Do trees die of old age, or can they live forever? Answered

You may read that “oak trees have a lifespan of 300-400 years”. But then, there are many oak trees that have lived for more than 1000 years. And the oldest tree in the world is the Great Basin Bristlecone Pine, which has seen 5000 years pass on earth. How on earth does that work? Do trees die of old age, or can they even life forever?

phoenix tree regeneration
‘Phoenix regeneration’ – seemingly dead, new life has emerged from this tree’s living roots. Phoenix Tree by Glyn Baker, CC BY-SA 2.0, via Wikimedia Commons

Trees can die of ‘old age’ in theory, but in practice a tree’s lifespan is limited by environmental factors such as availability of water and nutrients, damage, disease and overgrowth. Trees can however form genetic clones which can outlive their parent and therefore, in a sense, live forever.

Having a scientific background, I’m excited to bring you this article where I’ll explain what I mean by this. It certainly is a complicated subject but I’ve undertaken some academic research, and combined with my hands-on tree experience, I can now bring you the most complete answer possible.

How do trees die of old age?

A mature tree that looks very much alive is actually almost totally made up of ‘dead’ cells – in fact only 1% comprising living tissue – a microscopically thin layer under the bark (the cambium), the root tips, branch tips, buds and leaves. Also a branch, or part of the roots, can ‘die’ and remain attached to a tree even though there is no further living activity in it. Half of a tree may ‘die’ even – following a lightning strike, while half survives and continues to grow.

So we cannot think of the term ‘death’ in the same way that we think of it for humans, or animals.

It follows that ‘aging’ is a different concept as well, but there are some similarities. Aging theory in trees (as in humans) focuses on genetic material called telomeres, which are lengths of DNA at the end of chromosomes. Every time a living cell divides through the life of an organism, these telomeres lose a short length of the DNA. It follows that there is a maximum number of times that a cell can divide before the telomere runs out, and no further division is possible (a state called senescence). This would mean that in theory, there is a ‘biological limit’ on longevity. Telomere discoveries won a Nobel Prize in 2009!

However, humans and plants do not typically live long enough to actually exhaust all of their telomere length. What we may call ‘dying of old age’ in humans is almost always death from disease – such as pneumonia, cancer or heart disease.

Indeed, what we may call dying of old age in trees, is actually:

  • Death from lack of resources, i.e. water, sunlight and nutrients – often in competition with other plant life
  • Death from environmental damage, e.g. frost, wind, fire; animals or insect consumption
  • Death from disease – e.g. Dutch Elm Disease, Emerald Ash Borer, Oak Wilt or a myriad of others

Trees actually may have more capacity to regenerate their telomeres than humans do, so in practice trees don’t reach the ‘biological limit’. However, there is experimental evidence supporting the idea that the most long-lived bristlecone pine trees seem to have exceptionally long telomeres, so this theory still holds some weight in tree longevity. Here’s an in-depth academic review of where we are currently with plant life and telomeres.

One of the limitations to conducting research in this area is the extreme timescale involved. It’s simply impossible to observe a tree over 1000 years – which is very much longer than the career of the average scientist – so there is no observational data proving that a tree can live infinitely… and there never will be!

Lifespan of individual trees vs clonal trees

Methuselah Grove, containing some of the world’s oldest individual trees – bristlecone pines which are approaching 5000 years old. Oke, CC BY-SA 3.0, via Wikimedia Commons

Trees often reproduce by forming suckers – new shoots from the base of their trunk or the root system. In time, the new shoot can become a titan in its own right. When a tree does this, it has created an identical clone of itself. Sometimes a tree can have so many different stems that it’s difficult to tell where the original tree ends and the newer parts begin.

Here’s where the longevity issue becomes unclear. Is the new growth a brand new tree, or simply an offshoot from the old tree? What if the ‘original’ tree is cut down, and the new tree survives? Has the first tree really died?

To put the question another way: when I take a cutting from a branch, place it in a pot and provide it with soil and water, and it grows – is the cutting still the original tree (just with 99% of the old tree removed)?

OK, we’re getting philosophical. The point is, there is a difference between dating an ‘individual’ tree – usually with a single trunk, and what’s called a ‘clonal colony’ – a growth of many tree trunks that all actually originated as offshoots of other trees, but which, sharing a root system and being genetically identical, can be regarded as a single living organism. Some of these clonal colonies are as big as forests.

Let’s compare to the Great Basin Bristlecone Pine mentioned earlier, at 5000 years is thought to be the world’s oldest individual tree. The longest-living clonal tree colony is believed to be a colony of Quaking Aspens in Utah known as Pando, or the trembling giant. It covers a vast area (over 100 acres), and although the individual tree trunks typically don’t live beyond 130 years, the organism as a whole has been aged at up to 14,000 years old!

Pando aerial view trembling giant clonal quaking aspen
Aerial view of Pando, The Trembling Giant, a clonal tree. Lance Oditt, Friends of Pando, CC BY-SA 4.0, via Wikimedia Commons

Do trees keep on growing forever?

As long as there are living cells in a tree, that means cell division can take place, so while growth slows in ancient trees, they never stop growing. However, the mechanics of how a tree functions place a limit on how tall it can grow.

Trees maintain a continuous upwards flow of water from roots to leaves. In the process of transpiration, water evaporates from the surface of leaves, thus drawing lower water upwards through a continuous column in the tree’s xylem tissue, which in turn causing the tree roots to absorb water. This process is essential for all trees to survive.

But the taller a tree gets, the longer the water needs to travel to get from the lowest roots to the highest leaves, and the more gravity makes this a challenge. Think of it like sucking water upwards through an incredibly long straw. The journal Nature published a study that used this concept to estimate a theoretical maximum tree height of 122–130m (400-426 feet), which is actually just a bit higher than the world’s tallest living tree, the coast redwood Hyperion. Beyond this height, the leaves would lose their turgor, and the flow of water through the tree would cease. Trees simply can’t grow any higher.

But they can grow outwards! Tree trunks will continue to thicken as the cambium divides, producing new wood on the inner aspect, and new bark on the outer. Branches reach outwards, seeking sunlight for their leaves. Ancient trees tend to have very thick trunks, long, sprawling branches, and sprawling root systems.

Trees: Life after death

When all’s said and done, what I love about trees is that even when they really do die, so much life abounds around them. Their hollowed-out trunk provides a habitat for insects and animals, and their wood is a substrate for years algae, fungi, and bacteria for many years. These contribute to wildlife and the regeneration of the forest floor, making an ideal place for more wonderful trees to grow. Certainly trees, in this sense, keep on giving life, forever.

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