Can Larix sp. Mill. Provide Suitable Habitats for Insects and Lichens Associated with Stems of Picea abies (L.) H. Karst. in Northern Europe?

: Recent observations suggest that climate change affects the growth conditions and range of tree species distribution in Europe. This may also have a major effect on communities of different organisms associated with these tree species. We aimed to determine whether Larix sp. could provide suitable habitats to insects and lichens associated with P. abies to conserve their biodiversity under climate change. The study sites were 10 Larix sp. and 10 P. abies forest stands in Lithuania. Both living and dead trees were included. Sticky traps, bark sheets, and exit hole methods were used for the assessment of insects. Independent plots on tree stems were established for the assessment of lichens. There were 76 and 67 different insect species on dead and living P. abies , respectively, using sticky traps. Similarly, there were 64 and 68 on dead and living Larix sp., respectively. The overall community of xylophagous insects consisted of nine and eight species, which were detected using the bark sheet and exit hole methods, respectively. The bark area colonized by lichens was 34.3% on dead P. abies and 63.2% on dead Larix sp., and 40.4% on living P. abies and 78.0% on living Larix sp. Taken together, the results demonstrate that native P. abies and introduced Larix sp. support similar diversity of stem-associated insect and lichen species.


Introduction
The ongoing process of climate change can be expected to have profound consequences for European forests, especially if species-specific climatic thresholds are surpassed. Prominent climatic changes, which are primarily affecting tree productivity, are mainly associated with increased droughts [1]. Droughts, especially in combination with different biotic factors, such as attacks by pests and pathogens, are known to make trees weaker or even cause mortality [1]. Consequently, the distributional range of different tree species and the composition of European forests can be expected to change in the future [2][3][4]. In north temperate and boreal European forests, the most economically, ecologically, and spatially important and abundant tree species are Scots pine (Pinus sylvestris, L.) and Norway spruce (Picea abies, (L.) H. Karst.), which are regionally experiencing increased mortality rates [2,[5][6][7][8][9][10][11][12].
Picea abies is one of the most canonical tree species in the forest ecosystems of Eurasia. The area of its natural distribution is vast and ranges from western Siberia to Fennoscandia and the mountain ranges of central Europe [13]. It grows under a wide range of climatic conditions and tolerates a cool and wet climate. It predominantly grows on fertile soils and is a relatively shade-tolerant tree species, forming pure or mixed forest stands with different tree species [14,15]. As it produces valuable timber and its stands are relatively easy to manage, P. abies has been extensively planted both within and outside the natural range of succession), and eco-evolutionary processes [40,41]. Epiphytic lichens are also an important component of forest biodiversity associated with coniferous forests in Europe [42]. Several studies have highlighted the importance of lichen diversity as an indicator of environmental change, which is based on their response to air pollution [43,44], climatic conditions [45][46][47], and forest structure and dynamics [48][49][50]. The specific association between certain epiphytic lichens and host trees was demonstrated by Roper [51], and this is probably due to differences in the structure and acidity of the bark, thereby leading to sharp differences in lichen cover and diversity between different tree species. Although Larix sp. is often considered as an alternative tree species to P. abies for the future, the comparative analysis of insect and epiphytic lichen diversity on the stems of these tree species is generally lacking.
The aim of the present study was to determine whether Larix sp. could provide suitable habitats to insects and lichens associated with P. abies to conserve their biodiversity under climate change.

Study Site and Observation
The study sites were in P. abies and Larix sp. forest stands at 10 different locations in Lithuania ( Figure 1 and Table 1). Larix sp. stands (L1-L10), and white color represents P. abies stands (S1-S10).
The identification of larch species is problematic due to their frequent hybridization [52,53], so in this study, they are referred to as Larix sp. At each site, there was one P. abies and one Larix sp. stand, which were within a radius of 200 m, so they were within the same geographical area and exposed to similar climatic conditions. The topography was similar in these areas. Information on the stand and site characteristics is in Table 1. Larix sp. stands (L1-L10), and white color represents P. abies stands (S1-S10).
The identification of larch species is problematic due to their frequent hybridization [52,53], so in this study, they are referred to as Larix sp. At each site, there was one P. abies and one Larix sp. stand, which were within a radius of 200 m, so they were within the same geographical area and exposed to similar climatic conditions. The topography was similar in these areas. Information on the stand and site characteristics is in Table 1.
Study sites were selected based on forest inventory data from the State Forest Cadastre database. The criteria used for the selection of each study site were: (i) P. abies or Larix sp. trees were the prevailing species at the site; (ii) similar soil type [54]; and (iii) similar vegetation type [55]. Most of the study sites were characterized by soils of moderate fertility and normal humidity and by an oxalidosa vegetation type (Table 1).

Assessment of Insects Associated with Tree Stems
Three different methods were used for the assessment of insects: (i) using sticky traps, which were used to capture insects occurring on the surfaces of living and dead P. abies and Larix sp. trees, (ii) recording signs of xylophagous insects under the bark of dead trees, and (iii) recording exit holes of xylophagous insects on the bark of dead trees. For the capture of insects on the surface of tree stems [56], two sticky traps, which were made of 20 × 20 cm polyethylene sheets treated with non-drying glue (Pestifix, "Flora", Talinn, Estonia), were attached to each of the five living and five dead P. abies standing tree stems. Both traps were placed at the same height of ca. 1.5 m above the ground to prevent interference from grasses and shrubs. Sticky traps on living and dead Larix sp. tree stems were established in the same way. The assessment of insects using sticky traps was carried out between May and August 2019. During this period, sticky traps with trapped insects were collected once a month and replaced with new ones, which resulted in three time points (June, July, and August). Collected sticky traps were transported to the laboratory the same day and stored at 5 • C until the identification of insect species using a binocular Zeiss Stemi 2000-C microscope (Oberkochen, Germany) and morphological insect identification keys [57][58][59][60][61][62]. Many insects were identified to the species level, while others were identified to the order, family, or genus level. Several insects remained unidentified, which was largely because they were missing body parts or were heavily covered by glue from sticky traps, thereby making reliable identification impossible. After the identification of insect species, accidentally trapped insects, i.e., species specifically associated with the tree crowns or non-target species, the development and feeding of which are not dependent on tree stems, were excluded from further analyses. However, predators and parasites of insects associated with tree stems were included in analyses.
Xylophagous insects, which are wood-and bark-boring insects, were assessed on dead P. abies and Larix sp. trees, and the signs and areas of their activity under the bark, i.e., larval tunnels, pupal chambers, and adult holes in the wood, were recorded. This was carried out once in August 2019 by removing a 20 × 20 cm bark sheet at a height of ca. 1.5 m above the ground [56]. In each study site, one bark sheet was removed from each of the five dead trees of each tree species, resulting in a total of 0.4 m 2 bark area in each site. The area of removed bark was photographed, signs of insect activity were analyzed in the laboratory, and insect species were identified.
For the assessment of exit holes of adult insects and the identification of their species, five dead trees of P. abies and Larix sp. per study site were visually inspected in August 2018. On each tree stem (1-1.5 m above the ground), insect exit holes were recorded on five plots, each 0.01 m 2 in size, and were situated along the stem and from four different geographical directions (N, S, E, and W), resulting in a 0.2 m 2 area per tree in total. This method was adopted from Asta et al. [63]. The number of exit holes was recorded for each insect species separately. Insect species for which exit holes were clearly species-specific, e.g., Ips typograhus and Pityogenes chalcographus, were identified to the species level, while others were identified to the family or genus level.

Assessment of Epiphytic Lichens
Epiphytic lichens were assessed at the same study sites in August 2018 ( Figure 1 and Table 1). In each P. abies or Larix sp. study site, five healthy and five dead trees were selected. The selected trees were ca. 18-20 cm in diameter at a height of 1.3 m above the ground, stem inclination was not more than 20 • , trees were without wounds on the stem, and bark structure and bark thickness were similar for all trees of each tree species. For the assessment of lichens, on each tree, four independent plots, each 10 × 10 cm in size and each facing a different geographical direction (N, S, E, or W), were established at ca. 1.5 m above the ground [63]. All lichen species present within each plot and the area covered by each of them were recorded. Most of the lichen specimens were identified to the species, genus, or family level, but several species remained unidentified.

Statistical Analysis
Differences in the richness of insect or lichen taxa between dead or living trees of P. abies and Larix sp. were compared by nonparametric chi-square test [64], taking into account the Bonferroni correction. The Shannon diversity index, qualitative Sorensen similarity index, and nonmetric multidimensional scaling (NMDS) in Canoco 5 [65][66][67] were used to characterize the diversity and composition of insect and lichen communities.

Insects
In total, there were 20,226 insects trapped using sticky traps (Table 2). When all sites were taken together, on dead P. abies, there were 76 different insect species identified among 10,858 (53.3%) insects trapped, while on dead Larix sp., there were 64 different species identified among 1017 (5%) insects trapped. Consequently, the chi-square test showed that the richness of insect species was significantly higher on dead Larix sp. than on dead P. abies (p < 0.0001). Similarly, on living P. abies, there were 67 different insect species among 7420 (36.7%) insects trapped, while on living Larix sp., there were 68 different species among 931 (4.6%) insects trapped. The richness of insect species was significantly higher on living Larix sp. than on living P. abies (p < 0.0001).
Many insect species were shared between dead trees and between living trees of both tree species. Among the 95 insect species identified, 5 were unique to living P. abies, and 4 were unique to living Larix sp. trees. Similarly, two insect species were unique to dead P. abies, and five were unique to dead Larix sp. (Figure 2).
Consequently, the Sørensen similarity index of insect communities was moderate when compared between dead trees of both tree species and living trees of both tree species ( Table 2). The Mann-Whitney test showed that the Shannon diversity index of insect communities was similar between dead trees (p > 0.05) and between living trees (p > 0.05) when compared between P. abies and Larix sp., respectively. NMDS showed that insect communities on living P. abies and living Larix sp. were partially overlapping (Figure 3a). By contrast, insect communities on dead P. abies and dead Larix sp. were separated along the diagonal (Figure 3a). However, NMDS showed that there was a partial overlap between insect communities on living P. abies and dead Larix sp. (Figure 3a). Assessments that were conducted in June, July, and August showed that there were only minor variations in the abundance of dominant insect species on both living and dead P. abies and Larix sp. (Table 3). The most common insect species on P. abies were Crypturgus pusillus, Ichneomonidae sp., and Eucnemidae sp. 1, while on Larix sp., they were Ichneomonidae sp., Eucnemidae sp. 1, and Eucnemidae sp. 2 (Table 3). All insect species detected using sticky traps are in Table S1. 4 were unique to living Larix sp. trees. Similarly, two insect species were unique to dead P. abies, and five were unique to dead Larix sp. (Figure 2). Consequently, the Sørensen similarity index of insect communities was moderate when compared between dead trees of both tree species and living trees of both tree species ( Table 2). The Mann-Whitney test showed that the Shannon diversity index of insect communities was similar between dead trees (p > 0.05) and between living trees (p > 0.05) when compared between P. abies and Larix sp., respectively. NMDS showed that insect communities on living P. abies and living Larix sp. were partially overlapping (Figure 3a). By contrast, insect communities on dead P. abies and dead Larix sp. were separated along the diagonal (Figure 3a). However, NMDS showed that there was a partial overlap between insect communities on living P. abies and dead Larix sp. (Figure 3a). Assessments that were conducted in June, July, and August showed that there were only minor variations in the abundance of dominant insect species on both living and dead P. abies and Larix sp. (Table 3). The most common insect species on P. abies were Crypturgus pusillus, Ichneomonidae sp., and Eucnemidae sp. 1, while on Larix sp., they were Ichneomonidae sp., Eucnemidae sp. 1, and Eucnemidae sp. 2 (Table 3). All insect species detected using sticky traps are in Table S1. The overall community of xylophagous insects consisted of nine species detected using bark sheets and eight species detected using the exit hole method (Table 4). When all sites were taken together, the colonized bark area was 39.0% on dead P. abies and 47.3% on Larix sp. trees. Consequently, the chi-square test showed that the bark area colonized was significantly higher on dead Larix sp. trees than on P. abies (p < 0.0001). The number of exit holes of xylophagous insects was 2509 (75.4%) on dead P. abies and 819 (24.6%) on Larix sp. The number of exit holes was significantly higher on P. abies than on Larix sp. trees (p < 0.0001) ( Table 4). More importantly, communities of xylophagous insects detected using bark sheet and exit hole methods were similar when compared between P. abies and Larix sp. (Figure 3b,c). In support, the Sørensen similarity index was 0.80, showing high species similarity between all dead P. abies and Larix sp. trees using both methods ( Table 4). The Mann-Whitney test showed that the Shannon diversity index of xylophagous insect communities was similar using bark sheet (p > 0.05) and exit hole methods (p > 0.05) when a comparison was made between dead P. abies and Larix sp., respectively.
However, the species composition of xylophagous insects was quite different when compared between bark sheet and exit hole methods ( Table 5). The most common xylophagous insects detected using bark sheets on P. abies were Polygraphus poligraphus  Table 5).

Lichens
The overall lichen community detected in the present study consisted of twelve species, among which eight were on dead P. abies, ten were on dead Larix sp., ten were on living P. abies, and eleven were on living Larix sp. ( Table 6). The bark area colonized by lichens was 34.3% on dead P. abies and 63.2% on dead Larix sp., and 40.4% on living P. abies and 78.0% on living Larix sp. (Table 6). ANOVA showed that the bark area colonized by lichens was significantly larger on dead and living Larix sp. trees than on corresponding P. abies trees (p < 0.0001). The Mann-Whitney test showed that the Shannon diversity index of lichen communities was similar between living P. abies and Larix sp. trees (p > 0.05) and between dead P. abies and Larix sp. trees (p > 0.05) ( Table 6).
The most abundant lichen species was Lepraria sp., which composed 69.9% of the total bark area colonized by lichens. The relative abundance of this species was 60.4% and 59.9% on dead and living P. abies, respectively, and 76.4% and 71.4% on dead and living Larix sp., respectively ( Table 7). The other most common lichen species detected on dead and living P. abies were Phlyctis argena (13.6% and 19.9%, respectively) and Lecidea elaeochroma (6.7% and 7.7%, respectively), while those on dead and living Larix sp. were Hypogimnia physodes (14.3% and 6.5%, respectively) and Unidentified sp. 1 (4.3% and 6.7%, respectively). Unidentified sp. 2, with a relative abundance of 7.0%, was detected only on living Larix sp. trees (Table 7). NMDS showed that lichen communities associated with dead and living trees of P. abies and Larix sp. were largely the same and thus overlapping (Figure 4). In agreement, the Sørensen similarity index of lichen communities was 0.86 between P. abies and Larix sp. trees, showing a high species similarity (all study sites combined) ( Table 6).

Discussion
The results demonstrate that the two coniferous tree species, namely, native P. abies and introduced Larix sp., support a similar diversity of stem-associated insect and lichen communities, but the species composition was only partially overlapping (Figures 2-4 and Tables 3-7). Consistently, for both insects and lichens, the Sørensen similarity index ranged from moderate to high, while the Shannon diversity index was similar between the two tree species. Therefore, Larix sp. has the potential to provide suitable habitats for some insect and lichen species associated with stems of P. abies. However, other organisms associated with these tree species should also be considered, as the replacement of native tree species by introduced ones is a drastic event and may affect biodiversity at both local and regional scales [68]. In addition, different groups of organisms may respond to introduced tree species, i.e., to a new habitat, in different ways [69]. Several studies have evaluated the effect of introduced trees on a particular group of organisms, e.g., plants [70], insects [29,30,71,72], or birds [73,74]. Nevertheless, similar studies that simultaneously as-

Discussion
The results demonstrate that the two coniferous tree species, namely, native P. abies and introduced Larix sp., support a similar diversity of stem-associated insect and lichen communities, but the species composition was only partially overlapping (Figures 2-4 and Tables 3-7). Consistently, for both insects and lichens, the Sørensen similarity index ranged from moderate to high, while the Shannon diversity index was similar between the two tree species. Therefore, Larix sp. has the potential to provide suitable habitats for some insect and lichen species associated with stems of P. abies. However, other organisms associated with these tree species should also be considered, as the replacement of native tree species by introduced ones is a drastic event and may affect biodiversity at both local and regional scales [68]. In addition, different groups of organisms may respond to introduced tree species, i.e., to a new habitat, in different ways [69]. Several studies have evaluated the effect of introduced trees on a particular group of organisms, e.g., plants [70], insects [29,30,71,72], or birds [73,74]. Nevertheless, similar studies that simultaneously assessed different groups of organisms are scarce but can be particularly valuable [75], especially if a number of factors, such as the age of forest stands, microclimate conditions, the type of forest management, or the volume of deadwood, are taken into consideration [76][77][78][79], as these may also have a strong impact on associated biodiversity [80].

Insects
In the present study, the use of different assessment methods (sticky traps, bark sheets, and exit holes) provided a comprehensive comparison of the diversity and composition of stem-associated insects (Tables 3-5 and Figure 3), thereby allowing the overall insect diversity to be estimated [39,81,82]. Among these methods, sticky traps represent one of the most commonly used types of passive traps [83], but additional methods are often needed, as these may provide valuable complementary information [84,85]. However, to increase the accuracy of species identification, additional methods such as DNA sequencing may be needed, as for several insects trapped, the species identity could not be established using morphological methods (Table 3). Nevertheless, sticky traps allowed the collection of important and host-tree-specific insect species but, in some cases, also resulted in unspecific individuals, e.g., Dalopius marginatus L., Conoderus sp. (Coleoptera: Elateridae),  (Table 3). Although the latter insects are abundant in the Palaearctic and Nearctic regions and play important roles in forest ecosystems [86], they are not specifically associated with P. abies or Larix sp. Beetles dominated insect communities in sticky traps, among which two species of aggressive bark beetles, i.e., Polygraphus poligraphus and Pityogenes chalcographus, which regularly attack and can kill living trees, were detected (Table 3). Interestingly, P. poligraphus and P. chalcographus are among the phloeophagous insect species, which are known to be specifically associated with the genus Picea [87], but in the present study, these were detected on both P. abies and Larix sp. (Table 3). Similarly, on both tree species, there were also several secondary bark beetle species, which are deadwood-dependent and colonize trees following attacks by aggressive bark beetles. These included Crypturgus pusillus, Hadrobregmus pertinax, Anobium rufipes, and Trypodendron lineatum (Table 3). Among these, C. pusillus is known to be a P. abies-dependent species that colonizes trees following attacks by I. typographus [87,88].
Interactions between xylophagous insects and their predators are common in nature and may have a direct effect on the health and sustainability of forest stands [89]. Several predators were detected, among which probably the most interesting was Nemozoma elongatum (Coleoptera: Trogossitidae), as it is one of the most important predators of P. chalcographus [90]. However, N. elongatum was captured in low abundance and only on dead and living P. abies (Table 3), even though its trapping coincided with the flying period (June-July) of P. chalcographus [91]. Zahradník and Zahradníková [92], using pheromone baited traps, showed a strong positive correlation between the abundance of P. chalcographus (1-4%) and N. elongatum (up to 60%). Among other predatory insects captured on P. abies and Larix sp. tree stems were Thanasimus formicarius, Tillus elongatus, Glischrochilus hortensis, and Anthribus nebulosus (Table 3). Thanasimus formicarius and Glischrochilus hortensis are predators of many different bark beetle species from the subfamily Scolytinae, including I. typographus [89,[93][94][95][96]. Tillus elongatus is also a predator of bark beetles, attacking them in larval tunnels [97]. Anthribus nebulosus is a predator of soft-scale insects from the family Coccidae [98]. Despite the detection, the relative abundance of predator insects was low, and in many cases, the host insects were absent, suggesting that their trapping could be accidental. The use of sticky traps also revealed the presence of Tetropium gabrieli on Larix sp. trees (Table S1), which is an important secondary pest of Larix sp. in Europe and was detected for the first time in Lithuania [99]. Among the deadwood-dependent insects, there were two species from the family Eucnemidae, which were trapped on both P. abies and Larix sp. (Table 3). Larvae of these insects develop in the wood of dead or dying deciduous or coniferous trees [85,100]. Eucnemidae may play an important role in the interactions between trees, fungi, and forest regeneration and can be used as an indicator species of forest biodiversity [101].
Many previous studies have shown that deadwood is one of the most important substrates in forests and supports a high diversity of xylophagous insects [88,[102][103][104]. Indeed, deadwood as a substrate is required for many species of beetles, bees, wasps, ants, flies, mosquitoes, and other invertebrates [100,[105][106][107][108][109][110]. It may serve as a resource for feeding, breeding, overwintering, or refuge [39,88,111,112]. In the present study, xylophagous insects detected in dead trees of P. abies and Larix sp. using bark sheet and exit hole methods were rather different as compared to those detected using sticky traps (Tables 3-5), thereby repeatedly showing that all of these methods complemented each other. However, the diversity of xylophagous insects detected using bark sheet and exit hole methods was generally limited (Table 4). In comparison, other studies have shown a much higher diversity of xylophagous insect species associated with deadwood of P. abies. For example, there were 47 insect species reported by Jonsell and Weslien [113] and 66 species reported by Seedre [87]. The lower diversity of xylophagous insects could be due to specific stand characteristics, i.e., middle-aged monocultures with routine and intensive forest management and a relatively low occurrence of dead trees (Table 1). Intensive forest management was also shown to have a negative impact on the diversity of xylophagous insects [114]. In addition, the diversity and composition of xylophagous insects may also depend on other factors, such as tree species, degree of decay, and the cause of tree death [115].
Although on P. abies and Larix sp., the diversity and composition of xylophagous insects were similar (Figure 3b,c), the use of bark sheet and exit hole methods showed certain specificity, which can probably be attributed to the biology and ecology of specific insect species. For example, adults of P. chalcographus and H. palliatus make numerous exit holes on tree stems, but their larvae are relatively small and colonize a relatively small area as compared to large larvae of Callidium sp., which was detected using the bark sheet method (Table 5). Furthermore, Trypodendron lineatum was abundantly detected on both tree species, but only using the exit hole method ( Table 5). Galleries of T. lineatum are found ca. 7 cm deep in the wood and are undetectable using the bark sheet method [116]. Similarly, Sirex juvencus was detected on both P. abies and Larix sp. using the exit hole method, as its larvae occur ca. 15-30 cm deep in the wood and leave no signs of activity under the bark [117]. Siricidae woodwasps make circular and smooth-edged exit holes of ca. 4-10 mm in diameter, making identification of the species relatively easy [118,119]. Despite the importance of the conservation of many xylophagous insect species and the promotion of deadwood habitats, the risk of bark beetle outbreaks should also be considered [120]. Bark beetle species such as I. typographus, P. polygraphus, or P. chalcographus, which usually colonize weakened and/or dying trees, can cause extensive damage [121]. In the present study, these were mainly associated with P. abies, suggesting that Larix sp. under the given conditions was less susceptible to their attack (Table 5). However, it was shown that Larix sp. can be vulnerable to attacks by bark beetles of the genus Ips, including I. typographus and I. cembrae [34,122]. Therefore, slight differences in the composition of xylophagous insects between Larix sp. and P. abies trees can probably be explained by certain host specificity. Xylophagous beetles colonizing fresh wood or dying trees need to overcome the tree resistance in the form of chemical barriers [123] and, therefore, are much more host-adapted than those of later decomposition stages [102]. Interestingly, Muller et al. [124] showed a low ranking of Larix decidua as the host, which was due to a generally lower number of herbivorous species, including saproxylic beetles, colonizing this tree species as compared to other coniferous tree species, e.g., P. abies.

Lichens
In the present study, the diversity of epiphytic lichens was generally low on both P. abies and Larix sp. trees (Tables 6 and 7). By contrast, Giordani et al. [50] reported a relatively high diversity of lichen species in mixed P. abies forests, but this diversity was similar between P. abies and Larix decidua trees. It is known that epiphytic lichens can be sensitive to several abiotic factors, such as light [125], temperature and annual precipitation [45][46][47]126], pH value and nutrient availability on the tree bark [42], and air pollution [43,44]. Forest structure and dynamics are among other determinants of the diversity of epiphytic lichens [48,49,127,128]. In addition, the diversity and biomass of epiphytic lichens appear to be higher in unmanaged old-growth forests than in managed ones [128]. Indeed, Marmor et al. [125] showed that on P. abies, the diversity of lichen species significantly increases with the age of trees. In the present study, similarly to insect species (see above), specific characteristics of P. abies and Larix sp. stands (Table 1) were likely among the main determinants of the low diversity of epiphytic lichens.
In agreement with results of the present study, Hauck [42] and Marmor et al. [125] showed that Lepraria sp. and H. physodes were among the most dominant lichen species on P. abies in boreal forests of Europe. Interestingly, both lichen species showed a higher preference for dead or living Larix sp. than for corresponding P. abies (Table 7). By contrast, P. argena showed a higher preference for dead or living P. abies than for corresponding Larix sp. (Table 7). Several studies have emphasized the effect of the tree species on the diversity and composition of lichen communities, e.g., [129,130]. This effect appears to be mainly due to species-specific differences in chemical and physical traits of the bark, e.g., [129,131]. Bark pH, which is usually between 3.0 and 4.0 for different conifer tree species [42], is among the principal factors that determine the occurrence and abundance of epiphytic lichens [132]. As the pH of the bark for both P. abies and Larix sp. was shown to be similar [133,134], this has likely led to the overlap of lichen communities associated with dead or living P. abies and Larix sp. (Figure 4). Consequently, the detected lichen species appear to be generalists, i.e., adapted to different tree species, as they only showed a preference for a particular tree species to a small extent. On the other hand, the larger bark area colonized by lichens on dead and living Larix sp. than on corresponding P. abies (Table 6) shows that the growth of lichens is faster on the former tree species.
In summary, the results revealed that P. abies and Larix sp. share a large number of stem-associated insect and lichen species. As climate change can be expected to have a strong negative effect on P. abies in the area, its gradual replacement by Larix sp. is likely to provide appropriate habitats for investigated insects and lichens, thereby supporting forest biodiversity. However, the possibility should not be excluded that some wood-boring insect species will not be able to jump between host tree species and may be lost if the mortality of P. abies drastically increases in the future.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/d14090729/s1. Table S1: Relative abundance (%) of insect species detected using sticky traps on dead and living trees of Picea abies and Larix sp. S1-S10 and L1-L10 denote different study sites.