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Quantifying lateral root reinforcement in steep slopes—From a bundle of roots to tree stand
Abstract
A review of present modelling approaches for root reinforcement in vegetated steep hillslopes reveals critical gaps in consideration of plant–soil interactions at various scales of interest for shallow landslide prediction. A new framework is proposed for systematic quantification of root reinforcement at scales ranging from single root to tree root system, to a stand of trees. In addition to standard basal reinforcement considered in most approaches, the critical role of roots in stabilizing slopes through lateral reinforcement is highlighted. Primary geometrical and mechanical properties of root systems and their function in stabilizing the soil mass are reviewed. Stress–strain relationships are considered for a bundle of roots using the formalism of the fiber bundle model (FBM) that offers a natural means for upscaling mechanical behavior of root systems. An extension of the FBM is proposed, considering key root and soil parameters such as root diameter distribution, tortuosity, soil type, soil moisture and friction between soil and root surface. The spatial distribution of root mechanical reinforcement around a single tree is computed from root diameter and density distributions based on easy to measure properties. The distribution of root reinforcement for a stand of trees was obtained from spatial and mechanical superposition of individual tree values with regard to their positions on a hillslope. Potential applications of the proposed approach are illustrated in a numerical experiment of spatial strength distribution in a hypothetical slope with 1000 trees randomly distributed. The analyses result in spatial distribution of weak and strong zones within the soil where landslide triggering is expected in large and continuous zones with low reinforcement values. Mapping such zones would enhance the quality of landslide susceptibility maps and optimization of silvicultural measures in protection forests. Copyright © 2010 John Wiley & Sons, Ltd.
... In addition, plant fibers and adhesion binders can also provide slope reinforcement strength (Oña et al., 2011). In slope revegetation engineering, plant roots can improve slope stability by exerting shear and tensile effects (Preti and Giadrossich, 2009;Schmidt et al., 2001;Schwarz et al., 2010). Compared to traditional slope protection methods, this technique not only guarantees the stability of the slope and the natural ecological landscape but also has a significant contribution to local ecosystem protection. ...
... The reinforcement effect of plant roots on slope stability of revegetation engineering has been extensively studied from the viewpoint of the relationship between pullout resistance and plant characteristics (Fan et al., 2011;Preti and Giadrossich, 2009;Schmidt et al., 2001;Schwarz et al., 2010). This includes the mechanical characteristics of a single root (Li et al., 2006), Dorgan's root (Mickovski et al., 2007), root moisture (Zhang et al., 2019), surface microstructure features of roots (Vannoppen et al., 2016), biological indexes (Li et al., 2006), root cellulose content (Zhang et al., 2014), the spatial distribution properties (De Baets et al., 2008) and angle between lateral roots (Mickovski et al., 2007). ...
... The structural equation modeling could identify the complex associations among variables, including the target variable. Specifically, an a priori model was first established based on our current knowledge according to previous studies (Fan et al., 2011;Preti and Giadrossich, 2009;Schmidt et al., 2001;Schwarz et al., 2010;Li et al., 2006;Mickovski et al., 2007;Zhang et al., 2019;Vannoppen et al., 2016) (Fig. 4a). ...
... Yet, the overall impact of roots is a significant contribution to slope stability. The most effective mechanism relevant to shallow land sliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Schmidt et al., 2001;Roering et al., 2003;Schwarz et al., 2010b). ...
... The most effective mechanism relevant to shallow land sliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Schmidt et al., 2001;Roering et al., 2003;Schwarz et al., 2010b). The influence of root reinforcement on soil shear strength is usually expressed as an additional cohesion term (root cohesion c r ) (Wu, 1984) in the Coulomb failure criteria ...
... These are invalid assumptions, however, since breakage often occurs below the shear surface where the root diameter is smaller than at the shear plane (Hubble et al., 2010;Giadrossich et al., 2013Giadrossich et al., , 2017Vergani et al., 2016). These limitations led to the development of field pull-out or shear tests and of root bundle models, which consider the dynamic stress-step (Pollen and Simon, 2005) or strain-step loading of a root system Schwarz et al., 2010bSchwarz et al., , 2013. The advantage of these models is that they consider root strength as a function of displacement. ...
... The efficacy of a root system in reinforcing soil is influenced by root properties and soil conditions. The former includes root geometry [18,29,52,70], root tensile strength [42,44], root hair bonding friction [52] and root spacing [17], while the latter comprises soil saturation [52] and matric suction [36,41,48]. ...
... The efficacy of a root system in reinforcing soil is influenced by root properties and soil conditions. The former includes root geometry [18,29,52,70], root tensile strength [42,44], root hair bonding friction [52] and root spacing [17], while the latter comprises soil saturation [52] and matric suction [36,41,48]. ...
... The efficacy of a root system in reinforcing soil is influenced by root properties and soil conditions. The former includes root geometry [18,29,52,70], root tensile strength [42,44], root hair bonding friction [52] and root spacing [17], while the latter comprises soil saturation [52] and matric suction [36,41,48]. ...
Landslides due to catastrophic weather events, especially heavy rainfall, have risen significantly over the last several decades, causing significant damage and affecting the health and livelihoods of millions of people. Using tree roots to bio-engineer shallow slopes has been proven to be a cost-effective, sustainable measure and thus has gained increasing popularity. As slope failure often occurs under heavy precipitation, it is important to understand the mechanical interactions in the soil matrix surrounding a root to better estimate the reinforcement capacity of a root system, especially as the soil undergoes wetting from drier conditions. However, very few studies of root reinforcements have considered the effects of degree of saturation on behaviour. In this study, steel wires are used as a root analogue to explore the impact of root geometry, soil dilation and soil saturation on the pull-out behaviour of a root and three commonly used unsaturated soil strength models have been used to interpret the pull-out results. It was found that roots with larger diameter did not contribute to additional resistance. Also, a linear relationship between degree of saturation and pull-out strength was identified over a large range of suctions and one of the unsaturated soil strength models seemed to provide a more reasonable interpretation. The results will help future bioengineering slope design by improving the understanding of soil-root interface behaviour, including the effect of root diameter in slippage failure and greater emphasis on the importance of taking degree of saturation into account in unsaturated soil strength models.
... A physical, process-based approach using quantitative measures for root reinforcement to quantify the increase in soil cohesion and slope stability achieved (Giadrossich et al., 2017;Schwarz et al., 2010b;van Zadelhoff et al., 2021). ...
... This is achieved through an apparent cohesion produced by root fibre reinforcement in soils, which is a function of the tensile strength of the roots in combination with the binding action in the root fibre/soil composite and adhesion of soil particles to roots (Reubens et al., 2007;Roering et al., 2003;Schmidt et al., 2001;Sidle and Ochiai, 2006;Styczen and Morgan, 1995). The primary mechanical function relevant to shallow landsliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Roering et al., 2003;Schmidt et al., 2001;Schwarz et al., 2010b). ...
... The primary mechanical function relevant to shallow landsliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Roering et al., 2003;Schmidt et al., 2001;Schwarz et al., 2010b). The influence of root reinforcement is expressed as a cohesion term (root cohesion ) in the Coulomb failure criteria where the soil increases the soil-root composite shear strength: ...
Landslide erosion results in loss of productive soils and pasture. Moreover, sediment delivered to streams from landslides can contribute to the degradation of freshwater and marine receiving environments by smothering benthic habitats and increasing turbidity, light attenuation, and sediment-bound contaminants. Silvopastoralism is an important land management practice used to combat landslide erosion and improve the health of downstream aquatic ecosystems. Yet, the effectiveness of widely spaced trees in reducing shallow landslide erosion and sediment delivery at hillslope to catchment scales remains largely unknown. Previous studies have been limited by scale (e.g., hillslope) or method (e.g., univariate analyses). This research aims to develop spatially explicit modelling to assess the impact of differing tree species, planting densities, and individual tree location, on rainfall-triggered landslides and sediment delivery while accounting for varying environmental conditions, such as slope gradient, lithology, or soil type. As such, this thesis combines geospatial methods and statistical models to address key challenges related to erosion and sediment control in New Zealand’s pastoral hill country.
First, using a study area in the Wairarapa, located in the southeast of the North Island, New Zealand (840 km2), a method was developed using open-source remote sensing products to generate high-resolution individual tree influence models for the dominant tree species. The objective was to generate a spatial explicit representation of individual trees for landscape-scaled statistical slope stability modelling. The combined hydrological and mechanical influence of trees on slopes was inferred through the spatial relationship between trees and landslide erosion. These spatial distribution models for individual trees of different vegetation types represent the average contribution to slope stability as a function of distance from tree at 1-m spatial resolution. The normalised models (0-1) largely agree with the shape and distribution of force from existing physical root reinforcement models. Of exotic tree species that were planted for erosion and sediment control, poplars (Populus spp.) and willows (Salix spp.) make up 51% (109,000) of trees located on hillslopes at a mean density of 3 trees/ha. In line with previous studies, poplars and willows have the greatest contribution to slope stability with an average maximum effective distance of 20 m. Yet, native kānuka (Kunzea spp.) is the most abundant woody vegetation species on hillslopes within the study area, with an average of 24 trees/ha, providing an important soil conservation function. A large proportion (56% or 212.5 km2) of erosion-prone terrain in the study area remains untreated.
In a world-first, this allowed the influence of individual trees to be included in a statistical landslide susceptibility model using binary logistic regression to quantify the effectiveness of silvopastoral systems at reducing landslide erosion and to support targeted erosion mitigation. Models were trained and tested using a landslide inventory consisting of 43,000 landslide scars mapped across the study area. Model performance was very good, with a median Area Under the Receiver Operating Characteristic curve (AUROC) of 0.95 in the final model used for predictions, which equates to an accuracy of 88.7% using a cut-off of 0.5. The effect of highly skewed continuous tree influence models on the maximum likelihood estimator was tested using different sampling strategies aimed at reducing positive skewness. With an adequate sample size, highly skewed continuous predictor variables do not result in an inflation of effect size.
Application of the landslide susceptibility model was illustrated using two farms from within the study area (Site 1: 1,700-ha; Site 2: 462-ha) by quantifying the reduction in shallow landslide erosion due to trees. Compared to a pasture only baseline, landslide erosion was reduced by 17% at Site 1 and 43% at Site 2 due to all existing vegetation. The effectiveness of individual trees in reducing landslide erosion was shown to be less a function of species than that of targeting highly susceptible areas with adequate plant densities. The excellent model performance means spatial predictions are precise, which has implications for land management as the maps provide greater certainty and spatial refinement to inform landslide mitigation. The terrain occupied by the “high” susceptible class – defined as the terrain where 80% of mapped landslides were triggered in the past – occupies only 12% of Site 1 and 7% of Site 2. This suggests there is great potential for improved targeting of erosion mitigation to these areas of the farms where landsliding may be expected in the future.
To enable biological mitigation to be targeted to critical source areas of sediment, determinants of sediment connectivity were investigated for a landslide-triggering storm event in 1977. In a first of its kind, a morphometric landslide connectivity model was developed using lasso logistic regression to predict the likelihood of sediment delivery to streams following landslide initiation. An experiment was undertaken to explore a range of connectivity scenarios by defining a set of sinks and simulating varying rates of sediment generation during runoff events of increasing magnitude. Sediment delivery ratios for the 1977 event ranged from 0.21 to 0.29, equating to an event sediment yield of 3548 t km-2 to 9033 t km-2. The likelihood of sediment delivery was greatly enhanced where debris tails coalesce. Besides scar size variables, overland flow distance and vertical distance to sink were the most important morphometric predictors of connectivity. When scar size variables were removed from the connectivity model, median AUROC was reduced from 0.88 to 0.75.
By coupling landslide susceptibility and connectivity predictions in a modular form, we quantified the cost effectiveness of targeted versus non-targeted approaches to shallow landslide mitigation. Targeted mitigation of landslide-derived sediment was found to be approximately an order of magnitude more cost-effective than a non-targeted approach. Compared with a pasture-only baseline, a 34% reduction in sediment delivery can be achieved by increasing slope stability through spaced tree planting on 6.5% of the pastoral land. In contrast, the maximum reduction achievable through comprehensive coverage of widely spaced planting is 56%. The coupled landslide susceptibility and connectivity predictions (maps) provide an objective basis to not only target mitigation to areas where future shallow landslides are likely to occur, but – perhaps more importantly – target future tree planting to locations that are likely to be future sources of fine sediment. In this way, the research presented in this thesis is both methodologically novel and has immediate application to support land management decisions aimed at creating a more sustainable socio-ecological landscape.
... However, it is assumed that the tensile properties of all roots can be fully exploited, and the roots break at the moment of reaching the maximum tensile strength, which makes the model significantly overestimate the contribution of roots to soil shear strength [32][33][34]. In order to more reasonably evaluate the root's mechanical reinforcement for soil, researchers have established new theoretical models or carried out modification research of Wu's model from different entry points, such as the fiber bundle model considering dynamic fracture of roots [34,35], fiber reinforced model based on the tensile force-displacement relationship of roots [36], the modified bundle model considering the root system displacement [37], shear strength model of root-permeated soil based on roots shear displacement and deformation [32], energy-based fiber bundle model considering both force and displacement drivers implicitly [38], and fiber bundle model considering the probabilistic distribution of root failure [39]. These models have further studied the root-soil interaction mechanism and considered the force transmission mechanism between roots and soil to some extent. ...
... Pollen and Simon [34] proposed a fiber bundle model ( Figure 2) based on stress redistribution, considering the dynamic movement of roots' tensile force and progressive fracture of roots during the deformation process of root-permeated soil. Schwarz et al. [36] developed a fiber-reinforced model that controlled the loading process by displacement by establishing the relationship between tensile force and tensile displacement of roots (3)). However, the fiber bundle model assumes that the driving force of the soil and the stress of the broken root from the newly distributed stress were all transferred to the roots in root-permeated soil and ignores the important root-soil interaction. ...
... Pollen and Simon [34] proposed a fiber bundle model ( Figure 2) based on stress redistribution, considering the dynamic movement of rootsʹ tensile force and progressive fracture of roots during the deformation process of root-permeated soil. Schwarz et al. [36] developed a fiber-reinforced model that controlled the loading process by displacement by establishing the relationship between tensile force and tensile displacement of roots (Equation (3)). However, the fiber bundle model assumes that the driving force of the soil and the stress of the broken root from the newly distributed stress were all transferred to the roots in root-permeated soil and ignores the important root-soil interaction. ...
Roots can be seen as natural soil reinforcement material. The prediction and quantitative evaluation of the shear strength of root-permeated soil is the focus of vegetation slope protection, in which the bonding effect of the root-soil interface is the key factor. Taking the roots of Chinese fir trees as an example, the shear resistance test of root-soil interface bonding strength and the direct shear test of root-permeated soil with different root area ratios and inclination angles were carried out. The results indicated that the bonding strength of the root-soil interface could be quantified by interfacial cohesion and friction angle. The shear strength of root-permeated soil increased with the root area ratio, and its relationship with the inclination angle of root relative shear direction was: 45 • > 90 •. In addition, an estimation model of the ultimate shear strength of root-permeated soil was developed, in which the bonding effect of the root-soil interface was quantified by the interface bonding strength parameters. The soil stress, root diameter, root length, and the initial angle between the root and shear direction can be considered in the estimation model. The rationality and accuracy of the estimated model were verified through the comparison of experimental results and Wu's model. The proposed model can be used to calculate the stability of the biotechnical reinforcement landslides and evaluate the shear strength of the root-permeated soil.
... Yet, the overall impact of roots is a significant contribution to slope stability. The most effective mechanism relevant to shallow land sliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Schmidt et al., 2001;Roering et al., 2003;Schwarz et al., 2010b). ...
... The most effective mechanism relevant to shallow land sliding is through basal root reinforcement, which is achieved when roots penetrate the shear plane, thus increasing the basal shear strength (Schwarz et al., 2010b). Lateral root reinforcement governs the onset and size of shallow landslides (Schmidt et al., 2001;Roering et al., 2003;Schwarz et al., 2010b). The influence of root reinforcement on soil shear strength is usually expressed as an additional cohesion term (root cohesion c r ) (Wu, 1984) in the Coulomb failure criteria ...
... These are invalid assumptions, however, since breakage often occurs below the shear surface where the root diameter is smaller than at the shear plane (Hubble et al., 2010;Giadrossich et al., 2013Giadrossich et al., , 2017Vergani et al., 2016). These limitations led to the development of field pull-out or shear tests and of root bundle models, which consider the dynamic stress-step (Pollen and Simon, 2005) or strain-step loading of a root system Schwarz et al., 2010bSchwarz et al., , 2013. The advantage of these models is that they consider root strength as a function of displacement. ...
... Both very detailed 3D explicit models (e.g., see Postma et al. (2017)) and simplified, physically-based (either laterally or vertically integrated) 1D models exist. Whilst the first are typically useful to investigate water uptake and the biomorphology and architecture of roots, their complexity is often unnecessary and excessively costly computationally (Schwarz et al., 2010a). For practical applications including the creation of spatial hazard maps (e.g., see Gasser et al. (2019)), 1D models providing analytical formulas for either the lateral root distribution (Schwarz et al., 2010a(Schwarz et al., , 2012 or the vertical root profile Tron et al., 2014) are more suitable. ...
... Whilst the first are typically useful to investigate water uptake and the biomorphology and architecture of roots, their complexity is often unnecessary and excessively costly computationally (Schwarz et al., 2010a). For practical applications including the creation of spatial hazard maps (e.g., see Gasser et al. (2019)), 1D models providing analytical formulas for either the lateral root distribution (Schwarz et al., 2010a(Schwarz et al., , 2012 or the vertical root profile Tron et al., 2014) are more suitable. Such models are parameterized with physically-based (and commonly measured) soil, hydrological and biological variables. ...
... In this work, three existing 1-D plant root models are linked into a single 2D modelling framework, which provides the explicit distribution of the below-ground biomass variables along the lateral (horizontal) and the vertical (depth) directions. The 1D models are those Schwarz et al. (2010aSchwarz et al. ( , 2012, Laio et al. (2006) and Tron et al. (2014Tron et al. ( , 2015, whose main characteristics are hereafter briefly reviewed. ...
The ability to predict the spatial distribution of tree root system variables (e.g., the Root system Area (RA), the maximum root diameter, the number of roots in diameter classes, the density of fine roots, etc.) under different environmental conditions is relevant to several scientific disciplines and to engineering practice. In this work, three well known analytical models from the literature are assembled into a unique framework called the Root Distribution Model (RDM). RDM models the expected vertical and horizontal distribution of coarse and fine root system variables for mature plants growing in different environmental conditions ranging from moderately humid to arid climates. All soil and moisture dynamic parameters are physically based, which make the model straightforward to calibrate via a single tuning parameter. At this investigative stage, it is shown that the model has the flexibility to represent a broad range of situations where soil moisture may result from precipitation inputs or from water level fluctuations due to either the presence of a water coarse or of deep aquifers or both. Accordingly, the distribution of the sectional RA may be either positively or negatively skewed, as well as show a peculiar bi-modal structure. The model can be used to study the impact of changing scenarios affecting precipitation, aquifer and channel hydrology.
... Studies focusing on the role of tree roots in slope stabilisation have revealed that the effectiveness of root systems for soil layer reinforcement is highly spatially heterogeneous and does not remain constant over time. For example, several studies have demonstrated that the abundance of roots in the soil layer and the mechanical strength of the entire root system are strongly dependent on tree species, size, and density [21][22][23]. Some intensive research in managed plantation forests showed that the effectiveness of tree root reinforcement decreased progressively over a period of approximately 10 years after logging due to the decay of felled tree roots, but recovered to the same level as before logging within 25 years due to the growth of replanted tree roots [5,10,24,25]. ...
... Possible reasons for slope destabilisation due to orchard development include reduced influence of vegetation on hydrogeomorphological processes by clearing former forest vegetation and impacts of alterations to the topography, foundation, and hydrological properties through new land creation. Although trees can increase slope stability by removing soil moisture through evapotranspiration and providing root cohesion to the soil layer [3,4], their effectiveness is highly dependent on tree species, size, and density [21][22][23], which determine root system expansion and canopy development. Therefore, fruit trees in orchards may have only a limited slope stabilising effect due to their small size and low planting density. ...
Land cover changes in mountainous regions are potential precursors to landslide disasters. However, the effects of past long-term land cover changes on the characteristics of recent landslides remains underexplored. We studied land cover evolution over a 56-year period on Omishima Island in western Japan to examine the spatial relations of landslides in the July 2018 storm event based on rainfall, land cover trajectories, and topography. We generated land cover maps for 1962, 1981, and 2018 by aerial photo interpretation. We also identified 512 new landslides. Based on 47-year precipitation records, we estimated the return periods of 1- to 264-h rainfalls during the storm using the generalised extreme value (GEV) distributions. Return periods showed wide variation when the derived GEV distributions were applied to 1-km grid rainfall distributions. Despite such pronounced spatial variations in rainfall, we did not observe a clear correlation between rainfall intensity and landslide distribution. In contrast, land cover trajectories had a pronounced effect on landslide occurrence. Landslides were more concentrated on slopes that experienced land cover changes after 1962. A comparison of slopes on farmland developed between 1962 and 1981 (mainly citrus orchards) indicated that landslide density and area ratio were significantly lower on slopes that had reverted to forests than on those remaining as farmland. However, the values of the reforested slopes exceeded those of forests and farmlands that remained since before 1962. Our geospatial analysis revealed that even if the field had shifted to forests, the effects of reduced slope stability due to orchard development had remained for at least 37 years. This suggested that the impacts of converting forests to orchards last longer than harvesting in managed plantation forests.
... But this model is affected by the change of root density. Schwarz32 proposes the Root Bundle Model, assuming that the friction between the root and loam varies linearly from 0.1 to 10 kPa with the soil volume moisture content. In recent years, studies on atmosphere-vegetation-soil coupling have been developed33,34 and Ng et al. ...
... By comparing formula (32) and (33), we can obtain: www.nature.com/scientificreports/ The samples are all composed of frictional elements after complete damage. ...
In seasonally frozen soil, soil sometimes is affected by freeze–thaw cycles and root systems. In order to study its mechanical characteristics, a series of consolidation drained triaxial tests under different confining pressures (25, 50, 100, 200 kPa), different freeze–thaw cycles (N = 0, 1, 5, 15) and different root-containing conditions (r = 0, 1, 3) were carried out. The test results show that the specimens exhibit strain softening behavior and volumetric dilatancy phenomena and shear failure under lower confining pressure, and strain hardening and volumetric contraction, bulging failure under higher confining pressure. With the increase of freeze–thaw cycles, the bearing capacity of the sample decreases and the volume strain increases. With the increase of volume ration of roots in the sample, the bearing capacity increases and the volume strain decreases. Based on the binary medium model, the soil is abstracted into bonded elements and frictional elements. At the same time, the bonded elements are transformed into frictional element when the bonded elements are broken during the loading process. Also, the root is abstracted into another non-destructive bonded elements material, which bears the load together. The linear elastic constitutive model is used for root and bonded elements, and the double-hardening model is used for friction elements. Considering the influence of freeze–thaw cycles, the extended binary model is derived here. Finally, the experimental results show that the predicted results of this model are in good agreement with the experimental results, and the new model can relatively well simulate the strain softening and volumetric dilatancy phenomena.
... landsliding), and also act as reinforcement of soil. They could play a prominent role in risk mitigation strategies (e.g. as a nature-based solution, or NBS) through modifying the groundwater regime [42] and/or through direct mechanical soil strength improvement [19,36,43]. With the increased frequency of windstorms [35] responsible for recent damage in European forests [14,26], root-soil interaction is of interest in Forestry, where it is important to understand how forest management plans can be adapted to increased natural hazards [12,30]. ...
... With the increased frequency of windstorms [35] responsible for recent damage in European forests [14,26], root-soil interaction is of interest in Forestry, where it is important to understand how forest management plans can be adapted to increased natural hazards [12,30]. It is also of interest in Civil Engineering, where trees could provide protection against earthquake-induced landslides [22] or be employed as costeffective protective barriers [36] to protect sensitive structures from wind loading or debris flow [6]. ...
For smaller plant roots in coarse-grained soils, large relative size of soil particles compared to roots can affect their axial resistance. Even for the larger roots of trees, particle size effects may be important, e.g. when testing 1:N scale models of tree uprooting in a geotechnical centrifuge. In this study the distinct element method (DEM) was used to investigate this problem. The sinker root of a centrifuge model tree root system under axial loading was analysed, with its simulated behaviour compared with finite element method (FEM) simulations, where the soil was modelled as a continuum and hence did not incorporate particle size effects. Both were validated against laboratory tests. Considering the same prototype size and soil particle size distribution, different scale factors/g-levels were applied to model roots, hence varying the ratio of root diameter (\({d}_{\rm r}\)) to mean particle size (\({D}_{50}\)). Even at the lower \({d}_{\rm r}/{D}_{50}\) values investigated (6–21), particle size effects on end-bearing capacity were negligible upon push-in of the root. In contrast, effects on shaft resistance (for pull-out) were significant and were interpreted by a simplified analytical model developed in this study using a combination of cavity expansion and root-particle size ratio. The absolute size of the root analogues considered was also representative of small diameter roots present in other plant species at 1:1 scale, making the analytical model also applicable to crop-lodging problems and for defining input parameters for analyses of nature-based solutions (NBS) using vegetation (e.g. for slope stabilisation).
... Changes in tree species composition and tree density also have an impact on the root reinforcement in forests (Roering et al., 2003;Genet et al., 2008). The influence of vegetation on landslides has been intensely studied on steep slopes in the European Alps (Bischetti et al., 2009;Vergani et al., 2014), the Oregon Coast Range (Schmidt et al., 2001;Roering et al., 2003), southern California (Terwilliger and Waldron, 1991), northern Italy (Borga et al., 2005;Schwarz et al., 2010b), New Zealand (Giadrossich et al., 2020 and China (Genet et al., 2008); however, little effort was directed to understanding the influence of vegetation on landsliding on lower-inclined hillslopes such as scarplands in southern Germany (e.g. Thiebes et al., 2014) or in the Flemish Ardennes (e.g. ...
... Dense thickets of Norway spruce occurred on Fürstenanger slopes above road cuts and on all landslide toes (Fig. 6c) and provide high root density that would enable sufficient stabilization. When shear planes exceed rooting depth, lateral root cohesion can have a stabilizing effect (Schwarz et al., 2010b) by affecting the onset and size of shallow landsliding (Schmidt et al., 2001;Roering et al., 2003) as indicated by tensed roots observed at Putzenstein (Fig. 4b). To stabilize shallow landslides with shear planes up to 1.5 m, our calculations showed that a cohesion between 1 and 4.5 kPa would be required (Fig. 10). ...
Landslides are important agents of sediment transport, cause hazards and are key agents for the evolution of scarplands. Scarplands are characterized by high-strength layers overlying low-inclined landslide-susceptible layers that precondition and prepare landsliding on
geological timescales. These landslides can be reactivated, and their role
in past hillslope evolution affected geomorphometry and material properties
that set the framework for present-day shallow landslide activity. To manage
present-day landslide hazards in scarplands, a combined assessment of
deep-seated and shallow landsliding is required to quantify the interaction
between geological conditions and vegetation that controls landslide activity. For this purpose, we investigated three hillslopes affected by
landsliding in the Franconian scarplands. We used geomorphic mapping to
identify landforms indicating landslide activity, electrical resistivity to
identify shear plane location and a mechanical stability model to assess the
stability of deep-seated landslides. Furthermore, we mapped tree distribution and quantified root area ratio and root tensile strength to assess the influence of vegetation on shallow landsliding. Our results show that deep-seated landslides incorporate rotational and translational movement and suggest that sliding occurs along a geologic boundary between permeable Rhätolias sandstone and impermeable Feuerletten clays. Despite low hillslope angles, landslides could be reactivated when high pore pressures develop along low-permeability layers. In contrast, shallow landsliding is controlled by vegetation. Our results show that rooted area is more important than species-dependent root tensile strength and basal root cohesion is limited to the upper 0.5 m of the surface due to geologically controlled unfavourable soil conditions. Due to low slope
inclination, root cohesion can stabilize landslide toes or slopes undercut
by forest roads, independent of potential soil cohesion, when tree density
is sufficient dense to provide lateral root cohesion. In summary, geology
preconditions and prepares deep-seated landslides in scarplands, which sets
the framework of vegetation-controlled shallow landslide activity.
... In terms of theoretical research on the mechanical properties of roots, some researchers have quantified the force that roots contribute to the soil [21][22][23][24][25][26][27][28][29][30][31][32][33]. For example, Wu et al. [23] and Waldron [24] believed that the root system altered the cohesiveness of the soil and proposed a vertical root system model based on the Mohr-Coulomb strength criterion, which has been gradually refined and adjusted [25,26]. ...
... However, the FBMs model undervalues the significance of the root system to some extent, which ignores the effects of variables such as root volume and root branching [30,31]. In view of this, in recent years, an extension of the FBM has been proposed; researchers have suggested root bundle models (RBMs) that can take into account the impact of the lateral roots of various diameters on the shear strength of the soil [30,32] and other researchers have proposed new energy-based FBMs (FBMs-W) [33], so as to further enhance the theoretical study of soil reinforced by vegetation roots. ...
Vegetation slope protection has been widely utilized as an eco-friendly approach for slope stability. Up to now, research on the slope protection effect of shrubs and herbaceous vegetation is more than those of trees, which can be attributed to the challenge of evaluating the slope protection effect of tree root systems that can be influenced by many factors, such as root morphology, root volume, plant spacing, and planting patterns. Therefore, this study takes tap-like tree root systems as the research object, constructs the corresponding root-soil composite model by using 3D printing technology, and carries out a series of physical model experiments on slopes supported by tap-like tree root systems, examining the anti-sliding force, slope surface displacement, sliding range, and slope cracks throughout the entire process of deformation and the damage to shallow slopes, to finally evaluate the effectiveness in the slope protection effect of tree root systems from multiple perspectives. The results indicate that: (1) the peak anti-sliding force of the slope supported by tree root systems correlates positively with the root volume and negatively with plant spacing generally, and the influence of tree plant spacing on the peak anti-sliding force is weaker than that of the root volume; (2) the displacement of slopes supported by tree root systems in the square planting pattern is generally less than that in the staggered planting pattern, and the displacement of slopes has a negative correlation with the root volume and a positive correlation with the plant spacing; (3) the sliding range of the slope supported by tree root systems is significantly reduced compared with that of the unsupported slope, and the tree root system can prevent the occurrence of slope surface cracks to a certain extent, which makes the sliding-body show better integrity. The above understanding enriches the study on the slope protection effect of the tree root system, reveals the influence of the tree root volume, plant spacing, and planting pattern (square distribution and staggered distribution) of the tap-like tree root system, and offers some guidance for the engineering application of tree slope protection in practice.
... Vegetation cover is the key factor for slope stabilization in forested hillslopes (Sidle, 1991). Vegetation root systems reinforce soil structures and thereby improve slope stability (Schwarz et al., 2010). This reinforcement depends on many contributing factors but is mainly affected by the characteristics of the root system architecture including root orientation and mass. ...
... Forest density and distance between vegetation greatly influence the overlapping length (e.g., Babi et al., 2019), affecting root reinforcement on the slope. Indeed, previous study suggested that the change in forest density from 400 to 1000 stems/ha elevated root reinforcement by 1.6 times because of more overlapping roots (Schwarz et al., 2010). Since root reinforcement affects slope stability, the change in forest density consequently alters landslide frequency (e.g., Imaizumi et al., 2007). ...
... Although the results on the accuracy of root breakage models compared to experimental and field tests varies widely (see Table 1 in Ji et al. [43]), it is well established that the WWM overestimates the root strength [12,14]. Schwarz et al. [17] report that for lateral reinforcement along a landslide scarp, the WWM overpredicts root cohesion by approximately a factor of three, a finding supported by other studies [12,39,44]. Our results also indicate that progressive failure models produce scarp-averaged cohesion values that are 18% and 26% of the WWM-estimated value for RBMw and FBM, respectively, a result which is approximately consistent with these studies and Figure 13. ...
... Although the results on the accuracy of root breakage models compared to experimental and field tests varies widely (see Table 1 in Ji et al. [43]), it is well established that the WWM overestimates the root strength [12,14]. Schwarz et al. [17] report that for lateral reinforcement along a landslide scarp, the WWM overpredicts root cohesion by approximately a factor of three, a finding supported by other studies [12,39,44]. Our results also indicate that progressive failure models produce scarp-averaged cohesion values that are 18% and 26% of the WWM-estimated value for RBMw and FBM, respectively, a result which is approximately consistent with these studies and others [15]. ...
Although accurate root cohesion model estimates are essential to quantify the effect of vegetation roots on shallow slope stability, few means exist to independently validate such model outputs. One validation approach for cohesion estimates is back-calculation of apparent root cohesion at a landslide site with well-documented failure conditions. The catchment named CB1, near Coos Bay, Oregon, USA, which experienced a shallow landslide in 1996, is a prime locality for cohesion model validation, as an abundance of data and observations from the site generated broad insights related to hillslope hydrology and slope stability. However, previously published root cohesion values at CB1 used the Wu and Waldron model (WWM), which assumes simultaneous root failure and therefore likely overestimates root cohesion. Reassessing published cohesion estimates from this site is warranted, as more recently developed models include the fiber bundle model (FBM), which simulates progressive failure with load redistribution, and the root bundle model-Weibull (RBMw), which accounts for differential strain loading. We applied the WWM, FBM, and RBMw at CB1 using post-failure root data from five vegetation species. At CB1, the FBM and RBMw predict values that are less than 30% of the WWM-estimated values. All three models show that root cohesion has substantial spatial heterogeneity. Most parts of the landslide scarp have little root cohesion, with areas of high cohesion concentrated near plant roots. These findings underscore the importance of using physically realistic models and considering lateral and vertical spatial heterogeneity of root cohesion in shallow landslide initiation and provide a necessary step towards independently assessing root cohesion model validity.
... Some investigations integrating root distribution across depth with root-soil mechanical models have highlighted the significant improvement in soil strength due to root reinforcement (Pollen 2007;Schwarz et al. 2010b;Giadrossich et al. 2019). The root reinforcement is controlled by the root distribution (Deljouei et al. 2023;Schwarz et al. 2010a). However, the number of roots exhibits a decreasing tendency with depth (Farooq et al. 2019;Ng et al. 2022). ...
The assessment of landslide susceptibility often overlooks the influence of forests on shallow landslide mobility, despite its significance. This study delved into the impact of forest presence on shallow landslide mobility during intense rainfall in Mengdong, China. Field investigations were coupled with the analysis of pre- and post-rainfall remote sensing (RS) images to delineate landslides. The ratio of landslide height (H) to travel distance (L) from a digital elevation model (DEM) were used to calculate landslides mobility. Preceding the event, forest coverage was evaluated using the normalized difference vegetation index (NDVI) derived from multiband RS image. The research identified 1531 shallow landslides in the area, revealing a higher concentration of landslides on slopes with elevated NDVI. Results indicated that disparities in soil permeability and cohesion, generating pore water pressure (PWP), triggered clusters of shallow landslides. Shallow landslides exhibit a higher propensity on slopes with elevated NDVI. The dimensions (height and area) of these identified shallow landslides typically exhibit a positive correlation with NDVI, consequently resulting in longer travel distances for landslides occurring on higher NDVI slopes. The average H/L ratio of all identified landslides was about 0.63. H/L generally increases with NDVI and decreases with landslide area. However, due to river channel restrictions, the H/L increases with slope gradient. The findings suggest that the high permeability of areas with tree roots poses a risk to the shallow stability of slopes, yet trees contribute to mitigating landslide mobility.
... Moreover, roots increase water infiltration and soil hydraulic conductivity (Beguería 2006). Vegetation also has a mechanical effect on the subsoil as roots create an apparent cohesion via root fiber reinforcement th at promotes slope stability (Roering et al. 2003;Schwarz et al. 2010). Consequently, the conversion from forest to pastures and agriculture lands results in a rapid decline of the sub soil cohesion. ...
Slope movements are dangerous geohazards causing serious socio-economic damages on unstable slopes. In the last decade, the number of landslides research studies has increased rapidly because of their complexity, involving multiple parameters varying in time and space, their great potential to hinder the socio-economic development and especially the high budgets invested in risk mitigation interventions worldwide. In active mountain belts as the Pre-Rif unit, both conditioning and triggering factors are present and human activity is often involved either through land use favouring instability, or the disturbance of hillslopes, and without omitting the vulnerability presented by certain urban and peri-urban extensions. Nowadays, hazard mapping and its integration into approved land-use planning documents is one of the preliminary and most effective means of mitigating and managing natural hazards. Hence, the public authorities have launched several tenders aimed at producing maps of suitability for urbanization (CAU), particularly regarding the risk of slope movements, in several of the kingdom's provinces. Within the framework of these projects, the present work has been carried out. Numerous approaches are used in landslide studies, heuristic, deterministic and statistical depending on the geomorphic context, scale, data availability and especially the objectives targeted. In the present research work, three types of approaches are elaborated to investigate landslide hazard in the Fez-Moulay Yacoub region. The deterministic methods developed have proved their effectiveness and complementarity in the study of this hazard in densely urbanized areas and at the scale of detail, providing precise information on the extent and kinematics of landslides affecting the urban center of Moulay Yacoub. As for the heuristic methods, the mapping of the susceptibility to ground movements at a broad scale gave results of high quality and of crucial utility. the analysis and evaluation of the conditioning parameters revealed that the anthropogenic factors are strongly involved, notably the use of land and the proximity to the road network, in addition to the classic factors of predisposition (slope, proximity to the hydro network, etc.).
Several statistical methods have been used in this work to investigate the impact of
topographic growth conditioned by active tectonics on the magnitude of ground movements in the southern Riffian front. The results showed the difference in terms of typology and slope dynamics between the southern edge of the Prerif and the hilly landscape dominating the province of Moulay Yacoub. Finally, the analysis of the impact of landslides carried out on several urban extensions showed that human activity is strongly involved in the instability of the slopes, especially because it presents a high vulnerability. Moreover, among the areas investigated, the urban center of Moulay Yacoub as well as the urbanized outskirts of the city of fez proved to be the most vulnerable to slope movements and highly exposed.
... Performing a root pull-out test is a straightforward way to evaluate the tree uprooting resistance provided by such an anchoring effect. Many pull-out studies have been carried out using experimental, analytical or numerical methods (e.g., Schwarz et al., 2010;Mickovski et al., 2010;Liu et al., 2014;Giadrossich et al., 2017). For example, after a root pull-out numerical model is developed and validated, it may be used to evaluate the effect of different tree species or soil types on tree stability, contributing to risk-based and safety-oriented urban tree management. ...
Tree collapse due to uprooting under extreme winds has caused enormous economic losses and human injuries, even casualties, in many coastal cities like Hong Kong. It is therefore of great interest to assess the tree uprooting resistance for evaluating the risk of typhoon-induced tree failures and improving urban tree management. However, modeling the process of tree uprooting is challenging as it involves complex large-deformation behavior within a root-soil system. Previous simulations often used finite element methods (FEM), which suffer from severe mesh distortion problems during large deformation and cannot simulate the pull-out process along the root-soil interface. In this study, a numerical model that employs the material point method (MPM) with a multi-level background grid is developed to simulate, for the first time ever, the whole large-deformation process of root pull-out, including root breakage. The developed model is firstly validated against existing centrifuge test results. Then some new insights into the root pull-out during the large deformation stage are revealed. The results show that the sticking root-soil contact model is more suitable than the frictional contact model when conducting large-deformation modeling of root pull-out. A parametric study performed shows that the root pull-out resistance increases with increasing soil dilation angle. Finally, an illustrative example is presented to demonstrate the ability of the developed model in simulating root breakage failure during pull-out.
... Performing a root pull-out test is a straightforward way to evaluate the tree uprooting resistance provided by such an anchoring effect. Many pull-out studies have been carried out using experimental, analytical or numerical methods (e.g., Schwarz et al., 2010;Mickovski et al., 2010;Liu et al., 2014;Giadrossich et al., 2017). For example, after a root pull-out numerical model is developed and validated, it may be used to evaluate the effect of different tree species or soil types on tree stability, contributing to risk-based and safety-oriented urban tree management. ...
Tree collapse due to uprooting under extreme winds has caused enormous economic losses and human injuries, even casualties, in many coastal cities like Hong Kong. It is therefore of great interest to assess the tree uprooting resistance for evaluating the risk of typhoon-induced tree failures and improving urban tree management. However, modeling the process of tree uprooting is challenging as it involves complex large-deformation behavior within a root-soil system. Previous simulations often used finite element methods (FEM), which suffer from severe mesh distortion problems during large deformation and cannot simulate the pull-out process along the root-soil interface. In this study, a numerical model that employs the material point method (MPM) with a multi-level background grid is developed to simulate, for the first time ever, the whole large-deformation process of root pull-out, including root breakage. The developed model is firstly validated against existing centrifuge test results. Then some new insights into the root pull-out during the large deformation stage are revealed. The results show that the sticking root-soil contact model is more suitable than the frictional contact model when conducting large-deformation modeling of root pull-out. A parametric study performed shows that the root pull-out resistance increases with increasing soil dilation angle. Finally, an illustrative example is presented to demonstrate the ability of the developed model in simulating root breakage failure during pull-out.
... Performing a root pull-out test is a straightforward way to evaluate the tree uprooting resistance provided by such an anchoring effect. Many pull-out studies have been carried out using experimental, analytical or numerical methods (e.g., Schwarz et al., 2010;Mickovski et al., 2010;Liu et al., 2014;Giadrossich et al., 2017). For example, after a root pull-out numerical model is developed and validated, it may be used to evaluate the effect of different tree species or soil types on tree stability, contributing to risk-based and safety-oriented urban tree management. ...
... This explains why shallow movements are mostly found on cereal-growing slopes while wooded areas present relatively deeper processes (Appendix 2 and 3). Vegetation also has a mechanical effect on the subsoil as roots create an apparent cohesion via root fibre reinforcement that promotes slope stability (Roering et al., 2003;Schwarz et al., 2010). Consequently, the conversion from wooded areas to pastures and agriculture lands results in a rapid decline of the sub soil cohesion. ...
In the recent decades, the growth of population, man-made facilities, infrastructures, and lifelines at the expense of landslides prone areas has been responsible for an exponential increase in human and economic losses in many parts of the world. In the Moulay Yacoub region, where marly hills dominate, the interaction of the semi-urban and rural socioeconomic development and landslides significantly increases, which urges identifying and prioritizing areas of risk in order to maximize harm reduction and to avoid the disastrous outcomes as is the case of Moulay Yacoub town. This paper aims to develop a landslide susceptibility map in a highly affected sector of the province, where no previous landslide data have been produced, and to find the most involved parameters. This goal will be attained using two robust methods, the Frequency Ratio and the GIS Matrix Method. Before that, the correlation of 11 factors was tested. The results show that the anthropogenic factors, particularly the agricultural practices, were highly involved, and the field investigation proved that cereal farming slopes were the most affected. The success rate was about 0.75 (75%) for both models showing good quality results for the two susceptibility maps. Therefore, the two models could be efficiently used, and the new agricultural projects located in landslide-prone areas of the province must include such reliable methods of landslide risk analysis to minimize the triggering probabilities, which would put human lives, ecosystems, food production, and infrastructure at risk.
... In addition to the root size effects on soil-structure interaction properties explored in the previous sections, roots themselves typically exhibit material properties which are dependent on diameter (Schwarz et al. 2010;Mao et al. 2012): ...
For smaller lateral plant roots in coarse-grained soils, a potentially large relative size of soil particles compared to the roots may affect their transverse resistance. Even for the larger roots of trees, particle size effects may be important, e.g. when testing 1:N reduced scale models in a geotechnical centrifuge. The Discrete Element Method (DEM) was used to investigate this problem. A rigid lateral root segment under transverse loading in plane strain was simulated and compared with Finite Element Method (FEM) simulations, where the soil was modelled as a continuum (no particle size effects). Even at the lower root/particle diameter ratios (dr/D50) investigated (6 to 21), particle size effects on transverse capacity were negligible upon push-in, while during uplift, they were observed for dr/D50 < 8, arising from the dimension of the uplifted soil volume above the root. The material properties of roots are also typically diameter dependent. Further simulations of long flexible roots subject to end rotation were performed employing a beam-on-non-linear-Winkler-foundation approach, using p-y curves obtained from the DEM or FEM simulations. Compared with particle-size related effects, diameter-dependent variation of material properties had a much larger controlling effect on root capacity and stiffness as relevant for plant/tree overturning.
... Despite an abundance of studies on the thermal dynamics of silty clay, a discernible focus is evident: investigations predominantly target singular soil types, often overlooking the interplay of biology, particularly plant root systems [15][16][17][18][19]. Moreover, extant literature is replete with experiments, often sidelining methodical theoretical analyses [20][21][22][23][24][25][26][27]. ...
... Present research on plant-stabilized slopes, mainly focus on mechanical reinforcements, and their effects on the root system, which mainly depend on the tensile strength to improve the shear strength of the soil [12][13][14][15]. To quantify the shear strength of root soil, researchers have carried out root tensile [16,17] and in situ shear tests of soil [18], and successively proposed the Wu-Waldron [19], fiber bundle [20], root bundle [21], and other mechanical models, experimentally and theoretically, to measure the shear resistance of root soil intensity. These studies showed that the root system significantly improved the shear strength of the soil. ...
Rainfall-prone shallow landslides account for one-fifth of the global land area, and rainfall is critical to the mechanics and hydrology of shallow slopes. In typical geological disaster-prone areas, the hydrodynamic responses of slopes with different vegetation types under rainfall conditions require further study. The purpose of this study was to analyze the hydraulic stability of soils with different vegetation types under rainfall conditions and their effects on slope stability. Thus, the soil–water characteristic curves and water-stable aggregate characteristics of soils with three vegetation types were analyzed. A two-dimensional finite element model was used to simulate the slope stability of extreme rainfall environments with different rainfall durations. The results showed that the matric suction of soil with trees was less affected by rainfall with a better stability of water-stable aggregates than that of soil with shrubs and grass. The plastic strain cloud map showed that the maximum plastic strain occurred at the toe of the slope. In addition, the potential slip depth of slopes with trees was smaller than that of slopes with shrubs and grass. Under the two rainfall durations, the factor of safety (FoS) of slopes with trees changed by 0.06, whereas that of slopes with shrubs and grass changed by 0.1. The findings of this study provide valuable insights into changes in the stability of slopes with different vegetation types under varying rainfall conditions. It is of great significance to provide a scientific basis for the application of ecological measures in the prevention and control of mountain disasters and guide the implementation of appropriate land management measures.
... However, recent studies have shown that the reinforcing effect due to root systems varies spatially and temporally [10,11]. The effect of root systems is distinguished by the direction of stress on the shear plane and is divided into vertical roots that pass through the landslide plane and horizontal roots that pass through the sidewalls around the landslide [4,12]. The reinforcing effect of vertical roots is most effective when uniformly present on the landslide plane [4], and the reinforcing effect of horizontal roots is reported to depend on the spatial distribution of roots [13]. ...
Climate change has increased the frequency and scale of heavy rainfall, increasing the risk of shallow landslides due to heavy rainfall. In recent years, ecosystem-based disaster risk reduction (Eco-DRR) has attracted attention as one way to reduce disaster risks. Vegetation is known to increase soil strength through its root system and reduce the risk of shallow landslides. To reduce the risk of shallow landslides using vegetation, it is necessary to quantitatively evaluate the effects that vegetation has on shallow landslides. In this study, we constructed a generalized linear model (GLM) and random forest (RF) model to quantitatively evaluate the impact of differences in the vegetation, such as grasslands and forests, on the occurrence of shallow landslides using statistical methods. The model that resulted in the lowest AIC in the GLM included elevation, slope angle, slope aspect, undulation, TWI, geology, and vegetation as primary factors, and the hourly rainfall as a trigger factor. The slope angle, undulation, and hourly rainfall were selected as significant explanatory variables that contribute positively to shallow landslides. On the other hand, elevation and TWI were selected as significant explanatory variables that contribute negatively to shallow landslides. Significant differences were observed among multiple categories of vegetation. The probability of shallow landslide in secondary grasslands was approximately three times that of coniferous and broadleaf forests, and approximately nine times that of broadleaf secondary forests. The landslide probability of shrubs was approximately four times that of coniferous and broadleaf forests, and approximately ten times that of broadleaf secondary forests. The results of constructing the RF model showed that the importance was highest for the hourly rainfall, followed by geology, then elevation. AUC values for the GLM and RF model were 0.91 and 0.95, respectively, indicating that highly accurate models were constructed. We quantitatively showed the impact of differences in vegetation on shallow landslides. The knowledge obtained in this study will be essential for considering appropriate vegetation management to reduce the risk of future shallow landslides.
... However, the impact of vegetation should be assessed with caution. The resistance provided by roots is only available in the reinforced region (Genet et al. 2008;Schwarz et al. 2010;Ji et al. 2012), and the above beneficial hydrological effects potentially disappear in a rainstorm (Reid and Lewis 2009;McGuire et al. 2016;Zheng et al. 2018). Furthermore, water intercepted by the canopy can flow down the trunk and along the roots, affecting the hydrological process (Johnson and Lehmann 2006). ...
Typhoon-induced deep-seated landslides are prone to cause mass casualties to coastal regions once they occur. On 10th August 2019, typhoon “Lekima” triggered the deadliest landslide in recent decades in Southeast coastal China (32 deaths). To investigate the possible failure mechanism of this catastrophic event, we conducted in situ investigations and image analysis to determine the geological characteristics and performed numerical modelling of transient variation in slope stability throughout the typhoon. Generalized modelling analysis was further designed to research the impacts of some landslide settings and forcing factors on this type of geohazard. Our findings show that cracks within the weathered rock played vital roles in the landslide initiation. Cracks-induced strength reduction made the saturated rock provide insufficient resistance to stabilize the slope. Preferential infiltration promotes the saturation of sliding mass within a short-duration typhoon, responsible for the rapid failure. Furthermore, the impact of trees on slope stability depends heavily on landslide settings. Constrained by the limited growth depth of roots, though both beneficial and detrimental mechanisms of trees show minor impacts on the development of a deep-seated sliding surface, trees and geological settings converged to produce the failure mode of the Yongjia landslide. Roots significantly enhanced the shallow soil layer and the soil–rock interface is simulated to be the potential sliding surface in the absence of trees. Results suggest future landslide risk assessments in Southeast coastal China to specify the meteorological and geological conditions, which would greatly improve the aim to predict the spatiotemporal distribution of landslides throughout a typhoon.
... The extra resistance provided by roots disappears below the rootreinforced area (Schwarz et al., 2010), and the beneficial hydrological effects potentially disappear in a rainstorm (McGuire et al., 2016). Therefore, these contributions hardly work below the rooting zone during a typhoon. ...
Typhoon‐induced landslides are widespread, damaging and deadly. In this study, we elucidated their spatiotemporal features through investigations in Southeast coastal China and proposed a numerical model that involves typhoon wind load, preferential infiltration and root reinforcement. The wind load is calculated through a combination of the autoregressive method and a multi‐degree‐of‐freedom tree swaying mode, transmitting to the slope through the wind–tree interaction. Taproots of trees are modelled as piles, and root–soil interfaces are modelled as preferential infiltration boundaries. Using the numerical model, we quantitatively assessed the impacts of wind load, rainfall infiltration and root reinforcement on the slope stability and compared results with cases in Waipaoa and Wairoa (New Zealand). Results suggest that the impacts of trees depend heavily on meteorological and geological conditions. Trees play a destabilizing role in Southeast China during a typhoon but a stabilizing role in the cases in New Zealand. For slopes in Southeast China with a thick soil layer (>root depth), strong wind load and preferential flow resulting from root–soil interfaces, rather than slope surface infiltration, significantly decrease the slope stability in a short time. Slope failure occurred in all scenarios that account for the preferential infiltration, and its combination with a Force 14 typhoon wind load can fail the slope at 10 h after the typhoon initiation. Differently, tree roots in cases in New Zealand can penetrate through the thin soil layer (<1 m) and provide considerable additional resistance to stabilize the slope.
... The results also indicated that not all root systems contributed to the hydrological connectivity pathway and roots with a smaller diameter (D < 1 mm) contributed more to water infiltration. The mass distribution of fine roots depends on the growth and mortality of individual root tips and has a high turnover rate, depending on local conditions and seasons (Schwarz et al., 2010). Some studies have shown that root systems with a large diameter (D > 5 mm) squeeze the soil as they grow in soil, which can lead to smoother soil channels and have the greatest impact on soil hydrological connectivity. ...
Under the combined effects of topography and vegetation, hydrological connectivity characteristics of riverbank slopes become complex and unclear, which limit the utilization and protection of riparian zones. To quantify the hydrological connectivity in root–soil composites, we conducted dyeing and tracing experiments in a high elevation plot and a low elevation plot on the bank of the Fenhe River. Soil and root properties and hydrological connectivity indexes in the plots were measured and analyzed. The results showed that the soil dyeing area ratio was approximate 1 in the soil depth of 0–5 cm and then decreased to 0.1 from 5 cm to 25 cm. The dyeing area ratio, maximum dyed depth, length index, peak value and non-uniformity coefficient of the high plot (Pc2) were 27%, 26%, 5%, 40% and 45% greater than those of the low plot (Pc1). The index of hydrological connectivity (IHC) of Pc2 was 7%, 44% and 71% greater than Pc1 in the soil depths 0–10 cm, 10–20 cm and 20–30 cm respectively. There was no significant correlation between the IHC and the physical properties of the soil at different depths, and the soil hydrological connectivity was closely related to the plant roots with diameter less than 1mm. The study primarily explored the characteristics of hydrological connectivity in root–soil composites. The results provide a scientific basis for exploring hydrological connectivity of riparian zones, which can support future riparian zone protection and restoration efforts in similar regions.
... Root distribution is modeled using the Root Distribution Model (RDM) described by Schwarz et al. [52]. The RDM estimates the number of roots in diameter class size i that cross a 1 m width vertical soil profile at a distance d from an isolated tree stem with the diameter at breast height (DBH, in [m]) φ t following the equation: ...
Populus nigra ita. is an important tree species for preventing rainfall-triggered shallow landslides and hydraulic bank erosion in New Zealand. However, the quantification of its spatial root distribution and reinforcement remains challenging. The objective of this study is to calibrate and validate models for the spatial upscaling of root distribution and root reinforcement. The data were collected in a 26-year-old “Tasman” poplar stand at Ballantrae Hill Country Research Station in New Zealand. We assessed root distribution at different distances from the stem of four poplar trees and from eleven soil pits along a transect located in a sparse to densely planting poplar stand. 124 laboratory tensile tests and 66 field pullout tests on roots with diameters up to 0.04 m were carried out to estimate root mechanical properties. The results show that the spatial distribution of roots can be well predicted in trenches of individual tree root systems (R2 = 0.78), whereas it tends to overestimate root distribution when planting density was higher than 200 stems per hectare. The root reinforcement is underestimated within single tree root systems (R2 = 0.64), but it performs better for the data along the transect. In conclusion, our study provided a unique and detailed database for quantifying root distribution and reinforcement of poplars on a hillslope. The implementation of these models for the simulation of shallow landslides and hydraulic bank erosion is crucial for identifying hazardous zones and for the prioritization of bio-engineering measures in New Zealand catchments. Results from this study are useful in formulating a general guideline for the planning of bio-engineering measures considering the temporal dynamics of poplar’s growth and their effectiveness in sediment and erosion control.
... Figure 2.4 shows the flowchart of the RipRoot model. After Pollen and Simon (2005), the FBMs method was extended and increased the accuracy of perdition of root cohesion by several researchers (Pollen and Simon 2009;Schwarz et al. 2010cSchwarz et al. , 2010bSchwarz et al. , 2013. These models differ significantly in terms of the load-contributing hypothesis among roots, resulting in disparate root failure orders and cr estimations. ...
Plant roots provide mechanical reinforcement to soils upon shearing and seismic loading. The effects of roots distribution and their orientations on any changes in soil anisotropy in terms of shear strength, critical state and maximum shear modulus (Gmax) are crucial to engineering design and analysis but have not been thoroughly studied. Existing failure criteria of rooted soils, that are predominantly derived based on the test results of direct shear, could not capture the strength anisotropy of rooted soils under general loading conditions. Moreover, whether, and how, roots provide resistance to soil liquefaction upon cyclic loading and the mechanisms of dissipated energy involved at liquefaction state have rarely been studied. Two series of drained and undrained monotonic triaxial tests upon compression and extension stress path were conducted on soils reinforced by the roots of a deep-rooted species (vetiver grass, (Chrysopogon zizanioides L.) to investigate the shear strength of soil and to aid the development of a new generalised 3-D anisotropic failure criterion for rooted soils. The underlying mechanism of liquefaction of rooted soil was investigated by the data of a series of undrained cyclic triaxial tests on rooted soils. A series of bender element tests on rooted soils were carried out following the isotropic loading and unloading paths, aiming to explore the effects of roots on Gmax and to develop new semi-empirical equations of Gmax for rooted soils.
It was discovered that the root reinforcement effect was anisotropic and path dependent. Roots with predominant orientation aligning in the tensile strain direction contribute the most to soil strength. In the case of vetiver grass, which has a taproot system, their roots show the strongest reinforcement effect in conventional triaxial extension path, in which the maximum portion of roots are subjected to tension. Comparing to bare soil, rooted soils were less compressible, and its recoverable elastic deformation were also smaller. Interestingly, the critical-state lines (CSLs) of bare and rooted soils in the υ-Lnp' space for the compression and extension paths converged. However, the gradient of the CSL in the q-p' space is dependent upon the stress path due to the fabric anisotropy and anisotropic distribution of roots in the soil.
A new generalised 3-D anisotropic failure criterion was derived for rooted soils. The projection of the microstructure fabric tensors of soil and root network on stress tensors to address the anisotropic effects of root network and soil fabric on the shear strength parameters of rooted soils upon various effective stress paths. This model addressed anisotropies of both cohesion and friction angle and explains why most of studies by direct shear reported that roots affect the cohesion but not the friction angle. Indeed, this stress paths are within section I of the deviatoric plane, where the effects of soil anisotropy on friction angle are not noticeable.
The presence of roots in cultivated samples increased the Gmax, whereas that in the artificially rooted samples reduced its Gmax. A new semi-empirical model was derived to address the effects of roots on Gmax of high compressible cultivated rooted soil by incorporating elastoplastic constitutive relationships, which took account the effects of mean effective stress and plastic volume change hardening. Additionally, A new equation was devised for sandy soil since the cultivated Gmax model was ineffective for artificially rooted soil due to difficulties in locating the isotropic consolidation line and negligible impact of over consolidation on Gmax.
The liquefaction resistance of artificial samples was improved with an increase in root volume, and this improvement was more remarkable at higher cyclic stress ratios. The grass roots, beyond certain volume, prevented the soil from experiencing limited flow failure that occurred in unreinforced sand, and switched the failure mode to cyclic mobility. Normalised cumulative dissipated energy (ΣΔW/σc’) of rooted soil depended on cyclic stress ratios (CSRs) and RVRs and this energy is uniquely correlated with the cyclic resistance ratio at the cycle number of 15 (CRR15). It was discovered that roots that were predominantly orientated in the direction perpendicular to the major principal stress of extensive path reduced soil anisotropy upon cyclic loading. Besides, the ΣΔW/σc’ was linearly correlated with the normalized cumulative strain energy (Σ4W/σc’) with a gradient of approximately 2.
... In addition, root data need to be collected in a format suitable for inclusion in slope stability and/or soil reinforcement models (e.g., Schwarz et al. 2010b), many of which suffer from a paucity of, and lack of consistency in, the types of data collected to date. The description of root systems and their analysis in New Zealand has largely focused on spatial distribution as a function of diameter. ...
Background: Trees and forests have been used in New Zealand to reduce erosion, particularly from rainfall–triggered landslides, gullying, and earthflows. Most New Zealand tree root research has been conducted during the life of the New Zealand Journal of Forestry Science, with much published in it. Methods: We undertook a retrospective ‘review’ of New Zealand tree root research focusing on soil reinforcement and its application for erosion control, slope stability assessment, and understanding tree stability in forests. The published and grey literature was searched using common search terms and relevant papers assessed. The international literature was not reviewed but helped provide context for the New Zealand studies. Results: Results were aggregated into broad topic areas and key findings summarised. Where multiple studies existed for a particular species, results are presented by species. Selected data are presented to enable inter-species comparisons, and the reader is directed to additional data or the original study. Conclusions: New Zealand tree root research has focused mostly on root description or simple measurements to support applied studies of root structure and function. Nonetheless, such research has made a valuable global contribution in addition to improving the understanding and management of New Zealand’s forests. Studies show that generally, exotic species outperform indigenous species for most empirical root metrics other than root tensile strength. A combination of both lateral and vertical roots provides the best soil reinforcement and contribution to slope stability. Future research should focus on acquiring more field data and improvements in dealing with spatial and temporal variability in model development. Practical tools for land managers to target the right places with the right vegetation (species, amount, density) are a pressing need as changing climate is changing the way we manage natural hazards like landslides, floods and wildfires.
... OF THE ROOTSBiomechanical properties of the roots mostly reported and used in studies assessing slope stability are the tensile strength and elastic modulus(Loades et al. 2013;Stokes et al. 2009;Schwarz et al. 2010;). Root tensile strength is regarded as the maximum force per unit area required to break the root(Genet et al., 2005). ...
The potentials of the roots of Pennisetum purpureum and Mangifera indica for use in sustainable stabilization of slope were evaluted. The plant roots were sampled close to an erosion site in Nwonta Isuikwuato local government of Abia State Nigeria. The tensile strength of the roots segments was determined using the Universal Testing Machine (UTM) at the Michael Okpara University of Agriculture, Umudike. The effect of water content and dehydration due to adverse weather condition was studied by tensile testing the roots after different drying periods (30 minutes, 1 hour, and 24 hours). Laboratory experiments was performed to evaluate the engineering properties of the soil at the erosion site. The mean tensile strength of the Pennisetum purpureum obtained were 5.30 MPa (hydrated roots), 12.49 MPa (30 minutes dried) and 17.13 MPa (1 hour dried). After 24 hours drying the roots of Pennisetum purpureum which is a grass species was not strong enough to be tested for tensile strength. For the Mangifera indica the mean tensile strength obtained were 17.41 MPa (hydrated roots), 18.73 MPa (30 minutes dried) and 21.84 MPa (1 hour dried) and 34.10 (24 hours dried). The roots show that root tensile strength increased as the root moisture content reduces. The heightened rate of soil erosion in the study area could be due to the fact that the soils are predominantly sand with no cohesion as observed from the laboratory test. Hence, planting these vegetations can significantly lead to the improvement of the strength properties of the soil and minimise the rate of soil erosion and slope failure.
... Plant roots could increase slope stability by enhancing the shear strength of soil [56]. The reinforcement of plant roots can be considered as an additional cohesion [57], which has been addressed by different root reinforcement models, such as the Wu model [49], the fiber bundle model [58], the root bundle model [59] and the energy approach model [60]. In this study, we used the Wu model to estimate additional cohesion from plant roots, due to use of the Wu model by many other studies in the same study area. ...
Vegetation can play a crucial role in stabilizing slopes through their hydrological and mechanical properties, yet few studies have systematically compared their effects on soil moisture resistance and slope stability. To investigate this, four steep slopes covered by different forests were analyzed in terms of climatic conditions, soil moisture dynamics, root strength and soil physical properties. The results revealed that the roots of Phyllostachys pubescens forests had a higher number and were deeper than the main plant species in the other three forests. Although the root tensile strength of Phyllostachys pubescens was not the strongest, its additional cohesion contributed more to hillslope stability. In the other three forests, suction stress was the main factor contributing to hillslope stability. The soil moisture change rate in Phyllostachys pubescens was found to be the smallest among the four forests studied, indicating that it had the greatest rainfall interception ability. The stability of the slope land covered by shrub forest was found to be more variable than the other three lands in high temperature conditions. Through its soil moisture reducing ability, root characteristics and magnitude of safety factor, Phyllostachys pubescens was identified as a suitable species for slope stabilization in the study area. The findings of this work may provide useful insights for local forest management in terms of selecting suitable plant species to reduce shallow landslides.
... where k = 1.2 is a factor accounting for the root orientation [14], k a reduction factor accounting for sequential root mobilisation (often quantified using a fibre bundle model, e.g., [18,19]), t r,i is the tensile strength of a single root i, A r,i the cross-sectional area of root i, A the shear area of the soil,t r the average root tensile strength and R ra the root area ratio, i.e., the fraction of a soil cross-section occupied by roots. Root sampling however is time-consuming, and because of the complicated mechanical interaction between soil and root such models often incorporate large assumptions, reducing their reliability. ...
Roots can help to stabilise slopes against landslides and anchor trees against wind loading, but their mechanical contribution to the strength of soil is difficult to rapidly quantify under field conditions. A new field measurement method, quantifying the shear strength of rooted soil by measuring the resistance against extraction of soil cores using a large corkscrew device, was tested across three heterogeneous slopes (unforested, forested and clearfelled) in Scotland. The presence of roots significantly increased the measured shear strength in the surface layer of the Sitka spruce forested slope. Differences in strength between the three areas were however not significant. This could be attributed to the large variation in the soil component of the combined root–soil shear strength, which was strongly affected by variations in both soil density and gravel content. Measured strength on these natural slopes were much more variable compared to previously investigated sites. These results highlight the importance of investigating the variation in soil strength during root-reinforcement measurements, and furthermore demonstrate the need for a sufficiently large number of tests to address this variation. The corkscrew provides rapid estimation of root-reinforced soil shear strength on sites with difficult accessibility. Compared to the more conventional shear vane method, which yielded comparable soil strength results, the corkscrew proved more suitable in stony soil layers and has the additional benefit of simultaneously extracting small (rooted) soil samples that could be used for further root and soil analysis. It therefore proved a useful and effective field tool for use when a rapid estimation of root-reinforced soil shear strength is required.
... The ability of plants to develop adequate rooting resistance depends on several factors. Plant anchorage strength is affected by the depth and the spatial branching density of the root system (Ennos and Pellerin, 2000;Schwarz et al., 2010b) and the moisture and texture of the soil (Pollen, 2007). Hence, the reintroduction of riparian plants in corridor ecosystems requires a full understanding of the candidate plant species and its interaction with river hydrology and geomorphology. ...
Successful establishment of riparian vegetation on riverbanks and bedforms
depends on river hydrology and related flow and sediment erosion processes.
Extreme flow-induced erosion events can uproot vegetation, leading in some cases
to failure of bank protection and river restoration schemes. This thesis uses
experimental, analytical, and numerical approaches to examine key aspects of
the mechanisms of vegetation uprooting by flow. First, the ability of riparian
vegetation to respond to different water table regimes is investigated in terms
of root growth and resistance. To this purpose, small-scale Salix cuttings
were allowed to grow under different water level regimes. At the end of the
growing period, extracted samples, obtained through pullout tests, were analysed
in terms of root biomass distribution and resistance to external forces. The
results demonstrate the driving influence of water and oxygen availability on the
vertical configuration of below-ground biomass and thence on uprooting resistance.
Second, a free-body model is derived to predict the critical rooting length – a key
parameter that determines the probability of flow-induced uprooting of flexible
plants at different erosion stages. Model validation is achieved using laboratory
and field-scale data. Third, the dynamics of mobilization of stranded living
wood logs from alluvial bedforms is investigated experimentally. Pullout test
results are used to assess the root resistance of small-scale wood logs at several
stages of growth. Trends in below- and above-ground biomass, together with
the free-body model, enable detection of ‘biological time windows’ within which
re-mobilization becomes possible. The results illustrate that uprooting occurs within two time-lapses, which coincide with particular growth stages of the plant.
Finally, a combined analytical and numerical model is derived. This model
uses the probability of flow-induced plant uprooting as a proxy to study how
perturbations to the natural flow regime may drive riparian ecosystem dynamics
towards new and potentially irreversible statistical equilibrium states. The model
is applied to an actual case study, in which dam impoundment of a reach of the
Maggia River, Switzerland, has led to intense riparian vegetation encroachment
with consequent river narrowing. The output of the model sheds light on the type
of irreversibility that may arise in riverine ecosystems of severely impounded river
basins. The theoretical and experimental results presented in the thesis should be
useful to river engineers and managers responsible for river restoration projects,
natural flood management schemes, and optimal dam regulation strategies.
... Root cohesion can be differentiated into basal and lateral root cohesion (Schwarz et al., 2010a). The basal root cohesion is characterized by roots crossing the shear plane of landslide at a depth z. ...
Landslides are important agents of sediment transport, cause hazards and are key agents for the evolution of scarplands. To analyse geologic and vegetation control on landsliding, we investigated three landslides in the Franconian scarplands. We used geomorphic mapping, soil analysis, electrical resistivity and a mechanical stability model to quantify the stability state of the landslides. Furthermore, we mapped tree distribution, quantified rooted area and root tensile strength to assess the influence of vegetation on shallow landsliding. Our results show that landslides are deep-seated incorporating rotational and translational movement with sliding along a geologic boundary between permeable Rhätolias sandstone and impermeable Feuerletten clays. Despite low slope angles, landslides could be reactivated when high pore pressures could develop due to geologic conditions. In contrast, shallow landsliding is controlled by vegetation. Our results show that rooted area is more important than species dependent root tensile strength and limited to the upper 0.5 m of the surface due to geologically controlled unfavourable soil conditions. Due to low slope inclination, root cohesion can stabilize landslide toes or slopes undercut by forest roads, independent of potential soil cohesion, when tree density is sufficient dense. Forest management currently adapts forests to climate change by diversifying tree species and introducing European beech, which would increase slope stability when sufficient rooted area is reached. Forestry activities should aim to keep a certain tree density to enable sufficient root cohesion that prevent landslide activity between harvesting or adaption periods. In summary, geological conditions in scarplands favour landslide activity and influence vegetation control on landslide activity, which suggest a combined forest and hazard management should be applied to prevent future landsliding.
... Geology, morphology, vegetation, and human activities are factors affecting landslide hazards [5,6], and land cover is a key determining factor. Forests and trees play important roles in reducing landslide risk mainly due to shear strength enhancement by roots on soil particles of slip surfaces [7,8]. A comparison of landslide distribution among land use types muddy turbidite, and turbidite soil. ...
Landslides often cause deaths and severe economic losses. In general, forests play an important role in reducing landslide probability because of the stabilizing effect of the tree roots. Although fruit groves consist of trees, which are similar to forests, practical land management, such as the frequent trampling of fields by laborers and compression of the terrain, may cause such land to become prone to landslides compared with forests. Fruit groves are widely distributed in hilly regions, but few studies have examined their role in landslide initiation. This study aims at filling this gap evaluating the predisposing and triggering conditions for rainfall-triggering landslides in part of Uwajima City, Japan. A large number of landslides occurred due to a heavy rainfall event in July 2018, where citrus groves occupied about 50% of the study area. In this study, we combined geodata with a regression model to assess the landslide hazard of fruit groves in hilly regions. We developed maps for five conditioning factors: slope gradient, slope aspect, normalized difference vegetation index (NDVI), land use, and geology. Based on these five maps and a landslide inventory map, we found that the landslide area density in citrus groves was larger than in forests for the categories of slope gradient, slope aspect, NDVI, and geology. Ten logistic regression models along with different rainfall indices (i.e., 1-h, 3-h, 12-h, 24-h maximum rainfall and total rainfall) and different land use (forests or citrus groves) in addition to the other four conditioning factors were produced. The result revealed that “citrus grove” was a significant factor with a positive coefficient for all models, whereas “forest” was a negative coefficient. These results suggest that citrus groves have a higher probability of landslide initiation than forests in this study area. Similar studies targeting different sites with various types of fruit groves and several rainfall events are crucial to generalize the analysis of landslide hazard in fruit groves.
Root pullout performance of plants is an important mechanical basis for soil reinforcement by plant roots in the semi-arid areas. Studies have shown that it is affected by plant factors (species, ages, root geometry, etc.) and soil factors (soil types, soil moisture, soil bulk densities, etc.). However, the effects of loading rates on root pullout performance are not well studied. To explore the mechanical interactions under different loading rates, we conducted pullout tests on Medicago sativa L. and Hippophae rhamnoides L. roots under five loading rates, i.e., 5, 50, 100, 150, and 200 mm/min. In addition, tensile tests were conducted on the roots in diameters of 0.5–2.0 mm to compare the relationship between root tensile properties and root pullout properties. Results showed that two root failure modes, slippage and breakage, were observed during root pullout tests. All M. sativa roots were pulled out, while 72.2% of H. rhamnoides roots were broken. The maximum fracture diameter and fracture root length of H. rhamnoides were 1.22 mm and 7.44 cm under 100 mm/min loading rate, respectively. Root displacement values were 4.63% (±0.43%) and 8.91% (±0.52%) of the total root length for M. sativa and H. rhamnoides, respectively. The values of maximum pullout force were 14.6 (±0.7) and 17.7 (±1.8) N under 100 mm/min for M. sativa and H. rhamnoides, respectively. Values of the maximum pullout strength for M. sativa and H. rhamnoides were 38.38 (±5.48) MPa under 150 mm/min and 12.47 (±1.43) MPa under 100 mm/min, respectively. Root-soil friction coefficient under 100 mm/min was significantly larger than those under other loading rates for both the two species. Values of the maximum root pullout energy for M. sativa and H. rhamnoides were 87.83 (±21.55) mm•N under 100 mm/min and 173.53 (±38.53) mm•N under 200 mm/min, respectively. Root pullout force was significantly related to root diameter (P<0.01). Peak root pullout force was significantly affected by loading rates when the effect of root diameter was included (P<0.01), and vice versa. Except for the failure mode and peak pullout force, other pullout parameters, including root pullout strength, root displacement, root-soil friction coefficient, and root pullout energy were not significantly affected by loading rates (P>0.05). Root pullout strength was greater than root tensile strength for the two species. The results suggested that there was no need to deliberately control loading rate in root pullout tests in the semi-arid soil, and root pullout force and pullout strength could be better parameters for root reinforcement model compared with root tensile strength as root pullout force and pullout strength could more realistically reflect the working state of roots in the semi-arid soil.
The simple-shear condition is closer to reality than the direct-shear condition for simulating the mechanical behavior of vegetated soil slope under shallow failure. However, study on simple-shear characteristics for vegetated slope is still insufficient, and there lacks intuitive comparison of characteristics between these two shear conditions. In this study, large-scale simple-shear and direct-shear experiments were conducted on soil permeated by roots of Amorpha fruticosa to investigate the shear strength and stiffness. The stress-displacement relationship of each sample was obtained and further normalized to unify the influence of root content. The results reveal that the direct-shear condition overestimates the shear strength of root-permeated soils (by 41%) and thus the estimation of slope stability based on the parameters of direct-shear condition is not conservative. Furthermore, the initial stiffness of root-permeated soil under simple-shear condition is 34% lower than that under direct-shear condition. The higher strength and stiffness under direct-shear condition are caused by the following reasons: the shear plane does not have the lowest strength, the shear area is decreasing, and the shear zone is thinner. The significant deformation (lower stiffness) revealed by the simple-shear condition facilitates the application of early warning for vegetated shallow landslides.
The effect of roots on slope stability has been discussed and documented by many earth science and geotechnical researchers. It requires explicit knowledge of the soil-vegetation relationships and the incidence of water as a trigger for landslides. Historically it has been a controversial issue, but most researchers recognize and accept the role of vegetation cover in erosion control. By consulting scientific information in the Web of Science database, with the keyword EFFECT OF ROOTS ON SLOPE STABILITY, 425 documents were selected and evaluated using open-source tools. It was found that the most significant scientific production was concentrated between 2012 and 2020; the USA is the most cited country,
despite having less scientific production than China and Italy. The most cited authors highlight the positive effect of vegetation on mechanical reinforcement, the reduction of pore ressure, and the increase in soils' shear resistance. However, they also recommend strengthening research processes to improve knowledge about the effect of the morphometry and density of the root system, the diameter, and resistance to traction of the roots and their spatial distribution, increasing slope stability. The researchers' main challenge is to incorporate the distribution of the roots, the stress-strain behavior of the reinforcement of the roots, and the resistance of the roots to compression into the slope stability models.
The root plate of trees, from a Geotechnical point of view, plays the role of the “living” foundation of a tall structure, subject to complex loading histories essentially deriving from environmental actions (e.g., wind loads). Its mechanical response to toppling loads (which represent a noticeable source of risk in urban areas), is not yet fully understood by the current interpretative models. In the paper, with the aim of highlighting the main features of the mechanical behavior of such systems, some small-scale 1g tests are presented. A simplified root prototype has been conceived, by combining elementary structural elements aimed at exhibiting both flexural and pullout properties of real-like flat root systems. Toppling loads (at constant vertical load and zero horizontal load) have been applied to the prototype and three different granular materials have been employed to reproduce the soil layer. The results highlight a certain positive correlation between a representative secant stiffness of the tests and their ultimate toppling resistance in case of very deformable soils. More uncertain trends are instead observed in case of stiffer granular materials. Although still not referred to real working conditions of tree roots, the experimental campaign may contribute to a deeper geotechnical understanding of the tree toppling phenomenon.KeywordsSmall scale teststree toppling mechanismlateral loadfoundations
The use of vegetation roots as a nature-based solution against landslides and erosion requires the definition of sample preparation protocols and adoption of equipment that allows testing representative elementary volumes of the whole soil-root system. For this purpose, large cell triaxial compression tests were carried out on fallow and vegetated samples at different degrees of saturation. Samples were prepared by static compaction of a silty sand and seeded with Cynodon dactylon . The hydraulic state during plants growth was controlled and reproduced on bare soil samples. After isotropic compressions, the shearing phase was carried out at very low confining stresses (i.e., below 50 kPa). Tests were deemed to be comparable by assessing the normalised volume of roots with respect to soil, after shearing. For a given confining stress, soil samples with higher matric suction exhibited higher shear strength, furtherly increased by roots. The stress-strain behaviour observed in the vegetated soil systematically changed, when comparing tests at low and high matric suction values, due to the different mechanisms of vegetation reinforcement depending on the hydraulic state at the soil-root interface. The results were successfully interpreted within a failure criterion and skeleton stress framework for partially saturated soils, considering soil suction, degree of saturation, soil microstructure and the normalised volume of roots.
In seasonally frozen soil, soil sometimes is affected by freeze-thaw cycles and root systems. In order to study its mechanical characteristics, a series of consolidation drained triaxial tests under different confining pressures (25,50,100,200 kPa), different freeze-thaw cycles (N = 0,1,5,15) and different root-containing conditions (r = 0,1,3) were carried out. The test results show that the specimens exhibit strain softening behavior and volumetric dilatancy phenomena and shear failure under lower confining pressure, and strain hardening and volumetric contraction, bulging failure under higher confining pressure. With the increase of freeze-thaw cycles, the bearing capacity of the sample decreases and the volume strain increases. With the increase of volume ration of roots in the sample, the bearing capacity increases and the volume strain decreases. Based on the binary medium model, the soil is abstracted into bonded elements and frictional element. At the same time, the bonded elements is transformed into frictional element when the bonded elements is broken during the loading process. Also, the root is abstracted into another non-destructive bonded elements material, which bears the load together. The linear elastic constitutive model was used for root and bonded elements, and the double-hardening model was used for friction elements. Considering the influence of freeze-thaw cycles, the extended binary model was derived here. Finally, the experimental results show that the predicted results of this model are in good agreement with the experimental results, and the new model can relatively well simulate the strain softening and volumetric dilatancy phenomena.
Purpose Root reinforcement is a key parameter in slope stability analysis, but is difficult to be effectively included at the hillslope-scale due to the complexity of root systems. As a result, hillslope-scale analysis of root reinforcement still requires high levels of field validation to account for variability in root properties as a function of topography, ecology, and soil properties. This study investigated root distributions and estimated root reinforcement at an unprecedent scale of field and laboratory measurement, using this to understand differences among species (Carpinus betulus and Fagus orientalis), diameter at breast height (DBH), slope position, altitude, vertical and horizontal distances from trees in Hyrcanian temperate forests, Iran. Method We excavated 1080 profile trenches 0.5 m wide, 1.0 m length, and 1.0 m deep upslope and downslope from trunks of C. betulus and F. orientalis with a range of DBH (7.5–82.5 cm) at three different altitudes (400, 950, and 1300 m a.s.l.). We assessed the effects of different forest coverage on slope stability via a 3-D limit equilibrium-based slope stability model where parameter uncertainties are explicitly accounted for using Monte Carlo Simulation.
Results The Root Area Ratio (RAR) of C. betulus is always higher than F. orientalis. RAR of F. orientalis is higher in upslope, whereas RAR of C. betulus is similar in both positions. Higher RAR contributed to higher root reinforcements for C. betulus when comparted with F. orientalis. Additionally, after accounting for DBH influences, altitude significantly affects the root reinforcement of C. betulus. The results of slope stability analysis showed that the most stabilizing species is C. betulus in a mature growth condition, maintaining an instability probability of~18.3%.
Conclusion C. betulus is preferable to F. orientalis for increasing slope stability. Forest managers should consider this outcome when developing strategies for silvicultural treatment and reforestation projects in mountainous areas of temperate regions.
Slopes along the highway and railway routes are subjected to not only static loads but also dynamic loads generated by vehicles and trains. The induced excessive deformation potentially poses a threat to slope stability. In terms of the extensive application of ecological slope protection, plants play a critical role in slope stability, as the roots can enhance the shear strength of the soil. This study aims to investigate the influence of different root distribution patterns on the dynamic characteristics induced by cyclic loading. By conducting a group of dynamic triaxial tests, the results indicate that the root system can significantly enhance the liquefaction resistance of the soil when the soil is subjected to lower dynamic loads, and the cross arrangement has a better-reinforced effect than the mixed arrangement. The reinforced effect was not obvious when the soil was subjected to a dynamic load with a larger stress amplitude. In addition, based on the validation of the seed model, a new pore water pressure development model was proposed according to the test results. Overall, the research provides a new model and some innovative observations to better understand the dynamic behavior of root-reinforced soil.
The SHETRAN model for simulating the sediment yield arising from shallow landslides at the scale of a river catchment was applied to the 45-km(2) Ijuez catchment in the central Spanish Pyrenees, to investigate the effect of loss of forest cover on landslide and debris flow incidence and on catchment sediment yield. The application demonstrated how such a model, with a large number of parameters to be evaluated, can be used even when directly measured data are not available: rainfall and discharge time series were generated by reference to other local records and data providing the basis for a soil map were obtained by a short field campaign. Uncertainty bounds for the outputs were determined as a function of the uncertainty in the values of key model parameters. For a four-year period and for the existing forested state of the catchment, a good ability to simulate the observed long term spatial distribution of debris flows (represented by a 45-year inventory) and to determine catchment sediment yield within the range of regional observations was demonstrated. The lower uncertainty bound on simulated landslide occurrence approximated the observed annual rate of landsliding and suggests that landslides provide a relatively minor proportion of the total sediment yield, at least in drier years. A scenario simulation in which the forest cover was replaced by grassland indicated an increase in landsliding but a decrease in the number of landslides which evolve into debris flows and, at least for drier years, a reduction in sediment delivery to the channel network.
Horizontal distribution of fine root density and fine root biomass net production were studied in six gaps and adjacent stands of pure Norway spruce using soil cores and in-growth cores. A statistical based model that does not require identifying the specific tree to each fine root was used to predict fine root distribution. This method calibrates fine root biomass functions of single trees by comparing fine root distributions with tree distributions via a least squares fit analysis. The variation of fine root density increases from 100 per cent within the stand to 250 per cent in gaps. The greatest root expansion distances regarding single tree positions were found in 35-year-old stands and in larger gaps. In-growth cores and root change maps based on model predictions show corresponding net growth rates. Net growth rates of fine root biomass differ significantly with distance to the gap edge (α-value of 0.10) with maximum values occurring in a circular strip 2.2 m from the gap edge. Our findings indicate that gaps with an average diameter <15 m should probably not be artificially regenerated because of rapid, below-ground gap closure.
The fine roots of trees are concentrated on lateral branches that arise from perennial roots. They are important in the acquisition of water and essential nutrients, and at the ecosystem level, they make a significant contribution to biogeochemical cycling. Fine roots have often been studied according to arbitrary size classes, e.g., all roots less than 1 or 2 mm in diameter. Because of the size class approach, the position of an individual root on the complex lateral branching system has often been ignored, and relationships between the form of the branching root system and its function are poorly understood. The fine roots of both gymnosperms and angiosperms, which formed ectomycorrhizae (EM) and arbuscular mycorrhizae (AM) fungal associations, were sampled in 1998 and 1999. Study sites were chosen to encompass a wide variety of environments in four regions of North America. Intact lateral branches were collected from each species and 18 561 individual roots were dissected by order, with distal roots numbered as first-order roots. This scheme is similar to the one commonly used to number the order of streams. Fine root diameter, length, specific root length (SRL; m/g), and nitrogen (N) concentration of nine North American tree species (Acer saccharum, Juniperus monosperma, Liriodendron tulipifera, Picea glauca, Pinus edulis, Pinus elliottii, Pinus resinosa, Populus balsamifera, and Quercus alba) were then compared and contrasted. Lateral roots <0.5 mm in diameter accounted for >75% of the total number and length of individual roots sampled in all species except Liriodendron tulipifera. Both SRL and N concentration decreased with increasing root order in all nine species, and this pattern appears to be universal in all temperate and boreal trees. Nitrogen concentrations ranged from 8.5 to 30.9 g/kg and were highest in the first-order "root tips." On a mass basis, first-order roots are expensive to maintain per unit time (high tissue N concentration). Tissue N appears to be a key factor in understanding the C cost of maintaining first- and second-order roots, which dominate the display of absorbing root length. There were many significant differences among species in diameter, length, SRL, and N concentration. For example, two different species can have similar SRL but very different tissue N concentrations. Our findings run contrary to the common idea that all roots of a given size class function the same way and that a common size class for fine roots works well for all species. Interestingly, fine root lateral branches are apparently deciduous, with a distinct lateral branch scar. The position of an individual root on the branching root system appears to be important in understanding the function of fine roots.
Coring methods cannot reveal the distribution of roots with depth in rocky soil, and fine roots are typically sampled without regard to the location of trees. We used quantitative soil pits to describe rooting patterns with soil depth and distance to trees in northern hardwood stands. We sited three 0.5 m2 quantitative soil pits in each of three young (19-27 years) and three older (56-69 years) stands developed after clear-cutting. Live roots were divided into diameter classes delimited at 0.5, 1, 2, 5, 10, 20, and 100 mm; dead roots were not distinguished by size. Mean total live-root biomass was 2900 ± 500 g·m-2 in older stands and 1500 ± 400 g·m-2 in young stands. The root mass in the 2-20 mm class was 2.7 times greater in the older stands (p = 0.03); fine-root (
The influence of forest clearing on landsliding is central to long-standing concern over the effects of timber harvesting on slope stability. Here we document a strong topographic control on shallow landsliding by combining unique ground-based landslide surveys in an intensively monitored study area with digital terrain modeling using high-resolution laser altimetry and a coarser resolution regional study of 3224 landslides. As predicted by our digital terrain based model, landslides occur disproportionately in steep, convergent topography. In terrain predicted to be at low risk of slope failure, a random model performs equally well to our mechanism-based model. Our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing. Our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the Pacific Northwest.
The influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies, yet few studies have examined how local vegetative patterns influence slope stability. Because root networks spread outward from trees, the species, size, and spacing of trees should influence the spatial distribution of root strength. We documented the distribution and characteristics of trees adjacent to 32 shallow landslides that oc- curred during 1996 in the Oregon Coast Range. Although broadly classified as a conifer-dominated forest, we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest (Mapleton) that experienced widespread burning in the 19th century. In industrial forests that were burned, selectively harvested, and not replanted (Elliott State Forest), swordfern was ubiquitous near landslides, and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps. We demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry-based index of root network contribution derived from mapping the size, species, condi- tion, and spacing of local trees, indicating that root strength can be predicted by mapping the distribution and charac- teristics of trees on potentially unstable slopes. In our study sites, landslides tend to occur in areas of reduced root strength, suggesting that to make site-specific predictions of landslide occurrence slope stability analyses must account for the diversity and distribution of vegetation in potentially unstable terrain.
An infinite slope stability model is proposed which incorporates changes in root cohesion and vegetation surcharge through several timber management cycles along with the stochastic influence of rainfall on pore water pressure. Recovery of rooting strength and tree surcharge following timber harvest are simulated by a sigmoid relationship, while root deterioration of harvested vegetation is described by an exponential decay function. The effects of long-term timber management on probability of failure are simulated by overlaying the impacts of a prior vegetation removal on a more recent removal. For each year the critical pore water pressure (ucrit) needed to trigger slope failure is computed. An empirical function relating piezometric level to antecedent rainfall, storm intensity, and total precipitation is presented to assess the probability of occurrence of ucrit, based on historical rainfall records for a site in coastal Alaska. Simulations of probability of failure indicate that alternate thinnings and clear-cuts and clear-cuts alone produce less stable conditions than shelterwood harvesting systems and partial cuts. Repeated harvesting cycles with progressively shorter rotations, reduced regeneration potential of new vegetation, and destruction of understory vegetation during logging or site preparation can all increase the probability of failure. These analyses can provide land managers with options for acceptable vegetation management strategies on potentially unstable hillslopes.
Research is being carried out at Technion in Israel to study the influence of plant roots on the stability of slopes. The present paper describes the part of the investigation concerned with the determination of the additional shear strength contributed to soil by roots. Specifically, results of the following studies are presented: tension tests on roots, pull-out tests of roots from the soil, and direct shear tests on soil and root-reinforced soil. The quantitative results obtained in these investigations provide data which may be used in calculations of slope stability, although this should be done with caution, as pointed out in the paper.
Des recherches sont effectuées en ce moment chez Technion en Israël pour étudier l'influence des racines de plantes sur la stabilité des pentes. Cet exposé décrit la partie de cette investigation concernant le calcul du gain de résistance au cisaillement donné au sol par les racines. Plus spécifiquement, nous présentons les résultats des études suivantes: essais de traction sur les racines, essais d'extraction des racines hors du sol, essais de cisaillement direct sur le sol et sur un sol renforcé par des racines. Les résultats quantitatifs émanant de ces investigations ont produit des données qui pourront être utilisées dans le calculs de stabilité de pente mais, comme nous le soulignons dans cet exposé, ceci devra être fait avec prudence.
Root systems of 16-year-old, plantation grown, Sitka spruce of a range of sizes were excavated by hand from a peaty gley soil. The length of each root segment and position of each branching point, bend, fork and proliferation was measured for four root systems using a plumb-bob after placing them in a rigid, metal framework. On three root systems measurements were made by tape and protractor of numbers of roots, branching points, bends, root lengths, branching angles and distribution of root origins around the bole.
Statistical analysis of these measurements revealed the root branching process as inherently regular. First-order roots tended to be regularly distributed around the main stem. The orientation of roots was determined by their initial direction either from the mainstem or where they were formed as laterals, and by changes in direction either from the mainstem or where they were formed as laterals, and by changes in direction at bends or branching points which tended to be alternately clockwise and anticlockwise. Lateral branches subtended larger angles from their parent roots than the angles between the two arms of a root fork and, irrespective of the length of a root segment, the laterals arising from it were evenly distributed along its length. The angles of lateral roots were generally more steeply downward than those of their parents. Root angles and directions changed where changes in soil structure or ditches were encountered.
We suggest that the root growth of Sitka spruce is inherently regular in that the species may possess mechanisms which ensure that its structural root system is more extended and more evenly spread than would result if growth was at random. However, root growth takes place in a heterogenous environment and it is this which causes the variability in final root pattern.
A traction effect refers to the mechanical effect of lateral to horizontal roots, normally in shallow soil, to enhance the in-plane tensile strength of soil in the rooted soil zone. It is one way in which plant roots can contribute to lateral reinforcement of a shallow soil mass. To verify whether or not a traction effect exists in the root system of a pine forest (Pinus yunnanesis French) in the Hutiaoxia Gorge, southwest China, and evaluate the magnitude of any such effect, an experiment-based modelling and a direct in situ test were conducted at the Erdui site in the gorge. The modelling prediction indicates that the lateral roots produce tremendous tractive resistance in the upper soil (0–60 cm below the surface), with a magnitude of 4169 N, on a vertical cross-section area of 105 mm2 at the top depth interval of soil (0–20 cm). The direct in situ test shows that the tractive resistance exerted by the lateral roots averages 561 N in the top depth interval and for the same vertical area, or an increase of sliding-pulling resistance by 38%. Under the influence of this tractive resistance, the tensile strength of the upper rooted soil was increased by at least 5.7 kPa. Together with the strong vertical anchorage of the taproot of the pine and sinker roots, the lateral roots are able to stabilise the shallow soil mass to a certain degree.
Root cohesion is an important parameter governing slope stability in steep forested terrain. Forest harvesting impacts root cohesion, and although the temporal effects have been noted, this dynamic parameter is often assumed to be spatially uniform. A model was developed to simulate the variation in root cohesion on a hillslope resulting from various forest management treatments. The model combines physical data on the horizontal rooting distribution of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) together with a temporal relation of root cohesion decay. Harvesting methods examined include clear-cutting, single-tree selection cutting, and strip-cutting. Model outputs are analysed qualitatively for regions of root cohesion minima and quantitatively for the average root cohesion within the simulated hillslope. A selection cutting simulation maintained the highest average root cohesion value, decreasing to only 81% of the preharvest condition. In contrast, the minimum root cohesion following clear-cutting declined to 38% of the preharvest value. Selection and strip-cutting scenarios resulted in smaller areas of reduced root cohesion that were adjacent to areas with high root cohesion. Such partial cutting methods shorten the period of reduced root cohesion following timber harvesting compared with clear-cutting.
Tree roots provide important soil reinforcement that improves the stability of hillslopes. After trees are cut and roots begin to decay, the frequency of slope failures can increase. To more fully understand the mechanics of how tree roots reinforce soil, fine sandy soil containing pine roots was placed in a large shear box in horizontal layers and sheared across a vertical plane. The shapes of the deformed roots in the sheared soil were explained satisfactorily by an equation that had been developed to model the deformed shape of artificial reinforcement elements, such as wood dowels, parachute cord, Bungy cord, and aluminum rods. Root deformation in sheared soil is influenced by the diameter and concentration of roots. A model is proposed that uses root strain to estimate the shear stress of soil reinforced by roots. The shear resistance measured from the shear tests compared quite well with the model simulation.
This study investigated stand factors associated with the rate of root graft formation in lodgepole pine stands. Forty plot areas, each containing 10 trees, were excavated in pure, even-aged pine stands in western Alberta. Exposed root systems were examined for grafts and various stand measurements were recorded at each plot. Results indicate that the number of grafts per square metre is controlled by plot tree density and tree diameter. Also, the percentage of grafted trees increased with both increasing tree age and decreasing distance between trees. Grafts also appear to form relatively early in stand development; the majority of grafts in the present study had formed by the time roots were 20 years old and 50 mm in diameter. These results suggest that grafting is a common occurrence in lodgepole pine stands where trees are <109 cm apart, which translates to a density of approximately 8500 stems/ha (based on even tree distribution). However, even clumps within relatively low density stands are likely to be grafted from a relatively early stage of development.Key words: Pinus contorta, graft formation, stand dynamics.
Decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes. Forests, however, have variations in vegetation species and age which can dominate the local stability of landslide-initiation sites. To assess the influence of this variability on root cohesion we examined scarps of landslides triggered during large storms in February and November of 1996 in the Oregon Coast Range and hand-dug soil pits on stable ground. At 41 sites we estimated the cohesive reinforcement to soil due to roots by determining the tensile strength, species, depth, orientation, relative health, and the density of roots ≥1 mm in diameter within a measured soil area. We found that median lateral root cohesion ranges from 6.8-23.2 kPa in industrial forests with significant understory and deciduous vegetation to 25.6-94.3 kPa in natural forests dominated by coniferous vegetation. Lateral root cohesion in clearcuts is uniformly ≤10 kPa. Some 100-year-old industrial forests have species compositions, lateral root cohesion, and root diameters that more closely resemble 10-year-old clearcuts than natural forests. As such, the influence of root cohesion variability on landslide susceptibility cannot be determined solely from broad age classifications or extrapolated from the presence of one species of vegetation. Furthermore, the anthropogenic disturbance legacy modifies root cohesion for at least a century and should be considered when comparing contemporary landslide rates from industrial forests with geologic background rates.
The mechanics of root reinforcement have been described satisfactorily for a single root or several roots passing a potential slip plane and verified by field experiments. Yet, precious little attempts have been made to apply these models to the hillslope scale pertinent to landsliding at which variations in soil and vegetation become important. On natural slopes positive pore pressures occur often at the weathering depth of the soil profile. At this critical depth root reinforcement is crucial to avert slope instability. This is particularly relevant for the abandoned slopes in the European part of the Mediterranean basin where root development has to balance the increasing infiltration capacity during re-vegetation. Detailed investigations related to root reinforcement were made at two abandoned slopes susceptible to landsliding located in the Alcoy basin (SE Spain). On these slopes semi-natural vegetation, consisting of a patchy herbaceous cover and dispersed Aleppo pine trees, has established itself. Soil and vegetation conditions were mapped in detail and large-scale, in-situ direct shear tests on the topsoil and pull-out tests performed in order to quantify root reinforcement under different vegetation conditions. These tests showed that root reinforcement was present but limited. Under herbaceous cover, the typical reinforcement was in the order of 0.6 kPa while values up to 18 kPa were observed under dense pine cover. The tests indicate that fine root content and vegetation conditions are important factors that explain the root reinforcement of the topsoil. These findings were confirmed by the simulation of the direct shear tests by means of an advanced root reinforcement model developed in FLAC 2D. Inclusion of the root distribution for the observed vegetation cover mimics root failure realistically but returns over-optimistic estimates of the root reinforcement. When the root reinforcement is applied with this information at the hillslope scale under fully saturated and critical hydrological conditions, root pull-out becomes the dominant root failure mechanism and the slip plane is located at the weathering depth of the soil profile where root reinforcement is negligible. The safety factors increase only slightly when roots are present but the changes in the surface velocity at failure are more substantial. Root reinforcement on these natural slopes therefore appears to be limited to a small range of critical hydrological conditions and its mitigating effect occurs mainly after failure.
Highway embankments and cutting slopes in the United Kingdom, particularly in the South East of England, are often constructed of or within stiff over-consolidated clays. These clays are prone to softening with time leading to shallow slope failures and costly repairs. Reinforcement by natural vegetation is potentially a cost-effective method of stabilising these types of slopes over the medium–long term. However, there is a lack of information on how natural vegetation reinforces and stabilises clay slopes. To investigate this problem, the potential reinforcement of selected oak (Quercus robur L.) and hawthorn (Crataegus monogyna Jacq.) roots was assessed by conducting in situ root pull-out experiments on a London Clay cutting in south-east England. Pull-out tests were carried out using specifically designed clamps and either a hand pull system with a spring balance and manual recording of force for oak roots or a jacking system with electronic data logging of applied force and displacement for hawthorn roots. Oak roots had a mean pull-out resistance of 7 MPa and that of hawthorn roots was 8 MPa. The electronic data logging of applied force (pull-out resistance) and displacement of the hawthorn roots provided additional data on the failure of branched roots which could be correlated with variations in root morphology. The failure of the roots can be categorised into three modes: Type A: single root failure with rapid rise in pull-out resistance until failure occurs; Type B: double peak failure of a forked or branched root and Type C: stepped failure with multiple branches failing successively. The different types of root–soil bonds are described in relation to root anchorage and soil stability.
The ability of vegetation to stabilize soils is frequently employed in water resource management and slope stabilization projects. The purpose of this investigation is to provide data about the contribution of plant roots to soil shear strength. Soil block samples permeated with roots of four plant species commonly used for remediation and habitat restoration purposes were tested in a large direct shear apparatus. Shear stress results of rooted soils were compared with results of unvegetated soil blocks with similar soil types. The plant species used were: tussock sedge (Carex stricta), switch grass (Panicum virgatum), common cottonwood (Populus deltoides) and black willow (Salix nigra). Shear strength increase was determined by comparing shear stresses at specific horizontal displacements. ANOVA and Tukey tests were conducted. The relative strength increase at the same displacement was 472% for switchgrass, 445% for black willow, 262% for tussock sedge, and 216% for cottonwood. Switchgrass roots increased soil shear resistance by 31.2 kPa compared to 6.6 kPa for unvegetated soil at a displacement of 7 cm. Switchgrass and black willow caused the largest increases in shear stress. The root systems of these plants resulted in a five-fold increase as compared with fallow soils. Tussock sedge and cottonwood increased the shear stress by a factor of about 3 to 5, but these results were not significantly different from the unvegetated treatment. The shear stresses in most of the rooted blocks were still increasing at the end of the test (maximum displacement of about 15 cm), indicating that root tensile failure did not occur during the shear tests. Root elongation or slippage rather than breakage was the most common condition during failure. The mode of failure appears to allow for the survival of the herb species after a slope failure. These conservative root cohesion values can be used in the qualitative or semiquantitative assessment of the stability of existing vegetated slopes and in the design of vegetated riverbanks, shorelines, embankments, cut slopes, retaining walls, landfill caps, and other reclamation applications where the mechanical contribution of root reinforcement is important to predicting soil behavior.
Streambank instability poses a number of economic and ecological problems. As sediment has been reported to be one of the principal contaminants of rivers in many areas, a high priority of river managers is to stabilize streambanks to prevent additional sediment being added to the channels. Riparian vegetation plays a number of roles in the protection of streambanks from erosion by the processes of particle entrainment and mass wasting, and its use in stabilization has a number of possible benefits, but is often neglected in favor of hard stabilization measures, such as concrete and riprap, that are more easily quantifiable and remain constant over time. This study seeks to investigate the importance of the assumptions previously made in calculations of soil reinforcement by roots, and aims to study how root networks, and the contribution to soil strength made over time varies both mechanically and hydrologically. Results show that previous methods used to estimate root reinforcement may have overestimated values by up to 91%. A new Fiber-Bundle model (RipRoot) is proposed here to reduce reinforcement overestimations. Hydro-logic reinforcement by evapotranspiration may also be important. Results show that soil cohesion values were increased by 1.0 to 3.1 kPa due to reductions in mattic suction by trees of just two-years old. The net effects of mechanical and hydrologic reinforcement have also been investigated, with results showing that even during the wettest time of the year, when evapotranspiration effects are negligible, the mechanical reinforcement from the root networks maintains some degree of stability.
Environmental conditions and nutrient supply do not only affect the above-ground growth of forest trees, but also the below-ground growth. In model experiments, N additions to soil can result in both increases and decreases in root dry weight or root length. More consistently, in most cases a decrease in the root/shoot biomass ratio is observed as a result of increased N supply in soil (George and Seith 1998; see also Chapin III et al. 1987). Some nitrogen fertilisation experiments suggest that much of the increased above-ground production may be due to a carbon translocation from below-ground to above-ground parts (Linder and Axelsson 1982). Although this change in carbon allocation is not necessarily harmful to the tree, on low-nutrient soils a nutrient imbalance may occur in the tree as a consequence of the decreased root/shoot ratio, and this may be one of the factors causing forest decline symptoms. Such effects can also be studied in the field in N-fertilisation experiments (Ahlstrom et al. 1988; Persson et al. 1995a) or N-addition and removal experiments (Clemensson-Lindell and Persson 1995; Persson et al. 1998). In the present experiment, we compared root growth of Norway spruce (Picea abies) and European beech (Fagus sylvatica) at different sites with contrasting climate and N deposition.
Stability analyses of vegetated hillslopes are usually carried out by the limit equilibrium (LE) method where shear displacement is not taken into account. Experiments show that soil with roots produces a shear stress - displacement curve with higher peak shear stress at larger shear displacements than fallow soil. If the safety factor is obtained by the LE method, the ability of soil with roots to resist large shear strains due to soil-root interaction may be underestimated. A new approach is proposed that incorporates, within the stability analysis, the ability of soil with roots to withstand strain. It is based on a consideration of the energy consumed during the shearing process of the soil-root system. This is developed using characteristics of the shear stress - displacement curve of a soil-root system obtained from in situ direct shear tests under simulated overburden pressure and pore-water pressure conditions. The method is limited to vegetated hillslopes where the stability analysis can be approximated by a simplified infinite slope model. Shear stress - displacement data for two tree species were obtained from hillslopes where shallow landslides commonly occur in rainstorms under near-saturated conditions. Using these results the energy approach (EA) and LE methods are compared. A procedure is also outlined to predict the safety factor for hillslopes with different plant densities. Extension of the EA method for general two-dimensional slope stability analysis involving nonlinear shear planes is also explained.
The increased shear resistance of soil due to root-reinforcement by four common Australian riparian trees, Casuarina glauca, Eucalyptus amplifolia, Eucalyptus elata and Acacia floribunda, was determined in-situ with a field shear-box. Root pull-out strengths and root tensile-strengths were also measured and used to evaluate the utility of the root-reinforcement estimation models that assume simultaneous failure of all roots at the shear plane. Field shear-box results indicate that tree roots fail progressively rather than simultaneously. Shear-strengths calculated for root-reinforced soil assuming simultaneous root failure, yielded values between 50% and 215% higher than directly measured shear-strengths. The magnitude of the overestimate varies among species and probably results from differences in both the geometry of the root-system and tensile strengths of the root material. Soil blocks under A. floribunda which presents many, well-spread, highly-branched fine roots with relatively higher tensile strength, conformed most closely with root model estimates; whereas E. amplifolia, which presents a few, large, unbranched vertical roots, concentrated directly beneath the tree stem and of relatively low tensile strength, deviated furthest from model-estimated shear-strengths. These results suggest that considerable caution be exercised when applying estimates of increased shear-strength due to root-reinforcement in riverbank stability modelling. Nevertheless, increased soil shear strength provided by tree roots can be calculated by knowledge of the Root Area Ratio (RAR) at the shear plane. At equivalent RAR values, A. floribunda demonstrated the greatest earth reinforcement potential of the four species studied.
An inventory of 61 landslide scars in part of the central California Coast Ranges is used to document sites of instability and to infer conditions necessary for failure. Scar dimensions cluster around widths of 7-10 m, lengths of 10-20 m, and depths of 0.7-1.1 m. Simple theoretical analyses indicate that root strength along the margins of a potential landslide imposes constraints on landslide size; typical scar size may reflect the size of a deposit required for instability at sites with typical vegetation, slope gradients, soil texture, and hydrology. Hollows are the main source of landslides, consistent with the convergence of shallow groundwater flow and the long-term accumulation of colluvium in hollows, but conditions of sufficiently thick soil and high pore pressures for failure are also attained on side slopes.
A model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near-surface through flow and slope stability models. The hydrologic model TOPOG (O'Loughlin, 1986) predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries. The slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity. The steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding. The spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high-resolution digital elevation data are available: Tennessee Valley in Marin County, California; Mettman Ridge in the Oregon Coast Range; and Split Creek on the Olympic Peninsula, Washington. Model predictions in each of these areas are consistent with spatial patterns of observed landslide scars, although hydrologic complexities not accounted for in the model (e.g., spatial variability of soil properties and bedrock flow) control specific sites and timing of debris flow initiation within areas of similar topographic control.
We propose an analytical model to relate the vertical distribution of plant roots in water controlled ecosystems to the local climatic and pedologic conditions. We find that the shape of the root profile is determined by the distribution of the incoming rainfall pulses, and that the rooting systems are deeper where the soils are coarse-textured and the evaporative demand slightly exceeds precipitation.
Mechanical reinforcement which stabilizes soil on slopes has been attributed to plant roots. To measure such reinforcement, direct shear tests were made on 25‐cm diameter root‐permeated soil columns. Roots of alfalfa ( Medicago sativa ), barley ( Hordeum vulgare ), and yellow pine ( Pinus ponderosa ), each increased the shear resistance of homogeneous and compacted layers of silty clay loam at 30‐cm depth. One‐year‐old alfalfa had a much greater reinforcing effect than pine trees 16 months after transplanting or barley at its maximum growth. Barley had a greater effect in the clay loam than pine, but its effectiveness decreased as depth increased from 15 to 30 to 45 cm. Alfalfa roots were more effective than either pine or barley roots in increasing the resistance to shearing between a dense gravel‐sand layer (simulating weathered rock) and the overlying soil, increasing shearing resistance to as much as 5 times that of fallow soil. A model is presented of soil reinforced by nonrigid roots. Calculations are given of slope safety factor increases from root reinforcement.
Summary The arrangement of a plant's roots in the soil determines the ability of the plant to resist uprooting. We have investigated the influence of root morphology on anchorage using simple patterns of root systems and numerical simulation. The form and mechanical properties of roots were derived from results found in the literature. Major parameters determining soil characteristics, root patterns and strength were varied so that their influence could be evaluated. The design of the experimental method we used generated an optimal number of configurations of different root systems, the tensile resistances of which were calculated by two-dimensional finite element analysis. We could quantify the influence of specific parameters, e.g. branching angle, number of lateral roots and soil cohesion, as well as global parameters such as total contact area, basal diameter and volume of the whole root system. We found that the number of roots and the diameter of roots were major components affecting the resistance to uprooting. The combination of topology and biomass explained 70% of the variation of tensile resistance.
A root-soil model developed previously has been extended to predict the amount of increase in soil shear resistance (root reinforcement) produced by stretching, slipping, and breaking roots of various sizes. We measured Young’s moduli, tensile strengths, and diameters of pine and barley roots, finding that both moduli and strengths decreased with increasing root diameter. These data and root diameter distributions in the shear zone of 0.25-meter diameter (pine) and 0.1-meter diameter (barley) soil columns were applied to the model. Comparison of model simulations with experiments showed that ø’, the strength of the soil-root bond, is the most important unmeasured model parameter. Its value, rather than root strength, limited root reinforcement in saturated clay loam with both plant species and was of the order of 25 grams per square centimeter.
Summary Horizontal distribution of fine root density and fine root biomass net production were studied in six gaps and adjacent stands of pure Norway spruce using soil cores and in-growth cores. A statistical based model that does not require identifying the specific tree to each fine root was used to predict fine root distribution. This method calibrates fine root biomass functions of single trees by comparing fine root distributions with tree distributions via a least squares fit analysis. The variation of fine root density increases from 100 per cent within the stand to 250 per cent in gaps. The greatest root expansion distances regarding single tree positions were found in 35-year-old stands and in larger gaps. In-growth cores and root change maps based on model predictions show corresponding net growth rates. Net growth rates of fine root biomass differ significantly with distance to the gap edge (α -value of 0.10) with maximum values occurring in a circular strip 2.2 m from the gap edge. Our findings indicate that gaps with an average diameter <15 m should probably not be artificially regenerated because of rapid, below-ground gap closure.
Soil reinforcement by roots is studied by considering the contribution of the tensile force in a root segment that intersects a potential slip surface in a soil-root system. To evaluate the tensile force when the system is subjected to a shear displacement, the root segment is analyzed as a beam on elastic-plastic support and as a cable for small and large displacements, respectively. Equilibrium and displacement compatibility at a branch point are used to analyze the distribution of forces between two root branches of a root system. Laboratory model tests and in situ root tests were performed to verify the analytical models. The analytical models and in situ root tests are used as means of evaluating the shearing resistance of the reinforced soil.
Forces applied to plants will subject many of the roots to tension, which must be transferred to the soil via shear if uprooting is to be prevented. The stress distribution will depend on the relative stiffnesses of the earth and root, and the mode of failure will depend on the relative strength of the soil and of the root soil bond. This study of the anchorage of sunflower radicles combined uprooting tests performed by a tensile testing machine with mechanical tests on the roots and soil.
The maximum extraction force increased with length to an asymptotic value and was reached at a very low displacement. Root hairs and soil particles covered the tapered top 20 mm of extracted root, but the lower cylindrical region was bare. The soil was stiffer than the root, so shear stress was initially concentrated at the top of the root, soil strength over the top 20 mm resisting uprooting. Lower regions of the root were stressed later, their sparser root hairs being sheared off, and resist uprooting only by friction. In a further lest upper and lower regions of radicles were uprooted separately. As predicted, the upper region generated much greater resistance to uprooting per unit length, and at much lower displacements than the lower region.
The top of the radicle is well adapted for anchorage, the profuse root hairs and mucigel it produces glueing the root to the soil. The lower regions are thus protected from damage.
Seasonal changes and vertical distribution of fine (< 2 mm diameter) and coarse (2-10 mm diameter) root mass of Pinus kesiya and fine root and rhizome mass of herbaceous species, and root production were studied in the 6-, 15- and 23-year old Pinus kesiya forest stands at Shillong, in the Meghalaya state of north-east India. Maximum fine and coarse root mass of P. kesiya, and fine root and rhizome mass of the ground vegetation were recorded during the rainy season. The contribution of the tree fine roots in 0-10 cm soil layer declined from 51% in the 6-year old stand to about 33% in the older stands. The major proportion (63-88%) of herbaceous fine root and rhizome mass was concentrated in this soil layer in all the three stands. The majority (36-57%) of tree coarse roots were present in the 10-20 cm layer in all the stands. The biomass and necromass values in the case of fine roots were more or less equal in a given stand, but the coarse roots had 5 to 9 times more live than the dead mass. The proportion of herbaceous fine root mass to the total fine root mass declined from 54% in the 6-year old stand to 30-32% in the 15- and 23-year old stands. The mean total fine root mass (pine + herbaceous species) decreased from 417 g m(-2) in the 6-year old stand to 302 in 15-year and 322 g m(-2) in the 23-year old stand. Annual fine root production showed a marked decrease from 1055 g m(-2) in the 6-year old stand to 743 g m(-2) in the 23-year old stand, but coarse root production increased from 169 g m(-2) in the 6-year to 466 g m(-2) in the 23-year old stand; the total root production thus remained approximately constant. (C) 2001 Editions scientifiques et medicales Elsevier SAS.
Below-ground characteristics of five Populus clones, belonging to different species and parentages, were studied during the second growing season of the third rotation of a high-density coppice culture. Size (length, area and volume), biomass, nitrogen and carbon concentrations of three classes of fine roots (diameter classes of 0–1, 1–2 and 2–5mm) were determined for four different soil layers. Fine root biomass varied significantly among clones and among soil layers. Clone Primo (Populus deltoides×Populus nigra) had the highest root biomass and the longest fine roots, while clone Hazendans (Populus trichocarpa×P. deltoides) had the lowest root biomass and shortest fine roots. The topsoil layer (0–5cm) was very rich in fine roots; the fine root biomass and distribution of all clones decreased with increasing soil depth. Fine root area index (diameter classes of 0–1 and 1–2mm) varied among clones, with higher values for clones Wolterson and Primo (3.6 and 3.7, respectively), while clones Hazendans and Columbia River had lower values (1.7 and 2.2, respectively). The absence of a significant correlation between fine root traits and above-ground biomass leads to the conclusion that above-ground biomass was not a reliable indicator of below-ground biomass in poplar, probably because of the age of the plantation in our study (stump age of 10 years). Fine root area index was positively correlated with leaf area index for all clones and at all soil depths, i.e., clones with a high fine root area index also had a high leaf area index. We conclude that leaf area index can be an indicator of root area.
We excavated and mapped the lateral extension of 32 creosotebush shrubs (Larrea tridentata) in the Chihuahuan desert of New Mexico to examine the effect of neighborhood interaction on root distribution. The smallest closed-angle polygon encompassing all roots of an individual was taken as a representation of its root system. Several geometrical characteristics of these polygons were measured and compared to interference vectors based on the location and size of the neighbors. We found that root systems were more developed away from the maximum competitive pressure of neighbors. Relation between root system shape and pressure from neighbors was stronger when the competitive vectors were integrating effect from all neighbors. Size of neighbors did not appear to contribute significantly to the relation. The resulting spatial pattern tended to reduce the overlap between neighboring root systems. Two conceptual models of root growth response to neighbors appear to explain our results. Both models assume that when the root system of neighbors meet, root growth is impaired or ceases at the zone of contact. In the nonoverlapping, non-compensatory model, the decrease in root growth between two close neighbors is not compensated elsewhere, possibly affecting the overall plant performance. In the non-overlapping, compensatory model, a plant with a close neighbor responds by investing in root growth away from the competitive pressure or simply in zones free of neighbors. Under this model, two plants can be close to each other and not compete. Competition in the population is for space and only occurs when a plant root system is crowded on all sides.
Most distributed slope-stability models for shallow landslide prediction neglect the forces acting on the sidewalls of landslide scars. Back-calculations and field observations, how- ever, show that lateral root strength is a primary control on size and location of shallow land- slides in soil mantled hillslopes. Here we report a theory that estimates landslide width, assuming that root strength acts primarily through a perimeter boundary. The model predicts that landslide width increases with increasing root strength and decreasing slope, as larger masses of soil are needed to overcome resisting forces. Perhaps surprisingly, the drier the soil, the larger the land- slide mass (and width), whereas the water table rise reduces the size needed for failure. The comparison of the model results with field data suggests that landslide size is controlled by the local patchiness of soil thickness, root strength and topographically-driven relative saturation.
Extraction force and displacement were measured on roots pulled horizontally from the sides of a pit made in the soil after
removal of the tree and the bulk of its root system. Measurements were made on a brown earth and a deep peat, in 24- and 27-
year-old crops respectively. The brown earth soil was drier and more deeply rooted than the peat, but the root diameter at
the pulled end, the length of root extracted and the root displacement at maximum extraction force were each similar on both
soils. Extraction force was related to the root cross-sectional area at the pulled end and regressions showed that roots required
significantly more force for extraction on the peat than on the brown earth. However, the difference was small, and any differences
in tree stability between the two sites would have to be explained by other features of the anchorage. Data are also presented
on effects of root morphology and depth on extraction force.
Direct shear tests were run on a dry sand reinforced with different types of fibers. Both natural and synthetic fibers plus metal wires were tested. Experimental behavior was compared with theoretical predictions based on a force equilibrium model of a fiber reinforced sand. Test results showed that fiber reinforcement increased the peak shear strength and limited post peak reductions in shear resistance. The fiber reinforcement model correctly predicted the influence of various sand‐fiber parameters through shear strength increases that were: (1) Directly proportional to concentration or area ratio of fibers; (2) greatest for initial fiber orientations of 60° with respect to the shear surface; and (3) approximately the same for a reinforced sand tested in a loose and dense state, respectively. The findings of this study are relevant to such diverse problems as the contribution of roof reinforcement to the stability of sandy, coarse textured soils in granitic slopes, dune and beach stabilization by pioneer plants, tillage in root permeated soils, and soil stabilization with low modulus, woven fabrics.
The stability of slopes before and after removal of forest cover was investigated. Porewater pressures and shear strengths were measured and the soil properties were determined by laboratory and in situ tests. A model of the soil-root system was developed to evaluate the contribution of tree roots to shear strength. The computed safety factors are in general agreement with observed behaviors of the slopes. Decay of tree roots subsequent to logging was found to cause a reduction in the shear strength of the soil-root system. Refs.
Altogether 15 root systems, five at each of three plots (north- and south-facing slopes and plateau), of 40-year-old Picea abies (L.) Karst. trees with different symptoms of forest decline were excavated down to a root diameter of 0.5 cm. The object was to investigate the variability of root morphology and to assess the influence of environmental variation on the architecture of the woody root system. For each tree, total height, diameter at breast height, and needle and twig biomasses were determined, and for each root system, biomass, growth, length, cross-sectional area, number and initial direction of branches, and branching forms were determined. The differences in many of the wood parameters within and between the plots were relatively few, so that forest decline symptoms determined at the crown could not be sufficiently related to the root system architecture. The results suggest a small influence of microsite conditions on the structural root systems, an influence of stand density on root distribution and soil exploitation, and a functional difference between horizontal and vertical roots that points out the importance of extensive long vertical roots, which insure a sufficient water and nutrient uptake.
In situ shear tests were performed on soil blocks that contained roots to study the contribution of roots to the shear strength in a case where the shear deformation is not constrained to a thin zone. The shearing resistance of the soil-root system, the tensile force in selected roots, and the deformation of the soil block were measured. The roots were exposed after the test and their positions were determined and used to estimate the initial positions. The root force and the shearing resistance of the soil-root system were estimated with known solutions and compared with measured root force and shearing resistance. None of the roots that passed through the shear zone failed in tension at the maximum displacement. As a consequence, the root resistance is much less than that found in a case where the failure surface is restricted to the boundary between a weak soil and a firm base and where roots are anchored in the firm base and fail in tension. Simplified procedures for estimating root forces are suggested for the case of a thick shear zone.Key words: in situ test, roots, shear strength, slope stability, soil reinforcement, soilroot interaction.
The modulus of elasticity and the tensile strength were determined for a sample of live Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) roots collected in the Oregon Coast Range. Most of the roots displayed both a "form" modulus of elasticity and a "material" modulus of elasticity. The form modulus occurred as a tortuous root straightened out, whereas the material modulus developed following this initial straightening as the wood fibers within the root directly resisted elongation. The average form and material moduli of elasticity were, respectively, 185 and 503 MPa, whereas the average tensile strength was 17 MPa.
Whole, juvenile root systems of four south-east Australian riparian tree species (Casuarina glauca, Eucalyptus amplifolia, Eucalyptus elata and Acacia floribunda) have been measured to determine the distribution Of root material beneath them. Mathematical expressions describing the lateral and vertical variation in the aggregate cross-sectional area of root material have been developed for application in river bank stability modelling. A reduction in the root material with increasing depth and lateral distance from the tree stern was observed for all four species with approximately 80% of the root material occurring in a near-surface zone with a thickness between 15% and 25% of the maximum vertical depth of the root system. A two-dimensional model of root distribution beneath trees is presented and used to calculate the root area ratio and root cohesion on potential slip surfaces that intersect a root system. Estimates of the soil shear strength enhancement by root systems of full-sized mature trees are also presented and indicate that the increase in soil shear strength provided by the roots is highly variable over the full extent of a tree's root system. Of the species Studied, the two eucalypts demonstrated a greater earth-reinforcing potential than the others. E. clam presents larger values of root area ratio in soil zones beneath it, while E. amplifolia has the capacity to reinforce a larger volume of soil. (C) 2009 Elsevier B.V. All rights reserved.