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Ecology of wildfires in Yosemite

Part 2 of 3

YOSEMITE -- Wildfires may be vital to the ecology of Yosemite National Park, but they also can help humans as well. More water is available when fire reduces the amount of small trees and brush.

Kristen Shive, Yosemite’s fire ecologist, said they have been studying the Illilouette Basin since 1974 when they began allowing fires to burn through that area.

“Yosemite is unique in that it is one of the few places in the west that, in the ‘70s, started doing controlled burns and allowing some lightning fires to burn for resource benefit; and most of that started in the Illilouette Basin.

“There’s an increased water yield coming out of that basin, because there are fewer trees sucking water out of the ground in these healthy, restored ecosystems. In a time of drought, we also found that there is less extensive tree mortality in Illilouette compared with other basins that are similarly situated.”

Before European-Americans came and started suppressing fire, these areas burned quite frequently, says Shive, probably every 5 to 15 years in the lower elevations, and every 20 to 30 years up higher.

Studying fire intensively over the last half-century has taught park officials a lot about the plants and animals that are adapted to ecosystems maintained by fire.

“The cones of the giant sequoias don’t open without fire,” says Shive. “There are some species of birds that will only use the openings created by fire. We are seeing a more diverse group of species in the basin, including a higher diversion of pollinator species, of bee pollinator species, of animal and plant diversity — we have a whole slate of species that are adapted to these different stages, all of which are maintained by fire.”

The lack of fire has had a dramatic effect on ecosystems as well as how fires burn today. Shive says that decades of allowing fires to burn in Illilouette has shown that suppression has not only blocked the benefits of fire, but has led to larger, more intense wildfires.

The 2013 Rim Fire scorched over 257,000 acres on the Stanislaus National Forest and in Yosemite. “When a fire burns small patches of trees, that isn’t necessarily a bad thing,” says Shive. “But when you get an area that’s over 5,000 acres in which more than 90 percent of the vegetation is destroyed, that brings really dramatic changes to the ecosystem. That’s total stand replacement; and because the trees only seed so far from their parent tree – and you have no live trees in an area – that’s not going to be a forest again for a very long time.”

One tool Yosemite fire managers use to reduce fuel loads, restore forest structure, and thereby reduce the intensity with which wildfires burn — is prescribed fire. Crews go out and put fire down on the ground, but do it under controlled conditions.

“That’s the primary tool we use at the lower elevations,” says Shive. “We do that in the late fall or early spring when temperatures are lower and humidities higher. Fire doesn’t burn as hot or as severe, and that’s what we want — for fire to just go through and clean it up. We’re setting the landscape up so it can handle a fire in the middle of the summer when things are really hot and dry.”

Shive says that opening up the forest by taking some of the small trees and brush out, allows more sunlight to reach the forest floor.

“A dense forest is very shady, and lots of things can’t grow. Also, by burning up litter and duff, you open up the soil. There are some species that will only germinate with totally bare soil. You also recycle nutrients that other species need to survive. There are some species – like a lot of the shrub species you see in the park – whose seeds are cued by fire.”


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