Eucalypt Open Woodlands and Tropical Savannas


Map 1 Distribution of eucalypt
woodlands and open-woodlands in the
Northern Territory (Wilson et al. 1990)

Most of the northern half of the Northern Territory is cloaked in a woodland of eucalypt trees, with an understorey of perennial and annual grasses. This grassy woodland is often referred to as "tropical savanna", and tropical savannas cover much of northern Queensland and the Kimberley, as well as the Top End. In fact, while the component species may differ, very similar savanna vegetation is found throughout the tropical regions of South America, Africa and Asia. Most of the other major environments of the northern NT (such as monsoon rainforest, riparian areas, floodplains and sandstone ranges) represent small areas of contrasting habitat that are embedded in the vast matrix of the savanna.

This page describes environments where the tree layer is sufficiently sparse to be considered a "woodland" (less than 30% foliage cover) or "open-woodland" (less than 10% cover). It is only near the higher-rainfall coastal margins of the Top End that the tree canopy is sufficiently dense and tall to be regarded as an "open-forest". These forests support a sufficiently distinctive wildlife to be considered a separate major environment (Open forests of the Top End).

A wide variety of eucalypt species may be the dominant species in tropical savannas, and the composition of the grassy layer is similarly variable. As a result, a large number of vegetation communities have been described - the NT vegetation map (Wilson et al. 1990) has 38 map units that are eucalypt woodland or open woodland. The composition and structure of the savanna is largely determined by the moisture and nutrients available to the plants, and this depends on rainfall, topography and properties of the soil.

Thus, different plant communities are generally found on rocky, sandy, loam or clay soils. The very pronounced north-south gradient of annual rainfall found in the Northern Territory is reflected in the structure of the savannas, with trees gradually becoming shorter and more widely spaced as you travel inland from the northern Top End. However, much of this variation is not obvious to visitors to the Northern Territory, who perceive a monotony of bushland separating scattered sites of scenic interest.

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The most frequent dominant tree species of the tropical savannas are the Stringybark Eucalyptus tetrodonta and the Darwin Woolly Butt E. miniata. These occur on a wide variety of soil types, with Stringybark becoming more dominant on well-drained, sandier soils. Some species, such as Silver Leaf Box E. pruinosa and Bloodwood E. terminalis are characteristic of extensive undulating plains in the south of the savanna zone. Others are largely restricted to skeletal soils on rocky hills and rises, such as Snappy Gums E. brevifolia & E. leucophloia (in the drier areas) or Salmon Gum E. tintinanns & Scarlet Gum E. phoenecia in the Top End.

Relatively tall woodlands of Ghost Gum E. papuana and Swamp Bloodwood E. polycarpa are often found on alluvial plains and adjacent to drainage lines, while Coolibah E. microtheca is the characteristic species of open woodlands in heavy clay soils in the southern savanna zone. Other species that are dominant over extensive areas include Northern Box E. tectifica, Variable-barked Bloodwood E. capricornia and Weeping Box E. patellaris. Non-eucalypt trees may also form a significant part of the canopy in some communities, most notably the Ironwood Erythrophleum chlorostachys, which occurs widely through the tropical savannas.

Variation in the composition of the grass layer is not necessarily closely associated with changes in tree composition in these woodlands. While many species occur within any community, the grass layer can be most simply characterised as dominated by annual Sorghum species; by perennial tussock grasses (such as perennial Sorghum, Chrysopogon, Heteropogon, Aristida or Themeda); or by hummock grasses (particularly Curly Spinifex Triodia bitextura). A Triodia understorey is typically found in woodlands on stony hills and sandstone ranges, while annual Sorghum is characteristic of woodlands on lateritic soils in the northern Top End.

The eucalypt woodlands support a great diversity of wildlife. In part, this reflects their enormous extent, as small patches of woodland may not contain as many species as comparable areas of distinctive habitats such as sandstone ranges, riparian areas or monsoon rainforest. Much of the wildlife of the woodlands occurs widely throughout the savannas of northern Australia, the species composition changing only very gradually with geographic separation and variation in annual rainfall. Other species are more restricted, occurring only, for example, in rocky hills or in association with the sandier soils.


Photo 1 Eucalypt woodland on stony hills
in Stage III of Kakadu National Park

The most visible component of the woodland fauna is the birdlife, which includes many species of honeyeater, finch, pigeon and parrot. In the Top End, characteristic species include the Brown Honeyeater Lichmera indistincta & White-throated Honeyeater Melithreptus albogularis; Double-barred Finch Taeniopygia bichenovii & Long-tailed Finch Poephila acuticauda; Peaceful Dove Geopelia stricta & Bar-shouldered Dove Geopelia humeralis; Red-winged Parrot Aprosmictus erythropterus & Rainbow Lorikeet Trichoglossus haemotodus. In the more southern open-woodlands, many of the common species are also found in the arid zone, such as the Grey-headed Honeyeater Lichenostomus keartlandi & Singing Honeyeater L. virescens; Zebra Finch Taeniopygia guttata; Diamond Dove Geopelia cuneata & Crested Pigeon Ocyphaps lophotes; and Galah Cacatua roseicapilla & Cockatiel Nymphicus hollandicus. Other frequently seen (or heard) woodland birds include the Black Kite Milvus migrans, Whistling Kite Haliastur sphenurus, Brown Falcon Falco berigora, Red-tailed Black-Cockatoo Calyptorhynchus banksii, Pheasant Coucal Centropus phasianinus, Southern Boobook Ninox novaeseelandiae, Red-backed Kingfisher Todiramphus pyrrhopygia, Red-backed Fairy-wren Malurus melanocephalus, Striated Pardalote Pardalotus striatus, Weebill Smicrornis brevirostris, Friarbirds Philemon spp., Yellow-throated Miner Manorina flavigula, Grey-crowned Babbler Pomatostomus temporalis, Varied Sittella Daphoenositta chrysoptera, Crested Bellbird Oreoica gutturalis, Cuckoo-shrikes Coracina spp. Woodswallows Artamus spp., Pied Butcherbird Cracticus nigrogularis and Torresian Crow Corvus orru.

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The mammal fauna in the woodlands is largely nocturnal and inconspicuous, apart from macropods such as the large Antilopine Wallaroo Macropus antilopinus and smaller Northern Nailtail Wallaby Onychogalea unguifera. Some rodents, such as the Western Chestnut Mouse Pseudomys nanus have a wide distribution; while others such as the Pebblemound Mice P. calabyi and P. laborifex have much more restricted distributions. Some species, such as the Northern Quoll Dasyurus hallucatus are now absent from the drier woodlands, although they remain patchily common in the Top End. Arboreal mammals are generally sparse or absent, although many species of small, insectivorous bats occur in the woodlands.

The most frequently seen reptiles in the woodlands are a variety of small (Carlia spp.) and medium sized (Ctenotus spp.) skinks that are active during the day, along with small agamids (Diporiphora spp.) and a number of goanna species (Varanus spp.). A variety of snake species are present, but are largely nocturnal and are most often seen while driving at night (such as the spectacular Black-headed Python Aspidites melanocephala). The night-time woodlands are also the domain of many gecko species (Gehyra, Diplodactylus and Oedura spp.). Away from the creeklines and waterholes, the woodlands appear an inhospitable environment for frogs during the long Dry season. However, in some areas several species of burrowing frog (Cyclorana spp.) occur, emerging to breed as soon as temporary pools form. The calls of the Green Tree Frog Litoria caerulea may also be heard echoing from tree hollows throughout the year.


Photo 2 Eucalypt woodland on the Sturt Plateau
(south of Katherine), with a dense perennial grass understorey)

Information about the invertebrate fauna of the eucalypt woodlands remains sparse. termites are a conspicuous component, hollowing the trunks and branches of many trees and consuming a large amount of the annual production of grass and litter. Ants are also a diverse and important part of the fauna, with up to 85 species recorded from a single hectare of woodland.

Although the eucalypt woodlands may appear monotonous, an important part of their ecology is related to variability - both the patchy distribution of different vegetation communities and the marked variation through time. The latter includes the relatively predictable annual variation resulting from a highly seasonal climate, with the wet season witnessing the sprouting of the annual Sorghum, the brief appearance of many annual herbs, and an abundance of fruits and insects - with many species breeding at this time to take advantage of this flush of resources. But there is also a less predictable variation associated with the timing and amount of the annual rains; and the timing and extent of fires, that occur patchily throughout the woodlands each year.

Many animal species, particularly birds, are sufficiently mobile to track the patchily-available resources in the savanna through space and time. For example, there are substantial annual movements of honeyeaters across the Top End, as different tree species flower at different times of year. Fire has a significant role in many aspects of the ecology of this environment, influencing the structure of the tree layer, the composition of the grass layer and the distribution of many plant and animal species.

There is a very long history of fire in this environment - lit by lightning strikes and by Aboriginal people - and most species are adapted to periodic burning. However, European settlement and changes in land management practices has substantially altered the pattern of fire in much of the tropical savanna. The eucalypt woodlands and open-woodlands are relatively well-represented in Northern Territory conservation reserves, with large areas in parks such as Kakadu, Litchfield and Gregory. However, some of the vegetation communities making up this environment remain poorly represented in reserves. Most of the eucalypt woodlands are under pastoral or Aboriginal land management. While their vast extent gives the impression of an healthy, intact landscape, this is not necessarily the case.

Subtle but significant perturbation of the ecology of this environment is witnessed by the landscape-scale decline of granivorous birds (such as the Gouldian Finch Erythrura gouldiae and Partridge Pigeon Geophaps smithii) and some mammals (such as the Northern Quoll and Brush-tailed Phascogale Phascogale tapoatafa). The major threatening processes in this environment are probably changes in the fire regime, feral carnivores and the impacts of cattle grazing. More localised threats are associated with the spread of weeds, land clearing for agricultural development, and mining.

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Photo 3 Corymbia capricornia open-woodland
with a hummock grass (Triodia) understorey

References