How to Increase Biodiversity of Saproxylic Beetles in Commercial Stands through Integrated Forest Management in Central Europe

Due to traditional forest management, the primary goal of which is the production of raw wood material, commercial forest stands are characterized by low biodiversity. At the same time, commercial forests make up the majority of forests in the Central European region, which means a significant impact on the biodiversity of the entire large region. Saproxylic species of organisms are a frequently used criterion of biodiversity in forests. Based upon the analysis of 155 scientific works, this paper defines the fundamental attributes of the active management supporting biodiversity as well as the preservation of the production function. Using these attributes, a model management proposal was created for three tree species, which takes into account the results of research carried out in the territory of the University Forest Enterprise of the Czech University of Life Sciences Prague, since 2019. The optimum constant volume of deadwood in commercial stands was set at 40–60 m3/ha, 20% of which should be standing deadwood. The time framework is scheduled for an average rotation period of the model tree species, while the location of deadwood and frequency of enrichment must comply with the rate of decomposition, the requirement for the bulkiest dimensions of deadwood possible, and the planned time of tending and regeneration operations in accordance with the models used in the Czech Republic. The goal of active management is to maintain the continuity of suitable habitats for sensitive and endangered species. The estimates of the value of retained wood for decomposition can be as high as 45–70 EUR/ha/year for spruce and beech, and about 30 EUR /ha/year for oak.


Introduction
The importance of deadwood for the biodiversity of saproxylic species of insects and fungi, as well as for the natural functioning of forest ecosystems, has long been the subject of research. Over the last 20 years, this topic has become the focus of attention for commercial forests too, as deadwood is no longer seen as a product of poor forest management. However, this issue has not been comprehensively settled in order to be a tangible and acceptable forestry practice. To date, there have been isolated research studies [1][2][3] or various original experiments, e.g., [4][5][6][7][8]. Saproxylic organisms are dependent on deadwood at all stages of their development, and throughout any stage of wood decomposition [1,[9][10][11]. The largest groups bound to deadwood are fungi and insects [12,13]. Fungi are the most important factor in the decomposition process [14,15], especially the division Basidiomycetes [16], and insects are the most important vector with active wood-seeking movement, while their way of life helps to spread fungi to more distant places [17][18][19]. Saproxylic beetles are very popular because they provide reliable data on the preservation of the environment and are often used as indicators of forest biodiversity [2,9,13,[20][21][22]. Nature reserves are one of the options to preserve and create conditions for many specific species of animals. A forest area excluded from management, however, may not always be the most advantageous environment for saproxylic beetles. In this respect, suitable habitats for a non-intervention regime are found especially at higher and middle altitudes in stands predominantly consisting of three main tree species-Norway spruce, European beech, and silver fir [23]. The non-intervention regime is also suitable for extreme positions, steep slopes, and drying sites where the canopy is not fully closed, and the stands remain strongly differentiated [24]. Nevertheless, a conservation (non-intervention) strategy is inappropriate for lowland forests where local species depend on sunny habitats, e.g., oak forests [25,26]. The absence of management would lead to the homogenization of species composition, the closure of the canopy, and a strong reduction of species richness of saproxylic beetles [24,[27][28][29][30][31][32][33]. In addition, there are a number of typical attributes of the natural forest in the reserves, towards which the development spontaneously leads. In particular, we are talking about large volumes of coarse woody debris (CWD), spatial heterogeneity, and the limited use of tree species, among other factors [1]. Societally, these facets are mostly considered as beneficial, but the owners to some extent view them as negative [1]. It is the wood production function that the owners perceive as positive, while it is excluded in nature reserves. Therefore, a compromise is sought between wood biomass production, and the expansion of the typical characteristics of natural forests, as high volumes of deadwood are problematic for the economy of forest enterprises [34]. One way to combine the production functions of forests and high biodiversity is functionally integrated forest management, with an emphasis on active enrichment of stands with deadwood [6], or retention management, which is preferred in Scandinavia, e.g., [35]. The implementation of different methods of management depends on several factors: socio-cultural, economic, and political [36]. In contrast to nature reserves, the goal of silvicultural interventions in functionally integrated forest management is the gradual increase in stand volumes and the improvement of production quality, accompanied by active enrichment with wood necromass [37]. Active enrichment with wood necromass may be in fact faster than the natural increase of deadwood volumes in newly established reserves [4][5][6][7]. While conventional forest management reduces the amount of deadwood, the number of microhabitats and the diameter differentiation of trees [37], functionally integrated management seeks to take into account all of these attributes [4]. However, it must be remembered that in commercial forests, it is still necessary to observe the basic principles of forest protection and the struggle against pests with special regard to climate change, reflected in rising temperatures and the uneven distribution of precipitationincluding periods of intense drought, which has manifested itself over the last years in Central Europe. These factors induce long-term stress on forest stands, reducing the natural resistance of forest tree species, and conversely, increasing the risk of an outbreak of insect pests, e.g., [38][39][40][41].
The application of scientific findings on the importance of deadwood in the management of production forests, which form the main share of woodlands in Central Europe, is essential for the biological diversity in the wider region of forests. Based on a thorough analysis of scientific findings, this work aims to define the attributes of functionally integrated forest management supporting the biological diversity of the saproxylic beetle species. The goal is to propose appropriate management measures in the context of common forestry practice in the Central Europe region. This paper is focused on commercial forests and does not discuss other types of management, such as game parks, grazing forests, pheasantries, orchard meadows, and other agroforestry systems, or similar entities in the category of special-purpose or protection forests. Although these are undoubtedly the most important areas for saproxylic insects in general, their share in Europe is very low [42,43]. It is necessary to find out how much deadwood there is in natural forests, where it is, and what its time dynamics are, in order to apply the findings for forest management [1]. For long-term sustainability, it is necessary to take into account the spatial and temporal relativity of deadwood habitats [44]. It is also necessary to determine production loss estimations for forest owners, and thus quantify the compensation associated with the application of the specific deadwood-enrichment management [45]. In the analytical part, the work aims to answer questions related to the deadwood management with an exclusive focus on creating suitable conditions for saproxylic beetles: • What is the optimal volume of deadwood? • What provides effective enrichment? • How to maintain the continuity of deadwood? • Where is the best place to start enrichment?
In the synthetic section of the paper, an active management model was proposed to increase biodiversity for three important tree species in Central Europe.

What Is the Optimal Constant Volume of Deadwood?
Discussions are often held on the amount of deadwood left to decompose, which is needed to comprehensively fulfill all its functions, while at the same time balance the reproductive offer for the widest possible range of saproxylic beetles. This question is important for the planning of wood-necromass management, as the number of saproxylic beetles are known to increase with the amount of deadwood [6,[46][47][48][49][50][51][52] and wood-inhabiting fungi [23,53,54]. The number of large logs in the late stage of decay, and the constant volume per hectare are the variables that best explain the species richness of this group of beetles [55][56][57] and fungi [54]. With each m 3 of deadwood per hectare, the number of saproxylic beetle and fungal species increases on average by an additional 1.2 species [5]. To some extent, even the exact optimal volume is relative, as it encounters acceptable economic loss and other risks associated with deadwood [1]. In some published studies, we can find a specified universal volume of deadwood for increasing and maintaining the biological diversity of saproxylic beetles in commercial forests. This volume most often fluctuates between 20 and 60 m 3 /ha [34,44,47,58,59].
These values correlate to the diversity of wood-decaying fungi, for which the optimal volume of deadwood might exceed 100 m 3 /ha, and only from 20 m 3 /ha do the first endangered species begin to appear [60]. Wood-decaying fungi generally require high volumes of necromass, sometimes exceeding 300 m 3 /ha [23]. Many saproxylic beetles are linked to wood-decaying fungi [7,17,61,62]. The solar influence is essential for the abundance and diversity of saproxylic beetles [28,63,64], and at the same time, sun exposure can compensate for the amount of deadwood [23]. A greater amount of insolation in sunny stands reduces the required volume of deadwood for saproxylic beetles, and vice versa in shady cold stands [47,65,66]. The strength of the effect of sun exposure on species richness of saproxylic beetles is more likely related to the local climate-the importance of sun exposure diminishes and may not become a limiting factor in warmer climatic conditions. By contrast, wood-decaying fungi react negatively to changed or opened canopy [23]. Humid and warmer environments are very important factors for many fungal decomposers [53,67,68]. Thus, in the case of high volumes of deadwood, sun exposure may be a limiting factor for biodiversity [48]. At the same time, several large logs cannot be replaced by a larger number of thin logs or even smallwood [69]. Logs are more valuable than branches [49], as numerous species cannot live on small dimensions of dead biomass and have a set minimum threshold diameter [49,70]. Similarly, the number of saproxylic beetle species increases with biomass diameter [71,72].
If we compare these recommended volumes of deadwood in managed stands to the current state of forests in 19 European countries, where there is an average of 15.6 m 3 /ha of deadwood, we find significant differences [73]. More precise volumes of deadwood identified in commercial forests are given in Table 1. The small volume of deadwood is the main reason why commercial stands are very poor in saproxylic beetles [44,74,75]. The same applies to the most endangered species in forest ecosystems, where these volumes are well below the limits at which they begin to appear [44], e.g., typically over 60 m 3 /ha [34,48,65]. Up to three times higher volumes of deadwood were detected in commercial stands in mountain areas [81]. However, even this condition strongly limits the biodiversity of saproxylic beetles because in such cold locations, a higher volume of deadwood is required compared to warm ecosystems [47,65]. The diversity of deadwood is also important [66]. Increased volumes of deadwood-and consequently, the saproxylic beetles' diversityare highly correlated with the diversity of other taxonomic saproxylic groups, which leads to an increase in the overall multidiversity of saproxylics [74]. This is due to the inhabitation of the same or similar types of deadwood microhabitats, e.g. by fungi, lichens, and mosses [34,54,82], also the group of Diptera [83]. This is also confirmed by the proven causality in the number of nesting birds in tree snags only when robustly colonized by saproxylic beetles [84].

How to Effectively Enrich the Stands
It is simple to enrich the stands just by leaving the felled logs, felling residues, and fallen wood in place [1]. The enrichment of forest stands with only small fractions of wood biomass is insufficient in terms of increasing biodiversity [3]. It is necessary to focus on bulky specimens of deadwood, as this type of wood is certainly missing in the commercial stands [85]. Therefore, thick wood fractions need to be applied [2,86]. The minimum diameter is 15 cm [87,88], but at the same time, the increasing diameter of deadwood increases the possibility to host bigger species of beetles [49] and a larger number of saproxylic beetles [72] and fungi [54]. Also, Grossner et al. [34] recommend preserving deadwood of larger dimensions in the stands, with a diameter of 50 cm and more, as much as possible. Primeval forest relics, which are species that depend upon forest habitats without interrupting the continuity of the forest with large amounts of bulky (>40 cm) deadwood [11,[89][90][91][92][93].
Diameters over 70 cm of veteran trees have a demonstrably positive effect on all saproxylic beetles [48]. Sizeable deadwood deposits can host more saproxylic species, both endangered and common, simultaneously [94]. However, such large fractions are very scarce in forests [95], while they are essential for highly endangered and rare saproxylic beetle species [11,34,59,86,92,93]. Although smaller wood necromass also hosts many saproxylic beetles [72,96], these are mainly groups of common species [59,96]. However, some studies still favor smaller deadwood mass over large fractions [97]. The difference is probably due to the greater abundance of early species of saproxylic beetles rather than the later species, which are dependent upon the later stages of wood decomposition.
Active enrichment brings about a more complicated issue-standing deadwood, the so-called snags, or microhabitat trees [4,5]. Good results in terms of biodiversity are provided by so-called ring-barked trees [98] or stumps of up to 4 m high (Figure 1), often used and studied in Scandinavian countries [35,[99][100][101]. High stumps are parts of the trees that remain in the stand after they have been felled at a greater height. Normally, trees are felled at a height just above the ground utilizing the lower part of the trunk to produce wood products. The importance of standing deadwood and its greater impact on biodiversity than that of lying logs, especially in endangered species, is illustrated in many cases [56,69,87,102,103]. They carry the highest number of microhabitats per unit area [104]. For this reason, it is necessary to focus on standing deadwood. The simplest, safest, and at the same time the most economically viable way seems to be the formation of high stumps, when at this height, we can then talk about the equivalent of snags. Nevertheless, leaving live trees in the stands is the most frequently recommended method [1], even though there is a far greater safety risk due to the unpredictable fall of a dying tree [3]. Leaving live coniferous species in commercial stands does not develop high-quality microhabitats for saproxylic organisms even after an extended period of time. It is much more convenient to leave deciduous tree species to die naturally in the stands [105] due to their high potential for microhabitat formation [3,106].
Active enrichment brings about a more complicated issue-standing deadwood, the so-called snags, or microhabitat trees [4,5]. Good results in terms of biodiversity are provided by so-called ring-barked trees [98] or stumps of up to 4 m high (Figure 1), often used and studied in Scandinavian countries [35,[99][100][101]. High stumps are parts of the trees that remain in the stand after they have been felled at a greater height. Normally, trees are felled at a height just above the ground utilizing the lower part of the trunk to produce wood products. The importance of standing deadwood and its greater impact on biodiversity than that of lying logs, especially in endangered species, is illustrated in many cases [56,69,87,102,103]. They carry the highest number of microhabitats per unit area [104]. For this reason, it is necessary to focus on standing deadwood. The simplest, safest, and at the same time the most economically viable way seems to be the formation of high stumps, when at this height, we can then talk about the equivalent of snags. Nevertheless, leaving live trees in the stands is the most frequently recommended method [1], even though there is a far greater safety risk due to the unpredictable fall of a dying tree [3]. Leaving live coniferous species in commercial stands does not develop high-quality microhabitats for saproxylic organisms even after an extended period of time. It is much more convenient to leave deciduous tree species to die naturally in the stands [105] due to their high potential for microhabitat formation [3,106]. In high stumps, it will take longer for the wood to become attractive to saproxylic species of the later stages of the decomposition process, because the decomposition of higher stumps takes longer [68]. In some cases, decomposition took up to 3 times longer, and was due to substantially lower moisture of the snag's wood [76,[107][108][109]. The point of the attractiveness to beetles was confirmed by Jonsell and Weslien [99]. It was found that even man-made stumps that are several years old match the species richness of natural stumps [100]. For species of the early stages of deadwood decomposition, insolation is more important than the diameter of the stump [101]. As the decomposition phase of In high stumps, it will take longer for the wood to become attractive to saproxylic species of the later stages of the decomposition process, because the decomposition of higher stumps takes longer [68]. In some cases, decomposition took up to 3 times longer, and was due to substantially lower moisture of the snag's wood [76,[107][108][109]. The point of the attractiveness to beetles was confirmed by Jonsell and Weslien [99]. It was found that even man-made stumps that are several years old match the species richness of natural stumps [100]. For species of the early stages of deadwood decomposition, insolation is more important than the diameter of the stump [101]. As the decomposition phase of deadwood progresses, the species dependent on these stages of deadwood decomposition will also occur [17]. With larger dimensions, however, deadwood in the form of trunk torsos and high stumps can host more microhabitats such as cavities, and thus be a hotspot for saproxylic beetles as well as nesting birds and bats. The larger the diameter of the tree, the more species of common and endangered beetles [110][111][112], as well as Picidae birds are found [84]. By contrast, fungi prefer lying deadwood [54].
It has been confirmed that the volume of lying wood has effectively increased since the introduction of integrated management, but special support for standing deadwood is still a necessity. Even after the introduction of integrated management, the number of torsos has not increased, while habitat trees have even decreased [4]. The share of standing deadwood is 20-30% (mode) of the total volume of deadwood, exceeding 100 m 3 /ha. This share was found in natural forests and old reserves [78,104,108,[113][114][115]. Too many snags on the local level (e.g., in one stand) can reduce the occupancy of individual snags, as resource availability would be greater than the ability of beetle communities to colonize these habitats [98]. At the same time, keeping in mind that isolation is negative, it is important to maintain the connection between these habitats [111], preferably in groups [2]. Functionally, the trunk torsos and veteran trees are also suitable for biological forest protection due to their great host potential. They often host large numbers of predatory, parasitoid insects, birds, and bats, and the synergistic effect of these groups can inhibit the growing number of pests to some extent. Deadwood itself is the host of many pest antagonists [87]. The enrichment strategy and the subsequent change over several years were evaluated by Doerfler et al. [5], who found that deadwood mass in production forests increased from 8 m 3 (set 1) and 18.9 m 3 (set 2) to 13.6 m 3 and 67.9 m 3 (set 1 ≥ 20 cm, set 2 ≥ 12 cm of deadwood for timber inventory). In standard production management, the annual increase in the average level of deadwood is only 0.18 m 3 /h [76]. As a result of active enrichment, saproxylic species respond positively to the increased amounts of deadwood in stands [5][6][7]116] and simultaneously increase multidiversity, including nonsaproxylic insect species [5]. It has even been found that the biodiversity of the saproxylics in production stands has exceeded recent reserves [8], or at least equalized them [7]. This can be proof of how suitable integrated forest management is for supporting an abundance of insects without the need for permanent and strict conservation activities that exclude the production function of forests. A positive effect of close-to-nature integrated management on saproxylic beetles is also confirmed by Jacobsen et al. [117].

Maintaining Constant Volume Continuity
The continuity of deadwood over time is essential for the maintenance of biodiversity, so that the supply of microhabitats for invertebrates is constantly evolving and emerging [50,118,119]. If a species does not find their particular type of microhabitat to establish in the landscape, they will persist in their current location, and upon losing it, they will completely disappear and become regionally extinct [120]. Therefore, it is necessary to supply deadwood more often than only once during the entire production period (rotation), as the decomposition process of wood is relatively short in some cases. The metadata assessment shows that wood of European beech (Fagus sylvatica, L.) decomposes fastest, in 20-60 years. The wood of Norway spruce (Picea abies, L. H. Karst) decomposes within 50-100 years on average, and wood of silver fir (Abies alba, Mill.) within 70-110 years, depending on the conditions and dimensions [67,68,108,109,[121][122][123][124][125][126][127][128]. By contrast, oak (Quercus sp.) is rather stable, and its wood standardly decomposes for at least 90 years in all circumstances [129].
For this reason, it is necessary to carry out the enrichment at least three times within the European beech rotation period, two times in the case of Norway spruce, and once is sufficient for the average oak rotation period, which is 115 years in the Czech Republic (Information on Forest and Forestry 2019). Similarly, enrichment in the range of 25-40 years is recommended by Přívětivý et al. [67]. Alternatively, it is recommended to keep standing trees in the stand to decay naturally, to bridge the period when deadwood is not left purposely in place after regeneration felling [1,3]. In order to create a practical methodology for forest owners-in connection with financial compensation-it will be necessary to calculate in more detail the necessary enrichment phases according to the site conditions and their original rotation periods.

Where to Enrich the Stands?
To select appropriate deadwood management, it is necessary to consider the mobility of species [81]. Generally, beetles disperse poorly due to their seasonal development, and because the stage of the active adult is relatively short [119]. Thus, saproxylic insects have a low ability for dispersion, decreasing with the rarity of individual species [130]. It also weakens the mobility of endangered and primeval forest relics [9,50,131]. It has been found that the continuity of microhabitats and large pieces of deadwood in forest stands perhaps play a more important role for saproxylic beetles, fungi, and lichens than the amount of wood necromass alone [119,131], or at least the continuity has an additive effect [50,57]. In production forests, however, the continuity of deadwood has disappeared, and so the implementation of integration principles based on increasing necromass begins at "square one" (Figures 2 and 3). For this reason, it is advantageous to start enriching the stands with deadwood in production stands near natural forests, old reserves, and other forest refugia of rare saproxylic beetle species. The distance must be as close as possible to the boundaries of these locations because the mobility and dispersal space capacity of saproxylic beetles, fungi, and lichens were found to be very low. The extent of dispersion activities is reported for 100-150 m [118,132], 20-200 m [82,133], and 100-400 m 2 [59,134]. Cavity-living species only occur in close proximity to the cavities, some of them not leaving them at all, e.g., [135,136].   Regardless of location, however, it has been documented that the very enrichment in shelterwood and selection-managed production beech stands has led to an increase in saproxylic insects [5,7], as beetles respond very positively to an increase in deadwood in all area scales [82]. Integrated management with an emphasis on deadwood accelerates high biodiversity and reinforces ecosystem services in production stands, whose fulfillment of these attributes so far has been rather problematic, e.g., [7]. At present, it is forest reservesoften former production forests that have been taken out of management-which serve as biodiversity hotspots. However, in newly established forest reserves, biodiversity does not significantly increase until at least 40 years of development [74]. If close-tonature integrated management is applied on a larger scale, forest reserves can be used to spread endangered saproxylic organisms that necessarily need specific types of deadwood microhabitats. At the same time, the reserves will be used to monitor the natural dynamics of the development of forest stands left to natural influences. Even after the widespread use of integrated principles in production forests, it is not intended nor economically feasible to increase deadwood to 130 m 3 /ha, as is the average in beech forest reserves-hence the unfailing importance of reserves [78], Table 2. Such high volumes of deadwood are necessary for the life of many species of primeval forest relics.    [115] 109 Mix Austria Tree species-the main tree species occurring in studied forest stands. Mix: forest stands with coniferous and deciduous tree species combined, * recent reserve.  Deadwood is one of the features typical of natural forests [1]. From this point of view, even commercial stands with rather substantial amounts of deadwood cannot fully substitute natural forests with all their attributes [141]. On the other hand, the artificial enrichment of stands with deadwood increases biodiversity relatively quickly compared to newly established reserves, while maintaining the important wood production function of the forest [5][6][7]. Larrieu et al. [114] found that in lowland oak-beech stands left to spontaneously develop for over 10-50 years, the volume of deadwood increased by 40 m 3 /ha, yet after 65 years of development, the incidence of larger pieces of deadwood (diameter > 30 cm) was very scarce. Fallen logs are mostly small in size ≤10 cm [95]. This low number of thick dead logs, even after such a long period of spontaneous development, is a problem for primeval forest relics and other highly endangered saproxylic beetles that are dependent on large fractions of wood necromass. Within integrated management, it is possible to intentionally enrich with deadwood, even those with large dimensions, quickly and easily, but on the other hand, it significantly increases the economic loss for the forest owner, even though the stands will perform this ecosystem function for a very long time.

Synthesis-A Case Study of Active Management
The attributes of forest management supporting biodiversity were derived from the above-mentioned extensive analysis of published scientific findings. The attributes of active forest management were defined for three important tree species in the commercial forests of the Czech Republic and wider Central Europe: Norway spruce (Picea abies [L.] H. Karst.); European beech (Fagus sylvatica L.); and oaks (Quercus sp., including Quercus robur L. and Quercus petraea [Matt.] Liebl.). These are the conditions of a temperate forest at the altitudes of 100-1200 m a.s.l., which correspond to the average annual temperature: 3-10 • C, and the annual total precipitation of 450-1300 mm. Active management is designed to create the best possible conditions in terms of biodiversity, which means leaving a significant volume of deadwood in the stands. However, its practical application may be less extensive, depending on the potential and motivation of the forest owner. Management measures are designed for all three tree species groups within the whole range of site conditions. The model is developed for three site indexes (SI): 34 (30), 28 (24), and 18 (14,16), which represent the best, average, and least favorable growth conditions of the model species. The SI number stands for an average height of the tree species at 100 years of age. The time scheme of active management aimed at increasing the diversity of saproxylic beetle species is designed for one average rotation period of commercial stands (100 years for beech and spruce/fir, 130 years for oak), which is based on Decree No. 298/2018 Coll. (Ministry of Agriculture 2018). The enrichment of forest stands with necromass is planned for the scheduled period of tending and regeneration interventions in accordance with the used models of tending and regeneration of commercial stands in the Czech Republic [142][143][144]. Potential yields of forest production, corresponding to the volume of wood left in forest stands under active management, were calculated on the basis of the yield tables [145,146]. The design of active management is shown in the diagram (Figures 3 and 4). The visualization respects the different rates of decomposition and the need to maintain the overlap of decomposition stages during the enrichment with wood at the age of thinning (pole stage), and at the time of regeneration felling (stemwood stage). The quantification of enrichment is given for the particular tree species in Tables 3-5. The calculations also reflect the different production conditions of the sites, SI: 34 (30), 28 (24), and 18 (14,16). The model does not expect deviations from the usual silvicultural practice applied for the particular tree species; the difference compared to the standard management lies in leaving a certain amount of wood in the stands to decompose.

Norway Spruce
A necromass enrichment model was designed for this tree species in accordance with the expected decomposition time, which corresponds to approximately half of the rotation period. With the planned continuous volume of 50 m 3 /ha, it means leaving 100 m 3 /ha in the

Norway Spruce
A necromass enrichment model was designed for this tree species in accordance with the expected decomposition time, which corresponds to approximately half of the rotation period. With the planned continuous volume of 50 m 3 /ha, it means leaving 100 m 3 /ha in the stands for the entire rotation. The first stage of enrichment with deadwood occurs during the major harvest, when 60 m 3 /ha of wood is left to decay. This is about 7% of the mature stand volume in the SI 34, 10% in the SI 28, and 20% in the SI 16. Another enrichment with deadwood begins at the age of 55 (20 m 3 /ha) and then at the age of 70 (75) (again, 20 m 3 /ha), when thinning is carried out (Figure 4, Table 3). Preferably, sterile snags and trees of low economic value are left to decompose. If there is a risk of bark pest infestation, it is necessary to debark the wood mass, which significantly increases the costs. In terms of forest protection, Norway spruce is the most delicate tree species in deadwood enrichment. For this reason, active enrichment with standing spruce deadwood is not computed.

European Beech
Due to the fact that beech wood decomposes rather quickly, it is necessary to supply it quite often to ensure the planned continuous volume and to leave about 150 m 3 /ha during the rotation period, which is the largest volume among the examined tree species. The first phase of enrichment with deadwood occurs during the major harvest, when 60 m 3 /ha of wood is left to decay. This is about 9.3% of the mature stand volume in the SI 34 and 12.6% in the SI 28. In the worst sites represented by ASI 18, it was necessary to increase the enrichment at the time of the major harvest to 93 m 3 /ha (39.7% of the mature stand volume), because the available volume of wood from standard thinning would not be enough to provide the necessary amount of wood to decay. At the time of the major harvest, approx. 15 m 3 /ha of the planned volume of trees would be left standing to die naturally and then decompose, which represents approximately 3-5 trees. Standing trees can be combined with at least 4 m high stumps. Another enrichment with deadwood would begin at the age of 35, 45, and 55 years (always 20 m 3 /ha) and then at 70 years (30 m 3 /ha), at the time of standard thinning ( Figure 4, Table 4). In the poorest sites, further enrichment would be limited to thinning at the age of 50, 65, and 80, every time leaving approx. 20 m 3 /ha to decay. During the last enrichment, about 15 m 3 /ha of the planned volume would be left standing to die naturally. Trees with low economic value (trees with cavities and visible rot, crooked, forked, and damaged trees) would preferably be left to decay.

Oak
Unlike spruce-and especially beech-oak wood decomposes relatively slowly. For this reason, it is sufficient to enrich the oak stands with the smallest volume of deadwood, which is equal to the recommended amount of 50 m 3 /ha during the rotation period. The first stage of enrichment with deadwood takes place during the major harvest, when 30 m 3 /ha of wood is left to decay. This is about 5% of the mature stand volume in the SI 30 and 7% in the SI 24. In the poorest sites, represented by SI 14, it was necessary to increase the enrichment during the major harvest to 33 m 3 /ha (18% of the mature stand), because the available volume of wood from the standard thinning would not be sufficient to ensure the necessary amount of wood to decompose. At the time of major harvest, approx. 10 m 3 /ha of the intended volume of trees would be left standing to die naturally and then to rot, which represents approximately 1-3 trees. Standing trees can be combined with at least 4 m high stumps. Further enrichment with deadwood should take place at the age of 65 (70), and 80 (85) years (always 10 m 3 /ha) in accordance with the tending models ( Figure 4, Table 5). In the poorest sites, further enrichment might be limited to thinning at the age of 55 and 75, leaving 11 m 3 /ha and 5 m 3 /ha. Trees with low economic value (trees with cavities and visible rot, crooked, forked, and damaged trees) would preferably be left to decay.

Discussion and Conclusions
The proposed management model suggests a relatively rapid enrichment of stands with deadwood, which is a key factor in increasing the diversity of saproxylic organisms. At the same time, the production function of forest stands-and thus the use of woodis not suppressed. Management is designed to be applicable to current standard forest management models. However, it is obvious that alternative, close-to-nature silvicultural methods are also important for increasing biodiversity in commercial forests. Changing silviculture systems are creating different conditions for many invertebrate communities [128,147] and the landscape is regionally heterogeneous [148]. Based on an analysis of scientific findings, the volume of wood mass actively left to decay, which is supposed to be continuously present in forest stands, was calculated at 40-60 m 3 /ha. This is a volume attempting to balance a wide range of requirements for saproxylic beetles, other ecosystem functions and economic considerations. This volume will ensure the survival of many endangered species, with the exception of critically endangered primeval forest relics, for which this volume may be insufficient, but is already approaching a stage where they are likely to occur [44,65]. Compared to actual volumes in Europe, there is at least 3 times less dead biomass on average in commercial stands [149]. The actual enrichment process is assessed in relation to the rate of decomposition of the examined tree species deadwood, and on the models of tending and regeneration silviculture measures. The most intensive management model is designed for beech stands (150 m 3 /ha per rotation period), while the least intensive enrichment is designed for oak stands (50 m 3 /ha per rotation period). Enrichment during the stand development is shifted to more advanced stages, when it is possible to leave wood of larger dimensions in the stand, which is especially important for increasing the diversity and abundance of rare species of saproxylic beetles. Deadwood over 15 cm of diameter has the best properties. These larger fractions maintain suitable conditions for extended periods of time. Standing deadwood should be kept in groups, with at least a few individuals representing a share of 20-30% of the total volume of deadwood. High stumps are available and safe standing deadwood. Due to economic aspects, labor intensity, and the effect on biodiversity, it is clear that creating such high stumps should be performed on trees with diameter at breast height (DBH) >35 cm. When created by harvester technology, even occupational safety precautions are met. In terms of the diversity of dead biomass, it is necessary to naturally preserve dead trees and their parts, as they are often damaged and contain important microhabitats-provided that there is no threat to forest stands by pests. This type of deadwood can be expected in the longer term from the trees that will be left standing to die naturally in the stand. Advantageously, a tree left in the stand gains space and time to grow to large dimensions, thus increasing the number of microhabitats [106,150,151]. A future veteran tree growing outside the closed stands will be preferable to saproxylic species, as evidenced by many studies, e.g., [27,32,112], while numerous rare species live only on these veteran trees. Considering the significant decline of old trees in the landscape [152,153], it is also possible to slightly reverse the unfavorable situation. However, the forest manager will find it difficult to select convenient places where these unique trees can be left to die naturally. Suitable sites include places where a substantial loss of the stand-growing area does not pose a problem, and where no significant crown projection is an obstacle to the growth of the new forest. From a safety perspective, it is necessary to avoid places frequently visited by people.
A continuity of nutrient/residence stability is undoubtedly a key issue. To maintain stability, it is necessary to enrich several times during the rotation period, depending on the type of species. It is three times for F. sylvatica, two times for P. abies, and once for Quercus sp. during their rotation period. In terms of standing wood, the production of high stumps in regeneration felling will be combined with leaving several live trees in place to die naturally. To conserve rare early-successional species, it is necessary to ensure a continuous input of dying trees by prolonging rotation times in mature forests [154] and other rare species [92]. The starting point of functionally integrated management is not tied to specific sites or conditions. This management concept can be used almost wherever the diversity of the landscape is observed. However, from the point of view of saproxylic beetles with rather limited mobility, it is appropriate to start enrichment in the immediate vicinity of forest reserves or other important refugia of endangered beetles. This will create a backbone network in the sense of a green corridor to more distant localities for easier dispersal of these species into the landscape. As a matter of fact, reserves are not a complex solution in the landscape where an area several times larger is occupied by commercial stands.
The proposed management for increasing the biodiversity of saproxylic species reduces timber production. Nevertheless, our concept of active forest management can be classified as Medium-Combined Objective Forestry in the sense of the Duncker et al. [155] classification, as it attempts to combine the fulfilment of multiple functions and needs in a single stand. The absolute amount of loss is influenced by the proposed management parameters, which reflect the tree species. The lowest production loss is in oak because the proposed volume of wood to be left in the stand is the smallest (50 m 3 /ha for 130 years). In contrast, the highest loss is recorded for beech (150 m 3 /ha for 100 years). The relative amount of loss in relation to the total volume production is significantly affected by the quality of the site. It varies between 6.3% (oak, SI 34) and 47.3% (beech, SI 18). An approximate estimate of the financial damage for the given tree species and the circumstances of the Czech Republic can be made using the study by Pulkrab [156], who calculated the economic effect of the timber-production function for all main tree species and habitat types. If we assume that the level of monetization will be the same, then the potential economic loss would be in the range of ca 20 EUR/ha.year (oak) to 45 EUR/ha.year (spruce). However, when applying the recommended methods in the management proposal, a reduction of this loss can be expected due to the preferential leaving of low-value trees to decay. While the economic loss in the application of the proposed model is comparable between sites, the share of this loss in total profit varies considerably, from ca. 10% (SI 30, oak) to ca. 100% (ASI 28, beech). This financial loss is somewhat less than reported by Doerfler et al. [5] and Roth et al. [7], who documented a loss of 80-90 EUR/ha·year in Germany. In our case, however, we calculated the actual potential loss by subtracting the estimated costs of harvesting and transporting the wood to the customer from the value of the deadwood left. If we considered only the value of the left wood, we get an amount of 45-70 EUR/ha.year for spruce and beech, and about 30 EUR/ha.year for oak. Moreover, in the poorest beech and oak habitats (SI 14, 18), standard management is unprofitable, so the application of the proposed management in payments for ecosystem services could mean a positive economic outcome and therefore a high incentive for forest owners. These values correspond to the potential ecosystem function of these stands and could be used to derive payments for this service to the forest owner. By comparison, these amounts are equivalent to 20-30% of the flat rate payment in 2020 for agricultural land in the Czech Republic, or 15-25% of the subsidy for organic farming. When applied generally to all commercial forests in the Czech Republic, this would mean an annual payment of approximately 1.9 billion CZK, which corresponds to about 76 million EUR. Still, these financial values are only indicative ones; it is necessary to provide a more precise analysis in a separate economic study.
Author Contributions: Conceptualization, V.Z. and J.R.; methodology, V.Z. and J.R; analysis, J.R. and K.P.; investigation, V.Z. and J.R.; resources, V.Z.; writing-original draft preparation, V.Z. and J.R.; visualization, V.Z. and J.R. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The study (review) was based on the data provided in the publications cited in the paper.