1. Introduction
According to Burkhart [
1], stand density is a crucial aspect of forests, mainly employed to promote wood growth [
2]. Studies have indicated that the management of forests, encompassing tree growth [
3], soil nutrient cycling [
4], and forest regeneration benefit from the control of stand densities [
5]. Optimal forest stand structures are essential for fostering robust plant growth, optimizing soil nutrient utilization, augmenting site productivity, and guaranteeing the sustained advancement of cultivated forests [
6]. Moderate stand density in conifers significantly influences tree growth and has a beneficial effect on the enlargement of tree diameter [
7]. Reduced stand density has been shown to improve soil qualities, including N, P, K, and many other vital nutrients for plant growth [
8]. A reduction in the density of tree stands frequently leads to a decrease in competition among trees for soil nutrients such as nitrogen (N) and phosphorus (P) [
9].
The Korean pine (
Pinus Koraeinsis Sieb. et Zucc), a prominent species in the temperate forests of northeast China, is economically valuable because it provides food, lumber, and habitat for wildlife. However, overharvesting has led to a reduction in its dominance in recent decades [
10]. According to earlier research, stand density and tree growth are connected. Higher quality timber production was noted in stands with less density [
11]. The regulation of stand density in forests is well understood, but further research is needed to understand its impact on understory vegetation and soil properties [
12].
Additionally, research in various species has proposed density reduction as a way to enhance tree growth, as the diameter increment of Aleppo pine (
Pinus halepensis Miller) was more noteworthy in a stand with a lower density [
13]. However, the dynamics and functions of forest stands are significantly influenced by silvicultural techniques and management activities, which must be appropriately carried out to preserve the sustainable performance of forests [
14]. Nonetheless, the primary objectives of wood production are to sustainably increase the managed stand’s size or enhance its quality. Managing stand density or starting planting densities is the primary strategy to do this [
15]. Accurately measuring stand density is essential for forecasting forest development and productivity [
1].
Soil nutrient levels and other physical and chemical indicators are frequently employed to evaluate the condition of the soil [
16]. According to a common notion, suppressed trees are expanding within stands toward their lower survival threshold at the tree level [
17]. When stand density (thinning) decreased, tree DBH increased [
18]. Soil, primarily responsible for delivering water and nutrients to agricultural plants, also regulates water flow and quality and provides habitat for soil biota [
19]. Meanwhile, little research has been carried out on the characteristics of the soil beneath various forest stands, which is crucial for sustaining ecosystems [
20].
However, most Korean pine plantations face environmental problems due to inappropriate starting densities. While there are still a lot of unanswered questions regarding the best stand densities to maximize ecological benefits, these species must continue to develop sustainably. According to a report, understory biodiversity is crucial in maintaining forest ecosystems and forest variety [
21]. The variety of understory vegetation displays how an organism dynamically adapts to its environment and represents the make-up, arrangement, and stage of development of community structures [
22]. Research has confirmed that a higher species diversity contributes to the stability of forest ecosystems [
23]. The Simpson, Shannon–Wiener, and Pielou indices are commonly employed to quantify vegetation diversity [
24,
25]. It was reported that, in a high-density stand, restricted tree growth is most likely the result of intense competition that slows down tree growth [
26].
Moreover, the effects of stand density regulation on tree growth, soil qualities, and understory vegetation are a topic of ongoing research and have yet to be sufficiently clarified. Therefore, the present work is undertaken to evaluate the effect of stand density on the growth of Pinus koraiensis, soil properties, and understory flora diversity and to determine the optimal stand density that could be appropriate for these factors. We hypothesized that the medium stand density would have the maximum total volume and diameter of trees, nutrient contents, as well high biodiversity status since the medium stand will encourage favorable environmental factors and less competition.
2. Materials and Methods
This research was carried out in Qingping Forest Farm, part of the Dahailin Forestry Bureau, Heilongjiang Province, northeast China. It is located southwest of Mudanjiang City. The location is 508 m above sea level and lies in Changting Town, Hailin County (44°03′–44°41′ N and 128°02′–129°01′ E) (
Figure 1). The climate in this region is a sub-temperate monsoon. The lowest temperature in the area is −35 °C and the highest is 33 °C, which occur in January and July, respectively. The plantation was established in 1976 with an initial planting distance of 1.5 m × 2 m. The plantation was 43 years old at the time of the thinning operation carried out in 2019. The local forestry authorities decided to implement a thinning operation on the 43-year-old Korean pine (
Pinus koraiensis) plantation due to the initial high density of trees planted during afforestation and following standard silvicultural practices and a comprehensive forest management plan. The purpose of this decision is to optimize the development, vitality, and overall condition of the trees by reducing competition among them. Thinning was carried out as a management practice to pave the way for desirable species. The study area covers a total of 8.6 hectares, within which the local authorities have designated six adjacent plots for research purposes. The most dominant shrub species was
Lespedeza bicolor Turcz, which also had the highest species important value (IV) index. However, the species with the highest (IV) under the herb layer were
Athyrium brevifrons Nakai ex Kitagawa (48.90),
Athyrium brevifrons Nakai ex Kitagawa (32.27), and
Festuca rubra L. (62.20) in low, medium, and high stand densities, respectively. Detailed information about the species’ important value is presented in
Section 3.9.
2.1. Experimental Design
In this study, we used 6 permanent circular plots with each having a size of 0.06 ha following the research methods in [
27]. A total of 3 stand density levels, low stand density (LSD 716 trees ha
−1), medium stand density (MSD 850 trees ha
−1), and high stand density (HSD 916 trees ha
−1), were selected in the third week of September 2023 in a 43-year-old middle-aged (
Pinus koraeinsis Sieb. et Zucc) plantation. The 43-year-old Korean pine plantations were selected as they represent the middle-aged plantation [
28] for this species. This allows us to analyze the development and durability of the ecosystem at its peak. Our research primarily centers on the mid-life period of Korean pine plantations. This stage is crucial for understanding the transition from fast expansion to senescence. Furthermore, the specific management technique or silvicultural treatment that we aim to evaluate in this study is associated with the age of 43 years. Variation in stand density levels was created during the thinning operation in 2019. The plantation is homogeneous in environmental conditions such as (stand type, temperature, aspect, slope, elevation, and soil type). The six neighboring plots were carefully selected to minimize bias. Grouping of stand density types was carried out a posteriori based on the number of trees per plot (
Table 1). Low stand density ranged from 700 to 716 trees/ha, stand density for medium class ranged from 833 to 850, and high stand density ranged from 883 to 916 trees/ha.
2.2. Vegetation Data Collection
All trees were measured at breast height at 1.3 m above ground, and their height was recorded. Tree volume was calculated using the formula reported in [
29].
where V is tree volume (stem volume over bark), D is the diameter at breast height, and H is total tree height.
Understory vegetation was assessed in two layers: shrubs and herbs. Shrub species were recorded from five (5 m × 5 m) quadrats located at the four corners and one in the center of each plot, while herb species were recorded from five (1 m × 1 m) quadrats located at the four corners and one in the center of each plot. Species diversity indices were used as a measure of community species diversity of each plot, using species richness (S), Shannon–Wiener (H), Simpson (D), and Pielou’s equitability (Jsi) reported in [
30,
31]. Species diversity, species richness, and Simpson, Shannon–Wiener, and Pielou indices were determined using the following formulas:
where
Ni = number of individuals for each species present in the sample area. D is the Simpson index,
H’ is the Shannon–Wiener index, E is the Pielou index,
lnS stands for the natural logarithm of species richness, S is the number of plant species in the standard area (the species richness index), and
pi is the proportion of individuals belonging to the i-th species (
ni/
n).
The species’ important value was calculated using the formula mentioned in [
31]. Species important value (IV) = (RD + RF + RC)/3.
Note: IV = species important value, RD = relative density, RF = relative frequency, and RC = relative cover
2.3. Soil Sample Collection
Soil samples were randomly collected using a corer, and a soil profile was dug at five points per plot in three soil depths (0–10 cm, 10–20 cm, and 20–30 cm) [
27]. The number of soil samples collected per plot was 15 (5 random locations × 3 depths), with 90 soil samples collected in the study area. All soil from each layer per plot was kept in sealed plastic bags to form composite soil samples for analysis of pH, Soil Moisture content (SMC), Soil Organic matter (SOM), Bulk Density (BD), Total Nitrogen (TN), Available Nitrogen (AN), Total Potassium (TK), Available Potassium (AK), Total Phosphorus (TP), and Available Phosphorus (AP). The soil samples were air-dried at room temperature and then crushed and passed through sieves of 2 mm and 0.15 mm mesh size for subsequent analyses. Every measurement of both physical and chemical properties was carried out in the College of Forestry’s Silviculture laboratory at Northeast Forestry University in China (
Table 2).
2.4. Statistical Analyses
Differences in tree growth parameters, understory biodiversity (Shannon–Wiener, species richness, species evenness, and Simpson’s diversity indexes for shrub and herb layers), and soil physical and chemical properties among the three stand densities were analyzed using one-way ANOVA and LSD (least significant difference) tests (significant at
p < 0.05). Analyses were carried out in SPSS 27.0.1 [
6,
32].
5. Conclusions
This study examined the impact of stand density on the growth of trees, soil characteristics, and the diversity of understory species in a middle-aged Pinus koraiensis plantation following a brief period of selective thinning. The results indicated that a stand density of 833–850 trees per hectare is the most favorable for attaining a harmonious combination of optimizing tree development and improving the health of the soil and understory vegetation in the study area. The findings further revealed that the density of Korean pine stands had a notable influence on the overall volume, particularly in stands with medium density. Additionally, stand density had a substantial impact on TN, AN, TK, AK, and SMC. However, TP, AP, pH, SOM, and BD were not shown to be influenced by stand density. The majority of the soil parameters examined were influenced by the depth of the soil and exhibited a declining pattern along the soil profile. Furthermore, our research demonstrated that the density of stands has a significant influence on biodiversity, with the greatest impact detected in stands of medium density. Therefore, the impact of medium stand density may contribute to preserving and improving biodiversity, soil qualities, and sustainable tree growth in middle-aged Korean pine plantations. These findings provide valuable knowledge for forest management, emphasizing the significance of continuous, long-term, and site-specific research.