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Article

Effect of Caragana microphylla Lam. on Desertified Grassland Restoration

by
Tiantian Zhu
and
Qinghe Li
*
Chinese Academy of Forestry, Beijing 100091, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(10), 1801; https://doi.org/10.3390/f15101801
Submission received: 12 September 2024 / Revised: 11 October 2024 / Accepted: 11 October 2024 / Published: 14 October 2024
(This article belongs to the Section Forest Soil)
Browse Figures
Versions Notes

Abstract

:
Background: The restoration of the degraded sandy grasslands in Hulun Buir is crucial for maintaining the local ecological balance and sustainable development. Caragana microphylla Lam., a shrub species widely employed in the restoration of sandy vegetation. It is essential to understand its impact on the understory vegetation and soil properties during this process. Methods: This study employed ANOVA, Pearson correlation, and redundancy analysis to systematically analyze the impact of C. microphylla on the three critical stages of desertified grassland vegetation recovery: semi-fixed dunes, fixed dunes, and sandy grasslands. It provided strategies for the restoration of desertified grassland vegetation and offered additional theoretical evidence for the role of vegetation in promoting the recovery of sandy lands. Results: (1) As the degree of vegetation recovery in desertified grasslands increases, the species richness of understory vegetation, Shannon–Wiener index, community height, and biomass also increase. Both the community height and biomass within shrublands are higher than outside, with species richness within the shrublands being higher than outside during the semi-fixed and fixed-sand land stages. (2) In both the 0~10 cm and 10~20 cm soil layers, soil water content showed an increasing trend, peaking in the sandy grassland stage (1.2%), and was higher within the shrublands than outside. The soil water content at 10~20 cm was higher than in the 0~10 cm layer. In both layers, clay and silt content gradually increased with the degree of vegetation recovery in the sandy land, and higher within the shrublands than outside, while the opposite was true for sand content. (3) In both soil layers, soil organic carbon gradually increased with the degree of vegetation recovery, peaking in the sandy grassland stage (4.12 g·kg−1), and was higher within the shrublands than outside. Total nitrogen increased from the semi-fixed-sand land stage to the fixed-sand land stage, with higher levels within the shrublands than outside at all stages. Soil pH within the shrublands decreased as the degree of vegetation recovery increased. There was no significant change in the total phosphorus content. (4) In both soil layers, soil physicochemical characteristics accounted for 59.6% and 46.9% of the vegetation changes within and outside the shrublands, respectively, with the main influencing factors being the soil particle size, total nitrogen, soil water content, and soil organic carbon. Conclusions: In the process of sandy grassland restoration, C. microphylla facilitates the growth and development of vegetation by enhancing the underlying soil physicochemical properties, specifically regarding the soil particle size distribution, soil water content, soil organic carbon, and total nitrogen.

1. Introduction

Grassland desertification is a major ecological and environmental issue faced by the world today [1]. It poses a significant threat to ecological functions and people’s living standards and severely disrupts the balance of the natural environment. The process of grassland desertification not only causes fundamental changes in the structure of vegetation, such as in the transition from perennial to annual plants, but also leads to the severe depletion of soil resources, including the loss of nutrients, changes in the soil particle composition, and a decline in water retention capacity, all of which exacerbate the expansion of bare ground [2]. For a long time, many countries around the world have suffered from the problem of grassland desertification. Studies have shown that over the past 15 years, the degradation of grasslands in Central Asia has intensified, and desertification has been expanding northward, with the southern and northern regions of Kazakhstan being the most sensitive, and it is expected that this phenomenon will worsen in the future [3]. Since the 1980s, China has faced the issue of desertification caused by human destruction and climate change, with the grasslands showing a particularly notable trend of desertification, mainly in Xinjiang, Inner Mongolia, Qinghai, and Gansu [4]. To effectively curb the spread of desertification, the Chinese government has implemented a series of proactive measures, including the “Returning Pasture into Rangelan” policy and the “ Three-North Shelterbelt Forest Program” project, which have significantly slowed down the expansion of sandy lands [5].
These projects pivot around the planting of shrubs and subshrubs with exceptional wind-erosion resistance and high stress tolerance, aiming to establish robust plant communities that effectively halt the further encroachment of desertification and gradually restore the health and stability of ecosystems [6].These shrubs, with their robust root systems, effectively curtail sand movement, mitigate wind erosion, and minimize water evaporation from the soil surface, thereby enhancing the reciprocal relationship between plants and soil that improves soil conditions. Moreover, their canopies moderate the microclimate beneath them, intercept seeds, and subsequently influence the germination and growth of understory vegetation, thereby impacting community composition and function [7]. The facilitation of plants plays a crucial role in global sand dune restoration projects, such as in tropical coastal dunes, where existing vegetation is used to nurture seedlings, mitigating the negative impacts of unstable soils, harsh microclimates, and limited resource availability, thereby increasing the success rate of vegetation restoration [8]. On the northern Brittany coast of Brittany in France, the dune restoration program implemented since 1988 has made significant progress by stabilizing dunes with the planting of Ammophila arenaria [9]. Consequently, elucidating the effects of shrubs on understory vegetation and soil not only aids in understanding the mechanisms of sandy grassland restoration but also holds significant importance for maintaining regional ecosystem stability and enhancing ecological functions.
The interaction between shrubs and understory vegetation, which can range from competitive to neutral to facilitative under varying stress conditions, has garnered considerable attention from scholars in terms of how it influences community composition, diversity, and development trajectory, with investigations conducted through modeling and controlled experiments [10,11,12]. The Stress Gradient Hypothesis (SGH) posits that facilitation among plants in shrub communities strengthens as biotic and abiotic stress increase [13]. However, some researchers have proposed a hump-shaped model, suggesting that facilitation is dependent on the position of the community along an environmental gradient. It enhances the niche of low-stress-tolerant species under moderate to high stress, but this facilitative effect wanes when environmental conditions become severely adverse [14]. Despite decades of field experiments and debates among ecologists, a consensus on the universality of the SGH has yet to be reached. Hence, there is a pressing need to evaluate the potential of shrubs to facilitate ecological restoration in environmentally stressed and arid ecosystems.
Hulunbuir, nestled at the junction of arid and semi-arid regions in northern China, has long been renowned for its pristine and unspoiled natural beauty. Regrettably, over the course of the last century, a confluence of environmental degradation, unchecked development, concentrated livestock grazing, frequent excavations, and a surge in human activities collectively contributed to the deterioration of this vast prairie, birthing the environmental challenge known as the Hulunbuir Sandy Land [15]. Since the 1990s, the Chinese government has embarked on a large-scale ecological conservation and restoration initiative, strategically centering on afforestation, complemented by aerial seeding techniques and stringent grazing prohibitions, in a steadfast effort to combat desertification [15]. These relentless endeavors have significantly improved the condition of sandy lands, successfully transitioning them from having highly mobile to semi-fixed and ultimately stable fixed sands. Despite the remarkable achievements of early rehabilitation efforts, recent climatic changes and persistent anthropogenic disturbances continue to pose substantial challenges to vegetation regeneration, jeopardizing the stability and longevity of restoration work. This scenario renders Hulunbuir a precious natural observatory for studying the interactions between dominant species and understory vegetation during the recovery process. Such research not only enhances our understanding of the mechanisms by which facilitative factors operate in desert ecosystem restoration but also holds immeasurable practical significance and strategic value for devising efficient management strategies, guiding the targeted remediation of degraded grasslands, and advancing the application of vegetation restoration technologies.
At the study site, C. microphylla, is a spiny leguminous shrub, has been widely planted locally to fix sandy land. Previous studies have shown that C. microphylla can ameliorate soil fertility and raise moisture conditions, providing a suitable microenvironment for understory plants [16,17]. In addition, C. microphylla belongs to the Fabaceae family, to helping ecological restoration in degraded ecosystems [18,19,20,21] not only because of their canopy role but also because of their additional input of nitrogen, which is an essential limiting nutrient in terrestrial ecosystems. And the ability of the Fabaceae to transfer some of the nitrogen they have immobilized to the non-Fabaceae plants they are associated with facilitates the growth of coexisting species [22]. However, the current research on how C. microphylla affects understory vegetation and soil characteristics is relatively limited. Given its key role in the restoration of sandy vegetation, there is an urgent need to delve deeper into the mechanisms by which it improves the characteristics of understory vegetation and soil.
This paper focuses on C. microphylla artificially introduced in the northern sand belt of the Hulunbuir Sandy Land. By selecting sites with similar site conditions representing three stages of sandy grassland vegetation restoration (semi-fixed dunes, fixed dunes, and sandy grasslands), we conducted comparative research on the herbaceous plant communities and soil physicochemical properties both within and outside C. microphylla. The objective was to reveal the impact of C. microphylla on the understory vegetation across different stages of sand-land vegetation restoration. Ultimately, this research aims to provide theoretical support for vegetation restoration and community maintenance mechanisms in this region.

2. Materials and Methods

2.1. Study Site

The study was conducted at the heart of the Hulunbuir Sandy Land, specifically in the Gongwan Forest Farm (49°13′ N, 118°27′ E), located approximately 617 m above sea level, under the jurisdiction of Chenbarhu Banner, Hulunbuir City. It serves as a prototypical example of grassland degradation and desertification within the Hulunbuir Sandy Region. This region experiences a typical temperate continental climate, characterized by an annual average temperature of −0.5 °C, a mean frost-free period of roughly 109 days per year, an annual precipitation of around 351.3 m predominantly concentrated between June and August, an annual evaporation of 1385.8 m, and an average annual sunshine duration of 1414 h [23]. According to the WRB classification, the zonal soil of the study area is kastanozem, but in the desertified areas, it has evolved into arenosols and is concentrated in the desertified grassland areas [24].
To curb the exacerbation and spread of desertification, since 2005, the area has consistently undergone aerial seeding operations. Additionally, from 2005 to 2015, C. microphylla were systematically planted to stabilize the sandy lands. These measures effectively promoted the solidification of mobile sands and accelerated vegetative succession. Presently, the region’s sandy lands have experienced various degrees of restoration, resulting in a mosaic vegetation pattern consisting of semi-fixed dunes (with vegetation coverage of 15% to 40%), fixed dunes (coverage of 40% to 70%), and sandy grasslands (coverage exceeding 70%) [25]. Semi-fixed dunes feature sparse shrubs, substantial ground exposure, absence of biological soil crusts, and herbaceous layer dominated by annual and perennial. In the fixed dune stage, sporadic patches of drifting sand can be observed, with psammophytes acting as companion species. Finally, in the sandy grassland phase, there is a high understory vegetation cover beneath the shrubs, virtually eliminating any signs of desertification [23].
The original zonal vegetation of the study area belongs to the typical steppe type; however, in the desertified regions, most of the vegetation has already degraded into desert vegetation [24]. The main vegetation in this area now consists of sand dune shrubs and herbaceous understory plants including C. microphylla Lam., Corethrodendron fruticosum (Pall.) B. H. Choi & H. O., and Artemisia halodendron Turcz. ex Besser. Among the prevalent herbaceous species are Corispermum maorocarpum Bunge., Artemisia frigida Willd., Lespedeza davurica auct. non (Laxm.) Schindl.: V. N. Vassil., Oxytropis hailarensis Kitag., and Elymus dahuricus Turcz.

2.2. Experimental Design and Data Collection

At the end of the 2022 growing season (late August), based on comprehensive field surveys and previous researchers’ findings [25], sites covered with C. microphylla were selected. According to the degrees of vegetation recovery in sandy grasslands, three stages of sandy grassland restoration within the study area were chosen as the subjects of research: Semi-fixed Sandy (SF), Fixed Sandy (FS), and Sandy Grassland (SG) [15], with sampling subsequently completed.
For each type of sandy grassland vegetation recovery, one large plot of 50 m by 50 m was evenly established. Within each plot, 15 Caragana shrubs of similar size were randomly selected (Table 1 and Figure 1), ensuring a canopy distance of over 3 m between each shrub. A 1 m × 1m square frame was placed within the crown area of each shrub and an identical frame was positioned outside the canopy (at least 1 m away from the shrub edge) to sample herbaceous plants. For each sand fixation stage, 30 such quadrats were set up, totaling 90 quadrats across all stages.
Within these quadrats, plant species, their counts, and heights were recorded, and aboveground biomass was collected for later laboratory processing, involving initial wilting at 105 °C followed by drying at 72 °C for 72 h until reaching constant weight. Meanwhile, soil samples were gathered from two layers (0~10 cm and 10~20 cm) next to both the shrubs and the herbaceous plots within each site after removing the surface litter. The soil samples were mixed, quartered to discard excess, and collected in aluminum boxes for moisture content measurement. Separate samples from each layer were taken back to the laboratory for analysis of physical and chemical soil properties.
Soil water content (SWC) was determined drying fresh soil samples at 105 �C until they reached a constant mass. Soil pH was measured within a 1:2.5 water–soil suspension. SOC was determined using potassium dichromate oxidation [26]. Soil total nitrogen (TN) content was quantified by elemental analyzer (Vario EL, Heraeus, Hanau, Germany). Total phosphorus (TP) content was measured using the molybdenum blue method [27]. Soil particle size distribution was analyzed using the MS2000 Laser Particle Size Analyzer (Malvern Method, Malvern Panalytical, Malvern, UK).

2.3. Data Analysis

In each herbaceous quadrat, species richness was calculated total number of species. The community average height and species Shannon diversity index (H) of the communities were calculated according to Stirling and Wilsey (2001) [28].
H = ∑ H i n
H 1 = - ∑ 1 S P i ln P i
Here, S is the number of species in the quadrat and Pi is the proportion of the number of plants of the i plant in the total number of plants.
Data analysis and graphical representations were conducted using R software (R Core Team, 2014) and Origin (2018). Two-factor Analysis of Variance (ANOVA) and three-factor ANOVA were employed to examine both significant differences (α = 0.05) in vegetation characteristics (species richness, Shannon–Wiener index, community height, and biomass), both inside and outside shrubs across different sand fixation stages, as well as variations in soil physicochemical properties (soil water content, particle size, organic carbon, total nitrogen, total phosphorus, and pH) across different soil layers. Subsequently, Least Significant Difference (LSD) tests (α = 0.05) were carried out for multiple comparisons while Pearson correlation analysis was used to reveal correlations between vegetation traits and soil physicochemical properties. Redundancy Analysis (RDA) was performed using the “vegan” packages in R to quantify the rates of contribution of soil physicochemical factors to aboveground vegetation and to identify key influential factors.

3. Results

3.1. Impacts of C. microphylla on Understory Vegetation Community Characteristics at Different Stage of Vegetation Restoration in Desertified Grassland

The results of the two-way ANOVA indicated that different vegetation restoration stages in desertified grasslands significantly affected the characteristics of the understory plant community (richness, Shannon–Wiener index, community height, and biomass), and the effects of shrubs inside and outside on the richness, community height, and biomass of the understory vegetation were significant, while the interaction effect was only significant for richness and biomass (Supplementary Table S1).
The Shannon–Wiener index and community height of the vegetation within the shrubs increased with the degree of vegetation restoration, showing a pattern of SG > FS > SF. As the restoration process transitioned from SF to FS, species richness and biomass significantly increased by 90.2% and 119.2%, respectively; however, upon further progression to SG, both species richness and biomass showed a significant decline (Supplementary Table S1 and Figure 2).
Significant differences in richness, community height, and biomass were observed inside and outside the shrubs during the three stages of vegetation restoration (p < 0.05) whereas the differences in the Shannon–Wiener index were not significant. At the SF and FS stages, the species richness of the vegetation inside the shrubs was higher than that outside, with increases of 30.1% and 48.2%, respectively. In contrast, during the sandy grassland stage, the richness outside the shrubs (6.73) was significantly higher than under the shrubs (6.07). Across the three stages of vegetation restoration, both the community height and biomass inside the shrubbery were higher than outside, with the community height increasing by 51.2%, 42.0%, and 31.6% and biomass increasing by 110.7%, 110.1%, and 18.0% compared to outside the shrubs (Supplementary Table S1 and Figure 2).

3.2. Effects of C. microphylla on Soil Physicochemical Properties across Various Stages of Vegetation Restoration in Desertified Grassland

3.2.1. Analysis of Soil Physical Properties

The results of the three-way ANOVA indicate that the stages of vegetation restoration in desertified grasslands, the insides and outsides of shrubs, and soil layers significantly affect the SWC and particle size composition. The interactions between the stages of restoration and the insides and outsides of shrubs significantly affect the composition of soil particle sizes. The interaction between the insides and outsides of shrubs and soil layers significantly affects the content of silt and sand particles in the soil. The interaction of the three factors significantly affects the content of silt in the soil (Supplementary Table S2).
In the two soil layers, soil moisture content showed a gradually increasing trend with an increase in the vegetation restoration degree, with the highest value being at the sandy grassland stage, at 1.2%. In all stages of vegetation restoration, the SWC was higher inside the shrubs than outside (p < 0.05), with increases of 96.6%, 28.4%, and 44.7% (Supplementary Table S2 and Figure 3).
The SWC in the 10~20 cm layer was higher than that in the 0~10 cm layer, with significant differences noted between different soil layers inside and outside the shrubs at the SF stage and inside the shrubs at the FS stage (Supplementary Table S2 and Figure 3).
In the two soil layers, clay and silt contents increased with the degree of vegetation restoration on sandy land, with the highest content being at the SG stage, averaging 21.0% for clay and 39.0% for silt; sand content gradually decreased, with the highest content being at the SF stage, at 77.0% (Supplementary Table S2 and Figure 3).
In the two soil layers, the contents of clay and silt inside and outside the shrubs showed significant differences at SG (p < 0.05), with higher contents being noted inside the shrubs than outside, while the proportion of sand was higher outside the shrubs than inside (p < 0.05) (Supplementary Table S2 and Figure 3).

3.2.2. Analysis of Soil Chemical Properties

The results of the three-way ANOVA indicate that the stages of vegetation restoration, the insides and outsides of shrubs, and soil layers significantly affect the SOC, TN content, and pH values. The stages of vegetation restoration and the insides and outsides of shrubs do not significantly affect soil TP content while soil layers have a significant impact on soil TP content. The interactions between the stages of restoration and the insides and outsides of shrubs significantly affect the SOC, TP content, and pH. The interaction between the stages of restoration and soil layers significantly affects TN and TP. The interaction of the three factors significantly affects TN and TP (Supplementary Table S3).
Significant variations were observed in the SOC across different stages of vegetation recovery, with levels gradually rising as the restoration advanced. The highest value was recorded at 4.12 g·kg−1. Additionally, the SOC was consistently higher inside the shrubs than outside, with increases of 56.5% and 93.9% inside compared to outside during the SF and FS stages, respectively. However, this difference was not statistically significant in the SG stage (Figure 4).
In the two soil layers, the soil total nitrogen content significantly increased from the SF stage to the FS stage during different stages of vegetation restoration, with an increase of 73.0% in the 0~10 cm soil layer and 20.5% in the 10~20 cm soil layer. Soil TN content was higher inside the shrubs than outside, with an increase of 60.0% in the 0~10 cm soil layer and 64.0% in the 10~20 cm soil layer (Figure 4).
There were significant differences in the pH at different stages of vegetation restoration, and as the degree of vegetation restoration increased, the pH inside the shrubs decreased (Figure 4).

3.3. Correlation between Soil Physicochemical Properties and Vegetation Characteristics

The Pearson correlation analysis revealed that, in the 0~10 cm soil layer (Figure 5a), the SWC, silt content, SOC, and TN content were significantly or highly significantly positively correlated with vegetation biomass (p < 0.05). SWC, clay and silt content, SOC, and TN content were significantly associated with community height (p < 0.01) whereas sand content showed a highly significant negative correlation with community height (p < 0.01). Both the Shannon–Wiener index and species richness of vegetation were significantly positively correlated with soil clay, silt content, SOC, and TN content (p < 0.01) and were highly significantly negatively correlated with soil sand content and soil pH (p < 0.01).
In the 10~20 cm soil layer, SWC, SOC, TN content, and pH showed significant positive correlations with vegetation biomass (p < 0.05). Clay content, SOC, and TN content in the soil were significantly positively correlated with community height (p < 0.05). Clay and silt content, SOC, and TN content exhibited significant positive correlations with the vegetation’s Shannon–Wiener index (p < 0.05) whereas sand content and pH were significantly negatively correlated with it (p < 0.05). SWC, clay, silt content, SOC, and TN content had significant positive correlations with species richness (p < 0.01) while sand content showed a significant negative correlation with species richness (p < 0.01).
The results of the RDA indicated that in the 0~10 cm soil layer, soil physicochemical properties explained 59.6% of the variation in vegetation characteristics both inside and outside shrubs (Figure 6a, with RDA1 contributing 47.5% and RDA2 12.1%. The first two axes effectively illustrated the relationships between vegetation attributes and various soil environmental factors, highlighting soil particle composition and total nitrogen content as the key factors significantly influencing the changes in the aboveground vegetation community (Figure 6b). In the 10~20 cm soil layer, soil physicochemical properties accounted for 46.9% of the variance in vegetation changes (Figure 6c, with RDA1 explaining 37.7% and RDA2 9.2%. Here, soil moisture content, total nitrogen content, and soil organic carbon were identified as the principal factors significantly affecting the shifts in the aboveground vegetation community characteristics (Figure 6d).

4. Discussion

The facilitative role of shrubs in promoting vegetation restoration primarily operates through canopy effects and soil amelioration [29,30,31]. Our study has revealed that during different stages of sandy grassland restoration, the vegetation community within C. microphylla shrubs exhibits greater heights and biomasses compared to areas outside the shrubs. Species richness and Shannon–Wiener indices were higher inside shrubs during the semi-fixed and fixed dune stages but lower within shrubs during the sandy grassland phase, aligning with the findings of Siddharth B. I. et al. [32]. Throughout the restoration process, the canopy acted as a barrier, intercepting airborne seeds, buffering sand burial disturbances, mitigating extreme solar radiation, and reducing soil evaporation, thereby creating a conducive environment for seed germination and establishment within shrubs [33]. This led to significantly higher species richness, height, and biomass within shrubs during the early stages of restoration in this study. However, in the sandy grassland stage, with the aging of C. microphylla and an expanding canopy, dense shading hampered photosynthesis within the shrub interior, and intensified competition among plants adopting different survival strategies further exacerbates conditions [25]. Consequently, species richness and diversity were higher outside shrubs in the later stages of dune stabilization, and the biomass within shrubs was notably less than in the fixed dune period.
The Stress Gradient Hypothesis (SGH) posits that as environmental stress increases, positive interactions among species also increase [34]. Contrary to SGH predictions, our results show that while shrubs facilitate species richness and Shannon–Wiener indices in the semi-fixed and fixed dune stages, the opposite is true in the sandy grassland phase. Relevant studies suggest that positive interactions can vary with changing environmental pressures, turning into negative correlations when pressures ease, as shrubs and the species they shelter may compete more intensely [13]. This explanation accords with our observation in the sandy grassland phase, where species richness and Shannon–Wiener indices were higher outside the shrubs, reflecting a shift from facilitation to competition under reduced environmental stress.
In this study, both soil moisture content and clay content within C. microphylla were higher than those outside, and both contents increased with the advancement of vegetation restoration, indicating that C. microphylla shrubs help reduce soil water loss and break down coarse soil particles, facilitating the transformation of coarse sand into finer particles. This suggests that the shrubs alter the proportions of soil particle sizes, consistent with the findings of Su Yong Zhong et al. [35]. During the restoration of sandy grasslands, shrubs not only buffer the wind erosion of surface soil but also trap dust, leading to changes in soil particle composition. Moreover, our study showed that as vegetation recovery progressed, soil organic carbon content increased, with higher levels of organic carbon and total nitrogen found within the shrubs compared to outside areas.
As a leguminous shrub, C. microphylla accelerates the accumulation of soil organic carbon through litterfall. Its roots host abundant rhizobia that fix atmospheric nitrogen, thereby enhancing the soil nitrogen content [27]. Additionally, this fixed nitrogen can be transferred to neighboring plants, influencing community composition and structure [22]. Erfanzadeh Reza [36], in analyzing the impact of various shrubs on soil characteristics, demonstrated that the “fertility island effect” of shrubs commonly leads to nutrient accumulation, with increased litter from both shrubs and herbaceous plants providing opportunities for C and N buildup. Furthermore, the nitrogen-fixing activity of microorganisms and the abundance of root-zone microorganisms in C. microphylla thickets enrich the nutrients in the rhizosphere soil, facilitated by the shrub’s robust root system, ensuring its growth in harsh environments [37]. Thus, the process of vegetation restoration is intimately linked with changes in soil physicochemical properties.
Vegetation–soil synergy and feedback have long been recognized as crucial mechanisms in the shrub-dominated facilitation of vegetation community restoration in sandy grasslands [38], constituting a significant focus of research in the ecological recovery and succession of vegetative cover in sandy ecosystems. Consistent with this, our findings illustrate a significant correlation between changes in soil physical and chemical properties and vegetation characteristics, with soil particle composition and total nitrogen content primarily driving aboveground vegetation changes in the 0–10 cm soil layer. Conversely, in the 10–20 cm layer, soil moisture content, total nitrogen content, and soil organic carbon emerge as pivotal factors influencing vegetation community characteristics. This suggests that C. microphylla’s influence on soil particle size alteration is more pronounced in the topsoil, and while chemical property changes in the 0–10 cm layer have a smaller impact on vegetation shifts compared to those in the 10–20 cm layer, it is the moisture, total nitrogen content, and organic carbon content in the latter that critically shape vegetation dynamics.
Scholars argue that plant interactions drive community assembly through both above- and belowground ecological processes [39]. On one hand, C. microphylla’s canopy reduces wind velocity; traps dust and litter, thereby accumulating organic matter within the shrub; and, during rainy periods, redistributes intercepted water, culminating in the formation of a “fertility island” effect, effectively conveying nutrients from the vegetation to the soil [35]. On the other hand, the accumulation of soil nutrients reciprocally impacts the growth of aboveground vegetation [40]. Research has shown that rhizobia associated with leguminous shrubs and the process of “nitrogen transfer” exert a profound influence on the pattern and distribution of aboveground vegetation [18]. These interwoven processes form a feedback loop, highlighting the intricate interplay between vegetation and soil in driving ecosystem restoration in sandy grasslands.
During the process of sandy-land vegetation restoration, water is a critical factor influencing vegetation succession [17,41]. In the early stages of sandy grassland restoration, dominant shrubs utilize their canopy and soil effects to facilitate the formation and development of soil crusts, hindering evaporation from the surface soil layer. This leads to a concentration of soil moisture in the shallow layer beneath shrub canopies, thereby accelerating the growth of herbaceous plants with shallow root systems. However, in later stages of restoration, the degradation of deep-rooted shrubs due to inadequate moisture availability promotes the succession and development of herbaceous vegetation [17]. In this study, our sampling and analysis were confined to the physical and chemical properties of the 0–20 cm soil layer. While the results indicate that soil moisture content in the 10–20 cm layer has a greater impact on vegetation changes compared to that in the 0–10 cm layer, this does not sufficiently elucidate the disparities between shallow and surface soil properties. In summary of the above analysis, our study systematically combined the changes in vegetation communities with the changes in soil physicochemical properties and quantitatively investigated and emphasized the key role of C. microphylla in the process of sand vegetation restoration. In this process, the dominant role of the shrub cannot be overlooked. Further investigation into the changes in deeper soil layers’ physicochemical properties and their effects on aboveground vegetation is warranted to clearly identify the key mechanisms constraining plant growth during different stages of vegetation restoration.

5. Conclusions

Planting C. microphylla is an effective measure to prevent the spread of desertification and to accelerate the succession of vegetation restoration in desertified grasslands. During the process of vegetation restoration, C.microphylla can enhance the biomass, height, and richness of the vegetation community within the shrubs, especially during the semi-fixed- and fixed-sand land stages. At the same time, it can increase the SWC and clay content and reduce the sand content. As the restoration progresses, the SWC, silt and clay particles, SOC, and TN content all increase, especially under the shrubs, while the TP content changes little. In the 0~10 cm soil layer, the soil particle size and TN content are the main factors affecting the changes in aboveground vegetation, while the SWC, TN content, and SOC are more critical to the characteristics of the vegetation community in the 10~20 cm soil layer. Therefore, C. microphylla increases the species richness, height, and biomass of vegetation by improving soil’s physical and chemical characteristics, which promotes the restoration of desert vegetation and is an important driving force for the succession of desert vegetation restoration.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f15101801/s1, Table S1: The results of the two-way ANOVA on the effects of vegetation restoration stages and locations (inside or outside shrubs) on vegetation characteristics. Table S2: Results of three-way ANOVA of vegetation restoration stage, location (inside or outside shrubs) and soil layer on soil physical characteristics. Table S3: The results of the three-way ANOVA on the effects of vegetation restoration stages, locations (inside or outside shrubs), and soil layers on soil chemical characteristics.

Author Contributions

Conceptualization: methodology, Q.L. and T.Z.; data analysis, investigation, writing—original draft preparation, and writing—review and editing and visualization, T.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 31470622).

Data Availability Statement

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

Acknowledgments

Wang Haolu provided invaluable assistance during the field sampling for this study and Jin Weilin offered significant help in the selection of field plots and species identification.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the study site in northeast China and photographs of the three stages of vegetation restoration in sandy areas.
Figure 1. Location of the study site in northeast China and photographs of the three stages of vegetation restoration in sandy areas.
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Figure 2. Changes in vegetation community characteristics under and outside the C. microphylla at different stages of vegetation restoration in desertified grassland. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location.
Figure 2. Changes in vegetation community characteristics under and outside the C. microphylla at different stages of vegetation restoration in desertified grassland. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location.
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Figure 3. Changes in soil water content and particle size distribution within and outside shrubs at different stages of vegetation restoration in desertified grassland. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location. The symbol * indicates a significant difference among different soil layers at the same position and vegetation restoration stage (* p < 0.05, and *** p < 0.001).
Figure 3. Changes in soil water content and particle size distribution within and outside shrubs at different stages of vegetation restoration in desertified grassland. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location. The symbol * indicates a significant difference among different soil layers at the same position and vegetation restoration stage (* p < 0.05, and *** p < 0.001).
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Figure 4. Changes in soil chemical characteristics within and outside shrubs at different vegetation restoration stages. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location. The symbol * indicates a significant difference among different soil layers at the same position and vegetation restoration stage (** p < 0.01, and *** p < 0.001).
Figure 4. Changes in soil chemical characteristics within and outside shrubs at different vegetation restoration stages. Note: The data in the figure are means � standard deviations. Different lowercase letters indicate significant differences at 0.05 level among different locations (under or outside shrubs) in the same sandy revegetation stage. Different capital letters indicate significant differences at 0.05 level among different sandy revegetation stages in the same location. The symbol * indicates a significant difference among different soil layers at the same position and vegetation restoration stage (** p < 0.01, and *** p < 0.001).
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Figure 5. Results of the Pearson correlation analysis of soil physicochemical properties and vegetation community characteristics in the 0~10 cm soil layer (a) and the 10~20 cm soil layer (b). Note: * p < 0.05 and ** p < 0.01.
Figure 5. Results of the Pearson correlation analysis of soil physicochemical properties and vegetation community characteristics in the 0~10 cm soil layer (a) and the 10~20 cm soil layer (b). Note: * p < 0.05 and ** p < 0.01.
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Figure 6. Redundancy analysis (RDA) bioplot of vegetation characteristics and indicators of soil physical and chemical characteristics and significance of each factor. Note: Figure (a,b) show the analysis results of vegetation and soil characteristics in the 0~10 cm soil layer while Figure (c,d) show the results for the 10~20 cm soil layer. Red arrow lines indicate vegetation characteristics and blue arrow lines indicate soil environmental factors; * p < 0.05.
Figure 6. Redundancy analysis (RDA) bioplot of vegetation characteristics and indicators of soil physical and chemical characteristics and significance of each factor. Note: Figure (a,b) show the analysis results of vegetation and soil characteristics in the 0~10 cm soil layer while Figure (c,d) show the results for the 10~20 cm soil layer. Red arrow lines indicate vegetation characteristics and blue arrow lines indicate soil environmental factors; * p < 0.05.
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Table 1. Changes in selected shrub size and community profiles by sand revegetation stage.
Table 1. Changes in selected shrub size and community profiles by sand revegetation stage.
Revegetation StageLength/(cm)
N = 15		Width/(cm)
N = 15		Height/(cm)
N = 15		Cover/(%)
N = 15
SF118.5 � 16.04 b89.70 � 19.87 b112.13 � 13.76 a25.2 � 8.21 c
FS161 � 23.55 a131.17 � 23.70 a150.53 � 41.49 a48.5 � 10.54 b
SG122.12 � 22.12 b98.27 � 27.93 b149.00 ± 36.30 b83.5 ± 4.57 a
Note: Different letters in the same column meant significant difference at 0.05 level.
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Zhu, T.; Li, Q. Effect of Caragana microphylla Lam. on Desertified Grassland Restoration. Forests 2024, 15, 1801. https://doi.org/10.3390/f15101801

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Zhu T, Li Q. Effect of Caragana microphylla Lam. on Desertified Grassland Restoration. Forests. 2024; 15(10):1801. https://doi.org/10.3390/f15101801

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Zhu, Tiantian, and Qinghe Li. 2024. "Effect of Caragana microphylla Lam. on Desertified Grassland Restoration" Forests 15, no. 10: 1801. https://doi.org/10.3390/f15101801

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