Relationships between leaf and root area indices and soil suction induced during drying–wetting cycles

Abstract

The stability and serviceability of geotechnical infrastructure may be affected by plantinduced soil suction during drying-wetting cycles, because an increase in suction would reduce hydraulic conductivity and also increase shear strength. Recent studies have been conducted to quantify suction induced during evapotranspiration (ET) and ponding in soil vegetated with non-crop species that are used for the ecological restoration of geotechnical infrastructure. However, induced suction distribution under drying-wetting cycles has not been investigated. The objectives of this study are to (1) quantify suction induced by a noncrop tree species, Schefflera heptaphylla, under ponding-drying-ponding cycles and (2) correlate intercepted radiant energy, tree leaf area index (LAI), extinction coefficient (k) and root area index (RAI) with suction. In total, 18 vegetated soil samples with LAI ranging from 0.9 to 3.1 and three bare soil samples (control) were tested and subjected to identical cycles of ET and ponding. Energy balance calculation was performed to determine the percentage of interception of radiant energy. An almost linear relationship can be seen between the percentage of energy intercepted (from 7% to 42% ± 4%) and LAI (from 0.9 to 3.1 ± 0.09) for S. heptaphylla. The measured value of k for S. heptaphylla (0.13) was found to be much lower than that of some crop species (0.4-1.6) reported in the literature. Peak suction is always identified at the depth, where RAI is maximum. It was further demonstrated that the tree-induced suction has a strong linear correlation with both the RAI and LAI.

Investigation of plant growth and transpiration-induced matric suction under mixed grass–tree conditions

“…No correlation can be found in the present study, regardless of the presence of grasses, as well as the study carried out by Ng et al (2016b). This appeared to be contradictory with the results reported by Ng et al (2016a), who showed that for single species condition, there was a linear correlation between suction preserved and RAI. In an attempt to explain such contradiction, the suction preserved was related with ks in Fig.…”

“…This suggests that upon wetting, the effects of preferential water flow created by the decayed roots due to tree-tree or tree-grass interaction affected the suction preserved much more prominently than the effects of RAI. Since there was no plant-plant interaction for the single species case tested by Ng et al (2016a), no significant preferential flow took place and this explains why RAI in their case was the predominant factor governing the suction preserved.…”

“…At present, it is not well understood whether the same conclusions can be drawn under mixed tree-grass conditions. It is also not clear whether tree spacing would affect the grass and tree traits, some of 5 which are known to have direct influences on plant root-water uptake and induced matric suction (López et al 2001;Garg et al 2015b;Ng et al 2016a, b).…”

“…Grass that has relatively shallow root system may be effective for soil erosion control, while tree has relatively stronger and deeper root system which may be used for mechanical soil stabilisation. Despite of rather different root systems, transpiration would happen for both types of species, causing a change in matric suction (Pollen-bankhead and Simon 2010; Garg et al 2015b; Leung et al 2015b;Ng et al 2013Ng et al , 2014Ng et al , 2016a and consequently the soil hydrology (Simon and Collison 2002;Smethurst et al 2015;Ng et al 2016c). The increase in matric suction due to plant transpiration would result in a decrease in soil hydraulic conductivity (Ng and Leung 2012) and also an increase in soil shear strength (Ng and Menzies 2007).…”

Correlating hydrologic reinforcement of vegetated soil with plant traits during establishment of woody perennials

“…Extensive field and laboratory studies have shown that transpiration-induced suction could be maintained in the soil during and after rainfall Ng et al 2014;Rahardjo et al 2014;Leung et al 2015a;Ng et al in press). Recent research also argues that the presence of roots could affect the soil water retention properties and hence the suction responses (Bengough 2012;Carminati and Vetterlein 2013;Scholl et al 2014;Leung et al 2015b;Ng et al 2016aNg et al , 2016b. The ability of plants to preserve/maintain suction has important implications for slope stability.…”