Inf luence of f ire on soil temperatures of pine forests of the middle taiga, central Siberia, Russia

Irina Nikolaevna Bezkorovaynaya ·Pavel Albertovich Tarasov · Irina Gennadievna Gette ·Irina Andreevna Mogilnikova

Abstract Temperatures of sandy podzols of middle taiga pine forests with moss and lichen ground cover were analyzed which had been exposed to ground f ires of low to medium intensity. In general, temperatures in lichen and moss plots of the pine forests under study, are close to each similar, but in the f irst year after a f ire a noticeable contrast was observed. The reasons are an increase in the amplitude of daily temperatures on the soil surface and stronger heating of upper mineral layers. Temperatures in the mineral layer with depths up to 30 cm depend on the thickness of the forest f loor. Analysis of the results show that the duration of postf ire Effects in pine forests with sandy podzols is determined by a number of factors: the intensity of the f ire, the degree of erosion of the ground cover and litter, and the recovery rate of these components.

Keywords Ground f ire · Pine forests · Soil temperature

Introduction

The currently f ixed steady surge in surface temperature is one of the leading factors determining not only the condition and productivity of forest ecosystems, but also the frequency and range of forest f ires. In this regard, recently collected data conf irm the trend of increasing number of f ires and their frequency in mountainous forests in Central Siberia(Loupian et al. 2006; Ponomaryov and Kharuk 2016).

Fires are considered a powerful and inf luential environmental factor in soil formation as they have a complex and multifaceted impact on ecosystem processes such as erosion,subtraction (runoff ) of organic matter and plant succession(Baldock and Smernik 2002; Certini 2 005, 2014; Cerdà and Doerr 2008; Guénon et al. 2 013). Numerous studies of the inf luence of the pyrogenic factor on soil systems distinguish two types of this inf luence (Sharrow and Wright 1977; Bezkorovaynaya et al. 2007; Tsibart and Gennadiev 2008; Krasnoschekov and Cherednikova 2012; Smits et al.2016; Dymov et al. 2018; Deviatova et al. 2019): (1) pyrogenic transformation or the combustion by heat of soils as a result of direct pyrolysis, i.e., change of physical and chemical properties, strengthening of processes of mineralization of organic matter, increasing the amount of water-soluble compounds, decreasing acidity, decomposition of aluminosilicates, changes in granulometric composition, modif ied water and thermal modes; and, (2) pyrogenic transformation of soil formation factors, i.e., creation of a secondary postf ire relief resulting in the complexity and microcomplexity of soils, a lower level of perennial permafrost, swamping,natural draining of swamps, erosion, changes in the nature of the accumulative process, and the intensif ied processes of eluviation-illuviation.

After a f ire, there is an increase in ash content (Pereira et al. 2013) and leaching of nutrients into deeper soil horizons (Bodi et al. 2014), compaction of the upper mineral layers and their structural transformation due to sintering or compaction of f ine fractions into dense, stable aggregates(Ulery and Graham 1993; Ketterings et al. 2000; Mataix-Solera et al. 2011; Dymov et al. 2018). Such post-pyrogenic changes in soil structure also alter thermal properties (Certini 2005; Smits et al. 2016). In the f irst years following a f ire, there are temperature increases, especially in the upper mineral layers (Vermeire et al. 2005; Bezkorovaynaya et al.2007; Santana et al. 2010; Krasnoschekov and Cherednikova 2012). A rise in post-f ire soil temperatures is a signal for biological processes to become activated (Bezkorovaynaya et al. 2005; Allison et al. 2010; Guénon et al. 2013) and can stimulate the vigorous growth of seedlings, playing a central role in the recovery of plant communities (Santana et al. 2010).

In spite of the aforesaid common features, the impact of f ires on soil properties vary and may depend on physiographic conditions, forest and habitat types, initial soil properties, as well as f ire type and its intensity (Conard and Ivanova 1997; Bezkorovaynaya et al. 2015; Dymov et al.2018).

One of the limiting factors for the development and functioning of boreal forests is soil temperature. The temperature of the active soil layer is directly dependent on the degree of development of the thermo-insulating moss-lichen cover and the thickness of the forest f loor (Prokushkin et al. 2002;Tarasov et al. 2011). Against the background of an increased frequency of forest f ires and shortened intervals between f ires in the territory of Siberia (Ponomaryov et al. 2019),there are a number of issues to consider concerning the emergence of the soil temperature regime in the post-f ire period; precisely how much the heat supply of the active soil layer changes, how long the post-f ire Effects remain, and what they depend on.

The purpose of this study is to analyze and assess the dynamics of soil temperatures in post-f ire pine forests growing on sandy podzols in the Central Siberian middle taiga.

Materials and methods

Study area

Research was carried out on the Ket-Sym lowlands on the left bank of the Yenisei River in the middle taiga pine forests on sandy podzols. The relief of the area is a chaotic alternation of f lattened hills, walls, hillocks with short, shallow ravines, rill channels and shallow gullies. The climate is continental with temperatures reaching 90-95 °C. However,this is a temperate cold humid area and the sum of temperatures above 10 °C is 800-1200 °C; Selyaninov’s hydrothermal coeffi cient is 1.2-1.6 and characterizes the level of moisture and precipitation values in the territory, calculated as the ratio of the sum of precipitation (mm) for a period with average daily air temperatures above 10 °C to one tenth of the sum of temperatures for the same period. Depending on landscape features, the average annual temperatures vary from − 5.4 to − 3.1 °C, and annual rainfall from 400 to 600 mm. Average minimum and maximum air temperatures are −48 °C and + 31 °C, respectively. The area covered by forest is 84%, with a large area occupied by swamps (10.5%).Pine forests with moss and lichens account for 40% of the area. The frequency of f ires in the forests under study is high, with a maximum number in June. In summer, this frequency is conditioned by long, dry periods during which forest areas are subject to extensive f ires. Therefore, there are numerous, large f ires at this time (Valendik 1995). For the landscapes of the West Siberian Plain near the Yenisei River, the average inter-f ire period is 57 years, and for the Central Siberian middle taiga subzone, it varies from 25 to 40 years (Furyaev 1996).

Experimental sites are post-f ire and close to other areas with lichen and moss pine forests(Pinus sylvestris-Pleurozium+Cladoniassp.): 1, 2, 4, and 5 years after the f ire (60°38′ N 89° 41′ E); 8 years after the f ire (60° 47′ N, 89° 21′E). All were ground f ires of medium and low intensity. The control included pine forests which had not experienced f ires(80-100 years). All areas are similar, average diameter and height of the stems were 25-30 cm and 17-20 m, respectively. Because of the thick bark at the base of the trunks,f ire did not have an impact on tree canopies. The mapping of the canopies showed that the f ires in this area were the same as the unburned control area and reached 8000 m 2 per 1 ha.

The surface of all sites is f lat with a small number of rises 30-40 cm high and potholes of the same depth. Moss,Pleurozium schreberi(Brid.) Mitt.with lichensCladina rangiferina(L.) Nyl. andCl. alpestris(L.) Rabenh., dominate in the ground cover. The soil is homogeneous, represented by sandy podzols on alluvial f ine non calcic sand.The prof ile of the podzols is diff erentiated into horizons:O-E-BF1-BF2-BF2-C (Bezkorovaynaya et al. 2005). The forest f loor is heterogeneous due to its transformation of its constituent material and contains small coals. All mineral horizons consist of f ine sand with the distribution of clay by prof ile, which is typical of podzols. The minimum clay content is in the podzol horizon E (4.3%), and the maximum in the upper part of the illuvial-iron podzol BF1 (8.4%).The soil has low humus content (up to 0.5%) and accessible forms of nutrients (5-10 mg/kg), as well as unsaturated bases and high acidity (pHКC1less than 4). These indicators decrease markedly down the prof ile. The extremely low content of f inely dispersed fractions results in a low absorption capacity of the podzol (4-5 m mol/100 g) which contributes to the rapid downward movement of soil formation products.

Methods

At each experimental site, at randomly selected points in moss and lichen plots, forest f loor thickness and stock were measured. Forest f loor samples were taken from a 25 cm × 25 cm area with a metal frame (n = 10) overlaid onto the forest f loor and above-ground parts of plants were cut with scissors. The forest f loor was then cut with a knife along the inner edge of the frame; the thickness of the forest f loor was measured along each of the four sides and the sample packed into a cloth bag. In the laboratory, the samples were dried at 90-100 °C and weighed. Density was calculated by dividing mass by volume, the derivative of the frame area multiplied by thickness of the forest f loor. The results were processed by standard methods of mathematical statistics.

Using the values of thickness and density of the forest f loor, the areal stock was calculated according to:

whereM-forest f loor stock, g/m 2 ;dy-density of forest f loor,g/cm 3 ;h-thickness of forest f loor, cm.

Ash content of forest f loor was calculated according to formula 2:

where A is the ash content, % of absolutely dry mass; OMcontent of organic matter, % of absolutely dry mass.

The organic matter content of the forest f loor was determined using the calcination method. A porcelain crucible was f illed with chopped forest f loor material and weighed,and calcined (oxidized) in a muffl e furnace at 800°C for 1 h. The crucible was cooled in a desiccator with calcium chloride and weighed again. The organic matter content was calculated using formula 3.

whereOM-the content of organic matter, % of mass of dry sample; m0-mass of an empty crucible, g;m1-mass of a crucible with air-dry mass, g;m2-mass of a crucible with calcined sample, g; CH2O-coeffi cient of soil hygroscopy.

The hygroscopic factor was determined by the thermal balance method using a glass weighing bottle f illed with a crushed sample of air-dry forest f loor material and weighed.The bottle was dried to a constant weight at 105 °C, cooled in a desiccator with calcium chloride and weighed again.The hygroscopic moisture content of the sample (formula 4)and the coeffi cient of the forest f loor hygroscopy (formula 5) were then calculated:

whereW his the hygroscopic moisture content (weight), %of the dry sample mass;m0the mass of an empty weighing bottle, g, m1the mass of a bottle with an air-dry sample, g;and m2is the mass of a bottle with a dry sample, g.

where CH2O-coefficient of hygroscopy;W h-hygroscopic moisture content (weight), % of dry sample mass.

An M-69 mobile albedometer was used to calculate the albedo in relation to the forest f loor thickness on experimental sites.

The soil temperature was measured with a mobile soil thermometer in lichen and moss plots at noon on the surface of the forest f loor, in the middle part of the litter and in the mineral layers at depths of 5, 10, 15, 20, and 30 cm. In total,10 measurements were made in each plot. All measurements were made in the f irst half of August.

To measure the surface temperatures of the forest f loor,maximum and minimum thermometers were used; they were and set at a distance of 5-6 cm from each other from west to east. The minimum thermometers were placed horizontally on the surface, while the maximum thermometers were placed with a slight inclination.

For each variable, the average value ± SE was calculated.Comparison of the main forest f loor characteristics and temperatures in moss and lichen plots in the control and post-f ire areas was carried out using t-value Student test. The conf idence probability wasP= 0.95.

Results

In forest ecosystems, the temperature of the active soil layer is directly related to the extent of development of the thermo-insulating moss-lichen cover and the thickness of the forest f loor (Sharrow and Wright 1977; Prokushkin et al. 2002; Ponomaryov et al. 2019). The non-uniformity of the typical soil cover of the pine forests of this area(Kovaleva and Ivanova 2013) is responsible for the diff erences in the reserves and forest f loor density of moss and lichen plots. In the control area, lichen plots have higher density (0.083 g m −3 ), reserves (4150 g m −2 ) and ash content(23.4%) (Table 1). However, the diff erences in these indicators with moss plots are not reliable (P= 0.6).

Table 1 Post-f ire transformation of basic forest f loor parameters with diff erent plots in lichen-moss pine forests a year after the f ire(mean ± SE)

The same holds true for most post-f ire parameters of the forest f loor one year after the f ire. Thus, their analysis revealed a fall in stocks, as well as an upsurge in ash content and litter density in both plots (Table 1), yet reliable differences (P= 0.95) can be found only in litter density. This stems from the specif icity of combustion in lichen plot, as a result of which the forest f loor “contracts”, which signif icantly increases its density (Table 1). In addition, compaction of the forest f loor after running ground f ires is caused by burning of the topmost and loosest layer, and also by essential increase in ash content.

The thickness of the forest f loor and its density is of particular importance in the processes of heat exchange between the soil surface and mineral layers. In the control area, the average thickness is 3.9 cm for lichen and moss plots. Following combustion, part of the forest f loor loses its thickness by more than a third in the f irst two years after the f ire,but by eight years after, it approaches its original thickness(Fig. 1).

Due to the pyrogenic transformation of the forest f loor,which plays an important role in the heat exchange between the atmosphere and soil, the temperature conditions of the latter change noticeably.

Fig. 1 Dynamics of forest f loor thickess in lichen and moss pine forests in the post-f ire period (average values of forest f loor thickness are given due to the absence of reliable diff erences between the forest f loor of moss and lichen plots)

It is f irst manifested in the contrast of the temperature range on the surface of the burned areas. On one hand, this derives from a signif icant reduction of albedo from 18-20%to 10-13%, and on the other, from more active heat radiation at night according to the Steff an-Boltzmann law. As a result, daily temperature f luctuations on the surface f loor of the pine forests exposed to f ires exceeded 40-50 °C on some days (with maximums of 44-55 °C, and minimums of 2.4-6.5 °C), while in the control area these indicators were 25-35 °C; 28-36 °C and 4.5-10 °C, respectively. Thus, surface temperatures of the burned areas show sharper contrast after the f ire.

The analysis of post-f ire temperature dynamics of the forest f loor and upper mineral layer in the 5 cm layer measured in the f irst part of August showed that, within the f irst years after the ground f ire, there was an average temperature increase by 1.5-2 °C (Fig. 2). A year after the f ire, higher surface temperatures coupled with an increase in thermal conductivity of the forest f loor add to greater heating of the mineral layer. As a result of these changes, one year after the f ire, the average temperature diff erence of the upper 30-cm layer compared to the control does not exceed 2 °C.This diff erence gradually declines with depth (Fig. 3), and maybe explained by the inf luence of two factors: the f irst is an increase in albedo of the burned surface close to initial values caused by the restoration of the live ground cover and the abundant fall of post-f ire needles; the second is an increase in the thickness of the forest f loor which reduces of its thermal conductivity.

Fig. 2 Dynamics of soil temperature in the f irst years after the f ire(measurements were made in the f irst part of August). Average temperatures for moss and lichen plots, n = 10

Fig. 3 Soil temperatures of various plots in the f irst decade of August, °C, n = 10

It can be assumed that, due to the high thermal conductivity of sand podzols (Gael and Smirnova 1999), their highest warming in a post-f ire period can reach a depth of about 1-m. Such a depth was noticed in the research on increases in soil temperatures of burned areas in pine forests on sandy soils of the Transbaikalia and Altai regions (Evdokimenko 1979; Bekhovykh 2002).

Comparison of soil temperatures in moss and lichen plots has shown that lichens before and after the f ire heat more,but the temperature diff erences between moss and lichen plots are not reliable (P< 0.95).

Eight years after the f ire, due to the restoration of the live ground cover that shades the surface and to an increase in the forest f loor thickness because of litter fall, temperature differences between the controls and burned areas have levelled out and account for tenths of a degree.

Other researchers have also pointed to a post-f ire increase in soil temperatures (Sharrow and Wright 1977; Vermeire et al. 2005). The persistence of such a post-fire effect depends on the development and the speed of regeneration of vegetation cover and the forest f loor and on the physical properties of soil. For example, in the f irst 12 years after a f ire in the northern larch forests of Central Evenkia (Russia), temperatures in the forest f loor and at a 10-cm depth of the mineral layer increased by 10-15 °C and only after 25 years did they correspond to pre-f ire values (Prokushkin et al. 2002; Bezkorovaynaya et al. 2015).

Analysis of the data in this study revealed that forest f loor temperatures are vaguely dependent on its thickness, while the inf luence of the thickness on temperatures of the mineral layers is more pronounced and increases with depth (Fig. 4).

Conclusion

Changes in the temperature regime of sandy podzols in pine forests exposed to ground f ires are conditioned by pyrogenic transformation of the ground cover and forest f loor,which exert a signif icant impact on the processes of heat exchange between the atmosphere and the mineral layers.These changes result in increased contrast of temperature conditions on the soil surface and greater heating of upper mineral layers.

This research has shown certain diff erences in soil temperature indicators for lichen and moss plots, which are different in their structure. To a considerable extent these diff erences are attributed to these biotopes determining the degree of pyrogenic impact on the forest f loor, and on the living ground cover and their subsequent post-f ire restoration.

Post-pyrogenic changes of temperature conditions in soils of lichen and moss pine forests cannot be estimated unequivocally. On the one hand, the sharpened contrast of temperature conditions of the surface of the burned areas makes their natural renewal more diffi cult, and on the other hand, better warming of the soil promotes the activation of many important physiological processes contributing to soil development.

Fig. 4 Relationship of soil temperatures at diff erent depths with forest f loor thickness

The temperature regimes of sandy podzols in the middle pine forests change insignif icantly after ground f ires of low and medium intensity. Positive change in heat supply of the active soil layer is observed only within the f irst two years after the f ire, and in eight years, it approaches its initial. Thus, the duration of post-f ire inf luence on temperature conditions of sandy podzols under lichen and moss pine forests is regulated by the intensity of f ire inf luence, which sets the extent of pyrogenic damage to the living ground cover and forest f loor, and also determines the speed of restoration of these components.

Acknowledgements The research was supported f inancially by the Russian Foundation for Basic Research, the Government of Krasnoyarsk Krai, and the Krasnoyarsk Regional Foundation for Science as a part of the scientif ic Project No. 18-44-243007 “Evaluation of stress proteins content and photosynthesis intensity of the pine needles(Pinus Sylvestris)in the post-pyrogenic period in the Krasnoyarsk foreststeppe”; Grant of the East Siberian Oil and Gas Company aimed at supporting scientif ic research of applied importance in 2020.

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