Around 70 years ago, the wisdom about old eucalypt trees was
very different – “They should be eliminated as soon as possible and replaced
by a useful crop, even coppice for firewood …” (Growth Habits of the Eucalypts,
Jacobs 1955).
One has to wonder whether we really have come very far
in the intervening years – “Gum tree removal causes stir in Drouin” (Warragul Gazette May 19th 2020).
But just how old is ‘a large old gum tree’? Determining the
age of standing trees is an inexact science.
Some references give a ‘rough estimate’ of the age of a
eucalypt being the same number as the diameter of the trunk at breast height in
centimetres – a 50cm diam eucalypt would be 50 years old. Another reference
says the same, but to multiply the diameter in centimetres by 0.98 – a 50cm
diam eucalypt would be 50x0.98 = 49 years old. Both are very crude methods
subject to enormous error tolerances.
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Left: The Railway Giant on the corner of Albert Rd and
Francis Ave in Drouin.
Right: The rare Strezlecki Gum at McNeilly wetlands.
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Ring counting or dendrochronology is often referred to as
the safest and most accurate method of dating a tree. This is fraught with
problems in that the tree has to be fallen on the ground or drilled for a core
sample so that the rings can be accessed. It is now accepted that there are
several opportunities for error in actually counting the rings. Some trees are
known to form several rings per season for example or sometimes to not set any
rings for several seasons and some seasonal rings are not clearly defined.
Carbon dating of a core sample is another method that
requires accessing the centre of the trunk.
The growth rate of any tree is influenced by many factors;
species, climate, soil condition, root stress, competition, disease, insect
attack, etc. Big trees are not necessarily old trees, they might be just very
healthy fast-growing individuals.
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The stand of remnant Mountain Grey Gums in Princes Way and
Hearn Park contribute to a magnificent gateway to Drouin.
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Another technique often used by forest researchers is called
the growth model method – a combination of dendrochronology, historical
evidence, known species growth rates, location (climate, soil, etc) and in the
case of eucalypts, the number and size of any hollows.
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The Settlement Giant on the corner of Settlement Rd and
Springwater Dr is one of our oldest eucalypts. This old Mountain Grey Gum
contains more than a dozen significant hollows.
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The time taken for a eucalypt to begin developing a hollow
varies from species to species but is considered to be from around 100 years to
about 200 years. Large, old eucalypts with numerous or large hollows are
probably several hundreds of years old at least.
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The Mountain Ash is the world’s tallest flowering plant.
Credit: ABC News.
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The Ada Tree, a giant Mountain Ash, is considered to be one
of Victoria's largest living trees. It is estimated to be over 300 years old
and towers over the surrounding rainforest in the headwaters of the Little Ada
River between Powelltown and Noojee.
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A Sulphur-crested Cockatoo ‘at home’ in a large eucalypt at
Picnic Point, Longwarry North.
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Large old trees are among the biggest organisms on Earth. The
number of large old trees is rapidly declining in many parts of the world, with
serious implications for ecosystem integrity, biodiversity and landscape
aesthetics.
Post script:
The community of a certain town greatly valued its eucalypts
that were planted in 1870. They had the trees designated as “heritage trees” in
1975 under a local law. That local legal status did not protect them from
several attempts by developers and road authourities to destroy the
trees. The people came to the defence of the trees and were eventually
successful in getting permanent legal status to protect 3.5 km of the
trees. That section of eucalypt-lined road was placed on the national
historical register in 2012 and the trees are now permanently protected. Where
was this? No, not Drouin, it was Burlingame, California!
