Survival is not enough: The effects of microclimate on the growth and health of three common urban tree species in San Francisco,California

Urban forest managers must balance social, economic, and ecological goals through tree species selection and planting location. Ornamental trees are often popular in tree planting programs for their aesthetic benefits, but studies find that they have lower survivability and growth compared to larger shade trees. To maximize ecosystem services within these aesthetic preferences, it is important to select species carefully based on their ability to grow in each particular climate. However, little locality-specific and species-specific data exist on urban trees in many regions. This study examines the growth, survival, and vigor of three common ornamental street trees in San Francisco’s three different microclimate zones after over 16 years since planting. While we found over 70% survival for all three species throughout the city, there were significant differences in health and vigor among microclimates for each species, likely due to differences in drought-tolerance. While Arbutus had the greatest proportion of healthy trees in the Fog Belt and Sun Belt zones, Prunus cerasifera had the greatest proportion in the Sun Belt, and Prunus serrulata had the greatest proportions in the Transition and the Sun Belt zones. This species-specific and climate-specific information will better equip urban foresters to target both planting and tree-care of these popular species appropriately to maximize the benefits provided by these street trees while still maintaining a diverse canopy. Finally, we argue that simple survival calculations can mask more complex differences in the health and ability of different urban tree species to provide ecosystem services.     

20 years later: Does reduced soil area change overall tree growth?

Urban conditions have been thought to affect tree growth, but there is little conclusive evidence as to the severity of those influences or how various species respond differentially to urban stress. Reduced growth expectations are important to understand, because they affect design choices for the urban tree canopy, particularly as required by legislative mandate. Five tree species (Acer rubrum, Prunus serrulata, Pyrus calleryana, Quercus pallustris and Zelkova serrata) grown in parking lots ranging from 18 to 23 years old in central and northern New Jersey, USA were studied. Tree height, diameter at breast height (DBH), and canopy radius were measured, as was apparent plant available soil (nonpaved planting zone area). Tree DBH, commonly recorded for many municipal inventories, was found to be a useful predictor of canopy area. Data were normalized within site, to facilitate multiple site analysis. Across different parking lots, reductions in tree size were consistently associated with reduced apparent soil access. A previous study from Florida, USA was used for comparison of regional data, permitting conclusions on canopy reductions, relative to specification of design space for tree establishment.     

Trees in planters – Growth,structure and ecosystem services of Platanus x hispanica and Tilia cordata and their reaction to soil drought

Urban environments are often characterized by extensive paved surfaces, exacerbating the urban heat island effect. At the same time, limited root space due to underground infrastructure poses a challenge for planting new trees in these areas. Trees in planters have emerged as popular design elements, offering innovative and sustainable greening solutions, particularly in urban environments with limited rooting space. However, growing conditions in planters may strongly impact tree growth and the provision of environmental ecosystem services (ES). In this 3-year study, we analyzed tree growth and ecosystem services (cooling by shading, CO₂-fixation) of London plane (Platanus x hispanica Münchh.) and small-leaved lime (Tilia cordata Mill.) in four planting treatments: in-ground (G), planters in the ground (PG), non-insulated plastic planters (P), and insulated planters (PI). We also recorded soil temperature throughout the experiment and implemented soil drought conditions by reducing soil irrigation for half of the trees after one year. Our findings revealed higher thermal fluctuations in soil temperature within non-insulated plastic planters (P), reaching a maximum of 45 °C, surpassing the critical temperature threshold for plant growth (>38 °C). In contrast, insulated planters (PI) effectively mitigated soil temperatures, staying below 33.8 °C. When planted in the ground (G), P. x hispanica exhibited a significantly higher stem diameter increment (52–66%) compared to other planting treatments, aligning with the provision of ecosystem services. However, T. cordata trees showed a more moderate response to planting treatments in terms of growth and ecosystem service provision. Furthermore, the implementation of soil drought conditions resulted in a reduction of up to 34% in stem diameter increment for P. x hispanica and up to 25% for T. cordata. Our results underscore the necessity of tree species-specific knowledge about growth responses to different planting treatments for effective urban planning perspectives, as the provision of ecosystem services may be influenced differently.     

Understanding the stability of Samanea saman trees through tree pulling,analytical calculations and numerical models

There have been several cases of tree failure in Singapore. Many studies have shown that soil properties and root architecture of trees are important factors that govern tree stability. Twenty Samanea saman trees were planted in different soil media, which were original in situ soil, top soil, mixture of 50% granite chips and 50% top soil, and mixture of 80% granite chips and 20% top soil. The objectives of this study were to investigate tree overturning processes and also to compare the results of tree pulling tests with results from an analytical calculation and numerical modeling for different soil types. The results showed that stability of the trees were not governed by the shear strength of the soil. Factors that affected tree stability included cross-sectional area of the roots and root plate area. Tree pulling test and numerical modeling results showed that there were two modes of failure which occurred when a tree was overturned. The first mode was the shear failure of soil and the second was a combination of shear failure of soil and root breakage. The maximum overturning force obtained from the tree pulling test was in the same order of magnitude as the maximum overturning forces obtained from the analytical calculation and numerical modeling.     

Predicting canopy and trunk cross-sectional area of silver linden (Tilia tomentosa) in confined planting cutouts

Urbanized land is characterized by the dominance of paved surfaces. Increasing tree canopy in urbanized areas has been identified as an effective way to reduce stormwater runoff, sequester carbon, improve air and water quality, and otherwise mitigate the environmental impacts and increase the livability of cities. However, attaining sufficient tree canopy in urban areas remains an elusive goal. Site design characteristics such as cutout size may limit urban tree growth and complicate efforts to predict future canopy, especially in highly paved systems such as parking lots. We studied 25 silver lindens (Tilia tomentosa Moench) grown for 14 years at one site, in pavement cutouts of various sizes. Regression analysis, even on these limited data, indicated a strong relationship between tree size and canopy projection area and unpaved soil surface area, but not soil depth. Cutout size explained 70% of the variability in tree canopy projection area and 77% of the variability in trunk cross-sectional area. The addition of other variables, such as soil bulk density, did not improve the model. Trees growing in parking lot cutouts <5.3 m² attained only limited size, regardless of the level of soil compaction. In larger cutouts, however, increases in soil bulk density from 1.1 to 1.5 Mg/m³ were associated with a 70% reduction in trunk cross-sectional area. In order to create urban sites with a sustainable tree canopy, site design must provide large areas of uncompacted soil for trees and protect this soil from compaction during use. Urban tree growth models that incorporate cutout characteristics are needed to predict future canopy area with confidence.     

Growth, survival, and root system morphology of deeply planted Corylus colurna 7 years after transplanting and the effects of root collar excavation

Urban trees are frequently planted with their root collars and structural roots buried well below soil grade, either because of planting practices, nursery production practices, or both. These deeply planted structural roots can impair tree establishment and are thought to reduce tree growth, lifespan, and stability, although research has provided few and contradictory results on these questions to date. This study examines container-grown (55 L) Turkish hazel trees (Corylus colurna L.), planted either at grade, 15 cm below grade, or 30 cm below grade into a well-drained silt loam soil, over nearly 8 years. Five years after planting, in 2004, remediation treatments (root collar excavations) were performed on two replicates of each below-ground treatment. Subsequently, all trees were subjected to flooding stress by being irrigated to soil saturation for approximately 6 weeks. In 2006, flooding stress was repeated. Trees root systems were partially excavated in 2007, and root architecture was characterized. Deep planting did not affect trunk diameter growth over 8 years. Survival was 100% for the first 5 years; however, one 30 cm below grade tree died after flooding in 2004 and another died after the 2006 flooding. Photosynthesis was monitored during the 2004 flooding and all trees experienced decline in photosynthetic rates. There was an apparent slight delay in the decline for trees with excavated root collars and those planted at grade. Girdling roots reduced trunk taper and occurred primarily on unremediated trees planted 30 cm below grade.

Selected individual roots were excavated and followed from the root ball and were observed to gradually rise to the upper soil regions. Analysis of roots emerging from excavation trench faces indicated that vertical root distribution at approximately 1.25 m from the tree trunks was the same regardless of planting depth. Longterm consequences of planting below grade are discussed.     

Growth and stability of deep planted red maple and northern red oak trees and the efficacy of root collar excavations

Trees with root systems established well below grade due to deep planting or soil disturbance are common in urban landscapes, yet the long term effects of buried trunks and subsequent remediation strategies, such as root collar excavation are poorly documented. We evaluated the consequences of deep planting over a 10-year period on tree growth and stability, with and without root collar excavation, for red maple [Acer rubrum L. Red Sunset^® (‘Franksred’)] and Northern red oak (Quercus rubra L.) planted at grade or 30-cm below grade. Sleeves to prevent soil-trunk contact were installed around trunks on a subset of deep trees. Root collar excavations were made during the 6th growing season for both species and trees were grown for an additional 4 and 3 growing seasons for red maples and Northern red oaks, respectively. Within two weeks of root collar excavations, pulling tests compared the effect of treatments on stability of red maples. Deep planting generally slowed growth of red maple but had no clear effect on Northern red oak. Root collar excavation had no lasting effect on growth of either species. Approximately 55% of deep red maples and 33% of deep Northern red oaks had roots crossing and in intimate contact with buried trunks, suggesting a potential for future girdling roots. Approximately 25% of deep maples had substantial adventitious rooting. All deep Northern red oaks had new roots emerging just above the first original structural roots but none were clearly adventitious. Trunk sleeves had no effect on growth for either species. Neither deep planting nor root collar excavation resulted in a loss of tree stability compared to trees planted at grade, although failure patterns varied among treatments. Overall, the biggest long term concern for deep-planted trees is the potential for girdling root formation.     

Porous-permeable pavements promote growth and establishment and modify root depth distribution of Platanus × acerifolia (Aiton) Willd. in simulated urban tree pits

In dense urban areas with heavy pedestrian traffic, current trends favor covering tree pits with porous-permeable pavement over installing grates or leaving the soil exposed. However, pavement cover potentially modifies soil moisture and temperature, altering tree growth and overall resilience, especially when coupled with heat stress and drought in a changing climate. This study evaluated the response of newly planted London plane (Platanus × acerifolia ‘Bloodgood’) trees to porous-permeable resin-bound gravel pavement and associated alterations in soil water distribution and temperature, in two distinct physiographic regions in Virginia, USA. Simulated urban tree pits were either covered with porous-permeable pavement or left unpaved, and root growth and depth, soil water content and temperature, and tree stem diameter measured over two growing seasons. At both sites, trees in paved tree pits grew larger than trees without pavement. Stem diameters were 29% greater at the Mountain site and 51% greater at the Coastal Plain site, as were tree heights (19% and 38% greater), and above ground dry biomass (67% and 185% greater). Roots under pavement developed faster and shallower, with many visible surface roots. In contrast, unpaved tree pits had almost no visible surface roots, and at the Mountain site only occupied an average area of 7 cm² within the 1-m² tree pits, compared with 366 cm² in paved tree pits. Pavement may have extended the root growing season by as much as 14 days, as the average soil temperature for the month of October was 1.1 °C and 1.2 °C higher under pavement than in unpaved pits. Porous-permeable pavement installations in tree pits accelerated establishment and increased growth of transplanted trees, but may result in shallower root systems that can damage pavement and other infrastructure. In addition, shallow root systems may prevent water extraction from deeper soils, compromising drought resilience.     

Defect-disorder and risk assessment of heritage trees in urban Hong Kong

Heritage trees in a city, echoing factors conducive to outstanding performance, deserve special care and conservation. To understand their structural and health conditions in urban Hong Kong, 30 defect-disorder (DD) symptoms (physical and physiological) subsumed under four tree-position groups (soil-root, trunk, branching, and crown-foliage) and tree hazard rating were evaluated. The surveyed 352 trees included 70 species; 14 species with 233 trees were native. More trees had medium height (10–15 m), medium DBH (1–1.5 m) and large crown (>15 m). In ten habitats, public park and garden (PPG) accommodated the most trees, and roadside traffic island (RTI) and public housing estate (PH) had the least. Tree dimensions and tree habitats were significantly associated. The associations between the 2831 DD and tree-position groups, tree habitats and tree hazard rating were analyzed. Fourteen trees from Ficus microcarpa, Ficus virens and Gleditsia fera had high hazard rating, 179 trees from 22 species moderate rating, and 159 trees from 55 species low rating. RTI, roadside tree strip (RTS), roadside tree pit (RTP), roadside planter (RP) and stone wall (SW) had more moderate hazard rating, and PPG, roadside slope (RS) and government, institutional and community land (GIC) more low rating. Redundancy analysis showed that DD were positively correlated with RTS, RTP, RP and SW, but negatively correlated with PPG, RS and GIC (p < 0.05). The DD significantly increased tree hazard rating and failure potential. Future management implications for heritage-tree conservation and enhancement focusing squarely on critical tree defect-disorder in urban Hong Kong were explored, with application to other compact cities.     

Contrasting natural regeneration and tree planting in fourteen North American cities

Field data from randomly located plots in 12 cities in the United States and Canada were used to estimate the proportion of the existing tree population that was planted or occurred via natural regeneration. In addition, two cities (Baltimore and Syracuse) were recently re-sampled to estimate the proportion of newly established trees that were planted. Results for the existing tree populations reveal that, on average, about 1 in 3 trees are planted in cities. Land uses and tree species with the highest proportion of trees planted were residential (74.8 percent of trees planted) and commercial/industrial (61.2 percent) lands, and Gleditsia triacanthos (95.1 percent) and Pinus nigra (91.8 percent). The percentage of the tree population planted is greater in cities developed in grassland areas as compared to cities developed in forests and tends to increase with increased population density and percent impervious cover in cities. New tree influx rates ranged from 4.0 trees/ha/yr in Baltimore to 8.6 trees/ha/yr in Syracuse. About 1 in 20 trees (Baltimore) and 1 in 12 trees (Syracuse) were planted in newly established tree populations. In Syracuse, the recent tree influx has been dominated by Rhamnus cathartica, an exotic invasive species. Without tree planting and management, the urban forest composition in some cities will likely shift to more pioneer or invasive tree species in the near term. As these species typically are smaller and have shorter life-spans, the ability of city systems to sustain more large, long-lived tree species may require human intervention through tree planting and maintenance. Data on tree regeneration and planting proportions and rates can be used to help determine tree planting rates necessary to attain desired tree cover and species composition goals.     

Genetic structure of Buxus sinica var. parvifolia, a rare and endangered plant

Buxus sinica var. parvifolia, a rare and endangered tree species in some semitropics alpine areas of China, plays an important role in the maintenance of the landscape and ecosystem. In this study, RAPD and ISSR markers were used to investigate the genetic diversity and structure of five natural populations and one tamed population of B. sinica var. parvifolia. 21 RAPD primers amplified 209 bands with 167 (79.90%) polymorphic and 21 ISSR primers amplified 518 bands with 467 (90.15%) polymorphic. The genetic diversity, estimated by Shannon’ index, was 0.4343 (by RAPDs) and 0.3661 (by ISSRs). Both RAPD and ISSR analyses revealed a high level of genetic diversity in natural populations of B. sinica var. parvifolia. Furthermore, analysis of molecular variance (AMOVA) was used to apportion the variation within and between populations. The proportion of variation attributable to within-population differences was very high (69.2% by RAPDs; 84.51% by ISSRs). Moderate differentiation was detected among populations using RAPDs (30.80%), while only a small amount of variation (15.49%) was detected among populations using ISSRs. We suggest that the present genetic structure is due to high levels of environmental variability and gene flow, which still need further study to confirm. Conservation measures are suggested, including in situ and ex situ strategies, based on the observed population genetic information.     

Reponses of mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding in calcareous soil

Physiological and growth responses of ‘Pantin’ and ‘Magana’ mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding were studied in a series of experiments. Trees were grown in containers in a very gravelly loam soil and were subjected to continuous flooding of the root zone for 30–66 days (Experiments 1 and 2) or alternating flooding–unflooding cycles for 50 days (Experiments 3–5). For all experiments, the control treatment consisted of nonflooded trees. Net CO₂ assimilation (A) and stomatal conductance (g_s) decreased within 3 days of continuous flooding and internal CO₂ concentration was significantly higher in leaves of flooded than nonflooded plants. In the cyclic flooding experiments, trees were flooded in 3- to 6-day cycles and then unflooded for the same time periods. Stomatal conductance and A decreased within 3 days of flooding, leaf epinasty occurred between days 5 and 10, leaf senescence and abscission occurred between days 15 and 30, and branch dieback and tree death occurred between days 30 and 60. Three cycles of 3-day flooding and 3-day recovery of trees had little effect on leaf gas exchange of ‘Magaña’ trees. Similarly, ‘Pantin’ trees survived 3 cycles of 6 days of flooding interspersed with 3–6 days of recovery despite consistent decreases in g_s and A during flooding. Stomatal conductance and A of both mamey sapote cultivars decreased within a few days of flooding and this species appears to have intermediate flooding tolerance compared with other tropical fruit crops based on tree survival.     

How good are containerized trees for urban cooling?

The capacity of urban trees in mitigating urban heat is well-known. As space is often limited, one feasible option for increasing the urban green would be containerized plants. Nevertheless, for optimizing the vitality and benefits, detailed knowledge on tree growth reactions in different types of containers is missing. We designed an experiment with two commonly planted but ecologically contrasting urban tree species Tilia cordata and Platanus x hispanica planted into the ground and in containers according to four different planting types, with or without drought stress. Along with the meteorological variables, continuous soil moisture and temperature at 25 cm depth, sap flow, as well as measurements of leaf physiological responses i.e. stomatal conductance, mid-day leaf water potential and chlorophyll content were measured three times on sunny and warm summer days during 2020 and 2021. P. hispanica showed more than double diameter increment at breast height in the ground than in containers; however, the growth trend was relatively better for T. cordata in containers. While comparing different container types and species reactions, it was clear that soil temperature within the plastic containers were significantly higher, whereas insulation is not enough to reduce either the temperature or slowing down the soil drying out. Where both the species showed lower stomatal control over atmospheric demand, P. hispanica showed leaf transpiration energy loss of around 300 W m⁻² when planted in the ground and T. cordata trees around 260 W m⁻² when planted in non-insulated containers, which are comparable to the energy loss from the street trees. Therefore, a strategy of mixed planting with faster growing species such as P. hispanica that provide stronger cooling at the initial stage in the containers to be complemented and eventually replaced with medium growing species T. cordata for relatively longer time period could be suggested.     

Response of apple trees to boron fertilization under conditions of low soil boron availability

The aim of the study was to examine the effect of boron (B) fertilization on the vegetative and the reproductive responses of apple (Malus domestica Borkh.) trees grown at low soil B availability. The experiment was carried out in 2005 under a greenhouse on 5-year-old ‘Jonagold’ apple trees/M.9 EMLA planted singly in 50-L containers filled with a sandy loam soil with hot water-soluble B concentration of 0.32 mg kg⁻¹. The trees were fertilized with B as foliar or soil application. Foliar B sprays were applied at the stage of pink bud, beginning of flowering, petal fall, and 10 days after flowering, at a solution concentration of 0.03%. Soil B fertilization was done at the bud break stage at a rate of 2 g per tree (27 mg B kg⁻¹ soil). The trees untreated with B served as the control. The results showed that soil B fertilization improved root development and tree vigor. Leaves of trees supplied with B to the soil had higher B concentration and chlorophyll, net photosynthetic rate, stomatal conductance, and activity of catalase and glutathione reductase than those of the control plants. Boron fertilization, regardless of application mode, increased fruit yield; the efficiency of foliar B sprays was higher than soil B application. Apple fruits of trees fertilized with B to the soil were bigger, more colored, richer in B, and had higher soluble solids concentration, and titratable acidity compared to those of the control trees.     

不同树龄苹果园土壤微生物的差异及对苹果砧木生长的影响

以平顶海棠(Malus robusta Rehd.)为试验材料,以4个不同树龄苹果园的土壤为对象,研究了不同树龄苹果园土壤对再植平顶海棠幼苗生长的影响。结果表明,在前茬树龄为3、8、15和24 a苹果园土种植的平顶海棠幼苗比正茬土对照的平均株高分别减少31.22%、46.38%、57.66%和63.10%;再植平顶海棠幼苗地上部干质量分别减少0.96%、57.58%、76.88%和78.10%;地下部干质量分别减少40.38%、48.79%、64.03%和79.43%。在24 a生苹果树土壤中再植海棠幼苗校正发病率达到了72.73%,而在3 a生苹果树的土壤中再植海棠幼苗校正发病率只有18.18%。对不同树龄的苹果园土壤根际细菌和真菌群落结构的差异进行分析,其结果为,随着苹果树龄的提高,根际土壤中真菌种类和数量显著增加,而细菌的种类和数量则随着树龄的增加显著减少。     

Factors influencing street tree health in constrained planting spaces: Evidence from Kyoto City,Japan

Street trees are threatened by multiple stresses from biophysical and anthropogenic factors. This situation can be extremely challenging in highly developed urban areas with limited space for tree planting. Asia has some of the most densely populated cities globally, but there is a lack of data on factors affecting street tree health in the region. This study aims to examine the impact of constrained planting environments on the health condition of street trees through a case study in Kyoto City, Japan. The health condition of 1230 street trees distributed throughout the city was assessed from June to October 2018. Additionally, several tree- and site-related variables were collected to identify their impact on tree health. Trees that were in excellent and good condition accounted for 19.9 % and 32.0 % of the sample population, respectively. Multivariate linear regression (N = 1139) revealed that tree health condition was significantly related to pruning intensity, tree pit size, adjacent land use, presence/absence of tree grate or guard, width of sidewalk, tree height, presence/absence of dedicated cycle route, tree pit pattern, crown light exposure, DBH and tree pit type. Platanus × acerifolia and other trees with large diameters exhibited relatively poor condition, along with those in tree pits with concrete paving, without tree grates, or in industrial areas, whereas trees planted in strips exceeding 1.8 m in length and exposed to weak pruning showed the best condition. These results imply the potential for healthy growth of street trees in the restricted planting spaces of Kyoto City, which suggests appropriate management and planting practices. Moreover, our empirical data can inform urban tree managers to support their efforts in making decisions on the better matching of species tolerances with urban site conditions for future street tree plans.     

Radial growth response of horse chestnut (Aesculus hippocastanum L.) trees to climate in Ljubljana,Slovenia

Horse chestnut (Aesculus hippocastanum L.) is a common urban tree species in Ljubljana, the capital of Slovenia. This area is forecast to experience a general reduction in precipitation and an increase in temperature, which increases water demand in plants. Because A. hippocastanum is known for its drought vulnerability, the question of the future suitability of this urban tree species in Ljubljana has arisen. To investigate how climate has influenced A. hippocastanum radial growth and how trees responded to extreme climatic events, standardized precipitation-evapotranspiration index (SPEI) was used as a proxy for water demand. Climatic signal and its stability through time were calculated using Pearson’s correlation coefficient. Additionally, to investigate whether the trees had a common response to extreme climatic events, pointer years were calculated using Cropper values. We sampled 19 trees that were growing in Tivoli Park in Ljubljana. After successful cross-dating of 15 trees, the ring count showed that the trees had up to 201 tree-rings and had 130 on average. Climate-tree growth analysis showed that in July, 3-month SPEI had the strongest influence on radial growth, but its influence on radial growth decreased over time, possibly due to the die-off process of trees. The narrowest tree-rings were a result of unusually dry periods at the time of cambium activity and/or new cell growth. With the forecast of longer, more frequent summer drought periods in Ljubljana, soil moisture stress will increase, and as a result, a decrease in radial tree growth of A. hippocastanum trees from Tivoli Park is expected.     

High carbon losses from established growing sites delay the carbon sequestration benefits of street tree plantings – A case study in Helsinki,Finland

We assessed the net carbon (C) sequestration dynamics of street tree plantings based on 10 years of measurements at two case study sites each with different tree species in Helsinki, Finland. We assessed C loss from tree soils and tree C accumulation, tested the applicability of pre-existing growth and biomass equations against observations, and estimated the time point for the beginning of net C sequestration for the studied street tree plantings. The tree woody biomass C accumulation in the first 10 years after planting was 18–32 kg per tree. At the same time the C loss from the growth media was at least 170 kg per growth media volume (25 m³) per tree. If this soil C loss was accounted for, the net C sequestration would begin, at best, approximately 30 years after planting. Biomass equations developed for traditional forests predicted more stem biomass and less leaf and branch biomass than measured for the species examined, but total aboveground biomass was generally well predicted.     

Does street canyon morphology shape particulate matter reduction capacity by street trees in real urban environments?

Urban street canyon morphology plays an important role in outdoor air quality and should be considered in tree planting schemes. However, the air pollutant reduction rate by street trees in different types of street canyon has rarely been analyzed for real urban environments. Therefore, this study conducted field investigation of 15 street canyons in residential areas to assess the reduction rate of particulate matter (PM) by trees in canyons with varying aspect ratio (AR) and orientation. The species of trees planted in these streets were Sophora japonica, Populus alba 'Berolinensis' L., Salix babylonica, Fraxinus chinensis, Pinus tabulaeformis, and Ulmus pumila. In the presence of trees, the mass concentration of fine PM (PM₁) decreased most in narrow canyons (AR = 1.37–3.02), while the concentration of coarse PM (PM₁₀) decreased most in wide canyons (AR = 0.45–0.69). The PM concentration increased most with tree planting in medium canyons (AR = 0.79–1.08). Additionally, street trees reduced fine PM concentration more in canyons with a 45° angle to the prevailing wind than in canyons aligned parallel to the prevailing wind. But they reduced coarse PM more in canyons aligned parallel to the prevailing wind than in canyons with a 45° angle to the prevailing wind. In comparison with tree-free cases, tree planting weakened the correlation between AR and coarse PM concentration, whereas no difference in correlation was found between AR and fine PM concentration. Overall, street canyon morphology should be considered seriously in developing tree planting guidelines for built-up environments.     

Long-term impact of road salt (NaCl) on soil and urban trees in Edmonton,Canada

The application of de-icing salts for winter road maintenance is recognized as a major contributor to the decline of urban trees. We conducted a long-term monitoring program across several locations in the City of Edmonton (Alberta, Canada) to evaluate the impact of roadway salt application on tree species widely planted in boulevards and right-of-ways: Ulmus americana, Fraxinus pennsylvanica, Pinus contorta, and Picea glauca. Soil and leaf samples were collected from a total of 16 sites over six years. There were four sites selected for each tree species: three mid- to high- traffic roadside sites that received regular winter maintenance and one non-serviced site (control). Sampling was performed three times per year from late spring to late summer. Airborne salinity was assessed in four locations at different distances from the road. In 50% of the roadside sites, soil electrical conductivity (EC) values exceeded 2 dS m⁻¹. Soil pH in all of the roadside sites was also significantly higher than in the control sites, with values ranging from 7.6 to 8.5. In all four species, trees growing in sites with high soil EC had increased leaf Na concentrations and reduced leaf chlorophyll concentrations. Among the airborne monitoring sites, Na deposition in high traffic locations was over four-fold higher than those measured in the control location. Furthermore, Na levels remained relatively high at 20–50 m from the main road. Our data suggest that while soil salinity is among the main stressors affecting roadside trees in Edmonton, salt spray deposition may also have a significant impact on trees located close to high vehicle traffic areas and dense road networks. Our study highlights the importance of collecting data over several years and from multiple locations to account for the spatial and temporal heterogeneity of the urban environments in order to better evaluate the impact of road salt application on urban trees.