The understorey shrub layer is an important component of many vegetation communities across the Northern Territory. Unlike grassy savannas, many forest and woodland communities feature either an open or more closed understorey shrub layer, especially in the Top End. Sometimes the shrub layer is the dominant vegetation stratum, particularly in the arid zone. In general, the structure, density and composition of the shrub layer are largely determined by rainfall, soil type and management history.
The shrub layer contributes to overall plant diversity and is important to wildlife in numerous ways. Many animals depend on shrubs for shelter and shade, nest sites or food – including nectar, fruit, seed and sap. The foliage of shrubs (for example Wattles) support insect populations which are also food for many wildlife species. Emus and Arnhem Rock-rats feed on the fruits of many shrubs. Possums and gliders, including the vulnerable Common Brushtail Possum also take leaves, flowers, and nectar. Tall understorey shrubs are utilised as perches by birds such as flycatchers, fantails and robins, searching for food or prey items which can be airborne, amongst the shrubs or on the ground.
Shrub layers of different densities suit different species. Many birds seek protection from predators by sheltering amongst the shrub layer or conceal their nests amongst shrubs. Some fairy-wrens, thornbills and whistlers utilise dense shrub layers for foraging, shelter and nest sites. The threatened Thick-billed Grass-wren lives in saltbush shrublands and Yellow Chats may perch in shrubs, but foray from them to feed on extensive grassy flats. Many lizards also shelter beneath shrubs including the vulnerable Great Desert Skink.
Fire, weeds and pest animals are the key issues affecting the understorey shrub layer. Land management practices impact on shrub cover, particularly fire management. Depending on their frequency, intensity and season, fires can either promote or remove the shrub layer.
Some shrubs such as Palm Valley Myrtle and Pink Myrtle are killed by fire but germination of their seed may be promoted. This can cause an increase in shrub cover after fire, especially where grazing or drought has resulted in reduced competition from grass cover. However, fire may delay flowering and fruiting and many shrubs need several years between fires to ensure they produce adequate flower and fruit crops. If fires are too frequent, shrubs that rely on seed for their replacement are not able to replenish the seed bank and local populations can be eliminated.
Even shrubs that resprout after fire benefit from fire free periods to maintain vigour.
Outside urban areas in the Top End, Common Brush-tailed Possums and Black-footed Tree-rats are only
found in long unburnt areas that have a dense, shrubby understorey. Fires that are too frequent can open-
up, modify and degrade the understorey shrub layer and the habitat becomes unsuitable for these threatened mammals.
The shrub layer can be out-competed by infestations of woody weeds such as Parkinsonia or Prickly Acacia. These weeds form dense thickets and exclude native plants, to the detriment of native animals that use these areas for habitat and shelter.
Shrubs can be important sources of forage for grazing animals, especially in times of drought. However, overgrazing, particularly by sheep, camels, goats and rabbits can reduce shrub cover and prevent shrub regeneration. Mulga is especially sensitive to sheep grazing.
A well-managed fire regime that leaves patches of country unburnt for a few years will allow shrubs to flower and fruit, and continue to support dependent animals. Fruit-eating possums and gliders are most likely to be found in areas that have not been burnt for some years.
Controlling feral animals and stock movements can help to maintain a healthy shrub layer, as can weed control programs.
Some pastoralists see shrubs such as Turkey Bush and Wattles as a nuisance, competing with grasses and making mustering more difficult. These species can dominate after disturbance created by overgrazing or earthworks, forming a ‘derived’ or secondary shrubland. Even if shrub cover is reduced through use of fire or grazing, it is important that areas with shrubs are left in parts of the property.