She-oak (Allocasuarina verticillata
) is a small, drooping tree. It is the most drought-resistant tree in Tasmania. Because it reaches a lower maximum height and has a slower growth rate than eucalypts it usually only dominates native vegetation in places where eucalypts find it hard to grow. These are generally north-facing slopes with shallow and rocky soils in areas receiving less than 700 mm of rainfall. Therefore, she-oak woodland and forest is widespread in dry eastern Tasmania and on the eastern Bass Strait islands, most commonly near the coast.
She-oak can be found as the dominant tree in woodland and forest. However, it can also form a major understorey component of eucalypt forest. The recent dieback of white gum (Eucalyptus viminalis
) in the lowlands has extended the areas occupied by she-oak woodland to include areas where she-oaks previously formed the understorey. The presence of a dense she-oak understorey increases the rate of dieback amongst emergent eucalypts during drought.
She-oaks do not respond to fire and grazing in the same way as eucalypts. Frequent fires will eliminate she-oaks while allowing eucalypts to survive. She-oak seedlings are more palatable to stock than eucalypt seedlings so under light grazing regimes eucalypts may re-establish successfully but not she-oaks.
She-oak woodland and forest varies from an almost closed forest that has little else beneath the trees but needles, to a woodland in which umbrella-shaped trees are interspersed in a species-rich sward dominated by tussock grasses. She-oak woodland and forest is widespread in dry eastern Tasmania and on the eastern Bass Strait islands, most commonly near the coast. Little of this community has been cleared since European settlement. In fact, it may occupy a greater area now than it did under Aboriginal occupation because of changed fire regimes.
Where to see she-oak woodland and forest
A fine example of she-oak forest can be seen on the hill to the east of the Midlands Highway between the rest stop north of Oatlands and the ruin of Antill Ponds. Here, the emergent white gums have recently died leaving the she-oak to dominate. Excellent examples of she-oak woodland and forest can also be seen on the northern slopes of the Domain in Hobart and in Cataract Gorge in Launceston.
Biodiversity values of she-oak woodland and forest
She-oak woodland and forest is not a high conservation priority in Tasmania as it is well reserved, has suffered no reduction in area, and contains few threatened species.
Management issues in she-oak woodland and forest
She-oak is a preferred firewood as it burns slowly and intensely and produces little ash. She-oak forest is of little value for grazing as the tree litter suppresses the growth of grass beneath the canopy. However, she-oak woodlands are widely used as rough grazing country. Because they occur on some of the least productive sites for pasture, and often exist within paddocks that contain more productive pasture, it is common to see bare red ground between the trees in summer and signs of erosion. There is usually no regeneration of she-oak in these grazed woodlands and many of the trees are beginning to die.
She-oak woodlands are more valuable for nature conservation than she-oak forests, which are less rich in native species. The main conservation management goals with she-oak vegetation are to maintain the woodland structure and the health of the understorey.
In urban remnants there is also a problem with woody weed invasion, the most threatening being boneseed (Chrysanthemoides monilifera
), gorse (Ulex europaeus
), and briar rose (Rosa rubiginosa
Maintaining the health of the dominant layer of she-oak woodland and forest is relatively simple:
- Exclude stock until the regenerated shrubs and trees are out of the reach of stock, assuming that seed is held on the trees and that no young trees are already present. If there is no seed on the trees refer to the publication Revegetating Your Farm (PDF) for revegetation guidelines.
Maintaining or creating a woodland structure where she-oak is regenerating and the stand is thickening or has thickened is more complicated. There are several management regimes that can be used.
- Option one: thin trees followed by spelled grazing. This is appropriate for dense stands. Thinning will create a woodland structure and grazing will prevent she-oak regeneration.
- Option two: thin trees, followed by fires every 1-5 years. This is also appropriate for dense stands. Care is needed to ensure that the trees left after thinning are not killed by the first fire. This could happen if slash from the thinning is allowed to lie beneath the retained trees, or if the fire is lit in hot and windy conditions. After the first fire the fuel build-up will not generally be sufficient to kill the adult trees if the recommended fire frequency is used. However, frequent fires will prevent new trees establishing.
- Option three: increase the grazing pressure. This is appropriate for woodland stands that are thickening. Graze the bush to a level that maintains the understorey but removes the she-oak seedlings.
- Option four: burn once every 1-5 years. This is most appropriate for woodland stands that are thickening. This range of fire frequencies should prevent thickening of the she-oak canopy while maintaining a healthy tussock grass understorey. Using this fire regime in she-oak forest may succeed in creating a woodland structure if the fire behaviour in the first blaze allows some of the larger trees to survive. However, if fuel loads are high and conditions on the day are hot and windy all the adult trees are likely to be killed. In this situation a 1-5 year fire regime is likely to remove the species from the site.