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Influence of plant root system morphology and architectural traits on soil shear resistance

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Abstract

Background and aims

Vegetation can be used to stabilise slopes with regard to shallow landslides, but the optimal plant architecture for conferring resistance is not known. This study aims at identifying root morphological traits which confer the most resistance to soil during shearing.

Methods

Three species used for slope stabilisation (Ricinus communis L., Jatropha curcas L. and Rhus chinensis Mill.) were grown for 10 months in large shear boxes filled with silty clay similar to that found in Yunnan, China. Direct shear tests were then performed and compared to fallow soil. Root systems were excavated and a large number of traits measured.

Results

Shear strength and deformation energy were enhanced by the presence of roots. Regardless of confining pressure, R. communis conferred most resistance due to its taprooted system with many vertical roots. J. curcas possessed oblique and vertical roots which created fragile zones throughout the soil profile. The least efficient root system was R. chinensis which possessed many horizontal lateral roots. Soil mechanical properties were most influenced by (i) density of roots crossing the shear plane, (ii) branching density throughout the soil profile, (iii) total length of coarse roots above the shear plane and (iv) total volume of coarse roots and fine root density below the shear plane. During failure, fine, short and branched roots slipped through soil rather than breaking.

Conclusion

Root morphological traits such as density, branching, length, volume, inclination and orientation influence significantly soil mechanical properties.

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Abbreviations

A:

Aerial parts of the plants

BD:

Root branching density (Nb of branches mm−1)

bw:

Backward from the shear direction (i.e. opposite to the direction of shear)

cR:

Coarse roots

CSA:

Cross sectional area (mm2)

D:

Root density (g mm−3)

d:

Dry

diam:

Root diameter (mm)

DL:

Dead leaves

GL:

Green leaves

F:

Fruits

fR:

Fine roots

fwd:

Forward to the shear direction (i.e. in the direction of shear)

GL:

Green leaves

inf:

Below the shear plane

L:

Root length (mm)

m:

Mass (g)

MC:

Moisture content (%)

Mdiam:

Root mean diameter (mm)

Nb:

Number

perp:

Perpendicular to the shearing direction

2 :

Partial η-squared statistical indicator

shp:

Shear plane

S:

Stem

sup:

Above the shear plane

V:

Root volume (mm3)

v:

Soil volume (mm3)

w:

Wet

[0–30°[:

Root inclination from 0° (horizontal) inclusive to 30° exclusive

[30–60°[:

Root inclination from 30° inclusive to 60° exclusive

[60–90°]:

Root inclination from 60° inclusive to 90° (vertical) inclusive

c′:

Effective cohesion of the material (kPa)

γ :

Tangential displacement (mm)

J :

Total energy per unit shear area and unit displacement (J m−2 m−1 = kPa)

ε :

Tangential strain (%)

ε yield :

Tangential strain at yield point (%)

T :

Tangential force (N)

Φ′:

Effective internal friction angle of the material (°)

G :

Shear modulus (kPa)

ρ d :

Soil dry bulk density (g cm−3)

ρ w :

Soil wet bulk density (g cm−3)

σ n :

Normal stress at yield point (kPa)

τ :

Tangential shear stress (kPa)

τ res :

Residual tangential stress (kPa)

τ yield :

Tangential shear stress at yield point (kPa)

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Acknowledgments

We thank François Pailler and Jérôme Nespoulous (INRA, UMR AMAP), Laurent Peyras, Nadia Benhamed, Faustine Byron, Yves Grémeaux, Guillaume Nunes (IRSTEA), for help with measurements and shear tests. We are grateful to Patrice Brahic and his team at the Aix de Milles nursery, for looking after our plants and John Dickie and Kenwin Liu from the Seed Conservation Department, Kew Royal Botanic Garden, U.K. for advice on seed scarification. Funding was provided from INRA (Jeune Equipe), CNRS (Ecopente project) and Ingecotech (CATAR project). AMAP (Botany and Computational Plant Architecture) is a joint research unit which associates CIRAD (UMR51), CNRS (UMR5120), INRA (UMR931), IRD (2 M123), and Montpellier 2 University (UM27).

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Correspondence to Murielle Ghestem.

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Ghestem, M., Veylon, G., Bernard, A. et al. Influence of plant root system morphology and architectural traits on soil shear resistance. Plant Soil 377, 43–61 (2014). https://doi.org/10.1007/s11104-012-1572-1

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