ObjectiveCrown fire is a serious type of forest fire, which plays a leading role in the spread of forest fires, especially to coniferous forests. This study aims to provide scientific basis for preventing and saving crown fire through the investigation of daily variation trend of the potential fire behavior and the canopy hazard index of stand.

MethodThe principle coniferous forests (Pinus tabuliformis and Platycladus orientalis) of Bejing mountain area were selected as main research object of the study. The establishment of the canopy risk index was based on critical meteorological factors for occurrence and development of crown fire, which was calculated by the survey data of combustibles load, physicochemical properties and distribution which can count the trend of variation of the potential fire behavior with wind speed. The type, time, duration of crown fire could be judged by meteorological data in the region.

ResultThe difference in physicochemical properties between the two stands was small, but there was a certain gap between the fuel load and distribution rules, especially the crown base height; at a high wind speed (40 km/h) condition, the active crown fire occurred with spread rate of 17.0 m/min and flame intensity of 16 079 kw/m in Pinus tabuliformis forest, and the passive crown fire occurred in Platycladus orientalis forest with 11.4 m/min, 5 290 kW/m. From the trend of potential fire behavior with wind speed, Pinus tabuliformis and Platycladus orientalis forest reached high-intensity fire at wind speeds of 30 km/h and 37 km/h, respectively; the canopy hazard index TI and CI of Pinus tabuliformis forest fluctuated over time 17.1-29.6 km/h and 33.9-38.8 km/h, respectively, while the range of Platycladus orientalis forest was 11.9-21.1 km/h and 47.5-54.9 km/h, the percentage of time of occurrence of surface fire, passive crown fire, and active crown fire in Pinus tabuliformis forest within 48 hours accounted for 87%, 9%, and 4%, respectively. The maximum speed and maximum flame intensity were 17.5 m/min and 8 598 kW/m, the Platycladus orientalis forest could only occur with surface fires and passive crown fire, the proportions of which were 56% and 44%. The maximum potential fire behavior indicators were 14.5 m/min and 4 506 kW/m.

ConclusionIn two kinds of stands, CBH is one of the most important influencing factors of crown fire; the overall potential fire behavior of the stands changes significantly with wind speed, and it can form high-intensity forest fires at moderate wind speed (level 5); it is easier to form a crown fire in Platycladus orientalis forest, and there is a high possibility of extremely high-intensity fires in Pinus tabuliformis.