A good introduction to the flora of the sandstone
country is provided in an essay by Dunlop & Webb (1991). The
following account is adapted from that essay.
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Sandstone habitat can be found in massive
plateaux or in isolated outcrops like this one near Mt Bundey,
NT
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Much of the sandstone flora of northern Australia originated in
the eastern part of the ancient conglomerate landmass,
Gondwanaland, now represented by the Australian continent and
close-by islands. These plants are mainly endemic to Australia or
the Australasian region. Well known examples include the eucalypts,
tea-trees, banksias, grevilleas and boronias. The features of these
plants include the lack of deciduous species and development of
'scleromorphy' in the form of hard, stiff, tough leaves which are
thought to have developed because there are few nutrients in the
soils.
Many of the sandstone plateaus in Arnhem Land are rugged and
access is difficult. The highest, most dissected and best watered
part of the plateau is in the north-west part of Arnhem Land.
Kakadu National Park lies at the western edge of this. This area
has the greatest diversity of habitats and richest flora. Over the
past 30 years helicopter surveys have surfaced many new species and
increased knowledge as to the distribution of plants. These surveys
have provided links between the Arnhem Land plateau and other
sandstone regions of north Australia and areas in the south of the
continent.
There are various habitats throughout the sandstone, one being
the bare stone pavements which hold water for a short time after
rain and do not accumulate soil. Such habitats are where a very
tough native grass species (Micraira) can be found. This
plant is exceptional in that it is capable of 'resurrecting' its
leaves soon after rain.
Resurrecting means the plants leaves can dry out and lose their
chlorophyll, yet remain attached to the plant where they will green
up and become functional again within 24 hours of soaking rain. In
a region where the wet season may have varying periods of drought,
the ability to switch on or off according to moisture availability
is an extraordinary advantage and critical for survival.
Other plant habitats of the sandstone areas include rock faces,
sheltered ravines and sandy soils which have formed from the
sandstone rocks. These areas also support many plants which are
capable of coping with drought; drought-resistance is achieved
through features like having expanded stems instead of leaves, or
having reduced, narrow leaves. Such adaptations enable the plants
to resist drying out in the harsh environment of the sandstone
country.
One very significant plant that dominates much of the rainforest
of the broken escarpment of Kakadu and the north Western-Arnhem
Land plateau is the endemic tree, Allosyncarpia ternata ,
These trees provide an evergreen sheltered canopy for many plants
and insects, and which birds, mammals and reptiles regularly visit
to feed, forage, nest or shelter.
Fire-sensitive species
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Sandstone country can offer protection for
fire-sensitive species - but when fire does spread to these
habitats it can have a severe impact on plants like obligate
seeders.
J. Russell-Smith
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A further feature of the sandstone country is that many
component shrub and tree species are known to be particularly
fire-sensitive. Unlike the great majority of woody species growing
in the savannas which can resprout from stem bases or roots when
their aerial parts have been killed by fire, many woody species in
sandstone habitats can only regenerate from seed. Such species are
called obligate seeders, and include well-known species such as the
Cypress pine (Callitris intratropica), and some acacias,
grevilleas and various other shrubs which grow in heathy (shrubby)
habitats. Obligate seeders are particularly susceptible to frequent
fires given the developmental time required from post-fire
germination to the attainment of reproductive maturity and the
replenishment of seed stocks.
Such seed stocks are called seed banks, and in northern
Australia these seed banks mostly comprise dormant seeds stored in
the soil. Available information indicates that as much as 50% of
Arnhem Plateau heath shrub species are obligate-seeders and, while
the majority of these are likely to attain reproductive maturity
within three years after a fire event, others require fire-free
intervals of at least five years, particularly those occupying
harsher, rocky sites.
More information is given in two papers by Russell-Smith et
al . (1998, 2002).
