Study on the Morphology and Mechanical Properties of Cynodon dactylon in the Riparian Zone Slopes of a Large Reservoir

Abstract

The stability of riverbank slopes is crucial in watershed ecology. The morphology and tensile strength properties of plant roots play a significant role in slope stability, which is of great importance for the ecological stability of riverbanks. The Jinsha and Yalong River basins are the largest hydropower bases in China and are in the ecologically fragile areas of the dry and hot river valleys, yet fewer studies are available on these basins. Further studies on the growth morphology and root mechanical properties of plant roots in the riparian zone at different elevations have not been reported. Therefore, we selected the dominant species of Cynodon dactylon root as the research subject, analyzed the root morphology, and conducted indoor single-root tensile tests to study its root structure and mechanical properties at various elevations. The results showed that the root morphology of Cynodon dactylon was positively correlated with elevation. Compared to low elevations (L and M), the root length increased by 57.3% and 21.47%, the root diameter increased by 24.85% and 13.92%, the root surface area increased by 93.5% and 67.37%, and the total root volume increased by 119.91% and 107.36%. As the elevation gradient increased, the flooding time decreased, leading to more developed plant roots for Cynodon dactylon. The Young’s modulus ranged from 148.43 to 454.18 MPa for Ertan Cynodon dactylon roots and 131.31 to 355.53 MPa for Guanyingyan Cynodon dactylon roots. The maximum tensile strength, ultimate tensile strength, ultimate elongation, and Young’s modulus of the plant root of the Cynodon dactylon showed a power function relationship with the diameter. The maximum tensile strength increased as the diameter increased, while the remaining properties decreased following a power function relationship. The maximum tensile strength, ultimate tensile strength, and Young’s modulus of Cynodon dactylon were positively correlated with elevation, while the ultimate elongation was negatively correlated with elevation. The results elucidate the influence of elevation on the root morphology and mechanical properties of dominant riparian species. This provides a theoretical basis for managing and protecting riparian slopes in ecologically fragile areas.

1. Introduction

In recent years, scholars have increasingly focused on studying plant roots and their role in soil consolidation and erosion resistance in the riparian zone. Severe soil erosion has become a significant ecological and environmental problem in the riparian zone [1]. The management of the riparian zone primarily involves restoring vegetation. Plant roots play a crucial role in consolidating soil and enhancing soil erosion resistance [2,3]. An important indicator for evaluating the soil consolidation ability of plant roots is the increase in the soil’s shear strength due to the presence of plant roots [4]. Herbaceous plant roots, as natural bio-fibers, possess a certain tensile strength and contribute to reinforcing the soil to enhance its shear strength. Plants exhibit good soil consolidation and erosion resistance, which is evident in both their above-ground and below-ground roots. On one hand, plants serve a buffering function by obstructing rainfall, runoff, and waves with their above-ground parts, thereby reducing the intensity of erosion. On the other hand, plant roots interlace and intertwine in the soil to withstand external erosive forces through a network structure, retaining and anchoring the soil, thus stabilizing it and enhancing its resistance to erosion [5].

Most current studies have primarily focused on herbaceous plants like Cynodon dactylon in the riparian zone of the Three Gorges reservoir. These studies have mainly analyzed the relationship between root morphology and root mechanical properties of the dominant species in the riparian zone. For example, Li L. W. et al., 2023 [6] and Yildiz A. et al., 2018 [7] demonstrated that plant roots could significantly improve the water erosion resistance and shear strength of soil. Li L. W. et al. conducted hydrostatic disintegration tests, while Yildiz A. et al. performed direct shear tests on soil samples containing roots. Qihong Y et al., 2022 [8] and Bo W et al., 2021 [9] studied the tensile properties of herbaceous roots and predicted the soil-fixing ability by using the single-root tensile test. They pointed out that herbaceous roots had excellent soil-fixing ability. Among them, the Pioneer species of Artemisia capillaris and Leymus secalinus had a high soil detachment capacity 6.32–12.57 times greater than that of late-succession species. The highest root tensile strength measured was for P. paspaloides (62.26 MPa), followed by Cynodon dactylon (51.49 MPa), H. compressa (50.66 MPa) and H. altissima (48.81 MPa). The high root cohesion of H. altissima and H. compressa means that they are suitable for revegetation [10,11]. Fewer studies have been reported on plant investigations in the riparian zone of reservoirs in the ecologically fragile areas of the dry and hot river valleys in the Yunnan–Guizhou Plateau region. Further research on plants in the riparian zone with varying inundation gradients is needed to analyze their root growth morphology and root mechanical properties in response to changes in inundation time.

In this paper, we investigated the interrelationship between these characteristics and elevation by studying the root morphology and root tensile resistance characteristics of the dominant species, Cynodon dactylon. The focus was on the representative Ertan Reservoir in the Yalong River riparian zone and Guanyinyan Reservoir in the Jinsha River riparian zone. Our research results will provide important references for ecological restoration and construction at different riparian elevations.

2. Materials and Methods

2.1. Study Area Profile

The Jinsha River is located in the northern part of Yunnan Province and belongs to the upper reaches of the Yangtze River. The main stream is 3481 km long, with an altitude of 2000~3500 m and a basin area of 502,000 km². The climate of the Jinsha River Basin is southern subtropical according to the Köppen climate classification. There is sufficient sunshine, with an average annual temperature of 20 °C to 23 °C. The evaporation is high, with an annual rainfall of 600 mm to 800 mm, and the annual evaporation is 3 to 6 times that of annual rainfall. The main type of soil along the Jinsha River is Ferralsols, according to WRB soil classification standards, and the other soil types are Luvisols. Ertan Hydropower Station is one of the hydropower stations in the Yalong River step development. Its construction was completed in 2000 with an installed capacity of 3300 MW. Guanyingyan Hydropower Station is the last step hydropower station in the middle reaches of the Jinsha River Hydropower Base, with an installed capacity of 3000 MW. The dispatching mode of the Jinsha River basin is a joint dispatching mode of reservoir groups. Typically, the water level begins to decrease in February every year. In June and July, the flood control water levels of Ertan Reservoir and Guanyinyan Reservoir are 1134 m and 1200 m, respectively. Then, the Ertan Reservoir will start to fill up to 1200 m in early September, and the Guanyinyan Reservoir will gradually fill up to 1122 m in early August. These reservoirs lead to the formation of seasonal riparian zones that contradict the natural flood and dry law [12].

2.2. Selection and Sampling of Sample Plots

In early July 2022, samples were collected from the riparian zones of the Ertan Reservoir (26°48′39″ N, 101°43′36″ E) along the Yalong River and the Guanyinyan Reservoir (26°26′01″ N, 101°28′13″ E) along the Jinsha River, respectively (Figure 1). To minimize the impact of land use, soil type, topography, geomorphology, vegetation, and human activities on the research, dry red soil and gently sloping sample strips in a near-natural state were selected. These sample strips had similar habitat characteristics in terms of slope direction, slope gradient, and land-use history. Soil samples were then collected. Before the anthropogenic clearing of the reservoir, the land-use types in the sample zone were all dry land. After inundation, the land-use types changed to natural grassland dominated by Cynodon dactylon. Both Ertan and Guanyinyan Reservoir fallout zones exhibit the anti-seasonal hydrological situation of “winter water, summer land”. Based on the historical water level data from the Yangtze River Hydrological Network of the People’s Republic of China (http://www.cjh.com.cn/swyb_syqbg.html, accessed on 16 October 2023), three sampling intervals were established for the elevations in the study area. The samples at elevation L were flooded for more than 7 months, those at elevation M were flooded for 5–6 months, and those at elevation H were flooded for 3–4 months. The horizontal distance of each elevation was divided into three sections, and three sampling points were set up to obtain the fresh roots of the plant using the total excavation method. The Cynodon dactylon at different elevations was then grouped and placed into labeled gauze bags (refer to Table 1) and transported back to the laboratory for preservation and subsequent processing. This process ensured the timely completion of root morphology measurements and tensile tests on Cynodon dactylon.

2.3. Parameter Determination of Plant Root Geometry

The roots collected at various elevations from Ertan and Guanyinyan reservoirs were brought back to the laboratory, cleaned with water, and then placed in a clean and highly transparent scanning dish. Pure water was added to the dish, and the roots were gently spread out using a brush to maximize their extension in the tray. The Epson 12000XL scanner (Seiko Epson Corporation, Japan) was used to scan in grayscale mode at 400 dpi, and then the plant root data were analyzed using analysis software (WinRHIZO Pro2007, Regent Instruments, Canada). Numerous studies have shown that it is mainly the fine roots that play the role of root fixation and slope protection in plants [9,10]. For comparison purposes, the statistics of root-related parameters in this experimental study did not take into account the underground stems of Cynodon dactylon roots.

2.4. Plant Root Stretching Experiment

The single-root tensile test method was adopted. The root tensile test was conducted using a WDW-10 electronic universal testing machine (MTS Systems Corporation, USA) and Smart Test terminal measurement and control software (MTS TestSuite TW, USA), with the relevant test data automatically obtained using a computer. Before conducting the tensile test on the root, upper and lower marks were placed 5 mm from the middle section of the root. Subsequently, the diameter of the root was measured at five positions between the upper and lower marks using electronic vernier calipers, and the average value was recorded as the single-root diameter. To prevent the plant root from sliding or shifting within the fixture, medical tape was utilized to secure both ends of the fixture, enhancing the friction between the plant root and the fixture. The spacing between the upper and lower fixtures was adjusted to 50 mm, followed by securing the plant root to ensure it remained straight without applying excessive force. During the experiment, the lower fixture descended at a rate of 5 mm/min. The data showing that the plant root was broken only in the middle one-third of the root were selected as valid data.

Calculation method of ultimate tensile strength [13,14]:

(1)$P=\frac{4F}{\pi D^2}$

where P is the ultimate tensile strength (MPa); F is the maximum tensile strength (N); D is the average diameter of the plant root (mm).

Calculation method of elongation [13,14]:

(2)$\epsilon =\frac{∆L}{L}\times 100\%$

where ε is the ultimate elongation, %; ∆L is the elongation of the plant root when stretched, in mm; L is the original length of the root segment, in mm, which in this study is the standard distance, taken as 50 mm.

Calculation method of Young’s modulus [13,14]:

(3)$E_r=\frac{200F}{\pi D^2∆L}$

where Er is Young’s modulus of elasticity, in MPa; F is the maximum root tension, in N; D is the average diameter of the plant root, in mm; ∆L is the elongation of the plant root when stretched, in mm.

2.5. Data Statistics and Analysis

SPSS 25.0 (IBM, USA) software was used to assess the existence of significant differences between the data at different altitudes before analysis. To confirm the existence of significant differences in the experimental data, a one-way ANOVA was performed to analyze the growth pattern, tensile strength, elongation, and modulus of elasticity of Cynodon dactylon roots at different altitudes. The results were plotted using Origin 2019 (OriginLab, USA) software.

3. Results

3.1. Relationship between Root Morphological Indexes and Elevation of Cynodon dactylon Roots

Morphological indicators of plant roots, such as root length, root diameter, root surface area, and root volume, directly influence the plant’s ability to absorb soil nutrients and water, as well as to anchor the soil. To investigate the effect of elevation on the growth of Cynodon dactylon, this paper analyzed the root scanning data of Cynodon dactylon at different elevations (low, medium, and high) in the Ertan and Guanyinyan reservoirs. The analysis results showed that the root length, surface area, average diameter, and total root volume of Cynodon dactylon exhibited a certain range of variation under different elevation conditions (Figure 2). Specifically, the root length varied from 17.1920 to 28.0508 cm, the surface area varied from 1.9814 to 4.1623 cm², the mean diameter varied from 0.3607 mm to 0.4898 mm, and the total root volume varied from 0.0212 to 0.0508 cm³. Under all elevation conditions, Cynodon dactylon located at Ertan Reservoir elevation (H) exhibited the highest root characterization index. Specifically, its root length was 28.0508 cm, the root surface area was 4.1623 cm², the average diameter was 0.4898 mm, and the total root volume was 0.0508 cm³. Compared to low elevations (L and M), the root length increased by 57.3% and 21.47%, the root diameter increased by 24.85% and 13.92%, the root surface area increased by 93.5% and 67.37%, and the total root volume increased by 119.91% and 107.36%, respectively. This result indicates that elevation had a significant effect on the plant root characteristics of Cynodon dactylon. In addition, the pattern of changes in the root characteristics of Cynodon dactylon with elevation in the Guanyinyan Reservoir was consistent with that of the Ertan Reservoir. However, compared with the Ertan Reservoir, the length, diameter, surface area, and total volume of the Cynodon dactylon system in the Guanyinyan Reservoir area decreased by 4.38%, 5.99%, 6.43%, and 6.65%, respectively. This result suggests that the environmental conditions of different reservoirs affect the plant root characteristics of Cynodon dactylon.

Based on previous studies on plant roots in the riparian zone [11], this paper conducts a detailed hierarchical analysis of plant roots at different gradients. It classifies the root diameters into four grades and performs an in-depth study of the length, surface area, and volume of each grade. The detailed data are shown in Figure 3 and Figure 4. It is evident from Figure 3 and Figure 4 that the Cynodon dactylon in Ertan Reservoir and Guanyinyan Reservoir both exhibited a higher percentage of plant roots with diameters exceeding 0.4 mm across various elevation gradients. Among them, the percentage of water storage in the Ertan Reservoir ranged from 42.85% to 100%, while that of the Guanyinyan Reservoir ranged from 30.71% to 100%. This indicates that the diameter of the mature Cynodon dactylon system is generally larger than 0.4 mm. The length, surface area, and volume of Cynodon dactylon system with rhizomes ranging from 0~0.4 mm accounted for 0% in both reservoirs under the H gradient. This indicates that under the higher elevation gradient, the root growth of Cynodon dactylon was mainly concentrated in the larger-diameter part. Under the L and M gradients, the length, surface area, and volume of the Cynodon dactylon system in Guanyinyan Reservoir were greater than the corresponding morphological indicators in Ertan Reservoir. In the range of 0.4 to 0.5 mm of rhizomes, the difference between the two reservoirs under the low (L) gradient was not significant. However, under the medium (M) and high (H) gradients, the proportion of the Cynodon dactylon system in Guanyinyan Reservoir was considerably greater than that in Ertan Reservoir. For the portion of the plant root with a diameter class exceeding 0.5 mm, the length, surface area, and volume share of the Cynodon dactylon system in Ertan Reservoir were much larger than that in Guanyinyan Reservoir under the low (L), medium (M), and high (H) gradients. This suggests that the growth of Cynodon dactylon in Ertan Reservoir is superior across a wider range of root size classes. In this study, plant root diameters were classified based on previous research. The roots of Cynodon dactylon at various gradients were thoroughly analyzed morphologically. The results showed that elevation had a significant effect on the root growth of Cynodon dactylon, and the environmental conditions of different reservoirs also had a certain effect on the root characteristics of Cynodon dactylon.

3.2. Relationship between Tensile Force and Tensile Strength of Cynodon dactylon Plant Root and Elevation

A total of 166 roots were successfully tested in the root tensile test of Ertan Reservoir and 158 roots were successfully tested in the root tensile test of Guanyinyan Reservoir, with an average success rate of 39.17%. The root stems ranged from 0.21 mm to 0.74 mm. The relationship between the mechanical properties of the plant root and the elevation obtained is listed in Table 2.

After analyzing the tensile force and ultimate tensile strength of Cynodon dactylon roots, it was found that there is a power function relationship between the two and the root diameter, as illustrated in Figure 5 and Figure 6. Specifically, the tensile strength of Cynodon dactylon roots increased with an increasing diameter, while the ultimate tensile strength decreased with a larger diameter. In addition, it was observed that the tensile force and ultimate tensile strength of Cynodon dactylon increased with an increasing elevation gradient. This result indicates that the elevation gradient has a significant effect on the mechanical properties of Cynodon dactylon. By comparing the tensile strength of Cynodon dactylon roots from Ertan Reservoir and Guanyinyan Reservoir, we found that the roots from Ertan Reservoir exhibited higher single tensile strength and maximum tensile stress. This difference may be attributed to the fact that the Ertan Reservoir was impounded one month later than the Guanyin Yan Reservoir, which allowed the Cynodon dactylon to grow for an additional month during the optimal growing season. The difference in growth conditions led to the roots of Cynodon dactylon from Ertan Reservoir being superior to those from Guanyinyan Reservoir in terms of both root morphology and mechanical properties.

3.3. Relationship between Ultimate Elongation and Young’s Modulus of Cynodon dactylon Plant Root and Elevation

The relationship between the ultimate elongation of a single root and rootstocks of Cynodon dactylon roots was modeled using power functions (Figure 7 and Figure 8). The fit (R²) was small for the relationship between ultimate elongation and diameter. However, a trend can be observed where the ultimate elongation decreased with increasing diameter. At different elevation gradients, the ultimate elongation followed the order L > M > H, and the average elongation exhibited significant variations across different elevation gradients (p < 0.05). The Young’s modulus ranged from 148.43 to 454.18 MPa for Ertan Cynodon dactylon roots and 131.31 to 355.53 MPa for Guanyingyan Cynodon dactylon roots. The Young’s modulus of Ertan Cynodon dactylon roots was greater than that of Guanyingyan at all three elevation gradients. In addition, the Young’s modulus of Cynodon dactylon roots decreased as a power function with increasing diameter and increased with elevation.

4. Discussion

4.1. Effect of Elevation on Root Morphology

Plant root morphology and distribution are primarily determined by the genetic characteristics of the plant itself and are also influenced by the soil ecological environment [15]. Changes in the environment have a significant impact on the plant’s root system [16]. The larger the surface area and volume of the plant roots, and the longer they are, the more advantageous it is for the plant to absorb soil water and nutrients, enabling better adaptation to the growing environment [17]. In this study, Cynodon dactylon grew in various elevation gradients and was exposed to differences in soil moisture, sunlight, and nutrients. Consequently, the root morphology indexes of the same plant varied significantly.

The root morphology of R. Cynodon dactylon exhibited significant superiority across various elevation gradients in Ertan and Guanyinyan Reservoirs, with the most notable performance observed in the H elevation gradient, where it secured the top position. This observation revealed a clear trend: as the elevation gradient increased, i.e., the corresponding shortening of the inundation time, the root development of Cynodon dactylon became more vigorous. This finding is highly consistent with the findings of Loreti E et al., 2016 [18]. They found that as the elevation decreased and the flooding time extended, the root morphology of Cynodon dactylon underwent significant changes. These changes were characterized by a reduction in the total number of roots, the total length of the plant roots, the average diameter, and the root dry weight. Flooding induces anaerobic respiration, a process that depletes energy stores and leads to a reduction in root diameter. When these energy substances were depleted, the plant root became less vigorous and eventually lost its vitality, turning black and detaching from the plant. This not only led to a drastic reduction in the total number of roots and total length but also resulted in a decrease in the average root diameter, total number of roots, and total length, which further triggered a reduction in root dry weight. Over time, the flooding led to a significant negative impact on the root morphology of Cynodon dactylon. Moreover, this is supported by the studies of Zhang Q 2017 [19] and Chen L 2021 [20], who found that key growth indicators such as stem length, stem width, and the number of divisions of Cynodon dactylon showed a significant downward trend when the flooding time reached or exceeded 40 days. The present study further confirmed this observation through empirical experiments and quantified the specific effects of flooding time on the root morphology of Cynodon dactylon: root length decreased by 36.43%, diameter by 24.46%, surface area by 50.2%, and total volume by 54.89%. These quantitative data provide strong support for a deeper understanding of the effects of the flooded environment on the growth of Cynodon dactylon roots.

The length of the plant root was determined by a combination of the length of individual roots and their number, while the surface area and volume were influenced by a combination of three factors: length, number, and diameter. Under the H elevation gradient, the plant roots of R. Cynodon dactylon exhibited a significant morphological advantage, with a larger average diameter and longer length. This led to a significant increase in the surface area and volume of the plant roots. A noteworthy phenomenon observed in the Cynodon dactylon root samples analyzed in this study is that plant roots with diameter classes ranging from 0 to 0.2 mm were absent, irrespective of the elevation gradient they were located in. This may be because the selected samples had all undergone a growth cycle of more than two years. The distribution of root size classes of Cynodon dactylon showed a clear pattern at different elevation gradients. At the low elevation gradient, the major root size classes were concentrated at 0.3 mm. At the M and H elevation gradients, the major root size classes were 0.4 mm. At the H elevation gradient, the smallest root size class observed was 0.5 mm, and there were no roots smaller than 0.4 mm. These observations suggest that the root diameter of Cynodon dactylon gradually increased with an increasing elevation gradient. In addition, as a whole, plant roots with diameters greater than 0.3 mm accounted for the majority of the total volume. Since the morphology of the plant root approximates a cylinder, its volume increases proportionally as the diameter increases. Also, plant roots with larger diameters are usually longer, which further emphasizes the significant impact of diameter size on root volume. In summary, the root morphology and growth of Cynodon dactylon showed significant differences across elevation gradients. As the elevation gradient increased, the root morphology and growth of Cynodon dactylon showed a tendency to increase, reach an optimum at elevation gradient H, and then decrease. The root morphology and growth of Cynodon dactylon exhibited a pattern of increasing and then decreasing as the elevation gradient increased. These findings provide important insights for a deeper understanding of the growth and acclimatization mechanisms of plants in the riparian zone.

4.2. Effect of Elevation on Mechanical Properties of Plant Root

The diameter is one of the most significant factors influencing the mechanical properties of plant roots [9,10,11,21]. There is an evident size effect on both the single-root tensile force and single-root tensile strength concerning root diameter. The single-root tensile force increases with root diameter, while the single-root tensile strength decreases. The relationship between these two root indexes and root diameter can be modeled by a power function, aligning with findings from other studies. In this experiment, the average diameter and total root volume of Cynodon dactylon roots increased with a higher elevation and a shorter flooding time. This implies that the maximum tensile strength will increase with elevation, and the ultimate tensile strength will also increase with elevation. Both strengths will be correlated with the diameter in a power function. This result is consistent with the findings of numerous national and international studies on the root systems of herbs, shrubs, and other plants [13,21,22,23,24]. Genet et al., 2005 [25] concluded that the fiber content within the plant root is closely related to the root diameter and tensile strength, which are functions of the root diameter. The internal morphology of the plant root is a complex laminar structure composed of cells whose microstructure and chemical composition (e.g., cellulose, hemicellulose, and lignin) synergistically affect the tensile mechanical properties [26]. The elongation and modulus of elasticity of Cynodon dactylon decreased as a power function with the increasing root diameter, which is consistent with the findings of Leung et al., 2023 [27]. As the root diameter increases, the cellulose and hemicellulose content within the plant root also increase, leading to a decrease in the ultimate elongation and modulus of elasticity of the plant root.

The maximum tensile force, ultimate tensile strength, and Young’s modulus of Cynodon dactylon increase with elevation, while the ultimate elongation decreases with elevation. The extreme hydrological conditions in large reservoirs, characterized by long-term flooding exposure–flooding alternation, have a significant impact on the plant biology in the floodplain. This variation also results in differences in the physicochemical properties and water content of the soil across various elevation gradients, as well as during different degrees of dry and wet cycles. These changes in soil properties and the surrounding conditions alter the tissue composition of the Cynodon dactylon system, ultimately leading to variations in elevation. The microstructure and chemical composition of individual roots of the same species vary across different elevation gradients. These differences are the primary factors contributing to the significant variations in tensile strength characteristics observed in individual roots of Cynodon dactylon across different elevation gradients [13,28]. Flooding affects the maximum tensile strength of plant roots primarily by altering root vigor, and this impact is most pronounced at the onset of flooding [18]. Roots with high vigor have high maximum tensile strength, while those with low vigor have low maximum tensile strength. Flooding causes Cynodon dactylon roots to undergo a large amount of anaerobic respiration. The ethanol produced is somewhat toxic to the Cynodon dactylon roots themselves, which reduces root vigor, leading to a significant reduction in the maximum tensile strength of Cynodon dactylon roots [18]. Zhao C 2011 [29] and some other scholars believe that the mechanical properties of plant roots are closely related to the cellulose, hemicellulose, and lignin present inside the roots. Lignin, etc., and different elevation gradients would lead to varying plant growth, as well as different cellulose, hemicellulose, and lignin content. Therefore, it is significant and valuable to further characterize the tensile strength of the chemical composition of the Cynodon dactylon system at different flooding gradients.

4.3. Application to the Southwest Reservoir Riparian Zone

Due to the prolonged inundation of the riparian zone resulting from the anti-seasonal water level regulation in the Southwest Reservoir Area, the original vegetation ecosystem has sustained significant damage, leading to the near-complete disappearance of native vegetation. To restore the ecosystem of the floodplain and control soil erosion, scholars have proposed a program to artificially restore the vegetation of the floodplain in the southwest reservoir area. Cynodon dactylon is one of the primary vegetation types that naturally reestablished in the riparian zone of the Three Gorges Reservoir following its formation and prolonged environmental selection. Studies have shown that the synergistic effect of plant roots, aboveground stems, and leaves can effectively reduce soil erosion. Cynodon dactylon, in particular, has a significant impact on reducing erosion [30]. However, this thesis concluded that the enhancement of soil shear strength by Cynodon dactylon is relatively small. This is primarily due to the fact that the soil and water conservation effect of plants relies on the combined action of their above-ground parts and below-ground plant roots. Although the root system of Cynodon dactylon mentioned in this paper has a lower soil-fixing effect compared to other plants, field investigations have revealed that its stolon growth is vigorous and its above-ground portion is well developed. This results in the formation of a denser vegetation layer on the soil surface, providing effective protection and significantly reducing the soil damage caused by external forces. In the southwest reservoir area of the riparian zone during the summer dew period, which is the most concentrated period of rainfall throughout the year, precipitation and the generation of slope runoff lead to significant splashing and scouring of the soil in the riparian zone, resulting in severe soil erosion issues. Therefore, Cynodon dactylon is of great value in vegetation restoration and soil erosion control in floodplains.

During the summer, the peak growth period of herbaceous plants enhances their overall water and soil conservation effect on both the aboveground and underground root systems. This effectively reduces soil erosion caused by rainfall runoff and prevents the loss of soil and nutrients in the riparian zone. During the winter flooding period, although most aboveground parts of plants may die, their underground root systems remain active, continuing to play a significant role in soil stabilization. This helps protect the surface soil of the riparian zone from erosion caused by reservoir water waves or reduces the extent of their impact. It is worth mentioning that the roots of Cynodon dactylon remain active in winter, thus performing their soil conservation function throughout the entire growth period. The water level in the southwest reservoir area is gradually regulated in a fluctuating manner, rather than rising instantly. During the rising water process, vegetation that is not submerged or re-emerges after short-term submersion continues to play a role in water and soil conservation. Furthermore, the vegetation in the water–land transition zone also plays a crucial role in attenuating waves, thereby reducing wave erosion on reservoir banks. In conclusion, this article discusses the strong root system stabilization ability of Cynodon dactylon. Combined with its good flood tolerance and ability to reduce concentrated flow erosion, this plant can be utilized in vegetation restoration and soil conservation projects in the riparian zone of the southwest reservoir area. Additionally, the excellent characteristics of Cynodon dactylon make it potentially suitable for promotion and application in other reservoirs or lake riparian zones.

5. Conclusions

Indoor single-root tensile tests were conducted on the plant roots of Cynodon dactylon on the slopes of the banks of the Jinsha River to analyze the effects of different elevation gradients on the root morphology and mechanical properties of Cynodon dactylon roots. The following conclusions were drawn.

The root morphology of Cynodon dactylon was positively correlated with the elevation. The root morphology indexes of the H gradient were all greater than those of M and L, indicating a relationship of H > M > L. This suggests that as the upper gradient increases, the plant root of Cynodon dactylon becomes more developed with shorter inundation times. The major root diameter class of L was 0.3 mm~0.4 mm and the major root diameter class of M was 0.4 mm. The minor root had a diameter of 0.5 mm, while the main root had a diameter of 0.4 mm~0.5 mm, and there was no diameter class in 0~0.4 mm.

The maximum tensile strength, ultimate tensile strength, ultimate elongation, and Young’s modulus of the plant root of Cynodon dactylon exhibited a power function relationship with the diameter. The maximum tensile strength increased with the diameter, while the other properties decreased as a power function.

The maximum tensile strength, ultimate tensile strength, and Young’s modulus of Cynodon dactylon were positively correlated with the elevation, while the ultimate elongation was negatively correlated with the elevation. At elevation H, Cynodon dactylon exhibited the highest resistance to tensile forces and the greatest consolidation capacity, followed by M and then L, which showed the weakest performance.

This study on the morphology and mechanical properties of Cynodon dactylon roots on the Jinsha River Basin riparian slopes offers valuable insights for ecological restoration and for the construction of riparian slopes at various elevations in the Yunnan–Guizhou–Chuan Plateau. The findings can be extrapolated and utilized in the Three Gorges Basin of the Yangtze River and other reservoirs in riparian zones with comparable conditions, serving as a theoretical foundation for riparian slope management in the future.

Author Contributions

P.W. proposed the review and organized the review content; P.Z. and S.J. were responsible for writing and revising the manuscript; S.J. performed the experimental analysis; P.Z. performed the data analysis. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Sampling points of Jinsha River riparian zone.

Figure 2. (A) root length, (B) root diameter, (C) root surface area, and (D) root volume of Cynodon dactylon in the hydro-fluctuation belt of reservoirs GYY and ET, respectively. The lowercase letters indicate statistically significant differences between water-level elevations at each reservoir, which are depicted by different lowercase letters next to the symbols (p < 0.05). T1, T2, and T3 depict three elevations from low to high.

Figure 3. Proportion of each index to the total amount for different diameter classes of plant roots in Ertan Reservoir. L, M, and H represent three elevations from low to high. (a): Length was used as the root structure index; (b): the surface area was used as the index of root structure; (c): volume was used as the index of root structure.

Figure 4. Proportion of each index to the total amount for different diameter classes of plant roots in Guanyinyan Reservoir. L, M, and H represent three elevations from low to high. (a): Length was used as the root structure index; (b): the surface area was used as the index of the root structure; (c): volume was used as the index of the root structure.

Figure 5. Relationship between different elevation gradients of Ertan and the tensile force and tensile strength of Cynodon dactylon (a): tensile resistance; (b) tensile strength.

Figure 6. Relationship between different elevation gradients of Guanyingyan and the tensile force and tensile strength of Cynodon dactylon (a): tensile resistance; (b) tensile strength.

Figure 7. Relationship between different elevation gradients of Ertan and the elongation and Young’s modulus of Cynodon dactylon root (a): Ultimate elongation; (b): Young’s modulus.

Figure 8. Relationship between different elevation gradients of Guanyingyan and the elongation and Young’s modulus of Cynodon dactylon root (a): Ultimate elongation; (b): Young’s modulus.

Table 1

Root sampling of Cynodon dactylon roots.

Location	Geographic Coordinates	Elevation Gradient	Elevation Gradient Interval	Number of Samples
Ertan	26°48′39″ N, 101°43′36″ E	L	1150.579~1155.963 m	131
		M	1155.963~1159.279 m	144
		H	1159.279~1163.424 m	134
Guanyinyan	26°26′01″ N, 101°28′13″ E	L	1090.180~1092.934 m	137
		M	1092.934~1095.143 m	143
		H	1095.143~1097.380 m	129

Note: The data of elevation gradient interval are all based on the Yellow Sea elevation.

Table 2

Relationship between mechanical properties of Cynodon dactylon roots and elevation.

Location	Elevation Gradient	Average Tension/N	Average Tensile Strength/Mpa	Average Elongation/%	Average Young’s Modulus/Mpa
Ertan	L	6.01 ± 1.40 c	54.12 ± 15.43 b	24.95 ± 2.59 a	213.72 ± 41.89 c
	M	8.59 ± 1.99 b	58.14 ± 14.03 ab	21.03 ± 2.56 b	274.44 ± 45.62 b
	H	12.26 ± 2.40 a	61.69 ± 17.16 a	18.39 ± 2.11 c	330.76 ± 61.58 a
Guanyinyan	L	5.02 ± 0.86 c	52.92 ± 12.13 b	28.19 ± 2.05 a	186.25 ± 32.10 c
	M	6.95 ± 1.21 b	56.83 ± 12.81 ab	24.24 ± 2.66 b	232.56 ± 33.61 b
	H	9.63 ± 0.83 a	60.65 ± 8.24 a	21.27 ± 1.05 c	284.78 ± 32.83 a

Note: The data in the table are mean ± standard deviation. The same letters indicate no significant difference between the two (p > 0.05), while different letters indicate significant difference between the two (p < 0.05).

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