Do fuel treatments modify fire behavior in the sagebrush steppe?

Do fuel treatments modify fire behavior in the sagebrush steppe?

By Lisa Ellsworth, Assistant Professor and Rangeland Fire Ecologist, Oregon State University

Invasive species, land cover change, altered fire regimes, and a changing climate interact to imperil sagebrush steppe ecosystems that are critically important for local economies, as well as for species of conservation concern (e.g. Greater sage-grouse). One of the major challenges in sagebrush steppe conservation is altered fire regimes and the resultant uncharacteristic fire behavior now widely exhibited across the sagebrush biome. The increasing emphasis on sagebrush conservation and the reduction of large, invasive grass-fueled wildfires suggests that increased use of fuel treatments could be beneficial. While expansion of invasive annual grasses is creating more fire-prone situations across the sagebrush biome, increasing shrub cover threatens to outcompete understory native herbaceous vegetation in other areas. In these areas, fuel treatments may have the added benefits of breaking up continuous woody cover, providing anchor points for fire suppression, and reducing flammable fuel loads.

Fuel treatments are activities that reduce burnable material with the ultimate goal of decreasing fire behavior. In the SageSTEP plots, fuel treatments included mechanical thinning, application of tebuthiuron (herbicide), and prescribed fire. Untreated control plots were also measured. Mechanical treatments (i.e. mowing) were used to remove top growth of sagebrush and other shrubs, reducing shrub canopy cover and enhancing herbaceous understory. Applications of tebuthiuron were applied to partially kill sagebrush canopies and enhance understory vegetation. Plants rooted within a 50 cm radius of application were impacted, but plants farther away are unharmed, giving a patchwork thinning treatment.  Prescribed fire broke up woody fuel continuity, reduced overall fuel loads, and enhanced herbaceous vegetation.

Despite the need for designing and employing effective fuel treatments to mitigate wildfire, little is known about the actual effects of fuel reduction treatments on fire behavior in sagebrush ecosystems, or for how long following treatment these impacts last. Using the SageSTEP fuels data set, we described fuel structure and accumulation for ten years following fire, mechanical, and herbicide treatments as well as in unburned control plots in Wyoming big sagebrush communities. We then used the fire behavior model Fuel and Fire Tool to quantify how those patterns affected future fire behavior (i.e. flame length, rate of spread).

Fuels

In control plots herbaceous biomass ranged from 293-584 kg ha^-1, litter was 212-453 kg ha^-1, woody surface fuel loads (downed woody debris) were 947- 4962 kg ha^-1,and shrub biomass was 4094-5567 kg ha^-1 across the 10 years sampled. Fire treatment resulted in a reduction in standing herbaceous biomass by 30- 60% the first year after fire. By ten years post-fire standing herbaceous biomass had increased four-fold. Shrub biomass was reduced by 75-85% the year after fire treatment. In the years following fire treatment, shrub biomass slowly increased and reached 25-35% of the pre-treatment level after ten years (Figure 1).

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Figure 1. Herbaceous, litter, downed woody debris (woody), and live shrub biomass in control, prescribed fire, mechanical, and tebuthiuron treatment plots at seven Wyoming big sagebrush sites across the Great Basin, USA. Error bars represent standard error for total fuel load.

Mechanical treatment initially reduced herbaceous biomass by about 30%, but by 10 years post-treatment herbaceous biomass had increased about three times above pre-treatment levels. Ten years after mechanical treatment shrub biomass was on average 50-55% of the pre-treatment level. Tebuthiuron treatment had a delayed effect on vegetation response compared to fire and mechanical treatment. No change in herbaceous biomass was observed the first couple of years post-treatment.  After ten years the increase in standing herbaceous biomass was similar to that observed ten years after fire or mechanical treatment. Shrub biomass also decreased slowly over the first six years of treatment; at the sixth year post-treatment shrub biomass was 50% of the pre-treatment level. Ten years post-treatment, shrub biomass had begun to increase again and was about 55% of the pre-treatment level.

Fire behavior

Before treatment, modeled fire rate of spread averaged 13.6 m min^-1 when herbaceous fuels were fully cured (ie. late in the fire season). In the first post-treatment year, fire and mechanical treatments reduced rates of spread to 2.2-4.0 m min^-1; in contrast, tebuthiuron treatments did not reduce rate of spread. By 10 years after treatment, fire and mechanical treatments continued to reduce fire behavior, with rates of spread of 1.0-2.2 m min^-1. Untreated control and tebuthiuron treatments had rates of spread of up to 2.6-3.0 m min^-1 when fully cured (Figure 2).

Figure 2. Rate of spread (m/min) in control, control+imazapic, prescribed fire, prescribed fire + imazapic, mechanical treatment, mechanical + imazapic, tebuthiuron, and tebuthiuron + imazapic plots in a fully cured moisture scenario in SageSTEP plots across the Great Basin, USA.

In control plots, flame lengths averaged 2.1 m when herbaceous fuels were fully cured.  In the first post-treatment year, fire and mechanical treatments reduced flame lengths to 0.6-1.0 m, but tebuthiuron treatments did not reduce flame length. By 10 years post-treatment, fire and mechanical treatments continued to reduce fire behavior, generating flame lengths of 1.0-2.2 m.  Untreated control and tebuthiuron treatments had higher flame lengths, at 2.6-3.0 m when fully cured (Figure 3).

Figure 5. Flame length (m) in control, control+imazapic, prescribed fire, prescribed fire + imazapic, mechanical treatment, mechanical + imazapic, tebuthiuron, and tebuthiuron + imazapic plots in a fully cured moisture scenario in SageSTEP plots across the Great Basin, USA.

These differences in flame length have strong implications for fire management. The hauling chart, a standard for how firefighters can approach a fire, sets thresholds at 4 and 8 ft (1.2-2.4 m) flame lengths. Below flame lengths of 4 ft, fire crews can fight the fire with just hand tools. Between 4-8 feet, heavy machinery, such as dozers, and air resources such as helicopters and retardant drops from airplanes are used. Above 8 ft flame lengths, fire control is extremely difficult and spot fires, crown fires, and extreme fire behavior are expected. Prescribed fire and mow treatments maintained fire behavior below this threshold for extreme fire behavior, and in early years even kept it within the 4 ft control mark. Control and tebuthiuron treatments can be expected to have fire behavior that is more difficult to control.

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