New England

These Trees Say So Much: Hubbard Brook Experimental Forest in North Woodstock, New Hampshire

Why a little-known New Hampshire forest is one of the most studied and important in the world.

By Yankee Magazine

Sep 03 2023


U.S. Forest Service research ecologist Lindsey Rustad at Hubbard Brook Experimental Forest, a living laboratory for long-term studies that have helped transform the field of ecology and environmental policy—including the discovery of acid rain.

Photo Credit : Joe Klementovich

By Nina MacLaughlin

Early afternoon, high summer, and a blazing sun lights a span of the White Mountains in North Woodstock, New Hampshire, and it glows a deep green-gold. Then shadow washes across the mountainsides, as though someone has pulled a curtain on the day. The clouds aren’t cottony fluffers on their easy way, but heavy, thick, with eggplant underbellies and gray-cream tops carrying threat of storm. So it shifts, between beaming sun and shadow, back and forth, raising a question only time would answer.

I slip into the forest. Out from under open sky, the canopy eclipses most of what’s above, save a patchwork brightness. It’s cooler in the woods. Moss throbs off rocks, turns earth to pillow, softens the edges of the path. A sweep of ferns ruffles a bank around a corner. To the left, unseen but heard, the chuckling of a brook, water bouncing downhill over rock. The trees, thick and thin, uncowering, stand as bridges between the dark wet soil below and the sky above. The air has a floral smell, lilies and vanilla, and below that, the lactic tang of decay, that autumnal scent, aggressive but not unappealing, like a challenging cheese, musky, rotting, and seductive all at once. A hawk skreees from somewhere up the hill. A chatter of round little chickadees bounces between low branches. Forest air feels more nutritious to breathe in. You’ve felt this, too, maybe, the way the blood answers to it differently. We take the woods inside of us and it changes what we’re made of, an experience easier sensed than understood.

What we sense, what we know, what we wonder and try to find the answers for—the forest stirs it in us. We sense: a great net of relationships and connections, invisible strands aglow above us and below. We know: that’s paper birch, that’s goldenrod, that’s toad, salamander, slug. And we wonder: how long has it been here, how’s it changing, what’s happened, what’s happening, what’s going to happen?

Those questions drive the scientists and researchers at Hubbard Brook Experimental Forest, one of the longest-running research forests in the country. It looked that day like the forests that I know, familiar in its New England way, the birch and beech and honeysuckle, the now-and-then scurry of a chipmunk or a squirrel, puddles of light on the leafy floor. A forest like other forests, and unlike any forest anywhere. Its 7,800 acres—nearly 10 times the size of Central Park—and the trees, soil, birds, beetles, microbes, leaves, lichens, rock, and water that it holds have been under the watchful, patient eyes of an international and intergenerational team of scientists and researchers for nearly 70 years. These are some of the most scrutinized woods on the planet, and the span and depth of data collected here impacts environmental policy, land management, fire safety, recreation, the way weather is reported, logs are harvested, laws are passed. And it informs what we know—and, more important, what we don’t—about our changing climate.

* * * * *

Not a lot of people know about the place. Did you know Hubbard Brook is where acid rain was discovered in North America? I didn’t. On a July day in 1963, a rainwater sample taken there raised the eyebrows of the scientists. The pH levels were between 10 and 100 times more acidic than what would be expected for the region. What was causing it? Was it specific to this area or something more widespread?

To study the effects of severe drought on forest health, Hubbard Brook researchers built a platform that blocks about half the water that would typically fall onto the forest floor.
Photo Credit : Joe Klementovich

Nine years of research later, Hubbard Brook published a paper on the issue. The initial hypothesis, that acid rain was caused by the combustion of coal and oil in electrical plants, gave way as they discovered it was much more so the result of nitrogen oxide emissions from cars and trucks. Which led to another question: Will acidity lessen if we reduce emissions? The Clean Air Act in 1970, and its amendments in 1990, proved that yes, it will, directly and significantly. None of this happened overnight. “It required 18 years of continuous measurement from the beginning of the Study before a statistically significant downward trend in acidity at Hubbard Brook was observed,” as Richard Holmes and Gene Likens write in Hubbard Brook: The Story of a Forest Ecosystem. “For enthusiasts of monitoring, this is sobering. But it shows the critical value of long-term data.”

Part of what’s involved in the long-term data at Hubbard Brook is the handing down of information to younger generations, the sharing of data and wisdom, allowing new questions and new experiments to rise from what’s come before; it’s a palpable spirit of the place. At the annual cooperators meeting last summer, Hubbard Brook scientists, students, and researchers gathered in person for the first time in two years, filling round tables in a large conference room at Plymouth State University. Sport sandals, hiking boots, pants with ample pockets—it was a wardrobe that suggested a group more comfortable tromping through woods than sitting in a conference room under drop ceilings around laminated tables, but spirits were high, and an energy of support and collaboration apparent.

Nicholas LoRusso, 31, grew up on the Saint Lawrence River, swimming, fishing, having conversations with his father and his grandparents about water levels, pike populations. That connection to the outdoors animated his research at Hubbard Brook, studying the processes of dissolved organic matter within calcium-treated watersheds at the forest. He spoke about the evolution of the work being done. “One generation has their ideas; interests grow and become interconnected.” A complex ecosystem of collaboration and connection develops among ecologists, biologists, entomologists, geologists, chemists, policy makers, musicians, artists.

For LoRusso, who was about to start a job at Harvard, this forest “builds love and community like no other place that I’ve experienced.” Everyone does their specific work, and all the work connects, overlaps, grows, not unlike the forest itself.

* * * * *

“We’re changing the seasons,” says Lindsey Rustad, research ecologist and U.S. Forest Service team leader for Hubbard Brook who’s been studying the forest since 1997. We’re in the forest on that summer day as the sun comes in and out of clouds. Lightly freckled, in a dark green Forest Service shirt, Rustad speaks with her whole body and moves with nimble ease through brush and over forest tangle, with a spark that often lights the eyes of people who spend a lot of time outdoors. She looks younger than 64, in part for the enthusiasm, the almost kid-like glee, she has when she explains what’s afoot in these woods.

Weather conditions are being manipulated here, something that ends up being both simpler and more complex than one might expect. To accelerate the arrival of spring warmth, snow cover is shoveled off a particular section and heaped somewhere else. To approximate a drought, a translucent platform is built over a plot of forest; sun gets in, rain does not.

USFS technician Amey Bailey amid the archive of water samples that have been collected at Hubbard Brook for more than 60 years.
Photo Credit : Joe Klementovich

Tucked away in another corner, researchers are studying what happens when the forest soil warms. Plastic laundry bins sit here and there upside down. Certain trees wear belts of metal. To an outsider’s eye, it looks haphazard—messy yard as opposed to science lab. NASA, it is not. But the simple-looking above-ground tools belie a sophisticated and long-running study. Underneath the surface, Rustad explains as we step through this area, heating coils warm the earth the way radiant heat warms a kitchen floor. This experiment began in 2014 and required a year of careful monitoring and measuring before the switch was flipped and the coils were turned on. How is the forest system responding? What’s changing in the make-up of the soil, the growth of the trees, the chemistry of the leaves? “It doesn’t look bothered now,” Rustad says, “but at some point it’ll make a difference.”

At what point? When? And what will “bothered” look like? Rustad uses the word “resilient” to describe the forest again and again, and each time it comes from her mouth with a sense of reverence, almost awe. But it’s resilient to a point. “Our concern is not having knowledge of what the tipping points are.” That’s one of the things they’re trying to figure out.

Researchers clear-cut a swath of forest to see how that altered the water. Nitrogen levels rose 50 times higher. But in three years, raspberry, pinchberry, and birch grew; the zone biochemically reinvented itself, and normal levels returned. “When you’re measuring over 50 years, you see how fast the forest regulates itself,” Rustad says. In another forest section, they powdered areas with calcium to see what it did to growth; elsewhere, they lit fires and let land burn. Life came roaring back.

Extremes are part of what drives the work in the woods. Storms are getting bigger, stronger, longer. There are more droughts and fires and floods. Heat waves and hurricanes. For the northern forest, it’s the ice storm that has the potential to become more intense, and more damaging, as warm air travels farther north and bashes against polar air drifting down from the Arctic. Back in the late ’90s, Rustad and her team wondered, Can we manufacture an ice storm? The answer: yes. On a series of frigid nights, using firefighting equipment, they sprayed three sections of the woods, each the size of a basketball court, seeing what would happen with less than a quarter inch of ice, a half inch, and three-quarters of an inch.

Using fire hoses and high-pressure pumps, researchers sprayed water 100 feet into the air during subfreezing temperatures to simulate one of nature’s most extreme winter events: an ice storm.
Photo Credit : Joe Klementovich

They wanted to see how much the forest could take. In the quarter-inch section, there’s almost no noticeable difference. Walk to the half-inch section, and you’ll see some trees arched and bent as though picking up a sock or bowing down in prayer. We move toward the three-quarter-inch section. Light starts to shift on the forest floor. “Look up,” Rustad says. Open sky. A great gap in the canopy, not a patchwork of light through leaves, but raw blue, up and up. It’s as though a bus-size meteor burned a hole through this one section of the woods. Limbs cracked. Trees downed. “After half an inch of ice,” Rustad says, “is when it starts to get catastrophic.”

They don’t just look at what happens the day after, or week, or month. This experiment began in 2009. They’ve been monitoring the impact of these ice storms over years. What dies? What’s resurrected? Do the prayer-bowed trees rise again? What happens to the nitrogen levels in the soil? The data impacts how emergency alerts are issued and when people start getting a heads up. The results have real-world impact, and these kinds of storms, altering, cataclysmic, are on the rise.

* * * * *

It’s easy to forget these threats when walking through the woods, surrounded by the pulse and throb of undeniable vitality. All this life, all this movement, all this light and shadow; how many greens, how many browns, roots tangling below, leaves swaying above, two orange slugs slicking over a soft and rotted stump, two bright red toadstools bulging at its base. Life, everywhere you look.

Rustad and I sit down on a bed of copper needles overlooking the brook. She sets a timer for four minutes and we sit together on the earth in silence. The water speaks, the trees speak, the water and the trees speak in prophecies; they tell us what’s to come and we’re learning how to listen.

Walt Whitman writes of the tree, “It is,yet says nothing.” I disagree with him. The tree says so much. It carries information from below the earth up through the body into the branches and into the leaves into the air into the sky into our lungs and our blood. In its quiet eloquence it brings the news we need, it breathes with us. It is, it is, yes, and it says, it communicates. Maybe one reason we’re so drawn to trees is that they resemble our neurons, which have rooted clusters, slender trunks, and branching spreads. Charged and pulsing, they carry information around the human body. To look into a forest is to see the trees repeating what exists in the wilderness of our minds, our own and actual selves. Is that why being in the woods alters us? Eases our nervous system, makes us more open, alert, and calm than the frantic, hungering, distracted selves we first enter with? Because we see the inside on the outside? Maybe it’s a reason so many fairy tales are set in the forest, enchanted, frightening, altering; it’s where we enter our shared mind.

Rustad’s timer sounds. Four minutes goes by quickly. We look at each other and smile gently. When we speak again, we speak more slowly, our voices are quieter, as though coming from a deeper place in us. Rustad speaks of “warm knowing.” There’s the hard data, the figures and measurements, the precision of the tools, the meticulous monitoring, and she’s proud of bringing the forest into the digital era. And there’s what we know in our bodies. We do not enter the woods and exit the same. Even a short loop on a familiar route does something to our brains, which is to say, it does something to our bodies, which is to say, it does something to the deepest places in us and some might call that soul, I don’t know the word, that might be it.

* * * * *

The trees and dirt and plants and birds get attention at Hubbard Brook, and so does the water. A big barn holds the Hubbard Brook archives. Rows of shelves are filled with bottles holding water samples from every day the experimental forest has been running. (“Find your birthday,” Rustad says.) There’s so much information. And so many hands required to gather it over all these years, people kneeling on the bank to fill a bottle with water.

As we sit by the brook, it pools in places, held by bowls of rock. Water bugs shift on their suction paws like stars which skim across the night. The water seems swift and smooth, a gentle benevolence. But the power of water moving over earth is hard to overstate. It has strong jaws. It chews through stone. It carves mountains. Like the trees, like the neurons, like blood, it carries with it so much information.

The water moving down the brook is decomposition, is dirt, is tree and root, is light and path, is the rock it caresses, tumbles, shapes, is rush, is lapping, is turtle shells, is pollywogs, is broth, is a history of cells. Fed by numerous tiny trickling tendrils, the water moves down the long hillside slow in gurgle and trickle and rush, over rock and twig and root and branch, over moss and lichen, picking up information, absorbing what’s there, over mud and sand, down and down to a place where it merges with the Pemigewasset River when the little brook becomes bigger than itself, spreading and widening and getting absorbed into the Pemigewasset, an Abenaki word that means “swift current and where the side current is.”

USFS hydrologist David Zietlow checks streamflow measurements during an extreme rainfall event.
Photo Credit : Joe Klementovich

Blueberry, elderberry, and shadbush grow on its banks. The Pemigewasset shifts and swifts and drifts and joins the Winnipesaukee River in Franklin, New Hampshire, to form the Merrimack River, which curves over 117 miles, through Concord, Manchester, Nashua, through Tyngsborough, Chelmsford, Lowell, through Dracut, Tewksbury, Andover, Methuen, Lawrence, North Andover, and on past the red brick of the old textile mills, through Haverhill, Groveland, West Newbury, Merrimack, and Amesbury, where it reaches its mouth, its spilling wide-open mouth, where what was once the chuckling trickle of a brook is taken up into the Atlantic Ocean.

And from there, from there, the molecules of brook are swished and swallowed into the sea. And at some point a wind comes. Hot air meets cool, water is pulled from the ocean and rises into the air, rises into the sky, gets absorbed with other water molecules on the move, there, above, we can’t see it happening, but we can when a cloud forms, and it drifts over the land, swelling as it moves, shifting in the wind, as it takes up more water, as it absorbs what’s there, what we’ve put there, and it moves through the sky, the tiny bit that came from the brook, combined with so much else, it shifts shape and it moves, casting shadows on the land, passing between the sun and a mountainside on a summer afternoon, and you look up and wonder, Is it going to rain?

To learn more about Hubbard Brook research, go to