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Spring-cleaning for the forest: Fire ecology in Yosemite
November 13, 2013 - 3:34pm
“How many of you have heard of the Rim Fire?” I ask.
Ten hands shoot up, dwarfed by the auburn red of giant Sequoias in the background; our trail group of fourteen students is in the Merced Grove. After taking a few moments to gaze up into the dense canopy, we are discussing fire ecology.
The Rim Fire began in late August of this year and was one of the largest in California history, covering 402 square miles and burning at high intensity for several weeks. It affected roughly 10% of Yosemite National Park and was covered extensively in the media.
The Sierra Nevada is an ecosystem forged by fire. Generally resulting from summer lightning strikes, fires have historically been relatively small and of low intensity. Fire is ubiquitous and a vital component of this particular ecosystem.
Many organisms native to the Sierra Nevada are adapted to these disruptions with fire-resistant adaptations such as tanic acid within their bark, which acts as a fire retardant. Sequoias and other conifers have the ability to survive partial burns and even scar over and close off exposed wounds. In fact, the mighty Sequoia is a pyrophyte—a “fire plant”—its seeds need fire to sprout, and the young trees thrive on open patches of forest ground cleared by fire of competing vegetation.
Fires sweep through the forest understory, burning up debris and ladder fuels, clearing out clusters of vegetation, or taking out crammed seedlings competing for sunlight. Think of it as spring-cleaning for the forest, like removing all the clutter from your garage.
Even fires that leave no more than precarious and charred snags can increase long-term resilience by adding to the patchiness of the landscape. Exposed soil often leads to a jump-start of diverse plant communities—lupine, low-level shrubs, and sun-loving deciduous trees precede the conifers.
Before the 1840s and active fire suppression, most fires in the Sierra Nevada were of low intensity, with burns every 7–11 years. There is convincing historical evidence that Native American groups used fire regularly to manage for desired plant compositions.
Fires of the size and intensity of the Rim Fire are a relatively new phenomenon in the Sierra Nevada, the result—most foresters will tell you—of more than a century of fire suppression and the long-term trends of climate change. Both have resulted in a build-up in available fuels (dense undergrowth, more numerous and smaller trees, and uncleared snags), and thus more often than not, when the forest burns these days, it burns long and hard.
The extensive coverage of the Rim Fire, however, has provided a fantastic opportunity to peak interest and raise awareness of fire ecology in the Sierra Nevada. My trail group examines pictures of Yosemite Valley throughout the decades, noting differences in conifer density and species variety:
“The Ahwahneechee used fire to manage for deer and oak trees, while now we like big trees in our parks and suppress fires,” Josh says.
Which is better? It depends on your objective.
I ask the students: Do we manage to maximize black oak acorns? To attract tourists? Or to prevent catastrophic fire? Perhaps all three.
After our brief discussion, Leia rushes to the front of the trail group as we walk through a stand of conifer trees licked by fire scars, excitedly discussing how we would manage this forest, if given the chance.