Influence of Picea Abies Logs on the Distribution of Vascular Plants in Old-Growth Spruce Forests

: The deadwood contributes to an increase in soil heterogeneity due to the changing the microrelief (by the formation of windthrow-soil complexes), as well as changes in physical and chemical characteristics of decaying wood directly during xylolysis. We hypothesized that fallen logs as an element of microrelief influence the species composition and cover structure of vascular plants. We studied the influence of Picea abies (L.) Karst fallen logs of moderate and advanced decay stages on the horizontal distribution and heterogeneity of vascular plant cover in different microsite types (small boreal grass type, blueberry type, small boreal grass-blueberry type, herbs, and blueberry type) in old-growth middle taiga spruce forest in the Kivach State Nature Reserve (Republic of Karelia, Russia). The fallen deadwood acts as a factor of heterogeneity, causing reversible changes in the homogeneity of the original plant cover. The decaying logs influence the horizontal distribution of small herbs by changing the occurrence and density of shoots of Oxalis acetosella L., Maianthemum bifolium (L.) F.W. Schmidt, Vaccinium myrtillus L., and Vaccinium vitis-idaea L., as well as the occurrence of Luzula pilosa (L.) Willd. and Calamagrostis arundinacea (L.) Roth. Its impact on the heterogeneity parameters can be traced up to 20 cm from the log. The differences in vascular plant cover between fallen logs and the surrounding forest floor depend on the soil conditions of the microsite. The heterogeneity of conditions created by the logs smoothed out with increasing decay class, resulting in decreasing differences in the heterogeneity parameters of vascular plant cover between deadwood and forest floor. The changes in the homogeneity of the initial vascular plant cover by deadwood and the gradual smoothing of heterogeneity between the logs and the forest floor in rich and poor conditions have different, mainly opposite, trends. Finally, the structure of the vegetation cover reaches a state that is typical of particular growth conditions beyond deadwood.


Introduction
Natural old-growth forests are complex structured adaptive self-regulating systems [1][2][3].Deadwood plays a key role in the functioning of such forest ecosystems worldwide [4,5].The addition in the deadwood pool, represented by non-living woody biomass located on the forest floor or in the soil [6,7], is a continuous long-term process occurring because of small-scale and catastrophic disturbances in the tree stand structure [8].
The death of mature trees for various reasons contributes to an increase in environmental heterogeneity by changes in microrelief [37,38].Decaying logs and stumps increase the forest floor heterogeneity by creating microhighs with relevant positive impacts on forest biodiversity [5].Such microhighs may cover about a quarter of the total area of the forest [39,40].Fallen deadwood influences microclimatic conditions by smoothing temperature and moisture smoothing regimes [41,42].The physical and chemical transformation of fallen logs during decomposition [43][44][45] has a strong influence on the composition and biological activity of the soil directly under the logs [43][44][45], as well as on the surrounding area [46].The microrelief diversity contributes to differences in the plant's habitat conditions.As a result, the microrelief may determine the vegetation cover structure and the spatial distribution of plants [47].
In addition to microrelief transformation, deadwood increases the environment heterogeneity over time due to its decomposition and, as a result, structural and chemical composition changes.The xylolysis of woody substrates is a multi-stage complex process determined by the saproxylic community, mainly by wood-decaying organisms.Deadwood is a complex substrate that forms a wide variety of unique microhabitat conditions determined by its tree species identity, age, death causes, and decomposition duration.The physicochemical properties [48], nutrient [35,46,49], and moisture [29,34] contents of deadwood change during the decomposition.These changes reflect on the taxonomic structure of the saproxylic community inhabiting it [5,[50][51][52].Fungi are the first to colonize deadwood [44,53,54] and use its nutrients for biomass growth [14,44], also creating an environment for bacteria [55] and acting as a feed source for various invertebrates [56].The nutrient content of saproxylic species biomass is much higher compared to that of colonized woody substrate [57].The nutrient translocation occurs due to the mycelial transfer from the soil [58,59], the activity of specific cyanobacteria groups in association with bryophytes [60,61], as well as commensal interactions between bacteria and fungi [53,54,62].
The colonization of deadwood by plant species begins at the early decay stages: lichens and liverworts are dominant at the beginning, and then they are replaced by mosses [63][64][65].Moss cover helps retain water and creates valuable conditions for the vascular plant's colonization [66] already at the early decay stages [9,25].The plant species succession occurring in deadwood [9,23,28,67] increases the substrate complexity.Although deadwood is inhabited by plant species typical for a particular forest community [9,67], the plant species composition differs on deadwood and the forest floor at the beginning [68] and then tends to gradually converge along the decomposition [21].
Previous studies have focused on differences in the vascular plant diversity between deadwood and forest floor, as well as between deadwood of different decay stages [9,21,25,28,67,69,70].We hypothesized that fallen logs of Picea abies (L.) Karst of moderate and advanced decay stages strongly influence the distribution of vascular plants in an undisturbed old-growth middle taiga spruce forest.We aimed to determine: What is the size of the influence zone of a decaying fallen log on the ground vegetation structure?2.
Is the homogeneity of vascular plant cover altered by the presence of deadwood?3.
How do vascular plant species occurrence and shoot density change with a distance from decaying logs? 4.
Does (and how does) the homogeneity of vascular plant cover in the influence zone of deadwood change with an increased decay stage?

Experimental Sites Description
The study was conducted in an old-growth spruce forest in the Kivach State Nature Reserve (Republic of Karelia, Russia, 62.295 • N, 33.779 • E) in the summer of 2023 (Figure 1).The Reserve covers an area of 10,930 ha.The average annual air temperature is +2.4 • C, the average annual precipitation is 625 mm, and the duration of the growing season is 90 days [71].Object selection and sampling were carried out on permanent sample plots of the Forest Research Institute of the Karelian Research Center (IL KRC RAS), established in middle taiga spruce forests of blueberry and sour-blackberry forest types on podzolic soils with podzol microprofile (Glossic Stagnic Retisol (Siltic, Cutan-ic)) and eluvial-metamorphic typical (Albic Stagnosol Loamic) soils [72].

Experimental Sites Description
The study was conducted in an old-growth spruce forest in the Kivach State Nature Reserve (Republic of Karelia, Russia, 62.295° N, 33.779° E) in the summer of 2023 (Figure 1).The Reserve covers an area of 10,930 ha.The average annual air temperature is +2.4 °C, the average annual precipitation is 625 mm, and the duration of the growing season is 90 days [71].Object selection and sampling were carried out on permanent sample plots of the Forest Research Institute of the Karelian Research Center (IL KRC RAS), established in middle taiga spruce forests of blueberry and sour-blackberry forest types on podzolic soils with podzol microprofile (Glossic Stagnic Retisol (Siltic, Cutan-ic)) and eluvial-metamorphic typical (Albic Stagnosol Loamic) soils [72].

Data Collection
We selected 24 sample plots (SP) associated with fallen dead P. abies logs in 3-5 decay classes according to the decay class system described in Shorohova and Shorohov [73] with modifications (Supplementary Materials Table S1).Highly decomposed logs were divided into class 5a (the contours of the log are visually recognizable) and class 5b (the log is practically compared with the ground surface, and its contours are difficult to distinguish).The forest canopy determines the habitat conditions and, as a result, influences

Data Collection
We selected 24 sample plots (SP) associated with fallen dead P. abies logs in 3-5 decay classes according to the decay class system described in Shorohova and Shorohov [73] with modifications (Supplementary Materials Table S1).Highly decomposed logs were divided into class 5a (the contours of the log are visually recognizable) and class 5b (the log is practically compared with the ground surface, and its contours are difficult to distinguish).The forest canopy determines the habitat conditions and, as a result, influences the deadwood decay rate [74].We selected three replicates with a specific decay class and canopy position-in intercrown (the SP is located outside the horizontal projection of tree crowns) and undercrown (the SP is located under tree crowns) spaces.Each SP was characterized by habitat conditions (Supplementary Materials Table S2).
The mean basal area of living trees by species age group was calculated from the relascope plot measurements.The age group was identified visually; 3-5 trees were randomly selected from each group and cored to estimate tree ages.The mean DBH and height of the 3-5 measured trees were calculated for each tree species group.The stock of each tree group was calculated by multiplying basal area by mean species-specific height [75] and then summed.The site index and the relative stand density were determined, according to Tretyakov et al. [76].
The P. abies logs within the SP were at least 20 cm in diameter.The SP was plotted on both sides of the fallen log in a straight line using a tape measure.The borders of SP were marked by colored pegs and presented by two sides of 1 × 1 m and the log between them.To record the coverage and number of plants, we used a plastic grid with a single cell size of 10 × 33 cm, where each cell represented one counting site (CS) (Figure 2).The species composition of vascular plants and the number of shoots of each species were described in each CS on the forest floor and on the surface of the log within the SP.Habitat conditions were evaluated for each CS using the Ellenberg scales for light (L), moisture content (F), substrate response (R), and substrate nitrogen (N) availability [77][78][79].A total of 54 forest floor descriptions were conducted, with 27 on each side, and the nine descriptions were conducted on the log, with three from the frontal surface and two from each lateral surface for each SP Descriptions were conducted in six-fold repetition at varying distances from the deadwood (0-10, . . .90-100 cm).
the deadwood decay rate [74].We selected three replicates with a specific decay class and canopy position-in intercrown (the SP is located outside the horizontal projection of tree crowns) and undercrown (the SP is located under tree crowns) spaces.Each SP was characterized by habitat conditions (Supplementary Materials Table S2).
The mean basal area of living trees by species age group was calculated from the relascope plot measurements.The age group was identified visually; 3-5 trees were randomly selected from each group and cored to estimate tree ages.The mean DBH and height of the 3-5 measured trees were calculated for each tree species group.The stock of each tree group was calculated by multiplying basal area by mean species-specific height [75] and then summed.The site index and the relative stand density were determined, according to Tretyakov et al. [76].
The P. abies logs within the SP were at least 20 cm in diameter.The SP was plotted on both sides of the fallen log in a straight line using a tape measure.The borders of SP were marked by colored pegs and presented by two sides of 1 × 1 m and the log between them.To record the coverage and number of plants, we used a plastic grid with a single cell size of 10 × 33 cm, where each cell represented one counting site (CS) (Figure 2).The species composition of vascular plants and the number of shoots of each species were described in each CS on the forest floor and on the surface of the log within the SP.Habitat conditions were evaluated for each CS using the Ellenberg scales for light (L), moisture content (F), substrate response (R), and substrate nitrogen (N) availability [77][78][79].A total of 54 forest floor descriptions were conducted, with 27 on each side, and the nine descriptions were conducted on the log, with three from the frontal surface and two from each lateral surface for each SP Descriptions were conducted in six-fold repetition at varying distances from the deadwood (0-10, ... 90-100 cm).

Data Analysis
The Statistical analysis of data was performed using 'Statistica 10.0'.To systematize the SP, we conducted the a cluster analysis based on the standardized data of the total number of shoots of each vascular plant species found at the 24 sites.We used the hierarchical association method and the Ward's method for this analysis (Figure 3).

Data Analysis
The Statistical analysis of data was performed using 'Statistica 10.0'.To systematize the SP, we conducted the a cluster analysis based on the standardized data of the total number of shoots of each vascular plant species found at the 24 sites.We used the hier-archical association method and the Ward's method for this analysis (Figure 3).   1.
The identified clusters characterize the conditions of the vegetation microsite and the dominance of species by the number of shoots in each SP (Table 1).Three clusters were identified at the Euclidean distance value of 15.The first cluster comprises seven SPs that are characterized by the active participation of Oxalis acetosella L. and grass species (further in the text, in tables and figures-the P microsite type).The third cluster includes only two SPs that are dominated by O. acetosella and large herb species typical for nemoral herbaceous spruce forests (the HM microsite type).The second largest cluster is dominated by Vaccinium myrtillus L. and other boreal species.It can be divided into two subclusters: 2a, consisting of six SP, which are typical bilberry microsites with the active participation of Vaccinium vitis-idaea L. (the M microsite type), and 2b, consisting of eight SP, which are the same as 2a with the addition of O. acetosella (the OM microsite type).1.
The identified clusters characterize the conditions of the vegetation microsite and the dominance of species by the number of shoots in each SP (Table 1).Three clusters were identified at the Euclidean distance value of 15.The first cluster comprises seven SPs that are characterized by the active participation of Oxalis acetosella L. and grass species (further in the text, in tables and figures-the P microsite type).The third cluster includes only two SPs that are dominated by O. acetosella and large herb species typical for nemoral herbaceous spruce forests (the HM microsite type).The second largest cluster is dominated by Vaccinium myrtillus L. and other boreal species.It can be divided into two sub-clusters: 2a, consisting of six SP, which are typical bilberry microsites with the active participation of Vaccinium vitis-idaea L. (the M microsite type), and 2b, consisting of eight SP, which are the same as 2a with the addition of O. acetosella (the OM microsite type).
Each cluster was characterized by the content of basic nutrient elements in the soil (Table 2).At each SP, the subsoil mineral layer was sampled without the influence of deadwood.Total carbon (C) and nitrogen (N) contents were determined on completely Forests 2024, 15, 884 6 of 23 dry material using the Unicube Elementar (Elementar GmbH, Germany), calibrated with a certified acetanilide standard (200007435 Acetanilide Elementar Analysensysteme).All laboratory studies were carried out using the scientific equipment of the analytical laboratory of the Forest Research Institute (Karelian Research Centre of the Russian Academy of Sciences).The log influence on the horizontal distribution of vascular plant species within the SP was assessed by the shoot occurrence and density.The species occurrence was calculated as the proportion (%) of SP with shoots of a species out of the total number of SP.The species occurrence sampling is a sequence of scores at each distance (0-11, . . .88-99 cm) from the log (0-absent, 1-species present).The shoot density is the number of shoots of a species per CS (0.036 m 2 area).A scoring system has been developed to assess species abundance based on the species occurrence and the projective cover.The system uses a scale of 0 to 6, where: 0-indicates that the species is absent, 1-the presence of single individuals, 2-the rare occurrence with insignificant coverage (maximum 5%), 3-the rare occurrence with few individuals, but minimal coverage is 20%, or minimal occurrence is 20%, but maximal coverage is 7%.4-the minimal occurrence is 20%, the projective coverage in the interval of 7%-20%.5-the minimal occurrence is 50%, and the minimal projective coverage is 20%.6-the minimal occurrence is 75%, and the minimal projective coverage is 30%.
We analyzed the heterogeneity of vascular plant cover on the SP by examining the average number of species, cover homogeneity values [80], and Simpson's dominance index.The vascular plant cover homogeneity value is the difference between the maximum (one) and the mean normalized.The Euclidean distances are calculated from the participation rates at each ADL.To calculate the homogeneity and the dominance index, we used vascular plant species participation coefficients, which represent the ratio of the number of shoots of a species to the sum of shoots of all species in each CS.Differences in heterogeneity parameters were assessed at the log and forest floor of the areas distance from log (ADL) (0-20, 20-55, 55-100 cm) with increasing decay class (from the decay class 3 to the 5b) (Figure 2).Previously, some studies have shown changes in soil physicochemical properties at similar distances from the deadwood [41,43,81].Based on this, we analyzed the difference in heterogeneity parameters of vascular plant cover on the deadwood and the adjacent forest floor within the same decay class and microsite type.The dynamics of heterogeneity parameters of vascular plant cover were assessed within each study plot, depending on microsite type and with increasing decay class of the logs.
In this study, we used the term 'xylolytic substrate' (XS) to refer to the substrate that results from the multistage biogenic transformation of wood in natural conditions.This transformation is mainly enzymatic and is carried out by a saproxylic community that includes fungi, bacteria, archaea, insects, and other organisms.These organisms modify the structure and chemical composition of the original wood substrate.The log impact of the horizontal distribution parameters and cover heterogeneity of vascular plants was assessed using the non-parametric Kruskal-Wallis test; the reliability of differences was determined using Dunn's criterion.The results were found to be significant at p > 0.05.In the first step, we analyzed the occurrence and density of vascular plant species based on the SP cardinal directions orientation.The identified differences were either unsystematic or insignificant.Obtained from each SP data were combined as a total sample and analyzed based on the decay class and cluster of microsite types (Table 2).
Geobotanical descriptions of the sample plots were ordinated based on the principal component analysis (PCA) using the PC-ORD program (version 6.0, MjM Software Design).The coverings of grass-shrub species were taken into account in the analysis.

Results
We found the 37 vascular plant species in the 24 SP (Table 3).The soil of the P and the HM microsite types contained a high level of nutrient elements (Table 3).According to the Ellenberg ecological scales, these habitats are classified as medium-humid with moderately acidic and moderately nitrogen-supplied soils.Furthermore, the highest number of species was encountered here.The M and OM microsite types were found to have nitrogen-poor acidic soil with low nutrient content.The M microsite type was assessed as a dry habitat, while the OM microsite type was assessed as a slightly moist habitat.Floristically, these microsite types were found to be less diverse compared to the others.

The Impact of P. abies Fallen Logs on the Horizontal Distribution Vascular Plants
The impact was observed for some microsites and only certain species (Figure 4).The distribution parameters (occurrence and/or density) of Oxalis acetosella changed with distance from the deadwood in all microsite types, except the HM type (Figure 5).In the HM microsite type, no change in the occurrence and density of shoots of all species was encountered.Changes were observed in the distribution parameters of Maianthemum bifolium (L.) F.W. Schmidt and O. acetosella in the P microsite type.The occurrence of M. bifolium shoots increased at a distance of 30 cm from the log at the 5a-th decay class, whereas the density of its shoots increased at a distance of 40 cm.In contrast, the density of O. acetosella shoots decreased at a distance of 30 cm from the logs at the 3rd and 5a-th decay classes.In the M microsite type, only the distribution parameters of O. acetosella changed.The density of shoots of this species decreased at a distance of 20 cm from the log at the 3rd decay class, whereas the occurrence of its shoots decreased at a distance of 30 cm from the log.Both the occurrence and density of O. acetosella shoots decreased at a distance of 40 cm from the logs at the 4th decay class.In the OM microsite type, the horizontal distribution of many species changed.The density of V. myrtillus shoots increased by 30 cm from the log at the 3rd decay class, while the density of V. vitis-idaea shoots increased by 70 cm from the log at the same decay class.At a distance of 55 cm from the log at the 5a-th decay class, the occurrence of Luzula pilosa (L.) Willd.decreased, while the occurrence of Calamagrostis arundinacea (L.) Roth.increased, both with no change in shoot density.Additionally, the shoot density of O. acetosella decreased at a distance of 10 cm from the log at the 4th decay class and at 20 cm from the log at the 5a-th decay class.At a distance of 90 cm from the log at the 4th decay class, there was an increase in the shoot density of M. bifolium.Furthermore, no changes were observed in the occurrence of O. acetosella and M. bifolium.

The Impact of P. abies Fallen Logs on the Homogeneity of Vascular Plant Cover
The impact varied across microsite types.In the HM and P microsite types with initially rich soil conditions and high species diversity, the P. abies logs at the 3rd decay class did not affect the average number of species and the homogeneity of vascular plant cover at 1 m distance (Table 4).The structure of the vascular plant cover on the deadwood differed from that on the forest floor by a higher value of the Simpson's index.The deadwood at the 4th decay class influenced the adjacent forest floor at a distance of 0-20 cm, as a higher dominance index value was indicated.These changes in the index value were associated with the change in shoot density of O. acetosella.Thus, the mean value of the shoot density of this species on the log at the 3rd decay class was 31.0 ± 8.1, which was 90% of the total shoot density of all species.On all ADL, the mean density of O. acetosella decreased to 12.9 ± 1.8, which is only 66% of the total shoot density.At a distance of 0-20 cm from the logs at the 4th decay class, the mean shoot density of O. acetosella was 32.8 ± 3.1, which is 81% of the total shoot density, whereas at a distance of 55-100 cm, the density of this species decreased to 15.8 ± 2.7 which was only 60% of the total shoot density on the SP.We noted differences in the homogeneity and Simpson's index values on the SP in the P microsite type (Table 4).The mean number of vascular plant species did not change depending on the ADL.In the P microsite type logs at the 3rd decay class had lower homogeneity values, but the Simpson's index did not change.The main shoot density is composed of O. acetosella: on deadwood-50% (15.9 ± 3.2), at the distance of 0-20 cm from the log-74% (15.3 ± 1.4).On deadwood, the shoots of V. vitis-idaea (6.9 ± 2.0) and P. abies (5.4 ± 0.9) had the largest proportion (39% of the total).At a distance of 0-20 cm, 18% of the total species composition is composed of Calamagrostis arundinacea (1.5 ± 0.5), Convallaria majalis L. (1.3 ± 0.3), and V. vitis-idaea (0.9 ± 0.3).The species composition of vascular plants on the logs at the 5a-th decay class differed compared to this on the forest floor: O. acetosella was dominated on the deadwood.The fallen logs at the 5a-th decay class influenced the ground cover at a distance of 0-20 cm: the homogeneity value was lower, whereas the dominance index value was higher compared to those at the distances of 20-55 and 55-100 cm.The plant cover on the logs represented by O. acetosella (10.5 ± 3.2 or 60% of the total shoot density), P. abies undergrowth (1.4 ± 0.3), V. myrtillus (1.1 ± 0.3), Rubus saxatilis L. (1.1 ± 0.3), C. arundinacea (0.9 ± 0.6), M. bifolium (0.6 ± 0.4), V. vitis-idaea (0.3 ± 0.1), Trientalis europaea L. (0.3 ± 0.1).At a distance of 22-99 cm from the deadwood, O. acetosella accounted for only 22% (3.1 ± 0.4), whereas 57% were distributed among shoots of C. arundinacea (3.4 ± 0.6), M. bifolium (2.5 ± 0.2) and V. myrtillus (2.1 ± 0.3).The SP associated with the logs at the 5b-th decay class did not differ in the heterogeneity parameters.In the P microsite type, the vascular plant cover on both the deadwood at advanced decay classes and the forest floor was similar.
In the M microsite type, we noted the increase of Simpson's index with the distance from the deadwood.The homogeneity value of vascular plant cover on the log and adjacent ground (up to 1 m distance) did not differ, at least, within the same decay class (Table 4).The structure of vascular plant cover on deadwood at the 3-5b decay classes differed compared to that on the adjacent ground by decreasing dominance of V. myrtillus and V. vitis-idaea.The mean number of vascular plant species was lower on the deadwood at the 3rd decay class compared to that on the forest floor.The dominance index on the forest floor increased compared to that on the deadwood at a distance of 55-100 cm from it, mainly due to an increase in the shoot density of V. myrtillus and V. vitis-idaea.On the logs at the 3rd decay class, around 88% of the total shoot density was composed of only three species: P. abies undergrowth (mean shoot density per CS was 1.4 ± 0.6), V. myrtillus (0.8 ± 0.4) and V. vitis-idaea (0.6 ± 0.3).At a distance of 55-100 cm from the deadwood, 75% of shoots were represented by only two species-V.myrtillus (3.6 ± 0.5) and V. vitis-idaea (2.5 ± 0.3).On the SPs associated with the log at the 4th, 5a-th, and 5b-th decay classes, we observed the changes only in the dominance index values.Deadwood of the 4th decay class affected the ground cover at a distance of 0-20 cm by decreasing the dominance index value compared to that at a distance of 55-100 cm.Higher dominance index values were associated with an increase in the shoot density of V. myrtillus and decreasing in the shoot density of O. acetosella and V. vitis-idaea.On both deadwood and forest floor at a distance of 0-20 cm, the main proportion of shoots (63%) was represented by three species: O. acetosella (2.9 ± 0.7), V. myrtillus (2.5 ± 0.4) and V. vitis-idaea (1.2 ± 0.3).At a distance of 55-100 cm, 50% of the shoots were represented by only V. myrtillus (3.6 ± 0.4), whereas remaining 47% were represented by a large number of species: Linnaea borealis L. (1.5 ± 0.4), V. vitis-idaea (0.7 ± 0.2), C. arundinacea (0.6 ± 0.2), O. acetosella (0.3 ± 0.1), M. bifolium (0.3 ± 0.08), and L. pilosa (0.1 ± 0.04).On the logs at the 5a-th decay class, the Simpson's index was significantly lower compared to that at a distance of 55-100 cm from the log, mainly due to the shoot density of O. acetosella.On deadwood, 93% of shoots were represented by V. vitis-idaea (7.6 ± 4.2), V. myrtillus (3.2 ± 1.2), and O. acetosella (1.7 ± 0.9).At a distance of 55-100 cm from the deadwood, only two dominant species-V.vitis-idaea (5.1 ± 0.6) and V. myrtillus (3.1 ± 0.3), accounted for 87%.The dominance index on the deadwood at the 5b-th decay class was lower compared to that on the forest floor, independent of the distance from the log.The increase in the Simpson's index was associated with an increase in the V. myrtillus shoot density.On the deadwood, 87% of shoots were represented by V. myrtillus (4.1 ± 1.5) and V. vitis-idaea (2.2 ± 1.3), whereas on the forest floor within the SP, around 72% of shoots were represented only by V. myrtillus (5.7 ± 0.3).Finally, the deadwood at the 5a-th and 5b-th decay classes did not affect the surrounding ground cover.
In the OM microsite type, all heterogeneity parameters differed within the SP (Table 4).The vascular plants composition on the deadwood of the 3rd decay class was characterized by a smaller number of species, a lower homogeneity index value, as well as a lower dominance of O. acetosella compared to that on the forest floor.The fallen logs influenced the ground cover up to a distance of 0-20 cm by decreasing the species number and the homogeneity value.On the deadwood at the 3rd decay class and at a distance of 0-20 cm, the mean number of species and homogeneity value were lower compared to that both at the distances of 20-55 cm and 55-100 cm.We noted the clear differentiation of the SP into two parts: (1) deadwood itself with the forest floor at a distance of 0-20 cm from the log and (2) the forest floor at a distance of 20-100 cm from the log.The mean number of vascular plant species at a distance of 20-100 cm was 2.2 times higher compared to that on deadwood and 1.3 times higher compared to that at a distance of 0-22 cm.The dominance index was higher at a distance of 0-20 cm compared to that at a distance of 55-100 cm.On the deadwood and at a distance of 0-20 cm, around 68% of the total number of shoots was represented by V. myrtillus (2.8 ± 0.5) and O. acetosella (2.4 ± 0.5).At a distance of 20-55 and 55-100 cm, the shoots of V. myrtillus (4.3 ± 0.2) and O. acetosella (2.8 ± 0.5) represented 49% of the total shoots.Almost half (46%) was composed by a large number of species: C. arundinacea (3.2 ± 0.7), V. vitis-idaea (0.6 ± 0.1), T. europaea (0.4 ± 0.06), C. majalis (0.4 ± 0.05), Melampyrum pratense L. (0.4 ± 0.1), L. pilosa (0.4 ± 0.05), M. bifolium (0.3 ± 0.1), Gymnocarpium dryopteris (L.) Newm (0.2 ± 0.05), Orthilia secunda (L.) House (0.2 ± 0.05).Within the SP associated with the deadwood at the 4th decay class, we found no differences in vascular plant cover heterogeneity.On the SP associated with the logs at the 5a-th decay class, deadwood had a lower number of species: it was 1.6 times lower compared to that on the forest floor.At the same time, the deadwood at the 5b-th decay class did not affect the surrounding ground cover.
In the OM microsite type, the mean number of species and the homogeneity value increased on deadwood with increasing decay class, whereas the dominance index value did not change (Figure 6).The shoots of O. acetosella made up the largest part of the total number of species on the logs at the 3rd and 4th decay class (64% and 69%, respectively).
O. acetosella and V. vitis-idaea represented 58% of the total number of shoots on the logs at the 5a-th decay class.The homogeneity value did not change with increasing decay class and distance from the log.Proceeding from the 4th to the 5a-th decay class, the mean number of vascular plant species increased, while the dominance index value decreased, which was associated with the changes in the shoot density of O. acetosella.At a distance of 0-20 cm from the logs in the 3rd decay class, 69% of the shoots were represented by two species-V.myrtillus (2.8 ± 0.5) and O. acetosella (2.4 ± 0.5).On the SP associated with the decayed wood of the 4th decay class, 67% of the shoots were represented only by O. acetosella (9.3 ± 1.5).When the deadwood reached the decay class 5a, O. acetosella shoots accounted for 45% (6.7 ± 1.0), while 47% were represented by M. bifolium (1.9 ± 0.5), V. myrtillus (1.8 ± 0.3), V. vitis-idaea (1.7 ± 0.5), L. pilosa (1.0 ± 0.3) and R. saxatilis (0.8 ± 0.3).At a distance of 22-55 cm from the log at the 3rd decay class, 80% of the shoots were composed of three species-V.myrtillus (3.9 ± 0.4), C. arundinacea (2.8 ± 0.7) and O. acetosella (2.2 ± 0.3).On the SP associated with the 4th decay class O. acetosella sprouts accounted for 51% (4.0 ± 0.7), on the SP associated with the 5a-th decay class but only 22% (3.0 ± 0.5), while 61% were distributed between M. bifolium (2.8 ± 0.5), V. myrtillus (2.4 ± 0.4), C. arundinacea (1.8 ± 0.5) and L. pilosa (1.3 ± 0.2).At a distance of 55-100 cm from dead wood at the 4th decay class, 83% of the plant cover consisted of shoots O. acetosella (2.0 ± 0.3), C. arundinacea (1.3 ± 0.5), M. bifolium (1.2 ± 0.2), V. myrtillus (1.1 ± 0.2).On the SP associated with the 5a-th decay class, 84% of the plant cover was represented by shoots more species: In the M microsite type, we noted the opposite trends: as the decay status of deadwood increased, the mean number of vascular plant species decreased, whereas the dominance index value increased (Figure 6).On the deadwood along the decomposition, the homogeneity and dominance index values did not change, whereas the mean number of species increased on the logs from the 3rd to the 4th decay class.On the log at the 3rd decay class, 43% of the total number of shoots was represented by P. abies (1.4 ± 0.6) and 44%-by V. myrtillus (0.8 ± 0.3) and V. vitis-idaea (0.6 ± 0.3).On the logs at the 4th decay class, 77% was composed by shoots of V. myrtillus (2.8 ± 0.9), V. vitis-idaea (2.0 ± 0.5) and O. acetosella (1.7 ± 0.9), on the logs at the 5a-th decay class-81% by V. myrtillus (3.2 ± 1.2) and V. vitis-idaea (7.6 ± 4.2), on the logs at the 5b-th decay class-87% by V. myrtillus (4.1 ± 1.5) and V. vitis-idaea (2.2 ± 1.3).The mean number of species of ground cover surrounding the deadwood increased in the SP associated with the logs at the 5b-th decay class compared to that at the 3rd decay class.At a distance of 0-22 cm from the deadwood, the homogeneity and dominance index values were higher on the SP associated with the logs at the 5b-th decay class compared to that at the 3rd and 4th decay classes.At this distance from the logs at the 3rd decay class, 43% of the total shoots were represented by O. acetosella (6.0 ± 1.4), and the other 50%-by V. myrtillus (3.6 ± 0.8), V. vitis-idaea (1.5 ± 0.3), and C. arundinacea (1.8 ± 0.9).On the SP with the logs at the 4th decay class, 57% of the total shoots were represented by V. myrtillus (3.5 ± 0.8), V. vitis-idaea (2.1 ± 0.6), and O. acetosella (3.6 ± 0.8).On the SP associated with the deadwood at the 5th-a decay class, 73% of the total shoots were represented by V. vitis-idaea (2.7 ± 0.7) and V. myrtillus (2.5 ± 0.4), whereas on the SP with the logs at the 5th-b decay class only V. myrtillus (4.9 ± 0.6) represented 71% of the total shoots number.At a distance of 20-55 cm from the deadwood with progressing from the 4th to 5a-th decay class, the homogeneity value increased, while the dominance index value did not change significantly.At a distance of 55-100 cm from the deadwood, the homogeneity value decreased with progressing from the 3rd to 4th decay class and then increased with progressing from the 4th to 5a-th decay class.The dominance index value increased on the SP with the deadwood at the 4th decay class compared to that at the previous decay class, and it did not change compared to that at advanced decay classes.At a distance of 55-100 cm from the deadwood at the 3rd decay class, 75% of the total shoots were represented by V. myrtillus (3.6 ± 0.5) and V. vitis-idaea (2.5 ± 0.3), at the 4th decay class-50% by V. myrtillus (3.6 ± 0.4), and at the 5b-th decay class-77% of V. myrtillus (6.4 ± 0.3).In the M microsite type, we noted the opposite trends: as the decay status of deadwood increased, the mean number of vascular plant species decreased, whereas the dominance index value increased (Figure 6).On the deadwood along the decomposition, the homogeneity and dominance index values did not change, whereas the mean number of species increased on the logs from the 3rd to the 4th decay class.On the log at the 3rd decay class, 43% of the total number of shoots was represented by P. abies (1.4 ± 0.6) and 44%-by V. myrtillus (0.8 ± 0.3) and V. vitis-idaea (0.6 ± 0.3).On the logs at the 4th decay class, 77% was composed by shoots of V. myrtillus (2.8 ± 0.9), V. vitis-idaea (2.0 ± 0.5) and O. acetosella (1.7 ± 0.9), on the logs at the 5a-th decay class-81% by V. myrtillus (3.2 ± 1.2) and V. vitis-idaea (7.6 ± 4.2), on the logs at the 5b-th decay class-87% by V. myrtillus (4.1 ± 1.5) and V. vitis-idaea (2.2 ± 1.3).The mean number of species of ground cover surrounding the deadwood increased in the SP associated with the logs at the 5b-th decay class compared to that at the 3rd decay class.At a distance of 0-22 cm from the deadwood, the homogeneity and dominance index values were higher on the SP associated with the logs at the

Discussion
The decaying fallen log of P. abies acts as a factor of the ground cover heterogeneity, causing reversible changes in its original structure.The changes occur both on the log itself and in the surrounding area.In our study, all fallen logs at 3-5b decay classes were inhabited by vascular plants (Table 2).All plant species noted in all 24 sample plots are typical for middle taiga spruce forests.The colonization of deadwood occurs by species typical for a particular forest community [9,67,70] and no obligate epixyl species among the species are found [21,24].Our findings showed that the list of vascular plant species on the fallen logs at the 3rd decay class in the P microsite type, as well as at 3-4th decay classes in the M and OM microsite types, differs from that of ground cover surrounding the deadwood, but these differences disappear with decay class progression (Figure 6).
The diversity of conditions across ADL can be illustrated by an ordination plot mapping the variation of the vascular plant species cover depending on the log decay class and the distance from the logs in the factor space (Figure 7).In the M microsite type, the differences in vascular plant cover are particularly clear between two areas: (1) deadwood at decay classes 3 and 4 and the forest floor at the 0-20 cm from the log and ( 2) the plots at distance of 20-55 and 55-100 cm from the log.Some authors showed similar patterns [21,25].The vascular plant species composition changes [69,82,83] and their richness increases with the decay class of deadwood [9,[21][22][23]67].Plant species with low nutrient demand after dying on the fallen log surface contribute to the accumulation of organic matter, which creates favorable conditions for the colonization of more nutrient-demanding species [4].
Deadwood is an important component of forest ecosystems as a habitat [5,[50][51][52], a key factor of biodiversity [4,5,[9][10][11], and an essential link to the nutrient cycle [84].Our study demonstrates a previously undetected impact of the P. abies fallen logs on the distribution of vascular plants on its adjacent forest floor.We noted the influence of a decaying log on the ground cover at a distance of up to 20 cm (Table 4, Figure 7).Fallen logs, as a microrelief factor, create a certain microclimate in its surrounding area [42], mitigating temperature fluctuations and maintaining stable humidity and soil enzyme activity even under summer droughts [46].In addition to their direct impact, decaying log changes the surrounding environment indirectly due to its physical and chemical transformation during decomposition [46], and, as a result, initiates succession processes on both the log and ground cover (Figures 4-7).Deadwood contributes to the growth of fungal [45] and microbial biomass [44], as well as alters the composition and/or activity of microcommunities [85,86].The strong stimulating effect of decaying logs on soil biological activity [43] enhances the mineralization of soil organic matter [45].Our data indicate that the deadwood impact on the uniformity of vascular plant cover has opposite trends depending on the condition richness (Table 4).The influence of deadwood on surrounding soil varies depending on its type and properties [45].In the poor conditions at the M microsite type, the decaying logs are probably capable of creating local patches with nutrient enrichment, which can increase the activity of soil microorganisms [87].A possible increase in soil enzyme activity and, as a result, higher decay rates of forest litter may also indicate the strong impact of the deadwood [46].Soil biological activity near decaying logs is higher Some authors showed similar patterns [21,25].
The vascular plant species composition changes [69,82,83] and their richness increases with the decay class of deadwood [9,[21][22][23]67].Plant species with low nutrient demand after dying on the fallen log surface contribute to the accumulation of organic matter, which creates favorable conditions for the colonization of more nutrient-demanding species [4].
Deadwood is an important component of forest ecosystems as a habitat [5,50-52], a key factor of biodiversity [4,5,[9][10][11], and an essential link to the nutrient cycle [84].Our study demonstrates a previously undetected impact of the P. abies fallen logs on the distribution of vascular plants on its adjacent forest floor.We noted the influence of a decaying log on the ground cover at a distance of up to 20 cm (Table 4, Figure 7).Fallen logs, as a microrelief factor, create a certain microclimate in its surrounding area [42], mitigating temperature fluctuations and maintaining stable humidity and soil enzyme activity even under summer droughts [46].In addition to their direct impact, decaying log changes the surrounding environment indirectly due to its physical and chemical transformation during decomposition [46], and, as a result, initiates succession processes on both the log and ground cover (Figures 4-7).Deadwood contributes to the growth of fungal [45] and microbial biomass [44], as well as alters the composition and/or activity of microcommunities [85,86].The strong stimulating effect of decaying logs on soil biological activity [43] enhances the mineralization of soil organic matter [45].Our data indicate that the deadwood impact on the uniformity of vascular plant cover has opposite trends depending on the condition richness (Table 4).The influence of deadwood on surrounding soil varies depending on its type and properties [45].In the poor conditions at the M microsite type, the decaying logs are probably capable of creating local patches with nutrient enrichment, which can increase the activity of soil microorganisms [87].A possible increase in soil enzyme activity and, as a result, higher decay rates of forest litter may also indicate the strong impact of the deadwood [46].Soil biological activity near decaying logs is higher compared to that at a distance of 1 m [43].More favorable conditions contribute to a higher vascular plant species diversity and decrease the dominance index.
In the rich P microsite type, the changes in vascular plant cover near fallen logs have different trends compared to the poor conditions (Figure 5).Nutrient-rich soils respond to decaying logs less strongly compared to nutrient-poor soils [45].The decomposition of P. abies logs under soil conditions with sufficient nutrient supply is likely capable of creating local specific sites different to the initial ones.Changing conditions, in turn, affect the initial species composition structure by a decrease in the species diversity and an increase in the dominance index.
The changes in the vascular plants cover heterogeneity with decay class progression in studied microsite types have different trends (Figure 6).We found the same trends on both XS and the forest floor at a distance of 0-20 cm from the log.In the P microsite type, the average number of vascular plant species increases, and the dominance index decreases with increasing decay class of the logs (Figure 6).In the M microsite type, we note the opposite trend: the species number decreases, and the dominance index increases with the decay class (Figure 6).The structure of plant cover is different on the deadwood and the forest floor [68].At earlier decay stages, a decaying log may be inhabited by plant species that are non-typical [25].As deadwood decomposes, the vascular plant composition gradually approaches to typical one for particular conditions [21].In rich and poor conditions, the species heterogeneity decreases archives by the oppositely directed processes (Figure 6).For instance, in the initially rich conditions of the P microsite type, the homogeneous vascular plant cover is formed by the density of shoots of many species.The fall of a tree affects the surrounding space: the fallen log changes soil properties and creates new specific conditions.As a result, it destroys the initial uniformity of the vascular plant cover at a distance of up to 20 cm from the log (Figures 6 and 7).The changes in the vascular plant cover on the decaying log and the forest floor within this distance have similar trends.The average plant species number decreases, and the dominance index increases due to an increase in the density of O. acetosella shoots.In all studied types of microsites, the changes in the heterogeneity parameters of the vascular plants covered with the distance from logs are associated, to a greater or lesser extent, with the density of O. acetosella shoots: with the distance increase the number of shoots of O. acetosella decreases (Figure 5).Probably, the decreasing of soil pH [88] due to the leaching of organic matter from deadwood to soil [89,90] creates favorable conditions for O. acetosella.The vascular plant cover gradually changes with an increase of the log decay class: the species number increases, whereas the dominance index decreases.Shoots of V. vitis-idaea, V. myrtillus, M. bifolium, as well as undergrowth of P. abies, R. saxatilis appear on the XS; shoots of V. myrtillus, M. bifolium, C. arundinacea, V. vitis-idaea, R. saxatilis, T. europaea, C. majalis, L. pilosa, G. dryopteris, Carex sp., Fragaria vesca appear on the forest floor at distance 0-20 cm from the log.With the log decomposition, the heterogeneity of conditions is smoothed out: the differences between decaying logs and forest floor are gradually decreased.In poor conditions, with the initially high uniformity of the vascular plant cover due to a small number of dominant species (V.myrtillus and V. vitis-idaea), the decaying deadwood improves nutrient availability.On the XS and the forest floor at a distance of 0-20 cm from it, the species number increases, and the dominance index decreases.It occurs due to the growth of the P. abies and O. acetosella shoots on the XS and the O. acetosella and C. arundinacea shoots on the forest floor.With the decomposition, the structure of the vegetation cover comes to a typical state for particular growth conditions: the total species number decreases, and the dominance of V. myrtillus and V. vitis-idaea returns.

Conclusions
The fallen P. abies logs act as one of the factors of heterogeneity of the forest floor and reversibly reduce its homogeneity.The heterogeneity of forest floor reduces with the log decomposition, and its structure becomes more typical for particular forest growth conditions.Deadwood influences the horizontal distribution of some vascular plant species in an old-growth spruce forest in the middle taiga: O. acetosella, M. bifolium, V. myrtillus, and V. vitis-idaea change their shoot densities with the distance from the logs, L. pilosa and C. arun-dinacea change their occurrence.The increase in density of M. bifolium starts at a distance of 80-90 cm from the log in the P and OM microsite types.The shoot density of V. myrtillus and V. vitis-idaea increases in the OM microsite type at distances of 40 and 70 cm from the log, respectively.In all types of microsites, the parameters of the horizontal distribution of O. acetosella invariably decrease with the distance from the log.In the HM microsite type, the decrease in density starts at a distance of 80 cm, in the M microsite type-20-40 cm, and in the OM microsite type-10-20 cm.
The changes in the homogeneity of the initial vascular plant cover by deadwood and the gradual smoothing of heterogeneity between the logs and the forest floor along the decay process in different, rich and poor conditions have different, mainly opposite, dynamics.In the HM and P microsite types, the influence of deadwood on the vascular plant cover homogeneity is represented by a decrease in the species number and an increase in the density of shoots of some of them.The heterogeneity decreases with the log decomposition due to an increase in the density of the shoots of many plant species.In the M microsite type, a decrease in the plant cover homogeneity occurs due to the appearance of new species and, consequently, an increase in the density of their shoots.With decay class progression, typical dominant plant species displace other species.The composition and structure of the vascular plant community on deadwood at the 5th decay class do not differ from those outside the zone of xylolytic substrate influence.
We observed the influence of the deadwood on the vascular plants of the forest floor at a distance of up to 20 cm from the log, which was represented by the same trends of heterogeneity indicators as on the log itself.In all microsite types, most of the changes in vascular plant cover with distance from the log are mainly associated-to a greater or lesser extent-with the change in shoot density of O. acetosella.

Figure 1 .
Figure 1.Research area: (А).Position of the Kivach Nature Reserve on the map of biogeographic areas of Europe (https://www.eea.europa.eu/data-and-maps/figures/biogeographical-regions-ineurope-2; the access date 18.05.2024),copyright holder: European Environment Agency (EEA).;(B).Location of the survey sites in relation to each other in the natural spruce forests of the Kivach Nature Reserve (Google image); (C,D)-appearance of the forest site and studied plots with the deadwood.

Figure 1 .
Figure 1.Research area: (A).Position of the Kivach Nature Reserve on the map of biogeographic areas of Europe (https://www.eea.europa.eu/data-and-maps/figures/biogeographical-regionsin-europe-2;accessed on 15 May 2024), copyright holder: European Environment Agency (EEA); (B).Location of the survey sites in relation to each other in the natural spruce forests of the Kivach Nature Reserve (Google image); (C,D)-appearance of the forest site and studied plots with the deadwood.

Figure 2 .
Figure 2. Description scheme one SP associated with the fallen P. abies log.

Figure 2 .
Figure 2. Description scheme one SP associated with the fallen P. abies log.

Figure 3 .
Figure 3. Results of the cluster analysis: (a) dendrogram; (b) distribution of the shares of shoots of species from the total number of shoots of vascular plants on the SP in four selected clusters.The cluster structure and description are represented in Table1.

Figure 3 .
Figure 3. Results of the cluster analysis: (a) dendrogram; (b) distribution of the shares of shoots of species from the total number of shoots of vascular plants on the SP in four selected clusters.The cluster structure and description are represented in Table1.

Figure 4 .
Figure 4. Changes of the average density (pcs/0.036m 2 ) shoots of some vascular plant species at different distances from the P. abies logs in different microsite types: (A)-gradient on the average SP, (B)-chart of the average value change.Different letters denote statistically reliable differences in the parameter (p < 0.05).

Figure 4 .
Figure 4. Changes of the average density (pcs/0.036m 2 ) shoots of some vascular plant species at different distances from the P. abies logs in different microsite types: (A)-gradient on the average SP, (B)-chart of the average value change.Different letters denote statistically reliable differences in the parameter (p < 0.05).

Forests 2024 , 23 Figure 5 .
Figure 5. Changes the average density (pcs/0.036m 2 ) shoots Oxalis acetosella different distances from the P. abies logs in different microsite types: (A)-gradient on the average SP, (B)-chart of the average value change.Different letters denote statistically reliable differences in the parameter (p < 0.05).

Figure 5 .
Figure 5. Changes the average density (pcs/0.036m 2 ) shoots Oxalis acetosella different distances from the P. abies logs in different microsite types: (A)-gradient on the average SP, (B)-chart of the average value change.Different letters denote statistically reliable differences in the parameter (p < 0.05).

Figure 6 .
Figure 6.Increasing the log decay class change heterogeneity vascular plant cover on xylolytic substrate (XS) and the areas distance from the log (ADL).Different letters denote statistically significant differences (p < 0.05).

Figure 6 .
Figure 6.Increasing the log decay class change heterogeneity vascular plant cover on xylolytic substrate (XS) and the areas distance from the log (ADL).Different letters denote statistically significant differences (p < 0.05).

Figure 7 .
Figure 7. Ordination plot of the vascular plant species cover in the M microsite type depending on the log decay class and the distance from the log.The differences in tested parameters are noticeable on deadwood at the 3rd and 4th decay classes and on the forest floor in close proximity to the logs (outlined with an oval).

Figure 7 .
Figure 7. Ordination plot of the vascular plant species cover in the M microsite type depending on the log decay class and the distance from the log.The differences in tested parameters are noticeable on deadwood at the 3rd and 4th decay classes and on the forest floor in close proximity to the logs (outlined with an oval).

Table 1 .
Formation of the analyzed parameters of the horizontal distribution and heterogeneity of vascular plant cover parameters.

Table 1 .
Formation of the analyzed parameters of the horizontal distribution and heterogeneity of vascular plant cover parameters.

Table 2 .
The microsite types (clusters) are characterized by the content of biogenic elements in the soil and in relation to environmental factors (based on Ellenberg ecological scales).

Table 3 .
Vascular plant species abundance was encountered at the 24 SP.
Note: XS-xylolytic substrate, S-soil cover without XS; the system of abundance indices of 0 to 6, see in the text.

Table 4 .
Differences in the heterogeneity indices of vascular plant cover between the xylolytic substrate (XS) and the areas distance from log (ADL) in the different microsite types.