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Health and climate related ecosystem services provided by street trees in the urban environment Salmond, Jennifer A; Tadaki, Marc; Vardoulakis, Sotiris; Arbuthnott, Katherine; Coutts, Andrew; Demuzere, Matthias; Dirks, Kim N; Heaviside, Clare; Lim, Shanon; Macintyre, Helen; McInnes, Rachel N; Wheeler, Benedict W Mar 8, 2016

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REVIEW Open AccessHealth and climate related ecosystemservices provided by street trees in theurban environmentJennifer A. Salmond1*, Marc Tadaki2, Sotiris Vardoulakis3,4,5, Katherine Arbuthnott3,5, Andrew Coutts6,7,Matthias Demuzere6,7,8, Kim N. Dirks9, Clare Heaviside3,5, Shanon Lim1, Helen Macintyre3, Rachel N. McInnes4,10and Benedict W. Wheeler4From The 11th International Conference on Urban HealthManchester, UK. 6 March 2014AbstractUrban tree planting initiatives are being actively promoted as a planning tool to enable urban areas to adapt toand mitigate against climate change, enhance urban sustainability and improve human health and well-being.However, opportunities for creating new areas of green space within cities are often limited and tree plantinginitiatives may be constrained to kerbside locations. At this scale, the net impact of trees on human health and thelocal environment is less clear, and generalised approaches for evaluating their impact are not well developed.In this review, we use an urban ecosystems services framework to evaluate the direct, and locally-generated,ecosystems services and disservices provided by street trees. We focus our review on the services of majorimportance to human health and well-being which include ‘climate regulation’, ‘air quality regulation’ and‘aesthetics and cultural services’. These are themes that are commonly used to justify new street tree or street treeretention initiatives. We argue that current scientific understanding of the impact of street trees on human healthand the urban environment has been limited by predominantly regional-scale reductionist approaches whichconsider vegetation generally and/or single out individual services or impacts without considering the widersynergistic impacts of street trees on urban ecosystems. This can lead planners and policymakers towards decisionmaking based on single parameter optimisation strategies which may be problematic when a single interventionoffers different outcomes and has multiple effects and potential trade-offs in different places.We suggest that a holistic approach is required to evaluate the services and disservices provided by street trees atdifferent scales. We provide information to guide decision makers and planners in their attempts to evaluate thevalue of vegetation in their local setting. We show that by ensuring that the specific aim of the intervention, thescale of the desired biophysical effect and an awareness of a range of impacts guide the choice of i) tree species, ii)location and iii) density of tree placement, street trees can be an important tool for urban planners and designersin developing resilient and resourceful cities in an era of climatic change.Keywords: Street trees, Ecosystems services, Health impacts, Climate* Correspondence: j.salmond@auckland.ac.nz1School of Environment, University of Auckland, Private Bag 92019, Auckland1142, New ZealandFull list of author information is available at the end of the article© 2016 Salmond et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.Salmond et al. Environmental Health 2016, 15(Suppl 1):36DOI 10.1186/s12940-016-0103-6BackgroundUrban tree planting initiatives are being actively promotedas an urban planning solution to reduce the environmen-tal degradation caused by urbanization, enhance urbansustainability, mitigate and adapt to climate change and toimprove human health and well-being [1, 2]. The publicperception of the value of green spaces and green infra-structure (especially trees) within cities has prompted anumber of initiatives to promote the ‘greening’ of citiesthrough urban reforestation and protection programs toincrease the percentage of tree canopy cover, such as theNew York City ‘Million Trees’ program [3], or the City ofMelbourne’s 40 % tree canopy cover target. Such projectshave stemmed from a wide range of different organisa-tional bodies encompassing local to international-scalegovernance, community based, charitable and regulatoryapproaches. Here, the broader arguments for increasedtree density stem from benefits for public health and qual-ity of life, and the sustainability and resilience of cities inlight of climate change [4].However, two issues immediately arise. First, oppor-tunities for urban greening remain limited in cities. Landis expensive and trees require economic and environ-mental resources to survive as assets in the harsh envir-onmental conditions characteristic of urban areas.Careful thought needs to be put into considering theirplacement, their beneficiaries, viable alternatives, who isresponsible for ongoing costs and maintenance, and po-tential co-benefits with urban planning objectives atmultiple scales. Second, urban trees do not provide ubi-quitous ‘good’ for all actors in all contexts. The complexphysiology and ecological functioning of trees mean thatefforts to optimise for one ‘good’ (such as less leaf litteror shade) can produce undesirable effects (such as in-creased aero-allergens) for different sites, scales and socialgroups. Thus, key questions remain in urban design andplanning as to how to invest in green urban infrastructurein ways which incorporate the large body of scientific un-derstanding of multiple biophysical and social processes inways relevant to human decision making.The application of urban climate, environmental andsocial sciences in this field is in its infancy, and few stud-ies have sought to integrate understanding of the phys-ical world with the social and cultural contexts of urbanenvironments. Given the heterogeneity and complexityof the processes which determine the environmental andsocial impacts of urban vegetation, it is not surprisingthat there have been few attempts to synthesise thecurrent knowledge about the net impact of trees on thephysical, public health and cultural aspects of the urbanecosystem. Current research in this field often empha-sises a singular benefit and direct planners towards asingle-variable optimisation strategy. This becomesproblematic when a single-variable intervention offersdifferent outcomes and has multiple effects and potentialtrade-offs. For example, current preference for male overfemale trees of the same species in many NorthAmerican and European cities to reduce mess fromseeds and fruit can result in higher pollen loads inthe atmosphere [5].There is a pressing need for holistic assessments of thehealth impacts of climate change mitigation/adaptationpolicies such as the promotion of street trees. Vegetationprovides shade and humidity thereby reducing surfaceand air temperatures at local scales and thus is a poten-tial adaptation strategy in an era of climate warming.Given that increasing vegetation density also has the po-tential for significant co-benefits to be realised across arange of public health arenas, exploring the two themesof health and climate enables a broader appreciation ofthe complexity of the issues and services realised at dif-ferent scales in different urban settings. We focus ontrees along streets, as street trees represent a particularmode of greening urban areas which offer particular ser-vices and functions [6, 7]. As such, there is significantinterest in the potential of street trees as a tool in urbandesign to mitigate against a number of climate-relatedurban problems.This paper provides a critical review of the potential ofstreet trees as an urban planning (or engineering) solu-tion to improve human health and well-being through‘climate regulation’, ‘air quality regulation’ and ‘aestheticsand cultural services’. These are themes that are com-monly used to justify new street trees or street tree re-tention initiatives. We seek to match changes in thesebiophysical processes resulting from street trees withhealth impacts (such as physical health, mental healthand the well-being of residents) at relevant scales.We utilize an urban ‘ecosystem services’ (ESS) frame-work [4, 8] as a platform through which to synthesizecurrent knowledge, and assess the holistic value of streettrees by thinking through the different processes andfunctions that street trees perform which are of humanvalue in the spheres of climate and health. While mostESS typologies often present the potential climate, airquality and cultural-aesthetic benefits of trees in a ‘list’fashion, these are rarely discussed in sufficient detail tohighlight contradictions and the place-specific context ofresults. We identify the limitations of promoting invest-ment rationales for street trees drawn from single-issuemodelling studies that highlight a single benefit oreven co-benefit (e.g. Jim and Chen [9]). This leads usto propose some methodological recommendationsabout how the impact of street trees on urban ESScould be approached differently, and how future ana-lyses might be oriented to facilitate dialogue aboutthe diverse meanings of trees and green space inurban environments.Salmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 96 of 171An urban ecosystem services approach (ESS)Much research and advocacy has focussed on document-ing the human benefits arising from integrating variousforms of ecological restoration (such as urban tree-planting) into urban design and planning [10, 11]. The‘ecosystem services’ approach is increasingly being utilizedby researchers, advocates and policy makers to highlightand evaluate the human benefits received through theecological functioning provided by urban trees and othersuch ‘ecological infrastructure’ [4, 10, 12]. Ecosystem ser-vices refer to the subset of ecological functions that aredirectly or indirectly linked to human benefits or well-being [13]. What is crucial about the ecosystem servicesframework is that it analyses the relationships betweenspecific ecological processes and attributes, and specificoutcomes of value to humans. Analytically, this means fo-cussing on identifying, quantifying and modelling the hu-man benefits (and costs) of ecological and biophysicalprocesses relating to urban green infrastructure.What constitutes ‘best practice’ in identifying and clas-sifying ecosystem services (ESS) has been debated, con-tested and refined over the years for various purposes[14, 15]. In mainstream ESS thought, a four-part typ-ology of services distinguishes: provisioning services(direct outputs of human value, such as food), regulatingservices (maintenance of valuable processes, such aswater purification by wetlands), supporting services (pro-cesses indirectly valued, such as pollination) and culturalservices (providing valued social and spiritual meanings)[16]. Some scholars have developed more specific classifi-cations of ESS for urban environments. One study [12]provided an early and simple categorization of ESS uniqueto urban ecosystems and environments, highlighting howurban green infrastructure provides benefits to humanhealth in the forms of micro-climate regulation, air filtra-tion, noise reduction, rainwater drainage, sewage treat-ment and cultural values. Another [10] expanded thistypology and situated a range of urban ESS underneatheach of the four major classes used in the MillenniumEcosystem Assessment (see Table 1).While urban ESS classifications and lists of the envir-onmental services and disservices provided by streettrees (provided in reviews elsewhere [2]) provide usefulheuristics for highlighting the potential services providedby urban ecological infrastructure, detailed reviews areneeded to assess the weight of evidence, contextual vari-ability and robustness of the relationships that have beendocumented linking specific urban design elements tospecific human benefits in particular urban contexts.This review embraces the ESS framework to criticallyreview the literature pertaining to the potential benefitsof street trees for urban design and human well-being.We view street trees as a specific ‘ecosystem component’involved in the delivery of services [17]. As noted in theIntroduction, street trees are increasingly viewed as aplanning solution to urban problems; they are being in-cluded as integral components for climate sensitiveurban design, for urban liveability and environmentaljustice [6]. By critically reviewing the scientific literaturefor a range of often-proposed ESS for street trees, weaim to inform and advance dialogue in urban planningabout the role/s that street trees might play in pursuinga range of societal objectives.We use the ESS framework to organize our reviewaround the services (and disservices) provided by streettrees, emphasising the regulating and supporting ser-vices identified by Gomez-Baggethun et al. [10] whichare relevant at local scales to climate mitigation and hu-man health. However, the framework also brings intofocus three further points. First, it has been well ac-knowledged that much ESS work is reductionist, in thatit focusses on one or two elements or services (such asclimate regulation provided by trees) ignoring otherfunctions or processes of potential value to humans. Ithas been argued that ESS has become a ‘complexityblinder’ [18] that conceals as much as it reveals aboutwhich ecological processes (should) matter to humans.Second, while we take street trees as a useful startingunit for analysis, the ESS literature sensitizes us to thescale-dependent provision of services [1]. That is, thebenefits provided by a unit of street trees may bedependent upon whether street trees and/or other re-lated green infrastructure are providing similar servicesnearby. Third, and relatedly, the ESS framework high-lights how ‘benefits’ are social constructs that are con-text specific [19]; what is beneficial in one context maynot be in another, and what is seen as ‘beneficial’ by onesocial group may not be seen as beneficial by another. InTable 1 Urban ecosystem services relevant to human health. Classification adapted from [8]Service class Specific servicesProvisioning services Food supply, water supplyRegulating services and relatedhealth benefitsUrban temperature regulation, noise reduction, air quality improvement, moderation of climate extremes,runoff mitigation, waste treatment, pollination, pest regulation, seed dispersal, global climate regulationSupporting (habitat) services Habitat for biodiversityCultural services Recreation, aesthetic benefits, cognitive development, place values and social cohesionSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 97 of 171summary, ESS analyses need to be grounded in theirparticular biophysical and social contexts; our review at-tends to these insights as relevant for street trees.We also draw on the cultural ecosystem services litera-ture as a framework for thinking about the diverse ways inwhich street trees are meaningful to human subjects [1].We approach cultural ecosystem services broadly as the“contributions of ecosystems (or nature) to human well-being via nonmaterial connections” [20]. This definitionemphasizes the importance of meaning to human actors(i.e. the ‘nonmaterial connections’). This aspect is import-ant from a human well-being point of view, but is less tan-gibly connected to notions of physical environment.The following sections provide a discussion of a selec-tion of the relevant literature to highlight the challengesassociated with determining the impact of street treesboth on the local-scale physical processes operatingwithin urban ecosystems and also the social, culturaland health aspects. The literature on these topics is vast.We have been very selective in our use of case studiesand examples and do not claim to provide an exhaustivereview or systematic list of all services and disservices(see Roy and Pickering [2] for this). Rather we are per-forming a wider information-organizing function forprospective decision makers to help make sense of 1)the diversity in ESS for urban street trees, as well as 2)the importance of tree species, density and location inservice provision for any given location, and 3) the im-plications and potential health and societal effects ofoptimising for a singular service.The role of street trees in provision of regulating servicesa) Micro-climateAs a result of the extensive replacement of natural soilsand vegetation with impervious surfaces, cities havewarmer drier climates than their rural counterparts atlocal, urban and regional scales, especially at night [21].Increasing vegetation cover in urban areas leads to re-duced ambient and surface temperatures and increasedevapotranspiration, precipitation interception and re-duced runoff. Increasing the vegetation density is there-fore considered an effective option for mitigating urbanheat and thereby adapting to climate changes causedboth by regional-scale changes in land use and global-scale changes in atmospheric composition [22]. How-ever, little is known about the general effects of changingthe density of street trees on urban climates at regionalor local scales.Most studies of heat effects on health are undertakenat regional scales and use mean daily temperature ormaximum daily temperature as the most relevant pre-dictor for mortality or morbidity [23–25]. From a healthperspective, urban residents are particularly at risk ofsuffering from heat stress, especially during extreme heatevents as locally generated heat exacerbates the effectsof regional scale heatwaves [26]. Typically, urban climatemodelling studies at similar scales employ urban landsurface schemes which categorise vegetation cover gen-erally rather than specifically street trees. Such studiesdo show that increased vegetation cover results in redu-cing both mean air temperatures [27, 28] and extremetemperatures during heat waves [29]. Some studies havealso shown that the cooling effect of vegetation at a re-gional scale is more pronounced at night [29]. This issignificant from a health perspective since minimumtemperature has also been strongly associated with mor-tality due to the inability of the body to recover fromheat stress during the night time period [30].Where predicted temperature changes have been re-lated to changes in health parameters, simple statisticalcorrelations are often used which cannot easily be ap-plied in other contexts. For example, it has been foundthat a 20 % increase in vegetation cover resulted in a7.18 % decrease in 24-h average temperature in Phoenix,Arizona, where hot dry conditions dominate [31]. Thiswas then projected to reduce average annual heat-related emergency calls by 11 % [31].While such regional-scale research highlights the po-tential mean temperature reduction from increasingvegetation, modelling studies generally employ a reso-lution of around 1-5 km and are unable to capture thetype of vegetation or exactly where it is placed (e.g.parks or street trees). This general approach to repre-senting ‘vegetation’ may therefore bias results and notprove accurate for predicting the local effect of streettrees. In one rare study of the impact of increasing juststreet trees on temperatures at these urban to regional-scales [32] showed only a very small reduction in theaverage air temperature at 1500 h of between 0.2 and0.5 °C during heat waves in New York City. However,again, the results are specific to the local characteristicsof urban form and general climate zone.To understand the underlying processes which relatechanges in tree cover to changes in climate, local-scaleprocesses need to be characterised and understood.Trees provide shade, blocking solar radiation fromreaching pedestrians [33] and limit solar heating of im-pervious surfaces with high heat capacity and thermalconductivity (such as concrete), reducing heat storage.Vegetation can increase urban albedo (compared to darkasphalt surfaces), and vegetated surfaces have lower ra-diative temperatures than impervious surfaces with thesame albedo [34, 35].At local scales, extensive tree coverage can deliver sig-nificant benefits to outdoor human thermal comfort (ameasure of the temperature and humidity of the envir-onment in relation to the body’s ability to maintain acomfortable core temperature) and result in lower heatSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 98 of 171stress levels [36, 37], especially during extreme heatevents [38]. At these scales, the changes in temperatureobserved from the presence of street trees can be muchlarger than regional effects, but are highly variable anddifficult to generalise. For example, in Bangalore, India,an experimental study showed that afternoon ambientair temperatures were 5.6 °C lower in roads lined withtrees, and road surface temperatures 27.5 °C lower thanthose measured in comparable tree-less streets [39]. Ob-servations from a courtyard in Israel with shade treesand grass showed reduced air temperatures of up to2.5 °C [40]. The impact on local climate is dependent onthe prevailing regional climatic context, geographic set-ting of the city, urban form, the density and placementof the trees, species type, age and the health of the tree.However, even when average air temperature reduc-tions from street trees are small, the net benefits of treesfrom shading effects for human thermal comfort can besubstantial. Shading is critical for improving humanthermal comfort, particularly via reductions in mean ra-diant temperature which is the dominant influence onoutdoor human thermal comfort under warm, sunnyconditions [40, 41]. Shashua-Bar and Hoffman [34] alsonote that within the urban canyon, as much as 80 % ofcooling from trees comes from shading.The presence of street trees can also modify indoortemperatures by shading buildings and significantly re-ducing the risk of indoor overheating) [42]. This canbenefit human health where economic resources are un-available to cool buildings or could provide further co-benefits by reducing energy demands for building cool-ing [43]. One study shows that tree shade can reducewall temperatures by 9 °C and air temperatures by up to1 °C [44]. It also argues that it is very difficult to general-ise the impact of trees on building thermal performanceas there is very limited data available and the impactsare dependent on materials, architecture and design,geometry, tree species, aspect and season.However, the positive summertime effects of streettrees during the daytime need to be counter-balanced bytheir night and wintertime impacts. At night, althoughthe presence of trees may reduce local-scale heat storageand hence release at night, street trees trap radiationwithin the canyon and reduce ventilation, preventingthe dissipation of sensible heat that has built up dur-ing the day. Therefore, while an extensive tree canopycover may be beneficial during the day, there is a riskof restricted nocturnal longwave cooling leading toslightly higher and more uncomfortable indoor tem-peratures during the night [38]. It should also benoted that trees change aerodynamic resistance toheat diffusion, and may limit the penetration ofbreezes and cooling of buildings through open win-dows at night during summer.While the health effects of increased heat are dam-aging, the majority of deaths caused by temperature inurban areas around the world are associated with mod-erately cold weather rather than heat [25, 45, 46]. There-fore a drop in ambient temperature during the wintercaused by shading from ever-green street trees couldhave a negative effect on health. Reduced light levels inthe winter time could also have an impact on mentalhealth for individuals sensitive to Seasonal Affective Dis-order [47]. Increased shading can also result in lower in-door temperatures, increasing mould and dampnesswithin buildings and increase energy consumption forbuilding heating in winter.There is a synergistic relation between trees and cli-mate. Water has an important role to play in maintain-ing full and healthy, actively transpiring tree canopies.Urban environments can place additional pressures onstreet trees [48] that may not be experienced by theirrural ‘forest tree’ counterparts. Elevated urban tempera-tures, dry air and soils and large radiative loads (espe-cially on isolated street trees) can lead to a very highevaporative demand [49, 50]. Without alternative irriga-tion sources to increase soil moisture and support streettrees, as well as to dissipate high heat loads [51], theirhealth and capacity to cool urban environments can beimpaired. This could be particularly significant in manyurban areas given projected climate change patterns.Trees generally increase humidity, acting as channelsfor water loss to the atmosphere [51] with their rootsdrawing moisture from deeper layers of the soil. Watersensitive urban design, storm water harvesting andrecycled water can all provide a means for increasing soilmoisture levels in cities where water availability is anissue. Biofiltration systems and irrigation from rainwatertanks can deliver substantial increases in evapotranspir-ation as a result of stormwater retention [52]. Such mea-sures have additional eco-hydrological benefits includingreducing run-off (which benefits downstream water-ways), and improving soil drainage and soil erosion con-trol [53]. Street trees intercept and store rainfall, filterrunoff in the canopy and in the root-zone, and drawmoisture from the soil, increasing the soil water storagecapacity for rainfall events [54]. Trees also modify thebelow-ground environment, improving the permeability ofsoils [55]. In these ways, indirect health benefits from re-duced 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 no-tion that increasing vegetation density in urban areas canlead to positive changes from both the local climate andhealth perspectives. However, most studies linking climatevariables to health have been undertaken at regionalscales, and little is known about the underlying biophys-ical processes or causal pathways which specifically linkSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 99 of 171street trees to health effects at local scales. Thus, as dem-onstrated in the next sections, the evidence for the directeffect of street trees on health remains poor. Although atlocal scales the effects of street trees on climate and hencehuman health is context specific, some generic recom-mendations can be made when just considering direct cli-mate effects and health. For example, during the day,street trees tend to be more effective in cooling streetswhich 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 ratioincreases, the role of building shade and thermal mass be-gins to overwhelm the contribution of street trees in cool-ing [38]. Clustering trees into lines or small groups [58]interspersed with open areas in a ‘savannah’-type arrange-ment [59] can help reduce the radiative load [51], provideshade, and allow longwave cooling at night. Large, widetrees with dense canopies could be considered for streetswith low H:W, while taller narrower trees could be con-sidered for streets with high H:W. However, uncertaintyremains in the literature, as it has been suggested that thecooling effects of trees is related mostly to planting densityand canopy coverage [56], while others note that attributesof tree species like leaf colour and leaf area index can alsostrongly influence cooling [60].b) Air quality and noise regulationThe potential impact of street trees on air quality re-mains one of the most poorly understood aspects of thestudied ecosystem services and benefits [61]. Street treeshave the potential to regulate air quality by absorbingpollutants and increasing pollutant deposition. Theyemit pollutants and pollutant precursors in the form ofbiogenic volatile organic compounds and pollen andmay also regulate the soundscape of the city. However,the plethora of processes operating at different scalesmake it very difficult to predict the net effect of streettrees on air quality in any given environment. The ESSframework is important here in assisting with matchingscales of study with outcomes.c) Deposition and dispersionThe health effects of air quality regulation by trees inthe urban environment have mainly been studied at re-gional scales using modelling approaches which have notbeen extensively validated with field trials. Most studiesat regional or city scales show a modest modelled reduc-tion in pollution concentration of less than 5 % resultingfrom urban vegetation [62, 63]. Trees increase both thesurface roughness (slowing air flow thus enhancing de-position and absorption pollutant removal processes)and the area of the ground surface that atmospheric pol-lutants come into contact with (acting as biological fil-ters, enhanced by the properties of their surfaces) [64].Trees absorb CO2 and gaseous pollutants such as O3,NO2, SO2 primarily by uptake via leaf stomata or sur-face, and accumulate airborne particulates (by intercep-tion, impaction or sedimentation) more effectively thanother urban surfaces [65–67].Estimates of the resulting modelled improvements in airquality from vegetation are generally extrapolated at re-gional scales in association with health metrics usinglarge-scale epidemiological approaches, and few studiesspecifically focus on urban greening. For example, it hasbeen suggested current woodland cover (non-urban) inGreat Britain mitigates between five and seven deaths andfour and seven hospital admissions annually due to re-duced PM10 and SO2 concentrations [68]. However, simi-lar to the pitfalls associated with assigning a monetaryvalue to the economic benefits of street trees [69, 70], suchcalculations are dependent on the accuracy of the under-lying assumptions used in the methodological approaches.At local scales there is little evidence to link air qualityregulation from vegetation with improved health out-comes. Indeed at local scales, studies are less conclusiveas to the direction of the relation between vegetationand pollution, possibly because the interplay betweenurban form and vegetation becomes important. At localscales, the characteristics of the tree canopy, tree densityand proximity to other urban structures influence theability of plants to remove pollutants [71, 72]. The rateof pollutant removal is species dependent, and trees witha large leaf surface area can remove 60 to 70 times moregaseous pollutants a year than small ones [69]. However,the extent to which particle concentrations can be re-duced via deposition is more controversial, as particlescan be washed off and re-suspended [73]. Besides beingaffected by particle size (see Janhäll [67] for a compre-hensive review), plant species differ in their ability toscavenge dust-laden air due to their differing featuressuch as habitus, canopy height, or position, size, of themorphology (shape, texture, roughness) of leaves (e.g.[62, 72, 74, 75]).At local scales, changes to the urban air flow regimesfrom the tree canopy may also reduce the horizontal andvertical exchange of both clean and polluted air betweenthe urban canyon and its surroundings (also referred toas the ventilation hypothesis [76]). Many depositionalstudies do not take this into account and therefore mayunderestimate the effective deposition rate.Similar challenges are associated with attempts toquantify the effect of street trees on canyon-scale pollu-tant dispersion processes. This makes it difficult to gen-eralise the net impact of street trees on local airpollution concentrations. A plethora of wind tunnel andcomputational fluid dynamics (CFD) studies have beenperformed on idealized urban geometries with trees tocharacterise the under-lying processes which determineSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 100 of 171local dispersion effects on one (see Moonen et al. [77]and references therein) or two intersecting street can-yons [78–80]. Unlike the studies which focus on depos-ition and removal processes, most of these dispersion-led studies report a localised increase in traffic-relatedgaseous pollutant and particulate matter concentrationsassociated with increased tree cover. The results remainconsistent when scaled up to neighbourhood areas withone study [81] reporting an increase in average pollutantconcentrations of 1 % associated with every 1 % increasein tree crown volume fraction relative to the tree-freesituation for occupation fractions of 4-14 %. It is there-fore unclear to what extent this impact of street trees onair quality remains valid for ‘real’ street canyons. In acombined modelling and field study, one study con-cluded that excluding the effect of vegetation results innon-negligible errors in pollutant predictions andresisted attempts to generalise the local impacts of treeson air quality [78].A limited number of experimental studies have attemptedto quantify the net change in pollutant concentrationsresulting from street trees (e.g. [76, 82–84]). The resultsfrom these studies provide mixed answers as to whethertrees provide a net benefit in regulating air quality, pointingto local factors as important determinants of the local ef-fects. For example, a seasonal investigation of six street can-yons in residential Shanghai (China) revealed that in thepresence of street trees, the rate of decrease in concentra-tion of PM2.5 with height was much lower compared totree-less streets [85]. In comparison, another study showedthat sections of major highways in Queens New York(USA) which had trees planted perpendicular to the streethad fewer spikes in PM2.5 concentration but higher meanbackground concentrations, indicated reduced dispersioncompared to grass-covered sections [86]. But, while treeswhich form a continuous tunnel or canopy within a streetpromote pollutant storage of pollutants emitted within thecanyon, they can also reduce transport of pollutants fromother locations within the city.One study has examined experimentally the impact ofstreet trees on indoor air quality by temporarily install-ing a line of young trees (silver birch) outside a row ofterraced houses in a heavily trafficked street in Lancaster(UK) [87]. Their results indicated that rather than in-creasing total urban tree cover, single roadside tree linesof a selected, high-deposition-velocity, PM-tolerant spe-cies appear to be optimal for PM removal. However, fur-ther experimental research into vegetated streets isnecessary to verify these results [88].In summary, it remains challenging to quantify therate of deposition using either modelling or measure-ment approaches. Large uncertainties remain and theranges reported vary significantly, especially at localscales [63]. The rate of deposition also depends on thechemical species in question. For example, SO2 morereadily deposits to surfaces (as do other acidic gases),whereas PM may be less so (and may actually be resus-pended from the vegetated surface). At local scales, thespecific combination of tree species, canopy volume,canyon geometry, and wind speed and direction must beaccounted for on a case-by-case basis [89].d) Emission of biogenic volatile compoundsOther ecosystem (dis)services associated with streettrees include the direct emission of gases which act asprecursors to the formation of secondary pollutants suchas ozone in urban atmospheres. Trees emit biogenicvolatile organic compounds (bVOCs) as a reaction tostress in their environment, such as high light intensitiesand/or temperatures or low water availability [90, 91].Isoprene is the most abundantly emitted bVOC [92]. Inthe presence of NOx and sunlight, isoprene contributesto ozone formation, which may accumulate locally whenventilation is limited [93, 94]. Other types of bVOCs,such as monoterpenes and sesquiterpenes, are also emit-ted, but unlike isoprene, these continue to be emitted atnight. In addition to contributing to ozone formation,terpenes can also contribute to particulate formation(Secondary Organic Aerosol – SOA) as they chemicallydegrade in the atmosphere [95]. Due to their very com-plex reactions, quantifying their contribution to pollut-ants is still an active area of research [96].A recent study provides an extensive review on theemission of bVOC by street trees and their impact onO3 concentrations [94]. They argue that due to the lim-ited availability of studies at the urban level, a number ofkey processes are still poorly understood, including theamount of bVOCs emitted by street trees, the inter-action between bVOCs and urban pollution and their in-fluence on O3 formation, and the effects of O3 on thebiochemical reactions and physiological conditions lead-ing to bVOC emissions. It should also be noted that theproduction of ozone from bVOC emissions may be out-weighed by the reduction in ozone due to depositionand uptake by the tree, though this will depend on thespecifics of the scenario. For example bVOCs from streettrees may increase ozone concentrations within traf-ficked street canyons due to the high concentrations ofNOx, but are less likely to have a significant effect inareas with low NOx concentrations.Tree/plant species and environmental stresses (such asdrought, heat, and pest infestation) influence the amountand type of bVOC emission. Temperature increase hasimportant direct influence on rates of bVOC emissions,gas-phase chemical reaction rates, and O3 dry depos-ition, which could result in higher O3 levels under cli-mate change conditions [97]. Also, here, a properselection of tree species is relevant; a recent studySalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 101 of 171indicates that planting one million low bVOC-emittingtrees compared to, for example, one million English oaktrees (high emitters) in Denver (USA), is equivalent ofpreventing emissions from as many as 490,000 cars [98].Donovan et al [99] developed an urban tree air qualityscore that ranks trees in order of their potential to im-prove urban air quality. Of the species considered, pine,larch, and silver birch have the greatest potential whileoaks, willows, and poplars can worsen downwind airquality if planted in very large numbers. To summarise,since bVOC emission (which may lead to ozone produc-tion) can vary with species, as can the effectiveness ofpollutant dispersion and/or uptake, the particular treespecies as well as the environment it will be sited in,need to be considered carefully to balance any benefit inpollution reduction with the potential for enhancedozone production and altered dispersion of pollutants.More detailed studies are required to specifically linkthe health effects to air quality regulation from trees atlocal scales. Further, although the importance of thecommuter micro-environment is well known in deter-mining personal exposure, little is known about the roleof street trees in determining personal exposurewhilst moving around the city using any mode oftransport. Cyclists, motorcyclists and pedestrians aremost susceptible to exposure to peak concentrationsdue to a lack of physical barrier between them andthe source [100, 101].e) Noise attenuationA further atmospheric service that is often consideredalongside air pollution is noise pollution. Noise in urbanareas has been associated with annoyance, self-reportedsleep disturbance and hypertension [102]. Little is knownabout the specific value of street trees in reducing noisepollution in street canyons, although there is certain evi-dence that trees can attenuate traffic noise roadside ofopen busy streets [103].More significant is the role that urban trees may playin the masking of urban noise. Almost universally,people rate the quality of natural sounds more highlythan anthropogenic sources [104]; the source of thesounds is as important as the actual intensity level. Forexample, the introduction of natural sounds, in urbanopen spaces have been shown to improve the perceptionof the quality of the soundscape [105–108]. While muchof the focus has been on the role of water features [107],the introduction of trees within a street canyon also hasthe potential to significantly alter the soundscape bygenerating sounds associated with the rustling of leavesin response to wind, and attracting bird wildlife soundsthat would be rated more positively than a street canyondominated by road traffic noise.f) PollenExposure to allergenic pollen from trees is associatedwith a range of health effects, including allergic rhinitis,exacerbation of asthma in susceptible individuals, andeczema. These pollen grains are produced in the flowersof trees, and the timing of their release varies dependingon the tree species and environmental conditions. Treepollen is spread by the wind and its dispersion isdependent on a number of environmental factors, in-cluding the local meteorological conditions. Individualscan be sensitive to pollen from one or more differentspecies of trees. Estimates of the levels of tree pollen al-lergies in the population range from around 5 % to over50 % in Europe [109]. As such, it is a significant environ-mental health issue.Some species of trees are more highly allergenic thanothers. Most of the allergenic tree pollen in Europe isproduced by Betula (birch), and in Mediterranean re-gions, Olea eropaea (olive) (found mostly in agriculturerather than in cities) and Cupressus (cypress) [109]. Des-pite being highly allergenic, Betula is popular for orna-mental planting in cities and streets [110]. In Europe,the largest proportion of the population with a positiveskin prick test to Betula allergens was 54 %, recorded inZurich, Switzerland [109]. In the city of Cordoba, Spain,Cupressaceae pollen accounts for 30 % of the totalpollen count during winter and is responsible for allergicrhinitis at a time when no other allergenic plants areflowering [109, 111]. Cryptomeria japonica (Sugi orJapanese cedars) has been shown to be highly allergenicwith large health effects found in populations [112, 113].This species can be found planted in cities both in Asiaand in North America. Jianan et al. [114] offer a reviewof allergenic planting in urban areas, with a focus onspecies planted in China.The effect of interacting environmental and meteoro-logical conditions on the production and release of aller-genic tree pollen is highly complex. It is thereforeunclear what effect climate change will have on pollen,although there is some evidence that it may result in earl-ier seasonal appearance of respiratory symptoms and lon-ger duration of exposure to pollen [115]. The productionof tree pollen is dependent not only on the current me-teorological conditions (including day length, temperature,precipitation, and wind speed/direction), but also on theconditions and water availability experienced in the yearprior during which pollen is formed [116]. Any changes inthese conditions affect the phenology of the tree and thusthe timing of the onset of pollen release, the total volumeof pollen produced, and the length of the flowering season[117]. Several studies have measured the diurnal cycle oftree pollen, and have found that different species exhibitdifferent daily cycles. Ščevková et al. [118] found that treepollen tends to peak in the afternoon, with lowest levelsSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 102 of 171observed throughout the night. Significant variations areobserved between species. However, another found thatBetula resulted in peaks throughout the day and night. Itis unclear from the literature how the urban environment,particularly the light, water and temperature modificationin streets, might affect both the timing of onset of releaseand the diurnal pattern of pollen release [119].There is also a synergistic effect between pollutantconcentrations and the health response to pollen. Peoplewho live in urban areas have been shown to be more af-fected by pollen allergies (asthma and allergic rhinitis)than those who live in rural areas [109, 120, 121]. Urbanstreets with high levels of vehicle emissions have beenshown to coincide with increased pollen-induced respira-tory allergies. There is suggestive evidence that exposureto air pollution prior to pollen exposure can exacerbatesymptoms and lower the threshold of pollen required totrigger symptoms in allergy sufferers [122, 123]. To fullyunderstand and quantify the effect of exposure to both al-lergenic tree pollen and traffic-related pollutants, it is ne-cessary to determine the effect on both the allergenicity(such as increased allergenicity of pollen which had beenexposed to NO2 found by Cuinica et al. [124]) and the vol-ume of pollen grains released under increased air pollu-tion. It is also important to consider the health impacts ofall these factors in high co-exposure areas such as traffic-heavy urban streets. The co-exposure of pollen and airpollutants (ozone, NO2, SO2, PM2.5 and PM10) is currentlyan active area of research [125, 126].In some instances there may also be a tension betweenthe choice of tree species to mitigate air pollution andpollen production. For example London Plane Trees(Platanus x acerifolia) are a commonly cited source ofallergy-producing pollen [127, 128], however these trees,with their large leaves, are likely to be very effective atremoving pollutants from the air.It is also important to note that, as with air quality,there are a number of feedback loops and synergistic ef-fects which make it very difficult to predict the net effectof increasing street tree density on pollen production es-pecially when changing climates are taken into consider-ation. The local effect of climate change on pollenproduction, release timing, transport and depositionfrom urban street trees is highly complex, and its impacton pollen allergies is very uncertain. Plants may releasepollen earlier and for longer periods in warmer climates[122]. Increases in atmospheric CO2 concentration maylead to great pollen release through increased plantproductivity, but plants may also be limited by other fac-tors such as water stress.In summary, few studies examine the complex rela-tions between urban vegetation, urban form and airquality, especially at a local scale [8]. Thus, the trade-offbetween increased deposition and removal processeswhich act to reduce pollution concentrations against re-duced horizontal and vertical dispersion, and increasedbiogenic (bVOC) emissions and pollen, remains poorlyunderstood. To date, the empirical evidence available islimited in spatial and temporal extent, and is stronglydependent on case-specific local characteristics, makinggeneral conclusions difficult to justify (see Fig. 2 in Jimand Chen [8]). This is further exacerbated by the factthat street trees affect local air quality in a number ofways, driven by a complex interplay of physical andchemical processes and by variable emission sources andprevailing (urban) meteorological conditions.Cultural values, ecosystem services and the meanings ofurban treesUrban street trees mean different things to differentpeople. For some, they might contribute to ‘connectingwith nature’, to others, they may be a nuisance (see Royet al. [2]). These meanings can be explored quantitativelyand qualitatively, and at different scales, with differentapproaches making different assumptions about both theecosystems and social groups being studied or repre-sented. We present this section as a survey of ap-proaches rather than as a comprehensive summary.a) Quantitative approachesQuantitative approaches to understanding the meaningsof urban ecosystems for human subjects are often tar-geted at documenting the psychological, recreational andaesthetic benefits of natural environments to humanhealth and well-being [20, 129, 130]. Psychological re-search on these topics has focused on relating access to‘green space’ to proxies of human well-being such asself-reported levels of stress and workplace productivity[20]. Whilst the evidence is somewhat mixed, these ben-efits are thought to arise through mechanisms includingopportunity and motivation for physical activity, stressrecovery, cognitive restoration and social contact [131].Overall, there has been limited work to date that focuseson street trees in particular (but see Schroeder et al.[132]. Tzoulas et al. [129] reviewed three dominantquantitative approaches to evaluating the relationshipsbetween urban green space and human psychologicalwell-being outcomes: observational epidemiological stud-ies, surveys and experimental trials.Observational epidemiological studies have been usedto examine the relationships between green infrastruc-ture and social variables (such as human health indica-tors and income), using population samples andstatistics to hypothesize causal relationships betweenthem. In this context, these are often ecological in de-sign, in other words, exposures or outcomes are aggre-gated at population or group level. For example, a recentecological cross-sectional study using data for LondonSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 103 of 171(and controlling for other confounding variables) sug-gested that antidepressant prescribing rates (as an im-perfect proxy for depression/anxiety amongst the localpopulation) were slightly lower in areas with greaterstreet tree density per length of street [133]. A differentstudy in the Netherlands was not specifically focused onstreet trees, but audited ‘streetscape greenery’, and foundpositive associations with self-reported general health,mental health and acute health-related complaints [134].Similarly, Lovasi et al. [135] found an inverse associationbetween density of urban trees and the prevalence ofchildhood asthma (but not with hospitalisations due toasthma). Although this analysis controlled for populationdensity, socio-economic characteristics (e.g. proportion ofpopulation living below the poverty line) and proximity tosources of air pollution, residual confounding in thisstudy, and other observational studies, remains possible.Practitioners in health, environmental and social sci-ences are increasingly mapping and investigating thespatial relationships between trees and social groups andpractices, generating estimates of environmental ‘expo-sures’ and supporting new questions and research pro-jects. Foremost among these could be recent work bypolitical ecologists exploring the links between streettrees and social inequality [6, 136].Experimental studies seek to control how exposures(e.g. to street trees) are distributed across study participantsin order to determine causal relationships. For example, re-cent laboratory-based studies exposed participants to dif-ferent imagery of street scenes, with results suggesting thatstreets with greater tree coverage promote stress-recovery(based on standard self-report measures), although the as-sociation was non-linear [137]. A similar study suggestedthat this stress-recovery benefit may be gender-specific,finding a benefit only amongst men [138]. Bowler et al.[130] reviewed only experimental studies which sought tolink human psychological health and the natural environ-ment, and found a small number of generalizable relation-ships (e.g. positive effects on activities such as walking),calling for more rigorous experimental designs [139].Surveys can be used to understand individuals’ interac-tions with – and attitudes towards – urban trees. Avolioet al. [140] surveyed five counties in California (n: 1029surveys) about attitudes to and uses of urban trees, andrevealed significant regional differences in desired treeattributes. Residents living in hotter areas value treesmore for shade, and desert area residents valued treesmore than those who live near natural forests. Surveys canalso be used to document preferences for future desiredoutcomes. For example, Giergiczny and Kronenberg [7]used an economic choice modelling survey of urban resi-dents to elicit their willingness to pay (in the form of ahypothetical tax) for planting trees in different spatialareas. They found a high willingness to pay for greeningthe streets in general, but the strongest preferencewas for greening those streets which currently havefew or no trees.A fourth quantitative approach (which we add to thethree identified by Tzoulas et al. [129]) is city- or region-wide valuation studies. These use meta-data to presentan administrative logic for valuing urban trees and in-creasing tree density. Many economic studies embracethis approach, which:1) treats urban trees as if they produce a series ofeconomically valued goods, such as carbon dioxidesequestration or air pollution reduction,2) estimates prices for these ‘goods’ (e.g. through thecost of substitutes to do the same function),3) adds these prices together to provide the totaleconomic ‘benefit’ provided by trees, and thensubtract the costs of producing and maintaining theurban treescape.This procedure will produce the ‘net benefit’ of urbantrees to a region in financial terms. Maco and McPherson[141] followed this logic to produce a benefit-cost ratio of3.8:1 for urban trees in the city of Davis, California, con-cluding that further plantings and rejuvenation of urbantreescapes will produce net societal gains. Soares et al.[142] used a similar approach in Lisbon on urban streettrees, arriving at a benefit-cost ratio of 4.48:1.b) Qualitative approachesWhere quantitative approaches seek to gauge how the‘magnitude’ of a specific relationship (e.g. a magnitudeof preference for a particular type of tree) changes acrossspace and across social groups, this requires that the re-lationship be specified by the analyst in advance. It as-sumes that the analyst knows which relationships are(most) important a priori. Qualitative approaches, incontrast, seek to understand which relationships andmeanings matter to participants, be they urban resi-dents, policymakers, scientists or activists. Such ap-proaches seek to understand the personal and historicalmeanings of urban trees in specific urban contexts, andcan include interviews, textual analysis, focus groups,participant diaries and open-ended surveys. Two exam-ples provide an indication of the insight and utility ofqualitative approaches. In the first example, Peckham etal.’s [143] semi-structured yet open ended approach tothe diaries of residents in Halifax and Calgary revealed adiversity of ways in which urban trees were meaningfulto participants. Some went out of their way in theircommutes to walk through urban green space, and manyhighlighted the peacefulness of the songs of birds. In asecond example, Heynen et al. [144] demonstrated thesocio-economic disparity in the location and density ofSalmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 104 of 171urban trees in Milwaukee. Owing in part to differencesin capacities for tree maintenance, residents in poorerareas found urban trees to be a nuisance and a financialliability. Here, the ecosystem disservices of trees (such asinfrastructure damage, fruit and leaf waste and attractionof pests, difficulties in navigation or reduced visibility, orincreased economic, energy or water costs with treemanagement) assume more significance [144]. Plantingtrees in these communities would have further marginal-ized the views and aspirations of these communities,and certainly would not have helped lessen the environ-mental injustice insofar as justice relies on the disadvan-taged feeling empowered and represented in urbandevelopment decisions. In both of these examples, thevalue of qualitative methods comes through their abilityto understand the local and social-political meanings ofurban trees.While studies linking urban nature to human well-being are illuminating and valuable, care needs to betaken in making generalizations about these relation-ships across urban environments and across social andeconomic groups. Qualitative and mixed methods re-search in particular have demonstrated that assuming‘positive’ relations between urban street trees and psy-chological well-being can be politically problematic andnot just empirically unwarranted. For example, extrapo-lating the preferences of white middle-class urbanites tosocially and economically marginal groups (as in theMilwaukee example) could be seen as ethically and polit-ically irresponsible [144].Clear links between the underlying processes need tobe established in order to understand apparently contra-dictory results. For example, epidemiological cross-sectional studies, such as that of Lovasi et al. [135],found an inverse association between density of urbantrees and the prevalence of childhood asthma (but notwith hospitalisations due to asthma). Although the ana-lysis controlled for some confounding factors, perhapsdue to the scale of the study, clear physical, environmen-tal or psychological mechanisms were not identified.Similarly, Donovan et al. [145] showed that a loss oftrees in the neighbourhood resulted in increased mortal-ity related to cardiovascular and lower-respiratory-tractillness, but no mechanism was suggested. Scale can alsobe important in interpreting apparently conflicting re-sults in the literature. For example, regardless of themethod, the evidence supporting the value of vegetationin promoting increased physical activity has producedmixed conclusions [146]. Understanding the conflict be-tween viewing trees as a beneficial environmental featuresupporting the ‘walkability’ (and hence physical activitypromoting nature) of urban areas [147, 148] versus no-tions of reduced visibility and fear need to be under-stood in local neighbourhood contexts. Furthermore, thelocal role of environmental factors may be important asshading from tree canopies may be desirable in warmerclimates but less so in cooler climates or on cold days.c) ImplicationsWhat is at stake in these choices about how to modelthe cultural ESS produced by street trees? Clearly, theESS literature does not provide a ‘universal list’ of cul-tural services, and this review suggests that practitionersshould be sceptical of using one, even if one is proposed.Rather, these choices about methodological approach areabout connecting ESS analysis to the political contextsand social groups who will make use of the research.The social meanings of urban trees are not pre-given ornon-political; the meanings of urban trees are historical,they are symbolic, and they are differentiated across so-cial groups. Ignoring the context of decision making canlead to outcomes that may produce net costs for manyor all involved. Kirkpatrick et al. [149] highlight thatplanning for urban trees needs to consider the distributionand dynamics of residential ownership and regulationsupon private property. Any coherent environmental just-ice strategy built around equitable access to urban greenspace needs to fully consider the dynamics driving thepresent and future distribution of environmental out-comes. Wolch et al. [150] further warn that strategies toincrease access to urban green space for poor neighbour-hoods can paradoxically result in higher property valuesand gentrification (displacement of poorer residentsthrough higher rents). It is crucial then to understand thelocal contexts and meanings of urban street trees whenconducting analyses, rather than assume that such mean-ings will follow the quantitative predictions derived fromsurveys of narrow social groups and locational contexts.Conclusions and recommendationsAs urban greening initiatives continue to be mobilisedinto planning agendas and narratives of liveability, healthand well-being, researchers can strengthen and shapethese conversations by providing supporting inter-disciplinary analysis. Our review of ESS provided bystreet trees reveals that the relationships between thebio-physical properties of trees and human benefits areboth complex and context-dependent. While some ofthe biophysical functions of trees can be summarisedand described ‘in general’, the particular meanings, valuesand societal implications of street trees for a particularsetting need to be evaluated scientifically and justifiedpolitically in place. Our review did not attempt to com-pile a master list of services and disservices for urbanand street trees (for this we refer readers to Roy, et al.,[2]). Rather, we have selected a number of well-knownESS for urban street trees and evaluated the extent towhich these ESS relationships are in fact generalizable.Salmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 105 of 171Through reviewing the evidence for the ESS provided bystreet trees in the context of climate change, air qualityand cultural ecosystem services, we conclude that the‘benefits’ produced by street trees are shaped by variousscales of biophysical context, as well as social meanings,histories and inequities that give street trees meaning totheir local communities.The challenges of translating the (physical and social)science into local policy are complex. This review dem-onstrates that over-emphasizing a single process in justi-fying urban trees (such as air pollution abatement orclimate change mitigation) can have unintended conse-quences (such as increased pollen). The current evidencebase also does not allow the impact of greening inter-ventions to be reliably predicted from general rules ortop-down frameworks. Such frameworks may supportthe accumulation of knowledge ‘in general’ but do notprioritise careful place-based understanding of the urbanbiophysical and social contexts of urban tree plantinginitiatives. Single-issue optimization and modelling ap-proaches that make decisions based on the modelling ofindividual ‘(dis)services’ of street trees risk 1) benefitingonly a small number of stakeholders, 2) reproducing re-lationships of power and marginality in the community,and 3) opening the potential for mal-adaptation.Our review, in agreement with other papers in the ESSliterature (e.g. Andersson et al. [151]) has alsohighlighted the importance of scale when determiningthe effect of trees on climate and health. Whilst much ofthe research to date has focussed on the regional andurban scale effects of vegetation on climate and health,it is much less clear what the impacts of street trees areat local scales where the result of the intervention ismost clearly felt. Similarly, the net effect of individualpollutants on population health has been widely re-ported at regional scales, but little is known about thecombined direct health effects of air pollution, pollenand temperature. This makes quantifying the resultinghealth impacts particularly challenging. Feedback loopsalso exist as a result of changes in energy consumptionand carbon sequestration which can exacerbate or miti-gate climate change processes.There is a strong practitioner desire for prescriptiveuniversal templates (which quantify the financial costsand benefits) when it comes to decision making. Institu-tions and governmental organisations that manage streettrees often have a limited budget which requires seekingthe largest possible benefit from the trees for the cost ofplanting, maintenance and protection of trees. Given thecost of planting initiatives and the potential lifespan ofthe trees, consideration also needs to be given to the ex-pected changes in urban form and function with timeand space. Clear aims are required to ensure success of agiven intervention at local scale.From our review, we argue that decision makingframeworks need to be locally tailored and embeddedinto bottom-up decision making processes. This enablescommunities to articulate what matters to them abouturban trees, and not just have technical scientific mean-ings used to justify ecological interventions (e.g. Tadakiet al. [152]). Urban greening initiatives should be pur-sued through a process where the multiple meanings ofurban trees (cultural as well as scientific) can be articu-lated and deliberated together. A universal list of poten-tial societal benefits provided by urban trees (such asthose listed by Roy, et al. [2]) can provide a startingpoint for conversation with affected stakeholders abouthow urban trees might become meaningful to the futureof a particular community, but scientific lists and frame-works should not be used instead of meaningful engage-ment from diverse community voices and perspectives.Frameworks such as the ‘Right Tree Right Place’ check-list for urban trees in London [153] can provide sensitiz-ing questions that draw on accumulated scientificknowledge, while also requiring and supporting context-ually specific and locally justified responses.Where modelling is required, systems dynamics ap-proaches could also be used to capture the complexityand dynamic interactions occurring within urban systems,and has been used previously to integrate informationfrom different disciplines and sectors whilst maintaining ahealth focus. Other participatory modelling approacheswhich take account of different outcome goals and criteria[154–156] (within an urban area or more widely) allowthe assessment of policy options and the priorities of var-ied stakeholders to be taken into account. Suchapproaches provide a practical resource which local au-thorities can use to guide how science can best informpolicy for maximising the benefits of street trees, whilstavoiding potential maladaptation issues.There is a clear need for in situ validation of theseprocesses to better parameterise the underlying effects.However, attempts to seek and claim a ‘net impact’ ofstreet trees, even for a local context, should be treatedwith caution. This approach implies that we know (andknow how to value) all of the different effects in timeand space to produce a single ‘net’ value. Finally, it isworth remembering that environmental justice concernsunderlie all of these conversations about how and forwhom urban greening should be done. As scientists andcitizens, these opportunities to green our cities can alsobe seen as opportunities for creating more just socialand environmental places.This review has intended to sensitize decision makersto concerns and issues that can help develop place-specific knowledge and strategies. On the one hand, pre-scriptive ‘check lists’ are one useful way of accumulatingand organizing knowledge about the ESS of urban trees.Salmond et al. Environmental Health 2016, 15(Suppl 1):36 Page 106 of 171There remains a legitimate scientific project to compileand review accumulated knowledge about the effects ofurban trees at different scales. We need to bring thisknowledge together, evaluate its coherence, and assessthe robustness of generalizable claims. On the otherhand, simply applying generalised checklists is no substi-tute for meaningful policy development with diversestakeholders about future urban environments and theirmeanings. We cannot assume that there are or will berobust relations across all contexts. Rather, as our reviewhas shown, there is a need to develop reflexivity abouthow urban trees produce ESS for different social groupsat different scales.Additional fileAdditional file 1: Peer review reports. (PDF 326 kb)Competing interestsThe authors declare they have no competing interests.Authors’ contributionsCorresponding and first author JS led the development of the manuscript,managed the team of authors and drafted the bulk of the manuscript. MTplayed an important role in developing the argument of the paper, narratingkey concepts, editing drafts and led the ecosystems services contribution. SVco-led the development of the key ideas and overall theme of the paper,managed co-authors and assisted with editing. KA and CH were responsibleresearching and drafting the content on health impacts, AC and MD for airpollution and meteorology, KD and SL for noise and general editing andresearch of parts of the script, HM for air pollution chemistry, RM for pollen andBW for social science. All the authors read and approved the final manuscript.AcknowledgementsThis research was partly funded by the National Institute for Health ResearchHealth Protection Research Unit (NIHR HPRU) in Environmental Change andHealth at the London School of Hygiene and Tropical Medicine inpartnership with Public Health England (PHE), and in collaboration with theUniversity of Exeter, University College London, and the Met Office. Theviews expressed are those of the authors and not necessarily those of theNHS, the NIHR, the Department of Health or Public Health England. M.Demuzere is funded by the Flemish regional government through a contractas a Fund for Scientific Research (FWO) post-doctoral position. J. Salmondwas funded by the University of Auckland. The Cooperative Research Centerfor Water Sensitive Cities is a Commonwealth of Australia supportedprogramme. We also acknowledge the support of Healthy-Polis: InternationalConsortium for Urban Environmental Health and Sustainability.DeclarationsThis supplement has not been supported by sponsorship. Funding forpublication is provided by Research Enhancement funds for JenniferSalmond provided by the University of Auckland.This article has been published as part of Environmental Health Volume 15 Suppl1, 2016: Healthy-Polis: Challenges and Opportunities for Urban EnvironmentalHealth and Sustainability. The full contents of the supplement can be found athttp://www.ehjournal.net/supplements/15/S1.Peer reviewPeer review reports for this article are attached as Additional file 1.Author details1School of Environment, University of Auckland, Private Bag 92019, Auckland1142, New Zealand. 2Department of Geography, University of BritishColumbia, 1984 West Mall, Vancouver, BC V6T 1Z2, Canada. 3Centre forRadiation, Chemical and Environmental Hazards, Public Health England,Chilton OX11 0RQ, UK. 4European Centre for Environment and HumanHealth, University of Exeter Medical School, Knowledge Spa, Royal CornwallHospital, Truro, Cornwall TR1 3HD, UK. 5Department of Social andEnvironmental Health Research, London School of Hygiene and TropicalMedicine, 15-17 Tavistock Place, London WC1H 9SH, UK. 6School of Earth,Atmosphere and Environment, Monash University, Melbourne, Victoria 3800,Australia. 7Cooperative Research Centre for Water Sensitive Cities, Australia.8Department of Earth & Environmental Sciences Physical and RegionalGeography Research Group - Regional climate studies Celestijnenlaan 200E,KU Leuven, 3001 Heverlee (Leuven), Belgium. 9School of Population Health,University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.10Met Office Hadley Centre, FitzRoy Road, Exeter, Devon EX1 3 PB, UK.Published: 8 March 2016References1. 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