Mechanical Pull-Out Capacity and Root Reinforcement of Four Native Tree and Shrub Species on Ecological Rehabilitation of Roadside Slopes in Hong Kong

Ecological rehabilitation using woody plants on roadside slopes has been gaining momentum in promoting urban biodiversity since 21 st century. Many native tree and shrub species have been used for planting on roadside slopes. Yet, there is limited information in the local forestry and engineering literature on the mechanical properties and their relationship with above-ground characteristics, and the establishment method of native woody plants for shallow slope stabilisation. In this study, field pull-out tests were conducted to investigate the anchorage ability of two shrub (Rhodomyrtus tomentosa and Melastoma sanguineum) and two trees (Schefflera heptaphylla and Reevesia thyrsoidea) species. They have been commonly used in roadside slope rehabilitation in recent years. Samples were taken from planted and wild individuals. The plant pull-out resistance quantifies the degree of root reinforcement. The results showed that around 1 to 6 kN was required to uproot a 2 m plant. Trees exhibited better pull-out performance than shrubs. The planted trees had significantly stronger anchorage than natural ones. All the potential native plant species for slope rehabilitation should be screened by field pullout test, to ensure their achievement of both urban biodiversity enhancement and slope stabilisation purposes.