Trees along a highway: Species composition, risk rating, and tree performance

Species composition is an important aspect of urban forestry. Some urban tree stands may be monospecific. It is important to identify factors explaining and predicting the occurrence of monospecific stands in previous less-explored space, such as transport land use. A large-scale tree survey was conducted, covering 53 slopes and 52 verges along San Tin Highway in Hong Kong. The aim of this study was to examine the occurrence of monospecific stands in relation to the biodiversity and the structure of urban forests adjacent to transport routes. 7,209 trees were found in the surveyed sites. However, the tree stock was dominated by Corymbia citriodora (72%) which was featured in monospecific tree stands, especially for the highway verges. Using chi-square tests, significant associations were found between monospecific stands, habitat type, and tree risk rating. A logistic regression model was constructed to predict the occurrence of monoculture. Every metre increase in maximum tree height, the odds of a stand being monospecific would be 1.22 times greater. Stands on verges had 5.26 times greater odds of being monospecific against slope. The associations and relationships were attributed to the dominance of C. citriodora . By boosting the logistic model, model reliability increased as kappa rose from 0.51 to 0.63, while balanced accuracy improved from 0.72 to 0.85. The occurrence of monospecific stands could be reliably predicted using maximum tree height and habitat type of tree stands. The findings could assist urban forest management in terms of species selection and biodiversity enhancement.