HALM Research Team's San Tin Highway Tree Management Research Project
- San Tin Highway connects San Tin to Yuen Long. It is one of the important expressways in Hong Kong and provides essential transportation links for the western part of the New Territories. In order to provide quick establishment of vegetation cover along the highway, the Highways Department (HD) planted about 9,000 Eucalyptus trees as pioneer trees along the slopes and verges of the highway when it was constructed about 30 years ago.
- The stressful growing environment and the significant height of Eucalyptus trees together affect the tree stability, posing a safety hazard to the road users.
- Therefore, the HD commissioned the teachers and students of the Bachelor of Science (Honours) in Horticulture, Arboriculture and Landscape Management (HALM) of the Technological and Higher Education Institute of Hong Kong (THEi) to conduct a research to enhance the tree management system of San Tin Highway, aiming to proactively enhance road safety and reduce the probability of tree accidents.
- Dr. Allen ZHANG Hao, Associate Professor of the Faculty of Design and Environment, THEi, pointed out that Eucalyptus trees are tall trees that can grow to more than 20 meters tall and are allelopathic. The low suitability of their seeds and fruits as food resources and habitats for local birds and insects limits their contribution to biodiversity. To improve biodiversity and increase the wind resistance of urban trees to mitigate the tree risk in the typhoon season, the research team has set up different thinning experiments along the San Tin Highway in the past two years and planted a variety of native saplings and shrubs on the slopes. Ms. Kyle LAU, graduate of HALM who participated in the research, said that the team went to the highway every month to monitor the growth condition of the saplings. Different tree parameters like the height, crown spread, diameter at breast height were measured to evaluate the health condition and calculate the growth rate. The survival rate of the different species were also compared.
- The research team observed that native species like Ilex rotunda var. microcarpa, Machilus chinensis, Schefflera heptaphylla, and Sterculia lanceolate can better adapt to the environment of San Tin Highway. They all demonstrated a better survival rate, health status and growth rate. The findings also revealed Eucalyptus trees had a high failure risk under severe weather conditions when the crown spread exceeded 3 meters, diameter at breast height exceeded 24 cm, and tree height exceeded 12 meters.
- The thinning experiments showed that 60% thinning intensity was more conducive to the growth of the understory native trees on the slopes. The team is now gearing up for the second phase of the study, which will involve the continuous observation of the planted trees and collection of more long-term and comprehensive data to support the finding. The team also put forward a suggestion of improving the highway vegetation management system. It proposed that the slopes and verges of different sections of the highway should be categorised into high, medium and low priority levels according to the quantitative tree risk assessment, which are based on factors such as trees risks and potential impacts. The categorisation can enhance the management efficiency through identifying the priority areas for resource and manpower allocation. The results of the study and the proposed mid- and long-term management plans have been submitted to the HD. The research team hoped that the findings and proposal can help strengthen the effectiveness of urban tree management.