Street Trees in Paved Urban Environments – The Benefits and Challenges

Simon Beecham and Terry Lucke Centre for Water Management and Reuse School of Natural and Built Environments
University of South Australia Mawson Lakes, SA 5095, Australia
Phone +61883025141, email : [email protected]
Storm water Research Group, University of the Sunshine Coast, QLD, Australia
e-mail : [email protected]

Abstract

Street trees have long been associated with increased residential house prices and provide many environmental, economic and social benefits in both residential and commercial precincts.However, increase durbanisation has led to an exponential growth in impermeable surfaces which can increase the environmental stresses on street trees. This can often lead to tree roots spreading to areas that have more favourable growth conditions, which in turn can cause infrastructure damage and pavement uplift. The costly results of this natural growth have led researchers to investigate a range of preventative measures to both reduce pavement damage and to sustain tree health. This paper provides are view of the benefits provided by street trees, their perceived community values, the costs associated within appropriate root growth and pavement damage, and most importantly the latest research on verified ameliorative measures for preventing pavement damage and improving tree growth.

Introduction

Urbanisation results in increasing areas of impervious surfaces within a catchment and this can change the natural drainage characteristics of the catchment. Pavements are an everyday part of the urban landscape that can have a significant environmental impact. Typically two-thirds of all the rain that falls on potentially impervious surfaces in urban catchments is falling on pavements (Ferguson,2005), which are responsible for the generation of excess runoff hat is often contaminated with heavy metals and hydrocarbons(Fletcheret al.,2005;Hattetal.,2009). Impervious surfaces also inhibit ground water recharge and this can result in local water shortages and other water balance problems. Pavements are very much at the forefront of the planning process for developers and local authorities because impervious surfaces can have such a major impact on downstream flooding,receiving water quality and on the health of natural ecosystems.

Street trees are often incorporated in tour ban developments because of the many environmental, economic and social benefits that they can provide. However, changes in the urban environment, particularly due to increasing impervious surface areas can place increasing stresses on urban forests and ecosystems (Martinet al.,2012). The central function of urban street trees has changed considerably over the last two to three decades. Their primary function has changed from a purely aesthetic role of beautification and ornamentation to a role providing significant environmental, economic or social benefits (Seamans, 2013).

Street trees have an important role in providing healthy urban communities and they can produce significant social impacts by improving human health, reducing crime,increasing community interaction and boosting property values (Burden, 2006). They also provide benefits such as energy conservation, storm water reduction and increased air quality (Mullaneyetal.,2014). Although street trees can provide multiple environmental, social and economic benefits, they can also cause disruptive and costly damage to pavement and other civil infrastructure.

This paper discusses the benefits of street trees and the challenges of growing trees in urban environments. It also describes a current field investigation into street tree irrigation using harvested road run off. The paper should provide a practical resource for use by urban landscape designers, engineers, and council parks and gardens staff.

Benefits of Street Trees

Street trees improve the live ability of towns and cities in a number of ways including reducing stormwater run off, increasing air quality, storing carbon, providing shade,and reducing urban heat-island effects. They can also enhance bio diversity by providing food, habitat and landscape connectivity for urban fauna (Burden2006; Rhodes etal., 2011).

Research has shown that most urban residents have a positive view of street trees, and they believe that the benefits that street trees provide clearly outweigh any detriments (Sommeretal.,1989;Schroederetal., 2006). Despite identifying potential problems such as falling branches, leaf litter, tree debris and infrastructure damage residents attitudes to street trees remain positive. The aesthetic and practical attributes of street trees such as beautification, shade provision, increased property values, added privacy and noise reduction are rated highly by most city residents (Summit&McPherson,1998;Flannigan,2005;Zhangetal.,2007;Moskell & Broussard Allred,2013).

Social Benefits

Green space within a city’s boundaries can improve the urban environment by providing recreational opportunities and promoting contact between community residents. This encourages physical activity, reduces stress and stimulates social cohesion (Van Dillenetal., 2012). The presence of street trees have also been linked to reduced crime and increased public safety trees (Kuo&Sullivan,2001;Tarran,2009). For example, urban areas with higher levels of vegetation can have approximately 50% lower crime levels than areas with low levels of vegetation (Kuo & Sullivan,2001). Street trees also act as a visual and physical barrier between motorists and pedestrians. Trees can help motorists assess their vehicular speed and provide a physical defence for pedestrians against vehicle injury (Tarran, 2009).

Stormwater Benefits

Increases in impervious surface are as due to urbanisation, can reduce water infiltration into the soil as well as increase stormwater runoff volumes and peak flow rates. Planting street trees in urban environments can significantly improve the overall water balance within a catchment. For example, depending on the site characteristics and the tree species, street trees have been shown to intercept large volumes of rainwater (McPhersonetal., 2005; Bonifaci,2010; Soaresetal., 2011)and this can significantly reduce stormwater runoff volumes.

Street trees canal so increases oil infiltration as leaves and branches intercept, absorb and temporarily store water before it evaporates from tree surfaces or gradually infiltrates into the soil. Increased soil infiltration due to interception by street trees also reduces stormwater runoff. Lower stormwater runoff volumes also directly reduce down stream pollution levels and minimise the need for stormwater treatment systems,which are often expensive and difficult to install.

Estimated annual reductions in stormwater runoff volumes range from 3.2kL to 11.3kL per tree, and the annual values assigned to stormwater reduction vary from A$3.4 to A$58 per tree (Figure1). Five of the studies place the value assigned to stormwater reduction below A$8.50 per tree per year. However, three of the studies valued the trees at between $18 and $58 per tree. The reasons for these much higher values were not clear but maybe due to differences in how the values were assigned. Either way, all of the studies reviewed in Figure1 demonstrate that street trees can provide a significant reduction in stormwater management costs.

Air Pollution Benefits

Traffic emissions and other fine particulate air pollution can cause serious health effects, including premature mortality, pulmonary inflammation, accelerated a the rosclerosis, and altered cardiac functions. However, street trees can be particularly effective at capturing airborne pollutants in urban areas (Tallisetal.,2011). Some of the pollutants removed by street trees include ozone, nitrogen oxides, sulphur oxides, sulphur dioxides, carbon monoxide, carbon dioxide (CO2). It has been estimated that large healthy trees can remove between 60 and 70 times more air pollution than smaller trees (McPhersonetal.,1994). Street trees are an effective tool in reducing air pollution and creating healthier urban environments (Nowak etal.,2013). Reduction in energy use due to street trees also leads to reduced emissions of CO2, nitrogendioxide, veryfine particulate matter and volatile organic compounds.

The economic benefits from removing air pollution range from $0.34 to $42/tree/year (Figure 2). The large variation in the results is possibly due to different locations, treesizes and treespecies.

Carbon Benefits

The “green house effect” is caused by CO2 and other gases trapping heat generated from the earth in the atmosphere and prohibiting the heat from being released into space. Urban forests and street trees can help to improve our air quality by removing (sequestering) CO2 from the atmosphere during photosynthesis (Ferrini & Fini,2010). This process produces carbohydrates required for tree growth and returns oxygen back into the atmosphereasaby-product. Roughly half of the green house effect is caused by CO2. Therefore, trees act as carbon sinks, alleviating the green house effect.

To put this into context, Moore (2009) estimated that the inner-city tree population of Melbourne, Australia (~100,000 trees) had sequestered more than one million tonnes of carbon since they were planted. The economic benefit of CO2 reduction by street trees is less than that of other benefits. However, the values of street tree carbon sequestration still range between $0.4 and $6 per tree/per year(Figure 3).

Energy Benefits

Street trees provide energy savings through their shading and cooling effects in summer and the wind-chill protection they offer in winter. The cooling effect provided by trees is directly related to tree size,canopy cover, tree location, and planting density. As much as 80% of the cooling effect of trees results directly from shading (Shashua-Bar & Hoffman, 2009). Street trees can reduce day time temperatures by between 5°C and 20°C, making everyday activities more pleasurable and healthier (Killicoatetal.,2002;Burden,2006).

Energy cost reductions due to street trees have been estimated at between $2.6 and $77 per tree/per year (Figure4). The reasons for the particularly high value of $77.44 by Killicoatetal. (2002) were not clear but may be due to the very-different energy use assumptions and assigned cost values used in the study. Average savings in electricity due to street trees have been estimated at 95kWh/tree/year, equating to anannual saving of US$15/tree/year(McPhersonetal.,2005). A later study calculated a power saving of 30kWh/tree/year(Moore, 2009).

Benefit Rankings

The ranked estimated annual street tree benefits are in shown in Figure 5.

Overall Economic Benefits

Functional benefits of trees such as the removal of air pollution by leaves, and there duction of stormwater flows through root and leaf uptake, increase as tree canopy cover increases.Therefore,the economic benefits of street trees often correlate with physical tree variables such as trunk diameter and leaf surface area (Killicoatetal.,2002; McPhersonetal.,2002; Bonifacietal.,2010). Figure6 shows the influence that street tree size has on the general economic benefits of the tree.

Street trees canal so provide other, less expected benefits, For example,trees caping has been shown to increase business income by 20% (Burden,2006).While this may seem surprising, consumers have been shown to willingly pay more for the same item in a retail development that includes street trees compared with the same item in non-trees caped retail outlets (Wolf,2005). This amenity value has also been observed for house prices in Perth,Australia,which were shown to be an average of 20–30% higher when there was tree cover on the public space next to,or near, the property (Panditetal.,2012). Although there are a variety of different ways to assign value to trees, urban street trees clearly generate significant economic benefits for communities and local governments, regardless of the reporting format. Taking estimated values from a range of studies, the annual net benefit per tree seems to lie between $45 and $242 (Figure 7).

Challenges of Growing Trees in Urban Environments

Despite the advantages mentioned above,ensuring the survival of street trees is often challenging,and urban designers need to consider critical issues in their landscape design including site conditions,space availability, and maintenance requirements. The healthy growth of trees can be disrupted by abroad range of biotic (living) and abiotic (non-living) factors,and indeed more than one factor can affect the health of a tree at any time. Biotic factors that can adversely affect tree growth include (Boa, 2003):

• Fungi
• Bacteria
• Viruses
• Phytoplasmas

Insects

• Mites
• Parasitic plants
• Weeds
• Larger animals, such as deer and other mammals.

A biotic factors that can adversely affect tree growth include (Boa, 2003):

• Soil and water chemistry
• Mechanical agents, including poor pruning, construction equipment and malicious human damage
• Soil conditions, including soil type, volume and porosity
• Water availability
• Fire damage
• Weather conditions, including heat and frost

Tree growth is influenced by a biotic factors including air quality, irradiance, soil chemistry, soil moisture, soil volume and soil porosity (Iakovoglouetal.,2001;Morgenrothetal.,2013). A change in availability of these a biotic factors in the urban environment can result in costly damage to infrastructure as tree roots proliferate in otherwise-undesirable areas that provide sufficient water, nutrients and oxygen for tree survival and growth. The use of pervious surfaces to improve the health of urban street trees has been the focus of recent research (Volderet al., 2009; Morgenroth & Visser, 2011; Mullaneyet al.,2012). Our research has also investigated whether permeable pavements, which allow water and oxygen to infiltrate through the pavement surface and into the soil, extend the life of street trees (Mullaney & Lucke, 2014).

Some studies have investigated whether permeable pavements minimise damage to pavements and other urban infrastructure (Mullaney & Lucke,2014). The underlying hypothesis of these studies is that the integral drainage layers required underneath permeable pavements may effectively create a root barrier beneath the pavement surface, forcing roots to grow at greater depths. These layers can potentially also increase the pavement’s water storage capacity, promoting tree health directly while minimising pavement damage.

CaseStudy

A research project is currently underway at the University of South Australia investigating street tree irrigation using harvested road runoff. The study involves the use of “TREENET Inlets” (kerbsidetreeinlets), which area relatively new technology introduced by TREENET Inc. The inlets are currently adopted by a number of councils in South Australia and inter state. The inlets form the system by which road runoff can be readily harvested for street tree irrigation. Various back end distribution systems have also been introduced by council engineers and landscape designers. These systems within lets aim to harvest road runoff with very low maintenance. This is because there is a perception in the industry that maintenance costs are high for stormwater harvesting schemes.

The experimental site is located in the City of Mitcham, Kingswood. In total 28 inlets and distribution systems were installed between July and November 2014. The three key components of this study are: (1) water quality; (2) infiltration (water quantity); and (3) life cycle assessment of the systems with different back fill media.  The inlet and distribution system design is based on a “leaky well” which has been tested at several sites in the City of Mitcham.  It is a very simple design to harvest the first flush of storm water, which has a higher nutrient content.  There are four different media types located in the distribution component of the leaky well system.  The different substrate types were randomly distributed at the different locations. The four types of media include 14 mm dolomite gravel(G), water treatment solids(W) from Happy Valley Reservoir, sandy loam soil (S)and control media(C) units which were backfilled with native soil.

The systems were constructed by contractors to the council.  Twenty-one distribution systems, each 440mm in diameter and 1 m deep were dug using a hydrovac system. There mainings even control systems were backfilled with native soil backfill. The leaky well design is shown in Figure 8.

The study will collect inlet and outlet water samples from one of each of the fourty peso fleaky well unit. Runoff events will be targeted upto and including the 2 year ARI. BOM weather forecasts and rainfall radar will be monitored for the local area and on this basis the experimental site will be visited before a storm to collect the inflow and outflow samples. A total of 4 critical storms will be selected after atleast 6 days’ dry period. A tipping bucket pluvio meter will be installed to measure the rain fall intensity. Samples will be collected at 15 minute intervals for a maximum of 2 hours (depending one vent duration and presence of sample). Each inflow and outflow samples will be tested for EC, pH, totalnitrogen, totalphosphorous, zinc, copper, lead, cadmium and potassium.

This field study will also provide information on the quantitative performance of the media and leaky well distribution system. Four leaky well systems (back filled with each of the four different media) have been selected to determine the capacity of the systems. Each of the four sites will be tested once every three months onsite. The catchment area and runoff generated by the contributing impervious area will be calculated using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC).Water will be poured into the systems and the infiltration rate will be measured over a given time period. The effect of sediment and organic matter will also be investigated. For this purpose,two additional kerbside in lets will be selected to test with and without sediment and organic matter settled near the kerb side inlet.

Interms of life cycle cost assessment, the construction and maintenance costs are considered over the lifetime of the project. Life cycle assessment of the TREENET inlet and leaky well distribution systems will include the performance of the various media types, the size of the leaky well distribution system, and the size and number of TREENET inlets. These will be combined to provide a full economic model that can describe the costs and benefits of street trees.

Conclusion

Street trees and other vegetation in the urban environment all help to secure and provide vibrant ecosystem services. Local councils and communities recognise the importance of street trees in the urban environment but these trees often suffer from inadequate water supplies intimes of drought. The concept of WSUD, and in particular storm water harvesting and reuse, can be used to develop sustainable solutions to the significant challenges that trees face in an urban environment. However, there are many barriers to adoption and implementation of this concept including funding for implementation, policies and regulations, political support and most importantly lack of institutional support and research. This research study will provide the base line information on the performance of road-based stormwater harvesting systems.

 Acknowledgments

This research project is supported by the Australian Research Council through grant LP120200678 and the support of the industry partner Sunshine Coast Council is gratefully acknowledged. Further financial and in- kind support for this research program has been provided by the City of Mitcham, the Adelaide and MtLofty Ranges Natural Resources Management Board, the Environment Protection Authority of South Australia and the TREENET organization. The authors also gratefully acknowledge the important contribution and guidance provided by Professor Randy Stringer from the University of Adelaide, Tim Johnson from City of Mitcham, David Lawry from TREENET and Baden Myers, Tim Golding and Victor Vorel all from the University of South Australia.

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