Study on the Limit of Moisture Content of the Sub-Surface Fires Converted to the Surface Fires in the Boreal Forests of China

A sub-surface forest fire is a type of smoldering combustion with a slower spread rate, longer combustion time, and lower combustion temperature compared with flame combustion. Sub-surface fires are usually accompanied by surface fires, and the surface fires' conversion from sub-surface fires has great uncertainty. Therefore, there are considerable difficulties in monitoring and fighting sub-surface fires. However, there are few studies on the conversion from sub-surface fires to surface fires, and the mechanism and influencing factors of the conversion remain unclear. This study focuses on Larix gmelinii forests, which are representative of the boreal forest of China and hot spots of sub-surface fires, studies the moisture content limit of sub-surface fires' conversion to surface fires by simulating a smoldering experiment, and establishes a monitoring model of sub-surface fires and an occurrence probability prediction model of sub-surface fires' conversion to surface fires. The results showed that the moisture content limit of the conversion was 25% in the grass-Larix gmelinii forest and Ledum palustre-Larix gmelinii forest and 20% in Rhododendron dauricum-Larix gmelinii forest. There was a significant positive correlation between the time and temperature caused by the smoldering. The monitoring model of the sub-surface fires based on the surface temperature and moisture content had a good fitting effect (p < 0.01). The occurrence probability prediction model of the sub-surface fires' conversion to surface fires, based on a logistic regression model, had high prediction accuracy (AUC = 0.987). The lower the moisture content of the humus, the closer the smoldering came to the surface and the higher the probability of conversion. This research could contribute to the study of the mechanism of sub-surface fires' conversion into surface fires.