Story Cores

STORY C RES

Story & photographs by Joshua Brown

In Professor Shelly Rayback’s lab, these tree cores— extracted from a forest on the side of Camel’s Hump—tell a tale in each ring, a story going back centuries.

At first, the noise coming out of the tree sounds like a slo-mo woodpecker, “thack…thack…thack.” Every few seconds, Professor Shelly Rayback twists the drill another turn, her forearm muscles flexing visibly, teeth gritted in determination, rain and sweat mixing on her forehead and running down her jacket onto the forest floor. “You gotta be in shape to do this,” she says, laughing. “This spruce is pretty easy. But you spend a day doing maple and it’s a workout.”

As the threads bite deeper into the wood, the metal auger squawks more loudly, now like a distressed bird, reverberating through this dripping stand of hardwoods—and a few large spruce—on the lower slopes of Camel’s Hump. “Sometimes it makes a much louder noise,” says research technician Chris Hansen ’04, G’15, turning his own drill on the opposite side the trunk, “like a moose.”

Some storytellers stare into the campfire. Shelly Rayback cores trees. Inside, written in annual rings of new wood, she and her students read a complicated tale that carries them back hundreds of years, revealing the rise and fall of temperatures, pollution, rainfall, drought. “Trees are archives on the landscape,” she says. “They’re recording what's going on around them in their environment.” And, Rayback’s research shows, what’s written in these rings may help us glimpse our climate-changed future too.

She gently pulls a half-round extractor tray out of the hollow auger, now almost wholly embedded in the tree. Resting on top is a glowing, pale-yellow tube of wood, more than a foot long and thinner than a pencil. “That’s a good one,” she says, as Hansen fishes in his pack for a plastic drinking straw to store this core in, to take back to the lab. Then Rayback holds the tray up close to her eye. “You can see the sapwood,” she says, pointing to a translucent section of the core, just in from the deep-brown spruce bark. This is the wet, diaphanous soul of the tree that carries water and minerals to the leaves and lays down a layer of new cells each year that will age and die and darken into heartwood.

Many people have squinted at a tree stump, counting the rings back in time. “Sometimes you look at an individual ring and say, ‘Oh my God, this was the year we signed the armistice for World War I—and the tree had already been living here for 300 years,’” Rayback says. She and Hansen may look at this core under a microscope back on campus and discover this muscular spruce was a quivering sapling in, say, 1776.

“Well, it doesn’t look particularly old,” says Hansen, peering at the rings. Sometimes a huge tree is less than a hundred years old, growing on a fine site after it was cleared; they’ve also tapped a scrawny spruce, in Vermont, no thicker than your leg, that was growing before George Washington was born. Hansen and Rayback examine their two cores more closely, looking to see if either of the lines they picked into the spruce hit the pith, the very heart of the tree when it first began to grow.

“Pretty close this time,” Hansen says.

“We’re looking for the curvature of the rings,” Rayback explains. As the rings get wider and more curved, the closer they are to center.

Rayback needs to know exactly how old this tree is and what year corresponds to each ring. But that data just calibrates the clock, really, for the richer story that she reads in the rings. Rayback, a professor in UVM’s geography department since 2005, is digging into these mountainside trees today—and hundreds of the oldest spruce trees she can find across the region. (Don’t worry. It doesn’t hurt the spruces any more than does tapping maples for syrup.) “Most everywhere was logged,” she says, “but we have found spruce that go back into the 1600s.” Her goal: uncover the story of past temperatures across Vermont, and the whole of eastern North America, stretching back five hundred years.

With a specialized drill called an increment borer, UVM geographer—and expert reader of tree rings—Shelly Rayback samples the heart of a large spruce on the side of Camel’s Hump. It’s hard work, but harmless to the tree.

Rayback tucks her tools away, pulls on her pack, and then cranks her head back to look up into branches. They extend outward in jagged dark lines, a green lacy pattern that softens away into bright, low-hanging mountain mist. “It’s not too hot today,” she says. But she knows that summer temperatures in Vermont will soon reach levels never experienced during the last century— bringing huge changes to the North Woods and the people that live here.

Instrument records of these past temperatures only go back into the late-1800s. “Earlier, we don’t have much of a clue,” Rayback says. That’s why the National Science Foundation has funded her work, with collaborators across New England and as far away as Indiana and Idaho. “If we want to understand climate change, this work helps us get a better grip on how unusual today’s temperature are over a longer timeline,” she says.

Rayback calls herself a dendrochronologist—a measurer of time in trees. But she’s a geographer first, which means she explores the world through patterns across the land too. “Trees are all over the place,” she says, and in great densities, making them an abundant source of data. Past temperatures and conditions greatly varied from a Connecticut River Valley hollow to a Northeast Kingdom upland bog to a brave wave of stunted fir and spruce near treeline on the side of Camel’s Hump. Trees everywhere respond to their environment. “The rings are like their memory,” Rayback says.

And the collective memory of these hundreds of trees can show, on a precise annual basis, how temperatures rose and fell over centuries, from coastlines to mountaintops—a complex pattern driven by many forces from local topography to regional ocean conditions to the swirling chaos of the global atmosphere. For Rayback, this view of the past, this basic science, is a fascinating end in itself—just as any good story provides its own justification. “But I’m not content to rest there,” she says. “Because of the urgency of needing to understand what is going on now with climate change, our work has to be a continuous thread from past to future.” If she and other paleo-scientists can tease out the effects of climate on these temperature-sensitive trees before the industrial age of humans, Rayback thinks, then we’ll be better able to understand the impact of our recent decades of dumping vast amounts heat-trapping carbon into the air. How does a tree ring—a pattern of summer growth and winter slumber—laid down in layers of cellulose and lignin, record the summer temperature, when, say, Samuel Champlain, in 1609, paddled into the huge lake the local Abenaki people called Pe-tonbowk? Each spring, a tree’s early growth tends to be made of larger cells with thinner walls. As summer progresses, the growth gets denser. A robust body of research shows that the maximum density of this summer growth—so-called “latewood”—reflects the summer average temperature in some tree species, including red spruce like those growing here on Camel’s Hump. Denser latewood, hotter summer. It used to be that this density could only be examined by expensive and laborious X-ray techniques. But Rayback has employed a new, faster, and cheaper technique using blue light and an off-the-shelf scanner. When Rayback and Hansen get the wet cores, now in their backpacks, back to UVM, they’ll be dried, mounted, and polished with hyperfine 800-grit sandpaper. Then they’ll measure blue light bouncing off this carefully prepared surface; the denser wood reflects less of the light.

A few hours and nine spruces later, in a tangled riot of soaking hobblebush and upstart maple, Rayback and Hansen circle around their last tree of the day, rising in a ponderous, three-foot-thick tower. They line up their augers at breast height and, again, begin drilling into the cracked plates of a spruce’s bark. “OK, in the most coarse view, we know it’s going to get hotter,” Rayback says a few minutes later, crossing her hands, taking a deep breath, and turning the blue handle one final turn. Then she quietly takes out another core, eases it into a straw, labels it with a sharpie, and packs it away—before finishing her thought: “But if modelers are going to build more accurate predictions of summer temperatures across the East and in Vermont—over the next decades and centuries—they need better understandings of historic variability.” The kind of historic variability written in ten thin tubes of wood she and Chris Hansen will soon carry down the side of Camel’s Hump. UVM