Savannah woodlands and open forests are the most widespread vegetation types within the wet-dry tropics of northern Australia (Dunlop and Webb, 1991) and are dominated by Eucalyptus species with a tall grass understorey. Dense Eucalyptus forest occurs along the northern and eastern coastal regions of Australia and tree density reduces to woodland and then to scattered trees as average annual rainfall decreases along southern and western gradients.
Rainforest occurs in some areas along the east coast of Australia, and in numerous small patches through the northernmost portion of the Northern Territory (NT) and Western Australia (WA). In the northern region of the NT and the Kimberley region of WA, there are 16,500 rainforest patches, which average <5 ha in area (Russell-Smith et al., 1992). These include moist rainforests of evergreen species and seasonally dry rainforests dominated by deciduous or semi-deciduous species. Many tree species in these rainforests have bat- or bird-dispersed seeds and are likely to have expanded into the NT from South-east Asia or north Queensland (Russell-Smith and Lee, 1992).
Rainfall is summer-dominated in northern Australia (Fig. 43.1). Darwin has an annual rainfall of 1700 mm, with >90% falling between November and April. On the north-east coast, Cairns also has a strongly summer-dominated rainfall pattern, but some rain still tends to occur during the winter months. The rainfall decreases to the south of Darwin and to the west of Cairns. Tennant Creek has a shorter summer wet season and a much lower annual rainfall, averaging 371 mm. Temperatures show only a slight decrease during the winter months at Darwin, but there is substantial variation between higher summer and lower winter temperatures further inland towards Katherine.
Fire is frequent in the wet-dry tropical areas of northern Australia and many areas are burnt annually. Most fire events are human-lit, either by pastoralists, fire managers or aboriginal traditional landowners. Fire intensity is low during the early dry season, but the high-intensity fires during the middle to late dry season can affect the seed production of trees (Setterfield, 1997). Fire in the late wet season and early dry season can destroy seeds of grasses and herbaceous plants before they are mature and incorporated in the soil seed bank.
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