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Portrait: Larissa Yocom
Melissa Hatfield Riggs

Lady of the Rings

Larissa Yocom in the NAU Ecology Lab

The polished cross-section of wood under the microscope is beautiful, with whorls of rich red-brown and tan interspersed with dark brown rings. It is mounted carefully on a piece of plywood and marked with identifiers revealing that it came from a log lying on the floor of a high-elevation pine forest in Mexico. This particular sample is Larissa Yocom’s favorite from her current research: the earliest ring dates to 1409, and the tree died in 1590. The wood is so impregnated with resin that it shows no sign of rot despite the hundreds of years it lay on the forest floor.

She carefully examines it under the microscope, noting distinctive growth patterns, years of drought or deluge, and the scars of five wildfires over its 180-year life. She dates the rings, measures them, analyzes for a host of more complex factors, and loads the data into the worksheet that is building a detailed picture of the forest’s climate and fire patterns over the centuries in this unstudied region of North America. When she is done with the 40+ boxes of wood samples she brought back to NAU from diverse sites hundreds of miles apart, including the western and eastern Sierra Madres and a central Mexican volcano, Yocom will be able to differentiate between local, regional, and continent-wide factors influencing wildfire occurrence.

Join this piece of the puzzle with other location studies and there will be new evidence to answer a foundational question on the causes of wildfire: bottom up or top down? In other words, is wildfire occurrence most strongly regulated from the top down by climatic events like El Niño that can affect an entire continent, or from the bottom up by site-specific characteristics like fuel availability, topography, and forest structure and species?

Yocom, an NAU doctoral student in forestry, received the 2010 Research and Creative Activity (RCA) Award for Most Promising Graduate Researcher from the Office of the Vice President for Research in April. The award came hard on the heels of her National Science Foundation (NSF) Doctoral Dissertation Enhancement grant earlier this year. She is also the co-principal investigator on a new NSF grant proposal for a large-scale, multi-year expansion of her fire/climate research in Mexico and Central America. The first chapter of her dissertation has already been published in Ecology, the most prominent journal in her field.

Her work will supply hard evidence for predicting how global climate change is likely to affect wildfire occurrence in North America and how we can mitigate it, as well as improving natural resource policy and generating new data for use in ecosystem maintenance and restoration. Understanding how often and under what conditions fires burned historically, for example, helps managers make decisions about prescribed burning, letting wildfires burn unchecked, and implementing thinning projects.

Pete Fulé, an international expert on wildfire and a director of NAU's Environmental Research Institute (ERI), is Yocom’s advisor and colleague. “Larissa is among the finest scholars and contributors to the NAU academic community,” he says. “She is an exceptional scientist working in an important and challenging line of research, and one of the rare people who can negotiate the differences in culture and language that too frequently divide us from our neighbors.”

Yocom’s work is pioneering in several ways, Fulé says. Fire patterns have barely been studied in northern Mexico. Her work forms a key part of an international partnership that includes scientists and students in Mexico, Central America, and the U.S. And her research has important implications for our country as well. Parts of the remote Mexican forests in which she works have been spared logging, grazing, and fire suppression, offering examples of near-natural ecosystems that are rare in the U.S.


Ring Lore

Yocom and ERI undergrad student discuss a fire-scarred pine

Yocom grew up in southwest Washington on a small farm surrounded by timber, living intimately with western natural resource issues. She came to Northern Arizona University after master’s work at Yale University’s School of Forestry and Environmental Studies, previous graduate studies in Latin America (she is fluent in Spanish), and professional work in the American northwest and Australia.

Because of the nature of her work, she is CPR-certified, a Wilderness First Responder, and certified to fight fire. Her undergraduate training as a cross-country athlete prepared her well for the strenuous demands of working at high elevations and in rough terrain carrying chain saws and wood samples.

But her fascination is not so much fire as climate. And one of the best ways to determine historical climate patterns is through dendrochronology: the science of following tree-ring patterns back through time. “Unlike words,” says a modern poet, “tree-rings never lie.”1 It’s not quite that simple, of course.

"Taking samples does not harm the tree," Yocom says.
Yocom taking wood samples in Mexico

Yocom first collects samples from fire-scarred live trees, dead trees, stumps, and logs. She determines the precise year of each fire scar and maps every fire-scarred tree to determine the spatial extent of the fires. She analyzes the years of widespread fires statistically in relation to yearly climate variables and collects information on local factors such as fuels and tree species, density, and height. She notes elevation, slope, and aspect, then analyzes all these factors in relationship to historical fire occurrence. Yocom and/or other scientists now have the data necessary to use fire behavior simulation software to model area fire risk in future climate scenarios.

With this level of involvement in dendrochronology, Yocom is frequently asked to guest lecture for graduate-level courses at the School of Forestry and has co-taught a semester-long NAU graduate seminar on the subject. She has presented at two Mexican universities as well as the Ecological Society of America and the Association for Fire Ecology.

One of her most meaningful presentations, however, came last year when she and two other members of her research team spoke at an impromptu public seminar in Ciudad Serdán, Pueblo, Mexico near the highest mountain between Colombia and the Yukon. The hotel manager where they were staying was so interested in what they were doing and why that he decided the whole town needed to know. He publicized the presentation over the next few days and approximately fifty people attended.

(All photos in the field courtesy of Larissa Yocom)
Speaking in Ciudad Serdán;

“The research I am working on is scientifically fascinating to me personally, and hopefully useful for conservation professionals, natural resource managers, and fire professionals,” Yocom says. "But it’s important to me that I am doing a job that is important to other people. It has to matter to the local community.”


Circles of Life

Tree rings are circles with a common center. Like ecosystems. Like communities.

For Yocom, the secrets are in the circles.


—Melissa Hatfield Riggs, Graduate College


1Quoted from "The Oldest Tree on Earth: The Curse of Methuselah" by British poet Roger McGough.