Mechanical properties of thin roots of tree species for shallow landslide protection along an altitudinal gradient

93 Mechanical properties of thin roots of tree species for shallow landslide protection along an altitudinal gradient Laura Sanchez-Castillo1, Edmar Melendez-Jaramillo1, Santos Daniel Chicas2, Ma. Teresa de Jesus Segura Martinez1, Marisol Gonzalez-Delgado3 1 Faculty of Engineering and Sciences, Autonomous University of Tamaulipas, Centro Universitario, CP 87120 Ciudad Victoria, Tamaulipas, Mexico 2 Department of Agro-Environmental Science, Faculty of Agriculture, Kyushu University, Fukuoka, Japan 3 Centro de Producción Agropecuaria, Universidad Autónoma de Nuevo León, Carretera Nacional km. 143, Linares, Nuevo León, CP. 67700, Mexico Corresponding author: Laura Sanchez-Castillo ([email protected]) Copyright: © Laura Sanchez-Castillo et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Shallow landslides and erosional problems in mountain areas are a recurrent issue that can represent a threat to human life and impact landscape ecology in many ways. Soil bioengineering is an approach that uses biological and ecological knowledge and engineering design principles to restore shallow-landslide-affected slopes. Tree species can be used as elements of soil bioengineering to reduce erosion and provide reinforcement through their roots. For this reason, we estimated the mechanical properties of thin tree roots from three vegetation communities along an elevational gradient. Our results showed that Piper amalago and Pinus teocote consistently exhibited higher mechanical properties than all species analyzed. The absence of a consistent altitudinal trend suggests that local site conditions and species-specific traits have a stronger influence on root mechanics than elevation alone. Key words: Elasticity, forest, root reinforcement, tensile strength, thorn scrub Introduction Mountain environments are ecologically unique and play an important role in maintaining species biodiversity; however, they are also very susceptible when hydro-meteorological events occur. Rainfall-induced shallow landslides are a very important and dangerous hazard in mountainous areas. Shallow landslides are defined as the movement of soil along a slope with a sliding surface up to 2 m (Schaller et al. 2025). The investigation of the mechanical contribution of plant roots to soil has significant implications. From an environmental point of view, plant-based soil stabilization represents a solution for erosion control and ecosystem resilience, contributing to the restoration of degraded landscapes while maintaining soil health and plant species biodiversity. From a societal standpoint, the use of plants to reduce shallow landslides protects human lives and infrastructure, reducing economic losses in vulnerable rural mountainous regions. Soil bioengineering is a nature-based solution that can be an alternative or complement to the conventional hydraulic or civil engineering approaches of Academic editor: Pavel Stoev Received: 26 September 2025 Accepted: 23 November 2025 Published: 11 December 2025 Citation: Sanchez-Castillo L, Melendez-Jaramillo E, Daniel Chicas S, Segura Martinez MaTdeJ, GonzalezDelgado M (2025) Mechanical properties of thin roots of tree species for shallow landslide protection along an altitudinal gradient. BioRisk 23: 93–109. https://doi.org/10.3897/ biorisk.23.173254 BioRisk 23: 93–109 (2025) DOI: 10.3897/biorisk.23.173254 BioRisk 94 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection soil stabilization (Kettenhuber et al. 2022). Using biological and ecological knowledge and engineering design principles, this discipline allows plants to be used as living structures or in combination with plant community propagation to naturally control soil erosion, sediment flows, and flooding (Stokes et al. 2009; Bischetti et al. 2012; Jiang et al. 2024; Suresh et al. 2024). The development of natural vegetation on this structural intervention is of fundamental importance for soil erosion protection (Romano et al. 2022), especially on mountain slopes. Tree species can help prevent soil erosion and shallow landslides in two ways: modifying the soil moisture regime via evapotranspiration and providing root reinforcement to soil (Stokes et al. 2009; Cao et al. 2025; Zhu et al. 2025). Soil root reinforcement is a very significant contribution by tree species to soil stability. If soils are shallow, roots can reach deeper to more stable substrates, and dense lateral root systems in upper soil horizons can form a mesh that stabilizes the soil. According to Wu et al. 1988, the root systems of trees can provide reinforcement in potential slope failures (Flepp et al. 2021; Huang et al. 2025; Kong et al. 2025). In mountainous areas, tree basal areas, organic matter, and dense root systems are of interest to prevent surface erosion problems (Schmidt et al. 2001; Hao et al. 2021; Gobinath et al. 2022; Liu et al. 2022). The effect of roots on soil fixation has been studied and reported by many authors in many countries over the years (Genet et al. 2006; Abdi et al. 2018; Valizade and Tabarsa 2020; Masi et al. 2021; Meijer 2021; Su et al. 2021; Zhu et al. 2022). Models of root mechanical contribution to soil were created, usually overestimating the effect of roots on soil cohesion by assuming that during failure the roots break at the same time (Bischetti et al. 2009; Schwarz et al. 2013) or that roots are oriented perpendicular to the slip surface (Nghiem et al. 2003; Pollen and Simon 2005; Danjon et al. 2007). Quantifying the exact additional soil shear strength gained by the presence of roots is difficult to achieve. For tree root systems, there must be a distinction between thin roots (<10 mm) and thick roots (>10 mm). When soil failure occurs, thin roots can act as flexible elements of a mesh to contain soil, and the thick roots of trees can act as structural pillars adding longitudinal strength (Reubens et al. 2007). Previous research efforts indicate that thin roots are significantly stronger than thick roots, mainly because of differences in cellulose content (Genet et al. 2005). Some thin roots showed tensile strength values over 700 MPa, surprisingly stronger than steel (Bischetti et al. 2005). Therefore, the thickness of roots and how flexible the roots can be is a very important factor to consider, as it can indicate their ability to penetrate soil and reinforce it against failures. Soil characteristics vary widely across regions and are highly susceptible to rapid modification under anthropogenic disturbances and meteorological events. As a result, estimating root reinforcement potential while assuming that plant species behave similarly across different soil types can lead to significant underestimation or overestimation of their capacity as soil bioengineering agents. Therefore, advancing this field requires the development of local databases documenting the mechanical properties of native plant species. Such information can subsequently be integrated with local soil characteristics and root area estimations from the sites to enable a more accurate assessment of their potential for use in soil bioengineering strategies aimed at stabilizing degraded mountainous regions. 95 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Consequently, the main objective of this study is to estimate the mechanical properties—tensile strength and elasticity—of thin roots from tree species across three distinct vegetation communities distributed along an elevational gradient. Methods Study area The study area is in the Natural Protected Area Altas Cumbres in Ciudad Victoria, Tamaulipas, Mexico. The altitude fluctuates between 350 m a.s.l. and 2180 m a.s.l. The highest elevation is found in the northwest, and the lowest elevation is in the eastern part of the area. The Natural Protected Area Altas Cumbres covers around 30,327.85 hectares and was declared in 1997 with the purpose of protecting and conserving the hydrological watershed, vegetation species, and biodiversity. The geomorphology of the area is characterized by secondary limestone formations folded into sharp anticlines and synclines. The mountain range is rugged and highly dissected along most of its length, with less pronounced escarpments toward the north. Slopes range from 0° to 58°, creating zones with high susceptibility to landslides. The rich biodiversity of both plant and animal species further highlights the ecological value of the region and underscores its importance for conservation. The climate is highly variable depending on altitude, with a prevailing humid subtropical climate (Cwa) according to the Köppen climate classification. The average monthly temperature ranges from 20 °C to 35 °C, with occasional extreme values of 10 °C in January and 48 °C in July. The study was performed in three sampling sites: 1) Tropical semi-deciduous forest (TSdF) located at 23°45.30'N, 99°18.39'W at an altitude of 340 m a.s.l.; 2) Tamaulipan Thornscrub (TTS) located at 23°46.32'N, 99°14.96'W at an altitude of 550 m a.s.l.; 3) Oak–Pine Forest (OPF) located at 23°46.62'N, 99°12.55'W at an altitude of 1100 m a.s.l. (Fig. 1). Root sampling and processing The tree specimens chosen were representative species of the area. The root system of the specimens was cleared from the soil. After the thin root area was completely exposed, it was examined for external damage, and segments of thin roots were collected for further analysis. The classification of roots in this study was defined based on Stokes’s (2009) list of desirable plant root traits for protecting natural slopes, which follows the processes governing soil fixation and divides thin roots into three classes depending on diameter: fine roots (>0.0–2.0 mm), thin roots (>2.0–10.0 mm), and thick roots (>10.0 mm). For this study, because of variation in root diameter among species, we statistically separated three categories of roots: small-diameter roots, medium-diameter roots, and large-diameter roots, all belonging to the thin part of the tree root system (Fig. 2). The contribution of thin tree roots is especially useful because, according to Dupuy et al. (2005), this group contributes to enhanced soil fixation due to bending and branching patterns. The tests were carried out using a digital dynamometer with a 500 N capacity (Baoshishan ZP) placed on a test stand consisting of a clamp system on each side (Fig. 3). Root samples were positioned and clamped carefully to the experimental device at both extremities, avoiding damage to the root samples. 96 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Figure 1. Study area and location of sampling sites. A. Location of Tamaulipas in Mexico; B. Location of the Natural Protected Area Altas Cumbres; C. Location of the sampling sites with different vegetation. Figure 2. Diagram of the position of thin roots tested in this study. The elastic properties of each root sample were determined using the following formula: where Ts is the peak maximum force recorded by the dispositive, Cs is the cross-sectional area, L1 is the change of length of root sample, and L0 is the original length (Fu et al. 2024). 97 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Diameters were measured with a digital caliper (Mitutoyo) at three different points (upper, middle, and lower) to calculate a mean diameter corresponding to the breaking point of the samples. A constant speed of 10 mm/min was maintained for each test, and the dynamometer recorded the maximum tensile strength at root breakage. Tree species The tree species selected were healthy and growing on the slopes of the study area. For the taxonomic identification of the specimens, the work of Stubbendieck et al. (2003) was consulted. Likewise, the botanical nomenclature was homogenized using the International Plant Name Index (International Plant Name Index 2023). The description of the species found in the study sites is as follows: Ficus cotinifolia Kunth (common name: amate) is a tree that can reach a height of 20 m, beginning as an epiphyte and becoming a strangler. Young branches are ribbed, floppy, and light gray. Leaves are obovate, ovate to elliptic, 5.5–13 cm long, and 3.5–6.5 cm broad. It is distributed in warm climates and is usually associated with disturbed vegetation of tropical deciduous forest (Ibarra-Manríquez 1992; Piedra-Malagon et al. 2006). Annona globiflora Schltdl. (common name: anona) is a small tree of 1–4 m tall. Young branches, buds, and petioles are densely pubescent, with brownish trichomes that are appressed or slightly raised. Persistent leaves are 4–8 cm long and 1–3 cm wide, rounded at the base. It is frequent in tropical deciduous forests and can be found with flowers and some fruits from April to August and only with fruits toward the end of the year (Ortiz-Rodriguez et al. 2015). Taxodium mucronatum Ten. (common name: ahuehuete) is a majestic-looking tree that reaches up to 40 m in height. It has light brown bark, straight leaves 8–200 mm long, and inflorescences 15–30 cm long. It is a species of riparian habitats with large water requirements, located in permanent or semi-permanent streams and occasionally in places without direct water contact but with a shallow groundwater table (Martínez-Sifuentes et al. 2021). Figure 3. Device used for testing. A. Dynamometer; B. Clamp system; C. Test stand. 98 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Sommera grandis Standl. (common name: palo colorado) is a tree that reaches a height of 8 m, with reddish-brown branches. Leaves have thick petioles of 0.8–2.5 cm, are light green and pale on the underside, and have subsessile flowers or a fruiting pedicel 3–8 mm long (Borhidi and Stranczinger 2012). Piper amalago L. (common name: cordoncillo o higuillo de limón) is a small tree or shrub 1–6 m tall, profusely branched. Stems are green, internodes 3.5–5.5 cm long, and leaves are uniform in shape and size along all axes, symmetrical, and immensely ovate. It lives in warm, semi-warm, and temperate climates. It is a wild plant that grows along roads and is associated with disturbed vegetation of deciduous, subdeciduous, and evergreen tropical forests (Santos et al. 2015). Randia obcordata S. Watson (common name: altanisa) is a small tree or shrub that grows primarily in the seasonally dry tropical biome. It is very dominant in Tamaulipan thornscrub, reaching heights of around 3 m (Villaseñor 2016). Celtis laevigata Willd. (common name: palo blanco) is a medium-sized tree with narrow and jagged leaves. It grows in mountain zones, and the leaves contain chemicals that inhibit the germination of other plant species (Midolo 2024). Zanthoxylum fagara L. Sarg. (common name: limoncillo) is a tree belonging to the family Rutaceae. It reaches up to 30 m in height. Its trunk is covered by abundant, very strong conical spines. Leaves are spirally arranged and have 3–7 leaflets. It inhabits regions from Mexico to Central America (Valdes-Alameda et al. 2020) Karwinskia humboldtiana Schult. Zucc. (common name: tullidora) is a shrub or small tree 1–8 m tall, with a trunk that can measure 20 cm in diameter. Leaves are small (1–1.5 mm long), glossy green, paler below, with black spots on the main veins, opposite, and with linear stipules. It is found in various habitats, from deciduous tropical forest and dry scrub to oak forest and grasslands, between 250 and 2200 m a.s.l. (Fernandez-Nava 1996). Quercus rysophylla Weath. (common name: encino colorado) is an evergreen tree of rapid growth that can reach 25 m in height. Leaves are alternate, simple, rigid, and elliptic, 7–25 cm long and 2–8 cm wide. Acorns are ovoid, mucronate, and glabrous at maturity, 1–1.7 cm long, single or in pairs, and sessile. The cupola, golden-colored and silky when young, occupies one-third of the acorn and matures after 2 years. This species is endemic to Mexico and found mostly at mid-altitudes in the Sierra Madre Oriental (Sanchez-Castillo et al. 2016). Pinus teocote Schltdl. & Cham. (common name: ocote) is an evergreen tree that reaches 10–20 m in height, with a canopy 12–14 m wide and a trunk 0.65 m in diameter. The bark is fissured and grayish brown; leaves are 10–15 cm long. The fruits are ovoid cones 3.5–6.5 cm long. Naturally, its foliage tends to be rounded, but in cultivation it can take on different shapes (Farjon et al. 1997). Liquidambar styraciflua L. (common name: liquidambar) is a mediumto tallsized tree, 20–35 m but occasionally up to 41 m, with a trunk up to 1 m (and sometimes more than 2 m) in diameter. Leaves are palmate and lobed, 7–19 cm long and broad. It was well known as a medicinal plant by Native Americans. Data analysis The values of tensile strength (Ts) and elasticity (E) of the roots recorded were classified into three categories according to root diameter (Dm); this was done for each of the selected species using Sturges’ rule. Subsequently, to identify 99 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection which of the recorded variables (Ts or E) had the greatest correlation with respect to Dm, a Spearman correlation analysis was carried out. This analysis was performed in the Statistica 13.3 program (TIBCO Software Inc. 2017). To identify the species that best characterize each Dm category, a non-metric multidimensional scaling (NMDS) analysis was performed using Euclidean distance as the similarity matrix. A PERMANOVA was also performed to test differences in species composition between sites. Both analyses were performed using the Vegan package (Oksanen et al. 2019) in R 3.5.3. Results A total of 224 roots from 12 tree species were evaluated, corresponding to 12 genera and 11 families. Five species belong to the TSdF, four to the TTS, and three to the OPF. Piper amalago L. turned out to be the species with the highest Ts for the TSdF, while Randia obcordata S. Watson was the species with the highest Ts for the TTS, and Pinus teocote Schltdl. & Cham. for the OPF. On the other hand, for E, the species with the highest values for both the TSdF and OPF were the same: Piper amalago L. for the TSdF and Pinus teocote Schltdl. Table 1. Parameters evaluated by species in the Peregrina Canyon, Mexico. LDm = Large diameter; MDm = Medium diameter; SDm = Small diameter. Taxon Diameter Min Diameter Max Ts E TSdF Annonaceae Juss. Annona globiflora Schltdl. (Aglo) SDm 1.50 2.23 31.59 327.13 MDm 2.23 2.97 14.60 127.42 LDm 2.97 3.70 8.30 24.72 Moraceae Gaudich. Ficus cotinifolia Kunth (Fcot) SDm 2.10 2.97 54.82 240.11 MDm 2.97 3.83 60.60 283.55 LDm 3.83 4.70 42.11 75.12 Piperaceae Giseke Piper amalago L. (Pama) SDm 2.30 3.70 99.80 640.88 MDm 3.70 5.10 102.40 161.32 LDm 5.10 6.50 83.25 117.62 Rubiaceae Juss. Sommera grandis (Bartl. ex DC.) Standl. (Sgra) SDm 1.30 2.87 54.96 361.59 MDm 2.87 4.43 36.10 116.22 LDm 4.43 6.00 21.17 44.16 Cupressaceae Gray Taxodium mucronatum Ten. (Tmuc) SDm 1.50 3.20 72.90 550.32 MDm 3.20 4.90 61.42 116.93 100 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Taxon Diameter Min Diameter Max Ts E LDm 4.90 6.60 48.45 101.51 TTS Cannabaceae Martinov Celtis laevigata Willd. (Clae) SDm 2.00 3.30 33.57 223.36 MDm 3.30 4.60 22.83 58.76 LDm 4.60 5.90 21.42 24.81 Rhamnaceae Juss. Karwinskia humboldtiana (Schult.) Zucc. (Khum) SDm 2.00 3.00 41.23 267.35 MDm 3.00 4.00 22.68 60.88 LDm 4.00 5.00 22.36 41.11 Rubiaceae Juss. Randia obcordata S. Watson (Robc) SDm 2.00 3.00 45.95 201.39 MDm 3.00 4.00 28.83 75.07 LDm 4.00 5.00 25.48 74.31 Rutaceae Juss. Zanthoxylum fagara (L.) Sarg. (Zfag) SDm 1.60 2.97 33.63 181.50 MDm 2.97 4.33 45.00 83.75 LDm 4.33 5.70 19.72 17.75 OPF Altingiaceae Horan. Liquidambar styraciflua L. (Lsty) SDm 2.20 3.27 38.41 133.31 MDm 3.27 4.33 14.40 23.64 LDm 4.33 5.40 30.78 38.14 Pinaceae Spreng. ex Rudolphi Pinus teocote Schltdl. & Cham. (Pteo) SDm 1.40 2.87 51.94 415.84 MDm 2.87 4.33 33.88 114.72 LDm 4.33 5.80 19.42 48.63 Fagaceae Dumort. Quercus rysophylla Weath. (Qrys) SDm 1.40 2.60 39.05 319.77 MDm 2.60 3.80 28.26 86.12 LDm 3.80 5.00 29.48 58.41 & Cham. for the OPF. However, for the TTS, Karwinskia humboldtiana (Schult.) Zucc. turned out to be the species with the highest E recorded (Table 1). The E was the variable that best correlated with root diameter. This correlation turned out to be negative but significant (p < 0.05). The PERMANOVA test detected significant differences in species composition among all sites (SStotal = 1366000; SSwithin-group = 628500; F = 3.52, P < 0.001). The sampled plant communities formed separate groups in the NMDS diagram (Stress = 0.11) (Fig. 4). Piper amalago L., Sommera grandis (Bartl. ex DC.) Standl., and Taxodium mucronatum Ten. were the species with the highest 101 BioRisk 23: 93–109 (2025), DOI: 10.3897/biorisk.23.173254 Laura Sanchez-Castillo et al.: Mechanical properties of root tree species for landslide protection Figure 4. NMDS for plant species elasticity by root diameter categories. TSdF (green), TTS (yellow), and OPF (red). The meaning of the abbreviations is presented in Table 1. Table 2. Order of maximum values recorded for tensile strength and E of each species. Dm TSdF Ts SPiper amalago >Taxodium mucronatum > Sommera grandis > Ficus cornifolia > Annona globiflora MPiper amalago > Taxodium mucronatum > Ficus cornifolia > Sommera grandis > Annona globiflora LPiper amalago > Taxodium mucronatum > Ficus cornifolia > Sommera grandis > Annona globiflora ESPiper amalago > Taxodium mucronatum > Sommera grandis > Annona globiflora > Ficus cornifolia MFicus cornifolia > Taxodium mucronatum > Piper amalago > Sommera grandis > Annona globiflora LPiper amalago > Taxodium mucronatum > Ficus cornifolia > Sommera grandis > Annona globiflora Dm TTS Ts SRandia obcordata > Karwinskia humboldtiana > Zanthoxylum fagara > Celtis laevigata MZanthoxylum fagara > Randia obcordata > Celtis laevigata > Karwinskia humboldtiana LRandia obcordata > Karwinskia humboldtiana > Celtis laevigata > Zanthoxylum fagara ESKarwinskia humboldtiana > Celtis laevigata > Randia obcordata > Zanthoxylum fagara MZanthoxylum fagara > Randia obcordata > Karwinskia humboldtiana > Celtis laevigata LRandia obcordata > Karwinskia humboldtiana > Celtis laevigata > Zanthoxylum fagara Dm OPF Ts SLiquidambar styraciflua > Quercus rysophylla > Pinus teocote MPinus teocote > Quercus rysophylla > Liquidambar styraciflua LLiquidambar styraciflua > Quercus rysophylla > Pinus teocote ESPinus teocote > Quercus rysophylla > Liquidambar styraciflua MPinus teocote > Quercus rysophylla > Liquidambar styraciflua LQuercus rysophylla > Pinus teocote > > Liquidambar styraciflua E and small Dm in the TSdF. 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