Which fuel type transition results in a fourfold increase in rate of spread?

• A. Brush to grass
• B. Timber mixed with brush to grass
• C. Timber and second growth to brush
• D. Timber and second growth to grass

Answer: (D)

Rate of spread rises dramatically when the fuel becomes finer and more continuous. Grass is fine, lightweight fuel that ignites and preheats the ahead fuels much more quickly than timber or second-growth, and it tends to burn in a continuous layer, allowing the flame front to move forward rapidly. So switching from heavy timber and second-growth to grass creates the largest, most rapid boost in spread, often described as a fourfold increase. While transitioning to grass from other fuel types (like brush) also increases spread, none match the magnitude seen when moving from heavy, bulky fuels to grass.