Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move

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Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move Ina Sa¨umel a,b, Frauke Weber a, Ingo Kowarik a,c,* a

Department of Ecology, Chair of Ecosystem Science/Plant Ecology, Technische Universita¨t Berlin, Rothenburgstr. 12, 12165 Berlin, Germany Department of Ecology, Chair of Ecological Impact Research and Ecotoxicology, Technische Universita¨t Berlin, Ernst Reuter Platz 1, 10587 Berlin, Germany c Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), 14195 Berlin, Germany b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 January 2015 Received in revised form 28 October 2015 Accepted 18 November 2015

Fostering ecosystem services in urban road corridors is an important challenge for urban planning and governance because residents are often exposed to environmental pressures in these ubiquitous open spaces. We here aim at illustrating multiple ecosystem services that may be underpinned by roadside vegetation. Previous work is broadly scattered in papers from the natural and social sciences and biased by a focus both on regulating services (temperature regulation, air filtration, carbon sequestration) and also on trees. We provide a first synthesis that illustrates (i) the multi-functional capacity of green elements in streetscapes to deliver various ecosystem services; (ii) the relevance of planted and wildgrown herbaceous vegetation as well as trees; and (iii) trade-offs between certain ecosystem services as well as risks related to disservices. Trees and herbaceous road vegetation can mitigate adverse environmental conditions in road corridors, which is particularly important in vulnerable neighborhoods that are undersupplied with green spaces. Enhancing the amenity value of streetscapes might also positively influence public health by promoting physical activity. However, significant knowledge gaps exist, e.g. on the contribution of biodiversity to ecosystem services and on the valuation of green street components by different sociocultural groups. Our synthesis illustrates management options that can support planning and governance approaches toward more livable streetscapes by fostering ecosystem services and counteracting disservices. ß 2015 Elsevier Ltd. All rights reserved.

Keywords: Ecosystem service Disservice Climate regulation Cities Pollution Urban green infrastructure

1. Introduction The majority of humans are exposed to urban environmental conditions that often challenge human health and well-being and also threaten natural resources (Elmqvist et al., 2013). Ecosystem services (ES), conceptualized as benefits from ecosystems to human well-being (TEEB, 2011) are increasingly acknowledged to meliorate urban living conditions (Tzoulas et al., 2007; Elmqvist et al., 2013). Understanding, quantifying and fostering ES in urban land use types is thus highly relevant for urban planning, governance and management (Gaston et al., 2013; Haase et al., 2014). Given that cities are ecological-social systems (Pickett et al., 2011) approaches from the natural and social sciences are needed because the question of whether a particular ecosystem function is regarded as a benefit depends largely on societal demands (HainesYoung and Potschin, 2010).

* Corresponding author at: TU Berlin, Rothenburgstr. 12, 12165 Berlin, Germany. E-mail address: [email protected] (I. Kowarik).

A recent review revealed an increasing number of studies on urban ES but also identified important limitations (Haase et al., 2014): (i) Most studies address regulating ES, in particular temperature regulation and carbon sequestration/storage, while provisioning ES and cultural ES are highly understudied. (ii) Most studies refer to larger spatial scales, i.e. to parts of or entire cities, and less to ES flows at the local scale. (iii) Most studies at the habitat level focus on urban forests or parks; other land use types are understudied. (iv) Despite the multifunctionality of urban green spaces, most studies address only a single type of ES. Yet it has been stressed that it is necessary to consider trade-offs between multiple ES for enhancing human well-being in general (Haines-Young and Potschin, 2010) and in cities in particular, where highly heterogeneous green infrastructure components come up against large numbers of decision makers, land managers and social groups with divergent valuations of urban spaces and goals for planning and management (Elmqvist et al., 2013; Gaston et al., 2013). Developing and maintaining a multifunctional green infrastructure thus requires considering a range of ES, ecosystem disservices (ED; Lyytima¨ki and Sipila¨, 2009; von Do¨hren and Haase, 2015) and underlying societal demands and conflicts.

http://dx.doi.org/10.1016/j.envsci.2015.11.012 1462-9011/ß 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Sa¨umel, I., et al., Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move. Environ. Sci. Policy (2015), http://dx.doi.org/10.1016/j.envsci.2015.11.012

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Because urban growth often causes a decrease in green space per capita, ES provisioning will increasingly depend on informal green spaces (Fuller and Gaston, 2009; Rupprecht and Byrne, 2014). In this study we consider urban road corridors, the ubiquitous open spaces in cities worldwide that often include trees and other green elements. Within the last decade, studies in road ecology mainly focused on negative impacts on biodiversity and human health (Foreman et al., 2003; Dratva et al., 2010). This paper highlights beneficial effects of roadside vegetation that might help to mitigate environmental pressures in road corridors. While most previous studies have focused on trees, we address roadside vegetation in a broad sense, comprising all types of cultivated or wild-grown plant assemblages growing in road verges, medians, swales, tree pits or paving joints. Yet managing ES in streetscapes presents some challenges because aims related to the transportation of humans and goods can conflict with ideas for developing streetscapes as attractive parts of the urban green infrastructure. Moreover, decisions as to where to establish which kinds of green elements and how to manage them depend on a large number of stakeholders with diverging valuations and goals. The aims of this paper are to (i) elucidate potential benefits to urban dwellers from a range of roadside vegetation types; (ii) identify possible trade-offs between different types of ES and ED; and (iii) illustrate management approaches that foster ES and counteract ED in urban streetscapes, to inform planning and governance strategies aimed at creating more livable urban streetscapes. 2. Methods To review research on roadside vegetation related to human well-being, we apply the concept of ecosystem services (TEEB, 2011). We screened articles in the Web of Science by using keywords covering the four main categories of ES (regulating, provisioning, cultural, habitat services; see Table S1). A keyword search (January 2013) in the Web of Science revealed >20,000 references related to ES in the ‘‘topic’’ or ‘‘title’’ fields, but only 5% of these had an urban context; >90% of the latter were published since 2006. Only 33 papers were identified that directly address ES of urban roadside vegetation. We also included scholarly books and other papers found by cross-references in our research. In addition to studies that explicitly address roadside vegetation, we also considered work on other urban green elements (e.g., parks, gardens) that indicate ES of analogous elements in streetscapes. Moreover, based on existing literature, we sketch management approaches to foster ES and encounter ED (Table 1, last row). Supplementary Table S1 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.envsci.2015.11. 012. 3. Results Based on our qualitative review, we illustrate multiple ES as well as ED that can be underpinned by urban roadside vegetation. We highlight studies that explicitly address elements of roadside vegetation, mention studies on other components of urban green systems that indicate potential benefits to urban dwellers from a range of roadside vegetation types and identify gaps of knowledge. These results are synthesized in Table 1. 3.1. Regulating services and disservices 3.1.1. Air filtration Traffic-related emissions are a major health problem for city dwellers worldwide, and action is increasingly required because

emission limits have been greatly exceeded in many cities (UNEP, 2007; Chiesa et al., 2014). In consequence, the capacity of plants to immobilize particulate pollution has gained more attention (Escobedo et al., 2011; Langner et al., 2011). The dust deposition per leaf area decreases exponentially with increasing distance from the emission source (Jim and Chen, 2008; Litschke and Kuttler, 2008). Roadside vegetation can thus offer some relief from particulate loads due to its location close to traffic and the affected population. Most studies have focused on the role of woody species in immobilizing airborne particles (e.g., Beckett et al., 1998; FreerSmith et al., 2004). The filtration potential of herbaceous vegetation, in contrast, is understudied, although it has been hypothesized to supplement woody vegetation by binding particulates that have been re-suspended or washed off of trees (Gorbachevskaya et al., 2007). As a first study, Weber et al. (2014a) assessed the immobilization function of herbs and grasses along streets and the role of different plant traits (e.g., surface types) in binding particulate matter. Results suggest that herbaceous vegetation that is diversely structured in terms of plant height, branching pattern or leaf traits can reduce pollution loads, likely better than monotonously structured vegetation, where pedestrians are exposed to major emission sources. Moreover, vegetation barriers may function as shields (Al-Dabbous and Kumar, 2014), resulting, for example, in reduced pollution loads in vegetables grown in high traffic areas (Sa¨umel et al., 2012). Still ED related to roadside vegetation must be considered. Trees or dense shrub layers can reduce wind speed and nearsurface air exchange in narrow streets and thereby increase local air pollution (Buccolieri et al., 2009; Salim et al., 2011; Vos et al., 2013). Hence, greening initiatives that only aim to increase street tree coverage can be misleading (Pugh et al., 2012). The challenge thus is to use both trees and herbaceous vegetation, optimized to local conditions. Moreover, some plants decrease air quality by emitting biological aerosols (volatile organic compounds, aeroallergens; D’Amato, 2000; ˜uelas and Staudt, 2010; Simpson and McPherson, 2011). While Pen biogenic volatile organic compounds can contribute to ozone formation it remains challenging to balance ozone mitigation versus ozone formation by urban trees (Calfapietra et al., 2013). Plant stressors such as air pollution and increased temperatures can induce increased levels of allergenic proteins in the pollen – and thereby related health problems (Rogers et al., 2006). 3.1.2. Temperature regulation Global warming will exacerbate existing adverse impacts of urban heat islands on human health and the associated macroeconomic costs (Townsend et al., 2003; Gabriel and Endlicher, 2011). Traffic increases heat stress and air pollution in streetscapes, and cumulative effects of both stressors are a major health risk for urban dwellers (Burkart et al., 2013). Plants reduce elevated temperatures by shading and evapotranspiration – given that enough water is available for evapotranspiration. While most studies measured air temperature within parks or beneath trees (Bowler et al., 2010), only a few studies have addressed roadside vegetation explicitly (Shashua-Bar and Hoffman, 2000; Gulya´s et al., 2006; Johansson and Emmanuel, 2006; Leuzinger et al., 2010; Hunter et al., 2014). Shading by trees is important, but short vegetation also contributes to cooling by evaporation as illustrated by positive effects of short vegetation cover compared to concrete, asphalt, or bare soil (Yilmaz et al., 2008; Bowler et al., 2010; Onishi et al., 2010). However, the context matters. Trees in parks were significantly cooler than street trees surrounded by sealed ground, and leaf morphology modulated these effects since small-leaved trees remained cooler than large-leaved trees (Leuzinger et al., 2010). Moreover, vegetation increases human comfort by reducing

Please cite this article in press as: Sa¨umel, I., et al., Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move. Environ. Sci. Policy (2015), http://dx.doi.org/10.1016/j.envsci.2015.11.012

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ENVSCI-1683; No. of Pages 10 I. Sa¨umel et al. / Environmental Science & Policy xxx (2015) xxx–xxx Table 1 Ecosystem services [ES, (+)] and ecosystem disservices [ED, ( )] provided by roadside vegetation and proposed management approaches. ES (+) or ED ( )

Examples

Management approaches

Regulating (+)

Improving air quality, air filtration

Removal and immobilization of pollutants

Reducing air quality

Emission of biogenic volatile organic compounds (BVOC) that contribute to urban smog Allergic problems caused by some windpollinated plants Increased pollution levels due to reduced air exchange, blocked wind

Develop structurally diverse species assemblages along roads, with a variety of plant species and morphologies Foster vegetation at the ground level, i.e. at the interface between traffic-related pollution sources with people Increase the total plant surface area, e.g. by transforming lawns into meadows or by allowing spontaneous vegetation at road verges Consider risks of BVOC-producing plants in plant design schemes

( )

(+)

Temperature regulation

Cooling by shading and/or evapotranspiration

(+)

Shielding function

(+)

Carbon sequestration Carbon emission Noise reduction

Shielding adjacent parts of the city from air pollution Carbon capture and storage in biomass

( ) (+)

(+)

Regulation of water cycling

(+)

Water purification

Provisioning (+) Food supply

(+)

Genetic resources

(+)

Groundwater recharge

Habitat (+)

Habitat provision

( ) (+)

Ecological corridor or stepping stone for animal and plant species

( )

Cultural (+)

( )

( )

Psychological, esthetic or recreational benefits Psychological disservices; fear of crime Reduction of permeability

Carbon emission by maintenance measures Physical absorption of noise Reduced perception of noise due to exposure to biodiversity Storm water management by reducing runoff

Consider risks of allergenic plants in plant design schemes Control weeds of allergenic species (e.g. Ambrosia species) Design greening measures to maintain air exchange, e.g. by enlarging distances between trees in narrow streets or by replacing trees by ground vegetation Enhance plant biomass in road corridors, from surface to tree layers Design greening measures to shade paved or other built surfaces Implement water-sensitive urban design to enhance evapotranspiration (see water cycling below) Design greening measures to maintain cooling by allowing air exchange Develop greening measures to enable shielding effects to benefit adjacent housing areas or gardens Optimize plant choices and consider holistic approaches to maximize urban carbon pool Minimize fossil fuel use for maintenance of green elements Enhance vegetation structures at noisy locations Enhance biodiversity (plants, nesting or feeding habitats for birds) in road corridors Implement water-sensitive urban design and green streetscape principles (e.g. swales, planters, vegetated curb extensions, rain gardens, pervious paving for storm water management)

Removal and immobilization of pollutants

Horticulture or agriculture along urban roads for local food production

Heritage trees as genetic resources in streetscapes Unpaved areas of streetscapes allow infiltration of runoff

Consider different pollution loads when designing plantings (e.g. distance to roads, barriers) Adapt choice of crops in regard to pollution loads (e.g. berries instead of vegetables) Protect and propagate heritage trees Enhance unpaved areas to foster infiltration of runoff Consider potential pollution loads of runoff and establish bioretension swales for decontamination

Habitats for unwanted species associated with invasion or health risks Dispersal corridor for species supporting ES or for species of conservation concern

Reduce impervious surfaces to foster habitat development Re-introduce native species (e.g. grassland species at road verges, wetland species in swales, native tree species) Work to accept spontaneous vegetation development Protect habitat structures in old trees (e.g. cavities) Develop biodiversity-friendly management (e.g. mowing regime; selection of plant species as food source for animals) Manage habitats to counteract risks of unwanted species Control weeds Link streetscapes with urban habitat networks Encourage dispersal of native plant species by setting initials

Dispersal corridor for unwanted species of animals and plants associated with invasion or health risks

Control weeds Enhance native vegetation to slow down dispersal or migration of invasive species along roads (e.g., cane toad in Australia)

Attractive streetscapes promote social cohesion and physical outdoor activities and reduce stress

Develop multifunctional ‘‘livable’’ streetscapes by enhancing green elements Design green elements according to demands of local people

Traces of neglect, too dense or dark vegetation structure

Ensure a minimum level of design and maintenance Add esthetically attractive herbs to wild grown vegetation

Reduced view or passage way for road users

Ensure a minimum level of maintenance

Habitat for esthetically attractive native or nonnative species or for species of conservation concern

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4 Table 1 (Continued ) ES (+) or ED ( )

(+)

Damage on infrastructure, security risks Property values

(+)

Cultural heritage

( )

( )

Examples

Management approaches

Damage to built structures by roots; security risks due to fallen wood

Ensure a minimum level of maintenance

Attractive public street scapes increase sales prices of adjacent houses Alle´es, veteran tress and other green elements as drivers of regional identity

Enhance attractiveness of public streetscapes by developing green elements Protect heritage trees, traditional alle´es and other historical green elements in road corridors Re-plant traditional native or introduced tree species Adapt traditional design concepts to new roads Assess trade-offs between different ES and ED delivered by invasive trees Replace risky alien trees with native species Integrate wild-grown vegetation into traditional green design schemes Implement low impact maintenance Use roadside vegetation for environmental education and Citizen Science Integrate biomonitoring into urban monitoring programs as an effective low-cost method

Invasion risks by traditionally planted alien ornamentals

(+)

Educational services

Experience and valuation of biodiversity where people live and move

(+)

Bioindication

Using plants for indicating pollution loads

long-wave emissions and limiting the reflection of solar radiation from sealed surfaces (Shashua-Bar et al., 2011). Yet as a disservice, a reduced air exchange due to dense tree stands could decrease cooling effects. Vegetation-mediated cooling effects also matter in terms of environmental justice. Compared to wealthier areas, low income neighborhoods often have fewer green streets, thereby increasing environmental pressures on human well-being (Harlan et al., 2006; Jenerette et al., 2007; Kirkpatrick et al., 2011). 3.1.3. Carbon sequestration Trees are important urban carbon pools (Nowak and Crane, 2002; Pataki et al., 2006; Strohbach and Haase, 2012), with a minor contribution of street trees compared to other trees (Escobedo et al., 2010). Fossil fuel-dependent maintenance can reduce net effects of urban vegetation (Pataki et al., 2006). Thus, street trees that are usually highly managed can be associated with a higher CO2 emission than lesser maintained trees, for example in urban woods. Since carbon storage depends on tree age, this ES is limited due to an often reduced life span and health of trees in urban streetscapes (Vaughn et al., 2014). When managing street trees for climate change mitigation multiple ES, costs, community needs, and preservation of existing forests should thus be considered (Escobedo et al., 2011). 3.1.4. Noise reduction Vegetation can attenuate noise via diffusion depending on the shape of vegetation barriers; leaf size and branching characteristics affect resonant absorption properties (Aylor, 1972; Fang and Ling, 2003). Attenuation effects are larger in woody vegetation compared to grasslands or fields (Kragh, 1981), and increased with density and width of woody vegetation belts (Fang and Ling, 2003). At the same time, the perception of natural soundscapes such as bird song along roads can decrease the perceived level of traffic noise (De Coensel et al., 2011; Hong and Jeon, 2013). 3.1.5. Regulation of water cycling and water purification Tree plantings, green filter strips, rain gardens and bioretention swales (Fig. 1) are effective measures for controlling urban storm water by reducing volume and peaks of runoff (Marsalek et al., 1993; van Roon, 2007). Urban runoff also collects various pollutants as it travels toward a treatment plant or water body (Johnson et al., 2011; Zhao et al., 2011); the decontaminating effects of infiltration and absorption by vegetation and soil can reduce associated risks (Fach and Geiger, 2005). Increasing such

Fig. 1. Multifunctional swales in urban streetscapes in Berlin (A: Rummelsburg; B: Kladow; C: Weißensee). Such swales are established to mitigate stormwater events by increasing infiltration, groundwater recharge and biological uptake of pollutants. Depending on design and management, swales can enhance biodiversity, landscape quality, and air cooling and filtering.

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functions is a timely challenge for water-sensitive urban design techniques (Brown et al., 2009; Apul, 2010; Kazemi et al., 2011). 3.2. Provisioning services 3.2.1. Food production In contrast to practices in most industrialized countries, millions of people in other parts of the world depend on crops and animals raised in cities, often close to roadsides (Egziabher et al., 1994; Nsangu, 2009). Overall, (peri-)urban agriculture can contribute to reducing poverty and increasing food security by providing an estimated 10% of the global food supply (Graefe et al., 2008). In some African countries, considerable amounts of food are being produced along roadsides (Freeman, 1991; Nsangu, 2009); tree products are frequently used as well (Kaoma and Shackleton, 2014). Vegetables produced next to roads may be loaded with toxic contaminants (Gnandi et al., 2008; Sa¨umel et al., 2012). However, barriers between roads and sites of cultivation (e.g., dense plantings of woody species) can shade vegetables from trafficborn pollutants (Sa¨umel et al., 2012). In contrast, nuts, berries, pome and stone fruits from high traffic areas are safe when washed before consumption (von Hoffen and Sa¨umel, 2014). 3.2.2. Genetic resources Veteran trees along roads (‘‘heritage trees’’) provide an ecological legacy of genetic material from the regional gene pool (Jim, 2004) that can help restore forests in deforested regions. 3.2.3. Water recharge Unsealed surfaces enhance rain water infiltration in streetscapes and can contribute to groundwater recharge. While runoff often contains pollutants, bioretention swales (Fig. 1) can support decontamination (see above). 3.3. Biodiversity and habitat services Urban streetscapes can harbor a considerable variety of animal and plant species that colonize existing microsites or use road corridors as part of urban habitat networks for moving. About a quarter of the total urban flora thrives along roadsides in Berlin (Langer, 1994). Small patches around trees on sidewalks, a globally distributed habitat type, (i) can be rich in species, with highly variable species assemblages and a large stock of common roadside species (Ise, 2006; Wittig and Becker, 2010), whereby species richness (ii) increases with decreasing maintenance; (iii) is higher in residential areas than in urban cores (Langer, 1994; Ise, 2006), but (iv) is lower than in other urban habitats (Lososova´ et al., 2011). Adjacency to parks and gardens increases the abundance of introduced species along roads (Kalwij et al., 2008). Urban road verges can be refuges for native plant species such as grassland species (Langer, 1994; Cilliers and Bredenkamp, 2000), with habitat age and intensity of maintenance driving species composition (Jantunen et al., 2006). Small grassland patches in roundabouts or other road enclosures can support abundant insect populations (Helden and Leather, 2004; Koivula et al., 2005). Since urban habitats are usually highly fragmented, vegetated road corridors can function as parts of urban biotope networks by linking urban habitats (e.g., parks with forest remnants) or urban green spaces with rural areas (Ignatieva et al., 2011). Such functions have been shown for birds (Ferna´ndez-Juricic and Jokima¨ki, 2001), bats (Oprea et al., 2009) and mammals (Way and Eatough, 2006; Munshi-South, 2012). However, at the same time, roads can be barriers for less mobile groups of animals such as amphibians (Ignatieva et al., 2011). Native trees and diverse roadside vegetation have been shown to support avian biodiversity (Ikin et al., 2013).

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Yet habitat provision for unwanted species can be perceived as ED (Bixler and Floyd, 1997). In many cities non-native trees line roads, which results in huge propagule pressure and strengthens related invasion processes (Kowarik et al., 2013; Dickie et al., 2014). Within and outside cities, roadside floras are usually rich in alien plant species (Langer, 1994; Gelbard and Belnap, 2003). Vehicles have been shown to move seeds of non-native species over longer distances than seeds of native species (von der Lippe and Kowarik, 2008), and some invasive species spread frequently along roads (Brown et al., 2006; Essl et al., 2009). However, results on the functioning of roads as invasion corridors are ambiguous (Gelbard and Belnap, 2003; Kalwij et al., 2008). 3.4. Cultural services Urban roads are not merely transport corridors but public spaces where people engage in recreational and social activities (Bosselmann et al., 1999). Developing livable streets that encourage social functions and human health has thus been considered a main challenge of urban design (Jacobs et al., 2002; Frank et al., 2003; de Vries et al., 2013). 3.4.1. Psychological, esthetic and recreational services Many studies suggest an important role for urban nature elements in human well-being (Sullivan et al., 2004; Dean et al., 2011). Despite the omnipresence of urban streetscapes, the way roadside vegetation affects human well-being is clearly understudied (but see van Dillen et al., 2012; de Vries et al., 2013). Trees have a great influence on preference for types of street greening (Todorova et al., 2004), and adding trees to urban streets strongly increases the perceived quality of life (Sheets and Manzer, 1991). Weber et al. (2008) consistently identified vegetation as a fundamental property influencing esthetic judgments of urban streetscapes. However, vegetation may be also perceived as an ED when associated with traces of neglect or growing wild (Nassauer, 1995; Tzoulas and James, 2010; Pellegrini, 2012), but ‘‘cues to care’’ (Nassauer, 1995) created by human interventions can enhance acceptance (Tzoulas and James, 2010; Hofmann et al., 2012). In a perception study in Berlin, wild grown roadside vegetation met with high approval, although planted and maintained vegetation was slightly preferred. The majority of respondents were, however, aware of a range of ES provided by both categories of vegetation (Weber et al., 2014b). Another study revealed the potential of wild vegetation around street trees to attenuate negative valuations of litter and dog’s mess in streetscapes (Gerlach et al., 2013). Because car-dependent lifestyles negatively affect human health, research on environmental characteristics and physical activities that foster healthier lifestyles is gaining importance (Cervero and Duncan, 2003; Srinivasan et al., 2003). Urban streets are being used for leisure and health-related physical activities (Powell et al., 2003; de Vries et al., 2013), and sidewalks and enjoyable scenery likely foster such activities (Brownson et al., 2001). A range of studies address the role of neighborhood characteristics for stimulating physical activities (Frank et al., 2003; Hanibuchi et al., 2011). However, how different types of roadside vegetation affect such activities is unknown, despite anticipated effects on physical comfort and positive environmental feedback due to fewer automobile trips. 3.4.2. Enhancing property values Presence and quality of urban greenery affect property values as reported for properties adjacent to greenbelts or parks (Crompton, 2005). Consistently, studies reveal a greater willingness-to-pay for goods and services with increasing amounts of roadside vegetation (e.g., Sander et al., 2010). However, greening measures can also

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lead to a ‘green gentrification’ and a displacement of those residents which were anticipated to benefit from greening. Consequently, planners should aim at both social and ecological sustainability (Wolch et al., 2014). 3.4.3. Cultural heritage Historical alle´es belong to the cultural heritage of many cities, structure urban space and create iconic pedestrian realms (Jacobs et al., 2002). Streetscapes can also harbor heritage trees, i.e. old trees celebrated for their age, appearance, or cultural significance (Jim, 2004). In Guangzhou (China), streets accommodate more heritage trees than parks or religious sites (Jim, 2004). Yet as the age of a street tree increases so too does the potential for safety hazards due to fallen wood (Sreetheran and Yaman, 2010). Non-native trees have traditionally been planted in many cities and deliver both regulating and cultural ES in streetscapes. Some species, however, can induce severe invasion impacts, necessitating assessments of the trade-offs between ES and ED that acknowledge multiple objectives related to biodiversity conservation and urban greening (Dickie et al., 2014). 3.4.4. Educational services Urban dwellers increasingly feel disconnected from nature, leading to a loss of natural experiences (Miller, 2005). In consequence, children’s understanding of the natural environment has clearly declined in recent years (Pretty et al., 2009). Due to its location in the space where urban dwellers interact daily, roadside vegetation offers manifold opportunities for linking humans with biodiversity because it is easily accessible. In many cities, guerrilla gardening has transformed road verges and tree planting sites into flourishing flower beds or vegetable gardens (Reynolds, 2008). Such informal activities foster the involvement of residents in shaping urban environments and enhance, as a side-effect, experience with natural elements. 3.4.5. Bioindication Using plants along roads to monitor local conditions via indicator values or by measuring pollution levels and related impacts on biological receptors is an effective low-cost method (Oliva and Ferna´ndez Espinosa, 2007). Measuring dust deposition on plant surfaces (Weber et al., 2014a) or magnetic properties to leaves (Rai et al., 2014) can reveal pollution patterns. 4. Discussion

Fig. 2. Examples of wild growing road side vegetation in Berlin. Such vegetation is easy to maintain, contributes to temperature regulation and demobilization of pollutants and provides access to biodiversity in ubiquitous urban sites.

To approach the ideal of livable cities, planners and decision makers need to be informed about both ES and ED associated with the urban green infrastructure (Elmqvist et al., 2013; Gaston et al., 2013). Our review illustrates a broad range of ES that can be delivered by roadside vegetation and help mitigate environmental pressures, which is specifically relevant for socioeconomically disadvantaged groups with limited access to parks or gardens (Kirkpatrick et al., 2011). The results summarized in Table 1 represent a first integrative synthesis because existing studies mainly address individual ES and are broadly scattered in papers from the natural and social sciences. Previous work appears to be biased twice over in its focus: first with a bias toward regulating services (Haase et al., 2014), and second toward trees. As an important result, our review indicates that the potential of ground vegetation to complement the services of trees is likely underestimated. Herbaceous plants bind particulates close to both major emission sources and humans (Weber et al., 2014a) and help counteract temperature stress (Shashua-Bar et al., 2011; Fig. 2). Such effects can be increased by synergies between water- and temperature-related ES, e.g. in bioretention swales, which enhance cooling through greater

evaporation while potentially fostering biodiversity (Kazemi et al., 2011; Fig. 1). It is thus an important direction for future research to assess the relative contribution of ground versus tree vegetation to ES/ED in streetscapes across scenarios that differ, for example, in width and depth of street canyons and in features of vegetation structure and composition. Introducing native plant species to highly modified urban sites is a promising measure to enhance urban biodiversity (Fischer et al., 2013). However, since swales are categorized as civil engineering structures, related design and maintenance are often strongly regularized. The Berlin Standard for Swales, for example, comprises 60+ rules, including a rather limited standard for greening (BWB, 2012). By further developing such regulations, biodiversity-friendly interventions can be promoted to enhance the multi-functionality of urban swales (e.g., by allowing seeding or planting a broad array of native species). Herbaceous vegetation does not negatively affect air exchange in streetscapes—a potential ED resulting from dense tree plantings in narrow streets which is not yet generally acknowledged (Vos et al., 2013; Ng et al., 2015). It is thus promising to complement

Please cite this article in press as: Sa¨umel, I., et al., Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move. Environ. Sci. Policy (2015), http://dx.doi.org/10.1016/j.envsci.2015.11.012

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policies on decreasing urban stressors at the source (e.g., Chiesa et al., 2014) with ES-based mitigation approaches. The scale of urban road networks offers considerable development potential for green elements. We argue for enhancing roadside vegetation due to the many synergies among regulating, habitat and cultural services and sketch management approaches in Table 1 that may inform urban planners or decision makers on how to strengthen ES, or counteract ED, in urban streetscapes. In particular, herbaceous roadside vegetation offers unexploited opportunities as it is easy to grow, flexible in its lay-out, and can be easily incorporated into existing infrastructure without disrupting major transport functions of roads. Allocating funds toward green streetscapes is likely efficient because these urban spaces are highly visible and accessible in the daily lives of urban dwellers. Still our review indicates gaps of knowledge that challenge an informed ES management. Ecosystem services other than regulating services, in particular cultural ES, are strongly understudied, as are tradeoffs between different ES because most papers address single ES. While some papers also stress the general need to consider ED related to green elements (Lyytima¨ki and Sipila¨, 2009; Pugh et al., 2012), specific information on streetscapes is limited (Table 1). In contrast to many studies on human preferences for urban parks and forests, the valuation of roadside vegetation types has rarely been addressed (but see Todorova et al., 2004; Weber et al., 2014b; Ng et al., 2015). How different sociocultural groups value vegetation elements in urban streetscapes is an intriguing field of research because results would strongly inform design and maintenance approaches at the local scale. Moreover, testing different roadside greenings as stimulus for physical activities is a promising perspective for trans-disciplinary approaches that would link urban design and maintenance approaches with sustainability and health issues. Yet integrative approaches to consider multiple ES also require integration at the administrative level, because competences for streetscapes are often divided. In Paris, for example, streetscape responsibilities involve the departments of the public road network, urban waste management and city planning (Pellegrini, 2012). Alley greening programs in the U.S. show that a range of collaborating agencies and organizations beyond public administration can be involved in funding and implementing ES-oriented concepts in streetscapes (Newell et al., 2013). In Germany, many planners are aware of the ES concept, but lack support from higher levels as well as the appropriate resources to be able to implement related concepts (Albert et al., 2014). A suite of papers illustrate that streetscapes serve as habitats for plants and animals. However, the extent to which distinct species, communities, or ecosystems function as ES providing units (Kremen, 2005) is an open yet important question because specific answers would help link approaches toward human wellbeing with biodiversity conservation. Biodiversity in streetscapes might yield psychological benefits, but such effects still need testing (Keniger et al., 2013). Novel species assemblages with many non-native plants challenge traditional conservation strategies aiming at restoring native biodiversity. Because novel plant communities are usually well adapted to urban conditions (Kowarik, 2011), different strategic approaches to creating and managing road greening should be considered. One goal is to counteract ED at a specific level, e.g., by regulating species that pose health risks or preventing plantings of highly invasive species close to dispersal corridors that lead to susceptible natural areas. Another approach might emphasize that both native and introduced species can deliver ES, in particular at severely changed sites. Greening approaches beyond tree plantings could thus include several options such as (i) working with spontaneous, wild-grown assemblages of native and non-native species (Ku¨hn, 2006; Del Tredici, 2010), (ii)

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incorporating native species into design schemes (Ignatieva et al., 2008; Fischer et al., 2013) or (iii) establishing mixtures with esthetically attractive native and non-native species (Hitchmough and Wagner, 2013). 5. Conclusions Developing and managing ES in streetscapes is challenging for urban planning and governance because of limited access to knowledge about particular ES and ED, including the trade-offs among these. Our review highlighted that several functions of roadside vegetation are potentially conflicting or can be addressed both as ES or ED, depending on local settings and valuations of citizens (see synthesis in Table 1). Applying the principle of multifunctionality to roadside vegetation thus requires integrating a large set of environmental, social, economic, cultural and esthetic functions in developing livable green streets. As a consequence, integration is also required at the administrative level. The aim of enhancing livable streetscapes would benefit from the cooperation of different stakeholders which often stick to separated sectors such as traffic planning, tree plantings, health, security, or cleanliness issues. Moreover, developing concepts for roadside greening would profit from participatory processes because demands on specific ES likely differ among social groups at the local scale. Participation could help reconcile conflicting demands from urban dwellers and other stakeholders that might focus, for example, on traditional design, ecological aims or other socio-economic goals such as safety. Awareness about the value of multifunctional ES in streetscapes can therefore inform planning and decision processes and lead to the development of more livable streets. Acknowledgements Parts of this study were funded by GREEN SURGE, EU FP7 collaborative project, FP7-ENV.2013.6.2-5-603567, and by KURAS project (BMBF, INIS program). We thank Kelaine Ravdin for improving our English. References Albert, C., Hauck, J., Buhr, N., von Haaren, C., 2014. What ecosystem services information do users want? Investigating interests and requirements among landscape and regional planners in Germany. Landsc. Ecol. 29, 1301–1313. Al-Dabbous, A.N., Kumar, P., 2014. The influence of roadside vegetation barriers on airborne nanoparticles and pedestrians exposure under varying wind condition. Atmos. Environ. 90, 113–124. van Dillen, S.M.E., de Vries, S., Groenewegen, P.P., 2012. Greenspace in urban neighbourhoods and residents’ health: adding quality to quantity. J. Epidemiol. Community Health 66 (6), e8. Apul, D., 2010. Ecological design principles and their implications on water infrastructure engineering. J. Green Build. 5, 147–164. Aylor, D., 1972. Noise reduction by vegetation and ground. J. Acoust. Soc. Am. 51, 197–205. Beckett, K.P., Freer-Smith, P.H., Taylor, G., 1998. Urban woodlands: their role in reducing the effects of particulate pollution. Environ. Pollut. 99, 347–360. BWB [Berliner Wasserbetriebe], 2012. Regelblatt 600: Mulden-Rigolen-SystemeGrundsa¨tze 7p. Bixler, R.D., Floyd, M.F., 1997. Nature is scary, disgusting, and uncomfortable. Environ. Behav. 29, 443–467. Bosselmann, P., Macdonald, E., Kronemeyer, T., 1999. Livable streets revisited. J. Am. Plan. Assoc. 65, 168–180. Bowler, D.E., Buyung-Ali, L., Knight, T.M., Pullin, A.S., 2010. Urban greening to cool towns and cities: a systematic review of the empirical evidence. Landsc. Urban Plan. 97, 147–155. Brown, G.P., Phillips, B.L., Webb, J.K., Shine, R., 2006. Toad on the road: use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia. Biol. Conserv. 133, 88–94. Brown, R.R., Keath, N., Wong, T.H.F., 2009. Urban water management in cities: historical, current and future regimes. Water Sci. Technol. 59, 847–855. Brownson, R.C., Baker, E.A., Housemann, R.A., Brennan, L.K., Bacak, S.J., 2001. Environmental and policy determinants of physical activity in the United States. Am. J. Public Health 91, 1995–2003.

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Please cite this article in press as: Sa¨umel, I., et al., Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move. Environ. Sci. Policy (2015), http://dx.doi.org/10.1016/j.envsci.2015.11.012

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Zhao, H., Li, X., Wang, X.M., 2011. Heavy metal contents of road-deposited sediment along the urban–rural gradient around Beijing and its potential contribution to runoff pollution. Environ. Sci. Technol. 45, 7120–7127.

Frauke Weber is a landscape planner and PhD student within the Postgraduate Research and Study Program (DFG – GRK 780) ‘‘Perspectives on Urban Ecology III’’. Her scientific research focus is on cultural and regulating ecosystem services of urban roadside vegetation.

Ina Sa¨umel is a biologist and geographer with a research focus on urban ecology and sustainable land use. She works as an assistant professor at Technische Universita¨t Berlin and currently leads a Research Group that aims to develop strategies toward multifunctional, biodiverse and sustainable productive landscapes in South America, based on analyses of grassland modifications by forestry and agriculture.

Ingo Kowarik is professor of Ecosystem Science/Plant Ecology at Technische Universita¨t Berlin. His research interests include understanding biodiversity patterns, underlying mechanisms and related ecosystem services in urban landscapes as well as developing conservation approaches. He also serves as an honorary State Commissioner for Nature Conservation and Landscape Management of the federal state of Berlin.

Please cite this article in press as: Sa¨umel, I., et al., Toward livable and healthy urban streets: Roadside vegetation provides ecosystem services where people live and move. Environ. Sci. Policy (2015), http://dx.doi.org/10.1016/j.envsci.2015.11.012