Some trees can live for thousands of years, but we may not be around long enough to really know whether they can die of old age.
Trees do not pay taxes. Some seem to avoid death as well. Many of the world’s most ancient organisms are trees, including a 3,600-year-old cypress in Chile and a sacred fig in Sri Lanka that was planted in the third century B.C. One bristlecone pine known as Methuselah has been alive for nearly five millenniums, standing in a forest in what is now called California.
But according to a paper published Monday in the journal Trends in Plant Science, time ravages us all in the end. The paper, “Long-Lived Trees Are Not Immortal,” argues that even the most venerable trees have physiological limits — though we, with our puny life spans, may never be able to tell.
Sergi Munné-Bosch, a plant biologist at the University of Barcelona, wrote the article in response to a January study on ginkgo trees, which can live for over a thousand years. The study found that 600-year-old ginkgos are as reproductively and photosynthetically vigorous as their 20-year-old peers. Genetic analysis of the trees’ vascular cambium — a thin layer of cells that lies just underneath the bark, and creates new living tissue — showed “no evidence of senescence,” or cell death, the authors wrote.
Dr. Munné-Bosch said he found the paper “very interesting,” but disagreed with how some readers of the study in popular media and beyond had interpreted it.
“In my opinion at least, there is no immortality,” he said.
Those tree species that can live for centuries or millenniums have a lot of tricks for staying youthful. They have simple body plans, and develop modularly, so they can replace parts they lose. They also build on their own dead tissue, which provides support and volume at a low metabolic cost. The trunk of a very old tree might be 95 percent dead, Dr. Munné-Bosch said, a strategy used also by other plants.
For these reasons, it’s much more likely that such a tree will die of external causes than age-related ones. In some populations, this can result in “negative senescence” — a phenomenon where the durability of older trees means they actually have a greater chance of survival than younger ones, Dr. Munné-Bosch said.
Still, “everything seems to indicate” that individual trees are mortal, he said.
But others have a different take.
“A modular organism such as a tree could hypothetically live forever,” said Peter Brown, a forest scientist who runs an ancient tree database called the OldList. “I don’t think there is any real physiological or anatomical limitation for them not to just keep going.”
In practice, though, “something always comes along” and interrupts, whether that’s a windstorm, a logging harvester or a swarm of bark beetles, he said. Many trees on the OldList won the placement lottery, Dr. Brown said — they’re rooted deep into rocks, hard to get to with an ax, and far enough from other trees that pests can’t spread.
Dr. Munné-Bosch points to some potential limits. For instance, the vascular tissue that ginkgos produce gets thinner and thinner each year. At some point, it could become too thin to function, killing the tree, he said.
Ginkgos also suffer more physiological stress as time goes by, along with a depleted supply of growth hormone. Despite their miraculous vascular cambiums, “it’s probable that even ginkgo trees may die from ‘natural causes,’” said Richard Dixon, one of the authors of January’s ginkgo paper.
Dr. Brown and Dr. Munné-Bosch agree that the question is almost impossible to answer experimentally. Very old trees are rare, and the same tricks that allow for their long-term survival make them hard to find. (The oldest age group in the ginkgo study contained just three trees, all younger than 700.) So it’s difficult to design a comprehensive study on them.
Plus, our own life spans are simply too short. Even if a scientist dedicated her whole career to very old trees, she would be able to follow her research subjects for only a small percentage of their lives. And a long enough multigenerational study might see its own methods go obsolete.
For these reasons, Dr. Munné-Bosch thinks “we will never prove” whether long-lived trees experience senescence, he said. So in his own experimental work, he now focuses on shrubs with more manageable life spans, of around 30 years.
“I think at the end,” he said, “we have to accept that we will all die.”