Vegetation and Fuel Responses to Linear Fuel-Break Treatments in and around Burned Sagebrush Steppe
After the Soda Wildfire burned nearly 280,000 acres of public and private land in southwest Idaho, the Bureau of Land Management’s (BLM) Emergency and Rehabilitation and Stabilization program (ESR) applied herbicide and seeding treatments to prevent exotic annual grass invasion and restore native vegetation. To reduce the threat of future wildfire, the BLM also constructed a network of linear fuels treatments, or fuel-breaks, along existing roadways within and surrounding the area. In conjunction with the Boise District Fuels Office, we began evaluating the fuel-break network in 2018 to assess risks of treatment implementation, such as exotic annual grass invasion, against the intended benefits of moderated fire behavior.
Background
Post-fire landscapes in the American west are at high risk of wildfire and post-fire erosion and invasion from exotic annual grasses (EAGs), especially in sagebrush steppe. Post-fire rehabilitation investments are often implemented by Bureau of Land Management’s (BLM) Emergency Stabilization and Rehabilitation program (ESR). Following the 2015 Soda wildfire, the BLM’s ESR program implemented a large suite of herbicide and seeding treatments over the burned area to address soil stability and exotic annual grass invasion threats. The BLM then constructed a network of roadside fuel-breaks within and surrounding the recovering and rehabilitated area to protect the investment. Fuel-breaks alter vegetation to improve fire suppression efficacy and to increase firefighter safety.
The fuel-break treatments consisted of mowing or hand-cutting shrub fuels, herbicide spraying, and drill seedings of perennial vegetation along roadways. When herbaceous fuels such as perennial vegetation burn, they tend to have smaller flames that don't burn as long as shrub fuels and are safer for ground crews to extinguish.
To evaluate fuel-break effectiveness, the USGS Fires, Invasives, and Restoration Ecology in Sagebrush Steppe (FIREss) team collected data from 40 plots on fuel-breaks, as well as adjacent untreated “control” plots. We surveyed for perennial bunchgrasses, exotic annual grasses, overall biomass, and the height of the leaf litter layer. This data will be used to assess the costs and benefits of the fuel-breaks relative to EAG invasion and fire moderation. Treatment effects on fire behavior are determined primarily from computer simulations, and a major part of the research is determining how to adapt simulations for use in sagebrush steppe and validating their accuracy.
Objectives
- Identify the short-term effects of herbicides, shrub reductions, and drill seeding treatments on native and exotic plants and fuels, then determine how these responses will impact fire behavior
- Monitor the changes in vegetation, fuels, and simulated fire behavior over time, on or adjacent to roadside fuel breaks
- Determine how these changes are influenced by site climate, post-treatment weather, re-treatments, landscape management choices, and the presence of exotic annual grasses.
- Complete our ongoing assessment of optimal field-sampling approaches and use of remotely sensed data to improve simulation methods and fire behavior predictions
Results
Vegetation Responses to Fuel-Break Treatments
Pre-emergent herbicides reduced exotic annual grass and total herbaceous cover without affecting native perennial bunchgrasses for 2-3 years, in areas dominated by exotic annual grasses prior to fuel break construction. However, exotic annual grass cover was still over 30% after herbicide application, meaning it is likely they will reinvade and dominate in the future.
In areas that were less invaded and had more native plants, mowing or hand-cutting shrubs to half their size did not increase exotic annual grasses. Cutting did lead to an undesirable increase in fuel continuity by increasing other herbs and distributing wood fragments to the soil surface.
Drill seedings that followed fuel treatments successfully established native perennial bunch grasses, but were also linked to increases in exotic forbs like Russian thistle.
Fuel and Wildfire Behavior Responses to Treatments
Using information on the fire fuels collected in the field at fuel-break sites, we used a fire behavior simulator to predict wildfire behavior. In areas dominated by exotic annual grasses, simulated flame length, rate-of-fire spread, and heat of fire reaction were reduced in areas treated with herbicides. However, simulated fire behavior was still above levels desired for fire management. Fuel treatments also modestly reduced predicted fire behavior at sites with more shrub cover and achieved fire management targets.
Implications for Land Management
Predicted fire-moderation benefits over the first four years of fuel break implementation were modest and varied with site. In general, increases in exotic annual grasses and the fire risks they are associated with were not observed. Our results suggest that fuel treatments which reduced shrub and increased herbaceous fuels could help achieve fuels-management goals. Fuel treatments that result in herbaceous fuel dominance likely improve fire suppression efficacy and wildfire fighter safety. Though herbaceous fuels, especially grasses, are well known to increase the frequency and size of wildfires in sagebrush steppe, fires in herb-dominated areas can be easier to control because they can be more effectively covered by flame retardants, are easier to extinguish, and are more amenable to creating backfires to protect firefighters.
After the Soda Wildfire burned nearly 280,000 acres of public and private land in southwest Idaho, the Bureau of Land Management’s (BLM) Emergency and Rehabilitation and Stabilization program (ESR) applied herbicide and seeding treatments to prevent exotic annual grass invasion and restore native vegetation. To reduce the threat of future wildfire, the BLM also constructed a network of linear fuels treatments, or fuel-breaks, along existing roadways within and surrounding the area. In conjunction with the Boise District Fuels Office, we began evaluating the fuel-break network in 2018 to assess risks of treatment implementation, such as exotic annual grass invasion, against the intended benefits of moderated fire behavior.
Background
Post-fire landscapes in the American west are at high risk of wildfire and post-fire erosion and invasion from exotic annual grasses (EAGs), especially in sagebrush steppe. Post-fire rehabilitation investments are often implemented by Bureau of Land Management’s (BLM) Emergency Stabilization and Rehabilitation program (ESR). Following the 2015 Soda wildfire, the BLM’s ESR program implemented a large suite of herbicide and seeding treatments over the burned area to address soil stability and exotic annual grass invasion threats. The BLM then constructed a network of roadside fuel-breaks within and surrounding the recovering and rehabilitated area to protect the investment. Fuel-breaks alter vegetation to improve fire suppression efficacy and to increase firefighter safety.
The fuel-break treatments consisted of mowing or hand-cutting shrub fuels, herbicide spraying, and drill seedings of perennial vegetation along roadways. When herbaceous fuels such as perennial vegetation burn, they tend to have smaller flames that don't burn as long as shrub fuels and are safer for ground crews to extinguish.
To evaluate fuel-break effectiveness, the USGS Fires, Invasives, and Restoration Ecology in Sagebrush Steppe (FIREss) team collected data from 40 plots on fuel-breaks, as well as adjacent untreated “control” plots. We surveyed for perennial bunchgrasses, exotic annual grasses, overall biomass, and the height of the leaf litter layer. This data will be used to assess the costs and benefits of the fuel-breaks relative to EAG invasion and fire moderation. Treatment effects on fire behavior are determined primarily from computer simulations, and a major part of the research is determining how to adapt simulations for use in sagebrush steppe and validating their accuracy.
Objectives
- Identify the short-term effects of herbicides, shrub reductions, and drill seeding treatments on native and exotic plants and fuels, then determine how these responses will impact fire behavior
- Monitor the changes in vegetation, fuels, and simulated fire behavior over time, on or adjacent to roadside fuel breaks
- Determine how these changes are influenced by site climate, post-treatment weather, re-treatments, landscape management choices, and the presence of exotic annual grasses.
- Complete our ongoing assessment of optimal field-sampling approaches and use of remotely sensed data to improve simulation methods and fire behavior predictions
Results
Vegetation Responses to Fuel-Break Treatments
Pre-emergent herbicides reduced exotic annual grass and total herbaceous cover without affecting native perennial bunchgrasses for 2-3 years, in areas dominated by exotic annual grasses prior to fuel break construction. However, exotic annual grass cover was still over 30% after herbicide application, meaning it is likely they will reinvade and dominate in the future.
In areas that were less invaded and had more native plants, mowing or hand-cutting shrubs to half their size did not increase exotic annual grasses. Cutting did lead to an undesirable increase in fuel continuity by increasing other herbs and distributing wood fragments to the soil surface.
Drill seedings that followed fuel treatments successfully established native perennial bunch grasses, but were also linked to increases in exotic forbs like Russian thistle.
Fuel and Wildfire Behavior Responses to Treatments
Using information on the fire fuels collected in the field at fuel-break sites, we used a fire behavior simulator to predict wildfire behavior. In areas dominated by exotic annual grasses, simulated flame length, rate-of-fire spread, and heat of fire reaction were reduced in areas treated with herbicides. However, simulated fire behavior was still above levels desired for fire management. Fuel treatments also modestly reduced predicted fire behavior at sites with more shrub cover and achieved fire management targets.
Implications for Land Management
Predicted fire-moderation benefits over the first four years of fuel break implementation were modest and varied with site. In general, increases in exotic annual grasses and the fire risks they are associated with were not observed. Our results suggest that fuel treatments which reduced shrub and increased herbaceous fuels could help achieve fuels-management goals. Fuel treatments that result in herbaceous fuel dominance likely improve fire suppression efficacy and wildfire fighter safety. Though herbaceous fuels, especially grasses, are well known to increase the frequency and size of wildfires in sagebrush steppe, fires in herb-dominated areas can be easier to control because they can be more effectively covered by flame retardants, are easier to extinguish, and are more amenable to creating backfires to protect firefighters.