Increasing vegetation cover in urban areas leads to reduced ambient and surface temperatures and increased evapotranspiration, precipitation interception and reduced runoff. Increasing the vegetation density is therefore considered an effective option for mitigating urban heat and thereby adapting to climate changes caused both by regional-scale changes in land use and global-scale changes in atmospheric composition [ 22 ]. However, little is known about the general effects of changing the density of street trees on urban climates at regional or local scales.
Most studies of heat effects on health are undertaken at regional scales and use mean daily temperature or maximum daily temperature as the most relevant predictor for mortality or morbidity [ 23 ]—[ 25 ]. From a health perspective, urban residents are particularly at risk of suffering from heat stress, especially during extreme heat events as locally generated heat exacerbates the effects of regional scale heatwaves [ 26 ].
Typically, urban climate modelling studies at similar scales employ urban land surface schemes which categorise vegetation cover generally rather than specifically street trees. Such studies do show that increased vegetation cover results in reducing both mean air temperatures [ 27 ], [ 28 ] and extreme temperatures during heat waves [ 29 ]. Some studies have also shown that the cooling effect of vegetation at a regional scale is more pronounced at night [ 29 ].
This is significant from a health perspective since minimum temperature has also been strongly associated with mortality due to the inability of the body to recover from heat stress during the night time period [ 30 ]. Where predicted temperature changes have been related to changes in health parameters, simple statistical correlations are often used which cannot easily be applied in other contexts.
However, again, the results are specific to the local characteristics of urban form and general climate zone. To understand the underlying processes which relate changes in tree cover to changes in climate, local-scale processes need to be characterised and understood.
Trees provide shade, blocking solar radiation from reaching pedestrians [ 33 ] and limit solar heating of impervious surfaces with high heat capacity and thermal conductivity such as concrete , reducing heat storage. Vegetation can increase urban albedo compared to dark asphalt surfaces , and vegetated surfaces have lower radiative temperatures than impervious surfaces with the same albedo [ 34 ], [ 35 ].
At these scales, the changes in temperature observed from the presence of street trees can be much larger than regional effects, but are highly variable and difficult to generalise. For example, in Bangalore, India, an experimental study showed that afternoon ambient air temperatures were 5. Observations from a courtyard in Israel with shade trees and grass showed reduced air temperatures of up to 2. The impact on local climate is dependent on the prevailing regional climatic context, geographic setting of the city, urban form, the density and placement of the trees, species type, age and the health of the tree.
However, even when average air temperature reductions from street trees are small, the net benefits of trees from shading effects for human thermal comfort can be substantial. Shading is critical for improving human thermal comfort, particularly via reductions in mean radiant temperature which is the dominant influence on outdoor human thermal comfort under warm, sunny conditions [ 40 ], [ 41 ]. The presence of street trees can also modify indoor temperatures by shading buildings and significantly reducing the risk of indoor overheating [ 42 ].
This can benefit human health where economic resources are unavailable to cool buildings or could provide further co-benefits by reducing energy demands for building cooling [ 43 ]. It also argues that it is very difficult to generalise the impact of trees on building thermal performance as there is very limited data available and the impacts are dependent on materials, architecture and design, geometry, tree species, aspect and season. However, the positive summertime effects of street trees during the daytime need to be counter-balanced by their night and wintertime impacts.
At night, although the presence of trees may reduce local-scale heat storage and hence release at night, street trees trap radiation within the canyon and reduce ventilation, preventing the dissipation of sensible heat that has built up during the day. Therefore, while an extensive tree canopy cover may be beneficial during the day, there is a risk of restricted nocturnal longwave cooling leading to slightly higher and more uncomfortable indoor temperatures during the night [ 38 ].
It should also be noted that trees change aerodynamic resistance to heat diffusion, and may limit the penetration of breezes and cooling of buildings through open windows at night during summer. While the health effects of increased heat are damaging, the majority of deaths caused by temperature in urban areas around the world are associated with moderately cold weather rather than heat [ 25 ], [ 45 ], [ 46 ].
Therefore a drop in ambient temperature during the winter caused by shading from ever-green street trees could have a negative effect on health. Reduced light levels in the winter time could also have an impact on mental health for individuals sensitive to Seasonal Affective Disorder [ 47 ]. Increased shading can also result in lower indoor temperatures, increasing mould and dampness within buildings and increase energy consumption for building heating in winter. There is a synergistic relation between trees and climate. Water has an important role to play in maintaining full and healthy, actively transpiring tree canopies.
Elevated urban temperatures, dry air and soils and large radiative loads especially on isolated street trees can lead to a very high evaporative demand [ 49 ], [ 50 ]. Without alternative irrigation sources to increase soil moisture and support street trees, as well as to dissipate high heat loads [ 51 ], their health and capacity to cool urban environments can be impaired. This could be particularly significant in many urban areas given projected climate change patterns. Trees generally increase humidity, acting as channels for water loss to the atmosphere [ 51 ] with their roots drawing moisture from deeper layers of the soil.
Water sensitive urban design, storm water harvesting and recycled water can all provide a means for increasing soil moisture levels in cities where water availability is an issue. Biofiltration systems and irrigation from rainwater tanks can deliver substantial increases in evapotranspiration as a result of stormwater retention [ 52 ]. Such measures have additional eco-hydrological benefits including reducing run-off which benefits downstream waterways , and improving soil drainage and soil erosion control [ 53 ].
Street trees intercept and store rainfall, filter runoff in the canopy and in the root-zone, and draw moisture from the soil, increasing the soil water storage capacity for rainfall events [ 54 ]. Trees also modify the below-ground environment, improving the permeability of soils [ 55 ]. In these ways, indirect health benefits from reduced flooding and storm water damage can be achieved.
However, these effects are difficult to quantify [ 1 ]. In summary, there is some evidence to support the notion that increasing vegetation density in urban areas can lead to positive changes from both the local climate and health perspectives. However, most studies linking climate variables to health have been undertaken at regional scales, and little is known about the underlying biophysical processes or causal pathways which specifically link street trees to health effects at local scales.
Thus, as demonstrated in the next sections, the evidence for the direct effect of street trees on health remains poor. Although at local scales the effects of street trees on climate and hence human health is context specific, some generic recommendations can be made when just considering direct climate effects and health. For example, during the day, street trees tend to be more effective in cooling streets which are exposed to large amounts of solar radiation wide open streets of low height-to-width H:W ratios [ 56 ] and those oriented east-west [ 57 ].
As the H:W ratio increases, the role of building shade and thermal mass begins to overwhelm the contribution of street trees in cooling [ 38 ]. Large, wide trees with dense canopies could be considered for streets with low H:W, while taller narrower trees could be considered for streets with high H:W.
However, uncertainty remains in the literature, as it has been suggested that the cooling effects of trees is related mostly to planting density and canopy coverage [ 56 ], while others note that attributes of tree species like leaf colour and leaf area index can also strongly influence cooling [ 60 ]. The potential impact of street trees on air quality remains one of the most poorly understood aspects of the studied ecosystem services and benefits [ 61 ].
Street trees have the potential to regulate air quality by absorbing pollutants and increasing pollutant deposition. They emit pollutants and pollutant precursors in the form of biogenic volatile organic compounds and pollen and may also regulate the soundscape of the city. However, the plethora of processes operating at different scales make it very difficult to predict the net effect of street trees on air quality in any given environment. The ESS framework is important here in assisting with matching scales of study with outcomes.
The health effects of air quality regulation by trees in the urban environment have mainly been studied at regional scales using modelling approaches which have not been extensively validated with field trials. Trees increase both the surface roughness slowing air flow thus enhancing deposition and absorption pollutant removal processes and the area of the ground surface that atmospheric pollutants come into contact with acting as biological filters, enhanced by the properties of their surfaces [ 64 ].
Trees absorb CO 2 and gaseous pollutants such as O 3 , NO 2 , SO 2 primarily by uptake via leaf stomata or surface, and accumulate airborne particulates by interception, impaction or sedimentation more effectively than other urban surfaces [ 65 ]—[ 67 ]. Estimates of the resulting modelled improvements in air quality from vegetation are generally extrapolated at regional scales in association with health metrics using large-scale epidemiological approaches, and few studies specifically focus on urban greening.
For example, it has been suggested current woodland cover non-urban in Great Britain mitigates between five and seven deaths and four and seven hospital admissions annually due to reduced PM 10 and SO 2 concentrations [ 68 ]. However, similar to the pitfalls associated with assigning a monetary value to the economic benefits of street trees [ 69 ], [ 70 ], such calculations are dependent on the accuracy of the underlying assumptions used in the methodological approaches. At local scales there is little evidence to link air quality regulation from vegetation with improved health outcomes.
Indeed at local scales, studies are less conclusive as to the direction of the relation between vegetation and pollution, possibly because the interplay between urban form and vegetation becomes important. At local scales, the characteristics of the tree canopy, tree density and proximity to other urban structures influence the ability of plants to remove pollutants [ 71 ], [ 72 ].
The rate of pollutant removal is species dependent, and trees with a large leaf surface area can remove 60 to 70 times more gaseous pollutants a year than small ones [ 69 ]. However, the extent to which particle concentrations can be reduced via deposition is more controversial, as particles can be washed off and re-suspended [ 73 ]. At local scales, changes to the urban air flow regimes from the tree canopy may also reduce the horizontal and vertical exchange of both clean and polluted air between the urban canyon and its surroundings also referred to as the ventilation hypothesis [ 76 ].
Many depositional studies do not take this into account and therefore may underestimate the effective deposition rate. Similar challenges are associated with attempts to quantify the effect of street trees on canyon-scale pollutant dispersion processes. This makes it difficult to generalise the net impact of street trees on local air pollution concentrations. A plethora of wind tunnel and computational fluid dynamics CFD studies have been performed on idealized urban geometries with trees to characterise the under-lying processes which determine local dispersion effects on one see Moonen et al.
Unlike the studies which focus on deposition and removal processes, most of these dispersion-led studies report a localised increase in traffic-related gaseous pollutant and particulate matter concentrations associated with increased tree cover. In a combined modelling and field study, one study concluded that excluding the effect of vegetation results in non-negligible errors in pollutant predictions and resisted attempts to generalise the local impacts of trees on air quality [ 78 ].
A limited number of experimental studies have attempted to quantify the net change in pollutant concentrations resulting from street trees e. The results from these studies provide mixed answers as to whether trees provide a net benefit in regulating air quality, pointing to local factors as important determinants of the local effects. For example, a seasonal investigation of six street canyons in residential Shanghai China revealed that in the presence of street trees, the rate of decrease in concentration of PM 2.
In comparison, another study showed that sections of major highways in Queens New York USA which had trees planted perpendicular to the street had fewer spikes in PM 2. But, while trees which form a continuous tunnel or canopy within a street promote pollutant storage of pollutants emitted within the canyon, they can also reduce transport of pollutants from other locations within the city. One study has examined experimentally the impact of street trees on indoor air quality by temporarily installing a line of young trees silver birch outside a row of terraced houses in a heavily trafficked street in Lancaster UK [ 87 ].
Their results indicated that rather than increasing total urban tree cover, single roadside tree lines of a selected, high-deposition-velocity, PM-tolerant species appear to be optimal for PM removal. However, further experimental research into vegetated streets is necessary to verify these results [ 88 ]. In summary, it remains challenging to quantify the rate of deposition using either modelling or measurement approaches. Large uncertainties remain and the ranges reported vary significantly, especially at local scales [ 63 ]. The rate of deposition also depends on the chemical species in question.
For example, SO 2 more readily deposits to surfaces as do other acidic gases , whereas PM may be less so and may actually be resuspended from the vegetated surface. At local scales, the specific combination of tree species, canopy volume, canyon geometry, and wind speed and direction must be accounted for on a case-by-case basis [ 89 ]. Other ecosystem dis services associated with street trees include the direct emission of gases which act as precursors to the formation of secondary pollutants such as ozone in urban atmospheres.
Isoprene is the most abundantly emitted bVOC [ 92 ]. In the presence of NOx and sunlight, isoprene contributes to ozone formation, which may accumulate locally when ventilation is limited [ 93 ], [ 94 ]. Other types of bVOCs, such as monoterpenes and sesquiterpenes, are also emitted, but unlike isoprene, these continue to be emitted at night. In addition to contributing to ozone formation, terpenes can also contribute to particulate formation Secondary Organic Aerosol — SOA as they chemically degrade in the atmosphere [ 95 ].
Due to their very complex reactions, quantifying their contribution to pollutants is still an active area of research [ 96 ]. A recent study provides an extensive review on the emission of bVOC by street trees and their impact on O 3 concentrations [ 94 ]. They argue that due to the limited availability of studies at the urban level, a number of key processes are still poorly understood, including the amount of bVOCs emitted by street trees, the interaction between bVOCs and urban pollution and their influence on O 3 formation, and the effects of O 3 on the biochemical reactions and physiological conditions leading to bVOC emissions.
It should also be noted that the production of ozone from bVOC emissions may be outweighed by the reduction in ozone due to deposition and uptake by the tree, though this will depend on the specifics of the scenario. For example bVOCs from street trees may increase ozone concentrations within trafficked street canyons due to the high concentrations of NOx, but are less likely to have a significant effect in areas with low NOx concentrations. Temperature increase has important direct influence on rates of bVOC emissions, gas-phase chemical reaction rates, and O 3 dry deposition, which could result in higher O 3 levels under climate change conditions [ 97 ].
Also, here, a proper selection of tree species is relevant; a recent study indicates that planting one million low bVOC-emitting trees compared to, for example, one million English oak trees high emitters in Denver USA , is equivalent of preventing emissions from as many as , cars [ 98 ].
Contributions of nitrogen deposition and forest regrowth to terrestrial carbon uptake
Donovan et al [ 99 ] developed an urban tree air quality score that ranks trees in order of their potential to improve urban air quality. Of the species considered, pine, larch, and silver birch have the greatest potential while oaks, willows, and poplars can worsen downwind air quality if planted in very large numbers.
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More detailed studies are required to specifically link the health effects to air quality regulation from trees at local scales. Further, although the importance of the commuter micro-environment is well known in determining personal exposure, little is known about the role of street trees in determining personal exposure whilst moving around the city using any mode of transport.
Cyclists, motorcyclists and pedestrians are most susceptible to exposure to peak concentrations due to a lack of physical barrier between them and the source [ ], [ ]. A further atmospheric service that is often considered alongside air pollution is noise pollution. Noise in urban areas has been associated with annoyance, self-reported sleep disturbance and hypertension [ ]. Little is known about the specific value of street trees in reducing noise pollution in street canyons, although there is certain evidence that trees can attenuate traffic noise roadside of open busy streets [ ].
More significant is the role that urban trees may play in the masking of urban noise. Almost universally, people rate the quality of natural sounds more highly than anthropogenic sources [ ]; the source of the sounds is as important as the actual intensity level. For example, the introduction of natural sounds, in urban open spaces have been shown to improve the perception of the quality of the soundscape [ ]—[ ]. While much of the focus has been on the role of water features [ ], the introduction of trees within a street canyon also has the potential to significantly alter the soundscape by generating sounds associated with the rustling of leaves in response to wind, and attracting bird wildlife sounds that would be rated more positively than a street canyon dominated by road traffic noise.
Exposure to allergenic pollen from trees is associated with a range of health effects, including allergic rhinitis, exacerbation of asthma in susceptible individuals, and eczema. These pollen grains are produced in the flowers of trees, and the timing of their release varies depending on the tree species and environmental conditions. Tree pollen is spread by the wind and its dispersion is dependent on a number of environmental factors, including the local meteorological conditions.
Individuals can be sensitive to pollen from one or more different species of trees. As such, it is a significant environmental health issue. Some species of trees are more highly allergenic than others. Most of the allergenic tree pollen in Europe is produced by Betula birch , and in Mediterranean regions, Olea eropaea olive found mostly in agriculture rather than in cities and Cupressus cypress [ ]. Despite being highly allergenic, Betula is popular for ornamental planting in cities and streets [ ]. Cryptomeria japonica Sugi or Japanese cedars has been shown to be highly allergenic with large health effects found in populations [ ], [ ].
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This species can be found planted in cities both in Asia and in North America. Jianan et al. The effect of interacting environmental and meteorological conditions on the production and release of allergenic tree pollen is highly complex. It is therefore unclear what effect climate change will have on pollen, although there is some evidence that it may result in earlier seasonal appearance of respiratory symptoms and longer duration of exposure to pollen [ ].
Any changes in these conditions affect the phenology of the tree and thus the timing of the onset of pollen release, the total volume of pollen produced, and the length of the flowering season [ ]. Several studies have measured the diurnal cycle of tree pollen, and have found that different species exhibit different daily cycles. Significant variations are observed between species. However, another found that Betula resulted in peaks throughout the day and night.
It is unclear from the literature how the urban environment, particularly the light, water and temperature modification in streets, might affect both the timing of onset of release and the diurnal pattern of pollen release [ ]. There is also a synergistic effect between pollutant concentrations and the health response to pollen. People who live in urban areas have been shown to be more affected by pollen allergies asthma and allergic rhinitis than those who live in rural areas [ ], [ ], [ ]. Urban streets with high levels of vehicle emissions have been shown to coincide with increased pollen-induced respiratory allergies.
There is suggestive evidence that exposure to air pollution prior to pollen exposure can exacerbate symptoms and lower the threshold of pollen required to trigger symptoms in allergy sufferers [ ], [ ]. To fully understand and quantify the effect of exposure to both allergenic tree pollen and traffic-related pollutants, it is necessary to determine the effect on both the allergenicity such as increased allergenicity of pollen which had been exposed to NO 2 found by Cuinica et al.
It is also important to consider the health impacts of all these factors in high co-exposure areas such as traffic-heavy urban streets. In some instances there may also be a tension between the choice of tree species to mitigate air pollution and pollen production. For example London Plane Trees Platanus x acerifolia are a commonly cited source of allergy-producing pollen [ ], [ ], however these trees, with their large leaves, are likely to be very effective at removing pollutants from the air.
It is also important to note that, as with air quality, there are a number of feedback loops and synergistic effects which make it very difficult to predict the net effect of increasing street tree density on pollen production especially when changing climates are taken into consideration. The local effect of climate change on pollen production, release timing, transport and deposition from urban street trees is highly complex, and its impact on pollen allergies is very uncertain.
Plants may release pollen earlier and for longer periods in warmer climates [ ]. Increases in atmospheric CO 2 concentration may lead to great pollen release through increased plant productivity, but plants may also be limited by other factors such as water stress. In summary, few studies examine the complex relations between urban vegetation, urban form and air quality, especially at a local scale [ 8 ].
Thus, the trade-off between increased deposition and removal processes which act to reduce pollution concentrations against reduced horizontal and vertical dispersion, and increased biogenic bVOC emissions and pollen, remains poorly understood. To date, the empirical evidence available is limited in spatial and temporal extent, and is strongly dependent on case-specific local characteristics, making general conclusions difficult to justify see Fig.
This is further exacerbated by the fact that street trees affect local air quality in a number of ways, driven by a complex interplay of physical and chemical processes and by variable emission sources and prevailing urban meteorological conditions. Urban street trees mean different things to different people. These meanings can be explored quantitatively and qualitatively, and at different scales, with different approaches making different assumptions about both the ecosystems and social groups being studied or represented.
We present this section as a survey of approaches rather than as a comprehensive summary. Quantitative approaches to understanding the meanings of urban ecosystems for human subjects are often targeted at documenting the psychological, recreational and aesthetic benefits of natural environments to human health and well-being [ 20 ], [ ], [ ]. Whilst the evidence is somewhat mixed, these benefits are thought to arise through mechanisms including opportunity and motivation for physical activity, stress recovery, cognitive restoration and social contact [ ].
Overall, there has been limited work to date that focuses on street trees in particular but see Schroeder et al. Tzoulas et al. Observational epidemiological studies have been used to examine the relationships between green infrastructure and social variables such as human health indicators and income , using population samples and statistics to hypothesize causal relationships between them. Pleim, J. Pouliot, G. Development of a biomass burning emissions inventory by combining satellite and ground-based information. Journal of Applied Remote Sensing, 2 1 , Baltimore, Maryland.
Development of the crop residue and rangeland burning in the National Emissions Inventory using information from multiple sources. Quantification of emission factor uncertainty. Schwede, D. Changes to the biogenic emissions inventory system version 3 BEIS3. Wilkins, J. The impact of US wildland fires on ozone and particulate matter: a comparison of measurements and CMAQ model predictions from to International Journal of Wildland Fire, 27, Xing, J. Observations and modeling of air quality trends over — across the Northern Hemisphere: China, the United States and Europe.
Historical gaseous and primary aerosol emissions in the United States from to Yienger, J. Empirical model of global soil-biogenic NO x emissions. Zhou, L. Modeling crop residue burning experiments to evaluate smoke emissions and plume transport. Science of the Total Environment , , Contact Us to ask a question, provide feedback, or report a problem. Jump to main content. An official website of the United States government. Contact Us. Marine Vessel Sources Anthropogenic emissions are emitted by marine vessels.
Mobile Sources Anthropogenic emissions are emitted by mobile sources such as cars, trucks, and buses. Industrial Sources Industrial processes produce anthropogenic emissions. Forests Forests are a type of biogenic emission source and can contribute to the emissions of VOC, NOx, and greenhouse gases. Windblown Dust Windblown dust has important effects on the atmosphere e. Wildland Fires Fire sources are event-based sources and can be classified as wildfires, prescribed fires, crop residue burning, and range land burning. Sea Spray Sea spray is an important component of particles in the atmosphere of coastal locations.
Agricultural Sources Agricultural lands can contribute to biogenic emissions. Dry Deposition Dry deposition is the gravitational sedimentation of particles during periods without precipitation. Emissions processes Anthropogenic sources Biogenic and natural sources F ire sources Windblown dust sources Ocean and sea spray sources Deposition processes Dry deposition Wet deposition Bidirectional exchange processes Related links References Emissions processes All emission sources can be classified as either natural or anthropogenic.
Emission sources for all of these gases are natural processes occurring: In vegetation and soils In marine ecosystems, caused by geological activity like geysers or volcanoes The of meteorological activity, such as lightning From fauna, such as ruminants and termites Although emissions resulting from activities associated with the agriculture industry are human-created, they are also included in the natural source category.
Scientific approach The NEI is a comprehensive listing by sources of six common air pollutants over the United States for a specific time interval. Top of Page Biogenic and natural sources Biogenic Sources are sources that originate from land based vegetation such as trees, shrubs, soil and crops. Scientific approach Biomass burning, including forest fires, burning of agricultural wastes and other prescribed burning, is sometimes included as a natural source of VOC emissions.
Top of Page Fire sources Fire sources are event-based sources and can be classified as wildfires, prescribed fires, crop residue burning, and rangeland burning. Wildfire: An uncontrolled fire that burns any area and are often called forest fires, grass fires, peat fires depending on what is being burnt. Prescribed fire: A fire that is planned for a predetermined area, under specific environmental conditions, to achieve a desired outcome.
Pre-harvest burning for removal of leaves and other biomass sugarcane. Post-harvest burning for removal of ground-level senescent vegetation. CMAQ first calculates friction velocity at the surface of the Earth. Once this friction velocity exceeds a threshold value, saltation, or horizontal movement, flux is obtained.
Their Deposition and Interception
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