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What a year with a single tree reveals about climate change

A century-old oak in The Harvard Forest in Petersham offers lessons about global warming — and life on earth.

Lynda V. Mapes (far left) spent a year in the Harvard Forest studying one red oak and kept meticulous notes. In the oak, climbing instructor Melissa LeVangie (left) shows novice Mapes the ropes. Two webcams keep an eye on seasonal changes in the witness tree canopy. Lynda Mapes

MY HAMMOCK RODE the wind in the tree’s topmost branches, its rocking embrace some 80 feet up amniotic and primal.

This was my triumphal tree climb, a capstone of a year spent with a single 100-year-old oak at the Harvard Forest in Petersham. My climbing instructors had slung the hammock, and I had hauled a picnic in my backpack up into the tree to share: roast chicken, a side of incendiary dhal, and, to top it all off, a big slab of dark chocolate.

We ripped into the food, then quieted, just enjoying being in the top of the tree. A black and yellow swallowtail butterfly cruised past my shoulder; chickadees called sweetly, alert to our presence. The leaves stirred in every direction, and the tree moved with the breeze, up, down and sideways, all at once.


Feeling both at home and distinctly a visitor, I thought what familiar and alien things trees are. They remain wild, essentially other, a kingdom apart. We need them, but they do not need us.

I had lived this tree’s seasons, from its hail of acorns in fall to the sharp glitter of burning stars through its bare branches on winter nights. I had watched its spare winter geometry soften into the filigree of first leaves in spring, that time we so yearn for, the season of fresh starts and new life. I had endured biting black flies as the tree’s deep-olive-green summer leaves grew resplendent, and reveled in the glow of yellow and red maples lighting the autumn wood.

As a journalist, I’d set myself the task of a fresh look at climate change beyond dueling politicians, press releases, or marches for the environment. Instead, I sought the quiet counsel of living things. My oak, it turned out, was the perfect teacher.

The tree first came into my life along with John O’Keefe, a biologist who for the past 25 years had studied the same 50 trees at the Harvard Forest, less than two hours west of Boston. I was a Knight Fellow in science journalism at MIT in 2013-2014, sitting in with professor Andrew Richardson’s lab at Harvard University. Andrew and his colleagues were onto something new, taking data from John’s daily walks as the literal ground truth to enhance a new observational method that Andrew and his collaborators had devised: a network of bank security cameras, of all things, mounted on towers to keep an eye on the tree canopy. Here was a way to see the pageant of the seasons John was watching, but on a far bigger scale.


Known as phenology, the observance of seasonal changes in nature is perhaps one of humanity’s oldest biological records. In the eighth century, Japanese monks made notes on the first cherry blossoms; closer to home, Henry David Thoreau kept records on the first flowering of plants on his daily walks around Concord.

Disregarded as a mere hobby by mainstream science for generations, phenology has been rediscovered as researchers explore old records to find evidence of our changing climate. Nature, John’s records attest, is an articulate witness.

The observations from John’s forest walks showed in pointillist detail a changing world. He noticed everything: the mineral scent of the soil released in the spring thaw, the first call of wood frogs, the arrival of ice on the puddles. Recorded year after year with his No. 2.5 pencil, his hyperlocal focus brought home a global reality: The seasons are not what they used to be. On average, spring is earlier. Fall is later. And winter is getting squeezed on both ends.


I’d heard of John’s work and asked if I could join him on his surveys. Not long into our adventures, I sent him an e-mail. “John,” I said. “I need a tree.” I wanted a frame through which to probe global questions, from the changing climate to shifting seasonal timing and the evolution of the New England landscape and how these can be revealed through the life of one forest and even one tree.

“Here,” John said, putting his hand on the bark of a big red oak — more than 80 feet tall with a 65-foot crown spread — as we walked his survey loop one day. “This might be a good one for you.”

Sprouted beside a stone wall in what used to be a pasture now grown back to woods, the oak had stood witness as the people who worked the land left for jobs in towns and factories, making the carbon dioxide emissions that are altering our world. Borrowing a term from the trees that settlers once used to mark the bounds of changing landscapes (the living landmarks turn up in survey notes and maps still kept in town records all over New England), the big oak would be my “witness tree.”


Before long, I had moved to the woods as a Bullard Fellow in forest research to live with my tree. My base camp was an apartment in Community House, a glorious, many-times-remodeled 18th-century New England farmhouse rented by Harvard to house visiting scholars. There was even a troupe of cows in the pasture out my front window for company. My tree was just steps away.

Biologist John O’Keefe (left, with Lynda V. Mapes) has studied the same 50 trees for 25 years. doug macdonald

WHILE IT IS A NATURAL New England wood, signs that the Harvard Forest, founded in 1907, also is an outdoor classroom and laboratory are never far from view. Trees bristle with tags and flagging, and the forest floor is studded with equipment. On just one tract of the 4,000-acre forest, where my big oak stood, more than 40 experiments were underway, involving dozens of scientists from Harvard University and around the country and world.

In hiking boots, snowshoes, bare feet, snow boots, muck boots, sandals, bedroom slippers, and waders, I explored the forest in every kind of weather, at every hour. There was so much to learn about something seemingly so simple: a tree.

Scientists are still probing the brilliance of trees’ photosynthetic alchemy, turning sunlight, water, and air into the substance of their leaves, roots, and branches, a feat mere people have never been able to re-create. But what of the poetics of trees, how they move, breathe, and command such fluent agency? I truly had no idea; to know even one tree well, I discovered, is to be dazzled.

Trees are not just standing there. They are busy, productive, consummate diplomats, managing a suite of relationships both collaborative and combative. Although they may look sedate and aloof in the forest, seemingly each trunk to itself, up in the canopy and below ground the real story is revealed.


I had noticed particularly when I climbed the oak that each branch tip reached just exactly to the edge of the next tree’s branches, as close as possible, yet chastely not touching its neighbor, a phenomenon scientists charmingly call canopy shyness. In this way, each tree, I discovered, claims exactly its place, presenting an interwoven, interspecies community of branches to the sun and wind.

Below ground, a network of fungal filaments less than one-20th the width of a human hair thrums with the exchange of nutrients amid an uncountable constellation of microbial life.

And silent? Trees are anything but. In addition to the wind songs in their branches, their creaks and cracks, oaks have a language all their own of pheromones that can call out a sharp warning to neighbors if under attack by packs of caterpillars devouring their tender leaves. They can even summon an airstrike of beneficial wasps to rid them of the chewing horde. As if that wasn’t enough, for the attentive interpreter, trees also depict the seasonal gyre, from the plainsong of winter dormancy through each station of nature’s procession.

Yet in many people’s lives today it is easy to forget the seasons, and even our connection with nature itself. Eating diets detached from calendar or place, living in climate-controlled enclosures, and defeating the cycles of light and dark with ceaseless light, we are deracinated, disconnected, unmoored from the seasons, one of our deepest chronologies.

But not John O’Keefe, whose immersion in the rhythm of 50 trees over time has allowed scientists like Andrew to combine ground-level phenological observations with satellite imagery, drone photography, and data from cameras, together with other measurements, to explore the effect of climate change on forest ecosystems.

Not surprisingly, Andrew and his collaborators are still figuring out what to do with so young a method. Their work keeps turning up surprises, from learning that red oaks like mine at the Harvard Forest are growing faster and more efficiently than at any time in the last 20 years to discovering that the growing seasons now are lasting even longer than the leaves on the trees.

Leaves fall off, their season complete, even as the weather remains fine, the frost date stretching deeper into the calendar. The trees don’t have to shut down for the year, but they do.

There are several possible explanations. “Plants know from the history of their ancestors how long their timeline is,” says Trevor Keenan, now at Lawrence Berkeley National Laboratory and lead author with Andrew on a paper exploring spring and fall seasonal timing. “So it makes sense they would have some mechanism built into their optimum function, to have a preprogrammed senescence. . . . The question is, how quickly can they learn to change and detect that the environment around them has changed?”

Another theory is that once trees have filled up their carbon stores, they are finished with their work. “They have been as productive as they need to be for the year,” Trevor says. “They are done.”

For me the idea of seasons lasting longer than the leaves could stay on the trees was a lot to take in. There is something unnatural about it — because, of course, it is unnatural. It’s a human forcing of the climate system, imposed on a natural physiological cycle with its own timing. There are two calendars now: the seasonal timing evolved within living things and the seasons cooked up by us.

The big oak, in full summer regalia.lynda v. mapes

AMONG CLIMATE SCIENTISTS, climate warming is not controversial. They are in near-universal agreement that global warming is underway, and due primarily to human activity, particularly the burning of fossil fuels.

It follows from the laws of physics that the more greenhouse gas there is in the atmosphere, the higher the earth’s average annual surface temperature will be. So no one should be surprised that the earth logged its warmest year ever recorded for the third year in a row in 2016.

Yet climate is not weather, and the two can be easy to confuse — or be used to sow confusion by those seeking to deny the scientific consensus on climate change. I experienced this while I was at the forest, during a record cold winter with snowfalls so mighty that in Boston the National Guard had to dig out the snowbound rails of the T. Yet back at my home in Seattle, my husband was cutting the grass during a record early spring.

Extremes are the new normal as the changing climate works its way on the landscape with varied effects. How trees are faring in the face of it is not one story, even in one forest. At the Harvard Forest, red oak, the dominant tree in these woods, is surging, at least for now, but warmer winters also have set invasive woolly adelgids on the march, expanding their range in a rampage expected to take out most of the eastern hemlock at the Harvard Forest and beyond, as the bugs literally suck the life out of them.

The future, climate scientists warn, may bring storms, droughts, fires, pest outbreaks, floods, and species extinctions, scaling ever upward in severity according to our failure to reduce carbon emissions and stop making our problem worse.

Perhaps there will be a technological fix. Perhaps we will figure out how to break the connection among prosperity, comfort, and carbon. But this much is for sure: In an uncertain world, forests can help. They are a repository of only good verbs: Shelter. Nurture. Moderate. Cleanse. Regenerate. Provide. Connect. Sustain. And in the resilience of the oak, more is revealed of the climate change story than the usual gloom-and-doom scenarios to which we have all become so accustomed.

I take heart in the journals of Thoreau. How he lamented in the 1850s about living in a time in which the big trees were already gone, and the “nobler” animals with them. But it is worth noticing that some things, many things, are much better now than when he was writing, or when my big oak sprouted. The regrowth of forests on former agricultural land over the six-state region of New England and beyond amounts to one of the great, accidental rewildings of our time. With the trees, Thoreau’s animals have returned to their forest home: bear, moose, deer, and more.

Harvard professor Andrew Richardson scales a tower to adjust webcams that keep an eye on seasonal changes in the trees. See the images at harvardforest.fas.harvard.edu/webcams. lynda v. mapes

MORE THAN BUILDING material, fuel, and carbon-storage utilities, forests are foundational to life on the earth, refuge for countless animals and an endless source of human joy, renewal, and refreshment. The big oak and its forest were certainly all of that for me.

Particularly when climbing it. I had realized early on that just walking below my tree would not be enough. So it was my great good luck that Melissa LeVangie, the tree warden of Petersham, and her twin sister, Bear — professional arborists, competitive tree climbers, and instructors with national reputations — lived up the road and were willing to take on a rank beginner such as myself.

On a frigid morning as winter began to grit its teeth, Melissa joined me in the woods with the oak for my first climb. She sized it up with an experienced professional’s eye, gauging its safety for a climb. She noted its broad spreading crown and first branches, some 20 feet up the trunk, where the tree opens wide for big gulps of sun. Sure, she declared, this climb should be no problem.

Opening a heavy pack of gear, Melissa soon had me trussed like a turkey in a climbing harness, ropes, and helmet, all clipped together with a confusing array of carabiners and knots and pulleys that seemed quite important, in a life-or-death sort of way. I was noticing she had her blood type written on her helmet just as Melissa announced it was time to get my feet off the ground. Too stubborn to bail at this point, I gave the rope a pull, releasing my hold on earth’s dear, familiar gravity, and swung free in the air.

Melissa stuck close from the start, helping me into the first rank of branches. Exhausted, excited, I eventually let my feet rest in the big oak’s broad boughs.

From my perch, the connectedness of the oak to a vast community of lives was beautifully revealed. There were tapestries of lichen and moss. Insects on urgent errands. Birds aloft in their tiny treetop kingdom. And a sweep, as far as I could see, of trees of many arboreal nations: white pine, white birch, red oak, red maple, black birch, cherry, and beech. The oak was the largest tree in its grove, but it was a beneficent monarch. From more than 100 species of animals and insects that dined on its acorns and leaves to the vast network of fungi lounging all through its roots, the big oak was alive in so many dimensions and hosted a far larger menagerie of lives than I ever would have imagined.

It was on our last climb, finally all the way to the top of the oak with that picnic and hammock, that I thought it seems our task now is to live on this earth at least as successfully as this tree. It felt like a lesson, a personal reckoning, to simply grasp the reality of where we stand on this earth. We are not separate from nature; we are of it, and in it, and we need an ethical framework to match. We need a tree culture, a social and political act of biomimicry inspired by the genius of trees.

I had watched my tree through four seasons. I had seen trees change scientists’ understanding of the world. And the big oak had certainly changed me. I had learned many things, but most of all this: People and trees are meant to be together, and if we work at it, that’s how we will stay. Right here, dwelling in our common home on this beautiful earth, far into the future, amid the wonder of trees.

The author will read from her book at the Fisher Museum of the Harvard Forest in Petersham on May 2 at 7 p.m. and at the Arnold Arboretum in Boston on May 5 at 6 p.m.

Lynda V. Mapes is a reporter for The Seattle Times. This story is adapted from her new book, “Witness Tree: Seasons of Change With a Century-Old Oak.” Send comments to magazine@globe.com.