Between 1993 and 2007, lightning caused most of the wildfires in the Planning Area (118). The remaining fires were human caused (71) or, in a few cases, the cause of the fire was unknown (BLM 2004a).

A wildfire is one that burns outside the parameters defined in land use plans and FMPs for that location under current and expected conditions, such as fires burning in areas where fire is specifically excluded; fires that exhibit burning characteristics (intensity, frequency, and seasonality) outside prescribed ranges, including fires expected to produce severe fire effects; and fires that occur during periods of high fire danger. Wildfires are caused by lightning, volcanoes, or unauthorized and accidental human-caused action (e.g., arson, escaped camp fires, and equipment fires) or escaped prescribed fires.

However, a lightning-caused wildfire could still be used to meet fuels and ecosystem management objectives if:

A diversity of fuel types occur in the Planning Area, primarily because of its location in three physiographic areas (the Northern Shortgrass Prairie to the north and east, the Central Rocky Mountains on the west, and the Wyoming Basin south of and including the Planning Area). The Bighorn Basin is bounded by mountains on all sides; thus, there is a rain shadow effect in the basin bottom. This effect generally limits both wildfires and prescribed fires, except in river bottoms due to denser vegetation growth, by limiting the growth of vegetation to fuel wildland fires. Upslope from the basin bottom, fuel types and fire regimes are similar to those found in the physiographic areas, and nearly all wildfires and prescribed fires occur in these areas.

Table 3-21 lists the total burned acres in the Planning Area from 1993 through 2009, regardless of ownership. During the same period, more than 4,000 acres in the Planning Area burned twice. It is noteworthy that these areas, which experienced no major fires since European settlement, have now burned twice since 1996; this is largely a result of an increase in vegetative ground cover, including cheatgrass, in these areas.

It is expected that the average number and size of wildfires will increase compared to the period from 1993 through 2008 in most of the Planning Area (BLM 2009b). It has been documented that cheatgrass is more widespread, especially in the Nowater FMU, than it was 25 years ago (BLM 2009b). In all likelihood, fire size and frequency will increase in areas affected by cheatgrass. Another area of concern is the mixed conifer forest with health issues caused by bark beetles and blister rust. The Northern Zone FMP provides a more in-depth history of wildfire occurrence in each FMU from 1992 through 2003, the years reviewed for that plan (BLM 2004a).

Changes in temperature and precipitation can lead to fluctuations in wildfire occurrence. A series of years with below normal temperatures and above normal precipitation usually leads to a decrease in the number and size of wildfires. However, seasonal and yearly variations in temperature and precipitation have different effects on different fuel types. For example, above normal spring precipitation can increase fire danger in areas dominated by cheatgrass and at the same time decrease fire danger in timber types. Higher temperatures could increase the size and intensity of fires in timber and woodland types. Higher temperatures leading to sustained drought can actually decrease fire danger on some rangeland types due to lack of grass or fine-fuel production.