Street trees provide an opportunity to mitigate urban heat and reduce risk of high heat exposure

Spector J.T., Sampson L., Flunker J.C., Adams D., Bonauto D.K.

AbstractBackgroundInsufficient heat acclimatization is a risk factor for heat‐related illness (HRI) morbidity, particularly during periods of sudden temperature increase. We sought to characterize heat exposure on days before, and days of, occupational HRIs.MethodsA total of 1241 Washington State workers' compensation State Fund HRI claims from 2006 to 2021 were linked with modeled parameter‐elevation regressions on independent slopes model (PRISM) meteorological data. We determined location‐specific maximum temperatures (Tmax,PRISM) on the day of illness (DOI) and prior days, and whether the Tmax,PRISM was ≥10.0°F (~5.6°C) higher than the average of past 5 days (“sudden increase”) for each HRI claim. Claims occurring on days with ≥10 HRI claims (“clusters”) were compared with “non‐cluster” claims using t tests and χ2 tests.ResultsSeventy‐six percent of analyzed HRI claims occurred on days with a Tmax,PRISM ≥ 80°F. Claims occurring on “cluster” days, compared to “non‐cluster” days, had both a significantly higher mean DOI Tmax,PRISM (99.3°F vs. 85.8°F [37.4°C vs. 29.9°C], t(148) = −18, p < 0.001) and a higher proportion of “sudden increase” claims (80.2% vs. 24.3%, χ2[1] = 132.9, p < 0.001). Compared to “cluster” days, HRI claims occurring during the 2021 Pacific Northwest “heat dome” had a similar increased trajectory of mean Tmax,PRISM on the days before the DOI, but with higher mean Tmax,PRISM.ConclusionsOccupational HRI risk assessments should consider both current temperatures and changes in temperatures relative to prior days. Heat prevention programs should include provisions to address acclimatization and, when increases in temperature occur too quickly to allow for sufficient acclimatization, additional precautions.

Sousa-Silva R., Duflos M., Ordóñez Barona C., Paquette A.

Urban tree planting initiatives have been blooming worldwide to help tackle climate change and nurture healthy living environments for people and biodiversity. Many initiatives are characterized by ambitious targets based on the number of trees planted but are not defined by clear objectives, which hampers the success of these initiatives in achieving the desired benefits. Growing an urban forest is a long-term endeavour that requires sustained commitment and support by urban communities to fully realize the provision and even distribution of multiple benefits. In this perspective article, we discuss the characteristics of tree planting initiatives and argue they should be better connected to urban forest management objectives. We propose seven principles that can help improve the success of tree planting initiatives by ensuring that the right types of trees are planted where they are needed most and where they will have the greatest impact. These principles include: (1) connecting initiatives with long-term management, including defining desired benefits, objectives, targets, and indicators; (2) facilitating community engagement on tree planting and maintenance; (3) focusing on tree canopy cover targets rather than on the number of trees planted; (4) focusing on post-planting care and encouraging retention of existing trees; (5) monitoring tree losses and gains to determine whether the specified targets are being met; (6) increasing species diversity, through careful species selection, as well as age and size diversity to enhance urban forest resilience; and (7) addressing the inequitable distribution of tree canopy, specifically where low tree cover overlaps with socioeconomic needs.

White R.H., Anderson S., Booth J.F., Braich G., Draeger C., Fei C., Harley C.D., Henderson S.B., Jakob M., Lau C., Mareshet Admasu L., Narinesingh V., Rodell C., Roocroft E., Weinberger K.R., et. al.

AbstractIn late June 2021 a heatwave of unprecedented magnitude impacted the Pacific Northwest region of Canada and the United States. Many locations broke all-time maximum temperature records by more than 5 °C, and the Canadian national temperature record was broken by 4.6 °C, with a new record temperature of 49.6 °C. Here, we provide a comprehensive summary of this event and its impacts. Upstream diabatic heating played a key role in the magnitude of this anomaly. Weather forecasts provided advanced notice of the event, while sub-seasonal forecasts showed an increased likelihood of a heat extreme with lead times of 10-20 days. The impacts of this event were catastrophic, including hundreds of attributable deaths across the Pacific Northwest, mass-mortalities of marine life, reduced crop and fruit yields, river flooding from rapid snow and glacier melt, and a substantial increase in wildfires—the latter contributing to landslides in the months following. These impacts provide examples we can learn from and a vivid depiction of how climate change can be so devastating.

Flunker J.C., Zuidema C., Jung J., Kasner E., Cohen M., Seto E., Austin E., Spector J.T.

Occupational heat exposure is associated with substantial morbidity and mortality among outdoor workers. We sought to descriptively evaluate spatiotemporal variability in heat threshold exceedances and describe potential impacts of these exposures for crop and construction workers. We also present general considerations for approaching heat policy-relevant analyses. We analyzed county-level 2011–2020 monthly employment (Bureau of Labor Statistics Quarterly Census of Employment and Wages) and environmental exposure (Parameter-elevation Relationships on Independent Slopes Model (PRISM)) data for Washington State (WA), USA, crop (North American Industry Classification System (NAICS) 111 and 1151) and construction (NAICS 23) sectors. Days exceeding maximum daily temperature thresholds, averaged per county, were linked with employment estimates to generate employment days of exceedances. We found spatiotemporal variability in WA temperature threshold exceedances and crop and construction employment. Maximum temperature exceedances peaked in July and August and were most numerous in Central WA counties. Counties with high employment and/or high numbers of threshold exceedance days, led by Yakima and King Counties, experienced the greatest total employment days of exceedances. Crop employment contributed to the largest proportion of total state-wide employment days of exceedances with Central WA counties experiencing the greatest potential workforce burden of exposure. Considerations from this analysis can help inform decision-making regarding thresholds, timing of provisions for heat rules, and tailoring of best practices in different industries and areas.

Rempel A.R., Danis J., Rempel A.W., Fowler M., Mishra S.

Extreme heat events are becoming more frequent and more severe in the Pacific Northwest and in comparable dry-summer climates worldwide, increasing the occurrence of heat-related illness and death. Much of this risk is attributed to overheating in multifamily dwellings, particularly in neighborhoods with abundant asphalt, few trees, and limited financial resources. Air-conditioning expansion is problematic, however, because it creates vulnerability to operational costs and power outages, while expelled hot air intensifies urban heat island effects. In contrast, passive cooling strategies that deflect solar radiation and recruit the cool night air typical of Mediterranean, semi-arid, and arid climates are quite promising, but their abilities to improve residential survivability during extreme heat have not yet been explored. To understand this potential, here we investigate the extent to which well-controlled shading and natural ventilation, in some cases with fan assistance, could have diminished the hours in which indoor heat index levels exceeded ‘caution’, ‘extreme caution’, ‘danger’, and ‘extreme danger’ thresholds during the June 2021 heat wave in the Pacific Northwest; building thermal performance was simulated in EnergyPlus under conditions experienced by Vancouver BC, Seattle WA, Spokane WA, Portland OR, and Eugene OR. Strikingly, we find that in Portland, where the highest temperatures occurred, integrated shading and natural ventilation eliminated all hours above the danger threshold during the 3-day event, lowering peak indoor air temperatures by approximately 14 °C (25 °F); without cooling, all 72h exceeded this threshold. During the encompassing 10-day period, these passive measures provided 130–150h of thermal relief; baseline conditions without cooling provided none. Additionally, passive cooling reduced active cooling loads by up to 80%. Together, these results show the immediate, substantial value of requiring effective operable shading and secure operable windows in apartments in mild dry-summer climates with rising heatwave intensity, as well as public health messaging to support the productive operation of these elements. • Passive space cooling capacity in extreme Pacific NW heat is shown for the first time. • Without intervention, ‘dangerous’ indoor heat index hours predominated. • Integrated control allowed shading and ventilation to cool all dangerously hot hours. • Well-controlled shading and ventilation reduced air-conditioning loads by up to 70%. • Results are relevant to Mediterranean and cold semi-arid and arid climates worldwide.

Riedman E., Roman L.A., Pearsall H., Maslin M., Ifill T., Dentice D.

Urban tree cover is inequitable in many American cities, with low-income and non-white neighborhoods typically having the least coverage. Some municipal and non-profit tree planting programs aim to address this inequity by targeting low-income neighborhoods; however, many programs face lack of participation or resistance from local residents. In this study, we aimed to uncover the economic, social, cultural, and physical barriers that community leaders face in planting trees and fostering engagement in a neighborhood with low tree canopy. In collaboration with an urban greening nonprofit in Philadelphia, Pennsylvania (US), twenty in-depth interviews were conducted with community leaders in a low canopy neighborhood, North Philadelphia. Half of these leaders were already involved with local tree planting programs, while the other half were not. Findings reveal that despite broad appreciation for trees and greenspaces, there are concerns about the risks and costs residents assume over the course of a tree’s life cycle, the threat of neighborhood development and gentrification associated with trees, limited plantable space, and limited time and capacity for community organizations. Additionally, these barriers to participation may be amplified among low-income and communities of color who face the legacies of historical tree disservices and municipal structural disinvestment. Addressing community concerns regarding the long-term care of trees beyond the initial tree planting would likely require further programmatic support. Overall, this research highlights the complexity of addressing inequities in tree canopy and the importance of integrating resident and community leader perspectives about disservices and management costs into tree planting initiatives. • Urban tree planting initiatives sometimes target low-canopy and under-resourced communities. • Yet programs may face lack of participation or resistance from residents. • Interviews reveal how community leaders encounter various barriers in regards to urban tree planting initiatives. • Barriers to participation may be amplified among low-income and communities of color. • Further support is needed to ensure community concerns are addressed regarding the long-term care of trees beyond planting.

Huang X., Song J., Wang C., Chui T.F., Chan P.W.

The synergistic effect of urban heat island (UHI) and urban moisture island (UMI) aggravates the heat stress during hot summers. To investigate the mechanisms of UHI and UMI, we developed an advanced urban canopy model with more robust predictability of the urban surface heat and moisture budgets by considering dynamic building-tree-air interactions within an enhanced aerodynamic resistance network. Our results show that in a compact high-rise city, anthropogenic heat (AH) emission from building cooling systems is nonnegligible (up to 400 W/m 2 , including ~60% sensible heat flux and ~40% latent heat flux), which can further aggravate UHI and UMI with a 1.64 °C increase in air temperature and a 0.89 g/kg increase in air specific humidity, respectively. On the other hand, the impact of building-tree-air interactions on UHI and UMI within compact high-rise street canyons is complicated due to strong shading effect and hindered turbulent transport. In general, planting trees effectively reduces UHI (−2.90–0 °C) but leads to increased UMI (+0–1.66 g/kg), therefore an optimized tree planting strategy is needed to avoid heat stress aggravation due to high humidity. Specifically, tall trees with large and small crown areas are optimal tree types for street canyons with low and high aspect ratios, respectively. In addition to city greening, optimal building air-conditioning (AC) control in an energy-efficient mode can reduce outdoor maximum heat stress index by 1.10 °C. Furthermore, synergistic effects of optimal AC operation and optimal tree planting can effectively reduce pedestrians’ dangerous hours with heatstroke risks by 11%–35%. • We propose an advanced urban canopy model considering dynamic building-tree-air interactions. • Anthropogenic emissions from buildings aggravate UHI and UMI in compact high-rise cities. • Crudely planted trees may aggravate UHI and UMI in compact high-rise cities. • Two mitigation strategies are proposed, including optimal AC control and optimal tree planting.

Buxton J., Powell T., Ambler J., Boulton C., Nicholson A., Arthur R., Lees K., Williams H., Lenton T.M.

Nature-based solutions to climate change are growing policy priorities yet remain hard to quantify. Here we use remote sensing to quantify direct and indirect benefits from community-led agroforestry by The International Small group and Tree planting program (TIST) in Kenya. Since 2005, TIST-Kenya has incentivised smallholder farmers to plant trees for agricultural benefit and to sequester CO2. We use Landsat-7 satellite imagery to examine the effect on the historically deforested landscape around Mount Kenya. We identify positive greening trends in TIST groves during 2000–2019 relative to the wider landscape. These groves cover 27,198 ha, and a further 27,750 ha of neighbouring agricultural land is also positively influenced by TIST. This positive ‘spill-over’ impact of TIST activity occurs at up to 360 m distance. TIST also benefits local forests, e.g. through reducing fuelwood and fodder extraction. Our results show that community-led initiatives can lead to successful landscape-scale regreening on decadal timescales.

Li P., Wang Z.

Urban greening has been as a popular and effective strategy for ameliorating urban thermal environment and air quality. Nevertheless, it remains an outstanding challenge for numerical urban models to disentangle and quantify the complex interplay between heat and carbon dynamics. In this study, we used a newly developed coupled urban canopy-carbon dynamics model to investigate the environmental co-benefits for mitigating urban heat stress as well as the reduction of carbon dioxide (CO 2 ) emission. In particular, we evaluated the impact of specific components of urban greening, viz. fraction of the urban lawn, bare soil, tree coverage, and irrigation on heat and carbon fluxes in the built environment. The results of numerical simulations show that the expansion of urban green space, in general, leads to environmental cooling and reduced CO 2 emission, albeit the efficacy varies for different vegetation types. In addition, adequate irrigation is essential to effect plant physiological functions for cooling and CO 2 uptake, whereas further improvement becomes marginal with excessive irrigation. The findings of this study, along with its implications on environmental management, will help to promote sustainable urban development strategies for achieving desirable environmental co-benefits for urban residents and practitioners. • We quantified the co-benefit of heat and carbon mitigation by urban greening. • Shade trees can potentially suppress carbon uptake of ground vegetation. • There is a bi-modal pattern of latent heat in response to urban irrigation. • Excessive urban irrigation has marginal effect on heat or carbon mitigation.

Schramm P.J., Vaidyanathan A., Radhakrishnan L., Gates A., Hartnett K., Breysse P.

Record high temperatures are occurring more frequently in the United States, and climate change is causing heat waves to become more intense (1), directly impacting human health, including heat-related illnesses and deaths. On average, approximately 700 heat-related deaths occur in the United States each year (2). In the northwestern United States, increasing temperatures are projected to cause significant adverse health effects in the coming years (3). During June 25-30, 2021, most of Oregon and Washington were under a National Weather Service excessive heat warning.* Hot conditions persisted in parts of Oregon, Washington, or Idaho through at least July 14, 2021. The record-breaking heat had the largest impact in Oregon and Washington, especially the Portland metropolitan area, with temperatures reaching 116°F (46.7°C), which is 42°F (5.6°C) hotter than the average daily maximum June temperature.

Hsu A., Sheriff G., Chakraborty T., Manya D.

Urban heat stress poses a major risk to public health. Case studies of individual cities suggest that heat exposure, like other environmental stressors, may be unequally distributed across income groups. There is little evidence, however, as to whether such disparities are pervasive. We combine surface urban heat island (SUHI) data, a proxy for isolating the urban contribution to additional heat exposure in built environments, with census tract-level demographic data to answer these questions for summer days, when heat exposure is likely to be at a maximum. We find that the average person of color lives in a census tract with higher SUHI intensity than non-Hispanic whites in all but 6 of the 175 largest urbanized areas in the continental United States. A similar pattern emerges for people living in households below the poverty line relative to those at more than two times the poverty line. Individual exposure to heat is associated with adverse health and economic outcomes. Here, the authors show that people of color and people living in poverty bear a disproportionate burden of urban heat exposure in almost all major cities in the continental United States.

Wang X., Dallimer M., Scott C.E., Shi W., Gao J.

The Urban Heat Island Effect (UHIE) is a widely recognised phenomenon that profoundly affects the quality of life for urban citizens. Urban greenspace can help mitigate the UHIE, but the characteristics that determine the extent to which any given greenspace can cool an urban area are not well understood. A key characteristic is likely to be the properties of trees that are found in a greenspace. Here, we explore the sensitivity of the strength of the cooling effect to tree community structure for greenspaces in Changzhou, China. Land surface temperatures were retrieved from Landsat 7 ETM+ and Landsat 8 TIRS and were used to evaluate the temperature drop amplitude (TDA) and cooling range (CR) of 15 greenspaces across each of the four seasons. Tree community structure of the greenspaces was investigated using 156 sample plots across the 15 greenspaces. We found that a number of plant community structure indicators of greenspaces have a significant impact on the strength of the cooling effect. The Shannon-Wiener diversity index, tree species richness and tree canopy coverage of greenspaces are all positively correlated with the magnitude of the temperature drop amplitude, with the strength of their influence varying seasonally. We also find that mean crown width is positively correlated with cooling range in summer and autumn, while greenspace tree density is negatively correlated with cooling range in winter. Our findings improve understanding of the relationship between plant community structure and the cooling effect of greenspaces. In particular, we highlight the important role that tree species diversity provides for mitigating the UHIE, and suggest that if planners wish to improve the role of urban greenspaces in cooling cities, they should include a higher diversity of trees species. • Cooling effect of greenspaces varies seasonally • Tree diversity and tree coverage positively correlated with temperature drop amplitude • Tree crown width positively correlated with cooling range, and tree density negatively correlated with the cooling range. • Optimizing the plant community structure improves the cooling effect of greenspaces without increasing their size.

Locke D.H., Hall B., Grove J.M., Pickett S.T., Ogden L.A., Aoki C., Boone C.G., O’Neil-Dunne J.P.

Redlining was a racially discriminatory housing policy established by the federal government’s Home Owners’ Loan Corporation (HOLC) during the 1930s. For decades, redlining limited access to homeownership and wealth creation among racial minorities, contributing to a host of adverse social outcomes, including high unemployment, poverty, and residential vacancy, that persist today. While the multigenerational socioeconomic impacts of redlining are increasingly understood, the impacts on urban environments and ecosystems remain unclear. To begin to address this gap, we investigated how the HOLC policy administered 80 years ago may relate to present-day tree canopy at the neighborhood level. Urban trees provide many ecosystem services, mitigate the urban heat island effect, and may improve quality of life in cities. In our prior research in Baltimore, MD, we discovered that redlining policy influenced the location and allocation of trees and parks. Our analysis of 37 metropolitan areas here shows that areas formerly graded D, which were mostly inhabited by racial and ethnic minorities, have on average ~23% tree canopy cover today. Areas formerly graded A, characterized by U.S.-born white populations living in newer housing stock, had nearly twice as much tree canopy (~43%). Results are consistent across small and large metropolitan regions. The ranking system used by Home Owners’ Loan Corporation to assess loan risk in the 1930s parallels the rank order of average percent tree canopy cover today.

Lin J., Wang Q., Li X.

• Street tree measures of abundance, species diversity, and size structure are derived from multiple data sources. • GWR is employed to examine street tree inequalities among different socioeconomic groups. • Inequalities are prevalent based on tree abundance and species diversity. • Race- and education-based inequalities are more severe than age-, income-, and household characteristic-based inequalities. • Equitably allocate limited public resources to underprivileged populations and socially vulnerable areas is needed. Street trees are often unequally distributed in urban areas, and their physical and structural attributes, such as extent of canopy cover, species composition, and size distribution, are also spatially heterogeneous. Some studies report that inequalities are more prevalent in streetscapes than in private landscapes. Considering the existing inequality issues and public nature of street trees, street tree inequality studies warrant greater attention. However, most existing studies in this field focus heavily on the unequal distribution of tree canopy cover, while disregarding other tree attributes. In this study, seven street tree measures covering tree abundance, species diversity, and size structure were derived from high-resolution satellite images, Google Street View, and street tree census. We then applied the geographically weighted regression to these seven tree measures in New York City, United States, compared street tree inequalities among different socioeconomic groups, and identified inequality hotspots. Our results show that street tree inequalities are greatest with respect to tree abundance and species diversity. Furthermore, race-based and education-based inequalities are most notable, and age-, income-, and household characteristic-based inequalities were also detected based on tree abundance or species diversity. Socially vulnerable areas that suffer the most severe inequalities are clustered in Brooklyn and Queens. Disaggregated street tree inequalities, with explicit recognition of the differentiated distribution of limited tree resources among different social groups and across geographical areas, are critical for effective decision-making to alleviate environmental inequities.

Sharifi A.

Accounting for over 70% of global CO2 emissions, cities are major contributors to climate change. Acknowledging this, urban climate change adaptation and mitigation plans are increasingly developed to make progress toward enhancing climate resilience. While there is consensus that focusing on both adaptation and mitigation is necessary for addressing climate change impacts, better understanding of their interactions is needed to efficiently maximize their potentials. This paper, first, provides a bibliographic analysis to map existing knowledge regarding adaptation-mitigation interactions. This is done using methods such as bibliographic coupling, co-citation analysis, and co-occurrence analysis. Then, drawing on the literature, this study explores two types of interactions between adaptation and mitigation measures, namely co-benefits and synergies. These interactions are explored through analyzing evidence reported in the literature on different measures related to sectors such as energy, transportation, waste, water, green infrastructure, urban planning, and governance. Results of the bibliographic analysis show that there is a lack of research in the Global South. Results of the detailed content analysis show that many measures can provide co-benefits and synergies. Measures related to green infrastructure, buildings, energy systems, and, transportation are particularly capable of providing co-benefits. In addition, it was found that appropriate levels of density, promotion of public transportation, and urban greenery are measures that are more likely to provide synergistic benefits if combined with other adaptation and/or mitigation measures. This study highlights the need for more empirical research to better understand the magnitude of synergistic benefits between different measures.

Li Q., Gao J.

Kwong R., Victor T.E., Dijstelbloem N., Dong Z.Y., Robillard A.M., Gagnon W.A., Simon C.S., Radhakrishnan C.I., Peralta K.A., Youngstrom E.C., Terjesen E.C., Tsignadze M., Okuyama D., Ma M., Katz D.S.

AbstractThe effects of urban trees on public health should change over decades due to shifts in tree composition and abundance, environmental conditions, human demography, and disease incidence. However, there are few case studies documenting changes in the health-related ecosystem services and disservices provided by trees over time. Here we use seven tree censuses to quantify changes in the effects of city-owned trees on air pollution, allergenic pollen, hydrology, and heat due to shifts in tree abundance and environmental conditions in Ithaca, New York, a small city in the Northeastern United States. We also review how shifts in disease incidence have affected these trees’ health consequences. From 2005 to 2021, trees removed 20% more ozone and 268% more PM2.5; they also removed substantially more runoff and had higher cooling capacity. In contrast, the amount of certain air pollutants removed by trees dropped from 2005 to 2021 for sulfur dioxide (361 kg/yr to 8 kg/yr), carbon monoxide (65 kg/yr to 32 kg/yr), and nitrogen dioxide (380 kg/yr to 298 kg/yr) as their ambient concentrations dropped. Pollen production by street trees from 1947 to 2021 initially dropped due to Dutch elm disease but has quadrupled since the 1980s due to increases in several high-pollen producing genera. Overall, this study illustrates how the public health effects of trees vary over decades due to changes in tree composition and abundance, environmental conditions, and changes in disease incidence and human demography, emphasizing the importance of incorporating long-term perspectives into contemporary tree management decisions.Highlights● Urban trees health-related ecosystem services changed over decades in a case study● The removal of SO2, NO2, and CO by urban trees is diminishing due to cleaner air● Shifts in tree composition have had large effects on allergenic pollen production● Changes in human disease incidence over decades mediate trees health effects

Baradaran Motie M., Bemanian M., Yeganeh M.

Following the Industrial Revolution and the rise of automobiles in cities, the shape and geometry of cities underwent significant changes, which affected urban morphology. This unsustainable development disrupted the balance between built and natural environments, leading to many problems for cities and their citizens over time. Environmental problems are the most significant. Damage to green infrastructure, natural ecosystems, water and air pollution, and extreme heat are just a few examples. Numerous Nature-based solutions have been proposed to achieve these goals. One primary strategy is the development of urban green spaces, which serve as a platform for restoring balance to cities and reducing environmental harm. Recent studies show that simply adding green spaces to cities is not enough. Their design and placement need to be carefully considered to maximize their benefits. Not considering these properties in the design and development of microclimates not only does not improve urban resilience but can also exacerbate these harms. Therefore, this chapter focuses on understanding the behavior of urban green spaces and their influential properties on urban microclimates. These influential properties include planting design and layout, tree proportions and geometric form, density and permeability, and other physiological characteristics.

Beele E., Aerts R., Reyniers M., Somers B.

Urban heat poses significant challenges to public health, as exposure to high temperatures is associated to heat stress, resulting in heat strain, sleep deprivation, and cardiovascular morbidity and mortality. As the frequency of heat waves is increasing due to global warming, urban green spaces are often proposed as a nature-based solution to mitigate urban heat stress. This study investigated the impact of urban green space on perceived heat stress and sleep quality, using questionnaires and detailed land cover data. We surveyed 584 respondents during four heat and four control events in the summers of 2021 and 2022, assessing perceived heat stress, sleep quality, and mental health. Using structural equation models, this study analysed the influence of both tree cover and grass and shrub cover on perceived heat stress and sleep quality, while controlling for risk and vulnerability factors. The outcomes revealed that during heat events, enhanced tree cover was associated with reduced heat stress (B = -0.484, 95% CI [-0.693, -0.275], p = 0.001), while increased grass and shrub cover was associated with both reduced heat stress (B = -0. 361 [-0.529, -0.193], p = 0.000) and improved sleep quality (B = -0. 241 [-0.399, -0.083], p = 0.003). Conversely, during control events, stress indicators were more strongly associated with individual vulnerability factors rather than surrounding green space. These results emphasize the importance of combining trees with lower vegetation in urban planning to mitigate heat-related stress and enhance sleep quality, thereby improving overall well-being during heat events.

Chen X., Li Z., Wang Z., Li J., Zhou Y.

Trees positively improve the annual thermal comfort of the built environment in tropical areas, where climate change is slight throughout the year. However, for areas with high changes in climate all year, the current studies have only explored the summer cooling performance of trees without the impact of different types of trees on annual thermal comfort, especially in cold seasons. Therefore, to quantify the impacts and scientifically guide the optimization of green space layout in hot summer and cold winter areas, this study selected Changsha City as the study area and analyzed how the annual thermal comfort is affected by evergreen trees and deciduous trees, which are two common types of trees in hot summer and cold winter areas. The analytical results indicated that the difference in the effect of deciduous and evergreen trees on outdoor thermal comfort was insignificant in summer, where the difference in the monthly mean PET for the three summer months was slight, being 0.28 °C, 0.14 °C, and 0.29 °C, respectively. However, evergreen trees greatly exacerbated winter cold compared to deciduous trees, with a monthly mean PET decrease by nearly 1.0 °C and an hourly PET reduced by up to 3.57 °C. The difference is mainly attributed to the absorption and reflection of solar radiation by the tree canopy, as well as the cooling and humidifying effect of the tree leaf. In hot summer and cold winter areas, outdoor thermal comfort is still in the “comfortable” and “slightly warm” acceptable stage despite the warming effect of deciduous trees in the spring and autumn seasons. Planting evergreen trees is an inevitable thermal mitigation choice for tropical areas. However, for the areas with high annual climate change, such as hot summer and cold winter areas in China, a change in empirical tree planting patterns and selecting deciduous trees where appropriate will improve year-round outdoor thermal comfort.

Castelo S., Bussolotti V.M., Pellegrini I., Ferreira F., Ismail N.A., Poggi F., Amado M.

Nature-based solutions have been promoted as an effective strategy to address climate impacts, including urban temperature reduction. In this paper, we analyze the impacts of the introduction of street trees on temperature (Universal Thermal Climate Index, UTCI) for three different dates, 2000, 2023, and 2050. A 3D model was developed in Rhinoceros software for a part of George Town, on Penang Island. Four different sections of streets were simulated after integration of the model with the Grasshopper plug-in, where a parametric system was built for temperature measurements based on simulations in the Ladybug and Honeybee plug-ins. The tree species used were selected from a pool of tree species commonly planted in urban settings in Malaysia that have low and medium sensitivity to climate impacts. The results show a maximum reduction of 7 °C between 2000 and 2050, achieved on a street with an NW–SE orientation that was planted with three rows of trees. The minimum UTCI reduction achieved was 3 °C, between 2023 and 2050, in a street with NW–SE orientation that was planted with one tree row. The two streets with a SW–NE orientation showed a 5 °C temperature reduction between 2023 and 2050. Both streets have only one row of trees but different species and sizes, with the bigger trees reducing the temperature in a slightly larger area. The results show the importance of introducing and safeguarding street trees to reduce urban temperatures in the country, potentially keeping temperatures below life-threatening levels, thereby safeguarding urban health, while also reducing costs of energy consumption. Solar orientation, the number of tree rows, and their distribution impact the outcomes. The findings provide useful guidance for climate-conscious urban planning practices in Malaysia.

Berg E., Kucharik C.J.

Abstract Maintaining and increasing canopy cover is frequently promoted as a strategy for mitigating excess heat in cities. However, the impact of changing tree cover on surrounding air temperatures is often unpredictable and can depend on tree species, size, shape, and location. In this study, we explore whether the removal of ash trees in one downtown neighborhood in Madison, Wisconsin affected surrounding air temperatures at small spatial scales (15 to 35 m). We used a bicycle-mounted temperature and radiation sensor to repeatedly record high-frequency observations along four transects. We observed no discernible difference between daytime air temperatures near locations surrounded by tree removals and temperatures at locations without any nearby tree removals. Overall, across the four clusters of streets that were monitored, proximity to mature ash trees did not correlate to reduced air temperatures. Attempts to model temperature as a function of surrounding land cover and street tree characteristics all had poor predictive power (R2 ranged from 0.01 to 0.54), and explanatory variables related to tree cover were never statistically significant. The finding that the removal of ash trees did not impact daytime air temperatures patterns at the neighborhood scale suggests that the most densely developed streets in Madison may not easily support sufficient canopy cover to experience tree-induced cooling, underscoring the importance of pursuing a variety of strategies to mitigate urban warming.

Nicholson S., Nikolopoulou M., Watkins R., Löve M., Ratti C.

Lightweight shading devices like sails, canopies, and street-scale shelters are a key strategy for urban cooling. Studies indicate that interactions between characteristics such as height, thermal emissivity and color, significantly affect outdoor thermal comfort, and those effects vary considerably with local context and time. However, practical understanding of shading often focuses solely on blocking direct solar radiation. Integrating microclimate simulation into design workflows can optimize shade design by considering all thermal effects. This paper validates two workflows using Ladybug Tools (LBT) for modelling freestanding shade, proposing their design applicability. A novel workflow models shortwave and longwave effects of lightweight shade materials on outdoor thermal comfort, differing from LBT's standard approach by considering shade interactions in longwave radiation exchanges. Simulated mean radiant temperature (MRT) is validated against field measurements and compared across both workflows. Field measurements separate long and shortwave contributions for accurate MRT comparison, revealing LBT's tendency to overestimate shortwave radiation effects. The custom workflow is applied to a design case study in Turin, Italy, evaluating various shade materials and their effects on MRT. Results demonstrate how shade material, height, ground material, and time of day interact to influence cooling effects, emphasizing practical implications for optimizing shade design.