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Article

The Effects of Forest Gaps on the Physical and Ecological Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests

by
Xingjian Dun
1,*,†,
Yuchen Liu
2,†,
Fengjie Lian
2,
Wentai Zhao
3,
Wei Su
2,
Wei Zhao
2,
Zhihao Tian
2,
Yanhui Qiao
1,
Peng Gao
2 and
Zhenxiang Zhang
2
1
Shandong Academy of Forestry, Shandong Jinan Urban Ecosystem National Observation and Research Station, Ji’nan 250014, China
2
Mountain Tai Forest Ecosystem Research Station of State Forestry and Grassland Administration, Forestry College, Shandong Agricultural University, Tai’an 271018, China
3
Shandong Provincial Forestry Protection and Development Service Center, Ji’nan 250014, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2024, 15(10), 1784; https://doi.org/10.3390/f15101784
Submission received: 24 August 2024 / Revised: 28 September 2024 / Accepted: 2 October 2024 / Published: 11 October 2024
(This article belongs to the Section Forest Soil)
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Abstract

:
Forest gaps alter the environmental conditions of forest microclimates and significantly affect the biogeochemical cycle of forest ecosystems. This study examined how forest gaps and non-gap areas affect soil’s physical properties and eco-stoichiometric characteristics. Relevant theories and methods were employed to analyze the impact of forest gaps on nutrient cycling in Pinus densiflora Sieb. (PDS) and Robinia pseudoacacia L. (RPL) forests located in the Taishan Mountains. The results revealed that (1) forest gaps significantly enhanced the soil physical properties of PDS and RPL forests compared to non-gap areas (NPs). Notably, the bulk density of the soil decreased by 53%–12%, particularly in the surface layer (0–20 cm). Additionally, its non-capillary porosity increased by 44%–65%, while the clay and silt content rose by 39%–152% and 24%–130%, respectively. Conversely, the sand content decreased significantly, by 24%–32% (p < 0.05). (2) The contents of C, N, and P in the gap soil of PDS forests showed a significant increase compared to those in non-gap soil, with increases of 56%–131% for carbon, 107%–1523% for nitrogen, and 100%–155% for phosphorus. There was a significant drop of 10%–33% and 39%–41% in their C:N and C:P ratios, respectively (p < 0.05). The contents of C and P in the gap soil of the Robinia pseudo acacia L. Forest increased significantly, by 14%–22% and 34.4%–71%, respectively. Its C:P and N:P ratios significantly increased, by 14% to 404% and 11% to 41%, respectively (p < 0.05). (3) Compared with NPs, the forest gap significantly reduced the soil electrical conductivity and increased the soil pH. Additionally, compared to the soil at the gap’s edge, the surface soil in the gap’s center had noticeably higher concentrations of C, N, and P. (4) Key variables affecting the soil pH, silt content, bulk density, and overall porosity in forest gaps include the concentrations of carbon (C), nitrogen (N), and phosphorus (P) present and their ecological stoichiometric ratios. The findings showed that forest gaps had a considerable impact on the soil’s physical characteristics and ecological stoichiometry. They also had a high potential for providing nutrients, which might aid in the establishment of plantation plants.

1. Introduction

Forests account for about 31% of the Earth’s land area and play a vital role in regulating the water cycle, stabilizing the climate, and protecting biodiversity. Many forests functional processes are regulated by forest gaps. Forest gaps, which are crown openings caused by the death of one or more trees, are small-scale disturbances that are widely distributed in forest ecosystems.
At present, the concept of a forest gap has not been standardized worldwide. Different researchers have different definitions of forest gaps. There is a lack of uniform standards for the methods and norms of forest gap research. Specifically, coherence and the long-term multidirectional monitoring of a single region are lacking in comparisons of different types of regions, and this leads to variations in research focus amongst regions. Thus, it is essential to enhance our understanding of how forest gaps contribute to forest succession processes and the development of new forests [1]. Forest gaps play a key role in regulating the composition and structural diversity of forest ecosystems [2], cycling nutrients, maintaining site fertility [3], and promoting soil development and forest regeneration. The forest gap division hypothesis (GPH) shows that gaps produce a large resource gradient from the gap center to the understory. To gain a deeper insight into how forest gaps influence forest resources, it is important to conduct a comprehensive analysis of the soil’s physical properties, nutrient distribution, and ecological stoichiometry in different gap positions.
Forest gaps are crucial for the dynamic succession of forest ecosystems and the optimization of ecological functions. They are important drivers of forest regeneration and environmental improvement [4]. The creation of a forest gap alters the physical structure of the forest stand, serving as an essential mechanism that fosters regeneration. Additionally, because these gaps aid in the promotion of nutrient cycling and improve ecosystem functioning overall, they are essential to the general health of ecosystems. As matter of fact, changes in the physical and chemical composition of the soil can impact biological traits and the cycling of nutrients. Soil’s physical and chemical properties are important indicators that reflect soil function, improve water conservation functions and the stable growth of vegetation, and affect plant growth [5]. Forest gaps created by management practices can modify the physical and chemical properties of the soil by influencing the microclimate, enhancing soil texture, and increasing soil fertility, which in turn impact stand growth. Despite this, there is a notable lack of systematic and comprehensive research regarding the potential impact of forest gap positions. Specifically, the center of the gaps significantly increased the bulk density and porosity of the surface soil (0–10 cm) [6,7]. Furthermore, due to the higher levels of rainfall that reached the soil and the reduction in transpiration, a comparatively elevated soil water content was observed in the spaces between temperate deciduous forests, boreal coniferous forests, and humid tropical forests [8]. Forest gaps result in a reduced presence of roots and secretions in the soil, which in turn affects soil aggregates and influences the composition and distribution of soil particles to some extent [9]. When the surface soil fertility of forest gaps greatly improves, the biological activity and chemical element content in that soil increase or decrease, and these changes lead to changes in soil pH [10]. Soil electrical conductivity serves as an indicator of the current salinity levels within the soil, particularly under specific moisture conditions. This conductivity not only reflects the salt concentration but also plays a critical role in the transformation and availability of soil nutrients. Furthermore, it influences the state in which these nutrients exist, impacting their availability for plant uptake. Additionally, the level of electrical conductivity acts as a threshold that can restrict the activity of both plants and microorganisms. The forest gap causes the soil to be exposed, changes the water and heat balance in the soil’s natural state, and allows the soil to be fully leached by rainwater. Because of the lack of vegetation, evaporation is limited, which reduces the soil electrical conductivity [11]. However, the variation in soil physical properties within forest gaps and the effects of gap position remain ambiguous. Consequently, gaining a better insight into how gap position affects soil physical properties is advantageous for developing and sustaining management strategies that support essential soil processes.
Soil nutrients and ecosystem stability can be inferred from the abiotic stoichiometric properties of soil carbon (C), nitrogen (N), and phosphorus (P). When paired with external conditions, the distinct internal physiological and biological processes of different tree species affect the variations of C, N, and P in soil [12]. Forest gaps can promote early mass loss and carbon release from litterfall [13]. The rearrangement of the canopy layer, along with changes in solar radiation and precipitation (both rainfall and snowfall), creates diverse microenvironments within and around the gap. This leads to higher soil temperature and humidity at the center of the gap compared to non-gap centers. Wang Zhuomin [14] reported that organic matter decomposed more rapidly under forest gaps and nutrient mineralization was more rapid, which could effectively improve soil fertility. Without arborous plants in the gap to absorb nutrients, soil nutrient availability may increase [15]. However, some researchers argue that the absence of vegetation cover in the center of forest gaps, combined with direct rainwater exposure that erodes the soil, may result in nutrient loss, thereby decreasing soil nutrient levels. Moreover, forest gaps reduce soil carbon sequestration in high-altitude and frigid forests by reducing plant litterfall [16]. In addition, although soluble soil nutrients increase, the nutrient loss and denitrification caused by leaching also increase [17]. Compared with non-gap areas, the soil under a forest gap receives more light and precipitation, which changes the forest microclimate; affects the soil’s cycling of carbon (C), nitrogen (N), and phosphorus (P); and thus affects the C:N:P stoichiometry of the forest [18]. Thus, through modifying the temperature and moisture content, the gap’s location can directly affect the leaching and mineralization processes of carbon, nitrogen, and phosphorus in the soil while also indirectly affecting its carbon-to-nitrogen-to-phosphorus (C:N:P) stoichiometry characteristic [19]. Previous studies on forest soil’s ecological stoichiometry have focused mostly on closed forests [20,21]. As a result, it is still necessary to investigate the soil stoichiometry in various gap positions.
The Huangqian watershed, situated in Tai’an City, Shandong Province, is a tributary of the Yellow River Basin. It is a typical area of the Taishan mountainous area in northern China and a key state-level soil and water erosion control area. Pinus densiflora Sieb. and Robinia pseudoacacia L. forests are typical artificial forest types in the region and have important ecological protection functions such as preventing water and soil erosion, facilitating water conservation, and increasing carbon sequestration. In recent years, several researchers have examined the ecological stoichiometric properties of forest litterfall as well as the characteristics associated with rainfall runoff and the loss of nitrogen and phosphorus in forest ecosystems subjected to simulated rainfall conditions; the attributes of the soil microbial community undergo alterations throughout the process of litter decomposition, and the expansion of Robinia pseudoacacia L. forests has an impact on the distribution of soil particle sizes within this area [22,23,24,25]. However, our survey revealed that many natural and stable forest gaps have formed due to natural causes such as soil degradation and inadequate tending management, as well as natural aging, death, lodging, wind breaks, pests, and diseases. The physical qualities, ecological stoichiometric traits, and influencing variables of gap and non-gap soils in PDS and RPL forests have not been well studied in this field. The processes through which forest gaps influence the physical and chemical characteristics of soil remain poorly understood, making it challenging to provide a precise explanation of the impact of these gaps on the heterogeneity of soil properties.
In the study area, two types of trees (Pinus densiflora Sieb forest and Robinia pseudoacacia L. forest) have been growing for many years. The forest gaps of the treese species were also formed after a severe wind disaster in July 2008. Based on a previous plot survey, in July 2023, a formed forest gap and was selected in each of the Pinus densiflora Sieb. (PDS) and Robinia pseudoacacia L. (RPL) forests in the study area. Moreover, a non-gap plot with an area of 20 m × 20 m was set up under the closed forest at a distance of more than 20 m from the edge of the forest gap. Quantitative analysis and the methods of field fixed-point sampling were adopted, and ecological stoichiometry and soil fractal theory were used to study (1) the fundamental physical properties, including soil bulk density, total porosity, capillary porosity, and soil water content, along with the particle size distribution, of PDS and RPL forest gap soils compared to non-gap soils; (2) the carbon (C), nitrogen (N), and phosphorus (P) contents of soils in forest gaps and non-gap areas, as well as their ecological stoichiometric characteristics, which are important for understanding nutrient dynamics; and (3) the elements that affect the ecological stoichiometric features of forest gap and forest non-gap soils. This study provides basic data to support further research on the mechanisms governing soil’s physical properties and nutrient distribution patterns and the ecological stoichiometric characteristics of PDS and RPL forest gaps and establishes a scientific foundation for the sustainable management of resources and nutrients, the preservation of soil fertility, and tending measures for artificial forests.

2. Research Area and Methods

2.1. Introduction to the Study Area

This study was conducted at the Shandong Taishan Forest Ecosystem National Positioning Observation and Research Station, situated in the Huangqian watershed of Tai’an City, Shandong Province. Its geographical coordinates range from E 1171°04′39″ to 117°22′26″ and from N 36°17′58″ to 36°27′30″. The station is situated to the northwest of the mountainous region in the south–central section of the Earth-Rock Mountain area in northern China. (Figure 1). The rocks in the area are mainly granite gneiss weathering layers; the predominant soil type is brown soil characterized by a sandy loam texture; and the soil layer exhibits a relatively limited thickness, averaging between 30 and 40 cm. The pH of the soil is 5.5 to 6.5, which indicates that it is mostly mildly acidic. The climate of the area is warm temperate continental semi-humid monsoon, with 758 mm of precipitation on average each year. Notably, 75% of the rainfall in this region occurs from June to September. This study area is designated as an important national ecological protection forest zone, with a forest coverage rate of 89%, primarily consisting of warm temperate deciduous broad-leaved forest vegetation. The main species are Pinus densiflora Sieb. et ZuNG., Robinia pseudoacacia L., Quercus acutissima Carruth., Larix kaempferi (Lamb.) Carr., Lespedeza bicolor Turcz., Spiraea trilobata L., Themeda japonica (Willd.) Tanaka, Zoysia japonica Steud, etc. Moreover, many forest gaps have formed in these woodlands (such as in the Pinus densiflora Sieb forest and Robinia pseudoacacia L. forest) after a severe wind disaster in July 2008.

2.2. Sample Plot Setting and Sample Collection

Based on a previous plot survey, in July 2023, a formed forest gap and was selected from each of the Pinus densiflora Sieb. (PDS) and Robinia pseudoacacia L. (RPL) forests in the study area. Moreover, a non-gap plot (NG) with an area of 20 m × 20 m was set up under closed forest at a distance of more than 20 m from the edge of the forest gap. For each gap, we established a 20 m × 20 m plot at both the center of the gap (GC) and the edge of the gap (GE). The NG and GC were repeated three times, and the GE plot was repeated four times (east, west, south, and north) to eliminate errors due to different directions. The fundamental characteristics of the plots were assessed and are presented in Table 1.
We established 1 m × 1 m sampling points in each gap and non-gap plot. Soil samples were collected from two distinct layers (0–20 cm and 20–40 cm) at each sampling location using a five-point sampling method. The collected soil was packaged in a self-sealing bag, labeled, and carried to the lab. A cumulative total of 40 samples were obtained (2 stands × (3 centers + 4 forest edges + 3 non-gaps) × 2 soil layers).

2.3. Sample Processing and Determination

The soil samples were placed in a shaded, well-ventilated area to dry naturally. After drying, the material was processed using a 2 mm sieve. After grinding the soil samples, the pH and electrical conductivity of the soil were measured. The organic carbon content was determined by the potassium dichromate heating oxidation method. A certain amount of the soil sample was weighed into a digestion tube, and an appropriate amount of potassium dichromate solution and concentrated sulfuric acid were added, mixed evenly, and then placed on the heating device for heating digestion. After digestion, the organic carbon content was calculated by titration with specific reagents. We repeated the measurement three times and took the average value. The total nitrogen content was quantified by sulfuric acid digestion. A soil sample was placed in a digestion tube with sulfuric acid and a catalyst and digested at high temperatures. After digestion is complete, the total nitrogen content was determined by analysis with a specific instrument. We took three measurements of the same sample and created an average value. The total phosphorus content was determined using molybdenum-antimony reverse colorimetry. A certain amount of the soil sample was weighed, and specific reagents were added for digestion and color reaction. The absorbance was then measured using a spectrophotometer at a specific wavelength, and the total phosphorus content was calculated from a standard curve. We took three measurements and calculated the average value. Finally, a detailed regression analysis was conducted on the calculated data. Soil bulk density was measured using the knife ring method, and soil porosity was subsequently calculated. The soil moisture was measured with an aluminum box, and the diameter of the soil particles was assessed utilizing a Malvern laser particle size analyzer. The soil pH was measured with a 1:2.5 soil–liquid ratio pH meter. The conductivity was measured by a 5:1 liquid-to-soil conductivity meter. The organic carbon content was assessed using the potassium dichromate oxidation method combined with external heating. The total nitrogen content was quantified through the sulfuric acid digestion method. The total phosphorus content was determined by the molybdenum-antimony reverse colorimetric method.

2.4. Data Analysis and Processing

2.4.1. Soil Fractal Dimension

The fractal model of soil particle volume is used to calculate a single fractal dimension. The specific calculation method is as follows:
V r   <   R V T = R λ V 3 − D
R represents the arithmetic mean of the upper and lower limits of a particle size interval. V(r < R) denotes the cumulative volume of particles with sizes smaller than R, expressed as a percentage of the total volume. The entire volume of soil particle is denoted by VT. The cumulative percentage volume of soil particles smaller than R is denoted by V r < R V T . D is the soil fractal dimension. Additionally, λv represents the upper limit of all components and is numerically equal to the maximum particle size (in millimeters).

2.4.2. Statistical Analysis Method

Data preprocessing, statistical analysis, and drawing were completed in Microsoft Office Excel 2016, SPSS 20.0, and Origin 8.0 and R (4.0.5), respectively. The Duncan and least significant difference (LSD) methods with a one-way analysis of variance (ANOVA) were used to examine differences in soil physical properties, as well as the contents of carbon (C), nitrogen (N), and phosphorus (P) in the soil, along with the C:N, C:P, and N:P stoichiometries of PDS and RPL forests across various forest gap positions (GC, GE) and different stands. A significance threshold was set at p = 0.05. A principal component analysis was employed to analyze the relationships between the soils’ ecological stoichiometry and physical and chemical properties. The relationships between the C:N, C:P, and N:P stoichiometric ratios; soil physical and chemical parameters; and soil C, N, and P levels were assessed using a Pearson correlation analysis. The effects of stand, gap position, and soil layer interactions on the C, N, and P contents of the soil, as well as the C:N, C:P, and N:P stoichiometric ratios, were investigated using a three-way ANOVA.

3. Results and Analysis

3.1. Physical Properties of Soil in Gap and Non-Gap Forests of Pinus densiflora Sieb. and Robinia pseudoacacia L.

3.1.1. Soil Physical Indicators

In the center and edge of forest gaps in PDS forests, soil bulk densities at depths of 0 to 20 cm and 20 to 40 cm were significantly lower (p < 0.05) compared to that of the non-gap samples, showing reductions of 12% and 6% for the upper layer and 6% and 11% for the lower layer, respectively (Table 2). The non-capillary porosity of the soil in the gap center of PDS forests was substantially higher than that of the nonporous soil, by 60%–65% (p < 0.05). No substantial difference was observed in total soil porosity or capillary porosity between different forest gap positions and non-gaps. In PDS forests, the soil moisture content significantly decreased in the 0–20 cm and 20–40 cm layers at both the gap center and edge, with reductions of 32% and 13% and 21% and 10%, respectively (p < 0.05).
In RPL forests, the bulk density of the 0–20 cm soil layer was significantly lower in the gap center and edge compared to in non-gap forests, showing reductions of 6% and 5%, respectively (p < 0.05). Additionally, in the gap center of RPL forests, the total porosity, capillary porosity, and non-capillary porosity of the 0–20 cm soil layer increased by 8%, 6%, and 44%, respectively (p < 0.05). There were increases of 4%, 5%, and 20%, respectively, in the 20–40 cm soil layer (p < 0.05). At both the gap center and edge, the soil water content significantly increased, by 24% and 30% in the 0–20 cm layer and by 15% and 16% in the 20–40 cm layer, respectively (p < 0.05).

3.1.2. Soil Particle Composition and Distribution Characteristics

Compared with that in non-gap forests, the powder content in gap center and edge soil increased by 130% and 119%, respectively (p < 0.05). The sand content significantly decreased, by 35% and 32%, respectively (p < 0.05). In PDS forests, the 20–40 cm soil layer showed no discernible variation in the composition of its soil particles between different gap positions and non-gap areas.
In the gap center of RPL forests, the soil clay and silt contents in the 0–20 cm layer were significantly higher than those in non-gap soils, increasing by 39% and 24%, respectively (p < 0.05) (Table 2). The clay content within the 0–20 cm layer of soil from the gap center and the gap edge of the RPL forests decreased significantly, by 24% and 10%, respectively (p < 0.05). In the 20–40 cm soil layer of the RPL forests, the analysis showed no discernible difference in the composition of soil particles between the different gap positions and non-gap positions.
The distribution of the volume fractions of soil particles exceeding 50 μm in size within the PDS and RPL forest ecosystems in the study region was observed to be comparatively elevated; that is, the average sand content in the soil was 66% (Figure 2). The peaks of the frequency distribution curves appeared to be silt (2–50 μm) and coarse sand (500–1000 μm). Within the soil layer ranging from 0 to 20 cm in depth, the changes in the frequency distribution of the soil particle volume content in the gap center (3% and 3%) and the gap edge (3% and 5%) were smaller than those in the non-gap areas (5% and 5%) (p > 0.05).

3.2. Ecological Stoichiometric Characteristics of Soil in Forest Gaps and Non-Gaps of Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests

3.2.1. Soil Chemical Indicators and C, N, and P Contents

In PDS forests, the contents of soil organic carbon (C), total nitrogen (N), and total phosphorus (P) in both the gap center and edge were significantly higher compared to those in non-gap forests. In the 0–20 cm soil layer, the C content increased by 130.6% at the gap center and 85% at the gap edge. In the 20–40 cm layer, the increases were 61% at the gap center and 56% at the gap edge (p < 0.05) (Table 3). Similarly, the N content rose noticeably, with increases of 152% at the gap center and 107% at the gap edge in the 0–20 cm layer. In the 20–40 cm layer, the increases were 115% at the gap center and 130% at the gap edge (p < 0.05). The P content also demonstrated significant increases, rising by 132% at the gap center and 100% at the gap edge in the 0–20 cm layer, and by 155% at the gap center and 136% at the gap edge in the 20–40 cm layer (p < 0.05). Furthermore, within the 0–20 cm depth, the concentrations of C, N, and P were greater in the center of the gap than at the edge, with increases of 25%, 22%, and 16% seen, respectively (p < 0.05).
In the RPL forests, the soil carbon (C) content in the 0–20 cm layer significantly increased by 22% at the gap center and 14% at the gap edge compared to in non-gap forests (p < 0.05) (Table 3). There was no notable variation in the N content of the soil between different gap positions and non-gaps. In the RPL forests, the increase in nitrogen (N) content in the 0–20 cm soil layer was 34% at the gap edge and 50% at the gap center. For the 20–40 cm soil layer, the increases were 71% at the gap center and 52% at the gap edge (p < 0.05). The total phosphorus and soil organic carbon levels in RPL forests were greatly impacted by the forest gap’s position, while the overall N content was not considerably impacted.
The pH values of the soil in the center of PDS forests increased by 4% and 7%, respectively, compared to those in non-gap soils at the 0–20 cm and 20–40 cm depth intervals (p < 0.05). In the gap center of PDS forests, the electrical conductivity of the soil dropped by 36% and 29%, respectively, at the 0–20 and 20–40 cm depth intervals (p < 0.05) (Table 3). Table 3 also indicates a 13% decrease (p < 0.05) in the electrical conductivity of the soil at the gap edge of PDS forests at both the 0–20 cm and 20–40 cm depth intervals.
The soil pH values at both the gap center and edge increased significantly compared to non-gap samples, with an increase of 3% at the gap center and 1% at the gap edge in the 0–20 cm soil layer (Table 3). In the 0–40 cm soil layer, there was an increase of 4% at the gap center and 2% at the gap edge (p < 0.05). Additionally, in the RPL forests, the electrical conductivity of the soil decreased by 13% at the gap center and 10% at the gap edge within the 0–20 cm depth range (p < 0.05).

3.2.2. Characteristics of Soil’s Ecological Stoichiometry

The carbon-to-nitrogen (C:N) ratios of the soil in the 0–20 cm and 20–40 cm depth layers at the center and edge of gaps in PDS forests significantly decreased by 10% and 26% and 10% and 33%, respectively, compared to that in non-gap layers (p < 0.05) (Figure 3). Additionally, the carbon-to-phosphorus (C:P) ratio in the soil at depths of 20 to 40 cm at both the gap center and edge significantly decreased by 39% and 41%, respectively (p < 0.05). In PDS forests, there were no appreciable variations in the soil nitrogen-to-phosphorus (N:P) ratio between different gap positions and non-gaps.
The carbon-to-phosphorus (C:P) ratios in the soil layers at depths of 0–20 cm and 20–40 cm were measured at the center and the edge of the gaps within the RPL forests. They increased significantly, by 40% and 36% and 14% and 29%, respectively, compared with those in non-gap layers (p < 0.05). The nitrogen-to-phosphorus (N:P) ratio in the soil layers ranging from 0 to 20 cm and 20 to 40 cm at the gap center and edge of RPL forests increased significantly, by 33% and 41% and 11% and 35%, respectively (p < 0.05). Additionally, the carbon-to-phosphorus (C:P) and nitrogen-to-phosphorus (N:P) ratios in the 0–20 cm soil layer at the center of the gap in RPL forests significantly increased, by 23% and 19%, respectively, compared to the ratios at the gap edge (p < 0.05) (Figure 3).

3.3. Correlation Analysis between Soil’s Ecological Stoichiometry and Soil-Related Properties in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forest Gaps

A principal component analysis (PCA) was employed to visualize the relationships between the soil’s ecological stoichiometry and physical and chemical properties in forest gaps and forest non-gaps and in the presence of different tree species. The interpretation rate of PC1 was between 39% and 48%, and the interpretation rate of PC2 was between 22% and 25% (Figure 4). We found that the forest gap position significantly altered the ecological stoichiometric characteristics and physicochemical properties of the soil, thereby modifying the interrelationships among these factors.
Furthermore, a Pearson correlation analysis indicated that in the 0–20 cm soil layer of PDS forests, the concentrations of carbon (C) and nitrogen (N) showed a significant negative correlation with electrical conductivity, soil bulk density, moisture content, and sand content (p < 0.05) (Figure 5a). In these forests, a significant positive correlation was also found between the soil’s carbon and nitrogen contents and silt content (p < 0.05) (Figure 5a). In forests dominated by PDS, a significant positive correlation was observed between the soil’s carbon (C) and nitrogen (N) contents and the silt content (p < 0.05). Additionally, the nitrogen content exhibited a positive correlation with the single fractal dimension of the soil (p < 0.05). Conversely, the phosphorus (P) content and the nitrogen-to-phosphorus (N:P) ratio displayed a significant negative correlation with the soil bulk density (p < 0.05). In addition, both the carbon-to-phosphorus (C:P) and nitrogen-to-phosphorus (N:P) ratios were significantly positively correlated with pH value and total porosity (P < 0.05). In contrast, the carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) ratios in forests of RPL were significantly negatively correlated with the pH value (p < 0.05). Moreover, the nitrogen-to-phosphorus (N:P) ratio exhibited a significant positive correlation with the silt content, total porosity, capillary porosity, and moisture content (p < 0.05). The Pearson correlation analysis revealed that the soil’s ecological stoichiometry and physical and chemical properties differed across various soil layers. In the 20–40 cm layer, the phosphorus (P) content exhibited a significant positive correlation with total porosity (p < 0.05). Furthermore, the nitrogen-to-phosphorus (N:P) ratio was significantly positively correlated with both electrical conductivity and moisture content (p < 0.05).

4. Discussion

4.1. Effects of Forest Gaps on the Soil Physical Properties of Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests

The physical characteristics of soil serve as fundamental indicators of its functionality and are intricately linked to plant growth [26]. Soil bulk density plays an important role in water infiltration, root growth, and plant nutrient availability [27,28]. During plant growth, soil porosity directly influences the ease of root elongation and plays a crucial role in regulating water, heat, gas, nutrients, and microbial activity within the soil. The extent of soil porosity serves as an indicator of aeration and water permeability [29]. This study found that forest gaps (both gap centers and edges) significantly decreased the soil bulk density in PDS and RPL forests. Additionally, these gaps enhanced capillary porosity, total soil porosity, and non-capillary porosity, leading to a notable increase in surface soil porosity (0–20 cm). This phenomenon may be attributed to the reduction in stand density caused by the presence of the forest gap, which enhanced the light transmittance, accelerated the growth of understory vegetation, and developed the fibrous roots of most shrubs and grasses, because their growth range is shallow. The strong fibrous roots caused the soil surface layer to become loose and porous; moreover, the increase in light in the forest gap, the rise in surface temperature, the increase in the number of microorganisms and animals in the topsoil, and the improvement in activity promoted the breakdown of leaf litter, the augmentation of soil organic matter content, the acceleration of the transformation rate, and the reduction in soil bulk density, accompanied by an increase in total porosity and non-capillary porosity [6,30]. Moreover, we also found that the soil bulk density and porosity of RPL forests were greater than those of PDS forests. This may be because the litterfall of PDS forests has lasted for a long time and has decomposed slowly, which may ultimately lead to a decrease in the nutrient absorption capacity of trees. Moreover, desiccated pine needles may inhibit the decomposition of various forms of plant litter and their subsequent absorption into the soil.
This study revealed that forest gaps have a significant effect on the soil moisture content, but, due to the effects of different forest types, the moisture content of the soil greatly changes. The moisture content of the soil influences the absorption of soluble nutrients and the activity of microorganisms, both of which are contingent upon plant growth and are intricately linked to it [2]. The forest gap can receive more precipitation while reducing canopy interception and plant transpiration; as a result, the soil in the forest gap receives relatively more precipitation [31]. Meanwhile, the forest gap reduced the soil bulk density of RPL forests while enhancing their total porosity, capillary porosity, and non-capillary porosity, resulting in an improved capacity for soil water storage. Therefore, the soil moisture content of RPL forests was greater at the gap edge and gap center. However, the soil moisture content of PDS forests were greater in non-gap areas. This may be because the litterfall of PDS forests is difficult to decompose, and thicker litter layers accumulate at non-gaps, reducing soil moisture evaporation [32]. However, there was no discernible pattern in the soil moisture levels across various gap positions, which was consistent with the results of Shu et al. [33]. The impact of forest gaps on soil moisture content requires further study with respect to control variables.
Some researchers argue that forest gaps provide ample light and precipitation, which facilitate the decomposition of litterfall, enhance the thickness of the humus layer, and boost the organic matter content in the soil. These processes contribute to the alteration of soil particle composition [34]. Additionally, the diversity and abundance of shrubs and herbs in forest gaps surpass those found in non-gap areas [19,35]. Furthermore, the physical and chemical properties of plant root exudates play a significant role in improving soil particle fineness [36,37]. Specifically, root exudates have the capacity to enhance the cementation between soil particles, thereby facilitating the development of soil aggregates. This process leads to an improved bonding effect among fine particles [38]. Soil particle diameter is a crucial aspect of soil structure, influencing its physical and chemical properties. This characteristic is closely linked to the soil’s ability to retain water and nutrients, as well as its permeability [39]. Forest gaps improved the soil’s particle structure, reduced its gravel and coarse sand contents, and increased its fine particle content, thereby improving the soil structure. Moreover, the specific surface area of fine particles is large, their holding capacity is strong, and their nutrient content is high, which can improve the soil quality and make the soil more conducive to vegetation growth. However, the improvement effect of forest gaps on soil particle diameters was mainly concentrated within 0–20 cm of the soil surface. This phenomenon can be attributed to the shallow root systems of the shrubs and herbs found in forest gaps, with their active root layer primarily situated within the upper 0–20 cm of the soil profile. Analysis of the soil’s mechanical composition indicated that the predominant soil particles in this study area were sandy. The average soil sand contents in the forest gap and non-gap positions were 66% and 66%, respectively. This shows that the soil contains more coarse particles, that PDS forests are concentrated, that the texture of the soil is uniform, and that the soil particle diameters have obvious heterogeneity [40]. The change in the frequency distribution of the soil particle volume content at depths from 0 to 20 cm in the forest gaps was less than that in the non-gaps. In the surface soil of PDS forest gaps, the particle size distribution curve shows a decline. This results in a more uniform volume fraction across different particle classes, while simultaneously increasing the heterogeneity of soil particle distribution [25]. According to the results, we have discovered that forest gaps significantly improved the contents of clay and silt and reduced the sand content in soil. Our findings reveal that gaps significantly impacted the improvement of soil physical properties in both the 0–20 cm and 20–40 cm layers. However, the position of the gap affected only the physical properties of the soil within the 0–20 cm depth range. This may be attributed to the fact that the 0–20 cm soil layer is the primary zone for plant root distribution and serves as the main area where vegetation absorbs nutrients and water. Forest gaps can promote plant growth and litterfall decomposition, thereby affecting soil physical properties. Plant roots and activities had less of an impact on the 20–40 cm soil layer, which was deeper. Consequently, the impact of varying forest gap locations on the soil layers was less pronounced on this layer than on the 0–20 cm layer.

4.2. Effects of Forest Gaps on the Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests

Forest gaps are crucial for the carbon (C), nitrogen (N), and phosphorus (P) cycles within forest ecosystems. In this study, the concentrations of C, N, and P in the soil of PDS forests showed significant increases both within gaps and at various gap positions. This increase can be attributed to the fact that the formation of forest gaps significantly enhanced the light and moisture conditions in forested areas. An increase in temperature promotes the mineralization and availability of soil nutrients and microbial activity, thereby promoting the decomposition of litterfall and increasing soil nutrients [41,42,43]. The concentrations of C and P in the soil of RPL forests also showed a substantial increase. However, the impact of a forest gap on the soil N content in RPL forests was not significant. This may be because RPL is an excellent nitrogen-fixing tree species, and its nitrogen fixation ability can provide a stable nitrogen source for the soil. Therefore, even if small-scale disturbance to a forest gap can change light and precipitation and promote litter decomposition and nutrient release, RPL forests can still maintain relatively stable N and P contents in their soil. Therefore, disturbance from forest gaps did not significantly affect the N and P contents of Robinia pseudoacacia L. soil.
Soil’s ecological stoichiometry is a necessary indicator for determining soil nutrient cycling [44]. C:N and C:P ratios are used to determine the degree of the decomposition of soil organic matter and its potential contribution to soil fertility [45,46]. Lower C:N and C:P ratios suggest that organic matter undergoes more rapid mineralization, resulting in higher levels of available N and P [47]. The C:N and C:P ratios in the soil of PDS forests significantly decreased in forest gaps and different locations within them, indicating that forest gaps accelerated the breakdown process of soil organic matter in PDS forests [48], which is beneficial for enhancing soil nutrient content and augmenting the availability of nutrients. However, forest gaps significantly increased the C:P ratio within the soil of RPL forests but exerted no substantial influence on the C:N ratio. First, this may be a common result of the forest gap effect and the change in understory plant properties. The removal of vegetation biomass in forest gaps reduces primary productivity [49], resulting in a decrease in litterfall and rhizosphere deposition and a decrease in soil nutrient sources. Moreover, due to the shallow root distribution of understory plants, nutrient resources can be preferentially obtained. Therefore, the rapid renewal of understory plants consumes a large amount of soil nutrients and reduces soil nutrient availability. A similar conclusion was reached in a study of forests in Europe [50,51].
P and N, the most basic nutritional components in plants, are the most critical limiting factors in terrestrial ecosystems. The N:P ratio serves as a more sensitive indicator than other stoichiometric ratios, effectively reflecting the decomposability of organic matter and assessing the nutrient availability in soil for plant growth [18,44,52]. It is generally believed that the availability of P limits the growth of vegetation when the soil’s nitrogen-to-phosphorus (N:P) ratio is greater than 20, and that N limits the growth of vegetation when the N:P ratio is less than 10 [53,54,55]. In this study, the soil N:P ratio under the different treatments in the forest gap was less than 10, indicating that the growth of vegetation in the study area was constrained by the availability of N [56]. Therefore, the study area should be improved by increasing the application of nitrogen fertilizer to promote plant growth and increase the species diversity of forest gaps.
A significant positive correlation was found between the carbon and nitrogen contents of the soil and its silt content, whereas a notable negative correlation was observed with its sand content. Soil organic matter is typically closely associated with the surfaces of fine-grained minerals, and the specific surface area of these minerals is directly proportional to the amount of soil organic matter present. This study found that soil porosity and bulk density are the primary factors influencing the P content in the study area. Studies have shown that there is a notable positive correlation between the total phosphorus content and total porosity in forest soil, which is consistent with the conclusions of this study. A significant inverse relationship was observed between the carbon-to-nitrogen (C:N) ratio and the C:P ratio in RPL forests, as well as between the C:N ratio and the pH value. This finding is consistent with the results of the study conducted by Zhu Dehuang on a Phoebe bournei forest [57]. The C:N ratio in the soil there exhibited a significant correlation with the P content, as well as with the C:N and C:P ratios present in the litterfall. In contrast, the C:N and C:P ratios in PDS forests showed a significant positive correlation with pH, although they were influenced by the soil layer. This may be related to the relatively high organic matter decomposition rate and soil enzyme activity of RPL forests [58]. Moreover, through a three-factor analysis of the variance of the forest stand, forest gap, and soil layer (Table A1), it was revealed that the interactions among the forest, forest gap, soil layer, and F × S had little effect on the nutrient composition and ecological stoichiometry ratios within the study region. However, the N:P ratio was significantly influenced by the soil layer. A strong positive correlation was found between the N:P ratio and total porosity (except for in the 20–40 cm layer in PDS forests) and moisture content (excluding the 0–20 cm layer in PDS forests). This conclusion is also in line with the findings of our three-way analysis of variance, which showed that the soil layer, GHS, and F × G × S all have an impact on the N:P ratio. This result is the same as that of Zhou et al. [59]. By optimizing the gap structure and implementing close-to-nature forest transformation and forest tending strategies, the nutrient cycling ability of artificial forests can be significantly enhanced, thus effectively improving the ecosystem productivity of PDS and RPL forests.

5. Conclusions

Forest gaps had a significant effect on the physical properties and ecological stoichiometric characteristics of soil in PDS and RPL forests. The trees of the forest gaps fell with their roots, and then a depression was formed in the forest gaps, leading to the accumulation of more water there than in non-gap soil. Consequently, grass grew well and more humus was formed. Moreover, in the center of the forest gaps, these conditions were better and more soil carbon, nitrogen, and phosphorus were formed; this led to better porosity and other properties. In particular, the improvement effect of the gap center on the physicochemical properties of the soil surface layer (0–20 cm) was more significant than those at the gap edge.
Soil pH, silt content, soil bulk density, and total porosity are key factors influencing the contents of C, N, and P, as well as the ecological stoichiometric ratios, of forest gaps. In conclusion, the soil nutrient status in forest gaps is good, and they have a high nutrient supply potential for promoting the growth of trees. In the field of forest management, more attention should be paid to the role of forest gaps in forest ecosystems to promote the sustainable management of plantations.

Author Contributions

Conceptualization, X.D. and W.S.; methodology, W.S.; software, W.Z. (Wei Zhao) and Y.Q.; validation, Y.L., F.L. and W.Z. (Wentai Zhao); formal analysis, Z.T.; investigation, Y.L.; resources, W.S.; data curation, W.S.; writing—original draft preparation, W.S.; writing—review and editing, X.D.; visualization, Z.Z.; supervision, P.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Central Financial Forestry Science and Technology Promotion Demonstration Project (Grant number LU [2023] TG008); the Agricultural Science and Technology Fund of Shandong Province (No. 2019LY006); Monitoring project for combustible load and moisture content under typical forest areas (No. SDGP3700000002024020008201_001); and an open subject of the science and technology innovation platform of the Department of Natural Resources of Shandong province (2024).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author/s.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Three-factor analysis of variance of stands, positions of forest gap, and soil layers, and their interaction, on soil’s ecological stoichiometry.
Table A1. Three-factor analysis of variance of stands, positions of forest gap, and soil layers, and their interaction, on soil’s ecological stoichiometry.
Factorsdf CNPC:NC:PN:P
Forest1F0.12165.457 **48.241 **216.750 **232.887 **3.840
P0.731<0.001<0.001<0.001<0.0010.060
Forest Gap 2F54.350 **71.941 **31.574 **91.134 **85.589 **0.259
P<0.001<0.001<0.001<0.001<0.0010.773
Soil layer1F278.481 **291.262 **68.698 **2.35487.922 **21.844 **
P<0.001<0.001<0.0010.136<0.001<0.001
F × G2F36.935 **13.531 **9.334 **28.402 **11.768 **2.379
P<0.001<0.001<0.001<0.001<0.0010.111
F × S1F8.689 *2.5941.1844.633 *0.0670.071
P0.0060.1190.2860.0400.7970.792
G × S2F23.718 **18.701 **4.176 *10.388 **26.159 **10.172 **
P<0.001<0.0010.026<0.001<0.001<0.001
F × G × S2F17.694 **25.387 **3.390 *19.851 **10.791 **8.034 *
P<0.001<0.0010.048<0.001<0.0010.002
df: Degrees of freedom. Asterisks indicate significant correlation: * = p < 0.05; ** = p < 0.01.

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Forests 15 01784 g001
Figure 1. Map of study area location.
Figure 1. Map of study area location.
Forests 15 01784 g001
Forests 15 01784 g002
Figure 2. Soil particle distribution frequency curve in different gap positions of PDS and RPL forests. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate significant differences between different gap positions within the same tree species (p < 0.05).
Figure 2. Soil particle distribution frequency curve in different gap positions of PDS and RPL forests. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate significant differences between different gap positions within the same tree species (p < 0.05).
Forests 15 01784 g002
Forests 15 01784 g003
Figure 3. The ecological stoichiometry of soil in different gap positions of PDS and RPL forests. Subfigures (a), (b) and (c) represent C:N, C:P and N:P of PDS and RPL at different forest gap positions and different soil depths, respectively; PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate significant differences between different gap positions within the same tree species (p < 0.05).
Figure 3. The ecological stoichiometry of soil in different gap positions of PDS and RPL forests. Subfigures (a), (b) and (c) represent C:N, C:P and N:P of PDS and RPL at different forest gap positions and different soil depths, respectively; PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate significant differences between different gap positions within the same tree species (p < 0.05).
Forests 15 01784 g003
Forests 15 01784 g004
Figure 4. Principal component analysis (PCA) of soil’s ecological stoichiometry and physical and chemical properties. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.
Figure 4. Principal component analysis (PCA) of soil’s ecological stoichiometry and physical and chemical properties. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.
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Forests 15 01784 g005
Figure 5. Correlation analysis between soil’s ecological stoichiometry and physical and chemical properties. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L. **: highly significant correlation (p <0.01); *: significant correlation (p < 0.05).
Figure 5. Correlation analysis between soil’s ecological stoichiometry and physical and chemical properties. PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L. **: highly significant correlation (p <0.01); *: significant correlation (p < 0.05).
Forests 15 01784 g005
Table 1. Basic status of the study area.
Table 1. Basic status of the study area.
StandSample PlotForest Gap Size
(m2)		Elevation
(m)		Slope
(°)		Slope
Aspect		Longitude–LatitudeAverage Tree Height of Border Trees (m)Average Mean Diameter of Border Trees
(cm)		Average Age of Border Trees (a)Crown DensityStand Density
(tree•hm−2)
PDSG209.42802.021Southwest N 36°20′2″
E 117°6′46″		7.1922.1654//
NG/784.025West N 36°19′5″
E 117°6′4″		///0.6621
RPLG351.86768.122West N 36°20′2″
E 117°5′4″		13.8918.9851//
NG/771.427NorthwestN 36°20′24″
E 117°5′42″		///0.65760
Notes: PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; G: forest gap; NG: non-gap.
Table 2. Basic physical properties of soil in different gap positions of PDS and RPL forests (Mean ± SEM).
Table 2. Basic physical properties of soil in different gap positions of PDS and RPL forests (Mean ± SEM).
StandSoil Layer
(cm)		Gap PositionSoil Bulk Density
(g•cm−3)		Soil Total Porosity
(%)		Capillary Porosity (%)Non-Capillary Porosity (%)Soil Moisture Content (%)Clay (%)Silt (%)Sand (%)
PDS0–20GC1.21 ± 0.00 b50 ± 1 a42 ± 0 a8 ± 1 a16 ± 1 b2 ± 0 ab47 ± 4 a51 ± 4 b
GE1.30 ± 0.05 ab48 ± 1 a43 ± 1 a5 ± 1 b19 ± 3 ab2 ± 0 a45 ± 6 a53 ± 7 b
NG1.38 ± 0.02 a48 ± 1 a43 ± 1 a5 ± 0 b23 ± 0 a1 ± 0 b20 ± 2 b79 ± 2 a
20–40GC1.39 ± 0.03 a47 ± 2 a40 ± 0 a7 ± 2 a13 ± 0 a1 ± 0 a23 ± 6 a76 ± 6 a
GE1.32 ± 0.05 b45 ± 1 ab40 ± 0 a5 ± 1 a14 ± 1 a1 ± 0 a30 ± 3 a69 ± 3 a
NG1.48 ± 0.00 a44 ± 0 b39 ± 0 a4 ± 0 b15 ± 0 b1 ± 0 a31 ± 4 a68 ± 4 a
RPL0–20GC1.07 ± 0.01 b55 ± 2 a49 ± 0 a6 ± 2 a24 ± 0 a2 ± 0 a41 ± 2 a57 ± 3 c
GE1.08 ± 0.02 b51 ± 1 b47 ± 1 b3 ± 1 b22 ± 1 b1 ± 0 b31 ± 1 c68 ± 2 a
NG1.27 ± 0.00 a48 ± 2 b44 ± 0 c4 ± 2 b19 ± 1 c2 ± 0 b23 ± 1 b76 ± 1 b
20–40GC1.25 ± 0.05 a49 ± 1 a45 ± 1 a3 ± 1 a20 ± 0 a2 ± 0 b30 ± 3 a68 ± 3 a
GE1.14 ± 0.05 a47 ± 2 b43 ± 0 b3 ± 1 a18 ± 1 b2 ± 0 a38 ± 4 a59 ± 4 a
NG1.32 ± 0.01 a47 ± 1 b43 ± 0 b3 ± 1 b16 ± 0 c2 ± 0 b31 ± 3 a67 ± 3 a
PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate that there are significant differences between different gap positions within the same tree species (p < 0.05).
Table 3. Basic chemical properties of soil in different gap positions of PDS and RPL forests (Mean ± SEM).
Table 3. Basic chemical properties of soil in different gap positions of PDS and RPL forests (Mean ± SEM).
StandSoil Layer (cm)Gap PositionOrganic Carbon (g·kg−1)Total Nitrogen (g·kg−1)Total Phosphorus
(g·kg−1)		pHElectric Conductivity (μs·cm−1)
PDS0–20GC35.2 ± 1.8 a2.8 ± 0.2 a0.4 ± 0.0 a6.1 ± 0.0 a96.6 ± 3.1 c
GE28.2 ± 1.1 b2.3 ± 0.1 b0.4 ± 0.0 a5.8 ± 0.0 b130.3 ± 5.0 b
NG15.3 ± 0.7 c1.1 ± 0.0 c0.2 ± 0.0 b5.8 ± 0.0 b150.0 ± 4.0 a
20–40GC16.6 ± 0.6 a1.3 ± 0.0 a0.3 ± 0.0 a6.4 ± 0.0 a74.2 ± 3.9 b
GE16.1 ± 1.7 a1.4 ± 0.1 a0.3 ± 0.02 a6.0 ± 0.0 b112.5 ± 5.1 a
NG10.3 ± 0.76 b0.6 ± 0.1 b0.1 ± 0.02 b6.0 ± 0.0 b104.2 ± 5.1 a
RPL0–20GC26.2 ± 0.9 a2.5 ± 0.2 a0.5 ± 0.03 a5.9 ± 0.0 a67.4 ± 2.2 b
GE24.6 ± 1.4 b2.0 ± 0.2 a0.4 ± 0.02 a5.8 ± 0.1 ab65.6 ± 2.2 b
NG21.5 ± 0.2 b2.1 ± 0.1 a0.3 ± 0.01 b5.7 ± 0.0 b75.0 ± 1.4 a
20–40GC16.3 ± 0.6 a1.6 ± 0.1 a0.4 ± 0.01 a6.1 ± 0.1 a55.9 ± 4.4 a
GE13.4 ± 1.3 a1.4 ± 0.1 a0.3 ± 0.04 a5.9 ± 0.1 ab52.2 ± 4.1 a
NG14.5 ± 0.5 a1.4 ± 0.1 a0.2 ± 0.00 b5.8 ± 0.0 b62.9 ± 2.1 a
PDS: Pinus densiflora Sieb. Et Zucc.; RPL: Robinia pseudoacacia L.; GC: gap center; GE: gap edge; NG: non-gap. Different letters indicate significant differences between different gap positions within the same tree species (p < 0.05).
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Dun, X.; Liu, Y.; Lian, F.; Zhao, W.; Su, W.; Zhao, W.; Tian, Z.; Qiao, Y.; Gao, P.; Zhang, Z. The Effects of Forest Gaps on the Physical and Ecological Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests. Forests 2024, 15, 1784. https://doi.org/10.3390/f15101784

AMA Style

Dun X, Liu Y, Lian F, Zhao W, Su W, Zhao W, Tian Z, Qiao Y, Gao P, Zhang Z. The Effects of Forest Gaps on the Physical and Ecological Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests. Forests. 2024; 15(10):1784. https://doi.org/10.3390/f15101784

Chicago/Turabian Style

Dun, Xingjian, Yuchen Liu, Fengjie Lian, Wentai Zhao, Wei Su, Wei Zhao, Zhihao Tian, Yanhui Qiao, Peng Gao, and Zhenxiang Zhang. 2024. "The Effects of Forest Gaps on the Physical and Ecological Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests" Forests 15, no. 10: 1784. https://doi.org/10.3390/f15101784

APA Style

Dun, X., Liu, Y., Lian, F., Zhao, W., Su, W., Zhao, W., Tian, Z., Qiao, Y., Gao, P., & Zhang, Z. (2024). The Effects of Forest Gaps on the Physical and Ecological Stoichiometric Characteristics of Soil in Pinus densiflora Sieb. and Robinia pseudoacacia L. Forests. Forests, 15(10), 1784. https://doi.org/10.3390/f15101784

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