Translating Nature’s Historians: The Tree Ring Lab Turns 40

September 16, 2015
 

Throughout the Tree Ring Lab at Lamont-Doherty Earth Observatory, you’ll find pencil-thin wood cores under microscopes and growth rings being measured to precision. They come from trees all over the world, and together they hold thousands of years of climate history – when major droughts set in, when the planet cooled and when it started warming, even when earthquakes rattled the landscape and volcanoes erupted.

One famous collection tracks changing temperatures and rainfall across the Northern Hemisphere, from volcanic eruptions that led to fast cooling in the early 1800s, to a warming trend that started with the Industrial Revolution and continues today. Another holds evidence used to confirm that a mega-earthquake struck the Pacific Northwest in 1699-1700, likely creating the “orphan tsunami” that reached Japan. Teak samples from South Asia reveal trends in the changing monsoon season. Bristlecone pine cores from the American Southwest hold warnings of drought.

It all started 40 years ago this month, when Ed Cook and Gordon Jacoby established the Tree Ring Lab at Lamont with a grant from the National Science Foundation.

Tree ring research was a young field, evolving from just dating wood to tracking droughts and temperature swings. The Lamont Tree Ring Lab, among the first in the world, would expand dendrochronology’s capabilities over the years and become a global leader in research, training and technology.

“Silent though they are, the trees, it turns out, can speak volumes,” Jacoby, who recently passed away, wrote in 1999. “The size, density, anatomy and chemistry of each ring reflect the environmental conditions in the year in which it grew. So like ancient scribes, long-lived trees can sensitively record the environmental history of a given place and time.”

300 Years of Climate History

Jacoby and Cook would explore the world over the Lab’s first four decades, but they started out closer to home, working in the climate-sensitive boreal forests of North America to better understand forest dynamics and the physiological impacts of climate on tree growth.

With Rosanne D’Arrigo, the Lab’s first graduate student and now director of the Division of Biology and Paleo-Environment at Lamont, they would fly into remote regions of Alaska and Canada and camp for weeks as they collected samples at the tree line, where trees are at the edge of survival, and small changes in temperature or water stress will leave measurable markings.

Gordon Jacoby (pictured above) and Nicole Davi core trees in Alaska in order to reconstruct temperature  of Alaska and the Northern Hemisphere. Lab scientists and students often spend weeks hiking and camping to collect samples. Photo by Nicole Davi

Gordon Jacoby cores a tree in Alaska to reconstruct temperature through history. Photo: Nicole Davi

Back in the Lab, the team was developing new methods of analysis and software to process stable isotope ratios in tree cellulose to function as paleo-thermometers. They also turned their attention to acid rain in the 1980s, ruling out natural causes for the widespread deaths of red spruce trees across New England. At the same time, the scientists continued to build North American tree-ring chronologies.

By 1989, Jacoby and D’Arrigo were able to publish a reconstruction of temperatures across the Northern Hemisphere going back over 300 years. The tree ring data reflected a climate that had been cooler in the 1700s and early 1800s but then warmed as carbon dioxide emissions from industry and vehicles increased.

Their research was a breakthrough for climate science and dendrochronology. Earlier studies had found evidence of a warming climate in pockets, including a 1981 study by Jacoby and Cook tracking 400 years of climate change in the Yukon. The new chronology showed that that trend extended around the world.

“Prior to this study, tree-ring reconstructions of past temperatures were limited to the local and regional scale. This paper expanded those analyses to the hemisphere, which provided more relevance for interpretation of large-scale temperature trends, including recent anthropogenic warming,” D’Arrigo said.

In the 1990s, Jacoby, Cook, and D’Arrigo, along with Paul Krusic, Brendan Buckley and Nicole Davi, shifted their attention to Southeast Asia and Mongolia. Members of the team once trekked for 13 days across Nepal to sample ancient hemlocks that held a 1,200-year history of climate in the region. In Burma, they were on guard for tigers, and they risked arrest at checkpoints in politically unstable regions, but they made progress toward what would become a widely used atlas of drought in the Asian monsoon region.

Old Tools, New Technology

The tools that tree ring scientists hike in with today with aren’t much different than those used by foresters 200 years ago. Increment borers are still metal shafts best powered by human muscles that can feel changes in resistance while coring a tree.

Rose Oelkers, a technician at the Tree-Ring Laboratory, examines a cross-section of archeological wood under a microscope. Photo by Nicole Davi

Rose Oelkers, a technician at the Tree Ring Lab, examines a cross-section of wood. Photo: Nicole Davi

It is in the lab where new technology, increasing computing power and new methods of analysis have advanced the field by magnitudes.

Over the years, Cook and Krusic developed widely used methods of statistical analysis and tree ring software that they posted free for anyone to use, including a computer program that remains the standard for developing tree ring chronologies, ARSTAN (autoregressive standardization). Sharing knowledge and tools remains a founding principle for the lab. As Cook explained, “We always felt it was a service we should provide to the community.”

The lab scientists also advanced the use of technology such as X-ray densitometry to extract sensitive climate data. The team developed a low-cost, high-resolution ring-width measuring system that reduced the time to obtain accurate tree ring measurements, allowed them to increase sample analysis from hundreds to thousands and contributed to tree ring research around the world. Today, D’Arrigo, Laia Andreu Hayles, Nicole Davi and others are experimenting with blue light densitometry that measures ring density through imaging.

“The Tree Ring Lab at Lamont-Doherty has literally re-defined our knowledge of climate over the past 1,000 to 2,000 years,” said Henri Grissino-Mayer, a dendrochronology professor at the University of Tennessee whose software list for students is filled with Tree Ring Lab links. “Today, one cannot open a textbook and read about past climate without reading the accomplishments of the staff and scientists at the TRL.”

Fine-Tuning Climate Models

The advanced software and a growing network of samples and trained colleagues around the world helped the lab develop two widely used drought atlases: the Northern American Drought Atlas, published in 2004, and the Asian Monsoon Atlas, published in 2011.

The drought atlases and many of the lab’s studies have been crucial to the development of climate models.

Cook explained how his work with the long-lived, slow-growing bristlecone pines of the American Southwest first piqued the interest of climate modeler and Lamont colleague Richard Seager in the early 2000s.

In 2004, Cook showed that the ongoing drought in the U.S. Southwest paled in comparison to a drought about 1,000 years earlier. The paper suggested that the region is extremely vulnerable to disastrous drying during periods of climate warming. Seager followed with an influential 2007 paper suggesting that the region will dry significantly in the 21st century and that the transition to a long-term more arid climate may already be underway.

“We have trees in North America going back 1,000 years that show periods of incredible mega droughts. You throw on top of that the greenhouse forcing effects, and we could potentially see a phenomenally bad state of mega-drought. Whether it will happen, you can argue. Model projections suggest it could,” Cook said.

Brendan Buckley and Ed Cook stand on a ridge in Tasmania, where they were able to reconstruct 4,000 years of temperature history and develop an almost complete Holocene link of more than 10,000 years. Photo courtesy of Brendan Buckley

In Tasmania, Brendan Buckley and Ed Cook were able to reconstruct 4,000 years of temperature history and develop an almost complete Holocene link of more than 10,000 years. Photo: Courtesy of Brendan Buckley

Cook’s bristlecone pines have also helped the Lab reconstruct a 1,100-year history of El Niño Southern Oscillation fluctuations, supporting the idea that continued warming may lead to more extreme climate conditions around the globe.

The lab has turned attention more recently to the influence of climate change on drought in western North America, with Park Williams finding that warming increases forest drought stress, leading to decreased growth and decreased ability to recover after droughts. His latest paper explains how global warming is exacerbating the current drought in California, and will continue to exacerbate future droughts as warming continues.

The Tree Ring Lab has dated timbers from a ship found buried at the site of the World Trade Center; connected a warm, wet period in Mongolia to the rise of Genghis Khan; and found tree ring evidence of the 1815-1816 eruption of Tambora in Indonesia, the same period American colonists called “the year with no summer.” A recent study of tree rings from Mongolia dating back more than 1,000 years confirms that warming in central Asia has no parallel in any known record. Cook, D’Arrigo and Buckley are also pursing tree rings in the tropics, where the lack of seasons makes annual rings more difficult to find.

Along the way, the team has helped establish dendrochronology labs in Nepal, Bhutan, Thailand, Indonesia, Tasmania, New Zealand, Mongolia and across North America.

“We have helped train young scientists, young foresters, dendrochronologists from many different countries of the world,” Cook said. “We’ve probably done more of that than any other tree ring lab in the world because we wanted a global view. We wanted to see the bigger picture beyond the work in our own back yard.”

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Marie DeNoia Aronsohn
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