Historical drivers of street tree species selection: A comparative archival study of Canadian prairie cities

Anomalies in Australian municipal tree managers’ street-tree planting and species selection principles

Street trees provide ecosystem services such as heat mitigation, improved community well-being, and biodiversity conservation. At the wildland–urban interface (WUI), high-flammability street trees also provide a conflicting ecosystem disservice, heightening risks of wildfire spread into urban areas. We addressed this service–disservice conflict by assessing shoot flammability patterns in 10 street tree species, to identify low-flammability species that can potentially mitigate wildfire risks at the WUI. We found significant differences among species in flammability attributes including time-to-flame (TTF), flame duration (FD), number of flaming events (nF), and flame temperature (FT), and identified low-flammability species for each attribute. Overall, species’ rankings from least to most flammable differed considerably across the four attributes. For example, native water gum (Tristaniopsis laurina) had the slowest TTF, but had the longest FD. Among nine shoot traits, we found that high leafing intensity was the most frequent trait correlated with flammability. In particular, high leafing intensity was significantly related to fast TTF and high FT. Lack of coordination among flammability attributes suggests that, in general, selection of low-flammability street tree species should consider how each flammability attribute differentially contributes to wildfire spread risk. Nonetheless, native Tuckeroo (Cupaniopsis anacardioides) emerged as a potential candidate for further exploration as a low-flammability street tree as it had comparatively long TTF, short FD, and low nF. We found no consistent evidence that exotic species were less flammable than native species, and suggest that native trees be the focus of further research to identify low-flammability street trees.

Street trees in cities display a varied pattern that shape the city's physical fabric. This article presented the methodological aspects of a proposed selection process for the key urban street tree species. The process of this study was twofold: i¬rst it was aimed at selecting the appropriate tree species through field trip and data analysis of the street trees in streets and parks of the natural secondary forests, the ancient and famous trees in Hefei. Secondly, a selection among alternative tree species in the inventory was made using the Analytic Hierarchy Process (AHP) method and the expert knowledge approach. In the methodology, the weights of the selection criteria were determined by pair-wise comparison matrices of AHP. The priority ranking of the selection trees depended on expert evaluation of the comprehensive quality of the trees. Based on the use of subjective and objective data sources, the selection process could be adapted for use in the street tree planning of Hefei, as well as other cities. Key words: Urban tree, street tree, species selection, urban green space, AHP.

Tree abundance, species richness, or species mix? Exploring the relationship between features of urban street trees and pedestrian volume in Jinan, China

Identifying simple tree attributes of street trees, i.e., tree height, crown width and crown height obtained by unmanned aerial vehicles (UAV), plays a significant role in urban management to maximize the ecological benefits of street trees. However, simple attributes usually fluctuate over a wide range due to differences in tree-age and growing environment, leading to inconspicuous interspecific features and low classification accuracy. Composite attributes, expressed by two or more simple attributes, can be used to reduce the variability in simple tree attributes, thus providing an alternative to improve the accuracy of street tree classification. In this study, we examined the classification effects of simple attributes and simple-composite attribute combinations by back propagation (BP) neural network, K-nearest neighbor (KNN) and support vector machine (SVM). The results showed that (1) the values obtained by UAVs and observations were highly consistent and R2 values for tree height, east-west crown width, north-south crown width and crown height were 0.90, 0.87, 0.78 and 0.76, respectively. The relative errors of tree height were the most stable among different tree species, followed by the crown height, east-west crown width and north-south crown width. (2) Compared to simple attributes, composite attributes displayed significant differences among street tree species, and these differences were helpful for identifying street tree species that could not be identified with simple attributes. (3) The accuracy of tree species identification after including corresponding composite attributes can be improved by 29.7% (kappa coefficient improved by 0.34) compared with only using simple attributes. The results suggested that consideration of composite attributes in street tree species classification reduced the mistakes for identifying tree species, thus providing a new approach for identifying street tree species and managing street trees efficiently.

One major benefit of urban trees is the shade they provide on sunny days; this reduces the heat stored in engineered surfaces and lowers the heat load on people, increasing their comfort. This study compared the shading effectiveness of five small street tree species within the urban landscape of Manchester, UK. The area of shade produced by each tree during early and midsummer 2012 was calculated from morphological measurements, such as canopy height, width, and aspect ratio. The effect of tree shade on air, mean radiant and surface temperatures was also compared and related to the leaf area index (LAI) of the canopy. It was found that tree shade reduced mean radiant temperatures by an average of 4°C, though neither tree species nor LAI had a significant effect. Tree shade reduced surface temperatures by an average of 12°C, and the tree species and LAI both had significant effects. Tree species with higher LAI, Crataegus laevigata and Pyrus calleryana, provided significantly more cooling than the other species, and surface temperature reduction was positively correlated with LAI. This study has shown that trees are useful in improving both human thermal comfort and reducing surface temperatures in urban areas, and that selection of tree species with high LAI can maximize the benefits they provide.

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

Pragmatic factors outweigh ecosystem service goals in street tree selection and planting in South-East Queensland cities

Street trees provide essential ecosystem services and are important to local residents’ life, but there has not been much research on the structure of street trees in the city of Changzhou, China. This research surveys the structure of street trees in Changzhou with the aid of geospatial technologies and field sampling surveys. An investigation of the street trees on the 220 sampled transects of streets across the four districts of Changzhou reveals prominent problems with the species diversity, distribution, age structure, and density. Though street tree density in the city is approximately 177/Km, higher than many other cities in the world, with only 29 street tree species in 19 families and 27 genera, the diversity is lower. Among the tree species, only a limited few are planted widely. Young trees account for 43.2% of the total tree population, while large old trees decrease in number. Close spacing between trees, caused by unreasonably high density, results in enormous government expenses on pruning. These findings highlight the need for careful selection of appropriate tree species on the part of municipal administrators and planners. Suggestions are given on how to achieve a sustainable structure of street trees in Changzhou in the long run. Both the problems found and the corresponding suggestions are of referential meaning to the street tree development of other cities in China.

Developing sustainable urban areas in such a way that every citizens can enjoy a high quality of life and environment is one of the pressing environmental challenges we face today. This study was conducted to identify the major urban tree species and people’s perceptions towards urban forestry for sustainable urban development in Hetauda Sub-metropolitan City, Nepal. Mixed method of field assessment and questionnaire survey with urban households (N=125) was used to collect information regarding urban forestry by applying simple random sampling. Data were analyzed using descriptive and Likert scale rating through SPSS. The results showed that 46 different tree species were recorded in the study area with most frequent being Saraca asoca, Mangifera indica and Ficus religiosa. Based on local people’s preference Sparaco asoca, Nyctanthes arbortristis, and Thusa spp. are top 3 species for urban plantation. People opined that management of urban forest is not so good and they responded that poor management of roadside trees is due to lack of people participation with weighted mean value of 2.10. More than 90% of the respondents agreed that urban trees are essential for sustainable urban planning. Furthermore, rapid urbanization, lack of public awareness, and limited institutional support are major challenges while community engagement, ecological resilience, and government initiatives and policies are major opportunities for urban forestry development in the city. Therefore, appropriate selection of tree species, community participation and awareness regarding urban forestry should be emphasized. Furthermore, urban forestry program requires its prioritization and integration in the local and national policies.

"Background: In contrast to trees from northern hemisphere genera, there has been little research into the selection and vegetative propagation of Australian native tree species for use as street trees. Eucalyptus leucoxylon F. Muell. is one of a few eucalypts occurring in south-eastern Australia with bright coloured fl owers and is highly regarded as an ornamental tree that fl owers readily. It is propagated from seed, but progeny typically show seedling variability and diversity. Eucalyptus leucoxylon was identifi ed as the most widely planted eucalypt in the streets of the city of greater Melbourne, Australia. Methods: This research assessed 300 E. leucoxylon street trees growing across the city of greater Melbourne for their performance against arboricultural criteria relating to canopy structure and density, straightness of the trunk, health (assessed on canopy, trunk, and branch condition, production of exudates, and presence of fungal fruiting bodies), fl ower colour, and root systems. Results: The results showed that E. leucoxylon was a suitable street tree species with most specimens showing good habit, vigour, and health. Discussion: The trees had traits such as live crown ratio, height, fl ower colour, and capacity to cope with pruning that are considered appropriate for a street tree. Their dense canopies and high live crown ratios provide shade that can reduce the urban heat island (UHI) effect. Conclusion: This suggests the species has the potential to be a successful street tree not only in Australia, but in other parts of the world where it has been grown successfully in forestry plantations.

Afforestation can increase soil organic carbon (SOC) storage, but the selection of tree species may be critical. This study explored soil CO2 production and effluxes in relation to SOC contents in temperate forests. Sites included even-aged (38 years) first generation stands of eight coniferous and two deciduous species planted at three sites along a gradient in soil fertility. SOC stocks (forest floor + mineral soil 0–50 cm) differed significantly between tree species, but soil type influenced SOC stocks the most. SOC stocks were significantly higher for stands on low-fertility sandy soils (141 ± 31 Mg C·ha–1) than stands on fertile loamy soils (62 ± 14 Mg C·ha–1). Soil CO2 effluxes were measured on two occasions in 2002 and varied between 18.7 and 36.3 kg C·ha–1·day–1. Variability within temperature-adjusted fluxes was not clearly related to tree species or soil type. Potential CO2 production rates in a laboratory incubation experiment (reported as CO2 produced per unit soil C) were mainly affected by site and were two times higher for soils from the nutrient-rich site compared with the most nutrient-poor site. Thus, the combined effect of contrasting CO2 production rates and stocks of soil C may explain the similar CO2 effluxes measured in the field. Results suggest that selection of soil type and tree species are important for SOC sequestration in future afforestation projects.

Street trees make up an important part of the eco-environment and landscape of urban roads. The species of street trees significantly affect the green volume of urban roads. The leaf area index (LAI) is often adopted to measure the ratio of green volume for urban roads, laying a scientific basis for optimizing street trees. This paper measures and analyses the LAI and green plot ratio (GPR) of 14 common street tree species in Xinxiang, a city in Central China’s Henan Province. The results show that, except for evergreens, the LAI values of deciduous trees varied significantly from month to month, forming a single-peaked curve. The LAI values of street trees have a significant positive correlation with the day of year (DOY) (P<0.01). As for the roads with a single row of street trees, the highest mean annual GPR values were achieved by Juglans regia Linn., followed in turn by Ligustrum lucidum Ait., Sophora japonica L., Populus tomentosa Carrière, Fraxinus chinensis Roxb. and Platanus orientalis Linn. Among the 12 common types of double-row road tree combinations, the GPR values all increased first and then decreased; the largest annual mean value belonged to the combination “Sophora japonica L.+ Sophora japonica L.” In the same section, the annual mean GPR value of double-row road trees was 3-7 times higher than that of single-row road trees. Our research demonstrates that the GPR can quantify the differences between different street tree species and combination types, and help to optimize the greening arrangement and plant configuration.

Forests are the main components of terrestrial ecosystems and play an important role in the protection and construction of the national ecological security barrier. For a long time, China's large-scale afforestation had been practiced in areas with rainfall higher than the 400 mm threshold, but the issue of afforestation in high altitudes on the Tibetan Plateau remains elusive in both practical experience and theoretical exploration. It is worth thinking further about what principles should be followed in the selection of tree species and suitable altitudes for afforestation in high-altitude areas, as well as what experiences and lessons of previous afforestation efforts should be applied in high-altitude areas. As per the law of vegetation zonal distribution, this paper argues that afforestation at high altitudes should comply with the principle of vegetation zonal distribution and the low temperature limitation, and points out that afforestation is feasible only within the forest distribution area and below the altitudes of climate timberlines. Furthermore, we demonstrate the potential spatial areas of afforestation, and determine the local tree species that may be used for afforestation based on the existing problems of afforestation in eastern Tibet. In summary, afforestation in high-altitude areas of the eastern Tibetan Plateau must comply with the law of zonal vegetation distribution, focus on the upper limit of altitude and the selection of suitable tree species, and adopt only suitable native tree species.

Abstract. Cities have become increasingly interested in reducing their greenhouse gas emissions and increasing carbon sequestration and storage in urban vegetation and soil as part of their climate mitigation actions. However, most of our knowledge of the biogenic carbon cycle is based on data and models from forested ecosystems, despite urban nature and microclimates differing greatly from those in natural or forested ecosystems. There is a need for modelling tools that can correctly consider temporal variations in the urban carbon cycle and take specific urban conditions into account. The main aims of our study were to (1) examine the carbon sequestration potential of two commonly used street tree species (Tilia × vulgaris and Alnus glutinosa) growing in three different growing media by taking into account the complexity of urban conditions and (2) evaluate the urban land surface model SUEWS (Surface Urban Energy and Water Balance Scheme) and the soil carbon model Yasso15 in simulating the carbon sequestration of these street tree plantings at temporal scales (diurnal, monthly, and annual). SUEWS provides data on the urban microclimate and on street tree photosynthesis and respiration, whereas soil carbon storage is estimated with Yasso. These models were used to study the urban carbon cycle throughout the expected lifespan of street trees (2002–2031). Within this period, model performances were evaluated against transpiration estimated from sap flow, soil carbon content, and soil moisture measurements from two street tree sites located in Helsinki, Finland. The models were able to capture the variability in the urban carbon cycle and transpiration due to changes in environmental conditions, soil type, and tree species. Carbon sequestration potential was estimated for an average street tree and for the average of the diverse soils present in the study area. Over the study period, soil respiration dominated carbon exchange over carbon sequestration due to the high initial carbon loss from the soil after street construction. However, the street tree plantings turned into a modest sink of carbon from the atmosphere on an annual scale, as tree and soil respiration approximately balanced the photosynthesis. The compensation point when street tree plantings turned from an annual source into a sink was reached more rapidly – after 12 years – by Alnus trees, while this point was reached by Tilia trees after 14 years. However, these moments naturally vary from site to site depending on the growing media, planting density, tree species, and climate. Overall, the results indicate the importance of soil in urban carbon sequestration estimations.