A Biomechanical Study of Potential Plants for Erosion Control and Slope Stabilization of Highland in Thailand

: Soil bioengineering provides a sustainable method for erosion control and soil slope stabilization using vegetation with multiple co-benefits. This study evaluated ten plant species in Thailand’s highland regions for their soil bioengineering potential and additional benefits. Root architecture, tensile strength, and Young’s modulus were measured to compare biomechanical traits. G. sepium , F. griffithii , P. americana , B. asiatica , and C. arabica exhibited H-type roots with wide lateral spread, while M. denticulata and C. officinarum had VH-type roots with deep taproots and wide lateral extent. A. sutepensis showed M-type roots with most root matrix in the top 0.3 m, where C. cajan and C. sinensis had R-type roots with deep, oblique growth. Most species showed a negative power relationship between the root strength and Young’s modulus with the root diameter except C. cajan that showed a positive correlation. P. americana , F. griffithii , C. officinarum , and C. arabica showed relatively high values of 1 mm root tensile strength (exceeding 24 to 42 MPa), while M. denticulata , G. sepium , and B. asiatica exhibited intermediate root tensile strength (ranging from 8 to 19 MPa). A. sutepensis , C. cajan , and C. sinensis demonstrated the lowest root tensile strength, up to 7 MPa. It is advised to plan slope vegetation by selecting diverse plant species with varying root structures and benefits, addressing both engineering and socioeconomic needs of the sustainable nature-based solution.


Introduction
Due to the current climate change and extreme weather conditions, land degradation and rainfall-triggered landslide are one of the most pressing environmental concerns particularly in mountainous highlands.These problems are accelerating in recent years due to land scarcity and the increasing pressure for food security that lead local farmers in highland to follow unsustainable agriculture practice such as slash and burn, associated with annual crops.In order to tackle this problem, a sustainable approach that promotes societal development while addressing economic and environmental needs is followed by Royal Project Foundation and Highland Research and Development Institute (HRDI) in Thailand [1].Various long-term initiatives were developed to encourage local farmers in northern provinces to turn to more environmentally friendly agroforestry practice and reforestation.One of the philosophies proposed by the late King Bhumibol of Thailand regarding this issue is that of "three forests with four benefits".The three kinds of forests include species for (a) general usage such as timbers, (b) edible fruits, and (c) fuels, which can yield four benefits, including the three original purposes, plus the fourth one being soil and water conservation and reforestation.Such a sustainable approach of plant selection is mostly adopted in community capacity development projects in the northern part of Thailand.
The vegetation planted on slopes not only serves as soil and water conservation and erosion control, benefiting the environment, but also provides economic incentives to local farmers, making these practices sustainable.In this way, all three key aspects of sustainability can be realized, that is, the environmental, economic, and social development aspects.However, biomechanical studies on how these species reinforce the soil and their application in soil bioengineering are relatively scarce [2].A holistic discussion of all aspects of sustainability, combined with biomechanical studies of plant roots, is also rare.
Soil bioengineering is the use of plants for erosion control and shallow slope stabilization, normally combined with engineering techniques such as erosion control blankets, with some forms of retaining structures, namely gabion walls and soil bags [3][4][5][6].This technique has recently received considerable attention in infrastructure development due to its attributes of being relatively low cost, emitting low levels of carbon dioxide (CO 2 ), being aesthetically pleasing, and being environmentally friendly [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].Plant roots can stabilize soil by providing mechanical reinforcement and creating a composite material with improved properties, such as enhanced resistance to tension [8,18,19].The effectiveness of soil stabilization by plant roots depends on various factors including species, architectural shape, morphology, and interface properties of the root system, as well as their concentration, branching characteristics, and spatial distribution within the soil [20].Essentially, it is the combination of root orientation, root concentration, and root tensile strength characteristics that collectively gives rise to the overall capacity of the root-reinforced soil [4].
Root strength and architecture are influenced by local soil and environmental conditions.Yen [21] classified root growth types into H-type, R-type, VH-type, V-type, and M-type.Styczen and Morgan [20] further elaborated on these root growth types, providing detailed explanations to enhance comprehension.According to Styczen and Morgan [20], H-type and VH-type plant roots are suitable for slope stabilization and wind resistance, while H-type and M-type roots are recommended for soil reinforcement [22].Plant roots provide soil reinforcement by physically anchoring shallow soil to deeper layers, thereby preventing erosion and slope failures [23,24].The size of roots has a significant impact on their effectiveness in soil reinforcement, with fibrous roots being particularly effective in creating a root-soil composite similar to fiber-reinforced soil [25,26].Fibrous roots can exhibit greater cohesiveness compared with tap roots, even when considering the same root area ratio [27].Smaller roots tend to have higher tensile strength than larger roots primarily due to their biological components such as cellulose [4,28].Consequently, small roots can leverage their tensile strength to enhance soil-root friction, while larger roots intersect shear surfaces and mobilize soil-root friction force [24].Root mechanical reinforcement is influenced by various factors, including root system morphology [29] and growth conditions [30], the number and relative amount of fine and coarse roots, the diameter [25,27,29,31], water content and suction [8,[32][33][34], chemical composition [35], and tensile strength and stiffness, characterized by Young's modulus (the initial linear part of a tensile stress-strain curve) [19,25] and the loading rate of testing [36,37].It is noteworthy that root moisture content and root suction were found to have profound effects on root tensile strength as well as the stability of the root-reinforced slope subjected to long-term drought followed by heavy rain [8].
The most commonly utilized soil bioengineering method in Thailand and other Southeast Asian countries is the use of the vetiver (Chrysopogon zizanioides) system, known for its cost-effectiveness and environmental friendliness [38].In Thailand, the predominant species is Chrysopogon zizanioides, a lowland vetiver, while Chrysopogon nemoralis, an upland vetiver, is less favored due to its potential invasiveness and shorter root system [39].Despite its advantages, vetiver grass exhibits shade intolerance and is occasionally viewed as a monospecies solution.The incorporation of local plant species in addition to the vetiver system not only fosters biodiversity but also reduces expenses associated with harvesting, handling, and transportation [40].
This study aims to investigate locally cultivated plant species commonly found in landslide-prone regions within four HRDI project areas in northern Thailand for their potential use in erosion control and soil bioengineering applications, while also considering their additional benefits.The primary goal is to analyze the root characteristics of these species and assess their biomechanical properties, specifically focusing on tensile strength and modulus.Additionally, a comparative evaluation was undertaken to gauge the suitability of each species for soil bioengineering applications based on their biomechanical attributes and other benefits, presenting a promising avenue for future research in this field.

Selection of Plant Species and Sampling
In selecting plant species for soil bioengineering applications in this study, careful consideration is given to various factors, including not only their effectiveness in resisting mechanical stresses but also the habitat of the species being both native and exotic species that are available in the study areas as well as their other uses.Furthermore, observations from local farmers and expertise from relevant authorities (HRDI) contributed to the selection process, ensuring other tangible benefits such as economic value, timber resources, and soil and water conservation.Various plant species (10 species in total; details of which are shown in Tables 1 and 2) were selected for root architecture study and tensile strength testing based on their availability in four project areas of HRDI in northern provinces of Thailand.In Tak province (Mae Song project area), Cajanus cajan, Persea americana, Fraxinus griffithii, and Cinnamomum officinarum were tested.In Chiang Rai province (Mae Salong project area), Camellia sinensis, Aspidistra sutepensis, and Coffea arabica were featured.The Sop Moei project area, Ma Hongsorn province, included Macaranga denticulata and Buddleja asiatica.The excavation and washing methods were employed to examine the root architecture of the selected species from the study areas and to collect root specimens for further biomechanical analysis.In spite of the risk of breaking smaller roots, even with careful handling [41,42], this technique was considered the most reliable option given the resources and tools available during this study.It is a non-invasive exotic species and is cultivated for its seeds.Non-invasive species [43] Gliricidia sepium (Jacq.)Kunth Mexico to Colombia.It is a non-invasive exotic species.Non-invasive species [43] Macaranga denticulata (Blume) Müll.Arg.

S. China to Tropical
Asia.
Central & S.China to Tropical Asia and Marianas.
It is a native species.
Invasive species [43] Chrysopogon zizanioides (L.) Roberty Poaceae NE.India to Indo-China.It is a native species.Unknown [43] From Table 2, it is noted that the eight selected species-Persea americana, Fraxinus griffithii, Cajanus cajan, Cinnamomum officinarum, Camellia sinensis, Coffea arabica, and Gliricidia sepium-are all non-invasive exotic species.These plants have been primarily cultivated for various benefits, such as fruits, food, timber, and ornamental purposes, in addition to soil and water conservation.Over time, these exotic species have become naturalized to the environment in Thailand, integrating into the native ecosystem and demonstrating their ability to survive.This adaptation is likely reflected in their root tensile strength.

Measurements of Root Biomechanical Properties
After excavation and washing, the root specimens were placed in sealed plastic bags, transferred to the laboratory, and stored in a refrigerator at 8 • C. Subsequently, all specimens were tested at ambient temperature in the moisture condition they retained post-sampling.This condition is hereafter referred to as as-sampled moisture condition.However, for Cinnamomum officinarum, additional series of tensile strength tests were carried out on root specimens that had been dried for 7 days in order to explore the effects of drying.In the tensile strength test, the root specimen was gripped using a drill-bit clamp connected to a load cell, and tension was applied using a loading frame at a constant speed of 4 mm/min [8].To ensure secure gripping of fine roots, a customized gripping mechanism was employed, utilizing two chuck grips at the ends of the root.Epoxy resin or latex glue was applied to reinforce up to 5 mm at the root ends to prevent squeezing and pulling out during testing, and sometimes a rubber membrane and gauze cloth were also used between the root and the grip to avoid stress concentrations and improve the grip.After reinforcement of root ends (within 30 min), specimens were subjected to testing, with the root segment length between clamps set at 50 mm.Only test results of specimens with ruptures occurring near the middle part were considered reliable and included in the data analysis.In the case of any slippage, the tensile test was discarded.Figure 1 shows the complete assembly of the tensile testing machine used in this study.The root tensile strength (MPa) at maximum load and Young's modulus (MPa) were obtained from the stress-strain curve of the tested sample [30].The tensile strength (T r ) at maximum load was calculated using Equation (1).
where F is the maximum force (Newton) the root can sustain before breaking, and d is the root diameter (mm) (the arithmetic mean of three-point measurements made after the tensile test).Young's modulus (E r ) was calculated from the gradient of the stress-strain curve measured at the peak stress: where L o is the initial length (mm) of the root sample (i.e., 50 mm), and ∆L is the change in the root length (mm) at peak stress [27,30].The root diameter is, therefore, a key parameter that can affect both the values of root tensile strength and Young's modulus (Equations ( 1) and ( 2)).
Sustainability 2024, 16, 6374 5 of 19 clamp connected to a load cell, and tension was applied using a loading frame at a constant speed of 4 mm/min [8].To ensure secure gripping of fine roots, a customized gripping mechanism was employed, utilizing two chuck grips at the ends of the root.Epoxy resin or latex glue was applied to reinforce up to 5 mm at the root ends to prevent squeezing and pulling out during testing, and sometimes a rubber membrane and gauze cloth were also used between the root and the grip to avoid stress concentrations and improve the grip.After reinforcement of root ends (within 30 min), specimens were subjected to testing, with the root segment length between clamps set at 50 mm.Only test results of specimens with ruptures occurring near the middle part were considered reliable and included in the data analysis.In the case of any slippage, the tensile test was discarded.Figure 1 shows the complete assembly of the tensile testing machine used in this study.The root tensile strength (MPa) at maximum load and Young's modulus (MPa) were obtained from the stress-strain curve of the tested sample [30].The tensile strength (Tr) at maximum load was calculated using Equation (1).
where F is the maximum force (Newton) the root can sustain before breaking, and d is the root diameter (mm) (the arithmetic mean of three-point measurements made after the tensile test).Young's modulus (Er) was calculated from the gradient of the stress-strain curve measured at the peak stress: where Lo is the initial length (mm) of the root sample (i.e., 50 mm), and ΔL is the change in the root length (mm) at peak stress [27,30].The root diameter is, therefore, a key parameter that can affect both the values of root tensile strength and Young's modulus (Equations ( 1) and ( 2)).

Root Morphology and Architecture
The morphology and architectural shape of plant roots (Figure 2) were characterized using patterns of root growth outlined by Yen [21] and Styczen & Morgan [20] in this study.According to Yen's classification, Gliricidia sepium, Persea americana, Fraxinus griffithii, Coffea arabica, and Buddleja asiatica exhibit H-type roots.These roots are characterized by a wide lateral extent, with 80% of the root matrix concentrated in the top 60 cm of soil, and moderate depth for maximum root length.On the other hand, Macaranga denticulata and Cinnamomum officinarum have VH-type roots, similar to H-type but with a main taproot penetrating deep into the ground while lateral roots extend horizontally.Aspidistra sutepensis possess M-type roots, where the main root profusely grows under the stump with a narrow lateral extent, and the majority of the root matrix accumulates in the top 30 cm of soil.Lastly, Cajanus cajan and Camellia sinensis are characterized by R-type roots, which have deep maximum root growth but grow obliquely, with only 20% of the root matrix found in the top 60 cm of soil and with main roots extending at right angles to the slope.
Understanding the root architecture of different tree species can greatly aid in the strategic arrangement of vegetation planting on slopes.Planting only a single tree species (monoculture) or a certain type of root morphology on slopes can pose risks to slope stability.Monocultures tend to have uniform root patterns, lacking the diverse root systems of mixed-species forests, thereby reducing their resistance to a variety of slope failure mechanisms.They are also more susceptible to diseases and pests, which can devastate the entire tree population and further compromise slope stability.In contrast, a diverse array of plant species with varied root architectures (H-, VH-, R-, and M-types) can enhance slope stability by addressing different types of mass movement, both surface and deep-seated.Additionally, such diversity improves ecological values, decreases vulnerability to diseases and pests, and provides economic and societal benefits.It should be noted that while the site conditions across the study areas are similar, i.e., hilly terrain, there may be variations in soil types and nutrient availability from one site to another.These factors can influence root architecture and the extent to which root types vary with site conditions.Further detailed studies on specific species are needed to elucidate these variations.

Root Tensile Strength
This study investigates the root tensile strength across 10 plant species, ranging in diameter from 0.22 mm to 3.43 mm (Table 3).Average tensile strengths for all root diameters are also shown to vary across the species, with values ranging from 4.06 MPa (Aspidistra sutepensis, the lowest value) to 36.82 MPa (Fraxinus griffithii, the highest value) in an as-sample root moisture condition.Notably, Fraxinus griffithii demonstrated the highest strength per individual root section, reaching 89.12 MPa for a 0.612 mm diameter root segment.The effect of root drying was clearly evident by comparing the root tensile strengths of Cinnamomum officinarum in as-sampled and in 7-day drying conditions.The average tensile strength increased from 19.11 MPa to 33.37 MPa as the roots dried for 7 days.This trend of increasing strength with decreasing moisture content is in line with the behavior of other woody species observed by Hales and Miniat [33] and Boldrin et al. [32], and herbaceous plants reported by Zhang et al. [34], while it contradicted that of the grass species reported by Mahannopkul and Jotisankasa [8].It is noted that the range of root suction tested by Mahannopkul and Jotisankasa [8] was only up to 50 kPa, while the root suction during drying of Cinnamomum officinarum in this study was unknown.Table 3 presents the fitting equations based on the power law relationship, including the coefficients of determination (R 2 ) and p-values obtained from regression analysis and ANOVA in Microsoft Excel.For seven species-Persea Americana, Fraxinus griffithii, Cajanus cajan, Cinnamomum officinarum (7 day drying), Camellia sinensis, Coffea Arabica, and Buddleja asiatica-a significant relationship (p-value < 0.05) was found between the root diameter and tensile strength.In contrast, the relatively low R 2 values and higher p-values (>0.05) for Cinnamomum officinarum (as sampled), Aspidistra sutepensis, Gliricidia sepium, and Macaranga denticulata suggest that factors beyond the root diameter, such as root suction and moisture conditions, likely contribute to variations in tensile strength.Interestingly, this study identified two distinct trends in the tensile strength-diameter relationship across species, which may be described by two different types of fitting equations (Figure 3; Table 3).Among the species, 9 show a negative power trend in the tensile strength-diameter relationship, indicating a decrease in tensile strength with an increasing root diameter, while the Cajanus cajan exhibit positive relationships between strength and diameter and show an increase in tensile strength with diameter.The positive correlation can be attributed to several factors, including changes in root structure, namely their relative cellulose and lignin percentages, with root diameter [50].In the case that larger roots exhibit a higher proportion of the lignin/cellulose ratio, known to provide material strength, this results in positive correlations.For negative correlation, an increase in diameter may correspond to a lower lignin/cellulose ratio, typically associated with decreased strength as reported by Yang et al. [50].However, there is still no root cross-sectional image studies to confirm this hypothesis.Further studies are required, in particular, to understand the basic cellular structure of dicot and monocot roots of these species.
Figure 4a presents the tested root tensile strength as a boxplot.The largest variation in strength with root diameter was found in Fraxinus griffithii, which was also reflected in the high exponent value (1.414) of its negative power law curve fitting as shown in Table 3.In other words, an increase in the root diameter can bring about a faster rate of reduction in tensile strength for the roots of Fraxinus griffithii as compared with other species.The roots of Persea americana, Cinnamomum officinarum, and Coffea arabica exhibited a more moderate variation, while the others showed a narrow variation in tensile strength.However, it should also be noted that the range of root diameters was different among the tested species.The narrow variation in tensile strength for some species can be due to the fact that no smaller root size was presented in the specimens tested, or these smaller roots were too soft to handle during the tests.3) (Cinnamomum officinarum* tested after 7 days drying).

Young's Modulus of Roots
Figure 5 and Table 3 illustrate the relationship between Young's modulus of roots for each tested species.The average Young's modulus of various plant specimens significantly ranged from 62.90 MPa for Aspidistra sutepensis to 615.63 MPa for Fraxinus griffithii.For Cinnamomum officinarum under as-sampled condition, Young's modulus was   3 illustrate the relationship between Young's modulus of roots for each tested species.The average Young's modulus of various plant specimens significantly ranged from 62.90 MPa for Aspidistra sutepensis to 615.63 MPa for Fraxinus griffithii.For Cinnamomum officinarum under as-sampled condition, Young's modulus was measured at 269.28 MPa, a lower value than the drier roots, demonstrating the same trend as that of tensile strength as explained earlier.The tested species displayed a relationship between Young's modulus and the root diameter (Figure 5, Table 3), akin to trends observed in strength-diameter relationships.The modulus-diameter relationship, with p-values lower than 0.05 indicating significant relationships, was only found for Fraxinus griffithii, Cajanus cajan, Camellia sinensis, and Coffea arabica.Other species, including Persea Americana, Cinnamomum officinarum, Aspidistra sutepensis, Gliricidia sepium, Macaranga denticulate, and Buddleja asiatica, showing p-values higher than 0.05, demonstrated larger deviation of moduli from the fitting trend line, reflecting their intrinsic natural variability.Stiffer roots, characterized by higher Young's modulus values, are shown to effectively mobilize stress, potentially enhancing soil shear strength and stability.Figure 4b presents the tested root Young's modulus as a boxplot.A similar trend can be observed regarding the variation in the modulus as the tensile strength.The largest variation in the modulus with the root diameter was still found in Fraxinus griffithii followed by Coffea arabica, which showed a more moderate variation, while the other species exhibited a more constant modulus.Figure 6 focuses on comparing the root tensile strength among different plant species using a standardized root diameter of 1 mm.This 1 mm root tensile strength can be directly obtained from the fitting equations of trendlines as summarized in Table 3.The root diameter of 1mm is typically associated with fibrous root reinforcement, which contributes more significantly to the root reinforcement than thicker roots, as shown in Figure 6 focuses on comparing the root tensile strength among different plant species using a standardized root diameter of 1 mm.This 1 mm root tensile strength can be directly obtained from the fitting equations of trendlines as summarized in Table 3.The root diameter of 1mm is typically associated with fibrous root reinforcement, which contributes more significantly to the root reinforcement than thicker roots, as shown in previous studies [27,28].Notably, exotic species such as Cinnamomum officinarum, Persea americana, Fraxinus griffithii, and Coffea arabica demonstrate relatively higher values of 1mm root tensile strength while not being native to Thailand.These exotic species, showing a similar range of tensile strength to Chrysopogon zizanioides, would be able to contribute to not only soil erosion control but also their usage for foods and economic purposes.Additionally, this study categorizes species into intermediate (Macaranga denticulata, Gliricidia sepium, and Buddleja asiatica) and low root tensile strength groups, with species like Aspidistra sutepensis, Camellia sinensis, and Cajanus cajan exhibiting the lowest root tensile strength.However, it is noted that even species with relatively lower root tensile strength can contribute to soil stability if their density (root biomass per soil volume) is sufficiently high [51,52].Table 4 provides a concise overview of the soil reinforcing properties and diverse advantages of several plant species investigated in this study.Persea americana and Fraxinus griffithii stand out with higher tensile strength and are considered effective in soil and water conservation, with Persea americana also providing high economic and edible fruit benefits.Cinnamomum officinarum, also known for its commercial values and medicinal use, possesses moderate tensile strength and a dense root system, rendering it appropriate for reinforcing soil and conserving soil and water at a medium to high level.Cajanus cajan and Aspidistra sutepensis have low tensile strength but would still contribute to soil conservation, as their root density varies from medium to high.They also provide high edible fruit benefits.Coffea arabica is distinguished by relatively high tensile strength and root density, which shows its potential for soil reinforcement and conservation in addition to its well-known commercial use.Other species, like Gliricidia sepium and Buddleja asiatica, offer medium tensile strength and root density, with varying benefits for general use, economic use, and soil conservation.Table 4 provides a concise overview of the soil reinforcing properties and diverse advantages of several plant species investigated in this study.Persea americana and Fraxinus griffithii stand out with higher tensile strength and are considered effective in soil and water conservation, with Persea americana also providing high economic and edible fruit benefits.Cinnamomum officinarum, also known for its commercial values and medicinal use, possesses moderate tensile strength and a dense root system, rendering it appropriate for reinforcing soil and conserving soil and water at a medium to high level.Cajanus cajan and Aspidistra sutepensis have low tensile strength but would still contribute to soil conservation, as their root density varies from medium to high.They also provide high edible fruit benefits.Coffea arabica is distinguished by relatively high tensile strength and root density, which shows its potential for soil reinforcement and conservation in addition to its well-known commercial use.Other species, like Gliricidia sepium and Buddleja asiatica, offer medium

Conclusions
This study investigated ten plant species in Thailand's highland regions for their soil bioengineering potential and additional benefits.The root morphology along with the relationship between root tensile strength and Young's modulus with a diameter was extensively studied.The following conclusions can be drawn from this study: • G. sepium, F. griffithii, P. americana, B. asiatica, and C. arabica possessed H-type roots and wide lateral spread.M. denticulata and C. officinarum demonstrated VH-type roots with deep taproots in addition to their lateral extent.A. sutepensis showed M-type roots with most root matrix in the top 0.3m, whereas C. cajan and C. sinensis had R-type roots with deep, oblique growth.This information is crucial for strategically arranging slope vegetation, enabling a more robust bioengineering system through diversified root reinforcement patterns and a variety of plant usages.

Figure 4 .
Figure 4. Boxplots of root tensile strength (a) and Young's modulus (b) tested species for different species.The bottom and top of the boxes represent the 25th and 75th percentiles, respectively, while the line within the box marks the median and the circle represents outlier (Cinnamomum officinarum* refers to 7-day drying condition).

Figure 4 .
Figure 4. Boxplots of root tensile strength (a) and Young's modulus (b) tested species for different species.The bottom and top of the boxes represent the 25th and 75th percentiles, respectively, while the line within the box marks the median and the circle represents outlier (Cinnamomum officinarum* refers to 7-day drying condition).

Figure 5
Figure 5 and Table3illustrate the relationship between Young's modulus of roots for each tested species.The average Young's modulus of various plant specimens significantly ranged from 62.90 MPa for Aspidistra sutepensis to 615.63 MPa for Fraxinus griffithii.For Cinnamomum officinarum under as-sampled condition, Young's modulus was measured at 269.28 MPa, a lower value than the drier roots, demonstrating the same trend as that of tensile strength as explained earlier.The tested species displayed a relationship between Young's modulus and the root diameter (Figure5, Table3), akin to trends observed in strength-diameter relationships.The modulus-diameter relationship, with p-values lower than 0.05 indicating significant relationships, was only found for Fraxinus griffithii, Cajanus cajan, Camellia sinensis, and Coffea arabica.Other species, including Persea Americana, Cinnamomum officinarum, Aspidistra sutepensis, Gliricidia sepium, Macaranga denticulate, and Buddleja asiatica, showing p-values higher than 0.05, demonstrated larger deviation of moduli from the fitting trend line, reflecting their intrinsic natural variability.Stiffer roots, characterized by higher Young's modulus values, are shown to effectively mobilize stress, potentially enhancing soil shear strength and stability.Figure4bpresents the tested root Young's modulus as a boxplot.A similar trend can be observed regarding the variation in the modulus as the tensile strength.The largest variation in the modulus with the root diameter was still found in Fraxinus griffithii followed by Coffea arabica, which showed a more moderate variation, while the other species exhibited a more constant modulus.

Figure 6 .
Figure 6.Root tensile strengths of 1 mm diameter root size of the studied species (blue bars), including data of **Chrysopogon zizanioides (red bar) from Mahannopkul and Jotisankasa [8] for comparison (Cinnamomum officinarum* refers to 7-day drying condition; other cases referred to as as-sampled moisture condition).

Figure 6 .
Figure 6.Root tensile strengths of 1 mm diameter root size of the studied species (blue bars), including data of **Chrysopogon zizanioides (red bar) from Mahannopkul and Jotisankasa [8] for comparison (Cinnamomum officinarum* refers to 7-day drying condition; other cases referred to as as-sampled moisture condition).

Table 1 .
Selected plant species for this study and their family, tree height, age, perimeter, longest measurable root length, root type, and site coordinates.

Species Family Site Cordinates Age, Year Height, m Tree Perimeter, cm Longest Measurable Root Length, m Type of Root *
[21]ot types according to Yen[21].NA = Not applicable.

Table 2 .
Selected plant species for this study with additional information on vetiver, their distributions, origin, invasiveness, and references.

Table 3 .
Summary of the data on root tensile strength and Young's modulus (Mean) per tested species.The best-fit equations, R 2 , and p-values are given for the strength and Young's modulus-diameter relations.

Table 4 .
Summary of the strengthening properties of plant roots and their applications.

Table 5 .
Growth response to biotic and abiotic stresses.
• The root tensile strength and Young's modulus-diameter relationships of the ten species highlighted two different trends.For nine of the species tested, P. americana, F. griffithii, M. denticulata, G. sepium, A. sutepensis, C. arabica, C. sinensis, C. officinarum, and B. asiatica demonstrated a decrease in tensile strength with the diameter, the commonly observed negative power law relationship with the exponent values ranging from −0.187 to −1.983.Conversely, C. cajan exhibited an increase in tensile strength with the diameter, the positive power law with the exponent value of 1.5474 diverging from the typical trend.•Plantspecies with relatively high values of 1-mm root tensile strength (exceeding 24 to 42 MPa) include P. americana, F. griffithii, C. officinarum, and C. arabica.Species exhibiting intermediate root tensile strength (ranging from 8 to 19 MPa) comprise M. denticulata, G. sepium, and B. asiatica.Conversely, A. sutepensis, C. cajan, and C. sinensis demonstrate the lowest root tensile strength, up to 7 MPa.•It is recommended to design slope vegetation by strategically selecting species with diverse root morphologies and mechanical properties, as well as incorporating a variety of plant species that offer multiple benefits.This approach addresses both the engineering and socioeconomic requirements of the nature-based solution.Conceptualization, W.M. and A.J.; data curation, M.H., S.P. and S.S.; formal analysis, M.H. and A.J.; funding acquisition, A.J.; investigation, W.M., A.J., N.L., C.N., P.R., J.P., S.P. and S.S.; methodology, W.M., A.J. and N.L.; project administration, A.J. and J.P.; resources, A.J. and J.P.; supervision, A.J.; validation, A.J., N.L. and C.N.; writing-original draft, M.H. and A.J.; writing-review and editing, M.H., A.J., N.L., C.N. and A.N.F.A.All authors have read and agreed to the published version of the manuscript.The research grant was provided by Highland Research and Development Institute (HRDI) and the Faculty of Engineering, Kasetsart University, Thailand. Funding: