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Do trees die of old age?

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.

Why are there so many dying trees? What emerald ash borer damage looks like and what we learn from it

If you follow news about trees and gardening, you’ve probably been hearing for many years the ominous news of a devastating invasive insect called the emerald ash borer.

The emerald ash borer, a shiny green beetle destroying ash trees across North America, emerges fron a D-shaped burrow in ash tree trunks in early spring.

History of the emerald ash borer in North America

Ash trees are—or were—one of the most common tree types in North America. Our native ash species include green ash, black ash, white ash and a few others, with extensive natural ranges as one of the most dominant tree caregories in the eastern U.S. They’re also popular in yards for their dense shade and tolerance against late cold snaps, summer heat, periodic drought, and soil compaction found in developed areas or under pavement.

Ash trees, a genus with many species, grow wild across an extensive range spanning North America, particularly in the eastern part of the continent. Additionally, ash trees are one of the most common trees planted in urban areas.

In addition to the North American ash species, there are other ash tree species found around the world, including northwestern Asia where ash trees have contended with the emerald ash borer for millennia. The trees co-evolved with the beetles, making the trees resistant to severe infestation just as the beetles became completely dependent on the ash trees to complete their life cycle.

The pill-shaped, iridescent green flying beetle lays its eggs on bark. The larvae burrow beneath to feed on the carbohydrate-rich cambium layer, the green living tissue just under the bark that lays down new growth rings each year and carries sugars from leaves to roots.

After growing beneath the bark until fall, emerald ash borer grubs pupate through winter and emerge as beetles to infest other ash trees. When a beetle finds a fitting ash tree host, it produces pheromones to attract many others to come in, overwhelming the tree’s ability to drown them out with sticky sap.

In ash trees with no resistance to the emerald ash borer, the larvae burrow so extensively under the bark that they completely cut off the flow of carbohydrates through the vascular system to the roots.

Asian ash trees have long adapted to the presence of the borer; North American trees have their own species of slower-growing borers that can damage but rarely kill the trees. Asian ash trees survive infestation from the more aggressive borer species by producing chemicals that slow the insects’ digestion and thus slow their growth, among other adaptations. That way, they can create new tissue faster than the borers eat it, and achieve a healthy equilibrium with the beetles.

North American ash trees have no resistance to the Asian beetles, so when the first infestation was discovered in Michigan in 2002, it set off sirens for ecologists concerned about the future of several vitally important tree species.

The borer population exploded. Within years, ash tree forests across the Midwest and eastern Canada were wiped out. Entire pure stands of ash were converted into shrubby meadows, with ominous dead gray trunks standing over them, as if a wildfire had burned through. Shortly after, mixed forests lost their ash tree stocks. Traveling a few miles per year, the borers spread throughout the region, and occasionally hitchhiked on firewood or other human transport to arrive in other portions of the continent.

Ash borers invading the continent have resulted in vast swaths of dead forests in the Eastern U.S. and Canada.

Infestations in Boulder County, Colorado began several years ago, initiating a government quarantine against ash wood products being moved from Boulder to Denver or beyond. But when the borers were identified in the northern Denver suburbs in 2019, the quarantine was retired since there was now no way to stop them from flying tree to tree and infiltrating the entire metro area.

The documented extent of emerald ash borers in North America as of summer 2020.

In 2021, this is the first summer we’ve seen extensive damage to virtually all of the untreated ash trees in the region. While it’s uncommon to see adult beetles, the signs are impossible to miss.

How ash borers kill ash trees, and how to identify the damage

Emerald ash borers destroy the part of the tree that carries sugar from the leaves down to the roots (the outer layer of cambium or phloem), but they do not stop water from rising from the roots to the leaves, which travels through nonliving tissue (xylem) in the inner cambium. Because the xylem carries only water and minerals, it doesn’t have the carbohydrates borers need to grow and the grubs won’t eat it. That means that the tree can produce leaves for a while even after the infestation is advanced.

But without phloem to carry sugars made in the leaves down to the roots, the entire root system begins starving. It is unable to produce an adequate number of annual feeder roots that sprout from the larger woody roots. Unlike other tree diseases that may kill a section of the canopy at a time, the entire tree’s system for staying alive begins to fail. The appearance is similar to a tree that has a girdling root, or is being girdled by a grate or has experienced other extensive damage to the roots: the canopy thins from the top down and starts retreating.

An ash tree with an advanced ash borer infestation has already suffered extensive damage to roots, starved because the insects have cut their supply of energy from leaves. The canopy, in turn, dies back from the top to try to achieve a more manageable size. It happens on all branches at the same time rather than in sections, which would be a sign of a different kind of pathogen.

During the first season, an ash borer infestation can go completely unnoticed. The first subtle sign is either woodpeckers showing an unusual interest in the tree or small, D-shaped holes in the bark where the first generation of beetles hatched out to mate and lay more eggs.

Trees that are already catastrophically damaged by ash borer can continue to produce dwindling leaves for a year or two because the part of the circulatory system that carries water—located deeper in non-living wood—is still intact. But there’s no hormone communication between the roots and leaves, and no sugar making its way down to feed the roots. Soon, the roots die back and reduce their ability to absorb enough water for the whole tree at its current size.

A tree with inadequate roots, no matter the cause, thins out at the top (which is the the most distant point for water and hormones produced by roots to reach). Trees cope with severe root loss by trying to restructure themselves: they sacrifice leaves and branches that are no longer getting enough water and nutrients to photosynthesize, which starts at the top. Ash trees will produce new green shoots from large branches or trunk, and the new growths can be very vigorous and dense. However, their vigor is deceptive since the total number of leaves is much lower. This would effectively make a smaller tree, which in some scenarios would return to balance with the weakened root system and allow the canopy to regenerate, albeit with a very poor branch structure.

Unfortunately, in the case of an ash borer infestation, these attempts at restructuring the canopy are futile—the root system is not going to rebound since it has been separated from the canopy by ash borer tunnels. The root system is already dying by the time the top level of branches is completely bare.

Usually, the severe damage becomes obvious in late spring when higher portions of an ash tree will fail to leaf out or the tender new leaves die off shortly thereafter. You don’t see much progression in the middle of the growing season since the dying roots can continue to provide a limited water supply to lower leaves. Meanwhile, chunks of bark may begin to curl back or fall off since there’s nothing attaching them to the underlying wood.

You also don’t see much severe wilting and browning in the canopy from ash borer. Browning and singing of the leaves, which occurs, for example, in apple and pear trees affected by diseases like fire blight, suggests a cause that is affecting the ability for water to reach the leaves through xylem. Xylem exist below the upper cambium in channels made of columns of cells that have died and become like hollow straws (the wood’s grain). Bacteria or fungal diseases exist as single cells or filaments that are one cell wide, so they can grow right into the water-carrying xylem channels and shut them off. Infected twigs and branches wilt and turn brown in days.

But because the xylem on infested ash trees are still intact, leaves stay hydrated and don’t yellow or wilt dramatically. Yet, the tree, sensing a nonproductive branch, eventually cuts off the water supply to limbs and retreats further down toward its base.

Later stages of ash borer infestation in a tree

Even green shoots from the base of a mature ash tree infected by emerald ash borer will eventually die as the root system dies, but younger trees, or trees that produce shoots through the soil itself, can form permanent shrubby base growth as they die. The ash borers eventually leave and move on, since they only eat living trunk tissue, and only lay eggs on trunks and branches that are more than a few inches thick.

In trees with basal shoot growth that outlives the dying tree, it’s due to new root systems emerging from the base of each green shoot where they contact moist soil. These bushy, limited growths are vulnerable to drought until the roots grow larger, and they won’t be useful as shade trees. They will only be infested again if they reach a substantial size.

In the last phase of an ash borer infestation, the borers may move on since the part of the trunk they eat is now dead. The tree will produce new shoots from the base, but even these will continue to decline since the root system is so severely starved.

Mature, valuable trees can be protected with systemic insecticides applied before or early in the infestation. But trees that are completely girdled by borers are too far gone even if they still have living portions of canopy, since the roots are dying. Arborists can make a judgment call as to whether the tree is salvageable, usually deeming trees to be too far into the process if the top of the canopy has lost 50 percent or more of a healthy density of leaves.

Asian ash tree species, or hybrids between North American and Asian species, may have enough resistance to ash borers to survive after the first big wave of infestation has passed through a region. Horticulturalists and ecologists are already working hard to develop new ash tree stocks that can resist the borers and potentially restore some of the ash tree options for urban trees. They may even be able to develop strains of native trees to reseed American forests.

Lessons from the emerald ash borer

We as gardeners and tree lovers can take a few valuable lessons from this slow-moving disaster. First and foremost, we need to be conscious of the threat of invasive species. This is not the first time a wave of disease or pest insects threatens to wipe out a major class of North American trees, and since other types of tree epidemics are still making their way around the continent we know it won’t be the last.

Better inspections of plants coming into the U.S. can help, and by carefully observing the environment around us, we may be able to notice, slow down, or eradicate ecological threats before they become unstoppable. County extension offices, which are local branches of state agricultural universities that exist to communicate with the public about landscape health, can be a resource to funnel information to the attention of trained scientists.

A second lesson is to encourage a broader diversity of trees and plants in our urban landscapes. In the recent past, Denver’s urban canopy was dominated by a handful of tree species or families. A loss of one species means a loss of a huge portion of the mature urban canopy, leaving tremendous gaps that take decades to fill with newly-planted trees. By recruiting new species into cultivation (particularly natives in the region), and making sure that each park, yard or city block has several types of trees in it, we can hedge against the chance that a future tree epidemic leaves huge portions of the city bare.

Finally, we can be conscious of the benefits of inviting a diverse array of wildlife into our spaces. Although pests like the ash borer are sometimes unstoppable, they do have natural predators here: woodpeckers, with many native species in this area, eagerly eat emerald ash borer larvae and have actually grown in population in places where the ash borers provide a generous food supply for raising chicks. Other microbes, insects and birds can help compete against current and future destructive pests as long as they have a large enough base population to respond quickly to a major change.

By planting diverse seed-producing gardens, tolerating a moderate amount of native insect pests providing steady food sources, converting rooftops, pavement and gravel strips to open landscapes with lots of region-appropriate plant species, and limiting the use of chemical controls to occasional spot-treatments only, we can create a balanced, abundant urban ecosystem that provides shade and beauty to humans and animals alike, for generations to come.