Can a low intensity burn be too hot?

The fire trickling through the forest was not always hot enough to remove shrubs.

The fire trickling through the forest was not always hot enough to remove shrubs, but ironically, it seriously damaged large numbers of trees.

The answer is an emphatic YES! but it’s also a complex story. A small patch of forest on the slopes of Leerson’s Hill, at Toorour, was burnt a few weeks ago (by Hancocks Plantations, Victoria). From a fire planner’s point of view, conditions probably seemd good: no wind, dry fuel, mild day-time temperatures and cool humid nights.

The forest at this site is recovering from logging perhaps 40 years ago and is a mixed stand of stringybarks, gums and peppermints. Most of the trees are young, but there are/were some older trees and dead trees with hollows. The whole district had received very little rain since January.

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By all accounts the fire did get up into the canopy and flames were clearly visible above the tree-tops from a few hundred meters away. In these forests, fire gets into the canopy by crawling up the rough-barked eucalypts (Stringybarks and Peppermints) and perhaps flaring up in some ribbony bark or dry leaves. It might look like the tree crowns are burning, but that didn’t happen in this case.  The images below show that, although the fire got into the canopy, it didn’t burn much of the tree-crowns. The leaves are brown because of crown scorch (radiant heat and rising hot air from the fire, damaging, but not igniting them); they’ll all be shed in the next weeks and months.

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The fire’s behaviour at ground-level was very different (above). Where ground litter was sparse, the fire was weak and didn’t even burn the shrub-layer. Where there was litter build-up the fire burnt very hot and usually burnt all that fuel to ash, leaving the ground bare (below). This happened particularly around the bases of Blue Gums and Manna Gums; both are smooth-barked eucalypts that shed bark in ribbons and which accumulates around a tree’s base.

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Many of the Blue Gums and Manna Gums now have a ring of pale-coloured ash around their burnt and charred trunks (above), indicating that these spots were very hot and that the sap and vascular tissue just beneath the bark surface was probably cooked. No wonder the trees are now shedding their leaves – they can’t get any nutrients or water up to them. When this happens the tree must repair and re-grow it’s vascular (sap bearing) tissue (if it hasn’t been too badly damaged), which can take many months or even years if the damage is severe.

Even though the burn is still fresh, there is already considerable new fuel on the ground. Numerous trees, burnt out at their hollow bases, fallen branches and collapsed dead trees, now litter the ground.

Conclusions

Even though this was a fire that didn’t incinerate the forest (it was a relatively cool, low intensity burn and certainly one that forest managers would likely consider successful), it has had significant impact.

  • Hollow-bearing trees have clearly suffered, both from the fire and from the subsequent need to cut down substantial numbers of ‘dangerous’ trees.
  • Fire intensity at the bases of Blue Gums and Manna Gums will likely and unnecessarily, seriously damage and perhaps kill a good proportion of these trees, some of them being the oldest trees in the forest. It’s also likely that this fire, coming at the end of a very dry spell, burned plants when they were already stressed and will likely result in higher mortality that otherwise.
  • Fine fuel loads (leaves, twigs, bark) have been temporarily reduced, though leaf and branch fall, along with vigorous new growth in the next few years could soon see the accumulation of fuel loads at the same level or greater than those present before the fire.
  • Medium and coarse fuel loads are also likely to increase as more and more trees either collapse due to fire damage, or from being cut down from safety concerns.
  • The burn probably killed the majority of insects (eg beetles & moths), other invertebrates and fungi that play such a crucial role in decomposing forest ‘fuel loads’ and it will take some time for this ecosystem service to replenish.
  • The litter and humus build up in the soil and around trees has gone, making all plants more susceptible to moisture stress and making it easier for the forest to carry another fire, until those resources are replenished.

The net result, in terms of fuel reduction, is highly questionable. Within months, to a few short years, fuel loads in the forest will likely be similar to what they were before the burn, or even greater.

One big difference is that the forest will contain fewer big, gum-bark trees (Manna Gums and Blue Gums), and certainly fewer hollow-bearing trees, than it did before the burn.

All this, from a slow, patchy ‘fuel reduction burn’.

2 responses to “Can a low intensity burn be too hot?

  1. Pingback: Tames Rd, Strathbogie – another ‘fuel reduction burn’ | Our Strathbogie Forest

  2. Pingback: Planned burn – an ecological disaster? | Our Strathbogie Forest

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