Dynamics of Vegetation and Soil Cover of Pyrogenically Disturbed Areas of the Northern Taiga under Conditions of Thermokarst Development and Climate Warming

Abstract

Vegetation and soils of the North Taiga zone were studied in natural and thermokarst-disturbed areas of Yana-Adycha interfluve (northeastern Yakutia). Soil research includes a description and physicochemical analysis of samples. The objects of study were selected taking into account the landscape diversity of the area experiencing permafrost melting due to pyrogenic factors under global climate change: young thermokarst and taiga untouched by fires and within the thermokarst basin of early Holocene. It was determined that the permafrost melting is accompanied by the transformation of homogeneous soil cover. After a forest fire, thawing depth increases and occurs redistribution of moisture and water-soluble matters. As a result, on the drier tops of byllars, the formation of albic material under the organogenic horizon is observed in the calcic cambic cryosol, which indicates a fairly fast transformation rate. In depressions, the forest is not recovered. In the mature alas, the vegetation and soil cover has a belt structure, represented by a combination of cryosols, stagnosols, and gleysols. In contrast to the soils of the Central Yakutia alases, there are almost no signs of lacustrine redeposition of soil, which indicates a difference in the processes of alas formation in different parts of the cryolitozone.

1. Introduction

The permafrost zone is one of the most extensive components (landscapes) of the Earth and occupies about 20% of the land surface, including about 16.7 million km² in Eurasia [1,2]. According to scientists, cryolithozone contains about 33% of the world’s organic carbon in its thickness, which, during permafrost degradation, can affect the global carbon balance [3,4,5,6]. This study was carried out in the northeast of Eurasia, on the interfluve of the Yana and Adycha rivers (Figure 1). The entire territory is located in the zone of continuous permafrost with a wide development of ice complex (IC). The IC here is represented by thick syngenetic ice wedges in frozen loess-like loamy deposits of the Late Pleistocene. These deposits are often called “yedoma” in the literature [7,8,9]. Yedoma is an open and vulnerable system where, as the ground temperature rises, the ice begins to melt, which leads to the degradation of the soil cover and relief [10,11,12,13].

The thermal erosion of the yedomas IC, as a rule, is associated with physical soil erosion and redeposition of soil material and proceeds relatively quickly, leading to the formation of young thermokarst landscapes and the changes in the water balance of permafrost territories [6,14,15]. Thus, thermokarst destroys the natural soil profile, cardinally changing the flows of moisture, energy and carbon [16,17,18,19,20,21,22]. In the process of ice melt, in addition to releasing of previously stored in permafrost additional moisture, a deep thermokarst subsidence occurs over melting ice wedges [13,23]. The depth of subsidence of the land surface can vary from less than a meter to several tens of meters, and the areas of separate objects ranges from several square meters to hundreds of square kilometers [18,24]. The thermokarst process that has begun, can proceed without the influence of external forces fueled by a series of positive feedbacks, such as water accumulation, albedo, and an increase in soil temperature [13,25,26,27]. This process is unlikely to reverse under current climatic trends. The degradation of the yedomas IC will continue until the complete depletion of ice wedges [11,28,29,30].

On the upland of the Yana-Adycha interfluve, the northern taiga is located on soils with IC, the soil surface is complicated by cryogenic forms of micro- and macrorelief, ranging from permafrost cracking with the formation of polygons and rounded hillocks (byllars) to giant ravines and ancient vast alases.

Under modern conditions of climate warming, the degradation of permafrost is affected by an increase in air temperature, a change in precipitation and increased fire activity in the northern territories [31,32,33,34,35,36,37]. Thus, in the continental part of Yakutia, forest fires becoming one of the main factors contributing to the thermokarst degradation of permafrost [33,38,39]. Vegetation cover and soil organic layer can be partially or completely destroyed, because depending on the intensity of fire and leads to an increase in soil temperature and the deepening of active layer [15]. If the positive temperatures reach ice wedges, then start their melting and soil deformation begin. The main regularities and greatly varying rates of surface changes are associated with complex interactions of factors such as climate, terrain, soil formation, structure of IC, the nature and composition of soil-forming rocks, hydrology, and vegetation, which change over the time [6,30,39].

As in the entire subarctic permafrost zone, on the Yana-Adycha interfluve, the thermokarst subsidences of the Holocene age (8000–4500 years ago) are common [40]. The last Quaternary glaciation during the Sartan stade was limited to river valleys within the Chersky and Orulgan ranges to the east and west of the Yana upland [41]. Climate warming during the Pleistocene-Holocene transition caused the reduction in glaciers and the local development of thermokarst. Temperature changes at the beginning of the Holocene are indicated by data on vegetation reconstruction. According to palynological studies, the vegetation of the region during the Late Pleistocene is characterized by the predominance of herbaceous plants (Cyperaceae, Poaceae) and a fairly high content of Betula sect. Nanae and Salix pollen. Moreover, the amount of dwarf birch pollen increases up the section, while that of willow, on the contrary, decreases [42]. As the sampling depth decreases, the pollen composition is characterized by a higher pollen concentration and a higher content of pollen grains from Betula sect. Nanae and Alnus fruticosa, by the presence of Pinus and Picea pollen and some Larix gmelinii, now found much further to the west [40,42]. All this suggests that during the beginning of the Holocene, changes in the flora, which indicates warming, were observed. Sampling core diagrams in Chukotka show a rather well-defined Holocene climate optimum (8000–6000 years ago), known as the Atlantic, which is characterized by the dominance of alder pollen [42]. During this period, the northern border of the forest moved far to the north everywhere [43,44]. In numerous thermokarst depressions around the lakes, the vegetation was represented by forb-sedge-herbaceous associations with the participation of sphagnum mosses [42].

Thus, the Holocene optimum marked beginning of the IC melting and development of thermokarst over a vast area. Fluctuations in climatic conditions during this period led to the extensive development of thermokarst, caused by the melting of icy permafrost deposits and subsequent subsidence of the landscapes surface [12,45,46]. During the formation of alas depression, the original soils were mixed, redeposited, changing their morphological structure and physicochemical properties [47]. The development of alas soils on lacustrine-marsh deposits in mature thermokarst depression led to the growth of azonal vegetation (bogs, mesophytic and xerophytic meadows) in alas.

In Eastern and Northeastern Siberian taiga zone with permafrost, about 30 thousand forest fires occur annually on the area of approximately 5 million hectares. The Republic of Sakha (Yakutia) is among the regions with a high density of fires, where more than 600 fires are recorded per year. The forest fires quantity here are almost two times higher than the average forest fires in Russia. In the summer of 2021, the area of fires in Yakutia reached catastrophically record values—about 3.6 million hectares of forest burned down, which was a consequence of climate change and high summer temperatures.

On the territory of the Verkhoyansk region, forests occupy 17.2 million hectares. Thus, during the period from 1955 to 2011, a 967 forest fires occurred on the territory of the Verkhoyansk forestry, 369.4 thousand hectares of forests burned down. On average, 14 fires are recorded here annually, the average annual number of burned areas is 487 hectares [48].

The soil cover of the northern taiga zone has been poorly studied, not to mention the transformation of soils due to natural and anthropogenic processes. Soil formation here is extremely slow due to the severity of the climate and the short warm season. In this area, major changes in the soil cover due to thermokarst transformation occurred in the Holocene. Recently, due to climate change, increased anthropogenic influence, and more frequent fires in the northern taiga, modern changes in landscapes and soils are taking place. This paper considers changes in the vegetation and soil cover of byllars formed after a forest fire 20 years ago on the yedoma landscapes of the northern taiga in comparison with the soils of the zonal taiga and thermokarst basin formed in the boreal period of the Holocene. Thus, this study cover intact, intermediate (byllars) and final stages (alas) of thermokarst transformation of the landscape and soils of the northern taiga.

2. Research Object and Methods

2.1. Objects and Conditions of Research Area

The Yana-Adycha interfluve is located in the northeastern part of Siberia (Figure 1). The region is a part of vast area of extremely ice-covered cryogenic-aeolian deposits of Pleistocene glaciations distribution [49,50,51]. These deposits are the product of the accumulation of dusty particles brought by winds to the plains of the extraglacial zone during two cold epochs of the Pleistocene—in the Zyryan and Sartan stade. Cryogenic-aeolian deposits are represented by clay loess-like siltstones. They have a thickness of up to 92 m [52,53]. The main feature of the “yedoma ice-loess formation” is the cover nature of these deposits, which are distributed not only on terraces of different heights but also on interfluves. In the territory under consideration, the loessoids overlap watershed areas starting from low-mountain watershed slopes and up to altitudes of 500–550 m a.s.l. The capacity of alluvial deposits of different ages along river valleys does not exceed 60 m.

On the interfluve area, along the line of Batagai and Betenkes settlements, the altitudes of the watershed are 270–320 m a.s.l. The structure of the geological profile and the stratification composing the watershed were uncovered and studied on the example of Batagaika mega slump to a depth of more than 80 m. The uppermost layer of the section is a modern soil stratum (up to 1–3 m). The section of the slump wall consists of two contrasting thicknesses of loose sediments with different composition: the upper stratum is loamy (total 4 units), dissected by stratified loams, ice-rich; the lower stratum is yellowish-gray fine-grained sandy, underlain by dark gray sands, with a massive cryogenic texture [40,54,55].

The upper layer of the IC has a thickness of 20–44 m, according to Vasilchuk Yu.K. et al. [56] up to 30 m, the formation is dominated by syngenetic ice wedges of several meters wide, enclosing columns of sand with a diameter of 1 m to several meters. The age of the deposits based on the samples of plant remains was determined within the limits of MIS-3 which is27–34 thousand years [57] and 24.8 to 38.3 thousand years [58]. The upper layer of the IC located on sandy deposits with a thickness of 20–38 m. High and narrow syngenetic ice wedges (<0.5 m wide) with tube-like morphology located at a distance of ~1–4 from each other are present in the composition of the layer of sandy sediments throughout the thickness [40]. A layer of the lower IC has a thickness of 3–7 m, according to Vasilchuk Yu K. et al. [56] up to 15 m. The horizon has an interglacial status [40]. The age of this stratum is estimated as MIS-5 and MIS-7 with the age of 243–191 thousand years ago. The capacities of the selected layers do not coincide for different researchers because locations of profiles in different parts of the Batagai slump.

The climate of the studied territory belongs to the subarctic climatic zone, where the temperate air masses prevail in summer and Arctic air masses prevail in winter [59]. The air temperature at the Batagai meteorological station, located 29 km west of the studied area, has been observed since 1949, but precipitation data have very large measurement gaps (Figure 2). The coldest month of the year is January with an average long-term temperature of −43.4 °C, and the warmest month is July. Its temperatures varied from 11.6 to 21.9 °C with an average of 15.4 °C during the measurement period.

The average annual air temperatures for 74 years of measurements vary widely from −16.4 to −10.2 °C [60]. Since the observation (1949), there has been steady but more or less gradual warming until 2000. The average trend of air temperature increase during this period varies from −14.4 to −13.1 °C. It means a warming of 1.3 degrees for this period (1949–2000). However, since 2001, the average warming trend has been going up sharply, increasing from −13.2 to −11.1 °C. It means the increasing by 2.1 degree over the past two decades (Figure 2).

The average annual precipitation is about 194 mm/year. January, February and March are considered the driest months, which are characterized by precipitation of about 5 mm/month. Most of the precipitation falls in July—on average up to 37 mm [61].

Thus, the climate warming over the past 20 years on the yedoma area in combination with forest fires can cause irreversible changes not only in the soil cover but also in the entire landscape. An example of this is the Batagai mega slump formed after logging in the 1960s. In the 1970s, after forest fires, the sinkhole began to rapidly increase in size and by 2019 it reached an area of 81 hectares [54,62]. In our opinion, an abnormal increase in the average annual temperature to −11.0 °C (with an average of −13.7 °C) played an important role in accelerating the melting of ice in 1975 (Figure 2). It led to a sudden increase in the heat supply of soils and to an increase in the thawing depth. With increasing global warming, the average annual temperatures over the past 20 years have reached −12.1 °C with a maximum in 2020 (−10.2 °C). In July 2001, the average monthly temperature was anomalous 21.9 °C while long-term average is 15.4 °C. Such increase in air temperature provoked the occurrence of forest fires in the study area, which led to the thermokarst transformation of vegetation and soil cover considered in this work. In the same 2001 year, only 143 mm of precipitation falls, it is less than the annual for 27% (Figure 2).

According to the complex of natural and botanical features, the territory of the Yana-Adycha interfluve belongs to the Verkhoyansk district of the Northeastern sub- province of the North Taiga forest subzone [63]. This subprovince is dominated by Larix cajanderi. Mayr. According to the scheme of floristic regionalization of Northeast Asia, developed by B.A. Yurtsev [64], the studied area is included in the Orulgan-Moma subprovince of the Verkhoyansk province of boreal floristic region. In the flora of the Batagay vicinity, the predominance of species of the Eurasian fraction was noted, which is unusual for the hypoarctic floras of North Asia. Species of the boreal fraction dominate significantly, which also did not occur among the studied floras of the Far North [65]. Here the share of Arctic species is the smallest (7%) and the species of the meadow-steppe complex are widely represented (along the sides of the Yana River valley) [66]. In the basins of the Lena, Yana, Indigirka River, etc. the steppe areas are distributed in small hearth [67]. These landscapes belong to the relics of the Late Pleistocene epoch [68].

According to the soil-geographical zoning of Russia, the experimental site is located within the Verkhoyansk soil province of cold and very cold permafrost soils of the East Siberian permafrost-taiga soil-bioclimatic region [69]. This is the zone of gleysols, rendzic leptosols, calcaric cambisols, and calcic cambic cryosols distribution. Soil-forming rocks in the river valleys are alluvial gravel-sandy-sandy loam deposits, on the flat watersheds of the rivers—loess-like loamy deposits (loessoids), in the mountainous part—redeposited weathering products of dense rocks. Due to the climate severity, the degree of weathering and transformation of the source rocks minerals is low [70]. Permafrost is widespread throughout the all province [71]. The maximum depth of frozen rocks near the Batagai village, which is a geographically part of the Verkhoyano-Kolyma mountain country, is 350 m with an absolute altitude of 30 m [72].

Thermokarst processes have started in the study area as a result of seasonal thawing increase due to the fire occurred in 2001, which destroyed the natural buffer in the form of forest cover. On the places where the fire occurred, the initial forms of thermokarst degradation—baijerakhs, were formed within 20 years. The polygonal microrelief of baijerakhs includes elevated mounds—remnants of the original relief and the depressions between them formed due to the melting of ice wedges. In the future, under certain conditions, such areas evolve into thermokarst depressions and lakes.

The study objects were the vegetation and soil cover of two sites: No. 1—forest with an adjacent disturbed area with initial forms of thermokarst (Figure 3a); No. 2—extensive ancient alas (Figure 3b).

To compare the changes occurring during the thermokarst degradation of the northern taiga, the study of vegetation and soil cover was carried out taking into account the forms of meso- and microrelief. On Site No. 1, on the burns after the fires of 1999–2001, the intensive melting of ice led to the formation of polygonal microrelief. On this territory, vegetation and soil cover have been studied in the forest untouched by fires, at the top of byllar and in a depression between byllars. It should be noted that the successional processes for the restoration of vegetation cover were quite active here, as evidenced by the relatively dense larch moss-and-vaccinium young growth on byllars. The control soil profile was located in a section of the larch forest untouched by fire (profile F-1-20). On the post-fire elements of the microrelief disturbed by thermokarst, soil profiles are made at the top of byllar under young larch growth (profile B-1-20), and in the depression between byllars (profile D-1-20) (Figure 4).

The following three soil profiles are located within the ancient thermokarst basin—alas. The location of control points was chosen taking into account the water regime, relief, and vegetation changes. Profile A-1-21 was located on the waterlogged sedge-cotton-reed grass meadow. Profile A-2-21 is just below the level, on sedge-tussock meadow. Profile A-3-21 is described on the highest and driest area under dwarf birch thicket and willow with some larches.

2.2. Methods

At both sites, the method of trial plots was used [73]. On Site 1, two trial plots were established in the forest phytocenosis: in vaccinium larch forest and a moss-vaccinium larch young growth, each with an area of 20 × 20 m. Descriptions of trees, undergrowth, and living ground (grass and moss) cover were carried out on the site. The species composition of the tree stand was identified, the diameters were measured at a height of 1.3 m, the heights of 20–25 trees were measured, and the age of the trees was determined. Each type of tree has a tier marked (I, II, III…). On Site 2, inside of thermokarst depression, three sample plots 5 × 5 m was made: dwarf birch, sedge-tussock and cottongrass-reed grass-sedge meadows. The projective cover of higher plants was assessed using the 7-point Brown-Blanque cover class: class 5—projective cover of the species over 75%; class 4—projective coverage from 50% to 75%; class 3—projective cover from 25% to 50%; class 2—projective cover 5% to 25%; class 1—projective cover from 1% to 5%; +—species occurs several times; r—the species occurs singly [74]. On Site 2, the determination of aboveground phytomass was carried out by the method of square areas. Sampling was carried out on squares with a size of 50 × 50 cm in 3 replications on typical sites for community (9 samples). Plant samples were measured by air-dry method. Statistical analysis was performed by the correspondence analysis (CA) method in the PAST 4.0 software.

Soil studies were carried out taking into account landscape features. On the two experimental sites, which are described in more detail in the section on the objects of study, 6 soil profiles (including control) were made, followed by a detailed morphological description of their profiles. The distance between Sites No. 1 and No. 2 is about 1.5 km. The horizon-oriented selection of soil samples (23 samples) for physical-chemical analysis was carried out. To study the composition and properties of soils, standard analytical studies were performed: particle-size distribution (pyrophosphate method), pH (in water suspensions 1:2.5), organic carbon (Tyurin’s method, which includes the combustion of the organic matter with a 1:1 mixture of 0.14M K₂Cr₂O₇ and concentrated H₂SO₄ at 150 °C for 20 min and titration with ferrous sulfate solution), exchangeable bases (ammonium acetate method for carbonate soils), hydrolytic acidity, content of anions and cations in the water extract of soil [75,76,77]. Soil moisture was determined by the gravimetric method with the soil drying in a thermostat at 105 °C.

The fractions of particle-size distribution of soil were determined according to the Russian system of particle-size classes: sand, 1–0.05 mm; silt, 0.05–0.001 mm, and clay, <0.001 mm.

The names of soils are given according to the Classification and Diagnostics of soils in Russia [78] and the WRB system [79].

To visualize some of the findings, the main physicochemical parameters of the soil and vegetation data were statistically processed using correspondence analysis (CA) in the PAST 4.0 software [80,81]. Samples values with a high ignition loss were excluded from the soil sample data set, since a high content of undecomposed plant residues gives a strong variability in values.

3. Results

3.1. Site No. 1

The research area is a combination of forest spaces and a thermokarst-disturbed area with cryogenic forms of microrelief. Figure 1 clearly shows the boundaries of strong fire that occurred more than 20 years ago. Here a polygonal microrelief was formed, represented by a scattering of large byllars and narrow depressions between them. The byllars in the study area have an irregular shape with rounded edges and are quite large (20–40 m in diameter), the depressions between them reach a width of up to 5 m and have a depth of up to 1 m. A part of depressions contains shallow water. On the areas untouched by fires of 1999–2001, a larch forest of Larix cajanderi Mayr. with grasses is developed. The tree stand is the same aged. The following tiers are distinguished: tree, shrub, dwarf shrub, grass, and moss-lichen (

Table 1. Composition of woody and herbaceous species of study sites.

Profile No.	Tree Layer	Shrub Layer	Dwarf Shrub Layer	Herbaceous and Moss-Lichen Layers
Forest Site (No. 1)
F-1-20 (forest)	Larix cajanderi	-	Ledum palustre, Vaccinium vitis-idaea	Arctous alpina, Empetrum nigrum, Chamerion angustifolium, Calamogrostis langsdorffii, Anemone sylvestris, Equisetum arvense, Phlox sibirica, Bromopsis karavaevii, Lathyrus humilis
B-1-20 (top of byllar)	Larix cajanderi	Betula divaricata, Salix bebbiana	Vaccinium vitis-idaea	Arctous alpina Aulacomnium turgidum
D-1-20 (depression between byllars)	-	Salix bebbiana	-	Carex juncella, Chamerion angustifolium, Calamogrostis langsdorffii Aulacomnium turgidum
Alas Site (No. 2)
A-1-21 (meadow)	-	-	-	Carex juncella, Calamogrostis lapponica, Alopecurus roshevitzianus, Eriophorum angustifolium, Petasites frigidus, Ranunculus propinquus, Potentilla nivea, Dreponacladus aduncus
A-2-21 (meadow)	-	Betula nana, Salix brachypoda	-	Carex juncella, Rubus arcticus, Valeriana capitata, Polemonium acutiflorum, Acrtagrostis latifolia, Bromopsis karavaevii, Poa angustifolia, Aegopodium alpestre, Potentilla stipularis, Ranunculus propinquus, Veronica longifolia, Petasites frigidus, Potentilla nivea, Halenia corniculata, Stellaria kolymensis, Dreponacladus aduncus
A-3-21 (shrubs)	Larix cajanderi	Betula nana, Salix brachypoda	-	Arctous erytrocarpa, Carex juncella, Anemone sylvestris Peltigera aphthosa, Aulacomnium turgidum

). The shrub cover is represented by Ledum palustre L., Vaccinium vitis-idaea L., Arctous alpina (L.) Niedenzu, Empetrum nigrum L., Chamerion angustifolium (L.) Holub, Calamogrostis langsdorffii (Link) Trin, Anemone sylvestris L., Equisetum arvense L., Phlox sibirica L., Bromopsis karavajevii (Tzvel.) Czer., Lathyrus humilis (Ser.) Sprengel (Table 1). According to the structure of the tree stand, a maturing tree stand, undergrowth, and young growth were identified (

Table 2. The structure of larch forest tree stand.

Tree and Shrub Layers	Crown Density Index	Tree Stand Formulas	D (1.3) (cm)	H (д) (м)	Age Class
Larch forest herb. (F-1-20)
Maturing tree stand	0.7	10 L	10 cm	8	III
Young growth	0.2	10 L	5 cm	2	I
Undergrowth	-	-	-	-	-
Larch forest moss-vaccinium vitis-idaea (B-1-20)
Young forest growth	0.1	10 L	5	3	I
Youngester	0.8	10 L	3	0.75	I
Undergrowth	0.2	3B2S	1.5	0.5	I

). There are signs of previous fires in the soil and tree trunks. Probably the fires were not so intense as to destroy the stand. The permafrost pale illuvial-ferruginous gleyic soil (profile F-1-20), Reductaquic calcic cambic cryosol (Loamic, Siltic), is described here, the morphological profile of which is represented by the following horizons: Oao (0–3/15 cm)—BPLf,g (3/15-7/25 cm)—BCAg (7/25–70 cm)—Cca,g┴ (70–97 cm). Under the thin histic horizon lies “pale” cambic horizon (BPLf,g) with stagnic properties, which were formed in relatively reducing conditions due to poor drainage and high humidity. However, the cryic/calcic horizon occupies the largest part of the profile, also with stagnic properties in the form of bluish and ocher spots and streaks. The soil is light loamy with a predominance of coarse dust fraction (

Table 3. Granulometric composition of studied soils.

Profile No.	Depth, cm	Bulk Density	Particle Size (%), mm						Sum of Particles, <0.01 mm
			1–0.25	0.25–0.05	0.05–0.01	0.01–0.005	0.005–0.001	<0.001
Forest soils (Site No 1)
F-1-20 (forest)	3–7	2.65	1.3	33.3	36.8	4.9	8.9	14.8	28.6
	7–70	2.66	0.8	34.2	43.7	14.7	1.2	5.4	21.3
	70–97	2.63	0.9	33.7	43.7	8.2	10.6	2.9	21.7
B-1-20 (top of byllar)	4–15	2.66	1.8	41	33.9	3.6	10.6	9.1	23.3
	15–42	2.66	1.9	45.4	32.2	4.9	8.2	7.4	20.5
	42–105	2.68	2.6	50.8	26.9	4.1	7.8	7.8	19.7
D-1-20 (depression between byllars)	5–20	2.67	2.8	42.9	35	6.2	7.3	5.8	19.3
	20–98	2.68	1.3	35.8	41.2	6.1	8.2	7.4	21.7
	98–105	2.68	2.9	42.3	31.9	5.3	9.4	8.2	22.9
Alas soils (Site No 2)
A-1-21 (meadow)	7(11)–30(40)	0.65	0.9	7.5	64.8	6.8	11.2	8.8	26.8
	30(40)–80	2.66	0.4	39.5	48.6	2.4	4.5	4.6	11.5
A-2-21 (meadow)	13(15)–35(45)	2.64	0.8	12.4	56.8	10.4	8.4	11.2	30
	35(45)–87	2.65	0.5	36.3	46	5.6	3.6	8	17.2
A-3-21 (shrubs)	1(2)–15(18)	2.61	3.7	15.1	41.2	11.2	15.2	13.6	40
	15(18)–68	2.66	0.1	39.8	45.3	3.3	4.9	6.6	14.8

). The soil reaction is slightly acidic in the upper part of the profile (pH 6–6.7) and alkaline in the lower part (pH 8–8.4). Ignition loss of the organogenic horizon reaches 36% (

Table 4. Physical and chemical properties of studied soils.

Profile No	Depth, cm	pH (H2O)	Corg, %	Exchangeable Cations, mmoL/100 g			HA, mmoL/100 g	Sum of Salts, %	Moisture, %
				Ca2+	Mg2+	Na+
Forest soils (Site No 1)
F-1-20 (forest)	0–3	5.98	36.05 *	-	-	-	20.4		-
	3–7	6.68	1.18	9.44	3.84	0.09	1.56	0.091	22.9
	7–70	8.41	0.57	8.64	1.28	0.08	-	0.089	21.2
	70–97	8.07	1.07	8.8	2.08	0.07	-	0.09	27.6
B-1-20 (top of byllar)	0–4	5.91	53.36 *	-	-	-	22.4	-	-
	4–15	6.58	0.56	8.16	2.56	0.08	1.13	0.059	12.1
	15–42	8.14	0.59	8	2.24	0.09	-	0.096	10.3
	42–105	8.36	0.52	7.04	1.28	0.07	-	0.084	11.6
D-1-20 (depression between byllars)	2–5	6.27	42.14 *	-	-	-	16.7	-	-
	5–20	6.56	0.87	7.75	2.38	0.08	1.46	0.056	20.1
	20–98	8.3	0.47	8	0.88	0	-	0.084	16.5
	98–115	8.11	0.57	9.13	1.12	0.04	-	0.092	17.6
Alas soils (Site No 2)
A-1-21 (meadow)	2–7(11)	7.84	72.14 *	-	-	-	-	-	-
	7(11)–30(40)	8.25	1.13	8.48	4.64	0.33	-	0.217	42.2
	30(40)–80	8.29	0.77	4.96	3.52	0.12	-	0.151	34
A-2-21 (meadow)	0–13(15)	6.2	82.35 *	-	-	-	-	-	106,3
	13(15)–35(45)	8.1	1.31	5.92	4.8	0.2	-	0.143	25.5
	35(45)–87	8.23	1.02	5.76	2.72	0.14	-	0.129	29.9
A-3-21 (shrubs)	0–1(2)	6.58	43.14 *	-	-	-	-	-	41.3
	1(2)–15(18)	8.27	2.19	9.25	4.25	-	-	-	28.8
	15(18)–68	8.41	0.74	6.24	2.08	0.1	-	0.102	16.5

*—ignition loss; **—hydrolytic acidity.

). The sum of exchangeable calcium and magnesium in the mineral part reaches a maximum in cambic horizon (13.4 mmol/100 g), which is possibly associated with a relatively high content of organic carbon in this layer, and a minimum in the calcic horizon (10.0 mmol/100 g). In the suprapermafrost layer, against the background of an increase in pH and the amount of exchangeable bases, there is an accumulation of organic carbon on the surface of the permafrost layer up to 1%. The soil is highly saturated with bases, the permafrost (at 13 September 2020) was on the depth of 97 cm.

On the uneven top of byllar, with small cracks and tubercles, under a post-fire young larch forest was formed permafrost pale typical soil (profile B-1-20). The tree stand is the same aged. Young grouth is Larix cajanderi (Table 2). Undergrowth vegetation dominated by Betula divaricata Ledeb., Salix bebbiana Sarg. Vaccinium vitis-idaea, L, Arctous alpine (L.), and moss Aulacomnium turgidum (Wahlenb.) Schwagr. (Table 1). This soil calcic cambic cryosol (Loamic) (by WRB), with weak evidence of albic material accumulation under the organic horizon. The morphological profile consists of the following horizons: O (0–4/15 cm)—BPLe,f (4/15–15/30 cm)—BCAcr (15/30–42/50 cm)—BCca┴ (42/50–105 cm). The surface organogenic horizon consists of organic residues of varying degrees of decomposition, below is a cambic horizon, not reacting with HCl, with a slight presence of albic material, intertwined with roots and saturated with organic carbon. In the middle part of the profile, there is a cryic/calcic horizon violently reacting HCl, presents signs of cryoturbation in the form of weakly pronounced dark brown curves and ocher interlayers.

The granulometric composition of this pale soil is light loamy-sandy loam with a predominance of fractions of fine sand and coarse dust (Table 3). The pH is slightly acid-alkaline (5.9–8.4) with an increase down the profile (Table 4). The ignition loss in the thin organogenic horizon reaches 53%. In the mineral horizon underlying the organogenic layer, the content of organic carbon is quite low, and a sharp decrease in hydrolytic acidity is observed here. The sum of exchange bases is 10.8 mmol/100 g. The content of exchangeable calcium and magnesium decreasing down the profile, while increasing alkalinity and a uniform distribution of organic carbon. Probably such a picture is associated with the influence of seasonal thawing and freezing processes that pull soluble substances and, as a consequence, exchange cations to the middle part of the profile, which is typical for pale soils [82]. The permafrost table at 14 September 2020 was on the depth of 105 cm.

Permafrost pale muck-gleyic soil (profile D-1-20), Histic calcic cambic cryosol (Loamic, Siltic), develops in a relatively dry depression between byllars, under the small reed-sedge vegetation. The microrelief is tussocky. Carex juncella (Fries) Th. Fries are abundant in the grass cover. The soil profile is characterized by the following horizons: O (0–2 cm)—H (2–5/7 cm)—BPLf,g(5/7–20/37 cm)—BCAg (20/37–98 cm)–BCg,ca┴ (98–115 cm). Under the moss cover of Chamerion angustifolium, Calamogrostis langsdorffii a thin histic horizon was formed with layers of weakly decomposed moss and single inclusions of charcoal (Figure 4). Below is located a sandy-loamy-loamy cambic horizont (BPLf,g) that does not reacts with HCl and is heterogeneous in color with ocher spots due to the manifestation of stagnic properties and the accumulation of free iron oxides. Below, the soil is loamy, violently reacts with HCl, due to the high content of carbonates that is manifestation of the calcic horizon properties.

The soil is sandy loam with a predominance of fine sand and coarse dust (Table 3). In contrast to the previous soil profile, here is observed the inverse distribution of clay proportion. It increases from top to bottom. In general, the pH reaction in the upper layers is also subacid, even neutral (Table 4). The soil becomes alkaline below 20 cm from the surface at the maximum pH value in the horizon of BCAg (pH 8.3). The content of exchange cations is average—8.9–10.2 mmol/100 g. Ignition loss in a thin peat layer reaches 42%. Since there is no albic material in the profile, there is more carbon in the suborganogenic horizon here—0.9%. Base saturation is very high. Permafrost table at 15 September 2020 was on the depth of 115 cm.

3.2. Site No. 2

The large ancient alas is a mature thermokarst depression that occurred as a result of climate warming at the boundary of Holocene and Pleistocene. Unlike the alases of the Central Yakutia, it is characterized by the absence of clearly defined radial belted structure. The bottom of alas has a mosaic appearance due to a combination of fragments of birch and willow shrubs with the participation of a single larch and meadows of different humidity with a slight difference in absolute heights. The soil cover is represented by the complex aggregate of meadow-swamp and forest soils, the formation of which is associated with an uneven surface of the basin bottom.

The ring of a waterlogged cotton-grass-reed-sedge meadow borders a small lake at the bottom of alas. Here dominates Carex juncella—50% on the sedgy hummock bog, Calamogrostis lapponica (Wahlenb.) C. Hartm., Alopecurusro shevitzianus Ovcz.—30%, Eriophorumangus tifolium Honck.—15%, Petasites frigidus, Ranunculus propinquus, Potentilla nivea–5% (

Table 5. The structure of the herbaceous vegetation in alas.

Species	Projective Cover by Brown-Blanque Cover Class	H (cm)	Phenophase
Dwarf birch thicket
Arctous erytrocarpa	2 *	10	~ **
Carex juncella	r	20	~
Anemone sylvestris	r	25	v
Sedge-tussock meadow
Carex juncella	3	25	~
Rubus arcticus	2	20	~
Valeriana capitata	1	30	v
Polemonium acutiflorum	1	25	v
Acrtagrostis latifolia	1	55	v
Bromopsis karavaevii	1	30	~
Poa angustifolia	1	30	~
Aegopodium alpestre	+	25	v
Potentilla stipularis	+	20	~
Ranunculus propinquus	+	25	~
Veronica longifolia	+	25	v
Petasites frigidus	+	20	~
Potentilla nivea	+	30	~
Halenia corniculata	r	25	v
Stellaria kolymensis	r	15	v
Cottongrass-reed-sedge meadow
Carex juncella	4	28	~
Calamogrostis lapponica,	3	45	~
Alopecurus roshevitzianus	2	40	~
Petasites frigidus	2	25	~
Eriophorum angustifolium	1	35	#
Ranunculus propinquus	1	25	~
Potentilla nivea	1	35	~

*—Brown-Blanque classes: class 5—projective cover of the species over 75%; class 4—projective coverage from 50% to 75%; class 3—projective cover 25% to 50%; class 2—projective cover 5% to 25%; class 1—projective cover 1% to 5%; +—species occurs several times; r—species occurs singly, **—phenophase: #—shedding of seeds (fruits), ~—secondary vegetation, v—secondary vegetation.

). In a moss cover the coverage degree of Dreponacladus aduncus is 50%. The productivity of this meadow is 2.8 t/ha. The permafrost alas peaty-gley soil, Histic Calcic Gleysol (Gelic, Ruptic, Hydric) by WRB (Profile A-1-21), is developed under this meadow (Figure 1). The morphological profile has the following structure: O (0–2 cm)—T (2–7/11 cm)—G1ca (7/11–30/40 cm)—G2ca┴ (30/40–80 cm). Under a thin layer of moss litter with an admixture of marsh grassy litter, there is a histic horizon about 10 cm thick, consisting of organic remains of medium and high degrees of decomposition. The underlying horizon with strongly pronounced gleyic properties is subdivided into two subhorizons according to external features. The upper part (G1ca) has an inhomogeneous grayish-blue-gray color with rusty-ocher spots (up to 30%) and dark brown organogenic residues, its moderately reacts to HCl. The underlying part of the gley horizon (G2ca┴) is distinguished by an abundance of rusty-ocher spots (more than 50%), greater thixotropy, fluidity, and a weak reaction to HCl due to high moisture. Permafrost table at 28 July 2021 was on the depth of 80 cm.

These are the most alkaline soils in alas (pH 7.8–8.3). The content of physical clay in upper part of mineral layer reaches 27% (light loam) and in the lower part—12% (sandy loam). In general, it should be noted a rather similar pattern of particle distribution in the profile of each soil described within alas: the accumulation of physical clay in a layer at the depth of up to 15(35) cm and the increases of fine sand in a layer from 15(35) cm (Table 3). Ignition loss of the upper layer reaches 72%, the carbon content in the lower layers is not high—up to 1.1%. Low mineralization, a small amount of humus in the mineral part indicates the relative youth of this soil formation.

Sedge-tussock meadow occupies middle and not the most humid areas of alas. Sedge hummock from Carex juncella dominates here—45%, Rubus arcticus L. grow on hummocks. Singly growing Betula nana with average height of 80 cm and Salix brachypoda with height of 70 cm. Valeriana capitata Pall. ex Link, Polemonium acutiflorum Wild. ex Roem. et Schult., Acrtagrostis latifolia (R.Br.) Griseb., Bromopsis karavajevii, Poa angustifolia L., Aegopodium alpestre Ledeb., Potentilla stipularis L., Ranunculus propinquus C.A. Mey, Veronica longifolia L., Petasites frigidus (L.) Fries, Potentilla nivea L., Halenia corniculata (L.) Cornaz, Stellaria kolymensis Khokhr. are rare (Table 5). In a moss cover predominate Dreponacladus aduncus (Hedw.) Warnst., coverage degree is 40% The productivity of this meadow is 1.4 t/ha. The permafrost alas peaty-gleyic soil, Histic Calcic Stagnosol (Gelic, Magnesic, Ruptic) by WRB, is developed under this meadow (profile A-2-21). The morphological profile has the following structure: OT—(0–13/15 cm)—Bg,ca (13/15–35/45 cm)—BCg,ca┴ (35/45–87 cm) (Figure 4). The upper histic horizon, composed of peat and densely intertwined with small grass roots, abruptly passes into a horizon with a high content of carbonates and strongly pronounced stagnic properties, which increase with depth. The soil is medium loamy in the upper part and sandy loamy in the lower part, with a predominance of the coarse dust fraction, while the content of physical clay reaches a maximum under the surface horizon. The reaction is slightly acidic in the organogenic layer (pH 6.2), and strongly alkaline below the level of 13 cm (pH 8.2). Ignition loss in organogenic layer reaches a significant value (82%), which indicates a weak mineralization of peat. The humus content in the mineral part is average—1.0–1.3%. The sum of exchange cations reaches 8.6–10.9 mmol/100 g, which is an average value. Permafrost table at 27 July 2021 was on the depth of 87 cm.

On the high and dry peripheral (bordering on a forest) southern part of alas, the dwarf Arctic birches from Betula nana L. are common and occupy 30% of vegetation cover. Pure thickets of dwarf birches with an average height of 90 cm account for 75% of the total coverage, Salix brachypoda (Trautv. et C.A.Mey.) Com. with average height 80 cm—10%, spots on bunds Arctouse rytrocarpa small.—10%. Larix cajanderi, with an average height of 3 m, is represented singly. The species composition of higher vascular plants is given in Table 5. The coverage degree of the lichen-moss cover is 60%. It consists of Peltigera aphthosa (L.) Willd. (10%) and Aulacomnium turgidum (50%). The productivity of this community is 3.7 t/ha. On this belt is developed permafrost pale degraded soil, Eutric Cambic Cryosol (Magnesic, Raptic) by WRB (profile A-3-21). The morphological profile consists of the following horizons: O (0–1/2 cm)—[BPL-BCA]tr (1/2–15/18 cm)—B(BC)ca┴ (15/18–68 cm). A very thin surface horizon, consisting of organic material of varying degrees of decomposition, abruptly passes into a violently reacting with HCl cryoturbated layer, consisting of saturated carbonates material and fragments of the cambic horizon (Figure 4). Below located a transitional to the soil-forming rock horizon, underlain by permafrost. Permafrost table on 27 July 2021 was at a depth of 68 cm.

Pale degraded soil is medium loamy-sandy loam (Table 3). The accumulation of physical clay in the upper part of the profile suggests of pale properties of soil. In the lower layer, there is high participation of the fine sand fraction. The pH reaction and its distribution scheme along the profile has similar values with a forest profile (Table 4). Ignition loss of the organogenic layer reaches 43%, and the carbon content in the mineral part reaches 1–1.3%. The sum of exchangeable calcium and magnesium is equal to 8.3–13.5 mmol/100 g and has maximum values under the organogenic layer.

An analysis of the water extract showed that the soils of the first and second sites are not saline, but there is an increase in about 1.5–2 times of the total amount of salts in the soils of the alas depression.

4. Discussion

Studies on Site No. 1 showed that the soil surface with a completely disturbed forest stand after fires is deformed in a short period of time. After the disappearance of the trees canopy as a result of increased soil heating, the active layer depth is sharply increases in the first years [6,14]. When this process reaches the upper boundary of the IC, the melting of ice wedges and subsidence of the soil begin, forming a hillocky-depressed polygonal microrelief—byllar. Byllars on the study area are large, the depth of depressions reaches 1 m, the width of the rounded polygons is up to 40 m. The redistribution of moisture on new elements of mesorelief creates different conditions for vegetation regeneration. Young growth of larch forest with moss-vaccinium is currently growing on the hillocks, and a sedge-reed grass community is developing on the depressions. In the areas untouched by fires, the original type of vegetation is preserved—a larch forest from Larix cajanderi.

The resulting diversity in the structure of vegetation cover on hillocks and depressions in the post-fire period (more than 20 years) formed the soil cover with the participation of modified variants of natural soils (Figure 4). Soils of forests with undisturbed stands are represented by permafrost pale illuvial-ferruginous gleyic soil (Reductaquic calcic cambic cryosol) with profile structure: Oao (0–3/15 cm)—BPLf,g (3/15–7/25 cm)—BCAg (7/25–70 cm)—Cca,g┴ (70–97 cm). On the well-drained tops of byllars, the soil profile shows no signs of stagnic properties and profile structure is as follows: O (0–4/15 cm)—BPLe,f (4/15–15/30 cm)—BCAcr (15/30–42/50 cm)—BCca┴ (42/50–105 cm). In the byllar soils, the albic layer is a lighter horizon and they can be attributed to permafrost pale typical soil (calcic cambic cryosol) with a slight accumulation of albic material]. In the depressions of the microrelief, mineral horizons of the soil (below 5/7 cm) experience stagnic properties due to increased humidity, and the annual accumulation of herbaceous plant litter on the soil surface has led to the appearance of a humus horizon. Soil profile structure is as follows: O (0–2 cm)—H (2–5/7 cm)—BPLf,g(5/7–20/37 cm)—BCAg (20/37–98 cm)–BCg,ca┴ (98–115 cm). These soils should be classified as permafrost pale muck-gleyic soil (Histic calcic cambic cryosol).

All the soils studied on Site No. 1 have a light loamy granulometric composition, a slightly acid reaction of the medium in the upper part of the profile and an alkaline one in the lower part (Table 1 and Table 2). Ignition loss in organogenic layers is 36–53%, and the carbon content in the mineral part is low (0.5–1.2%).

Young growth of larch on the surface of the byllars indicates a satisfactory restoration of the forest after fires in the conditions of the northern taiga. When assessing the moisture content (Table 3) of the studied soils, an interesting picture can be observed. The wetter soils on Site No. 1 are soils under intact natural forest (F-1-20), and the moisture in it increases when moving down the profile. Perhaps this is due to poor lateral flow, as well as to the deterioration of growing conditions and inhibition of plant growth due to the growth of mosses and the process of self-thinning in developed forests. In such cases, the thawing depth is relatively decreases, same as the soil temperature, its moisture increases and aeration deteriorates [83,84]. The soil of byllar depression (D-1-20) has almost the same high moisture, but on the contrary in which, the maximum moisture is observed closer to the surface, what indicates that relatively excessive moisture was formed due to the location in the negative element of the microrelief. However, at the top of the byllar, due to the outflow of moisture into depressions, pyrogenic degradation of the litter, as well as the formation of a fairly closed young larch forest, conditions were formed for the drying of the soil. With relatively light particle size distribution, eluvial processes began to intensify, expressed by the appearance of albic material under a thin organogenic horizon. This is also indicated by a decrease in the content of water-soluble salts in the upper part of the profile, i.e., there is a slight differentiation of the profile compared to the soil of a slightly disturbed forest. Stagnic properties are presumably inherited to a greater extent from the soils of intact forests formed under conditions of relative oxygen starvation. If in the soil profile of byllar it is already now morphologically expressed weakly, then in the depression they are still quite clearly manifested.

The soil cover of the mature alas (Site No. 2) retains, although weakly expressed, quite noticeable patterns of the belt structure, which are especially pronounced around a small lake in the eastern part of the alas depression. According to the results of this study, in the highest parts of alas bottom under larch-dwarf birch associations (Betula nana with rare larch dominate), where signs of zonal soil formation have been preserved. In this place was described degraded permafrost pale soil (Eutric Cambic Cryosol (Magnesic, Raptic)). On the optimally moistened areas of alas, under the sedge-tussock meadows, the permafrost alas peaty-gleyic soil (Histic Calcic Stagnosol (Gelic, Magnesic, Ruptic)) is formed. Moreover, the permafrost alas peaty-gley soil (Histic Calcic Gleysol (Gelic, Ruptic, Hydric)) developed under the wettest cottongrass-reed-sedge meadows. Thus, we can observe a number of soils with increasing signs of stagnic properties as we approach the lake.

To compare the main physical and chemical properties of the studied profiles, an ordination analysis was performed (Figure 5).

Figure 5 shows that almost all soil samples taken in alas form a slightly scattered group with a relatively high proportion of magnesium, sodium, and water-soluble salts on the left side of the graph and are also characterized by maximum moisture. These soils have significant difference from the forest and byllar soils. At the same time, all samples taken on the top of byllar (in the upper right part of the graph) have increased values of calcium proportion, and are also the least moistened, as mentioned earlier. The soils of control and depression between byllars on the burned area can also be called the most similar in terms of indicators, which probably indicates that at a similar level of moisture, soils undergo changes in physical and chemical parameters more slowly even with a strong transformation of vegetation. However, desiccation (as in the byllar) contributes to the leaching of salts to the surrounding depressions.

Compared to forest soils, alas soils react with HCl below the organogenic horizon, and the proportions of exchangeable magnesium and sodium in the soil-absorbing complex also increase. At the same time, the content of exchangeable magnesium almost reaches that of calcium. There are no signs of salinization, but the content of water-soluble salts in alas soils increases by 1.5–2 times. The reason for this may be the redistribution of water-soluble salts over the relief elements due to the income of water-soluble substances with melt water from the surrounding upland, but relative salinization during the degradation of the IC, associated with the release of salts consisting in ice wedges and frozen grounds, is not excluded [17,18].

Despite the weak visual differentiation of alas soil profiles, the mineral sequence is characterized by a binary granulometric composition (medium loam in the upper part and sandy loam in the lower part). The reason for this may be both the processes of metamorphism and the specifics of the redeposition of the parent material during the thermokarst process, which has a more mechanical nature of formation than lacustrine-accumulative. So, for example, we attribute the uneven structure of alas depression to the fact that it was presumably formed as a result of rapid subsidence of the soil, similar to the Batagai mega slump. Moreover, it is in contrast to the same age alases of the Central Yakutia, which soils have multiple redeposition traces in profile.

In Figure 6, it can be seen that in terms of species composition, the most similar are the vegetation of the dwarf birch on the alas and the tops of the byllar. The vegetation of the sedge hummocky meadow of alas has the same species as the vegetation of the depression between the byllars. The control plot in the vaccinium larch forest is located separately in terms of species composition, which has a significant difference in the vegetation of the post-fire byllars (top and depression). As well, species of cottongrass-reed grass-sedge meadows of alas stand out in a separate group.

In modern conditions, as in the Pleistocene, herbaceous plants from the families Cyperaceae and Poaceae predominate in the vegetation of the studied Sites No. 1 and No. 2. On Site No. 2, under conditions of thermokarst development, a high abundance of Betula nana and Salix is noted. The woody vegetation of these sites is formed by Larix cajanderi. Although, as noted earlier, Larix gmelinii needles were found in the Holocene [43,44]. At present, this species is not represented in this territory, and the distribution boundary is located west of 120–123° E; that is, the cooling after the Holocene optimum shifted the distribution area of Larix gmelnii toward Europe.

According to the literature data, in Siberia, at least during the last 50 thousand years, there was no catastrophic change in flora. Moreover, despite the fact that the steady cooling of the climate, on average, over a given period led to a gradual depletion and transformation of the Late Pleistocene flora and the formation of modern ones, the cyclical nature of climatic changes gave this unidirectional course of vegetation evolution an undulating, pulsating character [68]. Therefore, for example, at present, under the conditions of global warming, it is possible to observe the penetration of some Eurasian plant species into the study area, which indicates the beginning of fluctuation changes in vegetation.

5. Conclusions

It has been revealed that in the last two decades, due to climate warming, the initial stages of thermokarst development are intensively formed in the vast expanses of pyrogenically disturbed areas of the northern taiga. According to Earth’s remote sensing data, the distribution of cryogenic forms of micro- and mesorelief clearly shows the areas and boundaries of past fires. A fire, disrupting the protective function of the forest, leads to the melting of ice wedges, causing subsidence of the soil above them. A system of expanding depressions appears on the soil surface, between which remain mounds and byllars untouched by thawing. As a result, in the first place, the structure of the vegetation cover is complicated—in the place of natural old larch forests, a complex of young larch stands is formed on the surface of byllars and sedge-reed grass communities confined to depressions. Thus, the thermokarst process disrupts the natural course of reforestation processes and the carbon cycle.

It is revealed that even the initial stages of thermokarst influence the morphological profile of permafrost soils. The process is accompanied by the transformation of the previously homogeneous soil cover with the formation of more complex soil cover with the participation of modified variants of natural soils. It has been determined that in the soils of cryogenically disturbed places, the depth of soil thawing increases by almost 10–20 cm. In addition, the permafrost pale soils on byllars become drier, presumably due to the improved drainage, and the outflow of moisture into numerous depressions around them is also more intense. Therefore, the albic material starts to form in such conditions. In the soils of byllars, a deepening of the upper boundary of the carbonate horizon is also observed. The combination of numerous hillocks and depressions creates conditions for the redistribution of moisture. Waterlogging of depressed areas against the background of climate warming and without the protective function of the forest will lead to an expansion of the area of depressions and, subsequently, to a deeper degradation of the IC, which can cause catastrophic subsidence of soil in a given territory over a wide area.

In Central Yakutia, the formation of alas has a clear evolutionary pattern, where byllars turn into a thermokarst lake and then accumulate specific lacustrine soil-forming material at the bottom of the basin. Here, the formation of alases often has a “drier” character, which is reflected in the studied soils of the alas, which were formed under similar conditions to the present in the Holocene during the climatic optimum. There are practically no signs of lacustrine redeposition in alas soils, they retain signs of zonal soil formation, and the bottom of alas has a weakly expressed ring structure. That is, the alas studied by us most likely formed during the rapid subsidence of the soil, as well as the mega slump located not far from the study areas.

Thus, one of the main reasons for the acceleration of thermokarst processes in the last 10–20 years under climate warming is the increase in the area and degree of forest fire in Yakutia recorded in recent years. This paper shows how soils can vary in different types of cryogenic landscapes in a small area. Moreover, when comparing the control soil with a young hilly-depression landscape, they change at the subtype level, which is already significant. The soils of mature alas have certain differences from the intact forests undamaged by fire at the type level, caused by changes in soil-forming conditions. Such studies of degrading areas soils on the permafrost zone were carried out for the first time and will help in the future in studying the dynamics of the soil and vegetation cover under the conditions of degradation of the IC.

The formation of a huge mega slump in the study area in the 1960s due to deforestation, as well as alases in the Holocene optimum, makes us think that fires under global warming conditions can lead to the same consequences, given the rate of formation of a pronounced cryogenic microrelief in a short time period (in our case, 20 years). The question also arises of how stable the system “depression-byllar” is in time. Because the renewal of the forest at the top of the byllar occurs quite quickly, but in the depression, the forest vegetation is not restored. Under modern climate warming, the expansion of depressions between byllars will lead to swamping and the subsequent formation of thermokarst origin lakes.