|
Vegetation
History
Of
The Atherton Tableland
Over the last 100 years forestry, agriculture, waterworks and urban development have
replaced or modified large areas of the forest, woodland and savanna which formerly
covered the Atherton Tablelands. Even so, we are able to reconstruct the vegetation
pattern seen by the earliest European settlers. We can do this by studying the surviving
vegetation, whether it be in large tracts or small patches, and by examining historical
records.
Rainforest and dry sclerophyll woodland are the two most important natural vegetation
types. Rainforest has tall trees which grow close together over an undergrowth of smaller
trees and climbers. The leaves of rainforest plants are of various sizes, some being very
large, and most are soft. Only a few of the many different kinds of tree shed all their
leaves once a year. One example is the Australian cedar (Toona australis). Rainforest has
many different kinds of animals as well as plants.
Dry sclerophyll woodland is very different. Its trees are smaller and less
closely packed,
and the undergrowth is a more diffuse assemblage of low shrubs and grasses. A relatively
small number of species occur (mainly eucalypts and their relatives), and leaves of most
of the woody plants are rigid and tough.
Other types of natural vegetation are found in the region, notably the taller, denser,
sclerophyll forest which inter-fingers with the rainforest. In addition, a great deal of
local variation occurs within these main sorts of vegetation. Human interference, such as
the logging of forest or clearing land for farms, may encourage some species at the
expense of others, or even create an entirely new vegetation. Fast growing trees, for
example, may occupy deserted farm land.
Although strongly affected locally by soil type and other variables, the natural boundary
between rainforest and dry sclerophyll woodland depends mainly on effective rainfall, the
amount of water available to plants after evaporation, soil percolation and run-off have
taken their toll. Within the region as a whole effective precipitation is dominated by the
total amount of rain, and this diminishes rapidly from east to west; 200 cm a year at
Gadgarra, 160 cm a year at Lake Barrine, 140 em a year at Yungaburra and 110 cm a year at
Herberton. The natural rainforest boundary lies roughly between 130 cm and 170 em of rain
a year.
to wake an intelligent guess about the origins Of the main vegetation types of the
Atherton Tablelands, we must use what little is known about the fossil history of
Australian plants, and we have to take into account theories concerning the movement -C
continents over the world's surface. Rainforest contains two main groups of plants The
west recent group consists of species which migrated into Australia during the past ten
million years from Southeast Asia. The more ancient group comprises the direct descendants
of forest plants which originated when Australia, Antarctica, South America and India were
joined into a single, great continent called Gondwanaland.
Australia and Antarctica, the last pieces of Gondwanaland to split apart, finally
separated 55 million years ago. Australia has drifted northwards since then, reaching
more-or-less its present position in relation to Southeast Asia 10 million years ago.
During this time the original forest stock evolved and many living families of plants have
been identified from the fossils of these times These include Sapindaceae (hops),
Araucariaceae (hoop pines) and Proteaceae (Australian honeysuckles).
Several factors, of which developing aridity was perhaps the most important, slowly
reduced to patches along the eastern seaboard of Australia, the area habitable by the
ancient rainforest.
One of the ancient Gondwanaland group of families is the Myrtaceae (myrtle family). This
is one of the families that was able to take advantage of progressively drier conditions
and poor soils by evolving sclerophyllous species suited for such conditions. Eucalypts,
Australia's most important plant group and the dominants of dry sclerophyll woodland, are
examples of this Australian group.
The distinction between the rainforest on the one hand and dry sclerophyll woodland on the
other Is therefore very ancient and of great plant-geographical interest.. Because the
rainforest - sclerophyll boundary seems to be controlled by climate, any change in its
position during prehistoric time could reflect a change in climate which might still be
continuing. Significant natural change in the range of forest trees takes place so slowly
that men are unlikely to notice it over their life-span. If we are to study long-term
changes in the distribution of vegetation we have to make use of fossil evidence. Pollen
grains, because of their wide dispersal, are excellent fossils and are commonly preserved
in lake muds.
Much of the Atherton Tableland consists of lava erupted during the last 3 million years.
On the Tableland are several volcanic craters, the ends of pipes from which gases, rock
fragments and lava were ejected. Some of these craters are edged by well defined ridges and
contain lakes. Barrine, Eacham, Quincan and Euramoo are good examples. Other, usually
larger, craters are not so well defined and have developed into swamps (eg. Lynch's
crater). Crater lakes are excellent sites for pollen analysis.
Material washed into a lake sinks and is deposited in the mud on the bottom. In deep, still lakes the mud, once deposited, remains undisturbed. Mud accumulates year
by year, that being deposited in any one year falling on top of the mud in
previous years. The youngest mud is that being laid down now at the top of the mud
column.
Mud is a mixture of plant and animal remains, clay and other mineral and organic detritus
(including pollen grains) blown into the lake or washed in from the lake edges. Because of
the way mud accumulates, a stratified deposit is so that the mud is progressively older as
one goes down from the mud-water The change in composition of mud from its surface can
itself yield important information about environmental change such as soil erosion or
chemical pollution. The history of the forests is told by the pollen grains preserved in
the mud
Most plants produce pollen grains which are small, more-or-less
spherical bodies about 2 hundredths of a millimetre across (Figure 1). Pollen serves to
carry the male reproductive cell from one flower to the female parts of another flower of the same
species. When pollen grains are examined tinder a microscope they are seen to come
in all imaginable shapes and forms. They may be spherical or angular,
they may have pores or furrows or bladders, they may have a silky, smooth or reticulate
surface pattern, and so on. All these characters are features of the very
resistant, outer-wall of the pollen grain. Each kind of plant produces pollen with a
particular wall-structure and we can tell what kind of plant produced a particular sort of pollen.
Sometimes we can identify the species of plant from which a pollen grain has come,
but more usually we can identify only the genus or family responsible.
Every year the plants growing in a particular area release millions of pollen grains into
the atmosphere. Some fall out of the air very quickly and are deposited on the ground
nearby. A very few may be carried high up into the atmosphere to be transported for
hundreds of miles before being washed to the ground in rain. Most will fall within a few
hundred metres of the plants releasing them. Some pollen from the forest around Lake
Barrine falls directly onto the lake surface and some is washed into the lake from the
surrounding land.
|
Australian Cedar
|
Eucalypt
|
Black pine
|
Scale: about one
hundredth
of a centimetre =
Figure 1. Sketches of some Australian pollen grains as seen through the microscope.
If we identify and count the pollen falling on Lake Barrine we can estimate the composition Of the
forest around the lake. And because pollen preserved in lake muds retains its identity, and grains
thousands of years old can be recognised as easily as newly deposited pollen, we can study the
composition of the forest formerly around the lake by examining pollen in mud deposited long ago.
Over the past few years we have used a variety of devices to collect mud cores from Lakes Barrine
and Eacham. We take sequential samples from along the length of a typical core, each sample being
about a cubic centimetre. Because each sample is younger than the samples beneath it we are
sampling through time. Every sample is subjected to a series of physical and chemical treatments
which concentrate the pollen grains and make them easily visible under a microscope. One sample may
contain many thousands of pollen grains. A representative fraction of these is identified avid the
quantity of each type identified is determined. The results of all samples are usually combined
into a pollen diagram (Figure 2) which shows changes In the quantities of each kind
of pollen
through the mud core, and therefore through time. Samples of mud from known positions in the core
are dated, often by radiocarbon assay. To provide a history of the vegetation around the lake the
pollen diagram is interpreted in the light of what we know about pollen production and dispersal at
the present day.
RAINFOREST /
SCLEROPHYLL
RATIO
Figure
2. A part of a pollen diagram
through the mud of
Quincan Crater from about 7000 years ago at
the bottom to about present day at the top.
Horizontal scales are measures of the abundances
of the named plants (after A.P. Kershaw).
Pollen
analysis has been carried out at a number of sites on the Atherton
Tableland. Different sites begin their record at different times: Euramoo and Bromfield
Swamp about 10,000 years ago; Quincan Crater about 7,000 years ago; and Lynch's Crater
more than 100,000 years ago. Together these sites provide one of the few long and
detailed records of vegetation history in the tropical world. The most striking events
of this history are summarized in Table 1.
|
Approximate years
before present |
Main vegetation characteristics
|
| 0
- 80 |
European Settlement
& Agriculture
|
|
80 -
10,000
|
Complex rainforest with some conifers
|
|
10,000 -
30,000
|
Rainforest with abundant conifers
|
| 30,000
- 80,000 |
Sclerophyll woodland
|
| 80,000
- 100,000 |
Complex rainforest with few
conifers |
Table 1. Main vegetation changes on the Atherton Tableland during the
last
hundred millenia
(information from A.P. Kershaw). |
There are three important points to note. First, the distribution pattern of
rainforest prior to European settlement had been stable for at least 7000 years.
The second point is that between 30,000 and 10,000 years ago, dry sclerophyll
wood-land occupied a large part of what had formerly been and was later to be, rainforest;
such a big shift in a major vegetation boundary must reflect a climatic change. Finally, the composition of the Tableland rainforest before this sclerophyll period
was different from that of later time. In particular, it contained more conifers
(eg. Araucaria) and even one (Dacrydium) which is now extinct on the Australian
mainland. Much more information can be gleaned from these pollen diagrams, for example about the stability of vegetation
(and therefore the animal which live in it) and about climatic and other factors which have changed the vegetation.
Rainforest is the most complex kind of vegetation in the world and the mechanisms by
which It maintains itself or changes are of the greatest importance to ecological
theory. Obviously, they are crucial to our understanding of tile best ways to maintain
timber yields and of
how to preserve areas for reserves or parks. One way of studying
these mechanisms is to find out how the populations of particular tree species fluctuate through time. Some notion of this can be obtained
by estimating the ages of living trees, so determining whether all the species have
reproduced together or separately, how long a plant stays in a reproductive stage and
so on. Another way is to try to refine the technique of pollen analysis so as to trace
the rises and falls in tree populations, at, say, decadal intervals during the last two
thousand years. This is methodologically difficult. It requires sites for which the
source area of the pollen is well in which mud accumulation has been regular and
undisturbed and for which detailed dating can be obtained. Lake Barrine is such a site.
Techniques have been developed which allow sufficiently undisturbed samples to be taken
from the mud beneath the 65 metres of water at its centre. These muds are very finely
layered. Each set of layers may represent a single year's accumulation so that, once
this has been proved, it will be possible to date any analysed sample very accurately
by counting the layers above it. Other problems also have to be overcome, both in
laboratory techniques and in statistical analysis.
The particular characteristics of the Lake Barrine site also provide good conditions for other kinds
of
investigation. These include studies of changes in the microscopic organisms and chemicals in the lake itself, which
indicate some of the things which have been happening in the soils surrounding it. The properties of some of the
minerals in the mud record changes in the direction and intensity of the earth's magnetic field. Studies of living plant
and animal populations, when integrated with the historical record from the lake mud, will indicate whether
disruptions to natural processes result from the isolation of rainforest communities. None of these studies can
effectively be carried out in isolation and all build on a general background knowledge of the geology, vegetation,
climate and history of the sites themselves and of the Atherton Tableland generally.
The main purpose of all this research is the advancement of human knowledge about the world in which we live.
More particularly it is intended to shed light on the processes of change in nature which are persistent, which have
produced the patterns of nature in the world as we know them and which we must reckon with In the future.
Additional reading
Breeden, S. and K. A natural history of Australia: 1, Tropical Queensland. Collins, Sydney, 1970.
Flenley, J.R. The equatorial rainforest: a geological history. Butterworths, London, 1979.
Kershaw, A.P. The changing vegetation in Northeastern Queensland. fin See vol. 2 eh 4, 1979 (in press).
Moore, P.D. and Webb, J.A. An illustrated guide to pollen analysis. Hodder and Stoughton, London, 1978.
Pike, G. Pioneers country. Published by the author, Mareeba, 1976.
Russell, K. and R. A lake in the forest. Hisine Technique, Herberton, 1976.
(Prepared by D. Walker, Australian National University, with assistance from colleagues and
A.P. Kershaw,
Monash University. Research at Lake Barrine was conducted by members of the Australian National University
with the collaboration of The National Parks and Wildlife Service of Queensland, CSIR0 Division of Forest
Research, the Queensland Department of Forestry and others. 1 September 1979).
|
