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While Our Backs Are Turned

Petersham, Massachusetts, my home for the past six years, hasn’t changed much since the early 1900s. If anything, the place has grown more tree-filled and sedate. A small rural town of twelve hundred people, it’s too far from Boston to accommodate commuters, and strict zoning statutes limit new construction to six houses per year. We don’t have a gas station or stoplights. It’s inexpensive to live here; homes purchased in the past year averaged just over $150,000. I’m not saying this to romanticize the place; economically, Petersham is not in its heyday.
What we do have are rich forests, streams, and the kind of ponds you can dive into. The majority of our land is permanently conserved by state agencies, nonprofit organizations, land trusts, and the Harvard Forest, a 3,700-acre research site and department of Harvard University, where I work. A large percentage of Petersham’s land base forms the shore of metro Boston’s water supply. The land’s future here is certain: bean counters in Boston value the free, 24 /7 water-cleaning infrastructure the undeveloped forest provides.
Like most of the Northeast, our landscape is blanketed by swaths of young to middle-aged oak, maple, birch, and white pine. Most of the trees are less than one hundred fifty years old—a byproduct of centuries of exploitative European settlement. Interspersed among those bright young forests are the older, darker hemlocks, which grow in thick patches where colonial farmers’ woodlots once stood.
Photo: Nicholas_T
Even if you don’t spend much time with trees, even with your eyes closed, you’re aware when you enter a hemlock forest that you’ve arrived someplace new. It’s ten degrees cooler than surrounding forests, for one thing, because of the dense shade: only 1 percent of the sun above an intact hemlock canopy reaches the ground at your feet. The ground itself feels different too—springier. Unlike the leaves of hardwood trees, which decompose within a few years, a mat of hemlock needles can be more than a foot thick, gradually decomposing over hundreds of years. The air pockets between the needles are like a natural trampoline.
A hemlock grove is so shady that it’s really only other hemlocks that can thrive in the understory. The rich greens and browns of hemlocks of all sizes stretch as far as you can see, punctuated by the occasional broad leaves and flowers of mountain laurel and hobblebush. Vibrant orange newts scramble around in the moist soil, navigating tiny thickets of wintergreen and brilliant red partridgeberry.
The few moose I have encountered in the forest have been standing among hemlocks, in the shade that helps keep their massive bodies cool. Many other creatures—black bear, bobcat, fisher, porcupine—move through the trees as well. Petersham’s streams and rivers are sheltered by hemlock branches, which keep the waters cool for sensitive species like trout, and for the anglers who pursue them up riverbeds in all seasons. And in contrast to broadleaf trees, which guzzle water from streams, hemlocks only use as much water as their thin, efficient needles require. Throughout the year, layers upon layers of hemlock branches intercept moisture and sound from the air, making everything calm, quiet, and green.
Which brings me to the birds that also call this place home.
In the slowed-down quiet of life in Petersham, I’ve gotten to know the comings and goings of warblers and waxwings, woodcocks and geese. Spring draws their fervent arrival, and in October they take their leave. Among the last birds to hit the road before winter are the black-throated green w­arblers. Their bright yellow heads and chipper zoo-zee calls disappear from hemlock boughs all at once in late fall. I don’t blame them for going. Winter can be mean around here.   
Come March, they’ll swoop back over three thousand miles, from whichever Central American clime where they’ve spent the winter scouting bugs. And once they get back to Petersham in May, they’ll head straight for eastern hemlock trees—the dark groves that Longfellow deemed “the forest pri­meval.” In the hemlocks’ shady boughs, in fastidious nests spun of twigs, moss, and spider silk, they’ll lay three or four speckled eggs, each about the size of a chickpea. The eggs will hatch in twelve days, and ten days later, stuffed to bursting with insects and spiders, the half-grown black-throated greens will streak around the forest, calling loudly after their parents. Year after year, the birds will return to the same breeding area to repeat this cycle.
Warblers are notoriously choosy about nesting sites, and studies show that some populations of black-throated green warblers depend specifically on eastern hemlocks—and, to a lesser extent, spruce—for the camouflage and structure they offer. In fact, black-throated greens are considered an indicator species for hemlock ecosystem health. Disrupt the hemlocks, especially in southern New England, and you’ll see a proportionate decline in the numbers of black-throated green warblers. Although some populations will nest elsewhere, even in deciduous forests, the populations local to my area may have evolved specific genetic adaptations to feeding and nesting in eastern hemlock.
The first sight that millennia of black-throated green warblers have encountered when they bob their tufted heads out of the nest has been an eastern hemlock branch. Until now. (Isn’t there always an “until now” in a story like this?) Every winter, while the w­arblers are migrating, a little more of their hemlock nesting ground disappears. Or, more accurately, their nesting ground is drained of its life force—by a bug so tiny that thousands could fit in the palm of your hand.
By now, the story of tree populations collapsing is a familiar one. In the early 1900s, millions of American chestnuts dropped dead from an exotic fungal disease. The sudden absence of the widespread, nut-bearing trees transformed most woods—and the timber economies that depended on them—east of the Mississippi River. A few decades later, a disease came for the sturdy American elm, one of the most widely planted urban street trees, stealing the shade from countless northeastern towns and cities. Today, pests and disease are decimating trees with unprecedented speed. In the western United States, mountain pine beetles have rendered millions of pines leafless and dead across a nineteen-state area, in what might turn out to be the largest insect blight in North American history. In the county where I live, an Asian beetle is wreaking havoc on city streets and surrounding forests, munching its way through twenty-four different species of deciduous trees. Tens of thousands of trees have been cut in an effort to contain the infestation.
All of these things I know. In my work as a science communicator, it’s my duty to explain them. I spent years as a field tech digging around in the data—really digging: one handful of bog mud and one decaying tree stump at a time. Now I spend half my time at a computer, linking local research results to the larger story of global change. I write fact sheets on watershed decline, tweet about the uptick in dangerous storms, and explain in detail why allergies will worsen as climate warms. The reality I regularly convey is that global change is all around us. And among other changes, massive populations of trees are dying.
But until now, none of those trees have been my trees. None of them were the hemlocks I have traipsed through in all weather, a grown-up country kid grasping for lucidity and finding it among the trees’ generous green.
My initial take on hemlocks was not one of affection. From 2007 to 2009, one of my field sites was a massive hemlock stand, where I studied soil chemistry, ant and spider biodiversity, carbon fluxes, and canopy light levels. Black flies and mosquitoes tormented me in summer. Autumn ticks infected me with Lyme disease. In winter, I developed second-degree frostbite. I ripped my way through many sets of expensive field clothes. But in those years, I also caught my first glimpses of porcupine, fisher, and black bear. I began to appreciate the way a hemlock sapling weighted almost flat to the ground with snow can release back to vertical in the spring.
The nearest thing to civilization around that particular hemlock site is a cloistered monastery. My hemlocks are at least a mile beyond those walls. You’d be hard pressed to find a quieter forest. The thick hemlock trunks—some that you need two sets of arms to get around—and their low-sweeping boughs absorbed all sound, except the huff of my own breath as I lugged field equipment up the steep slopes. And, of course, the occasional zoo-zee of a black-throated green warbler.
Over time, the thick boughs and the damp air of the hemlock forest settled around me like a protective shawl. I draped it over my shoulders and carried it with me out of the woods. And I came to rely on my regular return to those trees.

There’s one patch of hemlocks at the Harvard Forest where, if you stand in the middle and turn in a circle, you’ll see, on all the trunks, a tall, wide gash that runs from the ground up about ten feet, sometimes higher. I’m told that thirty years ago, lightning hit one tree, which shattered, and then zapped all the nearby trees—the electricity conducted underground through the roots. Incredibly, most of the trees survived.
Shortly after I discovered these trees, I got very ill and spent some time in the hospital. When I was finally well enough, I hobbled out to the hemlocks and ran my hand up the gash that had split the biggest tree in half. The tree was alive, and both sides were still growing. I held my hand against the split in the trunk for a long time.
You might suppose that this grove of two-hundred-year-old hemlocks, able to face lightning and keep growing, could stand up to a microscopic pest. Like humans, though, trees are incredibly resilient, except to the one thing that finally does them in. The hemlock woolly adelgid, a tiny aphidlike bug, can kill a full-grown hemlock tree in six to ten years—even more quickly in southern Appalachia. And although individual hemlocks can be treated with pesticides, there is no solution at the forest scale.
In the southeastern United States, including in the Smoky Mountains, hundreds of thousands of hemlocks have died since the now-infamous adelgid was accidentally introduced from Japan in the 1950s. In its home range, the adelgid is controlled by native populations of predatory beetles. They are part of the food web that coevolved with the hemlocks there. Not so in the United States. The woolly adelgid has spent thousands of years learning to infiltrate hemlock trees overseas, but our hemlocks are totally naive to its wiles. At first, the insect’s spread in the United States was slow, localized to the Southeast. But over the past several decades, the adelgid has crept its way into the Northeast and, very recently, into the Harvard Forest.
The hemlock woolly adelgid gets its name from the white, woolly material it builds around itself to protect its eggs. On a heavily infested tree, the dense layers of wax filaments can be seen as white dots on the undersides of branches from many paces away. The white masses of adelgid, and the gray corpses of hemlocks that have already succumbed, stand out starkly against the remaining green. The white wool muffles the few sounds associated with a hemlock forest; eventually it will muffle the hemlocks’ silence too.
One female adelgid (and all adelgid are female; they procreate through a kind of asexual reproduction) can lay several hundred eggs each year. The insects feed most intensively during the winter. In late October, they get to work, piercing the tree’s young twigs with their hollow mouth-tubes and sucking out the starch reserves that the tree has secured for winter survival. For good measure, they also inject a toxic saliva into the tree’s nutrient transport system, which further weakens the tree. It’s ingenious, really. Eastern hemlocks of all ages succumb to this strategy.
When I first moved to Petersham in 2007, we were at the invasion’s northern frontier. Bitter winters were keeping the adelgid in check. Now the insect has marched past Petersham, north into Vermont, New Hampshire, and Maine. Extreme cold spells can knock it back, but a warming climate is flinging open the doors of even our most northern climes. As the insect’s range expands, many landowners are preemptively clearing their hemlocks to salvage financial value from any as-yet undamaged trees. Given its recent rate of spread, some say the adelgid could engulf the eastern hemlock’s full range—from Minnesota, across to Nova Scotia, and down to Georgia—over the next few decades.

As a part of my job I lead tours through the Harvard Forest landscape—groups of students, policy makers, cub scouts, and artists, to name a few—so they can better understand our native ecosystems and the research we do there. These days as I bring people into the hemlocks, I have to take a few breaths to compose myself. The white wool is everywhere. I point it out, and explain it, but mostly I ask people to take in the living trees, to understand that this place will be very different even five years from now. An eye sensitive to these woods can already see how the lush green needles have taken on a grayish hue. When the wind blows, the needles fall audibly as a dry rain, leaving behind a shiver of bare twigs. The hemlocks’ tiny seed cones cast a doomed carpet over the walking trails. The few sounds in the forest no longer have a soft landing inside the curtain of trees. They seep out into the sky through the holes where the branches have gone bare.
Sometimes groups ask me what we are doing to fix the adelgid problem in Petersham. They are relieved to hear that it is possible to save one tree—that tall hemlock in their yard—by dumping pesticides over the tree’s roots, drawing the poison up into the tree’s own food system, to meet the insects when they tap in for a drink. People usually anticipate the second half of my explanation, though: those methods are just not workable (or safe) for a whole forest.
We’ve decided that the most effective thing we can do is document all aspects of the adelgid’s invasion: learn how the insect moves from tree to tree (it may hitchhike on birds, including warblers: a cruel irony), and note how the forest’s soil chemistry, hydrology, carbon and nitrogen budgets, temperature, vegetation, and wildlife (from moose to salamanders) respond as the hemlocks die. These are the types of meticulous measurements I was making in my early years at the forest, and they will be sustained by a parade of students and researchers until long after the hemlocks have returned to the soil. We do this partly so we can warn land managers north of us what’s coming. And we do it because this is one of the first invasions that scientists have had the time and ability to brace for. Already we know: birches and brambles will replace the hemlocks, and then a new wave of hardwoods will move in. The forest will go on. But tracking the specifics of the invasion will teach us important lessons about forest succession and resilience. The hemlocks will not collapse in vain.

This forest isn’t the first place I’ve seen transformed. When I first drove my now-wife through the m­iddle-class Tennessee town where I grew up, I asked her to close her eyes so she could envision what I remember of it. I described the miles of farm fields, the rows of knobby fenceposts punctuated by the occasional horse’s nose. I shouted for her to brace as we sped, airborne, over the train tracks on the steepest hill.
She was game for all this, but then, she has a strong imagination. Had she opened her eyes, we could have been almost anywhere in America. The rolling farms of my youth—incidentally, minutes from the country house where Johnny Cash and June Carter settled for thirty-five years—have been carved into a patchwork of big-box stores, subdivisions, and parking lots. Most of the remaining pastures are now listed on as “potential bank or office sites.” The population of the town—now a city—has doubled over the last thirty years, and the number of houses has quadrupled, which seems to have more than quadrupled the demand for Walmarts, Cracker Barrel restaurants, and Home Depots.
This story is perhaps even more familiar than the stories of tree populations collapsing. Like most kids in the United States in the 1980s, I grew up on the sprawl frontier. Our grassy lots and scrub woods, thick with lightning bugs, crawdads, and whippoorwills—the wild places in Sumner County where my friends and I caravanned our bikes, splashed in creeks, and shimmied up poplar trees—are unrecognizable today.
Places change when you leave them. I always assumed I had exaggerated this sense when I left for college—until I witnessed the transformation in annual increments on Google’s Earth Engine program online. Earth Engine can show you a time-lapse sequence of satellite photographs of just about any place, from 1984 to 2012. In Google’s bird’s-eye view of my hometown, the green of farm fields and pastures gives way to gray and white roofs. The sweeping curves of forests and wetlands are clipped to sharp corners by parking lots and culverts. I tend to pause the animation just before the early 2000s, when development really takes hold. I imagine the sound of whippoorwills calling, and the smell of onion grass on my hands as I coax lightning bugs back out into the night.
The Earth Engine view of Petersham, in contrast, shows little change over the past twenty years. There aren’t enough residents to attract one big-box store, let alone half-mile expanses of them. But if Google’s satellites keep watching, they will see what the warblers see—each spring, in the deep green hemlock canopies, patches of gray beginning to show, and to spread.

My colleagues tell me that five thousand years ago, during one mysterious, catastrophic century, most of the hemlocks in New England disappeared. The irrefutable pollen record tells us so, although it can’t tell us exactly why.
And it took the hemlocks two thousand years to return.
To accept that truth requires thinking like a forest, which grows and changes over hundreds and thousands of years. You can’t think like a human, who lives and dies in something around eighty years—who will watch eastern hemlocks decline and not return within imaginable time. And you definitely can’t approach the fact of hemlock’s disappearance from the perspective of a black-throated green warbler, which lives six years at maximum—the same amount of time it takes the hemlock woolly adelgid to cripple a stand of eastern hemlocks, covering their lush green branches with blinding, cottony white, then sucking them to brittle stems.
The places we love are bound to change while our backs are turned. Clouds shift in the gaps between seconds. Eggs become adolescent birds in the space of a month. Over a period of years, beavers can turn a forest to a meadow. And the speed and span of landscape change today has no analog in history. But that understanding doesn’t make the changes any easier to witness. It’s painful to watch a bulldozer roll through your playground. Knowing the adelgid is at work in a forest doesn’t lighten the branch breaking off in your hand.
Still, in the case of the hemlocks in my adopted home, we’ve resolved not to turn our backs. We’re watching the changes as closely as we can. We believe that, by knowing what came before, we can enrich the landscapes that will be.
Twenty years from now, black-throated green warblers will migrate north in spring to find, instead of hemlocks, a patchwork of sunny birches. Most will keep flying, to seek the tall, dark trees that fit the nests they are building in their minds. Their survival will depend on their ability to make do with what they find.

For now, some hemlock groves at the Harvard Forest are doing just fine. I bring tours through those places as well. We duck between the massive trees to follow newts scuttling through the brush. We dip our hands in the frigid, mossy streams, and hop light-footed across the soft, needled ground. We inhale the cool air, and do our best imitations of warbler and wood thrush.
Twenty years from now, I will lead tour groups among the young birches, and have them close their eyes and envision what I remember of the hemlocks that filled these woods. The ceiling of deep green branches. The sweet warbler calls. The dark, murmuring streams. Around our shoulders, a shawl of still, cool air. 

Photo: Nicholas_T