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Ghosts of Ancient Forests

May 14 2018

Scientists working on the outer coast of Southeast Alaska recently discovered an ancient forest exposed by glacial retreat. In late May, they’ll return to the site.

Could these trees reveal how life how life has responded to past warming? Could they provide insight into how life will respond today?

May 14 2018


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Preparation Stage


Rocks. Lake sediments. Tree-rings. Ice-sheets. Corals. Shells. Microfossils. What the paleoclimatologist (paleowhat?!) finds today paints a picture of the past and, perhaps, offers clues for how climate may shape life in the future.

“What’s unique about this place, this discovery” Dan says of the La Perouse forest, “is that we have a chance to look at two systems—both the forest dynamics and the glacial dynamics.” Typically, paleoclimatologists, like Ben, are looking to reconstruct past climate from proxies, such as rocks or tree-rings. But this project is about more than past climate. It’s also about how life responded.

“There’s a bunch of different ways for recording when glaciers got bigger,” Ben told me. Radiocarbon dating allows scientists to determine the age of an object containing organic matter based on its chemical properties. “You could date boulders, for example, to see when they were unearthed by the ice and started receiving direct sun,” Ben says. But tree ring dates are more precise.

Each line in a core documents a year of growth. Wider rings mark the years with conditions for better growth; narrow rings distinguish the times of stress and hardship. Take one tree and match the rings up with the next tree—maybe an older one—and you start building a chronology of events back in time. Finding young trees that are growing near the face of a glacier can indicate retreat. Abrupt changes in the symmetry of the cores in a forest can indicate tilting caused by advance, ice-contact, or even death. So Ben and the team are after slabs—not just simple cores—for the more complete records at La Perouse.

“It’s a bit circular,” Ben admitted to me about his thoughts on the study during recent preparations. “We’re looking for what the trees can tell us about the timing of glacial advance and retreat and advance and retreat again,” but also “for how the trees themselves responded to changing climate conditions.” The ice and trees—the forest still standing and the ancient layer below—hold the same climate history.

Preliminary data suggests the ancient forest below dates back to 600 A.D., not long after the end of the Roman Era. Then there are the dead trees above, crushed and tilted and scarred by the ice that came with the Little Ice Age just over a century and half ago. What’s in between lies the Medieval Warm Period, or what scientists call the MWP for short.

“Today’s warming is different,” Greg Wiles told me in conversation about the MWP. “The rates are quite a bit different. But the medieval warm period was a natural warm period relative to today.” Some scientists consider it the closest analog we have for considering the consequences of future warming.

Ben and the team’s field technician, Phillip, are in Fairbanks getting ready to head south the archipelago. Yesterday, they packed food and shipped saws to Yakutat. Today, they’re packing in fresh greens while they can. Weather permitting, they’ll load up Wednesday for the flight to La Perouse.

UP NEXT: More on the medieval warm period and the many confusing aspects of historical climate. And the gear? What’s on the list for La Perouse?



“Though a wanderer himself, John Muir didn’t ride a glacier from to Yosemite or Alaska. But in a metaphorical way, he did. Raised in northern geographies that harbored the ghosts of glaciers—Scotland and Wisconsin—he became a keen observer. He developed ice age eyes….He regarded the long views, the wisdom of rocks.” – Kim Heacox, John Muir and the Ice that Started the Fire

Most people think of John Muir (aka the “Father of National Parks”) and the California Sierra Nevada, but in the four expeditions he made to the north, Muir left behind a legacy in Alaska as well. In 1899, Edward Harriman, a wealthy American in the railroad industry, arranged for an expedition from Seattle to Siberia and back. Muir was one of his top picks for the interdisciplinary team of scientists, artists, and photographers on board. The descriptions and illustration the team put together are some of the earliest known records of the La Perouse glacier. Even back then, the team noted the ice abutting forest—and remnants of a forest shaped by the glacier coming and going.

The Harriman Expedition report from June of 1899 notes: “The remnant of timber standing east of the stream valley was separated from the glacier at the time of our visit, by a belt of barren ground from 100 to 200 yards wide. This ground was occupied by bouldery till containing bruised and macerated branches and trunk fragments, and the margin of the timber showed unmistakable evidence of recent attack by the ice.” What was curious to the explorers back then would only become all the more relevant to another interdisciplinary team one hundred and seventeen years later.

“We didn’t go there in 2016 looking for the forest,” Dan told me. “I was interested in the old stuff—a section of an old marine terrace that the geologist on the expedition also described.” Over his years of working in Alaska, Dan has searched for evidence of the route the people who first populated the Americas took from Siberia. He’s looking for when and where the “the ice gates open”—when deglaciation occurred on the order of 14,000 or 15,000 years ago (maybe even more).

But the creek was flooded that year, so Dan and the team couldn’t cross over to the terrace the way they’d planned. “We went over the glacier just to get there just an easier way, and when we came around the corner, it looked like a field of telephone poles there, sticking out of the ground. We just stumbled upon them.”

Ben says that when they got down from the ice and walked about amidst the trees, they see that perhaps there were two forests left behind. The first appeared to be a relatively recent remnant of the Little Ice Age, the second perhaps from long ago.

UP NEXT: A bit about the role of paleoecology in understanding climate as the team preps for the return to La Perouse. How does the science work? What can a tree core reveal about the past?



There’s pretty much one pilot you want if you’re going to try to a land plane on the outer coast anywhere within striking distance of Yakutat, Alaska. His name is Hans Munich, and his experience flying the northern reaches of the Alexander Archipelago once saved me from rationing a couple days of emergency food over a week of waiting. Dr. Ben Gaglioti, our project lead from Columbia University’s Lamont-Doherty Earth Observatory, calls him a “man of few words”. But then, who wants chatting for this kind of flying? Weather turns quickly. Rain comes and goes. Thick fog settles, rapidly restricting visibility across the mountainous terrain. Where Hans decides that he can land is where the La Perouse expedition begins. Then to reach the recently exposed forest, the rest is by foot—fording creaks, bushwhacking through downfall, or crossing the ice of the glacier itself.

Funded in part by National Geographic, our team brings together scientists with a range of expertise. Ben is a paleoecologist and paleoclimatologist, which means he studies climate from long ago, past environmental conditions, and their relationships with ancient organisms. He focuses on the last 40,000 years, mainly at high latitudes and works with lake sediments and tree-rings for a window into past. I’m on the other end of the spectrum when it comes to time. I focus on the present and the future in light of climate change—what impacts we’re seeing and will continue to see, how people (and other species) can adapt to a rapidly changing world. I’m an ecologist and social scientist by training.

“Tell me why you turn to the past,” I said to Ben over the phone during one of our recent conversations.

“I want to understand what happened the last time things suddenly warmed up," he replied, "and make discoveries about the Earth’s systems that aren’t available from short-term observations.” Ben acknowledges the role of human activity in the unprecedented rates of warming today but also sees an opportunity to learn from the past. Climate has always been changing. (More on this complicated topic later.)

Dr. Dan Mann from the University of Alaska Fairbanks is a geologist and paleoecologist who studies how climate changed during the last ice age and, to some extent, how it shaped the world we know today. Then there’s Dr. Greg Wiles, an expert in glacial geology and tree-ring analysis. Greg and I aren’t going to the coast for various reasons. His role comes into play once that bush plane makes it out with tree cores and slabs for analysis. I’ll help with the science from afar and write. I spent six years studying how forests on the outer coast of Southeast Alaska are impacted by climate change and how people are coping with the changing environment. Now I work on climate adaptation -- helping people plan for the future, or respond, today, to impacts in their local environment. So, I too, thought maybe there was something to learn from the past.

UP NEXT: What did the ice at La Perouse look like when John Muir traveled the rugged coastline in 1899? What were Dr. Mann and Dr. Gaglioti doing out there in 2016, when they first discovered the recently exposed forest?


I was recently reading "The Unnatural History of the Sea" and was struck how much we use old explorer logbooks as a source of natural history. Can't wait to read more of this story!

Expedition Background

In 2016, a team of scientists traveled to the outer coast of Southeast Alaska in search of evidence of a strip of land that was ice-free thousands of years ago. They were looking for clues to solve a persistent mystery: if people first populated the Americas by migrating from Siberia to Alaska, what route did they take? They made a surprisingly different discovery.

Forced by changing stream conditions to take an unexpected route along the La Perouse glacier, they spotted an ancient forest once buried by ice—now exposed by recent glacial retreat. Snapped in places and leaning away from the ice, giant ghosts of an old-growth forest remained.

Samples later confirmed that these trees, and others, well-preserved a layer below, recorded a long history of glaciers coming and going. Some trees by the La Perouse Glacier dated back to the Little Ice Age. Others lived over a millennium ago. The rings inside could be used like fingerprints trapped in time, revealing when they died, under what climate conditions, and where the glacier reached back then.

Now, researchers are headed back. The plan is to harvest cross-sections and core samples from the sentinels on the outer coast, and via bush plane, ship out the history preserved in the wood for study. The question at stake: What can we learn about our warming world from what the melting ice leaves behind?


Lauren this is really cool. I'm excited to be following along with the journey.