History of fire and drought shapes the ecology of California, past and future

Fire season has arrived in California with vengeance in this third year of extended drought for the state. A series of large fires east of Redding and Fresno, in Yosemite, and on the Oregon border prompted Gov. Jerry Brown to declare a state of emergency on Sunday, August 3rd.

As force of destruction and renewal, fire has a long and intimate history with the ecology of California. Ecological scientists will discuss aspects of that history in detail at the upcoming 99th Annual Meeting of the Ecological Society of America on August 10 -- 15th, 2014.

"Big fires today are not outside the range of historical variation in size," said Jon Keeley, an ecologist based in Three Rivers, Cal., with the U.S. Geological Survey's Western Ecological Research Center, and a Fellow of the Ecological Society. Keeley will present research on the "association of megafires and extreme droughts in California" at the Annual Meeting as part of a symposium on understanding and adapting to extreme weather and climate events.

He will synthesize his research on the history of wildfire across the entire state, contrasting historical versus contemporary and forested versus non-forested patterns of wildfire incidence. He and his colleagues reviewed Forest Service records dating to 1910, as well as a wealth of newspaper clippings, compiled by a Works Progress Administration archival project, that stretch back to the middle of the last century. Understanding historical fire trends, Keeley said, means recognizing that when we talk about wildfire in California we are talking about two very different fire regimes in two different ecosystems: the mountain forests and the lower elevation chaparral, oak woodlands, and grasslands.

The chaparral shrublands of southern California, and similar sagebrush ecosystems in the Great Basin, are not adapted to the kind of frequent fire typical of the mountain conifer forests in California. Fires in the lower elevation ecosystems are always crown fires, which kill most of the vegetation. In the millennia before humans arrived, these ecosystems burned at intervals of 100 to 130 years.

These lower elevation ecosystems experienced unprecedented fire frequency in the last century, with fire returning to the same area every 10 to 20 years, altering the ecology of the landscape.

"In Southern California, lower elevation ecosystems have burned more frequently than ever before. I think it's partly climate, but also people starting fires during bad conditions," Keeley said. Bad conditions include extended droughts and dry fall days when the Santa Ana winds blow through the canyons.

In high elevation conifer forests, spring temperatures and drought are strongly correlated with fire, and Keeley thinks climate change and management choices are likely playing a role in current trends. But in the hotter, drier valleys and foothills cloaked in grass, oak, and chaparral, human behavior dominates. Through arson or accident, in southern California, over 95% of fires are started by people, according to Cal Fire.

"Climate change is certainly important on some landscapes. But at lower elevation, we should not be thinking just about climate change," said Keeley. "We should be thinking about all global change." Land use change and population growth create more opportunities for fires to start.

The high frequency of fire has instigated a persistent switch from chaparral to grass in some areas. Frequent fire favors quick germination and spread of forbs and grasses. Most grasslands in California are not native.

Since the more recent arrival of immigrants from Europe and Asia, several of the exotic grasses they brought with them from the Old World have been quick to capitalize on the opportunities presented by fires to spread invasively throughout roughly a quarter of chaparral country. To Keeley, this means that prescribed fires in lower elevation ecosystems now have entirely different consequences for the regional ecology than they did when native Californian peoples set fires to manipulate resources.

"When the Native Americans did it, they did not affect native species so much, because native perennial bunchgrass and other herbaceous species grew in," said Keeley. "Once the aliens got here, it completely changed."
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References:

Science Daily. 2014. “History of fire and drought shapes the ecology of California, past and future”. Science Daily. Posted: August 6, 2014. Available online: http://www.sciencedaily.com/releases/2014/08/140806173838.htm