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Role of street trees in mitigating effects of heat and drought at highly sealed urban sites
Abstract
Abstract
Vegetation and trees in particular play a key role in mitigating thermal load in urban areas. The goals of this study were to analyse species-specific microclimatic effects and to identify the role of leaf-area-density, transpiration, and stomatal conductance for cooling effects at urban sites. Air temperature, relative humidity and surface temperature were used to study the effects of six tree species with similar age structure in a residential area with comparable site conditions. Transpiration, stomatal conductance and leaf-area density were measured during the summer months in 2013. Air temperatures and relative humidity were monitored for four trees per street in hourly intervals, and surface temperatures were recorded in the afternoon for three hot days on the shaded and unshaded asphalt surfaces beneath the trees.
Species differ significantly in their ability to reduce air and surface temperatures, as well as to increase relative humidity. Trees showing both a high leaf-area density and a high rate of transpiration are more effective in cooling the air temperatures. Differences in the surface temperatures of the tree shaded areas are more pronounced compared to the air temperatures. Thus, the surface temperatures of the full sun exposed areas are up to 15.2 K warmer than that in the tree shaded areas. The results provide information to decrease thermal load in urban areas for future tree planting by choosing species with high cooling potential. The highest cooling potential was measured for Corylus colurna and Tilia cordata ‘Greenspire’ and the lowest for Ulmus × hollandica ‘Lobel’.
... The role of urban trees in mitigating the UHI effect has also been well reported in many urban studies (Rahman et al., 2019;Pataki et al., 2011). Particularly, the potential of urban trees with respect to heat mitigation has been shown to be significant under extremely high temperatures (Gillner et al., 2015;Schwaab et al., 2021). Trees provide cooling through two main mechanisms. ...
... However, the specific effects of an urban environment on trees' response to stresses such as drought and heat are not yet fully known. A few studies have investigated the potential cooling effect of urban trees during heatwaves and droughts (Gillner et al., 2015) or the impact of heatwaves and drought on urban trees' functions (Rötzer et al., 2021), where urban trees' functions depend on the species, growing conditions, local climate, and water availability. Nonetheless, because of the complex urban stressors and the spatial heterogeneity of urban landscapes, further studies are needed to quantify the impact of extreme events on urban trees' functions. ...
... Lower transpiration in Betula pendula at the Forest site indicates that Betula pendula trees growing in an urban forest do not cool the environment as much as Tilia or Malus species. Several previous studies have reported that the transpirational cooling effect of urban trees during hot and dry days increases or sustains the transpiration rates to prevent excessive heat accumulation (Gillner et al., 2015;Duarte et al., 2016;Drake et al., 2018;Urban et al., 2017;Ibsen et al., 2021). Keeping the stomata open under hot and dry conditions cools down the internal leaf temperatures, enabling the maintenance of photosynthesis (De Kauwe et al., 2019;Urban et al., 2017;Drake et al., 2018). ...
Urban vegetation plays a role in offsetting urban CO2 emissions, mitigating heat through tree transpiration and shading, and acting as deposition surfaces for pollutants. The frequent occurrence of heatwaves and of concurrent drought conditions significantly disrupts the processes of urban trees, particularly their photosynthesis and transpiration rates. Despite the pivotal role of urban tree functioning in delivering essential ecosystem services, the precise nature of their response remains uncertain. We conducted sap flux density (Js) and leaf gas exchange measurements of four tree species (Tilia cordata, Tilia × europaea, Betula pendula, and Malus spp.) located in different urban green areas (Park, Street, Forest, and Orchard) in Helsinki, Finland. Measurements were made over two contrasting summers 2020 and 2021. Summer 2021 experienced a local heatwave and drought, whereas summer 2020 was more typical of Helsinki. In this study, we aimed to understand the responses of urban tree transpiration (measured with sap flux density) and leaf gas exchange to heatwave and drought conditions, and we examined the main environmental drivers controlling the tree transpiration rate during these periods. We observed varying responses of Js during the heatwave period at the four urban sites. When comparing the heatwave and no heatwave periods, a 35 %–67 % increase in Js was observed at the Park, Forest, and Orchard locations, whereas no significant change was seen at the Street site. Our results also showed that Js was higher (31 %–63 %) at all sites under drought conditions compared with non-dry periods. The higher Js values during the heatwave and dry periods were mainly driven by the high atmospheric demand for evapotranspiration, represented by the high vapor pressure deficit (VPD), suggesting that the trees were not experiencing severe enough heat or drought stress that stomatal control would have decreased transpiration. Accordingly, photosynthetic potential (Amax), stomatal conductance (gs), and transpiration (E) at the leaf level did not change during heatwave and drought periods, excluding the Park site where a significant reduction in gs was seen. VPD explained 55 %–69 % of the variation in the daily mean Js during heatwave and drought periods at all sites. At the Forest site, the increase in Js saturated after a certain VPD level, likely due to low soil water availability during these hot and dry periods. Overall, the heat and drought conditions were untypical of the region but not excessive enough to restrict stomatal control and transpiration, indicating that ecosystem services such as cooling were not at risk.
... To date, most of the studies on the impact of vegetation on the microclimate in a canyon street have been carried out at full-scale. In Dresden, Gillner et al. (2015) showed that, thanks to the shading effects, vegetation can reduce surface temperatures by 5.5 to 15.2°C and air temperature by 0.7 to 2.2 K depending on the tree species used. The authors also showed that trees can reduce asphalt temperatures by up to 4.6°C per unit of LAD Gebert et al. (2019) are some of the few authors to have measured the water content by volume in soil. ...
... These values correspond to differences at the time of temperature peaks. In their study, Gillner et al. (2015) showed that, thanks to the shading effects, vegetation can reduce surface temperatures by 5.5 to 15.2°C depending on the plant species used. The authors also showed that trees can reduce asphalt temperatures by up to 4.6°C per unit of LAD (Leaf Area Density). ...
... Indeed, a synthesis study carried out by Qiu et al. (2013) shows that vegetation can reduce the air temperature of the surrounding environment by 0.5°C to 4°C depending on the season, the vegetalization solution and the plant species. It has also been shown that, compared to a non vegetated surrounding area, vegetation can reduce air temperature by 0.4 to 6°C according to Coutts et al. (2016), up to 2.5°C according to Bowler et al. (2010), by 0.7 to 2.2°C according to Gillner et al. (2015), by 1.5 to 5.6°C according to Jamei et al. (2016) andup to 1°C according to Shashua-Bar andHoffman (2003). ...
Le changement climatique global et les épisodes extrêmes qu’il induit sont devenus l’un des enjeux majeurs de ce siècle. La compréhension du microclimat en milieu urbain suscite une attention croissante de la part des chercheurs depuis quelques années, en raison des phénomènes de surchauffe observés en ville et de la densité de population qui en font un environnement sensible aux vagues de chaleur. De nombreuses études ont montré que la végétation peut réduire la température de l’air en ville, mais ces bénéfices dépendent de l'environnement construit, et de nombreuses variables souvent non maitrisées en ville, comme la disponibilité de l'eau pour les végétaux. Dans ce contexte, ce travail de thèse vise à analyser et quantifier les services climatiques rendus dans une rue canyon par des arbres en confort hydrique. Elle s’appuie sur une double approche associant expérimentation et modélisation. Des campagnes de terrain ont été réalisées sur une maquette arborée à l’échelle (1/5) installée en milieu extérieur sur le site de l’Institut Agro, à Angers, France. Sur le plan numérique, des simulations 2D du climat distribué en régime instationnaire ont été réalisés selon une approche de type CFD. Entre autres résultats, les travaux de cette thèse ont montré que la rue canyon crée une surchauffe pouvant aller jusqu’à 2.8 °C pendant la nuit, et jusqu'à 2.4°C pendant la journée, et que les arbres peuvent réduire la température de l'air dans la rue de 2.7 °C pendant la journée et améliorer le confort humain thermique en réduisant jusqu’à 8 °C les valeurs de l’UTCI à la mi-journée. Ce travail fournit des éléments de quantification qui pourront aider les décideurs dans leur politique d’aménagement.
... Trees in street canyon are seen to modify airflow around their crowns, which involves complex airflow-leaves interaction (Fan et al., 2021a). Effects of trees have been investigated by wind tunnel measurements (Fellini et al., 2022;Gromke, 2011;Manickathan et al., 2018a;Zhao et al., 2023), field measurements (Armson et al., 2013;Gillner et al., 2015;Shashua-Bar et al., 2009) and Computational Fluid Dynamics (CFD) simulations (Manickathan et al., 2018b). Gromke (2011) applied a novel concept in wind tunnel modeling of street trees using fiber-like porous material to mimic artifical leaves. ...
... Some studies have shown an air temperature increase up to 0.6 • C at night (Konarska et al., 2016;Rahman et al., 2017;Shashua-Bar et al., 2009). Gillner et al. (2015) studied the diurnal cooling potential of six tree species on three hot days in Dresden, Germany. During the daytime, the local air temperature is reduced by 0.8 • C to 2.2 • C. The cooling effects vary, depending on different background climates and soil conditions, dominant plant species, plant morphologies, and plant densities. ...
... Urban trees growing in parks and gardens typically have access to relatively higher volumes of soil, potentially greater access to water and nutrients, and lower soil compaction (Konarska et al., 2016), compared to trees surrounded by pavements or asphalt (Mullaney et al., 2015b). Moreover, urban trees are exposed to more radiant heat from the surrounding asphalt and concrete in street-settings which, especially when water availability is limited, can exacerbate temperature and water stress in trees (Gillner et al., 2015) potentially limiting growth and evapotranspiration. Consequently, street trees in particular may have a lower potential to reduce surrounding temperatures via evapotranspiration, compared to those growing in less restricted soils or exposed to less re-radiated heat. ...
... Dense and wide canopies can intercept and reflect large amounts of incoming short-wave radiation (Pataki et al., 2011;Qin et al., 2014;Gillner et al., 2015;Kong et al., 2017). A comparable study in Taipei in sub-tropical Taiwan by Lin and Lin (2010) reported that air temperature decreased by 0.29 • C with each additional unit of LAI. ...
Urban warming affects many millions of city dwellers worldwide. The current study evaluated the extent to which trees reduce air and surface temperatures in urban settings across Greater Sydney, Australia. Summertime air and surface temperatures were measured directly in the shade of 470 individual trees planted in three contrasting contexts (parks, nature strips, asphalt) and compared with temperatures in paired adjacent areas receiving full sunlight. Differences between shade and sunlit temperatures were evaluated against measured morphological traits (leaf area index [LAI], clear stem height, crown depth, height and diameter at breast height) for all trees. On average, tree shade reduced mean and maximum air temperatures by 1.1 °C and 3.7 °C, respectively. Temperatures of standardised reference surfaces (black and white tiles and artificial grass) in tree shade were up to 45 °C lower compared to full-sun exposure, and were also lower in parks and nature strips compared to asphalt settings. The surface temperature of shaded natural grass was cooler compared to sunlit natural grass, although this difference did not vary between nature strip and park settings. The magnitude of air and surface temperature reductions due to tree shade was significantly, positively related to tree-level LAI and these relationships were stronger in asphalt and park contexts compared to nature strips. These findings can inform decisions made by urban managers and planners around the selection of tree characteristics to enhance cooling benefits in different contexts, as an important step towards more liveable and resilient cities.
... From a microclimate perspective, trees play an important role due to their cooling capacity through evapotranspiration and shading. Surface temperature can be reduced by up to 12-20 °C through the shading effect [60,61] and air temperature by up to 2.6 °C through evapotranspiration [62]. The combination of tree planting with the sponge city principle optimises the effect of both adaptation measures at all levels. ...
... From a microclimate perspective, trees play an important role due to their cooling capacity through evapotranspiration and shading. Surface temperature can be reduced by up to 12-20 • C through the shading effect [60,61] and air temperature by up to 2.6 • C through evapotranspiration [62]. The combination of tree planting with the sponge city principle optimises the effect of both adaptation measures at all levels. ...
The ongoing effect of climate change heating up urban areas is forcing cities to exploit the adaptation potential of their public open spaces. Streets and squares are important urban open spaces that can contribute to climate change adaptation through the targeted application of individual measures. In order to ensure the effective and appropriate application of climate-relevant measures for the public good, the city of Vienna relies on the development of a guideline that focuses on measures from the field of urban green and blue infrastructure (UGBI) (and a few technical measures (TM)) in the urban open space. In the future, this guideline will make it easier for city employees to select appropriate measures. In the context of an applied research project, existing and possible measures in Vienna were collected, examined, and assessed for their climate, ecological, and social sustainability based on the concept of ecosystem services (ES). The challenge here is to capture this broad topic of sustainability and climate change and to draw on a broad spectrum of knowledge from science and research, as well as directly from practice. The result is a methodological framework that can be used by other cities as a basis for the development of individual guidelines to foster climate-relevant measures and a critical analysis of the use of co-creation in the development of the framework.
... Recent research on cooling effects of urban forests has been notably focused on single or small groups of trees in highly sealed areas (e.g., PACE et al. 2021, RAHMAN et al. 2020b, HELLETSGRUBER et al. 2020, RÖTZER et al. 2019, LINDÉN et al. 2016, GILLNER et al. 2015, while research on larger forest stands located in the urban matrix has been only sparsely explored. If urban forest stands are studied, research is often based on remote sensing techniques, measuring the surface temperatures of the tree canopies instead of the air temperature itself (GAGO et al. 2020, REN et al. 2018, DU et al. 2017, ALAVIPANAH et al. 2015. ...
... To address these knowledge gaps, air temperature and humidity measurements were performed between April 2018 and May 2020 at the eleven monitoring sites already shown in the introduction (chapter 2.4). The applicability of iButtons for forestry microclimatic studies has been already shown by GREISER et al. (2018), HARDWICK et al. (2015 and GILLNER et al. (2015) amongst others. A self-made plastic enclosure wrapped in aluminum was used to protect the iButton from rain and direct radiation. ...
In the dissertation, the carbon sequestration and the cooling effect of urban forest types (semi-natural forests, urban greening forests, and succession forests) are quantified, and the influence of phytodiversity on the provision of these ecosystem services is analyzed. The investigations were carried out in the heat and dry period of 2018/2019. Therefore, differences in the resilience of the urban forests against these climatic extremes can be seen in the results. Overall, the semi-natural forests on mesophilic sites were most effective in providing regulating services and were more resilient to the occurring heat and drought than the other forest types. In contrast, the highest phytodiversity was found in the succession forests. The forest structure essentially determines the regulating ecosystem services provision of urban forests. An influence of phytodiversity on the services could not be found.
... Several strategies can be applied to mitigate the effects of heat and drought on soil: revegetation/afforestation, plantation of street trees, establishment of plant covers, green roofs, use of porous pavements in urban areas instead of asphalt or concrete, adoption of rainwater reservoirs, and rainwater harvesting techniques [9][10][11][12][13][14][15][16]. Regarding revegetation and afforestation, there is a need for technological solutions able to support planting and forestry operations, hence promoting the survival rate of growing plants [17][18][19]. ...
The aim of this study was to investigate the effect of different types of natural cellulose-based fillers on the properties of Xanthan gum (XG) in order to develop novel bio-based soil conditioners (SCs) that could be used in forestry and agricultural applications. Rheological measurements highlighted that SCs with cellulose fillers characterized by a high aspect ratio and low oxide ash content exhibited an average increase of 21% in yield stress compared to neat Xanthan gum. The presence of cellulose fillers in the composites resulted in a slower water release than that of neat XG, limiting the volumetric shrinkage during the drying process. Furthermore, an analysis of the water absorption and water retention capacity of soils treated with the different SCs was carried out, demonstrating that the addition of 1.8 wt.% of SC with optimized composition to the soil led to an increase in water absorption capacity from 34% up to 69%. From the soil water retention curves, it was observed that the addition of SCs significantly increased the amount of water effectively available for plants in the area between field capacity and permanent wilting point (100–1000 kPa). From practical experiments on grass growth, it was observed that these SCs improved the water regulation of the soil, thus increasing the probability of plant survival under drought conditions.
... The transportation sector in Egypt is responsible for almost 32% of total emissions, according to studies conducted in 2019 [5]. The construction of urban infrastructure using impermeable building materials like concrete and asphalt contributes to the urban heat island (UHI) effect which raises temperatures [6]. ...
... Forests 2023, 14, 671 2 of 22 effects through evapotranspiration and shading, thereby regulating local and regional climates [6][7][8][9]. ...
... Additionally to general temperature increase, climate change is predicted to lead to a higher frequency of extreme weather events such as heat waves and droughts (IPCC, 2021), that will negatively affect the growth of urban trees and reduce the provision of ecosystem services (Brune, 2016;Rötzer et al. 2021a;Rötzer et al. 2012). Ecosystem services (ESS) of urban trees, e.g., cooling or carbon fixation, proportionally increase with age as well as with crown volume (Rötzer et al. 2021b), leaf area density (Gillner et al. 2015) and leaf area index (Rahman et al. 2018) and in turn, are the highest for mature, healthy trees (Gillner et al. 2014). As ESS are linked to the health status of trees, changes in growth due to heat or water stress might cause relevant reduction of these ESS. ...
... Even during extreme times such as the COVID-19 pandemic, green and blue spaces within 61 the UGI were found beneficial to mental health of urban dwellers (e.g., Pouso et al., 2021). 62 Moreover, the regulatory functions of UGI can tame natural hazards such as floods and droughts 63 by facilitating infiltration and groundwater recharge (e.g., Gill et al., 2007;Zhang et al., 2012), as 64 well as heat hazards through the cooling effect (e.g., Gillner et al., 2015;Ghosh and Das, 2018). 65 Conversely, the different components of UGI are prone to drought and heat hazards and their 66 functions under these hazards can be affected (e.g., Brune, 2016;Allen et al., 2021). ...
Urban green infrastructure (UGI) is a prominent concept towards climate adaptation and urban resilience, but is also affected by droughts and heat. Hence, this study aims to advance the multi-assessment of drought and heat risks for UGI through a framework with conceptual and methodological features, paving the way towards knowledge creation and decision support. The framework was systematically developed, starting with defining the situation, analyzing concepts, and finally, constructing the framework. The situation is interpreted as a Coupled Human and Natural System to represent the biophysical and immaterial elements, processes and interrelations. Further, the concepts of risk, UGI and ecosystem services come together in a risk system showing the compound hazards, the exposure, and the cascading vulnerabilities of the UGI. The drought and heat risk assessment framework distinguishes two stages, multi-risk analysis, and multi-criteria risk evaluation. The analysis includes definition and interpretation of the UGI situation under drought and heat conditions, analyzing the hazards, exposure, and vulnerabilities of the system, and translating the risk system into an indicator-based information system. Hereby, the vulnerability analysis of the biophysical UGI aspects comprises the susceptibility and resilience of UGI entities, as well as the degree to which providing ecosystem functions and services can be affected. The multi-criteria risk evaluation covers the assignment of thresholds and weights for indicators, in addition to the aggregation methods. The resulting framework intends to support local actors in the risk assessment of current and future conditions, fostering evidence-based decisions and interventions to deal with risks.
... Trees provide economic, social, health, visual, and aesthetic benefits to cities (Roy et al. 2012). Research documents the role of trees and greenery in compact urban developments in reducing the urban heat island effect (Bowler et al. 2010;Gillner et al. 2015;Norton et al. 2015). On the other hand, trees in a compact urban built-up area have disadvantages, including maintenance costs, light attenuation, infrastructure damage, and causing allergies (Avolio et al. 2015). ...
Trees in the densely built-up historical cores of cities increase the aesthetic values of the cityscape as well as lower the effect of the urban heat island. The research aimed to determine the spatial distribution of tree populations in the medieval parts of cities. The investigation included three cities in Central and Eastern Europe: Poznań and Lublin in Poland and Lviv in Ukraine. The oldest parts of these cities share similar origins, the same continental biogeographical region, and comparable climatic conditions. This study considered indicators of occurrence, density, species composition, and spatial distribution of trees. The study identified different types of management concerning trees. In Poznań, trees are actively introduced in public spaces. In Lublin, there is the largest share of trees in the backyards and self-seeding is common. In Lviv, there are new plantings in private and public spaces. Lviv is distinguished by its large share of native trees, and in Poznan and Lublin, new plantings include artificially shaped, small spherical crowns.
... To continue, the leaf area density, a parameter reflecting the foliage characteristics of the modeled tree types, must be defined at different intervals heights, while also accounting for its seasonal variation for deciduous tree types [27]. However, in the existing literature, relevant LAD measurements are mainly available in summer, when the peak foliage density has been reported [28][29][30]. On the other hand, the seasonal variation in the foliage characteristics has rarely been investigated using on-site experimental campaigns [31], and thus there is insufficient knowledge on the LAD values during defoliation periods. ...
The current research proposes an integrated computational method to consider the effect of the urban microclimate and the higher urban air temperatures on the assessment of urban building energy demands on an annual basis. A one-way coupling procedure is established to generate datasets on typical weather years that can capture the particularities of the urban microclimate as a function of their morphological and geometrical characteristics, thus providing a global perspective of the annual building energy performance at a reasonable computational cost. The proposed simulation method, here applied for an energy performance analysis of generic, non-insulated building units located in four different urban sites of Thessaloniki, Greece, is based on the three tools: (a) the ENVI-met v.4 microclimate model, (b) the Meteonorm weather generator and (c) the dynamic BEPS tool EnergyPlus. The obtained simulation results indicate a decrease in the annual heating energy needs of the examined building units of 8.2–11.5% when the effect of urban warming was accounted for, along with a rise in the annual cooling energy needs of between 13.4 and 28.2%, depending on the case study area.
... Forests 2023, 14, 671 2 of 22 effects through evapotranspiration and shading, thereby regulating local and regional climates [6][7][8][9]. ...
... Forests 2023, 14, 671 2 of 22 effects through evapotranspiration and shading, thereby regulating local and regional climates [6][7][8][9]. ...
... Forests 2023, 14, 671 2 of 22 effects through evapotranspiration and shading, thereby regulating local and regional climates [6][7][8][9]. ...
... Forests 2023, 14, 671 2 of 22 effects through evapotranspiration and shading, thereby regulating local and regional climates [6][7][8][9]. ...
Citation: Amer, A.; Franceschi, E.; Hjazin, A.; Shoqeir, J.H.; Moser-Reischl, A.; Rahman, M.A.; Tadros, M.; Pauleit, S.; Pretzsch, H.; Rötzer, T. Structure and Ecosystem Services of Three Common Urban Tree Species in an Arid Climate City. Forests 2023, 14, 671. https:// Abstract: Urban forests play a critical role in improving the quality of life in cities, but in arid environments, little is known about the potential benefits and growth conditions of different tree species. Our study aimed to fill this gap by investigating the relationships between tree dimensions, above-ground biomass carbon storage, and shading potential in three common urban trees in the arid city of Jericho, Palestine, (i.e., Ficus nitida, Delonix regia, and Phoenix dactylifera). The trees were chosen according to their distribution in urban locations and tree vitality, with ages ranging from 20 to 90 years. Based on the results from tree structure measurements, the carbon storage and shading potential were calculated using the City Tree model. The results indicate a moderate to strong relationship between tree height, crown diameter, and crown volume for F. nitida and D. regia (R 2 = 0.28-0.66), but no relationship for P. dactylifera (R 2 = 0.03-0.06). The findings suggest that the analyzed tree species can considerably contribute to the potential benefits of trees in improving the climate of an arid city: D. regia shows a higher median of above-ground biomass carbon storage of 155 kg C tree −1 , while P. dactylifera 91 kg C and F. nitida 76 Kg C. D. regia and F. nitida have a higher median of shading potential, (31 m 2-41 m 2), respectively. Information on the ecosystem services from urban trees and their relationships in terms of species, age, and tree planting urban location are very important for city planners, in relation to sustainable urban green spaces in arid cities.
... The systematic GI intervention can be developed in conformity with the lower difficulty for GI retrofitting. Some specific trees with a lower crown temperature can be planted as urban tree canopy in the industrial zones in targeted response to urban heat island effect (Gillner et al., 2015), stormwater pond with high runoff control effectiveness can be developed to relieve the regional flooding pressure, and ecological network can be implemented to improve the landscape connectivity (Jalkanen et al., 2020;Yazdi, 2019). Overall, in the primary GI intervention zone, many types of GI can be woven to maximize its multifunctionality and to comprehensively improve the socioecological quality under lower GI retrofitting resistance. ...
The implementation of green infrastructure (GI) is crucial for improving the resilience of cities to the challenges of urbanization and climate change. However, the practicality of GI retrofitting is often neglected in traditional GI planning, which tends to focus mainly on the benefits of GI. In this study, we spatially quantify regional states from six aspects, including stormwater management, air quality, access to green space, urban heat island effect, habitat quality and development level on 1km² grids. While the first five indicators represent the benefits of GI, the last indicator represents the opportunity cost for GI retrofitting. These regional states are treated as driving factors to sway overall GI planning and are further integrated for synergy and trade-off analysis, priority ranking and grids clustering. The results reveal significant spatial unevenness of regional states, which indicates the necessity for targeted GI planning. The spatial pattern of synergy and priority ranking suggests that the city center in the eastern bank of the river should be prioritized for GI intervention. Adaptive GI strategies are proposed based on the results of grids clustering. The developed approach could be conducive for decision makers in implementing targeted GI planning for responding to spatial unevenness.
... As florestas urbanas oferecem diversos serviços ecossistêmicos importantes para manter a saúde ambiental e o bem-estar humano (Mullaney, Lucke & Trueman, 2015). Esses benefícios incluem aspectos ecológicos (Gillner, Vogt, Tharang, Dettmann & Roloff, 2015), sociais (Nowak & Dwyer, 2007) e econômicos (Pandit, Polyakov, Tapsuwan & Moran, 2013) exemplificados pelo suprimento de água de mananciais em áreas urbanas e entorno; sua conexão com a conservação de terras úmidas e sua biodiversidade; regulação do clima; proteção de áreas em risco; suprimento de energia; suporte para agricultura; prevenção da erosão dos solos, e oferta de áreas de recreação e inspiração cultural, são exemplos de serviços providos pelos ecossistemas, inclusive o urbano (Duarte et al., 2017;United Nations, 2018). ...
A revista Terr@ Plural tem a missão de publicar artigos científicos relacionados à área de Gestão do Território que contribuam com o desenvolvimento do conhecimento teórico e metodológico deste campo de saber. Além disso, visa estimular o debate acadêmico daqueles que atuam na temática e ampliar as relações com profissionais de outras regiões do Brasil e do exterior. A revista é uma publicação semestral e é composta pelas seções de artigos e resenhas.
... Previous studies pendula trees growing in an urban forest do not cool the environment as much as Tilia or Malus. Several previous studies have reported that the transpirational cooling effect of urban trees during hot and dry days increases or sustains the transpiration rates in order to prevent excessive heat accumulation(Gillner et al., 2015b; Duarte et al., 2016; Drake et al., 2018;Urban et al., 2017; Ibsen et al., 2021). Keeping the stomata open in hot and dry conditions cools down the internal leaf temperatures enabling maintaining photosynthesis(De Kauwe et al., 2019;Urban et al., 2017; Drake et al., 2018). ...
Urban vegetation plays an important role in offsetting urban CO2 emissions and mitigating heat through tree transpiration and shading. With frequent heatwave events and the accompanying drought, the functioning of urban trees is severely affected in terms of photosynthesis and transpiration rate. The detailed response is however still unknown despite tree functioning having crucial effects on the ecosystem services they provide. We conducted sap flux density (Js) and leaf gas exchange measurements of trees (Tilia cordata, Tilia × europaea, Betula pendula, Malus spp.) located at four types of urban green areas (Park, Street, Forest, Orchard) in Helsinki, Finland, over two contrasting summers 2020 and 2021. Summer 2021 had a strong heatwave and drought, whereas summer 2020 was more typical for Helsinki. In this study, our aim was to understand the responses of urban tree transpiration and leaf gas exchange to heatwave and drought and examine the main environmental drivers controlling the transpiration rate during these periods in urban green areas. We observed varying responses of tree water use during the heatwave period at the four urban sites. Js was found to be 35–67 % higher during the heatwave as compared to the non-heatwave period at the Park, Forest, and Orchard sites but no significant difference was found at the Street site. Our results showed that Js was higher (31–63 %) at all sites during drought as compared to non-dry periods. The higher Js during the heatwave and dry periods were mainly driven by the high atmospheric demand for evapotranspiration represented by the vapor pressure deficit (VPD), suggesting that the trees were not experiencing severe enough heat or drought stress that stomatal control would have decreased transpiration. Accordingly, maximum assimilation (Amax), stomatal conductance (gs), and transpiration (E) at the leaf level did not change at the four sites during heatwave and drought periods. However, gs was substantially reduced during the drought period at the Park site. VPD explained 55–69 % variations in the daily mean Js during heatwave and drought periods at all sites except at the Forest site where the saturation of Js at high VPD was evident due to low soil water availability. The heat and drought conditions were untypically harsh for the region but not excessive enough to restrict stomatal control and the increased transpiration indicating that ecosystem services such as cooling was not at risk.
... However, at the same time the model enabled us to predict air temperature reductions in a simple, straightforward, and scalable manner across a wide spatial area. Additionally, tree coverage cooling capacity might depend on other variables that were not considered in the model, such as type of trees planted (eg, in terms of leaf size and shape, 78,79 height, and crown width 80 ). We also acknowledge that we did not account for the uncertainties associated with each model input, specifically the data for the amount of water evaporated by trees, which were obtained from another model. ...
Background:
High ambient temperatures are associated with many health effects, including premature mortality. The combination of global warming due to climate change and the expansion of the global built environment mean that the intensification of urban heat islands (UHIs) is expected, accompanied by adverse effects on population health. Urban green infrastructure can reduce local temperatures. We aimed to estimate the mortality burden that could be attributed to UHIs and the mortality burden that would be prevented by increasing urban tree coverage in 93 European cities.
Methods:
We did a quantitative health impact assessment for summer (June 1-Aug 31), 2015, of the effect of UHIs on all-cause mortality for adults aged 20 years or older in 93 European cities. We also estimated the temperature reductions that would result from increasing tree coverage to 30% for each city and estimated the number of deaths that could be potentially prevented as a result. We did all analyses at a high-resolution grid-cell level (250 × 250 m). We propagated uncertainties in input analyses by using Monte Carlo simulations to obtain point estimates and 95% CIs. We also did sensitivity analyses to test the robustness of our estimates.
Findings:
The population-weighted mean city temperature increase due to UHI effects was 1·5°C (SD 0·5; range 0·5-3·0). Overall, 6700 (95% CI 5254-8162) premature deaths could be attributable to the effects of UHIs (corresponding to around 4·33% [95% CI 3·37-5·28] of all summer deaths). We estimated that increasing tree coverage to 30% would cool cities by a mean of 0·4°C (SD 0·2; range 0·0-1·3). We also estimated that 2644 (95% CI 2444-2824) premature deaths could be prevented by increasing city tree coverage to 30%, corresponding to 1·84% (1·69-1·97) of all summer deaths.
Interpretation:
Our results showed the deleterious effects of UHIs on mortality and highlighted the health benefits of increasing tree coverage to cool urban environments, which would also result in more sustainable and climate-resilient cities.
Funding:
GoGreenRoutes, Spanish Ministry of Science and Innovation, Institute for Global Health, UK Medical Research Council, European Union's Horizon 2020 Project Exhaustion.
... Trees in urban areas affect the bioclimatic conditions and can mitigate high levels of heat in buildings and cities, depending on their spatial distribution (Donovan et al., 2009;Mazhar et al., 2015). Moreover, street trees can be used for the cooling of the microclimate and the reduction of thermal loads during summer days (Gillner et al., 2015). ...
The ability to estimate the space volume that a tree occupies, in various heights, is a crucial factor in designing the street trees schedule in pavements of new urban infrastructures. The dimensions of Acer negundo tree crown in various heights can be the basis for a better space management in the pavements of cities. In this study, the height and the crown width of the A. negundo street trees in the Greek cities of Orestiada and Alexandroupoli and the allometric relations that can be found between them, were investigated. Data from 117 street trees growing in semi-permeable pavements of the two cities were used. In each selected tree, the total height (H), and the maximum and minimum crown diameter (CW) were measured. The selected model (CW-H) for Alexandroupoli exhibits a coefficient of determination (R ² ) of 0.81. The R ² of the model selected for Orestiada is lower (R ² = 0.66). The R ² of the model selected using the complete dataset is 0.77. A. negundo appears to have greater crown width in Alexandroupoli compared to that of Orestiada. In Orestiada the conditions of growth were variable since in many cases the measured trees were under side shade, while this not the case in the corresponding trees in Alexandroupoli. The better fit of the selected model in Alexandroupoli compared to that of Orestiada is probably due to the more variable growth conditions of Orestiada.
... A study using thermal satellite imagery in Terre Haute, Indiana, USA, showed that for every unit increase of LAI, the surface temperature is reduced by 1.2 °C [9]. Another study in the city of Dresden, Germany, showed that the surface temperature is reduced as the leaf area density (LAD) increases [10]. However, the study of Hien and Jusuf [11] shows that mature and bulk trees may lead to nighttime warming as the longwave radiation trapping is increased. ...
Nowadays, cities are frequently exposed to heatwaves, worsening the outdoor thermal comfort and increasing cooling energy demand in summer. Urban forestry is seen as one of the viable and preferable solutions to combating extreme heat events and urban heat island (UHI) in times of climate change. While many cities have initiated tree-planting programmes in recent years, the evolving impact of trees on street microclimate, in a time span of up to several decades, remains unclear. We investigate the cooling effects of linden trees in five groups, i.e., 10-20, 20-30, 30-40, 40-60, and 60-100 years old. The leaf area index (LAI) and leaf area density (LAD) vary nonlinearly as the trees grow, peaking at different ages. Computational fluid dynamics (CFD) simulations solving microclimate are performed for an idealized street canyon with trees of varied age groups. Turbulent airflow, heat and moisture transport, shortwave and longwave radiation, shading and transpiration are fully coupled and solved in OpenFOAM. The meteorological data, including air temperature, wind speed, moisture, and shortwave radiation of the heatwave in Zurich (June 2019), are applied as boundary conditions. The results show that young trees in the age group of 10-20 years old provide little heat mitigation at the pedestrian level in an extreme heat event. Optimal heat mitigation by trees is observed for the group of 30-60 years old trees. Finally, the potential impact of growing trees as a heat mitigation measure on air ventilation is evaluated.
... However, at the same time the model enabled us to predict air temperature reductions in a simple, straightforward, and scalable manner across a wide spatial area. Additionally, tree coverage cooling capacity might depend on other variables that were not considered in the model, such as type of trees planted (eg, in terms of leaf size and shape, 78,79 height, and crown width 80 ). We also acknowledge that we did not account for the uncertainties associated with each model input, specifically the data for the amount of water evaporated by trees, which were obtained from another model. ...
... The benefits of urban trees vary across various sizes, species, locations and other individual parameters of the trees (Escobedo, Kroeger, & Wagner, 2011;Escobedo & Nowak, 2009). For example, different tree species have different capabilities to mitigate air pollution and the greenhouse effect (Gillner, Vogt, Tharang, Dettmann, & Roloff, 2015). Different species of trees are also associated with different species of birds, small mammals and pests, which are important to biodiversity and the maintenance of urban ecological systems (Raupp, Cumming, & Raupp, 2006;Watson & Adams, 2010). ...
Trees in urban areas have diverse ecological, social, and health benefits. The establishment of up-to-date and accurate street-tree inventories that list the species and locations of individual street trees is critical to urban tree management and tree-planting campaigns. However, street-tree inventories are incomplete or lacking altogether in most cities. This is partly because conventional field assessment is laborious or expensive. In this study, we developed and validated a novel and cost-effective method to establish a city-wide tree inventory based on computer vision and freely available street view images (SVIs). Tree information such as species, height, crown diameter, and geographical coordinates at the individual tree level can be assessed. Based on an object detection model, we adopted a species-based loss function to address the challenges of long-tailed class distribution of species, which is caused by imbalance among sample size of different tree species and can lead to poor performance of the model. Compared with other research in urban tree species recognition, the modified model shows a higher accuracy. In order to calculate quantitative features of street trees, we employed a deep learning algorithm, which is pretrained on stereo dataset and validated on Google Street View images, to estimate the depth of each pixel in SVIs. Furthermore, as a demonstration, we established the citywide tree inventory and conducted tree diversity analysis for Jinan, China. Compared with new developed area, the old town has more street trees and more diverse tree species which can improve biodiversity and walkability. We also found that plane trees, which can cause allergic reactions, are dominant in northern new developed urban area.
... increment significantly declined in the paved land, as widely observed in previous studies with different pavements (Sanders and Grabosky, 2014;Sand et al., 2018). A higher LAD would block more amount of solar radiation and has a higher cooling effect on the surface temperature (Gillner et al., 2015). In addition, all materials of the paved surface in urban settings usually have a low albedo and thermal capacity, absorbing a large amount of solar radiation, and no evaporation can occur (Krayenhoff and Voogt, 2010;Fini et al., 2017), then the paved land heats up considerably. ...
Trees growing on paved lands endure many environmental stresses in the urban environment. However, the morphological and physiological mechanisms underlying tree adaptation to pavement in the field are less known. In this study, we investigated 40 sites where Ginkgo biloba and Platanus orientalis grow on adjacent pairs of paved and vegetated plots in parks and roadsides in Beijing, China. Relative to the vegetated land, the mean increments in the diameter at breast height and height in the paved land were significantly decreased by 44.5% and 31.9% for G. biloba and 31.7% and 60.1% for P. orientalis, respectively. These decreases are related to both the decrease in assimilation products due to the reductions in leaf area, leaf total nitrogen content, and chlorophyll content and the increase in energy cost due to the synthesis of more soluble sugar and proline for mitigating stress. The increase in leaf soluble sugar content, proline content, and δ13C indicated that trees could adapt to the paved land through the regulation of osmotic balance and the enhancement of water-use efficiency. Piecewise structural equation models showed that trees growing on the paved land are stressed by compounding impacts of the leaf morphological and physiological changes. Therefore, it is critical to explore the complex response of plant morphological and physiological traits to the pavement-induced stress for improving tree health in urban greening.
... Street trees are an important part of the eco-environment and landscape appearance of urban roads. Street trees have important roles in beautifying the environment [1], shading [2], improving microclimate [3], purifying air [4], intercepting rainwater [5], stagnating dust [6], improving biodiversity [7], etc. They have a significant impact on urban road landscape and urban ecology, providing important ecosystem service functions and values [8][9][10]. ...
Street trees make up an important part of the eco-environment and landscape of urban roads. The species of street trees significantly affect the green volume of urban roads. The leaf area index (LAI) is often adopted to measure the ratio of green volume for urban roads, laying a scientific basis for optimizing street trees. This paper measures and analyses the LAI and green plot ratio (GPR) of 14 common street tree species in Xinxiang, a city in Central China’s Henan Province. The results show that, except for evergreens, the LAI values of deciduous trees varied significantly from month to month, forming a single-peaked curve. The LAI values of street trees have a significant positive correlation with the day of year (DOY) (P<0.01). As for the roads with a single row of street trees, the highest mean annual GPR values were achieved by Juglans regia Linn., followed in turn by Ligustrum lucidum Ait., Sophora japonica L., Populus tomentosa Carrière, Fraxinus chinensis Roxb. and Platanus orientalis Linn. Among the 12 common types of double-row road tree combinations, the GPR values all increased first and then decreased; the largest annual mean value belonged to the combination “Sophora japonica L.+ Sophora japonica L.” In the same section, the annual mean GPR value of double-row road trees was 3-7 times higher than that of single-row road trees. Our research demonstrates that the GPR can quantify the differences between different street tree species and combination types, and help to optimize the greening arrangement and plant configuration.
... La especie puede diferir de manera importante en la habilidad para reducir la temperatura del aire y de la superficie, así como en incrementar la humedad relativa. Por ejemplo, las diferencias de temperatura superficial que se encuentran sombreadas por árboles, son más pronunciadas comparadas con la temperatura del aire de las superficies sin sombra por vegetación (Gillner, Vogt, Tharang, Dettmann, & Roloff, 2015). ...
El presente trabajo de investigación ha tenido por objetivo el desarrollar una metodología que permita llevar a cabo la evaluación de la sostenibilidad urbana dentro de la premisa del medio ambiente, a partir del análisis de las características climáticas y del medio físico construido de una ciudad cualquiera, con clima frío, templado cálido-seco y cálido húmedo; y así, determinar si la ciudad evaluada está creciendo o siendo planificada conforme a sus características climáticas. De lo contrario, poder realizar una serie de recomendaciones resultantes del proceso de evaluación para llevar a cabo una adecuada intervención urbana y procurar una configuración acorde al clima.
El trabajo se encuentra dividido en cuatro partes, conformadas por cinco capítulos: 1) La primera parte, Capítulo I, se basa en el desarrollo del estado de la cuestión en torno a las herramientas y modelos de evaluación de la sostenibilidad urbana que se emplean hoy en día para aplicarse en diversos ámbitos y escalas urbanas. De este análisis resultan únicamente 8 de ellas enfocadas en la evaluación a escala de ciudad; pero, ninguna de ellas contempla en su proceso metodológico el empleo de indicadores climáticos que se relacionen con la estructuración del entorno urbano construido. Por tanto, en el Capítulo II, se lleva a cabo la descripción de las implicaciones que tiene la conformación de este último en el microclima urbano y su impacto en el confort térmico. Asimismo, se presentan las características y diferentes enfoques en torno al urbanismo bioclimático como una herramienta de diseño, evaluación y planificación de las ciudades. 2) La segunda parte conformada por el Capítulo III, presenta la metodología de evaluación propuesta en este trabajo y todo su proceso de gestión de la información para poder llevarla a cabo. Esta incluye seis fases: establecimiento de los antecedentes; análisis climático; requerimientos del entorno; análisis del Medio Físico Construido (MFC); evaluación; y recomendaciones. En esta parte se proponen 21 indicadores para llevarla a cabo; 3 de localización, 6 climáticos y 12 del MFC. 3) Esta tercera parte la conforma el Capítulo IV y presenta la aplicación de la metodología propuesta en el capítulo anterior, al estudio de caso de la ciudad de Hermosillo, Sonora, México. 4) Finalmente, la cuarta parte presenta, a través del Capítulo V, las conclusiones generales y particulares de este trabajo de investigación, así como las aportaciones al estado de la cuestión y futuras líneas de investigación.
Así pues, el objetivo principal de este trabajo de investigación se ha podido lograr al haber desarrollado esta metodología, tomando como guía las premisas del urbanismo bioclimático para evaluar las ciudades, con relación a su clima y MFC, a partir de los 21 indicadores propuestos. Además de proponer una serie de herramientas de procesamiento de la información y de bases de datos de libre acceso, para facilitar lo mayormente posible, la gestión de la misma. Sin embargo, como se presenta de manera desarrollada en el capítulo V, estas pueden presentar una serie de limitaciones, tales como: El nivel de especificidad de los resultados obtenidos del procesamiento de las imágenes satelitales LANDSAT8/OLI; la falta de diferenciación entre cubiertas y vialidades; la disponibilidad y actualización de cartografía oficial; el acceso a bases de datos estadísticos, sociodemográficos y geográficos; etc.
... Urban street trees provide various ecosystem services through purifying air and water, regulating microclimate and stormwater runoff and increasing amenities (Nowak et al., 2006;Dobbs et al., 2011;McPherson et al., 2011;Gillner et al., 2015). Especially, tree species and canopy structural profile (hereafter referred to as 'tree profile') of urban forests is one of the most critical environmental factors in determining urban ecosystem services (Nowak et al., 2006;Dobbs et al., 2011). ...
Tree species and canopy structural profile (‘tree profile’) are among the most critical environmental factors in determining urban ecosystem services such as climate and air quality control from urban trees. To accurately characterize a tree profile, the tree diameter, height, crown width, and height to the lowest live branch must be all measured, which is an expensive and time-consuming procedure. Recent advances in artificial intelligence aids to efficiently and accurately measure the aforementioned tree profile parameters. This can be particularly helpful if spatially extensive and accurate street-level images provided by Google (‘streetview’) or Kakao (‘roadview’) are utilized. We focused on street trees in Seoul, the capital city of South Korea, and suggested a novel approach to create a tree profile and inventory based on deep learning algorithms. We classified urban tree species using the YOLO (You Only Look Once), one of the most popular deep learning object detection algorithms, which provides an uncomplicated method of creating datasets with custom classes. We further utilized semantic segmentation algorithm and graphical analysis to estimate tree profile parameters by determining the relative location of the interface of tree and ground surface. We evaluated the performance of the model by comparing the estimated tree heights, diameters, and locations from the model with the field measurements as ground truth. The results are promising and demonstrate the potential of the method for creating urban street tree profile inventory. In terms of tree species classification, the method showed the mean average precision (mAP) of 0.564. When we used the ideal tree images, the method also reported the normalized root mean squared error (NRMSE) for the tree height, diameter at breast height (DBH), and distances from the camera to the trees as 0.24, 0.44, and 0.41.
... Street trees not only evapotranspire, but provide shade to pedestrians, buildings, and heat-absorbing ground-level infrastructure, dramatically reducing radiation and consequently overall daytime heat exposure and nighttime heat release (Coutts et al., 2016;Oke, 1989). However, trees can warm temperatures at night (Gillner et al., 2015;Krayenhoff et al., 2020) and slow winds and prevent dispersion of pollutants emitted at a ground level (Santiago et al., 2017; P. E. J. Vos et al., 2013), such as those from vehicle tailpipes, and interfere with subsurface infrastructure. Surface and air temperature cooling from green roofs and low vegetation, and to a lesser extent, trees, is critically dependent on adequate soil moisture either from precipitation or irrigation (Heusinger et al., 2018;Krayenhoff et al., 2021). ...
Urban overheating, driven by global climate change and urban development, is a major contemporary challenge that substantially impacts urban livability and sustainability. Overheating represents a multifaceted threat to the well‐being, performance, and health of individuals as well as the energy efficiency and economy of cities, and it is influenced by complex interactions between building, city, and global scale climates. In recent decades, extensive discipline‐specific research has characterized urban heat and assessed its implications on human life, including ongoing efforts to bridge neighboring disciplines. The research horizon now encompasses complex problems involving a wide range of disciplines, and therefore comprehensive and integrated assessments are needed that address such interdisciplinarity. Here, our objective is to go beyond a review of existing literature and instead provide a broad overview and integrated assessments of urban overheating, defining holistic pathways for addressing the impacts on human life. We (a) detail the characterization of heat hazards and exposure across different scales and in various disciplines, (b) identify individual sensitivities to urban overheating that increase vulnerability and cause adverse impacts in different populations, (c) elaborate on adaptive capacities that individuals and cities can adopt, (d) document the impacts of urban overheating on health and energy, and (e) discuss frontiers of theoretical and applied urban climatology, built environment design, and governance toward reduction of heat exposure and vulnerability at various scales. The most critical challenges in future research and application are identified, targeting both the gaps and the need for greater integration in overheating assessments.
The urban heat island effect poses a growing threat to human society, especially in densely populated and developed megacities. With the introduction of the Local Climate Zones (LCZ) framework, new perspectives and findings have been brought to urban heat island studies. This study investigated the cooling effect of vegetation and albedo on the surface urban heat island (SUHI) in the classification system of LCZ during different seasons, using three Chinese megacities as case study areas. Single-factor linear regression and Pearson’s correlation coefficient were applied to analyze the seasonal cooling effect of both albedo and the NDVI on the SUHI within different LCZs. The results show that (1) the variability of the SUHI is reflected in its dominance and intensity within certain LCZs in different cities and in the efficiency of cooling factors; (2) the cooling effect of vegetation is dominant in each season, and the cooling effect produced by albedo within specific seasons can be differentiated by LCZs. This study provides valuable information for the mitigation of the SUHI magnitude in specific regions and at specific times of the year.
Urbanization offers various opportunities for people to lead a better and more comfortable life. On the other hand, it also comes with some costs and side effects, such as worsening climate conditions. In the local sense, the old climatic conditions changed by urbanization in rural areas can be called natural when compared with the new climatic conditions deteriorated by this widespread urbanization. One of the side effects of urbanization is thermal pollution caused by certain urban activities and sectoral urban designs and consequent patterns on the land surfaces of cities. Thus, thermal pollution changes city’s local climate over time and negatively affects the city’s comfort level at least locally. There are some studies that focus on the analysis of time series thermal data distribution covering an entire city and make a great contribution to the studies in this field. Here in this research is introduced and suggested a Simulated Single Image (SSI) method based on Simulated Single Data (SSD) statistical analyze approach for the studies based on time serious data. Therefore, SSI method using time series data was applied to Remote Sensing (RS) LANDSAT satellites’ bands especially to time series’ thermal bands to reveal where generally Urban Hot Spots (UHS) appear and Urban Heat Islands (UHI) develop in the city. Stereo representation of the study region is also used to visually examine the topographical effect on UHI distribution in the city. Since all these analyzes depend on the SSI analysis, which is a statistical result of time series data, they appear as results that have been repeatedly confirmed. The study clearly demonstrated that industrial regions and roads with large surfaces, bare lands with sparse bushes, empty or sparse grassy urban lands and more significantly the urban land parts faced to certain directions are the main urban land cover and structure types contributing UHSs to appear and UHI developments in the city. So, this means that the outcome is confirmed again and again over years since SSI data analyse technique depends on long term data and these contributing factors adversely affect the previous natural climate in the lands that latter swallowed by the city and then let UHSs to appear and UHIs to develop at and around where these urban land cover structures are located or seen in the city over time. These city sections are the most at-risk areas where city authorities must take serious steps to care about the chronic climatic (thermal) conditions of their cities and to return these areas to their former natural climatic and environmental conditions. There are also some nature-based solutions given and suggested in the concluding part of the article to reduce the impacts caused by these contributors in the city.
The report summarizes the findings of four subprojects related to sustainability in horticulture. The first subproject analyses the fertilizing value of secondary phosphorous materials for horticultural plants. The second subproject developes sustainability criteria for the value chain of flowering potted plants. The third subproject examined the usability of trees coming from more arid environments in urban environments. The fourth subproject deals with consumer aspects related to Fairtrade cutroses in Germany.
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Urban vegetation is valuable in alleviating local heatwaves. However, drought may decrease vegetation health and limit this cooling effect. Here we use the satellite-based Normalized Difference Vegetation Index (NDVI) and Palmer Drought Severity Index (PDSI) to investigate the sensitivity of urban vegetation to drought in the Coastal Greater Los Angeles from 2001 to 2020. We applied four statistical models to analyze the relations between 15 socioeconomic variables and the vegetation’s sensitivity to drought. We then examined the changes in the cooling effect of the urban vegetation during drought and non-drought periods using remotely sensed land surface temperature (LST) data. The results suggest that areas that are economically disadvantaged and have higher proportions of Hispanics and Blacks are typified by more sensitive vegetation to drought, which is likely linked to inequality in water use. Moreover, these populations experience a lower degree of vegetation cooling effects and higher exposure to heatwaves. The findings of this study imply that the potential of a community’s vegetation in mitigating heatwaves is significantly influenced by the socioeconomic conditions of the community. Increasing resilience of urban vegetation to drought over the disadvantaged communities may help promote environmentally sustainable and socially resilient cities under a warming climate.
p>Urban forests play a crucial role in the development of cities because of the urban ecosystem services they provide. Previous works have alleviated urban forest monitoring by discriminating tree species and performing tree inventories using street view images and convolutional neural networks. However, the characterization of trees from street-view images remains a challenging task. Determining tree structural parameters has been limited because of inaccurate tree segmentation caused by combined, occluded, or leaf-off trees. Therefore, the current work evaluates the potential of vegetation indices derived from red, green, blue, and synthesized near-infrared and red-edge spectral bands for urban tree segmentation. In particular, we attempt to show whether or not vegetation indices add relevant information to deep neural segmentation networks when there are low fine-tuning training samples. A conditional adversarial network generates red-edge and near-infrared images in urban environments, which retrieve an average structural similarity index of 0.86 and 0.81, respectively. Furthermore, we note that by using appropriate multispectral vegetation indices, one can boost the average intersection over the union between 5.07 % to 13.7 %. Specifically, we suggest the SegFormer segmentation network pre-trained with the CityScapes dataset and Red Edge Modified Simple Ration index for improving urban tree segmentation. However, if no multispectral data is available, the DeepLabV3 network pre-trained with the ADE20k dataset is suggested because it could achieve the best RGB outcomes average IoU value of 0.671. </p
p>Urban forests play a crucial role in the development of cities because of the urban ecosystem services they provide. Previous works have alleviated urban forest monitoring by discriminating tree species and performing tree inventories using street view images and convolutional neural networks. However, the characterization of trees from street-view images remains a challenging task. Determining tree structural parameters has been limited because of inaccurate tree segmentation caused by combined, occluded, or leaf-off trees. Therefore, the current work evaluates the potential of vegetation indices derived from red, green, blue, and synthesized near-infrared and red-edge spectral bands for urban tree segmentation. In particular, we attempt to show whether or not vegetation indices add relevant information to deep neural segmentation networks when there are low fine-tuning training samples. A conditional adversarial network generates red-edge and near-infrared images in urban environments, which retrieve an average structural similarity index of 0.86 and 0.81, respectively. Furthermore, we note that by using appropriate multispectral vegetation indices, one can boost the average intersection over the union between 5.07 % to 13.7 %. Specifically, we suggest the SegFormer segmentation network pre-trained with the CityScapes dataset and Red Edge Modified Simple Ration index for improving urban tree segmentation. However, if no multispectral data is available, the DeepLabV3 network pre-trained with the ADE20k dataset is suggested because it could achieve the best RGB outcomes average IoU value of 0.671. </p
Under the action of repeated load of vehicles, asphalt pavement tends to suffer rutting and other failures at high temperatures, which shortens the service life of the pavement. Meanwhile, asphalt pavement aggravates the urban heat island (UHI) effect, reducing the livability of the city. Research shows that lower road temperatures can mitigate the UHI effect. For this reason, researchers have launched lots of studies to cool asphalt pavement under high temperatures, such as reflective pavement and phase change pavement, etc. Although these technologies are effective at reducing the temperature of asphalt pavement, there are still some limitations. This paper mainly discusses the latest development of asphalt pavement temperature reduction strategies, classifies them according to different action mechanisms, and discusses the advantages and limitations of each strategy in detail. Suggestions for future research are given to address the limitations of each strategy, such as passive radiation technology can provide new ideas for reflective pavement, planting trees should consider plant physiological parameters and spatial distribution of planting, and phase change materials can be encapsulated by high thermal conductivity materials, etc. This study can provide a guidance for further development of asphalt pavement cooling technologies.
Cities in tropical regions are experiencing high heat risks by overlaying the urban heat island (UHI) effect. Urban green space (UGS) can provide local cooling effect and reduce UHI. However, there still lack a comprehensive exploration of the characteristics of UHI and cooling effect of UGS due to high cloud coverage and limited number of available remote sensing observations. In this study, the enhanced spatial and temporal adaptive reflectance data fusion method was employed to develop an enhanced land surface temperature data in winter seasons in three tropical megacities, Dhaka, Kolkata, and Bangkok. The spatiotemporal variations of surface urban heat island (SUHI) were explored from 2000 to 2020 with a 5-years interval. The optimal size of UGS associated with its cooling effects was assessed by using the threshold value of efficiency (TVoE). The relationship between the intensity and range of urban cooling island (UCI) and four landscape metrics of green space patches, total area (P_Area), shape index (P_SI), normalized difference vegetation index (P_NDVI), and land surface temperature (P_LST), were analyzed. The results show that the average SUHI intensity increased by 0.98°C, 1.42°C, and 0.73°C in Dhaka, Kolkata, and Bangkok, respectively, from 2000 to 2020. The maximum intensity of UCI ranges from 4.83°C in Bangkok to 8.07°C in Kolkata, and the maximum range of UCI varies from 300 m in Bangkok to 420 m in Kolkata. The optimal size of green space is 0.37 ha, 0.77 ha, and 0.42 ha in Dhaka, Kolkata, and Bangkok, respectively. The P_NDVI and P_Area had significant positive effects on UCI intensity and range, while the background temperature had significant negative effects. With higher background temperature, the optimal patch size of UGS is larger. This study provides useful information for developing effective heat mitigation and adaptation strategies to enhance climate resilience in tropical cities.
To reduce increasing human morbidity and mortality due to urban overheating, urban cooling strategies need to be targeted towards intra-urban hot spots. The application of findings from numerous urban heat studies is limited because methods do not measure air temperature relevant to the human experience with a sufficient level of data granularity. In this work, we developed and tested a unit consisting of a temperature data logger and a custom-made weather shield that is substantially less expensive than published alternatives and can be deployed with high spatial flexibility to measure canopy-layer (near-surface) air temperature relevant to people living in the built environment. Accuracy of results was compared with a more expensive, commercially available scientific instrument as well as official Bureau of Meteorology weather stations in Sydney, Australia. The unit costs around 15% of the price of the tested alternative and only 25% of the least expensive published alternative. Data quality was almost identical to that provided by more expensive scientific instruments and official weather stations. Temporal and spatial coverage and the resulting granularity of air temperature data were very high. The air temperature measurement method reported here can be used in future urban heat studies to determine intra-urban hot spots. Resultant knowledge can be used to target cooling strategies that maximise benefits to the human population, reducing heat-related illnesses and death in overheating cities.
Outdoor heat stress is a growing problem in cities during hot weather. City planners and designers require more pedestrian-centered approaches to understand sidewalk microclimates. Radiation loading, as quantified by mean radiant temperature (TMRT), is a key factor driving poor thermal comfort. Street trees provide shade and consequently reduce pedestrian TMRT. However, placement of trees to optimize the cooling they provide is not yet well understood. We apply the newly-developed TUF-Pedestrian model to quantify the impacts of sidewalk tree coverage on pedestrian TMRT during summer for a lowrise neighbourhood in a midlatitude city. TUF-Pedestrian captures the detailed spatio-temporal variation of direct shading and directional longwave radiation loading on pedestrians resulting from tree shade. We conduct 190 multi-day simulations to assess a full range of sidewalk street tree coverages for five high heat exposure locations across four street orientations. We identify street directions that exhibit the largest TMRT reductions during the hottest periods of the day as a result of tree planting. Importantly, planting a shade tree on a street where none currently exist provides approximately 1.5–2 times as much radiative cooling to pedestrians as planting the same tree on a street where most of the sidewalk already benefits from tree shade. Thus, a relatively equal distribution of trees among sun-exposed pedestrian routes and sidewalks within a block or neighbourhood avoids mutual shading and therefore optimizes outdoor radiative heat reduction per tree during warm conditions. Ultimately, street tree planting should be a place-based decision and account for additional environmental and socio-political factors.
Street trees are an important driver of street microclimate through shading and transpirative cooling, which are key mechanisms for improving thermal comfort in urban areas. Urban canopy models (UCM) with integrated trees are useful tools because they represent the impacts of street trees on neighborhood-scale climate, resolving the interactions between buildings, trees and the atmosphere. In this study, we present the results of a measurement campaign where vehicle transects were completed along two similar parallel streets of Barcelona with different tree densities, recording upward and downward radiation fluxes, air temperature and humidity at street level. These observations are used to evaluate and improve the multi-layer UCM Building Effect Parameterization with Trees (BEP-Tree). Prior simulations of the model revealed insufficient heat exchange between the canyon surfaces and the air at the lowest vertical levels inside the deep canyons, which we solve by including turbulent buoyancy driven wind velocity in the model. Air temperatures are on average 1.3 °C higher in the street with sparser trees when wind direction is perpendicular to the streets. The BEP-Tree simulations demonstrate good agreement with the observations in terms of temperature and radiation, and capture the diurnal evolution of temperature and radiation between the two streets.
Many studies have shown that planting trees near the west wall of a building is conducive to saving energy for cooling in summer. However, there is a lack of research quantifying the thermal radiation disturbance caused by the location of trees near the west wall. This paper studies the influence of long-wave and short-wave thermal radiation disturbance of trees at different locations around the west wall in summer through field experiments and ENVI-met simulations. It was found that the thermal radiation disturbance (TRD) of trees on the west wall of the building is mainly caused by short-waves which produce 2-3 times greater disturbance than that of long-waves. According to the fitting relationship between the change rate of daily thermal radiation disturbance, the view factor and the tree projection area, the functional formula between them is obtained. These findings provide a theoretical foundation for scientifically evaluating the influence of tree location and practical applications for guiding the planning and landscaping of vegetation around building walls.
The aim of this study was to provide new insights into how intraspecific variability in the response of key functional traits to drought dictates the interplay between gas-exchange parameters and the hydraulic architecture of European beech (Fagus sylvatica L.). Considering the relationships between hydraulic and leaf functional traits, we tested whether local adaptation to water stress occurs in this species. To address these objectives, we conducted a glasshouse experiment in which 2-year-old saplings from six beech populations were subjected to different watering treatments. These populations encompassed central and marginal areas of the range, with variation in macro- and microclimatic water availability. The results highlight subtle but significant differences among populations in their functional response to drought. Interpopulation differences in hydraulic traits suggest that vulnerability to cavitation is higher in populations with higher sensitivity to drought. However, there was no clear relationship between variables related to hydraulic efficiency, such as xylem-specific hydraulic conductivity or stomatal conductance, and those that reflect resistance to xylem cavitation (i.e., Ψ12, the water potential corresponding to a 12% loss of stem hydraulic conductivity). The results suggest that while a trade-off between photosynthetic capacity at the leaf level and hydraulic function of xylem could be established across populations, it functions independently of the compromise between safety and efficiency of the hydraulic system with regard to water use at the interpopulation level.
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The urban environment has distinctive biophysical features in relation to surrounding rural areas. These include an altered energy exchange creating an urban heat island, and changes to hydrology such as increased surface runoff of rainwater. Such changes are, in part, a result of the
altered surface cover of the urban area. For example less vegetated surfaces lead to a decrease in evaporative cooling, whilst an increase in surface sealing results in increased surface runoff. Climate change will amplify these distinctive features. This paper explores the important role
that the green infrastructure, i.e. the greenspace network, of a city can play in adapting for climate change. It uses the conurbation of Greater Manchester as a case study site. The paper presents output from energy exchange and hydrological models showing surface temperature and surface
runoff in relation to the green infrastructure under current and future climate scenarios. The implications for an adaptation strategy to climate change in the urban environment are discussed.
Sensitivity to magnesium chloride (MgCl2) was assessed on five common roadside tree species by maintaining soil concentrations at 0-, 400-, 800-, or 1,600-ppm chloride
via MgCl2 solution over four growing seasons. Evaluations of growth, leaf retention, foliar damage, and ion concentrations were conducted.
Water potentials were measured on two species. Foliar chloride and magnesium concentrations were positively correlated with
foliar damage in all species. Conifers exhibited mild damage during the first growing season but moderate to severe damage
during the first winter and second growing season. The two highest MgCl2 treatments caused leaf loss, severe damage, or mortality of Douglas-fir, lodgepole, and ponderosa pines after two seasons
of treatments and of limber pine after four seasons. Aspen also displayed foliar damage and crown loss but abscised damaged
leaves and flushed asymptomatic leaves throughout the growing seasons. The highest treatment caused mortality of aspen in
4 years. Approximately 13,000–17,000-ppm foliar chloride was associated with severe damage in conifers but ranged from 13,000-
to 33,000-ppm in fully necrotic leaves. Aspen foliage contained the highest concentrations of chloride (24,000–36,000-ppm),
and limber pine leaves had the lowest (2 caused reductions in leaf water potential, aspen and ponderosa pine did not appear to be under substantial moisture stress
and continued to take up ions. Mortality of common roadside tree species in 2 to 4 years can occur due to high MgCl2 soil concentrations, and transportation officials should consider these implications in their management plans.
Urban trees occupy a wide variety of habitats, from a single specimen competing in the urban jungle to extensive remnant or planted forest stands. Each is shown to produce distinct micro- to local scale climates contributing to the larger urban climate mosaic. These effects are discussed in relation to the radiative, aerodynamic, thermal and moisture properties of trees that so clearly set them apart from other urban materials and surfaces in terms of their exchanges of heat, mass and momentum with the atmosphere. Their resulting ability to produce shade, coolness, shelter, moisture and air filtration makes them flexible tools for environmental design.
Urban trees can potentially mitigate environmental degradation accompanying rapid urbanisation via a range of tree benefits and services. But uncertainty exists about the extent of tree benefits and services because urban trees also impose costs (e.g. asthma) and may create hazards (e.g. windthrow). Few researchers have systematically assessed how urban tree benefits and costs vary across different cities, geographic scales and climates. This paper provides a quantitative review of 115 original urban tree studies, examining: (i) research locations, (ii) research methods, and (iii) assessment techniques for tree services and disservices. Researchers published findings in 33 journals from diverse disciplines including: forestry, land use planning, ecology, and economics. Research has been geographically concentrated (64% of studies were conducted in North America). Nearly all studies (91.3%) used quantitative research, and most studies (60%) employed natural science methods. Demonstrated tree benefits include: economic, social, health, visual and aesthetic benefits; identified ecosystem services include: carbon sequestration, air quality improvement, storm water attenuation, and energy conservation. Disservices include: maintenance costs, light attenuation, infrastructure damage and health problems, among others. Additional research is required to better inform public policy, including comparative assessment of tree services and disservices, and assessment of urban residents and land managers’ understanding of tree benefits and costs.
The temperature cooling effects of ten urban parks on surrounding environments in Guangzhou, southern China, are analysed and quantified using Landsat Thematic Mapper data. The results show that there is a temperature rise (about 1.74°C) between green spaces of parks and bare-ground areas of the surroundings. For those parks whose green area percentage is more than 69% and length:width ratio is close to 1, the average temperature differences between boundaries and surrounding sites of parks have linear relationships with the green areas of parks (R > 0.82). Moreover, the nonlinear relationship between the average cooling distance of parks and green areas can be simulated very well using a logarithmic curve (R > 0.93). When the green areas of parks are smaller than 10 566 m, parks will have no temperature cooling effects on their surrounding environments. When the green areas of parks reach 740 000 m, the increase of temperature cooling distance is less than 1 m per 10 000 m increase of the green area. The most appropriate size of green areas of urban parks should fall between 10 566 and 740 000 m. For those parks with water areas larger than 128 889 m, the temperature cooling effects are usually more remarkable. When the length:width ratios of the green areas of urban parks are more than or equal to 2, their temperature cooling distances are always larger than those with length:width ratios equal to 1 given similar green area. Parks with larger green areas (37 163 m) or larger water areas (>128 889 m) will have more significant temperature cooling effects in June than in October.
This study examines the effects of single trees and small clusters of trees on the bioclimate of a city. Investigations of the thermal environment and air quality of the urban climate were carried out on September 19 and September 29,2000, at Fahnenbergplatz, in the northem city center of Freiburg in southwest Germany. The study area, approximately 1,700 m2 , contains 12 horsechestnut trees (Aesculus hippocastanum) of different ages and sizes. The positive effect of trees on the thermal environment and air quality component was confirmed by the study. In particular, the mean radiation temperature Tmn , and the human biometeorological thermal index known as the physiological equivalent temperature (PET) showed distinct differences between areas with trees and areas without trees, despite the small size of the investigation area. A high reduction potential for nitrogen oxides and ozone was found inside the tree crowns, but outside the crowns there was no measurable reduction. The most important result regarding volatile organic compounds (VOCs) was the absence of terpene emissions [rom the horsechestnuts. Similarly, no isoprene emissions from horsechestnuts were found. Therefore, horsechestnut trees have a very small ozoneforming potential.
Microclimate formation and its significance in urban planning was examined through two components that predominate in affecting the city's climate: built-up morphology and urban shade trees.
The methodological approach focus is on a generalization procedure for quantifying the thermal effect of any studied situation through parameterization of the vegetated variables and the built-up forms. The analysis is integrative, using empirical climatic data followed by an analytical study for generalization and sensitivity analysis using an integrative model, the Green CTTC model.
Three urban tree species predominant in the Tel Aviv gardens and streets, with different canopy characteristics, and three levels of building densities were analysed to determine their thermal effect on an urban street microclimate. The variables were parameterized according to six basic cooling attributes for the studied tree species in urban gardens in Tel Aviv, and according to three geometric built-up parameters for the studied urban street.
The integrative modelling approach of considering all changes simultaneously was illustrated on an urban boulevard in Tel Aviv. The analysis demonstrates the shortcomings of piecemeal modelling and the merits of the integrative approach.
The study indicates the importance of urban trees in alleviating the heat island effect in a hot and humid summer. The tree cooling effect was found to be strongly related to the built form geometry. In all the studied cases, the thermal effect of the tree was found to depend mainly on its canopy coverage level and planting density in the urban street and little on other species characteristics.
The methodology of analysis presented in this paper can be applied to develop an operational tool in assessing for an urban open space the integrative thermal effects of different tree species, and of the varied urban morphology and the interaction between them. Copyright
Trees in cities have an important positive effect on people’s lives. One such positive effect is the amelioration of microclimate.
The aim of this research is to assess the correlation between parameters that affect the microclimate of parks during the
summer. We measured air temperature, relative humidity and solar radiation in the sun and shade of the trees in urban parks
in Thessaloniki, Greece. The results indicate that: the air (A) temperature reduction percentage (dTair%), air relative humidity
increase (dTRh%), discontent index reduction percentage (DI%) (cooling effect) and solar radiation (L) percentage that passes
through the trees’ foliage to their shade creates an exponential function of dA% = a.e-bL. These functions are also applicable
to the limiting variation values of the parameters. If we use L = 0 (meaning Lightsh = 0, which is the case for an extremely
dense tree), then the values that we expect from this particular parameter are the maximum possible. These maximum values
are a characteristic feature of the parameter variation for this particular research area. These maximum values for the trees
in the parks of Thessaloniki are: maxdTair% ≈ 24%, maxdRh% ≈ 41% and maxdDI% ≈16%.
The objective of this article, which is being published in two parts, is to investigate the effects of global climate change on a conurbation (the Ruhr area) and to discuss local counter-measures.
Part 1 deals with the origins of the natural and anthropogenic greenhouse effects, presents the contributions of the two effects to global warming and discusses recent examples of climate development from the Ruhr metropolis. The climate changes projected by various numeric and statistical models include an increase in thermal stress for city-dwellers and a rise in convective precipitation with corresponding run-off peaks in summer. As regard air pollutants (e.g. O3, PM10), it will mainly be the concentration of the near-surface trace gas ozone which will be clearly increased with rising temperatures. In contrast, a close correlation between atmospheric fine dust concentrations (≤ PM10) and temperature changes has not yet been established.
Part 2 of the publication indicates how global climate change can be effectively combated at the local level using a number of application-oriented examples. These include measures affecting both individual properties and whole areas with a view to reducing urban overheating and CO2 emissions.
This research analyses upward long-wave radiation flux density from urban surfaces using a high-resolution thermal-infrared (TIR) camera and meteorological measurements in the city of Berlin, Germany. We report spatio-temporal patterns of the difference between upward long-wave radiation flux density from courtyard surfaces and the roof. For temporal analysis, the TIR camera recorded one TIR image per minute over a period of two days from 3rd to 5th May 2007. Three-hourly averaged thermal patterns show persistence effects due to shadow, sky-view factor (SVF) distribution in the courtyard, thermal properties of the surface materials, human activities and turbulence characteristics of the surface–atmosphere interface. The history of shadow influences the 24-hourly mean pattern. Shadow caused by temporarily parked cars results in a lower upward long-wave radiation flux density compared to the non-shadowed surface close to it. Immediately after car departure, this difference decreases. We propose a method to derive the thermal admittance of a concrete surface based on TIR data from this attenuation process. This study shows that ground-based high-resolution TIR imagery is highly suitable to investigate surface thermal properties and dynamic processes controlling thermal patterns within a complex three-dimensional (3D) urban structure.
Urban Ecology is the study of ecosystems that include humans living in cities and urbanizing landscapes. It is an emerging, interdisciplinary field that aims to understand how human and ecological processes can coexist in human-dominated systems and help societies with their efforts to become more sustainable. It has deep roots in many disciplines including sociology, geography, urban planning, landscape architecture, engineering, economics, anthropology, climatology, public health, and ecology. Because of its interdisciplinary nature and unique focus on humans and natural systems, the term "urban ecology" has been used variously to describe the study of humans in cities, of nature in cities, and of the coupled relationships between humans and nature. Each of these research areas is contributing to our understanding of urban ecosystems and each must be understood to fully grasp the science of Urban Ecology. Therefore, in Urban Ecology: An International Perspective on the Interaction Between Humans and Nature, we introduce students and practitioners of urban ecology to its roots, bases, and prospects by way of a diverse collection of historical and modern foundational readings. The editors are urban ecologists from the United States, Italy, and Germany who together view these readings as a fair representation of the importance of both natural and social sciences to Urban Ecology. This book presents important papers in the field of Urban Ecology that both set the foundations for the discipline and to illustrate modern approaches, from a variety of perspectives and regions of the world. The editors do this by reprinting important publications, filling gaps in the published literature with a few targeted original works, and translating several key works originally published in German. The aim of this collection is to provide students, practitioners, and professionals with a rich background in some of the core facets of Urban Ecology. © 2008 Springer Science+Business Media, LLC. All rights reserved.
At urban street tree sites the specific conditions restrict growth, health and longevity for many species. Highly sealed urban sites in particular, with a high rate of surface run-off, low infiltration into soil and strong re-radiation effects can be characterized by a high stress level e.g. atmospheric drought. Maintaining the ecologic and economic benefits of street trees throughout future decades, it is essential to establish species with a high tolerance to stressful urban environments.
We measured leaf-gas exchange of Acer platanoides, Acer pseudoplatanus, Platanus x hispanica, Quercus rubra, and Tilia platyphyllos to assess stomatal conductance, transpiration, and net-photosynthesis at highly sealed urban sites in the city of Dresden, Germany.
The results show significant higher leaf-gas exchange rates for the species Platanus x hispanica and Quercus rubra, compared to the species Acer platanoides and Acer pseudoplatanus. The significant higher mean values of water-use efficiency (wue) of Platanus x hispanica and Quercus rubra, and in particular the values during the selected periods with a high vapor pressure deficit (VPD) indicate more economical water consumption.
The temperature difference between an urban space and surrounding non-urban space is called the urban heat island effect (UHI). Global terrestrial evapotranspiration (ET) can consume 1.4803×1023 joules (J) of energy annually, which is about 21.74% of the total available solar energy at the top of atmosphere, whereas annual human energy use is 4.935×1020 J, about 0.33% of annual ET energy consumption. Vegetation ET has great potential to reduce urban and global temperatures. Our literature review suggests that vegetation and urban agricultural ET can reduce urban temperatures by 0.5 to 4.0°C. Green roofs (including urban agriculture) and water bodies have also been shown to be effective ways of reducing urban temperatures. The cooling effects on the ambient temperature and the roof surface temperature can be 0.24-4.0°C and 0.8-60.0°C, respectively. The temperature of a water body (including urban aquaculture) can be lower than the temperature of the surrounding built environment by between 2 and 6°C, and a water body with a 16 m2 surface area can cool up to 2 826 m3 of nearby space by 1°C. Based on these findings, it can be concluded that the increase of evapotranspiration in cities, derived from vegetation, urban agriculture, and water body, can effectively mitigate the effect of urban heat islands.
The frequency of extreme weather has been rising in recent years. A 3-year study of street trees was undertaken in Tokyo to determine whether: (i) street trees suffer from severe water stress in unusually hot summer; (ii) species respond differently to such climatic fluctuations; and (iii) street trees are also affected by nitrogen (N) deficiency, photoinhibition and aerosol pollution. During the study period (2010-12), midsummers of 2010 and 2012 were unusually hot (2.4-2.8 °C higher maximum temperature than the long-term mean) and dry (6-56% precipitation of the mean). In all species, street trees exhibited substantially decreased photosynthetic rate in the extremely hot summer in 2012 compared with the average summer in 2011. However, because of a more conservative stomatal regulation (stomatal closure at higher leaf water potential) in the hot summer, apparent symptoms of hydraulic failure were not observed in street trees even in 2012. Compared with Prunus × yedoensis and Zelkova serrata, Ginkgo biloba, a gymnosperm, was high in stomatal conductance and midday leaf water potential even under street conditions in the unusually hot summer, suggesting that the species had higher drought resistance than the other species and was less susceptible to urban street conditions. This lower susceptibility might be ascribed to the combination of higher soil-to-leaf hydraulic conductance and more conservative water use. Aside from meteorological conditions, N deficiency affected street trees significantly, whereas photoinhibition and aerosol pollution had little effect. The internal CO2 and δ(13)C suggested that both water and N limited the net photosynthetic rate of street trees simultaneously, but water was more limiting. From these results, we concluded that the potential risk of hydraulic failure caused by climatic extremes could be low in urban street trees in temperate regions. However, the size of the safety margin might be different between species.
Trees in urban environments are exposed to heat stress, low air humidity and soil drought. The increasing temperatures and the more frequent heat and drought events will intensify the stress level of urban trees. We applied a dendrochronological approach to evaluate the species-specific suitability under increasing risk of drought of five tree species at highly sealed urban sites in the city of Dresden (Germany).
Climate-growth correlation analyses show that temperatures and water availability from April to July in the current year and in summer and autumn of the previous year are the main determining factors for radial growth. However, distinct species-specific differences were found in the response to temperature, precipitation and the self-calibrated Palmer Drought Severity Index (scPDSI). During the study period, the influence of temperature and drought on radial growth during summer months increases for A. platanoides and A. pseudoplatanus, whereas no changes occurred for Q. petraea, Q. rubra, and P. x hispanica. Pointer year analysis and superposed epoch analyses revealed a species-specific response to extreme climatic events. While for A. platanoides and A. pseudoplatanus a higher number of negative pointer years and significant growth declines in drought years were found, Q. petraea and Q. rubra showed more frequent positive pointer years but no significant growth reductions during drought. Based on these response patterns we classified the studied tree species according to their suitability and drought tolerance for urban sites.
This study focuses on the climate and growth response of the sessile oak (Quercus petraea (Matt.) Liebl.) and the northern red oak (Quercus rubra L.) using dendrochronological methods. Tree-ring series of street trees in the city of Dresden, Germany and in a nature reserve close to the urban area were analysed, and the climate and growth response and moving correlation functions (CF) were computed. Moreover, the impact of 13 years of drought was examined by superposed epoch analysis (SEA). Our results show that differences emerge from peculiarities of the different site conditions, indicating that city oaks contain a higher level of noise in their ring width data due to human activities that degrade the strength of growth-climate associations. Importantly, climatic conditions during the previous summer and autumn and during the current spring season display significant correlations with annual growth. The stability of the climate-growth relationship was tested via moving correlation functions. A strong increase in the correlation with the temperature and precipitation in April was found for the trees of northern red oak in forests. Drought years induced significant reductions in radial growth for the northern red oak only. Nevertheless, the long-term impact of drought years must be considered as marginal because of the fast recovery of the radial growth after one to two years. In the future, the growth of the northern red oak may be restricted by an increased frequency of droughts and warmer and drier conditions in April.
1 Conducting Units: Tracheids and Vessels.- 2 The Vessel Network in the Stem.- 3 The Cohesion-Tension Theory of Sap Ascent.- 4 Xylem Dysfunction: When Cohesion Breaks Down.- 5 Hydraulic Architecture of Woody Shoots.- 6 Hydraulic Architecture of Whole Plants and Plant Performance.- 7 Other Functional Adaptations.- 8 Failure and "Senescence" of Xylem Function.- 9 Pathology of the Xylem.- References.
The bioclimatic situation in cities will be a major issue for future planning authorities. Especially cities characterized by dense urban structures and high rates of impervious surface coverage will have to deal with this issue, since thermal stress is most likely to increase in intensity and frequency due to climate change (, and ). Thus, it is essential to obtain profound knowledge and appropriate data regarding the thermal characteristics of settlement areas, on the basis of which the bioclimatic situation is assessed. This study presents a methodology for identifying sensitive settlement areas with regard to the bioclimatic situation in the city of Dresden, and introduces the Settlement Heat Sensitivity Index (SHSI) as a measure for thermal sensitivity. The methodology described takes into account the thermal characteristics of urban structures as well as demographic parameters, and is adaptable to certain age cohorts which are of special interest for urban planning, due to their potentially increased susceptibility to health risks related to thermal stress. As a result, sensitivity maps are produced, displaying the bioclimatic variability within the city of Dresden.
A three-dimensional micro-scale model is used to study the effects of various greenery on temperature in a built-up environment. Green design elements like roofs and facades, lawns in courtyards and single trees are studied individually as well as in various combinations. Measures for comparison are temperatures at 2 m height and mean temperatures for the urban atmosphere up to the building height. Different types of greenery can reduce local temperatures up to 15 K during specific daytime hours. However, this extraordinary effect is restricted to a short time and especially to the direct surroundings, while an impact over larger distances is small. Roof and facade greenery have hardly any influence on temperature at the 2 m level but reduce daytime heating of the urban atmosphere to a minor degree, while the relevance of trees is more or less limited to the shadow effect. A significant decrease in urban temperatures can be achieved only with a large number of very different individual green elements. The largest effect on the urban atmosphere was simulated for a change in albedo resulting in a temperature decrease of some degrees around noon.
We used a two-layer canopy model to study transpiration of tree species as affected by energy-balance properties of a vegetated and paved surface. During several dawn-to-dusk studies, tree transpiration, stomatal conductance, leaf temperature (Tl), and several microclimate variables, were measured over turf and an asphalt surface. Cumulative transpiration was estimated from a leaf energy-balance equation applied to a tree crown apportioned between sunlit and shaded layers. Afternoon asphalt surface temperatures (Ts) were 20–25°C higher than turf Ts in all studies. Air-temperature differences between sites were minimal due to the size and proximity of the two surfaces that resulted in mixing of air. Trees over asphalt had consistently higher T, than those over turf, apparently due to interception of the greater upwards long-wave radiation flux from higher Tl. In one study flowering pear over asphalt in a humid environment bad higher Tl resulting in one-third more total water loss compared to trees over turf. In other studies, however, water loss of green ash and Norway maple over asphalt in an arid environment was either equal to or less than that over turf. Less water loss was due to higher Tl over asphalt causing prolonged stomatal closure. Model manipulation indicated that tree water loss over asphalt will depend on the degree of stomatal closure resulting from how interception of increased energy-fluxes and ambient humidity affect leaf-to-air vapor pressure differences.
Please cite this article as: Berry R, Livesley SJ, Aye L, Tree canopy shade impacts on solar irradiance received by building walls and their surface temperature, Building and Environment (2013), doi: 10.1016/ j.buildenv.2013.07.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Two hundred and fifty urban woody species are used in Central European parks and gardens. Based on reliable publications we attempt, for the first time, to extensively classify and assess them with regards to their usability after predicted climate changes. A new Climate-Species-Matrix has been developed for this purpose. In a two-dimensional assessment, four degrees of drought resistance and winter robustness are the decisive criteria. The woody species (divided into trees of more than 10m in potential height and trees/shrubs of up to 10m in height) were placed into 16 categories by decreasing tolerance, ranging from 1-1 (very suitable) to 4-4 (very limited usability). For many of these species, as well as many others that are not mentioned in this study, not all questions could be answered. Nonetheless, this categorization provides a sound basis for decisions in planning the use of woody species in cities. This planning process must also include additional criteria, dependent on individual requirements such as, soil parameters, shade resistance, aesthetics, etc. This paper is therefore meant to be a basis for discussion and supplementation for further research.
The objective of this study is to quantify the effects of trees in a Midwest urban area on air temperature and humidity to determine if the effects are significantly different for: different species of trees, trees of the same species in different environments, and whether the effects can be ex- plained by physical characteristics of the individual trees. Replicate trees in each of five categories were studied: sugar maple, pin oak and walnut individuals overgrass, sugar maple individuals along streets over concrete, and sugar maple clumps over grass. All the trees show a consistent effect: temperatures are reduced and humidities are elevated under the canopies. The greatest cooling effect (0.7 -1.3°C) occurs in the early afternoon. The difference between species is insignificant, but street trees are significantly less effective in reducing temperature than either individual trees or clumps planted over grass. The clumps had no greater effect than the individual trees. The amount of cooling observed in this study was considerably less than that documented in many previous studies. No consistent linear relationships were determined between physical characteristics of the trees, such as leaf area index, and temperature reductions or humidity increases.
The processes involved in the phase of mortality affecting Scots pine (Pinus sylvestris L.) observed since the beginning of this century in the canton of Valais (Switzerland) are related to the vegetation history, the ecology and stand dynamics as well as the pollution history.
We measured air temperature in an urban green area that includes forest and grassland and in the surrounding urban area for a full year in Nagoya, central Japan, to elucidate seasonal variations of the difference in air temperature between urban and green areas. We determined the range of the “cool-island” effect as well as the relationship between vegetation cover and air temperature throughout the year. The temperature difference between urban and green areas was large in summer and small in winter. The maximum air temperature difference was 1.9°C in July 2007, and the minimum was −0.3°C in March 2004. The difference was larger during the day than during the night in summer, whereas in winter the opposite relationship was true. However, winter diurnal variation was not particularly noticeable, a behaviour thought to be related to reduced shading by deciduous trees in the green area. During the night, the cooling effect of the green area reached 200–300 m into the urban area. During the day, the cooling effect between August and October 2006 exceeded 300m and varied widely, although there was no correlation beyond 500m. The correlation between air temperature and forest-cover ratio within a radius of 200m from each measurement site was significant from 16:00 to 19:00. There was also a correlation during the night; this correlation was weakest in the early morning. The effect of the forest-cover ratio on air temperature was most pronounced in August 2006 and June 2007.
The cooling effect of small urban green wooded sites of various geometric configurations in summer is the object of this study. It was studied experimentally at 11 different wooded sites in the Tel-Aviv urban complex during the period July–August 1996. An empirical model is developed in this study for predicting the cooling effect inside the wooded sites. The model is based on the statistical analysis carried out on 714 experimental observations gathered each hour from the 11 sites on calm days, when urban climate is expressed. Two factors were found to explain over 70% of the air temperature variance inside the studied green site, namely, the partial shaded area under the tree canopy and the air temperature of the non-wooded surroundings adjoining the site. The specific cooling effect of the site due to its geometry and tree characteristics, besides the shading, was found to be relatively small, about 0.5 K, out of an average cooling of about 3 K at noon. The cooling effect of the green wooded areas on their immediate surroundings at noon was also analyzed. The findings corroborate earlier studies that the range is noticeable. At small green sites, the cooling effect estimated in this study is perceivable up to about 100 m in the streets branching out from the site. The empirical findings in this study permit development of tools for incorporating the climatic effects of green areas in the urban design. Some policy measures are proposed accordingly, for alleviating the “heat island” effect in the urban environment.
The present study is carried out for dry hot climate places, where excessive solar heating is felt throughout the year. The effect of tree shadowing buildings is found to reduce heating loads; hence trees have a beneficial effect in energy economics. The emerging economic value of tree shadows in hot climate cities grants the development of an appropriate simulation numerical method to establish relative advantages on energy savings related to dwelling envelopes. The results demonstrate that large trees can provide up to 70% shade during spring and autumn, thus saving a very large amount of energy along the whole year. Hence, economic value of larger trees is greater than that of younger species.
‘Urban greening’ has been proposed as one approach to mitigate the human health consequences of increased temperatures resulting from climate change. We used systematic review methodology to evaluate available evidence on whether greening interventions, such as tree planting or the creation of parks or green roofs, affect the air temperature of an urban area. Most studies investigated the air temperature within parks and beneath trees and are broadly supportive that green sites can be cooler than non-green sites. Meta-analysis was used to synthesize data on the cooling effect of parks and results show that, on average, a park was 0.94°C cooler in the day. Studies on multiple parks suggest that larger parks and those with trees could be cooler during the day. However, evidence for the cooling effect of green space is mostly based on observational studies of small numbers of green sites. The impact of specific greening interventions on the wider urban area, and whether the effects are due to greening alone, has yet to be demonstrated. The current evidence base does not allow specific recommendations to be made on how best to incorporate greening into an urban area. Further empirical research is necessary in order to efficiently guide the design and planning of urban green space, and specifically to investigate the importance of the abundance, distribution and type of greening. Any urban greening programme implemented would need to be appropriately designed and monitored to continue to evaluate benefit to human health through reducing temperature.
The air temperature pattern in three urban parks and their surrounding built-up areas was studied over a one and a half year period in Göteborg, Sweden. The measurements were made at mobile and permanent stations on nights with clear skies and light winds. The maximum temperature difference found between a park and a built-up area was 5.9°C (summer), and the extension of the cool park climate into the built-up area was over 1100 m from the park border. Both the extension and the magnitude of the temperature difference depended on the size of the park and the distance from the park border. The green area cooled at a faster rate than the built-up area, although there were large variations within each area. At the rural station, located at an open site, the cooling was less than at the open part of the green area. Differences in sky obstruction between the sites explained some of the variations in cooling and temperature. However, the relationship between the urban-park temperature difference and sky view factor was not statistically significant. © 1998 Royal Meteorological Society
Rapid urbanization of the planet is occurring at an unprecedented pace, primarily in arid and semi-arid hot climates [Golden, J.S., 2004. The built environment induced urban heat island effect in rapidly urbanizing arid regions – a sustainable urban engineering complexity. Environ. Sci. J. Integr. Environ. Res. 1 (4), 321–349]. This growth has manifested itself as a cause of various impacts including elevated urban temperatures in comparison to rural sites known as the Urban Heat Island (UHI) effect [Oke, T.R., 1982. The energetic basis of the urban heat island. Q. J. R. Meteor. Soc. 108, 1–24]. Related are the increased demands for electric power as a result of population growth and increased need for mechanical cooling due to the UHI. In the United States, the Environmental Protection Agency has developed a three-prong approach of (1) cool pavements, (2) urban forestry and (3) cool roofs to mitigate the UHI. Researchers undertook an examination of micro scale benefits of the utilization of photovoltaic panels to reduce the thermal impacts to surface temperatures of pavements in comparison to urban forestry. The results of the research indicate that photovoltaic panels provide a greater thermal reduction benefit during the diurnal cycle in comparison to urban forestry while also providing the additional benefits of supporting peak energy demand, conserving water resources and utilizing a renewable energy source.