Two-Year Post-Fire Abundance of Arthropod Groups Across Different Types of Forest in Temperate Central Europe

Abstract

Forest fires are commonly regarded as negative for ecosystems; however, they also represent a major ecological force shaping the biodiversity of invertebrates and many other organisms. The aim of this study was to better understand how multiple groups of invertebrates respond to wildfire across different forest types in Central Europe. The research was conducted following a large forest fire (ca. 1200 ha) that occurred in 2022. Data were collected over two years (2023 and 2024), from April to September. The research was conducted in coniferous forests and included six pairwise study types: burnt and unburnt dead spruce (bark beetle affected), burnt and unburnt clear-cuts, and burnt and unburnt healthy stands. In total, 96 traps were deployed each year. Across both years, 220,348 invertebrates were recorded (1.Y: 128,323; 2.Y: 92,025), representing 24 taxonomic groups. A general negative trend in abundance following forest fire was observed in the groups Acari, Auchenorhyncha, Blattodea, Dermaptera, Formicidae, Chilopoda, Isopoda, Opiliones, and Pseudoscorionida. Groups showing a neutral response included Araneae, Coleoptera, Collembola, Diplopoda, Heteroptera, Psocoptera, Raphidioptera, Thysanoptera, and Trichoptera. Positive responses, indicated by an increase in abundance, were recorded in Hymenoptera, Orthoptera, Lepidoptera, and Diptera. However, considerable differences among management types (clear-cut, dead spruce, and healthy) were evident, as their distinct characteristics largely influenced invertebrate abundance in both unburnt and burnt variants of the types across all groups studied. Forest fire primarily creates favorable conditions for heliophilous, open-landscape, and floricolous invertebrate groups, while less mobile epigeic groups are strongly negatively affected. In the second year post-fire, the total invertebrate abundance in burnt sites decreased to 59% of the first year’s levels. Conclusion: Forest fire generates a highly heterogeneous landscape from a regional perspective, creating unique ecological niches that persist more than two years after fire. For many invertebrates, successional return toward pre-fire conditions is delayed or incomplete.

1. Introduction

Fires have shaped the planet’s ecosystems for millions of years [1]. Forest fires were probably more frequent and widespread in the past than they are today [2], which is consistent with the evolved adaptations of many organisms to burnt sites. This type of adaptation, collectively known as pyrophily (fire-dependence), is particularly well documented in beetles [3]. Fungal species also exhibit pyrophilous traits, although these mechanisms are not yet fully understood [4], and similar adaptations occur in plants and trees [5]. A number of recent studies have reported declines in arthropod species across diverse ecosystems [6,7]. This trend may suggest that any disturbances (temporary changes in usual conditions causing a major change in an ecosystem), sensu lato, are perceived as undesirable both economically (in terms of land management) and socially (in public view). However, landscape variability, including disturbances, can offer many natural habitats for invertebrate species, such as those affected by wind disturbances [8], bark beetle outbreaks [9], and forest fires [10]. Forest fires remain a naturally occurring element in forest ecosystems [2]. However, climate change is increasing the frequency of fires, especially during prolonged dry periods, thereby making landscapes more prone to burning [11]. Several studies suggest that, in the near future (from a climate perspective), ecosystems and species are expected to face larger, more frequent, and more severe fires [12,13]. Nonetheless, this may reflect a reversion toward historical fire regimes, as forest fires are still considered to be occurring at a deficit compared with the deeper past [2,14].

Forest fires have been heavily suppressed in Europe for many decades, or even centuries, for valid reasons [15]. However, both fire exclusion and fire overexposure can negatively impact ecosystems. Long-term fire exclusion gradually increases the amount of available fuel, which is a fundamental prerequisite for the occurrence and intensity of fires [13]. However, most fires today are caused by human carelessness [16]. Forest fires affect all ecosystem components, including the water regime [17], air quality [18], soil conditions [19], vegetation [20], and—most notably—biodiversity, from soil microfauna to vertebrates [21,22,23,24,25]. Forest fires are a critical element in maintaining biodiversity [26]. Although forest fire research is increasing, its entomological aspects remain understudied [27]. In Europe, forest fires are most common in the Fennoscandian, Russian, and Mediterranean regions, whereas Central Europe experiences relatively few [28]. In some countries, prescribed burning is used to support biodiversity [29]. However, from an invertebrate perspective, these efforts may be insufficient, as fire severity plays a crucial role [29,30,31]. Fire severity refers to the degree of ecosystem change caused by a fire and is usually classified into low, medium, and high levels. Each severity level influences the post-fire landscape differently [10]. In the context of forest fires, severity is assessed exclusively after the event has occurred, based on observable impacts [30]. Prescribed burning can also provide socio-economic and ecological benefits, such as controlling invasive weeds [32], reducing tick populations [33], fuel reduction for wildfires [34], and maintaining biodiversity [35,36].

Arthropods are the most important animal group, comprising the vast majority of species on the planet. However, invertebrates are generally studied in the context of forest fires from a relatively narrow perspective. In Europe, research has primarily focused on specific orders, especially Coleoptera: Carabidae [10,25,37,38,39], as well as other beetle groups, such as saproxylic species [40,41,42]. Additional work has examined Hymenoptera, Lepidoptera, and other flower-visiting guilds [43,44,45], as well as soil-dwelling arthropods, including Collembola, spiders, and other macro- and microfauna [24,46,47]. Few studies have addressed a broader approach encompassing the wide range of invertebrates in Europe, e.g., [42]. Most existing research tends to focus on single forest fire events and specific forest types—such as clear-cuts or even-aged stands—along with their burnt counterparts. Our study is unique in addressing multiple disturbance agents and their burnt variants, a perspective missing from the current literature [46]. This research aims to fill that gap through two years of data collection, and to address the need mentioned by [10]: “However, scientists know very little about how insect communities respond to both prescribed fires and wildfires, since there is a mix of positive and negative results in the pyroentomology literature.” This study can thus contribute to a broader theoretical framework for preserving biodiversity in fire-affected ecosystems [48].

This study focused on the natural succession of invertebrates after a large forest fire, which is quite a unique event in Central Europe. In the Czech Republic, the long-term average size of a forest fire is small, around 0.3 ha [16]. In contrast, the forest fire in the study area affected a large, contiguous forest area of about 1200 ha, creating a natural experiment across multiple forest types. The area is characterized by different types of forest environments, which were affected by fires with different severities [49]. The majority of the area experienced low to moderate fire severity, while the highest severity was recorded in dead spruce stands previously impacted by bark beetle outbreaks [49]. The aim of this study was to describe the response of invertebrate abundance (number of individuals) following two years of post-fire data collection in different forest management types. We expect that our results will provide insights into how multiple invertebrate groups respond to forest fire in different forest types. This study seeks to address several questions.

(i) Which invertebrate groups are most abundant in burnt sites?

(ii) How does invertebrate succession progress over a two year post-fire period?

(iii) Are there differences in invertebrate abundances between forest management types?

2. Materials and Methods

2.1. Study Site and Design

This study was conducted within national parks in Central Europe, with the majority of plots (18 plots) located in the Bohemian Switzerland National Park (Czech Republic). Additionally, two study plots were situated in the neighboring Saxon Switzerland National Park (Germany). Standard coniferous commercial stands in the vicinity of the Bohemian Switzerland National Park were selected as reference plots (supplement). The study area is characterized by a cobble sandstone relief, with quartz sandstone rock formations and predominantly sandy soils, specifically sandy cambisols. Elevation across the plots ranged from 230 to 430 m above sea level. The unique forest fire with its size (ca. 1200 ha) resulted in a number of short-term studies on different topics, e.g., physical characteristics of the fire [49], the effect on plants and soil [20], and insect—Carabidae [25]. The majority of the forest area (60%) in these parks consists of non-native Norway spruce (Picea Abies L.) monocultures, replacing the native acidophilous European beech (Fagus sylvatica L.) forests of the Luzulo–Fagion association. The vast majority of the burnt area was covered with dead spruce trees affected by bark beetle outbreaks, along with beech stands and rock crevice vegetation [49]. For this reason, data were collected across different types of coniferous forests, which are quite prevalent in the area (Figure 1).

Each forest type in combination with a different fire effect was replicated in four plots (clear-cut burnt/unburnt, dead spruce burnt/unburnt, and healthy burnt/unburnt), with four traps per plot, two pitfall (P) and two window (W) traps, placed 20 m apart. In total, 96 traps (6 site types × 4 plots × 2P × 2W) were installed starting in April and remained continuously active until September. The traps were removed and reset annually in 2023 and 2024. Following the methodology of Zumr et al. [25], this study selected three categories of coniferous forest sites affected by fire (burnt sites), each matched with a corresponding unburnt control site of the same forest type. In total, six sites were sampled: (i) a burnt and (ii) an unburnt dead spruce site affected by a bark beetle outbreak (unlogged); (iii) a burnt and (iv) an unburnt clear-cut spruce site; and (v) a burnt and (vi) an unburnt healthy spruce site. A Scots pine (Pinus sylvestris L.) stand was used as (v) a burnt healthy site, since there were no fire-affected healthy spruce stands in the study area. The unburnt healthy spruce stands (=vi), with no signs of decline or severe defoliation and had not been impacted by fire, were located outside the national park near the border in commonly managed, even-aged forests. These control stands had a similar structure to those found within the national park.

2.2. Arthropod Collection

Arthropods were collected using two common trapping methods, window traps and pitfall traps. Pitfall traps are effective for sampling epigeic (ground-dwelling) animals [50,51], while window traps are designed to capture flying arthropods within the habitat and are particularly used to study beetles as well as many other flight-capable invertebrates [52]. Each plot included two subplots, each equipped with a pair of traps: one pitfall trap and one window trap (see Figure 2). The pitfall traps consisted of an opaque canopy covering a larger outer container (1020 mL), which housed a smaller inner container (450 mL), with a funnel placed inside. The preservative fluid used was 8% acetic acid (vinegar). For further details on pitfall trap design, see Zumr et al. [25]. Each window trap consisted of a roof, a plexiglass barrier, a funnel, and a collection container. The roof was made of a 45 cm diameter plastic dish. Beneath the roof, two perpendicular plexiglass panes acted as the intercepting surfaces. All traps used were installed at a standardized height of 1.5 m (flight interception corridor about 1.3–1.8 m). In clear-cut sites, traps were mounted on poles, while in dead spruce and healthy forest sites, they were attached directly to trees. The preservative solution used in the window traps was a propylene glycol solution (1:1.5), with a drop of detergent added to disrupt surface tension.

All traps were emptied in the field simultaneously and refilled with fresh preservative liquid every 3–4 weeks. Each sample was transferred into a glass container labeled with a unique ID. In the laboratory, all adult arthropods were counted and identified based on morphological characteristics. Only arthropod groups with at least five recorded individuals were included in this study. More than five individuals can better ensure variability in these data, and the hypotheses can be tested by analysis. The groups observed were: Isopoda, Acari, Araneae, Opiliones, Pseudoscorionida, Chilopoda, Collembola, Hemiptera: (1) Auchenorrhyncha, (2) Heteroptera, (3) Sternorrhyncha, Blattodea, Coleoptera, Dermaptera, Diptera, Hymenoptera: (1) Formicidae, and (2) Hymenoptera (mainly Aculeata and parasitic wasps), Lepidoptera, Neuroptera, Orthoptera, Plecoptera, Psocoptera, Raphidioptera, Thysanoptera, and Trichoptera. This study did not focus on species-level identification. The taxonomy of the arthropods corresponded with the concept of Zich O. (ed.), following the nomenclature presented in BioLib. http://www.biolib.cz (1 May 2025).

2.3. Analyses

The datasets obtained from paired traps (one WT and one PT) within the same subplot were pooled together and treated as one sample for analysis, corresponding to each study site. Each such sample was considered an independent observational unit.

Analyses were performed based on the abundance of arthropod groups (response variables) using the program R 4.3.1 [53]. To evaluate arthropod dynamics in relation to forest fire, generalized linear mixed-effects models (GLMMs) were applied using the “glmmTMB” package [54]. Models were fitted with a Poisson distribution and a negative binomial distribution. Plot identity (1|ID plot) was included as a random factor, and forest types were studied as fixed factors in all models. All models, using families of distributions such as nbinom1 or nbinom2, and Poisson, were evaluated for best fit by testing for overdispersion and examining residuals using the “DHARMa” package [55]. In the case of a match, the one with the lowest Akaike Information Criterion (AIC) was selected as the best-fitting model for each response group.

Two sets of results were generated for the arthropod groups studied:

(1) Burnt vs. Unburnt Comparisons by Year: Due to the low number of factors, individual models were performed for each comparison: burnt1.Y × unburnt1.Y; burnt2.Y × unburnt2.Y; burnt1.Y × burnt 2.Y; unburnt 1.Y × unburnt 2.Y.

(2) Forest Type Comparisons: A factorial model was used to analyze the three forest types (clear-cut, dead spruce, and healthy) and their respective burnt and unburnt variants. For multiple comparisons among categories, pairwise differences were assessed using the “emmeans” package [56] with post hoc Tukey HSD tests with Bonferroni adjustment. Differences were visualized using the “multcompView” package [57].

As an additional step, non-metric multidimensional scaling (NMDS) was performed to differentiate community composition across the studied forest types, based on abundance data. We used the “vegan” package with the metaMDS function [58], applying the “bray” dissimilarity matrix with three dimensions to keep stress values below 0.2. The global test of community differences was evaluated by permutation multivariate analysis of variance using distance matrices [59] via the “adonis2” function included in the “vegan” package, with 9999 randomizations.

3. Results

A total of 220,348 invertebrates were recorded during this study, distributed across 24 taxonomic groups. In the first year after the fire (1.Y), 128,323 individuals were collected, while 92,025 individuals were recorded in the second year (2.Y). Six groups accounted for 93% of the total invertebrates collected. The most abundant order was Coleoptera, with 69,106 individuals (1.Y: 40,817; 2.Y: 28,289), followed by Diptera with 52,965 individuals (1.Y: 34,440; 2.Y: 18,525), and Collembola with 43,213 individuals (1.Y: 26,947; 2.Y: 16,266). Other notably represented groups included Formicidae 18,450 (1.Y: 8402; 2.Y: 10,048), Araneae 16,836 (1.Y: 8046; 2.Y: 8790), and Acari 4488 (1.Y: 2777; 2.Y: 1711). The remaining 18 invertebrate groups collectively represented only 7% of the total.

Responses to fire varied among taxa. Figure 3 shows differences in abundance between burnt and unburnt areas across the two years. Hymenoptera and Orthoptera showed a significantly higher abundance in burnt areas in both years, while Lepidoptera, Diptera, Neuroptera, and Sterrhorhyncha responded positively in higher abundance to burnt areas, at least in the first year. Other groups, such as Araneae, Coleoptera, Collembola, Diplopoda, Heteroptera, Psocoptera, Raphidioptera, Trichoptera, and Thysanoptera, exhibited a neutral response, with no significant difference between burnt and unburnt sites. Conversely, some taxa responded negatively to fire, with higher abundances recorded in unburnt areas. These included Acari, Auchenorrhyncha, Blattodea, Dermaptera, Formicidae, Chilopoda, Isopoda, Opiliones, and Pseudoscorionida. Interannual changes in the abundance of groups were often neutral, with no consistent directional trend across years for most groups (Figure 3). The responses of the studied invertebrate groups to different forest conditions and their burnt and unburnt variants are presented in Figure 4. Acari was most abundant in unburnt areas, with the highest numbers in healthy stands. Araneae showed relatively little variation between burnt and unburnt areas, though their lowest abundance was consistently observed in healthy stands. Auchenorryncha and Blattodea showed a strong negative response to fire in all site types, with limited variation in abundance within both burnt and unburnt conditions. Coleoptera generally exhibited lower abundance in burnt areas, although the highest numbers were recorded in the first year (1.Y) post-fire in burnt healthy stands. Collembola displayed a frequently neutral response to fire, with the highest abundance observed in 1.Y in burnt dead spruce stands. Dermaptera were exclusively recorded in unburnt healthy stands. Diplopoda showed no significant differences in response to fire or between forest types. Diptera responded positively to burnt areas, particularly in the first year after the fire, reaching peak abundance in 1.Y in burnt clear-cut sites. Formicidae preferred unburnt sites, especially unburnt dead spruce stands. Hymenoptera showed a preference for burnt sites, with notably higher abundance in burnt clear-cut areas. In contrast, Heteroptera exhibited little difference between burnt and unburnt areas but tended to be more abundant in clear-cut areas overall. Chilopoda were least abundant in healthy stands and generally responded negatively to fire, with higher numbers in unburnt sites—except for healthy stands, where fire had no significant effect. Isopoda responded strongly and negatively to fire and were found exclusively in unburnt dead spruce stands and unburnt healthy stands. Lepidoptera showed a positive response to fire, mainly in burnt healthy stands.

Neuroptera did not show significant differences between sites, but were generally more associated with burnt areas. Opiliones responded negatively to fire across all forest types and preferred unburnt healthy stands. Orthoptera exhibited a positive response to burnt sites, especially in burnt clear-cut areas during the second year (2.Y). Pseudoscorpines showed a preference for unburnt areas, particularly unburnt dead spruce stands, although these differences were not statistically significant. Psocoptera displayed no significant differences among the studied factors. Similarly, Raphidioptera results were insignificant, but they were more likely to be recorded in burnt sites. Sternorrhyncha showed no difference between burnt and unburnt sites but exhibited a substantial increase in abundance in 2.Y. Thysanoptera and Trichoptera did not show significant differences in response to the studied variables.

Community

Invertebrate communities differed significantly in the global test (R² 0.27; p < 0.001). Communities clustered according to whether the forest type was burnt or unburnt (Figure 5). Overall, communities in burnt forest types exhibited high similarity to one another, indicating the formation of specific post-fire invertebrate communities. Conversely, unburnt forest types showed greater variation, with clear-cut sites hosting communities that appeared intermediate between the burnt and unburnt groups. Interannual differences within forest types were negligible.

4. Discussion

This study presents unique findings on the abundance responses of a wide range of invertebrates to different forest types and their burnt variants. Large differences in abundance were observed among the groups, each responding differently to fire and forest management types. Notably, a significant portion of the invertebrate groups included in this study remain underrepresented in the context of multi-disturbance research. However, a limitation of this study is the lack of species-level identification, as well as the absence of data on fire severity. Forest fire severity is a key factor influencing both the abundance and species richness of invertebrate communities [3,10], as well as soil microbial populations [60]. Moreover, a common challenge in forest wildfire research is the inability to design this study in advance; researchers must adapt to the post-fire conditions and tailor the study design accordingly. At the same time, the seasonality of fires could play a role. Most invertebrates show peak activity during May and June, with activity significantly declining in July and virtually ceasing by August. July and August are typically the hottest months, with the landscape becoming more arid, and the fire risk is therefore the highest. Given this mismatch between the invertebrate peak activity and the highest fire risk, the timing of fires can play a major role in the subsequent development of invertebrate communities. Fires occurring in the first half of the year may create fresh dead substrates for the mass development of saproxylic and phytophagous species, such as bark beetles, longhorn beetles, or jewel beetles, as well as promote a diverse herbaceous layer benefiting phytophagous groups. In contrast, fires in the second half of the year may offer limited benefits, as some invertebrates may find the habitat less suitable by that time, particularly pyrophilous beetles [25].

Overall, disturbances significantly alter community composition and abundance compared with unburnt, healthy reference sites. This positive effect of disturbance is confirmed by other studies [8,9] and appears to be a synergistic outcome of multiple factors. One key factor is the reduction in tree canopy cover, which creates homogenized, shady conditions for invertebrates [61,62,63]. Increased sunlight exposure promotes a richer herbaceous layer with blooming plants that serve as critical food resources for both larval stages and adult floricolous insects. Additionally, various disturbances, including windthrow, bark beetle outbreaks, and fire, contribute to the creation of deadwood microhabitats [64,65], which are essential for many rare species seeking new habitats [65,66]. The degree to which deadwood is burnt is also important; lightly burnt wood remains attractive to early successional cambiophagous groups for extended periods, as noted by Nappi et al. [67], whereas heavily burnt wood supports very few species [68].

Our findings indicate that fire disturbance had a convergent effect on invertebrate community composition across different forest types over the two years of this study. Following the fire, these distinct forest types hosted very similar invertebrate communities, likely reflecting the homogenizing physical landscape changes induced by the burn. The initial colonization of burnt areas is predominantly driven by mobile groups, while many epigeic groups exhibit very slow regeneration, consistent with previous studies reporting recovery periods on the order of decades [24,46,60].

Individual Study Groups

One of the groups that showed an increase in abundance was Hymenoptera, composed mainly of bees, wasps, bumblebees, and parasitic wasps. This trend is consistent with other studies [10,43] and is generally attributed to increased insolation and food availability, such as flowering plants as a feed and bare soil, which provide essential nesting sites for many solitary bee species. Charred or decaying wood, often present after wildfires, also supports numerous cavity-nesting species, including many red-listed taxa [43]. However, significant differences were observed among the different burnt forest types. The most pronounced increase in Hymenoptera abundance compared with their unburnt counterparts was recorded at the burnt healthy sites. Additionally, both burnt and unburnt clear-cut sites supported high Hymenoptera abundance, underlining the importance of open habitats for this group—similar findings have been reported by [69].

Orthoptera was another group that responded positively to burnt stands, especially in the second year after the fire. The post-fire conditions of the studied forest types likely resembled grassland and open habitats, which are typical environments for many Orthoptera species [70]. The importance of burnt areas for this group is also associated with the presence of bare ground, as very few individuals were recorded in unburned healthy stands.

Lepidoptera also showed a positive response to fire, contrasting with the findings of Mason et al. [10], who reported a neutral response in butterfly abundance to fire. This may be explained by the different forest types included in this study. The most positive increase in Lepidoptera abundance was observed in burnt coniferous healthy stands. In contrast, Mason et al. [10] found significantly reduced abundance in coniferous forests. In our study, the other forest types tended to show more neutral responses. For butterflies, interannual temperature fluctuations may also strongly influence population trends and cause their decline [71].

Diptera generally responded rather positively to post-fire areas, though their abundance varied considerably among sites. In all forest types, dipterans were most frequently captured in the first year post-fire at all sites, with the highest numbers recorded in clear-cut sites. These results align with findings from Thompson et al. [23], who reported Diptera as the dominant invertebrate group on burnt plots, particularly in the first post-fire year. The positive response of hoverflies (Syrphidae: Diptera) was similarly observed by [42]. Some dipteran species have developed perfect pyrophoric adaptations that make them well-suited to post-fire habitats [3]. Moreover, the increased abundance of dipterans likely reflects a temporary decline in predator populations, allowing rapid colonization of newly available habitat [24]. This pattern is particularly relevant for pyrophilic species [31].

Heteroptera, a group living predominantly on plants, exhibited a neutral response to fire. Nevertheless, clear-cut sites supported higher overall abundance, while the largest difference in favor of the burnt variant was observed in healthy stands. Such forest type-related patterns were not reported in [42].

Neuroptera and Raphidioptera, both predatory taxa, are known to reach their highest species numbers in shrub belts and forest mantles [72]. In our study, these groups showed a neutral or slightly increased abundance on burnt sites, which may share microclimatic characteristics with shrub belts and forest mantles, such as increased light availability and structural heterogeneity. This observation is consistent with the findings in [42], which also reported a neutral response of Neuroptera to fire.

Collembola were particularly prevalent in burnt dead spruce stands in both years of data collection. For example, Mälstrom et al. [46], who studied this group in clear-cut areas, observed a very low recovery of Collembola populations—remaining low even after 10 years compared with pre-disturbance levels. Similar findings were reported in an earlier study by Malmström [73], where springtail abundances were significantly reduced in clear-cut areas. Our study corroborates these results, as Collembola was least abundant in clear-cut sites, especially those affected by fire. In contrast, the highest numbers of springtails were captured in burnt dead spruce stands, where a substantial amount of charred or completely burnt wood was present. This aligns with the finding of [74], who reported that springtails are attracted to wood charcoal, which can increase their abundance. This may also support the idea that Collembola, as decomposers of organic material, rely on moist conditions [75], and charcoal has been shown to increase soil moisture [74]. Longeard et al. [29] also noted that springtail recovery post-fire is often associated with mature forests, where they may serve as the first major food source for predators. Moreover, springtails may take shelter under fallen logs to avoid both fire and direct sunlight, using these microhabitats as refuges and subsequent breeding sites.

Coleoptera is the most frequently studied group in the context of forest fire response, e.g., [3,10,25,31,40,76,77]. This is likely due to the fact that beetles are a dominant and diverse group, which is supported by our findings. Notably, while Coleoptera and Diptera together include almost thirty species known to be pyrophilous—species adapted to burnt vegetation—only a single such species has been reported for Hymenoptera [3]. The general trend observed in Coleoptera is a neutral response to fire when comparing burned and unburned areas, though variation emerges when considering different forest types. Both clear-cut and dead spruce sites showed a decline in beetle abundance after fire, while burnt healthy stands exhibited a significant increase in the first year post-fire, followed by higher abundance in unburned healthy stands in the second year. This highlights the importance of distinguishing among families that inhabit different forest habitats. For example, epigeic carabid beetles (Carabidae) are especially sensitive to habitat change. Studies by Zumr et al. [25] in the same region documented rapid declines in carabid abundance in clear-cut and dead spruce stands, but a neutral to slightly positive response in burnt healthy stands during the first post-fire year. This likely reflects a functional shift in the carabid community, as brachypterous (flightless) forest specialists disappear from burnt areas, likely because of direct fire mortality and habitat alteration, including the loss of typical food resources [37]. Conversely, pyrophilous and heliophilous species increase in abundance. Other studies [37,78,79] have found that pyrophilous carabids may constitute 40–90% of individuals in some burnt habitats, depending on site conditions and time since fire. Additionally, saproxylic (deadwood-dependent) beetles may benefit from fire-damaged trees, particularly xylophagous groups such as Scolytinae, Cerambycidae, and Buprestidae, which rely on severely weakened or dying trees [42]. This is likely to explain the significant increase in beetle abundance observed in burnt healthy stands in the first post-fire year.

Low-mobility groups, such as Acari, Blattodea, Dermaptera, Diplopoda, Chilopoda, Isopoda, Opiliones, and Pseudoscorionida, were strongly affected by fire across all studied sites. These groups mainly inhabit unburnt stands, likely because of fire-induced loss of organic litter Malmström [73]. Especially, Pseudoscorpionida are known to be more sensitive to fire and/or repeated fire passes than others [47], and Diplopoda are reported to exhibit a neutral response [47], a trend consistent with our findings. Many of these groups are considered reliable bioindicators for assessing environmental stress and disturbance [80].

However, fire does not always have a strictly negative impact. For example, Santorufo et al. [81] found that the effects of fire on soil arthropods can vary significantly depending on soil type and quality. Chilopoda, a predatory group living in the epigeic soil layer, declined in abundance post-fire in our study—mirroring observations by Trucchi et al. [82]. This decline is likely due to altered habitat conditions rather than direct fire mortality, as some individuals can escape into deeper soil layers during fire events [82]. Soil-dwelling detritivores in general are known to recover slowly, often over several years [24]. According to Longeard et al. [29], peak surface temperatures during prescribed burning can reach 700–800 °C, while just 4 cm below the surface, temperatures drop to around 200–400 °C. A few centimeters deeper, conditions may already be survivable for many soil invertebrates. Moreover, soil stoniness has been shown to increase the likelihood of survival for soil fauna by insulating against extreme heat [83]. Fire severity plays a key role in these outcomes. At our study site, Beetz et al. [49] confirmed that dead spruce stands experienced significantly higher fire severity than other forest types. This can be partly attributed to an increased fuel load, including contributions from wood-boring insects that facilitate flammability by weakening and perforating wood [84]. Notably, Chilopoda significantly preferred unburned dead spruce habitats—likely because of the ample presence of coarse woody debris, which provides both shelter and foraging opportunities. Araneae and Formicidae are among the more frequently studied arthropod groups in fire ecology, e.g., [80]. In our study, we observed different responses between these two mobile predator groups. For spiders (Araneae), the general effect of fire was indifferent or slightly negative, whereas ants (Formicidae) exhibited a significantly negative response to fire—consistent with findings by Moretti et al. [42].

Notable differences emerged among sites for spiders. Their abundance was generally higher in unburnt sites, with the exception of the burnt healthy in the second year (2.Y). This may suggest an influence of fire recurrence. Moretti et al. [42] noted that while single fires often lead to decreased spider abundance, repeated fires can, paradoxically, increase it. However, this is a somewhat controversial observation, as increased fire frequency is generally believed to reduce soil-dwelling fauna [47,80]. The relative resilience of spiders may be explained by their mobility and ability to recolonize burnt areas as independently active individuals. This could contribute to their neutral or only slightly negative fire response.

In contrast, ants as eusocial organisms face greater challenges in post-fire recolonization. If an original colony is destroyed by fire, recolonization depends on the presence of viable queen ants from nearby unburnt areas—without which new colonies cannot form. This underlines the importance of edge effects in post-fire recovery. As [24,29] have emphasized, proximity to unburnt habitat and the presence of microhabitats spared by fire are critical for successful recolonization. These factors are especially crucial for social insects such as ants, which require established nest infrastructure and coordinated colony behavior for persistence and recovery.

5. Conclusions

This study presents the results of two years of data collection across different forest types of their burnt and unburnt variants. The response of invertebrates varied significantly with both forest type and fire exposure, highlighting the importance of considering site-specific characteristics in pyroentomological research. Future studies should ensure detailed documentation of both burnt and unburnt stand conditions, as our results demonstrate that the nature of the habitat strongly influences post-fire invertebrate abundance. The key findings include: (i) Coleoptera, Diptera, and Collembola were most frequently recorded in burnt stands; (ii) Successional return to pre-burn states is delayed or incomplete for many invertebrates, particularly epigeic species, with generally poor recovery rates; (iii) Forest types (e.g., clear-cut, dead spruce, and healthy) significantly affect abundance outcomes, and these effects are further modulated by fire. Burnt sites often show a decline in predatory taxa, likely because of both direct mortality and post-fire habitat degradation, including reduced prey availability or changes in microhabitat structure [24,37]. Conversely, groups such as Hymenoptera, Orthoptera, and Lepidoptera appear to benefit from increased insolation, bare ground, and abundant floral resources following fire, leading to faster recovery and even dominance in some burned habitats. Overall, forest fires play a crucial role in shaping invertebrate communities and generating unique habitats that cannot be replicated by conventional forest management practices. For this reason, prescribed burning could be recommended as one of the management measures to mimic natural disturbances in Central European forests. In addition to the short-term effect of forest wildfires, it is important to monitor the longer-term impacts on invertebrates. Further research is therefore encouraged to explore these longer-term dynamics.