Quantification of Forest Sub-Surface Fire Suppression Risk Factors and Their Influencing Elements in Boreal Forest of China

Abstract

Forest sub-surface fires represent a challenging combustion phenomenon to control, and the process of smoldering is often overlooked in wildfire incidents. Traditional forest fire research has prioritized flaming combustion over smoldering dynamics, despite its critical risk factors including sustained high temperature and ground surface collapse that significantly endanger firefighter safety. This study focuses on The Daxing’an Mountains, a prime sub-surface fire-prone region in China, employing field investigations and controlled smoldering experiments to quantify the key risk factors for sub-surface fires suppression while elucidating moisture content’s regulatory effects. The results demonstrate that sub-surface smoldering fires maintain elevated temperatures with the surface peak temperature reaching 600.24 °C and sub-surface peak temperature up to 710.70 °C. The spread rate is relatively slow (maximum 27.00 cm/h), yet exhibits pronounced fluctuations along the vertical profile, creating a critical predisposition to overhanging collapse. The moisture content has extremely significant effects (p < 0.01) on key risk factors including surface temperature, sub-surface temperature, collapse time and ignition duration. Lower moisture content prompted earlier surface collapses, whereas higher moisture content displays delayed collapse but resulted in dangerously elevated temperatures at collapse points, presenting extreme suppression risks.

1. Introduction

Fire as an important ecological factor in the ecosystem, exerts fundamental regulatory control over forest succession dynamics, landscape configuration, and energy-matter fluxes within the ecosystem [1]. Forests play a significant role in the global carbon cycle, absorbing 33% of anthropogenic emissions, thus constituting a key factor in mitigating climate change [2]. However, recent anthropogenic land-use changes and climate change impacts have significantly increased the frequency and severity of global wildfire incidents [3,4]. For instance, about 15 million hectares of land were burned in Canada in 2023—exceeding historical averages by sevenfold [5]. When large-scale forest fires occur, they not only cause vegetation loss and ecosystem degradation but also create complex and hazardous fire environments that endanger firefighting personnel [6,7]. Therefore, the safety of firefighters has always been a matter of great concern for both wildfire management agencies and the scientific community. Forest fire combustion manifests in two distinct models: flaming and smoldering, both of which are present in most forest fires [8,9]. However, traditional forest fire research has focused more on flaming fires, often neglecting smoldering dynamics [10], which may lead to a “false suppression” phenomenon, where a flaming fire is extinguished, while sub-surface smoldering in humus or peat layers continues to spread and may turn back into a flaming fire after a long distance or time [11,12]. The sub-surface fire smoldering process can sustain prolonged high temperatures of 400–700 °C, leading to surface suspension, and the release of toxic gases such as CO and SO₂ [13,14]. Sudden ground collapse, sub-surface high temperatures, and the simultaneous release of large amounts of toxic gases, greatly increase the complexity of the fire scene and pose a serious challenge to the safety of firefighters [15,16]. Therefore, it is particularly important to actively research the key risk factors involved in sub-surface fire suppression (see Figure 1 for a schematic diagram of forest sub-surface fires suppression risks).

Monitoring, early warning and comprehensive prevention, and control capabilities for natural disasters have always been a key focus for both management authorities and researchers. Forest fires, as one of the world’s eight major natural disasters, have become a global challenge in their prevention and control, receiving significant attention from forestry departments worldwide. Due to the complexity of large-scale fire scenes and limitations in human resources, using satellite remote sensing and unmanned aerial vehicles to assess the risk of forest fire suppression and delineate safety zones based on real fire incidents are frequently used methods in current forest firefighting safety research [17,18,19]. However, because smoldering is highly concealed, the current ability to identify and manage the danger of its suppression is still insufficient. Smoldering is a complex phenomenon and exists in various scenarios. Safety concerns related to coal seam smoldering, wood smoldering, and polyurethane foam smoldering have been widely investigated [20,21,22]. In recent years, forest sub-surface smoldering has gradually gained attention. The toxic and hazardous gases released by the sub-surface smoldering, such as CO, CO₂, NH₃, and particulate matter, are considered significant threats to human health and environmental pollution. This feature has also become a research focus for scholars [23,24,25,26]. Due to the relatively late initiation of research on sub-surface fire smoldering, current studies have mainly focused on smoldering temperature, spread rate, influencing factors, and burning limits [27], but there is a lack of research on the safety of forest sub-surface fires suppression. Forest sub-surface fires represent a distinct phenomenon in wildland combustion, characterized by exceptional suppression challenges and high-risk coefficients [28,29,30,31,32]. Consequently, ensuring the safety of forest firefighters warrants more in-depth investigation while developing scientifically effective suppression methods for forest sub-surface fires.

Our study focuses on Larix gmelinii, a typical vegetation type of boreal forest in the Daxing’an Mountains—a region prone to frequent forest sub-surface fires in China. In recent years, due to climate change, the occurrence rate of forest sub-surface fires in boreal forest has been on the rise [33,34]. Under the combined effects of complex terrain, climate conditions, and fuel the suppression of forest sub-surface fires is extremely dangerous. This study employs actual humus of Larix gmelinii forest as experimental materials. Through large-scale laboratory-controlled burning experiments, we quantitatively analyzed critical risk factors that threaten firefighter safety during suppression operations, including surface temperature, surface collapse time, and sub-surface temperature patterns during sub-surface fire smoldering under various conditions. The research further identifies key driving factors influencing these risk parameters. These findings will contribute to reducing casualty risks during forest sub-surface fire suppression operations.

2. Study Site

Boreal forests represent one of the regions that is most susceptible to sub-surface fires [35]. The Huzhong National Nature Reserve in Daxing’an Mountains, Heilongjiang Province (122°39′30″−124°21′20″ E, 51°14′40″–52°25′00″ N) constitutes the largest coniferous forest ecosystem nature reserve in the cold temperate zone in China. The terrain slopes from higher elevations in the southwest to lower area in the northeast, with an average altitude of 812 m. The climate is a continental monsoon. The region experiences brief summers and prolonged, severe winters influenced by the Siberian-Mongolian high-pressure system. The average annual temperature is −4.4 °C and the average annual rainfall is 481.6 mm. The zonal vegetation type is cold-temperate coniferous forest dominated by Larix gmelinii, with associated species including Pinus pumila, Betula platyphyla, Populus davidiana, etc. The study area experienced frequent severe forest fires, the vast majority of which were lightning-ignited, creating favorable conditions for forest sub-surface occurrence. Most ignition sites are located in the primeval forests at high latitudes, presenting extreme suppression challenges and high risks, while causing the substantial ecological damage.

3. Methods

3.1. Humus Collection and Processing

During the fire prevention period, we conducted field surveys in the Huzhong National Nature Reserve in Daxing’an Mountains. The study focused on Larix gmelinii, which represent typical vegetation types prone to forest sub-surface fires in this area (Figure 2). Three 20 m × 30 m experimental plots were randomly established, and three 50 cm × 50 cm subplots were systematically arranged along diagonal transects of each plot. The average thickness of the humus layer was 23 cm, and all the sub-surface humus in the plots were collected for subsequent laboratory-controlled burning experiments. Three moisture content gradients (0%, 5%, 10%) were set up for the simulated ignition experiment. The collected humus samples were packed in archive bags and placed in a forced-air drying oven. The samples underwent continuous drying at 105 °C for 48 h with moisture content measurements taken every 6 h using a rapid moisture meter until the predetermined moisture levels were achieved. The humus with different moisture contents was individually sealed in plastic bags for controlled ignition experiments. Prior to formal testing, each sample’s moisture content was measured three times using a rapid moisture meter, and the average of the three measurements was taken as the actual moisture content of the ignition experiments. Final measurements confirmed mean moisture contents of 3.56%, 5.37%, and 10.57%.

3.2. Sub-Surface Fire Smoldering Simulation Experiment

The experiment employed a customized sub-surface fire temperature collection system (Figure 3). The system consisted of a smolder-fired reactor furnace, thermocouples, data acquisition modules, compensating cables, and a laptop, etc. The prepared humus with different moisture content gradients was placed in a cuboid smoldering reactor furnace (internal dimensions: of 30 cm long, 10 cm wide, and 20 cm high) constructed from high-temperature resistant, low-thermal-conductivity aluminum silicate ceramic fiber. A temperature monitoring matrix, consisting of 45 K-type thermocouples arranged in a 5 × 9 grid, was established on the side of the smoldering furnace. The thermocouples (300 mm long, 2 mm diameter) were inserted into holes drilled at 3 cm intervals along both the horizontal and vertical axes to monitor the temperature changes during humus combustion. The data acquisition module, consisting of NI9213 voltage acquisition board cards with DAQ-9174 chassis (4 card slots) connected via compensating cables, and the Labview2018 software was used to record temperature data at 10 s intervals, which enabled real-time visualization of the smoldering process through a dedicated monitoring interface. A total of 9879 sub-surface fire temperature data were collected throughout the entire experimental process. Smoldering is easily ignited by smoldering, similar to open flames [36]. Therefore, low-temperature cigarette butts that could smolder under oxygen-deficient conditions were selected as the ignition source in this study. The cigarette butts were shallowly buried 1 cm under the humus to simulate the smoldering process of sub-surface fires.

3.3. Monitoring of Risk Factors in Forest Sub-Surface Fires

During the simulated ignition experiment, real-time monitoring and data recording were performed using a FLUKE Ti400(manufactured by Fluke Corporation, USA) infrared thermal imager. The thermal imaging camera was mounted above the smoldering reactor to ensure complete coverage of the sub-surface fire combustion zone, enabling continuous monitoring of surface temperature variations induced by sub-surface smoldering. Simultaneously, the system recorded the dynamic process of lateral collapse of the surface, with all sequential images and data being systematically archived. In parallel, K-type thermocouples coupled with the data acquisition module were employed to record the temperature changes during the smoldering process of the sub-surface fire. This dual monitoring approach effectively captured both surface and sub-surface thermal characteristics and their evolving trends throughout the smoldering process (Figure 3).

3.4. Statistical Analysis

The collected data were statistically processed using Excel. Variation characteristics of the risk factors were graphically visualized using Origin 2024 software. Comparative analysis of moisture content effects on sub-surface fire suppression risk factors was conducted through ANOVA in IBM SPSS Statistics 27 software, with LSD employed for multiple comparisons at a significance level of p < 0.05.

4. Results and Analysis

4.1. Quantification of Risk Factors in Forest Sub-Surface Fire Suppression

Under different moisture content conditions, the peak surface temperature of humus remained within the range of 447.67 °C to 600.24 °C. As the sub-surface fire progressively developed, the smoldering phenomenon tended to stabilize, and the surface temperature of humus consistently maintained relatively elevated levels (Figure 4).

At 3.56% humus moisture content (Figure 4A), the peak surface temperature ranged from 453.41 °C to 551.96 °C, with localized maxima reaching 551.96 °C at 3 cm horizontal distance and 520.76 °C at 9 cm. When the moisture content of humus increased to 5.37% (Figure 4B), the surface temperatures exhibited a range (447.67–584.81 °C) with concentrated high-temperature zones, particularly peaking at 584.81 °C within the 3–15 cm horizontal range. The peak temperature was comparatively lower within the range of 18–27 cm, but it remained above 447.67 °C. The 13.56% moisture condition (Figure 4C) demonstrated superior thermal stability and intensity (468.15–600.24 °C). Compared to lower moisture contents, this condition produced both higher and more stable peak temperatures. At a horizontal distance of 9 cm, the peak surface temperature reached its maximum (600.24 °C), and at distances of 3 cm, 6 cm, 12 cm, 15 cm, 18 cm, and 21 cm, the peak temperature maintained above 504.22 °C. As the sub-surface fires propagated downward, the smoldering entered a stable state accompanied by heat accumulation, forming a distinct sub-surface thermal environment characterized by a persistent peak temperature gradient ranging from 277.53 °C to 710.70 °C. When overhanging structures formed at the surface layer and subsequently collapsed, the momentary exposure of the sub-surface heat reservoir generated instantaneous thermal shock, creating secondary hazard potential that significantly compromises firefighter safety during suppression operations.

With the increase in moisture content, the sub-surface fire spread rate decreased significantly. At a humus moisture content of 3.56%, the surface fire spread very rapidly, reaching a maximum velocity of 27.00 cm/h. A pronounced rate of discontinuity emerged with increasing depth, showing sharply reduced spread rates of 2.30–7.30 cm/h at 3 cm depth and a minimum observed rate of 0.90 cm/h at 12 cm depth (Figure 5A).

At a humus moisture content of 5.37%, the spread rate varied from 3.00 to 22.24 cm/h, exhibiting relatively rapid propagation at depth 0–6 cm before accelerating further at depth 6–9 cm, where it reached its maximum velocity of 22.24 cm/h at a depth of 9 cm, which was higher than the spread rate on the surface. As the sub-surface fires progressed deeper, their spread rate gradually stabilized. At a depth of 12–15 cm, the spread rate demonstrated minimal variation with rates maintained between 6.24 and 17.93 cm/h (Figure 5B).

When the humus moisture content was 13.56%, the initial spread rate was relatively low, indicating the inhibitory effect of moisture content on early fire development. However, as the sub-surface fires progressed, the spread rate increased significantly within a vertical distance of 3–15 cm. At a horizontal distance of 9–16 cm, the deep-layer propagation rates substantially exceeded those at the surface level. In addition, the most rapid spread was concentrated in the area spanning 18–27 cm horizontally and 1–8 cm vertically, with the fastest spread rate reached at 11.64 cm/h (Figure 5C).

When the humus moisture content was 3.56% (Figure 6A), the sub-surface fire propagated for 1.00 h before the first collapse occurred at a horizontal distance of 3 cm from the surface, and the highest sub-surface temperature at the collapsed site reached 502.43 °C. At 1.08 h of combustion, a second collapse occurred at 27 cm from the surface, with the corresponding highest surface temperature recorded at 423.01 °C. A third collapse occurred at 6 cm from the surface at 1.22 h. After 1.50 h, a collapse was observed at 24 cm, with the maximum surface and sub-surface temperatures at 302.31 °C and 577.64 °C, respectively. At 1.53 h, after the sub-surface fires spread, collapses occurred at 9 cm and 12 cm from the surface, with peak surface temperatures of 379.33 °C and 302.26 °C, while the highest sub-surface temperatures rose to 499.67 °C and 461.24 °C, respectively. Subsequently, at 1.57 h, collapses were observed at 15 cm and 18 cm. Finally, the last collapse occurred at 21 cm at 1.83 h, where the maximum surface and sub-surface temperatures recorded at 303.33 °C and 436.15 °C, respectively.

Under a humus moisture content of 5.37% (Figure 6B), the first collapse occurred at a horizontal distance of 3 cm from the surface after 0.88 h of sub-surface fire propagation, with maximum surface and sub-surface temperatures of 398.73 °C and 442.02 °C, respectively. As the sub-surface fires progressed, a second collapse occurred at a horizontal distance of 6 cm at 1.17 h, accompanied by a maximum surface temperature of 394.55 °C. By 1.20 h, a third collapse occurred at 9 cm, where the surface temperature peaked at 521.74 °C was detected. At 1.22 h, simultaneous collapses were recorded at 12 cm and 15 cm with the highest surface temperatures of 448.05 °C and 422.93 °C, while the maximum sub-surface temperatures reached 580.32 °C and 573.64 °C, respectively. Subsequent collapses followed at 18 cm (1.25 h) and 24 cm (1.37 h), with the highest surface temperatures of 403.82 °C and 423.45 °C and the peak sub-surface temperatures of 473.02 °C and 370.04 °C, respectively. After 1.86 h of combustion, collapse occurred at 21 cm, exhibiting maximum surface and sub-surface temperatures of 305.16 °C and 488.71 °C. After 1.91 h, the last collapse occurred at 27 cm, with the corresponding maximum surface temperature reaching 354.35 °C.

When the humus moisture content was 13.56% (Figure 6C), the initial collapse occurred at 3 cm from after 1.86 h of sub-surface fire propagation, with the highest surface and sub-surface temperatures reaching 469.85 °C and 472.43 °C, respectively. Subsequently a collapse was observed at 6 cm after 1.94 h of combustion. As the fire continued to spread, collapses occurred simultaneously at 9 cm and 12 cm at 2.16 h, where the maximum surface temperatures were 586.37 °C and 477.70 °C, respectively, while the maximum sub-surface temperatures were 624.92 °C and 486.03 °C, respectively. Subsequently, two additional collapse events followed at 15 cm (2.21 h) and 18 cm (2.60 h), with corresponding peak surface temperatures of 487.93 °C and 507.11 °C, respectively. At 2.75 h, collapse was observed at 24 cm, exhibiting maximum surface and sub-surface temperatures of 453.11 °C and 486.36 °C. The final collapse events occurred at 21 cm and 27 cm after 2.87 h of combustion, with the highest surface temperatures measuring 499.72 °C and 486.46 °C, and the sub-surface temperatures reaching 502.97 °C and 486.34 °C, respectively.

4.2. Key Factors Influencing Forest Sub-Surface Fire Risk Variation

When the humus moisture content was 3.56%, the surface temperature at collapse was the lowest (336.82 °C). As moisture content increased, the surface temperature at collapse exhibited a slight rise, exhibiting significant difference compared to moisture content at 5.37% and 13.56%. The highest surface temperature (397.73 °C) occurred at a moisture content of 13.56% (Figure 7A). With the progression of sub-surface fire, the highest sub-surface temperature (459.17 °C) at the collapse was recorded at a moisture content of 13.56%, followed by 5.37% (440.94 °C), both significantly higher than that at 3.56% (408.26 °C) (Figure 7B). The collapse time was longest (2.38 h) at 13.56% moisture content, showing highly significant difference compared to moisture contents of 3.56% (1.43 h) and 5.37% (1.34 h), and no significant difference was observed between 3.56% and 5.37% moisture levels (Figure 7C). Ignition time varied significantly with moisture contents. The highest moisture level (13.56%) required the longest ignition time (0.33 h), while decreasing moisture to 5.37% shortened ignition time (0.11 h), though still longer than that at 3.56% (0.06 h), which exhibited the shortest ignition time (Figure 7D).

With moisture content increased, both sub-surface and surface temperatures at collapse exhibited a rising trend. The highest temperatures were recorded at 13.56% moisture content, with the sub-surface and surface temperatures at collapse reaching 459.17 °C and 397.73 °C, respectively (Figure 8). Notably, sub-surface temperatures during collapse were consistently and significantly higher than corresponding surface temperatures across all moisture levels (p < 0.01). This thermal pattern indicates that during the spread of the sub-surface fire, the heat energy was mainly concentrated within the humus layer, resulting in substantially elevated sub-surface temperatures. The observed temperature gradient between sub-surface and surface layers represents a critical characteristic of sub-surface fire behavior, serving as a key indicator for assessing the fire intensity and evaluating the sub-surface fire risks.

5. Discussions

The top priority of forest fire monitoring, early warning and suppression is to ensure the safety of personnel’s lives and property [37], while research on the suppression risks of the smoldering of forest sub-surface fires—a concealed and prolonged special forest fire behavior—has not received sufficient attention. Our study, through indoor simulated ignition, quantified the risk factors involved in suppressing forest sub-surface fires and identified the influence of moisture content on the key risk factors affecting the spread of sub-surface fires (Figure 9). This study maximized consistency with actual field conditions in aspects such as the selection of experimental plots, treatment of humus, and design of ignition experiments—especially in terms of research scale and experimental setup. Currently, research on the smoldering of sub-surface fires is mainly focused on micro-scale and small-scale [38]. Moreover, the smoldering of sub-surface fires is a three-dimensional process that can spread both horizontally and vertically [39]. However, experiments with excessively small scales can only study the entire smoldering process by splitting it into separate parts, and the relevant conclusions drawn have advanced research in aspects such as smoldering characteristics and pyrolysis kinetics. Although the scale of this study has not reached to meters or kilometers, the experimental scale (6000 cm³ in this study) is far larger than other similar studies, such as Wu et al. [40] and Qin et al. [32] (1177 cm³ and 4000 cm³, respectively). Moreover, restoration of the three-dimensional process of sub-surface fire smoldering was attempted. The accurately quantified key risk factors for sub-surface fire suppression provide a reference for solving the difficult problems in practical suppression, such as strong concealment, difficult risk judgment, and high casualty risk, and further promote the development of large-scale sub-surface fire smoldering research.

Toxic and harmful gases produced during sub-surface fires are regarded as an important factor threatening the safety of firefighters [14], but the hazards caused by the high temperature generated by smoldering should not be underestimated. Studies have shown that when the combustion temperature reaches 115 °C, firefighters’ perceptual ability will be impaired [41]; when the temperature exceeds 300 °C, protective equipment will lose its protective capacity [42]; and when the temperature exceeds 500 °C, evacuation is mandatory, otherwise there is no possibility of survival [43]. Our study finds that the maximum temperature in a surface caused by smoldering sub-surface fires could reach 600.24 °C. Compared with the surface temperature, the sub-surface temperature is higher due to the rapid heat loss of the surface layer, reaching a maximum of 710.70 °C—both far exceeding the critical limit that endangers firefighters’ lives. When the humus moisture content is low, the surface spread rate is much higher than that of the sub-surface, indicating that sub-surface fires occurring at this time are more likely to be detected. While with the increase in moisture content, the sub-surface fire spread rate exceeds that of the surface, which is prone to cause the overhanging phenomenon. Nazaré et al. [20] and Huang et al. [44] also pointed out that the occurrence of the overhanging phenomenon in sub-surface fire combustion is mainly caused by the difference in spread rate between the upper and lower humus layers, as well as the interaction between oxygen supply and heat loss. Therefore, it is recommended to avoid entering the fire scene rashly when an sub-surface fire occurs—temperature changes on the surface and sub-surface caused by smoldering will quickly damage protective equipment and further scald firefighters. It is generally believed that higher fuel moisture content will weaken the fire development trend [45], but for sub-surface fires, it increases the risk of surface collapse. The suppression of sub-surface fires should focus on cutting off sub-surface fuel to prevent further spread and being equipped with high-temperature resistant equipment. Suppression and cooling should be achieved through remote water spraying, and the blocking and cleaning of fuel are recommended to be completed with large machinery.

We found that moisture content significantly affects the key risk factors for sub-surface fire suppression. Fuel moisture content has long been recognized as an important factor influencing the development and spread of forest fires [46]. Of course, fuel density, structure, and other factors can also affect the development and spread of sub-surface fires. However, for simulation experiments, multiple factors lead to more complex combustion, making it difficult to qualitatively and quantitatively analyze and summarize research results [47]. We adopted a controlled experiment method to study the impact of moisture content on hazardous factors. In the future, more factors will be incorporated to conduct more in-depth research on sub-surface fire smoldering. When the humus moisture content is low (3.56%), sub-surface fire smoldering causes surface collapse the fastest. Traditional forest fire suppression only focuses on surface fires, following the principle of “extinguishing fires upon detection” and actively carries out suppression in the early stage of fires under the assumption that the risk coefficient is low. However, for sub-surface fires—especially when the moisture content of dry humus is low—firefighters are prone to “underestimating risks in the early stage and entering rashly,” leading to collapse and casualties. It is recommended that while paying attention to surface open fires, equipment such as infrared thermal imagers are used to monitor surface temperature changes, so as to avoid sudden collapse accidents in the early stage where there are no obvious fire signs.

With the increase in moisture content, the combustion temperature of deep humus is high and decreases slowly. This may be because after the sub-surface fire spreads downward, the collapse of the upper layer allows oxygen to enter, intensifying the smoldering of the lower layer. Meanwhile, the falling debris prevents heat loss, resulting in a high combustion temperature and slow heat dissipation in the lower layer. Humus with higher moisture content consumes a large amount of heat during the dehydration and drying process, and the remaining heat cannot support the complete combustion of sub-surface fuel [24,25], leading to a large amount of remaining sub-surface fuel and slower temperature drop. Moreover, although the collapse occurs later, the higher the moisture content, the higher the surface temperature and sub-surface temperature at the collapse point. It is recommended that for the suppression of sub-surface fires with high humus moisture content, a “thermal shock warning line” should be set around the collapse area. After surface collapse, personnel are prohibited from approaching within 10 min to prevent burns from hot air currents. Additionally, after extinguishing surface open fires, surface temperature monitors and soil auger equipment should be equipped during fire scene cleanup. Attention should be paid to both surface temperature and temperature changes in the near-surface layer (3–6 cm) to avoid misjudging sub-surface risks by focusing attention only on the surface temperature. In particular, sub-surface temperature spot checks should be conducted in areas without surface open fires to prevent reignition caused by “false extinction”.

6. Conclusions

This study focuses on the representative Ledum palustre-Larix gmelinii forests in northern China, quantifying key risk factors during sub-surface fire suppression through simulated ignition experiments and revealing the influence of moisture content on these risk factors. When sub-surface fires occur in forests, both the sub-surface temperature (reaching up to 710.70 °C) and surface temperature (reaching up to 600.24 °C) are notably high, posing serious threats to firefighters’ safety. When the moisture content of humus is low, sub-surface fires are more easily detected, and the risk of collapse is reduced. However, when the moisture content of humus is high, the risk of surface collapse increases. As the moisture content of humus rises, although the time required for sub-surface fires to ignite and for collapse to occur becomes longer, both the surface and sub-surface temperatures at the moment of collapse are higher. Additionally, the collapse of the surface layer leads to sudden exposure to high sub-surface temperatures, posing a secondary risk of injury to firefighters. These findings emphasize that firefighters should not recklessly enter the fire scene to ensure their safety. The primary objectives of sub-surface fire suppression should be “containment and isolation”, and large mechanical tools should be used to complete them. This study also provides new evidence for the research and development of complexity and danger in fire extinguishing.