In the life of a forest, fire can be a frequent and demanding companion. How often the flames visit and whether they stay low, licking the tree trunks, or flare into the canopy, becoming what foresters call a “stand replacement fire,” can determine the character of the forest for centuries. Or until the next fire.
It’s a story that two OSU Ph.D. students are coming to know intimately. Alan Tepley (forest science and geosciences) and Jorge Ramirez (mathematics) are part of a research team that is taking a hard look at how fire affects the forests of Oregon’s western Cascades. In this steep ridge and valley terrain, the plot gets complicated. Fires can be fickle, burning one stand completely while leaving its neighbors untouched or lightly scorched. The result is a crazy quilt of stands with varying ages and compositions.
Trees Cores Tell Stories
Insects also take a toll on the forest. In his cores, Alan Tepley suspects he has found evidence of a spruce budworm outbreak in 1739, lasting until about 1750. The Douglas fir core (top), a host species, shows sharply reduced growth compared to a western hemlock (not a preferred host) at the same site. Black dots are made by researchers in counting the rings.
What controls these kinds of fires — and thus the structure of our forests — is of concern to policymakers as well as to scientists. The nation regularly spends more than $1 billion a year fighting forest fires, and western U.S. forests are expected to be at greater risk of burning in a warmer world. The long-term consequences of a more intense fire regime are poorly understood.
Through painstaking fieldwork and mathematical modeling, Tepley, Ramirez and their colleagues are demonstrating how forest stands evolve in the face of fires that occur repeatedly over centuries. Their work is supported by a National Science Foundation-funded education program, an innovative graduate student training program sponsored by the National Science Foundation. Known as the Ecosystem Informatics IGERT (Integrative Graduate Education and Research Traineeship), the program prepares students for science jobs by bringing them together with experts in a range of disciplines to analyze ecological processes.
Coring Into the Past
For his part, Tepley has looked to the forest. During the past two summers, he has pointed his gray Honda Civic toward the mountains, hiking deep into the woods. He has twisted tree-coring devices (breaking them more than once) into the hearts of centuries-old Douglas firs, hemlocks and red cedars.
The pencil-thin cores contain a record of growth rings that reveal a tree’s age and its yearly growth. Across the Fall Creek and Blue River watersheds, Tepley has collected data for almost 2,000 trees in 77 linear plots, each longer than a football field. Some of the sites are in one of the nation’s premier forest research facilities, the H. J. Andrews Experimental Forest, which also served as his base camp.
Back in Corvallis, Tepley is scrutinizing each core under a microscope. He has already identified six distinctly different forest histories, each representing what scientists call a successional pathway. Each is associated with a position on the landscape, from shady north-facing slopes to sunny hillsides. Tepley’s cores show that stands in the Fall Creek watershed tend to burn more frequently and less severely than those in the neighboring Blue River watershed. All stands in Fall Creek show evidence of fire within the last 200 years, but in Blue River, several stands show no fire evidence within the last 400 to 500 years.
Terrain turns out to be critical in setting the stage for how severely and how often fire visits a given stand of trees. “In certain locations, topography may consistently either reduce fire frequency or moderate fire severity,” Tepley has written in summarizing his work to date. “These places may function as refuges for old trees. Also, they may play important roles as a seed source for plant species or as temporary habitat for animal species following disturbance to the surrounding landscape.”
Tepley began his forest ecology career in Michigan where he has worked in oak-hickory, jack pine, and northern hardwoods forests. The son of a physicist and a social worker received his bachelor’s and master’s degrees at the University of Michigan. For the state’s natural heritage program, he reviewed land surveyor notes from the early 1800s, contrasting frequent observations of burned trees with modern forests in which fire had been suppressed. “For me, fire has always been a part of studying forests,” he says.
Insight From Mathematics
Since the 1970s, researchers have known that the western Cascades represent something special in forest fire science. That’s because many different forest histories exist side-by-side in these mountains, the result of what scientists call a “mixed-severity fire regime.” Tepley and his advisers — Julia Jones in the Department of Geosciences and Fred Swanson, a U.S. Forest Service scientist who conducted much of that work in the ’70s and is associated with the OSU Department of Forest Science — hope the tree core data will reveal just how such fire regimes influence the modern forest.
“There’s been a lot of controversy in the Pacific Northwest about ancient forests, which mainly are 500-year-old stands that were regenerated after fires during the 1500s. So the character of the landscape for which Northwesterners feel an affinity was shaped by the history of fire,” says Jones.
“We have a forest today that results from a particular, maybe even a peculiar, series of events,” she adds. “We tend to think of that as inevitable, but it wasn’t.”
In short, the modern forest could have turned out differently. Had fires burned more frequently or more intensely in the past, today’s forests could have stands with different ages or proportions of species. To think about what different fire histories might mean for forest development, the forest scientists have turned to mathematics and Jorge Ramirez.
Born and raised in Medellín, Colombia, Ramirez received his civil engineering degree at the Universidad Nacional de Colombia. Both his parents are mathematicians, and he equally enjoys soccer and collaborating with scientists to solve problems. For OSU fire researchers, the problem is how to relate the age structure of the forest to the frequency and intensity of fire.
Ramirez’s fire model — a set of equations that describe forest development as a function of fire — is intended to help scientists explore this issue by enabling them to see the consequences of different fire patterns. “This model is not for prediction. It is a conceptual model. The point is to get insight,” Ramirez says.
Ramirez started with simple assumptions. “We assume fires occur randomly in time at a fixed rate. We assume a fire will kill trees under a certain age. There will be some rules about how trees (regenerate) after a fire. Let’s see what that gives you,” he says. Mathematicians can help scientists reduce the complexity of what they observe in nature, “to bring it (the process) back to the bare bones.”
For Tepley, Swanson and Jones, modeling is also a tool for thinking about the future forest. “The data I have represent what has happened over the past 500 years. But ultimately it would be interesting to know what else could happen,” Tepley says. “Modeling has the ability to picture the range of possibilities.”
Knowing those possibilities could help guide forest management if climate change affects precipitation, temperatures, insect outbreaks and fire in the Northwest. “Were refuges buffered from past climate variability?” asks Swanson. “Might they be buffered in the face of future climate change? If we want to encourage old growth in the future, are those the places where we might have the best chance of being successful?