Aussie icons: Eucalypts are woody wonders in almost any climate
The Quintessential Australian Tree
Many of us regard eucalypts as the quintessential Australian tree. They are adaptable, resilient and persistent. They have proved themselves over millions of years of evolution to be a dominant and successful tree species, but they also have their place. Eucalypts tend to dominate the coastal fringe of Australia, where most of us live. As the climate becomes more arid and the soils more ancient and nutrient poor, eucalypts are replaced by Acacia species until they too can no longer cope with the harsh conditions, and woody trees and shrubs no longer dominate.
With about 800 species, most coming from the genera Eucalyptus and Corymbia, eucalypts are a remarkably diverse and versatile group. There is a eucalypt for almost any urban or gardening situation – tall, short, single or multi-trunked, varied leaf shapes and shades and spectacular or almost unnoticeable flowers. Eucalypts are essential components of Australian ecosystems and support many other plant and animal species that live around them. They are species that will be vital to the sustainability of both natural and urban ecosystems as climate change takes effect across Australia. They are often the remnant links and corridors that connect a disrupted, disjointed and diminished complex of landscapes and ecosystems across the continent.
Understanding Eucalypt Shedding
Given their urban familiarity, it is amazing how so few people understand eucalypt biology and surprising the antagonism that many people show towards eucalypts. One of the most common complaints about urban eucalypts is that they shed leaves, flowers, buds, fruits and small (and occasionally larger) branches. They do, but no-one asks why. The trees often shed more leaves when soils are dry. It is a common and efficient adaptation to aridity, but it also adds to the organic and nutrient content of often poor soils. The value of eucalypt litter should not be under-estimated as it can be crucial to the survival of the soil micro-flora and fauna and great for general soil health. Eucalypts are wonderful nutrient recyclers and this is essential for the survival of other species in their ecosystems.
The tough leathery leaves of eucalypts are described as sclerophyllous (plants or vegetation with hard, leathery leaves) and are a well-known adaptation to dry conditions and nutrient poor soils. So many Australian species exhibit sclerophylly that the Australian flora as a whole is often described as sclerophyllous. The tough leaves are also well adapted to fire. If you can keep soil moisture levels higher, many eucalypts will retain leaves for longer and you may avoid major leaf dumps over short periods of time. The sclerophylly of eucalypt leaves partly explains why they break down so slowly and why they can be slippery on paths and roads. If you grow your eucalypts in larger mulched beds, the shedding becomes part of a natural cycle of events that requires a minimum of human intervention.
Lignotubers and Epicormic Buds
Among the many adaptations that make eucalypts so resilient and persistent are their lignotubers and epicormic buds. Epicormic buds are dormant buds that grow each year just below the bark so you cannot see them and most people don’t even know that they are there. They only develop into shoots after damage to the canopy and are a rapid response system to replace photosynthetic foliage after major stresses such as fire, heavy insect or other grazing, storm damage or major branch shedding. Lignotubers at the base of the trunk are a store of dormant buds that again develop into shoots after an even greater level of stress. They have evolved to replace the trunk and branching canopy of a tree and many people would know of them from the fame in eras past of mallee roots as a source of fire wood. Nearly all eucalypts possess epicormic buds, but not all species or even all populations of a species possess lignotubers. Their presence in a species often depends on the levels of environmental stress a population faces or the different response strategies after fire.
Dormant buds are marvellous assets for those species that possess them. They allow rapid recovery from significant stress through the re-establishment of a functional tree canopy. One of the most spectacular examples is the very rapid production of epicormic shoots after a bushfire. About 6 weeks after a fire these shoots appear in great numbers, giving trees a toothbrush like appearance and providing an early indication of ecosystem recovery. This recovery benefits many other species after significant stress events by quickly providing food resources and habitat. Similar things happen when a tree or branch is heavily grazed or when there has been poor pruning or lopping of eucalypts.
Often Overlooked Benefits of Eucalypts
The possession of epicormic and lignotuberous buds is an adaptation that allows individual tree longevity, rapid stress recovery and resilience and persistence. Dormant buds are also wonderfully useful assets for arborists and horticulturists. With so many buds available it is possible to formatively and structurally prune eucalypts to minimise risk and prolong their useful lives, especially in gardens and urban landscapes. When mistakes are made or trees are damaged, eucalypts can be remarkably forgiving because they have the options of rapidly producing new shoots.
The bark of eucalypts is as diverse as the trees themselves – smooth-barked, gum-barked ironbark, stringybark and candlebark. The characteristics of the bark are often adapted to the environment within which a species grows and are often used to identify species. Some barks protect against grass and bushfires while others can protect against flooding, heat and cold. The bark provides habitat and refuge for a host of fungi, bacteria, insects and spiders, all of which are part of the rich web of life that eucalypts support.
The importance of eucalypt roots systems can easily be over-looked, but they are wonderfully adapted to Australian soils. Depending on the soil type, they can go deep or spread far and wide. Some species produce exudates that free up otherwise unavailable nutrients making them available not only to the trees themselves but other species that are dependent upon them. Without eucalypts many other species would be unable to survive.
The presence of lignotubers at the base of the trunk of so many eucalypts means that the buds within and the roots that supply them with water and nutrients are protected by a superb insulating layer of soil, which explains their near miraculous and speedy recovery from bushfires. Root systems consolidate soils on steep slopes preventing land slips and along waterways they can slow flood waters and reduce erosion of river banks. The roots and the other species of the rhizosphere can also sequester significant amounts of carbon.
Essential Trees for the Future
Not all eucalypts are tall, single-trunked and impressive; some are smaller multi-trunked mallee species. However, all sequester carbon above and below ground and they provide canopy shade and cooling through transpiration if soils are moist enough. Both can be important in reducing ambient temperatures and reducing the urban heat island effect (UBIE). Sometimes people argue that eucalypts only produce a mottled shade, but they forget that eucalypts can often continue transpiring in hot dry environments and so continue cooling when other species have closed stomata and ceased transpiring. Providing irrigation to eucalypts early in the morning of a very hot can not only help retain leaves but can maximise shade and cooling, both of which can be important for human health during heat waves.
While many people complain about the untidiness and shedding of eucalypts in their gardens, street and parks, and compare them unfavourably with some of the great exotic tree species from other parts of the world, I am amazed how well they do without the centuries, and sometimes millennia, of breeding and selection that has gone into the success of these exotic species. Australians have a history of exploiting our natural resources, including tree species, rather than investing in and developing them. Imagine how many great eucalypts we might have for urban use if we properly bred and selected them for specific sites and purposes.
Love them or loathe them, eucalypts will be essential parts of natural and urban ecosystems in many parts of Australia in future. They are capable of coping with climate change and will be a key to creating liveable and sustainable towns and cities across much of the continent. They are great assets in both natural and created ecosystems. In an urban context we should develop an understanding their superb biology and take the necessary steps to manage them properly so that they realise the full potential of their life spans and the many benefits that they provide.
Written by G M Moore. School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Burnley Campus, 500 Yarra Boulevard, Richmond, 3121, Australia.
Formative Pruning: How to Set Trees Up for the Long Term
Gardeners and landscapers with a passion for long term sustainability will be interested in formative pruning, which is all about the sustainability of urban trees for those who play the long game. In Australia, the term ‘formative pruning’ is used to describe the practice of correcting defects in young trees, but in other parts of the world formative pruning is part of the more general practice of structural pruning that corrects defects in trees of all ages.
Formative pruning can be defined as the selective removal of stems and branches early in a tree’s life to create a safer, stronger and more aesthetic structure. Many of the problems and defects of mature trees can be traced back to faults early in life (Moore 2012). Formative pruning aims to correct any and all defects in a young tree, up to about 6m tall. It is done from the ground using secateurs or pole saws and so is safe and cost effective. However, it requires genuine arboricultural expertise and knowledge of the young tree’s growth habit.
Formative Pruning has Many Benefits
The advantages of formative pruning are many, including:
- improving the structure and health of young trees
- improving their appearance
- reducing urban tree maintenance costs
- lowering the risks associated with large trees as they mature
- improving worker safety, and of course
- improving the sustainability of urban trees.
It is an impressive list of advantages, expanded on in the table below, so it is surprising that so few gardeners, even professionals, practice formative pruning as a matter of routine.
Table 1. Some of the many advantages of Formative Pruning (Modified from Moore 2012).
| Benefits of Formative Pruning | Advantages for the Gardener |
| Simple Equipment | Low cost, low energy use and low embedded tools |
| Greater Sustainability | Initially sustainable and increased sustainability as trees age |
| Small Wounds | Rapid recovery and growth, reduced risk of disease and decay |
| Earlier Intervention | Healthier and longer lived trees |
| Light Weights | Easier to prune and much safer for gardener |
| Quick and Easy | Quick operation with less effort for small branches and trunks |
| Saves Money | Cheap and low cost gardening |
| Better looking trees | Trees that have higher aesthetic and economic value |
| Safer trees | Trees with fewer structural defects are safer |
| Longer lived trees | Formatively pruned trees with fewer defects live longer |
What Can Formative Pruning Achieve?
There are many aspects of young tree structure that can be corrected by formative pruning (see table 2 below), but some are more common and more significant than others. Crossing and rubbing branches are obvious matters for correction, but perhaps the most significant structural matter as the tree matures and for its sustainability is the elimination of codominant stems, especially if bark is already being included.
These codominant stems can lead to trouble later on if bark is included and the two or more stems begin to split. This is not only problematic for the tree but presents an increased risk of failure in a mature tree. Codominant stems with included bark do not always fail, but if the opportunity for formative pruning exists early in the tree’s life to reduce the risk, it would seem prudent to do so.
Table 2. Common faults corrected by Formative Pruning (Modified from Moore 2012).
| Fault to be Corrected | Objectives of Formative Pruning |
| Co-dominant Stems/Trunks | Removes multiple leaders that may be prone to splitting |
| Included bark & Co-dominants | Removes twin or multiple leaders that have bark trapped or retained between the leaders which may increase risk of splitting |
| Rubbing Branches | Removes rubbing branches which can be weakened and provide entry for disease and decay |
| Crossing Branches | Removes branches that have the potential to become rubbing branches as the tree ages |
| Dead Wood | Removing dead wood removes potential risks, improves the appearance of the tree and promotes a healthier growth |
| Broken Stems/Trunks | Removes damaged plant tissue and promotes a healthier plant |
| Broken Branches | Removes damaged plant tissue and promotes a healthier plant |
| Epicormic Shoots | Removes epicormic shoots that may be weakly attached, unsightly or growing in the wrong place and could become co-dominant stems of rubbing or crossing branches |
| Low Branches | Removes low growing branches that cause obstruction or pose a hazard to pedestrians or property |
Who Should do the Work?
There is a tendency by some to think that formative pruning can be undertaken by lesser trained and inexperienced horticulturists or arborists, but this is folly. Working with young trees requires knowledge of young trees, but also of the mature specimen and the possible future risks.
Some local councils have formed formative pruning crews for their street tree plantings in particular, and the results are clearly evident within a few years; trees with a better structure and a better, longer, more sustainable future. The efficiency and speed of the work undertaken explains some, but not all, of the economic benefits of formative pruning.
Long-Term Savings on Tools and Equipment
Clearly, formative pruning requires equipment that comes at a lower economic and carbon cost than complex and reactive remedial work that might be required on a mature tree using trucks, chainsaws and elevated work platforms. Movement between trees, especially in larger street tree plantings, is usually by foot and so again there is a low carbon footprint. Working with young trees is much quicker for the same corrective procedure than work undertaken 10 or 20 years later on the tree when it has grown and matured.
For most formative pruning work, there will be no traffic management plan required, even for street trees, as work can be undertaken from the footpath or nature strip. The economic and sustainability benefits are substantial and continue to accrue over time. Formatively pruned trees will require less maintenance and fewer emergency arboricultural interventions during future storm events. These reactive unplanned arboricultural works are notoriously costly when compared with planned routine works of structural pruning.
Which Tree Species Benefit Most from Formative Pruning?
Not all species have the same requirements for formative pruning. In research undertaken in Melbourne, plane trees (Platanus x acerfolia) required most work, followed by Chinese elm (Ulmus parvifolia), callery pear (Pyrus calleryana), pin oak (Quercus palustris) with lemon scented gum (Corymbia citriodora) requiring least work. The cost of the work undertaken on the plane and pear in this study was greater than 3-4 times that for the lemon scented gum. Even the most expensive work undertaken was less than 10% of the same work required on a 20 year old tree. The economic case for formative pruning speaks for itself (Ryder and Moore, 2013).
Formative Pruning to Extend the Lifespan and Carbon Benefit of Street Trees
It is known that the closer you get to a central business district or major shopping strip or hub, the shorter the life span of the street trees that grow there. This is a world-wide phenomenon; not just something that occurs in Australia. There are two mortality peaks for urban street trees. One is in the first 2-4 years after planting, and a second peak occurs when trees are about 30 years old. This second peak again is a world-wide occurrence, but has significant impact on urban forest sustainability. While trees are great at sequestering carbon, street trees can take 25-30 years before they become carbon neutral and longer before they are positively sequestering. Think of the carbon and embedded energy in transporting and planting advanced street tree stock.
So if planted trees do not survive the first 2-4 years, and there is a second rise in tree deaths and removals around 30 years, many urban trees never reach carbon neutrality. This situation is unsustainable environmentally and ludicrous economically, but seems to go unnoticed. We accrue all of the costs of growing and planting these trees but recoup none of their benefits. However, both formative and structural pruning can reduce the carbon footprint, prolong the useful life and improve the sustainability of an urban tree. Formative pruning is one of those practices that make so much obvious sense and yet is so rarely implemented.
Dr Greg Moore is a Senior Research Associate , School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Burnley, and a member of SGA's Board.
Further Reading
- Ryder C and Moore G M (2013) The Arboricultural and Economic benefits of Formative Pruning Street Trees. Journal of Arboriculture and Urban Forestry 39 (1), 17- 23.
- Moore G M (2012) Formative Pruning: Good for the Gardener, Better for the Tree. Gardening Australia, July 36-7.
An Update on the Value of Urban Trees
It is almost 10 years since I last wrote on the economic value of urban trees for Sustainable Gardening Australia, and much has changed. As climate change takes greater effect on the continent, the economic value of trees has in some contexts soared, bolstered by a massive increase in tree related research. But urban trees continue to be under-valued. State governments in particular, and some more than others, place almost no value on urban trees in the face of development and a housing crisis that sees urban tree clearing on both public and private land at an unprecedented scale. This threatens the environmental and economic sustainability of the places where people will live under a warmer, dryer climate.
As research continues into the importance of urban trees for urban ecology, environmental sustainability and people’s health and well-being, the number and the diversity of published papers can be overwhelming. It is no longer possible for a single researcher to keep up to date with all aspects of the research undertaken, so it can be convenient to think of their contributions from two perspectives. One set of contributions comes from the environmental services trees provide and their economic value, and other contributions come from the perspective of human health and well-being, and the impacts that trees and treed greenspace have on State and Federal health expenditures.
Urban Trees Help to Cool our Cities
At present tree canopy cover is an important metric for those studying urban trees. In the face of climate change, the shade provided by trees is known to reduce ambient temperatures. However, many people are unaware of the cooling effect that evapotranspiration by trees has in cooling the environment around them, provided there is sufficient soil moisture. This cooling and shade can have a dramatic effect in reducing temperatures, particularly in heatwaves. This cooling may reduce heat related deaths. To be effective, using trees to cool our environment might require the use of irrigation at specific times of extreme temperatures, however this is not part of current government policies. The shade and cooling effects of large trees will diminish as uncontrolled urban tree clearing continues.
The urban heat island effect (UHIE) is of increasing concern as our climate changes and temperatures warm. With the prospect of many more days above 40OC and days approaching 50 OC, there are concerns about the economic impacts this could have on cities and towns. Air conditioners are not as effective when temperatures exceed 45OC, heat related hospitalizations and deaths increase as do their costs, school and workplace activities may be suspended for health reasons. All of these have an economic impact.
The simplest and cheapest way to reduce the influence of the UHIE is through treed greenspace and tree canopy cover. Concerns about the UHIE and heatwaves precipitated a review of tree canopy cover in Adelaide by the State Government. The city had lost over 11% of its tree canopy in a decade and was losing 75,000 urban trees per annum. Attempts are being made to preserve both trees and livability in Adelaide, but in other cities, such as Melbourne, where the losses have been greater, reductions in tree canopy cover continue unabated. It is possible to envisage a future heatwave claiming 500 to 1,000 lives at an enormous social and economic cost.
Other Economic Benefits
Other benefits from the presence of urban trees include removing common particulate air pollutants, which are still responsible for illnesses and deaths in Australia; the role of trees in minimizing local flooding and their use with porous paving and water sensitive urban design (WSUD) have been further documented. Some aspects of both WSUD and the use of permeable paving have now entered mainstream design, engineering and urban landscaping. However, other aspects such as the role of shade trees in prolonging the life of urban bitumen surfaces, stabilizing steep slopes and sequestering carbon are still under-appreciated. In States such as Victoria a powerline clearing regime that unnecessarily and harshly prunes trees around low voltage (LV) powerlines in low bush fire risk areas (LBRA) has been maintained by government despite a willingness by electricity distribution companies and other stakeholders to support change. Changing regulations that are clearly unfit for purpose would bring an immediate benefit of about $1billion per annum to Victorians and the increased shade would reduce fuel bills as electricity prices soar.
Health Benefits of Urban Trees
The disruption of Covid-19 had a significant impact on how treed urban greenspace is viewed by society. These spaces can be potent reminders of how important trees and the natural environment are to the health and well-being of humans. During the periods of lockdown, especially in Melbourne, visitation rates and the use of local parks, along waterways and in other greenspaces sky-rocketed. It was well understood by many that access to these spaces was vital to peoples’ health, well-being and resilience during stressful times. Even the State government recognized the importance of the connection during the period of lockdown. But a few years later, without an immediate crisis, greenspace is once again under threat and tree canopy cover is reducing at an increasing rate in suburbs, regional centres and country towns. It is curious and concerning that with threats of climate change and future pandemics, we will have fewer trees and less greenspace to allow people to cope with stressful change where they live.
Unsurprisingly, over the past decade the research into the human health benefits of urban trees has literally exploded. We now know, following a meta data study of the effects of living in a green and treed space on women’s health, that they recover more quickly when hospitalised, give birth to babies with higher birth weights, use fewer prescription drugs, have better general health and live longer. There are other benefits too in that people are more resilient under stress, have lower rates of type-2 diabetes and are less prone to blood pressure and cardiac related disease. Some of these health benefits derive from the exercise and lower rates of obesity that a treed environment and summer shade encourage. They are nonetheless benefits that are worth many billions of dollars to the nation’s health economy each year.
Recent studies here and overseas have emphasized the benefits or treed greenspace on the wellbeing, health and development of children. The effects can be subtle, ranging from the multi-tasking that children undertake as they move through a complex biological space using all of their senses and developing skills in interpreting complex spaces, to the health of their gut flora. Exposing primary school-aged children to a natural forest/woodland environment where there was leaf litter, humus and soil improved their gut microbiomes and for those with dermal conditions there was a significant improvement. Brief exposures to treed greenspaces can also assist students in focusing and concentrating on classroom activities and can reduce disruptive behaviours, and in some cases the need for medications. Once gain these benefits can be valued at millions of dollars per annum to the Australian economy.
Urban trees continue to provide high value services and functions in the places where Australians live. However, the full value of these services and benefits continues to be undervalued by most State governments. “You don’t know what you've lost till it’s gone” is a famous line from the Joni Mitchell song, but it could also be the epitaph for the loss of urban trees in many Australian States.
Selected References:
- Croeser, T, Ordóñezb, C, Threlfall, C, Kendal, D, van der Ree, R, Callow, D and Livesley, S J. (2020) Patterns of tree removal and canopy change on public and private land in the City of Melbourne. Sustainable Cities and Society, 56, 102096.
- James, P., Hart, J E., Rachel F. Banay,R F. and Laden F (2016) Exposure to greenness and mortality in a nationwide prospective cohort study of women. Environmental Health Perspectives, 124 (9) 1344-52.
- Mills J G. 2023) Urban Microbiome Restoration. Johnson T Editor, Proceedings of the Twenty Fourth National Street Tree Symposium, 17-23, University of Adelaide/Waite Arboretum, Adelaide.
- Moore, G M. (2022) Connected treed greenspace: a salve for climate change. TREENET.com
Why Living Trees Suddenly Fall Down
It is quite common that in winter or after big storms we wonder why living trees suddenly fall down. In June 2021 in Victoria, savage storms swept Victoria sending trees crashing down, destroying homes and blocking roads. This article, republished here with permission of the author and The Conversation, explores the reasons behind this phenomenon Some minor editorial changes have been made.
Under climate change, stronger winds and extreme storms will be more frequent. This will cause more trees to fall and, sadly, people may die. These incidents are sometimes described as an act of God or Mother Nature’s fury. Such descriptions obscure the role of good management in minimising the chance a tree will fall. The fact is, much can be done to prevent these events.
Why trees fall over
Firstly, in thinking about why living trees suddenly fall down, it’s important to note that fallen trees are the exception at any time, including storms. Most trees won’t topple over or shed major limbs. I estimate fewer than three trees in 100,000 fall during a storm.
Often, fallen trees near homes, suburbs and towns were mistreated or poorly managed in preceding years. In the rare event a tree does fall over, it’s usually due to one or more of these factors:
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Soggy soil
In strong winds, tree roots are more likely to break free from wet soil than drier soil. In arboriculture, such events are called windthrow.
A root system may become waterlogged when landscaping alters drainage around trees, or when house foundations disrupt underground water movement. This can be overcome by improving soil drainage with pipes or surface contouring that redirects water away from trees.
You can also encourage a tree’s root growth by mulching around the tree under the “dripline” – the outer edge of the canopy from which water drips to the ground. Applying a mixed-particle-size organic mulch to a depth of 75-100 millimetres will help keep the soil friable, aerated and moist. But bear in mind, mulch can be a fire risk in some conditions.
Root systems can also become waterlogged after heavy rain. So when both heavy rain and strong winds are predicted, be alert to the possibility of falling trees.
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Direct root damage
Human-caused damage to root systems is a common cause of tree failure. Such damage can include roots being:
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Photo: Greg Moore cut when utility services are installed
- restricted by a new road, footpath or driveway
- compacted over time, such as when they extend under driveways.
Trees can take a long time to respond to disturbances. When a tree falls in a storm, it may be the result of damage inflicted 10-15 years ago.
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Wind direction
Trees anchor themselves against prevailing winds by growing roots in a particular pattern. Most of the supporting root structure of large trees grows on the windward side of the trunk.
If winds come from an uncommon direction, and with a greater-than-usual speed, trees may be vulnerable to falling. Even if the winds come from the usual direction, if the roots on the windward side are damaged, the tree may topple over.
The risk of this happening is likely to worsen under climate change, when winds are more likely to come from new directions.
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Dead limbs
Dead or dying tree limbs with little foliage are most at risk of falling during storms. The risk can be reduced by removing dead wood in the canopy.
Trees can also fall during strong winds when they have so-called “co-dominant” stems. These V-shaped stems are about the same diameter and emerge from the same place on the trunk.
If you think you might have such trees on your property, it’s well worth having them inspected. Arborists are trained to recognise these trees and assess their danger.
Trees are worth the trouble
Even with the best tree management regime, there is no guarantee every tree will stay upright during a storm. Even a healthy, well managed tree can fall over in extremely high winds.
While falling trees are rare, there are steps we can take to minimise the damage they cause. For example, in densely populated areas, we should consider moving power and communications infrastructure underground.
You may be thinking large trees are just too unsafe to grow in urban areas, and should be removed. But we need trees to help us cope with storms and other extreme weather.
Removing all trees around a building can cause wind speeds to double, which puts roofs, buildings and lives at greater risk. Removing trees from steep slopes can cause the land to become unstable and more prone to landslides. And of course, trees keep us cooler during summer heatwaves.
It is a concern when a lot of trees fall but removing them is not the answer. Instead, we need to learn how to better manage and live with them.
The Things People do . . . . . . . . . . to Trees!!!
Given the splendour and usefulness of trees in the urban environment, it is really amazing what otherwise sensible and apparently sane people do to them. All too often people forget the basics – that trees are alive and should be treated as living things. This is one of the most important messages that I can give about trees. If taken to heart then trees will not only be treated with the respect that all living things deserve, but also we may avoid many common and potentially dangerous problems.
How often have you see signs nailed into a tree? I have seen a whole planting of mature trees seriously damaged by people nailing signs for weekend garage sales onto their trunks.
No one would approach something that they see as a living thing with a hammer and nail into its structure. These people are probably not villains! They just see the trees as lampposts with leaves, and don’t realise that they are doing serious long term damage.
As with all living things, trees grow and respond to changes in their environments. We all know this but when a blue gum (Eucalyptus globulus) is planted as a small tree from a nursery container, placing it 20 cm from a brick wall is not a good idea! This species can easily grow to 10m in a few years, on its way to a mature height of over 30m and with a trunk in excess of 1m in diameter. When it starts to lift the foundation and crack the wall, it is the tree that is blamed not the person who planted it. They forgot that it was alive and growing.
I have seen many trees planted under the eaves of a house or on the south wall in almost constant shade. These trees may grow away from 
the house leading to a sickly and poorly structured specimen, which is often later removed. What a waste of time, resources and opportunity! However, I have seen one specimen that grew under the eave and lifted the roof right off the wall, causing serious damage to the house. While a tree getting revenge has some appeal, who could forget that trees get taller as they grow?
How many trees have you seen planted or growing in paving, bitumen or concrete? Trees need space both above and below ground. A large tree always has a large root system, even if you can’t see it and much of it is in neighbouring properties. Most of the roots are close to the surface, so the root plate tends to be shallow and spreading. The root system develops this way because most of the roots will be where the moisture, nutrients and oxygen are located. So plant where there is sufficient space and don’t pave around or under the tree if you want to avoid trouble and expense.
The root systems of trees are often the victims of thoughtless actions. Most people still think that mature trees have a huge tap root that keeps them safe in the ground, but this is rarely the case. For many species the tap root, if it exists at all in urban areas, is a juvenile characteristic that is soon lost. So it is the extensive lateral roots with occasional descending roots that anchor the trees. So don’t cut lateral roots, especially if they are large (greater than 75mm) in diameter. This can have a serious impact on tree health and vigour as well as threatening the trees stability.
Each year, as a new tree grows over the old wood, the older roots gradually increase in diameter. Eventually they come to the surface and push up the surface soil. You can’t do anything about this – this is what trees do! So don’t cut the roots or mow over them re-wounding the roots every time you mow. This can cause the roots to decay, which again affects tree health and stability.
As often happens with trees in urban environments, the failure of trees has less to do with nature and more to do with our management practices. Too often the advice of horticulturists and arborists is either not requested or ignored by those managing our landscapes. The understanding of trees as living things and our knowledge of their root systems and response mechanisms has expanded enormously over the past decade or two. We can no longer use ignorance as an excuse for the often terrible things we do to trees in urban areas!
We have the knowledge to manage trees wisely and well to maximise both their longevity and the aesthetic contribution to our environment. By treating them as sophisticated living things, and providing appropriate growing conditions, we can avoid the high costs of the terrible things that people do to trees!
References
Harris, R.W., N. Matheny and J.R. Clark. 2004. Arboriculture. Integrated Management of Tree Shrubs and Vines in the Landscape, 4th Ed. Prentice Hall, New Jersey.
Anon, 2009. Protection of Trees on development sites, Australian Standard # 4970, Standards Australia, Sydney.
Article and photos copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne
Economic Value of Trees
Many people would take it as a given that parks, gardens and trees have their own aesthetic and intrinsic values. However, so often in capitalist societies, decisions are based on the economic or monetary value of systems and their components. In such systems, as soon as something performs a function or provides a service that is valued, then it too has an economic value. It worth thinking about the economic value of the benefits provided by our gardens and trees because it increases their status in decision making processes1.
Reducing Temperature
The shade from trees can lower temperatures by up to 8°C, which can reduce air conditioner use by 12-15% and also decreases carbon emissions from our largely brown coal generated electricity2. Two medium sized trees, planted in the right position (usually to the north or west) can save up to $180 per year off a domestic electricity bill through lower air conditioner use. The removal of large trees from school grounds after the Black Saturday fires removed shade and required the installation of large shade sails to protect students from excessive summer sun.
Moderating Wind
It is difficult to place a value on the role trees play in reducing wind speeds of up to 10% during storms and protecting property from hail damage. However, we do know that under climate change winds will be stronger and that severe storms will be more frequent, and that trees filter and deflect wind in they are planted in appropriate places. Indeed, it is often forgotten that properly planted and positioned trees can reduce the swirling of the wind during major fire events, making fire fighting safer.
Reducing Water Runoff
Trees are very good at holding and absorbing water during heavy rainfall events. This is now being recognized as an important part of storm water run-off management as it can reduce localized flooding. In many parts of Australia, while annual rainfall is likely to fall under climate change scenarios, the rain that does fall is likely to occur in heavier downpours. In many places, the storm water pipes are not large enough to cope with the extra volume of water and so water is now being diverted to open space and tree pits to ease the flow, which will reduce local flooding and save the cost of retro-fitting bigger pipes.
Increasing real estate value
Determining a monetary value that appropriate trees add to real estate can be difficult, but a conservative estimate of 5% of the total value has been used in the USA and a figure of about 5.4% has merged from research undertaken in Brisbane. Real estate agents have recognized the value of a good tree in a front garden at about $5,000 for a suburban property and earlier work had estimated that a tree-lined nature strip added 30% to properties. There is also an indirect but significant financial benefit for a local council in increased house prices, which are reflected in higher council property rates.
Excessive tree removal in part of a council’s area of control could impact on its income when properties are next valued for rating purposes. Given that these are significant financial considerations, it is not difficult to envisage a situation where the unnecessary removal of a safe and healthy tree from a streetscape could precipitate legal action by a resident for the loss of property value. I am often amazed at how seldom the effects of tree removal on real estate value are considered until after the tree is gone and the loss is realized.
Improving Health
Treed open spaces improve human heath, extend life spans, reduce violence and vandalism, and lower blood pressure. It is not for aesthetic reasons that hospitals now have gardens, even if they are many floors up. Their presence and visits by patients speed recover and shortens time until discharge, which reduce the costs of hospitalization. The vegetation also humidifies the air which can ease breathing and reduce the need for medication in those with respiratory difficulties. It is often forgotten that the fires of Black Saturday killed 172 people, but the heat wave surrounding it was responsible for 374 deaths. In reducing the urban heat island effect the shade from trees can substantially reduce excess deaths that occur3, predominantly among the elderly, during heat waves.
There is ample evidence that treed landscapes also foster both active and passive recreation, which in return significantly reduces annual medical expenses for each 1% increase in the population recreating1. Green and leafy environments are vital tools in dealing with children lacking exercise and becoming obese, encouraging an ageing population to exercise and curbing ever-increasing health costs. Indeed there are several studies indicating that interaction with trees is a necessary part of childhood development as well as being a foundation of the human condition.
In many Australian cities, new house blocks are becoming smaller and smaller and in the green and leafy suburbs where house blocks were larger land has been subdivided for townhouses, multi-unit developments and on a larger scale dense modern housing. While developers are maximizing their yields, at a time of climate change, it is worrying that the diminution of both private and public open spaces by urban development puts at risk long term sustainability. In many of these situations there is insufficient open space - public or private - for the planting of trees and the opportunities for the role of vegetation in ameliorating the heat island effect, lowering wind speed, providing shade and reducing energy use are lost. Even the roads are so narrow that nature strip trees often don’t survive the quest by residents for parking. This raises questions about the economic viability of such development, as well as their environmental sustainability.
Tree cover in many Australian cities is declining, particularly with the loss of private open space. While the damage and nuisance values attributed to trees are well-known, the benefits they provide are often under-appreciated. Cities are biodiversity hot spots due to the variety of habitats available in front and back yards. For most people planting the right tree in the right place at the right time for the benefit of future generations is a significant legacy. What else delivers so many benefits immediately: benefits that last centuries into the future, which prolong healthy lives and make cities both sustainable and livable?
Dr. Gregory Moore is a Senior Research Associate, School of Ecosystem and Forest Sciences, Burnley College, University of Melbourne.
References
1. McPherson E G (2007) Benefit based tree evaluation. Journal of Arboriculture & Urban
Forestry, 33 1–11.
2. Moore G M (2012) The Importance and Value of Urban Forests as Climate Changes
The Victorian Naturalist. 129 (5) 167-174.
3. Nicholls N, Skinner C, Loughnan M, Tapper T. (2008) A simple heat alert system for Melbourne, Australia. International Journal of Biometeorology 52, (5) 375-384.
Can’t see the Trees for the Lawns?
Each spring and early summer it is amazing how fast grass, weeds and lawns can grow. The combination of longer, warmer days and the occasional shower makes for ideal growing conditions, and there is always something to do in the garden.
For most of us, there will be the need to mow our lawns (if we have them) fortnightly, if not weekly, and if things have been a bit out of hand, we may even need to use a trimmer to cut back the weeds. You might begin to wonder if there is anything we can do to improve the situation?
Australians as a whole seem to be a group of lawn shavers rather than of lawn mowers. Many of us cut our lawns far too short! Grasses are living things and grass leaves grow from the base, which is just above the soil surface. So we should be cutting lawns higher, and although the height of the cut varies with the species of grass, soil type and weather conditions, in most instances we should not cut closer than 50mm.
Cutting lawns too short has a number of consequences, which are often unknown to home gardeners:
- Very short lawns use more water than longer lawns. With water utilities charging based on usage, leaving your lawn a little longer can reduce water consumption and lead to noticeable savings on your water bills. And, of course, the extra water used depletes water stores.
- Cutting too short often damages the grass, and can result in dead patches developing in a once lovely lawn
- Dead patches in the lawn allow weeds to invade, which leads to unsightly lawns and the need for tiresome weeding.
So try to remember not to cut your lawns too short.
While on the subject of lawns, I would remind everyone of the need to protect trees growing in lawns from trunk problems due to motor mower damage, as people try to cut too close to the trunk. Trees are major assets in any garden so don’t cut lawns around tree trunks!
Over the past few years the most common problem I have seen with trees is trunk damage from mowers and weed trimmers. Never cut closer than about 150mm, and remember that young, thin-barked trees are particularly vulnerable to weed trimmer injury.
The best way of dealing with grass or weeds growing around the trunks of trees is to hand remove material, use a suitable mixed particle size mulch.at a depth of 75mm or use an appropriate herbicide at the right concentration. In many instances organic mulches that you can produce yourself will prove ideal.
When you work in your garden remember that it is the trees that make a major contribution to its ambience and beauty and which shelter your garden and house from weather extremes. So make sure you don’t harm them when you are mowing your lawns and weeding. See trees as the assets they are and do all you can to protect them!
Your garden not only gives you pleasure, but adds real value to your home and the quality of your life. It is worth doing things properly in the garden as you will be amply rewarded for the time and money invested!
References
Harris, R.W., N. Matheny and J.R. Clark. 2004. Arboriculture. Integrated Management of Tree Shrubs and Vines in the Landscape, 4th Ed. Prentice Hall, New Jersey.
Articles in Gardening Australia Magazine
Article and photographs copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne
The value of trees in urban areas
It is a great pity that so many of our citizens think of our parks, gardens, streetscapes and urban landscapes only in terms of their aesthetics. While they are beautiful and decorative, these attributes often mask the many functions that they serve in our cities to the point where their economic and environmental benefits are often overlooked.
Urban landscapes and trees have been wonderfully silent assets in our cities for decades and even centuries. They are major urban infrastructure assets. Cities are biodiversity hot spots due to the variety of habitats available in public and private open space, including front and back yards. However as assets we may need to expend resources – labour, energy, and even water - on their proper management.
While I often hear it said that; “There are better things to use the water on than plants and gardens” I challenge you to name them. What else delivers so many benefits immediately, benefits that last centuries into the future, which prolong healthy lives and make cities both sustainable and livable?
At a time of climate change, it is worrying that both private and public open spaces are threatened by urban renewal and development that puts at risk long term sustainability. In many of these situations there is insufficient open space - public or private - for the planting of large trees and so there are less opportunities for the role of vegetation in ameliorating the heat island effect, reducing wind speed, providing shade and reducing energy. This outcome raises questions about the economic viability of such developments, as well as their long term environmental sustainability.
There is real economic value in the shade provided by trees that drop temperatures by up to 8°C. They can reduce air conditioner use by 12-15% which also decreases the carbon emissions from our largely brown coal generated electricity. While it is more difficult to place a value on reduced wind speeds of up to 10% due to the presence of vegetation, we do know that under climate change winds will be stronger and that Victoria has suffered the effects of three major wind storm events over the past few years.
Urban vegetation also has significance in removing atmospheric pollutants. In New York in 1994 it was calculated that the city’s vegetation provided US$10million of benefit in pollution removal. The planting of 11 million trees in the Los Angeles basin saves US$50million per annum on air conditioning bills. Sadly there are few similar studies for Australian cities, but in the only study of its kind, economists noted that an Adelaide street tree provides a minimum annual benefit of $200 per year and that it was a gross under-estimate of the real value.
There is also the role of vegetation under a changed climate in holding and absorbing water during more intense rainfall events. The economic value of reducing localized flooding could be substantial. So if we lose our urban trees and landscapes because we don’t think they are worthy of some of our resources we could pay a very high price indeed. We won’t know what we have lost till it’s gone.
I could go on for pages about the benefits of urban trees and landscapes. I haven’t even mentioned how gardens improve human heath, extend life spans, reduce violence and vandalism, lower blood pressure and save society a truck load on medical and social infrastructure costs. Isn’t it lucky that as we let all the turf in our parks and ovals die that we don’t have a problem with kids lacking exercise and becoming obese. If we did, we might be paying an even higher price for the loss of public and private space than we ever dreamed possible.
Natural turf is quite a complex ecosystem and has a significant effect on temperature and the heat island effect, and if properly managed also sequesters a considerable amount of carbon. Perhaps it is not the villain that many think when they consider only the water component of a more complex equation.
Melbourne is one of Victoria’s biodiversity hot spots. The parks, gardens, streets and front and backyards provide a very diverse range of plant species that generate a myriad of habitats and niches for wildlife. High density urban developments and inner city renewal make it virtually impossible to grow trees in places that were once green and leafy. We rarely ever see the real costs of such developments.
In the past decade tree populations in many Australian cities have declined, particularly with the loss of private open space. While costs, and the damage and nuisance values attributed to trees are widely known, the benefits they provide are often subtle and under-appreciated.
Urban vegetation provides economic and ecological services to society. They are assets which warrant the expenditure of resources such as labour, energy and water. Such expenditure is not wasted as trees and urban landscapes provide far more economically and ecologically than they use. In any comprehensive and fair calculation urban trees and landscapes are worth more than they cost.
Article and photos copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne