Urban Street Trees Research Articles

Deferring waterlogging through stormwater control and channelling of runoff

Stormwater control measures (SCMs) in the form of highly permeable tree pits can be useful in two ways. On one hand, they can help reduce surface runoff and on the other hand, the presence of trees in these pits can potentially help mitigate the urban heat island effect. Therefore, it is important to understand the pit designs that underpin tree health and growth performances. Here in this study, four pit designs namely, a control which did not receive runoff water, another that receive runoff into a sandy clay loam soil, yet another that was filled with structural soil, and a third that received runoff into structural soil with an underdrain system, used to test and identify which designs were best able to maintain hydrological balance and growth of Calophyllum inophyllum in an urban area dominated by sandy clay loam soil. Trees in pits which had structural soil and drainage installed had 6-fold more growth than the controls (that received no stormwater runoff), and 3-fold more growth when compared against street trees grown in pits that had sandy clay loam soil (that had run off channelled towards these pits). Treatments with structural soils also experienced more efficient exfiltration rates which helped avoid waterlogging which in turn, benefitted growth. The lower exfiltration rates of the poorly drained sandy clay loam soils resulted in periods of saturation and subsequent slower tree growth. Treatments with structural soil together with drainage and those that had sandy clay loam soils were found to have saturated water levels at a depth greater than 400 mm and between 100−200 mm (from the surface), respectively. The outcomes from this study indicated that tree growth can be improved by channelling stormwater into tree pits with structural soil and especially those that also have drainage installed. Noteworthy however, is that while waterlogging may be avoided through the application of structural soil and a drainage system, the high exfiltration rates may result in water stress when rainfall is scarce though this did not happen here. The need for hydrological balance will be the key to successful urban street tree planting.

The importance of prenol lipids in mitigating salt stress in the leaves of Tilia \xd7\xa0euchlora trees

Key messagePlants use multiple mechanisms to deal with salt stress. Salt stress increases the content of polyprenols inTilia’s leaves, which may mitigate stress.De-icing salt has been used on streets and pavements in most northern countries since the 1960s. Salt stress limits all vital functions of trees. Tilia × euchlora is planted in many cities given its unique decorative qualities. The aim of this study was to determine the tree strategy to mitigate salt stress due to the synthesis of polyprenols in leaves. Many years of observations have demonstrated that trees of the same species growing in the same street conditions may have extremely different health statuses. The study consisted of two experiments: a field experiment with urban street trees growing in saline soils and a controlled pot experiment with young trees exposed to increasing doses of salt. The differences between the young trees from the pot experiment and older trees from the field experiment were expressed in their ability to synthesize polyprenols. In urban conditions, the tree leaves with less damage contained significantly more polyprenols than did those with more damage. The salt stress mitigation strategy may be related to the ability to synthesize polyprenols. This ability can be acquired through adaptation by older trees. The mechanism involves limiting the transport of Cl− and Na+ to leaves. In the pot experiment, the young trees did not exhibit this ability.

Predicting Water Supply and Evapotranspiration of Street Trees Using Hydro-Pedo-Transfer Functions (HPTFs)

The climate, soil properties, groundwater depth, and surrounding settings in cities vary to a tremendous extent, which all lead to different growing conditions and health for street trees. Because of climate change, the availability of water in cities will undergo changes in the next decades. As urban trees have a very positive influence not only on microclimate but also on biodiversity and life quality in general, they need to be protected. Thus, we need to know how to measure and calculate the availability of water for street trees to optimize their site conditions and water supply. This study presents Hydro-Pedo-Transfer Functions (HTPFs) for predicting water supply and actual evapotranspiration of street trees for varying urban conditions. The HTPFs are easy to use, and the input parameters can either be mapped easily or taken from local climate agencies or soil surveys. The first part of the study focuses on the theoretical background and related assumptions of the HTPFs for predicting water supply, and on obtaining the potential and actual evapotranspiration of urban street trees using easily available data. The second part gives information and exemplifies how this input data can be measured, mapped, or predicted. Calibration of the HTPFs were done using the sap-flow measurements of three Linden trees (Tilia cordata). Exemplarily, the HTPF scenarios for the varying urban site conditions of Berlin are presented. The water supply and actual evapotranspiration of the street trees severely depend on the local climate (summer rainfall and potential evapotranspiration), site conditions (catchment area, soil available water, and degree of sealing), and on the tree characteristics (species, age, and rooting depth). The presented concept and the equations build a good and flexible frame that is easy to program using a spreadsheet tool or an R script. This tool should be tested and validated also for other cities and climate regions.

Effects of urban street trees on human thermal comfort and physiological indices: a case study in Changchun city, China

Planting trees along urban streets is one of the most important strategies to improve the urban thermal environment. However, the net impacts of urban street trees on human thermal comfort and physiological parameters are still less clear. On three similar east–west orientated streets with different degrees of tree cover—low (13%), medium (35%), and high (75%), urban microclimatic parameters and human physiological indices for six male students were simultaneously measured on three cloudless days in summer 2018. The results show that the differences in tree cover were predominant in influencing urban thermal environment and comfort. The street with the highest tree cover had significantly lower physiological equivalent temperature (PET) and more comfortable than the other two streets. The frequency of strong heat stress (PET > 35 °C) was 64%, 11%, and 0%, respectively, for streets with low, medium, and high tree cover. For the six male university students, human physiological indices varied greatly across the three streets with different tree cover. Systolic blood pressure, diastolic blood pressure, and pulse rate increased with decreasing tree cover. The results also suggest that urban thermal environment and comfort had considerable impact on human physiological parameters. Our study provides reasons for urban planners to plant trees along streets to improve the thermal environment and promote urban sustainability.

Improvement of the Planting Method to Increase the Carbon Reduction Capacity of Urban Street Trees

Improvement of the Planting Method to Increase the Carbon Reduction Capacity of Urban Street Trees

The Role of Green Infrastructure in Enhancing Microclimate Conditions: A Case Study of a Low-Rise Neighborhood in Abu Dhabi

Urban heat islands are characterized by the increased temperature in urban areas compared with the rural surroundings due to human-made interventions that replace natural lands with buildings and roads. This study focuses on the assessment and utilization of using local nature-based solutions such as trees, sensitive landscaping types and design strategies to enhance microclimate in neighborhood streets and the public realm in desert areas, taking Abu Dhabi as a case study. The research utilizes a design-based approach to propose landscaping and layouts of urban street trees in low-rise residential urban areas. In this study, two methods namely an on-site measurement using citizen science, and a numerical simulation model in the ENVI-met software are used. Site-measurements included the tree physical characteristics such as tree height, crown width (crown spread/diameter), and trunk height, and the use of technology (photography and the Fulcrum mobile application, Nikon Forestry pro Laser Rangefinder) and air temperature around trees. ENVI-met included four scenarios: 1—“no-vegetation”, 2—“grass-only”, 3—“existing conditions” and 4—“proposed landscape design”. Grass and three types of local street trees are used in the proposed scenarios including Ghaf, Poinciana, and Temple tree. In addition, a standard of 6 and 8 m spacing between each tree is applied to determine the effect of varying vegetation densities on the outdoor temperature. The combined results using citizen science and the model allowed the identification of particular urban tree species that show substantial cooling effects. This is the case of Poinciana trees, which decreased the air temperature up to 0.9 °C when spaced every six meters in pathways and open unshaded areas amongst alleys, improving the overall thermal conditions in neighborhoods of hot-arid landscapes.

Urban Environment Evaluation and Analysis Based on Street View Image

The real and intuitive reconstruction of the city street scene can not only give the viewer an immersive feeling, but also serve as a way to display information. As a mass data, it also provides unique decision-making ideas for recording phenomena, discovering rules and predicting results. This paper proposes a green visual rate to measure the green apparent area of urban street trees. For complex urban street landscapes, the green apparent area index can well reflect the visualization degree of green area of street trees and urban environment under street scenes. Research to Skesfehervar, Hungary as an example, it carried out the street tree green apparent area calculation model test research in different characteristics of road sections, different street scenes under the authenticity and adaptability.

Biodiversity of urban street trees in Italian cities: a comparative analysis

Street trees are key elements in urban ecosystems from several points of view, and different species can have different functions (i.e., ecological, socio-economic). As such, a careful biodiversity assessment is pivotal. We evaluated urban street trees in 15 Italian cities along a geographical and bioclimatic gradient. The data was obtained from scientific literature and Garden Service Offices of the cities. The distribution patterns of the species were analyzed in terms of taxonomy, chorology, and tree size. We registered 277 tree species belonging to 48 families. The species richness is very variable in analyzed cities, and it is not directly correlated with the city’s size. Only three species (Platanus × hispanica, Celtis australis and Quercus ilex) are common to almost all cities, while 41.52% of the species are present only in one city. The comparative analysis of species, considering the floristic and city distribution, showed different results, highlighting Pinaceae as the most common family. The chorological analysis shows a predominance of exotic species and a good relationship with the bioclimatic conditions of cities. A selection of the species, as street trees to maximize their ecological and socio-economic functions, is pivotal in urban landscaping and green management.

Mapping the diversity of street tree inventories across eight cities internationally using open data

Tree diversity, on a species-, genus-, and family-level, is an important factor in securing healthy urban forests and providing ecosystem services for billions of city dwellers. Using open-source data on global tree inventories, this study examines (1) the diversity of species, genera, and family of urban street trees in eight cities internationally; (2) how they score on diversity benchmarks and indices; and (3) the diversity variation inside and outside of cities’ centers. We hypothesized most cities would score poorly on diversity benchmarks and spatial patterns in species composition would exist, as illustrated by established relationships between urban density and urban tree diversity. Results indicate city centers were less likely to approach the proposed diversity benchmarks than outside the city center. Overall, both Shannon and Simpson diversity indices show greater diversity outside of the city center, especially at the species-level. Understanding street tree diversity and spatial variation patterns across cities internationally can offer needed evidence to back up heuristic benchmarks. The methodology and open-source data used in this study are intended to enable practitioners better target tree diversity efforts.

Urban street tree characteristics and benefits in city centre, Reykjavik, Iceland

Urban street trees are often overlooked as essential contributors to the total urban canopy. The city centre of Reykjavik, Iceland was surveyed using random plots throughout the central portion of Reykjavik to determine species composition, structure, potential conflicts with infrastructure, and environmental benefits. In this portion of Reykjavik, 11 tree species were observed, dominated by Populus trichocarpa. For all species, the density was approximately 67 trees/km, which when compared with other Nordic cities was quite high. However, in terms of number of species, diversity was much lower compared to other Nordic cities. Street trees in this portion of the city sequester approximately 793 Mg of carbon. Value to the city summed to 15,764,000 ISK (134,000 USD), which partitioned to 187,000 ISK per kilometer or 128 ISK per capita. There were also conflicts identified with buildings and with sidewalks buckling, particularly with P. trichocarpa. Given the density and value of the trees in this portion of Reykjavik, it is imperative for additional work to be undertaken to manage the urban canopy and mitigate any costs associated with conflicts between trees and infrastructure throughout the city.

An Over-Segmentation-Based Uphill Clustering Method for Individual Trees Extraction in Urban Street Areas From MLS Data

In this article, an over-segmentation-based uphill clustering method for individual extraction of urban street trees from mobile laser scanning data is proposed to solve the problem that the existing methods depend heavily on tree trunks and have poor extraction results in complex environments where the tree trunks are blocked by cars and green belts, and the crown touching or interlocking is large. First, supervoxels are generated by over-segmentation, so that the amount of original data is reduced and the boundaries of different objects are effectively preserved. Then, the potential tree crowns and trunks are obtained by extracting typical object structures. Finally, individual trees extraction is realized by extracting independent crowns from the potential crowns via uphill clustering and searching corresponding trunks from the potential trunks. The main contribution of this article is to propose an individual extraction method for street trees based on uphill clustering that does not rely on the extraction of tree trunks, which improves the completeness of extracted results in complex urban environments. The experimental results demonstrate that the proposed method effectively extracted the street trees individually from the test data, with the completeness of 100%, the correctness of 96.4%, and the F-score of 0.98. Moreover, the proposed method also achieves good result for the extraction of greening trees that are heavily blocked in the green belt areas. And the corresponding completeness, correctness, and the F-score are 94.6%, 83.3%, and 0.89, respectively.

Assessing the effects of urban street trees on building cooling energy needs: The role of foliage density and planting pattern

This study presents an one-way coupling approach between the ENVI-met microclimate model and the EnergyPlus building energy simulation program, to assess the effect of the urban greenery on the improvement of the buildings’ cooling energy needs, in a dense urban area in Thessaloniki, Greece. Three commonly encountered urban tree species with different foliage densities are analyzed, whereas 2 different planting patterns are also considered. The obtained results indicate that the potential of trees on cooling the ambient air temperature and regulating the buildings cooling energy needs is mainly attributed to the radiative shading and the respective reduction of the solar heat gains of the exposed building façades. Moreover, the reduction of the building’s cooling energy demand due to the addition of trees is directly related to their foliage density and their planting pattern. The higher energy savings up to 54 % have been achieved when the trees formed a continuous shading canopy and for the Leaf Area Density of 2.5 m2/m3. Yet, the cooling potential of street trees has been found rather minor when they were not tall enough to shade the biggest part of the outer building façade.

Factors determining on-site perception of ecosystem services and disservices from street trees in a densely urbanized area

Recent studies have increasingly explored peoples’ perceptions of urban greenspaces to incorporate their consensus and preference into a more feasible design and planning of urban ecosystems. However, little is known about the relative importance of several factors determining perceptions of urban street trees, which is fundamental to unveiling the full picture of perception. Here, we examined the influences of possible factors ranging from social to environmental ones on people’s on-site perception of street trees in the Minatomirai-21 district in the Tokyo-Yokohama region. Using a structured questionnaire, we conducted a survey to enquire about on-site perceptions of the ecosystem services and disservices potentially provided by street trees. Results suggested that attributes of urban building environments (street width and building height) and street trees (tree density and height, as well as dominant planted species) had little effect on the on-site perceptions of street trees. Instead, the results generally showed that on-site perceptions differed according to age, gender, and season. Relatively high perceptions of positive aspects of street trees, regardless of their attributes across sites, probably reflected people's satisfaction with street trees and streetscapes in the Minatomirai-21 district. Moreover, in the context of people's perception, our results suggested that the negative effects of street trees on the quality of life are limited. We argue that ensuring a certain number of street trees, even in a densely urbanized area, should be essential to benefit the quality of life among various stakeholders such as visitors, office workers, and residents.

Examining the spatial distribution and temporal change of the green view index in New York City using Google Street View images and deep learning

As an important part of the urban ecosystem, urban trees provide various benefits to urban residents. It is therefore important to examine the spatial distribution and the temporal change in urban tree canopies. Different from traditional overhead view remote sensing-based methods, street-level images, which present the most common view that people have of greenery, provide a more human-centric way to quantify street tree canopies. This study mapped and analyzed the spatial distribution and temporal change in the green view index, which represents the visibility of tree canopies along streets in New York City during the last 10 years using historical Google Street View images. Deep learning and computer vision algorithms were used to derive the quantitative information of street tree canopies from street-level images and map the spatial distribution of the green view index. This study further investigated the potential disparities in terms of green view index across different racial/ethnic groups by comparing with census data. Results show that non-Hispanic Whites tend to live in neighborhoods with higher green view index and Hispanics tend to live in neighborhoods with lower green view index. The green view index values in New York City have increased slightly in the last 10 years, and the change of green view index has no significant correlation with resident’s ethnic/racial status. This study proves the usability of historical Google Street View images for monitoring the temporal change of urban street tree canopy changes at large scale, and it also provides insights and a valuable reference for urban greening programs.

A Comparative Study on Phenotypic Plasticity of Seven Urban Street Tree Species in Two Contrasting Environments

A Comparative Study on Phenotypic Plasticity of Seven Urban Street Tree Species in Two Contrasting Environments

An Assessment of Street Tree Diversity: Findings and Implications in the United States

Integrating tree species diversity into urban forest management can help create resilient tree populations. In this study, the abundance of trees within families, genera, and species levels was determined through a system developed to evaluate the diversity of urban street trees at different scales. Municipal foresters were asked to report the six street tree species most commonly used in urban forests as an assessment of tree diversity. Through the use of this question in a series of urban forest management surveys, we were able to describe the diversity of urban street tree species in the 48 continental United States across time (1974 to 2014) and space (national, regional, and local levels). Throughout the United States the top six street tree species were distributed in 115 species, 71 genera, and 32 families. At the national scale, no one tree species or family dominated, but the Acer genus was 21.2% (0.8 standard error, or SE). At the regional level, 16.5% (3.3 SE) of all reported street trees were Acer platanoides in the Northeast region, and 8.1% (1.1 SE) of the South region were Quercus virginiana. At the local level, however, a lack of tree diversity becomes apparent. In any community, the top six reported tree species account for 61.5% (0.4 SE) of the total street tree population and the most common tree accounts for 23.7% (1.0 SE) of the total. When assessed over a 40-year period, changes in tree species and genera in each region were relatively minor. Among them, the Mideast and Northeast regions continued to be dominated by Acer and the South region maintained its preference for Quercus. This study provides a method to assess street tree populations when using surveys to quantify municipal forestry programs at state and national levels.

Open space networks can guide urban renewal in a megacity

As human populations move into cities they are increasingly isolated from the natural world, with associated negative impacts on health and well-being. However, as cities renew themselves through urban redevelopment and climate-adaptation, opportunities arise to improve people’s access to urban green areas that can be informed by modeling the network of urban open spaces. Recent research identified the need for multi-criteria indices of access to urban green spaces. Including open spaces such as empty lots, ground- and air-spaces surrounding buildings, and spaces associated with roads and other linear features can improve planning for urban greenspaces by identifying areas of opportunity for additional greening. Further, the gradient of interconnections among open spaces can be used to prioritize urban greening locations to build green networks. We modelled all open-space connections across 605 km2 in Seoul, population 10.3 million, using Omniscape, a landscape connectivity model. We combined the resulting open-space connectivity map with distance-based indices for existing urban parks and street trees. Combining these criteria permits rank-prioritization of locations where new green spaces would most improve residents’ access. We found 2910 of 3375 (86.2%) locations where urban green spaces already exist within 300 m for city residents. Of the remaining 465 locations, 276 are in areas with the lowest-open space connections. For urban street trees, 44.3% of the 2588 km of the city’s major roads are already planted with street trees. Of the remainder, 210 km (8.1%) are located in the areas with the least connections to green spaces. Nine new urban parks would provide relief for the most highly-impacted areas, where the flow of open space is lowest and where no green spaces are available within 300 m. The integration of a spatial model typically used for conservation assessments with city planning provides useful additional context for building urban health.

The benefits of tree wounds: Microhabitat development in urban trees as affected by intensive tree maintenance

In addition to social and economic benefits, urban trees provide habitats for many different species and contribute significantly to urban biodiversity. To prevent harm to humans or infrastructure, tree risk assessment is conducted regularly in urban areas. Resultant pruning activities are often criticized as leading to unnecessary injuries and artificial tree shapes. In this study we compared different tree maintenance levels and the occurrence of biodiversity related structures, so-called tree microhabitats, to provide another perspective on the consequences of pruning urban trees.For a sample of urban street and park trees in Montreal, Quebéc, Canada, we found that the positive relationship between tree size (diameter at breast height) and microhabitat abundance and diversity was less steep than in a nearby forest with comparable species composition. Intensive tree maintenance in urban trees led to levels of certain microhabitats such as cavities and injuries that were comparable to natural, unmanaged forests. Light maintenance of urban trees encouraged more crown deadwood than typical and intensive maintenance levels. Tree species as well as cardinal direction of the nearest building played a minor role in the occurrence and development of microhabitats.Our results underline the importance of conserving and maintaining large living trees, especially in urban areas to provide tree microhabitats. These results also demonstrate the important role of intensive tree maintenance in stimulating tree microhabitat development in urban areas. However, new ways have to be found to reconcile the need for microhabitats and the maintenance of structurally healthy urban trees.

Street trees in italian cities: story, biodiversity and integration within the urban environment

Since the nineteenth century, street trees have become a significant component of urban landscapes, with multiple ecological roles and social-economic functions. However, data about their abundance and diversity, and information about the criteria used for the species selection, are still limited. Besides providing a historical overview, we analyze and compare data on abundance, diversity and criteria of species selection of street trees in 15 Italian cities chosen considering their representativeness along a bioclimatic transect across Italy. The biodiversity of urban street trees resulted quite rich with 277 species (about 60% are exotic); among these, Platanus × hispanica Mill. ex Münchh., Celtis australis L. and Quercus ilex L. resulted as the most common species. Besides the general suitability for the local climatic conditions, several factors seem to concur in the species selection, such as their aesthetic appeal and dimension. Other selection criteria based on the species’ ecology or functionality were not evident.

The street as workspace: Assessing street vendors’ rights to trees in Hyderabad, India

Urban street trees provide critical ecosystem services such as shade and contribute to the health and well-being of urban residents - especially those who spend much of the day outdoors. Our paper focusses on one of the most vulnerable groups in a city: street vendors, who spend much of their time on streets, with or without trees. Most contemporary visions for green cities ignore street vendors and fail to consider their uses and expectations of public green spaces. Drawing from in-depth interviews with 75 street vendors in 11 market streets in the hot Indian city of Hyderabad, we discuss their perceptions of the importance of trees for their daily work and livelihood and describe the challenges that street vendors face in accessing shade in a hot city. In several instances, “development” of urban areas has led to gentrification, with the felling of trees and displacement of street vendors, bringing into question their ‘right to the city’. We argue that street vendors are largely left out of considerations of urban ecological planning, despite being one of the groups most affected by the availability of wooded streets in cities. Ecologists, urban researchers, planners and landscape designers must explicitly consider greening strategies from the perspective of street vendors for inclusive, sustainable urban development.