How old is that gum tree?

 

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. 

Left: The Railway Giant on the corner of Albert Rd and Francis Ave in Drouin.

Right: The rare Strezlecki Gum at McNeilly wetlands.

 

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. 

The stand of remnant Mountain Grey Gums in Princes Way and Hearn Park contribute to a magnificent gateway to Drouin.

 

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. 

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.

 

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.

The Mountain Ash is the world’s tallest flowering plant. Credit: ABC News.

 

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. 

A Sulphur-crested Cockatoo ‘at home’ in a large eucalypt at Picnic Point, Longwarry North.

 

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!