The Root Traits Reducing Landslide Susceptibility

• Stabilization of slope hillsides is one of the crucial ecosystem functions of roots.
• Roots modify mechanical and hydrological soil properties to reduce landslide susceptibility, which is one of the main benefits of root traits reducing landslide susceptibility.
• Roots reinforce soil, provide structural support, deepen anchoring, and help to dry soils quickly after rains.
• Several underlying root traits can act together to produce composite ones, and both categories of attributes determine the resistance that roots offer to landslides.

Shallow landslides are highly hazardous and can become destructive. Vegetation cover is a vital conditioning factor to reduce landslide susceptibility. Root reinforcement is the most relevant for slope stability of all the plant traits. Find out more about the effect of roots on soils that can be useful in mitigating landslides and the relevant root traits involved.

Land Cover for Stabilization

Landslides are triggered by intense rainfall, leading to multiple landslides in affected regions. Landslides quickly develop into debris avalanches with increased speed and material volume.

Land cover by plants can make slopes stable. Roots are the plant parts essential in stabilizing slopes through their functioning, sometimes combined with above-ground vegetation.

Soil reinforcement against landslides is one of the most critical ecosystem functions of root traits. Root traits can be morphological, architectural, physiological, and anatomical.

On the local scale, the mechanical effect of roots in soil reinforcement is crucial. On large scales, the hydrological impacts of rainwater interception and water infiltration into the soil, evapotranspiration, and suction that affect the soil water balance and runoff on slopes become crucial. See Figure 1.

Figure 1. “Mechanical and hydrological main effects of vegetation on slopes’ soil behavior.” Masi et al. 2021. (Image credits: https://doi.org/10.3390/geosciences11050212)

Mechanical Effects

Below are the ways in which plant root traits can reduce landslide susceptibility through mechanical means.

Soil Reinforcement: Roots increase soil shear and tensile strength through their root network or “root cohesion.” Soil reinforcement is the most critical mechanical factor that reduces landslide susceptibility. Plant root systems can enhance the shear strength of soils in two ways. Large woody roots that anchor trees to the ground also clamp the superficial layers of soil to stable substrates that cross potential planes of weakness, see Figure 2. Meanwhile, smaller roots boost the soil-root matrix cohesion by reinforcing the bonds between soil particles. Root reinforcement works on the basal and lateral plane failures of landslides.

Surcharge: The weight of vegetation, especially trees, increases the normal and tangential forces acting on slopes. However, in most cases, the impact of this additional weight on slope stability is minimal. So, it’s usually not a primary factor contributing to slope stability analysis.

Buttressing and Arching between Plants: Trees’ roots, stems, and branches function as arch abutments or buttress piles. In this capacity, they provide resistance against shear forces acting on slopes. This structural support can help stabilize the slope and reduce the risk of erosion or collapse.

Deep Anchoring: In shallow soils, plant roots that penetrate bedrock joints and fractures establish a deep and robust plant anchoring system. This deep anchoring can enhance the stability of slopes and prevent soil erosion.

Figure 2: “Interactions of small and large roots with soil particles and rocks that generate the root reinforcement effect. Masi et al. 2021. (Image credits: https://doi.org/10.3390/geosciences11050212)

Hydrological Effect

The hydrological effect of roots and above-ground canopy arises from delaying the soil from reaching critical saturation levels, which triggers mass landslide movements. The main plant processes are as follows:

Water suction: Vegetation influences soil moisture by absorbing water from the surrounding soil through their roots. This process is driven by transpiration, where moisture is taken up by the roots and released into the atmosphere through the leaves. Trees reduce soil moisture levels over an area three times their crown radius. Suction is helpful and effective in cases of moderate precipitation where evapotranspiration reduction of soil moisture levels before rainfall is enough to increase the capacity of the soil to store water. The soil moisture reduction is insignificant in regions where precipitation exceeds evapotranspiration consistently.

Interception: Vegetation, including branches and foliage, can intercept rainwater as it falls. It breaks the force with which water reaches the ground to cause erosion. Water retained on the canopy evaporates into the atmosphere or is released as it drips onto the ground. The extent of interception depends on factors like the plant species, its age, forest structure, and climate. Plants can intercept and evaporate all the rainwater only during light precipitation; only a portion is intercepted during heavy rainfall.

Infiltration and Subsurface Flows: Root systems affect water movement into the soil. Root decay leaves behind channels and pipes that allow for faster water drainage on slopes. However, these root channels can also increase the infiltration rate, which may raise the risk of landslides.

Destabilizing Effects

Vegetation cover can also have a few destabilizing effects, but they are not large enough, and roots’ overall effect is to stabilize soil. The significant destabilizing impacts are:

• Increasing the parallel stresses due to the plant’s weight.
• Transmitting bending force, due to canopy moment, to roots through the stems.
• Fracturing rocks by growing into rock cracks and causing bedrock weathering changes the strength of subsurface materials and increases landslide frequency, especially on steep slopes directly exposed to disturbances.

Root Traits Reducing Landslide Susceptibility

Figure 3. “Direct, indirect and hierarchical relationships between the (non-exhaustive) range of root traits linked to soil reinforcement against shallow landslides. Black arrows represent causal relationships. + and − represent the direction of the relationship. Trait position along the vertical axis depicts trait hierarchical relationships, with lower levels representing ‘composite’ traits and upper levels representing ‘underlying’ traits.” Freschet et al. 2020. (Image credits: https://doi.org/10.1111/nph.17072)

Traditionally, only root architecture and strength were the traits used to measure root reinforcement. Nowadays, root reinforcement magnitude is measured by several root traits.

Underlying root traits act independently and interact with other characteristics to produce composite traits. Figure 3 shows the combinations of some underlying root traits to make composite traits.

The composite root traits reducing landslide susceptibility are as follows:

• Mechanical resistance offered by roots depends on composite root traits like root bending resistance, tensile strength, and elastic modulus.
• Hydrological or drying ability and interaction with the mechanical influence of roots depends on mycorrhizal colonization ability, root hair length and density, and root hydraulic conductance.
• Traits related to the amount and position of roots, which can influence both hydrological and mechanical effects, are branching density, vertical root distribution index, root area ratio, root branching angle, and root lifespan.

The above composite traits can be derived and influenced by underlying root traits such as

1. Root diameter
2. Stele diameter
3. Root tissue density
4. Specific root length
5. Root mass
6. Root length density
7. Root exudates
8. Cortex area fraction
9. Total mycorrhizal hyphae length and diameter
10. Total mycorrhiza hyphae surface area
11. Total mycorrhiza hyphae volume
12. Total glomalin-related soil proteins

Factors Affecting Root Traits

The root traits are not the same in all plants. They differ based on several factors, as listed below.

• Growth forms and species have different trait syndromes and can have varying trait-to-function associations. Plant ontogeny and varying locations can produce variable root systems within a species.
• Land cover changes and management alter an area’s combination of species and effects on landslides. For example, grapevines with grass or woodlands have more soil stability than croplands or shrublands.
• Root types are vital, as the entire root system prevents soil collapse. So far, research has not focused on differentiating the effects of various root types.

Effective Mitigation Strategies

The number of root traits studied to monitor slope stability and develop models to mitigate landslides has increased in the recent past. Due to the species-specific and landcover-specific nature of root effects, scientists consider it necessary to study more species’ root systems to reduce landslide susceptibility. Precision tools like CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager used with minirhizotrons allow long-term non-destructive observation of root morphology and dynamics. They can significantly assist in studying slope stability and developing area-specific landslide mitigating models.

Sources

Alberti, S., Leshchinsky, B., Roering, J., Perkins, J., & Olsen, M.J. (2022). Inversions of landslide strength as a proxy for subsurface weathering. Nat Commun. 13;13(1):6049. doi: 10.1038/s41467-022-33798-5. Erratum in: Nat Commun. 2023 Mar 17;14(1):1509. PMID: 36229607; PMCID: PMC9561700.

Freschet, G. T., Roumet, C., Comas, L. H., … Stokes, A. (2021). Root traits as drivers of plant and ecosystem functioning: Current understanding, pitfalls and Future Research Needs. New Phytologist, 232(3), 1123–1158. https://doi.org/10.1111/nph.17072

Masi, E.B., Segoni, S., & Tofani, V. (2021). Root Reinforcement in Slope Stability Models: A Review. Geosciences 11, 212. https://doi.org/10.3390/geosciences11050212

Stokes, A., Atger, C., Bengough, A., Fourcaud, T., & Sidle, R. (2009). Desirable plant root traits for protecting natural and engineered slopes against landslides. Plant and Soil 324: 1–30.

Li, Y., Zhou, M., Zuo, H. et al. (2021). Root Traits and Mechanical Properties of Three Shrubland Species: Implications for Bioengineered Slope Stability. Environmental Engineering Science.1176-1187.http://doi.org/10.1089/ees.2020.0538