Have you ever paused to wonder how old the trees on your property are? Soon after purchasing my own property I remember marvelling at some of the trees and guessed that they may have predated European settlement, but it was just that….a guess.

The thought of ‘owning’ something that old didn’t sit comfortably with me, rather I felt a sense of responsibility to be a good custodian. But the question of just how old these trees were remained in the back of mind.

In the Australian bush the vast majority of seedlings die before reaching maturity with many only lasting a few years. For those that do survive their growth rate is influenced by many factors including individual species attributes, climate, water availability, geology, soil, root stress, drought, competition, disturbances and other factors such as disease.

So how can you determine the age of a tree? Most people will be familiar with the idea of counting growth rings (dendrochronology). A tree grows a little each year and in doing so it lays down a growth ring. A wide ring represents a wet year, and some ring variations can even be correlated with major events like droughts or wildfires. However this method is not considered reliable in the forests of Eastern Australia and besides, it’s not much use if the tree you want to age is still standing.

Knowing the disturbance history of a site can help to age a tree. By looking at historical aerial photos that show cleared areas which are now vegetated, the date of the photo will provide an insight into the age of the re-growth. Other tree ageing methods include radiocarbon dating and using growth models based on increments in tree diameter growth measured over time.

As a society we place significant value on 100 year old buildings, and yet we have trees in our landscape that are much, much older than this.

A recent scientific paper (Ngugi et. al., 2015) published in the Journal of Forestry Research drew on an impressive historical dataset to develop growth models that can be used to calculate the age of some trees (mainly commercial timber species).

The data included DBH (diameter at breast height) measurements collected for 75 years (1936-2011) on over 86,000 trees (155 species) in more than 640 permanent forest plots in South East Queensland. This allowed the authors to study trends in incremental growth rates in tree species across sites that receive similar annual rainfall. The findings of this study are considered consistent with other studies using tree core samples and carbon dating.

They found that stem diameter increments for species growing naturally in forests of subtropical Queensland ranged from 0.01 to 0.5 cm per year, with a mean DBH growth increment of 0.25 cm/yr.

Using this figure it would take a tree 120 years to reach the size (girth) of your average power pole (300 mm). This is slow compared to similar native species grown in plantations which can have DBH increments of 1-5 cm/yr. This faster growth rate for planted trees is presumably due to the application of horticultural practices and reduced competition compared to a native forest.

As you would expect, this research found that some species grow faster when they occur in an area with higher annual rainfall (e.g. Grey Ironbark, Eucalyptus siderophloia; Small-fruited Grey Gum, E. propinqua; Queensland Blue Gum, E. tereticornis; White Mahogany, E. acmenoides and Brushbox, Lophostemon confertus. Bucking this trend were Blackbutt (E. pilularis), Flooded Gum (E. grandis), Narrow-leaved Ironbark (E. crebra) and Poplar Box (E. populnea), which didn’t reach their maximum growth rates in the highest rainfall zones. Brown Bloodwood (C. trachyphloia) maintained relatively constant growth rates across all rainfall zones.

They also found that growth rates vary during the life of a tree, but across all rainfall zones most species go through their highest rate of growth between 20 and 60 cm DBH. As a tree gets older its growth rate slows and the authors warn that age estimates for large trees (>80 cm DBH) should be used cautiously because they had a limited representation in the dataset. Additionally, many older trees have bole deformities and/or swelling in the base of the tree (given the unfortunate name of ‘butt swell’). Such deformities can increase the margin of error when relying on DBH to calculate age.

 

 1030507090110130150170180
Blackbutt
(E. pilularis)
1600 mm rainfall
49607082991241662515091048
Spotted Gum
(C. citriodora)
800 mm rainfall
671292053801724
Spotted Gum
(C. citriodora)
1200 mm rainfall
58821051381953311046
Pink Bloodwood
(C. intermedia)
1200 mm rainfall
73139234532
Pink Bloodwood
(C. intermedia)
1600 mm rainfall
83103131179281644
Broad-leaved Ironbark
(E. fibrosa)
1200 mm rainfall
4384159608
Broad-leaved Ironbark
(E. fibrosa)
1600 mm rainfall
3274120203461
Narrow-leaved
Ironbark
(E. crebra)
800 mm rainfall
94211281327360384551
Tallowood
(E. microcorys)
1600 mm rainfall
7379108180588
Table: Predicted age (years) of tree species based on diameter at breast height (DBH) measurements and rainfall zones in SEQ. These figures have been calculated using growth models in Ngugi et. al., 2015.

 

For these larger trees radiocarbon dating of core samples provides a more accurate measure of age. A large Brushbox at Lamington National park has been dated using this method and was determined to be 1500 years old. While trees of this age are no longer common in the landscape, numerous other tree species of similar age are still standing in SEQ.

So why don’t we see more old trees? Well firstly, because most of them have been felled since European settlement, and secondly, many young trees die when they germinate in a location where there’s insufficient light, moisture or nutrients to sustain them into maturity. They may simply be eaten or succumb to disease, fungal attack, drought, fire, floods or wind.

Most Land for Wildlife members would know that tree hollows provide crucial habitat for wildlife and that older trees are more likely to have hollows. Research conducted in SEQ by Wormington & Lamb (1999) found that Blackbutt generally doesn’t produce hollows suitable for use by fauna until it is over 165 years old. Tallowood (E. microcorys) begins developing hollows at approximately 170-200 years and Scribbly Gum (E. racemosa) at 200-235 years. Large hollows in these species form after approximately 250 and 300 years respectively. To put this into perspective if you plant a Tallowood today (and assuming it survives) it will not provide a nesting hollow until about 2267!

Some Land for Wildlife members will be the current custodians of trees that are centuries, if not millennia, old. Knowing the age of a tree gives us an insight into the time scales of some ecological processes. Rarely have very old trees simply lasted by chance, they require a degree of luck, protection and importantly they require good custodians.

Today high intensity wildfires are a threat to many old trees. Habitat trees often have a hollow trunk and once alight these ‘pipes’ act as a chimney and will often burn until the tree falls.

The regular use of low intensity fire by Traditional Owners extended the life expectancy of many veteran trees in our landscape. Reducing the fuel around old trees where they are at risk of wildfire is one way you can help prolong their life. Other things you can do are to prevent soil compaction around the root zone by excluding stock and vehicles. Plan ahead to avoid future conflicts by not placing infrastructure close to large trees.

As a society we place significant historical value on 100 year old buildings, some are placed on a heritage register and protected, and yet we have trees in our landscape that are much, much older than this. These trees provide a valuable cultural link to the past. Despite this, in most areas of Queensland individual old trees are not afforded any protection.

As the urban fringes in SEQ continue to bulge, many of these old trees are deemed hazardous and removed. By having a deeper appreciation of the age of the trees around us, maybe we can learn to afford them the custodianship they will need to keep standing well into the future.

 

References & Further Reading
Brack C & Brookhouse M (2017) Where the old things are: Australia’s most ancient trees. The Conversation, ABC News, 19 April.
Gibbons P & Lindenmayer D (2002) Tree Hollows and Wildlife Conservation in Australia. CSIRO Publishing.
Koch A, Driscoll D, Kirkpatrick J (2008) Estimating the accuracy of tree ageing methods in mature Eucalyptus obliqua forest, Tasmania. Australian Forestry, 71 (2).
Ngugi MR, Doley D, Cant M & Botkin DB (2015) Growth rates of Eucalyptus and other Australian native tree species derived from seven decades of growth monitoring. Journal of Forestry Research, 26 (4).
Turner J (1984) Radiocarbon dating of wood and charcoal in an Australian forest ecosystem. Australian Forestry, 47 (2)
Veteran Tree Group Australia, www.veterantreegroup.blogspot.com.au
Wormington K & Lamb D (1999) Tree hollow development in wet and dry sclerophyll eucalypt forest in south-east Queensland. Australian Forestry 62 (4).

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Article by Nick Clancy
Land for Wildlife Officer
Sunshine Coast Council

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