Fires and Clear-Cuttings as Local Areas of Arthropod Diversity in Polar Regions: Khibiny Mountains

Abstract

The well-known phenomenon of attracting untypical animals to disturbed territories has been poorly investigated in the polar mountains. We studied arthropod diversity in self-healing industrial clear-cuts and burn areas in the Khibiny Mountains, Kola Polar region. Fieldworks were conducted at four sites, including a control mountain taiga forest and its three transformed variants: burnt forest, uncleared clear-cut, and twice-disturbed burnt clear-cut. Arthropods were collected using formalin traps 2–3, 5–6, and 8–9 years after industrial deforestation in 2012 and an extensive grass-roots fire in 2013. Out of 124 identified species (spiders—61; ground beetles—41; and rove beetles—22), 79 (or 64%) were collected in disturbed, primarily burned areas and were absent in control forest. We note ten species of rove beetles, nine species of ground beetles, and eight species of spiders for the first time in the well-studied arthropod fauna of the Khibiny Mts. We found that grass-root fires transform the soil vegetation covers in the polar mountain forests more powerfully in comparison with extensive deforestation and attract a greater diversity of arthropods with different preferences, enriching the fauna of the polar mountains and the Subarctic region as a whole. The attraction effect persists for, at least, a decade after the violations.

1. Introduction

Recognizing the evident negative consequences of industrial logging and fires on forest ecosystems and their soil fauna, the effect of attracting animal species with specific ecological preferences to these disturbed areas should not be overlooked. Studying these effects is particularly relevant in high-latitude regions, where species inhabit the northern periphery of their ranges in conditions of heat deficiency and their life cycles are synchronized with short summers, long low-temperature winters, and the frequency of the polar day and polar night. As a result, the faunas of the polar regions are characterized by low natural diversity, and ecosystems have weak compensatory mechanisms to maintain stability in case of disturbances. Due to the low diversity and elimination of sensitive species, ecosystems may lack «replacement species» that are capable of performing similar ecological functions. This will lead to a further decrease or loss in biodiversity. For the Kola Peninsula, the most industrially developed region in the Arctic zone (Figure 1a), over long-term research, numerous data have been obtained concerning the oppression of soil fauna in the vicinity of various enterprises [1,2,3,4,5,6,7,8,9]. As a rule, structural changes in soil communities have a similar nonspecific reaction, showing a reduction in taxonomic and trophic diversity and the total biomass of invertebrate animals.

In contrast to flat landscapes, richer plant and animal communities are formed in the intrazonal or azonal ecosystems of the polar mountains. This is determined by the diversity of habitats created by a complex of such specific factors as the mountain microclimate, the slope exposure receiving more solar radiation compared to the foothill valleys, the altitudinal zonality of soil and vegetation cover, and the ecotones [10,11]. For the Subarctic Khibiny Mountains, the rich taxonomic and zoogeographical diversity of fauna has been confirmed by more than a century of research, since the first scientific Finnish and Swedish expeditions in the «Kola Lapland» at the end of the 19th century [12,13,14,15,16,17,18,19,20,21,22,23] and including our soil zoological works for the last fifteen years [24]. For the protection of the reference polar mountains natural complexes with their unique biota and to limit industrial and tourism activities, the territory of Khibiny Mts with a total area of 84.8 thousand hectares, was declared a National Park of Federal Significance in 2018 (Figure 1b). Currently, 75 specially protected natural territories with a total area of about 200 thousand hectares are organized in the Kola Polar region, which is 13.6% of its area, and the Khibiny National Park accounts for 4.3% of them [25]. However, mountainous territories outside the park’s boundaries are undergoing a significant transformation due to mining activities whose production accounts for over 60% of the total industrial output in the Kola Polar region. Currently, minerals are extracted from six deposits in four mines in different parts of Khibiny Mts and transported via a wide dirt road network [26]. The total area of disturbed Khibiny landscape is estimated at 5%, which is comparable to the square of spruce forests in this massif [27]. Several other mineral deposits are under development. One of them, the phosphorus deposit of the Partomchorr Mt, is located in the intermountain valley of the Kuniyok River in the north of Khibiny Mts (Figure 1b).

In 2012, a mountain taiga forest was cut down here on an area of more than 100 hectares for the construction of an enrichment factory. Due to the conflict between industrialists and environmentalists concerning the borders of the “Khibiny” National Park, deforestation and the clearing of fallen trees were stopped. A year later, in summer 2013, an extensive grass-roots fire broke out in a clear-cutting littered with felled tree trunks and branches. It spread to the surrounding coniferous forest in the valley and the mountain taiga belt on the slopes. The total burned area covered 8 hectares.

The dirt road became a natural barrier to the spread of the fire and divided the intermountain valley into two sides: one burned and one unaffected by the fire. Thus, different types of disturbances in the mountain taiga forest arose almost simultaneously and within walking distance of each other, and we have organized four sampling sites for ecological monitoring of soil cover restoration. These were two pairs of adjacent sites, differing in the criterion of “burnt–unburned”, namely control and burnt forests, as well as unburned and burnt clear-cuttings (Figure 1c).

For ten years, these self-healing areas have been the objects of our comprehensive ecological monitoring, including periodical assessments of the chemical composition, CO₂ emissions, and temperature dynamics of the soil, as well as the abundance and diversity of the microbiota and the fauna of invertebrates [28,29,30,31,32,33,34].

In this article, we focus on the effect of long-term attraction through clear-cutting and fires of species that are not typical of the well-studied local fauna of the Khibiny Mts. We also want to emphasize the necessity of long-term monitoring studies and conservation efforts for fauna not only within the boundaries of the «Khibiny» National Park but also in disturbed areas outside its official zone.

Invertebrates inhabiting the soil cover of the Polar regions, are diverse from morphological, physiological, and ecological points of view, even with a low taxonomic composition. They belong to different size-weight and trophic groups, vary in abundance and ecological strategy, and are associated with different soil layers at certain life stages. Therefore, even in response to the influence of one prevailing factor disrupting the soil cover, animals show a dissimilar reaction from negative or neutral to positive. This has been illustrated on examples of epigeic and endogeic fauna in neighboring northern regions: in the burn forest of the Kostomuksha Nature Reserve (Karelia Republic) [35], clear-cuttings of the Komi Republic [36], as well as in our previous studies on the fauna of clear-cuttings and burn sites in the Khibiny Mts [28,29]. Therefore, we analyzed the responses of spiders, rove beetles, and ground beetles to the clear-cutting and burning of the mountain taiga forest.

These arthropods, firstly, quantitatively predominate among the soil fauna in these disturbed areas in different years of succession. Secondly, they are diverse and well-studied in Khibiny Mts in terms of the species number, and thirdly, they are non-specialized mobile predators, or generalists, capable of actively migrating to disturbed areas or, conversely, avoiding them. The dominance of these taxa in clear-cuttings and burning areas at different stages of succession has been repeatedly noted in the literature sources [35,36], which can be compared with our results in the Khibiny Mts. At the same time, these taxa differ in the degree of association with the soil cover. Namely, smaller rove beetles (Staphylinidae) are closely related to the soil at all stages of the life cycle, while larger and actively migrating ground beetles (Carabidae) are only at the larval stages, and spiders are confined not to the soil but to the litter and its surface.

Based on these facts, we hypothesized (1) that due to the specificity of stratification in the soil cover, these taxa will show different trends in abundance and species richness depending on the degree of disturbance on clear-cuttings and burnings. However, (2) their communities should be most similar to the control forest precisely in neighboring burnt forests due to the short distance between the selected sites (300 m) and available migration. Conversely, (3) the arthropod community should be most suppressed at the burned clear-cutting, disturbed for two consecutive years, and losing not only the tree canopy and litter layer but also the organogenic horizon. We wanted to check in the polar mountains (4) the well-known phenomenon of attracting rare and atypical arthropod species, including pyrophilous and meadow ones, to forest fires and clear-cuttings in the first years of succession, and how long this attraction effect persists during self-healing of extensive disturbed polar mountain areas. This would help to understand (5) whether it is worth spending physical effort and finances on ecological monitoring of these disturbed areas if they are not included within the official borders of the National Park and, therefore, do not fall under the protection regime and measures.

2. Materials and Methods

2.1. The Study Area

The Khibiny Mts, located 150 km north of the Arctic Circle, are the largest mountains in the Kola Polar region (Figure 1a). They have no analogues in combination of geological structure, difficult-cross terrain, humid microclimate, and altitudinal zonality of soil and vegetation cover [37,38,39]. The massif is characterized by a rounded shape with a diameter of about 50 km, featuring a central core reaching heights of 1000–1200 m above sea level and two semicircles (inner and outer) of lower peaks up to 600–800 m (Figure 1b). This geometry results in comparable proportions of slopes with different exposures: N, S, NW, and SE—approximately 13–14%, NE, E, W, and SW—around 11–12% [27]. Slopes with inclinations of 15°–35° predominate (34%), less steep (8°–15°) and relatively gentle (<8°) occupy 31% each, and the steepest (>35°) comprise only 2%.

The low mountain heights determine a simple altitudinal zonation of vegetation. The foothills up to heights of 300–350–400 m (depending on exposure) are occupied by rare-stand pine and spruce forests of the mountain taiga belt, transitioning to belts of birch crooked forests (up to 400–500 m) and mountain tundra (up to 900 m). The flat mountain peaks smoothed by glacial masses are cold rocky deserts, or «goltsy», with sparse moss and lichen pillows. Despite the low elevations, treeless tundra and cold, rocky deserts make up approximately 40% of the mountain landscape. A third is occupied by crooked and sparse birch forests (27%), and another third is a mountain taiga. Among the last, old-growth coniferous forests with trees aged 200–250 years have been preserved [27]. Pine forests are mainly found in the western, northwestern, and northern parts of the Khibiny, including the studied Kuniyok valley. Spruce forests are distributed in the south and southeast of Khibiny Mts, along the valleys of large mountain rivers.

Illuvial-humus podzols (or Albic Podzols) are typical soils in mountain forests, and podburs (Entic Podzols) in mountain tundra. These soils have formed on alkaline nepheline-syenite rocks with a rich mineral composition and differ from the soils of surrounding foothill plains by their low acidity and high humus content [37,40,41].

The research was conducted in the Kuniyok River intermountain valley in the north of Khibiny Mts, at coordinates 67°36′–67°50′ N, 33°39′–33°50′ E, and elevations from 220 to 236 m above sea level (Figure 1b). Two pairs of sites were laid out for monitoring: in the control (F) and burnt (Fb) pine forests, and in the unburned (C) and burnt (Cb) clear-cuttings (Figure 2). The distance was 300 m between sites in each pair and one kilometer between the forest and clear-cuttings (Figure 1c).

2.2. The Chemical Analysis and Temperature Measurements of Monitored Soils

The chemical analysis of soil from control and disturbed sites was conducted in the accredited analytical laboratory of INEP, Kola Science Center. The following soil parameters were determined (on an absolutely dry weight): pH value (by potentiometric titration), organic matter/ash content (by calcination in a muffle furnace at a temperature of 525 ± 25 °C for 2 h), the content of biogenic elements such as calcium and potassium (by atomic absorption spectrophotometry on the Aanalyst–800 device), phosphorus (using the photocolorimetric method), total nitrogen (using the Kjeldahl method), and organic carbon (using the Tyurin method) [42,43].

Year-round measurements of soil temperatures (T°C) were conducted at a depth of 5 cm to compare the processes of winter freezing, spring thawing, and summer warming in disturbed mountain soils. Programmable loggers with a measurement range from −40 to +40 °C and a recording frequency of 2 h were used for this purpose [44].

2.3. Collection of Arthropods in Monitored Sites

Arthropods were collected with a three-year interval: in 2015, 2018, and 2021, corresponding to 2–3, 5–6, and 8–9 years after the disturbances. Plastic pitfall traps of 500 mL containing 4% histological formalin were used in thirty replications at each site. They were set in three lines of 10 traps in line with a distance of 10 m between the lines and between the traps in line, which corresponded to the total trapping area of 0.18 ha (Figure 3). The traps were buried in soil up to their upper edge. The trapping period was at least 70 days, from the beginning of July to the second or third decade of September. It should be noted that the remote location of the monitoring sites in the northern tip of Khibiny Mts and the liberation of the mountain road from snowfields no earlier than the beginning of July did not allow organizing field work in May–June, during the breeding season with the greatest activity of arthropods. This, most likely, led to an underestimation of their real diversity and abundance. Generally, 120 traps were exposed at four sites in each monitoring year, with an operating period of at least 2100 trap days. Manual sorting of trap contents was carried out in the laboratory after washing with water and formalin. A total of 2.590 spiders (1.280 adults and 1.310 juveniles), 950 imago ground beetles, and 270 imago rove beetles were collected during the monitoring period.

The beetles and spiders were extracted from the traps by manual sorting in the laboratory. The collected specimens have been deposited in the Zoological Institute of the Russian Academy of Sciences, St. Petersburg (spiders), the Institute of Industrial Ecological Problems of the North (INEP), Apatity (ground beetles), and the Institute of Biology of the Komi Scientific Centre, Syktyvkar (rove beetles).

The influence of abiotic factors (soil temperature and chemical parameters) on the diversity and distribution of arthropods was evaluated using the Principal Component Analysis (PCA, Statistica-10), with covariance of variables and preliminary standardization of all measured soil and faunal parameters to eliminate differences in their scale.

3. Results

3.1. The State of Monitoring Sites by the End of the First Decade

3.1.1. Plant Cover

The control mountain taiga forest (F) is formed by a Laplandic form of Scots pine (Pinus sylvestris f. lapponica L.), Siberian spruce (Picea obovata), and curly birch (Betula tortuosa). It has a well-developed vegetation cover (Figure 2a). The dominant vegetation consists of ericaceous shrubs of the family Ericaceae (Vaccinium myrtillus, V. vitis-idaea, V. uliginosum, Empetrum nigrum, Ledum palustre), mosses Bryophyta, and lichens of the genus Cladonia. The thickness of the wet litter layer is, on average, 9.5 ± 1.3 cm.

Three disturbed sites, according to visual assessments of soil cover transformation degree and self-healing, in 2021, i.e., 9 years after the clear-cutting and 8 years after the fire, can be arranged as follows from least to most disturbed: C → Fb → Cb.

The burnt pine forest (Fb) as a result of an extensive grass-root fire lost the undergrowth of spruce and birch, and most of the pines have fallen their crowns (Figure 2b). The litter layer burned out, and the soil surface was dark-colored. Eight years later, the ash from the soil surface was washed away, and the site is almost completely overgrown with vegetatively renewable blueberries (Vaccinium myrtillus), which is typical for post-fire forests in the Kola Subarctic region [45]. Charred remains on tree trunks, which reached a height of 2.5–3.5 m after the fire, were still preserved at a height of 0.5–1.0 m despite the bark falling. The rare curtains of grasses genus Deschámpsia, herbs Chamaenerion angustifolium, lingonberry Vaccinium vitis-idaea in wet depressions, and young birch trees up to a height of 40–50 cm were observed sporadically.

Both clear-cuttings, C and Cb, uncleared since 2012, are littered with tree trunks and branches, making them difficult to traverse. The first one (C) saved the forest litter and a layer of shrubs, mosses, and lichens typical of the control mountain taiga forest (F) and was actively colonized by young birch trees (Figure 2c,e). The litter was wet, with peat formation in the lower layer. The second one (Cb) was damaged for two years running: the clear-cutting destroyed the tree stand in 2012, and a ground fire decimated the vegetation and litter layer in 2013, exposing the mineral soil horizons. Eight to nine years later, Cb appeared the most disturbed and overgrown weakly (Figure 2d,f). Birch undergrowth was sparse; its height was noticeably lower than in C. The eroded soil was covered by a thin layer of liverwort, Hepaticophyta. Perennial grasses and curtains of Deschampsia cereals were found among fallen, charred, and barkless tree trunks.

Both burned sites, Fb and Cb, were characterized by a diverse assemblage of xylotrophic aphyllophoroid fungi, including both annual species like Fomitopsis betulina, Cerrena unicolor, and Trichaptum fuscoviolaceum, and long-life species such as Fomes fomentarius, Fomitopsis pinicola, and Phellinus igniarius, as well as the agaricoid fungus Kuehneromyces mutabilis and the species of the genus Trametes. The area of Cb was a real kingdom of these wood-destroying fungi in 2018, or the fifth year after the fire. In Fb, the mass occurrence of fungi was observed later, in 2021, and they were less diverse and abundant compared to Cb.

3.1.2. Soil Properties

After the grass-roots fire, the content of organic matter and basic nutrients in the forest litter decreased. In 2015, the loss of organic matter in the burned sites Fb and Cb did not exceed 75%, compared to 88.4 ± 5.0% in the control site F. The reduction in the share of total carbon from 50% (F) to 36–39% (Fb, Cb) and nitrogen from 1.3 ± 0.1 to 0.8–0.9% led to an extension of the C:N ratio to 43 (Fb, Cb) compared to 37 (F). A significant increase in the ash content from 11.6 ± 5% (F) to 20–30% in the burnt soils led to the alkalization of the soil solution to pH_H2O values of 4.2–4.6 against 3.8 ± 0.1 (F).

A decade later, in 2021, the soils of all of the disturbed sites still differed from the control one, with lower levels of organic matter (64–83%), total carbon (37–44%), and nitrogen (0.8–1.2%) (Figure 4a–c). The soils of both burnt areas, Fb and Cb, yielded the unburned litters of F and C in terms of the carbon and nitrogen content, as well as important nutrients such as potassium (K) and phosphorus (P) (Figure 4d,e). The content of these elements, as well as magnesium (Figure 4f), was significantly lower in the twice-disturbed Cb. Conversely, the content of Ca, Mg (except for Cb), and ash was higher in disturbed soils (Figure 4g–h), resulting in their significant alkalization to pH_H2O values of 4.1–4.6 compared with 3.7 ± 0.1 in the control soil (Figure 4i).

The noticeable reduction in organic matter content, loss of humus throughout the soil profile, and changes in the morphological profile of forest soils due to the burning of litter and the upper humus horizon during grass-root fires have been repeatedly documented in the literature [46,47,48].

The more significant losses of K and P on the burned sites Fb and Cb compared to the unburned clear-cutting C confirm the primary influence of fire on reducing the accumulation of these biogenic elements in the disturbed litters and enhancing their leaching and washing out from the upper soil horizons under the humid conditions of the mountain microclimate and flushing soil regime [43,49,50,51]. Post-fire alkalinization of soil solutions, coupled with an increase in the ash content, further enhances the mobility and migration of K, P, and N and most cations downward through the soil profile [48,52].

The decrease in K and P, along with the increased levels of Ca and Mg in the upper layer of disturbed soils compared to the control forest, can also be explained by the replacement of coniferous to deciduous tree species during self-healing. It is proven that deciduous trees in the Kola Subarctic have a higher content of mineral elements in their assimilating organs and produce more alkaline litter compared to conifer ones. Among the ash-forming elements, Ca and K predominate in leaves, with Mg concentrations reaching 2.4 g/kg compared to 900 mg/kg in pine needles [53,54]. In Khibiny Mts, a biogeochemical peculiarity is the increased accumulation of K, Na, Mn, and P in spruce needles and Ca + Mg in birch leaves [51]. In disturbed areas of the Kuniyok valley, the cut down and burned coniferous forest was replaced by deciduous vegetation, particularly birch stands. The height and density of young birches are more significant in unburned clear-cuttings (C), which are also characterized by the highest content of K, Ca, and Mg in the litter.

3.1.3. Soil Temperature

Our measurements using autonomous loggers revealed significant variability in the soil temperature regime over the long-term dynamics. This did not allow for the identification of stable differences in the mountain soils depending on the type of disturbance. This can be explained by the location of the sites in the intermountain river valley, which, firstly, stretches in a meridional direction and is exposed to Arctic winds, and secondly, is influenced by air masses from the wide, cold Kuniyok River.

So, in the summer of 2015, two years after the fire, the dark-colored decomposed litter in Fb warmed significantly better than at the adjacent control forest F. The minimal, maximal, average, and cumulative soil temperatures were higher in Fb, which was also due to the sparse and crownless tree stands and burned shrub layer.

Over the 5 years, ash was washed out from the soil cover in the burned sites Fb and Cb, and in 2018, the temperature was significantly higher in the thick litters of the unburned sites F and C. These observations are consistent with classical notions about the thermoregulatory function of the forest litter [55,56,57,58,59]. The summer cumulative temperature was +927 and +954 °C at unburned sites F and C and 60–100 °C lower on the burned Fb and Cb sites.

In the hot summer of 2021, soils in all disturbed sites warmed better than in control forests. Its temperature was more strongly dependent on the dynamics of atmospheric air: the coefficient of correlation r was ≥0.99 for Fb, C, and Cb compared to r = 0.83 in F. The absolute maximums in the disturbed soils (+22.5–+29.4 °C) were the highest among all previously studied soils in the various high-altitude belts of Khibiny Mts. Both clear-cuttings warmed up 7–10 °C better than forest sites, reaching in July absolute maximums of +27.4 °C in C and +29.8 °C in Cb compared with +25.5 °C in Fb and +24.8 °C in the control site F, but cooled down earlier in August.

Thus, in different years, the soils achieved better summer warming in the burned sites, either burned forest Fb or burned clear-cutting Cb (

Table 1. Soil temperatures at monitoring sites in the Kuniyok valley in research years.

Temperature Parameter/Site	F	Fb	C	Cb
Annual temperature, °C
Average	+1.4–+1.5	+1.7–+1.9	+2.3–+2.8	+1.6–+2.6
Absolute maximum	+12.5–+20.4	+15.3–+19.9	+19.7–+29.1	+16.6–+28.3
Absolute minimum	−5.2–−2.6	−5.0–−3.4	−4.7–−2.6	−5.7–−3.4
Amplitude	15.2–25.5	20.3–23.3	22.3–33.7	22.3–31.7
Number of days with threshold temperatures
T < 0 °C	182–208	161–178	157–166	166–172
T ≥ 0 °C	83–109	113–130	125–133	120–125
0 ≤ T < +5 °C	27–53	47–63	58–69	54–65
+5 ≤ T < +10 °C	24–41	23–36	22–31	24–32
T ≥ +10 °C	11–32	30–34	34–44	28–41
Annual sum of threshold temperatures, °C
T < 0 °C	−239–−146	−188–−144	−173–−110	−211–−165
T ≥ 0 °C	541–665	690–710	786–976	677–913
0 ≤ T < +5 °C	47–110	44–80	47–76	40–96
+5 ≤ T < +10 °C	168–293	157–239	147–215	156–223
T ≥ +10 °C	122–450	366–476	495–779	359–713

Note: Site designations F, Fb, C, and Cb, as shown in Figure 2.

).

Overall, differences in the summer warming of soils both between years and depending on the type of disturbance were determined by the short period with active temperatures ≥ +10 °C. It occurred in July during warm years and shifted to August in cooler years due to the slow warming of moist mountain soils. In clear-cuttings, this period was recorded 7–17 days earlier compared to forested areas.

In autumn, all of the disturbed sites cooled down faster. The threshold temperatures < +5 °C started in the second decade of October and ten days later in the shaded control forest. Throughout autumn, the temperature was above +1 °C in the litter of the control site F, close to zero on the burned sites Fb and Cb, and dropped below (to −1.2 °C) only in the moist litter of the unburned clear-cutting C.

In different winters, when the atmospheric air in the valley cooled down to −22–−30 °C, different pairs of sites froze more deeply. It was either burnt soils in Fb and Cb, which reached absolute minimums of −5.7–−5.0 °C compared to −2.6 °C in unburned sites F and C, or conversely, unburned soils (−5.2–−4.7 °C compared to −3.4 °C in burned areas). A common winter feature of the Kuniyok valley was the additional freezing of soils to absolute annual minimums in early December, along with freezing in the first decade of February, typical for Khibiny mountain soils at different altitudes. The spring thawing of the upper 5 cm soil layer to a temperature > 0 °C occurred 9–11 days earlier in disturbed sites than in the control (by 13–15 May vs. 24 May).

3.2. Arthropod Communities

3.2.1. Spiders (Arachnida, Aranei)

A total of 61 spider species from 41 genera and 12 families were identified in the Kunyok valley. The families Linyphiidae, Lycosidae, and Gnaphosidae had the greatest diversity (30, 10, and 8 species, respectively), while the first two of them predominated quantitatively (

Table 2. Species composition of spiders (Arachnida, Aranei) at monitoring sites in the Kuniyok valley.

Taxon			Sampling Site
Family	Species	Acronym	F	Fb	C	Cb
Araneidae (1)	Cyclosa conica (Pallas, 1772) **	Cy.con	–	+	–	–
Lycosidae (10)	Acantholycosa lignaria (Clerck, 1757) *,**	Ac.lig	–	–	–	+
	Acantholycosa norvegica (Thorell, 1872) **	Ac.nor	–	–	+	–
	Alopecosa aculeata (Clerck, 1757)	Al.ac	+	+	+	+
	Alopecosa pinetorum (Thorell, 1856)	Al.pi	+	+	+	+
	Pardosa eiseni (Thorell, 1875)	Par.eis	–	+	+	+
	Pardosa hyperborea (Thorell, 1872)	Par.hyp	–	+	+	+
	Pardosa lasciva L. Koch, 1879 *	Par.las	+	+	+	+
	Pardosa lugubris (Walckenaer, 1802)	Par.lug	+	+	+	–
	Pardosa palustris (Linnaeus, 1758)	Par.pal	–	+	+	+
	Xerolycosa nemoralis (Westring, 1861)	Xe.nem	–	–	–	+
Linyphiidae (30)	Agnyphantes expunctus (O. Pickard-Cambridge, 1875)	Agn.ex	+	+	–	–
	Agyneta cauta (O. Pickard-Cambridge, 1903)	Ag.cau	–	+	–	–
	Agyneta gulosa (L. Koch, 1869)	Ag.gul	–	+	–	+
	Agyneta subtilis (O. Pickard-Cambridge, 1863) *,**	Ag.sub	+	–	–	–
	Bolyphantes luteolus (Blackwall, 1833)	B.lut	–	–	+	–
	Centromerus arcanus (O. Pickard-Cambridge, 1873)	Cen.ar	+	+	+	+
	Ceratinella brevipes (Westring, 1851)	Cer.br	+	+	–	–
	Diplocentria rectangulata (Emerton, 1915)	Dip.rec	+	+	+	+
	Drapetisca socialis (Sundevall, 1833)	Dr.soc	–	+	–	–
	Helophora insignis (Blackwall, 1841) *,**	Hel.in	–	+	–	–
	Hilaira herniosa (Thorell, 1875)	Hil.her	+	+	+	+
	Hypselistes jacksoni (O. Pickard-Cambridge, 1903)	Hyp.jac	+	–	–	–
	Leptothrix hardyi (Blackwall, 1850) *	L.har	–	–	–	+
	Macrargus rufus (Wider, 1834)	Mac.ruf	–	–	+	–
	Maso sundevalli (Westring, 1851)	Mas.sun	+	+	–	–
	Micrargus herbigradus (Blackwall, 1854)	Mic.her	–	–	–	+
	Minyriolus pusillus (Wider, 1834)	Min.pus	+	+	+	–
	Neriene clathrata (Sundevall, 1830)	N.cl	+	+	–	–
	Oreonetides vaginatus (Thorell, 1872) **	O.vag	–	+	–	–
	Palliduphantes antroniensis (Schenkel, 1933)	Pal.an	+	–	–	+
	Porrhomma pallidum Jackson, 1913	Por.pal	+	+	–	+
	Semljicola latus (Holm, 1939)	S.lat	+	+	–	–
	Tenuiphantes alacris (Blackwall, 1853)	T.al	+	+	–	–
	Tenuiphantes mengei (Kulczyński, 1887)	T.men	–	+	–	–
	Tenuiphantes nigriventris (L. Koch, 1879)	T.nig	+	+	–	–
	Tenuiphantes tenebricola (Wider, 1834)	T.ten	+	+	+	–
	Tibioplus diversus (L. Koch, 1879)	Tib.div	–	–	–	+
	Walckenaeria karpinskii (O. Pickard-Cambridge, 1873)	W.kar	+	+	–	+
	Walckenaeria nudipalpis (Westring, 1851)	W.nud	–	+	–	–
	Zornella cultrigera (L. Koch, 1879)	Zorn.cul	+	+	+	+
Liocranidae (2)	Agroeca cf. lusatica (L. Koch, 1875)	Ag.lus	–	+	–	–
	Agroeca proxima (O. Pickard-Cambridge, 1871)	Ag.pr	+	+	+	+
Miturgidae (1)	Zora nemoralis (Blackwall, 1861)	Zo.nem	+	+	–	–
Hahniidae (1)	Hahnia ononidum Simon, 1875 **	Ha.on	–	+	–	–
Thomisidae (4)	Psammitis sabulosus (Hahn, 1832)	Ps.sub	–	+	–	+
	Xysticus audax (Schrank, 1803)	Xy.aud	–	+	–	–
	Xysticus luctuosus (Blackwall, 1836) **	Xy.luc	–	+	–	–
	Xysticus obscurus Collett, 1877	Xy.ob	+	–	+	–
Cybaeidae (1)	Cryphoeca silvicola (C. L. Koch, 1834)	Cr.sil	+	–	+	–
Mimetidae (1)	Ero furcata (Villers, 1789) *	Er.fur	–	+	+	–
Salticidae (1)	Evarcha falcata (Clerck, 1757) **	Ev.fal	–	+	–	–
Gnaphosidae (8)	Gnaphosa lapponum (L. Koch, 1866) **	Gn.lap	–	–	+	–
	Gnaphosa montana (L. Koch, 1866) *,**	Gn.mon	–	+	–	–
	Gnaphosa sticta Kulczyński, 1908 *,**	Gn.st	–	–	–	+
	Haplodrassus signifer (C. L. Koch, 1839)	Ha.sig	–	+	+	–
	Haplodrassus soerenseni (Strand, 1900) **	Ha.soe	+	–	–	–
	Micaria aenea Thorell, 1871	M.aen	+	–	+	–
	Micaria alpina L. Koch, 1872	M.alp	–	–	+	+
	Micaria silesiaca L. Koch, 1875 **	M.sil	–	–	–	+
Theridiidae (1)	Robertus lividus (Blackwall, 1836) **	R.liv	–	–	–	+
In total (from 61 species found)			28	41	24	25
Species new for Khibiny Mts (among the nine species found)			2	4	2	4

Note: The Aranei nomenclature follows [61]. In parentheses—the number of species found in each family; one asterisk (*) denotes species not previously noted in Khibiny Mts; two stars (**) indicate species found in a single specimen; a plus (+) means the species was found on the site, and a dash (–) means species not found. Site designations F, Fb, C, and Cb, as shown in Figure 2.

). The disturbed areas Fb, C, and Cb were inhabited by 58 species from 40 genera and 12 families, which accounted for 34% of the known spider diversity in Khibiny Mts [60]. The burnt forest (Fb) exhibited high diversity, with 41 species from 30 genera and 10 families vs. 28 species from 24 genera and 7 families in the adjacent control site F. The highest number of unique species not found in other sites (13) was also collected in Fb, while only 3 unique species were collected at control site F. Both clear-cuttings, C and Cb, had a similar number of taxa (24–25 species from 18–20 genera and 7 families) but were inferior to both forest sites F and Fb. A higher number of unique species (8) observed in the burnt clear-cutting Cb.

Only 8 spider species out of 61, or 13%, were common to the two pairs of monitoring sites separated at a distance of 1 km from each other. Among them, the North European-Trans-Siberian boreo-montane species Pardosa lasciva (Lycosidae), with 17 collected specimens, is reported for the first time in the Khibiny Mts. One species (Agyneta subtilis) was recorded in the Khibiny Mts for the first time from the control pine forest and seven species from the three disturbed sites (Table 2). In particular, Helophora insignis and Gnaphosa montana were captured in Fb; Ero furcata in both Fb and C; Leptothrix hardyi, Acantholycosa lignaria, Gnaphosa sticta, and Micaria silesiaca in Cb. The finding of the European-West Siberian species Micaria silesiaca (2 specimens) is new for the Kola Polar region. As a result of research in the Kunyok valley, we have expanded the local spider fauna of Khibiny Mts to 169 species, compared with 161 previously known [60].

Adult spiders were most abundant in the litter layer of the control forest (36% of the total spider number in the valley), which is primarily due to web-building spiders (Linyphiidae mainly), which accounted for 83%. The dominant species was Hilaira herniosa (46%), and the subdominants were Tenuiphantes tenebricola (27%) and T. alacris (12%). On disturbed sites, these species were either scarce or absent, and the numbers of web-building spiders did not reach the control level, despite increased species diversity. Similar effects of clear-cutting on spiders were observed by F. Coyle in the southern Appalachians, where the diversity of certain ecological groups, including web-building spiders, decreased, while others (e.g., cursorial hunters) increased [62].

Juvenile spiders, on the contrary, were most abundant in the clear-cuttings, primarily due to the presence of linyphiid spiders. Almost 48% of the captured juveniles were concentrated in Cb, 28% in C, and only 17% in the control site F. In the burnt forest Fb, the abundance of both adult and young spiders was the lowest among the sites: 15% and 8%, respectively. Despite the highest overall diversity and the diversity of both unique and new for Khibiny Mts species, most of them had a low abundance, and 14 of the 41 species were singletons.

All disturbed sites differed from the control forest in terms of increased diversity and abundance of cursorial spiders: wolf spiders (Lycosidae), ground spiders (Gnaphosidae), and crab spiders (Thomisidae). The increase in the degree of sites disturbance was accompanied by a change in the dominants from forest-dwelling linyphiid species to active diurnal predators of the genera Pardosa, Alopecosa, and Xerolycosa (Lycosidae). In the control site, Linyphiidae associated with forest litter (Hilaira herniosa, Tenuiphantes tenebricola, T. alacris, and Centromerus arcanus) predominated. In the burnt forest Fb, only one lycosid species, Pardosa lugubris, dominated, along with T. tenebricola and C. arcanus. In the clear-cuttings, there were already five lycosid species (Pardosa hyperborea, P. eiseni, P. palustris, Alopecosa aculeata, A. pinetorum, and Xerolycosa nemoralis).

The change in the dominance of Linyphiidae wolf spiders in the control pine forest by Lycosidae species in the burned area over three summer seasons was noted by S. Kopponen in Finland [63]. Lycosidae, Gnaphosidae and Theridiidae were more species-rich at the burned than control forest site. The author also showed differences of ground-living spider communities in habitats which have been under different level of human activity [64]. Linyphiidae dominated, both at species and individual level, in the groves, Lycosidae were abundant on the wooded meadows and Gnaphosidae on the wooded pasture. Gnaphosidae species prefer open and warm habitats and have also been found in large numbers in dry and open habitats on the islands of the south-western Finnish archipelago [65]. A decrease in the number of spider species and an increase in the number of hunting species after a fire have also been described in a mountain mixed deciduous-coniferous forest in China [66] and, obviously, is a general pattern for ground-dwelling spider assemblages in disturbed territories [67].

3.2.2. Rove Beetles (Coleoptera, Staphylinidae)

Rove beetles were represented by 41 species from 23 genera and 8 subfamilies (

Table 3. Species composition of rove beetles (Coleoptera, Staphylinidae) at monitoring sites in the Kuniyok valley.

Taxon			Sampling Site
Subfamily	Species	Acronym	F	Fb	C	Cb
Omaliinae (7)	Acidota crenata (Fabricius, 1793)	A.cr	–	–	+	–
	Anthophagus caraboides (Linne, 1758)	An.car	+	–	–	+
	Anthophagus omalinus (Zetterstedt, 1828)	An.om	–	+	+	+
	Arpedium brachypterum (Gravenhorst, 1802)	Ar.bra	+	+	+	–
	Arpedium brunnescens (J. Sahlberg, 1871)	Ar.bru	+	+	+	+
	Arpedium quadrum (Gravenhorst, 1806)	Ar.qua	+	+	+	–
	Olophrum boreale (Paykull, 1792)	Ol.bor	–	+	–	–
Tachyporinae (8)	Carphacis striatus (Olivier, 1795) **	Ca.str	–	–	+	+
	Ischnosoma splendidum (Gravenchorst, 1806) **	Is.spl	–	–	+	+
	Lordithon trimaculatus (Fabricius, 1793)	L.tr	–	+	–	–
	Mycetoporus lepidus (Gravenhorst, 1806)	M.lep	+	+	+	–
	Mycetoporus longulus (Mannerheim, 1830)	M.lon	–	+	–	–
	Tachinus humeralis (Gravenhorst, 1802) *	T.hum	–	+	–	–
	Tachinus pallipes (Gravenchorst, 1806)	T.pal	+	+	–	–
	Tachinus proximus (Kraatz, 1855)	T.pr	–	+	–	+
Aleocharinae (12)	Aleochara brevipennis (Gravenhorst, 1806)	Al.bre	–	+	–	–
	Atheta brunneipennis (Thomson, 1852)	Al.bru	+	+	–	–
	Atheta fungi (Gravenhorst, 1806)	A.fun	–	+	–	–
	Atheta graminicola (Gravenhorst 1806)	A.gr	–	+	–	+
	Geostiba circellaris (Gravenhorst, 1806)	G.cir	–	+	+	+
	Liogluta alpestris (Heer, 1839)	L.alp	–	+	–	–
	Liogluta granigera (Kiesenwetter, 1850)	L.gr	–	+	–	+
	Liogluta micans (Mulsant & Rey, 1852)	L.mic	+	+	–	+
	Bolitochara pulchra (Gravenhorst, 1806)	B.pul	+	+	–	+
	Pella humeralis (Gravenhorst, 1802)	P.hum	+	+	+	+
	Oxypoda alternans (Gravenhorst, 1802) *	Ox.al	+	+	+	+
	Oxypoda annularis (Mannerheim, 1830)	Ox.an	+	+	+	+
Scaphidiinae (1)	Scaphisoma agaricinum (Linnaeus, 1758) *,**	S.ag	–	–	–	+
Steninae (3)	Stenus biguttatus (Linne, 1758) *	St.big	+	+	–	–
	Stenus palustris (Erichson, 1839)	St.pal	+	+	+	–
	Stenus tarsalis (Ljungh, 1810) *	St.tar	–	+	–	+
Paederinae (1)	Lathrobium brunnipes (Fabricius, 1792)	L.br	–	+	+	–
Xantholininae (3)	Othius lapidicola Kiesenwetter, 1848	O.lap	+	–	–	–
	Xantholinus linearis (Olivier, 1795) **	X.lin	–	+	–	–
	Xantholinus tricolor (Fabricius, 1787)	X.tr	–	+	+	–
Staphylininae (6)	Philonthus cephalotes (Gravenhorst, 1802) *,**	Ph.cep	–	+	–	–
	Philonthus politus (Linnaeus, 1758) *,**	Ph.pol	–	+	–	–
	Quedius boops (Gravenhorst, 1802) *,**	Q.boo	–	+	–	–
	Quedius limbatus (Heer, 1834) *,**	Q.lim	–	+	–	–
	Quedius semiaeneus (Stephens, 1833) **	Q.sem	–	–	+	–
	Quedius umbrinus Erichson, 1839 **	Q.um	–	+	–	–
In total (from 41 species found)			15	34	16	11
Species new for Khibiny Mts (among the nine species found)			2	8	1	3

Note: The Staphylinidae nomenclature is given by [70]. In parentheses—the number of species in each subfamily found; (*)—species was not previously noted in the Khibiny Mts; two stars (**)—species found in a single specimen; a plus (+) means the species was found on the site, and a dash (–) means species not found. Site designations F, Fb, C, and Cb, as shown in Figure 2.

). Among them, four subfamilies were the most diverse: Aleocharinae—twelve species; Tachyporinae—eight; Omaliinae—seven; and Staphylininae—six. These subfamilies, as a rule, prevail in the staphylinid fauna of Khibiny Mts and are typical for ecosystems of the northern taiga, southern tundra, and alpine belts [68,69]. Approximately one-third of the species of each subfamily known for Khibiny Mts were concentrated in the valley: Aleocharinae—28%; Tachyporinae—38; Omaliinae—29; and Staphylininae—40%. The subfamilies Euaesthetinae, Oxytelinae, and Proteininae, represented by one or two species in Khibiny Mts, were not found in the Kunyok valley. Subfamily Scaphydiinae with the small beetle species Scaphisoma agaricinum (Linnaeus, 1758), captured in Cb (three imago), we report in Khibiny Mts for the first time.

An additional eight species, new to the Khibiny Mts, belonging to the genera Tachinus, Oxypoda, Stenus, Philonthus, and Quedius, were identified in the Kunyok valley. Thus, the local rove beetles fauna of these polar mountains has been expanded to 127 species, one-third of which (or 32%) were recorded in clear-cuttings and burnt areas. Most of the new rove beetles species (7), like the spiders, inhabited burnt sites: five species in Fb, one in Cb, and two in both plots (Table 3). Only in Fb, two actively flying predators of the genus Philonthus (Staphylininae), Ph. cephalotes and Ph. politus, were caught in Khibiny Mts. for the first time.

The Fb was distinguished by the largest number of unique species (13 compared to 1–4 in other sites) and taxonomic diversity and abundance of rove beetles overall. Generally, 34 species from 17 genera and 7 subfamilies were recorded here, compared to 15–16 species in the control F and unburnt C, with the highest diversity of subfamilies Aleocharinae, Tachyporinae, Staphylininae, Steninae, and Xantholininae. Beetles of seven subfamilies were common to adjacent forests F and Fb, but the number of species in each subfamily was higher in the burnt forest, especially for Aleocharinae, Staphylininae, and Tachyporinae (on 6, 5, and 4 species, respectively).

Conversely, the rove beetle complex in the twice-disturbed Cb was the poorest. Only 11 species from 10 genera and 5 subfamilies dwell here; therefore, each genus was represented by a single species. More than half of the species (6) belonged to the subfamily Aleocharinae. However, three specimens of the new species Scaphisoma agaricinum were found only on Cb.

In the clear-cutting C with undisturbed litter, the number of rove beetle species and genera matched the control forest, but the number of unique species was higher in the C: four species (Acidota crenata, Carphacis striatus, Ischnosoma splendidum, and Quedius semiaeneus) vs. one (Othius lapidicola) in F.

Overall, among the 41 rove beetles species collected in the Kunyok valley, 40 species inhabited the three disturbed sites. All species belonged to the trophic group of zoophages. According to biotopic preferences, these were forest litters or eurytopic species, inhabiting the mountain-forest and mountain-tundra belts of Khibiny Mts. The ratio of these groups, equal in the control (50% of species in each), shifted towards the predominance of eurytopic species in disturbed sites: 59% in Fb and 63% in C.

3.2.3. Ground Beetles (Coleoptera, Carabidae)

Ground beetles in the Kunyok valley belonged to 22 species, 10 genera, and 9 tribes, with the highest diversity observed in the tribes Notiophilini, Zabrini, Harpalini (each with 4 species), and Pterostichini (3 species) (

Table 4. Species composition of ground beetles (Coleoptera, Carabidae) at monitoring sites in the Kuniyok valley.

Taxon			Sampling Site
Tribe	Species	Acronym	F	Fb	C	Cb
Notiophilini (4)	Notiophilus aestuans Dejean, 1826 *,**	N.ae	–	+	–	–
	Notiophilus aquaticus Linnaeus, 1758	N.aq	+	+	–	+
	Notiophilus biguttatus (Fabricius 1779)	N.big	+	+	–	–
	Notiophilus germinyi (Fauvel & Grenier, 1863)	N.ger	–	+	+	+
Carabin (2)	Carabus (Oreocarabus) glabratus Paykull, 1790	C.gl	–	+	+	+
	Carabus (Hemicarabus) nitens Linnaeus, 1758	C.nit	–	+	–	–
Broscini (1)	Miscodera arctica (Paykull, 1798)	M.ar	–	+	+	+
Bembidiini (1)	Bembidion (Ocydromus) grapii Gyllenhal, 1827 *	B.gr	–	+	–	–
Pterostichini (3)	Pterostichus (Bothriopterus) adstrictus Eschscholtz, 1823 *	Pt.ad	–	+	–	+
	Pterostichus (Cryobius) brevicornis (Kirby, 1837)	Pt.br	+	+	+	+
	Pterostichus (Bothriopterus) oblongopunctatum (Fabricius, 1787) *	Pt.ob	–	+	–	+
Sphodrini (2)	Calathus (Neocalathus) melanocephalus (Linnaeus, 1758)	C.mel	–	–	–	+
	Calathus (Neocalathus) micropterus (Duftschmid, 1812)	C.mic	+	+	+	+
Zabrini (4)	Amara (Amara) famelica (Zimmermann, 1832) *	A.fam	–	–	–	+
	Amara (Amara) lunicollis Schiødte, 1837 *	A.lun	–	+	+	+
	Amara (Amarocelia) erratica (Duftschmid, 1812)	A.er	–	+	+	+
	Amara (Paracelia) quenseli (Schönherr, 1806)	A.qu	–	–	–	+
Harpalini (4)	Dicheirotrichus (Trichocellus) cognatus (Gyllenhal, 1827) *,**	D.cog	–	–	–	+
	Harpalus laevipes Zetterstedt, 1828	H.lae	–	+	–	+
	Harpalus nigritarsis C.R. Sahlberg, 1827 *,**	H.nig	–	–	–	+
	Harpalus solitaris? Dejean, 1829 *,**	H.sol	–	+	–	–
Lebiini (1)	Cymindis (Tarus) vaporariorum (Linnaeus, 1758)	C.vap	–	–	+	–
In total (from 22 species found)			4	16	8	16
Species new for Khibiny Mts (among the 10 species found)			–	6	1	6

Note: The systematic and nomenclature of the Carabidae family is provided by [71]. In parentheses—the number of species in each tribe found; (*)—species was not previously noted in the Khibiny Mts; two stars (**)—species found in a single specimen; a plus (+) means the species was found on the site, and a dash (–) means species not found. Site designations F, Fb, C, and Cb, as shown in Figure 2.

).

Only two species were present in the thick, moist litter layer of the control plot F throughout the entire study period: the polyzonal forest species Calathus micropterus, the most common and abundant ground beetle species in Khibiny Mts, and the arcto-alpine species Pterostichus brevicornis, which is less abundant but inhabits all elevations in Khibiny Mts, including the cold rocky deserts on plateaued peaks [33]. The dominant species, C. micropterus, accounted for 83%, 90%, and 95% of the total number of ground beetles in 2015, 2018, and 2021, respectively. Both species inhabited three disturbed sites, but in smaller numbers compared to the control forest. C. micropterus maintained high dominance in unburnt clear-cutting C (57–72% in different years) among seven other ground beetle species captured here in one or two specimens. Among them, the rare tundra-taiga species Cymindis vaporariorum was caught only at this site.

Five species were common to all disturbed sites. Among them, three species, Amara erratica, Amara lunicollis, and Miscodera arctica, reached high abundances and dominated in Cb, while the large forest beetle Carabus glabratus dominated in the burnt pine forest Fb, and the small Notiophilus germinyi was present on both burnt areas, particularly five years after the fire. Additionally, A. lunicollis was recorded in Khibiny Mts for the first time.

Overall, ground beetles were more diverse and abundant on the burnt sites Fb and Cb (16 species in each) compared to the clear-cutting C (8 species) and the control F (4 species). Notiophilus aquaticus, Harpalus laevipes, Pterostichus adstrictus, and Pterostichus oblongopunctatus were captured in significant numbers only at the burnt sites. Both species from the genus Pterostichus were not previously recorded in Khibiny Mts. Additionally, only in Fb, the abundant Notiophilus biguttatus, rare species Carabus nitens, listed in the regional Red Data Book [72], scarce Bembidion grapii, and singular Notiophilus aestuans and Harpalus solitaris were found. The latter three species were also not previously recorded in Khibiny Mts. Only in Cb were the numerous Amara famelica and Amara quenseli, scarce Calathus melanocephalus, singular Harpalus nigritarsis and Dicheirotrichus cognatus captured, all of which, except C. melanocephalus, were noted in Khibiny Mts for the first time. In total, nine species of ground beetles, new to the carabid fauna of Khibiny Mts, were identified in the two burnt areas, whereas only one new species was found on the unburnt clear-cutting C (Table 4).

An increase in the number of ground beetle species across habitats, from control F (4 species) to C (8), Fb, and Cb (16 species in each), corresponded to an increase in the number of life forms: F (2) → C (6) → Fb, Cb (7 in each). This indicated a greater heterogeneity of the environment and a variety of habitat conditions at the disturbed sites. Across all sites, two groups of zoophagous beetles were represented: surface-litter-dwelling and litter-dwelling stratobionts.

On three disturbed sites, four additional life forms were added: a large walking zoophagous epigeobiont, Carabus glabratus (the largest ground beetle in the carabid fauna of the Kola Polar region), the small digging geobionts of the genera Bembidion and Dicheirotrichus, and two groups of mixophytophagous ground beetles: a running and burrowing geohortobiont, Miscodera arctica, and harpaloid geohortobionts of the genera Amara and Harpalus. On both burnt sites, the seventh life form was represented by surface and litter and soil-dwelling zoophagous stratobiont Pterostichus oblongopunctatus.

4. Discussion

During the monitoring period in the Kunyok valley, a total of 124 arthropod species were identified, including 61 species of spiders, 41 species of rove beetles, and 22 species of ground beetles. This corresponded to a decrease in the known diversity of these taxa in Khibiny Mts: spiders—169 species; rove beetles—128; and ground beetles—48 species ([33,60,73], our new data). Therefore, the proportions of each taxa in the intermountain valley were 36, 32, and 46%, respectively, of their diversity in Khibiny Mts. Of these, 79 species, or 64% (33 species of spiders, 26 species of rove beetles, and 20 species of ground beetles), were captured only on disturbed sites and were absent in the control mountain taiga forest, whereas only three species of spiders (Hypselistes jacksoni, Agyneta subtilis, and Haplodrassus soerenseni) and one rove beetle, Othius lapidicola, inhabited the control forest, and there were no ground beetles unique to control. Only eight species of spiders (or 13%), three species of rove beetles (7%), and two species of ground beetles (9%) were common to the four monitoring sites located within a radius of 0.5 km from each other (Figure 5, upper parts).

The low species similarity in all taxa between the monitoring sites, on the one hand, pointed to the specificity of habitat conditions depending on the type of anthropogenic impact, since the close location of sites within a radius of 0.5 km was not a barrier to the migration of mobile arthropods. On the other hand, the weak faunal similarity of disturbed sites with control forests indicates the slow recovery of disturbed northern taiga forests in the conditions of the polar mountains. This is confirmed by the results of the chemical analysis, according to which disturbed soils and, especially, burnt soils, did not return to the control values typical for mountain taiga soils by the end of the first decade of exposure (Figure 4).

The habitat specificity across different sites was confirmed by differences in the total number of species and abundance of arthropods, the number of unique species collected only on one of the sites during the monitoring period, and species that were new to the local fauna of Khibiny Mts. In each of these groups, more arthropod species were found on the burnt areas Fb and Cb compared to the unburnt areas F and C, and between the burnt areas, more species were found in the burnt pine forest Fb compared to the burnt clearing Cb (Figure 5, bottom).

The slow rates of both self-healing of disturbed mountainous territories and their colonization by invertebrates are also confirmed by the high number of arthropods in 2018 and 2021 (i.e., by 6–9 years of violations), compared to 2015, including species inhabiting open spaces. Among them, for example, three ground beetle species, Amara lunicollis, Bembidion grapii, and Pterostichus adstrictus, reached their greatest numbers in the burnt areas Fb and Cb in 2018, and the latter also in the burnt pine forest Fb in 2021.

Obtained faunal data allowed to contrast the monitoring sites based on the following criteria: «control—disturbed» (F–Fb, C, Cb), «burnt – unburned» (Fb, Cb–F, C), and «forests–clear-cuttings», i.e., wooded and open sites (F, Fb–C, Cb). Thus, all disturbed sites Fb, C, and Cb were characterized by a greater number of unique species and had 4–5 common species of spiders, rove beetles, and ground beetles that were not found in the control forest F. On the burnt areas, Fb and Cb, there were more common species of each arthropod taxa compared to the clear-cuttings C and Cb.

Among spiders, web-building Linyphiidae and Cybaeidae species showed a clear preference for the complex multilayered structure of the control taiga forest F and its thick, moist litter, formed by shrubs, lichens, and green mosses, being most abundant in this area. On all disturbed sites, hunters of Lycosidae and Gnaphosidae spiders, as well as Thomisidae, which do not build webs, were more diverse and abundant. Both clear-cuttings, C and Cb, differed from the forest habitats F and Fb in the greater number of dominant Lycosidae species and abundance of juvenile individuals of Linyphiidae spiders. Both burnt areas, Fb and Cb, had fewer unique species compared to forest habitats but had more common species and species that were new to the spider fauna of Khibiny Mts.

Among rove beetles, eurytopic species dominated those dwelling in the forest litter, on disturbed plots Fb, C, and Cb compared to the control forest F, and also in forest sites F and Fb compared to the clear-cuttings. The unburnt sites F and C had a similar species number (15 and 16) compared to the increased number in Fb (34) and the decreased one in Cb (11 species). Most species new to the rove beetle fauna of Khibiny Mts inhabited the burnt areas Fb and Cb.

Ground beetles were more diverse and abundant in burnt areas Fb and Cb (16 species in each) compared to the unburnt areas C and F (8 and 4 species). Both unburnt sites were characterized by a high dominance of the forest species Calathus micropterus.

Based on a set of data on chemical and temperature parameters of soils, discussed in Section 3.1.2 and Section 3.1.3 of the Results, the principal component analysis positioned the monitoring sites into four different quadrants of the projection space, highlighting factors specific to each site (Figure 6a,b).

In control forest F, these were the capacity of organic matter, total carbon, and nitrogen, corresponding to the thickness of the litter, and increased concentrations of K and P, which enriched the fall of coniferous trees. Clear-catting C, actively overgrown by yang birches, whose leaves are rich in Ca and Mg, was distinguished by the accumulation of these elements in the litter and its better warming to active temperatures ≥ +10 °C (Table 1). The soil cover of the burnt clear-cutting Cb, eroded to mineral mass, was characterized by the highest ash content and lowest acidity. The moist litter of the burnt pine forest Fb had a longer period and a higher sum of effective temperatures +5 ≤ T < +10 °C.

The PCA has defined the «burned–unburned» criterion as the first principal component (y-axis; strength of the factor influence F1 = 68%) and the «forests–clear-cuttings» criterion as a second one (x-axis; F2 = 18%), confirming the greater contribution of the ground fire to the transformation of the soil-vegetation cover in the mountain taiga forest compared to continuous uncleared clearings (Figure 6a).

The third factor, with the least influence at 14%, separated the sites into pairs Fb, C and F, Cb, with differences between them attributed to higher levels of two key mineral nutrients, Ca and Mg, in the soil of the sites in the first pair, overgrown with birch and grasses, compared to the control coniferous forest and sparsely overgrown burned clearings.

Obviously, the pyrogenic transformation has led to the creation of a more diverse and specific microenvironment in the forests Fb and clear-cutting Cb, suitable for the habitat of arthropod species with different ecological preferences. Significant accumulations of burnt and decaying tree trunks and branches formed a complex spatial structure of habitats and attracted an abundance of decomposing invertebrates and phytophages as a food resource for predatory arthropods [30]. Mosaic of vegetation provided uneven heating and moistening of substrates, while post-fire alkalinization led to an increase in the pH of acidic podzolic soils.

Many of the collected arthropod species turned out to be good indicators of changing conditions. For instance, all hygromesophilous rove beetle species (Olophrum boreale, Stenus palustris, S. biguttatus, S. tarsalis, and Lathrobium brunnipes) found in the litter of Fb experiencing overmoistening after the loss of the woody layer and root burnout [30], whereas mesophilous species inhabited other sites. The findings of the xerophilic and thermophilic European-West Siberian spider Micaria silesiaca (2 specimens), new to the Kola Subarctic, in the twice-disturbed Cb site corresponded to the maximum heating (up to +29.8 °C) and low humidity (up to 30%) of this sparsely vegetated area (Table 1) [30,31,32]. It has also attracted the highest number of thermophilic and heliophilic ground beetles inhabiting open landscapes, including field and meadow agrocoenoses. These were either ground beetles preferring rare birch groves and mountain tundra in Khibiny Mts (Calathus melanocephalus, Harpalus laevipes, and Notiophilus aquaticus) or previously unrecorded species such as Pterostichus adstrictus and species of the genera Amara and Harpalus (A. famelica, A. quenseli, H. nigritarsis, and Dicheirotrichus (Harpalus) cognatus). Only during the field season of 2018 at the burnt clear-cutting Cb and, to a lesser extent, in the burnt forest Fb, we caught a dozen ground beetle species new to Khibiny Mts, whereas during the ten-year period of studying the natural mountain ecosystems from 2008 to 2017, no more than 30 species were recorded [33].

The connection with open, dry, steppe, and mountain-steppe biotopes is a characteristic feature of ground beetles belonging to the genera Amara and Harpalus [74]. Another characteristic feature of these beetles is mixophytophagy, or the ability to consume not only animal but also plant food, including herbs and cereal seeds. In the first years after the fire, the burning clearing Cb was overgrown with Deschampsia cereals. In addition, the morphological feature of ground beetles of these genera is the adaptation of the first pair of legs, both for climbing on grasses and digging the soil [75]. The absence of litter and the soil erosion to the mineral mass on the Cb site made it the most suitable substrate for digging.

Post-fire alkalinization of the soil has become favorable for the development of larval stages of beetles, which are closer to the soil than adults. The larvae, both ground and rove beetles, were significantly more numerous in soil samples from burnt sites Fb and Cb, despite their different hydrothermal regimes [28]. The largest number of larvae were found in the eroded mineral soil with elevated pH values and the most contrasting temperature and humidity regimes. The fact of larvae development in the burnt areas is not surprising for ground beetles, which prefer open, sunlit, and warmed landscapes of mountain tundra and sparse birch-crooked groves, but interesting for rove beetles, which are closely related to the soil horizon at all stages of ontogenesis and confined, as a rule, to forest belts of Khibiny Mts [33,76].

It is impossible to consider the distribution patterns of all arthropod species caught in the Kunyok valley in one article; however, it is obvious that most of them prefer two burnt areas, especially the burnt forest Fb (Figure 6c–e). More than 70% of arthropod species from three disturbed sites were concentrated here, including 71% of all spider species, 88% of rove beetles, and 73% of ground beetles. The total number of unique species of three arthropod taxa was 31 in Fb compared to 14 on the burnt and 9 on the unburnt clear-cutting Cb and C, respectively, and the number of species newly recorded for the Khibiny Mts was 18 compared to 13 on Cb and 4 on C (Table 2, Table 3 and Table 4).

We associate the findings of most of unique and new Khibiny Mts arthropod species primarily in burnt areas, where they were more diverse and abundant six years after logging and five years after the fire, with the development of a whole complex of aphyllophoroid fungi that decompose wood. These wood-decaying fungi actively colonize charred trunks at this stage of succession (see Results, Section 3.1.1).

The diversity of beetles, trophically and topically associated with wood-decaying fungi at different stages of ontogenesis, amounts to over two thousand species [77]. In the former Soviet Union, such species were known in 25 families of beetles [78,79,80,81]. The family Staphylinidae is represented in fungal entomocomplexes by dozens of species, especially in disturbed forests at the initial stages of restoration [82,83,84]. Among them, the small rove beetle Scaphisoma agaricinum (Staphylinidae, Scaphidiinae), first recorded in Khibiny Mts on the burnt clear-cutting Cb (three adults), is a specialized mycetophagous and the most abundant species of mycetobiotes beetles in the forest zone of Europe, the European part of Russia, the Urals, and the Trans-Ural region [79]. It inhabits the fruiting bodies of xylophagous and mushroom-forming fungi during their growth and sporulation, feeding on fungal spores and participating in their dispersal.

The presence of mycetophilous and xylophilous rove beetle species with different trophic specializations (predators, sapro-mycetophages, or combining predation and mycetophagy) is also known in such genera as Acidota, Anthophagus, Ischnosoma, Carphacis, Arpedium, Lordithon, Tachinus, Aleochara, Atheta, Liogluta, Bolitochara, Oxypoda, Stenus, Lathrobium, Xantholinus, Philonthus, and Quedius [82]. Considering that from 28 to 40% of the species diversity known for these genera in Khibiny Mts was concentrated in disturbed sites, the findings of most of these species can be explained by their associations with ground and wood-decaying fungi actively colonizing the self-healing areas.

5. Conclusions

The occurrence of three adjacent variants of transformation of the mountain taiga forest (self-growing burnt forest, untreated clear-cutting, and burnt clear-cutting) allowed us to organize a system of monitoring sites for simultaneous observation of various successions of soil cover and its arthropod communities. Considering the vast areas covered by continuous logging and wildfire, this comparative observation has no analogues in the Polar regions and their mountains. Considering the vast areas of continuous clear-cutting and ground wildfire, this comparative observation has no analogues in the Polar regions and their mountains. He showed that the arthropod communities in all disturbed variants were far from the control mountain taiga forest for the entire first decade after exposure, but an extensive grass-roots fire led to a greater disruptive effect on the soil properties and vegetation cover as invertebrate habitats compared with continuous untreated clear-cutting. After the fire, more diverse and specific arthropod communities formed, including unique species found in Khibiny Mts for the first time. Significantly higher numbers of spider, rove beetle, and ground beetle larvae also indicated that burn sites are more favorable for the reproduction and development of these predators. Thus, the hypothesis of the greatest similarity between arthropod communities in two adjacent control and burned forest sites was not confirmed.

For twice-disturbed burnt clear-cutting, which has lost the tree canopy, vegetation cover, and organogenic soil horizon, the suppression of diversity and abundance was confirmed only for rove-beetles, closely related to the soil at all stages of the life cycle, as well as forest species of ground beetles. This most transformed and long-term non-overgrown site was attractive for untypical in Khibiny Mts light- and heat-loving spider and ground beetle species, preferring open habitats.

The greatest similarity with the control forest in terms of diversity, abundance, and dominance of forest-dwelling beetle and spider species was revealed in unburnt clear-cutting located 1 km away but having a forest litter with a developed moss-shrub layer. Thus, on the one hand, our research has shown the importance of preserving the forest litter under various types of impacts on the mountain ecosystems to speed up the restoration of the initial forest structure of soil communities and its ecosystem functions even after the destruction of the tree canopy. On the other hand, we have confirmed the classical view of the forest floor as an upper soil horizon maintaining a stable hydrothermal regime favorable for the restoration of the original forest structure of soil fauna.

The comparison of arthropod diversity and abundance between the four sites led us to an important methodological conclusion, which we recommend taking into account in similar studies. Namely, it revealed greater arthropod similarity between two sites located 1 km from each other but retaining the forest litter (control forest and clear-cutting) compared to pairs of adjacent sites in 300 m apart, which we selected at the start of monitoring on the principle of «burnt–unburnt» (i.e., control or burnt forests, and burnt or unburnt clear-cuttings).

Our research has shown that burnt mountain areas and, in a lesser extent, uncultivated continuous clear-cutting attract arthropod species with different preferences (pyrophilous, helio- and thermophilous, and, conversely, shade-loving and hydrophilous), including the species untypical for Khibiny Mts, thereby enriching the local fauna of these mountains and the Kola Polar region as a whole and expanding our understanding of the ecology of invertebrates at the northern periphery of the ranges. Due to the rounded shape of Khibiny Mts and the complex landscape limiting the arthropod penetration in their central parts and intermountain valleys, the attracting of uncommon species is most evident not in the first 1–2 years, as described in the literature, but later, and persists for at least ten years due to slow overgrowth rates of clear-cuttings and burnt areas in specific conditions of Polar latitudes and mountain microclimate. We observed the highest diversity and abundance of arthropods by 5–6 years after the disturbance. These parameters decreased by 8–9 years but remained higher compared to 2–3 years. These facts must be taken into account when planning conservation measures and designing routes to identify new habitats for rare and protected animal species. Ecological monitoring and species protection should be organized not only within the boundaries of the «Khibiny» National Park but also in disturbed areas outside its official zone, and this monitoring should be long-term to cover as many species as possible attracted by variants of disturbed sites at different stages of their succession.

As a result of zoological monitoring on three transformation variants of the mountain taiga forest in the Kuniyok valley in 2015–2021, the local fauna of the Khibiny Mountains is supplemented by eight species of spiders, nine species of staphylinids, and ten species of ground beetles and now includes at least 169, 127, and 48 species in these taxa, respectively. Checklists of these taxa from disturbed areas of Khibiny Mts are being published for the first time. Considering that the most arthropod species inhabiting northern latitudes have extensive ranges (Circumpolar, Holarctic, and Transpalaearctic), these lists can be used as reference material for ecological monitoring of the restoration of soil cover and its fauna in different types of disturbances in other polar mountains. Currently, the checklists are part of the author’s database, which includes information on chemical and temperature parameters of soil, gas emissions, and the abundance and diversity of soil fauna and microbiota. But we expect they can also serve as a basis for meta-analysis and the formation of a cross-regional information system on native and invasive species colonizing disturbed Polar ecosystems, especially mountainous ones.

6. Patents

• Certificate Number: 2021620847 [85].
• Certificate Number: 2022622566 [86].