Three hemlocks stand close in the small grove. They have grown up together, about 20 feet apart, their branches touching in places, their life histories intertwined. They have been through the same storms, felt the same winds, the same heat, and the same winters. They have shared creatures. Squirrels, chipmunks, warblers, flycatchers, thrushes, spiders, and dozens of other animals have moved between them. Deer, skunk, turkey, porcupine, bobcat, fisher, hare, bear, and perhaps a moose or few have moved beneath them. Hemlocks are among the longest-lived trees on the East Coast and in the right conditions—in cool damp, absent certain pests—these conifers can live at least 500 years and reach over 100 feet.

These three are about 70 years old and 80 feet tall. Even young hemlocks have gravitas. They are stunning dark trees, their bark deeply creviced and thick. Some hemlocks have bottom branches that droop toward the ground at the tips, creating around the trunk a circular open-air room with a low roof. These three have no lower branches that are easy to grab hold of. Their green-needled limbs start 15 feet up.

15 feet can be 15 feet too high when you are learning to climb trees, as I set out to do one May weekend at Camp Hi-Rock in Mount Washington, Massachusetts. Two dozen women and I had come to participate in the Women's Tree Climbing Workshop, an organization founded by identical twin sister arborists Bear LeVangie and Melissa LeVangie Ingersoll. I was there to be in trees and to better understand them. Trees and forests are facing existential threats because of climate change, but it can be a struggle to grasp the extent of the danger or to make sense of the complex and deadly interactions playing out between heat, drought, fire, water, disease, and pests. I hoped that being in trees would allow me to better see them through the eyes of people, like the LeVangies, who care for trees and who perceive so much more about trees and forests than many of us do. Forests are always shifting; they change form and location, they are in constant motion. At Camp Hi-Rock, I climbed a hemlock descended from trees that made their way here after glaciers started to recede some 20,000 years ago. Eastern hemlock, Tsuga canadensis, likely traveled up from various sites in what is now Georgia or North Carolina and started settling in New England about 10,000 years ago. Pollen from lake sediments show that white pine had by then replaced the spruce and jack pine that had become abundant in the area soon after the ice left. Hemlock arrived as the climate kept warming and rainfall increased as ice melted, refilling ocean basins. Beech, birch, and oak arrived too. By roughly 8,000 years ago, the forests of the region had many of the tree species common today.

Hemlock, slow-growing and shade-loving, gave rise to forests that were dark and cool, that held onto the moisture key to their survival. Hemlock forests lined creeks, streams, and rivers, keeping temperatures low and holding soil in place with their roots. In their shade, salamanders, frogs, and brook trout flourished. In their branches, ants, arboreal spiders, and dozens of birds found homes. The hemlock grew in clusters among other trees and in vast uninterrupted stands.

The hemlocks’ journey north is one of many arboreal migration stories that have helped scientists understand the climate and ecology of the postglacial period we live in. Hemlock have also helped unlock a fine-grained weather and climate record that tells us what trees have been though in the last several thousand years, what they have adapted to and survived, and how perilous and divergent their current experience is proving to be.

*

In the early 1970s, Edward Cook, now a researcher at Columbia University's Lamont-Doherty Earth Observatory, enlisted in the Army and was stationed at West Point, New York. He spent his free time driving around looking for rock-climbing sites and old trees. He had finished college at the University of Arizona, where he had been captivated by dendrochronology—the science of reconstructing past climate by examining tree rings—and now everywhere he went, he looked closely at trees. In the Shawangunks, which form a northern ridge of the Appalachian Mountains, he saw some ancient-looking gnarled pitch pines. From a dendrochronologist’s perspective, the older the tree, the better.

When the cambium comes to life each spring, it needs to get photosynthesis going fast. To do that, the leaves need a lot of water, which is elemental to breaking down carbon dioxide to make sugars. To get water quickly from roots to canopy, the cambium initially creates big xylem cells that stack vertically, forming long pipes running up the tree. These gallon-bucket cells form the early wood. As the growing season wanes, the tree starts to pack things up after the picnic and produces compact, thick-walled xylem cells that make up the late wood. The next spring, the process begins again, and the cambium comes to life between last year's late wood and the bark. The old dead or dying weed becomes the internal foundation on which the new live wood lies.

Cook wasn't sure how many rings—each one marking a year of early wood and late wood—he would see in the pitch pines. He used an increment borer to take his first samples. These devices only pierce a tiny bit of cambium, allowing dendrochronologists to see a tree's growth without causing it significant harm. They are elegant tools; two metal tubes, one nested inside the other. The tubes are assembled into a T, with the outer tube serving as a perpendicular handle. The extremely sharp end of the smaller tube is cranked until it reaches the heartwood or pith, the innermost and oldest part of the tree. Once the dendrochronologist has reached the heartwood, they often make a half reverse twist, which breaks adhesion between the metal tube and surrounding wood. Then they carefully pull out a tray that runs inside the boring tube, on which lies a delicate dowel-like core.

In a lab, the core is sanded and burnished to bring out the details of the bands of dark and light that to the expert eye can tell of rain or drought or fire, or radioactive fallout. Although the annual cycle of early wood, late wood, and dormancy is generally similar in temperate trees, every species has its own fingerprint: a maple and a spruce growing a few feet apart may tell the same story in different colors and textures. Pitch pines, like other conifers, have regular xylem cells, called tracheids. As Valerie Trouet writes in Tree Story: A History of the World Written in Rings, "In conifers, individual wood cells queue up in straight lines like Roman soldiers."

From the tidy light-dark bands of the Shawangunk pitch pines, Cook could see that they were what is called sensitive. That is, their rings varied in thickness, reflecting changing environmental conditions. Trees that are not as responsive and have consistently sized rings are termed complacent. He also learned that one of the trees was 400 years old, 200 years older than his textbook said pitch pines could be. That was the first surprise. On the Mohonk Preserve, also in the Shawangunk Mountains, Cook found another. There, on a rocky slope with virtually no soil, Cook caught sight of some hemlocks like the ones I’ve taken to climbing, with Bear and Melissa, in nearby Massachusetts.

*

Modern dendrochronology had emerged in the early twentieth century in the Southwest, in part because the desert simplified the yearly story trees told about the weather. There was plenty of sun. Plenty of warmth. But there was not plenty of rain. The trees of the region put on a lot of growth when the year was wet: they made bigger cells in those years. Wet and dry years were visible in the thick versus thin rings of ponderosa pine and giant sequoia that came to anchor the young field. The pine told of rainfall in northern Arizona going back 500 years, the sequoia of rainfall in the Sierra Nevada going back 3,000 years.

With a few exceptions, the field had remained largely fixed in semiarid landscapes in America. "There was this belief that dendrochronology couldn't reliably be done in the Northeast," Cook says. The concern was that the region had too many variables— rain, snow, seasonal extremes of temperature. It was too complex a place to provide a clear picture about any one aspect of climate. But when Cook saw the hemlocks growing on sere exposed rock, he realized that they might tell a story about rain. He learned that the Smiley family who owned Mohonk Preserve had been recording rainfall and temperature since 1896. Depending on the age of the hemlock, he might be able to fact-check the annual rings against the Smiley's data.

The hemlock turned out to be more than 300 years old and for the period they overlapped with the family's weather records, they matched. Eastern hemlock showed that decipherable information about rainfall, or not much rainfall, could be found outside semi-arid landscapes. They were one of the few regional tree species able to do this. "I knew that hemlock was potentially the single most important species because it was likely to be the most common old-growth community in existence, left in existence, in the forests in the Northeast," Cook says. Some hemlock had survived waves of colonial deforestation because its soft, light, easily decomposed wood made poor lumber or firewood.

In the 1800s, though, hemlock's thick bark was almost its undoing. In most trees, a corky outer layer of phloem becomes bark. In a beech or other smooth-boled tree, this cork cambium puts on little girth each year and the bark that it replaces dusts away. In rough-barked trees, the corky layer grows wildly and the trees package the growth increases and convolutions. That is why hemlock bark is a mesmerizing world of tectonic folds, of rivulets and ravines that visitors—an ant, a climber—can travel. In the case of hemlock, that profuse growth contains a high concentration of tannins, chemicals produced to stave off herbivores, but that can also render animal skins more water resistant. In the nineteenth century, the American tanning industry expanded and tanneries arose throughout the hemlock's range.

In the Catskill Mountains and on the ridges of the Shawangunks alone, an estimated 70 million hemlock were cut, their bark ripped off, their bare trunks left to decay.

Even so, pockets of inaccessible old growth persisted. Cook kept seeking out those stands and, in time, found hemlocks older than those at Mohonk. He read an article describing 500-year-old trees in the Tionesta area of Pennsylvania, so he set out with a friend to find them. The two found plenty of middle-aged hemlocks. They found a forest littered with pipelines from an era of oil exploration. They got lost. Then they happened into a hemlock grove that felt old. Old trees should not be stereotyped: they are not all massively tall, thick, moss draped. They can be small, their age revealed in bent posture and scars, in roots that run close to the surface, like the veins that rise on the backs of our hands as our skin thins with time.

They took cores from several Tionesta hemlocks, one of which, it turned out, had started growing in 1426. The records kept by the Tionesta hemlock joined those kept by the Mohonk hemlock—and Mohonk pitch pine, white pine, and chestnut oak. Those records were, in turn, joined by those of hemlock from Nova Scotia down to the Carolinas and as far west as Michigan. Hemlock became the backbone, as Cook describes it, of the North American Drought Atlas. The atlas, first published in 2004 and expanded and deepened since, is a database of tens of thousands of tree-ring records that reveal patterns of drought in hundreds of locations and as far back, in some regions, as two millennia. Cook's invention became a reference work for understanding historical drought and it allowed novel interdisciplinary work. Fire ecologists, anthropologists, historians, archaeologists, and modelers, among many others, have used the drought atlas to understand environmental history and cultural shifts as well as to fact-check climate models. Similar atlases have since been made for Europe, for South America, for Australia, for Mexico, for Asia.

Tree ring records cover a human-history timescale. The oldest living tree ring record is from a 4,800-year-old bristle-cone pine called Methuselah—a sapling during the world of Gilgamesh, of the Minoans and the Mayans, of the last mammoths. (The oldest fossil tree ring record goes back nearly 14,000 years.) Because trees are sessile for their lifetime, the information they hold is specific to a site. That specificity helps researchers know what has been true for a particular place and, when records can be combined across nearby areas, a particular region. Tree rings show that the Northeast, generally now a rain-rich place, underwent severe droughts in the 1500s and 1600s. In places where water is not in short supply but temperature changes, as in a wetland, tree rings can tell the story of temperature. They can also reveal global temperature patterns: tree rings document the "year without a summer" triggered by the explosion of Mount Tambora in Indonesia in 1815, which spewed ash and ejected sulfur particles into the stratosphere, where they reflected the sun's warmth back to space until some washed down as acid rain.

Increasingly, tree rings are adding detail about how we are changing the climate, making clear that we are in a new era. Valerie Trouet and her colleagues can see in European tree rings that the North Atlantic jet stream—the westerly wind that determines so much about formerly familiar weather patterns, season length, and temperature across the Northern Hemisphere—has become more erratic since the 1960s. Tree rings are helping scientists document precipitously declining snowpack in the Rockies and other mountain ranges. Tree rings are ground-truthing satellite observations of changes in forest extent and productivity. Tree rings reveal that the new character of wildfires—their intensity, their frequency—has no precedent in the last 2,000 years. And drought, the tree rings show, is going rogue with its coconspirator heat.

Dendrochronological drought atlases emerged in the Northeast, extended across the country and then across the world, in some small part because several wind-pummeled strong-rooted hemlocks on a talus in the Shawangunk Mountains shared their memories of the rain that came, or did not, for every one of their more than 300 years. And because amateur naturalists kept records down through generations and shared them with a young scientist who happened to show up one day. Mohonk Preserve and its trees remain deeply special to Cook. "Hemlock always has a very soft spot in my heart because it turned out to be such a pivotal species," he says.

The author in a hemlock.

Photo: Wenda Li

As far as I know, the three Hi-Rock hemlocks I’ve spent time with have never been cored. I don't know their exact age. The hemlocks must feel the seasons changing length, the winters becoming less cold, the summers longer and drier and hotter. They may feel a pull to migrate as their ancestors did and as some of their contemporaries have: in the last three decades, hemlock seeds carried by wind have been able to germinate and survive further north; hemlocks have shifted range by about eight miles over that time. It is entrancing to be in a huge hemlock and to feel its solidity and rootedness and to think about it as a traveler.

Climbing in the way that the LeVangies teach it—engendering a culture of consideration, attention, and care—can change people in surprising ways. As I learned to climb, I did things I never thought I could do. I overcame physical weakness and phobias. I found a new way to confront climate anxiety. And I came to know trees more deeply. I began to look closely, to see and to understand our profound connection to them.

Everything that flows from trees and forests—clean air, biodiversity, cooler temperatures, clean water, human health, carbon kept solid and out of atmospheric currency, rain, regional weather patterns, culture, history—is unraveling. If trees are to continue to sustain us and to help us address our apocalyptic mess, we need to pay them more attention, give them more care.

As Melissa and Bear have taught me, we need to take to the trees.

*

Four or so decades after his first visit to Tionesta, Edward Cook returned to western Pennsylvania to visit the hemlocks he hadn't seen since. He was with several colleagues, including Neil Pederson of Harvard University, who studies forest ecology. It was a difficult day. They spent many hours searching, passing through clear cuts. The forest was unrecognizable to Cook: there were so many dead beech trees, so much disturbance. Pederson felt saddened as well. "I am walking through a graveyard," he says. "Adelgid is on its way... the mature beech are gone."

The group made their way through thickets of beech saplings knowing that they too would be gone soon: they saw evidence of beech leaf disease in two places, following right behind the devastations of beech bark disease. "So that national landmark forest is doomed," Pederson says. Old-growth forests are not permanent, he adds, but "to me personally they feel as though they will always be there." Seeing forests change so radically during his lifetime has been devastating. And, he says, he was worried about Cook.

Pederson kept reliving in his mind a trip he had taken with his doctoral advisor, the late Gordon Jacoby, cofounder with Cook of Columbia University's dendrochronology lab. Jacoby had gone to revisit a tree in Mongolia. "It was the first old Siberian pine that he had cored that showed global warming in Mongolia, and it turned out that it was a stump and someone had cut it and it was gone," Pederson says. "I just didn't want to deal with that again. I couldn't deal with someone at the end of their career going back to see one of their favorite trees and..."

In the last hour of daylight, they found the hemlocks Cook and his friend had cored 40 years ago. "I had to run away, because I started crying and I was just like, Oh my God, he's reunited with his trees, you know," Pederson says. "They're not dead."

That the Tionesta hemlock were thriving despite woolly adelgid was a surprise. Another team of dendrochronologists had gone to a different site in the region where Cook had also found hemlock in the 1970s and those trees were gone. Cook discovered after the trip that the Pennsylvania Department of Conservation and Natural Resources has been treating the Tionesta hemlock, something that can be done with individual trees around homes and along streets, but that is extremely challenging to do in a forest. The department has been trying to save hemlock for about two decades.

It is an enormous undertaking, requiring a big and consistent budget and lots of trained people, and it entails the use of pesticides, which many states opt not to use. "Pennsylvania, historically in this program, has been willing to kind of do almost whatever it takes," says Sarah Johnson of the department's Bureau of Forestry. There are "key functions of hemlock that cannot be replicated by any other species." Where hemlock forests have disappeared, waterways heat up, embankments erode, and populations of birds plummet, including the black-throated green warbler. And the blackburnian warbler. And the hermit thrush. And the Acadian flycatcher. A tree is not just a tree. A forest is not just a forest. Both contain and sustain multitudes.

Johnson says that in the long term, the bureau's goal is to use chemicals less or not at all and to manage instead with other methods, including biocontrol—breeding and releasing non-pest insects to prey on pest insects—as well as genetic selection. A biocontrol program is underway: a fly and two beetles, one from the Pacific Northwest where they feed on a native woolly adelgid, have been released in some places. Johnson says she sees a lot of the beetles in the forest. "They're reproducing really well. We can find them very easily. They're moving very well," she says. But so far, in her view, "they're not helping trees very much." The second strategy, genetic selection, is moving slowly.

Tree genetics is a universe unto itself and is not always best understood in comparison with animal genetics. Unlike other plants and animals, trees can have enormous genetic variation within a single tree, especially if they are long-lived. The meristem of one branch may develop a mutation that every subsequent cell produced by that meristem will carry. Meanwhile, on another branch, perhaps a different mutation has occurred. Researchers studying Sitka spruce found one old-growth tree with 100,000 differences: needles, trunk branches... many were genetically distinct from one another. One tree can be many trees. In the Hi-Rock hemlock, the branch I sit on might be genetically distinct from the branch my foot rests on, from the branch hosting the lichen I am entranced by, from the branch my hand holds.

Trees can also have enormous genetic diversity at the species level. Some species have genomes that are fifty times as large as any animal's. Also, unlike many animals, plants often have more than two copies of a chromosome in each cell. At one end of the spectrum—the record holder for the moment, notes Kew Gardens Ilia Leitch—is a fern with ninety-six copies of its genome in each cell; near the other end are quaking aspen, which can be triploid, carrying three copies. Plant polyploidy, as multiple sets of chromosomes are called, arises from whole genome duplication and gives plants extraordinary genetic range and nimbleness—some mutation or some combination of genomes might solve an emerging problem. Diploid aspen seem to be more drought resistant than triploid aspen, for example. Many trees show great genetic variation both within a population and between populations, variation that reflects their migratory history, where they waited out glaciers or hot house times, how connected they were to other trees across the landscape, how they reproduced, who was eating them.

Eastern hemlock genes reveal their travels north from several southern refugial core populations. After the glaciers receded, some hemlock headed more centrally, toward today's Indiana. Others, including ancestors of the Hi-Rock hemlock, traveled along the Appalachian Mountains. The southern, the central, and the northern populations diverged genetically, as would be expected. Some of these lineages also reveal a pattern perhaps best explained by the hemlocks' rapid near disappearance 5,000 years ago. Eastern hemlock pollen vanishes from sediment records at that time, throughout the continent, likely because of infection or infestation, or both. A few trees lingered, genetic studies show, and, over several thousand years, restored hemlock populations. All the hemlock we know today are, in essence, descendants of those lingering hemlock.

The Hi-Rock hemlock I know do not look as though they are carrying a solution in their genes. They don't look like robust lingering hemlock. Melissa says these trees will be unclimbable soon. After that, the trio of big hemlock will need to be cut down for safety. I hope the wind has carried their secrets away, whispering them to hemlock in the nearby woods. ♦

Adapted from Take to the Trees by Marguerite Holloway. Reprinted by permission of W.W. Norton. Copyright © 2025 by Marguerite Holloway.