Sunday, 10 January 2016

The life of an average tree

How long do you think trees live on average? You know that some trees can live for centuries - the Methuselah Bristlecone pine is over 4,800 years old, but even that age has been eclipsed. In 2010, the claim for the oldest living tree was more than 9,500 years for a tree that has a single root system that old but which periodically pushes up new trunks that only last a few hundred years. But if we count different tree trunks with a common root as a tree, then the oldest and possibly biggest living organism in the world must be the Pando quaking aspen of Utah, USA. The Pando is a single genetic individual, currently more like a forest with over 47,000 trunks, whose genetic makeup could be up to one million years old!

Wollemi Pine clones in the National Arboretum Canberra. How "old" are these?

And what about the Wollemi Pine, also known as the Dinasour tree? There are less that 100 trunks standing in the wild and it seems they are all clones having vegetatively reproduced somewhat like the Pando quaking aspen. Until recently we thought the Wollemi pine were extinct with pollen records disappearing after their hey days 121 million years ago. Are these remaining trunks still a single genetic individual from something started that long ago?

But on average? Probably the average life span of a tree is less than 2 or 3 years.

If some trees can live hundreds, thousands or even for millions of years how can the average be so short? Well, thousands, millions even tens of millions don't live more than a few months. They germinate and are eaten. Or they germinate but there isn't enough rain or sunlight or soil to keep them going and they die before the next rain. They can even germinate but then be poisoned by their parents via a process called auto-allelopathy to make sure they don't crowd the parents out. Even when the seedlings do survive and start to grow, there may be thousands of them per hectare with only room for, at most, a couple of hundred after a few decades. So, they compete for the space and most are supressed by those trees which eventually dominate the site. The supressed ones die during a stage called stem exclusion. So, a lot of trees die very young while very few make it to old age - making the average life span quite short.

But the story doesn't end there. While above ground the trees appear to be competing and even killing each other off, there may be a different story below ground. While exploring all the soil available to them, roots often come into contact with each other ...and often these roots fuse or graft themselves together. If enough roots graft together, sharing the photosynthates produced by the leaves above and the water and nutrients gathered below, how long is it before they cease to be separate individuals? Maybe all forests end up similar to the Pando Quaking Aspen, sharing a common root system even though there are hundreds of separate individual trunks with different genes above ground.

Does it even make sense to talk about the age of a tree in a forest anymore? It is like the human body and its cells. No cell in your body is more than 7 years old even if "you" are 50 years old. Trees, like cells, come and go but the organism can go on.

Happy New Year.



  1. 47,000 trunks!
    Learnt a lot with this one as usual. You can still surprise me :-D
    Xo Jazzy Jack

  2. And those 47,000 trunks were counted over 40 years ago! Recon there are even more now ...and part of a tree that is 1,000,040 years old :-)

  3. one tree alone is beautiful - but what makes trees so marvelous is that they build forests.....
    wish you a wonderful new year with the woods! xxxx

    1. So right Beate! Beautiful and marvellous :-). We really must take the time to marvel at trees and their magic! I can't believe the number of people who just pass by and don't even see anything outstanding.

      And wishing you a wonderful welcome to th new year too.