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Article

Understorey Plant Functional Traits of Platycladus orientalis Depends on Crown Closure and Soil Properties in the Loess Plateau, China

by
Gaohui Duan
1,
Lifeng Liu
2,3,
Zhongming Wen
1,4,*,
Yu Tang
5 and
Boheng Wang
6
1
College of Grassland Agriculture, Northwest A&F University, Yangling 712100, China
2
Suide Administration and Supervision Bureau of Soil and Water Conservation of The Yellow River, Yulin 719000, China
3
Key Laboratory of State Forestry Administration for Soil and Water Conservation and Ecological Restoration in the Loess Plateau, Yulin 719000, China
4
Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, Yangling 712100, China
5
Zhejiang East China Forestry Engineering Consulting and Design Limited Company, Hangzhou 310000, China
6
East China Survey and Planning Institute of National Forest and Grassland Administration, Hangzhou 310019, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(6), 1042; https://doi.org/10.3390/f15061042
Submission received: 17 May 2024 / Revised: 9 June 2024 / Accepted: 14 June 2024 / Published: 16 June 2024
(This article belongs to the Special Issue Forest Disturbance and Management)
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Abstract

:
The crown closure of Platycladus orientalis forests has a wide-ranging impact on vegetation and soil, thereby affecting the overall functioning of the ecosystem. There is limited research on the effects of the Platycladus orientalis forest crown closure on changes in community plant functional traits, and their interactions are not yet clear. Therefore, we investigated 50 plots of different types of Platycladus orientalis crown closure, and we measured the functional traits of nine shrub species and 68 herb species in 50 plots under five different densities of Platycladus orientalis forests in the Loess Plateau. The consequence of Pearson’s correlation analysis showed significant positive correlations between LC and LTD, LN and LP, LN and LNP, LN and LV, LN and H, LP and LV, LP and H, and SLA and LV (p < 0.05). LC was significantly negatively correlated with LP, LC with SLA, LC with LV, LN with LTD, LP with LNP, LP with LTD, and LTD with H (p < 0.05). Only the soil phosphorus content (SP) and soil water content (SWC) showed a significant positive correlation with multiple plant functional traits. The crown closure of Platycladus orientalis forests increased significantly, as did the plant functional features. Changes in the Platycladus orientalis forest crown closure significantly increased the LC, LV, LN, LP, and SLA in plant functional traits. An increase in Platycladus orientalis forest crown closure significantly increased the soil organic carbon (SC), soil phosphorus content (SP), soil nitrogen content (SN), soil water content (SWC), field capacity (FC), and soil porosity (PO). Based on a structural equation model, we found that, while changes in the Platycladus orientalis forest crown closure did not directly affect plant functional traits, they could indirectly influence these traits through soil factors, primarily the soil water content (SWC) and soil phosphorus content (SP) (p < 0.05). Additionally, the mechanisms of the Platycladus orientalis forest crown closure’s impact on different functional traits vary. The research results provide scientific elements for the ecological restoration of Platycladus orientalis forests on the Loess Plateau.

1. Introduction

The functional traits of a plant are a set of qualities that plants develop in response to long-term environmental changes, including changes in their physiological and biochemical processes, growth and development, and morphological structures, ultimately forming the best suitability for external environmental changes [1,2]. These traits reflect the species’ adaptation strategies to environmental changes and can predict the direction of plant development under habitat changes [3]. Traits such as the leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen-phosphorus ratio (LNP), leaf volume (LV), plant height (H), and leaf tissue density (LTD) are often related to the structure and functioning of ecosystems.
Since every trait is associated with various aspects of ecological strategies, studying plant communities from a functional perspective requires a series of traits [4]. For example, it is generally recognized that nitrogen (N) and phosphorus (P) are crucial factors influencing plant growth [5]. Similarly, the content and stability of leaf carbon (C) have a significant impact on plant growth and yield [6]. Changes in leaf volume are an important indicator for optimizing plant photosynthesis, and increasing transpiration and nutrient absorption capacity [7]. Scholarly research has shown that plants with larger leaf volumes, under the same light and moisture conditions, may exhibit faster growth rates and higher yields due to their higher photosynthetic efficiency and transpiration capacity [8]. Additionally, because they can synthesize and store more nutrients, these plants can maintain relatively good growth even in nutrient-poor environments [9]; plant height affects the interaction between plants and their surrounding environment. Tall plants can better shield against wind and rain, providing shelter for shorter plants, and their root systems can also absorb water and nutrients from the soil more deeply, contributing to soil stability [10]. In ecosystems, plants of different heights form a rich hierarchical structure, which is beneficial for maintaining biodiversity and ecological balance [11]; the specific leaf area (SLA) is positively correlated with a plant’s light-capturing efficiency and comparative growth rate [12]. Vegetation with a smaller SLA has a comparatively weak light-capturing efficiency, slower growth rates, and longer lifespans [13]; the leaf tissue density shows the resources ratio invested in the plant, with plants having a higher leaf tissue density showing greater resistance to physical damage [14]. The various traits of plants are closely related to each other. The relationship reflects the synergistic or balancing effect of traits on the natural environment, and represents the growth and reproduction mechanisms of vegetation [15].
The mass ratio hypothesis advises that the impact of functional traits on ecosystems is closely connected with vegetation’s dominant position within the environment [4]. As an extension of the theory, to better characterize vegetation community responses to environmental changes, some of the research combined single plant functional traits with the relative abundance of species in the community, calculating the community-weighted mean (CWM) traits representing community-level traits [16]. With further study, CWM traits have been utilized as diagnostic indicators for determining plant responses to environmental filtration. Studying relationships between functional traits at the community scale is more efficient than at the species scale [17]. Community-level traits of plants have also been proven to be connected to environmental changes and ecosystem functions, making them crucial study indices in ecology [18].
The Loess Plateau, as one of the regions with the most severe soil erosion in the world, has long been troubled by soil erosion issues [19]. Long-time vegetation restoration projects, such as converting cropland to forest and the Three-North Shelterbelt Program, have played a positive role in improving the ecological environment of this region [20]. However, these initiatives have introduced some new challenges to the implementation process, such as the fact that a single afforestation measure not only consumes too much water, but also fails to properly control soil and water losses. The study discovered that restoring trees and grassland is the most successful method for ecological restoration on the Loess Plateau [21]. In addition, the crown closure of the forest has an important effect on the understory vegetation. Too much forest crown closure not only wastes groundwater but also causes soil dryness, whereas too little forest crown closure exposes the understory vegetation to sunlight, affecting the vegetation growth and ecosystem stability [22]. Therefore, during the vegetation restoration on the Loess Plateau, it is important to control forest crown closure reasonably [23]. This can effectively alleviate soil erosion, improve the ecological environment, and prevent issues such as the excessive consumption of groundwater and soil desiccation from occurring.
Platycladus orientalis is an evergreen tree, reaching a height of up to 20 m. Its bark is thin, grayish-brown, and longitudinally fissured into strips. The branches extend upwards or obliquely, giving the young trees a conical shape, while the mature trees develop a broad rounded crown. The leaves are scale-like, arranged in a flat plane, and the shoots with scale leaves are slender, extending upright or obliquely. Platycladus orientalis, as the main afforestation tree species in the Loess Plateau region, holds significant ecological and economic value [24]. Its distribution covers a wide range, spanning regions between 34–40° N latitude, 103–114° E longitude, and elevations of 1000–2000 m. Platycladus orientalis plays an important part in maintaining biodiversity, protecting water resources, and preventing soil erosion [25]. Additionally, it has economic significance due to its hard wood, widely used in industries such as construction and furniture manufacturing, and its branches, leaves, and fruits have medicinal value. However, excessive human interference, pests and diseases, and a dry climate have continuously disrupted and threatened the ecosystem functions of Platycladus orientalis forests. Crown closure changes due to natural drought are the main factors limiting Platycladus orientalis forests in the Loess Plateau [24]. Enhancing the various functions of Platycladus orientalis ecosystems in the Loess Plateau is of great practical significance for their sustainable use. Besides standard assessment approaches that utilize species diversity and structure, functional characteristic changes can be used to test ecosystem processes and evaluate future forest restoration [26]. Therefore, studying the impact of the Platycladus orientalis forest crown closure on community traits using trait-based methods is significant for improving ecosystem functions.
However, there is still a knowledge gap regarding the connection between Platycladus orientalis crown closure, soil properties, and functional traits, and the mechanisms through which Platycladus orientalis crown closure influences plant functional traits remain unclear. This study measured eight plant traits reflecting the resource acquisition and allocation strategies to calculate the response of community-level traits to different Platycladus orientalis densities across 50 sample plots. The main objectives of the study were as follows: (1) Which plant functional traits in Platycladus orientalis forests exhibit trade-offs or synergistic effects? (2) What is the correlation between crown closure, soil properties, and plant functional traits? (3) How does Platycladus orientalis crown closure influence plant functional traits? The conclusion will aid our understanding of how Platycladus orientalis community plant functional traits respond to community changes and provide a scientific foundation for the long-term management of Platycladus orientalis forests on the Loess Plateau.

2. Materials and Methods

2.1. Study Area

In order to maintain the uniformity of environmental factors and avoid interference from other variables in the experiment, we chose to set the study area in the Xindian National Soil and Water Conservation Park (110°17′20″ E, 37°31′09″ N), located in Yulin City, Shaanxi Province, China (Figure 1). In 2021, the annual average temperature is 8.3 °C (The lowest temperature is −7.5 °C and the highest is 38.4 °C), with an annual average rainfall of 486 mm, mainly concentrated in August, at an elevation ranging from 690 to 1016 m [19]. The total area of the Soil and Water Conservation Park is 1.44 km2, with Platycladus orientalis forests covering 0.49 km2, accounting for 34.3% of the total area. Platycladus orientalis is the main afforestation tree species in the watershed, along with Ziziphus jujuba Mill., Robinia pseudoacacia L., and others. The shrub layer consists mainly of nine species such as Caragana microphylla Lam., Clematis fruticosa Turcz., and Ziziphus jujuba var. spinosa (Bunge). There are 68 species in the herb layer, including Lespedeza davurica auct. non (Laxm.) Schindl., Artemisia stechmanniana Besser, Artemisia capillaris Thunb., Artemisia lavandulaefolia DC., and Stipa capillata L. According to the WRB system, the soil type is Calcaric Regosols.

2.2. Sample Design

Field surveys were conducted in August 2021. In the watershed, we surveyed 50 plots of Platycladus orientalis forests, with ten plots each for crown closure of 30%, 40%, 50%, 60%, and 70%. Each Platycladus orientalis forest plot has an area of 10 m × 10 m and contains one 5 m × 5 m shrub plot and three 1 m × 1 m herb plots (Figure 1). The survey covered all understory vegetation in the study area, including the 9 shrubs and 68 herbs mentioned above [27] (Figure S1).

2.3. Soil Properties

Soil properties include physical and chemical properties, including soil total nitrogen (SN), soil total phosphorus (SP), soil total carbon (SC), soil moisture content (SWC), soil bulk density (BD), field capacity (FC), particle density (PD), and soil porosity (PO) (Table S1). We used a soil spiral drilling method to collect samples at five points within each plot, sampling from 0 to 30 cm of the surface layer, mixing the soil thoroughly, sieving it through a 2 mm mesh screen, storing it in soil bags for later measurement of soil chemical properties. Additionally, a cylindrical metal sampler (100 mm2) was used to obtain soil samples for measuring physical properties such as BD and SWC. Finally, all soil samples were taken back to the lab for processing and analysis.
Soil nitrogen content (SN) is measured by Kjeldahl method using a FOSS Kjeltec 8400 Analyzer Unit (FOSS, Hillerod, Denmark). The SP was digested by H2SO4-HCIO4 and measured by spectrophotometer, and SC was determined using the dichromate oxidation method; the leaf C, N, and P contents were measured using the same method. Soil samples from the cylindrical metal sampler were analyzed by drying at 108 °C, soaking for 24 h, resting for one hour, and drying again to determine BD, SWC, FC, PD, and PO, among other physical properties [2].

2.4. Plant Functional Traits

The research looked at eight plant functional traits that should be connected with plant resource acquisition and utilization: leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen phosphorus ratio (LNP), leaf volume (LV), plant height (H), and leaf tissue density (LTD) (Table S1). Functional traits of plants were obtained using standardized methods [27]. Using a tape measure, we measured the height of herbaceous plants in the plots. To gather leaf samples, we chose five to ten individuals per species in the plots. From the individuals, we chose ten leaves exposed to sunlight. If the number of individuals was short, we averaged the samples from all individuals to constitute the desired number of leaves. Additionally, we collected approximately 20 g of leaf samples for subsequent chemical analysis.
The LI-300 Area Meter (LI-COR, Lincoln, NE, USA) was used to calculate the ten fresh leaf areas of various vegetation, and the water displacement method was employed to measure leaf volume. All fresh leaves were weighed on the day of collection using an electronic balance with an accuracy of 0.0001 g. Subsequently, the leaves were placed in the oven and kept at 60 °C for 48 h, after which the dry weight of the leaves was recorded. Upon completion of these steps, the samples were ground to a fine powder with a ball mill, and LC, LP, and LN were measured by the same method. SLA was the radio of fresh leaf area to dry weight, while LTD was calculated as the ratio of leaf dry weight to volume. The measurement of N, P, and C element contents followed the same method as soil property measurement, and LNP was calculated as the ratio of LN to LP.

2.5. Statistical Analyses

We used Pearson correlation analysis and principal component analysis (PCA) to look into possible correlations between plant functional features. In addition, Pearson correlation analysis was utilized to determine the relationship between plant functional traits and soil properties. Furthermore, we employed one-way analysis of variance (ANOVA) and LSD test to assess differences in plant functional traits across different degrees of forest canopy. Based on these analyses, ordinary least squares linear regression (OLS) was employed for simple regression analysis to predict the relationship between Platycladus orientalis crown closure and plant functional traits, as well as soil properties. Ultimately, building upon the aforementioned steps, this study established the structural equation models (SEMs) to delineate the causal relationship between Platycladus orientalis crown closure and soil properties on plant functional traits. In this study, the significant effects of Platycladus orientalis crown closure on soil properties and plant functional traits should be considered comprehensively when establishing SEM.
Principal component analysis (PCA), one-way analysis of variance (ANOVA), and ordinary least squares linear regression (OLS) were performed with R 4.1.2. SEM was performed with IBM AMOS 25.0. In general, a qualified SEM should match the criterion below: (1) non-significant χ2 test, namely, p > 0.05; (2) comparative fit index, CFI > 0.95; (3) incremental fix index, IFI > 0.9; and (4) root mean squared error of approximation, RMSEA < 0.05.

3. Results

3.1. Relationships among Plant Functional Traits

From Figure 2a, it can be observed that LC is significantly positively correlated with LTD (r = 0.41, p < 0.01), while LC is negatively correlated with LP (r = −0.28, p < 0.05), SLA (r = −0.33, p < 0.05), and LV (r = −0.39, p < 0.01); LN is significantly positively correlated with LP (r = 0.80, p < 0.001), LNP (r = 0.33, p < 0.05), LV (r = 0.29, p < 0.05), and H (r = 0.28, p < 0.05), while LN is negatively correlated with LTD (r = −0.35, p < 0.01); LP is significantly positively correlated with LV (r = 0.31, p < 0.05) and H (r = 0.35, p < 0.01), while LP is negatively correlated with LNP (r = −0.28, p < 0.05) and LTD (r = −0.29, p < 0.05); LTD is significantly negatively correlated with SLA (r = −0.75, p < 0.001), LV (r = −0.63, p < 0.001), and H (r = −0.28, p < 0.05); and SLA is positively correlated with LV (r = 0.35, p < 0.01). PCA axes 1 (PC1) and 2 (PC2) explain 56.2% of the trait variation and represent most of functional trait variation (Figure 2b). The relationships among functional traits obtained from the PCA are consistent with the Pearson correlation analysis (Figure 2b).

3.2. Relationships between Soil Properties and Plant Functional Traits

From Figure 3, it can be seen that H is significantly positively correlated with SN (r = 0.319, p < 0.05), SC (r = 0.277, p < 0.05), FC (r = 0.326, p < 0.05), and SP (r = 0.31, p < 0.05), and the same pattern applies to LV and SWC (r = 0.323, p < 0.05), while SLA is positively correlated with SN (r = 0.276, p < 0.05), SP (r = 0.279, p < 0.05), and SWC (r = 0.29, p < 0.05). Conversely, LTD is significantly negatively correlated with SN (r = −0.351, p < 0.01), SC (r = −0.295, p < 0.05), and SWC (r = −0.323, p < 0.05), and H is negatively correlated with BD (r = −0.308, p < 0.05). These significant correlations indicate that plant functional traits are strongly affected by soil properties. Based on these results, the SEM for the crown closure, soil properties, and plant functional traits will be established.

3.3. Plant Functional Traits under Different Types of Crown Closure of Platycladus orientalis (L.) Franco

In Figure 4, most functional traits show no significant differences under different densities (p > 0.05). However, we still observed some significant differences in the figure. Significant differences (p < 0.05) were detected in LN, LNP, LV, LC, and SLA between the 30% and 70% densities. We found that the largest median difference occurred in LN, and the median value of 70% crown closure was 1.47 times greater than that of the 30% crown closure. The median values of SLA at 50%, 60%, and 70% crown closure were 98.56, 107.19, and 112.41, respectively, and there was a significant difference between them, respectively (p < 0.05). The median values of LTD at 50% and 70% crown closure were 0.07 and 0.04, and there was a significant decreasing trend. Additionally, significant differences (p < 0.05) were detected in LV (median values = 2.92 and 3.40) and LC (median values = 463.25 and 451.39) between the 30% and 40% crown closure in Figure 4e,f. Finally, comparing community vegetation functional traits between the 60% and 70% crown closure, we found that only SLA significantly increased (p < 0.05), while other plant functional trait indicators showed limited increases or even significant decreasing trends.

3.4. The Effect of Crown Closure on Plant Functional Traits and Soil Properties

The results of the OLS regression of Platycladus orientalis crown closure on plant functional traits and soil properties are shown in Figure 5 and Figure 6, respectively. LC is positively correlated with Platycladus orientalis crown closure (p < 0.01), accounting for 36% of the variation in LC (Figure 5a). LV is also positively correlated with Platycladus orientalis crown closure (p < 0.05), accounting for 20% of the variation in LV. LN is positively correlated with Platycladus orientalis crown closure (p < 0.01), accounting for 45% of the variation in LN. LP is positively correlated with Platycladus orientalis crown closure (p < 0.05), accounting for 14% of the variation in LP. SLA is also positively correlated with Platycladus orientalis crown closure (p < 0.01), accounting for 36% of the variation in SLA. In terms of soil properties, SC (p < 0.01), SP (P < 0.05), SN (p < 0.01), SWC (p < 0.01), FC (p < 0.01), and P (p < 0.05) are positively correlated with Platycladus orientalis crown closure, and BD is negatively correlated with Platycladus orientalis crown closure (p < 0.01). Platycladus orientalis crown closure is responsible for 24% of the variation in SC, 19% in SP, 29% in SN, 31% in SWC, 48% in FC, 16% in P, and 49% in BD (Figure 6). In general, most plant functional traits and soil property indices are improved with the increase in crown closure; SLA exhibited the most noticeable improvement effect among all plant functional trait indices, while SWC had the most significant improvement among all soil property indices.

3.5. The Effect of Crown Closure and Soil Properties on Plant Functional Traits

The model results show that Platycladus orientalis crown closure indirectly affects plant functional traits through its influence on soil properties (Figure 7). SEM accounts for 38.4%, 25.7%, 25.8%, and 24% of the variation in LTD, H, SLA, and LV, respectively. The strongest relationships observed in the model are between Platycladus orientalis crown closure and SWC, SP (p < 0.05). The influence of Platycladus orientalis crown closure on LTD is primarily mediated by SP and SWC, and, additionally, SN and SC also have significant effects on LTD. In Figure 7b, Platycladus orientalis crown closure has a significant impact on SP and BD, while H is significantly influenced by SC, FC, SP, and BD. In Figure 7c, SP and SWC are the most significant factors affecting SLA, and the model results indicate that Platycladus orientalis crown closure’s indirect impact on SLA is also mediated through SP and SWC. Furthermore, in Figure 7d, we can observe that LV is only significantly affected by SWC, and there is no significant indirect effect between Platycladus orientalis crown closure and LV. Finally, according to the test results of the model (χ2 > 0.05, CFI > 0.95, IFI > 0.9, RMSEA < 0.05), we believe that the four SEMs are acceptable.

4. Discussion

In terms of functional features, CWM plant functional traits have gradually become the key attributes of ecosystem functions. This study clarified the relationships between plant functional traits, investigated the impact of forest crown closure on plant functional traits and soil properties, and explored how forest crown closure indirectly affects functional traits through soil properties. The results indicate that there is a basic equivalence between the trade-offs and synergies among various traits of forests on the Loess Plateau. In binary relationships, seven of eight soil properties and five of eight plant functional traits is significantly correlated with forest crown closure. In general, the association with soil properties is stronger than that with plant functional traits. Additionally, the pathways and intensity of forest-crown-closure-induced changes in plant functional traits are inconsistent. In conclusion, soil variations have a significant impact on many functional traits of plants. Therefore, we will discuss each theme separately.

4.1. Trade-Offs and Synergies between Plant Functional Traits

Studying the synergistic effects and trade-offs between functional traits is crucial for elucidating plants’ ecological strategies for coping with environmental stress, linking it to community structure and ecosystem functioning [28]. Plants interact with their environment over time, adapting to a variety of characteristic combinations in order to survive and reproduce in certain conditions [29]. Combined with the leaf economic spectrum of the traits, the strategy of plants’ functional traits shifts from resource acquisition to conservation in response to environmental stress, and generalizes this mechanism to the community level [30]. The co-ordination of plant functional traits is often the result of biophysical trade-offs under comprehensive ecological strategies [31]. LC is significantly positively correlated with LTD; studies indicate that an increase in LC content is often accompanied by thicker leaf cell walls, reduced cell spaces, and a denser arrangement [32]. LN is positively correlated with LP, LNP, LV, and H, and LP is also significantly positively correlated with LV and H, reflecting the important role of nitrogen in ecosystem processes; research also demonstrates that an increase in nitrogen promotes phosphorus absorption and utilization, thereby affecting leaf volume and the growth and development of the entire plant [33]. The specific leaf area (SLA) is significantly positively correlated with the leaf volume (LV); an increase in leaf volume is usually accompanied by an increase in specific leaf area [34]. Research indicates that plants optimize their photosynthetic efficiency by adjusting the leaf volume and specific leaf area [35]. Additionally, LTD is significantly negatively correlated with LN, LP, and H; this may be due to the barren and dry soil of the Loess Plateau, leading to overly dense leaf tissues that restrict the penetration and diffusion of nutrients, ultimately affecting plant growth [36]. SLA is significantly negatively correlated with LC and LTD, similar to the significant negative correlation between LTD and H. For instance, research by Duan et al. (2023) suggests that, in harsh survival environments, plants allocate more synthesized substances into leaf structures, increasing the distance for internal water diffusion to enhance drought resistance [19]. However, there is no correlation between H and LC, SLA, or LV, it may be that species with the independently evolved organs dominate the population.

4.2. Effect of Forest Crown Closure on Soil Properties

Controlling forest crown closure is a common forest management practice that significantly impacts many soil factors, especially in arid and barren loess plateaus [37]. In this study, forest crown closure has a significant positive impact on SC, SP, SN, SWC, FC, and PO, and a significant negative impact on BD. Firstly, among these binary relationships, forest crown closure has the greatest impact on BD. Soil bulk density reflects the structural condition of soil; a lower bulk density indicates loose and porous soil, which is favorable for the growth and development of vegetation roots [38]. In the loess plateau ecosystem, the soil bulk density is the primary factor influencing the ecosystem [39]. A good soil bulk density promotes the growth of vegetation roots, stabilizes soil, absorbs nutrients and water from deep soil layers, and benefits vegetation growth and development [40]. Secondly, improving the soil bulk density is also beneficial for soil water infiltration and retention, providing an adequate water supply for vegetation [41]. Additionally, a good soil structure helps in nutrient storage and release, and enhances soil fertility, thereby benefiting vegetation growth [42]. This may also be the reason for the improvement in FC and PO. Secondly, forest crown closure significantly impacts SWC. In ecosystems, SWC restricts plant transpiration and photosynthesis, impacting terrestrial surface water, energy, and biogeochemical cycles, which are critical factors affecting forest sustainability [43]. The observed changes in SWC in our study have clear ecological significance. An increasing forest crown closure thickens the canopy, facilitating water collection and increasing runoff. Many soil properties vary in a similar intuitive manner. The effects of forest crown closure changes mainly manifest in the influence of plant shade on soil surface temperature, the impact of the root on the soil water acquisition ability, the effects of vegetation patches and root channels on water infiltration, and the contribution of plants to evapotranspiration [44]. Lastly, forest crown closure significantly impacts SC; denser forests typically indicate more vegetation coverage, which increases the organic matter input into the soil [45]. Organic matter such as plant residues, fallen leaves, root exudates, etc., gradually decomposes and becomes part of the soil’s organic carbon. Additionally, denser forests host more biological activities, like soil micro-organisms and decomposers, which participate in the decomposition and transformation of organic matter, influencing the accumulation and stability of soil organic carbon [46]. In denser forests, trees’ root systems are more developed; roots secrete organic substances and promote the growth of soil micro-organisms, all of which contribute to the increase in soil organic carbon [47]. Furthermore, the role of tree roots also helps improve the soil structure and increase the storage space for soil organic carbon. Dense forest coverage can reduce sunlight exposure and wind erosion on the soil surface, benefiting the stability of soil organic carbon [48]. Additionally, the water retention capacity of forests can promote the accumulation of soil organic carbon since water retention helps slow down the decomposition rate of organic matter [49]. This is also a key factor leading to the increase in SN and SP.

4.3. Forest Crown Closure Indirectly Affects Plant Functional Traits through Soil Properties

At the community level, Platycladus orientalis forests on the Loess Plateau respond differently to soil factors; thus, the impact of the crown closure on different functional traits varies [50]. Overall, the PLS-SEM results indicate that Platycladus orientalis crown closure mainly affects functional traits through influencing soil nutrient cycling and physical properties [51]. The indirect effects of crown closure on LTD indicate that Platycladus orientalis crown closure can enhance the overall drought resistance, shade tolerance, and resistance to physical damage of Platycladus orientalis by increasing SWC and SP. The research by scholars indicates that stands with a higher canopy closure typically reduce direct sunlight exposure and wind speed on the soil surface, thereby lowering the rate of soil moisture evaporation [52]. Consequently, the soil water content in these areas is relatively higher, which is beneficial for plant root water absorption and maintaining internal plant water balance. An adequate water supply can promote plant growth and development, increasing the leaf tissue density in plants [53]. Areas with a higher stand canopy closure typically have better soil protection capabilities, reducing the soil erosion and phosphorus loss [54]. Additionally, a high canopy closure promotes the accumulation of organic matter in the soil, and phosphorus released from the decomposition of organic matter also increases the phosphorus content in the soil. An adequate soil phosphorus content can promote plant growth and nutrient absorption, thus increasing the density and quality of plant leaf tissues [55]. The indirect effects of crown closure on H indicate that Platycladus orientalis crown closure can increase H by enhancing SP and BD. Scholars studying the relationship between the forest stand structure and soil phosphorus content have found that an adequate phosphorus supply in the soil can promote plant growth and development, contributing to an increased leaf area [56]; scholars focusing on the effects of the stand structure on the understory vegetation and microclimate have discovered that a high canopy closure in forests reduces direct sunlight exposure and water evaporation from the soil surface, helping maintain soil moisture [52]. This may have a certain impact on plant growth, development, and leaf expansion. The indirect effects of crown closure on LV indicate that a higher Platycladus orientalis crown closure helps reduce soil water evaporation, provides sufficient water for vegetation, and transports it to the leaves, thereby promoting the leaf enlargement and volume increase in plants [57]. Lastly, studies have shown that Platycladus orientalis crown closure from 30% to 60% significantly improves vegetation functional traits, but, when the Platycladus orientalis crown closure reaches 60% to 70%, the enhancement in community plant functional traits is limited, possibly due to Platycladus orientalis consuming soil moisture reaching a critical point, constraining the growth of the understory vegetation.

5. Conclusions

The research results indicate that there are synergistic relationships between plant functional traits such as LC and LTD, LN and LP, LN and LNP, LN and LV, LN and H, LP and LV, LP and H, and SLA and LV in Platycladus orientalis forests on the Loess Plateau. Trade-off relationships are determined between LC and LP, LC and SLA, LC and LV, LN and LTD, LP and LNP, LP and LTD, and LTD and H. Platycladus orientalis crown closure affects LC, LV, LN, LP, and SLA in terms of plant functional traits. Soil factors including SC, SP, SN, SWC, BD, FC, and PO are all influenced by Platycladus orientalis, and the impact of Platycladus orientalis crown closure on soil factors is greater than its impact on functional traits, indicating the sensitivity of soil properties to changes in Platycladus orientalis crown closure. Therefore, we believe that Platycladus orientalis crown closure mainly affects functional traits by directly influencing the soil phosphorus content and physical properties, but the pathways of influence on different functional traits are different, indicating the complicated link between Platycladus orientalis crown closure, soil properties, and plant functional traits. Future research should employ model methods to study the impact of soil properties on plant functional traits. Considering how these traits respond to various environmental conditions, including more functional traits into ecological models would aid in understanding forest sustainable management and the practical application of study findings. Overall, the impact of Platycladus orientalis crown closure on plant functional traits linked to ecosystem function reflects that increasing the Platycladus orientalis crown closure is a reasonable forest management approach. Based on functional traits and current conditions in the study area, a Platycladus orientalis crown closure of 60% provides the best ecological benefits.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f15061042/s1, Figure S1: Flow chart of the research; Table S1: Details of variables used in the research.

Author Contributions

Conceptualization, G.D.; methodology, G.D.; software, G.D.; validation, L.L.; formal analysis, G.D.; investigation, Y.T. and B.W.; resources, Z.W.; data curation, L.L.; writing—original draft preparation, G.D.; writing—review and editing, G.D.; visualization, G.D.; supervision, Z.W.; project administration, Z.W.; funding acquisition, Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (No. 41977077).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors would like to thank the Council of China National Natural Science and Suide Administration and Supervision Bureau of Soil and Water Conservation of The Yellow River for their support in the project.

Conflicts of Interest

The authors declare that they have no known financial interests or personal relationships that could have influenced the word reported in this paper.

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Figure 1. Figure (a) shows the Xin Dian Gou watershed on the Loess Plateau; and Figure (b) illustrates the locations of Platycladus orientalis forests, shrubs, and grassland plots, along with the soil-sampling strategy.
Figure 1. Figure (a) shows the Xin Dian Gou watershed on the Loess Plateau; and Figure (b) illustrates the locations of Platycladus orientalis forests, shrubs, and grassland plots, along with the soil-sampling strategy.
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Figure 2. Relationships among plant functional traits in the Loess Plateau Platycladus orientalis forest, where (a) represents Pearson correlation analysis of eight plant functional traits. Symbols ***, **, and * indicate statistical significance at p < 0.001, p < 0.01, and p < 0.05 levels, respectively. (b) represents principal component analysis of the eight plant functional traits. Solid line arrows represent the direction and weight of the vectors corresponding to functional characteristics. Abbreviation: leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen phosphorus ratio (LNP), leaf volume (LV), plant height (H), and leaf tissue density (LTD).
Figure 2. Relationships among plant functional traits in the Loess Plateau Platycladus orientalis forest, where (a) represents Pearson correlation analysis of eight plant functional traits. Symbols ***, **, and * indicate statistical significance at p < 0.001, p < 0.01, and p < 0.05 levels, respectively. (b) represents principal component analysis of the eight plant functional traits. Solid line arrows represent the direction and weight of the vectors corresponding to functional characteristics. Abbreviation: leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen phosphorus ratio (LNP), leaf volume (LV), plant height (H), and leaf tissue density (LTD).
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Figure 3. Soil properties and Pearson correlation coefficients with plant functional traits. Symbol ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level. Abbreviation: leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen phosphorus ratio (LNP), leaf volume (LV), plant height (H), leaf tissue density (LTD), soil total nitrogen (SN), soil total phosphorus (SP), soil total carbon (SC), soil moisture content (SWC), soil bulk density (BD), field capacity (FC), particle density (PD), and soil porosity (PO).
Figure 3. Soil properties and Pearson correlation coefficients with plant functional traits. Symbol ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level. Abbreviation: leaf nitrogen concentration (LN), leaf phosphorus concentration (LP), leaf carbon content (LC), specific leaf area (SLA), leaf nitrogen phosphorus ratio (LNP), leaf volume (LV), plant height (H), leaf tissue density (LTD), soil total nitrogen (SN), soil total phosphorus (SP), soil total carbon (SC), soil moisture content (SWC), soil bulk density (BD), field capacity (FC), particle density (PD), and soil porosity (PO).
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Figure 4. The study investigated plant functional traits under different Platycladus orientalis: (a) leaf nitrogen concentration (LN), (b) leaf phosphorus concentration (LP), (c) leaf nitrogen phosphorus ratio (LNP), (d) leaf tissue density (LTD), (e) leaf volume (LV), (f) plant height (H), (g) leaf carbon content (LC), and (h) specific leaf area (SLA). Significance at the p < 0.05 level is indicated by different lowercase letters. Symbol ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level.
Figure 4. The study investigated plant functional traits under different Platycladus orientalis: (a) leaf nitrogen concentration (LN), (b) leaf phosphorus concentration (LP), (c) leaf nitrogen phosphorus ratio (LNP), (d) leaf tissue density (LTD), (e) leaf volume (LV), (f) plant height (H), (g) leaf carbon content (LC), and (h) specific leaf area (SLA). Significance at the p < 0.05 level is indicated by different lowercase letters. Symbol ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level.
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Figure 5. The link between the forest crown closure and plant functional traits was fitted with OLS. The shaded area represents the 95% confidence interval. (a) Leaf carbon content (LC), (b) leaf nitrogen phosphorus ratio (LNP), (c) leaf tissue density (LTD), (d) leaf volume (LV), (e) plant height (H), (f) leaf nitrogen concentration (LN), (g) leaf phosphorus concentration (LP), and (h) specific leaf area (SLA).
Figure 5. The link between the forest crown closure and plant functional traits was fitted with OLS. The shaded area represents the 95% confidence interval. (a) Leaf carbon content (LC), (b) leaf nitrogen phosphorus ratio (LNP), (c) leaf tissue density (LTD), (d) leaf volume (LV), (e) plant height (H), (f) leaf nitrogen concentration (LN), (g) leaf phosphorus concentration (LP), and (h) specific leaf area (SLA).
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Figure 6. The link between Platycladus orientalis crown closure and soil properties was fitted with OLS. The shaded area represents the 95% confidence interval. (a) soil total carbon (SC), (b) soil total phosphorus (SP), (c) soil total nitrogen (SN), (d) soil moisture content (SWC), (e) soil bulk density (BD), (f) field capacity (FC), (g) particle density (PD), and (h) soil porosity (PO).
Figure 6. The link between Platycladus orientalis crown closure and soil properties was fitted with OLS. The shaded area represents the 95% confidence interval. (a) soil total carbon (SC), (b) soil total phosphorus (SP), (c) soil total nitrogen (SN), (d) soil moisture content (SWC), (e) soil bulk density (BD), (f) field capacity (FC), (g) particle density (PD), and (h) soil porosity (PO).
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Figure 7. The direct and indirect effects of Platycladus orientalis crown closure and soil properties on plant functional traits were analyzed using a structural equation model. (a) Represents the impact path of forest cover on LTD, (b) represents the impact path of forest cover on H, (c) represents the impact path of forest cover on SLA, and (d) represents the impact path of forest cover on LV.Abbreviation: leaf tissue density (LTD), plant height (H), specific leaf area (SLA), leaf volume (LV), soil total nitrogen (SN), soil total phosphorus (SP), soil total carbon (SC), soil moisture content (SWC), soil bulk density (BD), and field capacity (FC). Symbol *** indicates statistical significance at the p < 0.001 level, ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level.
Figure 7. The direct and indirect effects of Platycladus orientalis crown closure and soil properties on plant functional traits were analyzed using a structural equation model. (a) Represents the impact path of forest cover on LTD, (b) represents the impact path of forest cover on H, (c) represents the impact path of forest cover on SLA, and (d) represents the impact path of forest cover on LV.Abbreviation: leaf tissue density (LTD), plant height (H), specific leaf area (SLA), leaf volume (LV), soil total nitrogen (SN), soil total phosphorus (SP), soil total carbon (SC), soil moisture content (SWC), soil bulk density (BD), and field capacity (FC). Symbol *** indicates statistical significance at the p < 0.001 level, ** indicates statistical significance at the p < 0.01 level, and * indicates statistical significance at the p < 0.05 level.
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Duan, G.; Liu, L.; Wen, Z.; Tang, Y.; Wang, B. Understorey Plant Functional Traits of Platycladus orientalis Depends on Crown Closure and Soil Properties in the Loess Plateau, China. Forests 2024, 15, 1042. https://doi.org/10.3390/f15061042

AMA Style

Duan G, Liu L, Wen Z, Tang Y, Wang B. Understorey Plant Functional Traits of Platycladus orientalis Depends on Crown Closure and Soil Properties in the Loess Plateau, China. Forests. 2024; 15(6):1042. https://doi.org/10.3390/f15061042

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Duan, Gaohui, Lifeng Liu, Zhongming Wen, Yu Tang, and Boheng Wang. 2024. "Understorey Plant Functional Traits of Platycladus orientalis Depends on Crown Closure and Soil Properties in the Loess Plateau, China" Forests 15, no. 6: 1042. https://doi.org/10.3390/f15061042

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