Territory Environments
Tussock Grasslands
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As the name suggests, tussock grasslands have no, or only a very sparse, tree and shrub layer, and a ground layer dominated by tussock-forming grasses (as opposed to the "hummock" or "spinifex" grasslands dominated by Triodia spp.). The tussock grasses are generally robust perennials, such as Mitchell grass Astrebla spp. and Bluegrass Dicanthium spp., although grasslands dominated by small short-lived perennials (such as Nine Awn Enneapogon spp.) or annual species also occur.
Tussock grasslands occupy c. 85000 km2 (6.3%) of the Northern Territory. A further 8000km2 of mixed grass-herb-sedgeland occurs on the northern floodplains, which are described on another page. Most of the tussock grasslands occur on heavy clay soils in the semi-arid region of the Northern Territory, although areas of grassland are also found on the coastal margins, in the wetter parts of the Victoria River District, and patchily on clay soils throughout the arid zone. Wilson et al. (1990) mapped 9 tussock grassland types in the Northern Territory, although many of these types intergrade. Depending on local variation in topography, soils and moisture regimes, tussock grasslands on heavy clay soils may merge into open woodlands (eg. dominated by Coolibah E. microtheca or Nutwood Terminalia arostrata) or Bluebush Chenopodium auricomum shrublands in drainage depressions.
The most extensive tussock grasslands are the Mitchell grasslands, dominated by Astrebla species, which occur on cracking-clay plains in the Barkly Tableland and Victoria River regions, and patchily throughout the arid zone. In areas of the Victoria River, Keep River and Gulf regions with higher annual rainfall, Mitchell grasses are replaced on heavy clay soils by grasslands dominated by Bluegrass (Dichanthium spp.), Golden Beard Grass (Chrysopogon fallax) and Sorghum (Sorghum timorense, S. plumosum). Elevated gravelly clay soils within the cracking-clay plains may support short grasslands dominated by Enneapogon and Sporobolus spp. Similar short grasslands also occur on skeletal loam soils in the Victoria River region. On some heavy clay plains in the arid parts of the Northern Territory, grasslands dominated by Never Fail Eragrostis spp. occur, intermixing with chenopod shrublands or Mitchell grasslands. Sparse grasslands dominated by annual species also occur on desert loam soils on the southern margin of the Barkly Tableland.
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In addition to the large northern floodplains, tussock grassland occur patchily around the Northern Territory coast, on dunes, coastal plains and on the fringes of salt flats and mangrove communities. Distinctive coastal grasslands include those dominated by Vetiveria elongata, Xerochloa spp. or Sporobolus virginicus.
The composition of many of the tussock grasslands communities is dynamic, reflecting the influence of rainfall, grazing and fire history. A series of wet or dry years may result in substantial changes to the relative frequency of the dominant species with, for example, Bluegrass replacing Mitchell Grass as the dominant species after a series of wet years. Perennial tussock grasslands usually have a diverse component of annual grasses and herbs which grow between the tussocks, but which is only evident following good rainfall. In some areas, prolonged heavy grazing has lead to a reduction in the density of perennial grasses and a dominance by annual species, such as Brachyachne convergens, Flinders Grass Iseilema spp. or weedy herbs. The role of fire in the ecology of the tussock grasslands is generally poorly understood, but in most areas fire has been suppressed since the advent of pastoral use. One consequence of this may be the increasing density of woody shrubs and trees observed in many areas of tussock grassland in the north of the NT, such as in the Victoria River region.
Tussock grasslands may show a moderate richness of plant species, due to the diversity of annual herbs and grasses that appear following rain. Many of the plant species that are common in the extensive grasslands on cracking-clay soils are also largely restricted to that environment. The diversity of vertebrate fauna in the grasslands is generally low, which can largely be attributed to low habitat complexity and relative homogeneity of the environment, although there is also a distinctive grassland fauna. The cracking-clay grasslands are notable for a number of endemic reptile species, such as the large goanna Varanus spenceri and the brown snakes Pseudonaja ingrami and P. guttata. Common bird species of the grasslands include the Singing Bushlark Mirafra javanica and Button-quails Turnix spp., but the iconic species of the grasslands is the Flock Bronzewing Phaps histrionica, a large pigeon that may still occasionally be seen on the Barkly Tablelands in aggregations of thousands of birds. The tussock grasslands also support a high diversity of raptors, including the rare Letter-winged Kite Elanus scriptus and Grass Owl Tyto capensis. These raptors increase greatly in abundance during plagues of the Long-haired Rat Rattus villosissimus, prompted by a flush of food resources after very good seasons. Other common mammals of the clay grasslands, such as the tiny Long-tailed Planigale Planigale ingrami are less conspicuous, being adapted to foraging in the broad cracks that form in the soil during the dry season. Red Kangaroo Macropus rufus are commonly seen in the tussock grasslands in the more arid part of the Northern Territory, while Nail-tailed Wallaby Onychogalea unguifera is common on the ecotone between grassland and woodland in the more humid areas.
Tussock grassland are one of the major environments of the Northern Territory that are most poorly represented in the reserve system, with only 0.3% of their total area within National Parks. This reflects both the low scenic and recreational value of this environment and the high value to the pastoral industry. The most extensive tussock grasslands, such as Mitchell grassland, are amongst the economically most important rangelands in the Northern Territory and are given over almost entirely to pastoral use. The development of a network of artificial waterpoints (dams and bores) throughout the grasslands means that virtually their entire extent is subject to grazing by stock.
The most pervasive threat to the biodiversity of the tussock grasslands arises from the dominance of pastoral use. Although these grasslands are generally resilient to the effects of grazing, studies have identified a proportion of the native plant and animal species that decline in abundance under heavy grazing. Such "decreaser" species may be threatened by a further intensification of pastoral use in their habitats. Species such as the Flock Bronzewing are known to have undergone historical declines over much of their range. Other conservation issues include the spread of noxious weeds (such as Parkinsonia and Prickly Acacia), the increasing density of native trees and shrubs in some areas ("woody thickening") and the reintroduction of appropriate fire regimes.
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References
- Perry RA & Christian CS (1954). Vegetation of the Barkly Region. In: Survey of the Barkly Region 1947-48. Land Research Series No. 3, CSIRO, Melbourne, pp. 78-108.
- Perry RA & Lazarides M (1962). Vegetation of the Alice Springs Area. In: Lands of the Alice Springs Area, Northern Territory 1956-57. Land Research Series No. 6, CSIRO, Melbourne, pp. 78-108.
- Perry RA (1970). Vegetation of the Ord-Victoria Area. In: Lands of the Ord-Victoria Area, WA & NT. Land Research Series No. 28, CSIRO, Canberra, pp. 104-125.
- Wilson BA, Brocklehurst PS, Clark MJ & Dickinson KJM (1990). Vegetation survey of the Northern Territory, Australia. Technical Report No. 49, Conservation Commission of the Northern Territory, Darwin.
