Spatial structure and morpholody of understory in the southern taiga zone of central Siberia

. The spatial structure of understory is influenced by various biotic and abiotic factors. The aim of the present study was to reveal the role of spatial distribution of the understory and its influence on the morphological and other forest characteristics in young stands. Visual and instrumental forest measuress were conducted to assess the spatial distribution of understory (forest inventory data for Boguchanskoye, Daurskoye and Manskoye forestries in 1965 – 1975 were used). In Central Siberia, the spatial structure of understory is significantly influenced by the number of understory trees and the share of conifers in the stand composition. Quality of the understory worsens with increase in its number and age. At the same time, the condition of young trees growing in groups is lower. Competition in groups accelerates growth in height, making trees growing in groups higher than those arranged uniformly after 30 years of age. It was confirmed that the spatial structure of understory changes from a group to a uniform one with age. The analysis of variance showed that group arrangement of the understory significantly influences its quality and the share that dominant forest-forming species (spruce) takes in the young generation. In the case of uniform arrangement, a significant characteristic is the number of understory trees. Group spatial structure significantly influences the quality of understory and its growth rate in the southern taiga zone of South Siberia. In Central Siberia, abandoned agricultural lands are restocked mainly with coniferous (Scots pine) and deciduous (birch) species.


Introduction
The spatial distribution of the understory influences the morphology of trees which is especially evident in habitats characterized by extreme thermal conditions [1][2][3][4].
When studying natural regeneration, the structure of the understory determines the probability and potential success of the restocking of forests [5][6].Ukhvatkina and Omelko [5] determined that in the mixed temperate broad-leaf/fir-/spruce-/cedar-dominated forests in the Russian Far East, the spatial distribution of understory was primarily associated with gaps in the canopy.An assessment of spruce regeneration under the canopy of deciduous forests [6] showed that the quantitative factor did not always indicate the success of regeneration.The most important factors include growth indicators of the understory and its spatial structure since group arrangement increases competitiveness and mortality rate (as was proved for thin spruce trees).
Some researchers consider the horizontal structure of undergrowth to be an important component of forest regeneration [7][8][9].Vozmishcheva, Lonkina and Krestov [7] found that habitat determined the spatial distribution and height of understory.Conditions are favorable for trees regeneration in gaps in the canopy.Ilchukov [8] studied young spruce trees in the Komi Republic and revealed that small trees were distributed in groups at the mortality rate of 8-20%.The larger understory was distributed sparsely at the mortality rate of 23-41%.Notably, young spruce trees can successfully grow in groups in the gaps in the canopy.Such a spatial structure of spruce understory is also typical of indigenous swamped spruce forests [9].
Various biotic and abiotic factors influence the spatial structure of understory [10][11].Ufimtsev [10] stated that the number and height of young trees depended on their confinement to phytogenic fields.Ilchukov [11] revealed that fires hampered forest regeneration in felling sites in spruce forests.Therefore, understory grows in groups more often in forest sites that have not been damaged by fire.
Studying forest regeneration processes on abandoned farmlads is highly relevant today.Researchers proved [12] that abandoned farmlands have been restocked by the main forestforming species (Scotch pine and European spruce) in the Leningrad Oblast.The new European Union Forest Strategy for 2030 aims to plant 3 billion trees on non-forested lands to mitigate climate change.The highest climate benefit was observed for larch, followed by European spruce, poplar, aspen and birch hybrid [13].Notably, the assessment of soil organic carbon stock is important for abandoned agricultural lands.A number of researchers stated [14] that afforestation accelerated soil carbon accumulation on agricultural lands in Taiwan due to carbon sequestration by the forest floor and tree biomass.In the Lithuanian semi-boreal zone, reforestation of pastures is effective only on mineral-poor soils [15].

Materials and methods
The aim of the study was to reveal the role of spatial distribution of the understory and its influence on the morphological and other forest characteristics in young stands.
Visual and instrumental forest measurements were conducted to assess the spatial distribution of understory (forest inventory data for Boguchanskoye, Daurskoye and Manskoye forestries in 1965-1975 were used).It should be noted that the current forest inventories (of recent years) do not contain information on the arrangement of the understory, limited to its number, species composition, average height, age and health status.
The understory spatial structure was described as sparse, uniform, non-uniform and group.To apply the analysis of variance, all forest characteristics, including the spatial distribution of the understory, were coded with natural numbers (Table 1).The following types of mathematical analysis were used: statistical, correlation, regression, factorial, discriminant.
Calculations were performed using the Microsoft Office software package and the STATGRAPHICS statistical package.

Results and Discussion
An analysis of the forest inventories made it possible to identify the stands characteristics in random forest compartments as follows.
Boguchanskoye forestry (Karabulskoye district forestry) is dominated by thin-leaved species and the following forest types: tall-herb birch forests, herb-rich birch forests, tallherb aspen forests, herb-rich aspen forests, and herb-rich pine forests.The stands age varies from 15 to 90 years.The stands are of the I-III bonitet classes.The average height ranges from 7 to 23 m.The average diameter ranges from 12 to 32 cm.The understory is arranged sparsely, non-uniformly and in groups.
Spruce forests of the Equisetum/feather moss forest type were analyzed individually.In total, 308 forest compartments were selected where group and uniform understory structure were observed (see below).
Daurskoye forestry (Kurtalskoye district forestry).The forest compartments are represented by coniferous and deciduous forest stands: Siberian stone pine stands with forest floor covered by reed grass, Siberian fir stands with forest floor covered by reed grass, reed grass/herb-rich aspen forests, reed grass birch forests.The age of deciduous stands varies from 20 to 90 years; the age of coniferous stands varies from 25 to 200 years.The stands are of the I-III bonitet classes.The average height ranges from 8 to 26 m.The average diameter ranges from 8 to 48 cm.The spatial distribution of understory is uniform and group.In the Siberian stone pine stands with forest floor covered by reed grass, the young trees are growing in groups.
Manskoye forestry (Ungutskoye district forestry) is represented by herb-rich birch forests, Equisetum/feather moss spruce forests, herb-rich larch forests, and herb-rich pine forests.The age of deciduous forest stands varies from 40 to 280 years, and the age of coniferous forests varies from 60 to 270 years.The stands are of the I-IV bonitet classes.Average height ranges from 12 to 27 m.Average diameter ranges from 8 to 44 cm.The spatial structure of understory is uniform and group.
The conclusion drawn preliminary indicated that the study area reflected the entire species diversity and forest types spectrum typical of Central Siberia.
Then, using forest characteristics (Table 1), a dispersion analysis was applied to assess the relation between characteristics of forest canopy and the spatial distribution of the understory (Table 2 shows the results).
There were no factors revealed that significantly influenced the spatial distribution of understory in the Boguchanskoye forestry.The closest in terms of significance was the stand age.For the Manskoye forestry, the number of understory trees turned out to be a significant factor.As for the Daurskoye forestry, significant factors were: the share of conifers in the stand composition and the number of understory trees (Table 2).
In general, it can be stated that the number of understory trees and the share of coniferous species in the stand composition have a significant impact on the spatial  At the next stage, understory in the spruce forests of the Boguchanskoye forestry was studied in detail.Statistical analysis and the criterion of significance made it possible to reveal the degree of difference in the understory characteristics in case of group and uniform arrangement (Table 3). of Siberian stone pine in the stand composition; SB -share of birch in the stand composition; SF -share of Siberian fir in the stand composition; SP -share of Scots pine in the stand composition; SA -share of aspen in the stand composition.
Notably, spatial distribution of the understory does not influence its quality significantly.The share of the dominant species (spruce) in the understory composition is higher in case of its uniform arrangement.The number of understory trees is larger in case of their group arrangement (5185 thousand trees per ha -1 ) than in case of uniform arrangement (3393 thousand trees per ha -1 ).Understory is typically growing in groups in birch, fir and aspen stands.A uniform structure of understory spatial distribution is typical of spruce, Siberian stone pine and Scots pine (Table 3).
Graphical analysis made it possible to study in more detail the influence that the spatial structure of understory has on its number, height and quality (Figure 1).Quality of the understory worsens with an increase in its number and age.In the study area, quality of the understory varies significantly (group arrangement 40-100 %, uniform arrangement 10-90 %).Nevertheless, the condition of young trees growing in groups is lower compared to those growing disorderedly (Figure 1c).
Competition in groups accelerates growth in height, making trees growing in groups higher then those arranged uniformly after 30 years of age (Figure 1a).The number of understory trees growing in groups is higher up to 28 years, but then a number of trees arranged uniformly becomes larger as they cover a larger area (Figure 1b).
Correlation analysis confirmed the difference in the relationships between the characteristics of the understory, depending on its spatial structure (Table 4).Correlations are higher when understory spatial structure is uniform, which is explained by the low level of group arrangement coefficients.Factor analysis made it possible to identify characteristics that determined the variation in the understory distribution (group -GS (1), uniform -US (2)).The characteristics (quality, share of the dominant species, area, age, height, number of young plants) altogether influence the spatial distribution of the understory in the study are at 76.3 %.The most variable characteristics are the average height of understory (53.0 %), followed by age (34.8 %).The other characteristics (quality and the share of spruce) influence the spatial distribution of the understory at 12.2 %.Thus, the spatial structure of understory changes from a group to a uniform one with age (with an increase in the number of understory trees).Notably, the average height of understory trees growing in groups is significantly higher than that of understory trees arranged uniformly.The quality of understory does not depend on its arrangement (5.7 %).
Linear discriminant analysis proved that 66.9 % of 139 cases of group arrangement of the understory were correctly predicted, and 33.1 % were predicted incorrectly.In the case of uniform distribution (169 cases), 72.2 % were correctly predicted, and 27.8 % were predicted incorrectly.The following predictive equations were used: Analysis of variance made it possible to determine the characteristics that significantly influenced the spatial distribution of the understory of each category of plants (in cases of both group and uniform arrangement).The studied characteristics altogether influence group arrangement of the understory at 36.8 %.There are two significant influencing characteristics: quality (3.3 % at pv = 0.04) and the share of spruce (4.1 % at pv = 0.01).The studied characteristics altogether influence uniform arrangement of the understory at 51.0 %.In that case, the only characteristic that has a significant impact is the amount of understory trees (25.2 % at pv = 0.00).All the estimates were obtained at a confidence level of 95.4 %.
In the European Russia [16] abandoned agricultural lands have been mostly restocked by deciduous species.Less often abandoned farmlands have been restocked by coniferous and mixed forests.Nowadays, 34.1 % of abandoned agricultural lands has been covered by forests.As for Central Siberia, abandoned farmlands have been mostly occupied by Scots pine and birch forests.According to scientific literature [17], natural reforestation of former agricultural lands covering a large area starts from an existing forest growing nearby and gradually occupies bigger area over time.

Conclusion
The following conclusions can be drawn from the present study.
-The most favorable conditions for the growth of understory are formed in gaps in the canopy.Small spruce understory is typically growing in groups, and large understory is arranged uniformly.
-In Central Siberia, the spatial structure of understory is significantly influenced by the number of understory trees and the share of conifers in the forest stand composition.
-A detailed analysis of spruce understory in the Boguchanskoye forestry showed that its quality did not significantly depend on the spatial structure.
-The share of the dominant forest-forming species (spruce) in the understory is higher in case of its uniform arrangement.A significantly larger number of understory trees was observed in case of its group arrangement.Notably, birch, fir, and aspen understory -Quality of the understory worsens with an increase in its number and age.At the same time, the condition of young trees growing in groups is lower.Competition in groups accelerates growth in height, making trees growing in groups higher than those arranged uniformly after 30 years of age.
-Morphological relationships are closer in case of uniform distribution, which is explained by low values of correlation coefficients that are high in case of the group spatial structure of understory.
-It was confirmed that the spatial structure of the spruce understory has been changing from a group to a uniform with age.
-Using the linear discriminant analysis, two linear equations were obtained to predict the spatial structure of the understory based on its characteristics.
-The analysis of variance showed that natural group arrangement of the understory significantly influenced the quality of the understory and the share that dominant forestforming species (spruce) took in the young generation.In the case of uniform arrangement, a significant characteristic is the number of understory trees.
-In Central Siberia, abandoned agricultural lands are restocked mainly with coniferous (Scots pine) and deciduous (birch) species.
Thus, it was found that the spatial structure of understory has been changing from a group to a uniform one with age.Group spatial structure significantly influence the quality of understory and its growth rate in the southern taiga zone of South Siberia.

Table 1 .
Coding the characteristics of forest canopy and understory ://doi.org/10.1051/e3sconf/202343101035distribution of understory.It was confirmed that the group arrangement of understory is typical of coniferous stands with a high density of understory. https

Table 2 .
Parameters in the analysis of variance (characteristics of forest canopy and understoryspatial distribution of the understory) А -the share of coniferous species in the stand composition; В -forest stand age; С -forest stand height; D -stand density; E -growing stock; F -number of understory trees; G -understory density; SS -the sum of the squared deviations; DF -degrees of freedom; MS -mean squared error; F -Fisher's criterion; p-v -significance level (a statistically significant test result at р < 0.05).

Table 3 .
The difference between the mean values of the understory characteristics in case of group and uniform arrangement in Equisetum/feather moss spruce forests Note: Q -understory quality; SS -share of spruce in the stand composition; А -age, years; Н -height, m; N -number of understory trees, thousand trees per ha -1 ; SSP -share

Table 4 .
Significant correlations between the characteristics of the understory, depending on its spatial structure

Correlation between the characteristics of the understory Correlation coefficient / Significance level Group arrangement Uniform arrangement
Note: Correlations are significant at р ≤ 0.05.
://doi.org/10.1051/e3sconf/202343101035typically grows in groups.Uniform arrangement of understory is typical of spruce and Siberian stone pine. https