The effects of fuel moisture on fire spread in shrub vegetation typical of upland heath systems in northern latitudes

Shrubs are the dominant fuel for wildfires in heathland systems. Despite this, there are relatively few studies which explore the processes of flame spread in shrub fuels. A series of laboratory flame spread experiments are used to identify the relationships between the fuel moisture content of the fuel components present in typical UK upland heath and the resulting fire spread dynamics. Measurements of energy release, flame spread rate and mass loss are made to characterise the burning of 2 m x 0.75 m fuel beds and heat flux measurements are made to record the magnitude of the propagating flux. Fuel moisture thresholds for fine and coarse fuel types required for fire spread are identified, and the magnitude of the in-bed heat fluxes are reported. From the observations, it is suggested that the leading edge of the flame front is driven by the moisture content of the fine dead material suspended in the heather canopy while the trailing edge is dominated by the burning of coarser fuels which supports the burning of fine green fuels. These findings allow further targeted experimental study and can be used to aid in determining fire effects.