Coordinator Arboriculture, Assoc. Dip App. Sc. (Arb)
City of Yarra
The intent of the paper is to explain and analyse tree root management practices within the City of Yarra, Melbourne. The high density of infrastructure, predominantly Victorian dwellings, reactive clay soils ‘Class H’ and the planting of trees often makes for conflict between infrastructure and the needs of the tree. Employed 12 years ago as the first arborist at Yarra it is my job to manage this conflict which involves the necessity of tree root management. This has resulted in practices being implemented that would normally never be recommended by an arborist with some unexpected results.
City of Yarra is approx. 20km2 and has a population of approx. 83,000 residents and is situated in the inner north east of Melbourne and includes suburbs of Richmond, Burnley, Cremorne, Fitzroy, Fitzroy Nth, Carlton Nth, Collingwood, Princes Hill, Abbotsford and Fairfield / Alphington South of Heidelberg Rd.
Yarra consists of high density infrastructure and housing. More than half the municipality consisted of light industry in the early to mid-20th century and has given way in the last 20 years to high density housing and a café culture.
Approximately %50 of the population changes houses every 5 years. The demographic who includes 38 different spoken languages includes both extremes of the socio economic scale i.e. Government housing to multi-million dollar apartments & mansions.
Until the late 1970’s the southern part of the municipality was seen as an undesirable place to live with the wealthier seeking the larger expanses of the suburbs.
Original residential dwellings are predominantly of the Victorian era, sitting on minimal footings of bluestone, brick or in many cases rubble. Very few nature-strips exist and asphalt and bluestone are prevalent.
The dominance of light industry and the influx of ethnic immigration after the 2nd world war did not see trees as a priority and minimal tree planting wasundertaken. The few trees planted at this time were species proven to survive, primarily of Platanus (London Plane), Melaleuca (Paperbark) &Ulmus (English Elm). There was no thought of the consequences in relation to size or location.
The change in demographic and greater understanding of the importance and benefits of trees in the landscape has the public demanding that council provide tree planting programs and that all trees be retained where possible.
The municipality has an estimated 30,000 street trees with additional 1000 planted annually. These are planted into hard asphalt surfaces overlaying highly reactive, Class ‘H’ Quaternary Basalt and Silurian clays with basalt floaters and bed rock in scattered locations. The soils readily expand and contract based on the amount of moisture they hold. The northern part of the municipality has a high water table and many new plantings fail due to waterlogging even in summer months
Trees planted in the middle part of the 20th century are now fully mature and many, predominantly Plane and Melaleuca trees, are impacting detrimentally on infrastructure. The need to preserve trees has resulted in what some may see as radical solutions to resolving the conflict between infrastructure and trees.
As arborists, do we know as much about trees as we think?
As arborists most of us have spent many years studying to obtain qualifications in arboriculture where the preservation of trees is paramount and the mantra of minimal interference to trees is prevalent to their retention. This includes the necessity to minimise root disturbance and the severance of tree roots that can have a detrimental impact on tree health.
The introduction of the Australian Standard for the ‘Protection of Trees on development sites’ – AS4970 was a huge step forward in the preservation of trees and advantageous document in the education of the building industry.
Unfortunately, in my situation I cannot apply the formulas as recommended. This would see nearly all trees I inspect contributing to infrastructure damage being removed on the grounds that they will become unstable or die.
The decision I have to make around the preservation of trees and any damage is simple. ‘Cut the roots or Remove the tree’.
This has led me to make decisions associated with tree root pruning based on my years of observed experiences and not on science or documented research. Roots are exposed using air knives and estimations are made of the extent of roots a tree may have and where these are expected to be located. Conclusions are then made of what roots can be cut that will not significantly impact on tree health or tree stability; stability being the priority due to safety reasons.I soon realised being an arborist had little to do with the decisions I was making.
Over the last 12 years I have instigated the pruning of roots on over 3000 trees. Many of these works I myself would class as barbaric and questionable. This has however, allowed me to see the consequences of my actions and the response from trees. My confidence in cutting larger and more roots has increased over the years and much to my surprise with minimal or long term impacts. Trees appear to either have no detrimental impacts or have sparse canopies for a few years and then develop full canopies. Minimal deadwood is observed. I call it sulking!
I have found trees to be significantly more resilient than we give them credit. They appear to be able to initiate feeder’s roots quickly in response to root severance as long as adequate quantities of water are available.
It leads me to question the following;
- Damage or sever a portion of the roots and a corresponding portion or the entire tree may die.
- Tree roots do not search for, sense or extend towards, anything. E.g. water, nutrients, drains or dripping taps.
- Tree roots cannot extend or grow in dry soil.
Tree Root Distribution
Tree roots are opportunistic and will grow wherever there is sufficient water, nutrients, air and non-compacted soils that allow for root extension.Where these are abundant roots will proliferate. This can be many times the height or with of the canopy away. In general they do not have a single tap roots but may established sinker roots in sandy soils or clay fissures to access deeper water reserves.
In an open ground where trees have ample room to grow with no impediments, i.e. building foundations, underground services etc. roots would tend to spread radially from the tree to a depth of around 600mm deep, where these elements are usually in abundance, as shown above.
In highly modified urban landscapes however with substantial underground impediments, this is not the case. Roots will grow wherever they can absorb sufficient elements for growth.
This is along the bluestone kerbs where water is plentiful, in the footpaths where soils are not severely compacted and in front yards where soil is friable and often regularly irrigated.
Often several meters underground growing in disturbed soil next to underground services or many tens of metres past the canopy i.e. in stormwater drains.
In urban environments Asphalt provides an ideal environment for surface rooting. It is porous and allows water and air exchange and holds heat. The interface between the asphalt and subgrade acts like a hot house.
Roots do not grow in the roadway due to compaction. This can be within 1m of the trunk. Root plates often don’t exist on the roadside of the tree.
Roots are often found where you don’t expect them.
Tree Root Associated Damage
Roots can often contribute to property and infrastructure damage. They can do this in two ways:
The movement of structures physically by tree roots.
This seldom occurs on buildings or major structures that have adequate foundations but regularly occurs to masonry fences, veranda returns, pipes, gutters that are not designed to carry significant weight or have force placed upon them.
The subsidence of soils by the absorption of moisture from the soil profile by tree roots.
Predominantly in clay soils that are prone to shrinkage and expansion based on water content. Trees can absorb sufficient amounts of water to make clay shrink, undermining footings. This is the main cause of private property damage claims in Yarra.
The suction effect of roots in clay soils through diffusion can result in soils drying out many meters from tree roots, where tree roots are not observed.
Evaluating Damage Related to Tree Roots.
As arborists we should not think that we are suddenly engineers and provide advice outside our field of expertise. However, we can give opinions based on experience.
Many factors can contribute to structural damage, including the following:
- inadequate or poor quality footings & construction,
- leaking water and sewerage pipes,
- seasonal shrinkage (drought),
- poor drainage, inadequate or damaged storm water discharge,
- poor property maintenance,
- privately owned trees,
- nearby construction disturbance,
- alterations to the property,
- service pits e.g. gas, Telstra
In recent years Victorian local government insurance has under gone changes from an equal lump sum payment made by all councils to a user pays scheme. The more claims made by an individual council/shire the higher the premium. This has highlighted tree root related claims as being some of the highest. In February the formation of a working group was instigated in collaboration with the Municipal Association of Victoria (MAV), Liability Mutual Insurance, Jardine Lloyd Thomson (Risk consultants) and arborists from 8 Victorian metro councils.
The intent to develop a consistent risk based approach to investigating possible tree related damage/claims. A draft template has now been developed that will be soon available to Victorian municipalities to trial and evaluate. It is hoped this will reduce claims and lead to the retention of more trees.
Minimising Infrastructure Damage.
Species selection – Based on soil and climatic conditions, and adequate space for its mature size, above and below ground.
Construction design – Ensure footings accommodate trees and their potential size and possible impacts. Modified footings that minimise soil disturbance and subsequently root damage i.e. screw piles, pier and beam, waffle slab, individual stumps. In urban environments there is always going to be a tree in the vicinity.
Root Grinding – The grinding of roots to allow for the reinstatement of kerbing, footpath etc. This leaves a substantial part of the root intact for stability and absorption. This is usually instigated by council civil contractors to undertake repairs to infrastructure
Selective Root Pruning – The severing and removal of sections of visible root/s that maybe causing physical damage e.g. a root lifting footpath, kerbing or masonry fence. Root pruning
Root Barriers – Designed to sever roots and to install a physical membrane to prevent roots growing towards infrastructure/property. These are often moisture membranes to prevent drying of soils around footings.
Often not seen as a long term solution they can prevent litigation for damages and preserve the life of the tree. Their success is based on location, size and quality of installation.
Barriers consist of high density polyethylene barrier to 1.5m deep (2m if roots discovered at this depth) backfilled with flowable stabilised sand, 3% cement. Roots are carefully exposed using a mini excavator and spotter and severed cleanly with cutting equipment, not ripped out. The 300mm bucket and subsequent trench allows access to seal around service and joins with butyl tape.Roots act similarly to human wounds in the sense that a clean cut will callus (heal) quickly. Ripped jagged surface are prone to disease and decay that can spread throughout the root system causing failure to callus and possible destabilisation in the future as the root plate decays.
The membrane is placed on the furthest side of the trench from the tree and backfilled with flowable sand on the tree side. This viscosity of the sand, which sets hard, allows it to creep into cracks, joins, around services etc. and provide an additional barrier. Initial inspections of barriers exposed after a few years shows flowable sand appears to stunt and restrict root development upon contact. Roots act like hyphae at the root sand interface.
They are instigated on property damage claims where it has been proven that the tree is having significant impact on the building. Proof is either by physically viewing roots causing the damage or with supporting evidence from the property owner i.e. soil engineers (GEO Tech) report that provides, soil moisture testing etc. This is beneficial as they identify all possible causes of any damage and footing types, depths.
Structural engineer reports are not excepted as they do not provide evidence and make assumptions that are not validated.
All significant root pruning is undertaken when tree are in advanced stages of autumn leaf drop or dormancy, usually in the months of May to October. This is climate and species dependant during autumn. Platanusinsularis – Cyprian Plane, did not lose its leaves till late July.
Service provider practices in response to tree involvement, previous and present.
Prior to Yarra employing an arborist trees were seen as a liability that caused damage and complicated repair works. The Arboricultural & Streetscape unit Council has slowly been able to educate the providers and council engineers and make them realise the importance of trees and how they are a valued essential council asset. Removing trees indiscriminately, ripping roots out with excavators and jack hammers was a common practice.
Thankfully it has now changed and service providers are very committed and understanding in minimising damage to trees. Aqua vacs that dislodge soil with high pressure and suck it out are always used when in the vicinity of a tree in Yarra unless council’s arborist approves excavation by machine under their supervision. The aqua vac provides the necessary water to minimise tree stress and assists in accurately locating roots and services that would otherwise be difficult and result in unnecessary damage to roots.
While this does cause some damage to tree roots by partially removing bark and sections of cambium from the roots it leaves much of the root intact. This ensures stability is maintained and also provides the necessary water to retained roots for quick recovery and minimise stress.
New innovations to protecting trees and minimising root damage.
To try and retain trees where root pruning is likely to significant impact on tree health and or stability, Yarra council is trialling a number of different strategies that minimise root severance or may prevent root damage to infrastructure.
Indenting kerbs to allow larger majority of roots to remain
Installing root deflectors at the time of planting to try and minimise future damage
Installing rubber footpaths to prolong footpath life and minimise trip hazards
Installation of sump holes to encourage draining and to prevent significant root pruning in gutters
Trees in an urban environment are an essential asset in its liveability; environmentally, aesthetically, socially and economically. The demand to plant more trees and retain the ones we have is becoming more and more difficult as the demand on opens space increases and the space available to trees diminishes.
It is essential that we retain as many larger trees as possible as these are the specimens that provide the greatest benefits and increase the likely hood of space being set aside for trees in the future. Past species and location selection practices make this task difficult and resourceful thinking is required. Innovation and sometimes debateable practices need to be implemented.
Tree managers need to keep educating engineers and civil contractors that trees are as important council asset as roads and signs and highly revered by the public.
We need to keep trialling and assessing new techniques in tree management to obtain a better understanding of trees and their responsiveness to enable larger mature trees to be retained longer in the environment for future generations to enjoy.