For nearly two weeks, the nation has been transfixed by wildfire spreading through Yosemite National Park, threatening to pollute San Francisco’s water supply and destroy some of America’s most cherished landscapes. As terrible as the Rim Fire seems, though, the question of its long-term effects, and whether in some ways it could actually be ecologically beneficial, is a complicated one.
Some parts of Yosemite may be radically altered, entering entire new ecological states. Yet others may be restored to historical conditions that prevailed for thousands of years from the last Ice Age’s end until the 19th century, when short-sighted fire management disrupted natural fire cycles and transformed the landscape.
In certain areas, “you could absolutely consider it a rebooting, getting the system back to the way it used to be,” said fire ecologist Andrea Thode of Northern Arizona University. “But where there’s a high-severity fire in a system that wasn’t used to having high-severity fires, you’re creating a new system.”The Rim Fire now covers 300 square miles, making it the largest fire in Yosemite’s recent history and the sixth-largest in California’s. It’s also the latest in a series of exceptionally large fires that over the last several years have burned across the western and southwestern United States.
Fire is a natural, inevitable phenomenon, and one to which western North American ecologies are well-adapted, and even require to sustain themselves. The new fires, though, fueled by drought, a warming climate and forest mismanagement — in particular the buildup of small trees and shrubs caused by decades of fire suppression — may reach sizes and intensities too severe for existing ecosystems to withstand.
The Rim Fire may offer some of both patterns. At high elevations, vegetatively dominated by shrubs and short-needled conifers that produce a dense, slow-to-burn mat of ground cover, fires historically occurred every few hundred years, and they were often intense, reaching the crowns of trees. In such areas, the current fire will fit the usual cycle, said Thode.
Decades- and centuries-old seeds, which have remained dormant in the ground awaiting a suitable moment, will be cracked open by the heat, explained Thode. Exposed to moisture, they’ll begin to germinate and start a process of vegetative succession that results again in forests.At middle elevations, where most of the Rim Fire is currently concentrated, a different fire dynamic prevails. Those forests are dominated by long-needled conifers that produce a fluffy, fast-burning ground cover. Left undisturbed, fires occur regularly.
“Up until the middle of the 20th century, the forests of that area would burn very frequently. Fires would go through them every five to 12 years,” said Carl Skinner, a U.S. Forest Service ecologist who specializes in relationships between fire and vegetation in northern California. “Because the fires burned as frequently as they did, it kept fuels from accumulating.”