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.
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 .
“
