Effects of Varying Flooding Durations on the Soil Reinforcement Capacity of Dominant Riparian Plants in the Yangtze River Basin

Abstract

This study aimed to investigate the relationships between the mechanical properties of plant roots and the soil reinforcement characteristics of the dominant species in the dominant riparian plants under various flooding durations. The objective was to comprehensively evaluate the optimal flooding duration for each plant under various flooding durations. This research was conducted to provide a scientific basis for plant restoration efforts. The primary focus of the study was on common species found in the middle and lower reaches of the Yangtze River, including Carex, Cynodon, and Eleusine. These species were cultivated in a local field setting and subsequently subjected to flooding tests of varying durations. The diameter of the root system gradually increases with prolonged flooding duration, while other root morphologies exhibit a trend of initially increasing and then decreasing. The flooding environment significantly influences the relationship between root diameter and the mechanical properties of the roots. This condition adversely affects Carex, whereas it has a beneficial impact on Cynodon and Eleusine. During the early stages of flooding, the shear strength of the plant root–soil complex increases; Carex is optimally applied in the restoration and protection of areas subjected to three to four months of flooding, with its ornamental value being particularly pronounced. Cynodon performs best in areas with up to six months of flooding, Eleusine is especially effective in regions with less than two months of flooding.

1. Introduction

Vegetation has been identified as a crucial factor in the prevention and control of soil erosion [1]. However, the quality and biodiversity of soils in the dominant riparian have significantly declined due to prolonged periods of water erosion and scouring. The presence of vegetation has been shown to increase soil erosion resistance through three different conditions within the soil–root interface: roots undergo tensile, pullout, and shear stresses [2]. Water flooding is a critical environmental factor that influences the survival and development of plants in dominant riparians, affecting light intensity, gas composition, soil nutrients, and other environmental factors essential for plant growth [3]. Under flooded conditions, the presence of water bodies obstructs direct contact between plants and the atmosphere, leading to a substantial reduction in the gas content and gas exchange rates of submerged plants [4]. This isolation from the external gaseous environment subject plants to prolonged anoxic conditions and low light levels, which inhibit their respiration and photosynthesis, ultimately resulting in adverse effects on plant growth. Compared with woody vegetation, herbaceous vegetation is more responsive to environmental changes [5]. Therefore, incorporating the ecological processes associated with these types of vegetation into research and methodological planning is essential [6].

The water level of the Three Gorges Reservoir fluctuates significantly on the basis of the reservoir’s operational mode. This variation leads to a period of exposure from April to September, followed by a water storage phase, resulting in a substantial fluctuation zone with a vertical height of 30 m [7,8]. The environmental conditions within this zone change drastically, while a limited number of dominant plant species can survive, most plants become submerged and decompose, releasing nitrogen, phosphorus, and mercury into the water [9]. This process creates a risk of environmental damage and indicates a certain degree of ecological vulnerability [10]. Changes in the vegetation in the dominant riparian affect not only buffering, filtering and especially the connectivity functions of the reservoir banks in the Three Gorges Reservoir area, but also the aesthetics of the environment and changes in ecological communities [11,12]. The original plants have difficulty tolerating prolonged flood stress in winter. Coupled with the summer recession period, the plants in the dominant riparian are exposed to a large amount of sunlight and undergo a certain degree of drought stress, resulting in a large area of plant recession in the reservoir area and a decrease in the biodiversity of the reservoir area; The structure of the plant community in the reservoir area is damaged and the structural community gradually becomes homogeneous. Soil erosion is promoted and there is serious land degradation and deterioration of the landscape, which in turn poses potential hazards related to shallow landslides and debris flow phenomena [13].

This study was conducted to investigate the relationship between changes in plant growth capacity and the soil fixation performance of the root systems of the dominant species in the dominant riparian plants under various flooding durations. Additionally, it aims to comprehensively evaluate the most temporally suitable areas for each plant under different flooding conditions. The individual mechanical properties, soil root content, root–soil interface characteristics, root distribution patterns, and other factors of roots within the root–soil complex are the primary influences [14,15,16,17]. Research has demonstrated [18,19] that variations in root distribution patterns, including combinations of horizontal, vertical, inclined, intersecting, and mixed alignments, have distinct effects on root strength. Not only that, there is also a link between the chemical composition (lignin, cellulose, and hemicellulose) of roots with their mechanical properties and, in particular, tensile strength and cohesion [20]. Flooding stress can diminish the soil consolidation and holding capacity of plants, thereby affecting the mechanical properties of the root–soil complex in the Three Gorges Reservoir area. It is crucial to further investigate the mechanisms by which flooding impacts plant soil consolidation and to evaluate the optimal flooding duration for each plant.

2. Materials and Methods

2.1. Study Area

The experimental area is situated in the South Experimental Base of Hubei University of Technology, located in the Hongshan District of Wuhan City, Hubei Province (Figure 1). Its geographical coordinates are 30°28′ N latitude and 114°18′ E longitude, with an average elevation of 30 m. This region falls within the subtropical humid zone, and is characterized by ample sunlight, abundant rainfall, and four distinct seasons. The lowest temperature recorded during the year does not drop below −5 °C, while the annual average precipitation ranges from 1150 to 1190 mm, with light and heat occurring in the same season. The soil in this area is yellow loam from the Wuhan South Lake base pit, which has a fine texture, high field water-holding capacity, and good water retention properties. However, it has poor water permeability, making it susceptible to soil erosion. The soil plastic limit is 23.0%, its liquid limit is 41.0%, and it has a natural density of 1.50 g/cm³. The maximum dry density is 1.750 g/cm³, with an optimum water content of 20.0% and an average organic matter content of 5.92 g/kg.

2.2. Plant Materials

In this research, Cynodon, Carex, and Eleusine were selected as test objects on the basis of field research conducted on the dominant plants in the dominant riparian. At the Plant Cultivation Base of Hubei University of Technology in Wuhan, the research team carried out a six-month planting trial starting in 15 March 2023. Carex exhibited slow growth under the prevailing soil conditions, with plants only reaching a height of 3 cm two months after sowing and failing to mature by October. Cynodon established ground cover one month after sowing and developed into a lawn after two months of growth. Eleusine, on the other hand, began to germinate within seven days of sowing, reached a length of 10 to 15 cm within a month, and matured after 45 days of growth.

2.3. Experimental Method

To simulate flooding conditions and control the duration of flooding in the dominant riparian, the flooding tolerance of typical riparian plants was tested. The test plants were uniformly cultivated and transplanted into pots (21 cm in diameter, 30 cm deep), and consistent growth was observed for more than 14 days. After the plant’s leaves remained bright green and some growth was observed, these plants were designated flooded plant material. The prepared experimental materials were uniformly placed into an experimental setup consisting of three large tanks (each measuring 980 mm in length, 760 mm in width, and 680 mm in height). The bottoms of these tanks were connected via a communicating vessel, and the top was covered with shading cloth that blocked 90% of light, with the flooding period set from 15 October 2023, to 15 April 2024. Sampling times of 0, 30, 60, 90, 120, 150, and 180 days were established for all three plant species. Changes in the morphology of the root system, the strength of individual roots, the root–soil composite, and the shear strength of the plant-free soil were monitored. To avoid secondary errors in the test results during the sampling process, all plant samples were placed under the same environmental conditions throughout the sampling period. The moisture content was monitored hourly, and in situ straight shear tests were conducted via a cross-plate shear apparatus when the moisture content stabilized at approximately 18%.

To ensure that the soil properties and root characteristics of each sample were as similar as possible, flooded samples were sampled in a manner that maintained consistent growth conditions at the time of sampling and ensured equal relative spacing between the plants, and the difference in root diameter at both ends did not exceed 10% of the average root diameter. The analysis of the plant root systems was performed using Epson 12000 XL root scanner, Suwa, Nagano, Japan: Epson Corporation, whereas the root strength was measured with an ETM 104B universal tensile testing machine, Shenzhen, Guangdong, China: Wance Testing Machine Co., Ltd. Soil strength was assessed using an in situ shear cross plate and a ZJ-type strain-controlled straight shear apparatus.

2.4. Single-Root Stretching Test for Plants

Fifty roots from each herbaceous plant, characterized by an intact epidermis, straightness, and uniform diameter, were selected for the single-root tensile test. The diameter of each root was measured using digital Vernier calipers, while the tensile strength was determined via a WDW-10 electronic universal (Jinan Kason Testing Equipment Co., Ltd., Jinan, China) testing machine in conjunction with Smart Test 8 control software. The test parameters included a set scale distance of 40 mm and a tensile rate of 5 mm/min. Prior to the test, a homemade plastic sheet was glued into the groove of the universal testing machine fixture to increase the friction between the fixture and the root system and to ensure that the root system was subjected to a uniform force. In the test, only the data measured when the root system broke in the middle 1/3 section were recorded. The tensile strength, ultimate elongation, and elastic modulus of the root system were calculated from the average diameter, tensile force, and displacement of the tested root section. The calculation method is as follows:

$$Tr={\displaystyle \frac{4\mathrm{F}}{\pi D^2}}$$

(1)

$$\epsilon ={\displaystyle \frac{\Delta \mathrm{L}}{50}}\times 100\%$$

(2)

$$Er={\displaystyle \frac{200F}{\pi D^2\Delta L}}$$

(3)

where Tr represents the tensile strength of the root system (MPa); F denotes the maximum tensile force of the root system (N); $D^2$ indicates the diameter of the root system (mm); ε signifies the longitudinal strain of the root system, specifically the ultimate elongation (expressed as a percentage); $\Delta L$ refers to the elongation of the root system when extracted (mm); Er is the elastic modulus (MPa).

2.5. Root–Soil Strength

The determination of the soil shear strength was conducted via a ZJ-type strain-controlled direct shear apparatus, incorporating vertical loads of 50, 100, 200, 300, and 400 kPa, in conjunction with a shear rate of 0.8 r/min. The shear strength of the soil (τf kPa) was calculated using Equation (4).

The soil shear strength is given by

$$\tau =\mathrm{c}+\sigma \tan \left(\theta \right)$$

(4)

$$\mathrm{c}={\mathrm{c}}_{\mathrm{s}}+\Delta \mathrm{s}$$

(5)

$${\tau }_{\mathrm{r}}\left(\mathrm{w}\mathrm{n}\right)={\tau }_{\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{d}}\left(\mathrm{w}\mathrm{n}\right)-{\tau }_{\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\text{-}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}\left(\mathrm{w}\mathrm{n}\right)$$

(6)

where c is the root–soil complex cohesion, σ is the vertical load (kPa), θ is the internal friction angle. The Wu and Waldron Model (WWM) is a pioneering analytical model used to assess the reinforcement effect of plant roots on soil shear strength [21]. In the WWM model, the quantification formula for Δs is the root cohesion, and c_s is the cohesion of the pure soil [22]. τ_r(wn) refers to the soil shear strength increment of the rooted soil, τ_rooted(wn) refers to the shear strength of the rooted soil under water content, and τ_root-free(wn) refers to the shear strength of the root-free soil under water content [23].

2.6. Statistical Analysis

Microsoft Excel 2021 was used for data collation and calculations; SPSS 26.0 was used for multifactor ANOVA and correlation analysis; Origin 2024 was employed for graphing.

3. Results

3.1. Response of Plant Root Growth to Varying Flooding Durations

3.1.1. Root Morphology

The response of various plant root morphology indicators to different durations of inundation was modeled via a quadratic function (y = ax² + bx + c) (Figure 2,

Table 1. Fitting equation for root morphology and flooding duration.

Species	Parameters	Equation	R2	Significance
Carex	Diameter	y = 0.62 + 4.45 × 10−4x − 7.5 × 10−7x2	0.74	<0.05
	Length	y = 686.79 + 8.28x − 4.02 × 10−2x2	0.83	<0.01
	Surface area	y = 140.99 + 1.69x − 8.17 × 10−3x2	0.94	<0.01
	Volume	y = 2.17 + 2.76 × 10−2x − 1.16 × 10−4x2	0.80	<0.01
Cynodon	Diameter	y = 0.64 + 1.90 × 10−4x − 2.51 × 10−7x2	0.58	>0.05
	Length	y = 735.79 + 3.55x − 1.88 × 10−2x2	0.82	<0.01
	Surface area	y = 149.00 + 0.47x − 2.45 × 10−3x2	0.77	<0.01
	Volume	y = 2.42 + 4.38 × 10−3x − 1.67 × 10−5x2	0.95	<0.01
Eleusine	Diameter	y = 0.67 + 4.37 × 10−4x − 4.97 × 10−7x2	0.90	<0.01
	Length	y = 394.49 − 0.16x − 1.93 × 10−3x2	0.84	<0.01
	Surface area	y = 68.78 + 0.40x − 2.03 × 10−3x2	0.89	<0.01
	Volume	y = 1.14 + 1.44 × 10−3x − 1.20 × 10−5x2	0.95	<0.01

). The total root system length of Carex initially increased but then decreased as the inundation duration increased, whereas that of Cynodon exhibited a similar trend, and that of Eleusine consistently decreased. The total root lengths of Carex, Cynodon, and Eleusine reached their respective maximum values of 1113.52 cm, 903.44 cm, and 384.86 cm at 103, 94, and 41 days of inundation, respectively. The total surface area of the root systems of the three species tended to increase but then decreased with prolonged inundation. The total root surface areas of Carex, Cynodon, and Eleusine peaked at 103, 94, and 41 days of inundation, respectively. The maximum total root surface areas for Carex, Cynodon, and Eleusine were 229.29 cm², 171.16 cm², and 88.2 cm², respectively, which were achieved at 103, 95, and 97 days of flooding, respectively. The total volume of the root systems also tended to increase and then decrease with increasing flooding duration for all three plants, with maximum values reaching 119, 131, and 88 days of flooding for Carex, Cynodon, and Eleusine, measuring 3.94 cm³, 2.71 cm³, and 1.37 cm³, respectively. The mean diameter of the root systems of the three plants gradually increased with increasing flooding duration, reaching maximum values of 0.68 cm, 0.67 cm, and 0.92 cm at 180 days for Carex, Cynodon, and Eleusine, respectively. Notably, the range of all the root morphometric indices for Eleusine was significantly different from that of the other two plants, with a linear range of 0.6–0.9 cm for Eleusine, while Carex and Cynodon exhibited a linear range of 0.6–0.7 cm (Figure 2A). The extremely high values of the three morphological indicators, excluding root diameter, for Eleusine were lower than the extremely low values for Carex and Cynodon.

3.1.2. Root Diameter Percentages

There were significant differences (p < 0.05) in the root proportions of the three plants: Carex, Cynodon, and Eleusine. The effects of varying flooding durations on the D5 diameter classes of the root systems of each plant were particularly pronounced (Figure 3). As the duration of flooding increased, the percentage of D1 (<0.2 cm) for the three plants initially increased but then decreased, reaching maximum values of 57.08%, 14.62%, and 6.51% after 90 days, respectively. The percentage of Carex in D2 (0.2~0.3 cm) increased and then decreased with prolonged flooding, while Cynodon tended to decrease, followed by an increase. Eleusine displayed a trend similar to that of Carex. The three species reached maximum values of 8.43%, 21.75%, and 16.3% at 120, 150, and 90 days, respectively. In D3 (0.3~0.4 cm), varying the flooding duration did not significantly affect the percentages roots of Carex or Cynodon, which reached maximum values of 8.41% and 22.31% at 90 and 150 days, respectively. The percentage of the D3 diameter class of Eleusine tended to increase, followed by a decrease as the flooding duration increased, with maximum values of 8.41%, 22.31%, and 23.01% at 90, 150, and 90 days, respectively. In D4 (0.4~0.5 cm), the maximum percentages for the three plants were recorded at 60, 150, and 90 days, with values of 8.28%, 15.05%, and 19.6%, respectively. In D5 (>0.5 cm), the percentage of Carex decreased initially but then increased with prolonged flooding, whereas Cynodon displayed an increasing trend followed by a decrease. Eleusine, on the other hand, showed an overall decreasing trend followed by increasing trend after reaching a maximum value at 30 days of flooding. The three species reached their maximum values at 180, 90, and 30 days, with percentages of 26.98%, 31.71%, and 50.37%, respectively. Notably, at the time of sampling, the above-ground portion of the 120-day-old Eleusine sample had completely ceased shedding and no longer exhibited growth with increasing flooding duration.

3.2. Response of Various Plant Root Mechanical Properties to Different Flooding Durations

3.2.1. Maximum Tensile Force of the Root

In this study, we performed fitting analyses on experimental data to evaluate the tensile force of individual plant roots with different diameters under varying flooding durations. The results demonstrated that flooding conditions significantly impacted larger-diameter roots, and a positive correlation was found between root diameter and individual root tensile force. Specifically, the maximum tensile force of Carex roots generally exhibited a declining trend with increasing flooding duration, whereas small-diameter roots (0–0.2 cm) initially showed an increase followed by a subsequent decrease (Figure 4A). The maximum tensile force of Carex peaked at 90 days of flooding, after which the fitting significance decreased sharply at 120 days (p > 0.05,

Table 2. Fitting equation for tensile force and root diameter.

Flooding Durations (Days)	Carex			Cynodon			Eleusine
	Equation	R2	Significance	Equation	R2	Significance	Equation	R2	Significance
0	y = 12.87x1.11	0.96	<0.01	y = 13.23x1.40	0.97	<0.01	y = 9.75x0.99	0.97	<0.01
30	y = 12.22x1.01	0.93	<0.01	y = 15.95x1.54	0.93	<0.01	y = 9.39x0.88	0.96	<0.01
60	y = 10.38x1.12	0.93	<0.01	y = 13.14x1.26	0.89	<0.01	y = 8.74x0.69	0.70	<0.01
90	y = 10.25x0.81	0.93	<0.01	y = 10.97x0.90	0.47	>0.05	y = 6.17x0.27	0.04	>0.05
120	y = 7.84x0.80	0.25	>0.05	y = 8.73x0.90	0.86	<0.01	y = 5.27x0.64	0.35	>0.05
150	y = 8.58x0.91	0.86	<0.05	y = 5.88x0.66	0.70	<0.01	y = 4.23x0.65	0.77	<0.01
180	y = 4.9x0.70	0.61	<0.05	y = 4.88x0.75	0.75	<0.01	y = 6.32x1.13	0.54	<0.05

), indicating instability in root tensile properties during this period. Similarly, the maximum tensile force of Cynodon roots exhibited a downward trend with prolonged flooding, accompanied by a gradual decline in the force of small-diameter roots (Figure 4B). Notably, Cynodon reached its maximum tensile force at 90 days of flooding, coinciding with a significant decrease in fitting significance (p > 0.05). As shown in Figure 4C, Eleusine exhibited considerable fluctuations in tensile force under varying flooding durations; the tensile force of its small-diameter roots increased rapidly after 60 days of flooding and then sharply declined, while the overall root tensile force gradually decreased over time. It is important to note that after 60 days of flooding, the above-ground parts of Eleusine almost completely died and were entirely shed by 150 days. This physiological response might have contributed to the observed changes in its root tensile properties. In summary, both Carex and Cynodon attained optimal tensile force at 90 days under flooded conditions, whereas Eleusine displayed the highest tensile force in the absence of flooding stress.

3.2.2. Tensile Strength of the Root

Under flooding conditions, all three plant species demonstrated a negative correlation between root diameter and tensile strength. Specifically, for Carex (Figure 5A), tensile strength initially increased and subsequently decreased with prolonged flooding duration. Nevertheless, small-diameter roots showed a relatively stable tensile strength compared to larger roots and the overall mechanical stability remained relatively consistent, as evidenced by high fitting significance across most flooding durations. For Cynodon (Figure 5B), a similar overall trend was observed; however, the differences in tensile strength among roots of various diameters under different flooding durations were not significant. Notably, at 90 days of flooding; the fitting significance markedly decreased (R² = 0.32; p > 0.05), suggesting notable mechanical instability at this stage. Eleusine (Figure 5C) exhibited a trend more similar to that of Cynodon, with significantly lower fitting quality and significance at 90 days (R² = 0.32; p > 0.05,

Table 3. Fitting equation for tensile strength and root diameter.

Flooding Durations (Days)	Carex			Cynodon			Eleusine
	Equation	R2	Significance	Equation	R2	Significance	Equation	R2	Significance
0	y = 10.76x−1.14	0.8	<0.01	y = 12.97x−0.72	0.81	<0.01	y = 12.42x−1.02	0.91	<0.01
30	y = 12.29x−1.05	0.98	<0.01	y = 19.99x−0.49	0.88	<0.01	y = 13.42x−0.97	0.91	<0.01
60	y = 10.79x−1.1	0.96	<0.01	y = 14.33x−1.1	0.94	<0.01	y = 19.37x−0.83	0.63	<0.05
90	y = 20.37x−0.89	0.87	<0.01	y = 15.73x−0.54	0.55	>0.05	y = 16.52x−1.07	0.32	>0.05
120	y = 16.57x−0.92	0.99	<0.01	y = 10.17x−1.12	0.90	<0.01	y = 7.98x−1.12	0.81	<0.01
150	y = 4.68x−1.67	0.9	<0.01	y = 11.04x−0.93	0.70	<0.01	y = 6.44x−1.62	0.95	<0.01
180	y = 2.87x−1.81	0.98	<0.01	y = 7.47x−1.1	0.96	<0.01	y = 8.1x−0.9	0.71	<0.01

). For both species, however, the negative correlation between root diameter and tensile strength remained robust across other flooding intervals. Overall; while Carex and Cynodon exhibited peak tensile strengths at intermediate flooding durations (90 days); Carex demonstrated superior mechanical stability under flooding stress compared to the other two species.

3.2.3. Elastic Modulus of the Root

The duration of flooding stress significantly influenced the mechanical characteristics of plant roots, and species-specific responses were observed in terms of elastic modulus. Specifically, Carex (Figure 6A) exhibited a notably higher elastic modulus values in smaller-diameter roots (<0.2 cm), decreasing gradually with prolonged flooding durations. Its elastic modulus sensitivity to diameter changes was enhanced in short- to medium-term flooding (as indicated by increased R² values compared to non-flooded conditions;

Table 4. Fitting equation for elastic modulus and root diameter.

Flooding Durations (Days)	Carex			Cynodon			Eleusine
	Equation	R2	Significance	Equation	R2	Significance	Equation	R2	Significance
0	y = 252.5x−0.69	0.62	<0.05	y = 348.3x−0.41	0.71	<0.01	y = 506.7x−0.33	0.84	<0.01
30	y = 219.6x−0.94	0.95	<0.01	y = 282.7x−0.34	0.60	<0.05	y = 276.3x−0.61	0.76	<0.01
60	y = 197x−1.03	0.93	<0.01	y = 270.15x−0.99	0.76	<0.01	y = 205.5x−0.68	0.45	>0.05
90	y = 239.9x−1.06	0.89	<0.01	y = 554.9x−0.99	0.24	>0.05	y = 331.4x−1.22	0.68	<0.01
120	y = 434.6x−0.63	0.64	<0.05	y = 241.8x−0.99	0.47	>0.05	y = 177.4x−1.65	0.92	<0.01
150	y = 489.1x−1.88	0.98	<0.01	y = 415.8x−0.59	0.43	>0.05	y = 289.6x−1.22	0.77	<0.01
180	y = 149.9x−1.41	0.72	<0.01	y = 382.5x−0.83	0.71	<0.01	y = 217.6x−1.21	0.81	<0.01

). The elastic modulus of Cynodon roots (Figure 6B) also decreased significantly with increasing diameter, yet exhibited a notable decline in fitting quality at 90 days of flooding (R² = 0.24, p > 0.05; Table 4), further indicating mechanical instability during this period. Similarly, Eleusine (Figure 6C) displayed a negative correlation between diameter and elastic modulus, yet its response to flooding differed somewhat. The significance of the fitting equation decreased notably at 60 days of flooding (p > 0.05), confirming potential structural adaptation or damage during early flooding stages. However, following prolonged flooding (150–180 days), the fitting accuracy and significance rebounded markedly (R² > 0.77, p < 0.01), indicating the gradual stabilization of root mechanical properties under long-term flooding stress.

3.3. Root–Soil Strength Test

3.3.1. Shear Strength and Increment

This study identified clear interspecific differences in the responses of plant root–soil composites to varying durations of flooding stress. The shear strength of the root–soil composite varied distinctly among species (Figure 7A,

Table 5. Fitting equation for soil shear strength and flooding duration.

Species	Parameters	Equation	R2	Significance
Carex	Shear strength	y = 49.43 + 7.2 × 10−2x − 4.38 × 10−4x2	0.82	<0.05
	Strength increment	y = 27.84 + 2.86 × 10−3x − 2.09 × 10−5x2	0.97	<0.01
Cynodon	Shear strength	y = 52.80 − 5.01 × 10−3x − 1.87 × 10−4x2	0.98	<0.01
	Strength increment	y = 15.49 + 0.18x − 8.63 × 10−4x2	0.84	<0.01
Eleusine	Shear strength	y = 52.50 − 0.27x + 6.79 × 10−4x2	0.96	<0.01
	Strength increment	y = 15.16 − 0.07x − 3.78 × 10−5x2	0.74	<0.05
CK	Shear strength	y = 37.31 − 0.19 × 10−2x + 6.78 × 10−4x2	0.96	<0.01

). Carex exhibited an initial increase in shear strength during the early flooding period, followed by a gradual decrease after 120 days. Despite the reduction under prolonged flooding, the overall decline in shear strength was moderate. In contrast, Cynodon showed the highest initial shear strength within 60 days of flooding; however, unlike Carex, its shear strength consistently declined throughout the entire flooding period. The decrease in shear strength was steady and gradual, resulting in a total reduction of 19.27% after 180 days. Eleusine demonstrated the most dramatic decrease in shear strength, dropping significantly after 30 days of flooding and eventually approaching values similar to the CK group.

In a flooded environment, the presence of vegetation can significantly enhance the soil’s shear strength. During the initial stages of flooding, all three types of plants can effectively contribute to an increase in soil strength. Specifically, the effects of Carex and Zoysia on the improvement of soil shear strength initially rise with the duration of flooding, followed by a decline; in contrast, Paspalum exhibits a gradually decreasing trend after the initial flooding period. After 120 days, 90 days, and 30 days of observation, Carex, Zoysia, and Paspalum achieved peak values of soil shear strength increase, measuring 27.59, 24.74, and 17.78 kPa, respectively (Figure 7B).

3.3.2. Friction Angle and Cohesion

The internal friction angle of soils under all treatments exhibited a decreasing trend with prolonged flooding duration (Figure 8A). Notably, Carex maintained the highest values throughout the experimental period, although it exhibited fluctuations rather than a steady decline. In comparison, Cynodon and Eleusine showed a more consistent and clear reduction over time, yet both remained consistently higher than the control group. Cohesion showed distinct species-specific responses to flooding duration (Figure 8B). Carex initially experienced an increase followed by a subsequent decline, demonstrating an adaptive capacity. Cynodon exhibited a steady, gradual decline throughout the flooding duration, maintaining relatively high cohesion values. Eleusine displayed the most rapid decline in cohesion after 30 days of flooding, with cohesion values eventually approaching those of CK. Overall (

Table 6. Fitting equation for soil strength indicators and flooding duration.

Species	Parameters	Equation	R2	Significance
Carex	Friction angle	y = 27.84 + 2.86 × 10−3x − 2.09 × 10−5x2	0.74	<0.05
	Cohesion	y = 23.08 + 6.92 × 10−2x − 42 × 10−4x2	0.83	<0.01
Cynodon	Friction angle	y = 23.54 − 7.77 × 10−3x + 1.28 × 10−5x2	0.92	<0.01
	Cohesion	y = 31.02 − 1.73 × 10−3x − 2 × 10−4x2	0.97	<0.01
Eleusine	Friction angle	y = 22.39 − 2.22 × 10−2x + 7.87 × 10−5x2	0.96	<0.01
	Cohesion	y = 32.04 + 0.25x + 5.99 × 10−4x2	0.94	<0.01
CK	Friction angle	y = 20.36 − 1.22 × 10−2x + 1.92 × 10−5 x2	0.97	<0.01
	Cohesion	y = 18.76 − 0.18x + 6.57 × 10−4x2	0.95	<0.01

), Carex showed greater stability and resilience under flooding conditions, while Eleusine demonstrated marked vulnerability under prolonged flooding stress.

4. Discussion

4.1. Varying Flooding Durations Affect Root Morphology

Under flooded conditions, plants make necessary adjustments to their growth characteristics and root morphology to mitigate the stress caused by flooding [24]. Different plant species exhibit varying response strategies to flooding stress. Although all three plants belong to the ‘static type’, which is better adapted to flooded environments—reducing their energy consumption while enhancing their adaptability to flooding stress without significant morphological changes [25,26], their root systems do not respond uniformly to flooding stress. Specifically, Carex and Cynodon reached their growth inflection points after 120 days of exposure, while Eleusine reached its inflection point after 60 days. There was a further increase in the proportion of smaller-diameter-class roots for Carex, along with an increase in the proportion of Cynodon roots greater than 0.5 cm in diameter. This suggests that both species experienced accelerated root growth and increased belowground biomass allocation under flooded conditions [27,28]. The two species differed in their growth patterns and root system distribution, with Carex producing more adventitious roots by promoting the growth of small-diameter-class root systems. With the exception of Carex, the small-diameter-class root system gradually dominated as a percentage of the total root system as the duration of flooding increased, while the opposite trend was observed for Carex. The total tensile strength and elastic modulus of the individual roots of the same plants increased with prolonged flooding duration; however, those of Eleusine gradually decreased, reaching a maximum value at 60 days. This trend in root system growth was also observed in Carex. The observed growth trend is responsible for the increase in the mechanical properties of individual roots within large-diameter root systems at the onset of flooding [29]. We speculate that the root systems of Carex and Cynodon were able to maintain some structural integrity and functionality to support plant growth and nutrient uptake in a flooded environment. In contrast, Eleusine exhibited a markedly different response strategy, showing an adaptive response within the first 30 days of flooding by eliminating small-diameter-class roots and further consolidating large-diameter-class roots. However, this adaptation was short-lived, and after 90 days of flooding, there was a significant decrease in activity and a substantial shedding of small-diameter roots.

4.2. Root Diameter and Varying Flooding Durations Affect Root Strength

The results of the root mechanics experiments indicate that variations in flooding duration lead to significant differences in the fitting equations between the root diameter and root mechanical properties. This may be attributed to the distinct ways in which the root diameter and root mechanical properties influence mechanical characteristics, as well as the differing overall impacts of the two on various mechanical indicators. However, the influence on diameter is not uniformly advantageous. Following exposure to a flooding environment, the root strength of sedge significantly decreased, while that of bermudagrass increased. The flooding environment directly influences the strength of the relationship between the root diameter and the mechanical properties of the root system. For Carex, this significance gradually diminishes with an increase in flooding duration. In contrast, both Cynodon and Eleusine exhibit a trend that initially increases before subsequently decreasing.

Plant root systems exhibit distinct behaviors in soil environments with varying water contents, and there is a correlation between the amount of water lost by the root system and the increase in root strength relative to root diameter [30]. One factor contributing to the high variability in the strength of small-diameter root systems is the water status of the root system, which may be influenced by the relatively large evaporative surface area associated with relatively small root diameters [31]. Under prolonged flooding, the tensile strength of the large-diameter root system of Carex gradually decreased, in the context of roots with a small diameter it is interesting the fact that at the smallest root diameters, grass roots have the highest tensile strength and shrubs the lowest [32]. The tensile strengths of grass and tree species tend to converge at root diameters above 5 mm. In contrast, Cynodon initially strengthened the large-diameter root system during the first 60 days of flooding before shifting its focus to enhance the small-diameter root system. Eleusine achieved optimal root system tensile strength after 60 days of flooding, particularly in the medium-diameter root system, which has a diameter range of 0.3–0.6 cm. The changes in root system elongation of Carex and Cynodon under flooding were more complex, with different species and varying flooding conditions leading to distinct responses for each [33,34]. Different durations of flooding significantly affected the variation in single root elongation in Eleusine. The results indicated that Eleusine was able to rapidly increase root elongation during the initial flooding period to penetrate deeper into the soil. However, over time, its root system may have undergone lignification, resulting in a significant decrease in elongation and loss of activity after 120 days of flooding. In the context of lignification, lignin acts as a matrix, increasing the mechanical strength of the cell wall, therefore cascading in the root systems and the soil–root interaction [35].

When combined with the study of changes in root morphology, it can be concluded that some smaller root systems with higher tensile strength may be limited by the larger root systems of the plant itself. The large-diameter roots, which were not well adapted to the flooded environment, became progressively softened by the water due to prolonged inundation, resulting in a decline in their strength. This decline may be attributed to the continuous weakening of the ability of the root system to evaporate water as the plant enters a persistently flooded environment. Additionally, gas exchange between the root system and the surrounding environment was hindered, leading to rapid oxygen consumption by both plants and microorganisms, which resulted in a hypoxic state in the root system. Under hypoxic conditions, aerobic respiration in the root system is inhibited. Plants can adapt to flooded and hypoxic conditions by developing aerenchyma tissues; however, this adaptation can interfere with the normal growth and development of the root system due to reduced energy metabolism, ultimately leading to changes in the structure of the root system and a further reduction in the size of the root system.

4.3. Root Strength Affects Root–Soil Strength

An increase in plant root strength not only enhances the root–soil complex but also improves soil cohesion. According to existing studies [36], plants can release specific substances into the soil, thereby modifying soil stabilization. The experimental findings suggest that in inundated conditions, vegetation can enhance the rate at which soil shear strength and cohesion decrease. Although their influence on the internal friction angle of the soil appears to be minimal, the internal friction angle may be more sensitive to the physical characteristics of the soil. These findings are still very important [37]. The findings of the six-month flooding study revealed that the shear strength of vegetated soils was significantly greater than that of unvegetated soils. Although the reduction in the shear strength of vegetated soils was greater than that of unvegetated soils, it decreased with an increasing duration of flooding. Carex was most effective at three to four months of soil consolidation, while Cynodon maintained a consistent performance throughout the entire period, and Eleusine was more suitable for short-term soil consolidation. Floodplain plants need to be flood-tolerant; however, once the water has receded, the ability of the plants to quickly recover growth indicators directly affects their survival in subsequent floods [38]. Under flood stress, Carex can maintain its leaf growth and ornamental properties in a flooded environment because of its good shade tolerance and strong self-seeding ability [39]. However, the relatively slow growth rate of Carex does not provide significant competitive advantages in the floodplain environment, especially compared with Cynodon and Eleusine [40]. Therefore, we consider Carex to be unsuitable for mixed planting with these two species. As a common slope protection plant and a dominant species in floodplains, most of the leaves of Cynodon rot and fall off during winter floods, affecting the aesthetics of the environment [41]. However, Cynodon is capable of rapid regrowth and can occupy the growing area once the water has receded [42], achieving high cover within a few months. Notably, while Eleusine is not suitable for prolonged flooding, its seeds are better adapted to the soil and germinate earlier than those of Cynodon [43,44]. Research has shown [45] that in mixed growth studies with Cynodon, Eleusine was able to enhance soil fixation and growth to a certain extent. Therefore, under short-term flooding conditions, a mixture of the two may further enhance flood tolerance.

Carex has excellent flood tolerance but a lengthy growth cycle, making it suitable for long-term ecological planning. In contrast, Cynodon possesses stable strength and a shorter growth cycle, making it more appropriate for a variety of protective applications. While Cynodon offers significant advantages in terms of root strength, its drawback is that it becomes unsightly when the water recedes. Conversely, Eleusine has distinct advantages in specific environments because of its remarkable growth rate and early maturity. During the ecological restoration process, appropriate plants can be selected at various elevations on the basis of the characteristics of different species [46]. This approach not only increase the effectiveness of plant restoration, but also improves soil shear strength and enhances the aesthetics of the affected area.

5. Conclusions

Short-term flooding promotes further growth of the plant root system, as evidenced by increases in root length, surface area, volume, and diameter. However, as the duration of flooding increased, the growth rate of the root system gradually slowed and eventually exhibited a decreasing trend with an increasing magnitude. As the duration of flooding increases, the proportion of Carex roots with a diameter greater than 0.5 cm initially decreases before subsequently increasing. Cynodon demonstrates a trend of first increasing and then decreasing, Eleusine also exhibits a pattern of initial decrease followed by an increase after reaching a peak in the early stages. These three plant species reached their extremes at 180 days, 90 days, and 30 days, respectively.

These three plant species exhibited significant negative correlations between root diameter and tensile strength under flooding conditions, but displayed distinctly species-specific patterns in their elastic modulus responses. The maximum tensile force and elastic modulus generally declined as flooding duration increased, while tensile strength initially increased before subsequently decreasing. Regarding the maximum tensile force, Carex and Cynodon both reached their optimal values at 90 days of flooding, whereas Eleusine peaked earlier, at 60 days. Tensile strength values for both Carex and Cynodon peaked at 90 days of flooding, while the peak tensile strength of Eleusine was observed under a shorter flooding duration (<60 days). These results highlight notable interspecific differences in root mechanical adaptability in response to prolonged flooding stress.

The root–soil composite shear strength of Carex exhibited an initial increasing trend during the early flooding stage but gradually decreased after 120 days. For Cynodon, shear strength consistently showed a stable and gradual decline throughout the entire flooding period. In contrast, Eleusine experienced a rapid reduction in shear strength shortly after flooding began (30 days), approaching levels comparable to the unvegetated control after 180 days. Both Carex and Cynodon maintained relatively high shear strengths even after prolonged flooding, whereas Eleusine demonstrated significant sensitivity and vulnerability under extended flooding stress. Clear interspecific differences were observed among the three plant species regarding their responses of root–soil composite shear strength, internal friction angle, and cohesion to prolonged flooding stress. In the simulated flooding environment of the Yangtze River Basin, the root–soil composite shear strength of Carex exhibited an initial increasing trend during the early flooding stage but gradually decreased after 120 days. For Cynodon, shear strength consistently showed a stable and gradual decline throughout the entire flooding period. In contrast, Eleusine experienced a rapid reduction in shear strength shortly after flooding began (30 days), approaching levels comparable to the unvegetated control after 180 days. Both Carex and Cynodon maintained relatively high shear strengths even after prolonged flooding, whereas Eleusine demonstrated significant sensitivity and vulnerability under extended flooding stress. Carex is optimally applied in the restoration and protection of areas subjected to three to four months of flooding, with its ornamental value being particularly pronounced. Cynodon performs best in areas with up to six months of flooding, Eleusine is especially effective in regions with less than two months of flooding.