In the US, fire suppression policy directed at putting out all fires as rapidly as possible has been practiced for much of the twentieth century. The effectiveness of fire suppression has varied temporally and spatially with relatively ineffective suppression early in the twentieth century but increasingly more effective later on. Fire suppression was immediately effective in areas of ready access, where fires could be discovered early and resources deployed quickly to extinguish them. In more remote areas, suppression policies did not have much impact on fire behavior until development of the fire lookout system in the 1930s facilitated early fire detection and the smoke jumping program in the 1940s facilitated access. Thus, in many remote locations fire suppression policies have significantly altered fire regimes only in the latter half of the twentieth century.
The extent to which fire suppression policy has affected ecosystems is linked to fire regime. Many coniferous forests historically exposed to frequent fires have had fire excluded for a century or more. This is well demonstrated by fire histories from fire-scarred trees (Figure 3) in southwestern ponderosa pine, where fire-scar records indicate frequent fires until the late nineteenth century, and then a cessation of burning throughout the twentieth century. Much of this is due to the unique conditions of these fire regimes. Fires typically burn surface fuels with short flame lengths that pose little threat to fire fighters. Also, throughout much of the Southwest fires are still largely ignited by lightning and under weather conditions particularly conducive to rapid suppression.
The effect of fire suppression has changed over time. Throughout the twentieth century the impact was to exclude fire. However, as more and more natural fire cycles were missed, forests have increased in tree density, and many of the saplings have remained suppressed in the understory. This has the unwanted effect of producing ladder fuels capable of carrying surface fires into the canopies of the dominant trees and converting surface fire regimes into crown fire regimes.
In contrast, southern California chaparral landscapes have not had fire excluded over most of the twentieth century. Despite the fact that these landscapes were managed by the same fire-suppression policy, and heroic efforts have been directed at trying to suppress fires, the ferocity of these crown fires, particularly when driven by the autumn foehn winds, has made total fire exclusion impossible. This, however, is not to say fire-suppression policy has had no impact on these landscapes. Quite the contrary, this landscape has had more than a century of an anthropogenic fire regime driven by human ignited fires. Without fire-suppression activities these landscapes likely would have had fire frequencies grossly in excess of natural and sustainable levels. On an average fire suppression has managed to maintain this landscape within the historical range of fire frequency, although many localized areas have been hammered with fire to such an extent that they have lost most of all of their native biota and been replaced by non-native grasses and forbs.
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