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Fire and Fuel in a Sierra Nevada Ecosystem

Early travelers and photographers in the mid-1800's recorded the forests of the Sierra Nevada as parklike, with little undergrowth and wide expanses of meadows. The forests were a mixture of conifers dominated by ponderosa pine, with some incense-cedar and California black oak at lower elevations and increasing numbers of sugar pine and white fir at the higher elevations. Beneath the larger trees, the forest floor was carpeted with needles, forbs, and grasses. The understory, where present, consisted of young trees and some chaparral shrubs. These open conditions were attributed to low-intensity surface fires set by lightning and augmented by Native Americans.


Lightning fires have unique spatial and temporal distribution patterns in relation to topography and vegetation. The ecological role of fire is a manifestation of those patterns. The simultaneous occurrence of a lightning strike, flammable fuel, and conducive weather determines the frequency, size, and intensity of a fire. The prevalence of lightning strikes and fires shows conclusively that fire is an integral and pervasive part of Sierra Nevada ecosystems rather than an external disturbance (van Wagtendonk 1994).


Nearly a century of fire control in the Sierra Nevada has led to conditions that now threaten the very forests they were designed to protect. Suppression of naturally occurring surface fires has allowed the forest floor to become a tangle of understory vegetation and accumulated debris. Open forests and meadows have been invaded by trees and chaparral. Thickets of shade-tolerant incense-cedar and white fir have increased and have deflected succession away from the less shade-tolerant ponderosa and sugar pines.


As undergrowth has increased, fuel volumes have expanded, and a continuous ladder of fuel extends from the ground to the forest canopy. The understory is now so thick with dead trees, branches, needles, and other debris that inevitable wildfires will soon reach catastrophic proportions. Today, millions of hectares of forest and grasslands in the Sierra Nevada face abnormally high risks of wildfire because of these altered conditions. This situation is worsened by the increasing numbers of vacation homes and other human developments within the forest and foothill vegetation.


The cycle can be illustrated by examining simulated fuel accumulation rates during periods before and after the initiation of fire suppression, by using graphs generated by the FYRCYCL model based on data collected in Yosemite National Park (van Wagtendonk 1985; Figure). As long as fires are suppressed, this new, longer cycle of extremely intense wildfires will continue.

Figure. Simulated fire cycle from Yosemite National Park as generated by the model FYRCYCL. Starting in 1790 with open ground, as might be expected following a stand-replacing fire, a forest began to grow and fuels accumulated. By 1820 enough fuel had accumulated to carry a fire, and lightning strikes occurred when weather conditions were conducive to burning. The resulting fire reduced fuels. According to the model, during the next 70 years, lightning-caused surface fires occurred in a cycle averaging one every 7 years. In 1890 a policy of total fire suppression was implemented, and all subsequent fires were extinguished. Without the frequent surface fires, fuels will have accumulated to such a volume by the years 2000 and 2055 that when fires do occur, they will be intense crown fires that exceed suppression capability. The amount of fuel on the ground is measured in units of heat per square meter.

If natural conditions and processes are to be restored and perpetuated in the Sierra Nevada, fire must be reintroduced. In large wilderness areas and parks, naturally occurring lightning fires should be allowed to burn under prescribed conditions. Where this is not possible because the area is too small or because other human factors (such as the presence of human dwellings, timber harvest areas, and so forth) preclude the implementation of a program to monitor wildland fires, surrogates for fire must be found. Prescribed burns, mechanical manipulation, and artificial cutting are possible options. In any case, it is important that naturally managed ecosystems not be denied ecologically significant processes such as fire.

Jan W. van Wagtendonk
U.S. Geological Survey
Biological Resources Division Western Ecological Research Center
Yosemite Field Station
El Portal, California 95318