Intensive livestock grazing throughout the US has contributed to altered fire regimes since at least the late nineteenth century. The primary effect of grazing is to reduce herbaceous fuels and in many forest types where fires are dependent on surface fuels, grazers outcompete fire for these herbaceous resources. Thus, livestock grazing has the capacity to exclude fire in much the same way as a fire-suppression policy can. In addition to diminishing fuels, livestock grazing reduces grass competition for woody species and thus enhances the recruitment of pines and other trees that contribute to dense thickets of saplings, which act as ladder fuels. Throughout western North America, grazing has been present much longer than fire suppression, and because 70% of the western USA wildlands are currently grazed, it should be considered a widespread factor affecting fire regimes.
Past logging practices have also altered fire regimes, not by excluding fire but by directly altering fuel structure. When the dominant trees in a forest are extracted for commodities, they open the way for extensive recruitment of smaller trees. Under natural conditions this dense thicket of trees would be thinned by fires; however, under modern fire suppression, these thickets remain and pose hazardous fuel conditions. Other effects of logging are to increase surface fuels from the 'slash' of dead branches and needles that may be left on site.
In some forests there is good evidence that fire severity depends more on past logging operations than fire suppression. In some cases this was due to residual flammable slash - for example, the most devastating fire of all times in North America was largely due to poor logging practices. Massive logging slash is considered to have been the major factor in the 1871 Peshtigo Fire that burned 500 000 ha and killed over 1200 people in Wisconsin forests. Thus, it should be recognized that contemporary efforts at reducing fire hazard by increasing logging is only likely to be successful if done in an appropriate manner where slash is removed or burned. Logging complicates our ability to make inferences about the impact of fire suppression on fire behavior since most of the western US ponderosa pine forests were logged at least once.
The fire description given above is generally valid for all fires including the frequent fires in Spain, Portugal, Greece, Australia, and South Africa.
See also: Chaparral.
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