Landscapes are often thought of and described in terms of their tree layer, however it is usually the understorey that supports the widest range of wildlife. Grasses and herbs comprise most of the plant diversity in the majority of terrestrial communities across northern Australia. This is particularly the case in the tropical savannas. Some studies estimate that up to 90% of biodiversity is found in the understorey.
The composition of the understorey depends on the climate, soil and management history. The understorey is typically made up of a mid layer dominated by shrubs; a ground layer of grasses and herbs; and logs and litter. The understoreys of woodlands in the Top End may have a well-developed shrub layer or be dominated by grasses with few or no shrubs at all. In contrast the arid zone features many vegetation communities dominated by shrubs or grasses or both, with few trees.
The shrub layer contributes to overall plant diversity, structural complexity and provides habitat resources such as shelter; protection from predators; food sources including nectar, fruits and insects; and nesting, roosting and perching sites for birds and other wildlife.
The ground layer provides critical habitat for a wide range of fauna, including many that are in decline across northern Australia. Such habitat components include grass seed and native herbs which are important and highly nutritious aspects of the diet of many granivorous and herbivorous animals.
Fallen logs and leaf litter are also an important component of the understorey. They help retain water and nutrients in the landscape and also provide food, shelter and nesting sites for many animals including insects, reptiles and small mammals.
There are significant pressures on the understorey including grazing from stock, pest animals and native herbivores; weed incursion; and altered fire regimes.
Grazing practices can heavily impact all layers of the understorey. The shrub layer can be reduced if overgrazed, particularly by sheep, camels, goats and rabbits. In the ground layer, overgrazing and trampling remove palatable species which creates space for weed incursion. The litter layer can be destroyed due to trampling by stock and pest animals, particularly under the shade of trees. This results in large areas of bare ground unsuitable for wildlife.
Weeds represent one of the greatest threats to understorey biodiversity. They replace native plants and reduce habitat for animals and can also increase the effects of fire. A number of woody weeds, such as Parkinsonia and Prickly Acacia can out-compete the shrub layer resulting in dense thickets. Such thickets reduce native plant diversity which also reduces habitat and shelter for wildlife. Some introduced pasture species can dominate the ground layer, at the expense of native species. These weeds can change the structure of the ground layer as well as the species composition. For example, Gamba Grass infestations leave little or no spaces between grass clumps. These spaces are important to many animals such as Gouldian Finch and Partridge Pigeon, which feed on seeds that have fallen in them.
Introduced grasses, such as Gamba, Mission, Guinea and Buffel Grass can also increase fuel loads which generate fire intensities that can kill some trees and other native plants and animals. Such intense fires also completely destroy fallen logs and leaf litter which reduces habitat for threatened species such as Yellow-snouted Gecko.
Frequent fires can reduce the shrub layer with negative impacts on the animals that depend on it for food and shelter. This happens if fires are too frequent and do not allow the soil seed bank to replenish. Even shrubs that resprout after fire can decline under frequent fires. Annual burning can also reduce ground cover plant diversity and prevent the accumulation of a well developed leaf litter layer.
Adverse changes to the understorey may result in a reduction in shelter, food resources and nesting sites. A change in shelter availability has occurred in some areas, including a reduction in tall, dense grass clumps of grass, perhaps as a result of altered fire management and high grazing pressures from feral animals, stock and native herbivores.
Management of the understorey requires skilful grazing, feral animal, weed and fire programs.
Spelling pasture from grazing after the wet season in the Top End or after good rainfall in the arid zone will allow plants of the understorey to regenerate. Control of feral animals will also reduce grazing pressure on native plants.
Avoid introducing pasture plants that out-compete native vegetation or increase fire intensity. Introduced species that have been highlighted for their negative impact on wildlife include Para Grass, Buffel Grass and Gamba Grass. Weeds that are already present should be identified and eradicated or controlled if possible.
Implementation of a fire regime to manage the understorey requires careful planning. Progressively ignited burns, with multiple ignition dates within the same general area, can increase the availability of grass and herb seed, which is a critical food source for many birds and small mammals. The result of progressive burning is typically better than burning a section of woodland all at once in the mid or late dry season, which can produce extensively burnt areas and a temporary drought of grass seed. Burning large areas in the late dry season is more likely to result in widespread loss of logs and leaf litter in the understorey. Fires at this time also remove grass cover which can cause erosion.
However, occasional more intense fires at the immediate start to the wet season can enhance recruitment of native legumes and some grasses, and benefit weed control efforts. Shrubs and other plants that rely on seed for replacement may benefit from remaining unburnt for a few years to allow the soil seed bank to build up.
The composition and intactness of the understorey is a significant contributor to overall land condition. Improvements in land condition across rangeland landscapes are likely to have positive biodiversity consequences.