A Passion For Plants: In Botanical Time

Discover the incredible longevity and resilience of these remarkable living beings.

Plants have played a vital role in shaping life on Earth, creating ecosystems and influencing our climate for millions of years. But how and why can some plants live to such a great age? Delve into the lifespan of plants and follow their fascinating evolution, from ancient single-celled ancestors to today’s rich variety.

 

The following excerpt is from In Botanical Time by Christopher Woods. It has been adapted for the web.

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The Incredible Lifespan of Plants

Our planet Earth is 4.5 billion years old, plus or minus 50 million years. Luca, an acronym for the last universal common ancestor—a single-celled bacterium-like organism, and the ancestor of all known life on Earth—is estimated to have been born 4.2 billion years ago.

How do we know the earth is 4.5 billion years old? The oldest rocks on Earth are from the Acasta Gneiss Complex of northwestern Canada. Uranium-lead dating places these rocks at 4.02 billion years old. Zircon crystals from the Jack Hills in Western Australia, aged at 4.4 billion years, currently hold the record for the oldest minerals on Earth. And analysis of isotopic ratios in meteorites, particularly using lead-lead dating, indicates the age of the Earth to be around 4.5 billion years.

In botanical time, plant life first emerged on land about 550 million years ago, during the Cambrian Period, around the same time as the first land animals began to emerge. Flowering plants arose more than 300 million years ago—some 50 million years before the rise of the dinosaurs. The plant fossil record is incomplete, no surprise with a 550-million year chronology, but we are getting very close.

It is thought that liverworts—along with hornworts and mosses, non-vascular plants (bryophytes)—may be the closest living relatives of the first land plants. These tiny plants, billions of them, transformed our planet, converting carbon dioxide and water into chemical energy and oxygen using light energy from the sun. This book is about the plants on Earth that have evolved to be the longest lived, with thoughts on how and why.

The Four Methods of Measuring Plant Age

There are four methods for measuring the age of living plants and they are: carbon dating, tree ring analysis, measuring the expansion rate of clonal organisms, and made-up stories.

CARBON DATING

Carbon dating of plants is used to determine the age of a plant as accurately as possible. It is a method used to measure the amount of decaying radioisotope. Carbon-14, a radioactive isotope of carbon, decreases steadily and at accurately measurable rates. Carbon-14 is ubiquitous in the environment. After it forms high up in the atmosphere, plants breathe it in and animals breathe it out.

When living things die, they stop taking in carbon-14 and the amount that is left in their body starts the slow process of radioactive decay. The isotopes of unstable radioactive elements eventually decay into other, more stable elements, in a predictable and precise amount of time called a half-life. The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay.

TREE RING ANALYSIS

Ed Cook, a climate scientist who specializes in dendrochronology at Columbia University avows, “The only way to truly determine the age of a tree is by dendrochronologically counting the rings—and that requires all rings being present or accounted for.” Each ring marks a complete year in the tree’s life (usually). Tree rings are taken by collecting a sample with an instrument called an increment borer. The borer extracts a thin strip of wood that goes all the way to the center of the tree. If the borer cannot reach the center, or if the center is nonexistent due to heartwood rotting or plain old age, the sample extracted will not be an accurate measure of the tree’s age.

But Jonathan Barichivich, a director of research at the Centre National de la Recherche Scientifique (CNRS) in France warns, “If tree rings are a book, then for forty years everyone’s just been looking at the cover.” So partial samples can, it follows, be used only to estimate the age of a tree—this happens with the Fitzroya cupressoides mentioned in this book.

The underlying patterns of wide or narrow rings also record year-to-year fluctuations in the growth of trees; trees grow more some years than others. The patterns therefore also reveal weather history in addition to age: a tree’s growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, such as droughts and harsh spring frosts, resulting in visible growth rings. Tree rings also provide information on the rates of change in the environment, and changes in climate over a prolonged period.

CLONAL EXPANSION

Cloning is the process by which individual organisms with identical genomes reproduce. With plants, organisms produce clones via asexual reproduction: this is called parthenogenesis. A colony or genet is a group of genetically identical individuals that all originated vegetatively, not sexually, from a single ancestor. In plants, an individual in such a population is referred to as a ramet. Clonal colonies are common in many plant species. Although many plants reproduce sexually, through the production of seed, most clonal reproduction occurs by underground stolons or rhizomes. Above ground, these plants most often appear to be distinct individuals, but underground they remain interconnected, one clone.

The Pando is the name given to an area of 106 acres (42.8 hectares) in Utah covered in quaking aspen (Populus tremuloides). It appears to be a large grove of 47,000 trees, but the trees are clones of one ancestor, making them all genetically identical, one tree with 47,000 above-ground manifestations.

Another example represented in these pages is King’s Lomatia (Lomatia tasmanica), endemic to Tasmania and estimated to be 43,600 years old. An underwater meadow in the Mediterranean Sea of the clonal marine grass Posidonia oceanica could be up to 100,000 years of age and possibly 200,000; I discuss its relative, Posidonia australis, the largest single organism in the world by area. Old Tjikko (Picea abies) is a 600-year-old Norway spruce. It has been cloning itself in a very harsh environment successfully for an estimated 9,550 years.

MYTHS AND LEGENDS

Like the children’s game of “telephone,” where the first player produces a message and whispers it to the ear of the second person down the line and so on, some information about the ages of trees has come down through generations of nearby inhabitants. But just as the statement announced by the last player usually differs significantly from that of the first player in the game, any data provided by “transmission chaining” as we call verbal information passed on from one person to another, so we must take into account how easily information can become corrupted by indirect communication and the unreliability of typical human recollection.

These myths are often fascinating, particularly when they give trees personalities or claim to imbue them with mystical energy, but have rarely been proven accurate with science. Nevertheless, they are worth recounting occasionally here as a record of how important certain trees have been to human culture, and often relate directly to modern conservation efforts to preserve not only the species, but the trees’ role in our cultures.

For example, the myth that living olive trees are thousands of years old—a popular claim throughout the Mediterranean—is at best romantic fabrication. It pleases us to think that a tree standing before us produced olives also consumed by the ancient Greeks and Romans, the Arabs, or the Turks.

Without accurate dendrochronology, however, the age of an “ancient” olive tree is impossible to measure. Olive trees’ heartwood rots as they age, making them hollow in the center and therefore impossible to date with a core sample.

Scientists have found that it is more likely the gnarled ancient trees glorified by the villages where they appear are actually only a few hundred years old. This does not detract from their beauty of course, even if the claims are untrue. In fact, they add mystery and myth to the beautiful reality and make us notice them: what stories are for.

Many are familiar with the Buddhist story of Gautama achieving enlightenment while sitting under a fig tree (Ficus religiosa, or pipal). This Bodhi tree is a major attraction for pilgrims, though most don’t realize that Sir Alexander Cunningham planted the tree we see today on the supposed original religious site in 1881. There is considerable doubt as to whether the present tree at Bodh Gaya is even a scion of the original Bodhi tree.

CLIMATE CHANGE

Climate change is often written off as fraud or fiction by many politicians and many corporations with a financial stake in climate denial. We know the climate is currently changing, however—the trees are recording it all for us with their habits.

Meteorologists recorded that 2024 was the hottest year since record keeping began, in 1880. In 2022 and 2023, Earth saw record increases in carbon dioxide emissions from fossil fuels. The concentration of carbon dioxide in the atmosphere has increased from preindustrial levels in the eighteenth century, at approximately 278 parts per million, to about 420 parts per million today.

Glaciers are melting before our eyes, and habitats for plants are also changing dramatically. Half of the world’s mountain glaciers are expected to disappear by the end of this century. As temperatures increase and soil moisture changes, plant and vegetative zones are shifting. Trees are forced to migrate to higher elevations to find cooler, more suitable climates for their survival, but there is only so much fertile real estate on a mountain.

Seas are predicted to rise a foot by 2050, regardless of whether we could even stop emitting carbon globally today. These various factors raise the question of whether trees can adapt quickly enough to environmental stressors to survive, not having been asked to do it so quickly ever before in the history of plant life on Earth.

Why Can Some Plants Live to Such a Great Age?

There is no single answer to why certain species develop what seem to be highly specific—if not seemingly outright bizarre— adaptations for longevity, but evolution through natural selection is the big-picture answer. And natural selection is determined by both environment and genetics.

Interestingly, most of the plants featured in this book grow in very adverse conditions. Not adverse enough to kill them, but enough to slow down their growth considerably. So adapting growth rates to the reality of available energy or nutrient sources seems to be an obvious but astoundingly effective means of survival—probably the one humans would do best to emulate ourselves.

Disturbance phenomena such as droughts, wildfires, and hurricanes, or infestations of bacteria, viruses, and fungi—and of course human impacts—represent the main causes of plant death, either immediate or indirect as natural defenses weaken. Plants have evolved many mechanisms to thwart insect and fungal invasion, few against droughts and wildfires, and none against human impact.

The least fertile sites tend to yield the oldest plants—where life is not easy, you tend to hunker down yourself rather than devote all your energy to procreation every season.

Precipitous cliffs harbor thousand-year-old individuals such as the Qilian juniper. Nutrient-poor sites, for instance in deserts, and alkaline mountain sites, produce some of the oldest plants. Take Welwitschia mirabilis, which grows in the Namib Desert, where in a good year it may receive 3 inches (7.6 centimeters) of rain. But water in the Namib Desert is not guaranteed: in some years, no rain falls. Yet the biggest Welwitschia plant is estimated to have endured for 1,500 years.

Bristlecone pines grow in the White Mountains of California, a land of bitterly frigid winter temperatures, strong winds, piercing sunlight, low rainfall, short growing seasons, and nutrient-deficient limestone and dolomitic alkaline soils. Growth is slow because of all this, but bristlecone pines are able to survive with only slivers of live growth in a mostly-dead tree.

Live wood snakes through the characteristically gnarled gray and white timber to a handful of tufts of waxy green needles and dark purple, bristled cones. Over the years, conditions make the trunks stunted and twisted, their hard wood sculpted by the ripping wind into eroded and fantastic shapes. Matching of dead trees’ growth rings with living trees’ gives us a 9,000-year-long, august history of the species.

Every living thing is born to die—but some take (much) longer to die than others. In the genome of long-lived plants we find many disease-resistant genes; how these were accumulated over the centuries is a subject of much study. Other mechanisms associated with longevity are gene duplication—a genetic backup system, and the active work of telomerases. Telomerase is an enzyme found inside cells; it adds short, repetitive “caps” to DNA strands. These caps are called telomeres.

Each time a cell divides, the telomeres become frayed and slightly shorter. Eventually, they become so short that the cell can no longer divide successfully, and the cell dies. Long-lived plants may have capabilities that prevent or slow down the fraying of the telomeres, thus contributing to the great age of some.

Aging is a complicated process!

A PASSION FOR PLANTS

I am now seventy-two years old and havespent most of my life working with plants, first as a gardener and then as a director of several public gardens in the United States. I retired from running gardens ten years ago, but I couldn’t let go of my passion for plants. So I started to write books about plants. This is my fourth. My interest has evolved through the process of each, from constructed landscapes to wild places—a personal evolution from a fascination with the man-made to the nature-made.

Whether being old myself qualifies me to write this book or piqued my interest in the topic, I can’t truly say. But I believe what has qualified me to be the reader’s guide through this topic is my passion for plants. I will continue to wander and wonder.

Read More: The Black River Cypress

The Black River cypress, at 2,624 years old, is the oldest known living tree in Eastern North America and the oldest wetland tree species globally. This deciduous conifer thrives in nutrient-poor, acidic swamps and has contributed significantly to the paleoclimate record of the region. The ecological integrity of the Black River ecosystem is evidenced by the longevity of its ancient cypress trees and the presence of rare species.  Keep reading….

 

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