Characteristics Of Canopy Fuel And Potential Fire Behavior In Major Coniferous Forests In The Mountainous Area,Beijing

Crown fire is a kind of forest fires that cause serious damage to forest ecosystems,especially to coniferous forests.Once crown fires occur,they play a leading role in the forest fires.In this paper,by understanding the characteristics of coniferous forest canopy,exploring the size and occurrence conditions of crown fire hazards,and calculating the dynamic changes of potential forest fire behavior,the scientific basis for preventing and fighting forest fire was provided.In this study,the main coniferous forests in Beijing,Pinus tabulaeformis,Platycladus orientalis and Larix gmelinii,were taken as the research objects to investigate their stand factors and fuel characteristics.The canopy fuel load was investigated and calculated by using standard branch method.The vertical distribution of canopy characteristics(canopy density,canopy fuel load)and its correlation with stand factors were studied.At the same time,the probability and potential fire behavior of crown fire in coniferous forest under different combustion conditions were studied by dividing combustion conditions of different gradients.Based on the potential fire behavior of canopy,a canopy fire risk index model with wind speed as the dominant factor was established by using different types of canopy fire critical index models.Based on the meteorological data in this area,the canopy risk index model was used to calculate the types,occurrence time,duration of crown fire and the dynamic changes of potential fire behavior of three forest types in different periods under actual meteorological conditions.This paper draws the following conclusions:(1)The types of canopy fire are closely related to the vertical distribution of canopy combustibles and the height of canopy base.Under low burning conditions,only 15%Platycladus orientalis of stands are likely to have canopy fires,whereas under medium burning conditions,the probability of continuous canopy fires(54%and 48%)is much higher in Pinus tabulaeformis and larch forests than in middle burning conditions because of the large amount and density of fuel loads in the lower layer of Platycladus orientalis forests,while only 13%and 11%in Pinus tabulaeformis and larch forests,and the canopy base height of Platycladus orientalis is the lowest.Platycladus orientalis(22%).(2)The potential fire behavior of continuous crown fire has the characteristics of fast spreading speed,high intensity of fire line and high flame height.Under high burning conditions,the maximum spreading speed,fire line intensity and flame height of crown fire of Pinus tabulaeformis forest were 91.3 m/min,80 627 kW/m and 56 m,respectively,while those of Platycladus orientalis forest and Larix gmelinii forest were 82.4 m/min,34 150 kW/m,28 m and 90.1 m/min,74 863 kW/n and 54 m.Extremely high intensity fires occurred in all three stands under high burning conditions.(3)Intermittent canopy fire is easy to occur in Platycladus orientalis forest,continuous canopy fire is easy to occur in Pinus tabulaeformis forest,and larch forest is in the middle.From the perspective of TI,the lowest wind speed is required for crown fire occurrence in Platycladus orientalis,but at the same time,because of the low canopy density,the crown fire spread index(CI)is the highest,so crown fire is the most prone to occur in this stand,and the intermittent crown fire is the main one;at the same time,the CI value of Pinus tabulaeformis forest is the lowest,that is,the wind speed needed to develop from intermittent crown fire to continuous crown fire is the highest.Low,continuous canopy fire is most likely to occur in the three forest types.(4)Under the actual meteorological conditions,the occurrence of canopy fire has a significant impact on the potential fire behavior of forest.Among the three forest types,once crown fire occurs,the potential fire behavior index of the stand is more than 10 times as high as that of the surface fire,and the fire intensity of the Pinus tabulaeformis forest is 30 times as high as that of the previous one,and high-intensity forest fire can be formed in a short time.