On Sunday morning, 2nd Aug, Field Naturalists were kindly invited to an event organised by the Friends of Aranda Bushland. It was a walk with fire ecologist, Michael Doherty, to assess and discuss a recent prescribed burn in Aranda Bushland.
Wildfire has always been a natural phenomenon in the Australian bush, favouring some species over others, and influencing species distributions. Notably, eucalypts seem to have evolved to encourage fire, as can be seen through the advance and retreat of eucalypt-rainforest ecotones over time. Generally, eucalypts have advantageous recovery, through their fast epicormic or lignotuber re-sprouting, compared to more fire-sensitive, light-sensitive and moisture-sensitive species. However, even some eucalypts, such as the ash species, can be disadvantaged if high-severity fires are too frequent, as their strategy relies on regrowth through seed drop from mature trees.
Likewise, fire management to modify vegetation has been an activity ever since the arrival of humans, whether to favour certain food sources, or more recently, to protect assets, or even for ecological reasons, further influencing species distributions.
If participants in the Aranda walk didn’t already know, they soon became aware of the extremely complex issues relating to fire in the Australian landscape.
This particular burn was on the western flank of the Aranda Bushland, in an area unburnt for 20 years. It was low-intensity and caused low-severity impact, with minimal crown scorch being apparent. As we walked, we noticed that the charring of stringy bark trunks tended to peter out about 2 m above the ground, and Michael wondered if that would have been considered satisfactory. We also talked about the importance of managing the actual burn, such as mopping up smoldering logs or trunks. Even a trickle burn can cause death to mature trees if smoldering continues.
There was discussion about some plants, such as orchids, being advantaged by fire and others being disadvantaged, and also the flow-on effects to other organisms, such as butterflies, eg. some of the rare ‘blues‘ losing their critical habitat. Discussion then turned to the impact of fire frequency and how ecologically damaging frequent fires can be. Michael estimated that in Aranda Bushland/Black Mountain the frequency was averaging about 8, or maybe 10 years, and this was less damaging than a 3-year frequency that had previously been envisaged. He also pointed out that a mosaic of patches with different times since last fire was desirable from an ecological perspective, but was often in conflict with management aims of ‘securing‘ large blocks. There was further discussion about recovery, and how this can be adversely affected if the post-burn weather is hot and dry.
John mentioned that asset protection has moved beyond housing and infrastructure, to include natural features such as sphagnum bogs and rare species. Also, the Friends of Aranda Bushland said there was a long-unburnt patch (at least since the 1960’s) that they wanted to have protected as a refence area, but hadn’t been entirely succeeding in doing so.
Later discussions took a step back, and turned to big questions about the rationale of the prescribed burn regime and its effectiveness in protecting the adjacent suburbs from wildfire.
The explicit goals of low intensity prescribed burns are to reduce fuel load and also fuel connectivity between the ground and canopy, in order to minimise wildfire intensity and to mitigate ‘crowning’. Success of such burns is determined as a measure of these explicit goals. However, how successful are these burns in relation to the obviously implicit goals of protecting life and assets? The answers are not immediately apparent. There is still so much that is unknown about fire behaviour and fire impacts. Research is on-going, and monitoring is so important. Michael has long-term monitoring plots in the Brindabellas, and this is a great example of what’s needed to provide deeper insights into the complexity of both wildfires and prescribe burns.
From my understanding, the simple assumption that the implicit goals naturally follow these explicit goals, is not necessarily the case. Things are much more complicated. Prescribed burns are often known by the misnomer, hazard reduction burns. They can reduce fuel, but they can also lead to an increase in fuel, depending on many variables. Immediately after a burn, there can be substantial leaf-drop, then within a couple of years, the growth of ‘fire weeds’ can start to be significant. After years of rising fuel loads, they eventually decrease and after a long-unburnt period, hazard ratings can be extremely low. So, depending on factors such as post-fire weather, vegetation type and time since burn at a site, prescribed burns can be hazard enhancement as well as hazard reduction operations.
Even more concerning to me, is an ‘elephant in the room’. The linkage between such explicit and implicit goals is more likely to be met when a wildfire has only moderate intensity, but paradoxically, this is when it has a better chance of being controlled anyway. Yet on ‘blowup’ days when wind is strong, humidity low and temperatures high, and it’s of paramount importance to meet the implicit goals, wildfires will invariably crown and not easily be controlled, if at all, no matter what the fuel load. This should pose questions about acceptable levels of risking ecological integrity when a prescribed burn is not an ecological burn or cultural burn. Hopefully, discussions and actions around trade-offs will become better-informed in the future.
What about actually meeting the implicit objectives? Research has shown that the most important variable for asset protection is the lack of vegetation in the immediate vicinity of the asset, especially in the fire direction. The significance decreases with distance, whereby the first 20 to 50 m in a buffer seems to be most crucial.
Irrespective of the wider debate about fire management, there was a great deal of interest in monitoring post-fire vegetation recovery over the coming months and years on the site of our walk. We noticed that already some ground covers were sprouting, such as honeydew, grasses were re shooting and there was a lot of moss on the ground. Clearly, conditions had been reasonably moist since the burn.
During the outing there were some nice bird highlights too. Clare pointed out a small flock of Varied Sitellas, which was considered an uncommon sighting here, and we heard several Olive-backed Orioles calling, which seemed fairly early for these largely latitudinal migrants.
Many thanks to Friends of Aranda Bushland for inviting FNAC to this thought-provoking event, and especially to Michael for sharing his much-appreciated expertise!
– John Stein
