No Such Thing as Frequent Fire in Sagebrush Ecosystems

Sagebrush and storm over Trout Creek Mountains, Oregon. Photo George Wuerthner.

An important question regarding sagebrush ecosystems, and species that rely upon them like sage grouse has to do with exactly what constitutes the fire rotation in sagebrush habitat? And a corrolary question is do current fire management policies emulate these historical conditions?

William Baker’s paper, Scaling Landscape Fire History: Wildfires Not Historically Frequent in the Main Population of Threatened Gunnison Sage-Grouse, is a significant contribution to our understanding of fire regimes in this crucial vegetation type.It is particularly relevant to the Gunnison Sage Grouse (GUSG) population, estimated to be about 5000 individuals, and listed as threatened under the Endangered Species Act. Baker’s findings have direct implications for current fire management policies.

The Gunnison Sage Grouse is found primarily in Colorado’s Gunnison Basin, dominated by mountain big sagebrush at its lower elevations. It is also found in small areas in other parts of western Colorado and nearby Utah. The Gunnison Sage Grouse looks similar to the more common Greater Sage Grouse but tends to be a third smaller.

Across the West, it is commonly asserted that, historically, wildfires in lower elevations were frequent (less than 25 years) and, therefore, kept fuels from accumulating. Consequently, fuels have built up, and wildfires are burning larger areas and tend to burn at higher severity, killing most of the vegetation.

Therefore, many agencies, academics, fire researchers, and others advocate for frequent burning of native vegetation, including sagebrush, assuming they emulate historic fire regime conditions.

The Baker study is crucial in understanding the historical wildfires in the Gunnison Sagegrouse (GUSG) habitat in mountain big sagebrush. It aims to determine the rate at which wildfires occurred historically and whether these rates were fast enough to be considered “frequent,” as described by other researchers. This understanding is essential for evaluating the effectiveness of current fire management policies.

This change in fire size and severity is usually attributed to two significant factors. One is government fire suppression policies, which some assert have led to fuel buildup.

The second reason is the elimination of Indian burning and the relegation of tribal people to reservations, removing their traditional ecological knowledge from the land. Lack of tribal burning has led, we are told, to unhealthy ecosystems.

Both factors are used to justify sagebrush elimination projects as well as forest thinning and burning in conifer forests on public lands.

The sagebrush steppe, one of the most significant vegetation communities in the West, deserves our attention. William Baker’s recent paper, Scaling Landscape Fire History: Wildfires Not Historically Frequent in the Main Population of Threatened Gunnison Sage-Grouse, provides a comprehensive overview of fire regimes in this crucial vegetation type.

Baker uses fire rotation, the expected period to burn across an area equal to a landscape of interest, although some reburning may occur. Fire rotations are calculated by summing fire areas over a landscape and period of interest, then dividing the period by the fraction of the landscape burned. For example, if 20% of a landscape burned in 30 years, the FR is 30/0.20 = 150 years.

The question that Baker attempted to answer is: What is the historical wildfires rotation in the GUSG habitat in mountain big sagebrush? What rate did these wildfires occur historically and were rates fast enough so that the fires could be considered “frequent” as described by Simic et al? Simic states, “tree-ring fire scars revealed a history of repeated low-severity fire at sagebrush–forest ecotones until 1892, followed by over a century without fire.”

The problem with fire scar studies like Simic is that they measure the number of fires, not the geographical extent of fire. In other words, you can document that a fire burned someplace in the study area every ten years, indicating a “frequent” fire regime. However, if each of those fires only burned a few acres (which is the typical situation), then most of the study area would not have burned at all.

You can have dozens of small fires that have almost no impact on the ecological function of an ecosystem. It is only the occasional large fire that counts ecologically.

The difference between Baker’s estimate that wildfire burned across the study area between 82 and 135 years is significantly longer than the tree fire scar method used in the Simic study.

These different conclusions about fire occurrence are a classic example of the problems resulting from tree scar studies, which exaggerate the frequency of wildfires in various ways.

As is typical with many fire scar studies, only a few trees were used to create a “historical fire regime profile,” which is then extrapolated to the larger landscape. As Baker noted, 10 fire-scar sites used to compile the study results were “insufficient to detect historical fire sizes and distributions across the large 168,753 ha sagebrush area.”

A further problem with studies that report on fire regimes in sagebrush is that they rely on fire scars in adjacent forested areas—which often do not have the same fire frequency or fire rotation as the sagebrush areas.

An earlier review of mountain big sagebrush fire rotation across the West found the fire rotation to be 150-300 years.

Baker utilizes three methods to reconstruct fire rotation in the area: the sagebrush recovery period, land-survey reconstructions, and scar-based spatial fire histories.

General Land Office (GLO) surveys were often done early in the settlement period across the West before there was any substantial fire suppression, and in many cases, while tribal people still roamed the landscape, setting cultural burns.

This GLO method has several benefits. The surveyors recorded the actual fire burns encountered during the surveys, including the length of the burn area and general fire severity. Since surveys were typically linear, there was no bias when selecting specific regions. These surveys are “line intercept” descriptions of the landscape.

As someone who worked as a cadastral surveyor in Alaska, I can verify that written descriptions of the terrain and vegetation passed along the transit line are written down. Thus, one does not have to interpret or infer whether a fire burned any area or the vegetation at the time of the survey. It is all recorded.

GLO reports have one problem: they are snapshots in time. The GLO surveys of the Gunnison Basin were done between 1876 and 1892. They reach backward in time since any evidence of a recent fire before the survey would be evident and likely recorded by surveyors. Nevertheless, they don’t provide long-term fire records.

However, long-term fire and vegetation changes can be determined when combined with other detection methods, like pollen and charcoal studies.

Baker reports that the GLO method detected several large fires, including one in 1880, which was likely 36,000 acres in size and was not even detected by the fire scar method in Simic’s study. Another 24,000-acre fire in 1876 or before was also not detected by the fire scar method. As these examples demonstrate, fire scar methods are ineffective for developing fire rotation for sagebrush ecosystems. These large fires would also seem to negate the assertion that fires were frequent and of low severity.

Simic previously studied the forest edge near the sagebrush but provided no direct evidence of the fire regime in the adjacent sagebrush habitat. Baker notes that “these detected fire years missed 91.2% of the total burn area found in sagebrush by the land surveys, even the likely largest fire in 1879/1880.”

Furthermore, the GLO methods found that most fires were small, but the last two burns were larger than 20,000 acres. Baker concludes that there is no evidence that wildfire was historically frequent or low severity in historical sagebrush landscapes in the study area.

Fire kills sagebrush. It doesn’t matter whether it is a fast or slow-moving, hot or intense, sagebrush will be killed.

George Wuerthner has published 36 books including Wildfire: A Century of Failed Forest Policy