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Monsoon Forests

As shown in the map below, while rainforests are not a dominant landscape in northern Australia, much of the far north has patches of rainforest — or 'monsoon' forest with extensive rainforest being found in north-east Queensland (from Russell-Smith & Stanton, 2002) . This section will focus on rainforests in the Northern Territory.

Rainforest Map

A good introduction to rainforests in the Top End of the Northern Territory is a short essay by Jeremy Russell-Smith in the book, Top End Native Plants, by John Brock (1988). The excerpt below is taken from that book.

“ The monsoon vine-forests and thickets of the Northern Territory occur typically as small, more or less discrete, and often widely separated patches distributed over a range extending from the islands to the north-west and north-east (11 degrees south) to around the latitude of Tennant Creek (18 degrees south). Within this range by far the greater proportion of patches, as well as the most structurally complex and floristically diverse vegetation, is confined to a narrow coastal/sub-coastal belt, becoming increasingly scattered and attenuated further inland. In no region however, does monsoon vine-forest constitute more than a very small proportion of total vegetation, save on smaller islands where vine-forest may comprise almost the whole vegetation cover. This pattern of vine-forest distribution corresponds broadly with the present day pattern of rainfall distribution. Patches are most concentrated in relatively high rainfall regions, becoming increasingly scattered and floristically less diverse along gradients of declining rainfall.

Vegetation

Monsoon vine-forests in the Top End of the N.T. occur in a variety of habitats, from situations favoured by year-round moisture availability, to sites where moisture is seasonally scarce. They also occur on a broad range of rock and soil types. As might be expected monsoon vine-forest vegetation is thus structurally very variable, ranging from tall evergreen forests over 30m in height, to deciduous thickets often only 2-3m in height on particularly harsh dry sites.

There are considerable differences in the species compositions of vine-forests associated with different habitat types. For example, vine-forests occurring at both wet and seasonally dry sites in scattered sandstone regions comprise many species not typically found in lowland or coastal situations. Nevertheless, over 20% of all N.T. Top End vine-forest species are known to occur in practically all types of vine-forest vegetation in the region.

The monsoon vine-forest vegetation of the N.T. Top End is described here in four habitat categories: 1. vine-forests occurring on sandstone substrates ; 2. vine-forests and thickets on rock outcrops, especially limestone and igneous rocks (e.g. basalt, granite); 3 . vine-forests of the lowland plains, especially patches fringing rivers and riverine floodplains; 4 . coastal vine-forests and thickets.

Sandstone vine-forests

“Extensive areas of exposed sandstone and other siliceous sedimentary rocks occur scattered throughout the Top End. Well known areas include the Arnhem Land escarpment and plateau, the Tolmer Range (included within Litchfield National Park), and many off-shore islands (e.g. Melville Is., the Wessels, and Groote Eylandt). Vine-forests in sandstone regions are essentially of two types, those associated with sites of year-round water availability and those occurring in seasonally dry situations.

Sandstone vine-forests associated with sites of perennial moisture availability occur typically along streamlines in protected gorges, at seepages on steep slopes or at the bases of cliffs and at headwater springs. Such patches vary in size from extensive streamline communities a number of kilometres long and 100m or so wide, to small spring forests much less than a hectare in extent. Good examples of these types may be seen in Litchfield National Park, also especially on Melville Is. In these situations the height of the canopy is often around 30m, dominated by evergreen trees.

Vine-forests on rock outcrops

Away from major sandstone regions vine-forest vegetation occurs on rock outcrops comprising a variety of rock types including igneous basalts and granites, metamorphics, sedimentary rocks, limestone, and lateritic ferricretes. In these situations soils are often non-existent and the mostly woody plants are rooted into or between exposed rock.

It requires little imagination to appreciate, therefore, that while moisture may be available for plants at depth in cracks and fissures in some rock formations (e.g. limestone), for many plants growing on rock outcrops moisture is likely to be a scarce commodity through the long hot dry season. This applies especially for vine-forest plants with shallow root systems, and seedlings.

Small patches of vine-forest, often much less than a hectare but occasionally up to almost a hundred hectares in extent, may be observed sprawling over rocky hills in most regions.

Good examples occur on limestone in the Katherine area, and on granite around Mt. Bundy on the Arnhem Highway.

The vegetation of rock outcrops is typically dense and thicket-like, with a low canopy often only a few metres in height. While vines are often abundant, especially with thorns the canopy comprises predominantly deciduous trees and shrubs.

Lowland vine-forests

In areas of relatively subdued topographic relief, such as occurs between Darwin and the Arnhem Land escarpment, vine-forests are associated with a number of landform types.

As with vine-forests in sandstone terrain, those in the lowlands are associated both with perennially moist sites as well as seasonally dry substrates.

Evergreen vine-forests are associated with rare perennial spring and seepage habitats, as well as along the margins of some perennial streams. Good examples in the Darwin region include Holmes Jungle, Berry Springs, Howard Springs and Black Jungle. In these situations soils are typically waterlogged with relatively deep organic loams overlaying grey clays. Under optimal conditions canopy height is seldom greater than 25m, the upper canopy dominated by evergreen trees.

Coastal vine-forests

Coastal vine-forests occur on a variety of mainly seasonally dry substrates, especially sandy or shelly beach ridges (e.g. parts of Casuarina Beach, Darwin), and lateritic landforms (e.g. East Point Reserve, Darwin). As with other types of vine-forest vegetation tolerant of seasonal dryness, coastal vine-forests show no particular requirement for deep soils as on substrates which offer no impediment to deep root development. The canopy of coastal vine-forests is typically less than 10m in height, although individual emergent trees, especially deciduous trees may attain heights of 16m or more.”

Rainforest history

A recent account of the history of rainforest across northern Australia, including the impact of fire, is given in an essay by Russell-Smith and Stanton (2002). In a concluding section of that essay, these authors make the following observations:

“Major contraction of rainforest across northern Australia occurred in the Late Tertiary [ie. especially over the last 10 million years] associated with developing rainfall seasonality and aridity. That trend has been amplified in the latter part of the Quaternary [ie. the last 2 million years], with mesic rainforest remnants in northeastern Queensland expanding in relatively wetter, warmer periods, and contracting at other times. Similar patterns, although on a much reduced scale, are likely to have operated elsewhere across northern Australia. While fire doubtless helped fashion regional savanna landscapes throughout the Quaternary, available data from northeast Queensland suggest that fire impact on rainforests was particularly prevalent just prior to the Holocene [ie. the last 10,000 years] under relatively low-rainfall, probably amplified seasonal moisture conditions. Burning by Aboriginal people possibly was involved in the replacement of araucarian [ie. hoop pine] rainforest with sclerophyll vegetation in the Late Pleistocene [ie. the whole period of the Quaternary, less the Holocene], and doubtless maintained sclerophyll vegetation and/or slowed rainforest expansion in the Holocene .