One Hundred Hikes in Yosemite

The Living Yosemite - The Role of Fire

The presence or absence of ground fires can really alter the system, for it significantly alters the populations of ground-dwelling plants and animals and everything associated with them. Fires were once thought to be detrimental to the overall well-being of the ecosystem, and early foresters attempted to prevent or subdue all fires. Until 1971, fire suppression was a general, though sometimes contested, Yosemite policy. This policy, however, led to the accumulation of thick litter, dense brush, and overmature trees-all prime fuel for a holocaust when a fire inevitably sparked to life. It also led to a change in the distribution of plant and animal species and encouraged root rot (see Hike 66).

Foresters now know that natural fires should not be prevented, but only regulated. These fires, if left unchecked, burn stands of mixed conifers—such as those found in Yosemite Valley—about once every 10 years. At this frequency, brush and litter do not accumulate sufficiently to result in a damaging forest fire; only the ground cover is burned over, while the trees remain generally intact. Thus, through small burns, the forest is protected from going up in smoke. Ultimately, however, stands of trees mature, the trees die, logs accumulate on the ground, and major fires occur, such as the 1988 Yellowstone fires.

Some trees are adapted to fire. The giant sequoia, for example, releases its seeds after a fire, as do numbers of conifers, shrubs, and wildflowers. Seeds of the genus Ceanothus are quick to germinate in burned-over ground, and some plants of this genus are among the primary foods of deer (see Hike 8). Hence, periodic burns will keep a deer population at its maximum. With too few burns, shrubs become too woody and unproductive for a deer herd. In like manner, gooseberries and other berry plants sprout after fires and help support a variety of different bird and mammal populations.

Without fires, a plant community evolves toward a climax, an end stage, of plant succession. Red and white firs are the main species in the climax vegetation that is characteristic of the Sierra's mid-elevations. However, a pure stand of any species invites epidemic attacks and therefore can be unstable. In the past, pure stands of Yosemite's lodgepoles have been severely attacked by lodgepole needleminers—the larval stage of a moth—which turned the living stands into "ghost forests."

Fire is also beneficial in that it unlocks nutrients that are stored up in living matter, litter, topsoil, and even rocks. Vital compounds are released in the form of ash when a fire burns plants and forest litter. Fire also can heat granitic rocks enough to cause them to break up and release their minerals. In one study of a northern coniferous forest it was concluded that the weathering of granitic rock in that area was primarily due to periodic fires. A post-fire inspection in Yosemite can reveal that fires do get hot enough to cause thin sheets of granite to exfoliate, or sheet off, from boulders.

Natural, periodic fires, then, can be very beneficial for a forest ecosystem, and they should be thought of as an integral process in the plant community. They have, after all, been around as long as terrestrial life has, and for millions of years have been a common process in most of the Sierra's plant communities.

Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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