Trees can live for CENTURIES thanks to an 'immune system' of protective chemicals that defends them against disease and drought in their old age

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The secret to how trees live for centuries and continuously grow has been solved by a team of scientists. 

Ancient trees have been found to strike the 'perfect balance between growth and ageing' which allows them to grow while fighting off disease. 

A primitive immune system keeps working for hundreds of years, with disease-fighting chemicals known as flavanoids protecting the tree. 

Researchers looked at gingko biloba, or 'maidenhair', trees between 15 and 667 years old and used DNA analysis to unpick its ageless secrets 

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Researchers looked at gingko biloba, or 'maidenhair', trees between 15 and 667 years old. They found evidence of a primitive immune system keeps working for hundreds of years, with disease-fighting chemicals known as flavanoids protecting the tree (file photo)

Dr Richard Dixon, co-author of the study from the University of North Texas, told the Daily Mail: 'It is incredible to think that there are trees which have been standing since before the Battle of Hastings, and trees in churchyards in the south of England which existed before the ancient churches they stand beside.

'They don't seem to be programmed to die, like most other living things. We don't know why they eventually succumb to a disease.

'We have only been able to get this insight on trees from sequencing their genes for less than a decade, and it is fascinating to learn they appear to be able to maintain their defences against microbial attacks for so long.'

Experts now understand that, unlike in humans, the 'immune system' of a tree does not slow down in old age, but keeps them protected against fungi and pathogens. 

Older trees cease growing upwards but their genes show they keep growing outwards, sprouting branches and developing thicker trunks.

This widening adds rings to the inside of the tree, but when a tree gets older the rings become smaller as nutrients diminish.  

To gain an insight into ancient trees, researchers sorted gingko biloba trees into three groups, of those around 20 years, 200 years and 600 years old.

Through genetic analysis, they found 41 genes important for fighting off plant diseases, of which 39 were still active in the oldest trees.

The study examined the 'vascular cambium', which is the tissue within the stems of trees which helps to grow its trunk and branches.

Genes for making flavonoids, the chemicals made from sunlight and air which keep trees healthy, were active in old trees as well as young ones. 

But when trees were centuries old, their genes which produce cell growth became less active, which might explain why tree rings showed they grew more slowly.

And despite diminishing resources, the cells in the cambium continue to divide and widen the tree. 

But they were still resistant to stress and their DNA showed their cells had not entered a 'senescent' phase where they started to deteriorate or die.

Dr Dixon said: 'We looked at these trees' resistance genes, which are like their immune system and provide their defence against the world.

'It was just as strong in older trees, where in older humans the immune system starts to be compromised. This could explain why trees are so successful.'  

Experts now understand that, unlike in humans, the 'immune system' of a tree does not slow down in old age, but keeps them protected against fungi and pathogens. Pictured, a young gingko biloba, or 'maidenhair', tree

Trees grow thanks to a unique tissue called meristem, which is similar to human stem cells.

It transforms into any plant tissue and allows plants to grow outwards.  

It is found in three places: the top of the tree, the tip of the roots and in a place called the vascular cambium. 

Older trees cease growing upwards as their apical meristem - located at the top of the tree - is destroyed or damaged over the years. 

However, their genes show they keep growing outwards.

The researchers discovered that this was due to how the vascular cambium changes over time. 

Inside the cambium are phloem and xylem, which transfer nutrients and water around the plant, respectively. 

When trees get older, they form what is known as secondary phloem and xylem, which are more commonly known as tree bark and wood. 

The study found this continues to happen in the cambium of ancient trees, and allows it to continue growing while simultaneously ageing. 

But the trees do grow old, and there are key clues which revealing the trees battle ageing just like humans do.    

One chemical produced by the tree is called IAA, an auxin which encourages growth, and the more a plant has of this the more it grows. In older gingko biloba trees, the concentrations of this decreases. 

Another key chemical produced by the vascular cambium is abscisic acid, which triggers leaves to die and also encourages the ripening of fruit. 

Low amounts of this chemical keep the tree healthy and growing. But the amount of this acid increases in older trees.