Ageless and timeless: the lords of the rings

Trees are fascinating. Despite the fact they grow in abundance all over the world, from the sweaty, dense rainforests of the tropics to the lively jungles of Africa and Asia and the vast, temperate forests of Patagonia, Canada and Siberia, we actually know relatively little about them.

We know that they are the tallest living creatures on Earth, with magnificent giant Sequoias and Redwoods dominating the foothills of Yosemite and adorning the American Pacific coast, but what are the theoretical limits to height?

We know that many shed their leaves in autumn, painting the woodlands of New England with a delicious smattering of golden yellow, luscious red, fiery orange and crispy bronze, but precisely why (we know how) they turn this vivid swathe of colours isn’t actually the easiest of questions to answer.

However, one area that we do know something about is how these stern, masterful products of nature’s shrewd artwork manage to grow so old; setting root and then thriving as time takes its course. Living through flooding and storms, forest fires and droughts, the bleakest winters and the most scorching summers.

Even the youngest of trees can live for several centuries, outlasting the most expansive and successful Empires of ages gone by. Some of the mighty African baobabs, amongst the oldest of the flowering trees, predate the Norman conquest of England.

However, there are some aged behemoths of nature that have survived not only the perils wrought by weather, climate, pests and disease, but have survived since before Egypt, Persia, Greece, Rome, and the dawn of Civilisation itself: the lords with the many, many rings.

There is quite some debate as to what the oldest trees are, and why.

We all know that you can, quite accurately, measure how old trees are by counting the rings in their trunk, by nature of the way that trees grow. Each year, trees make new rings of cells underneath the bark, which grow faster in the early season and more densely later on.

Some years trees might make more than one ring, but it’s a pretty decent way of estimating how old they are. Each new layer on the outside represents another year of successful growth. There’s tonnes of other information you can get from these rings, such as what the climate was like hundreds of years in the past.

It is using precisely this technique that has led researchers around the world to agree on what is the oldest tree, or at least, the oldest proper tree that can be called a tree.

The Methuselah Grove of the Ancient Bristlecone Pine Forest of California (apparently all the coolest trees grow in California), surviving millennia on mountains 3000 metres above sea level, is home to some staggeringly old creatures.

Methuselah herself has already reached a grand old age of 4850 years old, but even she is centuries younger than the oldest tree of them all - a Grand Basin Bristlecone Pine of 5068 years.

This ancient tree germinated in 3051 BC, practically as old as Ancient Egypt.

Closer to Egypt, there’s an olive tree in Portugal of 3350 years old, which, considering all of the other trees on the top ten list are conifers, is pretty impressive for a flowering plant (and a close relative of Europe’s more unfortunate ash trees).

It’s tempting to think that the Celts, later the Romans, even Napoleon’s invading armies and the freedom forces of Wellington, if not Christiano Ronaldo (he’s from Madeira anyway), all might have tasted the fruits of this ancient botanic marvel.

But rings on trunks and bitter old olives only tell us one part of the story, and much like an iceberg - sometimes it’s what lies beneath which truly reveals all.

Plants are as impressive underground as they are above it, with root systems penetrating the soil to branch out as broadly and as gracefully as the leaf-laden branches above.

Some root systems, however, are quite something else. Indeed, although the trunks of the Norway Spruce might live for mere centuries, it appears that their roots can last a significant amount of time longer.

Indeed, this is the story of Old Tijkko, whose root system has been shown by radiocarbon dating to have been growing since before Europe and Britain had a land bridge. This tree has lived through two pretty major Brexits either side of its almost 10 000 year lifespan.

However, this grand old ent cannot compete with the oldest on the world’s list of really, really old trees. The oldest of them all, quaking not through age but by name, is of the quaking aspen trees of Colorado.

Pando, Latin for “I spread,” a clonal tree of whopping proportions, is not only the largest living organism on Earth (give or take a hungry fungus in Oregon), but is also staggeringly old - estimated to have been sprouting up new trunks for at least 80 000 years. Estimates range up to 1 000 000 years for this truly ancient tree. Homo Sapiens had scarcely even considered leaving Africa when this plant first set root.

Alas, Pando seems to be slipping slowly away.

Humans are increasingly impressively long lived, but we start to shrivel up and wither away long before many trees have even begun to properly flourish. As the ends of our DNA start to fray, so too do our cells and eventually our bodies.

So, how is it that some trees, such as the magnificently old bristlecone pines, manage to survive for so long? The answer could be in their natural genetic longevity.

Trees, like humans, grow old. In biology, this is a process called senescence. It can apply in the short term, to things such as leaves which fall each autumn, or in the long term to an organism as a whole.

Trees, also like humans, have stem cells. In humans, these cells are famous for being able to differentiate into almost any cell type in the body, and are found in places such as the bone marrow.

In trees, these cells are found in such places as the tips of the shoot, the roots, in embryos and germinating seedlings, for example.

Even these stem cells, eventually, grow old. Indeed, with ageing, we tend to associate a buildup of mutations, which lead to the deterioration of DNA and therefore the cells that keep a large eukaryotic organism (algae, plants, fungi and animals etc.) youthful and alive. We assume that with age comes senescence.

This is where bristlecone pines seem to buck the trend. They just don’t get old. All of the things that you would assume would get worse with ageing just don’t get worse. From pollen viability to seed germination, seedling growth and frequency of mutations, bristlecone pines show no correlation with age.

This, however, we knew in 2001. More recently, it was shown why - or better still, how - that might be the case.

Back to the meristems of plants. This is where all the stem cells are. These are little cell manufacturing centres which very actively produce cells that can differentiate into any cell type. However, even these cells pick up damage over time.

In bristlecone pines, however, and indeed in giant sequoias, there is even more at work.

At the centre of these meristem regions lies a dormant “quiescent centre.” This quiescent centre contains stem cells that are “mitotically inactive,” meaning that they basically lie there and chill while all the cell division goes on around them.

Interestingly, they seem to be triggered into action only by exposure to stress-related plant hormones and DNA damage. It seems that, for the longest lived of the trees, they have a source of eternal youth lying at the core of their eternally young cells, which can replace even damaged stem cells.

This means that, throughout the centuries and millennia, the old bristlecones have stayed genetically unperturbed in the very regions from which they grow and multiply. This is quite something.

So, after all, it’s probably not a potion or elixir that will provide the key to eternal life, but the maintenance of genetic youth despite passing years. It’s not the avoidance of mutations, but the maintenance of a fresh pool of dormant, untainted DNA that can be spurred into life in light of times of stress and damage.

Mutations and senescence are in humans, currently, unavoidable. But who knows what the next era of the biological sciences might promise in the quest for everlasting youth.