Australia: The Land Where Time Began
Dry Rainforest - the Far North
The dry rainforests and low deciduous and semi-evergreen vine tickets of the far north of Australia occur in the form of diverse island-like patches, that act as fire-refugia, as a result of human land use, especially the use of fire.
Evidence can be seen, in areas of dark humid eroded soil, of the remains of megapode nests, made by the birds scraping the forest litter into mounds in which they incubate their eggs. This indicates that the area must have been covered by vine thickets and monsoon forests at some time in the not too distant past. Remnants of rainforest communities can also be seen among the boulders and rocky outcrops, as well as along the edges of streams and swamps, which, according to Meirer & Figgis (1985), can sometimes spread into the surrounding savanna at times when the fire frequency is reduced. In Arnhem Land fire refugia occur in the form of tall evergreen Allosyncarpia tenata forests. Also palms and cycads can often be seen the taller eucalypt sclerophyll forests, in places such as Melville Island and the Mitchell Plateau, places where water is more continuously available, as both rain and underground water, than in the drier interior savannas. Softwood remnants with boab (Adansonia gregorii) are sometimes present on boulder screes or outcrops, arid extremes of dry deciduous vine thickets. Northern cypress (Callitris intratropica) mixed with deciduous Terminalia species and the 'galip" nut tree (Canarium australasicum) or the Burdekin plum (Pleiogynium timorense) can be present on deep coastal sands. Other deciduous flowering species, such as the cotton tree (Cochlospermum fraseri) and the kurrajong (Brachychiton diversifolius) are found on shallow sandstone in such areas as the wetter extremes.
In the Yampi Sound area and protected areas of the islands of the northwest Kimberley area of Western Australia there are dense semi-evergreen and deciduous thickets on basalt outcrops. Kapok (Bombax ceiba) and cotton tree occur in dense deciduous vine thickets on the levees of some river systems in the Northern Territory, such as the Alligator, Victoria, Rapier and Daly River systems.
Gradations between monsoon forest and thicket can be seen in sandstone gorges of Katherine Gorge, some of these communities having close floristic ties with a similar vegetation type found in coastal eastern New Guinea.
Intact stands of monsoonal forest and low vine thicket are present on many foredunes in the Northern Territory. Groups of tea-tree (Melaleuca leucadendron) and pandanus palm swamps mixed with vine thickets are present near the mouth of the South Alligator River in the Gunn Point area. The scrub fowl (Megapodius freycinet) (dusky megapode or dusky scrub fowl) live in this forest, as do many large mammals.
In the savannas of northern Australia micro-refugia contain isolated plants of species normally found in thicket formations. Among the common genera are Buchanania, Brachychiton, Dolichandrone, Erythrina, Erythrophleum, Excoecaria, Gardenia, Gyrocarpus, Lysiphyllum, Pleiogynium and Terminalia.
Thicket remnants are sometimes found in open savanna, evergreen woody plants and dense sprawling shrubs of Atalaya, Carissa, Maytenus, and Owenia. There are fewer thickets at either end of their range in the east and west. There are climatic, soil and floristic differenced between Cape York Peninsula in the east and those of Melville Island in the west, though there are some species common to both locations. It has been suggested that east-west exchange may have been prevented by the outwash plains of the Gulf of Carpentaria, where there is open-fire savanna lands. It has also been suggested that narrow corridors for exchange may be provided by the fragmented band of thicket on the coastal dunes, which might allow the exchange of some plant and animal species from forests at either end of the range.
Along Princess Charlotte Bay and near Cooktown to the south, on Cape York Peninsula, there are extensive thickets growing on dunes, with some isolated pockets of semi-green vine thickets growing on deep sand, in more inland parts of the C ape, among what is otherwise normal eucalypt savanna. These thickets are unusual in that they accommodate fire. The low nutrient conditions of the soil means the fuel load for fire is low, leading to fires that burn at low intensity. It is the low intensity of the fires that is believed to allow the thickets to persist in spite of the presence of fire.
Along the banks of the major river systems of the Peninsula and far north, in areas that are less fire-prone, there are thickets and gallery forests. Very variable mixtures of rainforest species grow on the riverine alluvial soils. According to Meier & Figgis (1985), patches of dry rainforest are developing near Laura Station, along the levee system of the Normanby River. The species at this site include Acacia, Alphitonia, Barringtonia, Ficus, Gardenia, Mallotus, Melaleuca, Strychnos, Terminalia, and the palm Corypha elata.
Mixtures of open vegetation grow on shallow sandstone soils along the beds of creeks south of the township of Laura, made up of isolated dry rainforest species of cotton tree (Cochlospermum gillivraei), kurrajong (Brachychiton australis), quinine bush (Petalostigma astigma) and hard-leaved tea-tree (Melaleuca tamarascina and M. argentea), Kimberley heather (Calytrix exstipulata, and species of Acacia, Grevillea and Leptospermum. Cooktown orchid (Dendrobium bigibbum), the state emblem of Queensland, and the onion orchid (D. canaliculatum).
Author: M. H. Monroe Email: email@example.com
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