Combustion characteristics and occurrence probability of shallow underground fire in Larix gmelinii plantation

  Objective  Daxing’an Mountains of northeastern China is the hotspot of underground fire in whole country. Due to the influence of climate and topography, shallow underground fire is the main type in Daxing’an Mountains, and it is more likely to cause surface-fire and crown fire because of the rapid fire spread rate and thinner combustible materials. This study took Larix gmelinii plantations under five land types (sloping mountain, agricultural land, flat mountain, water wetland, Tatou wetland) in Daxing’an Mountains as the research object, and studied the combustion characteristics and occurrence probability of shallow underground fire in this area.

  Method  The simulated ignition experiment without destroying the original structure of combustibles was carried out to study the combustion characteristics of shallow underground fire. According to the loads and moisture content of litter and underground fuel, methods such as variance analysis, entropy weight method and cluster analysis were used to analyze and rank the occurrence probability of shallow underground fire in different land types.

  Result  During the combustion process of shallow underground fire in Larix gmelinii plantation, the temperature rised slowly at first. When the water in the combustibles evaporated mostly, the temperature rised rapidly, and stabilized after reaching a certain temperature, then the temperature decreased until it was extinguished. There were significant differences in the litter layer and underground fire combustible load and moisture content under the 5 land types (P < 0.05); the underground fuel load had the greatest impact on the probability of shallow underground fire in the 4 indicators; the occurrence probability of shallow underground fire in five types of land was as follows: water wetland, agricultural land, sloping mountain, Tatou wetland and flat mountain.

  Conclusion  The combustion process of shallow underground fire in Larix gmelinii plantation can be divided into four stages: ignition stage, combustion stage, steady stage, and descending stage, and 100 ℃ is the critical point. The moisture content of the litter layer on the sloping mountains (26.05%) is the lowest, and the moisture content of the underground fuel material in the agricultural land (40.69%) is the lowest. Agricultural land (3 805.31 kg/ha) has the highest litter layer load, and water wetland (27 956.73 kg/ha) has the highest underground fuel material load. The probability of shallow underground fire in 5 land types can be divided into 3 types. The probability of shallow underground fire is the highest for the water wetland, general for the sloping mountain and agricultural land, lower for the Tatou wetland and flat mountain.