Ash Wednesday 1983 – Bushfire mosaics.

There is a common belief that even the slightest bushfire in mountain ash forests (E. regnans) is catastrophic and uniformly kills every tree in its wake.

It’s true that mountain ash is very susceptible to bushfire, but the story is not that simple.

Fire behaviour and intensity depends on many things, like forest type and age, fuel dryness, fuel load and arrangement, wind, topography, slope, aspect, RH, temperature, day or night.

Under extreme conditions bushfires will often “hill surf”, and spot fires may start many kilometres ahead of the main front.

These new spot fires on hill tops and ridges may join up, or they may inexplicably leave large unburnt gaps behind. They can also trickle slowly down slope against the wind and into the wet treefern gullies and then fizzle out.

At the other extreme, crown fires are created when the ground fires underneath build up sufficient momentum and elevated fuels like bark and scrub act as ladders allowing the fire to surge into the tree tops and the highly volatile eucalyptus oil to explode in fire balls.

However, the leaves on standing trees will not, on their own, support a fire, and fires will not occur in tree crowns unless there is an intense fire burning on the ground beneath.

Once the crown fire has surged ahead of itself and there is no longer a ground fire underneath it will collapse and sometimes swirl back on itself. This frightening phenomenon creates what I call “damage waves” that remain visible in the forest canopy long after the fire has passed.

Unlike other Ash Wednesday bushfires at Lorne or Trentham, the entire eastern flank wasn’t lost at Warburton with the south west wind change. Large sections of edge near Gladysdale didn’t travel, either due to damp forests or stands of mature wattle which had originated after the 1926 and 1939 bushfires. This was probably the main reason the fire split in two.

And all this “cross-talk” leaves a complex mosaic behind.

The final condition of burnt mountain ash forest may range from completely defoliated with 100% of the trees killed, or patches which have been only lightly burnt where the trees will survive, through to islands which are miraculously untouched.

It’s also not uncommon to find areas of mixed aged forest, where the fire has burnt lightly through the understory, and many of the older trees have survived, which allow patches of younger regrowth to develop in the gaps.

This aerial photo of damage class mapping after the 1983 Ash Wednesday fires at Powelltown illustrates what I mean.

Ref: Woodgate (1984): https://www.crcsi.com.au/assets/Resources/Division-of-Forest-Management-Assessment-Section-Forest-Inventory-Report-No47.pdf