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Background and aims: Modelling and predicting soil reinforcement by roots is a central quest in eco-engineering to assess the effectiveness of vegetation in landslide or erosion mitigation. Several fibre bundle model algorithms (FBMs) have been successfully introduced for soil reinforcement estimation and have achieved satisfactory results. However, FBMs yield variable reinforcement estimates due to their different hypotheses on the progressive root failure mode in soil (e.g., failure driven by load, strength or strain of roots); thus, they lack consensus in mechanism and application. Herein, we proposed three new FBMs, namely, FBMs-W, in which the root failure mode is driven by energy (i.e., work), and compared them with others using ground truth data. These FBMs-W allow, to some extent, reconciliation of the previous assumptions in conflicts because both load and displacement of roots affect in root failure procedures. Methods: To measure soil reinforcement, we conducted in situ direct shear tests under both rooted and bare soil conditions and two soil moisture levels using living roots from four common tree or shrubby species (Robinia pseudoacacia L., Pinus tabulaeformis Carr., Syringa oblate Lindl., and Vitex negundo L. var. heterophylla (Franch.) Rehd.) on the Loess Plateau, China. The observed soil reinforcement was then compared with predicted ones using different FBMs for model assessment. Results: All the FBMs overestimated the soil reinforcement compared with the observed values. Among the FBMs, the three FBMs-W showed fairly conservative predictions of root cohesion (cr) that were close to each other. FBMs-W gave the most stable soil reinforcement estimates with the smallest standard deviations. The above results were consistent among the four species. Conclusion: Our findings highlight the high disparities in soil reinforcement estimates and bias from the observed values due to the choice of model algorithms. Due to the conservativeness and stability of the soil reinforcement estimates and good coherence with the observed root failure order, we recommend that FBMs-W should be good candidates to choose by future modellers and practitioners when evaluating the likelihood of the stability of a vegetated slope. [ABSTRACT FROM AUTHOR]

To investigate the effects of major forestry species roots on slope stability on Loess Plateau of China, we used monospecific stands of Robinia pseudoacacia and Platycladus orientalis as a case study. Tree roots provide positive mechanical influence (i.e. additional cohesion) on slope stability. We used two different methods to determine root additional cohesion in this research, i.e. Wu and Waldron's Model (WM) and revised WM (RWM). WM was developed based on limit equilibrium theory and assumed that all root in soil clods were mobilized in tension and fail simultaneously. Although WM approach was considered as a powerful and widely used method, it overestimated root additional cohesion due to all roots breakage simultaneously hypothesis. Therefore, based on many shear tests, a reduction factor for WM is introduced, which is RWM. The most critical parameters for WM and RWM were root area ratio (RAR) and root tensile strength. In this research, RAR was recorded on the soil trench profile, while root tensile strength was obtained by individual root tensile test. To evaluate tree roots effects on slope stability, a 2-D finite element model with terraced and contrast rectilinear surface shape of slope stability was developed and used to calculate the increase in factor of safety (FoS) due to root additional cohesion. Results showed that whether the land was prepared or not, afforestation can significantly increase slope stability. Moreover Robinia pseudoacacia roots were better that Platycladus orientalis roots on soil reinforcement. Terraced slopes were more stable than rectilinear slopes, regardless of the differences in hydrological regimes between these two terrain morphologies. It was also found that the percentage of FoS increase was larger when considering root additional cohesion simulated by RWM and virtual bare slope than root additional cohesion simulated by WM and RWM for both stands. Numerical sensitivity analyses for root additional cohesion illustrated that the relationship between FoS and additional cohesion was not linear, but exhibited as an asymptotic behavior. In detail, FoS value was stable when root additional cohesion reached the threshold value, which indicated that FoS was not sensitive to root additional cohesion calculation method. In addition, although root additional cohesion varied with the slope location, it was hard to find clear pattern to follow in our stands. However, roots in bottom part of slope always had stronger mechanical effects on slope stability. Therefore, more attention should be paid on the toe of slope and fully exerted its positive role for afforestation managers. This research can provide a basic theory of afforestation mode in spatial distribution and hence control shallow landslide. [ABSTRACT FROM AUTHOR]

Abstract: Vegetation is widely used for controlling shallow landslides. The mechanisms by which roots increase apparent soil cohesion is well documented and many values of root additional cohesion are available in the literature for different plant species. However, less information is given about the spatial variation of soil reinforcement by roots at a slope scale and its influence on slope stability, in particular in forest areas. The goal of this paper is to describe the spatial variability of root additional cohesion on two monospecific 17-y-old stands of Robinia pseudoacacia and Platycladus orientalis grown on slopes in the semiarid Loess Plateau of China, and to analyze numerically the effect of this variability on slopes stability. For this purpose, vertical trenches were dug at different distance and directions around trees situated at three different slope locations, i.e. up-, mid- and down-slope. Grids with a 10×10cm mesh were placed on vertical walls. Roots were counted within each grid cell and split according to their diameter class. Root area ratio (RAR) was estimated and compared among different positions around the trees and at three different locations along the slope. Roots tensile strength was determined with laboratory mechanical tests. RAR and root tensile strength were used as inputs in six different root reinforcement models to calculate root additional cohesion. A 2D finite element model of slope stability was developed and used to calculate the increase in factor of safety (FoS) due to root additional cohesion on rectilinear and terraced slopes. Results showed that both root tensile strength and Young''s modulus of R. pseudoacacia was about two times higher than tensile strength of P. orientalis. RAR distribution had a strong relationship with local soil moisture content measured in July during the raining season, and was significantly different with regards to tree location on the slope. The six theoretical models used to estimate the root additional cohesion gave different vertical profiles of root reinforcement distribution according to the underlying hypothesis on how forces are transferred to the roots. Theoretical analyses of slope stability showed that terraced slopes were 20% more stable than rectilinear slopes, disregarding the differences in hydrological regimes between these two terrain morphologies. Numerical sensitivity analyses also showed that the FoS reached an asymptotic value when increasing root additional cohesion. Actual additional cohesions of the two studied sites corresponded to FoS that were already close to this asymptotic values. Consequently variations of these actual root cohesions would not much affect slope stability. However it was showed that more attention should be given to the reinforcement of the bottom part of the actual slopes, where roots have a larger positive impact on the FoS. [Copyright &y& Elsevier]

Plant roots have considerable impact on the shear properties of soil, but to date the underlying mechanisms have been poorly quantified. In order to understand the fundamental mechanisms of soil reinforcement by Hedysarum scoparium roots and reduce the cost of the testing and relieve the destruction of environment due to digging roots, five-year plant specimens were collected from the Gaoshawo forest field (Northwest China) by in-situ excavation in this study. The shear properties of root-soil composite of Hedysarum scoparium were studied by the finite-element numerical simulation software. The influence of the root area ratio (RAR) and the vertical load on the shear strength of root-soil composite of Hedysarum scoparium was discussed in this study. The laboratory direct shear test was used to prove the reliability of the numerical simulation under the condition of the 7% soil moisture content and the RAR of 0.0034. The results showed that the shear strength of roots-soil composite conformed to the Mohr-Coulomb's yield criterion and the roots of Hedysarum scoparium could notably enhance the soil shear strength. It was also found that a strong correlation between the RAR and the root apparent cohesion. The root apparent cohesion increased with the increasing of the RAR according to a linear function (R2>0.9). Under the same RAR, the capacity of soil reinforcement by the roots was weakened with the increase of the vertical load, and a logarithmic function (R2>0.9) could be used to describe the relationship between the shear strength growth rate of root-soil composite to pure soil and the vertical load. Under the same vertical load, the growth rate of shear strength of root-soil composite to pure soil decreased linearly with the decreasing of the RAR (R2>0.9). The roots of Hedysarum scoparium played an obvious role to reinforce soil under the low vertical loads. The results of the study indicated that the peak value of the shear stress of root-soil composite of Hedysarum scoparium appeared later compared with that of pure soil. It implied that the root reinforcement did not occur until the significant plastic deformation appeared. Therefore, the roots seemed to have little influence on soil reinforcement for small strains acting on soil-root composite. The numerical simulation results were consistent with the results of laboratory test (the maximum relative error was only 4.26%). It was found that an increase in the vertical load of root-soil composite of Hedysarum scoparium made the contribution of roots to the shear strength increment of root-soil composite decrease. The differences of the cohesion stress and friction angle of root-soil composite of Hedysarum scoparium were only 0.6179 kPa and 0.0039° respectively based on the fitting equation between the vertical load and the shear strength. The fitting equation was developed from the numerical simulation and the laboratory direct shear test results. This paper presented a numerical simulation model capable of simulating the direct shear of root-soil composite of Hedysarum scoparium. The numerical simulation results could serve as the basis and reference for further studies on shear characteristics of root-soil composite. [ABSTRACT FROM AUTHOR]

Hedysarum scoparium and Salix psammophila have obvious effect on the fixation of mobile and semi-mobile dunes in the Mu Us Desert. In order to explore the friction characteristics between root and soil, 5-year-old Hedysarum scoparium and Salix psammophila roots were measured in the laboratory. The influences of different conditions such as species, soil moisture and vertical load were examined by using direct shear friction tests in the study. The finite element software was used to simulate the process of the laboratory experiments. The results showed that the cohesion stress of the root-soil interface of Hedysarum scoparium and Salix psammophila had a significant difference (P < 0.05). However, there was no significant difference between Hedysarum scoparium root-soil interface and soil-soil interface (P > 0.05). The cohesion stress of the root-soil interface of Salix psammophila and soil-soil interface had a significant difference (P < 0.05). The cohesion stress of the root-soil interface of Hedysarum scoparium ((1.51±0.65) kPa) was higher than that of Salix psammophila ((-0.92±0.50) kPa), and the cohesion stress of soil-soil interface ((3.22±0.55)kPa) was also higher than that of Salix psammophila. The friction angle of the root-soil interface of Hedysarum scoparium had a significant difference with Salix psammophila (P < 0.05) and it also had a significant difference with soil-soil interface (P < 0.05). However, there was no significant difference between Salix psammophila root-soil interface and soil-soil interface (P > 0.05). The friction angle of the root-soil interface of Hedysarum scoparium ((31.00±0.14)°) was higher than that of soil-soil interface ((30.30±0.25)°) and Salix psammophila((30.20±0.17)°). There was a significant difference between the cohesion stress and the friction angle of the root soil interface (P < 0.05) under 2% (dry season) and 22% (rainy season) soil moisture. However, there was no significant difference under 7%-17% soil moisture (P > 0.05). The cohesion stress of 2% soil moisture ((0.0021±0.34) kPa) was lower than that of 22% soil moisture ((3.16±0.57) kPa). The friction angle of 2% soil moisture ((29.80±0.38)°) was lower than that of 22% soil moisture ((30.92±0.59)°). The relationship between vertical load and shear strength of the root-soil interface obeyed the Mohr-Coulomb theory and the constitutive relation was hyperbola. The maximum relative error of the shear strength simulated by the finite element software was 9.54%. The results of the study indicated that the improvement of shear strength of the root-soil composite was not related to the cohesion stress but related to the friction angle. The shear strength of the root-soil interface of Hedysarum scoparium was stronger than that of soil-soil interface and Salix psammophila (P < 0.01). The change of soil moisture had a similar influence on the cohesion stress and the friction angle of the root-soil interface. The capacity of improving shear strength of soil by root reinforcement was significantly affected by the dry and rainy season (P < 0.05). The process of the direct shear friction tests of root-soil and soil-soil interface could be simulated by the finite element model established in the study. The simulation results were consistent with the laboratory tests. The results of this research can serve as a basis for the further studies on the friction characteristics of root-soil interface and root reinforcement. This study also can provide a reference for the selection of windbreak and sand-fixation tree species. [ABSTRACT FROM AUTHOR]

The Yellow River Basin (YRB) is an important region for both ecological and economic growth in China, and a healthy and stable ecological environment is fundamental to achieving high-quality development. Investigating the spatio-temporal changs of vegetation carbon sinks in the YRB is necessary to improve understanding of the regional carbon cycle. This study analyzed the YRB using the Google Earth Engine (GEE) platform. The basin was divided into sections based on ecological function regionalization schemes. We applied Theil-Sen median trend analysis and Spearman correlation analysis, along with the lasso regression model, to examine the NPP's temporal and spatial patterns and its driving factors in these regions. The objective was to examine the NPP' s temporal and spatial patterns and its driving factors in these regions, identifying commonalities within regions and differences between regions, while analyzing in a targeted manner the lack of vegetation carbon sequestration in ecological areas. This study highlighted the importance of ecological engineering in ecological civilization and carbon neutrality, providing scientific theoretical support and a decision-making basis for the realization of carbon neutrality. The research results indicate significant upward trends (p <0.001) in both the average annual NPP (AANPP) growth rate (57.98%) and the total annual NPP (TANPP) growth rate (41.9%) in the YRB from 2001 to 2020. The multi-year average NPP (MYANPP) exhibit a spatial distribution with high values in the south and low values in the north. The spatial distribution of NPP exhibits notable spatial aggregation and heterogeneity, where regions demonstrating improvement trends constitute 93.01% of the area, and approximately 79.73% of this region displays low volatility. A high-value area is concentrated in the Agroecological Zone of the Fenwei Basin (I-11), the Loess Plateau Agriculture and Grassland Ecological Zone (I-12), and the Qinba Mountains Deciduous and Evergreen Broad-leaved Forest Ecological Zone (I-15), with significant spatial differences. Temperature, precipitation, and forests are the foremost elements affecting NPP in the YRB, exhibiting positive correlations. Conversely, farmland displays negative correlations. Overall, NPP shows an increasing trend in all ecological function areas, and the NPP of the YRB shows a trend of improvement as a whole, as the ecological environment of the YRB has improved significantly. • Significant differences in NPP were observed among ecological functional zones. • The vegetation condition of the Yellow River Basin has been improved continuously. • Population, temperature, precipitation, forest were the main driving factor of NPP. • The average annual NPP and the total annual NPP increased significantly. [ABSTRACT FROM AUTHOR]