Tree composition and diversity in relation to urban park history in Hong Kong, China

In densely populated cities that are dominated by concrete buildings, urban parks serve as major green infrastructures for ecological and environmental functions. Trees are one of the important components that support these green infrastructures. Despite plenty of urban parks established in Hong Kong in the last 20 years, knowledge of tree composition and diversity is outdated. There were also no studies that investigated the differences in tree diversity in relation to park history. Therefore, this study aims to identify the temporary changes in tree composition and diversity in Hong Kong, by conducting a plot-based tree survey in 32 urban parks of different ages. Overall, 2801 trees belonging to 181 species were recorded in 319 plots across all the parks. A ridit analysis was conducted and it indicated the mature size of trees were not significantly larger in old parks. However, the linear mixed-effects models and the post-hoc tests suggested that DBH, tree height and the proportion of crown dieback for each class of tree size were greater in the old parks. Moreover, the composition of top-ranking dominant tree species varied substantially, where more ornamental and exotic trees were adopted in new parks. For species richness, the sample-based species accumulation curves of different park age overlapped when the horizontal axis of the curve was scaled by the average number of combined individual trees. When the horizontal axis was scaled by the number of plots, the curve for the old parks was above the curve for the new parks. The differences derived from these two accumulation methods indicated a higher tree density in old parks. For species evenness, both the rank-abundance curves and Rényi diversity curves depicted a similar low species evenness in old and new parks. These results suggested that species diversity remained largely unchanged from old parks to new parks though the dominant tree species varied. Greater attention should be paid to increase the species evenness in all urban parks, increase tree density in new parks and improve tree maintenance in old parks.