Sugar gum woodland in Wanilla Conservation Park, regenerating by epicormic regrowth, c. 3 years after the Wangary fire. Photo: Peter Lang.

In January 2005, the Wangary wild fire swept rapidly across southern Eyre Peninsula under conditions not dissimilar to those of the recent Kangaroo Island and Mount Lofty Ranges fires, and with reports of particularly intense and hot burns.

The Wangary fire burnt vegetation quadrats at nine different sites, which had been surveyed only the previous year as part of the Biological Survey of South Australia program. This provided an ideal opportunity to investigate post-fire recovery and changes to plant and animal species composition. The quadrats were re-surveyed using the same methods in 2007, three growing seasons after the fire. Findings were published by State Herbarium botanist Peter Lang and (the then) manager Peter Canty, together with Robert Brandle, in the following report:

P.J. Lang, P.D. Canty & R. Brandle, Biological impacts of the 2005 wildfire on southern Eyre Peninsula: monitoring post-fire recovery within three years using Biological Survey of South Australia sites. (12.7mb PDF)

Less than three years on, the vascular plant species richness had increased substantially from pre-fire levels in nearly all sites, with species losses outweighed by gains. The total species count for all sites rose by 43 (from 150) for indigenous species and by 19 for alien species (from 25). However, an index based on cover scores, showed a large disparity in responses of alien and indigenous species, with a post-fire jump of 136% for alien species compared to only 11% for indigenous species.

The report also documents and illustrates the regeneration modes observed — re-sprouting, seedlings or both (something that we plan to pursue in a future blog). Some sugar gums, for example, retained their major branches intact and had been able to regenerate quickly by epicormic growth. Some were killed in their upper parts and were re-sprouting basally, whilst others were completely destroyed and had to rely on seedling recruitment to regenerate.

Sugar gum open woodland with dense and diverse understorey, well recovered 15 years after Wangary fire, Wanilla Land Settlement Conservation Park. Photo: P. Lang.

Sugar gum (Eucalyptus cladocalyx) is a very distinctive eucalypt that is endemic to South Australia, with three isolated populations on Eyre Peninsula, Kangaroo Island and in the southern Flinders Ranges, now treated as different subspecies (see also the Flora of South Australia chapter on eucalypts; 33.8mb PDF). Recent DNA sequencing (both nuclear and chloroplast genes/markers) confirms that it has no close relatives. It is also ecologically significant, as a dominant tree for distinctive plant communities with varied and, often, species rich understories.

Concerns were raised about the impact of the Wangary fire on sugar gum plant communities, and these may be highly relevant for the recent Kangaroo Island fires, which also burnt large areas of sugar gum woodland. It seems that on certain soil types and where the fire was particularly intense, mass regeneration from seed occurred and that the highly successful adaptive response of this eucalypt may cause problems.

The following comments were made on page 39 of the report:

Sugar Gum forms a unique community both structurally and floristically that is of high conservation importance. It is valuable as plant and animal habitat, due in part to its structural characteristics in readily producing hollows, abundant fallen timber and, beneath its umbrella-like canopies, much open space which provides for a variety of diverse understorey types.

Depending on the severity and frequency, fire can have deleterious impacts by consuming a substantial amount of fallen timber and destroying hollows. In addition, where major seedling recruitment of Sugar Gum occurs, the structure of the resulting community will be changed substantially due to crowding and consequent overshadowing and nutrient/water competition. This effect has been observed for Sugar Gum regeneration in the Flinders Ranges over a 20 year period of following a severe wildfire in Mount Remarkable National Park. This fire led to the development of many dense stands of thin-stemmed trees, understorey suppression and a much reduced capacity for hollow formation. In both the Tucknott Scrub sites (KOP00501 and KOP00601), there was a dense and extensive establishment of seedlings from 10 cm to 2 m tall […]. Without intervention, it is expected that over the ensuing decades these will produce a similar crowded overstorey structure as observed at Mt Remarkable; indeed it is highly unlikely that natural thinning could produce a typical Sugar Gum community structure with well-spaced large trees in the lifetime of these stands. Failure to restore this structure will affect recovery of dependent wildlife species.

Mass recruitment of sugar gum seedlings (orange-coppery coloured foliage in mid-distance) amongst sparse existing trees, Tucknott Scrub Conservation Park, c. 3 years after the Wangary fire. (Foreground seedlings are golden wattle). Photo: P. Lang.

Recent observations on southern Eyre Peninsula, now 15 years on from the Wangary fire, show that those predictions are proving correct. In some places, particularly lowland areas with sandier soils and heathy vegetation, sugar gum plant communities have regenerated well and still retain their diverse open structure (see first two of above images). Elsewhere, however, in hilly areas such as in Charlton Gully, and the disturbed woodlands of Tucknott Scrub Conservation Park, the previous woodland structure with large well-spaced trees supporting diverse and species-rich understories has been lost. Instead there are now masses of closely crowded young erect trees resembling woodlots with understorey plants mostly eliminated by overshadowing and competition for nutrients and water.

Dense sugar gum regeneration near Tucknotts Scrub Conservation Park, 15 years after Wangary fire. Photo: K. Pobke.

Dense regeneration of golden wattle (Acacia pycnantha) has a similar effect, but only for a limited period due to its relatively short life span. It is a very different scenario though for sugar gums which can persist for centuries. While some natural thinning of sugar gum may still occur as the trees continue to age, we should not expect that they will return to the original structure as they reach maturity: it is well known in forestry practice that initial spacing affects resultant tree habit and size. As well as supressing understorey, the greater density of smaller and less spreading mature trees is likely to result in reduced and delayed hollow production, which is another concern since the availability of hollows can be a limiting factor for many wildlife species.

Observations of older post-fire sugar gum regeneration in Mount Remarkable National Park, and some massed post-grazing recruitment of red gum in the Mount Lofty Ranges, support the Eyre Peninsula observations that natural thinning is not going to return these formations to a more open woodland structure within the time frame of human life-spans at least, and probably much longer. This raises the issue of how the original open vegetation structure arose and was maintained, and whether active management involving selective thinning is now warranted as a conservation measure.

Dense sugar gum regenerated from seed 15 years after Wangary fires, showing deep leaf litter and lack of understorey, Tucknott Scrub Conservation Park. Photo: K. Pobke.

Prepared by State Herbarium botanist Peter Lang

(acknowledging helpful discussions with DEW ecologists Jason Van Weenen and Kat Pobke).