The effects of humus moisture content on underground fires in a Larix gmelinii plantation

Xiyue Han·Hening Xu·Tiantian Wang·Sainan Yin·Bo Gao·Yajun Wang·Yanlong Shan

Abstract Underground fires are a smoldering combustion with a slow spread rate,low temperatures and no flame.They can last from days to several months,and can even become overwintering fires.They are difficult to find,leading to considerable damage to the forests.The moisture content of combustible fuels is an important factor in the occurrence and persistence of underground forest fires.The Daxing’an Mountains are a hot spot for underground fires in China.This paper looks at the influence of different moisture contents on underground fire characteristics using simulation combustion experiments in the laboratory.The study showed that peak temperature and spread rate fluctuation of humus at different moisture levels increased with humus depth.Peak temperature and spread rate fulctuation of humus at different depths decreased with increased moisture;moisture content and depth of humus had a signifciant effect on peak temperature and spread rate fluctuation;peak temperature at different depths decreased with increased moisture;the spread rate in upper layers increased with moisture content,while the spread rate in the lower layers decreased with increased moisture content.

Keywords Daxing’an Mountains·Underground fire·Moisture content·Peak temperature·Spread rate

Introduction

Forest fire intensity,frequency and persistence have increased significantly in recent years due to climatic and anthropogenic impacts.Forest fires have become a global problem,seriously threatening life and property,and damages economic development and the ecological environment (Liu et al.2010;McClure and Jaffe 2018;Toledo et al.2018).Underground forest fires mainly occur in duff,humus and peat layers (Hartford and Frandsen 1992),so are referred to as humus fires or peat fires (Watts and Kobziar 2013).Underground fire is a smoldering combustion with a slow rate of spread rate,low temperatures and no flame,and may last for days to several months.An underground fire may even result in an overwintering fire and is difficult to detect and to extinguish.Large-scale underground forest fires in Southeast Asia and in northern areas have severely damaged ecosystems and caused widespread air pollution (Page et al.2020;Rein 2013;Huang and Rein 2017).These fires are sustained by the heat released,and moisture in the fuels or combustibles vaporizes when heated and is consumed by the heat (Frandsen 1987;Garlough and Keyes 2011).Therefore,the moisture content of combustibles is an important factor in the occurrence and persistence of underground forest fries(Frandsen 1987;Miyanishi and Johnson 2002;Chen et al.2015).

Wetlands or peatlands with rich organic matter are hot spots for underground forest fires,although underground fires are generally rare in these ecosystems because of their high moisture content which can be up to 300% (Moreno et al.2011;Turetsky et al.2015).However,the frequency,scale and damage by underground fires have increased in recent years (Page et al.2020;Turetsky et al.2015;Walker et al.2019) mainly due to drought caused by nature or humans (Rein et al.2008).The moisture content of combustibles varies with depth and so the moisture content threshold is different depending on the depth.Researchers have found that the moisture content threshold for frie propagation is 57% in the upper layer and 102% in the lower layer (Garlough and Keyes 2011).Smoldering combustion can occur with a moisture content between 3 and 300% at high fire intensities (Finney et al.2013;Jervis and Rein 2016).However,moisture content can affect combustion time,and the possibility of smoldering combustion in humus decreases by 19.3% for every 5% increase in moisture content (Reardon et al.2007).Current studies on the effect of moisture content during underground forest fires are based on using commercial peat or moss and the inhomogeneous texture of humus or other organics in the soil cannot be easily simulated,so the results of laboratory studies still greatly differ from reality (Reardon et al.2007).

Forest fires in boreal forest ecosystems have become more frequent and severe with the intensification of global warming (Ivanova et al.2019).The frequency and damage of underground forest fires will increase due to surface water loss and melting of subsurface permafrost as global warming continues (Hu et al.2018).The Daxing’an Mountains are an important component of boreal forests in China and a hot spot for underground forest fires.This research investigated the combustion of the humus layer of aLarix gmeliniiforest in the Daxing’an Mountains using simulation experiments.The purpose of this study was to determine:(1) the effect of humus moisture content on peak temperatures of underground fires;and,(2) the effect of humus moisture content on the spread rate of underground fires.

Materials and methods

Study sites

The Daxing’an Mountains are a temperate coniferous forest region in the northernmost part of China and the largest forest region in the country.They also span an ecologically sensitive area,being important ecological barriers to the Northeast Plain and the North China Plain and having a special ecological status (Frandsen 1997).The forests of the Daxing’an Mountains are highly fire prone with the largest annual areas burnt (He et al.2020).The study area is on the southeast slope of the Daxing’an Mountains,15 km south of the forest management technology extension station of Jiagedaqi (123°57??124°0?E,50°20??50°23?N).The region has a continental monsoon climate with four distinct seasons and a changeable climate.The temperature differences between the four seasons and day and night are significant.The forest management technology extension station was founded in 1973,and the north and western parts of the station are connected with the Dongfeng Forest Farm of the Jiagedaqi Forestry Bureau.The total operating area of the station is 7326 hectares.The main tree species areLarix gmelinii(Rupr.) Rupr.,Quercus mongolica(Fisch.) Turcz.,Betula platyphyllaSuks.,Populus davidianaDode.,andBetula davuricaPall.(Lu et al.2019).

Field survey

The field survey was carried out in the autumn of 2019.Underground fire occurred in the Tatou wetland-Larix gmeliniiplantation in 2008.Although the area of the fire was only 1.5 ha,it lasted for five days.Humus samples were collected from a 20 m×30 m plot where underground fires occurred;altitude,latitude and longitude,canopy density,and other basic information were recorded (Table 1).

Table 1 Sample plot information

Sample collection and processing

Three 0.5 m×0.5 m quadrats were selected on the diagonal of each sample plot.All humus in the quadrats were excavated after removing the litter layer.Dry branches,leaves and plant roots in the humus were removed,and the humus was put into file bags.The humus samples were dried for 48 h at 105 °C in an electric drying oven and crushed and screened by a standard inspection screen with a particle size ≤ 20 mesh to conduct the combustion simulation.Five moisture gradients (0%,5%,10%,15%,and 20%) were set in this study.Humus and distilled water were mixed and stirred evenly.The samples were sealed and allowed to stand fortwo months to fully mix the water and humus,and the final moisture contents were 0.6%,5.9%,9.1%,16.7%,and 18.6%.

Simulated combustion experiment

The simulated combustion experiment device was a self-assembled underground fire temperature acquisition system(Fig.1).The smoldering combustion furnace was cylindrical(20 cm high,10 cm bottom thickness,10 cm wall thickness,and 10 cm internal diameter).The material was aluminum silicate ceramic fiber,which has a good heat insulation.K-type thermocouples 30 cm in length and 2 mm in diameter were used to measure temperatures during the combustion process.The data were transmitted to the computer through the data acquisition module composed of two 16-channel NI9213 voltage acquisition board cards and a Daq-9174 case(4 card slots) produced by American NI Corporation.The data acquisition software was LabView 2018,which recorded the temperature change curves of each thermocouple.A far-infrared heating plate was used as the ignition device,and a temperature control meter was connected between the plate and power supply to keep the temperature of the heating plate constant.

Fig.1 Schematic diagram of the underground fire temperature acquisition system

Humus at different moisture contents was placed in a smoldering combustion furnace,and three repeated tests were conducted at each moisture content gradient.The side of the smoldering combustion furnace was drilled every 3-cm from top to bottom for a total of six small holes,and K-type thermocouples were inserted into the humus through the holes.The thermocouples and the data acquisition module were connected by compensating wires,and the temperature data transmitted to the computer every 10 s.The far-infrared heating plate was preheated to the set temperature,and the plate placed on top of a smoldering combustion furnace for 3 h at 500 °C.A 2-cm gap was kept between the plate and furnace to ensure air circulation.The heating plate was removed after heating for three hours with the humus maintaining the combustion process.

Data analysis

Statistical analysis of peak combustion temperature and spread rate was performed by SPSS.Statistical significance was accepted atP<0.05,least significant difference (LSD) was used in the multiple comparisons,and simple effect analysis was carried out when there was interaction.Box plots were finished by Origin;exponential equations between the peak combustion temperature or spread rate and moisture content of the underground frie at different depths were ftited;and the coefficient of variation was used to describe the fluctuation degree of the peak combustion temperature and spread rate.

The equation for the spread rate was as follows:

whereVis the spreading rate,Dthe distance between two thermocouples,andT1?T0the time difference between two adjacent thermocouples reaching the maximum temperature.

The equation for the coefficient of variation was:

whereCVis the coefficient of variation,SDthe standard deviation,andMNthe mean value.

Results

Effect of moisture content on peak temperature of underground fires

As shown in Table 2,with increasing combustion depth,the degree of fluctuation of the peak temperature between different humus moisture contents was greater,and the smallest degree of fluctuation was at a depth of 3 cm (CV=1.9%).The largest degree of fluctuation was at a depth of 12 cm(CV=9.4%),the lowest temperature was 570.1 °C,and the highest was 735.5 °C.With increased moisture content,the degree of fluctuation of the peak temperature at different depths decreased gradually,the biggest fluctuation was at a moisture content of 5.9% (CV=18.0%),followed by 9.1%(CV=16.9%) and 0.6% (CV=16.7%) moisture levels;the smallest degree of fluctuation was at 18.6% (CV=10.8%)moisture content.

Both moisture content and depth had a significant impact on the peak combustion temperature of humus,but the interaction between moisture content and depth was insignificant(F=1.55,P=0.159 >0.05).As seen in Fig.2,the highest combustion temperature was at 0.6% (mean=679.0 °C)moisture content level,followed by the 5.9% moisture content level;there were no significant differences between the two.Combustion temperatures were lower at the 16.7%(mean=607.1 °C) and 18.6% (mean=582.8 °C) moisture levels,and were significantly different from the other three moisture levels.The highest peak combustion temperature occurred at the 15-cm depth (mean=811.7 °C),which was significantly different from peak temperatures at the other depths.The second highest peak temperature occurred at the 12-cm depth (mean=665.5 °C),which was also significantly different from temperatures at the other depths.Peak combustion temperatures at 3 cm (mean=563.9 °C),6 cm(mean=562.5 °C) and 9 cm (mean=574.5 °C) depths were similar and not significantly different.

Fig.2 Effect of moisture content and depth on peak temperatures of underground forest fires;there are no significant differences between values that have at least one identical letter but there are significant differences between values that have no identical letter;this also applies in Figs.4 and 5

Peak combustion temperature of humus decreased with increasing moisture content at different depths (Table 3).The fitting curves are shown in Fig.3.

Fig.3 Fitting curves between peak temperatures and moisture content levels of underground forest fires at different depths

Effect of moisture content on the rate of spread of underground fires

As shown in Table 4,the degree of spread rate fluctuation at different moisture levels increased with depth,especiallyat the 15 cm depth (CV=56.4%).The fastest spread rate was 3.9 cm/h,and the slowest only 0.8 cm/h.The degree of spread rate fluctuation decreased with increased moisture at different depths;the variation coefficients were 102.4%and 103.6% with moisture levels of 0.6% and 5.9%,respectively.Spread rates in the upper layer (3 cm) were slower at 0.5 cm/h and 0.4 cm/h,respectively;spread rates in the lower layer (15 cm) were faster at 3.9 cm/h and 3.7 cm/h,respectively.The degree of spread rate fluctuation was lower at an 18.6% moisture content,but the fluctuation degree of the spread rate was greater than that of overall peak temperatures.

Table 2 Variation coefficients of the peak combustion temperature at different moisture contents and depths

Table 3 Equations for peak temperatures and moisture levels of underground fires at different depths

Table 4 Variation coefficients of the spread rate at different moisture contents and depths

Table 5 Simple effect analysis of moisture content and depth on the rate of spread of underground fires

Moisture content,depth and their interaction had significant effects on rate of spread.Simple effect analysis was performed (Table 5).There were significant differences in the spread rate when comparing different moisture content at the 15-cm depth;there were significant differences in the spread rate at the 0.6%,5.9%,and 9.1% moisture levels.

At the 15 cm depth,the fastest spread rate(mean=3.9 cm/h) was observed when the humus moisture was 0.6%,followed by a moisture content of 5.9%(mean=3.7 cm/h);there were no significant differences between the spread rates at these two moisture levels.The spread rates at the 16.7% and 18.6% moisture levels were not significantly different,but both were significantly different from the spread rates at the 0.6% and 5.9% moisture levels(Fig.4).

Fig.4 Effect of moisture content on the spread rate of underground fires

The fastest spread rates were at the 15 cm depth with moisture levels of 0.6%,5.9%,and 9.1%,and mean values of 3.9 cm/h,3.7 cm/h and 2.1 cm/h,respectively (Fig.5).The spread rate was significantly different from those at the other four depths,while the spread rates at the other depths were not significantly different.

Fig.5 Effect of depth on the spread rate of underground fires

The spread rates at 3 cm,6 cm and 9 cm increased with increasing moisture content,but at 12 cm and 15 cm,it decreased with increasing moisture (Fig.6,Table 6).

Table 6 Equations for the spread rate and moisture content at different depths of underground fires

Fig.6 Fitting curves comparing the spread rate and moisture contents at different depths of underground fires

Conclusions

Underground fire characteristics at different moisture contents

It is difficult to study the characteristics of occurrence and spread of underground fires because the smoldering process is complex (Rein 2009).Moisture content,density and organic matter of fuels affect the smoldering process,but the mechanisms and effects (spread speed,heat emission,combustion time) still need to be explored (Rein 2009;Pastor et al.2017).Moisture content of the fuel is an important factor in underground fires and influences fire behavior(Ertugrul et al.2019;Bilgili et al.2019;Zhang et al.2020).This study found the effect of moisture content on peak combustion temperature and spread rate of underground fires by studying the combustion characteristics of underground fires at different humus moisture levels.

With increased combustion depth,the fluctuation of the peak temperature of underground fires varies at different humus moisture contents;the degree of fluctuation of the peak temperature in the upper layer is smaller because the humus is affected due to its proximity to the fire (Ohlemiller 1985).Humus with a higher moisture content has the ability to absorb heat emissions (Garlough and Keyes 2011),so the degree of fluctuation of the peak temperature and spread rate is larger.Dehydration and drying are the first characteristics of underground forest fires,followed by fire spreading (Chen et al.2011;Alexander et al.2012),and with increased moisture content,the degree of fluctuation of the peak temperature at different depths gradually decreases.Humus with a lower moisture content easily dehydrates,while humus with a higher moisture consumes large amounts of heat in the dehydration and drying process.The remaining heat cannot support the complete combustion of humus (Frandsen 1987;Rein et al.2008) so the fluctuation is small.

Effect of moisture content on peak temperature and spread rate of underground fires

Moisture content and depth both have a significant impact on peak temperatures,and the highest peak temperature was detected at a 0.6% moisture content or a depth of 15 cm.Reardon et al.(2007),Ballhorn et al.(2009) and Huang and Rein (2015) also found that moisture content and depth affected underground combustion.Peak temperatures of underground fires at different depths decreased with increasing moisture content,and the results are consistent with Benscoter et al.(2011) and Huang and Rein (2017).With increased moisture content,it is more difficult for underground fires to occur and spread.Reardon et al.(2007) also pointed out that the occurrence probability of underground fires was reduced with increased humus moisture.The interaction of moisture levels and depth had a significant impact on the rate of spread of underground fires.The lowest spread rate was at a moisture content of 18.6% and 15-cm depth.Because the increased moisture in fuels hinder the combustion process,the results are consistent with other studies(Albini and Reinhardt 1995;Wang et al.2017;Cawson et al.2020).

The spread rate in the upper layer,in particular,increased with increasing moisture content.The spread rate of wildfires decreases with increasing moisture in general but He et al.(2014) found that the degree of spread rate fluctuation was not obvious at moisture levels between 3 and 21%.Huang and Rein (2017) also reported that increased moisture content could increase the spread rate of smoldering combustion.In addition to heat emissions,the oxygen content is also a condition to sustain the smoldering (Ohlemiller 1985;Rein 2016).Sufficient oxygen and a sufficient heat source in the upper layers and moisture content (Watts 2013)could lead to an increase in the spread rate.At the same time,the density of humus decreases with increasing moisture.Hartford (1989) and Garlough and Keyes (2011) found that a lower combustible density increased the possibility of combustion.The spread rate in the lower layers decreased with increasing moisture.Ash content of the upper layer gradually thickened to generate a thermal insulation layer with increasing combustion depth and obstructed oxygen(Grishin and Yakimov 2010).The increase in moisture content consumes large heat and decreases the oxygen content(Possell and Bell 2013),thereby decreasing the spread rate in the lower layer.

Prevention and countermeasures for underground fires based on moisture content

Humus accounts for most forest fuel (Finney and Martin 1993) and smoldering combustion seriously damages roots,leading to plant death and soil erosion (Varner et al.2005).The large-scale Tatou wetland-Larix gmeliniiplantation in the Daxing’an Mountains is a hot spot of forest underground fires under long-term drought conditions.The forest can maintain humus moisture at a high level and keep the ground moist to decrease the probability of underground fire if artificial precipitation,aircraft sprinkling,surface spraying and other methods are used.There is no significant difference in the combustion temperature of shallow underground fires at different moisture contents,and the small amount of water might increase the rate of spread of underground fires.Therefore,continuous and long-term use of highpressure water syringes on the humus could abate shallow underground fires.The fire ground should be entered cautiously when the underground fire has spread to lower layers because the high temperatures of deep underground fire will harm fire fighters.Fireproof ditches are a better method to prevent the spread of underground fires (Davies et al.2013),and by injecting water into these ditches,underground fires will be gradually extinguished because of the decrease in temperature and the spread rate with high moisture content.