| Wildfire is a primary source of natural disturbance in forest ecosystems, and it plays an important role in determining landscape structure and plant community composition. Wildfires are classified into ground, surface, and crown fires, based on the strata where the burning occurs. However, the shrub and small trees stratum acts as"ladder fuel," forming a continuous ladder that carries fire from the surface fuel up to the canopy fuel to link them and spread the burning across multiple strata. Forest fuel, weather, and terrain are key factors influencing wildland fire occurrence and behavior, and of these fuels is the only factor that can be controlled by humans. Forest fuels are not only the material basis of wildfire, but also one of the main determinants of burning. Different kinds of fuel types can be grouped into different complexes, which have different combustion characteristics and different characteristics of the fire behavior. In recent decades, high forest fuels accumulation has been coupled with a warmer and extreme climate, making it essential to study wildfire in order to address fire risk problems and to promote forest health.This study aimed to understand the forest fuel (arbor layer, shrub layer, herbaceous layer and dead furl layer) spatial distribution, properties and potential fire behavior. The study analyzed fuel loadings and their environmental gradient characteristics,examining the main coniferous forest using canonical correspondence analysis (CCA) for the southwest of Sichuan Province. In addition, the research analyzed the fire classes, potential fire behavior, and fire danger of different conifer forest stands at different spatial scales and three different burning conditions using BehavePlus. Finally, validity of firefighting measures was evaluated for different burning conditions in forest types according to the study results. The main conclusions from this study were:(1) There were significant difference in fuel loading, continuity and density of major coniferous forests in southwest Sichuan.Of the six forest types, the Tsuga chinensis forest had the highest surface fuel loading, at 2.34 kg.m-2,along with the lowest horizontal continuity; by contrast, the Platycladus orientalis forest demonstrated the lowest loading, at 0.23 kg.m-2, but had the highest horizontal continuity. The order of surface fuel loadings in the six forest types was:the Keteleeria fortune forest, the Tsuga chinensis forest, the P. yunnanensis and Platycladus orientalis (L.) Franco mixed forest, the P. yunnanensis Franch and K. fortunei (Murr.) Carr mixed forest, the P. yunnanensis forest, and the P. orientalis forest. Among these, the highest fuel vertical continuity was found for the P. yunnanensis and P. orientalis forest, while the lowest was the P. yunnanensis forest.(2) The fuel spatial distribution was significantly influenced by environmental and stand factors.The key factors affecting the spatial pattern of surface fuel loading were slope position (SP), age, canopy closure (CC), elevation, canopy width (CW) and canopy base height (CBH). Active crown fuels were correlated with age, diameter at breast height, CW and height; fine fuels were correlated with CC, SP, height, DBH, elevation, and SP; medium fuels were correlated with SP, age, SP, CC, and height; and thick fuels were correlated with height, elevation, CC, CBH, SP and age.(3) In the low burning condition, all forest types had surface fires with low speed and low intensity. The heat per unit area of the Tsuga chinensis forest was highest, at 90000 kJ.m-2. Heat for the P. yunnanensis and Platycladus orientalis forest was the second highest, at 45000 kJ.m-2, and heat for the other forest types was 22500 kJ.m-2. In the medium burning condition, all forest types had high-speed passive crown fires except the P. yunnanensis forest which had high-speed surface fires. The order of the transition ratio (TR) for the six forest types was:Keteleeria fortune forest, Tsuga chinensis forest, P. yunnanensis and Platycladus orientalis (L.) Franco mixed forest P. yunnanensis Franch and K. fortunei (Murr.) Carr mixed forest, and P. orientalis forest The heat per unit area of the Tsuga chinensis forest and P. yunnanensis-Platycladus orientalis forest were highest, at 180000 kJ.m-2, and the other forests were 45000 kJ.m-2 and 22500 kJ.m-2. In the high burning condition, all forest types had the high-speed and high-intensity active crown fires except the P. yunnanensis forest, which had high-speed surface fires. The order of the active ratio (AR) in the six forest types was:Keteleeria fortune forest, Tsuga chinensis forest, P. yunnanensis and Platycladus orientalis (L.) Franco mixed forest, P. yunnanensis Franch and K. fortunei (Murr.) Carr mixed forest, and P. orientalis forest The heat per unit area of the Tsuga chinensis and P. yunnanensis and the Platycladus orientalis forest types were highest, at 180000 kJ.m-2. The P. yunnanensis and K. fortunei mixed forest had the second highest heat per unit area, at 90000 kJ.m-2, and the other forest types demonstrated heat per unit area of 45000 kJ.m-2.(4) There were significant difference in validity of fire fighting measures in different burning condition, therefore, it is necessary to work out a plan according to forest fire behavior. Both the rate of fire spread and heat value per unit area increase with burning condition, so plans for fire fighting measures are different for different conditions. Artificial homemade tools were used to extinguish the fire when the surface flame length was less than 1 m; professional tools (such as bulldozers, pump truck, aircraft and flame retardant agent) were used to extinguish the fire when the surface flame length was between 1 and 2 m. Firefighters should pay attention to their own personal safety when the surface flame length is between 2 and 3 m, as wildfires may lose control; and all firefighters should be withdrawn from the security zone when the surface flame length is more than 3 m. |
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