Tim Johnson, TREENET Inc.
Local government urban forestry offers unique challenges and opportunities. The role is vital in the development of community and in our progress toward sustainability, though much of society doesn’t yet appreciate its value. The range of complex tasks performed by council arborists, the breadth of regulatory constraints under which they work and the limited information available to them make achieving desired outcomes difficult.
Local government is about community. Every task, function, service or governmental role performed by council should be for the good of the community. Urban forests should be established and maintained for the public good.
Public Trees and the Triple Bottom Line
The 1987 World Commission on Environment and Development (the Brundtland Commission) stressed the need for sustainable development. The 1992 United Nations Conference on Environment and Development (the Rio Earth Summit) championed ecologically sustainable development (ESD) based on a considered balance of social, environmental and economic goals.
Our national government, all states and territories and many local government authorities have declared their support for ESD goals. Their balance sheets should therefore use the “triple bottom line.” The costs of all goods, services and functions provided or performed by these organisations should be considered against social, environmental and economic outcomes. Thus local government should consider the economic, social and environmental costs and benefits of its arboricultural work. (For more information see http://www.ea.gov.au/industry/finance/publications/indicators/ )
The Benefits of Urban Trees
Reports over the last decade have analysed the values of urban trees, see Hewett, P. (2002); McPherson, E.G. (1996); McPherson et al (2000); Moore, G. (2000); Alexander, K. (2003). Researchers have also begun to present the environmental benefits of urban trees in financial terms.
The gross annual benefit of a typical Adelaide street tree has been calculated at $171 (Killicoat, P, et al 2002). McPherson (2001) analyses costs and benefits of small, medium and large trees and determines the average annual net benefit over a forty year period as -$2, $33 and $66 respectively (in US$).
Many local business operators consider the financial return derived through increased provision of parking space and “street exposure” to be of more value than amenity and environmental benefits provided by urban trees. A recent study by Wolf, K. (2003) suggests that this view is flawed. Wolf found that many clients were willing to travel further to and pay more for products and services in greener areas. Wolf’s findings included that “an urban forest canopy may enhance revenues for businesses” and “the urban forest should be a central element of retail place.”
An environmental concern that is considered in rural areas but not so much in major urban settings is rising saline groundwater. Experience in rural areas indicates that rising groundwater not only has significant impacts on native vegetation and biodiversity conservation but also on roads, buildings, gardens and sports turf (Department of Agriculture, Rural Towns Management Committee 2001). This report also identifies that trees are an effective element of groundwater management systems:
“…research indicates that the effect of trees on groundwater tables is very localised – to within 10 to 30m of the edge of the plantings. … The use of trees along road reserves is suggested because road damage represents the single largest cost arising from shallow groundwater. Trees planted near roads can be an effective method of preventing the ground beneath the road pavement from becoming saturated.”
The cost of road reconstruction in Adelaide (a 7m wide asphaltic concrete carriageway with concrete kerbing typical of minor residential streets) is approximately $1,200 per linear metre (Catinari, A., 2003 pers. com.). In groundwater affected areas the useful life expectancy of roads can be reduced to 20 years instead of the more typical 40 years. Groundwater impacts begin when water tables rise to less than six metres deep in reactive clay soils or 1 metre deep in sandy areas. Street trees should be considered essential in many areas on this basis alone.
Street trees can significantly extend the life of bituminous road seals by shading street surfaces. Periods between resealing road surfaces can be extended from approximately once per decade to less than once in every 20 years, with considerable environmental and economic savings resulting (McPherson et al 2001). Given the combined benefits from shading and groundwater reduction it is surprising that street tree planting is not more widely viewed as an essential part of road design and maintenance.
Impacts of rising groundwater are not restricted to public infrastructure. Costs to individuals for repairs to brickwork of private homes, drainage to prevent further damage, maintenance of underground services and increases in utilities charges (due to higher maintenance requirements) can be substantial. In areas where saline groundwater has reached road surfaces, vehicles have deteriorated to the point of becoming unroadworthy within a few years (Zidarich, K. 2002, pers. com).
Many of the economic benefits of tree planting are calculated on returns to regional commons such as water catchments, local government areas or even to global commons like the atmosphere. Though rising groundwater is a widespread problem, its potential economic impact on individuals might make it useful for raising the profile of related issues in urban areas and for gaining support for more intensive urban forestry.
Several recent studies extend previous works relating the human benefits of interaction with nature to societal functioning and the urban forest. Kuo, E. and Sullivan, W. (2001) detail the results of an extensive study of residents of architecturally identical dwellings with varying levels of surrounding vegetation. Kou and Sullivan found that incidences of violence were significantly reduced in both quantity and severity for residents living in better-vegetated areas. Similarly, anti-social behaviours such as littering, graffiti and vandalism were found to be reduced in areas with appropriate vegetation.
Westpahl, L. (2003) describes a range of social benefits derived from involvement in urban greening projects. Shorter periods recuperating in hospitals, reduced crime, improved business incomes, stronger club memberships, stronger community ties and improved cognitive functioning were observable in individuals and groups in the presence of higher levels of vegetation.
The Cost of Urban Trees
The cost of planting, establishing and maintaining urban trees is dependent on a range of factors. In 2001, staff of the City of West Torrens began a research project to determine the lifetime costs associated with several representative street tree species. Space available to trees, suitability to site conditions (such as rainfall and soil characteristics), mature size, growth rate, growth habit, life span and maintenance requirements were found to each have individual and interrelated influences on tree performance and cost.
Conflicts between trees and built infrastructure impact both on the health and maintenance cost of the tree and on the serviceability and maintenance cost of the hardscape. Failing to adequately plan sufficient space for urban trees increases cost. Guidelines on minimum space requirements for street trees suggest a planting space of more than two metres square is required, but that conflicts are likely in nature strips with less than three metres between the kerb and footpath, see O’Brien, D. (1993); Shigo, A. (1991); D’Amato, N. et. al (2003)
- Balancing the interactions of street trees and their planting sites is critical in minimising costs and maximising benefits. Both hardscape and trees are significant investments. Common sense suggests that both investments require simultaneous optimisation.
- The mature size of street trees largely determines the impact they have on street amenity. Planting smaller trees in preference to fewer large trees has been common practice to minimise hardscape maintenance and service impact costs. As tree purchase, planting and establishment costs are similar for large and small trees this practice increases costs to the community while reducing the benefits provided by the trees.
- Tree lifespan directly influences the period during which benefits can be derived and the frequency with which trees must be replaced. Maximum benefit is obtained from long-lived species. Frequent replacement of short-lived species exposes Councils to increased risk from deteriorating trees and reduces the period of benefit.
- Faster growing trees provide benefits sooner than slower species but the faster a street tree grows the more frequently it may require pruning to establish structure or to avoid problems. Some fast growing species develop into large specimens (forest trees) while others may be short lived (colonising species) so they may not make ideal street trees.
- Apically dominant tree species generally require minimal maintenance in street situations. Trees with decurrent habits or multiple leaders require frequent formative pruning, increased long-term maintenance and may have shorter useful life expectancies due to structural issues.
The Cost of Street Tree Establishment and Maintenance
Tree establishment and maintenance practices were documented throughout 2001-2002 to provide a basis for determining approximate lifetime costs for Callistemon ‘Harkness’, Celtis occidentalis, Platanus x acerifolius and Pyrus calleryana street trees. Maintenance time requirements for watering, fertilising, mulching, staking and formative/structural pruning were recorded for each species (Appendices). Gaps in pruning data were filled by pruning individual trees of various ages where recent history was known, eg: a 25 year old Celtis occidentalis that had not been pruned for three years was pruned in 45 minutes so an annualised requirement of 15 minutes (0.25 decimal hours) was recorded. Several gaps in data remained so observed trends were extrapolated.
Note that pruning times may vary due to travel distances, weather, community issues and other factors. Tables summarising the findings are presented in Appendices. Costs associated with tree purchase, planting, litter collection, hardscape repair or site specific works such as pruning trees to a boundary are not included in the figures given in the tables. Costs are calculated at $45 per hour.
|Species||Life Expectancy||Lifetime Maintenance Requirement||Annualised Cost|
|Callistemon Harkness||25 years||13.20||$594||$24|
|Callistemon Harkness||40 years||18.40||$828||$21|
|Celtis occidentalis||70 years||42.60||$1,917||$27|
|Platanus x acerifolius||100 years||39.25||$1,766||$18|
|Pyrus calleryana||50 years||16.10||$725||$15|
Table 1: Approximate lifetime tree maintenance costs for street trees in the City of West Torrens 2002.
Street Tree Planning
Table 1 suggests an economic justification for using larger, long lived, apically dominant species for street planting. The significance of the maintenance cost range becomes more apparent when the population size of urban forests is considered: the annual cost to maintain a forest of 50,000 street trees could vary from $750,000 to $1,350,000.
Consider the lifespan of the trees in table 1. A Platanus might realistically survive for 100 years in which time a nearby Callistemon might be replaced three times. The cost of four Callistemon would be $2,376 compared to the Platanus’ $1,766. The Callistemon would provide little benefit and require high maintenance four times (during the immature stages) compared to the Platanus’ once. These figures are compared graphically in figure 1.
Consider the mature span of the Callistemon, Pyrus, Platanus and Celtis. Developing a continuous canopy for an avenue one kilometre in length will require approximately 100 Platanus, 133 Celtis, 200 Pyrus or 400 Callistemon. The cost to maintain a continuous canopy with these species is summarised in Table 2 below.
|Canopy spread at maturity||5m||10m||15m||20m|
|Quantity per kilometre||400||200||133||100|
|Annualised cost per tree||$24||$15||$27||$18|
|Annualised cost per kilometre||$9,600||$3,000||$3,591||$1,800|
Table 2: Estimated annual maintenance cost of avenues of Callistemon, Pyrus, Celtis and Platanus.
Street Tree Planting and Replacement Rates
How many street trees should a council maintain? Is there an ideal number? The City of West Torrens has approximately 27,000 rateable properties, would one tree per property give a desirable urban forest density? West Torrens has 295 kilometres of council-managed roadway and 30 kilometres of state-managed roads. The maximum length of roadside available for tree planting is therefore 650km. The number of trees required to create continuous avenues in this length of roadway is dependent on spacing, as summarised below in Table 3.
|Quantity required in 650km|
|Annualised maintenance cost||$3,158,400||$999,000||$1,213,191||$613,800|
Table 3: Number of street trees required to create continuous avenues throughout West Torrens and their associated annual maintenance costs.
The number of trees required to “green” a city will obviously depend on the mix of species used, the spacing based on their mature size, and the length of roadway to be planted. Clearly the apically dominant, large species are significantly cheaper to maintain in the long term.
Tree replacement has the effect of increasing resource requirements for the urban forest as maintenance needs are highest for immature trees (see Figure 2). As the increased requirement compounds for each tree planted, keeping maintenance requirements to a minimum requires that the minimum number of immature trees be maintained. Annual tree planting rates should therefore be the minimum required to maintain the optimal number of trees for any given area.
Establishing an urban forest with trees of evenly distributed ages and life expectancies will allow for a range of benefits:
- maintenance costs and resources will be minimal
- replacement planting will match tree removal rate
- impact of tree replacement on amenity and environment will be minimal.
Thus if 68,000 trees are required in a given urban forest (as estimated for the City of West Torrens) and the trees have an average life expectancy of 50 years, 1360 trees should be established each year. A more intensive planting program will result in the desired number of trees being planted earlier, with the undesirable consequences of massive increases in short term maintenance needs and subsequent large-scale replacement programs required in the future (see Figure 3).
Planting large numbers of trees in the short term has the immediate disadvantage of high maintenance requirements and the long-term problem of large numbers of aged trees requiring replacement. Alternatively, planning long-term planting programs to establish and maintain an optimum number of trees allows for consistent staffing, maximum city-wide amenity, maximum environmental benefit and minimal cost to the community.
Annual costs of maintaining street trees vary from approximately $15 to $30 per year over the life of the tree (not including tree purchase, planting and removal). The annual net benefit provided by a typical small street tree in Adelaide is $171 (Killicoat, P. et al 2002). This figure does not include a range of the benefits such as:
- increased life of public infrastructure
- increased life of private property
- improved social networking and informal support mechanisms
- reduced crime rates
- improved cognitive functioning
Thus a conservative approximation of the value of investment in street trees (allowing $2,500 for planting and removal costs, annualised at $50) is between 214% and 260% return on investment.
Returns on tree assets are reduced in cases where there are conflicts between various infrastructure components. Clearance pruning of overhead power lines, removing tree root sewer chokes, replacing damaged concrete and paving, and early tree replacement costs are all met by the community whether through council rates or utilities bills. As many of these conflicts are managed through regulations under various Acts, local government is often unable to optimise urban forest benefits and meet the desires of the community with regard to tree planting and amenity.
Restricting the scale of urban trees and urban forests to reduce their impacts and costs is not the answer. Restricting urban forests restricts their cost-effective provision of essential ecological and social services. Urban forests must be expanded, but in ways where there is minimal conflict with other city infrastructure. The basic needs of street trees must be considered and met in the design and construction of our cities.
Plans for our cities must require the establishment and maintenance of sustainable urban forests. Rather than restricting urban forestry, legislation and regulation must establish a “level playing field” across local government boundaries to assist consistent urban forest management. It must consider long term costs and benefits as well as short term benefit associated with development value.
Effective urban forestry will play a major role in our future through its contribution to city sustainability. It will help us to enjoy this future by enhancing the functioning of our society. It will reduce the financial cost of our future through its contribution to the management of environmental issues. It will assist us to better appreciate and understand this future through its influence on cognitive functioning.
Local government urban forestry can help us to create a better future, to enjoy it more and in the process it can make us better individuals. Local government urban forestry offers unique challenges and opportunities.
- Alexander, Kathleen, 2003, Benefits of Trees in Urban Areas. www.coloradotrees.org/benefits.htm
- Catinari, Angelo, 2003, Senior Technical Officer, Infrastructure Services, City of West Torrens, pers com.
- D’Amato, Nicholas E.; Davis Sydnor, T.; Knee, M.; Hunt, R.; and Bishop, B., 2003. Which Comes First, The Root or The Crack? Journal of Arboriculture, Vol. 29, No. 1, January 2003.
- Department of Agriculture, Rural Towns Management Committee 2001, Economic impacts of salinity on townsite infrastructure. Government of Western Australia, Bulletin 4525, ISSN 1326-415X, June 2001, p4-5.
- Hewett, P. (2002) The Value of Trees – The Big Picture. TREENET Street Tree Symposium, 5th & 6th September 2002, Adelaide University Waite Campus, p6-14.
- Killicoat, P.; Puzio, E.; and Stringer, R.; (2002) The Economic Value of Trees in Urban Areas: Estimating the Benefits of Adelaide’s Street Trees, in TREENET Street Tree Symposium, 5th & 6th September 2002, Adelaide University Waite Campus, p94-106.
- Kuo, Frances E.; Sullivan, W. C.; Coley, R.L.; & Brunson, L., (1998) Fertile ground for community: Inner-city neighbourhood common spaces. American Journal of Community Psychology, 26, 823-851
- Kuo, Frances E. and Sullivan, William C. (2001) Aggression and Violence in the Inner City, Effects of Environment via Mental Fatigue. Environment and Behaviour, Vol 33 No. 4, July 2001 p543-571.
- Kweon, B.S.; Sullivan, W.C.; & Wiley, A. (1998) Green common spaces and the social integration of inner-city older adults. Environment and Behaviour, 30, 832-858
- McPherson, E.G. 1996., Urban forest landscapes, how greenery saves greenbacks. Wagner, C., ed. 1996. Annual Meeting Proceedings, American Society of Landscape Architects. Washington, DC. ASLA. p27-29
- McPherson, Gregory E.; Scott, K.I.; Simpson, James R.; Xiao, Qingfu; and Peper, Paula J. 2000, Tree Guidelines for Coastal Southern California Communities. Western Centre for Urban Forest Research and Education, USDA Forest Service, Pacific Southwest Research Station. Local Government Commission.
- McPherson, Gregory E.; Simpson, James R.; Peper, Paula J.; Qingfu, Xiao; Pettinger, Dennis R.; Hodel, Donald R. 2001, Tree Guidelines for Inland Empire Communities. Western Centre for Urban Forest Research and Education, USDA Forest Service, Pacific Southwest Research Station. Local Government Commission.
- Moore, Gregory, 2000, TREENET: A Management System and Choices for Australia. Proceedings Of The Inaugural Street Tree Symposium, 7th-8th September 2000, Adelaide University Waite Campus.
- O’Brien, David (1993) Street Trees for Cities and Towns Imago Press Pty. Ltd. p83
- Shigo, Dr Alex L., (1991) Modern Arboriculture Shigo and Trees: Associates Denham, NH 03824 – 3105 USA. p304
- Ulrich, Roger 1985. Human responses to vegetation and landscapes. Landscape and Urban Planning 13: 29-44.
- Westpahl, Lynne (2003) Urban Greening and Social Benefits: A Study of Empowerment Outcomes. Journal of Arboriculture, Vol 29, No. 2, March 2003.
- Wolf, Kathleen, 2003. Public Response to the Urban Forest in Inner-City Business Districts. Journal of Arboriculture Vol. 29, No. 3, May 2003.
- Zidarich, Kym, 2002, pers com.