A hazard tree in a burn zone does not fail on a convenient timeline. Root-compromised eucalypts and hollow stems come down during ignition, during mop-up, and days after everyone has gone home — into the ground your crew is working from.
Why fire changes the failure equation
Three mechanisms operate on trees during a burn, and only one of them is obvious.
- Root and butt consumption. Fire entering an existing basal cavity or burning into surface roots removes structural wood without changing the tree's appearance from head height. Smouldering can continue in a hollow stem — a “pipe” — long after the fire front has passed.
- Cambium kill. Heat girdles the stem, and the tree begins dying without immediately losing structural integrity. The failure comes later.
- Load change. Understorey removal alters wind exposure across the whole burn area at once.
The consequence is that the dangerous window is not the ignition. It is the mop-up and the re-entry, when crews are on the ground, tired, and the trees that were sound this morning are not.
What belongs in a burn plan
A burn plan without mapped hazard tree exclusion zones is an incomplete safety document. A pre-burn assessment should produce:
- A hazard tree register for the operational area — each identified tree located by GPS, with the defect recorded and a quantified risk of harm.
- Mapped exclusion zones. Operational doctrine sets the exclusion zone for a hazardous tree at a minimum of two tree lengths. That is a large footprint and it needs to be on the map before anyone plans a control line through it.
- Control line and anchor point review. Control lines routinely follow tracks that run directly beneath the largest, oldest, most defect-laden trees on the block, because that is where the tracks were cut.
- Marked trees on the ground. Standard marking is a yellow circle-and-cross symbol, around 30 cm in diameter, at roughly 1.5 m above ground, on two sides where it is safe to apply.
- A crew briefing document that names the hazards in plain terms and shows them on the map.
Not one. The exclusion zone accounts for the tree's full reach plus the distance debris travels on impact. Where terrain or a recognisable lean makes the fall direction predictable, the zone extends 180 degrees in that direction.
What crews should already be doing
Operational protocol requires work areas to be inspected before each shift and reassessed continuously as conditions change. Suspected hazardous trees are identified from a safe distance and escalated to the incident controller and safety adviser rather than approached.
The warning signs that doctrine lists are the same ones a QTRA assessment records formally: hangers and damaged limbs, new longitudinal cracks in timber or bark, excessive rot, exposed roots, stem damage, dead trees, excessive lean, and the two fire-specific hazards — widow makers (lodged limbs) and pipes (internally burning stems).
Where a pre-burn assessment adds something the crew brief does not
Dynamic risk assessment on the day is essential and it is not a substitute for pre-planning. A pre-burn assessment is done in daylight, without smoke, without time pressure, by someone whose only job that day is looking at trees. It produces a document that exists before the event rather than a judgement made during it.
It also produces the record. If something goes wrong, the question will be what was known and what was planned for. A dated hazard tree register with mapped exclusion zones answers it.
A note on who does this work
Assessing hazard trees in an operational fire context is not the same skill as assessing amenity trees in a suburb. It needs someone who understands fireground command structure, how a control line is chosen, how mop-up actually proceeds, and what a crew can realistically comply with — alongside the tree assessment itself. An exclusion zone that makes the burn undeliverable will be ignored, and an ignored control is worse than none.