Every other Friday, the Outside/In team here at NHPR answers listener questions about the natural world. Today’s question comes from Will Heap, calling from Coulterville, California, just outside of Yosemite National Park.”

“Driving past a beautiful forest yesterday, I had the realization that I don’t understand how trees age… So my first question is: How does aging work? Is it a breakdown of cells or in the efficacy of cells? And then my second question is: Do non-animal species such as plants, trees in particular, age the same as human cells?… What limits the age of a tree? 

Producer Justine Paradis counted tree rings to see what she could find.

Transcript

Justine Paradis: For many of us, aging is a matter of great concern. But it turns out it’s weirdly tough to define what aging actually is, even in humans.

Deborah Roach: Aging itself is the accumulation of damage and the decline of function with increasing age.

Justine Paradis: This is Deborah Roach, a biologist and professor emeritus at the University of Virginia. In her career, Deborah has studied the evolution of aging in plants. And it’s a topic that defies easy answers. 

Deborah Roach: Disappointingly, I think trees are a difficult species to begin to look at the questions of aging. 

Justine Paradis: Deborah studied a small leafy plant called Plantago lanceolata. She compared the performance of 1-year-old seedlings to 3-year-olds, and she found some differences based on age.

Deborah Roach: Seed size is smaller… germination is smaller. In other words, their offspring are lower quality.

Justine Paradis: But Plantago is a weedy, short-lived plant. It’s more challenging to study trees. 

Deborah Roach: You want to be able to follow individuals from the time of birth all the way through the time of death… and the trouble is that trees often live to, you know, 80 years, several hundred years, or even other species like the bristlecone pine trees, they live thousands of years. 

Justine Paradis: One measure of aging is a change in ability to reproduce. And in some tree species, scientists do observe a peak and then decline in reproduction as trees increase in size. But other tree species just keep on making babies. There’s a bristlecone pine known as Methusaleh which, at nearly 5000 years old, is the oldest known tree on earth. In the ‘70s, when a US Forest Service employee collected a pine cone from Methuselah, he found that its seeds had a 100 percent germination rate. They all grew into viable and healthy seedlings.

Deborah Roach: It’s not just that they can grow, not just that they can survive… these pine trees… can still make high quality offspring too.

Justine Paradis: Another measure of aging happens  on a cellular level. As human cells replicate, the DNA in our cells is copied, over and over. But over time, all that copying starts to degrade the DNA. When that happens in our stem cells, it’s a big deal. Stem cells are important to regenerate our tissues, blood, and neurons. And when they age, we notice. Our hair gets grey and our skin loses its elasticity. 

In plants, the equivalent to a stem cell is called a meristem. Meristems make new leaves and new branches. But unlike human stem cells, it looks like plant meristems may not age, in at least some species. So, “theoretically, trees can be immortal organisms.”  That’s a direct quote from a scientific review by a couple dendrologists published in 2020, titled “On tree longevity.”

The authors lay out evidence that trees don’t die because of genetically-destined cell decline. Instead, they’re killed by some external event, like a wildfire, insect attack, or the swinging of an ax. 

When it comes to super long-lived trees like Methuselah, they tend to grow very slowly in pretty harsh environments, where not much else can live.

In short, the reason that some trees can live for millennia is simple: they’re really good at surviving.  

If you’d like to submit a question to the Outside/In team, you can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org. You can also leave a message on our hotline, 1-844-GO-OTTER.

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