Environmental reporter Lynda V. Mapes spent a year at Harvard University’s Harvard Forest in Petersham, Massachusetts. There, she got up close and personal with a special red oak (Quercus rubra) that provided great insights on forest life and the growing effects of climate change on the natural world. This article is adapted from her recently published book, Witness Tree: Seasons of Change with a Century-Old Oak, which chronicles her experience.

2017. BLOOMSBURY ISBN 978-1-63286-253-2

I first met the oak in the fall of 2013, walking the Harvard Forest with John O’Keefe. A biologist given to wearing the same two sweaters all winter—that’s a long time in Massachusetts—and a slouchy rag wool hat, John has walked the same circuit of 50 trees in the Forest for more than 25 years.

John likes to say he started his long term survey of the timing of the seasons in the Forest, revealed in the budding, leaf out, leaf color, and leaf drop on the trees, as a way to get outside at least one day every week, then just never stopped. By now, he has compiled a valuable and unique record. Seasonal changes in nature are among the most readily observable clues to the biological effects of our changing climate, as warming temperatures reset the seasonal clock. In forests, water use, the growth rate of trees, the length of the growing season, and temperature all are connected. So John’s work, documenting the seasonal gyre of the woods, was a look, told through the language of leaves, at our changing world.

His foot survey is literally the ground truth for images of the tree canopy that are beamed over the Internet, continually recorded in daylight hours by surveillance cameras, watching these trees’ every move, from 120 feet overhead. With John’s tree-by-tree observations, the forest-level view from the cameras and other devices on observation towers, and even a drone used to fly regular photographic missions, these must be among the most closely-monitored trees in the world.

For while the Harvard Forest is a natural wood, reminders that it is also an outdoor laboratory and classroom are never long out of sight. Spread over nearly 4,000 acres, the Harvard Forest, founded in 1907 and with more than 100 years of research in the archives, has one of the longest records of some types of data anywhere.

Trees bristle with tags and flagging, and the Forest floor is studded with equipment. There are light sensors, and laundry baskets gathering leaf litter. Often, amid the birdsong, came sounds of science, from the buzz of a drone flying a photographic mission overhead, to the hum of motors, and fans. The reality is this forest is under a microscope. It’s the fulltime, year-round focus of a staff of about 40 to 45 biologists, modelers, GIS specialists, historians, ecologists, dendrologists, paleoecologists, information and communication specialists, policy experts, atmospheric chemists, research assistants, lab technicians, and administrative staff at the Harvard Forest with an operating budget from $4.5 to $6 million a year, and a larger cadre of visiting researchers from around the world.

On his weekly survey walks, John measured little but the occasional snow depth or length of an unfurling leaf. But what he does do is look closely at a set number of tracked trees, chosen to represent a range of species, heights in the canopy, and forest environments—dry, wet, open, and shaded. He makes systematic notes of his observations on data sheets he created for the purpose, filled out the same way each week, year by year.

Professor Andrew Richardson of Harvard University was among the first of many researchers to use John’s records in influential scientific papers about the effects of climate change on forests. I met Andrew in August 2013, when I first arrived in Cambridge as a Knight Fellow in Science Journalism at MIT. I wanted to explore new ways to tell the story of our changing climate—a yawner of a story for too many, if told as a distant debate about treaties, dueling politics, and doomsday scenarios. The stakes are high: species extinction, the function of natural processes, and viability of habitats. But the facts won’t matter if we can’t get anyone to pay attention.

I wanted to tell the story through the charisma of living things, and the compelling but largely overlooked drama of the delicate seasonal timing of the natural world and how it was being disrupted. So when Andrew took me up on my request to sit in with his lab, and John let me join his walks, I decided to dive deep. “John,” I wrote in an email not long after our first survey walk together, “I need a tree.”

We picked it not long after—a single, glorious, nearly 100-year-old red oak in his survey that I could use as a narrative frame for my own inquiry into the Richardson lab’s work. What where they learning? Could I see climate change at work in this forest, and even in this one tree? Just as settlers used notable trees, known as witness trees, to mark the metes and bounds of changing landscapes, could the big oak reveal the changing climate?

John’s walks were enthralling. He noticed everything, and with all five senses, creating in his field notes a portrait of the forest in Pointillist detail: how firm the tree’s buds were, or whether they had softened and were getting ready to crack open. The sound of the first call of wood frogs, the scent of mineral soil as the frost melted from the ground. The sight of the leaves’ first emergence; the filling and draining of puddles; the flow of the streams, and first unfolding of woodland flowers. The autumn colors of the leaves, the thunk of falling acorns; frost flowers and ice on the puddles, and the wintergreen taste of birch bark. Here was a place richly and closely observed, right down to the mud and black flies. With nothing more than a pair of binoculars, six-inch ruler, and clipboard, John, by walking the Forest again and again, amassed a detailed calendar of the seasonal year, his tiny handwriting in Number 2.5 pencil recording local events with planetary implications. His findings over the decades were clear. On average, spring is coming earlier. Fall is coming later. And winter is being squeezed on both ends.

Everything in the woods reflected these changes, from the level of water in the vernal pools and springs to when the black flies were biting, the ground frozen, or leaves budding out or finally coming off the trees. It wasn’t a matter of conjecture or political argument; the discussions of who does and doesn’t “believe” in climate change in editorial pages, news reports, and Congressional debates frames this all wrong. The changing climate, trees, streams, puddles, birds, bugs, and frogs attest, is not a matter of opinion or belief. It is an observable fact. Leaves don’t lie; frost isn’t running for office, frogs don’t fundraise, pollinators don’t put out press releases. What John compiles, while taking all these walks, is the testimony of an unimpeachable witness: the natural world.

Studying Phenology

Discerning the workings of the natural world in seasonal timing has a long history. The roots of the word are the Greek words phaino, meaning to show or appear, and logos, to study. It’s from phaino, too, that we get phenomenon, and traditionally phenology has consisted of the study of the timing of biological phenomena in nature and the relationship of these phenomena with Earth’s environment, particularly the climate. The Belgian botanist Charles Morren argued that like meteorology, botany, zoology, physiology, and anthropology, this merited being a scientific discipline unto itself: phenology. He is credited with the first use of the term at a public lecture at the Belgian Royal Academy of Sciences at Brussels in 1849.

Phenology’s roots are in old-style, hands-on observation like John’s, practiced long before the term phenology was invented. The longest continuous written phenological record is probably marking the first flowering of cherry trees at the royal court of Kyoto, Japan, dating back to AD 705. In Europe, French records of grape harvest dates in Burgundy stretch back to 1370, and have been used by scientists to reconstruct spring-summer temperatures back into the Middle Ages.

In England, Robert Marsham in 1736 began recording what he called his “Twenty One Indications of Spring” at his country estate in Norfolk. He tracked the seasonal stirrings of animal life: croaking frogs and toads, singing nightjars, pigeons and nightingales, arriving swallows and cuckoos, rooks building nests, and all manner of plant activity, from flowering snowdrops, wood anemones, and hawthorns to leafing birches, elms, oaks, beech, and horse chestnut. The recording duty passed from one generation of Marsham’s descendants to another until the death of Mary Marsham in 1958.

Mainstream science left phenology aside long ago. But it’s being rediscovered, as researchers look for evidence of climate change in the seasonal calendar of living things. Old photographs, records of birding and garden clubs, even art and literature reveal changes subtle in the moment but visible over time.

The daffodil of Shakespeare has advanced its bloom time so drastically as to no longer fit its literary frame: “Daffodils, That come before the swallow dares, and take The winds of March with beauty,” Shakespeare wrote in The Winter’s Tale. March. Not in January. And certainly not at Christmas, as happened in 2015 when the United Kingdom witnessed its warmest start to December in 50 years, The Guardian reported. At this rate, Britain’s native daffodil, the Lent lily [Narcissus pseudonarcissus]—named for its expected February–March bloom time—is going to need a new name. Of course this just confirms what the gardeners, the hikers, the outdoorsmen and women of every sort already know from their own sense of a fraying natural order. Reliable patterns of nature’s pageant are slipping their chronology.

Phenology Plus Technology

For Andrew Richardson, John O’Keefe’s records offer valuable data he uses to explore the effects of climate change on tree physiology and seasonal timing of the forest canopy. The object is to probe the forest at a variety of scales, from individual trees to the forest, region, and biosphere. The data from John’s weekly walks has also helped Andrew deploy phenology as a lens on the workings of the forest in a whole new way—and brought new relevance to John’s work.

It all got started when Andrew was at the University of New Hampshire, working with his colleagues making measurements of the daily and seasonal rhythms of carbon dioxide exchange between the trees and the atmosphere—the breathing of the forest. He was using instruments at the top of a 90-foot-tall tower in the Bartlett Experimental Forest in the White Mountains of New Hampshire. Then he had a hunch there were a lot of other things he could also be measuring to get a better idea of how the ecosystem worked. Which, in a project meeting one day, led to a conversation with one of Andrew’s collaborators. What, they wondered, about putting a camera on the tower, with the thought that at the very least they would get cool pictures of the forest canopy through the seasons for presentations at science talks?

They figured they would also probably be able to tell when the leaves came out and when they fell off, which would also be useful for estimating growing season length, key information for scientists studying how much carbon forests pack away. Within a few weeks they installed what was then a state of the art camera, beaming its images over a wireless connection back to a server on campus. When the first images came in over the Internet to their computers, they were delighted that, dinky as it was, the camera was performing just as they hoped. Suddenly, they could monitor their remote field site from their desks. That got Andrew thinking.

The next summer, Andrew asked a PhD student, Julian Jenkins, whether he thought he could use computer analysis to spot the beginning of spring green-up in the images. In just days, Julian created a computer program that converted the red, blue, and green pixels in the camera image to numeric values. He then could count the amount of greenness in an image. Voila: spring, pinpointed from the pixel mix. Now the team could track the development of the canopy all the way into summer, with every day’s incremental growth in the leaves showing up as increasing numbers of green pixels. And come fall, the camera’s pixilated signals of leaf coloring and drop were just as clear. Suddenly, big swaths of landscape could be remotely monitored for seasonal development, over the Web, from anywhere.

It was a breakthrough. Here was the possibility of creating a whole new kind of observatory: a remote, digital observatory, with a network of cameras that could monitor the rhythm of the seasons as they transformed the land, over as large an area as the cameras could be placed, with the information streamed to a central server where the data could be shared, archived, and analyzed. Andrew dubbed it the PhenoCam network. There had never been anything like it.

In less than a year, Andrew found funding to start a small PhenoCam network to observe forest phenology across northern New England and adjacent Canada. That was in 2007. Then the National Science Foundation (NSF) in 2011 provided money that allowed the team to expand the monitoring network. Next, in 2013, NSF kicked in more money that the team used to involve volunteers in interpreting and analyzing more than 5 million images streaming into a network by then grown to some 250 sites across North America, uploading images at least once an hour, seven days a week, during daylight hours. The cameras were all over the place, from instrument towers such as those in the Harvard Forest to weather stations and building tops, from forests to tundra to Hawaiian grasslands and the desert southwest. The PhenoCam network brought the phenological tradition of Robert Marsham, Thomas Jefferson, Henry David Thoreau, and Aldo Leopold into the digital age. What would Jefferson have given for a PhenoCam on his beloved gardens, instead of having to wait for letters from Monticello to fill him in on what was in leaf and in flower. We even put a camera for the network under my red oak. Visit it at http://harvardforest.fas.harvard.edu/webcams/witness-tree

Here was the ability to see the forest not only up close, from tree to tree, as John does, but at scale. The proverbial forest for the trees. Researchers are no longer limited only to what can be seen on foot, or the occasional imagery of a satellite, available only intermittently and from a great distance. Not surprisingly, Andrew and his collaborators are still figuring out what to do with so young a method. Their work keeps turning up surprises.

New Insights on Climate Change

Trevor Keenan, now at the Lawrence Berkeley National Lab, with Andrew published a paper in 2015 showing that the timing of spring and fall are connected, but not in the way widely supposed. Conventional wisdom—and many climate models—held that the warmer temperatures that brought on an earlier spring would also mean a later fall, and a longer and longer growing season. But Trevor and Andrew found out that the timing of autumn correlates more closely with the onset of spring than with temperature or day length. Spring, it turned out, exerted a strong control on the timing of fall, somewhat offsetting the effect of warming. The findings do not imply a growing season of fixed length, as the relationship between spring onset and autumn senescence they discovered was not one to one. Rather their results suggested that current models don’t include the effects of spring on autumn, leading to an over-prediction of the extension of the growing seasons by as much as 50 percent under future warming scenarios. “It was a eureka moment,” Trevor said. Struck by the importance of their initial findings, Trevor scaled up to investigate seasonal trends on the entire east coast. The same pattern still held true.

There are several possible explanations. “Plants know from the history of their ancestors how long their timeline is,” Trevor said. “So it makes sense they would have some mechanism built into their optimum function, to have a pre-programmed 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 for the year, even though the weather is still fine. “They have been as productive as they need to be for the year,” Trevor said. “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-caused forcing of the climate system, imposed on a natural physiological cycle with its own timing. There are two seasons now: the seasons of living things, and the seasons made by us. Trevor expected that in time the trees would catch up, using their ability to adapt to take advantage of longer growing seasons, as trees do further south. The question is how fast.

Long term carbon sequestration measurements at the Harvard Forest also show that trees at the Forest, dominated by red oak, have been growing faster since the 1990s, as global average temperatures and carbon dioxide levels began their most rapid rise. By now, red oak is putting on more mass than any other tree species in the Forest, and faster. True, that is partly just red oak’s nature. The relatively young age of the forest, still recovering from the deforestation of the nineteenth century, also makes for this strong growth. But the red oak’s surge is also the result of climate change, manifest in warmer temperatures on average in winter, increased rainfall, and growing seasons lasting longer than at any point in the last two decades.

With the millions of microscopic openings on their leaves, called stomata (from the Greek stoma, for mouth), trees also are speaking truth about the effects of the changing atmosphere. Water vapor, carbon dioxide, and oxygen all move in and out of leaves through these openings, creating a survival challenge. But Andrew and Trevor documented in another widely-read published paper that at higher carbon dioxide levels trees, including red oak at the Harvard Forest, are working more efficiently. They don’t open their stomata as much or as often to take in the carbon dioxide they need. That means they can make as much and even more food while using less water. It also suggests a shift in the physiology of trees, with profound implications for everything from water cycling to climate. Trees like my big oak are changing their inner workings, using less water even as they put on more growth as temperatures warm and carbon dioxide levels rise.

From the sky and its atmosphere to the seasonal timing and growth rate of trees and, deeper still, all the way into the photosynthetic process of individual leaves, human fingerprints are now on the most grand to the most intimate scales of our planet. You could see all this even within one tree. The big oak’s witness was clear: Our world is already changing.


The author is grateful to the Knight Program in Science Journalism at MIT for the purchase of the PhenoCam under the witness tree at the Harvard Forest and to the Harvard Forest for keeping it online. To learn more about the witness tree project see http://harvardforest.fas. harvard.edu/witness-tree


Keenan, T. F., G. Bohrer, D. Dragoni, J. W. Munger, H. P. Schmid, and A. D. Richardson. 2013. Increasing forest water use efficiency as atmospheric carbon dioxide concentrations rise. Nature 499: 324–327.

Keenan, T. F. and A. D. Richardson. 2015. The timing of autumn senescence is affected by the timing of spring phenology: implications for predictive models. Global Change Biology 21: 2634–2641.

Citation: Mapes, L. V. Witness Tree: What a single, 100-year-old oak tells us about climate change. Arnoldia, 74(4): 23–31.

Morton, O. Eating the Sun: How Plants Power the Planet. 2008. New York: Harper Perennial.

Richardson, A. D., T. A. Black, P. Ciais, N. Delbart, M. A. Friedl, N. Gobron, D. Y. Hollinger, et al. 2010. Influence of spring and autumn phenological transitions on forest ecosystem productivity. Philosophical Transactions of the Royal Society, Series B 365: 3227–3246.

Lynda V. Mapes is the environment reporter at the Seattle Times and the author of Witness Tree. Her research was supported by a Knight Fellowship in Science Journalism at MIT and a Bullard Fellowship in Forest Research at the Harvard Forest. For more on Lynda’s experience and the book visit http://lyndavmapes.com.