ROSTER OF THE ANCIENTS
How Old Is That Tree?
It is almost a marvel that trees should live to become the oldest of living things. Fastened in one place, their struggle is incessant and severe. From the moment a baby tree is born -- from the instant it casts its tiny shadow upon the ground -- until death, it is in danger from insects and animals. It cannot move to avoid danger. It cannot run away to escape enemies. Fixed in one spot, almost helpless, it must endure flood and drought, fire and storm, insects and earthquakes, or die.
-- Enos Mills, The Story of the Thousand Year Pine
Dendrochronology, from the Greek roots dendros (tree) and chronos (time), is the formal study of tree rings. As a dendrochronologist, I use trees to determine the timing of a wide variety of events relating to various problems in climatology, history, ecology, and even law (trees are often used as boundary markers). The most common problem I encounter, though, is a seemingly simple one: how old is a certain tree? You might suppose that it would be simple to answer this question: cut the tree down and count the rings. Well, for various reasons, it usually just doesn't work that way. In this article I will explain some of the various ways of determining a tree's age, and try to give some sense of the problems and uncertainties involved in aging trees.
Although tree age determination can be a fairly complicated process, there are basically two methods that can be used: (1) tree rings, and (2) everything else. We'll look at (2) first because it's a relatively short answer. I hope this break down is easy for you to follow and is informative.
The best way of determining a tree's age is to find out when it was planted. Obviously, this is frequently not practical, but it occasionally works if the tree was planted by humans and historical information can give us a date. For example, a forestry plantation, a tree in an extensive garden, or a non-native tree planted when an area was first colonized, could all be aged from historical data. For some kinds of trees, such as cycads, palm trees, yuccas or giant cacti, historical information may provide the only means of getting a good age estimate.
Dating a tree without rings can also be done by measuring its growth rate or by using a chemical analysis, such as radiocarbon dating. Growth rate measurements tend to assume that the growth rate measured over a given recent time period can be extrapolated to the entire lifespan of the plant. Such estimates can be wildly inaccurate. Nonetheless, extrapolation has been used to estimate ages of 1000 to 4000 (!) years for yew trees in England (Hartzell 1991), and it was used by one early researcher (Chamberlain 1919) to infer an age of 2000 years for a cycad based on counting the number of leaf scars on the trunk and multiplying by an estimate of how long it took the plant to produce a new leaf. A similar method has been applied to estimate the age of palm trees. One widely quoted age estimate of 200 years for a typical saguaro cactus (Carnegia gigantea) is based on observing how much a relatively large saguaro can grow over a period of a few years and then extrapolating to the observed size of full-grown cacti. There are numerous problems with extrapolation: trees change their growth rate in response to climate, disease, human activity, competition with other trees, disturbances such as fire, and even intrinsic factors related to the slowly changing size of the tree. Tree ring data show us that a tree may grow ten times as fast at some times as it does at other times. Therefore, age estimates based on extrapolation are basically nothing more than wild guesses. They provide no information.
Carbon dating has occasionally been used to measure tree ages. It is used surprisingly often by scientists who are unaware of the uses of dendrochronology, and is quite useful with certain trees native to the wet tropics, where there is little seasonal weather variation. In tropical climates, some trees never go through the seasonal period of reduced or halted growth that causes a tree ring to form. Radiocarbon (Carbon-14) dates can be reliable if the carbon in the heart of the tree is stable -- that is, if it has remained in place since the tree started its growth. This seems to be a valid assumption for a tree with no heartrot and intact heartwood. However, some trees—such as palms—do not have a single definable area of their trunk that dates to the tree's early years, and such trees cannot be carbon dated. Supposedly a radiocarbon age of 2000 years has been obtained for the tumbo, Welwitschia mirabilis, but I have been unable to find details on this. There is another drawback to radiocarbon: it costs several hundred dollars and takes several months to get a date.
Nearly all reliable estimates of tree age, especially for particularly old trees, are derived from tree rings. The principle used here is that in most trees that form rings, the rings are formed annually, so the number of rings in the tree will provide a fairly close approximation of the tree's age. In practice, there are a number of problems with this principle: (1) trees occasionally produce more than one ring a year; (2) trees occasionally go a year or more without producing a ring; (3) you have to somehow see the rings to count them, preferably without killing the tree; and (4) how was the tree first established and how fast did it grow in its earliest years? We'll look at these problems in turn, but first, a little information on how a tree grows.
Plants are not born in the same way that most animals are. At the very beginning, a tree might be born either from a seed, or from a growing part of an existing tree. In any event, at some point we can say there is a young plant growing, though its age may already be unclear. Let's say it's a young tree and it will be producing annual rings. As it gets bigger, it produces more and more foliage. That foliage requires more and more water and that requires wider and wider rings to carry the water from the roots to the foliage. If you look at the stump of a young tree, you will see this process written in its rings—the rings are very narrow when the tree is small, but they get wider and wider with each successive year. If the tree is growing in the open and has sufficient light and water, this process will continue for decades, carrying the tree through seedling and sapling stages until it's a fine tall tree.
At some point, the tree will start to grow more slowly. It may be getting close to its maximum height, or it may be encountering competition from other trees. Whatever the reason, eventually it stops putting on more and more foliage and reaches a relatively steady state. Every year, it carries about as much foliage and uses about as much water as the year before. Once the tree reaches this stage, each annual ring that it produces will have about the same cross-sectional area as the previous annual ring. However, because of the width of the ring, that area will be spread out around a tree that is a little bit larger. Consequently, each ring is a tiny bit narrower than the ring before it. For most big conifers, this process can go on for hundreds of years. This explains why it is foolish to extrapolate a tree's age on the basis of relatively recent growth—most fairly old trees are putting on narrower rings now than at any previous time in their lives.
Problem 1: Trees occasionally produce more than one ring a year.
Most tree rings are light-colored on the inside and dark-colored on the outside; this alternation of light and dark is what makes the ring easy to see. The change in color occurs because early in the growing season, the tree produces large cells; as the growing season goes on, drought stress causes the tree to produce smaller cells. Because the cells are smaller, there is proportionally more cell wall material, and this causes the cells to appear darker. If there is a period of renewed rainfall in the latter part of the growing season, the tree may start to produce big cells again, and then small cells a bit later on as drought stress resumes. The effect is to produce a second ring, commonly called a false ring. A striking example of this involves Caribbean pine (Pinus caribaea) growing in the Dominican Republic under a climate with very little seasonal variation. These pines put on a ring every time there is a wet spell, commonly making 4 to 5 rings a year. So, just counting rings on these trees could lead you to overestimate their age.
In temperate and subtropical climates it is usually possible to spot false rings by detailed microscopic examination of the cell structure of the tree ring. It's hard to describe exactly what you have to look for, but after you've seen it a thousand times, you get a pretty good idea of what it is. This is what graduate students in dendrochronology do to earn their keep.
Problem 2: Trees occasionally go a year or more without producing a ring.
This happens because the tree suffers some sort of severe stress. For example, the tree could be struck by lightning, burned by a fire, attacked by insects, injured by human activity, or under stress due to adverse weather (such as extreme cold or a severe drought). It can be very difficult to detect missing rings. It's done by means of a procedure called crossdating, which involves comparing ringwidth series from many different trees to identify common patterns. By crossdating, you can use trees that don't have missing rings to find where other trees DO have missing rings. Ah, the alert reader asks, "what if all of the trees are missing the ring for a year?" The answer is, that doesn't quite happen -- but it can come close. When they were first putting together a long bristlecone pine (Pinus longaeva) chronology, there was one year (609 AD) that was missing -- and it took hundreds of samples before that year finally turned up. It was a very dry, very cold year, but there were a few trees growing in sheltered locations that managed to form a ring anyway. Incidentally, some trees live in very harsh situations and have a lot of missing rings. If more than 10 percent of the rings are missing, it is very difficult to figure out the crossdating of a specimen and you are likely to underestimate the tree's true age.
Problem 3: You have to somehow see the rings to count them, preferably without killing the tree.
There are two common ways to get a look at tree rings. One is a saw. For many years (from 1965 to 2012) the oldest tree known was a bristlecone pine that was cut down to determine how old it was. It was found to have 4,844 rings. The remains of this little tree now reside at the Laboratory of Tree-Ring Research in Tucson, where they continue to inspire people to not cut trees down just to find out how old they are. However, saws are very useful for sampling trees that are already dead. For example, the oldest known examples of Pacific silver fir (Abies amabilis) are actually based on counts of tree rings from stumps in clearcuts. Of course, these trees are no longer alive, but the age data tell us how old they can get, and there are enough old-growth silver fir out there that comparably old trees are probably still alive.
The second way of seeing tree rings is with a tool called an increment borer (photo courtesy the Ultimate Tree-Ring Web Pages). It's a hollow drill that takes out a core about 4 mm in diameter and up to 50 cm long. There is a fair bit of debate about how much this hurts the tree. Without going into great detail, it apparently doesn't do much harm to a large and healthy tree, but may kill small or sickly ones. The rule of thumb is, do not core a tree without a very good reason, and then communicate your findings in a suitable forum (such as scientific journals) so that someone else will not have to repeat the damage a few years hence.
Finally, people have looked at tree rings without harming the tree by using techniques such as nuclear magnetic resonance tomography (there was a paper on this in Jacoby & Hornbeck (1987)). Being an incredibly complicated and expensive procedure, it never really caught on. Still, such methods may someday become more common.
Problem 4: How was the tree established and how fast did it grow in its early years?
Trees can establish either from seed or by vegetative means, growing from the branches, stem or roots of another tree. If a tree grows from a seed, then you can say that in one year there was no tree, and in the next year there was a tree; it had a definite beginning, and in theory we could determine when that was. If the tree arose from a growing part of another tree, then we can't even say for sure when the tree actually became a separate organism. Vegetative reproduction is common in trees, and in some groups it is much more common than reproduction from seed. For example, most of the giant Coast redwoods (Sequoia sempervirens) probably originated from roots of their ancestors. Aspens usually grow from the roots of their neighbors, and in fact it has been proposed that some of the largest and oldest organisms in the world are clones of aspen trees, which may contain thousands of individual stems and may live for thousands of years, even though individual aspen stems almost never live more than two hundred years. It is facts like this that demonstrate to me one of the most fundamental differences between plants and animals: the concept of an individual, that is born, lives, and dies, is usually irrelevant in the plant world. To plants, the individual is nothing; the genetic material, the DNA, is the defining thing that sets one apart from another.
Saplings may reach astonishing ages. Here in our Pacific Northwest rainforests, we have little trees that are called "advance regeneration." These trees live in the dark forest understory where they wait for big trees to die, to let a little sunlight through, so that the little tree can grow up into the forest canopy. One researcher found that Tsuga mertensiana seedlings less than 5 cm tall could be 20 years old, without having yet produced a single ring. Another researcher found that Abies amabilis saplings less than 1.3 m tall and 2.5 cm in diameter could have over 100 rings. So, it is very easy to underestimate the age of a tree by a century or more simply by failing to get the longest possible tree-ring sample. The center few centimeters of a tree may contain the record of most of its lifespan!
We do have some extremely old trees in the Pacific Northwest of North America. All of them are conifers, as are most extremely old trees the world over. Here are a few comments on our longest-lived trees:
Coast Redwood (Sequoia sempervirens): Besides being the largest tree ever thought to have lived (there are reliable records of sawmills processing redwoods larger than any tree now living), the redwood is also one of the oldest. Up until recently (2007), it was very difficult to get good age data on redwoods. Samples taken near the base of the tree reveal poor circuit uniformity, meaning that many rings do not go all the way around the tree. Therefore core samples have many missing rings. The only way to get accurate samples was by cutting cross-sections from trees. This is not acceptable with living trees, so most age data were taken from trees that were logged historically, or that died from natural causes. Unfortunately, such trees often have very extensive rot, so that they are hollow, and hundreds or perhaps even thousands of years of record have been lost. I have the oldest known redwood sample; it has over 2,000 rings, and it has many missing rings as well. I suspect that coast redwoods can live 4,000 years, but we may never know for sure. However, work by Allyson Carroll at Humboldt State University has recently shown that if you core coast redwoods above the base, meaning 10 meters or more above ground level, then the problems with circuit uniformity and missing rings disappear. These cores crossdate very well across more than 1,000 years of record. The next step is to try to collect some really long cores that will cover most of the tree's life span. In this way, we may soon be able to prove the existence of redwoods substantially more than 2,000 years old, and to extract environmental information covering that entire span of time.
Alaska yellow-cedar (Cupressus nootkatensis): The oldest known yellow-cedar sample contains 1,834 rings in a piece of wood about a meter long. Like redwood, yellow-cedars usually have severe heartrot. They live in some of the oldest forests in the world, on the west coast of Vancouver Island, in stands that have not been destroyed by fire, wind, tsunami or anything else for over 4,000 years. I suspect that some of those giant yellow-cedars are over 4,000 years old too, but we may never be able to prove it.
Western redcedar (Thuja plicata): The story is pretty much the same as for yellow-cedar. Redcedars grow to enormous sizes, sometimes over 6 meters in diameter, and they have similarly enormous heartrot.
Douglas-fir (Pseudotsuga menziesii): The oldest known Douglas-fir, 1,350 years, was a giant tree on Vancouver Island that blew down a few years ago. It yielded a tree-ring record that went to the solid heart of the tree. However, it is unlikely that Douglas-firs have ever lived much longer than this Methuselah. Sampling of thousands of old trees, from all over its vast range, has produced only a handful more than 1,000 years old.
Mountain hemlock (Tsuga mertensiana): There are not a lot of data on old mountain hemlocks, but what we do have suggests that these trees live at least 1,000 years under the right circumstances. The oldest trees start life as advance regeneration and live for a couple hundred years as seedlings and saplings. Then a gap opens in the forest canopy and they grow to the size of small trees before the gap closes. They wait then, dark and somber, for a couple hundred more years until another gap opens in the canopy and they can resume their skyward growth. The very oldest trees may repeat this process 3 or 4 times before they finally enter the forest canopy as full-fledged forest dominant trees at the venerable age of 700 or 800 years. Then, if they're lucky, they get a few hundred years in the forest canopy before lightning, fire, wind, or rot finally ends their career, and they return to the forest floor.