Vegetation is recognised to not only affect the soil suction of slopes, but also soil hydraulic properties, including soil water retention curves (SWRC) and soil permeability. This study aims to quantify and model effects of plant competition on (i) root characteristics, (ii) induced soil suction and (iii) changes in SWRC and soil permeability considering effects of planting spacing and mixed plant types.

A series of laboratory, field experiments and a theoretical modelling study were carried out. For the single plant type, a tree species, Schefflera heptaphylla, was planted at spacings of 60, 120 and 180 mm. For the mixed plant types, field monitoring was carried out in compacted soil that was vegetated with mixed species of the trees (with spacings of 120, 180 and 240 mm) and...[ Read more ]

Vegetation is recognised to not only affect the soil suction of slopes, but also soil hydraulic properties, including soil water retention curves (SWRC) and soil permeability. This study aims to quantify and model effects of plant competition on (i) root characteristics, (ii) induced soil suction and (iii) changes in SWRC and soil permeability considering effects of planting spacing and mixed plant types.

A series of laboratory, field experiments and a theoretical modelling study were carried out. For the single plant type, a tree species, Schefflera heptaphylla, was planted at spacings of 60, 120 and 180 mm. For the mixed plant types, field monitoring was carried out in compacted soil that was vegetated with mixed species of the trees (with spacings of 120, 180 and 240 mm) and a grass species, Cynodon dactylon. For the theoretical modelling, a new and simple SWRC model with only one root parameter, namely root volume ratio, for the root-permeated soil was proposed. Then effects of planting spacing on plant growth were modelled considering the principle of hydrotropism and plant competition. Moreover, effects of plant competition induced changes in soil hydraulic properties on slope stability were studied parametrically.

The test results showed that for the single plant type, 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 ET. Such tree-tree competition led to an obvious decay of roots. Upon the rainfall event, although most suction within the root zone was lost due to increased infiltration at 60 mm spacing, suctions in deeper depths below root zone were largely preserved. However, for the mixed plant types, the ET-induced peak suction within the root zone 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. During rainfalls, the highest suction was preserved for the case of 240 mm spacing due to the greatest reduction of soil permeability by roots.

The new and simple SWRC model showed its capability of predicting SWRC of silty sand vegetated with a tree species, S. heptaphylla, reasonably well. It had key abilities to capture a substantial increase in soil air-entry value (AEV) due to the presence of roots. Plant competition model can capture planting spacing effects on root characteristics during growth quite well. Size of root system decreased while root density increased at smaller planting spacing. The model can also simulate the induced soil suction during both evapotranspiration and rainfall events. There existed a threshold planting spacing, beyond which effects of planting spacing on root characteristics and induced soil suction became negligible. The parametric study showed that plant competition induced changes in soil permeability was the most predominant hydrological effect on slope stability.