Fire ecology and changing forests

For thousands of years, fire has shaped the ecosystems of the Sierra Nevada, Southern Cascades, and Klamath-Siskiyou ecosystems and mixed-conifer forests across the western U.S. Many common plants are adapted to fire, exhibiting traits, such as thick bark and fire-stimulated flowering, sprouting, seed release and/or germination.

From the foothills through the mixed-conifer forests, fire occurred on average about every 10 to 20 years in any given area during the centuries before extensive Euro-American settlement (known as presettlement, sometime between 1850 and 1900). Many Native American tribes used fire for various cultural purposes and were reliant on fire adaptive ecosystems.

Historically, local cool or moist areas, such as canyon bottoms which typically burned in lower-intensity fires, provided habitat for species that require dense forests and canopy cover. Warmer and drier areas on upper, south-facing slopes often had lower densities of trees.

In the 20th Century, fire suppression, livestock grazing, and the selective harvest of large conifer trees, especially pines, produced forests which are quite different from presettlement forests. Today's forests are denser with generally smaller trees and increased levels of fuels on the forest floor, as well as ladder fuels–small trees and brush which can carry fire into the forest canopy. Current land management objectives are aimed at restoring forests, so that they are more resilient to fire and other disturbances that will continue to influence forest conditions.

A comprehensive strategy for ecological restoration on an entire landscape includes reestablishing different stand structures (i.e. clumps of trees and more widely spaced trees intermixed with small gaps where young trees grow best) across the landscape using topographic variables (i.e., slope shape, aspect, and slope position) as a guide for supporting different densities of trees.

Collectively, management direction now emphasizes the ecological role of fire, changing climate conditions, sensitive wildlife habitat, and the importance of forest structure heterogeneity.