To cut or not to cut: Effect of vegetation height and bulk density on wildfire propagation under varied wind and slope conditions

The frequency, intensity and span of wildfires have surged in the past decades, mainly driven by changes in climatic patterns. Although grasslands cover ≈ 40% of the Earth's surface, they account for ≈ 80% of the burned area. Mowing is a common practice to reduce the Rate of Spread (RoS) and intensity of grassland fires. However, recent studies suggest the RoS may increase instead. This paper combines results from previous experimental studies in Australian grasslands and wheatlands with wildfire propagation simulations under a range of ambient wind speeds, vegetation heights (H _g), and terrain slopes to assess whether grass-cutting is an effective strategy to mitigate fire propagation. Previous investigations of the H _g–RoS correlation led to contradictory conclusions. The main finding here is a statistically significant negative correlation between the vegetation's bulk density (ρ _b) and the RoS/u ₁₀ ratio, where u ₁₀ is the wind speed 10m above ground. Seeking a relationship between H _g and RoS, we found positive correlations between H _g and the RoS/(u ₁₀M) and RoS/(u ₁₀R _H) ratios, where M (fuel moisture content) and R _H (atmospheric humidity) account for environmental conditions. These correlations seem to hold provided that ρ _b decreases with increasing H _g and that propagation is plume-driven. For wind-driven propagation, simulations show that decreasing H _g increases RoS/u ₁₀ for constant M and R _H. Further experimental research is needed to confirm this. Thus, grass-cutting is observed to effectively curb fire propagation, although it may be less effective under specific conditions such as high winds, as vegetation characteristics and propagation modes significantly affect fire dynamics.