Interactions Among Livestock Grazing, Vegetation Type, and Fire Behavior in the Murphy Wildland Fire Complex in Idaho and Nevada, July 2007
Karen Launchbaugh, Bob Brammer, Matthew L. Brooks, Stephen Bunting, Patrick Clark, Jay Davison, Mark Fleming, Ron Kay, Mike Pellant, David A. Pyke,and Bruce Wylie. 2008. Interactions Among Livestock Grazing, Vegetation Type, and Fire Behavior in the Murphy Wildland Fire Complex in Idaho and Nevada, July 2007. Open-File Report 2008–1214, U.S. Department of the Interior U.S. Geological Survey.
Prepared in cooperation with the Murphy Wildland Fire Grazing and Fuel Assessment Team
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Abstract
A series of wildland fires were ignited by lightning in sagebrush and grassland communities near the Idaho-Nevada border southwest of Twin Falls, Idaho in July 2007. The fires burned for over two weeks and encompassed more than 650,000 acres. A team of scientists, habitat specialists, and land managers was called together by Tom Dyer, Idaho BLM State Director, to examine initial information from the Murphy Wildland Fire Complex in relation to plant communities and patterns of livestock grazing. Three approaches were used to examine this topic: (1) identify potential for livestock grazing to modify fuel loads and affect fire behavior using fire models applied to various vegetation types, fuel loads, and fire conditions; (2) compare levels of fuel consumed within and among major vegetation types; and (3) examine several observed lines of difference and discontinuity in fuel consumed to determine what factors created these contrasts.
The team found that much of the Murphy Wildland Fire Complex burned under extreme fuel and weather conditions that likely overshadowed livestock grazing as a factor influencing fire extent and fuel consumption in many areas where these fires burned. Differences and abrupt contrast lines in the level of fuels consumed were affected mostly by the plant communities that existed on a site before fire. A few abrupt contrasts in burn severity coincided with apparent differences in grazing patterns of livestock, observed as fence-line contrasts. Fire modeling revealed that grazing in grassland vegetation can reduce surface rate of spread and fire-line intensity to a greater extent than in shrubland types. Under extreme fire conditions (low fuel moisture, high temperatures, and gusty winds), grazing applied at moderate utilization levels has limited or negligible effects on fire behavior. However, when weather and fuel-moisture conditions are less extreme, grazing may reduce the rate of spread and intensity of fires allowing for patchy burns with low levels of fuel consumption.
Introduction and Background
The sagebrush steppe ecosystem dominates about 73 million acres of western North America, but this amount is only about 55 percent of its historical potential (Connelly and others, 2004). Fire has been a major factor contributing to this change. More frequent and larger fires are a growing reality in the management of western rangelands. In Idaho and Nevada, the last decade (1997 to 2007) has yielded 18 fires greater than 100,000 acres. However, the size of these very large fires appears to be increasing given that 6 of the 10 largest fires of the decade occurred in 2006 and 2007 (National Interagency Fire Center [NIFC] records; http://www.nifc.gov). Impacts on natural and fiscal resources are high during those years when large acreages burn. Annual weather conditions undoubtedly contribute to the acreage burned in any given year, but other factors also may contribute to the risk of wildfire in the sagebrush steppe ecosystems. These factors include (1) changes in livestock management, such as reductions in stocking rates and changes in grazing seasons; (2) increased abundance of invasive species, such as cheatgrass; and (3) increased wildland-urban interfaces where human-derived ignitions can occur (Miller and Narayanan, 2008).
Heavy livestock grazing is thought to have affected fire regimes by severely reducing fuel loads and thereby reducing the potential for fires to sustain ignition and spread. The introduction of cattle, sheep, and horses to the Great Basin in the 1860s quickly created large ranching operations and excessive grazing pressure. The severe overgrazing removed fine fuels and resulted in a substantial reduction in the number of fires and the acres burned. Only 44 fires, burning a total of 11,000 acres, were reported from 1880 to 1912 in Great Basin rangelands (Miller and Narayanan, 2008). Evidence for reduced numbers of fires during this period is also deduced from the near elimination of fire scars on trees adjacent to sagebrush ecosystems during the late 1800s and continuing through most of the 1900s (Miller and Rose, 1999; Miller and Tausch, 2001).
The number of livestock in Great Basin and sagebrush ecosystems has dropped rapidly since the passage of the Taylor Grazing Act of 1934 (43 USC 315; http://www.blm.gov/wy/st/en/field_offices/Casper/range/taylor.1.html, accessed July 23, 2008). Livestock numbers in Idaho decreased in the 1950s primarily from loss of large sheep operations (indicated by changes in authorized use for grazing; fig. 1). Livestock numbers have fluctuated at or below this initial decrease through the remainder of the 1900s, with a steady conversion from sheep to cattle. In the last decade, a substantial decrease in authorized use on Bureau of Land Management (BLM) lands in Idaho has been recorded (fig. 1).
An important factor contributing to an increase in wildfires includes the expansion of cheatgrass (D’Antonio and Vitousek, 1992). Of the nearly 98 million acres of BLM lands in Idaho, Nevada, Oregon, Utah, and Washington, 17.3 million acres are believed to have at least 10 percent of the plant biomass composed of annual grasses, including cheatgrass or medusahead (Pellant and Hall, 1994). These annual grasses create fine-fuel loads that increase the probability of fire starts and the rate of fire spread in areas they dominate (Brooks and Pyke, 2001). …
The Murphy Wildland Fire Complex
On July 16 and 17, 2007, a series of wildland fires were ignited by lightning in rangelands near the Idaho-Nevada border southwest of Twin Falls, Idaho. The Rowland Fire (initiated west of Murphy Hot Springs, Idaho) and Elk Mountain Fire (initiated southeast of Three Creeks, Idaho) grew together and became known as the Murphy Wildland Fire Complex (fig. 2). The Scott Creek Fire (west of Jackpot, Nevada) also was ignited by lightning on July 17 and was later designated as part of the Murphy Wildland Fire Complex. Some of the fires in this complex burned for more than two weeks, and the complex was fully contained by August 2, 2007. This complex of fires burned across portions of three BLM FOs (Jarbidge, Bruneau, and Elko), portions of the Humboldt-Toiyabe National Forest, about 48 sections of land managed by the State of Idaho, and extensive stretches of private lands. A total of 652,016 acres was encompassed by this fire complex (NIFC data: http://www.nifc.gov).
These wildfires had tremendous impacts on the sagebrush steppe ecosystems of south-central Idaho and a portion of north-central Nevada. Seasonal and year-long habitats were altered for sage-grouse, mule deer, elk, bighorn sheep, pronghorn, Brewer’s sparrow, sage sparrow, other sagebrush-obligate birds, and many other wildlife species that use these rangelands. Severe impacts also were exacted on forage resources for livestock, cultural resource values, and watershed health and stability as a result of these fires. The ecological impacts of this fire will take several years to be fully realized and will vary depending on weather conditions in the coming years.
In the last three decades, several wildfires have occurred in the area that burned in the Murphy Wildland Fire Complex. Many of these burned areas were revegetated with perennial grasses, including both introduced and native species. Records from the Jarbidge Field Office (FO) of the BLM indicate that about 402,000 acres were seeded through the end of 2006. This number of acres represents 26 percent of the total public lands in the Jarbidge FO. Some land managers and livestock operators speculated that extensive seedings of perennial grasses following wildfires, without commensurate increases in livestock grazing, contributed to an increase in herbaceous production. The speculation in turn, considered the possibility that increased herbaceous production provided additional fuels for wildfire.
Purpose and Scope
In August 2007, a team3 of scientists, habitat specialists, and land managers was called together by Tom Dyer, Idaho BLM State Director, to examine initial information from the Murphy Wildland Fire Complex in relation to plant communities and livestock grazing patterns. This report is the result, which is presented to meet the following objectives:
Provide preliminary observations and recommendations regarding the effects, if any, of existing plant community composition (native rangeland and crested wheatgrass seedings) and current management of livestock grazing on fire behavior and rate of spread of the Murphy Complex Fires. Historical or potential vegetation composition, because it may have been influenced by historical livestock grazing levels or practices is, by necessity, background information and not the focus of this report.
Provide recommendations for long-term research or studies needed to address issues or remaining questions surrounding the use of livestock to reduce fuels while maintaining post-fire resource values in the area encompassed by the Murphy Wildland Fire Complex.
Discuss the potential application of the findings gleaned from the Murphy Wildland Fire Complex to other areas from a “lessons learned” perspective. …
This report focuses on the potential role that livestock grazing played in altering fuel loads and fuel types that affected the pattern and severity of fires in the Murphy Wildland Fire Complex. Because fire behavior, fire extent, and level of vegetation consumed result from many interacting factors, the specific role that grazing had on the fires was difficult to ascertain. The team preparing this report toured the area of the Murphy Wildland Fire Complex on August 28, 2007 and saw first-hand examples of completely burned areas, patchily burned mosaics, and contrasts where fires stopped at a fence-line or only fingered into the adjacent pasture. A reasonable explanation for these contrasts was a difference in the grazing management between the areas on each side of the fence-line (fig. 8).
Livestock operators in the area shared their knowledge of the pre-burn vegetation conditions, levels of grazing use, and on-site observations of fire behavior with the report team. These observations supported the possibility that livestock grazing resulted in a mosaic burn or observable fence-line contrasts that could be attributed to differences in utilization levels created by livestock grazing.
Major Findings and Lessons Learned
Much of the Murphy Fire Complex burned under extreme fuel and weather conditions. Weather conditions in the first four to five days of the fire were particularly dry, hot, and windy. It was during this period that between 75 and 90 percent of the total area burned. As confirmed by fire modeling, these extreme conditions likely overshadowed (or swamped) livestock grazing as a factor influencing fire extent and fuel consumption in many areas where these fires burned.
Level of fuels consumed (or burn severity) was affected mostly by the plant communities that existed on a site before fire (that is, shrubland communities can potentially experience a greater loss of biomass and vegetative structure than grasslands yielding higher burn severity values). Our study of fuel consumption and our field examination confirm that all vegetation types experienced a range of fuel consumption, including many acres in the low burn severity class, indicating patchy burning patterns or incomplete consumption of fuels. Greater proportions of plant communities characterized as grasslands were categorized in the low burn severity class than shrublands. This observation confirms that fuel consumption (or burn severity) is largely influenced by the kind of plant biomass and structure that exists before the fire.
There were many abrupt contrasts in fuel consumption (or, burn severity) primarily attributed to abrupt changes in vegetation type, such as a transition from seeded grasslands to shrubland communities. Burn severity contrasts throughout the Murphy Wildland Fire Complex were most strongly aligned with amount of shrub cover, current year’s biomass, and vegetation type. A few abrupt contrasts in burn severity coincided with apparent differences in actual use by livestock and other grazing factors, as illustrated by fence-line contrasts.
Potential effects for livestock grazing to reduce fuel and affect fire behavior were dependent on the vegetation type. Fire behavior in sagebrush vegetation types is driven by sagebrush cover and height, with the herbaceous component on which livestock focus their grazing, playing a lesser role. Consequently, opportunities to influence fire behavior through livestock grazing are greatest in grassland vegetation types. Fire modeling suggests grazing in grassland vegetation can reduce surface rate of spread and fire line intensity to a greater extent than in shrubland types where woody fuels generally are not reduced by cattle or sheep grazing.
Herbaceous biomass produced during one year and persisting into the next growing season contributes to the dead fine fuel load (that is, 1-htl fuels) in subsequent years. Livestock grazing that reduces the carryover of dead fuels from one year to the next can influence fire behavior, particularly under less intense fire conditions.
The potential effects of grazing on fire behavior are highly dependent on weather, fuel load, and fuel-moisture conditions. Extensive fires, such as those of the Murphy Wildland Fire Complex, generally result from a combination of many factors but are largely weather driven. Under such extreme conditions, grazing applied at sustainable utilization levels would have limited or negligible effects on the fire behavior. When weather and fuel moisture conditions are less extreme, grazing may reduce the rate of spread and intensity of fires allowing for more patchy burns with lower fuel consumption levels.
