Everything in nature dies. Human beings can live close to a century, but few do, and no one has lived past 122. Dogs live on average 8 to 14 years. Some tortoises can live to be 200. In vertebrates, changes on the cellular level give animals an upper limit on lifespan. Cells can only divide so many times before they’re compromised, making older individuals much more frail and vulnerable than youthful individuals.
Plants are fundamentally different: the larger and more established plants are, the higher their survival odds become. Out of 1,000 seedlings, only one may live to become a tree. But once a specimen is mature, it can be extremely resilient. Many common trees have potential lifespans in the centuries, towering over rivals and growing stronger every year.
Despite this trajectory, it’s uncommon to see single-stemmed trees grow past a thousand years old. So what limits them?
This is one of those questions I love, because there are so many angles to understand it from.
A tree is a system of redundancies
Unlike vertebrates, plant cells do not age. A rooted cutting from an ancient tree is identical, on the cellular level, to a young plant.
In fact, even in old trees, the live cells—leaves, roots and cambium—are young. Old leaves fall off, and new leaves grow. Feeder roots—the thin, water-absorbing tips and branches of the robust structural roots—also die and regrow cyclically, the same way leaves do.
The xylem—the tubes that carry water and minerals up a tree—are already dead. These cells are the most important type of tissue in determining what makes a tree a tree. Xylem cells form cylinders and die as soon as they mature, becoming conduits for fluid. Because they’re dead, xylem tubes cannot self-repair. Within a few years they develop tears and air pockets that stop them from working, so new cells replace them. At their final stage, defunct xylem cells are the wood. They are the old growth rings on the inside of the tree, existing only to hold it up. Their replacements, new xylem tubes, continually form on the outside of the trunk’s circumference, as new growth rings.
Phloem cells—which transport hormones and sugars from the foliage to the roots—also die annually. Unlike xylem, they are soft and rubbery, not rigid. They are also still living when they do their work. When spent, they die and collapse to a fraction of their former size, and are ultimately pushed outward as bark.
Between the phloem and xylem is a thin ring of tissue—the secondary meristem—which originates all the structural cells. It is rapidly dividing, and cells there migrate in to become xylem and eventually wood, or outward to become phloem and eventually bark.
Thus, all tissues in a tree are constantly turning over. The tree is less like a body, and more of a system. A tree’s young living layer clings to the skeleton of its own dead xylem, like a modern city on ancient ruins, or the live surface of a coral reef. All the while, it is growing in size. With time the system progresses through a structural evolution that defines the stages in its life cycle.
One mechanism that gives trees more resilience is redundancy. Remove a branch, and the loss of its hormone signal will resonate through the rest of the tree. Other branches grow more vigorously to correct for the loss, restoring the ratio between roots and shoots. If a fire or animal damages the vascular cambium on one side of a tree trunk, the cambium on the other side of the tree will thicken to compensate. Meanwhile, cells around the edges of the wound will grow inward to hopefully seal it off.
The hormone signals plant parts send throughout the plant are the only way a twig has any awareness, so to speak, as to whether it is a lower branch on a big tree, or a seedling on a forest floor. Aside from environmental stimuli—light, water, temperature and nutrients—hormones control how different sections of a tree cooperate. For example, if you remove a twig and root it, the hormone signals it receives will change. It is now much closer to cytokinin-producing roots and not getting suppressing auxins from higher foliage. In response, it will begin growing more vigorously, taking on characteristics of a seedling.
Now with that background info out of the way I can get more directly to the question: if the tissue in the tree does not age, and it continuously resets its behavior based on ongoing hormonal cues, why don’t trees live forever?
The problem is the accumulation of defects, which, over time, can overwhelm repair processes. The tree eventually reaches a point where it is much more difficult to keep things working properly.
Plants do not have cellular aging, but old trees suffer structural defects
• First: bigger trees usually have a higher ratio of unproductive (non-photosynthesizing) tissue compared to productive tissue (foliage). A seedling has leaves, a short stem, and roots. In a big tree, the leaves and branches are much higher and farther from the root tips. That means there is a long span of vascular tissue that must be kept alive. Trees expand their canopy as they grow, but the ratio between the massive cambium covering the trunk and branches, and the finite horizontal surface area determining the canopy’s access to sunlight, increases. The vascular cambium needs to maintain complete coverage over the heartwood to protect it from decay. Over time, there’s less energy to invest in new growth, so bigger, older trees grow more slowly. They’re also less able to correct structural defects like wounds.
• As a tree grows, the redundant parts communicate using hormone signals to work as a cohesive whole. Genetics, size and environmental stressors determine the size and circumstances in which the tree transitions to its mature stage. Gradually it uses resources to produce flowers, fruit, and seeds, investing less in growth. Some long-lived species may also invest more in energy storage when they get big, slowing growth further.
• The most important limit on the lifespan of old trees is decay in the heartwood. Even healthy middle-aged trees have a few pockets of decay here and there, but the tree’s ability to add new wood each year keeps pace so the tree remains strong. In very old, very massive trees, it is more difficult to defend a vast surface area, especially when decay pockets begin to coalesce inside the tree.
(One of the limits on the size and age of douglas firs, a very big and long-lived tree, is the fact that virtually all wild trees live with a decay fungus called dyer’s polypore. The fungi grows slowy, and the trees can be ancient before they eventually fall, so it’s not harmful to the species as a whole. But theoretically, if the polypore were not there, doug firs might be able to outgrow redwoods).
• In natural settings, competition limits the lifespan of old trees. Size provides incredible advantages reaching light and absorbing water, but there’s a point when the benefits max out. A big tree doesn’t have the opportunity to shrink its tissues to a more manageable size in a drought or disaster without exposing its dead heartwood to the environment and decay. Any loss of canopy requires a corresponding loss of roots, allowing other plants and trees to colonize soil and challenge the old tree’s dominance. Ancient trees tend to develop large sections of exposed, decaying heartwood, which means big portions will eventually break off. This means they often lose their tops, resprouting foliage from lower down. That costs them the advantage of height, while they still face the burdens of high tissue mass and very large sections of exposed heartwood that lead to continued breakage. An ancient tree may go through multiple cycles of breakage or dieback and regrowth. Each time, it accumulates a greater burden of decay, since the cambium is less and less of an intact cylinder covering the structure. Eventually, trees cannot compete against their less burdened neighbors, and die off while middle-aged trees assume dominance.
One other thing, more speculative on my part: plants get viruses, and viruses are not curable in plants. Usually, the plant continues to live, but less vigorously. Many plant viruses are asymptomatic, and their only effect is a metabolic burden leading to more stress and less growth. Luckily, most plant viruses do not get passed on to seeds. So there may be a point when the viral burden is high enough that the old tree is struggling too much and a seedling tree would be much better off. However, there would have to be more study into old trees and the presence of asymptomatic viruses for me to decide whether this is a realistic factor.