The Great Forest Migration

Present-day Labrador offers a glimpse at what the New England landscape would have looked like before trees took over. Photo by Frances Brann.

How New England's Forests Arrived, Where They Came From, and What it Means for The Future

Sometime around 12,000 years ago, the first human beings arrived in New England. We don’t know much about who they were. Evidence is limited to a few chipped stones and bits of charcoal, but we suppose that they arrived following mammoths and caribou herds. What’s remarkable to me is not just that humans trekked across thousands of miles of wilderness to get here, or that they survived the harsh climate once they arrived. After all, ancient humans had proven to be hardy travelers and survivors.

What I find particularly interesting is that these caribou hunters arrived before the forests did. I am used to thinking of humans as the invasive species, the ones who arrive in a pristine environment only to disrupt its equilibrium. The thought of a forest invading a land already inhabited by us is strange. Surely, the story of those humans who eked out a living on this land with primitive rock tools must be a remarkable one. But theirs was not the only great journey going on. What of the forests? Where did they come from? And how did they get here?

Forest migration is not a terribly new idea – it’s been around since at least 1837, when Louis Agassiz first proposed that much of the world had been covered by massive sheets of moving ice – but it is a counterintuitive one. Outside of fantasy novels, individual trees do not get up and move about. And if trees did migrate, it must have been a multigenerational process. Seeds from one generation would have to disperse and grow large enough to produce seeds of their own, in order to spread those seeds further still. Compared to the seasonal travels of birds or butterflies, tree migrations must be ponderously slow.

And just knowing that forest migration happened is a long way from knowing how it happened, or from understanding what adventures may have befallen the woody colonists along the way. And that story, the how story, is difficult to piece together – especially in New England, where ancient forests have left little trace in the region’s acidic soils. It’s a story that we couldn’t begin to wrap our minds around until the late 1930s, when a group of scientists at Yale University pioneered a new method of looking into the past – a method that basically consisted of collecting mud.

The Great Forest Migration Image

Sediment core. Photo by Andrew Barton.

Stories in the Mud

This wasn’t just any mud. It was mud from the bottom of lakes and ponds that had been deposited there over thousands of years. The method worked like this: The scientists would make their way out onto a pond either on rafts or winter ice. Then they would lower a tube-like drill down into the sediments below. After boring into these sediments, they would pull everything back up. Inside the bore would be a long cylinder of mud.

One of these scientists was Edward Deevey, whose graduate work at Yale consisted of lots of mud-gazing. He subjected mud cores to a battery of chemical tests, compiled exhaustive catalogs of every piece of algae, every dropped shell, and every fragment of insect that he found. But the real scientific pay dirt (so to speak) was the pollen – microscopic grains of pollen that had fallen on the lake’s surface each year for thousands of years; pollen that then sank to the bottom of the lake to be preserved in near-perfect chronological order. Deevey realized that what he was looking at was the entire history of northeastern forests since the retreat of the glaciers. For the first time ever, someone could peer into the forest’s past.

By the early 1950s, Deevey was at the head of a whole new generation of mud scourers – ranks that included Estella Leopold (Aldo Leopold’s daughter) and Margaret Davis. And this new generation brought a new technology to bear on the mud layers – radiocarbon dating. With it, scientists could tell which trees and plants had arrived first, which trees were relative newcomers on the scene, and how various species rose to prominence or fell on hard times. They had a timeline to put this story on – a sense of the pace of forest migration. The history of New England’s forests could finally be told.

Bryan Shuman, an associate professor of paleoecology at the University of Wyoming who has worked extensively in the Northeast, helped me understand the big picture. And the book The Changing Nature of the Maine Woods, by Andrew Barton, Alan White, and Charles Cogbill, helped fill in some details. The story goes something like this.

If you could turn back the clock on your favorite New England landscape by 14,000 years, chances are that place would be covered with up to a mile of ice. Glaciers scoured everything but a few high hilltops. But 14,000 years was a turning point, the last great glacial advance. After that, if we could watch the scene in fast-forward, the frozen world would transform.

As the ice melted, parts of the raw landscape saw the sun for the first time in 90,000 years. Some inland areas had been so compressed by the weight of the ice that seawater flowed in, creating huge inland seas (this explains why, in 1849, workers in Charlotte, Vermont, happened upon a whale skeleton in a farm field). The Gulf of Maine extended inland as far as Baxter State Park. But gradually, the Earth’s crust rebounded, rising and draining the land.

The first plants to invade this scoured landscape were grasses, sedges, and mosses, typical of what’s in the tundra today. These were tiny plants that could survive in the sand and scree that lay atop the permafrost, and eventually they attracted the herds of caribou pursued by New England’s first humans.

The first trees to arrive were spruces, firs, and pines. Jack pine and red pine came next, followed by balsam fir, larch, ash, and elm. At first it was not a forest, per se, but more of a patchwork of trees. Pockets of ice remained here and there, but as it melted north into what is now Canada, the trees thickened into New England’s first post-glacial forests, a boreal forest that would last for about 3,000 years. The landscape would have resembled present-day Labrador.

But forest advancement was far from linear. About 12,900 years ago, a remarkably abrupt interruption in the warming trend plunged the land to the brink of a renewed ice age, where it remained for another 1,300 years. Geologists call this period the Younger Dryas, and the forests stalled in their northward expansion. Maine went back to tundra conditions. Vermont, New Hampshire, and parts of New York were relegated to open spruce woodlands.

But the cold snap ended as abruptly as it began. After the Younger Dryas, the climate grew hotter and drier. In a single generation, the residents of New England saw the land’s most dominant tree, the white spruce, nearly disappear from the landscape and the white pine take over and create forests unlike anything we see in New England today, but which bear some similarity to the present forests of northern Minnesota. Charcoal residues in the pollen cores suggest frequent fires. Species that easily establish from seed after fire (like white pine and birch) and those that resprout from roots or stumps (like aspen) proliferated.

It is not until about 8,000 years ago, as the climate became cooler and moister, that the pines gave way to the deciduous invaders. The hemlock, beech, yellow birch, and maple that characterize today’s forests moved into northern New York and New England. Lakes rose. Oaks moved into southern regions. The forests took on a more familiar look. And while that basic pattern has remained the same since that time, the story of these forests is not without incident. About 5,000 years ago hemlocks mysteriously disappeared, not to return for more than a millennium. Chestnuts arrived late. Maples became much more abundant for several thousand years. A brief cool-down about 1,500 years ago allowed spruce to recolonize some of its lost land.

It’s an interesting story – full of mysteries and changing fortunes. Still, the overall trend follows a pattern that seems logical enough: as the glaciers retreated north, the plant communities followed them in an order that mirrors their current latitude. First comes tundra, then boreal forest, then mixed forest, then deciduous.

But when scientists finally placed these changes on a timeline using radiocarbon dating, their results highlighted a problem: the timing just didn’t add up.

The Great Forest Migration Image

Pine pollen on puddle.

Reid’s Paradox

The conundrum was first identified by an English geologist named Clement Reid. In the late nineteenth century, Reid worked for the Geological Survey of England, and his surveys brought him into regular contact with plant fossils and allowed him to observe a wide range of living plants, as well. These observations, coupled with his geologist’s understanding of glacial history, prompted him to ask an important question. How long had it taken for these plants to arrive in Britain from their refuges in continental Europe where they had waited out the last glaciation? To answer it, Reid spent much of his spare time studying how plants disperse their seeds – how abundant the seeds are, how far those seeds are dispersed from the parent, and how long it takes for the seeds to grow big enough to produce seeds of their own. In 1899, after 20 years of careful observation, Reid published what would become a definitive work on the subject of plant migration, The Origin of British Flora.

One might think that using Reid’s data to determine how plants moved would be a simple task of arithmetic. Just determine how far seeds could spread in one generation, figure out how long a generation is on average, and that should be enough to determine the speed at which each species of tree or herb could move north to occupy the new frontier habitats left open by the retreating ice. And Reid did just that. But there was a problem. If plants did advance at the rate Reid had determined, many of them would not yet be in Britain at all.

“The oak,” Reid wrote, “to gain its present most northerly position…probably had to travel a full six hundred miles, and this without external aid would take something like a million years.” A million years! Now, Britain didn’t begin to peek out from under the last ice sheets until almost 10,000 years ago. According to Reid, the oaks shouldn’t have been a widespread British tree, but there they are – dropping acorns all across the English countryside.

As you might imagine, this observation piqued the interest of scientists all over the globe. And since Reid first raised the point, it has been observed in plant after plant and in landscape after landscape. Trees, herbs, many fungi, and other creatures without legs or wings should migrate very slowly. But for some reason, they have raced to fill up available habitats far faster than standardized calculations would predict. Today, this is known as Reid’s Paradox, and it’s a dilemma that was faced by Edward Deevey and the other paleoecologists of his era as they brought radiocarbon dating to bear on their pollen-filled mud deposits. Like the oaks of jolly old England, the forests of New England were way ahead of schedule.

The Great Forest Migration Image

A hike to the top of Mount Mansfield today offers a glimpse at a tundra-type landscape.

Chance and Change

Reid’s paradox still intrigues scientists today. Even with modern instruments, computer models, and advances in dispersal calculations, questions remain about the ways that plants get around. There are two important hypotheses, but both of them rely on some rather uncommon coincidences.

In the first instance, we have to imagine that somewhere in the vast glaciated landscapes of the Ice Age, tiny pockets of plant life were scattered across the icy emptiness. Perhaps a mild valley or seep or some unusual combination of forces combined to allow a small patch of ground to escape the icy grip of the advancing arctic sheets. And so we might have had little refuges, where plants may have clung to life in such small number or in such low density that they left little trace. When the ice retreated, these plants may have emerged like Noah after the flood to be fruitful, multiply, and found a new northern forest – well ahead of their cousins from the southern latitudes. Such botanical arks, if they existed, must surely have been rare considering all the factors that would need to have aligned in order for them to persist during the thousands of years that ice covered the land.

It seems like a long shot.

But the other important hypothesis depends just as much on uncommon fortunes. The idea here is that plants and trees occasionally experience rapid, long-distance migration. It’s not that Reid’s calculations have been called into doubt. But when Reid crunched his numbers, he was basing his work not on what happens to all seeds, just what happens to most of them. To explain, let’s follow Mr. Reid’s lead and consider an oak tree. Most of its acorns will fall fairly close to the parent. But imagine you could somehow keep track of every single acorn that the mother oak produced: chase down every buried squirrel cache, follow all the ones that rolled downhill or got washed down a stream. To my knowledge no one has actually tried this, but it seems logical to assume that as you traveled in concentric circles farther and farther from the parent tree, you’d find fewer and fewer acorns. Michael Cain, an ecologist who has studied long distance seed dispersal, explains that a graph of this data “would look roughly like a classic bell curve cut in half. Most of the data would cluster near 0 and would drop off rapidly as you progressed. But there would be a long tail on that graph, and that tail is important.” Every once in a while, a few lucky acorns would beat the odds and travel some remarkable distances.

The Great Forest Migration Image

During the Younger Dryas period, forests in Vermont, New Hampshire, and parts of New York were dominated by spruce. Photo by Justin Meissen.

The stories behind far-wandering seeds would undoubtedly be fascinating – picked up by dramatic windstorms, washed miles away by floods. H. N. Ridley collected such stories in The Dispersal of Plants Around the World and describes some surprising examples. Wind-dispersed seeds have been carried off by birds for nesting materials. Some seeds have been observed traveling long distances on the backs of snails. In one recent example, a shrike was found with the digested remains of a lizard in its gut. The lizard had seeds in its own stomach. These seeds, even after being twice ingested, were still viable.

What are the chances against such things happening? A million to one? Tens of millions? Surely, such things must be improbably rare. The crux of the matter is, however, that plants produce massive numbers of seeds. If you’re a gambler, a million to one odds don’t seem all that favorable unless you are able to play ten million times. This is the strategy of most plants – just get lots of seeds in the game. If you’re lucky, one may even be swept dozens or hundreds of miles north to found a new population in some far-flung, recently deglaciated habitat.

No matter which of these hypotheses you consider more logical (and they are by no means mutually exclusive), it seems that the solution to Reid’s paradox lies in the power of unlikely coincidence.

The Great Forest Migration Image

After the Younger Dryas, dry conditions helped create pine-dominated forests similar to what we see today in Minnesota. Photo by Justin Meissen.

A Changing Future

In a world of rapidly changing climates, it is only natural to ask what might be in store for our forest’s future. If the changing ranges of plants do hinge on dramatic and fortuitous happenstance, what hope do we have of predicting the future of New England’s forests? It may be as difficult as predicting next month’s weather. Like the formation of clouds or the movement of air masses, the ranges of the trees in our forests are complex beyond the ability of our most powerful computer models to predict. Like the proverbial butterfly in China, whose flapping wing might change the fate of a hurricane thousands of miles away, our forests and their distributions are sometimes changed profoundly by seemingly tiny and insignificant causes. It seems safe to predict that our forests will change. But how? Forest communities do not migrate en masse. Each species moves at its own rate, governed by a unique set of environmental factors. And so we’re not necessarily just looking ahead to forests that move, but to forests that change completely. Cain notes that in various places across North America, we find the pollen fingerprints of forests that have no modern analog – incongruous mixtures of plants and trees that existed for some window of time in the past and are now gone.

Also, just because change is complicated doesn’t mean that it will be slow. As we learned long ago from pollen sediments, forest change can happen at a dramatic pace. Bryan Shuman at the University of Wyoming notes that the boreal forests of New England may have given way to their successors in as little as 200 years – only a handful of human generations. And through these changes, there’s evidence of dramatic resiliency, where some tree populations fall precipitously and then rebound.

The Great Forest Migration Image

Many early forests have no modern analogue. Photo by Eli Sagor.

As the forests change, so will we. We’ve had to do it before, and there is evidence that we have navigated those changes with some of the same resilience as the forests themselves. Coinciding with the boreal forest die-off 8,000 years ago is an abrupt change in the archaeological record. The few artifacts from that time, likely left by the people who had preceded the forests themselves, show a similarly dramatic change. Around that time, old tool types disappeared and were replaced by new ones, without antecedent in New England. Perhaps those early New Englanders weathered the dramatic changes in the land by changing how they lived. It’ll be up to us to muster a similar wisdom and ingenuity.

Benjamin Lord is a science teacher, naturalist, and writer from Putney, Vermont.

 
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