Small-Scale Fires Key To Understanding Forest Structure

"I'm interested in how fire shapes the landscape. Resource managers are interested in restoring forests to what they were like before the arrival of Euroamericans," says R. Matthew Beaty, graduate student in geography. "To restore forests, we need to understand their natural variability."

What Beaty and Dr. Alan H. Taylor, associate professor of geography, are finding is that variation on a fairly local scale is important and that the environment, especially the topography, is key.

The researchers examined pairs of aerial photographs from 1941 and 1993, and noted significant changes in the Cub Creek Research Natural Area of the Lassen National Forest in Northern California. These changes are remarkable because the Cub Creek area has never been logged or grazed. The only human impact in the area has been fire suppression.

The watershed is very rugged with two ridges that run east and west from its headwaters. The researchers divided the area into three regions based on differences in topography.

"In the 20th century, fire suppression has changed the density and species diversity of these forests, but differently in each area. The controlling factor seems to be the direction that the slope faces," Beaty told attendees at the annual meeting of the Association of American Geographers today (March 29) in Boston. "A slope's direction influences species distributions and fire vulnerability because of microclimates."

Fire suppression began in 1905 and these forests which had widely spaced larger trees became closed forests, densely packed with smaller trees, saplings and seedlings. Fire suppression has also changed the forest composition from mostly pine, which are fire tolerant, to firs which are more shade tolerant but fire intolerant. To learn about the pre-Euroamerican fire history, the researchers took wedges from existing trees to identify when past fires occurred. Because trees add one growth ring per year and because the width of these rings is affected by climate conditions, scientists can date fire years with accuracy.

"Some fires burned throughout the area," says Beaty. "In 1795, 1829 and 1883 there were major fires that burned everywhere and these were also very dry years throughout California."

The three areas investigated were a south-facing slope, headwaters and north-facing slope. Natural fire breaks separate these areas and the historic frequency and severity of fires differs in each.

The south-facing slope is sunny and dry and includes abundant pines. It last burned in 1926. The area averaged a fire about every 9 years. The north-facing slope has not burned since 1883 and is dominated by white fir.

In the past, the area burned on average every 35 years. The difference between the north-facing and south-facing slopes is large with respect not only to fire frequency but also to fire severity. While fires on the south-facing slope were light, fires on the north-facing slope were catastrophic, often producing brush fields.

The fire history of the headwaters area has characteristics of both of the other tow areas and burns every 17 to 25 years.

"One apparent outcome of fire suppression is that, without fire, each of these areas is becoming more homogeneous," says Beaty.

The historic diversity of fire regimes within this small watershed contributed directly to past biological diversity. Now, white fir has reached unprecedented dominance, while other species are in decline. If resource managers want to maintain the remarkable diversity of these landscapes, the restoration of diverse fire regimes is necessary.