Herbaceous Vegetation in Slope Stabilization: A Comparative Review of Mechanisms, Advantages, and Practical Applications

Abstract

Shallow slope instability poses a significant ecological threat, often leading to severe environmental degradation. While vegetation, particularly woody plants, is commonly employed in slope stabilization, herbaceous vegetation offers distinct and underexplored advantages. This paper reviews the role of herbaceous plants in enhancing slope stability, analyzing their mechanical and ecological mechanisms. Through an extensive review of the literature, this review challenges the prevailing view that woody vegetation is superior for slope stabilization, finding that herbaceous plants can be equally or more effective under certain conditions. The key findings include the identification of specific root parameters and species that contribute to soil reinforcement and erosion control. The review highlights the need for further research on optimizing plant species selection and management practices to maximize the slope stabilization effects. These insights have practical implications for ecological slope engineering, offering guidance on integrating herbaceous vegetation into sustainable land management strategies.

1. Introduction

Due to the concealed, cumulative, and long-term nature of shallow slope failures, this significant ecological issue is often overlooked. The factors affecting shallow slope failure mainly include terrain conditions, climate conditions, and human activities. According to the Food and Agriculture Organization of the United Nations (FAO) [1], sustainable land management strategies must fully consider the control of shallow slope failure. Sustainable soil and land use is pivotal for many Sustainable Development Goals [2].

Shallow slope failure is a common cause of slope soil erosion [3,4,5], characterized by the stripping away of surface vegetation and loose topsoil (or substitute materials). It primarily manifests as shallow landslides and shallow erosion:

1. Shallow landslides occur when the gravitational force on the slope exceeds its maximum internal resistance, typically triggered by increased pore water pressure in the topsoil and decreased soil cohesion [6], often occurring on weak sliding layers within the soil [7].

2. Shallow erosion is usually caused by disturbances such as snow movement [8], animal trampling [9], or human activities [10]. Excessive disturbance leads to vegetation degradation and surface tensile cracks, allowing rainwater or meltwater infiltration to create a sliding surface, resulting in surface vegetation damage and topsoil loss [11].

Shallow slope failure can lead to the loss of water and nutrients from agricultural land [12], degradation of aquatic ecosystem quality [13], damage to transportation infrastructure [14], and can even trigger geological disasters like landslides and mudslides [15,16]. Once the natural ecological environment is damaged, recovery is challenging and often takes decades if relying solely on natural restoration [4], with a high risk of secondary damage during this period [17]. Artificial restoration measures are costly and may not achieve the desired results, making the effective prevention and control of shallow slope failure particularly crucial.

The research indicates that appropriate ecological slope engineering prevention measures are more effective than post-disaster remediation [18]. Plants reinforce slopes by working together under harmful loads and coordinating deformation, effectively preventing shallow slope failure. The management of different plant configurations directly influences the probability of such failures [19]. Therefore, ecological slope engineering not only maintains ecosystem stability but is also a common method for preventing various landslides, significantly enhancing slope stability and inhibiting soil erosion [20].

However, ecological slope engineering measures can sometimes have counterproductive effects. On steep slopes with high humidity, dense root layers of herbaceous plants can promote local infiltration of precipitation, saturating and softening the surface root–soil layer. This can cause the slope to slide under gravity, leading to “peeling”-type shallow landslides [21].

In recent years, the research on ecological slope engineering has mainly focused on the slope stabilization mechanisms of shrubs and trees, achieving significant theoretical and practical progress [22]. In contrast, there has been less research on the mechanisms through which herbaceous plants enhance slope stability. Further exploration is needed to understand the mechanisms and dynamics of herbaceous plants in slope stabilization [23]. Compared to woody plants, herbaceous plants have higher species density and diversity, shorter growth cycles, and faster succession rates in the same growth habitat.

Conducting in-depth research on the slope stabilization effects of herbaceous plants is crucial for preventing shallow slope failures. This paper reviews the relevant research on the role of herbaceous plants in enhancing slope stability, compares them with woody plants, and explores the slope stabilization process and future research directions from both engineering and ecological perspectives.

2. Current Research on the Role of Herbaceous Plants in Slope Stability

Slope stability refers to the ability of a slope’s rock and soil to remain stable under certain height and gradient conditions, reflecting its mechanical resistance to failure [7]. When the gravitational force on a slope exceeds its maximum internal resistance, shallow landslides [24] or shallow erosion [25] may occur. Slope stability is mainly influenced by topographic features, hydrological conditions, soil physical and chemical properties, and vegetation parameters. Herbaceous plants improve slope stability by enhancing the hydrological conditions through ecological processes and reinforcing the mechanical strength through their root systems, making them one of the most effective ecological engineering management measures for slope stabilization.

2.1. Enhancing Topsoil Resistance

2.1.1. Mechanical Reinforcement

Herbaceous plants significantly contribute to the mechanical reinforcement of topsoil through their root systems. Their dense and fibrous root networks bind the soil particles together, increasing soil cohesion and enhancing resistance to erosion and shallow landslides [26]. The root–soil interaction can be understood through several key mechanisms:

(1) The roots of herbaceous plants penetrate the soil, creating a network that anchors the topsoil and reduces its mobility. This anchoring effect is particularly effective in stabilizing the upper soil layers where the majority of herbaceous roots are concentrated. Studies have shown that the root tensile strength and root density play critical roles in this process [27].

(2) The presence of roots increases the shear strength of the soil. This is because the roots act as natural reinforcements, distributing stress and reducing the likelihood of shear failure. Root-reinforced soils can withstand higher loads before failing compared to non-reinforced soils. The research indicates that the increase in shear strength is proportional to root density and root diameter [28,29].

(3) Roots exude organic compounds that enhance soil aggregate stability by promoting the formation of soil aggregates [30]. These aggregates improve soil structure, making it more resistant to erosion and compaction. Enhanced soil structure also facilitates better water infiltration and retention, further stabilizing the slope.

(4) The interaction between roots and soil creates a composite material with improved mechanical properties. This root–soil composite adheres to the principles of soil mechanics, enhancing cohesion and internal friction. The effectiveness of this composite material is influenced by factors such as root architecture, soil type, and moisture content [31,32,33].

2.1.2. Reducing Shallow Landslides

Shallow landslides are a major cause of slope instability, and herbaceous plants can effectively reduce their occurrence. From a mechanical perspective, herbaceous plant roots undergo non-elastic deformation under stress, allowing for the establishment of non-rigid root–soil reinforcement models [34]. Herbaceous plant roots develop dense and sparse root layers (Figure 1), with the root reinforcement enhancing the shear strength of root-containing soil [27], thereby improving mechanical slope stability [28] and inhibiting shallow landslides [31,35]. Besides vertical root reinforcement, horizontal roots, stems, and other interwoven plant fibers create a root–soil composite through the surface-mat effect [23]. This composite adheres to the Mohr–Coulomb law, altering the soil’s mechanical properties and providing additional cohesion [31,32]. Even minimal root penetration across potential shear planes can link surface soil with deeper layers, transferring localized shear stress from potential failure points to stable areas, thus enhancing the overall slope stability [23].

In Figure 1, the arrows represent the slope’s gravitational force along the slope surface, and the red dashed line indicates the potential failure surface. Area I is the stable slope zone, while Areas II–VI are potential shallow landslide zones. In Areas II and VI, extensive root penetration forms a stable root–soil composite, with root reinforcement and the surface-mat effect counteracting the landslide forces. In Areas III–V, fewer roots penetrate potential shear planes, reducing root reinforcement effectiveness, but the surface-mat effect continues to transfer some shear stress to more stable areas like Areas I, II, and VI.

In slope stability modeling, the limit equilibrium theory is typically used to calculate the safety factor. Most studies do not consider the effect of plant age on root tensile strength. Additionally, the existing methods often overlook factors such as root decay and root moisture content. Furthermore, many studies base additional root reinforcement measurements on point measurements or field shear tests without considering the spatial variability in root strength.

2.1.3. Mitigating Shallow Erosion

Numerous studies have indicated that surface runoff erosion [36], shallow subsurface flow erosion [37], snow avalanche erosion [38,39], and freeze–thaw erosion [40] are major factors contributing to shallow slope erosion. Intense rainfall during the rainy season increases the slope’s weight while reducing the soil strength. Rainwater runoff forms temporary surface runoff and internal seepage, which are primary causes of shallow slope erosion. Shallow erosion typically progresses from sheet erosion to rill erosion to shallow gully erosion, culminating in gully erosion [41]. Dense herbaceous plants retain rainfall, reduce raindrop kinetic energy, slow down surface runoff, redistribute erosive energy, and enhance soil erosion resistance [31,42].

In cold regions like northern areas and the Qinghai–Tibet Plateau, slopes face additional challenges from snow avalanches erosion and freeze–thaw erosion besides runoff erosion during the rainy season [43]. Heavy snow accumulations tend to slide down slopes under gravity [44], and snow slips or avalanches can easily cause shallow surface erosion [45]. Freeze–thaw erosion usually occurs in early spring when temperatures approach the melting point. Surface soil undergoes repeated freeze–thaw cycles, forming unstable melted layers. Snowmelt infiltration reduces soil cohesion and pulverizes the surface layer, lowering erosion resistance. The saturated, pulverized surface soil moves slowly downslope as a plastic meltwater flow [46,47]. Planting herbaceous plants on slopes can prevent direct contact between snow and the surface soil, increase surface roughness and sliding resistance, and reduce freeze–thaw damage, thereby mitigating shallow erosion.

2.2. Optimizing Hydrological Conditions

Changes in slope hydrological conditions, such as soil infiltration rates and moisture content, are major factors inducing shallow slope failures [48,49]. Herbaceous plants influence slope stability by altering the hydrological conditions in their coverage area through the following processes:

(1) Herbaceous plants increase surface roughness, indirectly enhancing soil infiltration and water retention. Rainfall or snowmelt infiltration raises the soil moisture content, increasing the soil weight, pore water pressure, and decreasing soil cohesion, ultimately reducing shear strength and slope stability [50].

(2) In regions with abundant rainfall, plants absorb soil water through root pressure and leaf transpiration, reducing the soil moisture content and thereby improving slope stability [48,51,52].

Optimizing slope hydrological conditions by controlling soil infiltration rates and maintaining a lower soil moisture content is an effective measure for sustaining slope stability [50]. Herbaceous slopes have different characteristics in altering hydrological conditions compared to shrub-covered or bare slopes:

(1) Dense herbaceous stems and leaves can retain some rainfall, weakening the erosive impact of raindrop splash and reducing soil aggregate breakdown [53]. Additionally, the high evapotranspiration capacity of dense herbaceous plants lowers the soil moisture content [51].

(2) The dense stems and leaves of herbaceous plants increase surface roughness, slowing down surface runoff and subsurface flow, thereby extending the water infiltration time and enhancing the soil’s water retention capacity [54].

Herbaceous plants also have varied effects on soil crack development. They can restrict the formation and expansion of shrinkage cracks, enhancing slope stability. In areas with a high clay content, drought or plant water uptake can cause surface shrinkage cracks, allowing for rapid water infiltration during subsequent rains and threatening the slope stability [55]. Studies have shown that drought-induced shrinkage cracks appear more frequently on sparsely vegetated or bare slopes than on densely vegetated slopes, although the latter’s cracks last longer [56]. Additionally, plant roots can accelerate rock and soil weathering through biophysical processes, while decaying roots create root–soil gaps, increasing soil porosity and forming preferential flow paths [57]. This enhances the soil’s permeability and moisture content, negatively affecting slope stability. Therefore, the impact of herbaceous plants on soil permeability should be considered in relation to the soil’s physical and chemical properties, plant species composition, slope geomorphology, and local climate characteristics.

In summary, plants affect slope stability by altering hydrological conditions, with both positive and negative impacts. The overall influence on slope stability is affected by the climate, soil physical and chemical properties, topsoil thickness, microtopography, soil biota [58], as well as ecological factors such as the species composition, plant configuration, and plant structure [59].

2.3. Advantages of Herbaceous Plants in Slope Stabilization

2.3.1. Enhanced Surface Protection

Compared to herbaceous plants, woody plants have longer lifecycles and greater seasonal stability, making them generally more suitable for slope stabilization [50]. Studies have shown that herbaceous plants are more effective in preventing sheet erosion and rill erosion, while woody plants are better at preventing large-scale slope failures [60]. Slopes covered with forests are less likely to experience landslides and tend to have steeper slope angles than those covered with grass [61,62]. However, comparing the two is challenging due to the difficulty in isolating the contribution of understory herbaceous plants and accounting for varying growth habitat properties such as terrain, soil quality, and the co-evolution of plants and landforms [4,61]. Additionally, shallow failures on forested slopes are often more concealed and thus underestimated, complicating objective comparisons [62].

Slope stability is influenced by root density and length. Woody plants only exhibit their anchoring advantages when their thick roots penetrate potential shear planes [63]. Under excessive sliding stress, these thick roots can detach from the surrounding soil, limiting their anchoring effectiveness. Moreover, the large aboveground biomass of woody plants can negatively impact slopes, as strong winds or storms may uproot them, causing additional slope damage [19]. The quality of the forest also significantly affects stabilization effectiveness; forests with large gaps, low underground biomass, and poor health are less effective at stabilizing slopes [50].

2.3.2. Rapid Establishment and Soil Improvement

Unlike the sparse, thick roots of woody plants, herbaceous plants have dense, fine root systems concentrated in the top soil layers (Figure 2), which are crucial for overall slope stability. These shallow root systems form root–soil composites that significantly enhance the soil’s shear strength and provide a stable habitat for soil microorganisms, promoting soil aggregate stability and positive ecological cycles [64]. In ecological slope engineering, the rapid establishment of vegetation is critical for quick root–soil interface fusion [65]. Herbaceous plants, with their fast growth and high density, are advantageous for rapid slope stabilization [66]. The community stability and biodiversity of herbaceous plants also play a significant role in slope reinforcement. Degraded grasslands can easily lead to sheet erosion, rill erosion, snowmelt erosion, and freeze–thaw erosion [17,23].

Herbaceous plants play a significant role in slope stability through erosion resistance and interception of debris or rockfall. Vegetation intercepts erosion in three ways: (i) by protecting soil particles from raindrop impact; (ii) reducing runoff volume by increasing surface infiltration via root systems; and (iii) decreasing sediment transportation through entrapment. A comparison of the erosion-resisting potential of roots and canopy cover can be seen in Figure 2B, with roots showing a higher potential against rill and gully erosion, while herbal leaves growth is more effective in mitigating splash and inter-rill erosion.

2.3.3. Hydrological Regulation

Soil loss due to erosion is generally predicted using the Revised Universal Soil Loss Equation (RUSLE). Erosion rates are typically expressed using Equation (1), with smaller erosion rates indicating higher erosion resistance.

$$A=R·K·LS·C·P$$

(1)

where A is the annual soil loss due to erosion [t/ha year]; R is the rainfall erosivity factor; K is the soil erodibility factor; LS is the topographic factor derived from the slope length and slope gradient; C is the cover and management factor; and P is the erosion control practice factor.

Research has shown the effects of grass roots on surface erosion, finding that as root mass density increases, the erodibility coefficient decreases, reducing surface erosion [67,68].

The applicability of slope stabilization strategies involving herbaceous and woody plants is highly dependent on the specific climatic and geographic conditions of the region. In humid climates, the rapid growth and high density of herbaceous plants can effectively prevent surface erosion, but their shallow root systems may lead to increased surface water infiltration, potentially destabilizing the slope under heavy rainfall. Conversely, in dry, arid regions, woody plants with deeper roots can access subsurface water, maintaining their stabilizing functions even during prolonged droughts. Therefore, the choice between herbaceous and woody vegetation—or a combination of both—must be tailored to the local environmental conditions. Moreover, the geographic context, including soil type and topography, further dictates the suitability of these plants for preventing shallow landslides and soil erosion.

In summary, the effectiveness of woody and herbaceous plants in slope stabilization cannot be simply compared. The stabilization effects vary greatly under different conditions and depend on factors such as the habitat conditions, species characteristics, hydrological conditions, and soil resistance [49,69]. Practical considerations, including the operability, cost-effectiveness, and management practices of ecological engineering, also play crucial roles (

Table 1. Comparison summary of woody and herbaceous vegetation for mechanical soil reinforcement.

	Woody Plants	Herbaceous Plants	Comparative Results
Mechanical Reinforcement	-Sparse taproot systems	-Dense fibrous root systems	-Different reinforcement mechanisms and significant interspecies differences -Woody root systems excel in anchoring when penetrating potential shear planes -Dense fibrous root systems provide better reinforcement than sparse coarse roots -Appropriate species should be selected based on the slope habitat
	-Most roots penetrate deeply	-Most roots penetrate shallowly
	-High tensile strength of single roots	-High tensile strength of interwoven fibrous tissues
	-Anchoring effect	-Root reinforcement and surface-mat effect
	-Prone to slippage	-Effective in shallow soil reinforcement
Optimization of Hydrological Conditions	-High individual plant transpiration capacity	-High overall transpiration capacity	-Woody plants generally outperform herbaceous plants -Significant interspecies differences
	-Root–soil gaps increase soil permeability	-High surface roughness increases soil permeability
Impact on Aboveground Biomass	-Heavy weight increases slope load	-Light weight	-Significant interspecies differences
	-Causes slope damage under strong winds	-Strong wind resistance
	-Retains rainfall	-Retains rainfall and reduces raindrop splash erosion
		-Prevents direct contact between snow and surface soil
Operability	-Low density	-High density	-Scientific management aids plant establishment -Reliability influenced by species and habitat differences -Quality of woodland or grassland affects slope stabilization effectiveness
	-Slow to take effect	-Quick to take effect
	-Low seasonal variability	-High seasonal variability
	-Slow response to management changes	-Quick response to management changes
Economic Benefits	-Low maintenance requirements	-Regular management (e.g., mowing)	-Scientific management helps improve economic benefits
	-Timber	-Feed or biogas material
	-Economic forests	-Medicinal herbs

). Ecological slope engineering should select appropriate plants for reinforcement based on the specific growth habitat.

3. Current Research on Herbaceous Plant Root Parameters

Plant root parameters are the most crucial factors affecting the slope reinforcement effectiveness. The measurement methods include indoor direct shear tests, field direct shear tests, root pull-out tests, permeameter methods, and auger drilling methods. Full-scale field monitoring is often expensive and time-consuming, so geotechnical centrifuge tests have been used in the past decade to study the impact of vegetation on slope mechanical strength. Roots can be simulated using live plants or root analogs, with the former mimicking the mechanical effects but are difficult to replicate and time-consuming to grow, while the latter offers high reproducibility, accurate root structures, and rapid production. Recently, dynamic finite element analysis validated by centrifuge test data has also made progress in studying the seismic performance of root-reinforced slopes. This section will explain the slope reinforcement effects of herbaceous plant roots from the aspects of root distribution characteristics, geometry, and tensile strength [34].

3.1. Root Distribution Characteristics

The root distribution characteristics include Root Length Density (RLD), Underground Biomass (UB), Root Area Ratio (RAR), and Rooting Depth (RD), which are key indicators for evaluating plant reinforcement effects on slopes.

(1) Root Length Density (RLD), defined as the total root length per unit volume of soil (m·m⁻³), serves as an indicator of root quantity and distribution, and is thus used as a measure of slope stability. However, due to the difficulty in measuring RLD, underground biomass is often used as a substitute indicator.

(2) Underground Biomass (UB) refers to the total root dry weight per unit volume of soil (kg·m⁻³). Aboveground biomass can be used to estimate the root biomass to some extent [65,70]. However, it is important to note that the root biomass metric does not distinguish between different root thickness classes and gradations. Given the same root biomass, a dense network of fine roots provides stronger soil reinforcement than sparse coarse roots [71].

(3) The root cross-sectional area ratio is defined as the proportion of the root cross-sectional area to the total area (m²·m⁻²), and it correlates with the tensile strength of the roots [72]. Similarly, the root area ratio fails to distinguish between different root thickness classes and gradations, making it challenging to quantify the roots with very small diameters (Φ < 0.25 mm). Consequently, it tends to underestimate the reinforcement capability of grasslands, which possess a substantial number of fine roots, in slope stabilization.

(4) Rooting Depth (RD) determines the range of reinforcement provided by plants [19,73]. The enhancement of soil shear strength and cohesion by herbaceous plants mainly occurs in the top 10–20 cm where their roots are concentrated [74,75,76]. Rooting depth is influenced by the species type, planting density, and soil physical properties. A high planting density induces competition for nutrients and water, promoting vertical root development [74]. However, hard soil layers or shallow bedrock can limit root growth, creating potential sliding surfaces between the bedrock and soil, increasing the slope failure risk [77,78].

The research on root classification methods has not yet established a unified standard, and field measurements of root distribution characteristics are challenging. There is a need to develop an effective research system to quantify the stabilizing effects of plant roots on slopes, providing adaptive management strategies for ecological slope engineering [79,80].

3.2. Root Geometry

Root geometry characteristics include the diameter–length ratio, curvature, and spatial distribution, which directly affect the soil shear strength and the effectiveness of root interweaving during rainfall [81].

Herbaceous plants can be categorized into taproot and fibrous root systems. Taproot systems have a high biomass but low RLD, while fibrous root systems are the opposite. Due to the high RLD of fibrous roots, they are generally considered more effective in slope reinforcement [82,83]. However, studies indicate that in biodiversity-rich areas, slopes dominated by taproot plants can also provide strong reinforcement [23]. Thus, the reinforcement effect of roots on slopes depends more on the plant types than root types.

The slope reinforcement effect also varies significantly between different root growth stages and parts [84]. Horizontal and lateral roots sometimes offer better reinforcement than vertical roots [70]. The mechanical interactions between roots and soil [85] and the enhancement of soil anti-disintegration properties [86] also vary significantly.

3.3. Root Tensile Strength

Root tensile strength refers to the ability of roots to resist breakage under static tension, defined as the maximum load a root can bear divided by its cross-sectional area (N·mm⁻²). This is another crucial indicator for evaluating the mechanical reinforcement effect of plants. Measuring root tensile strength is relatively simple but time-consuming. During measurements, it is essential to ensure that fixing the roots to the measurement system does not alter their properties, and that the roots are evenly stressed to reduce measurement errors [87].

Research has found that the tensile strength of roots varies significantly across species, ranging from several kilopascals (kPa) to hundreds of megapascals (MPa) [88,89]. The factors affecting root tensile strength include the habitat [20,72], plant density [77], rooting depth [90], root part [72], growth periods [72,91], root moisture [92], and cross-sectional microstructure [93]. Generally, root tensile strength is inversely proportional to root diameter [94] and directly proportional to soil shear strength [28,29], indicating that dense fibrous roots are more effective in slope reinforcement [87,95]. Therefore, in quantifying the reinforcement effects of roots on slopes, both RAR and root diameter gradation distribution should be considered.

4. Guidelines for Ecological Slope Engineering with Herbaceous Plants

Understanding the mechanisms and advantages of herbaceous plants in enhancing slope stability is crucial for their practical application in ecological slope engineering. This interdisciplinary field combines principles from ecology and engineering to significantly improve slope stability. Engineering measures and subsequent adaptive management practices continuously influence the slope environment and plant conditions, thereby altering hydrological conditions and topsoil resistance, ultimately affecting slope stability and ecosystem services (see Figure 3).

4.1. Species Selection

Ecological slope engineering typically involves three stages: plant establishment, positive community succession, and long-term stability management. Species selection is critical in this process, as different herbaceous plants exhibit varying root system utilization of soil layers and adaptability to the soil habitat conditions (such as moisture content, pH, nutrients, and microbiology). A single species may not fully exploit the topsoil space due to its root parameter limitations and may struggle to adapt to diverse soil environments [90]. Interspecies competition can increase the overall root biomass, density, and tensile strength in an area, leading to better slope reinforcement [65,96].

Additionally, the root parameters of different herbaceous plants vary significantly in the same growth habitat [69,97], and even the same species can exhibit considerable root parameter differences in different growth habitats [92,98]. Therefore, species selection and combination should be tailored to the specific slope growth habitat and potential shear failure planes [90,99]. In non-arid regions, a mix of species with a high underground biomass and a combination of taproots and fibrous roots should be prioritized to achieve functional and structural root diversity. This approach enhances the ecological service value and ecosystem resilience while providing comprehensive slope protection [71,100]. In arid or semi-arid regions, water-intensive plants should be avoided to prevent rapid groundwater depletion, and the planting density should align with the area’s ecological carrying capacity.

For mine tailings, waste dumps, or other artificial slopes, the complex terrain, poor soil, and harsh environment often hinder the formation of stable root systems [101]. In the early stages of plant establishment, the focus should be on improving the soil habitat. Selecting fast-growing native herbaceous plants as pioneer species, combined with artificial management measures, can accelerate plant establishment and positive community succession rates, thereby enhancing and improving the soil habitat [102].

4.2. Engineering Practice

Building on the existing theoretical foundations, ecological slope engineering increasingly emphasizes the role of herbaceous plants in slope reinforcement, integrating technologies from agronomy, grassland science, ecology, and horticulture.

In practice, ecological slope engineering leverages the high biodiversity, species density, short root system development cycle, and rapid succession of herbaceous plants, prioritizing the use of local dominant species [90,103]. During the plant establishment phase, the optimal plant configuration involves sequential planting of pioneer, transitional, successional, and climax species to quickly restore stable communities [104]. Adaptive management strategies, such as moderate grazing or mowing during succession and later management stages, help prevent biodiversity loss and the over-proliferation of undesirable species [105,106,107].

Where possible, selected species should also offer additional service values, such as water purification, carbon sequestration, economic benefits (e.g., food, fodder, medicinal materials, or paper raw materials), and aesthetic appeal [108]. Overall, ecosystems established through current ecological slope engineering practices exhibit strong stability and resilience, maintaining multidimensional ecological service functions with minimal maintenance.

5. Limitations and Future Directions in Plant-Based Slope Stabilization Research

5.1. Limitations in Current Research on Plant-Based Slope Stabilization

While significant progress has been made in understanding the role of vegetation in slope stabilization, several limitations in the current research warrant further investigation. One major gap lies in the long-term effectiveness of various plant species under differing climatic conditions. Most studies focus on short-term observations, often overlooking how factors such as seasonal variations, extreme weather events, and long-term climate changes may impact the root systems and overall stability provided by these plants. For example, the effectiveness of herbaceous plants in reducing soil erosion may diminish over time in arid regions due to water scarcity, while in humid regions, prolonged saturation could weaken root structures, leading to slope instability. Longitudinal studies that track the performance of specific plant species across multiple seasons and varying climates are needed to develop more reliable and sustainable slope stabilization strategies.

Another significant limitation is the challenge of accurately quantifying the synergistic effects of mixed vegetation on slope stabilization. While it is widely recognized that combining herbaceous and woody plants can enhance slope stability, the specific interactions between different plant types and their collective impact on soil reinforcement are not well understood. Current models often simplify or exclude the complex root–soil dynamics that occur when multiple species interact, leading to an incomplete understanding of their combined stabilizing effects. Moreover, the variability in root architecture, growth rates, and species-specific responses to environmental stresses further complicates the prediction of these synergistic effects. Future research should focus on developing advanced modeling techniques and field experiments that can capture the intricate interactions within mixed plant communities, providing clearer guidelines for practitioners aiming to maximize the ecological and mechanical benefits of diverse vegetation in slope stabilization projects.

5.2. Advances in Understanding Root–Soil Interactions

In recent years, significant progress has been made in understanding the complex root–soil interactions that contribute to slope stabilization. These interactions involve physical, chemical, and biological processes, which have been investigated through various experimental and modeling approaches [109,110].

One critical aspect of root–soil interactions is root reinforcement, which occurs when plant roots penetrate the soil, increase soil cohesion, and improve soil mechanical properties [111]. The study of root reinforcement has advanced with the development of new techniques for measuring root tensile strength and root–soil adhesion, as well as the use of advanced imaging methods like X-ray computed tomography to visualize root systems in situ [112].

Scholars have also been exploring the role of root exudates in altering soil properties. Root exudates are a complex mixture of compounds released by plant roots, including organic acids, sugars, and proteins. These exudates can promote the formation of soil aggregates and enhance soil stability by stimulating microbial activity and influencing soil chemical properties. Some researchers have suggested that root exudates might be a key factor in the stabilization of slopes by herbaceous plants, and further studies are needed to elucidate their precise role [113].

Another area of interest is the interplay between root systems and soil moisture. The presence of plant roots can alter the distribution of soil moisture, as roots uptake water and release it through transpiration. In some cases, this can lead to a more uniform distribution of soil moisture, reducing the potential for slope failure due to water-induced erosion. However, the specific effects of root systems on soil moisture dynamics are highly dependent on the plant species and habitat environment, and more research is needed to understand these relationships fully [114].

Biological processes, such as the activity of soil microorganisms, are also essential components of root–soil interactions. Microorganisms can directly and indirectly influence root growth and soil structure, and their activity is often modulated by the presence of plant roots. For example, mycorrhizal fungi, which form symbiotic associations with plant roots, can enhance soil aggregation and contribute to slope stabilization. Recent studies have also shown that the presence of certain microbial communities can increase the erosion resistance of soils, suggesting that they could play a role in slope stabilization [115,116].

In conclusion, advances in understanding root–soil interactions have provided valuable insights into the mechanisms by which herbaceous plants contribute to slope stabilization. Future research should continue to explore these interactions, with a particular focus on the role of root exudates, the relationship between root systems and soil moisture dynamics, and the influence of soil microorganisms on slope stability.

5.3. Improved Modeling and Simulation of Herbaceous Plant Effects on Slope Stability

Recent advancements in modeling and simulation techniques have improved our understanding of the effects of herbaceous plants on slope stability. Sophisticated numerical models, such as the finite element method (FEM) and discrete element method (DEM), have been employed to study the mechanical behavior of root–soil systems under various growth habitat loadings. These models can account for the complex geometry of root systems, as well as the interactions between roots, soil particles, and pore water, providing a more accurate representation of the factors influencing slope stability [35].

In addition to numerical models, researchers have utilized the root bundle model (RBM) and the root–soil plate model (RSPM), to better understand the contribution of root reinforcement to slope stability. These models are particularly useful for evaluating the effects of plant species and root system characteristics on the mechanical behavior of slopes.

Furthermore, recent advancements in remote sensing and geospatial analysis techniques have facilitated large-scale assessments of slope stability and vegetation patterns, allowing researchers to identify areas prone to slope failure and evaluate the effectiveness of herbaceous plant species in slope stabilization [117].

In conclusion, improved modeling and simulation techniques have significantly advanced our understanding of the effects of herbaceous plants on slope stability, providing valuable tools for researchers to study root–soil interactions and evaluate the effectiveness of vegetation in mitigating slope failure.

5.4. Integration of Herbaceous Plants with Other Bioengineering Techniques

The integration of herbaceous plants with other bioengineering techniques is crucial for maximizing slope stability and soil erosion control. Herbaceous plants, with their extensive root systems and adaptive growth, complement the mechanical reinforcement provided by techniques such as geotextiles, retaining walls, and terracing. Studies show that the combination of these methods leads to greater erosion resistance and slope stabilization than employing single techniques alone.

Scholars emphasize the importance of selecting appropriate herbaceous species for specific environments to optimize their effectiveness. For instance, grasses with fibrous root systems are ideal for shallow soil layers, while deep-rooted legumes can better penetrate and reinforce deeper soil layers. Furthermore, integrating herbaceous plants with other bioengineering techniques can also provide additional ecological benefits such as habitat creation, carbon sequestration, and improved water quality [118].

For example, in road construction projects where slopes are exposed to frequent disturbances and high loads, integrating herbaceous plants for immediate stabilization and woody plants for long-term protection can mitigate the risk of landslides and soil erosion. In the mining industry, where tailings dams and waste dumps require both surface erosion control and deep-root reinforcement, the application of mixed vegetation can improve the stability and safety of these sites. Furthermore, urban planners can utilize these findings to enhance the design of green spaces and urban slopes, promoting not only environmental stability but also biodiversity and ecosystem services.

In conclusion, the integration of herbaceous plants with other bioengineering techniques enhances slope stability and soil erosion control. The selection of suitable plant species and careful management of planting density and grazing intensity are key factors in ensuring the effectiveness of these integrated approaches.

6. Conclusions

Herbaceous plants have demonstrated considerable potential in enhancing slope stabilization through various mechanisms. Their extensive root systems contribute significantly to soil reinforcement, improving the shear strength and cohesion. This biological reinforcement is particularly effective in preventing shallow landslides and reducing surface erosion, which are critical concerns in slope management.

The integration of herbaceous vegetation in slope stabilization projects offers multiple ecological and engineering benefits. Firstly, herbaceous plants establish rapidly, providing immediate ground cover that protects the soil surface from raindrop impact and surface runoff. This rapid establishment is crucial in mitigating erosion during the critical period before more permanent vegetation can take root.

Secondly, the roots of herbaceous plants penetrate the soil, creating a dense network that binds soil particles together. This network enhances the structural integrity of the soil, making it more resistant to erosive forces and reducing the likelihood of slope failure. Furthermore, the diversity of root architectures among different herbaceous species allows for a comprehensive reinforcement effect at various soil depths, ensuring stability across the slope profile.

Additionally, the use of herbaceous plants in slope stabilization is sustainable and environmentally friendly. Unlike traditional engineering methods, such as concrete or synthetic reinforcements, herbaceous plants improve soil health over time by contributing organic matter and enhancing microbial activity. This ecological approach not only stabilizes slopes but also promotes biodiversity and ecosystem resilience.

The effectiveness of herbaceous plants in slope stabilization is well-documented, with numerous studies highlighting their role in improving soil cohesion and shear strength. For instance, research has shown that areas vegetated with herbaceous species exhibit significantly lower rates of erosion and higher soil stability compared to non-vegetated areas.

In conclusion, the application of herbaceous plants in slope stabilization offers a viable and sustainable alternative to conventional engineering methods. Their rapid establishment, extensive root systems, and ecological benefits make them an invaluable component of integrated slope management strategies. Future research and practical applications should focus on optimizing plant selection and management practices to maximize their stabilizing effects and long-term benefits.