Experimental Study on Effects of Moisture Content and Tree Height on Crown Fire Behaviors of Live Cypress Trees

Once a fire occurs in urban green belts, the live foliage of some tree and shrub species with larger moisture content than dead fuel can support fire spread, which poses a great threat to the wildland-urban interface (WUI). Cypress tree is widely used as an ornamental vegetation worldwide. This work aims to study the fire and spread behaviors through cypress trees. Experiments were conducted using three tree heights between 0.8 m and 1.6 m and five values of fuel moisture content (FMC) between 30% and 160%. The fuel mass loss, flame shape, vertical temperature distribution and outward vertical heat fluxes were measured. The results showed that for a given tree height, a small enough FMC promoted the burning process and decreased both the maximum heat release rate (HRR) and the time to reach peak HRR compared to a larger FMC. The tree height correlated strongly with the flame height, while the influence of FMC was much smaller. Then, a correlation between the normalized maximum flame height and the normalized maximum HRR was established by taking the crown height as the characteristic length. Reducing the FMC did not necessarily promote the fire spread and trees with large FMCs can support relatively large flame front spread rates. The effect of the tree height on the radiant heat flux was greater than the effect of FMC.