Invasion, succession, and modification of vegetation
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| 5A. Pristine riparian rainforest |
The biogeographical processes of invasion, succession and modification of vegetation
can come about in two ways: they can occur naturally and they can
be brought about by humans. Invasion refers to the process of one
type of plant species coming into an area and pushing other species
out. This tends to occur in Australia when weed species such as
lantana start to grow in our native forests. The lantana is able
to smother other species and become dominant.
Succession refers to the process where one type of plant species
over time is replaced by another. It might be that, over time,
climatic conditions change and a better adapted plant species
begins to grow in an area. Over time this species becomes dominant,
gradually filling the place of the former species as it becomes
less abundant. In other cases one plant species is removed by
humans and another grows (or is cultivated) to replace it.
Modification of vegetation is what occurs when humans impact
on the vegetation. It could be that the modification takes place
by invasion or succession but it is human induced, by humans.
In the Minnamurra Rainforest area the change in vegetation prior
to European arrival is difficult to assess, with even the information
we have about post-European arrival also somewhat sketchy.
Aboriginal use of the rainforest is largely deduced from educated
guesses based on the small number of archeological sites found
in the Illawarra area. It is not known whether Aboriginal people
used the rainforests as camp sites even though it is known that
Aboriginal people in the area did make use of the rich resources
the rainforest offered. It is thought that the damp and dense
nature of the undergrowth in the rainforest would make hunting
difficult within the forest and, more likely than not, Aboriginal
people would have hunted along the edges of the forest and along
the rivers, where the greater sunlight would have given rise to
more fleshy fruit and made hunting animals, such as the swamp
wallaby, possible.
It is well known that Aboriginal people from early times were
able to affect the distribution of rainforest on the Australian
continent through the use of fire. Based on this evidence it is
thought that this use of fire may have had some effect on the
Illawarra, but to what degree is really unknown. Perhaps the dense
parts of the forest were avoided by Aboriginal people, so this
may have given some of the rainforest protection from the effects
of fire.
Europeans, unlike Aboriginal people, were not so careful with
the rainforest's vegetation and rather than possibly modifying
small areas, they have modified large areas, so that the rainforest
now barely exists in these areas. This has given rise to the processes
of invasion and succession, where areas of rainforest have been
completely replaced with other vegetation such as lantana (invasion)
and European grasses (succession).
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| 5B. Outdoor classroom |
When the Illawarra, of which Minnamurra Rainforest is a part, was first settled,
very few records were kept. As a result, what we know about the
area is general to the whole district, and even then the information
is far from complete. We know that European settlement first occurred
in the area around 1815.
Between 1815 and 1822 the impact of European settlement was already
felt on an important part of the subtropical rainforest, with
red cedar trees (the tallest and the widest in buttress roots),
being removed in a wholesale fashion from the forest. This would
have led to an opening of sunlight in the forest, that would allow
invasion or succession of other species, beginning the modification
of the rainforest forever. The government of the time did recognise
that the removal of the resource was a problem; in 1819 it tried
to stop the removal of the wood through prosecution and in 1826
tried to regulate with permits the continued cutting down of the
trees .
By 1850, most of the cedar had been removed and the timber resource
destroyed. Although red cedar can be seen commonly in the area
today, none of the trees match the huge specimens described early
on.
The next period of modification of the vegetation of the rainforest
was dominated by the development of farming, which involved the
removal of much of the natural vegetation and its succession by
European grasses suitable to feed stock. This was exacerbated
for the subtropical rainforests in the area because they tended
to grow on the richer volcanic soils which are also good for farming.
The existence of Minnamurra rainforest today, with its core of
forest that has never been disturbed, is partly due to the rugged
nature of the slopes which had little attraction for farming.
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| 5C. Lantana growing next to
river |
On the outskirts of Minnamurra, cabbage tree palms can still be
seen in the middle of paddocks along the road into the National
Park. These palms were considered to be attractive along with the
large figs, hence they were left standing on their own, giving us
some information about the extent that the rainforest once had.
The bangalow palm was also left, but these trees were not able to
survive in the open and have died off.
From the time Europeans first entered the area around Minnamurra,
about 75% of the rainforest has been removed from the Illawarra,
being replaced or succeeded by pasture, invaded by weed species
such as lantana and modified significantly, so that the nature
of the rainforest has been affected. Today these changes continue,
although it is hoped that, with management, negative change can
be stopped and positive change assisted.
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