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London Plane trees (Platanus x acerifolia) before, during and after a heatwave: Losing leaves means less cooling benefit
Abstract
Platanus x acerifolia (London Plane) is a widely planted street tree throughout cities in temperate and Mediterranean climates. Heatwave intensity and frequency is likely to increase in these cities as a combined result of the urban heat island and climate change. High air temperature during heatwaves can lead to canopy leaf loss in vulnerable tree species, such as P. acerifolia. This rapid change in canopy cover may lead to subsequent reductions in ecosystem service benefits, such as shade cooling, human well-being, pollution interception and stormwater interception. This study investigates canopy leaf loss in two streets planted with P. acerifolia trees following a summer heatwave and the impacts upon street micrometeorological conditions and human thermal comfort. The plant area index of P. acerifolia, trees, and the micrometeorological conditions under and away from these trees was measured before and after a >43 °C heatwave in Melbourne, Australia. Physiological equivalent temperature was calculated from micrometeorological parameters to estimate human thermal comfort. Canopy loss was significant (30–50 %) and rapid and reduced thermal comfort benefits provided by canopy shade on two of the three warm summer days measured after the heatwave. However, the under-canopy areas of the street maintained a cooler micrometeorological conditions than areas in the open. This study suggests that as summer air temperature extremes and heatwave frequency increase in urban areas, more vulnerable urban tree species may experience sparse canopy cover throughout summer months reducing some ecosystem service benefits when they are needed most.
... Focusing on the US state of Indiana, Reynolds et al. [34] note that climate change is projected to change the species composition of its urban forests and will influence the services and disservices that they provide. Moreover, Sanusi and Livesley [35] established that, following a heatwave event in Melbourne, a 30-50% reduction in Plantanus x acerifolia leaf cover (a tree commonly planted in temperate and Mediterranean urban areas) was observed. Further, Dale and Frank [36] found that warming increases herbivorous insect pest fitness and abundance in urban forests, and that drought-stressed urban trees exhibit higher sugar and nitrogen concentrations that, in turn, make them more attractive to these pests. ...
... As the water content of the soil is affected throughout the year by incoming precipitation and outgoing evapotranspiration, a lack of precipitation can cause SWD. Following [35], after the end of the winter recharge (in December 2017). soils are at field capacity at the start of April 2018 (within the immediate suction zone of 5 kPa), and therefore adequate water is available for the grass to extract. ...
... Existing studies highlight how urban GI can be adapted to current extreme weather events, with particular attention paid to heat waves, e.g., [29,35]. Looking beyond the present day, adapting urban GI to extreme weather and climate change has received relatively limited attention. ...
The existing body of research into the environmental and socio-economic benefits of green infrastructure supports the case for it to be positioned as a form of critical infrastructure, particularly in urban settings. It is broadly recognized that extreme weather and climate change pose significant risks to critical infrastructure systems linked to the provision of services, including electricity, water, communications, and transport, and consequently risk assessments and associated adaptation strategies are common practice. However, although green infrastructure is also at risk from extreme weather and climate change, threatening the realization of benefits that it can deliver in urban settings, associated risks to green infrastructure are not widely understood or assessed in practice. This paper discusses the status of existing research on this topic and uses this as a foundation for a Greater Manchester (UK) case study that assesses the risk of low water availability to grassed areas, which represent a key element of the city-region’s green infrastructure. In doing so, the paper demonstrates how risks linked to extreme weather and climate change can be assessed spatially to inform green infrastructure planning. In summary, this paper aims to raise awareness of extreme weather and climate change risk to urban green infrastructure, present an empirical case study and associated methodological approach on this topic, and ultimately support efforts to enhance the resilience of urban green infrastructure to extreme weather and climate change.
... Typical regional forest species, such as Pinus sylvestris, Picea abies, or Fagus sylvatica, also showed low imperviousness in their surroundings due to their growing locations being mostly outside of the city center. Platanus × acerifolia, a typical street tree in temperate and Mediterranean cities [73], also showed very high imperviousness in its surroundings. On the tree pixel level, the linear model with T diff as the dependent variable and mean imperviousness within a 91 m window was highly significant (p < 0.001), with an R 2 of 0.16, indicating the climatic influence of sealed surfaces. ...
... Platanus × acerifolia, a typical urban tree species, also showed a strong change in the relative position to cooler temperatures compared to unadjusted T diff due to its characteristic urban location with high rates of impervious surfaces. Studies showed that this species provides cooling benefits even after heat and drought stress damage and temperatures above 43 • C, although these extremes reduce its cooling potential severely [73]. This species was also among the coolest species in other studies [18], but only in a park environment and not in urban regions as presented here. ...
The cooling potential of tree species in Central European cities is insufficiently studied during extreme heat and drought, although a stronger surge in heatwaves compared to the global average is observed in this region. Remote sensing-based thermal surveys are an important tool to shed light on the mitigation effects of green infrastructure, but approaches covering extreme events are scarce. In this study, we present a simple, low-cost thermal airborne methodology that covers the current daily heat record in 2022, after the second warmest and third driest spring-to-summer period since 1949, in the medium-sized German city of Forchheim. We found that in spite of record-breaking heat and drought conditions, trees still had a considerable cooling potential with surface temperatures of 2 °C to 6 °C below air temperatures. Tree species were characterized by substantial median differences in tree surface temperatures up to 3.64 °C. Conifers and drought-sensitive broadleaf species showed the highest temperatures during the extreme event, while riparian species with potentially good water provision showed the highest cooling potential. In addition to tree species, imperviousness and tree NDVI were important variables for urban tree surface temperature, showing positive (imperviousness) and negative (NDVI) correlations with tree surface temperatures. Our study provides a methodological remote sensing example for the spontaneous and rapid coverage of extreme events, documenting the benefits of tree species in the urban context.
... Trees also provide further benefits. Especially on warm summer days, the temperature below the canopy remains substantially lower than above exposed concrete surroundings, as the trees provide shading and a further cooling effect due to transpiration (Pace et al. 2020;Sanusi and Livesley 2020). Trees in urban areas also store significant amounts Communicated by Locosselli. of carbon-especially (old) trees in parks and cemeteries contribute to carbon storage (Kändler et al. 2011;Strohbach and Haase 2012;Richter et al. 2020). ...
... However, trees can only provide the services mentioned above if they are vital. For instance, the cooling effect of roadside Platanus × hispanica trees diminished during a heat wave, and as the trees shed up to 50 % of their leaves in response to this period of high temperatures and low water availability, the positive effect on the microclimate was reduced for several weeks, even though overall the conditions remained cooler compared to open areas (Sanusi and Livesley 2020). For seedlings of Tilia cordata subjected to a drought-stress experiment, a model simulation showed substantially lower CO 2 fixation, evapotranspiration and cooling than for control trees ). ...
Key Message
In an urban area, resistance and resilience of stem diameter growth differ substantially between tree species. Traffic emissions are reflected in wood nitrogen isotopes, but do not affect drought tolerance.
Abstract
With increasing drought and heat, the benefits of urban trees such as shading and cooling become more important. Yet, it is necessary to identify tree species able to withstand such extreme climatic conditions. We studied the resistance and resilience of stem diameter growth of five deciduous tree species in an urban area in Southwest Germany to three exceptional drought periods (2003, 2011 and 2015) for differences between and within species, especially considering the intensity of traffic emissions (NO x ). Analyses of the stable isotopic composition of carbon (δ ¹³ C) and oxygen (δ ¹⁸ O) as well as the intrinsic water-use efficiency (WUE i ) in the tree rings were carried out. Further, we investigated the stable isotopic composition of nitrogen in the wood (δ ¹⁵ N) to assess its potential as an indicator of NO x emissions from traffic. Stem diameter growth in all species was strongly limited by low water availability in spring, as was also reflected in elevated δ ¹³ C and δ ¹⁸ O values in Acer platanoides and Tilia cordata , which were particularly sensitive to drought. In contrast, growth of Platanus × hispanica and Quercus robur was less affected by drought, and resistance of Carpinus betulus ranged in between. Across species, δ ¹⁵ N was higher in trees located closer to roads and exposed to higher NO x traffic emissions. Unexpectedly, these conditions did not significantly affect drought resistance/resilience. Our study demonstrates the potential and interpretative challenges of coupled dendroecological and isotope analyses. It also indicates clear species-specific differences in drought tolerance and thus helps to identify suitable urban tree species.
... For instance, tree species with high LAI significantly enhance human thermal comfort by providing comfortable thermal conditions beneath their canopies (Gillner et al., 2015). However, under suboptimal growth conditions and drought-induced stress frequently observed in urban areas with high imperviousness, trees may face challenges in developing denser crowns , and might even resort to leaf shedding as an adaptive measure to tolerate heat stress (Sanusi and Livesley, 2020). Such an adaptive response, while helping trees endure harsh conditions, can also diminish the cooling benefits that were initially anticipated. ...
Highlights
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Tree growth and cooling provisioning affected by impervious surfaces, primarily due to low soil moisture.
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Tree species showed differing responses to imperviousness.
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Increased imperviousness is linked to higher PET under trees.
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Leaf Area Index is the most influencing tree trait for human thermal comfort.
... Serious droughts can occur in Melbourne and the last one has lasted for more than a decade from 1997 to 2009 (Grant et al., 2013). Droughts can lead to widespread decline in vegetation health (May et al., 2013) and their ecosystem benefits including their cooling effects in Melbourne (Sanusi and Livesley, 2020). Hence, irrigating urban green spaces in Melbourne is important to sustain the health and evapotranspirative cooling of its vegetation. ...
High temperatures in summer can prevent people from using urban green spaces. Irrigating urban green spaces is a promising strategy to reduce temperatures. In this study, we aimed to a) identify the proportional contribution of different irrigation cooling mechanisms and b) quantify the impacts of different irrigation amounts (from 2 to 30 mm d − 1) on the cooling effect of irrigating turfgrass in Melbourne, Australia. We first used a field experiment in Melbourne to provide empirical data to calibrate and verify the performance of an urban ecohydrological model, UT&C. Then, we used UT&C to predict the impacts of irrigating turfgrass on evapotranspiration, the energy balance and microclimate. UT&C predicted that irrigating turfgrass 4 mm d − 1 would increase the evaporation from grass canopy and soil surface by 0.2 and 0.6 mm d − 1 , respectively, whereas it would reduce transpiration by 0.6 mm d − 1 due to intercepted water covering part of the grass canopy following the irrigation. UT&C predicted that daytime (10:00-16:59) mean air temperature reductions would increase from 0.2 to 0.4 • C when the irrigation amount increased from 2 to 4 mm d − 1. However, increasing the irrigation amount beyond 4 mm d − 1 would not increase the cooling benefits.
... However, in an urban context, a deep root system will likely be constrained by the built environment (Day et al., 2010), potentially reducing transpiration-related cooling under hotter and drier conditions (Fig. 3). Water spenders may experience heavy leaf drop and thinning of canopy density in an attempt to survive periods of drought and heat in an urban environment (Sanusi and Livesley, 2020). Without active or passive irrigation using runoff from impervious surfaces (Thom et al., 2020), water spenders may not maintain good canopy quality through an entire summer and, as a result, may cool less than expected through both transpiration and shade. ...
Urban tree canopy cover is a promising solution for mitigating heat island. • Data-driven guidance on tree selection and planting locations is still limited. • Four research priorities are proposed, requiring a collaborative research effort. • Cross-climate morphological and physiological characteristics are desired. • Integration with atmospheric boundary layer models is suggested.
... Heat waves have been shown to reduce leaf photosynthetic rates (Duarte et al., 2016) and harm photosystem II (PSII; Guha et al., 2018). This can culminate in major effects such as extensive leaf loss (Sanusi & Livesley, 2020), dieback of trees (Marchin, Esperon-Rodriguez, et al., 2022) and tree mortality (Chaste et al., 2019), which influence urban greening and microclimate. Therefore, there is an urgent need to understand physiological and ecological tolerance of urban trees under such extreme conditions. ...
The increasing frequency and intensity of heat waves caused significant damages to urban woody species, and the different leaf structures between evergreen and deciduous species may be closely related to leaf heat tolerance. However, whether the different leaf structural traits of evergreen and deciduous plants contribute to their different responses under heat waves is still unclear.
During the record‐breaking and long‐lasting 2022 summer heat waves in China, we investigated the relationships between leaf thermal indices and leaf structural traits of 36 urban woody species in four cities along the Yangtze River.
We found that all the four thermal indices were significantly but weakly related with leaf damage status. The critical temperature that causes the initial 15% damage to photosystem II (Tcrit) may serve as a sensitive measure of heat tolerance. Evergreen species suffered less leaf damage during the heat waves and exhibited higher leaf heat tolerance, thicker leaves than deciduous species. Tcrit was significantly correlated with leaf mass per area, leaf thickness and thickness of spongy tissue.
Synthesis . Urban woody species with higher Tcrit, leaf mass per area and leaf thickness tend to be more tolerant to heat stress. This study provides insights for predicting leaf heat tolerance of urban woody plants in subtropical China and their physiological and ecological responses to severe heat waves.
Read the free Plain Language Summary for this article on the Journal blog.
... Urban green spaces have represented an important component of the landscape for centuries, and are claimed to provide a number of environmental and social services. These include pollution absorption, atmospheric cooling, reduction of building energy-use, habitat provision and enhancement of human mental and physical health and well-being (Van Leeuwen et al., 2010;Roy et al., 2012;Dadvand et al., 2015;Sanusi and Livesley, 2020). All these benefits for cities, and the constant population migration from rural areas to cities, underline the importance of increasing urban green spaces such as parks, gardens and street trees in the future (Ramaiah and Avtar, 2019). ...
Field surveys conducted in two urban green spaces in Athens (Greece) revealed the presence of Phytophthora-
related diseases on Platanus orientalis and P. x acerifolia trees. Declining trees showed a range of disease symp-
toms including root rot, stem bleeding cankers and extensive canopy dieback. Since there is little information
about the etiology of these diseases, a study was conducted from 2016 to 2021 to define the main pathogens
involved. A total of 71 Phytophthora isolates were obtained from bark tissues and rhizosphere samples collected
from symptomatic plane trees. Based on morphological traits and DNA sequence data, Phytophthora isolates were
identified as Ph. mediterranea (11 isolates) and Ph. nicotianae (60). Over a five-year period 41% of P. orientalis
trees associated with Phytophthora infections died and were removed from the park. Pathogenicity tests
confirmed that all Phytophthora species including Ph. cinnamomi used for comparison, are pathogenic on both
P. orientalis and P. x acerifolia trees. Phytophthora cinnamomi was shown to be the most aggressive species on both
P. orientalis and P. x acerifolia. Phytophthora nicotianae was the dominant species, whereas Ph. mediterranea was
associated with a few trees. Our finding has contributed to expanding knowledge on the host and geographic
range of Ph. mediterranea, an invasive pathogen with a high potential for diffusion in Mediterranean regions and
highlights the importance of enhancing biosecurity measures to prevent and limit the spread of invasive path-
ogens in urban and natural ecosystems.
... Especially, recent investigations have yielded contradictory conclusions concerning the cooling effect of urban vegetation during extreme heat. While some studies suggest an increase in the cooling effect (De Kauwe et al., 2019;Ibsen et al., 2021;Shiflett et al., 2017), others imply a decreasing trend with rising temperatures (Sanusi and Livesley, 2020;Wei et al., 2023). This uncertainty in the dynamic patterns of the urban vegetation cooling effect under extreme heat may arise from variations in measurement methods, climatic backgrounds, and species of urban vegetation (Gilabert et al., 2021;Li et al., 2021;Wang et al., 2019;Zhao et al., 2023). ...
Previous studies on the cooling of urban vegetation mainly focused on its transpiration or shading effect separately, neglecting to explore the combined evapotranspiration-shading cooling. Further, accurate quantification of evapotranspiration-shading cooling remains challenging due to heterogeneity of urban landscapes, which limits understanding of its high-resolution spatiotemporal patterns. Here, we integrate high-precision remote sensing data and the Soil-Canopy-Observations of Photosynthesis and Energy Fluxes (SCOPE) model to propose an optimised quantitative approach. The approach was used to investigate changes in evapotranspiration-shade cooling during extreme heat.Taking Shanghai metropolitan as case, the results show: (1) The cooling capacity of urban vegetation in nighttime (18:00 - 6:00) is enhanced during extreme heat, which is attributed to accumulated effect of shading and enhanced evapotranspiration due to elevated vapor-pressure deficit. (2) In densely built-up areas with limited vegetation, there is a significant lack of thermal regulation, especially in the early morning (7:00) and late evening (17:00), thus exacerbating thermal stress. (3) At midday (11:00-13:00) there was a slight decrease in evaporative cooling, probably caused by the behaviour of the stomatal closure at high temperatures. Concurrently, high radiation causes the shading effect of vegetation to become more prominent, amplifying the cooling contrast between areas with dense and sparse vegetation cover. Moreover, the study also highlights that grassland with more than 50% cover can provide cooling effects similar to that of forest land. Overall, our study not only enhances the understanding of urban vegetation's cooling effects but also underscores the importance of strategic urban vegetation planning in mitigating urban heat, particularly under the escalating frequency and intensity of heat waves.
... For instance, tree species with high LAI significantly enhance human thermal comfort by providing comfortable thermal conditions beneath their canopies (Gillner et al., 2015). However, under suboptimal growth conditions and drought-induced stress frequently observed in urban areas with high imperviousness, trees may face challenges in developing denser crowns , and might even resort to leaf shedding as an adaptive measure to tolerate heat stress (Sanusi and Livesley, 2020). Such an adaptive response, while helping trees endure harsh conditions, can also diminish the cooling benefits that were initially anticipated. ...
... London plane (Platanus × acerifolia) is one of the leading tree species in European cities capable to grow across a range of climate zones (Henry and Flood 1919;Browicz 1964;Mimet et al. 2009). It has been described to be resilient to urban pollutants, infestations, and severe drought (Dineva 2004;Ivanová et al. 2007;Pourkhabbaz et al. 2010;Gillner et al. 2015;Yang et al. 2015;Adamska 2019), and mitigates heat stress through transpiration-induced cooling, which is particularly effective during periods of high atmospheric evaporative demand (Bowden and Bauerle 2008;Ennos 2012;Lindén et al. 2015;Sanusi and Livesley 2020). Transpiration-induced cooling even remains operational during nighttime when stable nocturnal boundary layers typically limit air mixing (Lindén et al. 2016). ...
Background
Bark exfoliation is a common feature of London planes ( Platanus × acerifolia ) that reportedly increases during periods of drought-induced stem shrinkage. Here, we explore the spatial patterns and potential drivers of plane bark exfoliation in Mainz, a central European city of 220,000 inhabitants, following the exceptional summer drought of 2018.
Methods
We estimate the degree of bark exfoliation of 349 urban plane trees across the city and use stem microcores to analyze their tree-ring widths from 2006 to 2019. Further to impervious cover, settlement structure and vegetation cover in the vicinity of each tree, we investigate the relationships between bark exfoliation and tree, site, and climate factors.
Results
Results indicate that plane bark exfoliation correlates significantly with tree size and inner bark width (both p < 0.001) but is independent of impervious cover and local site conditions. Similarly, stem growth does not change within the city underlining the resilience of London planes to cope with highly diverse urban site conditions. Plane tree-ring widths were only weakly associated with exfoliation estimates ( p < 0.05) but strongly controlled by cold season temperatures ( p < 0.001).
Conclusion
As tree growth was also not affected by summer drought, potential detrimental effects by limited infiltration, increased runoff and altered evaporation are of less concern for the plane trees in Mainz. Projected winter warming is likely to enhance urban plane growth in upcoming decades.
... For irrigation alone, expertise from at least three diverse fields -social science, ecology, and behavioural studies -is thus required to develop a technically appropriate management strategy for nature-based cooling. Likewise, in empirical research in Melbourne, Australia, Sanusi and Livesley (2020) find a tree species selected for its hardiness and ability to survive in harsh urban environments -the London Plane -loses its leaves quickly in heatwave conditions. Sanusi and Livesley hence hold that a research evidence base spanning tree species selection, soil water availability, irrigation and street orientation among others is important if trees are to deliver cooling to residents. ...
Increased extreme heat events draw attention to the potential of urban nature as a heat adaptation strategy for cities. This is reflected in multiple scientific perspective pieces, policy documents and science media publications advocating for urban greening as a cooling approach. Although attention to the dangers of heat and the benefits of urban nature is welcomed, it is vital that nature-based approaches to cooling are underpinned by diverse knowledge and a sound understanding of what nature in cities can and cannot do. We explain why an evidence-driven and cautious approach to heat adaptation through urban greening is so important, and propose three actions that urban actors can take towards effective and equitable long-term cooling through urban nature: enabling dialogue between different sectors with multiple remits; including diverse knowledge systems in planning and governance processes; and investing in long-term stewardship for the climatological and societal conditions of the coming decades.
Policy and practice recommendation
• Create fora for dialogue between governments, residents, civil society and developers from planning stage for green cooling;
• Cooling through nature must be driven by expertise spanning diverse knowledge systems, combined with local knowledge and community needs;
• Consider future climates and stewardship when planning urban cooling via nature.
Science highlights
• Understanding link between urban thermal environment and nature is an inter- and transdisciplinary task;
• Critical need for evidence of how greening reduces heat impacts across different social and cultural contexts;
• Evidence of how species perform under future climates required for stewardship of urban nature.
... For example, a connection between cyclones/anticyclones and heatwaves was found in Melbourne (Australia), and reinforcing effects were found as a result of the pressure dipole 13 . Heatwaves and their response to vegetation changes were studied in London (U.K.) and central France where heatwave intensity was exacerbated due to vegetation losses 14,15 . Heatwave formation and duration are complex in the western U.S., especially over coastal and mountain areas where heatwave characteristics are strongly affected by marine meteorology and mountainous terrain 16,17 . ...
Extreme heat events are occurring more frequently and with greater intensity due to climate change. They result in increased heat stress to populations causing human health impacts and heat-related deaths. The urban environment can also exacerbate heat stress because of man-made materials and increased population density. Here we investigate the extreme heatwaves in the western U.S. during the summer of 2021. We show the atmospheric scale interactions and spatiotemporal dynamics that contribute to increased temperatures across the region for both urban and rural environments. In 2021, daytime maximum temperatures during heat events in eight major cities were 10–20 °C higher than the 10-year average maximum temperature. We discuss the temperature impacts associated with processes across scales: climate or long-term change, the El Niño–Southern Oscillation, synoptic high-pressure systems, mesoscale ocean/lake breezes, and urban climate (i.e., urban heat islands). Our findings demonstrate the importance of scale interactions impacting extreme heat and the need for holistic approaches in heat mitigation strategies.
... Moreover, A. hippocastanum poorly tolerates pruning operations and, therefore, it may be recommended in parks rather than in tree-lined avenues. Platanus x acerifolia could be used as street tree in cities but extreme attention must be paid to the widespread and devasting fungal disease Ceratocystis platani (Walter) Engelbrecht & Harrington as well as summer heatwaves (Sanusi and Livesley, 2020) inducing significantly canopy loss and consequently lower air pollutant removal efficiency. ...
... In the literature, there are many ways to define a heatwave or high-temperature event in the literature. Some researchers have used the thresholds of 30~47 °C of mean or maximum air temperature or LST(Buo et al., 2021;Guirguis et al., 2014;Lowe et al., 2011;Ossola et al., 2021;Raei et al., 2018;Sanusi & Livesley, 2020;Song et al., 2019;Williams et al., 2012). Here, a day with LST exceeding 40.6 °C (105 ℉) was defined as a high LST day and it represents an excessive heat warning threshold, which is harmful to both human and vegetation health.Please note that there are some differences between the remotely sensed LST and air temperature, and we discuss the uncertainties and limitations of this approach in the Discussion section. ...
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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.
... O 3 is also very evenly sequestered by trees. Platanus x acerifolia and Populus nigra take the lead in O 3 sequestration in both cities when considering the most frequent species (Tables 3 and 4) [54]. ...
In order to maximize ecosystem services (ES), a proper planning of urban green areas is needed. In this study, the urban greenery of two Italian cities (Milan and Bologna) exposed to high levels of atmospheric pollutants was examined. Vegetation maps were developed through a supervised classification algorithm, trained over remote sensing images, integrated by local trees inventory, and used as input for the AIRTREE multi-layer canopy model. In both cities, a large presence of deciduous broadleaves was found, which showed a higher capacity to sequestrate CO2 (3,953,280 g m2 y−1), O3 (5677.76 g m2 y−1), and NO2 (2358.30 g m2 y−1) when compared to evergreen needle leaves that, on the other hand, showed higher performances in particulate matter removal (14,711.29 g m2 y−1 and 1964.91 g m2 y−1 for PM10 and PM2,5, respectively). We identified tree species with the highest carbon uptake capacity with values up to 1025.47 g CO2 m2 y−1 for Celtis australis, Platanus x acerifolia, Ulmus pumila, and Quercus rubra. In light of forthcoming and unprecedent policy measures to plant millions of trees in the urban areas, our study highlights the importance of developing an integrated approach that combines modelling and satellite data to link air quality and the functionality of green plants as key elements in improving the delivery of ES in cities.
... Among meteorological factors of forest settings, temperature of an understory atmosphere is perceived by humans as a factor to incur emotional changes [32,33]. For example, the cooling effect under a canopy is usually perceived as a comfort feeling against the hot weather [34]. Rainfall is closely correlated with the temperature in forests subjected to warm climates [35]. ...
Degraded forests still retain a high dose of nature that may evoke positive sentiments of visitors. This function reminds policy makers to reevaluate the development of degraded forests by using their non-material services. Forest visitors have a general habit to take facial photos and share with internet friends. This results in the formation of a dataset that comprises scores of posted sentiments towards visitors’ experiences in degraded forests. People post facial photos with emotions exposed to a subjective extent which can be impacted by perceived experiences with jointly landscape and microclimate, but their combined effects have not been well demonstrated on a large geographical scale. In this study, a total of 30 degraded forests were selected from suburban areas of 22 cities in Southeast China. There were 2751 facial photos of forest visitors that were collected and screened from a database of social network platform of China, namely Sina Weibo. Happy and sad expressional scores were rated by the FireFACE software, and their spatiotemporal distributions were mapped. Both horizontal and vertical planes of objective forest landscapes were remotely evaluated by projected area and vegetation height, respectively. Microclimatic conditions were characterized by meteorological records on the same days when photos were posted. Exposed happiness was distributed as a geographical gradient from lower scores in the northeast region of study area to higher scores in the southwest. WV, TreeH, and elevation generated positive contributions to regressed happy score. However, combined low WV and high SunH benefitted the regression of higher sad scores. Our results revealed that people would like to pose more smiles in degraded forests with unhindered wind flows under tall and dense canopies located on highlands. Furthermore, policy makers could consider developing degraded forests as a type of infrastructure that can trigger the promotion of users’ mental well-being, instead of focusing only on negative consequences following ecological degradation.
... This indicates that most studies only relatively discussed this in general without actually estimating the impact on urban residents and the research was primarily based on a biophysical modelling approach. It has been known that many studies look at the importance of vegetation especially trees to provide ecosystem services such as mitigation of urban climate and improving human health and well-being especially for the reduction of heat-related illnesses (Chianucci et al., 2015;de Abreu-Harbich et al., 2015;Sanusi & Livesley, 2020). Therefore, without physically estimating the impact at the micro level, less evidence can be provided in estimating the human thermal comfort benefits when applying Blue-Green Infrastructure in the urban environment. ...
Forest in Indonesia is among one of the richest biodiversity in the world. However, this forest is currently under threat due to several factors such as illegal logging, forest fire, and forest conversion. Considering the ecological and economic importance of this forest, the local government is committed to its protection and sustainable use under the umbrella of “Green Regency” and has been working with several organizations to develop long-term landscape planning. This research focuses on Sintang, West Kalimantan, that has a vast tropical forest with high biodiversity of flora and fauna. In 2016, the Sintang Regency has declared its commitment as a sustainable regency and committed to protecting the forest within the framework of the Sustainable Development Goals (SDG). The research aims are to analyze land use and land cover (LULC) changes from 2006 to 2016 and forecast the land use and land cover on business as usual (BAU) and green scenario. The analysis used Land Change Modeler (LCM) module in TerrSet software to project the land cover. Based on the analysis of land cover change from 2006 to 2016, the secondary forest area experienced the largest decrease (−87,680 ha), while the plantation area had the largest increase (87,540 ha). The BAU scenario projected that plantations would grow to 253,571 ha in 2030 while only 180,300 ha under the Sintang Lestari scenario (green scenario). The results of the Sintang Lestari scenario showed that significant plantation development can still be achieved while protecting the remaining forest cover. However, limiting the expansion of plantation areas is only possible if current spatial plans are monitored and non-compliance with spatial planning is strictly enforced. The findings of the analysis can serve as a strong basis for future land development in Sintang Regency.
... This indicates that most studies only relatively discussed this in general without actually estimating the impact on urban residents and the research was primarily based on a biophysical modelling approach. It has been known that many studies look at the importance of vegetation especially trees to provide ecosystem services such as mitigation of urban climate and improving human health and well-being especially for the reduction of heat-related illnesses (Chianucci et al., 2015;de Abreu-Harbich et al., 2015;Sanusi & Livesley, 2020). Therefore, without physically estimating the impact at the micro level, less evidence can be provided in estimating the human thermal comfort benefits when applying Blue-Green Infrastructure in the urban environment. ...
Ecosystem services are processes in which the environment supplies benefits to humans and are classified into four major categories, including provisioning services, supporting services, regulating services, and cultural services. The increasing number of invasive pests and pathogens entering Malaysia signifies that the threat to forests is escalating along with climate change and globalization. In recent years, massive epidemics of forest diseases have devastated natural ecosystems and landscapes valued for timber and extended benefits to the community. Thus, an impeccable balance is required between food production to fulfill the global survival and maintenance of the other services supplied by the ecosystem services. This section reviews and discusses how the forest pathology system influences ecosystem services, specifically its effects on trees and forest diseases.
... Species composition is a key characteristic of street tree assemblages (Chimaimba et al. 2020;Zhao et al. 2020), and many studies have focused on one or two ecosystem services when selecting street tree species (Sanusi and Livesley 2020;Sgrigna et al. 2020;Zhang et al. 2020aZhang et al. , 2020b. We did not consider the species composition of the street tree assemblages in our study because a definitive variation trend along the urbansuburban continuum was not found (not showed in the script). ...
Urban afforestation is an important strategy for promoting sustainable urban development. In cities where large new green spaces are not available, the planting of curbside trees is deemed to be an important afforestation strategy. However, variations in the ecosystem services provided by street tree assemblages across socioeconomic gradients have been unexplored. We examined such variations in ecosystem services provided by street tree assemblages along an urban–suburban continuum. Our findings were as follows. (i) Not all ecosystem services showed increasing trends along the urban–suburban continuum. Some ecosystem services at the street tree assemblage level, such as air purification and rainfall interception were prominent in areas of high urbanization intensity. (ii) Diverse ecosystem service trends were found in relation to differential characteristics of street trees assemblages. Structural properties of street tree assemblages, such as tree density and age structure, are likely key factors influencing variations. (iii) Although street tree density could partially compensate for the loss of large old trees, the protection of such trees is important because of their close associations with key ecosystem services, such as total carbon storage. To maximize the value of street trees in promoting urban sustainable development, trade-offs among multiple ecosystem services should be integrated within the overall planning process and adjustments of planting regimes.
... Australia is experiencing more extreme weather than ever before, with record-breaking temperatures frequently reported over recent years (BM, 2018;BOM and CSIRO, 2020;Páscoa et al., 2022). As climate changes and temperatures continue to rise, the frequency and magnitude of heatwaves are predicted to increase (IPCC, 2021), with the potential for large, negative effects on urban forests (Esperon-Rodriguez et al., 2022a;Khan and Conway, 2020;Sanusi and Livesley, 2020). A large proportion of the tree species planted in Australian cities already experience climate conditions that exceed their physiological tolerance and the climate risk posed by warmer and drier conditions will be exacerbated in the future (Esperon-Rodriguez et al., 2022a;Esperon-Rodriguez et al., 2019;Zhang and Brack, 2021). ...
Urban tree inventories are useful tools to assess the environmental and socio-economic services provided by urban forests. These inventories enable the evaluation of the climate change risk to urban forests, and governments rely on such inventories for urban planning and management. Here, we assessed the future climate risk of Australia and the state of urban tree inventories across 116 local government areas (LGAs), representing 21% of the country’s LGAs and encompassing 55% of the national human population. We evaluated projected changes in temperature and precipitation by 2050 for each LGA and conducted a survey to obtain information on the extent and types of data available in existing urban tree inventories. Additionally, we compiled demographic, socio-economic, and geographical data for all LGAs to explore correlates with tree inventory status. Temperature increases in 2050 were predicted in all LGAs, with higher latitude and smaller LGAs identified to undergo greater increases in temperature compared to larger and lower latitude LGAs. Decreases in seasonal precipitation were predicted for 97 LGAs. Seventy-six (66%) of surveyed LGAs had urban tree inventories, which most commonly included trees along streets and in parks. Sixty-one LGAs record information on tree mortality, while 31 LGAs dynamically update their inventories. The presence of an inventory and the area it covered were positively associated with human population density. More than 30 years ago, in 1988, John Gray wrote that “insufficient statistics were available in Australia to provide an accurate picture of the urban forest estate”. Our research shows there has not been a significant advance in the adoption and use of urban forest inventories over the past three decades. Long-term, dynamically updated inventories are crucial for urban forest management to inform planting choices to support sustainable and resilient cities.
... Platanus acerifolia trees, as street trees are widely distributed in urban areas of cities globally. 29,30 The motivations of our study are to assess the role of leaves in the Platanus acerifolia tree for monitoring the pollution status of submicron soot particles and associated source contributions. Therefore, we developed a method to quantify the submicron soot particles on leaves of the Platanus acerifolia tree and measured the levels of submicron soot particles across four seasons at two sites in China. ...
Submicron soot particles (<1.0 μm in aerodynamic diameter) are responsible for global warming and health burdens worldwide. However, studies on bio-monitoring of submicron soot particles and their associated sources by using tree leaves are not comprehensively illustrated. Here, we determined the seasonal trends of submicron soot particles on the leaves of the Platanus acerifolia collected from two cities (Lu’an, Anhui Province, and Nanjing, Jiangsu Province) in the Yangtze River Delta region, China. The source apportionment of submicron soot particles was performed using stable carbon isotopic analyses. Significant seasonal trends of submicron soot particles were observed in two cities with averaged levels of 0.41-1.36 mg m-2 in cold seasons and averaged levels of 0.13-0.24 mg m-2 in warm seasons. The levels of δ13C for submicron soot at the suburban site of Lu’an city were observed to be in the range of -25.6‰ to -18.2 ‰ with fossil fuels dominated (~58%) in summer and -23.0‰ to -15.6 ‰ with biomass burning dominated in winter (~67%). In comparison, the ranges in the levels of δ13C in submicron soot were found to be from -26.5‰ to -20.4 ‰ in winter, and -24.2‰ to -17.9 ‰ in summer at the urban site of Nanjing. Fossil fuels accounted for a large fraction of submicron soot with average contributions of 53% in winter and 73% in summer, respectively. These findings demonstrate that Platanus acerifolia trees could be used as an effective and low-cost bio-monitoring tool for monitoring the pollution status of submicron soot and associated source contribution.
... Tree shade remains critically important for improving outdoor thermal comfort, particularly for trees with a high plant area index (de Abreu-Harbich et al., 2015). Urban trees may lose up to half of their leaves during heatwaves, thus undermining their cooling effects (Sanusi and Livesley, 2020). The foliage loss is likely due to high air temperature and aggravated by low soil moisture content (Tyree et al., 1993). ...
Backyards play important roles for individual households because they provide a private and safe green space for social and environmental interactions, relaxation, gardening and children's activities. The use of backyards is highly dependent on their thermal conditions. Turf is a common surface type in backyards but unirrigated turf can be as warm as pavement, bringing thermal discomfort and discouraging people from using them. Under certain conditions, turf irrigation provides an opportunity to reduce thermal stress by increasing evapotranspiration. This study aims to measure the impacts of turf irrigation on microclimate in a backyard environment in the warm season in Melbourne, Australia. The experiment consisted of four 6 m × 6 m turf-covered plots. Daily irrigation was applied at four amounts: 0, 2, 4 and 7 mm for six weeks. In Week 6, the 4-mm irrigation reduced daytime soil temperature, turf surface temperature, air temperature and universal thermal climate index by 1.7, 2.3, 0.6 and 0.4 °C, respectively. All daytime impacts were significant (p < 0.05, t-test). Irrigation has the potential to significantly improve the thermal conditions of backyards in combination with the use of tree shade.
... Broadleaf evergreen tree/shrub species with large, thin leaves are particularly vulnerable to extreme heat (Marchin et al., 2022). For example, Callery Pear (Pyrus calleryana) and London Plane Tree (Platanus x acerifolia) are widely planted in temperate cities but are vulnerable to leaf loss after extremely hot days (Sanusi and Livesley, 2020;Tabassum et al., 2021). Species with high leaf succulence or small, narrow, or divided leaves are more likely to survive extreme levels of thermal stress (Leuzinger et al., 2010;Leigh et al., 2017) and these traits are also related with drought resistance. ...
Modern cities are dominated by impervious surfaces that absorb, store and release heat in summer, create large volumes of runoff and provide limited biodiversity habitat and poor air quality can also be a health issue. Future climate change, including more frequent and extreme weather events will likely exacerbate these issues. Green infrastructure such as parks, gardens, street trees and engineered technologies such as green roofs and walls, facades and raingardens can help mitigate these problems. This relies on selecting plants that can persist in urban environments and improve stormwater retention, cooling, biodiversity and air pollution. However, plant selection for green infrastructure is challenging where there is limited information on species tolerance to heat and water variability or how these species can deliver multiple benefits. Therefore, we draw on research to illustrate how plant performance for green infrastructure can be inferred from plant attributes (i.e., traits) or from analysis of their natural distribution. We present a new framework for plant selection for green infrastructure and use a case study to demonstrate how this approach has been used to select trees and shrubs for Australian cities. We have shown through the case study and examples how plant traits and species’ natural distribution can be used to overcome the lack of information on tolerance to both individual and multiple stressors and how species contribute to the provision of benefits such as stormwater retention, cooling, biodiversity and air pollution mitigation. We also discuss how planting design and species diversity can contribute to achieving multiple benefits to make the most of contested space in dense cities and to also reduce the risk of failure in urban greening.
... Substantial declines in urban tree survival are predicted, however, due to climate change Esperon-Rodriguez et al., 2019). Extreme heat events are already causing extensive leaf loss in urban canopies (Sanusi and Livesley, 2020;Tabassum et al., 2021a), so urban practitioners need to prioritise selection of tolerant species to continue providing cooling benefits on hot summer days. ...
Cities have been described as ‘heat islands’ and ‘dry islands’ due to hotter, drier air in urban areas, relative to the surrounding landscape. As climate change intensifies, the health of urban trees will be increasingly impacted. Here, we posed the question: Is it possible to predict urban tree species mortality using (1) species climate envelopes and (2) plant functional traits? To answer these, we tracked patterns of crown dieback and recovery for 23 common urban tree and shrub species in Sydney, Australia during the record-breaking austral 2019–2020 summer. We identified 10 heat-tolerant species including five native and five exotic species, which represent climate-resilient options for urban plantings that are likely to continue to thrive for decades. Thirteen species were considered vulnerable to adverse conditions due to their mortality, poor health leading to tree removal, and/or extensive crown dieback. Crown dieback increased with increasing precipitation of the driest month of species climate of origin, suggesting that species from dry climates may be better suited for urban forests in future climates. We effectively grouped species according to their drought strategy (i.e., tolerance versus avoidance) using a simple trait-based framework that was directly linked with species mortality. The seven most climate-vulnerable species used a drought-avoidance strategy, having low wood density and high turgor loss points along with large, thin leaves with low heat tolerance. Overall, plant functional traits were better than species climate envelopes at explaining crown dieback. Recovery after stress required two mild, wet years for most species, resulting in prolonged loss of cooling benefits as well as economic losses due to replacement of dead/damaged trees. Hotter, longer, and more frequent heatwaves will require selection of more climate-resilient species in urban forests, and our results suggest that future research should focus on plant thermal traits to improve prediction models and species selection.
... Platanus spp. are widely planted as ornamental trees throughout cities in Mediterranean climates [23] with allergy risk generated by its pollen [24], due to the distribution of ornamental urban plane trees in the region [25] and the meteorological conditions. Hence, these trees are studied in urban green-space management plans to decrease the allergenic load, and thus, reduce the potential exposure to allergenic pollen and improve air quality in these spaces [26]. ...
Digitisation and building information modelling (BIM) technology are being adopted in the construction industry, but their application in the sustainable development of urban planning and civil engineering has been limited. A methodology was developed to generate 3D urban maps, which represent a potential environmental exposure related to urban green infrastructure. The methodology incorporated 3D geometric data obtained by Laser Imaging Detection and Ranging (LiDAR), location and information of green infrastructure, and meteorological data (direction and velocity of wind) into a single 3D BIM model to estimate the potential levels of exposure to urban green infrastructure by applying a modified aerobiological index to create risk maps for ornamental trees (AIROT). The BIM model facilitated a 3D visualisation of the exposure in urban environments by exporting information to GIS software, which could then be visualised on Google Earth. The main advance of the developed methodology is its applicability to any urban environment for determining and reducing aerobiological health risks in civil engineering.
... Leaf temperature is an important parameter; a decrease in g S can be associated with increases in leaf temperatures (Lu et al., 1994) and leaf temperatures can exceed air temperature by several degrees (Miller et al., 2021) leading to leaf damage (Hueve et al., 2011). Thus, identifying resistant species to high temperatures in water-limited environments is beneficial in urban environments because canopy loss can have negative environmental and social impacts (e.g., decrease in cooling benefits) (Sanusi and Livesley, 2020). ...
Urban forests play an important role in regulating urban climate while providing multiple environmental services. These forests, however, are threatened by changes in climate, as plants are exposed not only to global climate change but also to urban climate, having an impact on physiological functions. Here, we selected two physiological variables (stomatal conductance and leaf water potential) and four environmental variables (air temperature, photosynthetically active radiation, vapor pressure deficit, and water availability) to compare and evaluate the ecophysiological vulnerability to climate change of 15 dominant tree species from Mexico City’s urban forest. The stomatal conductance response was evaluated using the boundary-line analysis, which allowed us to compare the stomatal response to changes in the environment among species. Our results showed differential species responses to the environmental variables and identified Buddleja cordata and Populus deltoides as the least and most vulnerable species, respectively. Air temperatures above 33°C and vapor pressure deficit above 3.5 kPa limited the stomatal function of all species. Stomatal conductance was more sensitive to changes in leaf water potential, followed by vapor pressure deficit, indicating that water is a key factor for tree species performance in Mexico City’s urban forest. Our findings can help to optimize species selection considering future climate change by identifying vulnerable and resilient species.
... This result is advantageous for urban environments, as canopy loss during heatwaves can have negative environmental and social impacts (e.g. decrease in cooling benefits) (Sanusi and Livesley, 2020). Nonetheless, we acknowledge that irrigation requires strategic planning because it is often limited to an establishment period (Roman et al., 2014), is expensive, and is not feasible in locations where water shortages are increasing (Pauleit et al., 2002). ...
Extreme climate conditions, including more frequent and prolonged heatwaves, are increasing in many regions throughout the world. Urban trees can aid in mitigating the adverse impacts of climate extremes; however, their capacity to do so is limited by species differences in ability to maintain function and retain leaves to provide shade under extreme temperatures. To assess tree vulnerability to heatwaves, we used a common garden experiment in Western Sydney, Australia, which included four widely-planted tree species; two native evergreen and two exotic deciduous species. Data were collected during 2019 and 2020, the two hottest summers on local record with maximum air temperatures reaching 45°C. We monitored plant physiological status as stomatal conductance (gS) and midday leaf water potential (Ψmid). We determined heat tolerance by measuring the leaf critical temperature (Tcrit) for photosynthesis and leaf turgor loss point (πtlp) as thresholds for loss of function and calculated the thermal safety margin. Plant performance was assessed by measuring tree growth and leaf damage after heatwaves. Species responded dynamically with gS decreasing as air temperature and vapor pressure deficit increased during heatwaves. Exotic species had higher gS, Tcrit and πtlp than native species. Leaf damage under heatwave conditions was overall lower in native species. Our results highlight the impact of heatwaves on urban trees and the value of physiological metrics to inform tree species selection to create more resilient urban environments.
... In this way, forest interior temperatures are moderated across seasons , and temperature differences inside and outside forests are higher when macroclimatic conditions become more extreme . Structural characteristics that determine a within stand's microclimate include the predominant tree species (De Abreu-Harbich et al., 2015), tree vitality (Sanusi & Livesley, 2020), biomass volume (Norris et al., 2012) as well as elevation and canopy cover (Ma et al., 2010). ...
Forest management influences a variety of ecosystem structures and processes relevant to meso‐ and microclimatic regulation, but little research has been done on how forest management can mitigate the negative effects of climate change on forest ecosystems.
We studied the temperature regulation capacity during the two Central European extreme summers in 2018 and 2019 in Scots pine plantations and European beech forests with different management‐related structural characteristics.
We found that the maximum temperature was higher when more trees were cut and canopy was more open. Logging 100 trees per hectare increased maximum temperature by 0.21–0.34 K at ground level and by 0.09–0.17 K in 1.3 m above ground. Opening the forest canopy by 10% significantly increased Tmax, measured 1.3 m above ground by 0.46 K (including pine and beech stands) and 0.35 K (only pine stands). At ground level, Tmax increased by 0.53 K for the model including pine and beech stands and by 0.41 K in pure pine stands. Relative temperature cooling capacity decreased with increasing wood harvest activities, with below average values in 2018 (and 2019) when more than 656 (and 867) trees per hectare were felled. In the pine forests studied, the relative temperature buffering capacity 1.3 m above ground was lower than average values for all sample plots when canopy cover was below 82%. In both study years, mean maximum temperature measured at ground level and in 1.3 m was highest in a pine‐dominated sample plots with relatively low stand volume (177 m³ ha⁻¹) and 9 K lower in a sample plot with relatively high stock volumes of Fagus sylvatica (>565 m³ ha⁻¹). During the hottest day in 2019, the difference in temperature peaks was more than 13 K for pine‐dominated sample plots with relatively dense (72%) and low (46%) canopy cover.
Structural forest characteristics influenced by forest management significantly affect microclimatic conditions and therefore ecosystem vulnerability to climate change. We advocate keeping the canopy as dense as possible (at least 80%) by maintaining sufficient overgrowth and by supporting deciduous trees that provide effective shade.
... This may cause leaf temperatures to rise to critical levels, resulting in leaf death and defoliation (Allen et al., 2010;O'Sullivan et al., 2017, but see Drake et al., 2017. The resulting reduction in canopy cover is likely to reduce the cooling benefit provided by urban trees as well as decrease visual appeal of the urban forest (Sanusi and Livesley, 2020). Furthermore, the stress caused by experiencing recurrent drought and heatwaves can ultimately lead to increased vulnerability to pest and pathogen attack in trees and accelerated mortality (Bréda et al., 2006;Jactel et al., 2011). ...
Extreme heatwaves and drought have been shown to significantly affect urban tree survival, with potentially substantial economic costs for urban managers and local governments. During the 2019-2020 austral summer, the western Sydney Local Government Area (LGA) of Penrith experienced unprecedented high temperatures with less than 60% of average rainfall compared with the proceeding five summers. This culminated in the highest temperature ever recorded in greater Sydney, of 48.9 °C. It is increasingly important that trees for urban applications are selected to be able to withstand such conditions. In early 2020, we conducted a visual assessment of canopy damage on street trees found in the Penrith LGA following the 2019-2020 summer heatwaves. We assessed the health of over 5,500 trees and classified them as undamaged, lightly damaged, heavily damaged or defoliated. We found that more than 10% of all the trees assessed displayed some level of canopy damage, with exotic deciduous species showing the greatest proportion of canopy damage. A logistic regression revealed that for exotic deciduous species, the probability of having sustained no canopy damage was 79% lower than that for native evergreen species. Using these data, the economic costs to replace damaged trees was calculated using two scenarios that incorporated costs of tree planting and maintenance: low cost (replacing all heavily damaged and defoliated trees with juvenile trees) and high cost (replacing all heavily damaged and defoliated trees with advanced trees), with costs ranging from $500,000 to $800,000 (AUD). We also calculated the cost of replacing all individuals of the most damaged species with more climate-resilient species in order to secure the urban forest and found that the cost would be over AUD$1,000,000. This research highlights the importance of careful planning to ensure urban forest resilience and economic sustainability in the face of climate change.
... Nevertheless, irrigation of urban green spaces has been recognised as an important factor in mitigating urban heat stress and promoting the park cool island effect(Bowler et al., 2010; Crum & Jenerette, 2017;Ziter et al., 2019). Furthermore, since urban trees can experience significant leaf loss during a heatwave or drought(Sanusi & Livesley, 2020), leading to a reduced shading and cooling effect (Rahman et al., 2020), irrigating urban green space may create a synergistic cooling benefit by maintaining tree canopy cover and grass cover whilst promoting evapotranspiration cooling benefits of both. ...
Modern agricultural irrigation can produce extensive cooling that is strong enough to mask the current effect of global climate change. Irrigating urban green spaces therefore has the potential to mitigate heat stress in cities. However, the cooling potentials of irrigating urban green space in different climate regions of the world have never been estimated. Here we conducted a systematic literature review to determine air temperature reductions in past experimental, observational and modelling studies (N = 17). We developed an empirical model with the irrigation cooling effect as the dependent variable and background air temperature and rainfall of the study area as the independent variables. The model was subsequently used to estimate the cooling potential of irrigating green spaces in 100 global cities with arid, temperate and continental climates. We predict that 91 of the 100 cities will receive a cooling benefit from irrigating urban green space (mean =–1.09 °C), whereas the remaining nine cities will experience a slight warming effect (mean = +0.76 °C). The cooling potential of irrigating urban green space is greatest in arid cities (mean =–1.65 °C).
[Open access, please access full article at above DOI] Trait-based frameworks have received increased attention in biodiversity research for their capacity to streamline complex information for policy and management purposes. Such frameworks, however, are mostly developed for natural ecosystems, and social-ecological dynamics in urban ecosystems may present needs that are distinct from natural ecosystems. We proposed a framework for developing a social-ecological traits database for plant selection and spontaneous vegetation management using three major categories of social-ecologically relevant considerations: 1) Sustenance and maintenance effects; 2) Ecosystem service/disservice effects; and 3) Conservation and ecological-meaning effects. We further examined the potential of using the Flora of Taiwan and vascular plants of Taipei city parks as a case study for using Floras for database development, and compared the results with two global trait databases, TRY and GIFT. Issues in Flora data include: 1) systematic omission of horticultural species, 2) low species trait coverage, and 3) complexity in original data. TRY included more measurement-based traits imperative to functional interpretations; Flora had wider global coverage than TRY. GIFT was based on Floras, but included only functional traits and omitted traits of social importance identified in our framework. Information uniquely available in the Flora include information of local relevance (e.g. associated local landscapes and commonness/rarity within the region) and social aspects of ecosystem services (e.g., edibility and visual aesthetics). Maintenance considerations are lacking in all three databases. We suggest interdisciplinary collaboration to further advance the proposed social-ecological traits framework and database to enhance the functions and sustainability of urban green infrastructures.
The frequency and intensity of droughts combined with heatwave events have increased under climate change, increasing destruction in urban areas and leading to severe impacts on urban plants. These impacts remain poorly understood at the species level. Here, we investigate the effects of a drought–heatwave event on 12 urban woody species using in situ records of the dynamic changes in physiological traits in the field before, during and after the drought–heatwave event to assess resistance and resilience to hot drought. We found the following: (1) Hydraulic and photosynthesis traits showed an instantaneous decline during the hot drought event in the 12 species, with severe drought-induced xylem embolism in hydraulic systems and a high percentage loss of hydraulic conductivity (PLC). (2) The two conifer species were less resistant to hot droughts than broadleaves but capable of post-stress recovery, suggesting that conifers showed better resilience and that broadleaves showed better resistance under hot drought stress. (3) The evergreen species showed strong resistance, while three deciduous species showed strong resilience to hot drought stress. (4) The three shrubs may be more vulnerable to hot droughts than trees, as they showed lower resistance and were not able to recover the current year’s growth.
The co-occurrence of more frequent, intense and prolonged heat and drought events can lead to severe socioeconomic impacts. The integration of different types of urban green infrastructures (GIs; e.g., urban parks and building greens), and urban and building design measures (DMs; e.g., a network of cool urban spaces and passive cooling), at various scales, can help reduce outdoor air temperature, energy use and heat stress. Numerous accessible or emerging GI and DM solutions exist, highlighting the need for guidelines that evaluate their suitability in enhancing resilience to extreme heat. Those should consider broader aspects such as geographical conditions , urban morphology and development, social-economic background, governance and sustainability, as well as more specific considerations like maintenance, adaptability and soil and water resources. To facilitate decision-making, this study proposes guidelines based on a narrative literature review, discussing the potential impact of GIs and DMs and their role in enhancing the city's climate resilience. K E Y W O R D S climate change adaptation, design measures, heat-related events, nature-based solutions, sustainable development, urban drought, urban green infrastructures
Impervious surfaces create large volumes of stormwater which degrades receiving waterways. Incorporating trees into biofilters can increase evapotranspiration and therefore reduce stormwater runoff. Tree species with i) high water use, ii) drought tolerance and iii) rapid and full recovery after drought have been suggested for biofilters to maximise runoff reduction while minimising drought stress. Moisture availability fluctuates greatly in biofilter substrates and trees growing in biofilters will likely experience multiple, extended drought events that increase trade-offs between these traits. Providing an internal water storage has the potential to reduce tree drought stress and increase evapotranspiration. Two urban tree species (Agonis flexuosa and Callistemon viminalis) were grown in plastic drums with biofilter profiles. Three irrigation treatments were used: well-watered, drought with an internal water storage and drought without an internal water storage. Transpiration, leaf water potential and biomass were measured to determine the effect of biofilter internal water storage and repeated drought events on tree water use, drought stress and growth. Biofilter internal water storage improved water use and reduced drought stress for A. flexuosa, whereas C. viminalis reduced leaf loss but saw no change in water use or drought stress. A. flexuosa with biofilter internal water storage was able to recover transpiration to well-watered levels after repeated droughts, while C. viminalis experienced reduced recovery ability. It is recommended all biofilters planted with trees should have internal water storage. In systems with lower moisture availability a species with more stomatal control, such as A. flexuosa, is recommended. If selecting a species with less stomatal control, such as C. viminalis, the internal water storage volume needs to be increased to avoid drought stress.
In cities, urban heat island (UHI) effect is one of the most distinct climate issues where it is characterized by warmer conditions in city centres than the surrounding rural areas. This may influence the urban residents, especially on the health hazard issues such as thermal stress. Blue-Green Infrastructure is a nature-based solution that can improve the microclimate conditions of urban environments and improve human thermal comfort. However, the nature, imbalances and gaps of research on Blue-Green Infrastructure should be reviewed to see the extent of research on how its utilization can be linked to microclimate and human thermal comfort. Therefore, this review addresses this issue based on the previous literature. From 2018 to 2021, there was a gradual increase in the Blue-Green Infrastructure research. More research focused on Green Infrastructure followed by the combination of Blue-Green Infrastructure; however, limited studies were on Blue Infrastructure. Most studies used biophysical modelling followed by fieldwork, and only 36% studied human thermal comfort at micro level. This suggests that more studies should be done in the field to link the contribution of Blue-Green Infrastructure to urban microclimate and consequently its effect on thermal stress of urban residents. The review is concluded by highlighting aspects of Blue-Green Infrastructure research that can be further studied to improve future urban planning efforts for urban cooling.
In this study, the micromorphology and surface details of Pseudocercospora pseudostigmina‐platani conidia were investigated on American sycamore (Platanus occidentalis) leaves. Sooty films were collected from diseased leaves for fungal DNA extraction and observed using light and field‐emission scanning electron microscopy. Two types of conidia were present on the leaves: (i) hyaline and curved Cercostigmina‐like conidia and (ii) brown and straight Stigmina‐like conidia. The fungal pathogen was identified as P. pseudostigmina‐platani based on morphological characteristics and internal transcribed spacer region sequences. Some non‐glandular trichomes were found near the centre of the conidial masses on the abaxial surface of the naturally infected leaves. Leaf ultrasonication to dislodge conidia revealed sporodochia erupting through the stomata. Hyphae from sporodochia grew on the leaf surface and entered host tissues through the stomata. Conidiogenous cells had terminal rings and tapered towards the flat region where a conidium was attached. The conidia produced secondary conidia directly without hyphae. After dislodging the conidia by ultrasonication, scars were observed. These results indicate that the fungus has dimorphic conidia and microcycle conidiation for simple and rapid asexual reproduction.
Introduction
Sustainable cities ensure social inclusiveness for vulnerable communities through environmental solutions. Easy access to public facilities for all is a step to reduce inequalities and is linked to Goals 10 and 11 out of the Sustainable Development Goals. The main purpose of the study was to verify whether street trees in footways and footpaths affect walking.
Methods
Data was obtained from a Computer Assisted Web Interview (CAWI) conducted in January–February 2022 among 212 for people with a limited range of mobility (the users of walking frames, walkers, rollators, crutches, and manual or electric wheelchairs).
Results
The results indicate that although street trees may cause sidewalk tiles to bulge, aggravate allergies, or present elevated risk in the case of falling branches, they are ultimately a desirable element of the streetscape. People are ready to experience some inconveniences (sidewalk tiles with small holes) just to allow better habitat for street trees. Moreover, trees planted along the footways/footpaths stimulate physical activity and people prefer having big trees in streetscapes.
Conclusions
The results of the study suggest a need for in-situ research of the impact that small holes in sidewalk tiles (size, pattern, location in the tile) have on the vibrations experienced by users while getting around. Studies on the correlation between the perception of pedestrian circulation comfort and the speed of walking on the tiles with holes should be continued.
Evapotranspiration is an important cooling mechanism in urban green space (UGS). Irrigating vegetated surfaces with potable water, collected stormwater or recycled sewage water has the potential to increase the cooling effect of UGS by increasing evapotranspiration. Such cooling effect may not always be strong because evapotranspiration is dependent on local and regional factors such as background climate, seasonality and vegetation type. When using irrigation for cooling, city managers also need to consider management issues such as irrigation water supply and amenity use of the UGS. This study aims to develop a theoretical framework that explains the physical and energetic mechanisms of irrigation cooling effect and a framework to assist city managers to make decision about the use of irrigation for urban cooling. This is achieved by reviewing the impacts of irrigation on local climate reported in the literature and identifying the regional and local factors that influence irrigation cooling effect in warm seasons. The literature suggests that irrigation can potentially reduce daily maximum air temperature and ground surface temperature by approximately 2.5 and 4.9°C, respectively, depending on weather conditions and irrigation amount. Background climate is an important factor that influences the cooling potentials of irrigation. Cities with dry and warm climates have the highest cooling potentials from irrigation. The cooling potentials are also influenced by seasonality and weather, vegetation type, irrigation time of day and irrigation amount. Cities with a dry and warm season can consider using irrigation to mitigate urban heat within UGS because such climatic conditions can increase cooling potentials. To maximise irrigation cooling effect, cities with abundant irrigation water supply can use a soil moisture-controlled irrigation regime while those with limited supply can use a temperature-controlled regime. More studies are required to understand the cooling potentials of irrigating small, individual UGS.
As an important part of urban greening, the canopy of street trees has ecological benefits, such as oxygen production, noise reduction, and dust reduction. The living vegetation volume (LVV) can reflect the spatial structure of the canopy intuitively and enables the estimation of the ecological service value of street trees. Terrestrial laser scanning (TLS) has shown excellent performance for providing three-dimensional data of individual trees with high precision, enabling the accurate quantification of the LVV.
In this study, we divided the LVV into the total living vegetation volume (tLVV) and the effective living vegetation volume (eLVV); the latter does not include branches. The eLVV of 40 ginkgo trees separated in two roads in Nanjing was calculated from TLS data. A novel method named LAIM for accurate eLVV calculation based on point cloud data was proposed. The point cloud data of individual tree was segmented along the Z-axis and image processing methods were used. With this, eLVV of each tree was obtained. The results were compared with data obtained from a clustered point cloud generated using convex hulls. The Bland-Altman analysis was used to investigate the consistency of the two methods. Furthermore, we used correlation analysis and all-subsets regression to choose the variables, and the eLVV was fitted using six models. Finally, we evaluated O2 production, CO2 and SO2 absorption by the street trees based on eLVV, the ecological benefits of street trees were quantified.
The results showed the following: (1) The number of layers and the dilation size of the point cloud were crucial parameters in the LAIM. (2) For ginkgo trees, the mean difference between the eLVV obtained from the LAIM and the convex hull method was -0.53 to 0.19 m³, indicating that the results were highly consistent for the two methods. (3) The eLVV fitting performance was better for the exponential function model (R²=0.8523, RMSE=0.6838 m³) and linear model (R²=0.8361, RMSE=0.7224 m³). The tree height and crown width significantly affected the eLVV estimation. (4) The evaluation about ecological benefits of Zhaoyang Road was better than Cuizhu Road. The quantified ecological benefits were conducive to road ecological evaluation.
This study quantified the eLVV of individual trees using TLS, highlighting the importance of live vegetation in urban greening. The results can provide technical support for estimating the ecological service value of urban street trees.
Rainfall in cities can generate large volumes of stormwater runoff which degrades receiving waterways. Irrigating trees with runoff (passive irrigation) has the potential to increase transpiration and contribute to stormwater management by reducing runoff received by downstream waterways, but the stochastic nature of rainfall may expose trees with high transpiration to drought stress. We hypothesized that for success in passive irrigation systems, tree species should exhibit i) high maximum transpiration rates under well-watered conditions, ii) drought avoidance between rainfall events, and iii) high recovery of transpiration with rainfall following a drought. We assessed 13 commonly planted urban species in Melbourne, Australia against three metrics representing these behaviours (crop factor, hydroscape area, and transpiration recovery, respectively) in a glasshouse experiment. To aid species selection, we also investigated the relationships between these three metrics and commonly measured plant traits, including leaf turgor loss point, wood density, and sapwood to leaf area ratio (Huber value). Only one species (Tristaniopsis laurina) exhibited a combination of high crop factor (>1.1 mm mm⁻¹ d⁻¹) indicating high transpiration, small hydroscape area (<3 MPa²) indicating drought avoidance, and high transpiration recovery (>85%) following water deficit. Hence, of the species measured, it had the greatest potential to reduce runoff from passive irrigation systems while avoiding drought stress. Nevertheless, several other species showed moderate transpiration, hydroscape areas and transpiration recovery, indicating a balanced strategy likely suitable for passive irrigation systems. Huber values were negatively related to crop factor and transpiration recovery and may therefore be a useful tool to aid species selection. We propose that selecting tree species with high transpiration rates that can avoid drought and recover well could greatly reduce stormwater runoff, while supporting broader environmental benefits such as urban cooling in cities.
This book describes the detailed process behind the development of a comprehensive thermo-bio-architectural framework (the ThBA). This framework systematically connects the thermal performance requirements of a building to relevant solutions found in the natural world. This is the first time that architecture has been connected to biology in this manner. The book provides an in-depth understanding of thermoregulatory strategies in animals and plants and links these to equivalent solutions in architectural design. The inclusion of this fundamental knowledge, along with the systematic process of accessing it, should open up new avenues for the generation of energy efficient and sustainable buildings.
An anomalous event of drought and heat occurred in central Italy during the summer of 2017. Based on the SPI (Standardized Precipitation Index) and data from the European Space Agency, this event started in November 2016 and was characterized by a strong reduction of precipitation and soil moisture, especially in lowland areas with Mediterranean climate. The aim of this case report were to describe the impact of this event on representative forest communities in central Italy, to analyze the different responses of deciduous and evergreen tree and shrub species in contrasting environmental conditions and to assess their subsequent capacity of recovery or, if not, mortality. Trees suffered severe impacts consisting of widespread crown defoliation, leaf desiccation, crown dieback and whole tree mortality. Deciduous tree species (Fagus sylvatica, Quercus pubescens, Quercus cerris) shed their leaves during the summer, but apical buds and twigs were preserved. This allowed these species to produce new shoots in the following year (2018) and to restore the canopy closure of the stands. Mediterranean evergreen broadleaves, such as Quercus ilex and Phillyrea latifolia suffered of total or partial crown desiccation with wilting leaves and branch dieback. These species partially resprouted in 2018 from axillary and latent buds. The case presented here is discussed within the wider context of the impacts of climate change on Mediterranean forests. Future research directions should include an effective forest monitoring system that combines terrestrial and remote sensing surveys, ad hoc field climate change experiments and silvicultural trials from the perspective of proactive management for the adaptation of forests to future climatic conditions.
Background:
Observed increases in the frequency and intensity of heatwave events, together with the projected acceleration of these events worldwide, has led to a rapid expansion in research on the health impacts of extreme heat.
Objective:
To examine how research on heatwaves and their health-related impact is distributed globally.
Methods:
A systematic review was undertaken. Four online databases were searched for articles examining links between specific historical heatwave events and their impact on mortality or morbidity. The locations of these events were mapped at a global scale, and compared to other known characteristics that influence heat-related illness and death.
Results:
When examining the location of heatwave and health impact research worldwide, studies were concentrated on mid-latitude, high-income countries of low- to medium-population density. Regions projected to experience the most extreme heatwaves in the future were not represented. Furthermore, the majority of studies examined mortality as a key indicator of population-wide impact, rather than the more sensitive indicator of morbidity.
Conclusion:
While global heatwave and health impact research is prolific in some regions, the global population most at risk of death and illness from extreme heat is under-represented. Heatwave and health impact research is needed in regions where this impact is expected to be most severe.
Heatwaves are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. However, we have little information regarding the impact of extreme heatwaves on the physiological performance of large trees in the field. Here, we grew Eucalyptus parramattensis trees for one year with experimental warming (+3 °C) in a field setting, until they were greater than 6-m-tall. We withheld irrigation for one month to dry the surface soils and then implemented an extreme heatwave treatment of four consecutive days with air temperatures exceeding 43 °C, while monitoring whole-canopy exchange of CO2 and H2O, leaf temperatures, leaf thermal tolerance, and leaf and branch hydraulic status. The heatwave reduced midday canopy photosynthesis to near zero but transpiration persisted, maintaining canopy cooling. A standard photosynthetic model was unable to capture the observed decoupling between photosynthesis and transpiration at high temperatures, suggesting that climate models may underestimate a moderating feedback of vegetation on heatwave intensity. The heatwave also triggered a rapid increase in leaf thermal tolerance, such that leaf temperatures observed during the heatwave were maintained within the thermal limits of leaf function. All responses were equivalent for trees with a prior history of ambient and warmed (+3 °C) temperatures, indicating that climate warming conferred no added tolerance of heatwaves expected in the future. This coordinated physiological response utilizing latent cooling and adjustment of thermal thresholds has implications for tree tolerance of future climate extremes as well as model predictions of future heatwave intensity at landscape and global scales.
Background
Climate change can directly affect human health by varying exposure to non-optimal outdoor temperature. However, evidence on this direct impact at a global scale is limited, mainly due to issues in modelling and projecting complex and highly heterogeneous epidemiological relationships across different populations and climates.
Methods
We collected observed daily time series of mean temperature and mortality counts for all causes or non-external causes only, in periods ranging from Jan 1, 1984, to Dec 31, 2015, from various locations across the globe through the Multi-Country Multi-City Collaborative Research Network. We estimated temperature–mortality relationships through a two-stage time series design. We generated current and future daily mean temperature series under four scenarios of climate change, determined by varying trajectories of greenhouse gas emissions, using five general circulation models. We projected excess mortality for cold and heat and their net change in 1990–2099 under each scenario of climate change, assuming no adaptation or population changes.
Findings
Our dataset comprised 451 locations in 23 countries across nine regions of the world, including 85 879 895 deaths. Results indicate, on average, a net increase in temperature-related excess mortality under high-emission scenarios, although with important geographical differences. In temperate areas such as northern Europe, east Asia, and Australia, the less intense warming and large decrease in cold-related excess would induce a null or marginally negative net effect, with the net change in 2090–99 compared with 2010–19 ranging from −1·2% (empirical 95% CI −3·6 to 1·4) in Australia to −0·1% (−2·1 to 1·6) in east Asia under the highest emission scenario, although the decreasing trends would reverse during the course of the century. Conversely, warmer regions, such as the central and southern parts of America or Europe, and especially southeast Asia, would experience a sharp surge in heat-related impacts and extremely large net increases, with the net change at the end of the century ranging from 3·0% (−3·0 to 9·3) in Central America to 12·7% (−4·7 to 28·1) in southeast Asia under the highest emission scenario. Most of the health effects directly due to temperature increase could be avoided under scenarios involving mitigation strategies to limit emissions and further warming of the planet.
Interpretation
This study shows the negative health impacts of climate change that, under high-emission scenarios, would disproportionately affect warmer and poorer regions of the world. Comparison with lower emission scenarios emphasises the importance of mitigation policies for limiting global warming and reducing the associated health risks.
Funding
UK Medical Research Council.
This study assesses the impact of dust on surface solar radiation focussing on an extreme dust event. For this purpose, we exploited the synergy of AERONET measurements and passive and active satellite remote sensing
(MODIS and CALIPSO) observations, in conjunction with radiative transfer model (RTM) and chemical transport
model (CTM) simulations and the 1-day forecasts from the Copernicus Atmosphere Monitoring Service (CAMS). The area of interest is the eastern Mediterranean where anomalously high aerosol loads were recorded between 30 January and 3 February 2015. The intensity of the event was extremely high, with aerosol optical depth (AOD) reaching 3.5, and optical/microphysical properties suggesting aged dust. RTM and CTM imulations were able to quantify the extent of dust impact on surface irradiances and reveal substantial reduction in solar energy exploitation capacity of PV and CSP installations under this high aerosol load. We
found that such an extreme dust event can result in Global Horizontal Irradiance (GHI) attenuation by as much as 40– 50% and a much stronger Direct Normal Irradiance (DNI) decrease (80–90 %), while spectrally this attenuation is distributed to 37% in the UV region, 33% in the visible and around 30% in the infrared. CAMS forecasts provided a reliable available energy assessment (accuracy within 10% of that obtained from MODIS). Spatially, the dust plume resulted in a zonally averaged reduction of GHI and DNI of the order of 150W/m2 in southern Greece, and a mean increase of 20W/m2 in the northern Greece as a result of lower AOD values combined with local atmospheric processes. This analysis of a real-world scenario contributes to the understanding and quantification of the impact range of high aerosol loads on solar energy and the potential for forecasting power generation failures at sunshine-privileged locations where solar power plants exist, are under construction or are being planned.
The chief aim of this study was to investigate how different species of solitary trees in temperate urban areas vary in their branch structure during winter by assessing branch area indices (BAIs). The BAI data showed significant differences (P < 0.0001) between species and genotypes. The lowest mean BAI in the dataset was for Gingko biloba L., which had a BAI of 0.27. Pinus strobus L. 'Fastigiata' represented the largest mean BAI of 2.09. The results from the BAI analysis further indicate that within the same species group differences occur between genotypes. For example, the five genotypes of Acer platanoides L. range from a mean BAI of 1.77 for A. platanoides 'Globosum' to a mean BAI of 0.50 for A. platanoides 'Fassen Black'. A further aim was to apply the compiled BAI data in the computational modeling program of ENVI-met 3.1, which simulates the surface–air interaction and microclimates in complex urban settings. The simulations focused on mean radiant temperature and wind speed. Results illustrate how wind speed on the leeward side of the trees gradually decrease with an increasing BAI. With an increasing BAI, the Tmrt decreases to the leeward of the row of trees. The results are further discussed in the perspective of sustainable urban development (i.e., where, why, and how the species studied could be integrated in the urban fabric). This is of particular interest for the design of urban green space in densely built-up urban environments where space may be restricted.
Urban street trees provide many environmental, social, and economic benefits for our cities. This research explored the role of street trees in Melbourne, Australia, in cooling the urban microclimate and improving human thermal comfort (HTC). Three east-west (E-W) oriented streets were studied in two contrasting street canyon forms (deep and shallow) and between contrasting tree canopy covers (high and low). These streets were instrumented with multiple microclimate monitoring stations to continuously measure air temperature, humidity, solar radiation, wind speed and mean radiant temperature so as to calculate the Universal Thermal Climate Index (UTCI) from May 2011 to June 2013, focusing on summertime conditions and heat events. Street trees supported average daytime cooling during heat events in the shallow canyon by around 0.2 to 0.6 °C and up to 0.9 °C during mid-morning (9:00-10:00). Maximum daytime cooling reached 1.5 °C in the shallow canyon. The influence of street tree canopies in the deep canyon was masked by the shading effect of the tall buildings. Trees were very effective at reducing daytime UTCI in summer largely through a reduction in mean radiant temperature from shade, lowering thermal stress from very strong (UTCI > 38 °C) down to strong (UTCI > 32 °C). The influence of street trees on canyon air temperature and HTC was highly localized and variable, depending on tree cover, geometry, and prevailing meteorological conditions. The cooling benefit of street tree canopies increases as street canyon geometry shallows and broadens. This should be recognized in the strategic placement, density of planting, and species selection of street trees.
There is mounting concern for the health of urban populations as cities expand at an unprecedented rate. Urban green spaces provide settings for a remarkable range of physical and mental health benefits, and pioneering health policy is recognizing nature as a cost-effective tool for planning healthy cities.
Despite this, limited information on how specific elements of nature deliver health outcomes restricts its use for enhancing population health. We articulate a framework for identifying direct and indirect causal pathways through which nature delivers health benefits, and highlight current evidence.
We see a need for a bold new research agenda founded on testing causality that transcends disciplinary boundaries between ecology and health. This will lead to cost-effective and tailored solutions that could enhance population health and reduce health inequalities.
Heat-related mortality in US cities is expected to more than double by the mid-to-late 21st century. Rising heat exposure in cities is projected to result from: 1) climate forcings from changing global atmospheric composition; and 2) local land surface characteristics responsible for the urban heat island effect. The extent to which heat management strategies designed to lessen the urban heat island effect could offset future heat-related mortality remains unexplored in the literature. Using coupled global and regional climate models with a human health effects model, we estimate changes in the number of heat-related deaths in 2050 resulting from modifications to vegetative cover and surface albedo across three climatically and demographically diverse US metropolitan areas: Atlanta, Georgia, Philadelphia, Pennsylvania, and Phoenix, Arizona. Employing separate health impact functions for average warm season and heat wave conditions in 2050, we find combinations of vegetation and albedo enhancement to offset projected increases in heat-related mortality by 40 to 99% across the three metropolitan regions. These results demonstrate the potential for extensive land surface changes in cities to provide adaptive benefits to urban populations at risk for rising heat exposure with climate change.
Increases in drought and temperature stress in forest and woodland ecosystems are thought to be responsible for the rise in episodic mortality events observed globally. However, key climatic drivers common to mortality events and the impacts of future extreme droughts on tree survival have not been evaluated. Here, we characterize climatic drivers associated with documented tree die-off events across Australia using standardized climatic indices to represent the key dimensions of drought stress for a range of vegetation types. We identify a common probabilistic threshold associated with an increased risk of die-off across all the sites that we examined. We show that observed die-off events occur when water deficits and maximum temperatures are high and exist outside 98% of the observed range in drought intensity; this threshold was evident at all sites regardless of vegetation type and climate. The observed die-off events also coincided with at least one heat wave (three consecutive days above the 90th percentile for maximum temperature), emphasizing a pivotal role of heat stress in amplifying tree die-off and mortality processes. The joint drought intensity and maximum temperature distributions were modeled for each site to describe the co-occurrence of both hot and dry conditions and evaluate future shifts in climatic thresholds associated with the die-off events. Under a relatively dry and moderate warming scenario, the frequency of droughts capable of inducing significant tree die-off across Australia could increase from 1 in 24 years to 1 in 15 years by 2050, accompanied by a doubling in the occurrence of associated heat waves. By defining commonalities in drought conditions capable of inducing tree die-off, we show a strong interactive effect of water and high temperature stress and provide a consistent approach for assessing changes in the exposure of ecosystems to extreme drought events.
Trees form a significant part of the urban vegetation. Their meteorological and climatological effects at all scales in urban environments make them a flexible tool for creating a landscape oriented to the needs of an urban dweller. This study aims at quantifying the spatio-temporal patterns of canopy temperature (T
C) and canopy-to-air temperature difference (∆T
C) in relation to meteorological conditions and tree-specific (physiological) and urban site-specific characteristics. We observed T
C and ∆T
C of 67 urban trees (18 species) using a high-resolution thermal-infrared (TIR) camera and meteorological measurements in the city of Berlin, Germany. TIR images were recorded at 1-min intervals over a period of 2 months from 1st July to 31st August 2010. The results showed that ∆T
C depends on tree species, leaf size and fraction of impervious surfaces. Average canopy temperature was nearly equal to air temperature. Species-specific maximum ∆T
C varied between 1.9 ± 0.3 K (Populus nigra), 2.9 ± 0.3 K (Quercus robur), 3.2 ± 0.5 K (Fagus sylvatica), 3.9 ± 1.0 K (Platanus acerifolia), 4.6 ± 0.2 K (Acer pseudoplatanus), 5.0 ± 0.5 K (A. platanoides) and 5.6 ± 1.1 K (A. campestre). We analysed ∆T
C for a hot and dry period (A) and a warm and wet period (B). The range of species-specific ∆T
C at noon was nearly equal, i.e. 4.4 K for period A and 4.2 K for period B. Trees surrounded by high fraction of impervious surfaces showed consistently higher ∆T
C. Knowledge of species-specific canopy temperature and the impacts of urban structures are essential in order to optimise the benefits from trees in cities. However, comprehensive evaluation and optimisation should take the full range of climatological effects into account.
The present paper deals with the dependence of outdoor thermal comfort on the design of an urban street. The effects of the street vertical profile, including asymmetrical canyon shapes, the use of galleries and further shading devices on the façades, arranged in various orientations are assessed. The study is conducted by means of numerical modelling by using the three-dimensional microclimate model ENVI-met 3.0 which prognosticates the microclimatic changes within urban environments. Thermal comfort is evaluated for the daytime hours across the canyon in high spatial resolution and by means of the physiologically equivalent temperature PET.The results revealed that all design aspects investigated have a moderate impact on the air temperature and a strong effect on the heat gained by a human body and hence on the resulting thermal sensation. The larger the openness to the sky of the canyon, the higher the heat stress. For canyons with a smaller sky view, the orientation is also decisive: E–W canyons are the most stressful and deviating from this orientation ameliorates the thermal conditions. Basically, galleries and further shading through overhanging façades or vegetation enable a sensitive decrease of the period of time and of the area of thermal discomfort. Yet, this efficiency varies with the orientation and the vertical proportions of the canyon. Therefore, if appropriately combined, all investigated design elements can effectively mitigate heat stress in the summer and promote thermal comfort.
This paper presents a tool for quantifying the integrated thermal effect of built forms and of vegetation on the urban canopy layer (UCL) climate in design built-up alternatives. Three generic models were studied, representing the most common types of residential urban street: (a) the street form, a conventional type, with spacing between the houses, (b) the canyon form—a limiting case of the street form, (c) the courtyard house form. Recessed colonnades in streets and courtyards were considered in this study as the fourth generic model. The four models were analyzed hierarchically from shallow open spaces to deep ones. For each studied case, the built form effect, the vegetation effect and the colonnade effect were estimated using simulated data generated by the analytical Green CTTC model recently developed by the authors. Emphasis in this study is on the UCL air temperature variation at midday, in summer, in a hot-humid region, near the Mediterranean Sea coast (31–32°N). Eighty-six simulations were generated for estimating the various thermal effects. In addition, 100 experimental observations at 11 urban wooded sites were analyzed to confirm the simulated effect of the trees. Statistical analysis indicates that each of the thermal effects of the built form, vegetation and of the colonnade can be explained each by one linear relationship, common to all the studied built-up generic models, to a high degree of accuracy and confidence level. This provides a useful general design tool, as opposed to the analysis of a particular simulated case, to assess the potential thermal effects of control variables in different building configurations. The study also considers the extent of the thermal effects of built form, vegetation and colonnades, in streets and in courtyards. These effects are shown to depend, each on the envelope ratio, an overall geometry factor, and thus are interdependent.
This paper reviews the European summer heat wave of 2003, with special emphasis on the first half of August 2003, jointly with its significant societal and environmental impact across Western and Central Europe. We show the pattern of record-breaking temperature anomalies, discuss it in the context of the past, and address the role of the main contributing factors responsible for the occurrence and persistence of this event: blocking episodes, soil moisture deficit, and sea surface temperatures. We show that the anticyclonic pattern corresponds more to an anomalous northern displacement of the North Atlantic subtropical high than a canonical blocking structure, and that soil moisture deficit was a key factor to reach unprecedented temperature anomalies. There are indications that the anomalous Mediterranean Sea surface temperatures (SSTs) have contributed to the heat wave of 2003, whereas the role of SST anomalies in other oceanic regions is still under debate. There are methodological limitations to evaluate excess mortality due to excessive temperatures; however, the different studies available in the literature allow us to estimate that around 40,000 deaths were registered in Europe during the heat wave, mostly elderly persons. Despite previous efforts undertaken by a few cities to implement warning systems, this dramatic episode has highlighted the widespread un-preparedness of most civil and health authorities to cope with such large events. Therefore, the implementation of early warning systems in most European cities to mitigate the impact of extreme heat is the main consequence to diminish the impact of future similar events. In addition to mortality (by far the most dramatic impact), we have also analyzed the record-breaking forest fires in Portugal and the evidence of other relevant impacts, including agriculture and air pollution.
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.
In 2007, an extreme drought and acute heat wave impacted ecosystems across the southeastern USA, including a 19-year-old Liquidambar styraciflua L. (sweetgum) tree plantation exposed to long-term elevated (E(CO(2))) or ambient (A(CO(2))) CO(2) treatments. Stem sap velocities were analyzed to assess plant response to potential interactions between CO(2) and these weather extremes. Canopy conductance and net carbon assimilation (A(net)) were modeled based on patterns of sap velocity to estimate indirect impacts of observed reductions in transpiration under E(CO(2)) on premature leaf senescence. Elevated CO(2) reduced sap flow by 28% during early summer, and by up to 45% late in the drought during record-setting temperatures. Modeled canopy conductance declined more rapidly in E(CO(2)) plots during this period, thereby directly reducing carbon gain at a greater rate than in A(CO(2)) plots. Indeed, pre-drought canopy A(net) was similar across treatment plots, but declined to ∼40% less than A(net) in A(CO(2)) as the drought progressed, likely leading to negative net carbon balance. Consequently, premature leaf senescence and abscission increased rapidly during this period, and was 30% greater for E(CO(2)). While E(CO(2)) can reduce leaf-level water use under droughty conditions, acute drought may induce excessive stomatal closure that could offset benefits of E(CO(2)) to temperate forest species during extreme weather events.
Digital cover photography (DCP) is a high resolution, vertical field-of-view method for ground-based estimation of forest metrics, and has advantages over fisheye sensors owing to its ease of application and high accuracy. We conducted the first thorough technical appraisal of DCP using both single-lens-reflex (DSLR) and point-and-shoot cameras and concluded that differences result primarily from the better quality optics available for the DSLR camera. File compression, image size and ISO equivalence had little or no effect on estimates of forest metrics. We discuss the application of DCP for ground truthing of remotely sensed canopy metrics, and highlight its strengths over fisheye photography for testing and calibration of vertical field-of-view remote sensing.
The most important meteorological parameter affecting the human energy balance during sunny weather conditions is the mean radiant temperature T(mrt). It considers the uniform temperature of a surrounding surface giving off blackbody radiation, which results in the same energy gain of a human body given the prevailing radiation fluxes. This energy gain usually varies considerably in open space conditions. In this paper, the model 'RayMan', used for the calculation of short- and long-wave radiation fluxes on the human body, is presented. The model, which takes complex urban structures into account, is suitable for several applications in urban areas such as urban planning and street design. The final output of the model is, however, the calculated T(mrt), which is required in the human energy balance model, and thus also for the assessment of the urban bioclimate, with the use of thermal indices such as predicted mean vote (PMV), physiologically equivalent temperature (PET) and standard effective temperature (SET*). The model has been developed based on the German VDI-Guidelines 3789, Part II (environmental meteorology, interactions between atmosphere and surfaces; calculation of short- and long-wave radiation) and VDI-3787 (environmental meteorology, methods for the human-biometeorological evaluation of climate and air quality for urban and regional planning. Part I: climate). The validation of the results of the RayMan model agrees with similar results obtained from experimental studies.
Daily numbers of deaths at a regional level were collected in 16 European countries. Summer mortality was analyzed for the reference period 1998-2002 and for 2003. More than 70,000 additional deaths occurred in Europe during the summer 2003. Major distortions occurred in the age distribution of the deaths, but no harvesting effect was observed in the months following August 2003. Global warming constitutes a new health threat in an aged Europe that may be difficult to detect at the country level, depending on its size. Centralizing the count of daily deaths on an operational geographical scale constitutes a priority for Public Health in Europe.
Scientists rushed to study whether the scorching temperatures last week were linked to global warming. Scientists rushed to study whether the scorching temperatures last week were linked to global warming. People cool down in the fountains of Trocadero near the Eiffel Tower during a heatwave in Paris, France
A rapid rise of urban population is making cities denser. Consequently, the proportion of impervious surface cover has enlarged, increasing the amount and speed of runoff reaching urban catchment areas, which may cause flash flooding.
Trees play a key role to reduce runoff in the city, as they intercept rainfall and store part of it on their leaves and branches, reducing the amount and speed of water running onto impervious surfaces. Storage capacity will depend on the rainfall event, the climate conditions and tree characteristics and canopy density. These canopy characteristics vary greatly among different species and their phenology. Furthermore, these canopy characteristics can vary greatly among individual trees of the same age, size and species. This study tested how canopy density and leaf characteristics of three different tree species affect storage capacity under simulated rainfall conditions.
Three species were selected (Ulmus procera, Platanus x acerifolia, and Corymbia maculata), each being of the same height and similar canopy dimensions. Storage capacity was measured using a mass balance approach during a 15‐minute indoor, simulated rainfall event (2.54 mm/hr). Canopy metrics were estimated using a terrestrial laser scanner. Canopy surface area was measured through destructive harvest and leaf/twig/branch scanning. To investigate variations in the canopy leaf density, leaves were systematically removed to create four treatments: full, half, quarter and woody.
Canopy storage capacity was well correlated to plant surface area (m²), plant area index (m²/m²) and plant area density (m²/m³). All analyses indicated U. procera as the most efficient species for storing rainfall water within a canopy of equal volume or area index.
Results reveal the complexity of evaluating interception of rainfall by tree canopies. This study contributes to the discipline and practice by distinguishing how variation in the leaf density is important to consider when selecting urban tree species to be planted.
Based on the ENVI-met model v4.0 BETA, numerical simulations were carried out for five different sized building areas in the city of Stuttgart (Southwest Germany) on the heat wave day 4 August 2003. Human heat stress is primarily quantified by the physiologically equivalent temperature (PET). Additional background information is provided by both near-surface air temperature (Ta) and mean radiant temperature (Tmrt). The simulations concern five urban land use scenarios. Related to differences of simulation results between a scenario that only consists of asphalt surfaces and a green scenario only showing grasslands and trees, the resulting ΔTa, ΔTmrt and ΔPET values are interpreted as maximum extent of human heat stress reduction on the building areas by urban greening. To achieve a higher reliability for urban planning, the results are averaged over the period 10-16 CET. Exemplarily for one building area the results are presented in terms of grid-related absolute values. Besides mean absolute values for each building area whose magnitudes depend on the meteorological conditions of the simulation day, the results include mean relative ΔTa, ΔTmrt and ΔPET values. As verified by additional simulations for a current typical summer day, they can be regarded as representative for summer in Central Europe. Averaged over the five building areas mean ΔTa amounts to 1.1 °C (4%), mean ΔTmrt to 17.6 °C (26%) and mean ΔPET to 7.5 °C (16%). The results of further simulations point to the increase of human heat stress by the planning variants for the building areas compared to the maximum extent of human heat stress reduction by the green scenario. It reaches 0.4 °C (1%) for mean ΔTa, 4.9 °C (9%) for mean ΔTmrt, and 3.7 °C (9%) for mean ΔPET.
Aim
Urban macroecology studies can provide important insights into the impacts of climate change and human intervention in ecosystems. Current theory predicts that urban trees are constrained by temperature in very cold climates but not in other climates. Here we predict the climatic niche variables of planted urban tree populations from the realized climatic niche of native populations and explore whether niches are constrained across all temperatures.
Location
Global (182 cities across six continents).
Time period
Urban tree data: 1980–2016. Native tree data: 1950–2017.
Major taxa studied
Two hundred and three tree species.
Methods
We used urban tree inventory data and Global Biodiversity Information Facility occurrence data to compare the realized climatic niches of native and urban tree populations. Realized climatic niches are calculated by combining bioclimatic data with native tree and urban tree occurrence data. Regression is used to predict the climatic niche of urban tree populations from the climatic niche of native populations.
Results
The realized climatic niche of native tree populations was a good predictor of the realized climatic niche of urban tree populations, although climatic niches are attenuated in urban populations. Urban tree niches were 38–90% wider than native tree niches, with the mean annual temperature niche breath of urban tree populations 3.3 °C (52%) wider than native tree populations.
Main conclusions
Urban trees are planted in climates that are outside the realized climatic niche of native populations. Temperature remains a strong filter on urban tree populations across the full temperature range. Temperature increases attributable to the combined effect of the urban heat island and global climate change are likely to have a substantial impact on urban tree populations around the globe. This is particularly true for temperate cities, where cold climate trees are planted near the upper limits of their realized temperature niches.
Leaf area of urban vegetation is an important ecological characteristic, influencing urban climate through shading and transpiration cooling and air quality through air pollutant deposition. Accurate estimates of leaf area over large areas are fundamental to model such processes. The aim of this study was to explore if an aerial LiDAR dataset acquired to create a high resolution digital terrain model could be used to map effective leaf area index (Le) and to assess the Le variation in a high latitude urban area, here represented by the city of Gothenburg, Sweden. Le was estimated from LiDAR data using a Beer-Lambert law based approach and compared to ground-based measurements with hemispherical photography and the Plant Canopy Analyser LAI-2200. Even though the LiDAR dataset was not optimized for Le mapping the comparison with hemispherical photography showed good agreement (r² = 0.72, RMSE = 0.97) for urban parks and woodlands. Leaf area density of single trees, estimated from LiDAR and LAI-2200, did not show as good agreement (r² = 0.53, RMSE = 0.49). Le in 10 m resolution covering most of Gothenburg municipality ranged from 0 to 14 (0.3% of the values >7) with an average Le of 3.5 in deciduous forests and 1.2 in urban built-up areas. When Le was averaged over larger scales there was a high correlation with canopy cover (r² = 0.97 in 1 × 1 km² scale) implying that at this scale Le is rather homogenous. However, when Le was averaged only over the vegetated parts, differences in Le became clear. Detailed study of Le in seven urban green areas with different amount and type of greenery showed a large variation in Le, ranging from average Le of 0.9 in a residential area to 4.1 in an urban woodland. The use of LiDAR data has the potential to considerably increase information of forest structure in the urban environment.
Laboratory studies on artificial leaves suggest that leaf thermal dynamics are strongly influenced by the two-dimensional size and shape of leaves and associated boundary layer thickness. Hot environments are therefore said to favour selection for small, narrow or dissected leaves. Empirical evidence from real leaves under field conditions is scant and traditionally based on point measurements that do not capture spatial variation in heat load. We used thermal imagery under field conditions to measure the leaf thermal time constant (τ) in summer and the leaf-to-air temperature difference (ΔT) and temperature range across laminae (Trange) during winter, autumn and summer for 68 Proteaceae species. We investigated the influence of leaf area and margin complexity (NDMC) relative to effective leaf width (we), the latter being a more direct indicator of boundary layer thickness. NDMC had no or weak effects on thermal dynamics, but we strongly predicted τ and ΔT, whereas leaf area influenced Trange. Unlike artificial leaves, however, spatial temperature distribution in large leaves appeared to be governed largely by structural variation. Therefore, we agree that small size, specifically we, has adaptive value in hot environments, but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection.
The way a street tree is able to modify the local microclimate on pedestrian walkways may vary according to tree species according to key canopy and leaf characteristics, such as leaf angle, leaf size, canopy architecture or simply canopy density. Three similar north-south orientated streets, with three different tree species possessing different canopy and leaf characteristics were studied in summer 2014. Microclimatic parameters were measured on pedestrian walkways below and away from tree canopies between 06:00 and 20:00 on three cloudless days. Physiological Equivalent Temperature (PET) was estimated to indicate pedestrian thermal comfort. Microclimate conditions were measured below and away from trees at solar noon for a wide range of trees with different Plant Area Index (PAI) as determined using full-frame photography. In streets with Ulmus procera and Platanus x acerifolia trees, the microclimatic benefits were significantly greater than the street with Eucalyptus scoparia trees, however no significant differences in the estimated PET. Microclimate benefit increased with increasing PAI for all three tree species, however no significant difference in under-canopy microclimate amongst tree species when the PAI was similar. It appears that differences in PAI are paramount in determining the microclimatic and PET benefits. Obviously, certain tree species have a limit of the PAI they can achieve, and that should be considered when selecting or comparing tree species for shading and cooling benefits. This study assists urban planners and landscape professionals in selecting street tree species for cooling benefits based on the expected or managed tree canopy area.
The RayMan software is worldwide applied in investigations on different issues in human-biometeorology. However, only the simulated mean radiant temperature (T
mrt) has been validated so far in a few case studies. They are based on T
mrt values, which were experimentally determined in urban environments by use of a globe thermometer or applying the six-directional method. This study analyses previous T
mrt validations in a comparative manner. Their results are extended by a recent validation of T
mrt in an urban micro-environment in Freiburg (southwest Germany), which can be regarded as relatively heterogeneous due to different shading intensities by tree crowns. In addition, a validation of the physiologically equivalent temperature (PET) simulated by RayMan is conducted for the first time. The validations are based on experimentally determined T
mrt and PET values, which were calculated from measured meteorological variables in the daytime of a clear-sky summer day. In total, the validation results show that RayMan is capable of simulating T
mrt satisfactorily under relatively homogeneous site conditions. However, the inaccuracy of simulated T
mrt is increasing with lower sun elevation and growing heterogeneity of the simulation site. As T
mrt represents the meteorological variable that mostly governs PET in the daytime of clear-sky summer days, the accuracy of simulated T
mrt is mainly responsible for the accuracy of simulated PET. The T
mrt validations result in some recommendations, which concern an update of physical principles applied in the RayMan software to simulate the short- and long-wave radiant flux densities, especially from vertical building walls and tree crowns.
Greening is a useful mitigation strategy for planners mainly from a visual perspective. For high-density urban living environment such as Hong Kong, urban greening helps cooling the air and providing shade; it also helps lowering building energy consumption by providing a better outdoor boundary condition. Many researchers have also suggested that greening may be employed as a strategy for combating the ill effects of urban Heat Island (UHI). Working towards a set of better greening guidelines for urban planners, the current paper first provides a comprehensive review of planning with urban greening. It then describes parametric studies that have been conducted to investigate the preferred location, amount, and types of vegetation for urban planning. The parametric studies employed the numerical model ENVI-met, verified using field measurements, to simulate 33 cases with different combinations of factors. For benefiting urban activities, ambient air temperatures at the pedestrian level are compared among different greening strategies and building heights. For a city such as Hong Kong, which has a high building-height-to-street-width (H/W) ratio, the present study reveals that roof greening is ineffective for human thermal comfort near the ground. Trees are also suggested to be more effective than grass surfaces in cooling pedestrian areas. The amount of tree planting needed to lower pedestrians level air temperature by around 1 °C is approximately 33% of the urban area. The present study allows urban planners to identify more precisely the greening principles, amount and policies necessary for better urban living environment in high-density cities.
Heat wave frequency, duration and intensity are predicted to increase with global warming, but the potential impacts of short-term high temperature events on forest functioning remain virtually unstudied. We examined canopy processes in a forest in Central Ontario following three days of record-setting high temperatures (31-33°C) that coincided with the peak in leaf expansion of dominant trees in late May 2010. Leaf area dynamics, leaf morphology, and leaf-level gas-exchange were compared to data from prior years of sampling (2002-2008) at the same site, focusing on Acer saccharum Marsh., the dominant tree in the region. Extensive shedding of partially expanded leaves was observed immediately following high temperature days, with A. saccharum losing ~25% of total leaf production but subsequently producing an unusual second flush of neoformed leaves. Both leaf losses and subsequent reflushing were highest in the upper canopy; however, retained preformed leaves and neoformed leaves showed reduced size, resulting in an overall decline in end-of-season leaf area index of 64% in Acer saccharum, and 16% in the entire forest. Saplings showed lower leaf losses, but also a lower capacity to reflush relative to mature trees. Both surviving preformed and neoformed leaves had severely depressed photosynthetic capacity early in the summer of 2010, but largely regained photosynthetic competence by the end of the growing season. These results indicate that even short-term heat waves can have severe impacts in northern forests, and suggest a particular vulnerability to high temperatures during the spring period of leaf expansion in temperate deciduous forests. This article is protected by copyright. All rights reserved.
Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.
The plane tree (Platanus spp.) is one of the most frequently used ornamental trees in Spanish urban areas. For the past 30 years, the health of urban tree plantings in various parts of the Iberian Peninsula has been observed and a number of diseases, common to the majority of them, detected. The most frequently occurring diseases that affect Platanus spp. are anthracnose (caused by the fungi Sporonema platani Blaūml.) and powdery mildew (caused by Microsphaera alni (DC.) Wint.), as well as diseases caused by other minor pathogens. Since 1997, we have focused our research on surveying the state of health of urban trees in the town of Aranjuez (Madrid), as an example of an urban location in central Spain. Within the numerous tree species that exist in the town, a serious problem was detected among plane trees. The symptoms observed (young shoots and branches up to 3 years old growing in whorls, necrotic tissue close to phloem and xylem, premature defoliation) indicate the possible existence of Gnomonia veneta (Sacc. et Speg.) Kleb, anamorph Sporonema platani Blaūml. The majority of the trees show these symptoms to one degree or another. Applying diagnostic and identification methods in the laboratory, we were able to demonstrate the presence of S. platani in all of the samples collected from symptomatic trees. A detailed description of the two main diseases, anthracnose and powdery mildew, in Spain is presented, with special reference to symptoms and dispersal under Spanish environmental conditions. A discussion about the most appropriate control methods is included.
Summer 2003 was extremely hot in Europe. High light in combination with heat and drought exacerbates the generation of photo-oxidative
stress. Under these conditions photoprotective responses can be critical for plant survival. Photoprotection was analysed
in 2003 in several Mediterranean and Atlantic woody species. These data were compared with previous summers (1998, 1999 and
2001) to evaluate the potential acclimation for each species. A pattern of changes consisting on a decrease in chlorophyll,
ascorbate and Fv/Fm and an increase in tocopherol, xanthophyll cycle pigments (VAZ) and de-epoxidation index was regularly
observed. Acclimation potential was measured by the use of the plasticity index for each parameter. Mediterranean species
were more plastic than Atlantic ones. The latter were unable to increase antioxidant pools to the same extent or to down-regulate
the efficiency of light energy conversion. These results indicate that most Mediterranean species are able to perform an efficient
acclimation to heat stress, whilst Atlantic species will be more affected by climate warming.
The influence of several regulated deficit irrigation (RDI) strategies, applied under subsurface drip irrigation (SDI), on vegetative development and yield parameters in mature almond (Prunus dulcis (Mill.) D.A. Webb, cv. Cartagenera) trees was analysed during a 4-year field experiment. Five treatments were applied: T1 (100% crop evapotranspiration (ETc), full season); T2 (irrigated at 100% ETc except in the kernel-filling stage (20% ETc)); T3 (equal to T2 but in SDI); T4 (SDI, 100% ETc, except in the kernel-filling stage (20% ETc) and post-harvest (75% ETc)); T5 (SDI, 100% ETc except in the kernel-filling stage (20% ETc) and post-harvest (50% ETc). A close correlation between applied water, plant water status (pd) and tree growth parameters was observed. After four years, the vegetative development in T5 was reduced significantly due to a larger annual cumulative effect of water stress on growth processes, resulting in a smaller tree size (trunk and branch growth, canopy volume and pruning weight) compared to other treatments. Moreover, water stress during kernel-filling produced a significant reduction in the leaf expansion rate and a stimulation of premature leaf abscission, resulting in a smaller tree leaf area in this treatment. SDI produced a greater horizontal distribution of fine roots in the soil profile than surface drip system. The RDI practices applied under subsurface drip irrigation stimulated a deeper root development (40–80 cm) than surface treatments (0–40 cm), producing also a higher root density in the subsurface treatments watered the least (T4 and T5). Water stress during the pre- and post-harvest periods had no important effect on bud development, bloom, fruit growth or fruit abscission. Moreover, there were no significant reductions in kernel dry weight or kernel percentage. Reductions in kernel yield were significant compared to T1, being of 11% in T2, 15% in T3, 20% in T4 and 22% in T5. Water use efficiency (kg m–3) was increased significantly in the SDI treatments T4 and T5. A significant correlation between kernel yield and tree growth parameters was observed. We conclude that the application of overall reductions of water use of up to 50% during high water stress sensitivity periods (such as post-harvest) under SDI system, is a promising alternative for water management in semiarid regions in order to improve water use efficiency. Nevertheless prolonged water stress during kernel-filling and post-harvest can reduce excessively the vegetative development of almond, negatively affecting the long-term yield response.
On July 29, 1985, a hot summer day, biometeorological measurements were performed simultaneously in three different urban structures within the city of Munich and in the trunk space of a nearby tall spruce forest. Based on the results of these experiments the following thermophysiologically relevant biometeorological indices were calculated: Predicted mean vote, skin wettedness and physiologically equivalent temperature. These three indices are derived from different models for the human energy balance. They allow the assessment of the thermal components of the microclimates at the selected sites with regard to application in urban planning. The results quantitatively show the great heat stress in the urban structure “street canyon, exposed to south”, whereas in the “trunk space of the tall spruce forest” there is nearly an optimal climate even on hot summer days. Between these extremes the results for “street canyon, exposed to north” show a little higher heat load than for “backyard with trees”.
Trees are essential in a dense urban environment not only because of their aesthetic value, but also for their cooling effect during hot periods, which impacts directly on the local microclimate. However, certain trees cope better with high urban temperatures than others. Here, we report tree crown temperatures of 10 common tree species frequently planted in Central European cities (in part, supplemented with stomatal conductance data, gs). Parts of the city of Basel, Switzerland (7°41′E/47 °34′N) were scanned from a helicopter using a high-resolution thermal camera. A histogram of the composite image shows peaks at 18 °C (water), 26 °C (vegetation), 37 °C (streets) and a less obvious one at 45 °C (roofs). At an ambient temperature of c. 25 °C, tree crown temperatures ranged from c. 24 °C (Aesculus hippocastanum trees located in a park) to 29 °C in Acer platanoides trees, located in a street. Trees in parks were significantly cooler (c. 26 °C) than trees surrounded by sealed ground (c. 27 °C). The only coniferous species, Pinus sylvestris did not vary in temperature with location (park or street) and exhibited foliage temperature close to air temperature. Generally, small-leaved trees remained cooler than large-leaved trees. Stomatal conductance data collected during similar weather conditions suggest that there was no bias in crown temperatures due to locally different water supply between trees. Although the highest leaf temperatures of individuals of A. platanoides reached over 5 K leaf-to-air temperature difference (), we do not expect temperature stress to occur in these conditions. In order to estimate possible effects of future temperature extremes on , we evaluated the leaf energy balance for a range of stomatal responses and air temperatures, using leaf size, wind speed and the measured species-specific leaf boundary layer resistance. At an ambient temperature of 40 °C, ranged from 2 to 5 K when gs was assumed to drop linearly to 50% of its maximum value. When gs was compromised further (20% of species-specific maxima), the difference in between species became larger with rising ambient temperature (range 4–10 K). Those species with the lowest leaf temperatures at 25 °C were not necessarily coolest at 40 °C. Species-specific differences in under extreme temperatures as shown here may be useful for urban tree planning in order to optimise management cost and human comfort.
Street orientation and side of the street greatly influence the microclimatic benefits street trees can provide in summer
- Sanusi
Structural responses of Platanus orientalis L. leaves to elevated CO2 concentration and high temperature
- Koleva