers May Ameliorate Urban Heat Waves Better Than Broadleaf Trees: Evidence from Vancouver, Canada

Anthropogenic greenhouse gas emissions are increasing the frequency of deadly heat waves. Heat waves are particularly devastating in cities, where air pollution is high and air temperatures are already inflated by the heat island effect. Determining how cities can ameliorate extreme summer temperature is thus critical to climate adaptation. Tree planting has been proposed to ameliorate urban temperatures, but its effectiveness, particularly of coniferous trees in temperate climates, has not been established. Here, we use remote sensing data (Landsat 8), high-resolution land cover data, and Bayesian models to understand how different tree and land cover classes affect summer surface temperature in Metro Vancouver, Canada. Although areas dominated by coniferous trees exhibited the lowest albedo (95% CrI 0.08–0.08), they were significantly (12.2 °C) cooler than areas dominated by buildings. Indeed, we found that for conifers, lower albedo was associated with lower surface temperatures. Planting and maintaining coniferous trees in cities may not only sequester CO2 to mitigate global climate change, but may also ameliorate higher temperatures and deadly heat waves locally.

Ecosystem Services Provided by Urban Forests in the Southern Caucasus Region: A Modeling Study in Tbilisi, Georgia

All cities globally are growing considerably as they are experiencing an intensive urbanization process that leads to high soil consumption and pollution of environmental components. For this reason, cities are required to adopt measures to reduce these impacts and tree planting has been suggested as a cost-effective strategy. In our study, we implemented for the first time in a Southern Caucasus city the i-Tree Eco model to quantify the main ecosystem services provided by urban forests. Trees in two parks in Tbilisi, EXPO Park (694 trees) and RED Park (1030 trees), have been measured, and a model simulation was performed for the year 2018. These green infrastructures store large amounts of carbon in their woody tissues (198.4 t for EXPO Park and 126.5 t for RED Park) and each year they sequester 4.6 and 4.7 t of CO2 for EXPO Park and RED Park. They also remove 119.6 and 90.3 kg of pollutants (CO, NO2, O3, PM2.5, SO2), and reduce water runoff of 269.5 and 200.5 m3, respectively. This analysis highlights the key role of urban forests in improving the environmental sustainability of the city of Tbilisi and provides important decision support for tree species selection in this geographic area.

Hydraulic Efficiency of Green-Blue Flood Control Scenarios for Vegetated Rivers: 1D and 2D Unsteady Simulations

Flood hazard mitigation in urban areas crossed by vegetated flows can be achieved through two distinct approaches, based on structural and eco-friendly solutions, referred to as grey and green–blue engineering scenarios, respectively; this one is often based on best management practices (BMP) and low-impact developments (LID). In this study, the hydraulic efficiency of two green–blue scenarios in reducing flood hazards of an urban area crossed by a vegetated river located in Central Tuscany (Italy), named Morra Creek, were evaluated for a return period of 200 years, by analyzing the flooding outcomes of 1D and 2D unsteady hydraulic simulations. In the first scenario, the impact of a diffuse effect of flood peak reduction along Morra Creek was assessed by considering an overall real-scale growth of common reed beds. In the second scenario, riverine vegetation along Morra Creek was preserved, while flood hazard was mitigated using a single vegetated flood control area. This study demonstrates well the benefits of employing green–blue solutions for reducing flood hazards in vegetated rivers intersecting agro-forestry and urban areas while preserving their riverine ecosystems. It emerged that the first scenario is a valuable alternative to the more impacting second scenario, given the presence of flood control areas.

Towards a Standard Framework to Identify Green Infrastructure Key Elements in Dense Mediterranean Cities

Present-day dense cities are increasingly affected by the impacts associated with climate change. The recurrence of extreme climate events is projected to be intensified in cities in the next decades, especially in the most vulnerable areas of the world, such as the Mediterranean region. In this context, the urban green infrastructure (UGI) is presented as a nature-based solution that directly contributes to climate change mitigation in Mediterranean compact cities and improves health, social, welfare, and environmental conditions for inhabitants. This research sets out a manageable framework to define, locate, and categorize more functional green urban and peri-urban areas in a dense Mediterranean city. It takes spatial distribution, extension, and the capacity to improve inhabitants’ wellbeing through the provision of ecosystem services as classification criteria. Results show a scenario with a greater functional green surface available for the citizens to be managed. Identified areas have been categorized as cores, nodes, links, and green spaces defined as “other” areas. In particular, the latter play a significant role at social, structural, and ecological levels. The study showcases that rethinking urban design and strategic decision-making around these areas can enhance green equity in Mediterranean dense cities, their capacity to better deal with environmental extremes, and the inhabitants’ engagement with a culture of sustainability and wellbeing.