For a few wonderful minutes, David Quammen was perched amid the snow-clad branches of "the President," a soaring, sturdy sequoia that is acknowledged as the second-largest tree in the world.

Clinging to the crown of the President last February was a highlight of Mr. Quammen's assignment, one that was not unusually tricky or difficult, but thrilling just the same. On contract with National Geographic for three stories a year, the Montana-based writer jumped at the chance to provide the narrative in support of Michael "Nick" Nichols' images. The two had worked together before on stories, and their synergy is such that Mr. Quammen doesn't hesitate when offered an assignment with the photographer.

“Whenever they say to me, 'Nick is going to do blank, do you want to do it with him?' I simply say yes. 'Nick is going to do elephants of northern Kenya, Nick is going to do lions of the Serengeti, Nick is going to do sequoias, do you want to do it with him?' The answer is yes," the writer said during a phone call.

As a result, Mr. Quammen found himself trodding through the snowy fluff of Sequoia National Park to peer into research Humboldt Sate University scientist Steve Sillett was doing as part of the Redwoods and Climate Change Initiative. The story and supporting images claimed the cover of National Geographic's December issue.

It’s not quite the largest tree on Earth. It’s the second largest. Recent research by scientist Steve Sillett of Humboldt State University and his colleagues has confirmed that the President ranks number two among all big trees that have ever been measured—and Sillett’s team has measured quite a few. It doesn’t stand so tall as the tallest of coast redwoods or of Eucalyptus regnans in Australia, but height isn’t everything; it’s far more massive than any coast redwood or eucalypt. Its dead spire, blasted by lightning, rises to 247 feet. Its four great limbs, each as big as a sizable tree, elbow outward from the trunk around halfway up, billowing into a thick crown like a mushroom cloud flattening against the sky. Although its trunk isn’t quite so bulky as that of the largest giant, the General Sherman, its crown is fuller than the Sherman’s. The President holds nearly two billion leaves.

Trees grow tall and wide-crowned as a measure of competition with other trees, racing upward, reaching outward for sunlight and water. And a tree doesn’t stop getting larger—as a terrestrial mammal does, or a bird, their size constrained by gravity—once it’s sexually mature. A tree too is constrained by gravity, but not in the same way as a condor or a giraffe. It doesn’t need to locomote, and it fortifies its structure by continually adding more wood. Given the constant imperative of seeking resources from the sky and the soil, and with sufficient time, a tree can become huge and then keep growing. Giant sequoias are gigantic because they are very, very old.

Giant sequoias live at high elevations, enduring cold, heavy snows, lightning strikes—and growing bulky and strong, though not so tall as coast redwoods. This individual, the President, is the second most massive tree known on Earth. © Michael Nichols/National Geographic.

While Mr. Quammen has written about trees in the past -- bristlecones, to name one species -- he came away from this assignment with a botanical tidbit he hadn't encountered previously. That was "Steve Sillett’s wonderful discovery and documentation of the fact that the older these trees get, the more they grow," he explained. "And it completely explodes that whole idea of, or largely explodes that whole idea, of the senescence of forests, a forest being quote ‘overmature,’ which was a justification for short rotation for forestry.

“This is very long rotation, obviously," went on Mr. Quammen, referring to the sequoia forests. "The (President) is 3,200 years old, and yet each year it’s adding more and more wood, and therefore among other things, sequestering more carbon then at any other time of its life.”

Space limitations prevented the writer from delving into climate change and sequoias, though he agreed it's a topic that needs to be explored.

"This work that Sillett is doing comes out of a large grant, I mention it in the piece, the Redwoods and Climate Change Initiative. Certainly, that’s the driving concern, that these things live in a very narrow window, and that window could go away with climate change, and that’s the rationale of connecting this work to a climate change initiative," said Mr. Quammen.

“Now, Sillett is a very good, conscientious scientist, so he would not say that we should assume that climate change will adverserly affect these trees. We should investigate that hypothesis, and we should measure that," he said. "I think it would be premature to say that climate change will adversely impact these trees. But it’s important to investigate because climate change certainly could impact these trees.”

While the National Geographic team traveled to Sequoia in February for the story, that was more due to aesthetics than scientific research demands.

"The portrait was taken in the winter, not beause the science team was there then, but because Nick wanted to see the tree in the winter. He was thinking that that was just a very majestic and also revealing way to do a portrait of it," explained Mr. Quammen, "because they are in winter conditions for five or six months. It is important that they are adapted to survive those conditinos, and he thought, well, why not phoograph it in the midst of a snowstorm?"

While the writer heads to Tanzania next to research a story on lions in Serengeti National Park, the trek to the Sierra for the sequoia story wasn't any less fascinating or enjoyable.

"I love going places in the snow in winter. So being in the Sierra, which is a different kind of ecosystem from the mountain ecosystem I live in here in Montana, and going there in February and getting to snowshoe around suited me just fine," Mr. Quammen replied when asked how the assignment differed from his typical field work.

"There was nothing arduous about it. We were staying in a nice lodge, just snowshoeing a relatively short distance, a mile or so to get to the tree, but it was just a lovely environment. ... And as a super extra bonus, I got to climb that tree."