Northern Tree Care have compiled this report on instruction from Byron Shire Council. There are three (3) Liquidamber trees Liquidambar styraciflua growing in the footpath next to the Bottle shop in Byron St, Bangalow.

These trees have caused the pavers to move, creating a trip hazard in the footpath.

This report was compiled by Peter Gray Dip Hort (Arb). of Northern Tree Care. Peter Gray is an arborist with over 15 years experience. He has been providing tree reports for Local Government, State Government and private clients for over 12 years. The information contained in this report is true and accurate to the best knowledge of the author. Best professional judgement was used to make the recommendations contained in this report.

Peter Gray is a trained and experienced practitioner of Quantified Tree Risk Assessment (Ellison 2007) and is a registered consulting arborist with ISAAC (International Society of Arboriculture Australian Chapter).

Download Report:
Byron Bay Liquidamber Tree Report.pdf

The Blackbutt tree it fell off is part of a stand of mature aged trees. The trees are all growing on the road reserve beside the highway and  are Tweed Shire Council controlled trees.

These native forest remnants are common on the side of the road and form a very important part of the local environment. They are often the only relatively undisturbed areas of native forest growth left after clearing for farming and road building. There is pressure on them from a range of issues including weed invasions and pests and diseases. They are also sometimes removed because they are seen as a hazard to motorists using the highway.

There is no doubt that trees sometimes do fail and cause death and injury. I thought it would be interesting to examine this case and see just how dangerous these trees really are. Do we need to be doing more tree work to make our roads safer or is unnecessary work being carried out wasting resources and removing valuable trees for very little benefit?

The branch fell from a mature aged Blackbutt Eucalyptus pilularis. The branch was 300mm in diameter for approximately 3 metres and 7m long in total. The branch fell across the road and was cut and dragged off the road by a passing motorist. The branch fell off a tree that is part of an avenue of large mature aged trees Blackbutt trees. The branch broke 0.5m from the trunk and had no obvious signs of damage or hollow in the branch. The branch did show sign of some fungal activity. In fact the whole tree does show subtle signs of tree fungi activity. This is relatively common in these trees and most mature Blactbutt trees have some degree of tree fungi activity. The branch broke in moderate winds and there was no storm event at the time of the branch failure. No one was injured by the branch.

The tree the branch broke off has a similar branch in similar condition leaning over the road. There are also a number of trees over a distance of 500m that have large branches both dead and alive hanging over the road. In fact it is not uncommon in this local area to find similar trees with branches hanging over the road.

The risk assessment for this branch based on QTRA (Ellison M. 2011) is calculated using the the probability the branch failure causes death or serious injury or damage to property. The calculation takes into account the ‘target’ or what the branch has the potential to injure or damage, in this case most likely to be motorists and their motor vehicles. In the worst case the branch has the potential to land on a passing motorist caus- ing death or damage to the motor vehicle. The calculation also takes into account the size of the branch in this case known to be 300mm and finally the probability that the branch will fail in the next calendar year.

A probability is assigned to each aspect and the probabilities multiplied. So that Risk of Harm = Target x Size of Part x Probability of Failure.

QTRA assigns a probability of 1/2 for a branch of 300mm in diameter.

The target is calculated by examining the frequency of use of the road and the speed of the traffic on the road. It must be understood that the branch could cause injury or damage to the vehicle if it fell within the stop- ping distance of a vehicle travelling on the road. The road is a major road with a speed limit of 90km/hr and has a vehicle passing the tree every few minutes during the day and much of the evening. A typical vehicle spends 2.8 seconds in the zone where it would be likely to be affected by the falling branch. The total num- ber of seconds in a year is 31,536,000. If an average of 262,800 vehicles pass each year the total amount of time is 735,840 seconds spent is expressed as 1/42.

Tree Failure – Acceptable Risk

We know that tree branches do break and we know that they are more likely to fail if they are compromised by mechanical injury or decay. Mattheck C. (2003) describes how the mechanics of trees works and gives insights on the causes and processes of tree failure. Different species of trees have different likelihood of failure and the age and condition of the tree also have a strong influence. Where the values for size of part and the target can be calculated quite closely the likelihood of failure must be estimated by the tree inspector. QTRA advises the tree inspector to consider how many trees (or branches in this case) in the same condition would it need before you could expect to have one fail, is estimated to be in the order of 50,000.

Size of Part = 1/2
Target = 1/42
Probability of Failure = 1/50,000

Risk of Harm = 1/2 x 1/42 x 1/50,000 = 1/4,200,000.

Given the age and condition of the tree in this case it

So the risk of this branch firstly failing and then also causing death or serious injury is less than 1 in 4 million.

Even though we know the branch did fail it didn’t cause any damage in this instance (it fell but there was still only a 1 in 84 chance of it hitting a motor vehicle. We can see that after the event. What this does show is that to prevent branches from ever hitting motor vehicles a very large number of branches in this condition would need to be cut off.

It is interesting then to examine the issue from another angle. What does the risk from a branch failing have to be before we do need to do something about it. Studies have been done to look at what risk is acceptable in society and a figure of 1/10,000 of death or serious injury in one calendar year is accepted as the bench- mark. We know that this branch had a target and size of part ratio of 1/84. If the acceptable limit of risk is 1/10,000 then a branch would need to have a risk of failure of 1/119 to reach that threshold. A branch with a likelihood of failure greater than 1/500 would show very obvious signs of damage or decay.

This shows that the benefit of carrying out tree work on tree branches over the road is very low unless the branch is in obvious and very poor condition. In fact it is the case that tree work is being carried out in the name of tree safety that has no real relation to the risk and benefit involved. In other words time and money is being wasted cutting valuable trees for no good reason because poor and uninformed judgement is being used to determine just what constitutes a dangerous tree.

Ellison M. 2011. Quantified Tree Risk Assessment User Manual. QTRA. Cheshire.

Mattheck C. Breloer H. 2003. The Body Language of Trees. TSO. London

Alternatives to Pandanus Stem Injections

Rather than recommend the trees be treated by stem injection of the systemic insecticide Confidor 200SC, I recommended that, where practical, Confidor be applied by spraying the leaves with a misting machine or backpack sprayer. Confidor can be applied in this way; being a systematic pesticide, it will travel through the plant to reach the planthoppers which are causing the tree’s demise.

This type of pest control can be expected to last for several months. The alternative treatment, which has received considerable local media coverage, is the stem injection of Confidor. This treatment was developed and registered in Queensland and adopted for use in NSW.

Problems with Administering Pandanus Stem Injections

To explain my preference for mist spraying and my concerns about stem injection, especially multiple injections, I will set some context here. A few years ago there was an outbreak of disease among pandanus caused by the planthopper Jamella australiae first noticed in the Tweed in 2004. It occurred right through the North Coast from the Tweed Shire to the Ballina Shire.

In numerous cases, stem injections were done at or near waist height or into the main trunk of the plant, sometimes under pressure. Many injections were given to mature trees during a program controlled by the local authorities. In the most part it was successful in controlling the planthopper.

The treatment involved drilling a hole in the tree. Unfortunately, that hole remains in the tree and the resultant decay around the hole increases over time. The use of pressure injections increases the extent of decay around the drill hole. It is unlikely that any of the trees subjected to a single injection will die, however repeated injections will seriously compromise the trees’ long-term survival. Multiple injections create too many dead areas in the trunk, and as these areas merge, over time, the whole trunk will die. Hence, such ongoing injective treatments will be doing the trees no favours, long-term.

Fatal Yellow Disease in Pandanus

Recent laboratory tests have identified another pest attacking pandanus trees in the local area. The pathogen is the bacteria Phytoplasma australiense. It is soft-walled, bacteria-like and lives in the phloem (the soft tissue of the stems) of the plant. This pathogen also causes yellowing followed by death of the growing heads. The decline of the affected head is much faster than with jamella but otherwise similar. A number of monocotyledons (these comprise the majority of flowering plants in terms of biomass produced) are affected by phytoplasmas, notably palm trees, where this syndrome is known as “fatal yellow disease”.

There is no cure for fatal yellow disease although injections of the antibiotic oxytetracycline have been shown to slow it. The phytoplasma pathogen is transferred from plant to plant by the leaf-sucking Jamella, therefore and here is a key point controlling the Jamella is also the most useful way of preventing the spread of the fatal yellow disease. Finally, I would warn against the tendency to anthropomorphise trees in terms of the types of treatments that serve them best e.g. a quick shot or three into the “bloodstream” to reduce disease and revive health.

Biologically, plant life and human life bear many similarities, but equally there are significant differences that need to be recognised and respected when it comes to the care of trees.