Breaking the grass-fire cycle: modeled effects of post-fire treatments on risk of reburning, relative to fire-weather effects

Invasion by combustible exotic annual grasses and increased prevalence of extreme fire weather is causing shortened wildfire return intervals in drylands across the western United States. Seeding perennial vegetation and herbicide are used to “break” this exotic grass-fire cycle, but reburning often occurs in rapid succession. We asked how fuels, characterized as fire behavior fuel models (FBFMs), were affected by post-fire treatments. We also considered how the resulting landscape changes affected modeled probability of reburning under different weather scenarios. Treatment effects on FBFMs were determined by comparing plots that received a given treatment (drill seeding with perennial grasses, aerial shrub seeding, or herbicide spraying) during the ten years before monitoring to plots that had not received that treatment (n = 3777) across thirteen burned landscapes in the Great Basin, USA, using multinomial modeling. We used random forests models to map FBFMs in “true fuels” and the counterfactual “no treatment” (the hypothetical situation in which the landscape was not treated) scenarios and evaluated differences for three partially reburned landscapes. Reburn probability and fire size were simulated for each FBFM map using the fire-spread model FLAMMAP under five fire-weather scenarios. Grass fuel loading was greater where seedings occurred and less where herbicide was applied. The “true fuels” scenario resulted in slightly greater reburn probability under extreme fire weather conditions compared the counterfactual “no treatment” scenario. Treatments increased vegetation productivity, which increased fuel loadings and reburn probability under extreme fire weather. More detail in FBFMs and accounting for treatment effects on seasonal variation in fuel moisture may reveal greater benefits of the treatments in moderating reburn potential.