Martin Norris – Open Space Planner, Wellington Shire Council


Urban trees directly influence the lives of the 87% of Australian people that live in urban areas. Hence, urban trees are valuable community resources, and much literature has focused on this from both societal values (e.g., Kaplan 1977; Kuo 2003; Wolf 1998a) and via quantitatively measured values (e.g., Brack 2002; Maco & McPherson 2003; McPherson et al. 1999; Wee 1999). Changes that eliminate large trees can substantially reduce both values.

The management of aging urban trees has recently become a topical subject in Australia, however, Hitchmough (1994, p. 269) raised the issue in an Australian context over a decade ago, and the concerns he raised are still valid, the issues remain fundamentally unchanged and his comments are worth re-reading.

Numerous reasons could be suggested for this increased industry awareness of aging trees; greater numbers of better educated and arboriculturally focussed people employed by local government (e.g. tree planners); improved technology revealing the problems (e.g., trees inventories and GIS software), whilst these are all probably somewhat valid, it is more likely that the issue of aging trees has fundamentally been driven by the rise of a ‘risk society’ (‘risk society’ was a term created by Beck 1999.).

The demands of risk management have forced local governments to focus more closely on their urban tree population. Claims against local government have increased constantly (Chart 0) in the last decade (Trowbridge Consulting 2002). Council insurers are insisting on professional management of trees and in Victoria at least, conduct audits which are used to set premiums (Civic Mutual Plus 2003).

Has an increase in tree related claims or an increase in the number of tree removals (because of risk related concerns) also increased concerns about older trees? Technology has allowed data to be rapidly and inexpensively collected and detailed analysis undertaken. Once councils started collecting urban tree data, did numerous issues of aging trees become apparent?

Does a problem actually exist, if so what is the extent, and what should or can be done about it?


To set some context to the discussion, definitions are valuable. What are aging trees? Biologically all alive trees are aging, however for the purpose of this discussion, aging trees are interpreted to mean a shift in the overall proportion of trees in the landscape that  are considered mature or over mature; and this increase or perceived increase of trees reaching senescence is of concern.

Urban tree planting history

People across the world are becoming more urbanised, urban populations are growing 2.5 times faster than their rural counterparts.

Australia is one of the most urbanised countries in the world with some 87% of people living in urban situations (Australia’s Changing Population Distribution 2004; ABS 2003, 2004a, 2004b).


Trees have been a part of modern urban towns and cities for only the last few hundred years (Miller 1996). In Australia, urban trees have only been part of the urban fabric for a very short time, much of the urbanisation did not occur until post-1945 (very few 19th century streetscapes remain). In 1906 48% of the population lived in towns of less than 3000 residents, in 1996 53% of the population lived in cities of greater than 950,000 (five main centres). The population has grown exponentially from 3.8 million 1901 to 19.6 million in 2002 (ABS 2000). Clearly, this growth has been in urban areas. It is reasonable to assume that urban tree planting increased in line with this increasing urbanisation and population growth (albeit in a more fragmented and maybe less exponential manner).


Spencer (1988) details several distinct street tree planting phases that occurred in Australia.

  • 1850-70 fast growing evergreens (mostly conifers) and some natives trees (e.g. Eucalyptus globulus).
  • Post 1870s until World War II plantings followed in similar styles to North America and Europe with the introduction and wide scale planting of oaks, elms, planes and poplars, and some plantings of melia, silky oaks, kurrajongs and peppercorn trees.
  • Post-WWII as narrow suburban streets developed with increasing urban development many smaller species were introduced, e.g., Melaleuca spp., Prunus spp., Brush Box, and flowering gums.

In keeping with the rapid urbanisation of Australia, the majority of urban trees have been planted in the last 50 years; few are of significant age, nevertheless in numerous urbanised areas of Australia, trees are reaching a point of maturity where decline is inevitable. Whether this is a significant issue or merely a perception, is an interesting question.

The growth of aging trees

Why is the management of aging trees a problem, does a problem actually exist? What do the data show? If there is a problem – how significant is it and how is it best managed?

It seems reasonable to review the literature. In Urban Landscape Management Hitchmough’s (1994) contentions are interesting and intuitively comfortable, however they are not backed by any references or data. A few Journal of Arboriculture papers can be found (Clark 1991; McPherson et al. 1999), several conference papers (Moore 2004; Parker 2004a), and a ‘Google’ search will reveal numerous claims of short-lived trees and declining tree cover in urban areas. However, there does not appear to be any extensive research on the extent of aging urban trees in Australia, and the only major review of management options is in Matthew Parker’s PhD thesis (Parker 2004b).

Many Australian Councils have begun to address the numerous issues that arise when old and often venerated trees, particularly avenues, have reached the point that arboricultural and horticultural professionals believe that the trees require replacement. The recent Sydney debate in relation to trees in the Domain and the Melbourne City Council plans to replace some European trees with natives species are excellent high profile examples.

Obviously, aging trees exist in the community, and clearly they provide challenges, both societal and technical. Nevertheless, what is the extent of the problem?

The reality is that tree replacement has been an issue for many years; many early planted landscapes have been removed in the last 50 years, it is not a new phenomenon, what has really changed? Due purely to the passage of time there are greater numbers of large mature urban trees than at any other time in Australian history. Do we have far more trees that need replacement than ever before, maybe, however I would suggest what has really changed is the community. As communities become more affluent, dangers less obvious, and survival more assured, concerns turn to risks that would not have been considered significant even 100 years ago. Equally individuals in the community have far more voice than preceding generations, they are less likely to accept the word of ‘experts’ and are more likely to demand alternatives. The change has been to the attitudes of individuals in the community and the recognition of the heritage and social values of urban trees. The onus is on the managers (stewards?) of these urban trees to meet the challenges of a more knowledgeable and empowered community.  These changes have driven changes and improved the professional management of urban landscapes.

Managing Aging Trees

The call to manage aging trees should not be feared, it is a positive indication, as we can finally claim to have mature trees in the urban landscape. For many years we have been managing semi-mature landscapes. Are we merely going through a period where we as horticulturists have to develop? It could be suggested that the recent interest in managing and assessing mature aging trees reflects insecurity in the profession – this is the first time many have had to consider in any significant way the management issues of aging trees.

Trees co-exist in an urban environment via a complex web of relationships and interactions, vastly different to that found in other treed situations. Individual perception, (which is derived from a range of factors including personal beliefs, training, experience, and life influences) of trees in this urban web can strongly influence how trees are viewed, assessed and how the management of trees is conducted. Understanding the influence of one’s individual worldview and the numerous complex relationships and interactions that exist is critical to any person who is responsible for the management of urban trees.

Relationships with nature

Individuals manage trees according to their own worldview; this is influenced by factors such as education, experiences, cultural background and societal pressures.

Jorgenson (2004) adapting a study of Van den Born et al (2001) suggests people have one of two types of relationships with nature.



The technocrat adventurer

The manager-engineer

The steward of nature

The guardian of nature

A partner of nature

A participant with nature

Oneness with nature

Miller (1996, p. 19) cites a similar relationship with a nature model, consisting of a continuum of values, ranging from  ‘nature haters’ to those that ‘love and depend on nature’.

Where do urban tree management professionals fit within Jorgenson’s relationships? Where would other landscape professionals be positioned?  Anecdotally it could be suggested that many local government tree professionals would see themselves as ecocentric, whilst many engineers would be seen as anthropentric.

Relationships with nature models are important in helping to understanding that differing values can produce conflict between professionals and within the community.

Available approaches to managing aging urban trees

How are aging trees going to be managed in the future? Accepting that we understand the limitations of our own worldview, tree management could be considered from a various aspects.

  • Biological
  • Societal
  • Environmental
  • Engineering
  • Asset Management
  • Economic
  • Amenity
  • Ecological
  • Risk

Biological Consideration

A biological perspective considers trees as living growing complex biological organisms and applies management and assessment protocols and processes to maximise the potential of each tree.

Understanding tree biology is the key to maximising the potential for a tree to exist in any given environment. Tree biology is not fixed, as trees age various physiological and biological functions change, age related changes include reduced carbon-assimilation, decreased overall rates of growth; changes to mass:energy ratios, and reduce ability to respond to change (Clark 1991; Moore 2004; Shigo 1986).

Trees do not have a fixed life span (indeterminate growth), few urban trees die due to natural causes and decline and die due to ‘distinctive internally programmed degeneration’ – tree experience an increased venerability with age (Nooden cited by Clark 1991). The provision of a stable environment and a stable physical structure are critical to maximising biological potential (Clark 1991; Moore 2004).  In urban areas a stable environment can be difficult to achieve, due to other activities undertaken near trees (including, construction, compaction, irrigation, and pruning).

Assessments should include the potential of the site to meet the biological needs of the tree as it ages, and the potential of existing management to provide a stable environment and maintain a stable tree structure.

Older trees are the least tolerant of environmental changes. Management plans should focus on providing a stable environment. Failure to provide a stable environment and physical structure will lead to weakened defence systems, reduced health and vigour resulting in reduced lifespan, decreased amenity value, and fewer environmental and ecological benefits.

Economic Consideration

Understanding biology may be the key for maximising biological potential; however, considering the tree within an economic role is also a valid method of planning for an aging urban forest.

An economic view of assessing aging trees raises many interesting elements. Trees have been valued for amenity for decades and this amenity quantified as a financial value, many amenity valuation methods have been developed worldwide, significant variations occur between methods (for comparisons see Garner 1999; Watson 2002).

In the last decade a substantial effort has been expended to create models that financially quantify the benefits that urban trees bring to a community. Much of this research  has focussed on the direct environmental and physical benefits of urban trees, (Forests 2002; Greening Australia. 1994; Killicoat, Puzio & Stringer 2002; McPherson et al. 1999; Tyrvainen 1997; Wee 1999; Wolf 1998b) most of this work has been undertaken in the United States.  However, in Australia attempts have also been made to financially quantify the value of street trees, Killicoat et al. (2002) make a worthwhile effort at placing an economic value on urban street trees, Brack (2002) provides a model that estimates pollution mitigation and carbon sequestration by the urban forest of Canberra is worth US$20-67 million over  four years. Wee (1999) in her honour’s thesis identified that:

A single representative street tree in optimum condition has an internal rate of return of over 25%, and can produce almost $16,000 in cumulative benefits in its lifetime. The urban forest resource in Canberra as a whole has the potential to produce an estimated $45 to $95 million worth of benefits annually.

Whilst amenity and environmental benefits have become more financially quantifiable many other economic benefits can likewise be quantified. Wolf (1998a; 1999; 2003) conducted studies into consumer habits and concluded that a healthy urban forest can have a real and measurable positive effect on consumer spending.

Decision making using economic models

Financial analysis and justification is commonplace in business processes. Similar analysis and justification can provide a valuable contribution to the decision making process involved with managing and assessing older trees e.g., the use of cost benefit analysis.

The decision to remove urban trees is frequently made on the physical condition of the tree, it could be argued that an enhanced decision could be made by evaluating the decision to retain or remove by incorporating economic procedures (Scott & Betters 2000).

The difficulty in applying cost benefit modelling is in the provision of accurate estimates for the values used in a model. Scott and Better suggest using an accepted amenity valuation method to reflect benefits that urban trees provide and this would seem a reasonable approach, at least until valuations that include a wider range of benefits have been validated. Using various costs and benefit factors it is possible to provide a financial argument for retention, removal or replacement.

For instance using Scott and Better’s (Figure 1) PNV formula (also known as NPV – Net Present Value) it is possible to show that if recommended works to the tree in Table 1 resulted in a slight increase in useful life expectancy (ULE) and vitality it would be financially viable to undertake the works. However, if the works would not improve the amenity value of the tree and its future value would be less than the current valuation alternatives should be considered.  Scott also provides a model to compare the cost and benefits of retaining the tree compared to a replacement tree. Such economic modelling is common place in other industries that produce ‘crops’ e.g., forestry and agriculture.

Table 1

Ulmus procera (estimated age 125years)
Current amenity value (BO) $7,900
Cost of recommended works (years 0,1,5) (Ct) $1000
Future estimated amenity value due to works improved ULE and vitality, (value in 5years) (B1) $15,800
Net benefit (4% rate of return) (i) $4,900

Figure 1 Present Net Valuation model from Scott & Betters (2000)

Whilst economic analysis is not widely used in urban tree maintenance decision making, like any modelling it is limited by the assumptions embedded in the model and the uncertainty in the data used. Nevertheless, if tree managers accept the use of tree valuation methods (e.g., modified Burnley, Thyer), then they must accept that such values can be used to establish relative values against alternative actions and other community assets.

Economic arguments are powerful and important because as Archer and Beale (2004, p. 110) state ‘. . . we have reluctantly concluded that when we refer to ‘value’ we need to talk dollars to make sense’.

Assessment of older trees using financial modelling could provide new insights into the remove/retain/replace decision making process.

Societal Consideration

Societal values are those values that a community places on its trees, such as, cultural, historic, location, or age. Trees with high societal values tend to be those with strong connections to the community; these trees tend to be a product of time. The Sydney Domain trees are good examples of the conflict that occurs when high societal values exist.

Interestingly and perhaps not surprisingly they are also the values found in significant tree assessment criteria. Whilst many of our ageing tree population will never develop high levels of societal value, they do provide other worthwhile environmental, ecological, economic, and amenity values.

Assessment should determine the actual and potential societal values for individual trees and entire landscapes. Management plans should address the importance of societal values, and address and create the opportunity for such trees to be retained and opportunities for high societal value trees to develop.

Amenity Consideration

Considering trees from an amenity perspective tends to be the focus of horticulturists/arborists. The most obvious benefit trees provide is amenity – trees improve the physical and psychological ‘feel’ of urban areas. Amenity values frequently tend to be interwoven with societal values and trees with high amenity values often have high societal values. Many amenity tree valuation methods combine amenity and societal values.

Assessment of amenity can be quantified using amenity valuation methods. Most countries have a preferred often court sanctioned method (e.g., UK, USA, Germany). Unfortunately after more than 20 years of attempts Australia has not developed an accepted standard method, this has resulted in at least four methods being used across the country.

Size does matter and large trees provide the greatest amenity values (all tree valuation methods are very sensitive to the influence of size factors), although factors such as ULE, structure, and health can significantly modify values.

Removal of older large trees causes the greatest loss of amenity – replacement trees often take generations to provide the same amenity, the community and possibly some horticultural professionals do not tend to understand ‘tree time’.  Many of the tree replacement options available attempt to address the immediate loss of amenity, all replacement methods have various strengths and weaknesses (see Parker 2004b).

Assessment for amenity is relatively simple (although many models have been developed); however, professionals and community members may have significantly differing views.

Urban tree management plans must address both the issues surrounding the reduction of amenity from the loss of large trees and the more complex non-amenity issues. As more of the first generation planting of trees reach the end of their useful life, the loss of amenity and social values will produce conflict with the community, tree managers will need to improve their community communication processes; no longer will the ‘I am a professional, I know best’ approach be acceptable by an increasingly more educated and cynical public.

Environmental Consideration

Trees provide numerous environmental benefits, e.g., pollution remediation, storm water peak flow reductions, improved heating and cooling, etc. Many of these benefits can be quantified both with actual and financial values (e.g., tonnes of carbon sequestered, dollars of electricity saved).

To maximise the environmental benefits of trees, we need to define what benefits we wish to gain. For example deciduous trees provide the best opportunity to reduce heating and cooling costs; however, evergreen trees provide the best retention of stormwater. Large trees provide the greatest environmental benefits.

Aging trees provide both environmental challenges and opportunities; the decision to remove/replace/retain can have significant effects on the surrounding environment both in the short and longer term. In the United States a substantial effort appears to have been made to quantify environmental benefits of urban trees (e.g., City Green – American Forests 2002), although some attempts have been made to translate this process of quantification to Australia, it does not appear a suitable methodology has yet been developed.

Tree assessment incorporating environmental values is probably rarely considered or quantified. Few management plans consider the environmental effects that a changed urban tree age profile could produce.

Ecological Consideration

Ecological considerations take into account a tree’s relationships with its ecosystem, particularly the symbiotic or commensalism relationships with other organisms; besides the obvious macro-fauna (e.g. birds, possums) an individual tree can support hundreds of invertebrate species, numerous fungi, bacteria, lichen and mosses, and countless soil borne micro-organisms. The retention of older trees, complete with ‘defects’ e.g., deadwood, loose bark, and hollows, is vital to the development and the continued existence of many of these relationships and current tree management approaches do not adequately account for these complex relationships.

During the last decade, the social and ecological conservation values of veteran and ancient trees have been widely identified in the UK. A new term ‘ecological arboriculture’ has been coined, and it is defined as the management of trees for the benefit of not only the trees but all other associated organisms (Fay 2004). New and innovated pruning techniques have been developed that consider tree biology, the needs of other organisms and also reduce failure potential.

The greatest ecological potential occurs with indigenous trees, old remnant trees are the most obvious trees to consider from an ecological perspective. However, to sustain the organisms that depend on these older veteran trees for their existence, replacement trees must be allowed to develop (that is, a diverse age of trees is required).

It is doubtful that current assessments or management plans for urban trees consider the ecological impact of tree retain/remove/replace decisions. Management plans for veteran trees in the UK define treatments for the next 30-100 years.

Whilst, urban tree ecology has yet to become a significant concern in Australia, two issues should be considered, one, what damage is ignorance of the subject doing to the numerous species that depend on old trees for their existence, and two, what is going to be done once the issue has been raised.

Engineering Considerations

Parks departments have traditionally been organised similar to council engineering departments; this is not surprising given that the traditional parks and garden department was often part of the engineering department. Similarly, the grey and green infrastructure has developed together, and urban trees are currently seen to exist within an urban engineering framework (arguably, both are merely parts of an urban ecosystem).

Whilst horticulturists like to consider they hold the higher moral ground in this relationship, they cannot ignore the fact that trees have to co-exist in urban environments along with all of the other wants and needs of humanity e.g., roads, drains, electricity, etc.

Engineering has traditionally been the dominant ‘works’ function of local government. Most local government assets are run by engineers for good reason; they are organised, professional, outcome focussed, and have well accepted processes and systems in place. However, the construction/development focussed approach has lead to the current predicament where councils do not have adequate funds to renew declining assets; some of the issues of aging trees are probably related to this development focused engineering approach to urban tree planting. Many landscape architect or designer views would strongly align with the construction/development functions.

An engineering view of trees in an urban environment is probably one of function and outcome. Urban tree management could model itself on a professional engineering approach by improving the professional approach of tree managers and developing stronger, more widely accepted and used standards based on good science.

Tree assessment based on engineering principles, would consider how well existing plantings meet their design purpose, the remediation and replacement options; and quantification of opinion would be a strong feature of any assessment.

Management plans must address the biological standards the maintenance of mature trees requires. The engineer/horticulture relationship has traditionally been engineering focused leading to the poor understanding of the biological needs of trees and has arguably lead to most of the ‘grey and green’ problems.

Horticultural/arboricultural managers must be as professional as engineers in insisting on the infrastructure requirements of urban trees. Brisbane City Council’s ‘grey and green’ program is a good example of an engineering/arboricultural partnership (Foster 2003).

Risk management consideration

Risk management is currently the greatest driver of public tree management. From the perspective of tree retention/removal/replacement, risk assessment is the easy option and has been the major method of determining when a tree should be removed.

The risk management approach is based on the question ‘does the tree present an acceptable level of risk’, if not reduce the risk by remedial works, removing the tree, or reducing the risk target impact potential.

Most current tree risk assessment methods are qualitative and produce ordinal risk ratings. The use of qualitative risk assessment methods to determine removal/retain/replacement decisions is probably overrated due to the inherent limitations of most current risk methods. Most methods do not adequately address overall risk; the vast majority focus on what the arboricultural industry does best – identify defects in trees; the evaluation of risk target value, likelihood of damage, likelihood of impact, defining acceptable risk and dealing with bias and uncertainty are poorly considered.

Due to age, typically greater size and a greater perception of risk, assessor and assessment method bias could be a significant factor when assessing older trees. Pressure from some in the community and political factors could also be influences that unduly bias assessment and management of older trees.

Risk assessment is not an end unto itself; it should only be one part of the overall decision making process.

Management plans should address methodology and process, the acceptability of risk, and ensure that other important factors are considered as part of the overall decision-making process.

Asset Management Consideration

Public trees are ideally suited to being considered ‘assets’. Asset management is about knowing what you have, what condition it is in, its values (financial, heritage, cultural, environmental, etc.), overall performance, the costs of running that asset, the risk, asset life cycle (whole of life costs), etc. All of these factors should be able to be reported on.

The lack of full life cycle costing has become apparent since asset management processes have started to become integrated into local government operations. For instance the Local Government Association of South Australia stated that SA local government spends one third of the amount on asset maintenance it should to maintain existing assets and that until its 2004 study ‘. . . few councils could say what would need renewing when, or how much it would cost’ (LGASA 2004). This criticism is equally applicable to the management of urban trees; Parker (2004b) found that few Australian councils recorded any detailed tree records and even fewer recorded adequate details sufficient to identify individual tree costs, Norris (2005) identified that only 25% of council survey respondents had a complete or current tree inventory.

Asset management has the potential to supplant risk management as the greatest driver of local government. Asset renewal is the greatest problem facing local government.

Tree management should utilise the potential of an asset management approach. Trees compete alongside other council assets, the most obvious way of having them taking seriously is to ensure they are included in the asset register. Asset management systems can allow for the biological, economic, societal, amenity, environmental, ecological, and engineering functions required to adequately manage urban trees.

Asset management demands new skills that few in the horticultural industries have been taught and it is unlikely that many suitable asset management models have been created for urban trees. Nevertheless, asset management systems are ideal for management planning, works programming, recording of works and costs, and record keeping of the typically large numbers of trees that are the responsibility of local government tree managers. Tree assessments can be customised, standardised and analysed in most asset management software packages.


Whilst technology is not an approach to tree management or assessment per se, it is a valuable tool with which to gather data.  The tools to collect, mine, map, analysis, plan, etc., are readily and inexpensively available. Handheld data loggers can quickly and cheaply collect tree inventory data. Databases and particularly asset management software can hold vast amounts of data and make it readily available. Statistical tools (even Excel) can easily summarise and graphically display large datasets. GIS programs can graphically and spatially display data.

With currently available technology it is not possible to sustain the argument that it is not feasible to assess, plan or manage large tree populations.

Summary of considerations

We have considered a range of views and approaches that the urban forest can be compartmentalised into, all deserve consideration when assessing or evaluating the management of aging urban tree populations; most approaches are complex, all have validity and often the boundaries between overlap. Urban trees are part of a complex system where human, built habitats and non-human nature interact – an ‘urban ecosystem’ (Duryea, Binelli & Korhnak 2000; Dwyer et al. 2000; May 2004).  Ecosystems are complex and this complexity is one reason why managing aging trees is a difficult process.

We could continue to consider urban trees in isolation and each associated interaction as an isolated and hence quantifiable function, we could even simplify the relationships and represent various values as a mathematic formula e.g., Bi+Ec+So+En+Ec+En+As = decision. However, whilst such an approach is somewhat superficially appealing and is superior to merely considering single functions (e.g. risk), it still largely fails to place the tree in an ecosystem context.

If an attempt is made to understand the complexity by conceptualising urban areas as ecosystems, it will change the very way the industry and communities view and interact with the urban forest. Costs and values associated with a sustainable ecosystem will be quite different from that of the current individual tree view, or even the broader urban forest approach, particularly management and understanding of the development and restoration of the urban forest (Duryea, Binelli & Korhnak 2000).

Realistically communities are still some distance from considering urban areas as ecosystems, and it is easy for urban tree managers to take and ‘sell’ to communities a logical ‘engineering’ approach to older tree assessment and management.

Significant progress could be made by collecting and recording tree related data and developing management and assessment models based on analysis of such data. Existing technology allows the easy collection of data, and asset management systems provide the opportunity to store and analysis the data.

Local government will be adopting asset management as its major operational driver during the next decade; asset management is a tool that can be used to account for most if not all demands for managing urban trees. Professional trained people who understand the complexity inherent in managing trees in an urban environment and can combine this knowledge with assessment management models will be required if trees are to benefit from this increased asset management focus of local government.

Tree Assessments

Typically most assessments of public trees are undertaken for tree inventory data collection, works programming or risk assessment purposes.

It is not hard to assess an aging tree; it is no different than assessing a semi-mature tree. Assessment is merely part of an overall decision making process that is ‘tree management’. The tree management plan objectives should determine what data are collected and for what purpose.

Do people have more concerns when assessing older trees? It is important to understand what biases we are bringing with us when we are assessing trees; do we fear the increased uncertainty that may exist when assessing older trees; is it the greater perceived ‘damage’ that an aging (presumably larger) tree could cause; is it an issue addressing previous poor management practices or failure to plan or provide for the biological needs of the trees; or are we merely paralysed by the conflict created in our mind when we pronounce the ‘death’ sentence to and old and possibly venerated tree?

Individual assessments

The assessment of an ageing individual tree will generally consider one main question – how long can this tree remain here; however, the assessment should address biological, societal, amenity, environmental, ecological, engineering, risk issues, and consider these issues against the efficacy of possible modifications or remediation. Unfortunately this level of sophistication does not appear to exist in any tree assessment method and rarely would many people undertaking tree assessment have the time to consider all of these elements.

Predominantly urban tree removal decisions will be primarily controlled by the level of risk associated with each tree and the level of risk acceptable to the organisation. However, the removal option should be moderated by consideration of other factors and the methodology detailed in the management plan.

Landscape assessments

A more difficult assessment is the decision making process required when considering the future of amenity trees in a broader landscape context, for example the retain/remove/replace decisions for significant boulevard or avenue trees. In this situation individual tree assessment will typically show that trees in the planting vary widely in health, structure, useful life expectancy (ULE) and risk level. In some instances removal will not be due to declining condition, but for infrastructure development (e.g. new road) or a change in the landscape (e.g. urban renewal programs).

At what point are complete plantings, including healthy trees removed so that replacements can occur? Partial replacement (either individual trees, groups of trees or by inter-planting) is possible, however problems of amenity exist (the ‘complete’ avenue effect will not be present) and technical problems for example, competition for light, soil moisture and nutrients must be considered (Parker 2004b). Wholesale removal and replacement of entire streets is an issue for many in the community, particularly given that many trees will still be healthy and have a significant useful life expectancy.

It is possible to take an engineering view and replace trees when the majority of the population reaches a pre-determined point (the majority of the avenue reaches a ULE of <10years, or a risk level of x). Due to the subjective nature of this method it is doubtful that this approach will adequately address many of the community concerns and it ignores many of the non-amenity values of urban trees.

As the recent Sydney Domain tree removal decisions demonstrate, these issues can become political, and opponents of the decision will find other ‘experts’ to dispute the original findings. Whilst the assessment needs to be professional, rigorous, technically valid, and capable of validation, the decisions to remove/replace/retain is a management decision and the management focus needs to be on community communication.

Removal of entire streetscapes or large trees is never going to be acceptable to some in the community, and the horticultural/arboricultural industry should seriously question whether this architectural/engineering approach is sustainable and ecologically sound. The best solution is to have a well communicated tree management plan in place; this plan must document the assessment and decision making processes. Tree managers should be able to provide justification for the assessment methods chosen.

Wellington Shire’s Approach to ageing trees

For much of the 1990s we were removing more trees than we were planting, we knew little else about our urban tree population. There was a perception of many older over mature trees, and many ill-considered plantings from the 1970 and 80s.

In the late 1990s we started to develop a more professional approach to urban tree management with the adoption by Council of a tree management strategy with clearly defined goals and objectives. Fortunately risk management started to become an issue at the same time, this became a positive driver by placing trees on the political and senior management agenda. In Wellington this higher profile allowed us to change the way we managed our trees by developing a tree inventory, integrating a GIS and asset system and introducing a proactive maintenance program.

We took a very techno-scientific approach. Inventory data was collected and this formed the basis for initial works programs and tree replacement/planting programs.

Using inventory data it is simple to plot ULE, tree age, condition or any other variable onto a GIS layer and summarise the data statistically. This provides an overall picture and some accurate data as to the condition of the tree population. This simple approach allowed us to calculate the percentage of trees that would probably need replacing in the next, five, ten or twenty years. It is also possible to investigate the performance of individual species.

Surprisingly, our perceptions of an aging tree population were not strongly supported by the data. The data from our inventory suggested a reasonably normal distribution curve slightly skewed to mature. If this data had existed 20 years ago the curve may have been much more skewed to the younger tree end of the scale, nevertheless all we are seeing is the development of a normal distribution curve reflecting a street tree population that would be expected in a mature community.

We had some concerns in relation to the association between tree ages and useful life expectancies (ULE), plotting both sets of data demonstrated that the curves were not significantly different (Chart 2).

We have (had) about 10% of our street tree population with a less than five year life expectancy, this equates to about 2700 trees. If we use our nominated average life expectancy requirement of 50 years, this reflects a 2% per annum replacement need or 540 tree replacements pa, or approximately 10% over a five year period. Fifty-eight percent of the tree population will require replacement over the next 20 years, equivalent to a replacement program of 3% per annum. With these 58% replacement trees and approximately 9,000 planting spaces, at our current planting rate of 3.5% pa it will take approximately 35 years to completely plant the existing urban areas.Planting trees with a range of life expectancies greater than 50 years will improve the age profile over time, resulting in decreased required planting rates and hence costs.

Of course, this is simplistic mathematics and when managing biological organisms (that is tree and the community) and the political variability over 20 years it is not realistic to neatly compartmentalise into one-year timeframes. However this type of analysis is valuable because it allows the magnitude of the problem to be determined and goals set.

The Wellington tree inventory data would suggest that an aging tree population exists, however the age profile fits within a reasonably normal distribution, regardless it requires proactive management; and the data would suggest that the current tree planting program is barely adequate to meet replacement needs and slowly increase the overall population. Using combined data from the tree inventory it is possible to prioritise the planting program. The data do not provide solutions to addressing community concerns – data is merely an aid to the decision making.

Urban Tree Management Options

What is the role of arborists in local government – Matheny & Clarke (1991) state “the primary role of the arborist is to  develop management programs for urban trees” (emphasis added).

No doubt the increased number of professional arboricultural people employed in local government has improved the potential for planning to occur. Two real options exist, the do nothing model and the get proactive model. Generally resourcing and financial considerations are given as reasons for the do nothing option to exist. It is doubtful these reasons can be sustained as valid, and options will be discussed further.

Option 1 Do nothing

The easy option and one that is common in local government is to not plan or manage, merely have a reactionary approach. Planning is often considered a ‘nice to do’ in local government. Generally planning is only taken within a short timeframe, often related to political cycles.

Norris (2005) found that nearly 40% of Australian local government officers responding to an industry survey reported that they either inspected trees on a reactive (request driven) basis or did not inspect urban trees at all (Chart 4), and only 25% of councils reported a complete tree inventory (Chart 5).

Doing nothing is an option; however, it should not be made in ignorance.

Option 2 Get Proactive

The proactive approach is not about having substantial amounts of money, or masses of resources, it is about attitude and being a professional, in local government it is also about driving an agenda.

Tools in the form of technology and skills are readily available to allow proactive management.

In the last decade local government urban tree management has increased its profile significantly.  Risk has been a major ally in increasing this profile. Risk management of urban trees requires data; it is not possible to manage risk without understanding the number and the condition of trees, and the resources available. Proactive tree management requires the same data and incorporates risk management.

Whether an aging tree population exists or to what extent it is a problem cannot be determined by subjective opinion. Similar to risk management, management of aging trees is not a stand alone activity it is part of a proactive approach to urban tree management.

Urban tree management is complex, technically demanding, undervalued, and under-rated. As a reactive system it can be depressing, overwhelming and unrewarding. A proactive approach is empowering, rewarding and allows you to manage from a position of strength.


Trees cannot be managed in any sustainable way without knowledge. Many councils do not have any data on their tree population, therefore any knowledge will be assumptive. The often quoted limitation to developing a proactive approach to tree management is resources. These fall into four categories

  • Financial
  • Human
  • Skills
  • Technological

Financial Consideration

Trees are a major part of any local government infrastructure. Trees have a cyclic life just like other infrastructure, only far more complex. Asset management has become a major part of all councils, in Victoria (at least) councils are required to account for and manage their assets. Trees are merely another asset to manage and all assets require funds to provide sustainable maintenance.

The majority of public authorities actually have no data to confirm the belief that they have a problem of aging trees. Cost is often cited as a reason not to collect data; however, I do not believe that this is in reality a significant issue.

The cost of collecting data

The cost of collecting tree inspection or tree inventory data should not be financially constraining for councils.

Using Beer’s data (2001) an average Melbourne council has 46,000 street trees, Vial and Prior (2004) provide data that shows the mean per-tree budget for Melbourne Councils was $22.33/tree. Assuming a ‘consulting’ arborist costs $90/hr and inspects 400 trees/day; this translates to $1.80 per tree or 8% of the average available budget. Data can be collected over two financial years (collect autumn and spring) giving a 4% per year cost to the annual budgets.

This would make it difficult for a council to argue that at least biennial inspections were not achievable.

The cost of integrating data

It is easy to collect data; however it is only of any use if it is used. Whilst customised systems can be purchased or developed it is also possible to integrate this data into existing council systems at virtually no cost. For example, we did it at no direct cost, the data was imported into council’s existing asset management system (Conquest), and this in turn was linked to council’s corporate GIS system (Latitude), again no cost.

Very low cost options

For those with some statistical background a sample survey of a randomly selected number of trees could legitimately create a statistically valid picture of your tree population and from this survey data it could be established whether an aging tree population or any other issues exists. This would be an inexpensive exercise and could easily be done inhouse (may be by members of the tree crew on wet days). The data could easily be analysed with Excel.

Human Resource Consideration

Data collection can be collected using inhouse labour. However, using our average Melbourne council again, this would require some 130 days collecting data, or some 30% of one person’s available time spread over two years. Depending on structure it could be difficult for councils to ‘find’ a person to undertake this task. However, if data collection allowed a manager to determine the most urgent works, develop age and condition profiles, etc, and hence change from reactive to proactive works. Justification for such staff time would exist.

From a works on the ground perspective, we conducted a trial on the cost of reactive prunes versus a programmed approach; we found that the direct cost of a single reactive request to lift a street tree was 10 times greater than that of the same work conducted during a programmed prune.

We are currently on the first cycle of a proactive cyclic pruning program; this has been achieved without a substantial increase in the available budget. More tree work is being achieved with the same overall budget, whilst requests and after-hours callouts have been reduced. An alternative approach to cyclic pruning is to use the data to identify a range of tree works based on tree condition, health or age; this specific approach is possibly more efficient use of tree crews, however it does require more management/supervision/inspections to develop the works programs.


It appears that at least at the more senior end of the tree industry people are generally reasonably educated. A recent survey of council tree officers and consultants found that 59% of respondents have completed a Level V Diploma (VET/Tafe) or a higher qualification and many people had several qualifications (Norris 2005).

The skills to manage and assess trees and use current technologies are not complicated. Analysis of tree inventory data requires some basic statistical knowledge, and the ability to manipulate data is critical. Specialist software is not required and most of the skills can readily be learnt.


Poor or very limited record keeping was identified as a major limitation to tree management in Australian Councils (Parker 2004b). Existing technology allows data to be easily collected and records kept.

Technology can no longer be ignored. Most councils have computerised asset systems and GIS mapping software. Tree data can be integrated into these systems. Work undertaken to trees and tree planting details can be recorded electronically in the field; this data can be incorporated directly into an asset management system. Data can be sent and received via mobile phones.

The key to the management of urban trees is record keeping. Due to the complexity of tree management and the large numbers of trees involved it was previously difficult to keep such records, however, with currently available technology no council could claim that it was not ‘reasonable’ to collect and keep records.

Concluding Comments

Whilst mature and aging trees in the landscape are not new phenomena, the professional management of urban trees is a recent development. The management of urban trees is now commonly the preserve of arboriculturally trained and aware people. Arboriculture has until recently tended to focus on the individual tree; few arborists have been trained to manage or comprehend public urban trees as part of a dynamic diverse-aged forest, let alone part of an urban ecosystem.

No doubt, there are larger numbers of urban trees than at any previous time, most of these trees are first generation and many are mature or over mature, and few tree managers or communities have had experience in living in urban areas with significant numbers of mature trees or with wide-scale urban tree replacement programs.

It is easy to consider the management and assessment of urban trees from a techno-scientific/engineering perspective and it is easy to manage and assess trees as individuals. However, once the complexities involved with managing urban trees become obvious because of the inescapable interaction of humans, nature and built habitats, it equally becomes obvious that a conceptually a holistic approach that includes multidisciplinary management of an urban ecosystem is a logical approach to developing sustainable urban communities.

Costs and values associated with a sustainable ecosystem will be much different from the current individual tree view, or even the broader urban forest approach, particularly management (and understanding) of risk and development/restoration requirements (Duryea, Binelli & Korhnak 2000).

At the end of a somewhat long process I think I can condense the entire paper down to two points.

  1. Managing urban trees is a complex process that has for decades generally been ill-considered and undervalued. Many of the complex issues surrounding the management of ageing urban trees have resulted purely from a lack of planning, (the causes are complex and include lack of knowledge, skills and organisational influence). It is pointless to look at history – the development and rapid expansion of urban cultures and of urban tree management are very much recent phenomena. Management planning based on good data and a scientific approach is vital for future management of current and future urban tree populations. Current technology provides tools to collect, collate, analysis, and manage the massive amount of data that managing an urban forest requires. Until the community and industry are in a position to manage trees as part of an ecosystem management process, a whole life cycle approach is possible by using the tool of asset management.
  2. Whilst planning is important in managing the complex matrices involving biological entities; the future of the urban forest, whether the community has grand avenues of historical mature trees, has parks with venerated veteran trees, whether we manage our urban areas as ecosystems or merely have streets lined with the latest designer tree clone has nothing to do with biological, economic, engineering, amenity, etc issues; the single most important factor that will influence the urban forest that we have in the future rests solely on philosophy.

Philosophy reflects our attitudes, beliefs, values, and thinking. Once a philosophy is determined you can pick a management strategy that will deliver.

Managing aging trees is not difficult. It merely requires vision, that vision is a reflection of a philosophy. What is your philosophy going to be?


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