Comparisons of soil suction induced by evapotranspiration and transpiration of S. heptaphylla

et al. 2016

Ecological Engineering

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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.

Section: Test Proceduresmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Garg

et al. 2016

Ecological Engineering

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“…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).…”

Section: Introductionmentioning

confidence: 99%

“…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).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

Can. Geotech. J.

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Investigation of plant growth and transpiration-induced matric suction under mixed grass-tree conditions Ni, J. J.; Leung, Anthony; Ng, C. W. W.; So, P. S. General rightsCopyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain.• You may freely distribute the URL identifying the publication in the public portal. Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. AbstractAlthough evapotranspiration-induced matric suction for single species has been widely studied, little is known about how mixed-species planting would affect the plant growth and induced matric suction. This study aims to explore the effects of grass-tree interaction on their growth and induced matric suction during evapotranspiration (ET) and rainfalls. Field monitoring was carried out to measure matric suction responses in compacted soil that was vegetated with (i) single tree species, Schefflera heptaphylla and (ii) mixed species of the trees and a grass species, Cynodon dactylon. In each condition, three tree spacings (120, 180 and 240 mm) were planted. When tree spacing increased from 120 to 240 mm, the peak tree root area index (RAI, for fine roots with diameter < 2 mm) decreased by 16%, but the peak grass RAI increased by 29%. At mixed planting plots, the ET-induced peak matric suction for tree spacing of 240 mm was 20% higher than that for spacing of 180 mm because of increased contribution of grass root-water uptake as the trees were more widely spaced. Without grass, a reverse trend was observed as tree-tree interaction reduced at wide spacings. The peak ET-induced matric suction had a significant linear correlation with RAI. During rainfalls, the highest matric suction was preserved for the case of 240 mm spacing due to the greatest reduction of soil hydraulic conductivity by roots.

“…Although studies have been conducted to quantify evapotranspiration-induced suction by a single plant (Fatahi et al, 2010;Garg et al, 2015a;Leung et al, 2015b), seldom have the effects of planting density on suction responses in the vadose zone been investigated. Different plant spacings would result in different degrees of overlap among root systems, and hence different influence zones of induced suction.…”

Section: Introductionmentioning

confidence: 99%

Effects of planting density on tree growth and induced soil suction

et al. 2016

Géotechnique

114

Plant evapotranspiration is recognised to affect soil suction of slopes and landfill covers. Previous work has focused on evapotranspiration-induced suction by a single plant, with little attention paid to the effects of planting density. The aim of this study is to quantify any changes in tree growth and tree-induced suction during evapotranspiration and rainfall under different planting densities for non-mixed-species plantations. A tree species, Schefflera heptaphylla, which is commonly found in Asia, was planted in silty sand at spacings of 60, 120 and 180 mm, representing three different planting densities. For each case, three replicates were tested to consider tree variability. In total, the responses of suction for 297 seedlings subjected to drying and a rainfall event with a 10-year return period were measured. The test results show that reducing the tree spacing from 180 to 60 mm induced greater tree-tree competition for water, as indicated by a 364% increase in peak suction upon evapotranspiration. Such tree-tree interaction led to: (a) a 19-35% reduction in the leaf area index; (b) a 17-36% decrease in root length; and (c) an obvious decay of roots. Upon the rainfall event, the infiltration rate for vegetated soil with trees planted at a spacing of 60 mm was up to 247% higher than those for soil with a wider tree spacing, where mainly fresh roots were found. Although most suction within the root zone (i.e. top 100 mm) was lost due to increased infiltration at 60 mm spacing, suctions in deeper depths below root zone were largely preserved.