A recently published Government report confirms what many ecologists and rural people have suspected for a long time – planned burning (aka fuel reduction burning) is slowly and surely eliminating hollow-bearing trees (HBTs) from our forests.
Having conducted our own post-fire survey of a recent, “successful” planned burn in the Strathbogie Ranges, the results of this more comprehensive DELWP study are of considerable interest.
The DELWP ecological study, published earlier this year, was conducted across numerous sites in East Gippsland and examined in detail the impact of 30 planned burns (p. 17) on HBTs. While the report findings are alarming, the report is timely and provides valuable insight into the impact of planned burns on forest ecology across the State.
The study assessed a large sample of HBTs across burnt and non-burnt areas. “In total, 1575 HBTs were located and had their characteristics assessed in pre-treatment surveys. Of these, 666 were individually revisited in post- treatment surveys, while the remainder were located in plot types where repeat surveys of individual HBTs were not conducted” (p. 25).
The results showed that “From the sample of 273 HBTs that were directly exposed to fire, 70 collapsed and a further 73 were seriously structurally damaged, whereas these outcomes rarely occurred among HBTs not exposed to fire.” (p. 25) A ‘structurally damaged’ ” tree is one that has become structurally weaker .. and is at higher risk of collapse. (p. 18).
Perhaps the most telling summary of the report’s results is this diagram:
Within the planned burns assessed during the study, fire reached 273 HBTs (right-hand pie chart). Of these 273 HBTs, 70 collapsed (25%), another 73 (26%) were seriously structurally damaged and 130 (48%) remained undamaged. The report concludes that:
… planned burns in the study caused collapse or serious structural damage to 51% of the hollow-bearing trees that were reached by fire.
So, how do these East Gippsland research results compare with those of our smaller Tames Rd burn study? In summary our research showed:
- Half (51%) of all stags (dead standing trees; ≥ 70 cm DBH) in areas affected by the burn, were destroyed by the burn.
- Almost half, 47%, of the largest trees in the forest (≥100 cm DBH) in areas affected by the burn, were killed by the burn.
- The four biggest trees in the survey area, by a considerable margin (between 1.5m and 1.9 m DBH), were all killed and felled by the planned burn.
- 25% of all the trees (≥ 70 cm DBH) affected by the burn, were killed by the burn.
Whilst the two studies varied in their approach and size, the results are broadly similar and show just how destructive planned burns can be. Whilst planned burns have their place in bushfire mitigation, their negative impacts on big, old trees and forest health can’t be ignored. They must be used judiciously and strategically and in combination with other measures that increase community safety.
Why are hollow bearing trees important?
Large, old trees are critical ecological elements of a healthy forest, as well as being a broader surrogate for forest health and values that are difficult to assess. The large, old trees in Strathbogie forests:
- Produce numerous hollows for a range of specialized bird and mammal species: birds – e.g. tree-creepers, sittellas, owls, kingfishers, nightjars; mammals – e.g. phascogales, gliding-possums, pygmy-possums, Antechinus.
- Flower more reliably and produce more reliable flows of nectar and pollen for the numerous birds, mammals and invertebrates that rely on these resources.
- Shade the forest floor to keep the ground conditions cool and humid.
- Take 100-150+ years to develop the large hollows needed by hollow-dependent species like the Powerful Owl, Greater Glider and Yellow-bellied Glider.
- Were alive and thriving well before Archduke Franz Ferdinand of Austria was assassinated, which precipitated WWI and began the ANZAC legend.
- They are the dominant ‘mother trees’ of the forest, likely to be connected to every other tree and many non-tree plants in their area by fungal connections (mycorhizal fungi) in the soil. These interactions are poorly understood, but almost certainly increase the forests’ resilience in the face of unpredictable conditions (past and future climate change).