For WVU’s Amy Hessl, answer to Mongol expansion is in the wood

    Amy Hessl hoped it hadn’t all been for nothing.

    It was at her insistence that she and her colleagues drove for hours, carried supplies through an ancient lava field and traveled back to the nearest town for a replacement chain saw. One of her colleagues was sick, and they were truly in the middle of Mongolia.

    This side trip on a research project wasn’t based on a hunch. It wasn’t belief. And it wasn’t all luck. Hessl was an experienced scientist who saw a lava field covered in trees that she knew told an important story. She wanted to have the slices of old, tangled wood examined in a lab to see what their message might be.

    She had no idea that the team had picked the winning combination in the scientific lottery.

    This summer, Hessl and the team will be going back to continue what they started.

    What Hessl, a West Virginia University researcher, and her group found that day in the summer of 2010 was evidence that Genghis Khan had a valuable ally in his conquest of the largest contiguous empire in history. That ally was the weather, and this weather was rainy.

    It’s all about energy. ... Abundant rain made the grasses grow and grass powered the horses that grew the cavalry that conquered the region.

    —Amy Hessl
    WVU geographer on her Mongol discovery

    For hundreds of years, historians have examined and wrangled over just what motivated the Mongol hordes of the 13th century. But theories about the Mongol conquests have sparked almost as many questions as answers.

    Were they migrating in search of food? Were they behaving like landlocked pirates? Or were they satisfying a thirst for power in a wave of warfare that led to an empire straddling Eurasia?

    Mongol leader Genghis Khan forged an empire that eventually stretched from Asia’s Pacific coast to Eastern Europe and south into Persia and Southeastern Asia – a feat that may have cost more than 40 million lives.

    One of the most popular theories for the Mongol expansion was that the hordes started taking from their neighbors when they were forced to flee drought conditions.

    But Hessl, a WVU associate professor of geography in the Eberly College of Arts and Sciences, made a discovery that upsets conventional wisdom. She’s proposing that the spark behind the great Mongol empire expansion may have been rain, not drought.

    Hessl is a dendrochronologist – a trained expert in analyzing past climate conditions by studying the growth rings in trees. It is an activity that requires skill, patience, powers of observation on a microscopic level and a keen interest in the past.

    Journey to Mongolia

    In the summer of 2010 Hessl had the opportunity to work as the principal investigator on a National Geographic-sponsored project in Mongolia focused on how climate change might have affected the region’s risks of wildfire. She teamed with colleague, Neil Pederson of Columbia University’s Lamont-Doherty Earth Observatory, and Baatarbileg Nachin, a professor at National University of Mongolia, and a squad of his undergraduate students before heading out into a remote Mongolian countryside in search of tell tale wood samples.

    They journeyed to a high-elevation site that other researchers had found to contain trees that were hundreds of years old. They found that the site had burned, which gave them valuable data on the fire cycle in the region.

    • mongolia-slideshow-01

      Amy Hessl

      WVU professor Amy Hessl is a dendrochronologist – a trained expert in analyzing past climate conditions by studying the growth rings in trees. It is an occupation that sometimes requires powers of observation on a microscopic level which she pursues in the Montane Forest Dynamics Lab in Morgantown. The lab's work focuses on the interaction between ecosystem processes, climate variability and human activities in forested systems.

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      History in trees

      Tree rings can tell the tale of a region's environmental history. They can indicate whether abundant water supplies promoted growth or drought conditions inhibited it. At her WVU laboratory, Amy Hessl routinely unlocks that history through examination of the rings under the microscope.

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      Searching the lava flow

      One of Hessl's local assistants takes a break on the group's trek into the lava flows where wood samples were found that held surprises for environmental history researchers.

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      Home to Genghis Khan

      The Orkhon Valley, where Hessl journeyed in search of environmental history clues, is home to some of the best examples of Mongolia's historical sites, monuments and monasteries. The valley holds many religious and cultural antiquities that were an early backdrop to Hessl's foray into 7,000-year-old lava flows.

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      Hidden history

      On last summer's visit, Hessl took a moment to examine the trunk and roots of a tell tale pine tree near the lava flows where 7,000-year-old history was uncovered in the rings of ancient trees.

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      Professor and explorer

      Hessl's field research is expanding thanks to nationally recognized research results, the commitment of WVU to global engagement, and a society that recognizes the need to look to the past to prepare for the future.

    But on their way back, they stopped at an overlook and Hessl got an idea.

    “We looked out across this lava field and I said, ‘Oh, that looks like this place called El Malpais,” Hessl said.

    A colleague of Hessl’s had discovered ancient wood with an extensive fire history in El Malpais, a conservation area in New Mexico.

    She wanted to cross the river and explore immediately, but the rest of the group wasn’t as enthused. She made the case for returning to Mongolia’s capital for a few days of rest and then traveling back to the promising site.

    So they packed up their laptops, chain saws and survival gear and headed into the old lava flow near the ancient seat of the Mongol empire, the Orkhon Valley.

    The idea was to find tree samples dating back 500 years or so to fit in their fire history study.

    But it became hard to stay on track as equipment broke down, a colleague fell ill and dehydration became a problem. This spur of the moment excursion seemed less and less worth it.

    “It was this really long drive to get back to the lava flow and by that time everybody was sort of thinking, ‘Why are you wanting to go there?’” Hessl said.

    Morale improved as the slices of wood appeared useful. They didn’t seem too special but were salvaged trophies from the dramatic trip.

    That was until Pederson got the samples into the lab and started looking at them on a whim almost a year later. The wood fragments were not a few hundred years old. They were more than 1,300 years old, from 658 C.E.

    The team’s time in the 7,000-year-old lava field went from wasted trip to gold mine within moments.

    “We realized it was one in a million,” Hessl said. “I mean if you’d asked me five years ago if I would ever find anything like this, I would say ‘no way.’ It’s literally a needle in a haystack.”

    “We had all this environmental history all of a sudden that we never expected to have,” Hessl said. “It’s all about energy. What we are seeing in the rings is that around the time of the rise of the Mongols, there was abundant rain. Abundant rain made the grasses grow and grass powered the horses that grew the cavalry that conquered the region.”

    Hessl said energy was a motivation years later when the Mongols, after already establishing a massive empire, suddenly moved their capital. More evidence from the tree rings indicates that at the same time they moved their capital from Mongolia to Beijing, there had been a rapid decline in moisture in the Orkhon Valley.

    “The move was all about energy again,” she said in her Morgantown lab where a Mongolian flag hangs on the wall between shelves and shelves full of tree trunk slices. “The Mongols were forced to diversify when the grass became scarce. That had traditionally been their energy source for their horses.”

    She said that when the Mongols moved, the empire became less dependent on grass and used other energy sources instead: they developed a navy, raised rice products and pursed energy resources that were less grass- and horse-power based.

    “That’s where we learn from the history of past civilizations,” Hessl said. “Just as they diversified and switched energy sources in response to changing water quality and other environmental changes, we are seeing changes in our own civilization too. Right now we use fossil fuels but we will eventually have to find something else and adapt.

    “Exploring how the Mongols adapted might shed light on current challenges.”

    In an article about her work that appeared in the March 21 edition of Scientific American, Hessl stressed that she and her colleagues are not claiming that climate was the main factor in the rise and fall of the Mongols.

    “Genghis Khan was really the key to uniting many tribes together and spurred them to expand in a way that’s never been repeated,” she told Scientific American writer Charles Choi. “We just argue that it takes energy to create an empire, just as it does today and rains may have helped provide the grass that powered their horses. After Genghis Khan died, the empire became somewhat factionalized with most historians arguing that it became too large to effectively administrate. We’re saying maybe climate change may have made managing the empire difficult also.”

    Right now we use fossil fuels but we will eventually have to find something else and adapt. Exploring how the Mongols adapted might shed light on current challenges.”

    The research continues this summer.

    Hessl is going back to the Orkhon Valley with an expanded team and a refocused mission. National Geographic has awarded her another $20,000 grant as principal investigator to expand on the initial discovery. Recognizing the uniqueness of the work, its global significance, and its connection to the institution’s strategic goals of global engagement, exchanges of knowledge, and acceleration of quality research, West Virginia University Faculty Senate has also made a financial commitment of $12,000 to support the work and enable a return visit to Mongol territory.

    Hessl said that in addition to her graduate student John Burkhart of Morgantown, the team will expand to include researchers from other U.S. universities who can use Hessl’s tree ring data to estimate how many animals and resources the Mongols could have secured from the landscape. Hessl said historian Nicola Di Cosmo of the Institute for Advanced Study will join the effort to uncover written references to climate that coincide with the tree ring data.

    Avery Cook-Shinneman at the University of Washington will join the team to collect cores of sediment from lake bottoms in the region.

    “Her samples can show the history of the region in sediment,” she said while sitting at a microscope in the Montane Forest Dynamics Lab, the neatly organized research facility she operates in WVU’s Brooks Hall.

    “They will look for something called Sporormiella which are spores that thrived in livestock dung. The presence of Sporormiella in the samples can give us an idea of how much livestock the Mongols may have been able to accumulate in the time periods we are looking at.”

    The addition of a historian to the team makes sense because Hessl’s field of study has many implications for understanding the past. Matching the pattern in trees whose age is known to the pattern in wood found at an archaeological site can establish the age at which the wood was cut and thus the approximate date of the site. By comparing living trees with old logs and finding overlapping ring patterns, scientists have established chronological records for some species that go back as far as 9,000 years.

    Hessl and her graduate students have studied the influence of climate and land use history on fire regimes in the Appalachian Mountains, the Pacific Northwest and have developed millennial-length climate reconstructions for the Mid-Atlantic Region using the tree rings of ancient eastern red cedar collected in West Virginia. Her lab has also explored the relative impacts of climate variability and harvest strategies on carbon sequestration. In collaboration with the National Park Service the lab is exploring plant diversity on the cliffs of the Mountain State’s New River Gorge.

    “Earth’s citizens are faced with a host of environmental problems,” Hessl told National Geographic. “By looking at how the Earth has changed in the past and how peoples have responded to those changes, we can better find our way today.”

    By Gerrill Griffith
    WVU Research Corp.
    Diana Mazzella
    University Relations/News



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    • In the summer of 2010, WVU researcher Amy Hessl and her team collected wood samples in Mongolia.
    • When the team examined the samples a year later, they found that they dated back 1,300 years to the time of Genghis Khan.
    • Their research shows that climate, in the form of a rainy period that fed the plains, could have been a key factor in the expansion of Khan’s empire.
    • The team returned to Mongolia this summer as part of a National Geographic-funded research trip.