Roberto J. Marin

This is an automatically translated Manuscript of an article published by Revista Politécnica (ISSN: 1900-2351) on June 30, 2017, available at https://revistas.elpoli.edu.co/index.php/pol/article/view/1095

Marín, R. J., & Osorio, J. P. (2017). Efectos de la vegetación en la estabilidad de laderas: una revisión. Revista Politécnica, 13(24), 113–126. Recuperado a partir de https://revistas.elpoli.edu.co/index.php/pol/article/view/1095

Effects of Vegetation on Slope Stability: A review

EFFECTS OF VEGETATION ON SLOPE STABILITY: A REVIEW
Roberto J. Marín ¹, Juan Pablo Osorio²
¹ LandScient – Landslide Scientific Assessment, Medellín, Colombia. Email: rjose.marin@udea.edu.co
² Civil Engineer, PGDip, MBA, MEng, PhD. Email: juan.osorio@udea.edu.co
¹ ² GeoR – GeoResearch International Group, Environmental School, Faculty of Engineering, University of Antioquia – UdeA; Calle 70 No. 52-21, Medellín, Colombia.

Efectos de la vegetación en la estabilidad de laderas: una revisión

ABSTRACT
Slope stability is usually affected by factors that reduce soil strength and increase driving forces acting on the slope material, sometimes generating mass movements. Among these factors, vegetation has an important role related to hydrological (e.g., evapotranspiration and infiltration) and mechanical (e.g., tree surcharge and root reinforcement provided by trees) mechanisms. In this review, vegetation influences associated with slope stability are identified, describing the mechanisms involved in the occurrence of mass movements. Finally, the way these factors affect slope stability is explained according to soil properties, climatic conditions, and environmental characteristics.

Keywords: Vegetation, slope stability, landslides, trees, infiltration.

How to cite this article: R. J. Marín, J. P. Osorio, “Efectos de la vegetación en la estabilidad de laderas: una revisión,” Revista Politécnica, vol. 13, no. 24, pp. 113-126, 2017.

  1. INTRODUCTION

In recent decades, numerous studies have demonstrated that slope morphology and processes are significantly influenced by the distribution of vegetation on the terrain. Generally, vegetation has a positive impact on soil stability in sloped terrains [1], acting as a protective barrier between the soil and elements that provoke mass movements [2].

Mass movements are all processes that occur on slopes where gravity alone becomes the dominant agent transporting soil or rock masses [3]. The stability of a slope depends on the balance between resisting and destabilizing forces, which develop on a potentially unstable soil mass. Destabilizing forces generate shear stress that must be countered by the available shear strength. Based on this, the causes of instability in a sloped terrain can be subdivided into internal and external factors. Internal factors reduce the available soil strength, while external factors increase the destabilizing forces acting on the terrain mass [4].

One of the internal factors that have the most significant influence on triggering different types of mass movements is forest properties [5]. The influence of trees on stability can be divided into two types of mechanisms: hydrological and mechanical. Likewise, the way vegetation presence impacts stability includes elements that contribute to stability as well as those that favor terrain instability.

From a hydrological perspective, vegetation reduces the water available for infiltration and soil moisture levels through interception, rainfall evaporation in the tree canopy, and the transpiration process. However, it can increase the soil’s infiltration capacity by increasing roughness and promoting the formation of desiccation cracks.

Mechanically, vegetation contributes to stability by anchoring roots in more stable soil layers, laterally tying surfaces susceptible to failure, and providing a reinforcing membrane to the soil layer, thus increasing the terrain’s shear strength. Nevertheless, the weight of trees increases the normal and parallel force components on the slope, which in certain cases favors instability, as does the dynamic forces transmitted by the wind through tree trunks [6].

In this sense, the importance of the various mechanisms associated with vegetation presence and their role in slope stability has been considered by different researchers worldwide. Progress in studying these mechanisms over recent decades has reached the point where some parameters representing the effects of trees have been included in susceptibility models for mass movement occurrence on slopes or large terrain areas (e.g., [7] [5]). However, because many of these parameters have not been studied in detail and there are no tools available to quantify or include their contribution in stability studies, current models of these mechanisms have a long way to go to determine vegetation’s contribution to stability with more precise, detailed, and reliable approaches.

To advance slope stability evaluation methods, quantifying vegetation’s contribution requires expanding knowledge of these mechanisms and the techniques currently used to study them. In this sense, this review aims to contribute to understanding the mechanisms associated with vegetation cover presence in relation to slope stability. These mechanisms are conceptually described and explored, highlighting the relationships between them and their behavior influenced by other variables such as climatic, soil, and groundwater level characteristics, among other environmental factors.

Thus, to carry out this review, each of these mechanisms was identified, and a bibliographic database search was conducted to illustrate the current state of knowledge on them, evaluate the material found, and select information sources for the literature review. This started with the description of hydrological and mechanical mechanisms grouped in Table 1, an adaptation of that presented by Sidle and Ochiai [6], which is a summary of vegetation’s relative influences on slope stability. This summary is based on the one presented by Greenway [8], although widely modified.

  1. EFFECTS OF VEGETATION ON SLOPE STABILITY

The effects of arboreal vegetation on mass movement occurrence can be grouped into two main categories: hydrological mechanisms and mechanical mechanisms. Current approaches to the relationship of vegetation-related variables influencing the triggering of soil movements are based on numerous investigations carried out worldwide.

Table 1. Summary of vegetation influences on slope stability. ‘A’ denotes mechanisms adverse to stability, ‘MA’ denotes marginally adverse mechanisms, ‘MB’ denotes marginally beneficial mechanisms, and ‘B’ indicates beneficial mechanisms.

2.1 Hydrological Mechanisms

Vegetation has a significant influence on hillside hydrology, and this relationship is reciprocal; the characteristics of vegetation directly impact erosion activity and mass movements [2]. This influence is evident in the reduction of water available for infiltration due to processes such as rain interception by the tree canopy and its subsequent evaporation, decreased soil moisture levels due to the physiological processes of plants extracting water through transpiration, increased surface roughness due to the presence of plant components like stems and roots, and the formation of desiccation cracks due to moisture depletion.

2.1.1 Evapotranspiration

Evapotranspiration is a crucial component for maintaining the water and energy balance in forests [9]. It comprises different water vapor flux processes, including soil surface evaporation, plant transpiration, and evaporation of water intercepted by the tree canopy [10]. These processes are controlled by specific biotic and physical factors [11] [12].

Depending on soil characteristics, vegetation has a certain evapotranspiration potential, which, based on the availability of rainfall and groundwater, can achieve a moisture equilibrium [13]. Various vegetation covers have different balances of the main water vapor fluxes mentioned earlier, representing water losses concerning slope stability. Among these processes, transpiration and evaporation of water intercepted by vegetation are considered most important for stability. On the other hand, soil surface evaporation is controlled by the depth of the water table, pore pressure distribution, and local thermal balances. In terrains with extensive vegetation cover, these variations are small compared to other components affecting evapotranspiration [14] [15] [16] [6].

The evapotranspiration rate depends on turbulence, which results from wind distribution and surface roughness, allowing water vapor to dissipate into the air. Therefore, evapotranspiration is not constant over time; it varies significantly with atmospheric boundary conditions and the state of the soil surface and vegetation, directly influencing evaporation and transpiration rates [2].

Potential evapotranspiration refers to the maximum amount of water that can be evaporated under current atmospheric conditions on a soil or uniform water surface when there is no limitation in water availability [17] [18]. Due to the difficulty in separately measuring interception, transpiration, and evaporation fluxes, they are commonly grouped under the concept of evapotranspiration when assessing slope stability. Generally, evapotranspiration rates in temperate regions are very low in soils devoid of vegetation, several times higher in grasslands, and 5 to 10 times greater in forests [15] [6].

In temperate and dry regions, the evapotranspiration balance is influenced by seasons. In temperate climates, during the winter rainy season, evapotranspiration likely has little effect on antecedent soil moisture since soils are already near saturation, and transpiration is low [19] [20] [21], making the effect of evapotranspiration insignificant in landslide initiation. However, because most mass movements in this climate occur during prolonged winter rains, evapotranspiration may affect landslide occurrence if a storm occurs near the beginning or end of the rainy season [22] [23], as well as in dry conditions triggered by high-intensity rainfall events [6].

In tropical climates, evapotranspiration plays a more crucial role in altering soil moisture, as it is high in tropical forests year-round [6] [8] [23]. Although soil surface evaporation under the tree canopy is difficult to assess in experimental studies, in terrains with good vegetation cover, it can account for just over 10% of total evapotranspiration; meanwhile, in dry areas with sparse vegetation, higher surface soil evaporation rates may occur [24] [6]. Additionally, interception and subsequent evaporation of water from the vegetative cover is particularly significant in coniferous forests, where snow and rain losses from these dense foliage can represent between 30% and 50% of the gross annual precipitation [25] [6].

2.1.1.1 Rain Interception

In terms of vegetation effects on slope stability, canopy interception is defined as the amount of rain intercepted, stored, and lost by evaporation from the upper layer of tree leaves [26]. This water captured by vegetation represents a reduction in the amount of water available for infiltration. Similarly, the presence of vegetation influences the time it takes for water to reach the ground surface [6].

During a rain event, some drops fall directly to the ground, while others are intercepted by trees, grasses, and shrubs. The process by which water flows through the canopy or drips from the leaves is known as free runoff. Additionally, the process where water is channeled by stems and trunks to the soil surface is known as cortical runoff. Intercepted water is collected in leaves and branches, and water evaporated from vegetation into the atmosphere represents evapotranspiration losses [27] [28]. Interception losses are generally quantified as the difference between total precipitation and effective precipitation, which is the sum of free runoff and cortical runoff [29].

Interception losses mainly depend on the capacity of plant species to retain precipitation. Moreover, rain interception can occur at all types and levels of vegetation, including undergrowth and litter [2]. However, the amount of water that can be evaporated by grasses and shrubs is less since they receive less precipitation (when a tree canopy is present), have low surface roughness, and are protected from the sun and wind by trees [6]. These losses are significantly influenced by forest structure, precipitation characteristics, and climatic variables that govern the evaporation rate during and after rain events [30].

Trees with denser foliage tend to retain rain droplets for longer. Similarly, depending on rain intensity, more or less water is retained, with less in very intense precipitation events [13]. In terms of stability, the contribution of interception is not very significant in developing shallow landslides during extended rainy seasons, except in the tropics and subtropics, where evapotranspiration is high throughout the year [6] [23].

2.1.1.2 Transpiration

Vegetal transpiration is a phenomenon that occurs in plants, involving a physical and biological process where they convert liquid water into gas through their metabolism [31]. This phenomenon consists of water absorption from the soil by roots, transportation through the plant of organic compounds obtained from the soil, and the release of water vapor through leaf openings or stomata [6].

The plants’ capacity to consume soil moisture, measured by their transpiration rates, depends on solar energy availability, soil moisture, type, size, and age of species, depth of roots, vegetation density, leaf area index in species, leaf conductance, foliage albedo, canopy structure, soil characteristics, and other climatic and environmental factors [32] [6] [8] [13] [25]. Despite this, it has been observed that when water is freely available, similar transpiration rates generally occur [33] [34] [35] [6].

The reduction in soil moisture, mainly provided by the root system, positively influences slope stability in terms of developing both shallow and deep landslides. In terrains with dense vegetation, transpiration is the dominant process in water vapor conversion, being more important than the evaporation effect of water intercepted by the tree canopy and water evaporated from the soil surface [6]. The role of undergrowth in transpiration variation is more important when trees have deep roots and access to groundwater [16].

Although significant progress has been made conceptually in identifying the most and least influential variables in studying plant transpiration, measuring it remains complex, and it is difficult to separate transpiration and interception fluxes [6] [15].

Transpiration rates can be deduced from sap flow measurements. However, these measurements are feasible only for large trees in sparse terrain, making it difficult to capture the spatial and temporal heterogeneity of parameters related to transpiration action in terrains with extensive vegetation cover [2].

2.1.2 Infiltration

Infiltration is defined as the movement of water from the ground surface into lower soil layers through pores, interstices, and discontinuities in the soil mass [13]. Generally, the impact of rain infiltration on stability is evident in changes in suction and positive pore pressure, variations in the water table depth, increases in soil unit weight, and reductions in the shear resistance effect provided by rocks and soil [35] [37].

As described below, the presence of vegetation cover on a slope can favor infiltration due to increased surface roughness and the formation of desiccation cracks.

2.1.2.1 Increase in Surface Roughness

Vegetation presence increases surface roughness due to the arrangement and shape of the vegetative cover and the presence of roots, stems, and organic matter at the soil surface level. Greater surface roughness increases the soil’s water infiltration capacity [6].

As previously mentioned, surface roughness is related to turbulence, allowing water vapor to dissipate into the air. Therefore, surface roughness influences the evaporation process. This effect is more significant in isolated trees, where increased turbulence leads to higher evapotranspiration rates [2]. However, this is not the case with low-height vegetation covers like grasses and shrubs, as they have lower surface roughness and experience less turbulent air exchange above [6].

On the other hand, soil surface roughness affects runoff velocity, which, although also dependent on other factors like slope, rain intensity, and soil moisture [13], it can be said that greater terrain roughness reduces runoff velocity and, depending on rain conditions and soil infiltration capacity, allows greater water infiltration into the soil.

2.1.2.2 Generation of Drying Cracks in the Soil: Drying cracks form in the soil due to water loss, being common in clay-rich sediments that contract and develop fractures upon drying [38]. The presence of vegetation cover with deep roots can promote the formation of drying cracks due to moisture depletion in the soil, mainly in regoliths with high clay content, thereby allowing the creation of preferential pathways for rapid and deep infiltration [6]. This moisture depletion generally occurs during the season with dry climatic conditions, due to the action of evaporation and plant transpiration. These cracks develop vertically, potentially reaching widths of 20 cm and depths of 1 m. Repeated cycles of wetting and drying produce volumetric changes in the soils that lead to the formation of irregular surfaces [39] [2].

2.2 Mechanical Mechanisms
The mechanical properties of vegetation have beneficial and adverse effects on soil stability. On one hand, the presence of strong roots allows for soil reinforcement by anchoring in more stable soil strata; lateral roots tie the soil together by developing through potential failure planes. Likewise, root systems can provide a reinforcing membrane in the soil mantle, increasing its shear strength. Additionally, the roots and trunks of trees can provide support to the upper soil layer of the slope through bracing and arching. On the other hand, the weight of trees represents an overload that increases the components of normal and parallel forces to the slope, and the dynamic forces of the wind transmitted to the soil through the trunk and roots of the trees act as forces that can affect slope stability.

2.2.1 Anchoring in Deeper Soil Mantle: During slope failure, tree roots can provide an anchoring effect of the sliding mass to the stable part of the soil, helping to prevent further movement [40] [41]. In addition to the reinforcement they provide, the role of vegetation in anchoring the soil reflects a contribution to its stability, which depends on factors such as the morphology of the root system, strength, distribution, and interaction of the soil with the roots [42].
The anchoring that stumps or tree trunks can provide has been studied due to their frequent use as anchors in cable logging systems, such that their pullout resistance has been investigated [43] [44].
Furthermore, the anchoring of tree roots has been studied from the perspective of resistance to forces induced by wind that cause tree falls in forests [45] [46] [47] [6]. In general, there are numerous reports of the benefits of roots penetrating through relatively shallow soil mantles and anchoring in more stable strata [48] [49] [50] [6].
Studies on the effects of wind action on root growth showed that changes in the morphology and topology of the root system of young trees subjected to wind loads resulted in increased anchorage [51] [52] [2].
From a geomechanical perspective, roots penetrate shear surfaces, acting as individual anchors that extend through the soil matrix without failing, mobilizing a friction force at the soil-root interface. This is the second most important mechanism associated with the effect of the root system. The other mechanism is the increase in the shear strength of the soil due to the transfer of shear stresses through the root fibers [53].
Generally, these geomechanical effects are represented in slope stability models as an increase in the soil cohesion term in the Mohr-Coulomb failure criterion equation [54].

2.2.2 Connection through Weak Planes along Potential Failure Flanks: In slopes, the presence of lateral roots can provide reinforcement to the soil in this direction [55] [2]. The lateral connection of unstable layers through weak planes is a special case of lateral root reinforcement, which ties through the flanks of the potential sliding mass [6].
The presence of roots along the cutting zone, where stresses concentrate, tends to “sew” the elements that are separated with the displacement along the failure surface. Thus, two effects develop during failure: the direct one, which opposes the relative movement of the two blocks outside the cutting zone, and a second one, which is normal to the cutting plane, that tends to increase the resistance associated with friction on the failure surface [54].
In a study conducted in the Oregon mountain ranges, Schmidt et al. [56] concluded that the lateral cohesion of roots represented the dominant reinforcement mechanism in the shallow soils of this area. Likewise, numerous studies have noted the stabilizing effects of large roots, both along flanks and on sliding surfaces of potential landslides [57] [58] [59] [6] [48] [50]. On the other hand, other authors have described the benefits of lateral root reinforcement [60] [61] [20], with the widely accepted consideration that lateral roots provide greater protection against shallow landslides compared to deep failures [62]. Thus, although tree roots can provide stability in deeper soils through lateral reinforcement that crosses weak planes [63] [57], this beneficial effect would decrease with an increase in the area and depth of the potential failure [6].

2.2.3 Increase in Soil Shear Strength: Generally, the contribution of roots from vegetation cover to soil shear strength is recognized as more significant to slope stability than the losses due to evapotranspiration [64] [65] [6] [8] [48].
The stiffness and deformation of tree root fibers can significantly reinforce soil shear resistance [66] [67] [6]. Soil with the presence of tree roots acts as a composite material, in which roots embed in the soil matrix and contribute to increasing shear strength due to the high tensile strength developed by the roots. In contrast, the soil matrix is highly resistant to compression but weak in tension. Therefore, the combined effect of soil and roots results in reinforced soil [54].
Mechanically, under shear loading conditions, one of the effects of this composite material is the transfer of shear stresses developed in the soil matrix, which are transferred to the root fibers through the mobilized tensile resistance, generating an increase in the strength of the rooted soil [53] [54]. This mechanism represents one of the most significant geomechanical aspects associated with the effect of the root system on the soil.
In general terms, the influence of roots on soil shear strength is more significant at the moment of failure and after the critical phase, making the composite material of soil and roots more deformable. In contrast, in the phase prior to failure, the presence of roots is not significant from the perspective of contributing to the initial stiffness of the system [54].

2.2.4 Anchoring in Firm Stratum through Bracing and Arching: Bracing can be described as a phenomenon in which the presence of the root system and the trunk of trees, or the stem for vegetation in general, block soil movement. The action of these elements of vegetation cover, in contributing to soil stability through bracing, is more significant when both stems and roots have sufficient diameter to act rigidly in their anchoring [2].
In this phenomenon, the trunk provides stability to a column of soil located above it, in the direction of the slope, because the anchoring of the trunk acts as a buttress on the slope, exerting resistance to shear stresses and laterally restricting surface movement of the slope [68] [2]. The magnitude of the bracing contribution to the stability of the soil mass depends on both the depth of the soil mantle and the water table level, as well as the degree of penetration that roots develop in the bedrock [69] [70].
Arching is defined as a similar phenomenon that develops due to the presence of multiple trees and/or elements of vegetation. When there are two trees located a short distance apart horizontally on the slope (at the same elevation, separated laterally) and generate the bracing phenomenon in the soil, certain stability can be achieved in the soil located between both buttresses, which is not braced but can gain strength by deforming in an arch shape [70]. When an area of a slope experiences an arching condition, it can be stated that the growing trees act as anchored piles in a firm stratum of the subsurface [2] [64].
The combination of the forces exerted on the roots and the trunks of the trees, as a result of bracing and arching, is considered as static forces due to the gradual manner in which they increase in magnitude [2].

2.2.5 Overloading of Trees: In terms of the effect of vegetation on slope stability, overload can refer to both the individual weight of a tree on an inclined terrain and the combined weight of all the vegetation, from the perspective of a global analysis of slope stability [2]. In this context, the term overload can include any additional external load on top of the soil weight, such as the load generated by all types of existing or growing vegetation on the terrain, material deposited by the action of landslides or volcanic activity, or even loads contributed by water, as in the case of precipitation, surface flow, and groundwater flow [71].
The weight of trees on a slope, and thus the tree overload, depends on the species type, trunk diameter, height, density of timber trees, and their spacing in the terrain. However, a relatively dense forest represents only a small overload [7] [71].
Generally, tree overload is not considered a significant load in slope stability analysis, representing a minor mechanical component of instability [6] [13]. In fact, for most mature forests, the total weight of the soil on a potential failure plane often far exceeds the weight of all its trees, so any additional load contributed by them will not significantly affect slope stability [72] [73] [74] [75] [76].
Sidle [77], in a study on the relative importance of some factors in calculating the safety factor (FS) in landslide-prone soils on steep slopes, concluded that the weight of trees could not be significant enough to impact the shear strength of the soil. However, in some specific scenarios, tree overload can be an important factor that destabilizes slopes. This is especially true when trees have shallow roots, such as certain species with soft woods and that can suffer from overloading under extreme climatic conditions, which could generate situations of instability. In this sense, it has been shown that the stability of slopes with shallow-rooted trees can be affected more than expected by the weight of their branches, trunks, and leaves [75].

2.2.6 Wind Forces: The action of the wind and the efforts caused by dynamic loads reduce the shear resistance of the ground and increase shear stresses, potentially triggering mass movement events, which are favored when high levels of soil moisture are present [79][80]. The wind represents an external load that is transferred downward through the tree trunk and into the roots, from where it is efficiently transferred into the soil in cases where mechanical failure of the tree is prevented. If the root system is not adequately anchored, the tree may be toppled by bending or breaking at the root or base of the trunk, or it may even be uprooted [81]. When the soil is shallow, tall trees are more susceptible to falling during gusts of wind, causing a reduction in slope stability [2].

The mode of failure of a tree that is uprooted during a windstorm primarily depends on the morphology of the soil-root interface (i.e., roots and the soil adhered to them) and the type of soil. During the initial uprooting stage of the tree, the weight of the roots and the adhered soil provide initial resistance to tipping. If the force on the trunk exceeds the resistance of the soil-root interface, the tree is uprooted, and the surrounding soil experiences a rupture [2]. The tensile resistance of the roots on the side facing the wind provides high resistance to uprooting, while the bending resistance of the roots on the opposite side offers less resistance [82][83][2].

In some cases, and mainly in certain geographical areas, the occurrence of avalanches and debris flows in soils has been directly related to the action of the wind, which can cause the trunk of trees to be uprooted or broken. Such effects can substantially increase the susceptibility of slopes to landslides [79]. Likewise, fast and channeled debris flows develop, occurring on the side and front walls of ravines as a result of landslides triggered by wind action [80].

Trees can impose a small instability on slopes due to the effects of wind [6], considering wind loads relevant when analyzing the stability of individual trees. However, the action of the wind does not represent the same significance for the overall analysis of slope stability, as wind forces constitute a smaller proportion of the possible disturbing forces. Additionally, trees within the terrain are somewhat protected by those located at the edges, which are the ones most affected by wind action [84]. Nevertheless, the occurrence of mass movements triggered by gusts of wind is directly associated with the high exposure of trees found at borders, such as that caused by tree logging [80].

  1. CONCLUSIONS

The presence of vegetation on slopes affects stability in various ways and through different mechanisms. These mechanisms can be grouped into hydrological and mechanical mechanisms, which can constitute both adverse and beneficial effects on stability related to the occurrence of mass movements.

Hydrological mechanisms are primarily related to the infiltration of rainwater into the soil, in terms of the amount of water and its ease of reaching deeper strata and causing variations in pore pressures, which affects stability in the terrain. Among these mechanisms are rainfall interception, evaporation, transpiration, increased terrain roughness, and the generation of drying cracks. Mechanical mechanisms are mainly associated with the mechanical reinforcement provided by tree roots, although this group has included other effects such as wind forces and the mass of vegetation.

The described mechanisms show that, in general terms, the effects of vegetation on slopes tend to favor stability. Likewise, numerous studies demonstrate how the different effects of plant presence, primarily trees, contribute to slope stability. However, it is important to note that the complexity of the associated variables and the variability in the conditions of each slope lead to differing stability conditions for each case study.

There are few studies that point to the exact mechanism by which trees with different types of root systems fail during a mass movement. Field studies should be conducted after the occurrence of these events, involving the quantification of plant presence in assessing the type of failure. It is also considered important to include direct comparisons in future research between predictions from models that quantify the contribution of trees to stability (such as models of root reinforcement, evapotranspiration models, among others) and data obtained from efficient field studies [85]. In this regard, progress should be made in studying the morphology of plants and their root systems for selecting appropriate vegetation for stabilizing various terrains, how the diversity of vegetation influences stability, methods to increase soil reinforcement through combinations of plant species, considering the ages of the vegetation in these studies, and developing different strategies for establishing species in areas where soil erosion occurs [86][87].

REFERENCES

  1. Chirico, G. B., Borga, M., Tarolli, P., Rigon, R. y Preti, F. Role of vegetation on slope stability under transient unsaturated conditions. Procedia Environ. Sci., 19, 932–941, 2013.
  2. Stokes, A., Norris, J. E., van Beek, L. P. H., Bogaard, T., Cammeraat, E., Mickovski, S. B., … Fourcaud, T. How Vegetation Reinforces Soil on Slopes. En: Slope Stability and Erosion Control: Ecotechnological Solutions SE – 4, (Eds. J. Norris, A. Stokes, S. Mickovski, E. Cammeraat, R. van Beek, B. Nicoll, y A. Achim), Springer Netherlands, 65–118, 2008.
  3. Brunsden, D. Mudslides. Slope Instab, 363–418, 1984.
  4. van Beek, R., Cammeraat, E., Andreu, V., Mickovski, S. B. y Dorren, L. Hillslope processes: Mass wasting, slope stability and erosion. En: Slope Stability and Erosion Control: Ecotechnological Solutions SE – 4, (Eds. J. Norris, A. Stokes, S. Mickovski, E. Cammeraat, R. van Beek, B. Nicoll, y A. Achim), Springer Netherlands, 65–118, 17–64, 2008.
  5. Kim, D., Im, S., Lee, C. y Woo, C. Modeling the contribution of trees to shallow landslide development in a steep, forested watershed. Ecol. Eng. 61, 658–668, 2013.
  6. Sidle, R. C. y Ochiai, H., Landslides: Processes, Prediction, and Land Use. Wiley, University of Michigan, 2006.
  7. Hammond, C., Hall, D., Miller, S. y Swetic, P. Level I Stability Analysis (LISA) documentation for version 2.0, Level I Stability Analysis (LISA) documentation for version 2.0. US Department of Agriculture, Forest Service, Intermountain Research Station, 1992.
  8. Greenway, D. R. Vegetation and slope stability. Slope Stab. Geotech. Eng. Geomorphol., 187–230, 1987.
  9. Falge, E., Reth, S., Brüggemann, N., Butterbach-Bahl, K., Goldberg, V., Oltchev, A., … Bernhofer, C. Comparison of surface energy exchange models with eddy flux data in forest and grassland ecosystems of Germany. Ecol. Modell., 188, 174–216, 2005.
  10. Kool, D., Agam, N., Lazarovitch, N., Heitman, J. L., Sauer, T. J., & Ben-Gal, A. A review of approaches for evapotranspiration partitioning. Agricultural and Forest Meteorology, 184, 56–70, 2014.
  11. Scanlon, T. M. y Kustas, W. P. Partitioning evapotranspiration using an eddy covariance-based technique: Improved assessment of soil moisture and land–atmosphere exchange dynamics. Vadose Zone Journal, 11, 2012.
  12. Zhu, X.-J., Yu, G.-R., Hu, Z.-M., Wang, Q.-F., He, H.-L., Yan, J.-H., Zhang, J.-H. Spatiotemporal variations of T/ET (the ratio of transpiration to evapotranspiration) in three forests of Eastern China. Ecological Indicators, 52, 411–421, 2015.
  13. Suárez, J., Deslizamientos y estabilidad de taludes en zonas tropicales, Ingeniería de Suelos Ltda, 1998.
  14. Calder, I. R., Wright, I. R. y Murdiyarso, D. A study of evaporation from tropical rain forest — West Java. J. Hydrol., 89, 13–31, 1986.
  15. Jones, J. A., Global hydrology: Processes, resources and environmental management, Longman, 2014.
  16. Scott, R. The understory and overstory partitioning of energy and water fluxes in an open canopy, semiarid woodland. Agric. For. Meteorol., 114, 127–139, 2003.
  17. Doorenbos, J. y Pruitt, W. Guidelines for predicting crop water requirements. FAO Irrig. Drain. Pap., 24, 144, 1977.
  18. Brutsaert, W., Evaporation into the Atmosphere: Theory, History and Applications, Springer Netherlands, 2013.
  19. Harr, R. D. Water flux in soil and subsoil on a steep forested slope. J. Hydrol., 33, 37–58, 1977.
  20. Sidle, R. C., Pearce, A. J. y O’Loughlin, C. L., Hillslope Stability and Land Use, Wiley, Washington, 1985.
  21. Hetherington, E. D. The importance of forests in the hydrological regime. M.C. Heal. R.R. Wallace, 215, 179–211, 1987.
  22. Megahan, W. F. Hydrologic effects of clearcutting and wildfire on steep granitic slopes in Idaho. Water Resour. Res., 19, 811–819, 1983.
  23. Sidle, R. C. et al. Erosion processes in steep terrain—truths, myths, and uncertainties related to forest management in Southeast Asia. For. Ecol. Manage., 224, 199–225, 2006.
  24. Campbell, G. S., Soil Physics with BASIC: Transport Models for Soil-Plant Systems, Elsevier Science, 1985.
  25. Dingman, S. L., Physical Hydrology: Third Edition, Waveland press, 2015.
  26. Savenije, H. H. G. The importance of interception and why we should delete the term evapotranspiration from our vocabulary. Hydrol. Process., 18, 1507–1511, 2004.
  27. Crockford, R. H. y Richardson, D. P. Partitioning of rainfall into throughfall, stemflow and interception: effect of forest type, ground cover and climate. Hydrol. Process., 14, 2903–2920, 2000.
  28. Brauman, K. A., Freyberg, D. L. y Daily, G. C. Forest structure influences on rainfall partitioning and cloud interception: A comparison of native forest sites in Kona, Hawai’i. Agric. For. Meteorol., 150, 265–275, 2010.
  29. Fan, J., Oestergaard, K. T., Guyot, A. y Lockington, D. Measuring and modeling rainfall interception losses by a native Banksia woodland and an exotic pine plantation in subtropical coastal Australia. J. Hydrol., 515, 156–165, 2014.
  30. Muzylo, A., Llorens, P., Valente, F., Keizer, J. J., Domingo, F., & Gash, J. H. C. A review of rainfall interception modelling. Journal of Hydrology, 370, 191–206, 2009.
  31. Campos, D. F., Procesos del ciclo hidrológico, Universitaria Potosina, 1984.
  32. Reifsnyder, W. E. y Lull, H. W., Radiant Energy in Relation to Forests, U.S. Department of Agriculture, Forest Service, 1965.
  33. Monteith, J. L., Vegetation and the Atmosphere: Principles. Academic Press, 1975.
  34. Whitehead, D. y Jarvis, P. G. Coniferous forests and plantations. Water deficits plant growth, Ed. by Kozlowski, TT, Acad. Press. New York, NY, VI, 49–152, 1981.
  35. McNaughton, K. G. y Jarvis, P. G. Predicting effects of vegetation changes on transpiration and evaporation. Water deficits and plant growth, 7, 1–47, 1983.
  36. Yalcin, A. A geotechnical study on the landslides in the Trabzon Province, NE, Turkey. Appl. Clay Sci., 52, 11–19, 2011.
  37. Alimohammadlou, Y., Najafi, A. y Gokceoglu, C. Estimation of rainfall-induced landslides using ANN and fuzzy clustering methods: A case study in Saeen Slope, Azerbaijan province, Iran. CATENA, 120, 149–162, 2014.
  38. Carenas, B., Giner, J. L., González, J. y Pozo, M., Geología, Paraninfo, 2014.
  39. Cammeraat, L. H. A review of two strongly contrasting geomorphological systems within the context of scale. Earth Surf. Process. Landforms, 27, 1201–1222, 2002.
  40. Stokes, A. Biomechanics of tree root anchorage, Plant roots: The hidden half, 175–186, 2002.
  41. Abdi, E., Majnounian, B., Genet, M. y Rahimi, H. Quantifying the effects of root reinforcement of Persian Ironwood (Parrotia persica) on slope stability; a case study: Hillslope of Hyrcanian forests, northern Iran. Ecol. Eng., 36, 1409–1416, 2010.
  42. Reubens, B., Poesen, J., Danjon, F., Geudens, G. y Muys, B. The role of fine and coarse roots in shallow slope stability and soil erosion control with a focus on root system architecture: a review. Trees, 21, 385–402, 2007.
  43. Peters, P. A. y C. J. Biller. Preliminary evaluation of the effect of vertical angle of pull on stump uprooting failure. Proc. Improv. Product. through For. Eng., 90–93, 1986.
  44. Pyles, M. R. y Stoupa, J. Load-carrying capacity of second-growth Douglas-fir stump anchors. West. J. Appl. For., 2, 77–80, 1987.
  45. Somerville, A. Root anchorage and root morphology of pinus radiata on a range of ripping treatments. New Zeal. J. For. Sci., 9, 294–315, 1979.
  46. Cannell, M. y Courts, M. Growing in the wind. New Sci. ,777, 42–43, 1988.
  47. Watson, A. Wind-induced forces in the near-surface lateral roots of radiata pine. For. Ecol. Manage., 135, 133–142, 2000.
  48. Wu, T. H., McKinnell III, W. P. y Swanston, D. N. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geotech. J., 16, 19–33, 1979.
  49. O’Loughlin, C. y Ziemer, R. R. The importance of root strength and deterioration rates upon edaphic stability in steepland forests. Carbon Uptake Alloc. Subalp. Ecosyst. as a Key to Manag., 70–78, 1982.
  50. Riestenberg, M. M. y Sovonick-Dunford, S. The role of woody vegetation in stabilizing slopes in the Cincinnati area, Ohio. Geol. Soc. Am. Bull., 94, 506–518, 1983.
  51. Stokes, A., Fitter, A. H. y Courts, M. P. Responses of young trees to wind and shading: effects on root architecture. J. Exp. Bot., 46, 1139–1146, 1995.
  52. Stokes, A., Nicoll, B. C., Coutts, M. P. y Fitter, A. H. Responses of young Sitka spruce clones to mechanical perturbation and nutrition: effects on biomass allocation, root development, and resistance to bending. Can. J. For. Res., 27, 1049–1057, 1997.
  53. Waldron, L. J. The shear resistance of root-permeated homogeneous and stratified soil. Soil Sci. Soc. Am. J., 41, 843–849, 1977.
  54. Cazzuffi, D., Cardile, G. y Gioffrè, D. Geosynthetic engineering and vegetation growth in soil reinforcement applications. Transp. Infrastruct. Geotechnol., 1, 262–300, 2014.
  55. Zhou, Y. et al. The traction effect of lateral roots of Pinus yunnanensis on soil reinforcement: a direct in situ test. Plant Soil, 190, 77–86, 1997.
  56. Schmidt, K. M. et al. The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range. Can. Geotech. J., 38, 995–1024, 2001.
  57. Swanson, F. J. y Swanston, D. N. Complex mass-movement terrains in the western Cascade Range, Oregon. Rev. Eng. Geol., 3, 113–124, 1977.
  58. Ziemer, R. R. y Swanston, D. N., Root strength changes after logging in southeast Alaska. 306, Dept. of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station, 1977.
  59. Abe, K. y Iwamoto, M. Simulation model for the distribution of tree roots-application to a slope stability model. J. Japanese For. Soc., 72, 375–387, 1990.
  60. Tsukamoto, Y. y Minematsu, H. Evaluation of the effect of lateral roots on slope stability. Proc. 18th IUFRO World Congr., 1986.
  61. Tsukamoto, Y. Evaluation of the effect of tree roots on slope stability. Bull Exp. For., 23, 65–124, 1987.
  62. Swanston, D. N. y Swanson, F. J. Timber harvesting, mass erosion, and steepland forest geomorphology in the Pacific Northwest. Geomorphol. Eng., 199–221, 1976.
  63. Schroeder, W. L. The engineering approach to landslide risk analysis. Gen. Tech. Rep PNW-180, 43–50, 1985.
  64. Gray, D. H. y Megahan, W. F., Forest vegetation removal and slope stability in the Idaho batholith. 271, U.S. Dept. of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station, 1981.
  65. Phillips, C. J. y Watson, A. J. Structural Tree Root Research in New Zealand: A Review. Manaaki Whenua Press, 1994.
  66. O’Loughlin, C. L., Rowe, L. K. y Pearce, A. J. Exceptional Storm Influences on Slope Erosion and Sediment Yield in Small Forest Catchments, North Westland, New Zealand. First Natl. Symp. For. Hydrol., 84–91, 1982.
  67. Gray, D. H. y Ohashi, H. Mechanics of fiber reinforcement in sand. J. Geotech. Eng., 109, 335–353, 1983.
  68. John, J. R., Protección y estabilización de taludes para evitar deslizamientos, Universidad Austral de Chile, 2011.
  69. Tsukamoto, Y. y Kusakabe, O. Vegetative influences on debris slide occurrences on steep slopes in Japan. Proc. symp. Eff. For. L. use Eros. slope stability, Honolulu, Hawaii, 63–72, 1984.
  70. Morgan, R. P. C. y Rickson, R. J., Slope Stabilization and Erosion Control: A Bioengineering Approach: A Bioengineering Approach, Taylor y Francis, 2003.
  71. Van Den Eeckhaut, M., Poesen, J. y Hervas, J. Mass-Movement Causes: Overloading. Treatise Geomorphol., 7, 2013.
  72. Kawaguchi, T., A Study of Landslides on Mt. Akagi, Bull. of the Government For. Exp. Sta., Rept, 1951.
  73. Gray, D. H. Effects of forest clear-cutting on the stability of natural slopes: results of field studies: interim report, 1973
  74. Swanston, D. N., Mechanics of debris avalanching in shallow till soils of southeast Alaska, Pacific Northwest Forest and Range Experiment Station, Institute of Northern Forestry, Forest Service, U.S. Dept. of Agriculture, 1970.
  75. O’Loughlin, C. L. The effect of timber removal on the stability of forest soils. J. Hydrol., 13, 121–134, 1974.
  76. Ziemer, R. R. The role of vegetation in the stability of forested slopes. Memorias, Proc. First Union of For. Res. Org., Div. I, XVII World Congress, Kyoto, Japan 297–308, 1981.
  77. Sidle, R. C. Relative importance of factors influencing landsliding in coastal Alaska. Memorias, 21st Annual engineering geology and soils engineering symposium. University of Idaho, Moscow, 311–325, 1984.
  78. Wiel, M. J. y Darby, S. E. A new model to analyse the impact of woody riparian vegetation on the geotechnical stability of riverbanks. Earth Surf. Process. Landforms, 32, 2185–2198, 2007.
  79. Swanston, D. N. Mass wasting in coastal Alaska. U.S.D.A. For. Serv. Res. Pap. PNW-83, 15, 1969.
  80. Wilford, D. J. y Schwab, J. W. Soil mass movements in the Rennell Sound area, Queen Charlotte Islands, British Columbia. Memorias, Proceedings of the Canadian Hydrology Symposium-Associate Committee on Hydrology, 1982.
  81. Stokes, A. y Mattheck, C. Variation of wood strength in tree roots. J. Exp. Bot., 47, 693–699, 1996.
  82. Coutts, M. P. Root architecture and tree stability. Plant Soil, 71, 171–188, 1983.
  83. Coutts, M. P. Components of tree stability in Sitka spruce on peaty gley soil. Forestry, 59, 173–197, 1986.
  84. Norris, J. E., Greenwood, J. R., Achim, A., Gardiner, B. A., Nicoll, B. C., Cammeraat, E. y Mickovski, S. B. Hazard assessment of vegetated slopes. En: Slope Stability and Erosion Control: Ecotechnological Solutions SE – 4, (Eds. J. Norris, A. Stokes, S. Mickovski, E. Cammeraat, R. van Beek, B. Nicoll, y A. Achim), Springer Netherlands, 119–166, 2008.
  85. Schwarz, M., Preti, F., Giadrossich, F., Lehmann, P. y Or, D. Quantifying the role of vegetation in slope stability: a case study in Tuscany (Italy). Ecological Engineering, 36, 285-291, 2010.
  86. Genet, M., Stokes, A., Fourcaud, T. y Norris, J. E. The influence of plant diversity on slope stability in a moist evergreen deciduous forest. Ecological engineering, 36, 265-275, 2010.
  87. Norris, J. E., Di Iorio, A., Stokes, A., Nicoll, B. C. y Achim, A. Species selection for soil reinforcement and protection. En: Slope Stability and Erosion Control: Ecotechnological Solutions SE – 4, (Eds. J. Norris, A. Stokes, S. Mickovski, E. Cammeraat, R. van Beek, B. Nicoll, y A. Achim), Springer Netherlands, 167-210, 2008.