The effects of vegetation traits and their stability functions in bio-engineered slopes: A perspective review

Bio-engineered slopes use vegetation as “live” protection elements against the triggering forces of landslides, erosions and debris flows. In this paper, the effects of basic plant traits (both root and shoot) on their slope stability functions in compacted and planned green slopes have been reviewed. This review is based on over 200 papers on areas primarily related to unsaturated soil mechanics, plant biology, hydrology and ecology. The first section of this paper gives an overview of the plant-soil–water continuum, highlighting the key interactions that determine reinforcing actions of vegetation in sloped soil. The hydraulic pathway (water flow from roots to atmosphere) and plant response at varying soil stress (suction) is explained. Thereafter, the effects of inherent plant properties (bio-polymer composition, plant physiological parameters and root morphology) on soil reinforcement are discussed. The second section discusses the three stability functions (i.e., mechanical, hydrological and interception reinforcements) of vegetation in slopes. The first two subsections considering mechanical and hydrological reinforcement action are catalogued. In the third subsection, the interception functions of plant (both shoot and root parameters) against erosion and debris flow are illustrated based on experimental, numerical and field observations. In the last section, the knowledge gaps on the topic of bio-engineered slope stability are highlighted such as the consideration of plant age effect, plantation strategy and high suction response of vegetation. The possible negative effects of vegetation towards slope stability resulting from extreme climate conditions such as extended droughts, forest fires, freeze–thaw cycles and elevated CO2 concentration levels are discussed. The review identifies new research themes for developing futuristic bio-engineered slopes. The potential of real time monitoring and maintenance of bio-engineered slopes, redefinition of wilting point and concept of “plant sensors” are also put forth.