How do ancient trees grow so old?

Some large trees have steadfastly endured for hundreds, even thousands of years, surviving through generations of humans and other plants and animals – but how they do this has been a mystery.

Researchers have now shown that Ginkgo biloba trees, some of the oldest on Earth, don’t enter senescence, the declining division and growth of cells that inevitably results in aging and death, and retain strong resistance to external stressors.

“Ginkgo biloba is a living fossil, which originated about 280 million years ago,” says first author Li Wang from Yangzhou University in China; with no living relatives in the division Ginkgophyta, it has survived glaciations and lived alongside dinosaurs.

It grows widely at different latitudes and elevations, with many of the current specimens distributed in China, Japan and Korea, among other countries.

In China, especially, “hundreds of G. biloba trees aged over 1000 years still grow well and luxuriantly and produce large numbers of seeds,” says senior author Jinxing Lin from the Beijing Forestry University.

“This phenomenon is of great interest to us. How long can these old Ginkgo trees live?”

To explore how they survive so long, the team conducted RNA sequencing of the vascular cambium – the main growth tissue in the stem that produces inner wood cells and outer bark cells – in 15- to 667-year-old Ginkgo trees.

Most previous research on the aging of trees investigated the leaves, which is inappropriate with Ginkgo, a deciduous tree that loses its leaves each year.

“We therefore selected the vascular cambium,” says Wang, “which can better reflect the aging processes since it is indeterminate and continues to function throughout the lifespan of the tree.”

They expected that 600-year-old trees would show signs of senescence, along with marked differences to their 20-year-old juniors.

But, although several genes related to cell division, expansion and differentiation showed reduced expression in the old trees, there was no significant increase in the expression of genes related to senescence.

The old trees also had similar leaf areas, efficiency of leaf photosynthesis and seed germination rates to their younger counterparts and retained high expression of genes associated with disease resistance and synthesis of protective metabolites.

“It seems that the vascular cambium in G. biloba retains the capacity for continuous growth for hundreds of years or even millennia, and this may enable G. biloba to escape senescence at the whole plant level in the absence of outside accidents,” says Lin.

“On top of this, the remarkable maintenance of expression of genes associated with biotic stress was unexpected. In humans, for example, our immune systems gradually become compromised with age.”

The team expects that most ancient trees are likely to have this longevity mechanism whereby the continuous division of the vascular cambium can compensate for the aging process, similarly to cancer or germ cells in animals.

So, if the trees don’t enter senescence, how do they die?

“This is a difficult question,” says Wang.

Most trees die from diseases, pests and other environmental stressors such as extreme weather or bush fires.

These causes aside, plants and animals have dramatically different developmental patterns, explains co-senior author Richard Dixon from the University of North Texas, US, especially in their post-embryonic stage.

In most animals, organs and tissues show little difference over the lifespan, apart from growth, maturation and eventually senescence and death. Plants, on the other hand, constantly form new organs and tissues, which might help them avoid the whole-plant senescence process.

Senescence might still occur with trees older than 1000 years, a possibility that can’t be ruled out with the current study, notes Dixon.

More information should come to light with new methods for analysing an organism’s entire genome, as used in the present study, which have only recently become available.

It should be noted that the research, not easy to do with old trees because they grow slowly and their lifespans dwarf ours, is a “static snapshot” of the tree’s status at the time of harvest, says Dixon, noting that it would be more challenging to work out how different ages respond to stress in real time.

The paper is published in the journal Proceedings of the National Academy of Sciences.