Volume 1/Chapter 4/Fire And Fuels

* Critical Findings

ASSESSMENT

* Fire-Alternative Views

* Careless and Indiscriminate Fire Use

Strategies

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Effects of Human Activities Beginning in the Mid-1800s

Euro-American influence on fire in the Sierra Nevada began before the mid-1800s. By this time many Native American populations had been decimated by disease and genocide, and their traditional use of fire had been greatly reduced. The rapid influx of settlers into California following the discovery of gold, however, initiated more profound changes in the role of fire in Sierra Nevada ecosystems. Logging was undertaken initially to supply the mines and later to support the growing population of the new state. Timber volumes harvested in the Sierra Nevada continued to increase into the twentieth century, reaching a peak in the 1970s and 1980s. Typically, loggers harvested fire-resistant species and large trees, and these were replaced by greater numbers of much more fire-susceptible smaller trees. This pattern of biomass removal contrasted markedly with that of presettlement surface fires, which tended to kill (and later consume) small trees and leave many large trees to survive. Large quantities of debris left after logging led to severe fires, establishing vegetation patterns still evident today. A new pattern of ignitions, characterized in part by careless and indiscriminate burning, was introduced by miners, sheepherders, settlers, and loggers. In other areas there is evidence that heavy grazing by millions of sheep in the late 1800s may have effectively altered fuel conditions to reduce the influence or extent of fires.

The Role and Consequences of Fire Suppression

Suppression of wildland fires had been established as state and federal policy by early in the twentieth century. Following a series of disastrous fires in 1910 and a period of trial and debate about the merits of light burning as a management tool in forests and rangelands, intentional broadcast burning was repudiated and aggressive fire control became firmly entrenched. Only in recent decades have the benefits of prescribed fire become widely apparent.
Combined with the loss of ignitions by Native Americans, fire suppression resulted in significant reductions in area burned by wildfires during the twentieth century. For example, by comparing average annual acreage estimated to have burned during the presettlement period (based on fire history data) with twentieth-century fire-return intervals (table 4.1), we find that the annual area burned during this century has been reduced to approximately 10%, 3%, and 2% of presettlement values for the blue oak, mixed conifer, and red fir forest types, respectively.
The virtual exclusion of widespread low- to moderate-severity fire has affected the structure and composition of most Sierra Nevada vegetation, especially in low- to middle-elevation forests. Conifer stands generally have become denser, mainly in small and medium size classes of shade-tolerant and fire-sensitive tree species. Vertical fuels have become more continuous, contributing to more spatially homogeneous forests (figure 4.1).

Figure 4.1 (Actual View 72K)

Development of vertical fuels through ingrowth of white fir in a stand of mixed conifer as a result of fire suppression. (Photo by Constance I. Millar.)

Selective cutting of large overstory trees and the relatively warm and moist climate that has characterized much of the twentieth century may have reinforced these trends by producing conditions favorable to the establishment of tree seedlings and other plant species. Coupled with fire suppression, these conditions permitted the extensive development of dense, young forests. As a result, stands in many areas have experienced increased mortality recently from the cumulative effects of competition (primarily for water and light), drought, insects, disease, and, in some cases, air pollution. The increased density of young trees together with increased fuels from fire suppression and tree mortality have created conditions favorable to more intense and severe fires. Moreover, severe fires are more likely to be large because they are more difficult to suppress, although data on large fires in the Sierra indicate that current fire sizes vary greatly among national forests. While we cannot be sure whether more absolute area has burned in severe fires in the twentieth century than in pre-contact times, it is clear that within those areas that do burn, a greater proportion of fire is high-severity than in the past.
Several lines of evidence suggest that quantities of live and dead fuels have increased over the course of the twentieth century, although data from the early part of the century are not available to test this assertion directly. Over the same period suppression technology has improved, but in recent years available fire-fighting resources have declined. The net effect on a number of fire attributes has remained remarkably constant.